Four-coordinate titanium alkylidene complexes: Synthesis, reactivity, and kinetic studies involving the terminal neopentylidene functionality

1886

Organometallics 2005, 24, 1886-1906

Four-Coordinate Titanium Alkylidene Complexes:

Synthesis, Reactivity, and Kinetic Studies Involving the

Terminal Neopentylidene Functionality

Falguni Basuli, Brad C. Bailey, Lori A. Watson, John Tomaszewski,

John C. Huffman, and Daniel J. Mindiola*

Department of Chemistry and Molecular Structure Center, Indiana University,

Bloomington, Indiana 47405

Received August 3, 2004

A series of titanium complexes containing a terminal neopentylidene functionality have

been prepared by a one electron oxidatively induced R-hydrogen abstraction from the

t

corresponding bis-neopentyl precursor (Nacnac)Ti(CH₂Bu)₂(Nacnac^-) [Ar]NC(CH₃)CHC-

(CH₃)N[Ar], Ar ) 2,6-(CHMe₂)₂C₆H₃), among them (Nacnac)TidCH^tBu(OTf) and (Nacnac)-

TidCH^tBu(I). It was determined that bulky alkyl groups bound to titanium as well as a

bulky coordinating anion from the oxidant are needed to promote R-hydrogen abstraction.

Complex (Nacnac)TidCH^tBu(OTf) serves as a template for other four-coordinate titanium

neopentylidene complexes such as (Nacnac)TidCH^tBu(X) (X^-) Cl, Br, and BH₄). Complexes

(Nacnac)TidCH^tBu(X) undergo cross-metathesis reactivity with the imine functionality of

the Nacnac^-ligand forming the imido complexes (H^tBuCdC(Me)CHC(Me)N[Ar])TidNAr-

(X) (X^-) OTf, Cl, Br, I, BH₄). In addition, C-H activation of two tertiary carbons also takes

place to afford the titanacycles Ti[2,6-(CMe₂)(CHMe₂)C₆H₃]NC(Me)CHC(Me)N[2,6-(CMe₂)-

(CHMe₂)C₆H₃](X) (X^-) OTf, Cl, Br and η²-BH₄). Kinetic studies in C₆D₆reveal the formation

of (H^tBuCdC(Me)CHC(Me)N[Ar])TidNAr(I) from (Nacnac)TidCH^tBu(I) to be independent

of solvent (C₆D₆, Et₂O-d₁₀, THF-d₈) and the reaction to be first order in titanium (k ) 8.06

× 10^-4s^-1at 57 °C, with activation parameters ∆H^q) 21.3(2) kcal/mol, ∆S^q) -8(3)

cal/mol K). Compound (Nacnac)TidCH^tBu(OTf) reacts with various substrates to afford

products in which the alkylidene functionality has been significantly transformed. When

-

the alkylidene derivatives (Nacnac_tBu)TidCH^tBu(X) (X^-) OTf, I; Nacnac_tBu) [Ar]NC-

(^tBu)CHC(^tBu)N[Ar]) were prepared, the intramolecular cross-metathesis transformation

observed with (Nacnac)TidCH^tBu(X) was inhibited completely.

Introduction

lidenes is often restricted to this methodology, and

access to low-coordinate and redox-active systems pos-

sessing this functionality can be difficult or limited since

formation of the metal carbon double bond stems

typically from a high-oxidation-state (or alternatively

a singlet ground state) and often coordinatively satu-

rated precursors. For this reason an attractive entry to

the assembly of metal-ligand multiple bonds could

derive from a redox reaction. Only a handful of examples

of a one-electron oxidation leading to MdC bond forma-

tion have been reported.⁶

High-oxidation-state transition-metal alkylidenes play

an important role in industrial processes such as cross-

metathesis, ring-closing metathesis, ring-opening me-

tathesis, ring-opening metathesis polymerization, acy-

clic diene metathesis polymerization, acetylene polymeri-

zation, and Wittig-type or group-transfer reactions. The

increasing importance and need for metal alkylidene

complexes has been manifested in several reviews¹and

highlights.²To prepare high-oxidation-state metal alky-

lidenes, one must promote R-abstraction or R-deproto-

nation reactions from metal alkyl complexes lacking

â-hydogens.^1,3Promoting R-hydrogen abstraction to

yield high-oxidation-state metal alkylidenes is typically

induced thermally, photochemically, or with the aid of

Lewis bases.^1,3-5Hence the entry into d⁰-metal alky-

(3) (a) Schrock, R. R. J. Organomet. Chem. 1986, 300, 249-262. (b)

Schrock, R. R.; Fellmann, J. D. J. Am. Chem. Soc. 1978, 100, 3359-

3370. (c) Schrock, R. R.; Sharp, P. R. J. Am. Chem. Soc. 1978, 100,

2389-2399. (d) Rupprecht, G. A.; Messerle, L. W.; Fellmann, J. D.;

Schrock, R. R. J. Am. Chem. Soc. 1980, 102, 6236-6244. (e) Clark, D.

N.; Schrock, R. R. J. Am. Chem. Soc. 1978, 100, 6774-6776. (f)

Fellmann, J. D.; Ruprecht, G. A.; Wood, C. D.; Schrock, R. R. J. Am.

Chem. Soc. 1978, 100, 5964-5966.

(4) Beckhaus, R. Angew. Chem., Int. Ed. Engl. 1997, 36, 687-713.

(5) Baumann, R.; Stumpf, R.; Davis, W. M.; Liang, L.-C.; Schrock,

R. R. J. Am. Chem. Soc. 1999, 121, 7822-7836.

(6) For examples concerning oxidatively induced R-abstraction reac-

tions to prepare d⁰-complex containing metal carbon multiple bonds

see: (a) Duncalf, D. J.; Harrison, R. J.; McCamley, A.; Royan, B. W.

Chem. Commun. 1995, 2421-2422. (b) Artin˜olo, A.; Otero, A.; Fajardo,

M.; Garc´ıa-Yebra, C.; Gil-Sanz, R.; Lo´pez-Mardomingo, C.; Mart´ın, A.;

Go´mez-Sal, P. Organometallics 1994, 13, 4679-4682. (c) Jernakoff, P.;

Cooper, N. J. Organometallics 1986, 5, 747-751.

* To whom correspondence should be addressed. E-mail: mindiola@

indiana.edu.

(1) (a) Schrock, R. R. Acc. Chem. Res. 1979, 12, 98-104. (b) Schrock,

R. R. Acc. Chem. Res. 1990, 23, 158-165. (c) Schrock, R. R. Chem.

Rev. 2002, 102, 145-179. (d) Schrock, R. R. In Reactions of Coordinated

Ligands; Braterman, P. R., Ed.; Plenum: New York, 1986. (e) Feldman,

J.; Schrock, R. R. Prog. Inorg. Chem. 1991, 39, 1-74.

(2) (a) Rouhi, A. M. Chem. Eng. News 2002, 80, 29-38. (b) Fu¨rstner,

A. Adv. Synth. Catal. 2002, 344, 567. (c) Schrock, R. R. Adv. Synth.

Catal. 2002, 344, 571-572. (d) Grubbs, R H. Adv. Synth. Catal. 2002,

344, 569. (e) Herndon, J. W. Coord. Chem. Rev. 2004, 248, 3-79.

Publication on Web 03/17/2005

Four-Coordinate Titanium Alkylidene Complexes

Organometallics, Vol. 24, No. 8, 2005 1887

Despite rapid growth of research in this area, far less

attention has been applied to the synthesis and reactiv-

ity of terminal alkylidene complexes of titanium. Tita-

nium alkylidenes generated in situ are utilized com-

monly as stoichiometric reagents in organic synthesis.⁷

Their reactivity is typically manifested by the oxophi-

licity of the metal, thus leading to cross-metathesis

transformations (also referred to as Wittig-like). The

TidC bond is expected to be highly nucleophilic or

“Schrock-like”, as the ligand is bound to a very electro-

positive metal. Hence, complexes of this type are often

alkylidene complexes with high catalytic activity (e.g.,

group 5 and 6 transition metals) are four-coordinate.

In fact, even the late transition-metal complexes such

as the ruthenium-based carbene catalyst precursors

studied by Grubbs and co-workers have been described

as “active” when the system dissociates a labile ligand

to become four-coordinate.¹²The latter characteristic is

especially important since organotransition metal com-

plexes with low coordination numbers are inherently

reactive and consequently provide useful templates to

study processes such as small molecule activation and

catalysis.

In contrast to Schrock’s work on R-hydrogen elimina-

tion reactions to prepare TadC linkages, which involve

one-electron reduction of the corresponding alkyl halide

(referred to as R-H elimination, 1,2-H migration),¹³we

report the opposite, an oxidation reaction resulting in

R-hydrogen abstraction concomitant with formation of

a low-coordinate complex having a metal carbon double

bond.¹⁴Of particular interest was the seminal work

reported by Budzelaar^15aand Theopold^15fon redox-

active and stable early-transition-metal d^x(x ) 1-3)

complexes having metal alkyl ligands.

represented as M⁺-CR₂≈ MdCR₂much like ylides

-

would resonate.⁸Prototypical among nucleophilic alky-

lidenes are those that can readily react with Lewis acids

to make zwitterionic-like complexes.⁷ⁿUsing such a

characteristic as a basis, group 4 alkylidenes are

expected to be more reactive yet less tolerant to organic

group functionalities. However, a clear contradiction to

this dichotomy is Tebbe’s reagent.⁹Such a system has

been reported to tolerate a high degree of organic group

functionality, disfavor epimerization of optically active

substrates, and yet possess a very reactive alkylidene

functionality.⁷

Unlike groups 5 and 6, isolable and terminal alky-

lidene complexes of the group 4 metals remain very

elusive.¹⁰This fact likely arises from the lack of suitable

synthetic entries to the reactive alkylidene functionality.

Only a handful of group 4 complexes having a terminal

alkylidene are known, and the vast majority are tita-

nium based with coordination numbers g 5.^1c,4,5,11

Therefore one would argue that the quest for reactive

group 4 alkylidenes is most desirable for systems having

low coordination numbers since reported d⁰metal

Herein we describe a synthetic strategy leading to a

family of four-coordinate, terminal titanium(IV) alky-

lidenes and subsequent transformations of the corre-

sponding terminal TidCH^tBu functionality. The choice

of oxidant, alkyl group on the metal, and supporting

chelate ligand all have a profound effect in the forma-

tion, kinetic stability, and reactivity of the alkylidene

group.

Results and Discussion

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Looney, M. G. Tetrahedron Lett. 1992, 33, 671-674. (g) Petasis, N. A.;

Bzowej, E. I. J. Am. Chem. Soc. 1990, 112, 6392-6394. (h) DeShong,

P.; Rybczynski, P. J. J. Org. Chem. 1991, 56, 3207-3210. (i) Petasis,

N. A.; Bzowej, E. I. J. Org. Chem. 1992, 57, 1327-1330. (j) Petasis, N.

A.; Akritopoulou, I. Synlett 1992, 665-667. (k) Kauffmann, T.; Ennen,

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5400. (n) Crabtree, R. H. The Organometallic Chemistry of the

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tion Metals in the Synthesis of Complex Organic Molecules; University

Science Books: Mill Valley, CA, 1994.

Synthesis of Terminal and Four-Coordinate

Titanium Neopentylidene Complexes. Our ap-

proach for preparing low-coordinate and terminal tita-

nium alkylidene complexes involves an adaptation for

the synthesis of the precursor (Nacnac)TiCl₂(Nacnac^-

) [Ar]NC(Me)CHC(Me)N[Ar], Ar ) 2,6-(CHMe₂)₂C₆H₃)

complex reported previously by Budzelaar and co-

workers.^15aFollowing Budzelaar’s procedure we recrys-

tallized the THF base adduct (Nacnac)TiCl₂(THF) (1)

from toluene in 70% yield as dark green blocks.^14,15a

Isolation of 1 avoids lower yields as well as additional

steps in forming the THF-free complex (Nacnac)TiCl₂.

(11) (a) van Doorn, J. A.; van der Heijden, H.; Orpen, A. G.

Organometallics 1995, 14, 1278-1283. (b) Kahlert S.; Gorls, H.; Scholz,

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van der Heijden, H.; Orpen, A. G. Organometallics 1994, 13, 4271-

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Teuben, J. H. Organometallics 1997, 16, 4245-4247.

(12) Trnka, T. M.; Grubbs, R. H. Acc. Chem. Res. 2001, 34, 18-29.

(13) Fellmann, J. D.; Tuner, H. W.; Schrock, R. R. J. Am. Chem.

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Mindiola, D. J. J. Am. Chem. Soc. 2003, 125, 6052-6053.

(15) (a) Budzelaar, P. H. M.; van Oort, A. B.; Orpen, A. G. Eur. J.

Inorg. Chem. 1998, 1485-1494. (b) Kim, W. K.; Fevola, M. J.; Liable-

Sands, L. M.; Rheingold, A. L.; Theopold, K. H. Organometallics 1998,

17, 4541-4543. (c) Minhas, R. K.; Scoles, L.; Gambarotta, S. Organo-

metallics 1996, 15, 1113-1121. (d) Wills, A. R.; Edwards, P. G.;

Harman, M.; Hursthouse, M. B. Polyhedron 1989, 8, 1457-1459. (e)

Wills, A. R.; Edwards, P. G. J. Chem. Soc., Dalton Trans. 1989, 1253-

1257. (f) MacAdams, L. A.; Kim, W. K.; Liable-Sands, L. M.; Guzei, I.

A.; Rheingold, A. L.; Theopold, K. H. Organometallics 2002, 21, 952-

960. (g) Gibson, V. C.; Newton, C.; Redshaw, C.; Solan, G. A.; White,

A. J. P.; Williams, D. J. Eur. J. Inorg. Chem. 2001, 7, 1895-1903. (h)

Scoles, L.; Minhas, R.; Duchateau, R.; Jubb, J.; Gambarotta, S.

Organometallics 1994, 13, 4978-4983.

(8) Pine, S. H.; Shen, G. S.; Hoang, H. Synthesis 1991, 165-167.

(9) Tebbe, F. N.; Parshall, G. W.; Reddy, G. S. J. Am. Chem. Soc.

1978, 100, 3611-3613.

(10) (a) Fryzuk, M. D.; Mao, S. S. H.; Zaworotko, M. J.; MacGillivray,

L. R. J. Am. Chem. Soc. 1993, 115, 5336-5337. (b) Fryzuk, M. D.;

Duval, P. B.; Mao, S. S. H.; Zaworotko, M. J.; MacGillivray, L. R. J.

Am. Chem. Soc. 1999, 121, 2478-2487. (c) Fryzuk, M. D.; Duval, P.

B.; Patrick, B. O.; Rettig, S. J. Organometallics 2001, 20, 1608-1613.

(d) Barger, P. T.; Santarsiero, B. D.; Armantrout, J.; Bercaw, J. E. J.

Am. Chem. Soc. 1984, 106, 5178-5186. (e) Cheon, J.; Rogers, D. M.;

Girolami, G. S. J. Am. Chem. Soc. 1997, 119, 6804-6813. (f) Ivin, K.

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der Heijden, H.; Hessen, B. Chem. Commun. 1995, 145-146. (i)

Meinhart, J. D.; Anslyn, E. V.; Grubbs, R. H. Organometallics 1989,

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1888 Organometallics, Vol. 24, No. 8, 2005

Basuli et al.

Scheme 1. Synthesis of Four-Coordinate

Titanium Alkylidenes by a One-Electron Oxidation

Reaction

of the active product was hampered by its rapid decom-

position. Suspecting that electrophiles such as [CPh₃]⁺

might be good oxidants (E_red) -0.11 V vs FeCp₂/

+ 17

FeCp₂

)

and that Ti(III) are good reductants we

reasoned that oxidation of the metal was taking place

instead of abstraction of the alkyl group. In fact, a cyclic

voltammogram of a solution of 2 (THF/TBAH, TBAH )

n-tetrabutylammonium hexafluorophosphate) showed

one irreversible oxidation wave at -0.90 V (referenced

vs FeCp₂/FeCp₂⁺) for the Ti(III)/Ti(IV) couple, thus

suggesting that [CPh₃]⁺could be oxidizing the Ti(III)

center.¹⁴

Figure 1. DFT calculations on the model complex

(Nacnac*)TiMe₂and structural comparisons with 2. The

SOMO is also depicted along with the room-temperature

(298 K, top) and low-temperature EPR spectra (4.7 K,

bottom, g )1.963 and g_|) 1.955).

Chemically, it was found that treatment of 2 with

AgOTf or 0.5 equiv of I₂caused a rapid color change

from green to red-brown, concomitant with formation

of the alkylidene complexes (Nacnac)TidCH^tBu(X) (X^-

) OTf, 3-OTf, 89%; X^-) I, 3-I, 61%), as evaluated by

¹H and ¹³C NMR spectroscopic methods (Scheme 1).¹⁴

The moderate yield observed in the isolation of 3-I

reflects its thermal decomposition both in solution and

in the solid state (vide infra). On the NMR time scale

compounds 3-OTf and 3-I display spectroscopic features

consistent with the molecule retaining C_ssymmetry in

solution. A C_Rresonance centered at δ ∼271-272 with

a J_CHcoupling constant of 90-85 Hz is diagnostic of

both complexes having a terminal alkylidene functional-

ity.^4,5The J_CHcoupling from the ¹³C NMR spectral data

suggests significant R-hydrogen agostic interaction with

the electron-deficient metal center, especially for com-

1

Complex 1 was characterized by a combination of H

NMR spectroscopic methods, elemental analysis, and

single-crystal X-ray diffraction.¹⁴Ethereal solutions of

t

1 react rapidly with 2 equiv of LiCH₂Bu^3bto afford

t

emerald green solutions of (Nacnac)Ti(CH₂Bu)₂(2),

which was isolated as dark green blocks in 75% yield.

Complex 2 was fully characterized,¹⁴and the molecular

structure indicates no R-agostic interactions or remark-

able features different from that of the reported dimeth-

yl derivative.^15aComplex 2 is amazingly stable unlike

most low-coordinate bis-alkyl species of Ti(IV), and no

elimination or abstraction reactions are observed under

forcing conditions. In addition, complex 2 does not bind

Lewis bases such as THF or PMe₃.

EPR spectra of 2 are consistent with a d¹paramag-

netic complex. Room-temperature EPR spectra reflect

hyperfine coupling of the unpaired electron to titanium

(⁴⁷Ti, I ) 5/2, 7.4%; ⁴⁹Ti, I ) 7/2, 5.4%, 14.9 G), as well

as superhyperfine coupling to the two R-nitrogens (¹⁴N,

I ) 1, 96.63%, 4.6 G) and super superhyperfine couling

to four R-hydrogens (¹H, I ) 1/2, 99.99%, 2.4 G). Low-

temperature EPR spectra (77 K) display axial sym-

metry, suggesting that the orbital housing the unpaired

electron lies in a plane perpendicular to the z-axis (z-

axis is along the Ti-C_γof the NCC_γCN-Nacnac^-ring,

Figure 1).¹⁴In fact, DFT calculations on the model

complex (Nacnac*)TiMe₂(Nacnac*^-) [Ar]NCHCHCH-

N[Ar], Ar ) 2,6-Me₂C₆H₃) reproduced the key geo-

metrical features of the solid-state structure of 2. The

calculated SOMO (singly occupied molecular orbital) has

mostly d_xymetal character, hence is perpendicular to

the z-axis, which is clearly consistent with the aniso-

tropic EPR spectra for 2 (Figure 1).

1

plex 3-I.^1cIn compounds 3-OTf and 3-I, the H NMR

CH_Rresonance was located and differentiated unam-

biguously from the CH_γresonance for the Nacnac^-

backbone using HMQC NMR methods.¹⁸Complexes

3-OTf and 3-I are likely formed from the putative five-

t

coordinate and d⁰intermediate (Nacnac)Ti(CH₂Bu)₂-

(X)¹⁹(X^-) OTf, I), which then undergoes smooth

R-hydrogen abstraction to form the TidC linkage.

Formation of complexes 3-OTf and 3-I raised two

interesting questions in the R-hydrogen abstraction

process: the role of the anion present in the oxidant,

and the role of the alkyl group on titanium. Having

considered these, it was determined that OTf^-does in

fact promote R-hydrogen abstraction since oxidation of

2 with [FeCp*₂][B(C₆F₅)₄]^20-22affords a relatively stable

(17) Connelly, N. G.; Geiger, W. E. Chem. Rev. 1996, 96, 877-910.

(18) Summers, M. F.; Marzilli, L. G.; Bax, A. J. Am. Chem. Soc.

1986, 108, 4285-4294.

(19) For an example describing R-hydrogen abstraction stemming

from a five-coordinate complex see: Schrock, R. R.; Murdzek, J. S.;

Bazan, G. C.; Robbins, J.; DiMare, M.; O’Regan, M. J. Am. Chem. Soc.

1990, 112, 3875-3886.

(20) For synthesis of the analogous salt [FeCp*₂][B(3, 5-(CF₃)₂C₆H₃)₄]

see: Cha´vez, I.; Alvarez-Carena, A.; Molins, E.; Roig, A.; Maniukiewicz,

W.; Arancibia, A.; Arancibia, V.; Brand, H.; Manr´ıquez, J. M. J.

Organomet. Chem. 2000, 601, 126-132.

(21) Kitiachvili, K.; Mindiola, D. J.; Hillhouse, G. L. J. Am. Chem.

Soc., accepted.

Seminal work by Budzelaar and co-workers has

reported that titanium(III) bis-alkyl complexes react

with electrophiles (e.g., [CPh₃][B(C₆F₅)₄]¹⁶) to form

highly reactive zwitterion Ti(III) species.^15aAlthough

these species polymerize ethylene, the characterization

(16) Ihara, E.; Young, V. G., Jr.; Jordan, R. F. J. Am. Chem. Soc.

1998, 120, 8277-8278.

Four-Coordinate Titanium Alkylidene Complexes

Organometallics, Vol. 24, No. 8, 2005 1889

Scheme 2. Oxidation of 2 with [FeCp*₂][B(C₆F₅)₄]

and AgBF₄, and Oxidation of Budzelaar’s

(Nacnac)Ti(CH₃)₂with AgOTf

Figure 2. Molecular structures of [2]⁺and 4 showing the

atom-labeling scheme with thermal ellipsoids at the 50%

probability level. H atoms with the exception of the R-H

and aryl groups with the exception of the ipso-carbon on

the Nacnac^-ligand have been excluded for clarity. Crystal-

lographic data are reported in Table 3. Selected metrical

parameters: For [2]⁺: Ti1-N6, 1.934(3); Ti1-N2, 2.006-

(3); Ti1-C38, 2.009(5); Ti1-C33, 2.062(4) Å; N6-Ti1-N2,

95.2(4); N6-Ti1-C38, 115.5(7); N6-Ti1-C33, 109.3(7);

N2-Ti1-C33, 112.4(7); C38-Ti1-C33, 112.6(9); Ti1-

C33-C(34), 130.2(3); Ti(1)-C(38)-C(39), 145.6(3)°. For 4:

Ti1-N6, 2.039(3); Ti1-N2, 2.0409(13); Ti1-O35, 2.044(1);

Ti1-C33, 2.079(9); Ti1-C34, 2.097(7) Å; N6-Ti1-N2,

90.79(5); N6-Ti1-O35, 174.16(5); N2-Ti1-O35, 90.70(5);

N6-Ti1-C33, 94.66(7); O35-Ti1-C33, 90.09(7); N6-Ti1-

C34, 89.48(6); N2-Ti1-C34, 127.67(7); O35-Ti1-C34,

85.14(6); C33-Ti1-C34, 121.17(8)°.

t

bis-alkyl

cation,

[(Nacnac)Ti(CH₂Bu)₂][B(C₆F₅)₄]

([2][B(C₆F₅)₄]), which gradually decomposes in solution

at room temperature (Scheme 2). Lewis bases such as

THF or PMe₃appear to rapidly catalyze the decomposi-

tion of [2][B(C₆F₅)₄]. ¹H and ¹³C NMR spectra

(CD₂Cl₂, ranging from -60 to 25 °C) do not suggest the

presence of R-agostic interactions in solution under

these conditions and are consistent with two inequiva-

lent neopentyl groups. The molecular structure and

elemental analysis of [2][B(C₆F₅)₄] are also consistent

with the proposed connectivity (Figure 2). To the best

of our knowledge, four-coordinate and monomeric

Ti(IV) bis-alkyl cations have not been reported, which

is in sharp contrast to four-coordinate ion-paired or

neutral species described in the literature.²³The aver-

age Ti-C bond distance for [2][B(C₆F₅)₄] (∼2.03 Å)

compares well with the M-C bond length of electron-

deficient titanium zwitterions (2.081(5) Å).²³In the

solid-state structure of [2][B(C₆F₅)₄] it is difficult to

ascertain whether R-H agostic interactions with the

titanium center are taking place, but the discrepancy

between Ti-C_R-C_âangles (Ti1-C33-C34 ) 130.2(3)°

vs Ti1-C38-C39 ) 145.6(3)°) suggests that such an

interaction might be playing a role (Figure 2). Due to

the limited quality of the X-ray data for [2][B(C₆F₅)₄],

we are being conservative about the discussion of

structural details. Based on preliminary data reported

by Budzelaar and co-workers complex [2][B(C₆F₅)₄]

could be in principle the olefin polymerization catalyst

generated upon addition of the electrophile to the

Ti(III) precursor,^15aand we are currently examining this

possibility.

that bulky alkyl groups are also needed to promote

steric crowding and subsequent R-hydrogen abstraction.

Accordingly, oxidation of Budzelaar’s dimethyl complex

(Nacnac)Ti(CH₃)₂^15awith AgOTf does not lead to forma-

tion of the parent methylidene but instead affords the

five-coordinate Ti(IV) dimethyl triflato complex (Nacnac)-

Ti(CH₃)₂(OTf) (4) in moderate yield (Scheme 2). Ther-

molysis of 4 does not induce R-hydrogen abstraction, but

instead gives rise to a myriad of products, several of

which include Ti(III) compounds. Figure 2 displays the

molecular structure for 4 along with selected metrical

parameters.

The derivatives (Nacnac)TidCH^tBu(X) (X^-) Cl, 3-Cl;

Br, 3-Br; BH₄, 3-BH₄) can be readily prepared by the

corresponding salt metathesis with 3-OTf as depicted

in Scheme 3. Although the preparation of 3-Cl and

3-BH₄is straightforward and involves simple anion

24

exchange using MgCl₂and LiBH₄, the synthesis of

3-Br was somewhat less obvious. Greater yields of 3-Br

were obtained when the LiBr-ylide adduct Ph₃PdCH₂‚

LiBr²⁵was used in place of MgBr₂. The ylide salt adduct

is apparently a much more soluble Br^-source than

MgBr₂, and rapid Br^-exchange reduces the decomposi-

tion of thermally unstable 3-Br (vide infra). Table 1 lists

all the relevant spectroscopic features for 3-OTf, 3-I,

1

The anion present in the oxidant must be sterically

encumbering since oxidation of 2 with AgBF₄leads only

to oxidation and formation of (Nacnac)Ti(CH₂Bu)₂(F),

(2)-F (Scheme 2). The connectivity of 2-F has also been

confirmed by a combination of NMR spectra and single-

crystal X-ray diffraction studies (see Experimental

Section and Supporting Information). It was determined

3-Cl, 3-Br, and 3-BH₄. Judging from the H and ¹³C

NMR spectra, as well as J_C-Hcoupling constants of the

R-alkylidene C-H group, it is evident that electrophi-

licity of the metal center increases in the order 3-BH₄

< 3-Cl < 3-OTf < 3-Br < 3-I. The order of electrophi-

licity is reflective of the X^-ligand being a weaker

π-donor (excluding 3-BH₄); thus the metal participates

t

(22) Basuli, F.; Bailey, B. C.; Huffman, J. C.; Mindiola, D. J. Chem.

Commun. 2003, 1554-1555.

(23) For a comprehensive review on activated and low-coordinate

catalysts see: Chen, E. Y.-X.; Marks, T. J. Chem. Rev. 2000, 100, 1391-

1434. A three-coordinate titanium alkyl cation has been proposed, but

the geometry remains uncertain. Shafir, A.; Arnold, J. J. Am. Chem.

Soc. 2001, 123, 9212-9213.

(24) For a similar preparation concerning halide metathesis in low-

coordinate titanium complexes see: Cummins, C. C.; Schaller, C. P.;

Van Duyne, G. D.; Wolczanski, P. T.; Chan, A. W. E.; Hoffmann, R. J.

Am. Chem. Soc. 1991, 113, 2985-2994.

(25) (a) Schmidbaur, H.; Stu¨hler, H.; Vornberger, W. Chem. Ber.

1972, 105, 1084-1086. (b) Reger, D. L.; Culbertson, E. C. J. Organomet.

Chem. 1977, 131, 297-300.

1890 Organometallics, Vol. 24, No. 8, 2005

Basuli et al.

Scheme 3. Synthesis of Terminal Titanium

Alkylidene Complexes (Nacnac)TidCH^tBu(X) (X- )

Cl, Br, BH₄) by Salt Metathesis

Figure 3. Molecular structures of 3-X (X^-) OTf, I, Br,

BH₄) showing the atom-labeling scheme with thermal

ellipsoids at the 50% probability level. H atoms with the

exception of the R-hydrogens have been omitted for clarity.

Aryl groups with the exception of the ipso-carbon on the

Nacnac^-ligand have been also excluded. Table 2 lists

selected metrical parameters, and crystallographic data are

reported in Table 3.

Table 1. Spectroscopic Data for the Alkylidene

Complexes 3-X (X^-) I, Br, OTf, Cl, BH₄)

3-I 3-Br 3-OTf 3-Cl 3-BH₄

¹H NMR (δ) of Ti ) CH

¹³C NMR (δ) of Ti ) CH 272

J_C-H(Hz) 85

3.3

4.5

274

88

5.2

271

90

5.5

276

93

5.32

267

95

occurring in complexes 3-X arises from the low J_C-H

coupling (85-95 Hz, Table 1). Judging from the J_C-H

and the ¹H NMR resonance for R-H, complex 3-I appears

to have the greatest Lewis acidic character (Table 1).

The R-H agostic interaction observed in all complexes

(structurally or spectrocopically) is also substantiated

by the large Ti(1)dC(33)-C(34) angle of 160-164°. The

structure of 3-BH₄is nearly identical to that of the

more in R-agostic interaction with the alkylidene hy-

-

drogen. In the case of 3-BH₄, the hapticity of the BH₄

ligand remains uncertain. Although the borohydride

resonance was not located in the ¹H NMR, the ¹¹B NMR

spectrum displayed a quintet with J_B-H) 89 Hz,

-

consistent with a fluxional BH₄ligand.

Structural

Comparisons

of

(Nacnac)-

TidCH^tBu(X) (X^-) OTf, Br, I, and η³-BH₄). The

molecular structures for complexes 3-X (X^-) OTf, Br,

I, and BH₄) are isostructural and are shown in Figure

3. These systems are rare examples of terminal and low-

coordinate titanium alkylidene complexes that contain

very short TidC bonds (∼1.830 Å).^5,14,26,27Selected

metrical parameters are displayed in Table 2. The

molecular structures for 3-X (X^-) OTf, I, and Br) reveal

a four-coordinate titanium complex having C_ssym-

metry, and in all systems the tert-butyl group is along

the σ-plane bisecting the N-Ti-N angle and is oriented

syn with respect to the X^-ligand. The TidC_RH_Rhydro-

gen was located in the Fourier electron map for all 3-X

(excluding 3-I and 3-BH₄) systems and was refined

isotropically (Ti-H_Rdistances are reported in Table 2).

Even though there is a systematic error in detection of

hydrogens by X-ray diffraction²⁸the location of the

hydrogens in the Fourier map leaves little uncertainty

about their presence in each structure. However, stron-

ger evidence suggesting an R-H agostic interaction

-

halide and pseudohalide derivatives. Although the BH₄

1

protons were not observed by H NMR spectroscopy,²⁹

location and refinement of the hydrogens in the X-ray

structure suggest this ligand to be η³.³⁰We are however

doubtful of how accurate the hapticity of the BH₄ligand

really is in 3-BH₄. Solution IR and ¹¹B NMR spectros-

copy are also consistent with a tetrahydroborate ligand

being present.

Thermal Transformation of the Titanium Neo-

pentylidene Complexes 3-X. Although stable as solid

complexes, 3-X (X^-) OTf, Cl, Br, I, and BH₄) are all

kinetic products since these systems transform gradu-

ally in solution in the order 3-OTf < 3-Cl < 3-Br < 3-I

< 3-BH₄, as evidenced by ¹H and ¹³C NMR spectroscopy.

Heating benzene solutions of 3-OTf, 3-Cl, and 3-Br to

60 °C for several hours affords the titanium imido

triflato complex supported by the chelating anilide diene

ligand, (H^tBuCdC(Me)CHC(Me)N[Ar])TidNAr(X), 5-X

(29) We have not been able to locate the hydride resonance in the

¹H NMR spectrum, presumably due to broadening of the signal by

coupling of the hydride to the ¹¹B (I ) 3/2, 80%) quadrupolar nuclei.

(a) Harris, R. K., Mann, B. E., Eds. NMR and the Periodic Table;

Academic Press: London, 1978. (b) Akitt, J. W. NMR and Chemistry,

An Introduction to the Fourier Transform-Multinuclear Era; Chapman

Hall: New York, 1983; Chapter 4.

(30) For some examples of tridentate borohydride complexes of

titanium(IV) see: (a) No¨th, H.; Schmidt, M. Organometallics 1995, 14,

4601-4610. (b) Corazza, F.; Floriani, C.; Chiesi-Villa, A.; Guastini, C.

Inorg. Chem. 1991, 30, 145-148. (c) Marks, T. J.; Kolb, J. R. Chem.

Rev. 1977, 77, 263-293.

(26) A search of the Cambridge Crystallographic Database for

titanium-alkylidene bond lengths indicate values to be g 1.884(4) Å,

see ref 5.

(27) A four-coordinate titanium complex having a bridging alky-

lidene functionality has been reported: (a) Riley, P. N.; Fanwick, P.

E.; Rothwell, I. P. J. Chem. Soc., Dalton Trans. 2001, 181-186. A

transient three-coordinate titanium methylidene has been proposed

in a decomposition reaction: (b) Scollard, J. D.; McConville, D. H.;

Rettig, S. J. Organometallics 1997, 16, 1810-1812.

(28) Bau, R.; Drabnis, M. H. Inorg. Chim. Acta 1997, 259, 27-50.

Four-Coordinate Titanium Alkylidene Complexes

Organometallics, Vol. 24, No. 8, 2005 1891

Table 2. Selected Structural Parameters for the Alkylidene Complexes 3-X (X^-) I, Br, OTf, BH₄)^a

3-I^b

3-Br

3-OTf

3-BH₄

TidC_R

Ti1-C34, 1.829(4)

Ti1-N2, 2.008(3)

Ti1-N6, 2.029(3)

Ti1-I33, 2.6616(8)

n/a

Ti1-C34, 1.826(2)

Ti1-C33, 1.830(3)

Ti1-C33,1.840(8)

Ti1-N2, 2.039(3)

Ti1-N6, 2.069(4)

Ti1-B38, 2.200(2)

n/a

Ti-N_Nacnac

Ti-X

Ti1-N3, 2.032(5)

Ti1-N2, 2.012(3)

Ti1-N7, 2.013(6)

Ti1-N6, 2.025(3)

Ti1-Br2, 2.4452(4)

Ti1-H81, 1.99(2)

Ti1-O38, 1.957(2)

Ti-H (agostic)

C_R-H

Ti-C-C

C_R-Ti-X

N_Nacnac-Ti-N_Nacnac

C_R-Ti-N_Nacnac

X-Ti-N_Nacnac

Ti1-H87, 1.92(3)

n/a

C34-H81, 0.92(3)

C33-H87, 0.91(4)

n/a

Ti1-C35-C36, 163.8(4)

C34-Ti1-I33, 112.3(3)

N2-Ti1-N6, 93.6(1)

C34-Ti1-N2, 115.2(5)

C34-Ti1-N6, 105.7(5)

I33-Ti1-N2, 116.32(8)

I33-Ti1-N6, 111.46(7)

Ti1-C34-C35, 162.1(2)

C34-Ti1-Br2 112.43(8)

N3-Ti1-N7, 92.95(6)

C34-Ti1-N3, 105.90(8)

C34-Ti1-N7, 114.45(9)

Br2-Ti1-N3, 111.59(5)

Br2-Ti1-N7, 117.25(5)

Ti1-C33-C34, 163.9(3)

C33-Ti1-O38, 112.5(3)

N2-Ti1-N6, 94.2(1)

C33-Ti1-N2, 109.8(3)

C33-Ti1-N6, 111.2(3)

O38-Ti1-N2, 115.6(1)

O38-Ti1-N6, 112.7(1)

Ti1-C34-C33, 160.3(5)

C33-Ti1-B38, 108.23(9)

N2-Ti1-N6, 91.87(5)

C33-Ti1-N2, 110.39(7)

C33-Ti1-N6, 111.03(7)

B38-Ti1-N2, 119.50(7)

B38-Ti1-N6, 115.01(8)

^aBond lengths are reported in Å and bond angles in deg. ^bComplex 3-I suffers from disorder resulting from alternate positions of the

Ti and I atoms; hence bond lengths and angles may not be highly accurate.

Table 3. Crystallographic Data for Complexes [2][B(C₆F₅)₄], 3-X (X^-) Br, I, BH₄), and 4

[2][B(C₆F₅)₄]

3-Br

3-I

3-BH₄

4

empirical formula

fw

crystal system

space group

a (Å)

b (Å)

c (Å)

R (deg)

â (deg)

γ (deg)

V (Å³)

C

₆₃H₆₃BF₂₀N₂Ti

C₃₄H₅₁BrN₂Ti

615.58

C₃₄H₅₁IN₂Ti

662.57

C₃₄H₅₅BN₂Ti

550.51

C₃₂H₄₇F₃N₂O₃STi

644.68

1286.86

orthorhombic

Pbca

monoclinic

P2(1)/n

monoclinic

P2(1)/n

monoclinic

P2(1)/n

triclinic

P1h

21.529(3)

22.213(3)

25.426(4)

90

10.2863(4)

27.5797(11)

12.3294(5)

10.4564(8)

27.710(2)

12.3526(9)

10.522(2)

19.451(4)

17.009(3)

9.1297(11)

10.3411(12)

19.701(2)

78.359(3)

87.518(3)

64.550(3)

1643.0(3)

2

90

106.6320(10)

107.242(2)

102.739(6)

12159(3)

8

3351.4(2)

4

1.220

132(2)

3418.3(4)

4

1.287

113(2)

3395.4(12)

4

1.077

132(2)

Z

D_c(g‚cm^-3

T (K)

)

1.406

118(2)

1.303

135(2)

cryst size (mm)

solvent, color

0.30 × 0.25 × 0.15

0.30 × 0.10 × 0.10 0.15 × 0.15 × 0.14

0.25 × 0.25 × 0.20 0.30 × 0.25 × 0.25

Et₂O/TMS₂O, orange Et₂O, brown

pentane, dark brown hexane, brown

THF/pentane,

orange

-12 e h e 12

-14 e k e 14

-27 e l e 27

684

2.23-30.03

0.375

44 382

(h, k, l)

-23 e h e 23

-24 e k e 24

-28 e l e 28

5296

2.00-23.31

0.243

146 395

-14 e h e 14,

-38 e k e 38,

-17 e l e 17

1304

2.19-30.03

1.471

60 664

-8 e h e 12,

-32 e k e 32,

-14 e l e 14

1376

2.17-25.02

1.176

16 099

-14 e h e 14,

-27 e k e 27,

-23 e l e 22

1200

2.09-30.07

0.275

44 095

F(000)

θ range

linear abs coeff (mm^-1

total no. of reflcns

collected

)

no. of unique reflcns

F>4σ(F)

no. of obsd reflcns

R_int

8772

9808

6023

9938

9596

5144

0.1942

6897

0.0863

4145

0.0603

5681

0.1723

6863

0.0727

no. of data/params

R₁, wR₂(for I > 2σ(I))

GoF

8772/1036

0.0462, 0.0980

1.028

9808/547

0.0402, 0.0935

0.929

6023/553

0.0345, 0.0553

0.862

9938/563

0.0493, 0.1138

0.892

9596/567

0.0408, 0.0988

0.942

peak/hole (e/Å^-3

)

0.532, -0.337

1.948, -1.108

0.737, -0.360

0.755, -0.824

0.799, -0.431

(X^-) Cl, η²-OTf,¹⁴and Br, Scheme 4). From the 5-Cl,

5-OTf, and 5-Br derivatives only 5-OTf was isolated

in 65% yield.¹⁴Thermolysis of complexes 3-Cl and 3-Br

leads to several products, from which we identified 5-Cl

reported by Jordan and co-workers.³²Similar R-hydro-

gen agostic interactions have been reported for 1-aza,1,3-

diene ligands bound to titanium.³²However, the anilide

diene interaction in 5-X can also be viewed as a

delocalized charge about the NCCCH or CCH rings,

where a considerable amount of rehybridization of the

terminal carbon atom toward sp³has occurred. There-

fore one cannot exclude these binding options when

taking into consideration the small C-H coupling

constants for the terminal olefinic group (vide supra).³²

Complexes 5-X are formed likely by a cross-metathesis

1

and 5-Br based solely on H NMR spectroscopic com-

parisons with isolated samples of 5-OTf. In contrast to

3-X (X^-) OTf, Cl, and Br), complexes 3-I and 3-BH₄

decompose rapidly in solution at room temperature or

-35 °C to 5-I and 5-BH₄, respectively. In all systems

1

the H NMR spectrum for each complex also indicates

that only one diastereomer is present in solution at 25

°C. This feature is signified clearly by the R-hydrogen,

1

(31) For common J_C-Hvalues of sp²carbons see: Strothers, J. B.

Carbon-13 NMR Spectroscopy; Academic Press: New York, 1972.

(32) The low coupling value could also suggest considerable amount

of rehybridization of the terminal carbon atom toward sp³hybridiza-

tion. Such a spectroscopic feature has been suggested for 1-aza-1,3-

diene ligands containing R-hydrogens that are bonded to titanium.

Scholz, J.; Kahlert, S. Organometallics 1998, 17, 2876-2884. Stoe-

benau, E. J., III; Jordan, R. F. J. Am. Chem. Soc. 2003, 125, 3222-

3223.

which was detected in the H NMR spectrum (1.5-2.8

ppm) using HMQC experiments.¹⁸R-Hydrogen agostic

interaction is also present in isolated samples of 5-OTf,

5-I, and 5-BH₄, which is evident from the C-H coupling

constants of the terminal olefin group (126-129 Hz).³¹

The R and â olefinic carbon atoms display carbon NMR

resonances similar to those of group 4 alkene adducts

1892 Organometallics, Vol. 24, No. 8, 2005

Basuli et al.

Table 4. Crystallographic Data for Complexes 5-I, 5-BH₄, 7-OTf, 7-BH₄, 9‚3CH₂Cl₂, and 11

5-I

5-BH3

7-OTf

7-BH₄

9‚3CH₂Cl₂

11

empirical

formula

fw

cryst system

space group

a (Å)

b (Å)

c (Å)

R (deg)

â (deg)

γ (deg)

V (Å³)

C

₃₄H₅₁IN₂Ti

C₃₄H₅₅BN₂Ti

C₃₀H₃₉F₃N₂O₃STi C₂₉H₄₃BTi

C₇₅H₁₀₈Cl₆F₆N₄O₁₀S₂Ti₂

C

₄₂H₅₆F₃N₃O₃S₂Ti

662.57

triclinic

P1h

550.51

triclinic

P1h

612.59

monoclinic

P2(1)/n

9.2570(5)

17.9054(9)

18.0970(9)

478.36

orthorhombic

Cmc2(1)

19.268(9)

10.2436(9)

13.710(4)

90

1712.27

monoclinic

P2(1)/n

14.307(7)

25.734(3)

23.411(3)

819.92

triclinic

P1h

9.091(1)

11.000(2)

17.149(8)

102.356(2)

92.035(3)

99.780(3)

1646.2(3)

2

9.0020(3)

11.0365(4)

17.3981(6)

76.626(1)

89.1540(1)

78.3890(1)

1646.31(1)

2

10.662(4)

12.7780(4)

16.7565(6)

91.363(1)

94.131(1)

109.029(1)

2149.8(3)

2

1.267

132(2)

0.23 × 0.23 ×

91.720(1)

90

90.107(3)

90

2998.2(3)

4

2705.9(5)

4

8619(7)

4

1.320

120(2)

0.30 × 0.30 ×

0.18

Z

D_c(g‚cm^-3

T (K)

)

1.337

125(2)

1.111

129(2)

1.357

1.174

136(2)

0.30 × 0.30 ×

0.25

117(2)

cryst size

(mm)

solvent, color

0.26 × 0.25 × 0.21 × 0.18 ×

0.25 × 0.25 ×

0.20

0.13

0.04

0.20

toluene, dark hexane, orange- Et₂O, red

red brown

-11 e h e 11, -12 e h e 12,

-14 e k e 14, -15 e k e 15,

-22 e l e 22 -24 e l e 24

688

2.44-27.53

1.221

Et₂O, orange-pink CH₂Cl₂/Et₂O,

orange

-24 e h e 24,

-13 e k e 13,

-17 e l e 17

1032

pentane, black

(h, k, l)

-13 e h e 11,

-25 e k e 25,

-25 e l e 25

1288

-18 e h e 18,

-33 e k e 33,

-30 e l e 30

3592

2.30-27.60

0.487

-13 e h e 13,

-16 e k e 16,

-21 e l e 21

868

2.13-27.51

0.349

F(000)

θ range

600

2.03-30.02

0.283

2.27-30.01

0.407

2.11-27.52

0.335

linear abs.

coeff (mm^-1

total no. of

reflcns

collected

no. of unique

reflcns

F>4σ(F)

no. of obsd

reflcns

)

43 074

44 796

30 990

25 991

190 318

19 935

13 632

48 428

7574

9613

8731

3192

9878

6670

6795

5594

2679

8220

R_int

no. of data/

params

0.0413

6914/516

0.0963

9613/563

0.0678

8731/517

0.0583

3192/274

0.1067

19 935/1380

0.0548

9878/711

R₁, wR₂(for

I > 2σ(I))

GoF

0.0280, 0.0665 0.0402, 0.0935 0.0387, 0.0827

0.0398, 0.0647

0.0868, 0.2167

0.0309, 0.0791

1.022

0.972

0.917

0.423, -0.418

0.961

0.231, -0.242

1.107

1.417, -0.833

1.037

0.462, -0.285

peak/hole (e/Å^-3

)

0.766, -0.583 0.511, -0.350

reaction (or alternatively called Wittig-like) between the

titanium neopentylidene and the imine aryl functional-

ity of the Nacnac^-ligand. Not surprisingly, complexes

5-X seem to be thermodynamically driven due to forma-

tion of the strong TidN_imidobond.

(Figure 4). The molecular structures of 5-X (X^-) OTf,

I, and η³-BH₄) display only one of the possible diaster-

eomers, which is also consistent with solution NMR

spectra (vide infra). In the three crystal structures of

5-X the R-hydrogen was located in the Fourier electron

map and was refined isotropically (Ti-H_R, 2.05-2.09

Å). Such an R-hydrogen agostic interaction in all three

crystal structures might lock the olefinic arm, thus

preventing formation of diastereomers. In the crystal

structures of 5-OTf, 5-I, and 5-BH₄the titanium binds

to the R- and â-carbons. For instance, in the molecular

structure of 5-I the Lewis acidic metal center in

turn compensates by engaging strongly with the

^tBuHCdC(CH₃) group (Figure 4), and as a result, the

former CdC bond is elongated. This feature also cor-

relates with complex 5-I having the lowest R-C-H

coupling constant (vide supra). Since the pendant olefin

is regarded as a poor ligand, the low-coordinate titanium

center also compensates by forming a very strong bond

with the imido nitrogen (TidNAr average distance in

5-X is ∼1.72 Å). The molecular structures of complex

5-X (X- ) OTf, I, and η³-BH₃) are also interesting since

they depict a rare example of low-coordinate titanium

imido complexes.^14,24,33

The chelate ligand appears to behave “diene-like” in

5-X since the addition of a weak Lewis base such

as Et₂O to 5-OTf affords crystals of the adduct

(η¹-H^tBuCdC(Me)CHC(Me)N[Ar])TidNAr(OTf)(Et₂O)

(6-OTf) in 80% yield (Scheme 4).¹⁴In complex 6-OTf

the olefinic pendent arm of amide diene ligand has been

displaced completely by the Lewis base; thus we depict

the supporting ligand in 5-X as an anilide possessing a

1

pedant olefinic arm. H and ¹³C NMR spectra in C₆D₆

are relatively unchanged for the Lewis base adduct

6-OTf when compared to the Lewis base-free precursor

5-OTf, thus suggesting that the coordinated Et₂O might

be being displaced under these conditions. Mild heating

of 6-OTf under reduced pressure (50 °C, 2 h) regener-

ates 5-OTf quantitatively, showing that this process is

reversible (Scheme 4). As described in an earlier com-

munication, inspection of the structure of 6-OTf reveals

clearly that the olefinic arm of the former Nacnac^-

ligand is not interacting with the metal center (Ti-C_olefin

g 4.30 Å).¹⁴

Double C-H Abstraction Reactions in the Ther-

molysis of 3-X (X^-) OTf, Cl, Br, and BH₄). Attempts

to perform kinetic studies (60-70 °C) on the cross-

metathesis reaction leading to formation of 5-X were

Single-crystal structure determinations of 5-OTf, 5-I,

and 5-BH₄are also consistent with the proposed con-

nectivity suggested by the ¹H and ¹³C NMR spectra

Four-Coordinate Titanium Alkylidene Complexes

Organometallics, Vol. 24, No. 8, 2005 1893

Table 5. Crystallographic Data for Complexes 12‚Et₂O, 13‚C₅H₁₂, 14, and 15‚C₅H₁₂

12‚Et₂O

13‚C₅H₁₂

14

15‚C₅H₁₂

empirical formula

fw

cryst syst

space group

a (Å)

b (Å)

c (Å)

R (deg)

â (deg)

γ (deg)

V (Å³)

C

₅₀H₇₄F₃N₃O₄STi

C₄₅H₇₂F₃N₃O₃Sti

840.02

C₄₈H₆₁F₃N₄O₃Sti

878.97

monoclinic

P2(1)/c

12.7640(4)

17.7911(6)

20.8737(7)

C₅₀H₇₈F₃N₃O₃Sti

906.11

918.08

triclinic

P1h

triclinic

orthorhombic

P2(1)2(1)2(1)

14.4433(8)

17.842(1)

19.232(1)

90

4956.0(5)

4

P1h

12.152(5)

12.208(5)

18.140(2)

77.585(3)

75.257(3)

81.809(3)

2530.8(5)

2

15.851(3)

16.400(3)

18.475(3)

87.600(4)

82.003(4)

89.976(4)

4752(5)

98.537(1)

4687.6(3)

4

1.245

137(2)

Z

4

D_calc(g‚cm^-3

T (K)

)

1.205

136(2)

1.174

1.214

136(2)

cryst size (mm)

solvent, color

(h, k, l)

0.30 × 0.18 × 0.12

Et₂O, orange

-17 e h e 17,

-17 e k e 16,

-25 e l e 25

984

0.30 × 0.30 × 0.09

pentane, black

0 e h e 22

-23 e k e 23

-25 e l e 26

1808

0.30 × 0.30 × 0.25

toluene, dark red

-17 e h e 17,

-25 e k e 25,

-29 e l e 29

1864

0.25 × 0.25 × 0.18

Et₂O, red-brown

-18 e h e 18,

-23 e k e 23,

-25 e l e 24

1952

F(000)

θ range

2.33-30.16

0.265

2.18-30.11

0.275

2.49-30.02

0.283

2.28-27.57

0.268

linear abs coeff (mm^-1

)

total no. of reflcns collected

no. of unique reflcns F>4σ(F)

no. of obsd reflcns

R_int

39 113

51 532

103 943

110 696

14 802

27 772

13 681

11 423

8579

10 951

10 969

8317

0.1307

0.1200

0.0580

0.1065

no. of data/params

R₁, wR₂(for I > 2σ (I))

GoF

14 802/855

0.0543, 0.1139

0.927

27 772/1011

0.0896, 0.2059

0.835

13 681/812

0.0400, 0.1029

1.017

11 423/665

0.0489, 0.1305

1.005

peak/hole (e/Å^-3

)

0.778, -0.788

1.758, -0.700

0.929, -0.867

0.525, -0.338

Scheme 4. Thermal Transformation of the

Titanium Alkylidenes (Nacnac)TidCH^tBu(X) (X- )

OTf, Cl, Br, BH₄)

hampered by the presence of an additional product in

the mixture. Fortunately in the thermolysis of complex

3-OTf the second product was separated conveniently

from 5-OTf due to its limited solubility in pentane.

Washing of the red-brown solid with hexane and sub-

sequent recrystallization from Et₂O at -35 °C afforded

red prisms of the titanacycle Ti[2,6-(CMe₂)(CHMe₂)-

C₆H₃]NC(Me)CHC(Me)N[2,6-(CMe₂)(CHMe₂)C₆H₃]-

(OTf), 7-OTf, in 31% yield (Scheme 4). Thermolysis of

isolated 5-OTf under similar conditions failed to show

any conversion to 7-OTf, suggesting that complex 3-OTf

is in fact a precursor to 7-OTf. Increasing the temper-

ature above 70 °C in the thermolysis of 3-OTf did not

improve the yield of 7-OTf. Consecutive thermolytic

experiments yielded consistent results, thus suggesting

that an impurity (e.g., Ag⁺) could not be the reason for

the formation of 7-OTf. Formation of 7-OTf was unex-

pected, as C-H abstraction of two methine groups (3°

carbons) is exceedingly rare.^34-36Generation of 7-OTf

from the thermolysis of 3-OTf is selective inasmuch as

it is the only new product generated as a consequence

of activation of two tertiary C-H bonds of the formal

Nacnac^-ligand. The ¹H NMR spectrum of 7-OTf

displays one methine resonance integrating to two

protons, three distinctive methyl environments (one for

the â-carbon methyl of Nacnac^-and two for the C-H

activated isopropyl groups), two diastereotopic isopropyl

methyls, and one aryl environment, all being consistent

with the complex retaining C_ssymmetry in solution. In

addition, the ¹³C NMR spectrum of 7-OTf indicates the

(33) (a) Bennett, J. L.; Wolczanski, P. T., J. Am. Chem. Soc. 1994,

116, 2179-2180. (b) Cummins, C. C.; Baxter, S. M.; Wolczanski, P. T.

J. Am. Chem. Soc., 1988, 110, 8731-8733. (c) Schaller, C. P.; Wolc-

zanski, P. T. Inorg. Chem. 1993, 32, 131-144. (d) Schaller, C. P.;

Cummins, C. C.; Wolczanski, P. T. J. Am. Chem. Soc. 1996, 118, 591-

611. (e) Bennett, J. L.; Wolczanski, P. T. J. Am. Chem. Soc. 1997, 119,

10696-10719. (f) Schafer, D. F.; Wolczanski, P. T. J. Am. Chem. Soc.

1998, 120, 4881-4882. (g) Gade, L. H.; Mountford, P. Coord. Chem.

Rev. 2001, 216-217, 65-97. (h) Blake, A. J.; Collier, P. E.; Gade, L.

H.; Mountford, P.; Lloyd, J.; Pugh, S. M.; Schubart, M.; Skinner, M.

E. G.; M.; Tro¨sch, D. J. M. Inorg. Chem. 2001, 40, 870-877. (i)

Mountford, P. Perspectives in Organometallic Chemistry; Royal Society

of Chemistry, 2003; pp 28-46. (j) Blake, A. J.; Collier, P. E.; Gade, L.

H.; McPartlin, M.; Mountford, P.; Schubart, M.; Scowen, I. J. Chem.

Commun. 1997, 1555-1556. (k) Bashall A.; Collier, P. E.; Gade, L.

H.; McPartlin, M.; Mountford, P.; Pugh, S.; Radojevic, S.; Schubart,

M.; Scowen, I. J.; Tro¨sch, D. J. M. Organometallics 2000, 19, 4784-

4794. (l) Nikiforov, G. B.; Roesky, H. W.; Magull, J.; Labahn, T.;

Vidovic, D.; Noltemeyer, M.; Schmidt, H.-G.; Hosmane, N. S. Polyhe-

dron 2003, 22, 2669-2681. (m) Basuli, F.; Bailey, B. C.; Huffman, J.

C.; Mindiola, D. J. Chem. Commun. 2003, 1554-1555. (n) Basuli, F.;

Huffman, J. C.; Mindiola, D. J. Inorg. Chem. 2003, 42, 8003-8010.

(34) (a) Labinger, J. A.; Bercaw, J. E. Nature, 2002, 417, 507-514.

(b) Stahl, S. S.; Labinger, J. A.; Bercaw, J. E. Angew. Chem., Int. Ed.

1998, 37, 7, 2181-2192.

(35) Puddephatt, R. J. Coord. Chem. Rev. 2001, 219, 157-185.

(36) (a) Fekl, U.; Goldberg, K. I. J. Am. Chem. Soc., 2002, 124, 6804-

6805. C-H activation reactions have been reported for analogous

â-diketiminate Ir-hydride complexes: (b) Bernskoetter, W. H.; Lobk-

ovsky, E.; Chirik, P. J. Chem. Commun. 2004, 764-765.

1894 Organometallics, Vol. 24, No. 8, 2005

Basuli et al.

Figure 4. Molecular structure of 5-OTf, 5-I, and 5-BH₄with thermal ellipsoids at the 50% probability level. All H atoms

with the exception of R-hydrogens and aryl groups with the exception of ipso-carbons have been omitted for clarity.

Crystallographic data are reported in Table 4. Selected metrical parameters: For 5-OTf: Ti1-N25,

1.715(1); Ti1-N2, 2.015(1); Ti1-O38, 2.213(1); Ti1-O39, 2.254(1); Ti1-C6, 2.263(4); Ti1-C5, 2.524(4); Ti1-H48, 2.09(9);

C5-C6, 1.390(9) Å. For 5-I: Ti1-N26, 1.717(7); Ti1-N3, 2.015(7); Ti1-C7, 2.205(2); Ti1-C6, 2.527(2); Ti1-H10, 2.05(2);

C6-C7, 1.397(3); Ti1-I2, 2.6883(4) Å. For 5-BH₄: Ti1-N25, 1.716(2); Ti1-N6, 2.032(2); Ti1-C2, 2.230(4); Ti1-C3, 2.626-

(4); Ti1-H39, 2.08(7); Ti1-B38, 2.265(8); C2-C3, 1.403(2) Å.

presence of one quaternary and one methine carbon

environment centered at 88.78 and 106.2 ppm, respec-

tively. We also observe formation of 7-Cl, 7-Br, and

7-BH₄from thermolysis of their corresponding alky-

lidene precursors, but formation of these complexes is

based only on ¹H NMR spectroscopic comparisons with

isolated samples of 7-OTf. Attempts to isolate these

derivatives were hampered by a combination of low yield

and lypophilicity. Complex 7-Cl was also formed from

thermolysis of 3-Cl, but the reaction mixture contained

a myriad of additional products including 5-Cl (vide

supra). Independent studies have shown that 7-BH₄can

be prepared readily in 91% yield from 7-OTf and LiBH₄

in Et₂O. ¹H, ¹³C, and ¹¹B NMR spectroscopy, along with

single-crystal X-ray diffraction studies (Figure 5, vide

infra), confirmed the proposed connectivity (see Experi-

mental Section). The BH₄hydrogens were located and

refined isotropically in the crystal structure for complex

7-BH₄and suggest the ligand to be η². We have not

pursued the independent synthesis of 7-Br and 7-Cl

from anion metathesis of 7-OTf.

Single crystals of 7-OTf and 7-BH₄were grown from

a saturated Et₂O solution cooled to -35 °C, and the

molecular representations are depicted in Figure 5.

Complexes 7-OTf and 7-BH₄display a five-coordinate

titanium(IV) in a distorted square pyramidal environ-

ment with the pseudohalide ligand occupying the axial

site. The constrained geometry of 7-OTf and 7-BH₄is

likely a consequence of the ligand being macrocyclic, and

dihedral angles (N_Nacnac-C-C-C_R, range is 7.3(2)° to

-3.3(2)°) reflect the strain generated upon ring closure.

Similarly, the average acute angles Ti-C_R-C_âof 86°

also echo some strain in the metalacycle. The Ti atom

sits ∼0.82 Å from the mean plane bisecting the two

Figure 5. Molecular structure of 7-OTf and 7-BH₄with

thermal ellipsoids at the 50% probability level. All H atoms

with the exception of the η²-BH₄have been omitted for

clarity. Crystallographic data are reported in Table 4, and

complete metrical parameters are listed in the Supporting

Information.

is often encountered for the Nacnac^-derivative used in

the present study.³⁸Recently Goldberg and co-workers

reported C-H abstraction of both the methine and

methyl groups of the isopropyl of Nacnac^-via reductive

elimination of a Pt(IV) species and subsequent oxidative

addition leading to formation of an isopropenyl(hydrido)-

N

_Nacnacand C_Ratoms, and the structures of both 7-OTf

and 7-BH₄also reveal close arene interactions between

Ti and the ortho and meta carbons of the aryl groups

(average Ti-C_arenedistance is ∼2.58 Å). The molecular

structures for complexes 7-OTf and 7-BH₄are displayed

in Figure 5.

Previous work has documented C-H activation reac-

tions of the supporting Nacnac^-ligand in the ortho-aryl

positions, in the substituents on the â-carbon of the

backbone, and in 1° carbons attached to groups in the

ortho-aryl positions.³⁷In general C-H activation of 1°

carbons in the 2,6-dialkyl substituents on the aryl rings

(37) For the most current and complete description of chemistry

concerning â-diketiminate complexes see: (a) Bourget-Merle, L.;

Lappert, M. F.; Severn, J. R. Chem. Rev. 2002, 102, 3031-3066. (b)

Harder, S. Angew. Chem., Int. Ed. 2003, 42, 3430-3434, and references

therein.

(38) (a) Hayes, P. G.; Piers, W. E.; Lee, L. W. M.; Knight, L. K.;

Parvez, M.; Elsegood, M. R. J.; Clegg, W. Organometallics 2001, 20,

2533-2544. (b) Piers, W. E.; Emslie, D. J. H. Coord. Chem. Rev., 2002,

233-234, 131-155. (c) Knight, L. K.; Piers, W. E.; McDonald, R. Chem.

Eur. J. 2000, 6, 4322-4326.

Four-Coordinate Titanium Alkylidene Complexes

Organometallics, Vol. 24, No. 8, 2005 1895

Figure 6. (A) Concentration dependence at 57 °C for the thermal transformation of the titanium alkylidene (Nacnac)-

TidCH^tBu(I) 3-I. (B) Temperature dependence plot for the thermal transformation of the titanium alkylidene (Nacnac)-

TidCH^tBu(I) 3-I (temperature reported in °C). The complete data set for the run of 31.3 °C is not shown since the reaction

took over 314 min to go one half-life. (C) Eyring plot for the thermal transformation of the titanium alkylidene (Nacnac)-

TidCH^tBu(I) 3-I. (D) Solvent dependence plot for the thermal transformation of the titanium alkylidene (Nacnac)-

TidCH^tBu(I) 3-I at 31.3 °C

platinum(II) complex.^36aIn contrast our work estab-

lishes a C-H abstraction reaction by which two methine

groups are selectively cleaved under relatively mild

conditions.³⁹In the context of group 4 metal alkylidenes,

the addition of one C-H bond of a phenyl group in a

bulky (phenylphosphino)alkoxide ligand across the double

bond system of a transient titanium alkylidene has been

reported.⁴⁰

Scheme 5. Thermal Transformation of the

Titanium Alkylidene (Nacnac)TidCH^tBu(I), 3-I

Transformation of Complex 3-I to 5-I. Mecha-

nistic Investigation of an Intramolecular Cross-

Metathesis Reaction. Remarkably it was found

that complex 3-I cleanly transforms to solely one

product (31.3-65.5 °C), (H^tBuCdC(Me)CHC(Me)N[Ar])-

TidNAr(Ι), 5-I, without any formation of the hypotheti-

cal C-H abstraction derivative 7-I (Scheme 4). Appar-

ently the lower activation energy to form 5-I combined

with the greater Lewis acidity of the metal precludes

any secondary and indepenent side reaction from oc-

curring. Concentration-dependent experiments (by H

1

NMR spectroscopy, 0.225-0.0637 M) and fitting of the

data to a first-order decay plot determined the reaction

to be first-order in titanium with a rate constant k )

8.1(4) × 10^-4s^-1(Figure 6A). From the rate we calculate

the t_1/2for 3-I to be 14.4 min at 57 °C, which is relatively

faster than the estimated t_1/2values of the derivatives

3-X (X^-) OTf, ∼45.0 min at 57 °C; Cl, ∼16.9 min at 57

°C; Br, ∼19.2 min at 57 °C). Temperature dependence

studies (Figure 6B) allowed for extraction of the activa-

tion parameters (S^q) -8(3) cal/mol‚K^-1and ∆H^q)

21.3(2) kcal/mol) from the Eyring plot (Figure 6C).

Hence results indicate that formation of 5-I proceeds by

an intramolecular rearrangement stemming from 3-I.

Scheme 5 depicts a metalacyclobutane intermediate or

transition state species leading to formation of 5-I.

Kinetic studies also reveal formation of 5-I to be

(39) Double C-H activation of two 1° carbons has been documented

recently for electron-deficient Rh(III) systems: Dorta, R.; Stevens, E.

D.; Nolan, S. P. J. Am. Chem. Soc. 2004, 126, 5054-5055. Scott, N.

M.; Dorta, R.; Stevens, E. D.; Correa, A.; Cavallo, L.; Nolan, S. P. J.

Am. Chem. Soc. 2005, ASAP article.

(40) (a) van Doorn, J. A.; van der Heijden, H.; Orpen, A. G.

Organometallics, 1994, 13, 4271-4277. (b) van Doorn, J. A.; van der

Heijden, H.; Orpen, A. G. Organometallics 1995, 14, 1278-1283. Other

examples of C-H activation reactions involving transient titanium

alkylidenes have been reported: (c) Deckers, P. J. W.; Hessen, B.

Organometallics 2002, 21, 5564-5575. (d) Kickham, J. E.; Gue´rin, F.;

Stephan, D. W. J. Am. Chem. Soc. 2002, 124, 11486-11494. (f) Cheon,

J.; Rogers, D. M.; Girolami, G. S. J. Am. Chem. Soc. 1997, 119, 6804-

6813.

1896 Organometallics, Vol. 24, No. 8, 2005

Basuli et al.

Scheme 6. Cross-Metathesis Reactions of the

Titanium Alkylidene (Nacnac)TidCH^tBu(OTf),

3-OTf

Figure 7. DFT calculations on the model complex

(Nacnac*)TidCHMe(Cl) and structural comparisons with

3-OTf. The HOMO and LUMO are also depicted. Complete

geometrical features for the optimized geometry are in-

cluded in the Supporting Information.

independent of solvents such as C₆H₆, THF, and Et₂O

(Figure 6D). Therefore the mechanism leading to the

cross-metathesis reaction observed in 3-X (X^-) OTf,

Cl, Br, I, and BH₄) does not likely involve dissociation

of the Nacnac^-and X^-ligand or coordination of solvent.

Although enthalpy makes a favorable contribution to

the chelate effect, the main source of effect is found in

the entropy.⁴¹We cannot however rule out the possibil-

ity that a hapticity change in Nacnac^-could occur

during the transformation of 3-X to 5-X.

DFT Calculations of the Titanium Alkylidene

Complexes 3-X. Theoretical calculations were per-

formed on the titanium alkylidene model complex

(Nacnac*)TidCHMe(Cl) (Nacnac*^-) [Ar]NCHCHCHN-

[Ar], Ar ) 2,6-Me₂C₆H₃) using density funtional theory

(DFT) and the B3PW91 functional. The optimized

geometry reproduced the key features of the solid-state

structure of 3-OTf expectedly well (Figure 7). The

calculated molecular orbitals for the model (Nacnac*)-

TidCHMe(Cl) indicate the HOMO to be the π orbital

in the TidC double bond, while the LUMO is composed

predominantly of π* character localized about the N-C

bonds of the Nacnac^-backbone with a node at the γ-C

supplemented with some π* character in the TidC bond

(Figure 7).

The match in symmetry of the frontier orbitals

combined with the observed chemical reactivity suggest

tantalizingly that complex 3-X has a reactive alkylidene

functionality capable of both transferring the car-

benoid motif to give 5-X and activating inert carbon-

hydrogen bonds to afford titanacycles such as 7-X.

Kinetic results in combination with the theoretical

findings also suggest that the cross-metathesis trans-

formation occurs by an intramolecular [2+2] cycload-

dition in which the Nacnac^-ligand rearranges (without

dissociation) by placing the imine functionality across

the TidC bond, yielding the putative azametallacy-

clobutane complex. Electron shuffling then leads to the

final product 5-X (Scheme 5).

Reactivity of the TidC_alkylideneFunctionality in

Complex 3-OTf. Reactions of 3-OTf with Olefins.

Alkylidene 3-OTf failed to react with olefins such as

ethylene, styrene, trans-stillbene, 1-hexene, and chloro-

substituted alkenes. Electron-rich olefins such as 2,3-

and 2,5-dihydrofuran appear to react slowly with 3-OTf,

but attempts to isolate the product(s) only yielded 5-OTf

and 7-OTf. Internal alkynes did not react cleanly with

the TidC bond of 3-OTf. Attempts to carry out these

reactions at higher temperatures resulted only in rapid

transformation of 3-OTf to complexes 5-OTf and 7-OTf.

Thus, it appears that the reactivity of 3-OTf is ham-

pered significantly by its thermal rearrangement to

5-OTf and 7-OTf.

Reactions of 3-OTf with Polar Molecules. Com-

plex 3-OTf reacts rapidly and cleanly with 1 equiv of

benzophenone at room temperature to afford the olefin

H^tBuCdCPh₂and 0.5 equiv of the edge-sharing bio-

42

ctahedra titanium oxo dimer [(Nacnac)Ti(µ₂-O)(µ₂-OTf)]₂

(8)^14,43(Scheme 6). The reactivity observed between

3-OTf and benzophenone follows well-established “Wit-

tig-type” reagents studied in organic synthesis.⁷Like-

wise, CO₂also reacts with 3-OTf to yield the µ-oxo

-

titanium dimer [(L₁)₂Ti₂(µ₂-O)₂(µ₂-OTf)][OTf] (9) (L₁

)

[Ar]NC(CH₃)CHC(O)dC^tBuHC(CH₃)N[Ar]). Complex 9

results from a cross-metathesis reaction, leading to

formation of putative 8 and the ketene BuHCdCdO

t

(Scheme 6). Nucleophilic attack of the ketene generated

in the reaction by the γ-carbon of the Nacnac^-ring

yields complex 9. The ¹⁹F NMR spectrum displays two

independent resonances consistent with a bridging and

a nonbound OTf^-. The solid-state structure of 9 also

confirms these spectroscopic features as well as the

inferred degree of aggregation (vide infra). Indepen-

dently, we have also shown that complex 8 reacts with

the much more stable diphenylketene⁴⁴derivative to

yield a system analogous to 9, namely, [(L₂)₂Ti₂(µ₂-O)₂-

(µ₂-OTf)][OTf] (10) (L₂^-) [Ar]NC(CH₃)CHC(O)dCPh₂C-

(CH₃)N[Ar]) (Scheme 6). NMR (¹H and ¹³C) spectra for

9 and 10 are broader than usual and can likely be

attributed to a fluxional process occurring in solution.

Precedent in the literature concerning addition of elec-

trophiles to â-diketiminates has been described previ-

ously by us and others.^33n,45Complexes 9 and 10 have

been scrutinized by single-crystal X-ray diffraction

(42) The formation of alkene was confirmed by spectroscopic com-

parison with ¹H NMR shifts reported in the literature. Adam, W.;

Baeza, J.; Liu, J.-C. J. Am. Chem. Soc. 1972, 94, 2000-2006.

(43) The molecular structure of 8 showed an edge-sharing biocta-

hedra geometry composed of two bridging oxo and triflate ligands. See

ref 14.

(41) Cotton, F. A.; Wilkinson, G.; Murrillo, C A. Advanced Inorganic

Chemistry; John Wiley and Sons: New York, 1999.

(44) Taylor, E. C.; McKillop, A.; Hawks, G. H. Org. Synth. 1988,

50, 549.

Four-Coordinate Titanium Alkylidene Complexes

Organometallics, Vol. 24, No. 8, 2005 1897

TidNCHPh₂(NC^tBu)(OTf) (14) forms from NdN rupture

of the diazodiphenylmethane and subsequent alkylidene

and hydrogen atom transfer to yield TidN and CtN

bonds. These occurrences likely proceed through an

azine intermediate resulting from addition of N₂CPh₂

across the TidC bond. Under dynamic vacuum, complex

14 readily loses CN^tBu to yield the four-coordinate

titanium imide complex (Nacnac)TidNCHPh₂(OTf).

Scheme 7 displays the synthesis and structural diagram

for the isonitrile adduct 14.

Addition of N₃Ad to 3-OTf results in effervescence of

the mixture concomitant with formation of the four-

coordinate titanium imide (Nacnac)TidNCH^tBuAd(OTf)

(15) (Scheme 7). It is proposed that complex 15 results

from nucleophilic attack of azide to the metal center,

which leads to extrusion of N₂and subsequent adaman-

tyl (Ad) radical migration to the former alkylidene

carbon. As noted previously in the thermolysis of 3-OTf,

formation of complexes 14 and 15 appears to be ther-

modynamically driven due to the formation of a strong

TidN bond in a low-coordinate environment. Single-

crystal X-ray structures of 14 and 15 are displayed in

Scheme 7, and both systems reveal short TidN_imide

bonds (∼1.68 Å).

Figure 8. Molecular structure of complex [9]⁺with

thermal ellipsoids at the 50% probability level. All H atoms,

O and CF₃groups of the µ₂-OTf^-, and aryl groups with the

exception of ipso-carbons have been omitted for clarity.

Three disordered CH₂Cl₂molecules are omitted from the

structure of complex [9]⁺. Crystallographic data for 9 are

reported in Table 4, and complete metrical parameters for

both 9 and 10 are listed in the Supporting Information.

(Figure 8 displays the molecular structure of [9]⁺), and

complete structural data are included in the Supporting

Information.

Alkylidene 3-OTf was found to react with SCNPh to

afford the complex (Nacnac)Ti(η²-(S,N)-SCdCH^tBuNPh)

(11) in 88% yield. Complex 11 results from a [2+2]

cycloaddition of the SdC functionality across the

TidC bond and subsequent Ti-C bond breaking and

rotation of the S-C linkage (Scheme 7). Complex 11

is likely favored thermodynamically due to formation

Protonation of 3-OTf. The nucleophilic character of

3-OTf was demonstrated with a Bronsted acid. Ben-

zophenone imine cleanly reacts with 3-OTf to afford the

t

diphenylketiminide alkyl complex (Nacnac)Ti(CH₂Bu)-

(NdCPh₂) (16) in 73% yield (Scheme 7). Diagnostic

features for 16 include a ¹H NMR methylene resonance

for the neopentyl ligand at 1.88 ppm. In addition, the

former sp²alkylidene ¹³C NMR resonance has now

shifted to the titanium alkyl region at 53.1 ppm.

Treatment of 3-OTf with ammonium or anilinium salts

as well as other weak Brφnsted acids has resulted in a

mixture of products.

1

of a strong Ti-N bond. H and ¹³C NMR spectra of 11

are in accord with this molecule retaining C₁symmetry

in solution. Single-crystal X-ray diffraction studies of

11 also support η²-(S,N) coordination of the ligand

(Scheme 7).

The nitrile NCCH₂Mes (Mes ) 2,4,6-Me₃C₆H₂) un-

dergoes a cross-metathesis with alkylidene 3-OTf to

afford the four-coordinate titanium imido (Nacnac)-

TidN[CdCH^tBu(CH₂Mes)](OTf) (12) (Scheme 7). Com-

plex 12 forms from a putative azametalacyclobutene

intermediate. This type of reaction complements

Schrock’s original work on the reaction of alkynes and

nitriles with tantalum alkylidenes.^1a,46On the other

hand the isonitrile CN^tBu inserts into the TidC bond

of 3-OTf to afford the azaallene or N-alkylketenimine

complex (Nacnac)Ti(η²-(N,C)-^tBuNdCdCH^tBu)(OTf) (13)

Inhibiting an Intramolecular Cross-Metathesis

Reaction. Synthesis of the Four-Coordinate

Titanium Alkylidene Complexes (Nacnac_tBu)-

-

TidCH^tBu(X) (X^-) OTf, I; Nacnac_tBu) [Ar]NC-

(^tBu)CHC(^tBu)N[Ar], Ar ) 2,6-(CHMe₂)₂C₆H₃). Given

the facility for 3-X to undergo rearrangement to 5-X we

sought incorporation of a bulkier â-diketiminate ligand

which would inhibit formation of the putative azamet-

allacyclobutane complex resulting from intramolecular

t

[2+2] cycloaddition. Substituting Bu groups in the â-

carbon positions of the Nacnac^-ligand has been shown

to be effective at stabilizing low coordination numbers

on metal centers.^15a,37,38,47Unfortunately attempts to

1

in 88% yield as black crystals (Scheme 7). H and ¹³C

NMR spectra are in accord with the molecule having

C_ssymmetry. The η²-(N,C) coordination of the N-tert-

butylketenimine in 13 was confirmed by single-crystal

X-ray diffraction studies (Scheme 7).

Perhaps the most unusual transformation of 3-OTf

involves the reaction with N₂CPh₂to provide a titanium

imide with a coordinated nitrile. Complex (Nacnac)-

incorporate the â-diketiminate ligand Li(Nacnac_tBu

)

(Nacnac_tBu^-) [Ar]NC(^tBu)CHC(^tBu)N[Ar]) onto TiCl₃-

(THF)₃proved difficult and led instead to undesirable

products.^33l,48However, we found that room-tempera-

ture reaction of TiCl₃and Li(Nacnac_tBu) in Et₂O for 2

days led to isolation of (Nacnac_tBu)TiCl₂(17) in 23%

yield. Complex 17 has been fully characterized, and the

X-ray structure reveals a four-coordinate titanium

center, which is protected by the steric encumbrance of

the bulkier â-diketiminate ligand (see Supporting In-

formation). The widening of the N-Ti-N angle is in

turn compensated by the deviation of the metal center

(∼1.07 Å for 17) from the NCCCN plane of the

(45) Radzewich, C. E.; Coles, M. P.; Jordan, R. F. J. Am. Chem. Soc.

1998, 120, 9384-9385. Electrophilic substitution with Ph₂P⁺or PhClP⁺

at the γ-carbon of the Nacnac^-backbone has been recently reported.

(a) Hitchcock, P. B.; Lappert, M. F.; Nycz, J. E. Chem. Commun. 2003,

1142-1143. (b) Ragogna, P. J.; Burford, N.; D’eon, M.; McDonald, R.

Chem. Commun. 2003, 1052-1053, and references therein. In addition,

R-alkoxylithium-â-diimine ligands have been prepared from the reac-

tion of the lithium-Nacnac salt and the corresponding ketone. Carey,

D. T.; Cope-Eatough, E. K.; Vilaplana-Mafe´, E.; Mair, F. S.; Pritchard,

R. G.; Warren, J. E.; Woods, R. J. J. Chem. Soc., Dalton Trans. 2003,

1083-1093.

-

Nacnac_tBuligand, which is in contrast to the planar

(46) Schrock, R. R. Science 1983, 219, 13-18.

dichloro systems reported by Theopold’s ([Ph]NC(Me)-

1898 Organometallics, Vol. 24, No. 8, 2005

Basuli et al.

Scheme 7. Reactivity for the Terminal Titanium Alkylidene (Nacnac)TidCH^tBu(OTf), 3-OTf^a

a

Thermal ellipsoid plots are at the 50% probability level. H atoms with the exception of the formal alkylidene R-hydrogen and

aryl groups with the exception of ipso carbons on the Nacnac^-ligand have been omitted for clarity. Disordered solvent molecules

have been omitted from the structure of complexes 12, 13, and 15. Crystallographic data are reported in Tables 4 and 5, and

complete metrical parameters for the molecular structure of complexes 11-15 are listed in the Supporting Information.

15f

CHC(Me)N[Ph])TiCl₂and Budzelaar’s ([Mes]NC(R)-

CHC(R)N[Mes])TiCl₂(Mes ) 2,4,6-Me₃C₆H₂; R ) Me or

^tBu).^15a,49Complex 17 is closely related, structurally, to

d⁰scandium derivative (Nacnac_tBu)ScCl₂prepared by

Piers and co-workers in which the Sc atom sits 0.815 Å

out of the NCCCN plane.³⁸Unlike compound 1, THF

appears to catalyze the decomposition of 17.

t

Transmetalation of 17 with 2 equiv of LiCH₂Bu

leads to rapid formation of the bis-neopentyl complex

t

(47) (a) Smith, J. M.; Lachicotte, R. J.; Pittard, K. A.; Cundari, T.

R.; Lukat-Rodgers, G.; Rodgers, K. R.; Holland, P. L. J. Am. Chem.

Soc. 2001, 123, 9222-9223. (b) Smith, J. M.; Lachicotte, R. J.; Holland,

P. L. Chem. Commun. 2001, 1542-1543. (c) Smith, J. M.; Lachicotte,

R. J.; Holland, P. L. Organometallics 2002, 21, 4808-4814. (d) Bailey,

P. J.; Liddle, S. T.; Morrison, C. A.; Parsons, S. Angew. Chem., Int.

Ed. 2001, 40, 4463-4466. (e) Bailey, P. J.; Coxall, R. A.; Dick, C. M.;

Fabre, S.; Parsons, S. Organometallics 2001, 20, 798-801. (f) Knight,

L. K.; Piers, W. E.; McDonald, R. Chem. Eur. J. 2000, 6, 4322-

4326. (g) Spencer, D. J. E.; Aboelella, N. W.; Reynolds, A. M.;

Holland, P. L.; Tolman, W. B. J. Am. Chem. Soc. 2002, 124, 2108-

2109. (h) Hayes, P. G.; Piers, W. E.; McDonald, R. J. Am. Chem. Soc.

2002, 124, 2132-2133. (i) Andres, H.; Bominaar, E. L.; Smith, J. M.;

Eckert, N. A.; Holland, P. L.; Munck, E. J. Am. Chem. Soc. 2002, 124,

3012-3025. (j) Gibson, V. C.; Marshall, E. L.; Navarro-Llobet, D.;

White, A. J. P.; Williams, D. J. J. Chem. Soc., Dalton Trans. 2002,

4321-4322. (k) Aboelella, N. W.; Lewis, E. A.; Reynolds, A. M.;

Brennessel, W. W.; Cramer, C. J.; Tolman, W. B. J. Am. Chem. Soc.

2002, 124, 10660-10661. (l) Vela, J.; Smith, J. M.; Lachicotte, R. J.;

Holland, P. L. Chem. Commun. 2002, 2886-2887. (m) Holland, P. L.;

Cundari, T. R.; Perez, L. L.; Eckert, N. A.; Lachicotte, R. J. J. Am.

Chem. Soc. 2002, 124, 14416-14424. (n) Smith, J. M.; Lachicotte, R.

J.; Holland, P. L. J. Am. Chem. Soc. 2003, 125, 15752-15753. (o) Vela,

J.; Vaddadi, S.; Cundari, T. R.; Smith, J. M.; Gregory, E. A.; Lachicotte,

R. J.; Flaschenriem, C. J.; Holland, P. L. Organometallics 2004, 23,

5226-5239.

(Nacnac_tBu)Ti(CH₂Bu)₂(18) in 71% yield as green

blocks. Complex 18 is a close analogue to 2 and has no

unusual features. Scheme 8 depicts the reaction leading

to a more congested metal center in compounds 17 and

18, which is exacerbated by the sterically encumbering

^tBu group on the â-carbon position. This feature is

reflected by the more acute Ti-N-C_ipsoangle in the

(48) Addition of Li(Nacnac_tBu) to TiCl₃(THF)₃in toluene and reflux

for 3 days did not lead to the desire precursor (Nacnac_tBu)TiCl₂, but

instead afforded diamagnetic (Nacnac_tBu)TidNAr(Cl) in 39% yield. It

has been postulated in the literature that similar CdN bond cleavage

reactions using Nacnac^-occur through a putative (Nacnac)₂TiCl

intermediate (ref 33l). The organic byproducts generated from either

of these reactions have not been identified. Falguni, B.; Clark, R. L.;

Bailey, B. C.; Brown, D.; Huffman, J. C.; Mindiola, D. J. Chem.

Commun., in press.

(49) Significant deviation of the Ti atom from the NCCCN plane

has been observed with less bulky â-diketiminate systems of Ti(IV).

Kakaliou, L.; Scanlon IV, W. J.; Qian, B.; Baek, S. W.; Smith, M. R.,

III; Motry, D. H. Inorg. Chem. 1999, 38, 5964-5977. For the five-

coordinate titanium complex (Nacnac)TiCl₂(THF), the metal atom sits

∼0.778 Å from the NCCCN mean plane.

Four-Coordinate Titanium Alkylidene Complexes

Organometallics, Vol. 24, No. 8, 2005 1899

Scheme 8. Synthesis and Thermolysis of the Titanium Alkylidene (Nacnac_tBu)TidCH^tBu(X) (X- ) OTf,

19-OTf; I, 19-I)^a

^aComplete structural parameters for 17, 18, 19-OTf, and 19-I are included in the Supporting Information.

molecular structure of 18 (e.g., ∼124° for 2 vs ∼121°

for 18, see Supporting Information).

The titanium neopentylidene species (Nacnac)-

TidCH^tBu(X) are all kinetic products and have been

shown to undergo intramolecular cross-metathesis as

well as double tertiary carbon-hydrogen activation

reactions. In addition, these reactive compounds can

engage cleanly in cross-metathesis, cycloaddition, inser-

tion, and acid-base reactions involving the alkylidene

functionality.

One-electron oxidation of 18 with 1 equiv of AgOTf

or 0.5 equiv of I₂promotes R-hydrogen abstraction to

form the alkylidene species (Nacnac)TidCH^tBu(X) (X^-

) OTf, 19-OTf, 83% yield; X^-) I, 19-I, 75% yield,

Scheme 8). Spectral and structural features for 19-OTf

and 19-I are similar to their corresponding 3-OTf and

3-I derivatives (see Experimental Section and Support-

ing Information). Perhaps the most interesting feature

of the four-coordinate neopentylidene systems 19-OTf

and 19-I is the steric hindrance provided by the more

encumbering â-diketiminate ligand, which bans ther-

modynamic rearrangement to the titanium imide prod-

The steric encumbrance provided by the supporting

â-diketiminate ligand (Nacnac_tBu^-) has a tremendous

effect in the kinetic stability of the four-coordinate

titanium neopentylidene complexes by blocking the

intramolecular [2+2] cycloaddition reaction. We are

currently exploring what factors govern the activation

of activation of 3° carbons, as well as the reactivity of

ucts (H^tBuCdC(^tBu)CHC(^tBu)N[Ar])TidNAr(X) (X^-

)

-

OTf or I) (Scheme 8). In fact, exhaustive heating

(40-90 °C) of 19-OTf and 19-I did not lead to decom-

position. As a result, the putative azametalacyclobutane

complex formed in the thermolysis of alkylidene 3-X is

completely inhibited in complex 19, presumably due to

the steric repulsion among the proximal ^tBu groups. The

molecular structures of 19-OTf and 19-I are examples

of kinetically stable titanium complexes containing low-

coordination environments as well as a terminal and

highly nucleophilic neopentylidene functionality.

titanium alkylidene complexes having the Nacnac_tBu

skeleton, since these systems have proven to be much

more kinetically stable than the conventional Nacnac^-

analogues.

Experimental Section

General Considerations. Unless otherwise stated, all

operations were performed in a M. Braun Lab Master double-

drybox under an atmosphere of purified nitrogen or using high-

vacuum standard Schlenk techniques under an argon atmo-

sphere.⁵⁰Anhydrous n-hexane, pentane, toluene, and benzene

were purchased from Aldrich in sure-sealed reservoirs (18 L)

and dried by passage through one column of activated alumina

and one of Q-5.⁵¹Diethyl ether and CH₂Cl₂were dried by

passage through two columns of activated alumina.⁵¹THF was

distilled, under nitrogen, from purple sodium benzophenone

ketyl and stored over sodium metal. Distilled THF was

transferred under vacuum into bombs before being pumped

into a drybox. C₆D₆and CD₂Cl₂were purchased from Cam-

bridge Isotope Laboratory (CIL), degassed, and dried over 4

Å molecular sieves and CaH₂, respectively. CD₂Cl₂was vacuum

transferred from the CaF₂mixture and stored in a reaction

vessel under N₂. Et₂O-d₁₀and THF-d₈were purchased from

CIL and stored over Na film. Celite, alumina, and 4 Å

molecular sieves were activated under vacuum overnight at

200 °C. Li(Nacnac)⁵²(Nacnac^-) [Ar]NC(Me)CHC(Me)N[Ar],

Concluding Remarks

In this work we have reported a facile synthetic

method for generating a family of low-coordinate tita-

nium alkylidene complexes, all of which have allowed

unprecedented reactivity of the TidCH^tBu motif. Tita-

nium carbon double bonds were created by a one-

electron oxidation and subsequent R-hydrogen abstrac-

tion reactions. We determined that the nature of the

oxidant and the steric bulk of the alkyl substituent on

the metal center both influence the R-abstraction pro-

cess. It seems very logical to propose that the oxi-

datively induced R-abstraction reaction transpires in

three distinct steps: Outer sphere electron transfer

occurs first followed by anion binding to the electropos-

itive metal center to generate a coordinately saturated

species. The latter step leads to the rate-determining

step in the reaction, which is the R-abstraction process.

This process is consistent with the principle of sterically

induced R-hydrogen abstraction reactions promoted by

Lewis base addition. The key components facilitating

these sequential steps are the use of bulky alkyl groups

and congesting anions that have the ability to coordinate

to the metal center. It is unlikely that the anion from

the oxidant acts as a Brφnsted base during the R-ab-

straction process since F^-does not appear to catalyze

the formation of a neopentylidene titanium fluoride

complex.

Ar ) 2,6-(CHMe₂)₂C₆H₃), (Nacnac)TiCl₂(THF) (1),^14,15a(Nacnac)-

TiMe₂(THF),^15aLiCH₂Bu,^3bPh₃PdCH₂LiBr,²⁵Li(Nacnac_tBu

)

t

(Nacnac_tBu_{) [Ar]NC(}Bu)CHC(^tBu)N[Ar]),^15aN₂CPh₂,⁵³[FeCp*₂]-

-

t

[B(C₆F₅)₄],^20-22OC₂Ph₂,⁴⁴(Nacnac)Ti(CH₂Bu)₂(2),¹⁴(Nacnac)-

t

TidCH^tBu(OTf)(3-OTf),¹⁴(H^tBuCdC(Me)CHC(Me)N[Ar])-

(50) For a general description of the equipment and techniques used

in carrying out this chemistry see: Burger, B. J.; Bercaw, J. E. In

Experimental Organometallic Chemistry; Wayda, A. L., Darensbourg,

M. Y., Eds.; ACS Symposium Series 357; American Chemical Society;

Washington, D.C., 1987; pp 79-98.

(51) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.;

Timmers, F. J. Organometallics 1996, 15, 1518-1520.

(52) Stender, M.; Wright, R. J.; Eichler, B. E.; Prust, J.; Olmstead,

M. M.; Roesky, H. W.; Power, P. P. J. Chem. Soc., Dalton Trans. 2001,

3465-3469, and references therein.

(53) Miller, J. B. J. Org. Chem., 1959, 24, 560-561.

1900 Organometallics, Vol. 24, No. 8, 2005

Basuli et al.

TidNAr(OTf) (5),¹⁴(H^tBuCdC(Me)CHC(Me)N[Ar])TidNAr-

(OTf)(Et₂O) (6),¹⁴and [(Nacnac)Ti(µ₂-O)(µ₂-OTf)]₂(8)¹⁴were

prepared according to the literature. All other chemicals were

used as received. Solution infrared spectra (CaF₂plates) were

measured using a Nicolet 510P FTIR spectrometer. CHN

29.41 (CHMe₂), 28.99 (CHMe₂), 27.11 (CHMe₂), 24.76 (CHMe₂),

24.76 (CHMe₂), 24.40 (CHMe₂), 23.91 (C(Me)CHC(Me)). Anal.

Calcd for C₃₄H₅₁N₂BrTi: C, 66.34; H, 8.35; N, 4.55. Found: C,

66.61; H, 8.35; N 4.78.

Synthesis of (Nacnac)TidCH^tBu(I), 3-I. In a vial was

dissolved 2 [120 mg, 0.198 mmol] in 5 mL of toluene and the

solution cooled to -35 °C. To the solution was added a cold

toluene solution (5 mL) of iodine [27.6 mg, 0.109 mmol]. After

15 min the solutions turned violet and the reaction mixture

was pumped dry, then redissolved in pentane. The resulting

solution was filtered, concentrated, and cooled to -35 °C to

afford dark violet crystals of 3-I [80 mg, 0.121 mmol, 61%

yield]. ¹H NMR (23 °C, 399.8 MHz, C₆D₆): δ 7.24-7.04 (m,

C₆H₃, 6H), 4.79 (s, C(Me)CHC(Me), 1H), 3.64 (septet, CHMe₂,

2H), 3.42 (septet, CHMe₂, 2H), 3.26 (s, TidCH^tBu, 1H), 1.69

(d, CHMe₂, 6H), 1.50 (s, C(Me)CHC(Me), 6H), 1.38 (d, CHMe₂,

6H), 1.23 (d, CHMe₂, 6H), 1.09 (d, CHMe₂, 6H), 0.72 (s,

TidCH^tBu, 9H). ¹³C NMR (25 °C, 100.6 MHz, C₆D₆): δ 272.4

(TidCH^tBu, J_C-H) 85.0 Hz), 167.0 (C(Me)CHC(Me), 146.1

(ipso-C₆H₃), 143.0 (o-C₆H₃), 140.5 (o-C₆H₃), 127.2 (p-C₆H₃),

124.7 (m-C₆H₃), 124.2 (m-C₆H₃), 95.83 (C(Me)CHC(Me)),

49.84 (TidCHC(Me)₃), 31.38 (TidCHCMe₃), 29.85 (CHMe₂),

28.76 (CHMe₂), 27.30 (CHMe₂), 24.66 (CHMe₂), 24.56 (CHMe₂),

24.51 (CHMe₂), 23.91 (C(Me)CHC(Me)). Anal. Calcd for

C₃₄H₅₁N₂ITi: C, 61.63; H, 7.76; N, 4.23. Found: C, 61.61; H,

7.47; N, 4.40.

1

analysis was performed by Desert Analytics, Tucson, AZ. H,

¹³C, and ¹⁹F NMR spectra were recorded on Varian 400 or 300

MHz NMR spectrometers. ¹H and ¹³C NMR are reported with

reference to solvent resonances (residual C₆D₅H in C₆D₆, 7.16

and 128.0 ppm; CHDCl₂in CD₂Cl₂, 5.32 ppm and 53.8; proteo

THF in THF-d₈, 3.58, 1.73, and 67.4, 25.3; proteo toluene in

C₆D₅CD₃, 7.09, 7.00, 6.98, 2.09 and 137.5, 128.9, 128.0, 125.2,

20.4; proteo Et₂O in Et₂O-d₁₀, 3.34, 1.07 and 65.3, and 14.5).

¹⁹F NMR chemical shifts are reported with respect to external

HOCOCF₃(-78.5 ppm). ¹¹B NMR spectra were reported with

respect to external BF₃‚(OEt₂) (0.0 ppm). Magnetic moments

were obtained by the method of Evans.⁵⁴Cyclic voltammetry

was collected with the assistance of an E2 Epsilon potentiostat/

galvanostat with a PC unit controlled by Bioanalytical Systems

(BAS) software. Room-temperature and liquid helium tem-

perature X-band EPR spectra were recorded on a Bruker EMX

spectrometer. Acquisition and simulation were performed

using an integrated WIN-EPR software package (Bruker).

X-ray diffraction data were collected on a SMART6000 (Bruk-

er) system under a stream of N₂(g) at low temperatures.

Synthesis of (Nacnac)TidCH^tBu(Cl), 3-Cl. In a vial was

dissolved 3-OTf [180 mg, 0.262 mmol] in 5 mL of THF and

the solution cooled to -35 °C. To the solution was added a

cold THF (∼5 mL) suspension of magnesium chloride [75 mg,

0.787 mmol], which was broken into a fine powder. After 10

min the solutions turned red-violet and the reaction mixture

was allowed to stir for an additional 2 h. The solution was

dried, dissolved in pentane, and then filtered, leaving behind

a white precipitate. The solution was concentrated, then

cooled. Recrystallization from pentane at -35 °C afforded black

needles of 3-Cl [101 mg, 0.176 mmol, 67% yield]. ¹H NMR (23

°C, 399.8 MHz, C₆D₆): δ 7.2-7.1 (m, C₆H₃, 6H), 5.54 (s,

TidCH^tBu, 1H), 4.82 (s, C(Me)CHC(Me), 1H), 3.76 (septet,

CHMe₂, 2H), 3.20 (septet, CHMe₂, 2H), 1.71 (d, CHMe₂, 6H),

1.52 (s, C(Me)CHC(Me), 6H), 1.45 (d, CHMe₂, 6H), 1.19 (d,

CHMe₂, 6H), 1.18 (d, CHMe₂, 6H), 0.71 (s, TidCH^tBu, 9H).

¹³C NMR (25 °C, 100.6 MHz, C₆D₆): δ 276.0 (TidCH^tBu, J_C-H

) 92.78 Hz), 167.8 (C(Me)CHC(Me), 145.0 (ipso-C₆H₃), 143.1

(o-C₆H₃), 140.5 (o-C₆H₃), 127.1 (p-C₆H₃), 124.5 (m-C₆H₃), 124.2

(m-C₆H₃), 96.79 (C(Me)CHC(Me)), 48.91 (TidCHCMe₃), 31.24

(TidCHCMe₃), 29.01 (CHMe₂), 28.98 (CHMe₂), 26.77 (CHMe₂),

24.74 (CHMe₂), 24.65 (CHMe₂), 24.19 (CHMe₂), 23.77 (C(Me)-

CHC(Me)). Anal. Calcd for C₃₄H₅₁N₂ClTi: C, 71.50; H, 9.00;

N, 4.90. Found: C, 71.15; H, 8.96; N 5.26.

Synthesis of (Nacnac)TidCH^tBu(Br), 3-Br. In a vial was

dissolved 3-OTf [180 mg, 0.262 mmol] in 5 mL of ether and

the solution cooled to -35 °C. To the solution was added a

cold ether solution (5 mL) of triphenylmethylphosphonium

bromide [56 mg, 0.154 mmol]. After 15 min the solutions

turned red-violet and the reaction mixture was allowed to stir

for an additional 2 h. A white precipitate was filtered off, and

the solution was concentrated and cooled. Recrystallization

from Et₂O at -35 °C afforded in two crops brown crystals of

3-Br [100 mg, 0.163 mmol, 62% yield]. ¹H NMR (23 °C, 399.8

MHz, C₆D₆): δ 7.13-7.04 (m, C₆H₃, 6H), 4.84 (s, C(Me)CHC-

(Me), 1H), 4.57 (s, TidCH^tBu, 1H) 3.67 (septet, CHMe₂, 2H),

3.42 (septet, CHMe₂, 2H), 1.75 (d, CHMe₂, 6H), 1.55 (s, C(Me)-

CHC(Me), 6H), 1.46 (d, CHMe₂, 6H), 1.24 (d, CHMe₂, 6H), 1.19

(d, CHMe₂, 6H), 0.76 (s, TidCH^tBu, 9H). ¹³C NMR (25 °C, 100.6

MHz, C₆D₆): δ 274.2 (TidCH^tBu, J_C-H) 87.9 Hz), 167.6

(C(Me)CHC(Me), 145.5 (ipso-C₆H₃), 143.1 (o-C₆H₃), 140.5 (o-

C₆H₃), 127.1 (p-C₆H₃), 124.6 (m-C₆H₃), 124.3 (m-C₆H₃), 96.64

(C(Me)CHC(Me)), 49.21 (TidCHCMe₃), 31.38, (TidCHCMe₃),

Synthesis of (Nacnac)TidCH^tBu(BH₄), 3-BH₄. In a vial

was dissolved 3-OTf [200 mg, 0.29 mmol] in 10 mL of Et₂O

and the solution cooled to -35 °C. To the cold solution was

added a cold Et₂O suspension (5 mL) containing LiBH₄[6.36

mg, 0.29 mmol]. After stirring for 10 min the solution was

filtered and dried under reduced pressure. The brown powder

was extracted with hexane and filtered, and the filtrate

concentrated and cooled to -35 °C to afford in two crops brown

1

crystals of 3-BH₄[111.2 mg, 0.20 mmol, 68% yield]. H NMR

(23 °C, 399.8 MHz, C₆D₆): δ 7.17-7.05 (m, C₆H_3,6H), 5.32 (s,

TidCH^tBu, 1H), 4.96 (s, C(Me)CHC(Me), 1H), 4.11 (septet,

CHMe₂, 2H), 2.80 (septet, CHMe₂, 2H), 1.59 (s, C(Me)CHC-

(Me), 6H), 1.52 (br d, CHMe₂, 12H), 1.25 (d, CHMe₂, 6H), 1.07

(d, CHMe₂, 6H), 0.71 (TidCH^tBu, 9H). ¹³C NMR (25 °C, 100.6

MHz, C₆D₆): δ 266.7 (TidCH^tBu, J_C-H) 95 Hz), 167.9 (C(Me)-

CHC(Me)), 144.4 (ipso-C₆H₃), 142.92 (o-C₆H₃), 141.1 (o-C₆H₃),

126.8 (p-C₆H₃), 124.7 (m-C₆H₃), 124.2 (m-C₆H₃), 98.23 (C(Me)-

CHC(Me)), 46.83 (TidCHCMe₃), 31.54 (TidCHCMe₃), 29.16

(CHMe₂), 27.70 (CHMe₂), 26.17 (CH₃), 25.24 (CH₃), 24.51

(CH₃), 24.23 (CH₃), 24.18 (CH₃). ¹¹B NMR (23 °C, 128.4 MHz,

C₆D₆): δ -14.29 (quintet, BH₄, J_B-H) 89 Hz). Anal. Calcd for

C₃₄H₅₅N₂BTi: C, 74.18; H, 10.07; N, 5.09. Found: C, 73.81;

H, 9.79; N, 5.08.

t

Synthesis of [(Nacnac)Ti(CH₂Bu)₂][B(C₆F₅)₄], [2]-

[B(C₆F₅)₄]. In a vial was dissolved in 10 mL of Et₂O complex

2 [73 mg, 0.12 mmol] and the solution cooled to -35 °C. To

the cold solution was added a cold Et₂O suspension (∼5 mL)

containing [FeCp*₂][B(C₆F₅)₄] [119 mg, 0.12 mmol]. The solu-

tion rapidly turned orange and was allowed to stir for 3 min.

The solution was filtered and the filtrate dried under reduced

pressure. The orange powder was washed with pentane until

washings were clear, extracted with Et₂O, and filtered. The

filtrate was then concentrated, a few drops of (Me₃Si)₂O were

added, and the solution was cooled to -35 °C to afford orange

crystals of [2][B(C₆F₅)₄] [145 mg, 0.11 mmol, 92% yield]. Some

assignments of ¹H and ¹³C NMR resonances were deter-

mined by HMQC experiments.^{18 1}H NMR (23 °C, 399.8 MHz,

CD₂Cl₂): δ 7.53-7.39 (m, C₆H_3,6H), 6.20 (s, C(Me)CHC(Me),

t

1H), 3.33 (septet, CHMe₂, 2H), 2.95 (s, TiCH₂Bu, 2H), 2.90

t

(s, TiCH₂Bu, 2H), 2.44 (septet, CHMe₂, 2H), 2.33 (s, C(Me)-

CHC(Me), 6H), 1.59 (d, CHMe₂, 6H), 1.37 (d, CHMe₂, 6H), 1.29

(d, CHMe₂, 6H), 1.08 (d, CHMe₂, 6H), 0.95 (TiCH₂CMe₃, 9H),

0.62 (TiCH₂CMe₃, 9H). ¹³C NMR (25 °C, 100.6 MHz, CD₂Cl₂):

δ 170.9 (C(Me)CHC(Me)), 149.5 (B(C₆F₅)₄), 147.0 (B(C₆F₅)₄),

(54) (a) Sur, S. K. J. Magn. Reson. 1989, 82, 169-173. (b) Evans,

D. F. J. Chem. Soc. 1959, 2003-2005.

t

146.7 (TiCH₂Bu, based on HMQC), 141.8 (ipso-C₆H₃), 141.6

Four-Coordinate Titanium Alkylidene Complexes

Organometallics, Vol. 24, No. 8, 2005 1901

t

(o-C₆H₃), 139.7 (o-C₆H₃), 137.6 (B(C₆F₅)₄), 136.1 (TiCH₂Bu

based on HMQC), 135.2 (B(C₆F₅)₄), 130.6 (p-C₆H₃), 126.2 (m-

H^tBuC(Me)CHC(Me), 3H), 1.43 (d, CHMe₂, 6H), 1.29 (s,

H^tBuC(Me)CHC(Me), 9H), 1.17 (d, CHMe₂, 6H), 1.10 (d,

CHMe₂, 6H), 0.98 (d, CHMe₂, 6H). ¹³C NMR (25 °C, 100.5 MHz,

C₆D₆): δ 161.5 (C(Me)CHC(Me)), 158.0, 157.1, 147.6, 145.8

(C₆H₃), 143.7 (C₆H₃), 139.9 (C₆H₃), 134.2 (H^tBuCC(Me)CHC-

(Me), J_C-H) 129 Hz), 127.5 (C₆H₃), 125.5 (C₆H₃), 123.8 (C₆H₃),

C₆H₃), 126.1 (m-C₆H₃), 108.1, 100.2 (C(Me)CHC(Me), J_C-H

)

169 Hz), 43.96 (TiCH₂CMe₃), 41.80 (TiCH₂CMe₃), 32.30

(TiCH₂CMe₃), 31.23 (TiCH₂CMe₃), 29.37 (CHMe₂), 29.10

(CHMe₂), 25.28 (CH₃), 25.23 (CH₃), 25.14 (CH₃), 24.52 (CH₃),

23.93 (CH₃). ¹⁹F NMR (23 °C, 282.3 MHz, CD₂Cl₂): δ -133.45

(B(C₆F₅)₄), -163.95 (B(C₆F₅)₄), -167.77 (B(C₆F₅)₄). ¹¹B NMR

(23 °C, 128.4 MHz, CD₂Cl₂): δ -16.96 (B(C₆F₅)₄). Anal. Calcd

for C₆₃H₆₃N₂F₂₀BTi: C, 58.80; H, 4.93; N, 2.18. Found: C,

58.74; H, 4.96; N, 2.43.

123.6 (C₆H₃), 122.8 (br, CF₃), 97.25 (C(Me)CHC(Me), J_C-H

)

159 Hz), 35.99 (H(Me₃C)CC(Me)CHC(Me)), 32.18 (H(Me₃C)-

CC(Me)CHC(Me)), 29.15 (CHMe₂), 28.36 (CHMe₂), 24.89 (CH₃),

24.67 (CH₃), 24.47 (CH₃), 24.40 (CH₃), 24.26 (CH₃), 23.66 (CH₃).

¹⁹F NMR (23 °C, 282.3 MHz, C₆D₆): δ -75.6 (s, OSO₂CF₃). IR

(C₆H₆, CaF₂): 2964 (s), 2869 (s), 1619 (w), 1517 (m), 1463 (m),

1439 (m), 1363 (s), 1338 (m), 1239 (s), 1201 (s), 1167 (m), 997

(s) cm^-1. Anal. Calcd for C₃₅H₅₁N₂O₃SF₃Ti: C, 61.39; H, 7.51;

N, 4.09. Found: C, 61.36; H, 7.82; N, 4.02.

t

Synthesis of (Nacnac)Ti(CH₂Bu)₂(F), 2-F. In a vial was

dissolved 2 [130 mg, 0.214 mmol] in 10 mL of THF, and the

solution was cooled to -35 °C. To the cold solution was added

a THF suspension (5 mL) containing AgBF₄[41.7 mg, 0.214

mmol], causing precipitation of Ag⁰. After stirring for 20 min

the solution was filtered and the filtrate dried under vacuum.

The solids were extracted with Et₂O, and the solution was

filtered. The filtrate was concentrated under reduced pressure

and cooled to -35 °C to afford in two crops yellow crystals of

2-F [57.6 mg, 0.09 mmol, 43% yield]. Two minor impurities

are present with the product, and an attempt to separate 2-F

from the reaction mixture was unsuccessful (judged by ¹⁹F and

¹H NMR spectroscopy). Complete ¹H NMR assignment was not

possible due to impurities present in the reaction mixture. ¹H

NMR (23 °C, 399.8 MHz, C₆D₆): δ 7.28-6.88. (m, C₆H_3,6H),

5.11 (s, C(Me)CHC(Me), 1H), 3.31 (septet, CHMe₂, 2H), 3.16

(septet, CHMe₂, 2H), 1.73, 1.67, 1.56, 1.50, 1.39, 1.21, 1.17,

1.15 (TiCH₂CMe₃), 1.11, 0.79 (d, CHMe₂, 3H). ¹⁹F NMR (23

°C, 282.3 MHz, C₆D₆): δ -142.0 (m, Ti-F), and two additional

resonances observed at -140.5 (m) and -129.0 (m). Multiple

attempts to obtain satisfactorily elemental analysis were

unsuccessful.

Synthesis of (Nacnac)Ti(CH₃)₂(OTf) (4). In a vial was

dissolved (Nacnac)TiMe₂(THF) [270 mg, 0.47 mmol] in 10 mL

of pentane, and the solution was cooled to -35 °C. To the cold

solution was added a pentane suspension (∼5 mL) containing

AgOTf [128.4 mg, 0.50 mmol], causing precipitation of product

and Ag⁰. After stirring for 2 h the solution was filtered. The

solids were stirred in THF, and the solution was filtered. The

filtrate was concentrated under reduced pressure, layered with

pentane, and cooled to -35 °C to afford in two crops of orange-

red crystals of 4 [197 mg, 0.30 mmol, 64% yield]. ¹H NMR (23

°C, 399.8 MHz, C₆D₆): δ 7.20-7.12 (m, C₆H_3,6H), 5.18 (s,

C(Me)CHC(Me), 1H), 2.94 (septet, CHMe₂, 4H), 2.02 (Me₂Ti),

1.57 (s, C(Me)CHC(Me), 6H), 1.34 (d, CHMe₂, 12H), 1.03 (d,

CHMe₂, 12H). ¹³C NMR (25 °C, 100.6 MHz, C₆D₆): δ 168.2

(C(Me)CHC(Me)), 141.2 (C₆H₃), 128.28 (C₆H₃), 127.4 (C₆H₃),

125.2 (C₆H₃), 101.8 (C(Me)CHC(Me), J_C-H) 165), 88.89

(Me₂Ti, J_C-H) 124), 28.89 (CHMe₂), 25.40 (C(Me)CHC(Me)),

24.71 (CHMe₂), 24.16 (CHMe₂). ¹⁹F NMR (23 °C, 282.3

For 7-OTf: In a reaction vessel was dissolved 3-OTf [100

mg, 0.146 mmol] in 10 mL of C₆H₆and the solution was heated

at 70 °C for 2 h. The solution was dried under reduced pressure

and the red-brown solid washed with hexane to remove 5-OTf.

Recrystallization of the solid from Et₂O at -35 °C affords in

three crops brown crystals of 7-OTf [28 mg, 0.046 mmol, 31%

yield]. The hexane filtrate from the reaction mixture was

concentrated and the solution cooled overnight to -35 °C to

afford in two crops red-brown crystals of 5-OTf [62 mg, 0.09

1

mmol, 62% yield]. When monitoring the reaction by H NMR

at 70 °C, neopentane was observed (0.992 ppm).

For 7-OTf: ¹H NMR (23 °C, 399.8 MHz, C₆D₆): δ 7.59 (d,

C₆H₃, 2H), 7.52 (t, C₆H₃, 2H), 7.10 (d, C₆H₃, 2H), 5.69 (s, C(Me)-

CHC(Me), 1H), 2.74 (septet, CHMe₂, 2H), 2.58 (s, CH₃, 6H),

2.09 (s, CH₃, 6H), 1.21 (d, CH(CH₃)₂, 6H), 0.55 (d, CH(CH₃)₂,

6H), 0.51 (s, CH₃, 6H). ¹³C NMR (25 °C, 100.6 MHz, C₆D₆): δ

159.8 (C(Me)CHC(Me)), 142.7 (C₆H₃), 137.6 (C₆H₃), 137.6

(C₆H₃), 132.9 (C₆H₃), 130.9 (C₆H₃), 128.5 (C₆H₃), 127.4 (bs, CF₃),

126.6 (C₆H₃), 95.53 (C(Me)CHC(Me), J_C-H) 162 Hz), 88.78

(C(CH₃)₂), 27.93 (CHMe₂), 27.02 (CH₃), 26.76 (CH₃), 24.61

(CH₃), 22.41 (CH₃), 21.44 (CH₃). ¹⁹F NMR (23 °C, 282.3 MHz,

C₆D₆): δ -78.08 (s, OSO₂CF₃). IR (Et₂O, CaF₂): 3029 (w), 2967

(m), 2929 (m), 2867 (m), 2837 (m), 1540 (m), 1456 (w), 1420

(w), 1362 (s), 1239 (m), 1200 (s), 1069 (w), 999 (s) cm^-1. Anal.

Calcd for C₃₀H₃₉N₂O₃SF₃Ti: C, 58.82; H, 6.42; N, 4.57.

Found: C, 58.91; H, 6.75; N, 4.39.

Thermolysis of 5-OTf. In a NMR tube was loaded complex

5-OTf [27 mg, 0.039 mmol] in 0.8 mL of C₆D₆. The tube was

sealed under vacuum and the solution heated to 70 °C for 2

days. No decomposition of 5-OTf nor traces of 7-OTf were

1

observed to form based on H NMR spectroscopy.

Synthesis of (H^tBuCdC(Me)CHC(Me)N[Ar])TidNAr-

(Cl), 5-Cl, and Ti[2,6-(CMe₂)(CHMe₂)C₆H₃]NC(Me)CHC-

(Me)N[2,6-(CMe₂)(CHMe₂)C₆H₃](Cl), 7-Cl. 3-Cl [20 mg,

0.0350 mmol] was dissolved in benzene (∼675 mg) and the

solution placed in an NMR tube equipped with a Teflon

vacuum adapter. The NMR tube was heated to 45 °C in an oil

bath for 12 h, at which point no starting material remained,

as evidenced by ¹H NMR. Although there are two major

products seen by NMR which are consistent with 5-Cl and

7-Cl, assignments of the resonances was difficult due to the

presence of several other impurities in the reaction mixture.

Attempts to isolate either product were unsuccessful.

MHz, C₆D₆):

δ -77.69 (s, OSO₂CF₃). Anal. Calcd for

C₃₂H₄₇N₂O₃SF₃Ti: C, 59.61; H, 7.35; N, 4.35. Found: C, 59.80;

H, 7.46; N, 4.53.

Synthesis of (H^tBuCdC(Me)CHC(Me)N[Ar])TidNAr-

(OTf), 5-OTf, and Ti[2,6-(CMe₂)(CHMe₂)C₆H₃]NC(Me)-

CHC(Me)N[2,6-(CMe₂)(CHMe₂)C₆H₃](OTf), 7-OTf. In a re-

action vessel was dissolved 3-OTf [100 mg, 0.15 mmol] in 15

mL of benzene, and the solution was heated at 45 °C for 2

days. The solution was then filtered and dried under reduced

pressure. The red-brown solid was extracted with pentane,

filtered, concentrated, and cooled overnight at -35 °C to afford

in two crops red-brown crystals of 5-OTf [74.7 mg, 0.11 mmol,

75% yield]. If the thermolysis takes place at 60 °C, complex

5-OTf is formed over 2 h and is isolated in ∼65% yield. Some

assignments of ¹H and ¹³C NMR resonances were determined

by HMQC experiments.¹⁸

Synthesis of (H^tBuCdC(Me)CHC(Me)N[Ar])TidNAr-

(Br), 5-Br, and Ti[2,6-(CMe₂)(CHMe₂)C₆H₃]NC(Me)CHC-

(Me)N[2,6-(CMe₂)(CHMe₂)C₆H₃](Br), 7-Br. 3-Br [20 mg,

0.0325 mmol] was dissolved in benzene (∼675 mg) and the

solution transferred to an NMR tube equipped with a Teflon

vacuum valve. The NMR tube was heated to 45 °C in an oil

bath for 12 h, at which point no starting material remained

1

based on H NMR. Two products are observed by NMR, and

attempts to separate the two were hampered by their similar

solubilities. Integration is consistent with a ∼3:1 mixture of

5-Br and 7-Br.

For 5-OTf: ¹H NMR (23 °C, 399.8 MHz, C₆D₆): δ 7.20-

6.77 (m, C₆H₃, 6H), 4.67 (s, C(Me)CHC(Me), 1H), 3.87 (septet,

CHMe₂, 2H), 2.66 (s, H^tBuC(Me)CHC(Me), 1H), 2.14 (septet,

CHMe₂, 2H), 1.92 (s, H^tBuC(Me)CHC(Me), 3H), 1.57 (s,

For 5-Br: ¹H NMR (23 °C, 399.8 MHz, C₆D₆): δ 7.50-6.79

(m, C₆H₃, 6H), 4.90 (s, C(Me)CHC(Me), 1H), 3.87 (septet,

1902 Organometallics, Vol. 24, No. 8, 2005

Basuli et al.

CHMe₂, 2H), 2.49 (s, 3H), 2.16 (septet, CHMe₂, 2H), 1.92 (s,

CH₃, 3H), 1.88 (s, Ti-H^tBuCCMeCH, 1H), 1.48 (s, CH₃, 3H),

1.39 (d, CHMe₂, 6H), 1.21 (s, Bu, 9H), 1.08 (d, CHMe₂, 6H),

157.3 (C(Me)CHC(Me), 145.8 (C₆H₃), 143.0 (C₆H₃), 139.1

(Ti-CH^tBuC(Me)CH, J_C-H) 126 Hz), 127.4 (C₆H₃), 125.3

(C₆H₃), 123.42 (C₆H₃), 123.2 (C₆H₃), 122.7 (C₆H₃), 101.0 (C(Me)-

t

1.01 (d, CHMe₂, 6H), 0.93 (d, CHMe₂, 6H).

CHC(Me), J_C-H) 162 Hz), 35.94 (Ti-CHCMe₃), 32.38

For 7-Br: ¹H NMR (23 °C, 399.8 MHz, C₆D₆): δ 7.50-6.79

(m, C₆H₃, 6H), 5.60 (s, C(Me)CHC(Me), 1H), 2.72 (septet,

CHMe₂, 2H), 2.49 (s, CH₃, 6H), 2.08 (s, CH₃, 6H), 1.18 (d,

CHMe₂, 6H), 0.53 (d, CHMe₂, 6H), 0.58 (s, CH₃, 6H).

(Ti-CHCMe₃), 30.39 (CHMe₂), 28.46 (CHMe₂), 25.26 (CH₃),

24.88 (CH₃), 24.61 (CH₃), 23.88 (CH₃), 23.16 (CH₃), 22.14 (CH₃).

Anal. Calcd for C₃₄H₅₁N₂ITi: C, 61.63; H, 7.76; N, 4.23.

Found: C, 61.61; H, 7.47; N, 4.40.

Synthesis of (H^tBuCdC(Me)CHC(Me)N[Ar])TidNAr-

(η³-BH₄), 5-BH₄, and Ti[2,6-(CMe₂)(CHMe₂)C₆H₃]NC(Me)-

CHC(Me)N[2,6-(CMe₂)(CHMe₂)C₆H₃](η²-BH₄), 7-BH₄. In a

vial was dissolved in 10 mL of hexane 3-BH₄[120 mg, 0.22

mmol] and the solution stirred for 24 h. The solution was

concentrated until solid began to form and cooled to -35 °C

to afford in two crops red crystals of 5-BH₄[69.4 mg, 0.126

mmol, 58% yield]. Attempts to isolate pure 7-BH₄were

unsuccessful due to the presence of traces of 5-BH₄. However,

complex 7-BH₄was prepared independently by reaction of

7-OTf with LiBH₄in Et₂O. In a vial was dissolved 7-OTf [100

mg, 0.16 mmol] in 10 mL of Et₂O and the solution cooled to

-35 °C. To the cold solution was added a cold Et₂O suspension

(∼5 mL) of LiBH₄[3.50 mg, 0.16 mmol]. After stirring for 40

min the solution was filtered, the filtrate concentrated, and

the solution cooled to -35 °C to afford in two crops red crystals

of 7-BH₄[71.3 mg, 0.15 mmol, 91% yield].

Treatment of 3-OTf with CO₂. Synthesis of [(L₁)₂Ti₂-

(µ₂-O)₂(µ₂-OTf)][OTf] (9). In a reaction vessel was dissolved

in 10 mL of Et₂O 3-OTf [100 mg, 0.15 mmol] and the solution

cooled to -78 °C. To the cold and degassed solution was

syringed CO₂gas [3.9 mL, 0.16 mmol]. After stirring for 20

min an orange precipitate formed and the solution was allowed

to stir for an additional 20 min. The precipitate was collected

via filtration, washed with cold Et₂O, and dried under reduced

pressure. The orange powder was crystallized form dichlo-

romethane-Et₂O at -35 °C to afford in two crops orange

crystals of 9 [84 mg, 0.06 mmol, 82% yield]. Some assignments

of ¹H and ¹³C NMR resonances were determined by HMQC

experiments.^{18 1}H NMR (23 °C, 399.8 MHz, CD₂Cl₂): δ 7.23-

7.05 (m, C₆H_3,12H), 4.79 (s, C(Me)CHC(O)dCH^tBuC(Me), 2H),

4.71 (s, C(Me)CHC(O)dCH^tBuC(Me), 2H), 2.52 (overlapping

septets, CHMe₂, 8H), 2.05 (s, C(Me)CHC(O)dCH^tBuC(Me),

6H), 2.00 (s, C(Me)CHC(O)dCH^tBuC(Me), 6H), 1.19 (s, C(Me)-

CHC(O)dCH^tBuC(Me), 18H), 1.17 (br, CHMe₂, 12H), 1.07 (br,

CHMe₂, 12H), 0.97 (br, CHMe₂, 12H), 0.89 (br, CHMe₂, 12H).

¹³C NMR (25 °C, 100.6 MHz, CD₂Cl₂): δ 178.83 (C(Me)-

CHC(O)dCH^tBuC(Me)), 145.42 (C₆H₃), 140.47 (C₆H₃), 140.43

(C₆H₃), 140.37 (C₆H₃), 127.93 (C₆H₃), 127.59 (C₆H₃), 125.05

(C₆H₃), 124.1 (C₆H₃), 119.4 (C(Me)CHC(O)dCH^tBuC(Me), based

on HMQC, J_C-H) 157 Hz), 67.05 (C(Me)CHC(O)dCH^tBuC-

(Me), based on HMQC, J_C-H) 139 Hz), 32.69 (C(Me)CHC-

(O)dCH^tCMe₃C(Me)), 30.43 (C(Me)CHC(O)dCH^tCMe₃C(Me)),

28.46 (CHMe₂), 28.20 (CHMe₂), 27.39 (CH₃), 27.30 (CH₃), 25.36

(CH₃), 24.95 (CH₃), 24.77 (CH₃), 23.66 (CH₃). ¹⁹F NMR (23 °C,

282.3 MHz, C₆D₆): δ -75.71 (s, OSO₂CF₃), -79.72 (s, OSO₂CF₃).

Anal. Calcd for C₇₂H₁₀₂N₄O₁₀S₂F₆Ti₂: C, 59.33; H, 7.05; N, 3.84.

Found: C, 59.26; H, 7.23; N, 3.94.

For 5-BH₄: ¹H NMR (23 °C, 399.8 MHz, C₆D₆): δ 7.07-

6.85 (m, C₆H_3,6H), 4.89 (s, Ti-H^tBuCCMeCH, 1H), 3.81

(septet, CHMe₂, 2H), 2.77 (s, Ti-H^tBuCCMeCH, 1H), 2.26

(septet, CHMe₂, 2H), 1.93 (s, C(Me)CHC(Me), 3H), 1.54 (s,

C(Me)CHC(Me), 3H), 1.40 (d, CHMe₂, 6H), 1.26 (s, Me₃CHCC-

(Me)CHC(Me)N[Ar], 9H), 1.11 (d, CHMe₂, 6H), 1.06 (d, CHMe₂,

6H), 1.01 (d, CHMe₂, 6H). ¹³C NMR (25 °C, 100.6 MHz, C₆D₆):

δ 161.5 (Ti-H^tBuCCMeCHCMe), 158.6 (Ti-H^tBuCCMeCH-

CMe), 157.8, 145.6, 142.7, 139.7 (C₆H₆), 132.6 (^tBuHCC(Me)-

CHC(Me)N[Ar], J_C-H) 129 Hz), 127.1, 125.1, 123.6, 122.7

(C₆H₆), 101.7 (^tBuHCC(Me)CHC(Me)N[Ar]), 35.99 (Me₃CHCC-

(Me)CHC(Me)N[Ar]), 32.27 ((Me₃CHCC(Me)CHC(Me)N[Ar]),

29.47 (CHMe₂), 28.39 (CHMe₂), 24.94 (CH₃), 24.59 (CH₃),

24.45 (CH₃), 23.99 (CH₃), 23.81 (CH₃), 23.41 (CH₃). ¹¹B NMR

(23 °C, 128.4 MHz, C₆D₆): δ -13.5 (br, BH₄). Anal. Calcd for

C₃₄H₅₅N₂BTi: C, 74.18; H, 10.07; N, 5.09. Found: C, 74.16;

H, 10.23; N, 5.01.

Treatment of [(Nacnac)Ti(µ₂-O)(µ₂-OTf)]₂(8) with

OC₂Ph₂. Synthesis of [(L₁)₂Ti₂(µ₂-O)₂(µ₂-OTf)][OTf] (10).

To a vial was added 8 [50 mg, 0.040 mmol], and the solid was

dissolved in 2 mL of ether. In a separate vial was added

diphenylketene [9 mg, 0.046 mmol] and the oil dissolved in 2

mL of ether. Both solutions were cooled to -35 °C, and the

diphenylketene solution was slowly added to the stirring

solution of 8. The mixture was allowed to stir for 2 h. After

this time the solution changed to a dark red-brown and a

precipitate formed. The solution was filtered through a frit and

the precipitate washed with ether until washings were clear.

The orange solid was dissolved in CH₂Cl₂, concentrated, and

cooled to -35 °C to yield orange blocks of 10 [47 mg, 0.0284

For 7-BH₄: ¹H NMR (23 °C, 399.8 MHz, C₆D₆): δ 7.50-7.09

(m, C₆H_3,6H), 5.66 (s, C(Me)CHC(Me), 1H), 2.67 (septet,

CHMe₂, 2H), 2.52 (s, CH₃, 3H), 2.08 (s, CH₃, 3H), 1.18 (d,

CHMe₂, 6H), 0.51 (s, CH₃, 3H), 0.48 (d, CHMe₂, 3H), -1.82

(br, BH₄, 4H). ¹³C NMR (25 °C, 100.6 MHz, C₆D₆): δ 158.4

(C(Me)CHC(Me)), 142.8 (C₆H₃), 136.6 (C₆H₃), 132.4 (C₆H₃),

129.0 (C₆H₃), 127.9 (C₆H₃), 125.9 (C₆H₃), 105.1 (C(Me)CHC-

(Me), J_C-H) 162 Hz), 83.42 (CMe₂), 29.02 (CHMe₂), 27.86

(CH₃), 27.04 (CH₃), 24.31 (CH₃), 22.63 (CH₃), 21.90 (CH₃). ¹¹

B

NMR (25 °C, 128.4 MHz, C₆D₆): δ -12.3 (br quintet, BH₄, J_B-H

) 80 Hz). IR (C₆H₆, CaF₂): 2967 (s), 2831 (s), 2445 (w), 2398

(m), 2324 (w, ν_B-H), 2210 (w, ν_B-H), 2103 (w), 1960 (m), 1830

(s), 1537 (s), 1476 (s), 1376 (s), 1361 (s), 1352 (s), 1116 (m).

Anal. Calcd for C₂₉H₄₃N₂BTi: C, 72.81; H, 9.06; N, 5.86.

Found: C, 71.79; H, 8.69; N, 5.62.

1

mmol, 73% yield]. H NMR (23 °C, 400 MHz, C₆D₆): δ 6.85-

7.31 (br, aryl, 32 H), 4.78 (s, C(Me)CHC(Me), 2H), 2.26-2.53

(br, CHMe₂, 8H), 1.73 (s, C(Me)CHC(Me), 6H), 1.66 (s, C(Me)-

CHC(Me), 6H), 0.66-1.09 (br, CHMe₂, 48H). ¹³C NMR (25 °C,

100.6 MHz, C₆D₆): δ 178.0, 144.4, 140.4, 140.1, 138.4, 129.6,

129.5, 128.6, 128.4, 128.2, 127.9, 127.2, 124.4, 63.0, 28.46,

28.30, 28.24, 27.5, 25.3, 24.7, 24.6, 23.1. ¹⁹F NMR (23 °C, 282.3

MHz, C₆D₆): δ -75.6 (s, OSO₂CF₃), -79.7 (s, OSO₂CF₃). Anal.

Calcd for C₈₈H₁₀₂N₄O₁₀S₂F₆Ti₂: C, 64.07; H, 6.23; N, 3.40.

Found: C, 63.74; H, 6.29; N, 3.55.

Thermolysis of (Nacnac)TidCH^tBu(I), 3-I. Synthesis

of (H^tBuCdC(Me)CHC(Me)N[Ar])TidNAr(I), 5-I. In a vial

was dissolved 3-I (248 mg, 0.374 mmol) in 10 mL of hexane.

The solution was allowed to stir for 24 h, upon which a hexane-

insoluble precipitate emerged. The solution was filtered and

the precipitate collected. Recrystallization from toluene af-

forded dark red crystals of 5-I [165 mg, 0.249 mmol, 67% yield].

¹H NMR (23 °C, 399.8 MHz, C₆D₆): δ 7.10-6.83 (m, C₆H₃, 6H),

5.06 (s, C(Me)CHC(Me), 1H), 4.04 (septet, CHMe₂, 2H), 2.13

(septet, CHMe₂, 2H), 1.98 (s, C(Me)CHC(Me), 3H), 1.58 (s,

Ti-CH^tBuCMeCH, 1H), 1.55 (s, C(Me)CHC(Me), 3H), 1.44 (d,

CHMe₂, 6H), 1.25 (s, Ti-CH^tBuCMeCH, 9H), 1.14 (d, CHMe₂,

6H), 1.03 (d, CHMe₂, 6H), 0.98 (d, CHMe₂, 6H). ¹³C NMR (25

°C, 100.6 MHz, C₆D₆): δ 160.1 (C(Me)CHC(Me), 158.6 (C₆H₃),

Treatment of 3-OTf with SCNPh. Synthesis of (Nac-

nac)Ti(η²-(S,N)-SCdCH^tBuNPh) (11). In a vial was dis-

solved in 10 mL of pentane 3-OTf [100 mg, 0.15 mmol] and

the solution cooled to -35 °C. To the solution was added a

cold pentane solution (5 mL) of SCNPh [20 mg, 0.15 mmol].

After stirring for 2 h the solution was filtered, the filtrate

concentrated, and the solution cooled overnight at -35 °C to

afford in two crops dark blocks of 11 [105 mg, 0.13 mmol, 88%

yield]. ¹H NMR (23 °C, 399.8 MHz, C₆D₆): δ 7.17-6.91 (m, aryl,

Four-Coordinate Titanium Alkylidene Complexes

Organometallics, Vol. 24, No. 8, 2005 1903

9H), 6.19 (d, aryl, 2H), 5.28 (PhNCCH^tBuS, 1H), 4.29 (s, C(Me)-

CHC(Me), 1H), 3.27 (br, CHMe₂, 2H), 2.95 (septet, CHMe₂,

2H), 1.63 (s, C(Me)CHC(Me), 6H), 1.48 (br, CHMe₂, 6H), 1.27

(d, CHMe₂, 6H), 1.07 (d, CHMe₂, 6H), 1.01 (d, CHMe₂, 6H),

0.96 (s, PhNCCHCMe₃S, 9H). ¹³C NMR (25 °C, 100.6 MHz,

C₆D₆): δ 170.0 (C(Me)CHC(Me)), 149.1 (aryl), 146.3 (aryl), 145.6

(aryl), 142.3 (PhNCCH^tBuS), 142.12 (aryl), 129.2 (aryl), 128.2

(aryl), 126.9 (aryl), 125.2 (aryl), 124.6 (aryl), 124.0 (aryl), 112.9

(PhNCCH^tBuS, J_C-H) 152 Hz), 100.3 (C(Me)CHC(Me), J_C-H

) 162 Hz), 31.69 (PhNCCHCMe₃S), 31.16 (PhNCCHCMe₃S),

29.83 (CHMe₂), 28.95 (CHMe₂), 25.25 (CH₃), 25.02 (CH₃),

24.78 (CH₃), 24.75 (CH₃), 24.02 (CH₃). ¹⁹F NMR (23 °C,

282.3 MHz, C₆D₆): δ -77.52 (s, OSO₂CF₃). Anal. Calcd for

C₄₂H₅₆N₃O₃F₃S₂Ti: C, 61.52; H, 6.88; N, 5.13. Found: C, 59.7;

H, 6.88; N, 5.13.

Treatment of 3-OTf with NCCH₂Mes. Synthesis of

(Nacnac)TidN[CdCH^tBu(CH₂Mes)](OTf) (12). In a vial

was dissolved 3-OTf [100 mg, 0.15 mmol] in 10 mL of pentane

and the solution cooled to -35 °C. To the solution was added

a cold pentane solution (5 mL) of NCCH₂Mes [23.25 mg, 0.15

mmol]. After 5 min a red precipitate began to form and the

reaction mixture was allowed to stir for an additional 30 min.

The solid was collected by filtration and washed with cold

pentane. Recrystallization from Et₂O at -35 °C afforded in

two crops red crystals of 12 [92 mg, 0.11 mmol, 73% yield]. ¹H

NMR (23 °C, 399.8 MHz, C₆D₆): δ 7.06-7.19 (m, C₆H₃, 6H),

6.67 (s, C₆H₂, 2H), 4.96 (s, C(Me)CHC(Me), 1H), 3.41 (s,

NCCH^tBuCH₂C₆H₂Me₃, 1H), 3.2-3.1 (overlapping septets,

CHMe₂, 4H), 2.94 (s, NCCH^tBuCH₂C₆H₂Me₃, 2H), 2.04 (s,

p-Me₃C₆H₂, 3H), 2.01 (s, o-Me₃C₆H₂, 6H),), 1.73 (d, CHMe₂,

6H), 1.57 (s, C(Me)CHC(Me), 6H), 1.39 (d, CHMe₂, 6H), 1.33

(d, CHMe₂, 6H), 0.93 (s, NCCHCMe₃, 9H), 0.91 (d, CHMe₂, 6H).

¹³C NMR (25 °C, 100.6 MHz, C₆D₆): δ 168.2 (C(Me)CHC(Me)),

158.1 (NCCH^tBuCH₂C₆H₂Me₃), 144.7 (aryl), 142.5 (aryl), 140.6

(aryl), 136.9 (aryl), 135.3 (aryl), 132.6 (aryl), 129.0 (aryl), 127.9

(aryl), 124.9 (aryl), 124.1 (aryl), 120.1 (NCCH^tBuCH₂C₆H₂Me₃,

J_C-H) 151 Hz), 93.31 (C(Me)CHC(Me), J_C-H) 163 Hz), 37.79

(NCCH^tBuCH₂C₆H₂Me₃, J_C-H) 130 Hz) 31.92 (NCCHCMe₃-

BuCH₂C₆H₂Me₃), 30.97 (NCCHCMe₃CH₂C₆H₂Me₃), 29.57

(CHMe₂), 28.86 (CHMe₂), 26.13 (CH₃), 24.43 (CH₃), 24.26

(CH₃), 24.03 (CH₃), 23.96 (CH₃), 19.87 (CH₃). ¹⁹F NMR (23 °C,

282.3 MHz, C₆D₆): δ -78.19 (s, OSO₂CF₃). Anal. Calcd for

C₄₆H₆₄N₃O₃F₃STi: C, 65.46; H, 7.64; N, 4.98. Found: C, 65.43;

H, 7.77; N, 5.04.

the solution cooled to -35 °C. To the cold solution was added

a similarly cold Et₂

O solution (∼5 mL) containing N CPh

2

[28.64 mg, 0.15 mmol]. After stirring for 1 h the solution was

filtered and dried under reduced pressure. The orange powder

was washed with cold pentane, extracted with Et₂O, and

filtered. The filtrate was then concentrated and the solution

cooled to -35 °C to afford in one crop orange crystals of 14

1

[97 mg, 0.11 mmol, 73% yield]. Some assignments of H and

¹³C NMR resonances were determined by HMQC experi-

ments.^{18 1}H NMR (23 °C, 399.8 MHz, C₆D₆): δ 7.4-6.8 (m, aryl_,

16H), 6.23 (s, NCHPh₂, 1H), 5.43 (s, C(Me)CHC(Me), 1H), 3.70

(septet, CHMe₂, 2H), 2.51 (septet, CHMe₂, 2H), 1.71 (s, C(Me)-

CHC(Me), 6H), 1.24 (d, CHMe₂, 6H), 1.21 (d, CHMe₂, 6H), 1.09

(d, CHMe₂, 6H), 1.02 (d, CHMe₂, 6H), 0.788 (s, NC^tBu, 9H).

¹³C NMR (25 °C, 100.6 MHz, C₆D₆): δ 167.1 (C(Me)CHC(Me)),

147.3 (aryl), 143.7 (aryl), 143.1 (aryl), 142.4 (aryl), 128.8 (aryl),

128.2 (aryl), 126.7 (aryl), 126.6 (aryl), 124.8 (aryl), 123.9 (aryl),

101.1 (C(Me)CHC(Me)), 85.46 (NCHPh₂) 29.00 (NCCMe₃),

28.01 (CH₃) 26.81 (CH₃), 25.81 (CH₃), 25.32 (CH₃), 24.88

(CH₃), 24.41 (CH₃), 24.31 (CH₃), 23.95 (CH₃). ¹⁹F NMR (23 °C,

282.3 MHz, C₆D₆): δ -78.22 (s, OSO₂CF₃). Anal. Calcd for

C₄₈H₆₁N₄O₃SF₃Ti: C, 65.58; H, 6.99; N, 6.38. Found: C, 65.28;

H, 7.11; N, 6.29.

Treatment of 3-OTf with N₃(1-adamantyl). Synthesis

of (Nacnac)TidNCH^tBuAd(OTf) (15). In a vial was dis-

solved in 10 mL of pentane 3-OTf [100 mg, 0.15 mmol] and

the solution cooled to -35 °C. To the solution was added a

cold pentane solution (5 mL) of N₃Ad [28.5 mg, 0.16 mmol].

After stirring for 2 h the solution was filtered, the filtrate

concentrated, and the solution cooled overnight at -35 °C to

afford in three crops yellow crystals of 15 [100 mg, 0.12 mmol,

82% yield]. For 15: ¹H NMR (23 °C, 399.8 MHz, C₆D₆): δ 7.2-

7.11 (m, C₆H_3,6H), 4.92 (s, C(Me)CHC(Me), 1H), 3.34 (septet,

CHMe₂, 1H), 3.23 (septet, CHMe₂, 1H), 3.12 (septet, CHMe₂,

1H), 2.92 (septet, CHMe₂, 1H), 2.71 (s, TidN-CH^tBuAd, 1H),

1.91 (s, Ad, 3H), 1.74-0.86 (Ad and CH₃groups, 42H), 0.73

(s, CH₂CMe₃, 9H). ¹³C NMR (25 °C, 100.6 MHz, C₆D₆): δ 168.9

(C(Me)CHC(Me)), 167.7 (C(Me)CHC(Me)) 145.4 (C₆H₃), 145.2

(C₆H₃), 142.5 (C₆H₃), 142.4 (C₆H₃), 140.9 (C₆H₃), 140.7 (C₆H₃),

127.6 (C₆H₃), 127.5 (C₆H₃), 124.8 (C₆H₃), 124.7 (C₆H₃),

124.1 (C₆H₃), 123.9 (C₆H₃), 100.8 (C(Me)CHC(Me)), 93.49

(NCH^tBuAd), 41.47 (Ad), 41.35 (Ad), 38.46 (NCHCMe₃Ad),

37.25 (Ad), 30.35 (Ad), 30.15 (CHMe₂), 29.46 (28.95

(NCHCMe₃Ad), 28.91 (CHMe₂), 28.71 (CHMe₂), 25.85 (CH₃),

25.74 (CH₃), 24.95 (CH₃), 24.74 (CH₃), 24.62 (CH₃), 24.54 (CH₃),

24.51 (CH₃), 24.47 (CH₃), 24.18 (CH₃), 23.62. ¹⁹F NMR (23 °C,

282.3 MHz, C₆D₆): δ -78.54 (s, OSO₂CF₃). Anal. Calcd for

C₄₅H₆N₃O₃F₃STi: C, 63.38; H, 7.40; N, 5.41. Found: C, 64.80;

H, 7.98; N, 5.03.

Treatment of 3-OTf with CN^tBu. Synthesis of

(Nacnac)Ti(η²-(N,C)-^tBuNdCdCH^tBu)(OTf) (13). In a vial

was dissolved 3-OTf [100 mg, 0.15 mmol] in 10 mL of pentane

and the solution cooled to -35 °C. To the solution was added

a cold pentane solution (5 mL) of ^tBuNC [12.75 mg, 0.15 mmol].

After stirring for 2 h the solution was filtered, the filtrate

concentrated, and the solution cooled overnight at -35 °C to

afford in two crops dark blocks of 13 [99 mg, 0.13 mmol,

Treatment of 3-OTf with HNCPh₂. Synthesis of

t

(Nacnac)Ti(CH₂Bu)(NdCPh₂)(OTf) (16). In a vial was

1

dissolved in 10 mL of pentane 3-OTf [50 mg, 0.07 mmol] and

the solution cooled to -35 °C. To the solution was added a

cold pentane solution (5 mL) of Ph₂CNH [14 mg, 0.08 mmol].

After stirring for 2 h the solution was filtered, the filtrate

concentrated, and the solution cooled overnight at -35 °C to

afford in two crops light green crystals of 16 [46 mg, 0.05 mmol,

88% yield]. H NMR (23 °C, 399.8 MHz, C₆D₆): δ 6.96-7.05

(m, C₆H₃, 6H), 5.82 (s, C(Me)CHC(Me), 1H), 5.81 (s,

^tBuCHCN^tBu, 1H) 2.65 (septet, CHMe₂, 2H), 2.54 (septet,

CHMe₂, 2H), 1.78 (s, C(Me)CHC(Me), 6H), 1.48 (s,

^tBuCHCN^tBu, 9H), 1.38 (d, CHMe₂, 6H), 1.31 (d, CHMe₂,

6H), 1.16 (d, CHMe₂, 6H), 0.96 (d, CHMe₂, 6H), (0.83

^tBuCHCN^tBu, 9H). ¹³C NMR (25 °C, 100.6 MHz, C₆D₆): δ 170.0

(^tBuCHCN^tBu), 168.2 (C(Me)CHC(Me)), 146.5 (ipso-C₆H₃),

142.1 (o-C₆H₃), 139.8 (o-C₆H₃), 127.7 (p-C₆H₃), 125.0

(m-C₆H₃), 124.8 (m-C₆H₃), 122.6 (^tBuNCCH^tBu), 100.4 (C(Me)-

CHC(Me), 65.38 (Me₃CNCCH^tBu), 34.99 (^tBuNCCHCMe₃),

32.25 (Me₃CNCCHCMe₃), 31.88 (CHMe₂), 28.59 (^tBuNCCH-

CMe₃), 28.33 (CHMe₂), 25.64 (CH₃), 25.47 (CH₃), 25.16 (CH₃),

24.76 (CH₃), 24.38 (CH₃). ¹⁹F NMR (23 °C, 282.3 MHz, C₆D₆):

δ -77.80 (s, OSO₂CF₃). Anal. Calcd for C₄₀H₆₀N₃O₃F₃STi: C,

62.56; H, 7.88; N, 5.47. Found: C, 61.12; H, 7.97; N, 5.35.

Treatment of 3-OTf with N₂CPh₂. Synthesis of

(Nacnac)TidNCHPh₂(NC^tBu)(OTf) (14). In a vial was

dissolved 3-OTf [100 mg, 0.15 mmol] in 10 mL of Et₂O and

1

73% yield]. H NMR (23 °C, 399.8 MHz, C₆D₆): δ 7.15-6.86

(m, aryl_,16H), 4.92 (s, C(Me)CHC(Me), 1H), 3.07 (septet,

t

CHMe₂, 2H), 2.78 (septet, CHMe₂, 2H), 1.88 (s, CH₂Bu, 2H),

1.51 (s, C(Me)CHC(Me), 6H), 1.30 (d, CHMe₂, 6H), 1.25 (d,

CHMe₂, 6H), 1.19 (d, CHMe₂, 6H), 0.80 (d, CHMe₂, 6H), 0.46

(s, CH₂CMe₃, 9H). ¹³C NMR (25 °C, 100.6 MHz, C₆D₆): δ 168.0

(C(Me)CHC(Me)), 146.9 (aryl), 145.1 (aryl), 142.6 (aryl), 140.7

(aryl), 128.64 (aryl), 127.6 (aryl), 127.5 (aryl), 126.1 (aryl),

124.8 (aryl), 124.1 (aryl), 93.57 (C(Me)CHC(Me), J_C-H) 162),

t

88.76 (NCPh₂), 53.12 (CH₂Bu, J_C-H) 127), 31.81 (CH₂CMe₃),

31.14 (CH₂CMe₃), 29.83 (CHMe₂), 28.95 (CHMe₂), 25.24 (CH₃),

24.44 (CH₃), 24.38 (CH₃), 24.26 (CH₃), 23.89 (CH₃). ¹⁹F NMR

(23 °C, 282.3 MHz, C₆D₆): δ -78.23 (s, OSO₂CF₃). Anal. Calcd

1904 Organometallics, Vol. 24, No. 8, 2005

Basuli et al.

for C₄₈H₆₂N₃O₃F₃STi: C, 66.57; H, 7.22; N, 4.85. Found: C,

65.24; H, 7.91; N, 5.44.

formed with the Gaussian 98 package⁵⁵at the B3PW91⁵⁶level

of theory. Basis sets used included LANL2DZ for Ti and Cl

and 6-31G^*for C and N, and all hydrogens.⁵⁷The basis set

LANL2DZ is the Los Alamos National Laboratory ECP plus

a double-ú valence on Ti and Cl;⁵⁸additional d polarization

functions were added to the Cl atom in all DFT calculations

for model complex 3-OTf.⁵⁹All optimizations were performed

with C₁symmetry, and all minima were confirmed by analyti-

cal calculation of frequencies, which were also used to compute

zero-point energy corrections without scaling. All initial

geometries were adapted from the refined crystal structure of

the analogous compounds 2 and 3-OTf. For complex 2, the

neopentyls were replaced with methyls, and in complex 3-OTf

the triflate was replaced by a chloride atom. In both models

the hydrocarbon chains were replaced by CH₃or H where

indicated. Isopropyl groups on the 2,6-aryl positions of the aryl

rings attached to the â-diketiminate nitrogens were also

replaced with methyls to give the model complexes ([2,6-

Me₂C₆H₃]NC(H)CHC(H)N[2,6-Me₂C₆H₃])Ti(CH₃)₂and ([2,6-

Me₂C₆H₃]NC(H)CHC(H)N[2,6-Me₂C₆H₃])TidCHMe(Cl).

Kinetic Measurements. In a typical experiment, 10-30

mg of 3-I was dissolved in 675 mg of a stock solution (3 µL of

hexamethyldisiloxane, TMS₂O, as an internal standard in 675

mg of C₆D₆) and placed in an oven-dried NMR tube. The NMR

tube is then placed into a temperature-equilibrated NMR

probe (30-70 °C) and observed until ∼90% conversion to the

metathesis product 5-I occurred. No side products were

observed, as evidenced by ¹H NMR spectroscopy. The NMR

probe temperature was calibrated using an ethylene glycol

standard. The TMS₂O resonance was integrated to 1000, and

the decay of the hydrogen resonance for the γ-carbon of the

Nacnac^-backbone (5.08 ppm) was monitored over time. The

resulting data were fit to a first-order decay plot of the

alkylidene species 3-I. The rate constant is an average of 11

trials (8.06 × 10^-4s^-1( 4.06 × 10^-5s^-1) performed at 57.0

°C. Activation parameters were performed in triplicate for each

temperature run (∆H^q) 21.3 ( 0.2 kcal/mol, ∆S^q) -8 ( 3

cal/mol K). Solvent dependence tests were determined by

performing the above experiment with ∼20 mg of compound

in THF-d₈and Et₂O-d₁₀at 31.3 °C, and at approximately the

same concentration. Results for the calculated rates obtained

from the THF-d₈and Et₂O-d₁₀trials were within experimental

error of the C₆D₆trials.

Synthesis of (Nacnac_tBu)TiCl₂(17). In a reaction vessel

was suspended TiCl₃[1 g, 6.5 mmol] in Et₂O (100 mL), and to

the solution was added an Et₂O solution (20 mL) containing

Li(Nacnac_tBu) [3.6 g, 7.1 mmol]. After stirring for 2 days the

solution was filtered and concentrated, and the filtrate was

again filtered and cooled to -35 °C to afford green crystals of

17 [930 mg, 1.5 mmol, 23% yield]. ¹H NMR (23 °C, 399.8 MHz,

C₆D₆): δ 8.53 (∆ν_1/2) 21 Hz), 5.06 (∆ν_1/2) 38 Hz), 1.99 (∆ν_1/2

) 103 Hz). µ_eff) 1.89 m_B(C₆D₆, 23 °C, Evans’ method). Anal.

Calcd for C₃₅H₅₃N₂Cl₂Ti: C, 67.73; H, 8.61; N, 4.51. Found:

C, 66.19; H, 8.63; N, 4.40.

t

Synthesis of (Nacnac_tBu)Ti(CH₂Bu)₂(18). In a reaction

vessel was dissolved 17 [662 mg, 1.07 mmol] in 30 mL of Et₂O

and the solution cooled to -35 °C. To the cold solution was

t

added a cold Et₂O solution (10 mL) of LiCH₂Bu [175 mg, 2.24

mmol]. After stirring for 30 min the solution was filtered and

dried under reduced pressure. The green powder was extracted

with hexane and filtered, and the filtrate was concentrated

and cooled to -35 °C to afford in two crops green crystals of

18 [526 mg, 0.76 mmol, 71% yield]. ¹H NMR (23 °C, 399.8 MHz,

C₆D₆): δ 7.26 (∆ν_1/2) 13 Hz), 7.11 (∆ν_1/2) 45 Hz), 5.57 (∆ν_1/2

) 125 Hz), 2.98 (∆ν_1/2) 163 Hz), -0.06 (∆ν_1/2) 241 Hz). µ_eff

) 1.96 m_B(C₆D₆, 23 °C, Evans’ method). Multiple attempts to

obtain satisfactory elemental analysis failed due to the thermal

instability of complex 18.

Synthesis of (Nacnac_tBu)TidCH^tBu(OTf), 19-OTf. In a

vial was dissolved 18 [400 mg, 0.58 mmol] in 10 mL of THF

and the solution cooled to -35 °C. To the cold solution was

added a cold THF solution (5 mL) of AgOTf [147.69 mg, 0.57

mmol]. After stirring for 20 min the solution was filtered and

the filtrate dried under reduced pressure. The brown powder

was extracted with hexane and filtered, and the filtrate was

concentrated and cooled to -35 °C to afford in two crops dark

1

large blocks of 19-OTf [370.2 mg, 0.48 mmol, 83% yield]. H

NMR (23 °C, 399.8 MHz, C₆D₆): δ 7.08-7.01 (m, C₆H_3,6H),

5.50 (s, C(^tBu)CHC(^tBu), 1H), 4.06 (s, TidCH^tBu, 1H), 3.40

(septet, CHMe₂, 2H), 3.06 (septet, CHMe₂, 2H), 1.86 (d, CHMe₂,

6H), 1.46 (d, CHMe₂, 6H), 1.38 (d, CHMe₂, 6H), 1.26 (d, CHMe₂,

6H), 1.04 (s, C(^tBu)CHC(^tBu), 18H), 0.46 (TidCH^tBu, 9H).¹³C

NMR (25 °C, 100.6 MHz, C₆D₆): δ 259.6 (TidCH^tBu, J_C-H

)

92 Hz), 175.9 (C(^tBu)CHC(^tBu)), 147.2, 143.3, 138.7, 127.0,

124.3, 123.9 (C₆H₃), 92.43 (C(^tBu)CHC(^tBu), J_C-H) 154 Hz),

50.43 (TidCHCMe₃), 44.52 (C(CMe₃)CHC(CMe₃)), 31.98, 31.61,

30.27, 29.78, 28.98, 26.97, 26.11, 24.35 (CH₃). ¹⁹F NMR (23

°C, 282.3 MHz, C₆D₆): δ -77.05 (s, OSO₂CF₃). Anal. Calcd for

C₄₁H₆₃N₂O₃SF₃Ti: C, 64.04; H, 8.25; N, 3.64. Found: C, 62.78;

H, 7.94; N, 3.60.

Crystal Structure Determinations. Data Collection

and Refinement. Inert atmosphere techniques were used to

place the crystal onto the tip of a diameter glass capillary

(0.03-0.20 mm) and mounted on a SMART6000 (Bruker) at

113-140 K. A preliminary set of cell constants was calculated

from reflections obtained from three nearly orthogonal sets of

20-30 frames. Data collection was carried out using graphite-

Synthesis of (Nacnac_tBu)TidCH^tBu(I), 19-I. In a vial was

dissolved 18 [54 mg, 0.08 mmol] in 10 mL of toluene and the

solution cooled to -35 °C. To the solution was added a cold

toluene solution (∼5 mL) of I₂[9.9 mg, 0.04 mmol]. After

stirring for 21 min the solution was filtered and dried under

reduced pressure. The solid was extracted with pentane and

filtered, and the filtrate was concentrated and cooled to -35

°C to afford dark large blocks of 19-I [47 mg, 0.06 mmol, 75%

yield]. ¹H NMR (23 °C, 399.8 MHz, C₆D₆): δ 7.13-7.03 (m,

C₆H_3,6H), 5.56 (s, C(^tBu)CHC(^tBu), 1H), 3.97 (septet, CHMe₂,

2H), 3.24 (septet, CHMe₂, 2H), 2.33 (s, TidCH^tBu, 1H), 1.80

(d, CHMe₂, 6H), 1.40 (d, CHMe₂, 6H), 1.35 (d, CHMe₂, 6H),

1.27 (d, CHMe₂, 6H), 1.10 (s, C(^tBu)CHC(^tBu), 18H), 0.11

(TidCH^tBu, 9H). ¹³C NMR (25 °C, 100.6 MHz, C₆D₆): δ 259.6

(TidCH^tBu, J_C-H) 92 Hz), 173.45 (C(^tBu)CHC(^tBu)), 148.5,

143.1, 139.2, 126.7, 124.7, 123.8 (C₆H₃), 92.33 (C(^tBu)CHC-

(^tBu), J_C-H) 152 Hz), 49.68 (TidCHCMe₃), 44.89 (C(CMe₃)-

CHC(CMe₃)), 32.05, 31.01, 29.95, 28.73, 28.65, 26.01, 24.67,

24.57 (Me). Anal. Calcd for C₄₀H₆₃N₂ITi: C, 60.82; H, 7.84; N,

3.45. Found: C, 60.90; H, 8.03; N, 3.46.

(55) M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M.

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K. N. Kudin, M. C. Strain, O. Farkas, J. Tomasi, V. Barone, M. Cossi,

R. Cammi, B. Mennucci, C. Pomelli, C. Adamo, S. Clifford, J. Ochterski,

G. A. Petersson, P. Y. Ayala, Q. Cui., K. Morokuma, D. K. Malick, A.

D. Rabuck, K. Raghavavhari, J. B. Foresman, J. Cioslowski, J. V. Ortiz,

B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R.

Gomperts, R. L. Martin, D. J. Fox, T. Keith, M. A. Al-Laham, C. Y.

Peng, A. Nanayakkara, C. Gonzalez, M. Challacombe, P. M. W. Gill,

B. G. Johnson, W. Chen, M. W. Wong, J. L. Andres, M. Head-Gordon,

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D. J. Chem. Phys. 1993, 98, 1372-1377. (b) Becke, A. D. J. Chem. Phys.

1993, 98, 5648-5652. (c) Perdew, J. P.; Wang, Y. Phys. Rev. B 1991,

44, 13298-13307.

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P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299-310.

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Chem. Phys. Lett. 1993, 208, 237-240.

t

DFT Calculations for (Nacnac)Ti(CH₂Bu)₂(2) and

(Nacnac)TidCH^tBu(OTf), 3-OTf. All calculations were per-

Four-Coordinate Titanium Alkylidene Complexes

Organometallics, Vol. 24, No. 8, 2005 1905

monochromated Mo KR radiation with a frame time of 3 s with

a detector distance of 5.0 cm. A randomly oriented region of a

sphere in reciprocal space was surveyed. Three sections of 606

frames were collected with 0.30° steps in ω at different φ

settings with the detector set at -43° in 2θ. Final cell constants

were calculated from the xyz centroids of strong reflections

from the actual data collection after integration (SAINT).⁶⁰The

structure was solved using SHELXS-97 and refined with

SHELXL-97.⁶¹A direct-methods solution was calculated that

provided most non-hydrogen atoms from the E-map. Full-

matrix least squares/difference Fourier cycles were performed,

which located the remaining non-hydrogen atoms. All non-

hydrogen atoms were refined with anisotropic displacement

parameters, and all hydrogen atoms were refined with isotro-

pic displacement parameters (unless otherwise specified, vide

infra). We have avoided using the SQUEEZE algorithm in

some structures containing poorly modeled and ambiguous

solvent contributions in order to alter the original hkl data.⁶²

Most of the crystals examined are exceedingly reactive to O₂

and moisture and often suffer from degradation during the

mounting process and in some cases during the data collection.

For 3-I: Intensity statistics and systematic absences sug-

gested the centrosymmetric space group P2₁/c, and subsequent

solution and refinement confirmed this choice. A large peak

(ca. 3 e/A³) appeared in the difference map near C(34). Initially

it was thought that this might be a disorder in the -CH^tBu

group, but it was finally recognized that there was a second

form of the molecule superimposed in the lattice. Careful

examination located two alternate iodine positions and the

alternate titanium position. Hence, the structure consists of

two molecules occupying the same site. The primary compo-

nent is C₃₄H₅₁IN₂Ti (95%) and the minor component is

C₂₉H₄₁I₂N₂Ti (5%). Several models were refined in which the

minor component occupancies were varied with and without

constraints. Model reported constrained occupancies of Ti1A,

I33A, and I33B have the same occupancy, while Ti1, I33, and

the -CH^tBu group have 1 minus the occupancy of the minor

component. When all hydrogens were allowed to vary, those

on C36, C37, and C38 refined near reasonable positions but

with C-H distances of up to 1.28 Å. The model reported fixed

the hydrogen atoms on these three carbons during the refine-

ment. The largest peak in a difference Fourier at this point

was located adjacent to C34 and was assigned H80. The

refinement converged satisfactorily with this model. It should

be noted that the presence of the secondary form has distorted

some of the hydrogen positions, so the positions, while

qualitatively correct, are not highly accurate.

Fourier was featureless with the exception of one peak of 1.95

e/A³located in the vicinity of the Br atom. This may indicate

that there is a minor (less than 10%) substitution of another

ligand present in the lattice. The remaining electron density

is located on bonds.

For 5-I: Intensity statistics and systematic absences sug-

gested the centrosymmetric space group P1h, and subsequent

solution and refinement confirmed this choice. A slight disor-

der was present in two isopropyl groups, but was readily

modeled. With the exception of those associated with the

above-described disorder, all hydrogen atoms were located in

subsequent Fourier maps and included as isotropic contribu-

tors in the final cycles of refinement. The disordered hydrogen

atoms were placed in ideal positions and refined as riding

atoms with relative isotropic displacement parameters.

For 7-BH₄: Intensity statistics and systematic absences

suggested the non-centrosymmetric space group Cmc2₁, and

subsequent solution and refinement confirmed this choice. A

disorder was located in one of the ⁱPr groups. All non-hydrogen

atoms were refined with anisotropic displacement parameters.

The molecule lies on a mirror plane, and all hydrogen atoms

were located in subsequent Fourier maps and included as

isotropic contributors in the final cycles of refinement.

For 9‚3CH₂Cl₂: Intensity statistics and systematic absences

suggested the centrosymmetric space group P2₁/n, and sub-

sequent solution and refinement confirmed this choice. The

cation and anion are well defined, but there are three

disordered dichloromethane solvent molecules in the cell. All

non-hydrogen atoms were refined with anisotropic displace-

ment parameters. All hydrogen atoms were placed in ideal

positions and refined as riding atoms with relative isotropic

displacement parameters. The remaining electron density is

located in the vicinity of three dichloromethane solvent

molecules.

For 12‚Et₂O: Intensity statistics and lack of systematic

absences suggested the centrosymmetric space group P1h, and

subsequent solution and refinement confirmed this choice. A

disordered solvent molecule (Et₂O) was located and refined as

well. All hydrogen atoms with the exception of those associated

with the solvent were located in subsequent Fourier maps and

included as isotropic contributors in the final cycles of refine-

ment. All solvent hydrogen atoms were placed in ideal posi-

tions and refined as riding atoms with relative isotropic

displacement parameters. The remaining electron density is

located primarily on bonds.

For 13‚C₅H₁₂: Intensity statistics and systematic absences

suggested the centrosymmetric space group P1h, and subse-

quent solution and refinement utilized this choice. All hydro-

gen atoms were placed in ideal positions and refined as riding

atoms with relative isotropic displacement parameters. Two

crystallographically independent molecules were located in the

asymmetric unit along with two pentane molecules. A “ghost

image” corresponding to the two metal and two sulfur atoms

was present in difference maps translated ca. one-third along

b. In the final model the four atoms were included at 20%

occupancy. It is assumed that the ghost was due to the

twinning, but may be indicative of other problems in the

analysis. There is little doubt as to the structure of the sample,

but one should use caution in interpreting bond lengths. The

remaining electron density is located on bonds.

For [2][B(C₆F₅)₄]: Small orange crystals of this complex

are exceedingly reactive to traces of moisture and oxygen and

rapidly decompose at room temperature. For this reason,

crystal data suffer from a combination of crystal decay (during

crystal mounting) as well as weak diffraction of the single

crystal.

For 3-Br: Intensity statistics and systematic absences were

consistent with the centrosymmetric space group P2₁/n. Sub-

sequent solution and refinement confirmed this choice. All

hydrogen atoms were located in subsequent Fourier maps and

were refined as isotropic contributors. The final difference

(60) SAINT 6.1; Bruker Analytical X-Ray Systems: Madison, WI.

(61) SHELXTL-Plus V5.10; Bruker Analytical X-Ray Systems: Madi-

son, WI.

For 14: Intensity statistics and lack of systematic absences

suggested the centrosymmetric space group P2₁/c, and subse-

quent solution and refinement confirmed this choice. A disor-

der was present in the O₃SCF₃ligand. All non-hydrogen atoms

were refined with anisotropic thermal parameters. All hydro-

gen atoms were located and refined isotropically.

(62) There is no consensus in the crystallographic community about

the propriety of using SQUEEZE in the process of solving and refining

structures intended for publication. It is our own opinion that refine-

ments based on SQUEEZE data are rarely, if ever, appropriate for

publication, as the SQUEEZE procedure utilizes parameters derived

only from the experimental observations to produce a modified version

of the same observations. At best, this transformation introduces no

new information. In our view it is superior to provide an entirely

explicit model of the electron density, even if that model cannot be

interpreted exclusively in terms of discrete chemical moieties. We do

not see any value to the scientific community in disguising our results

so that they superficially appear more attractive.

For 15‚C₅H₁₂: Intensity statistics and systematic absences

suggested the noncentrosymmetric space group P2₁2₁2₁, and

subsequent solution and refinement led to satisfactory results.

It became apparent that a severe disorder was present in the

t

adamantane as well as the Bu group. The disorder was well

1906 Organometallics, Vol. 24, No. 8, 2005

Basuli et al.

resolved, however. In addition there is a disordered solvent

(pentane), which was modeled with partial occupancy on the

carbon atoms. All non-hydrogen atoms were refined with

anisotropic displacement parameters. All hydrogen atoms were

placed in ideal positions and refined as riding atoms with

relative isotropic displacement parameters. The remaining

electron density is located on bonds.

Foundation (CHE-0348941) for financial support of this

research. D.J.M. wishes to thank Prof. K. G. Caulton,

Prof. K. Meyer, Dr. Roger Isaacson, Dr. Maren Pink,

and Mr. Xile Hu for insightful discussions, and the

Indiana University Information Technology Research

and Academic Computing Services for access to the

high-performance computing system. L.A.W. acknowl-

edges the National Science Foundation for a pre-doctoral

fellowship.

For 19-OTf‚C₆H₁₄: Intensity statistics and systematic

absences suggested the centrosymmetric space group P2₁/n,

and subsequent solution and refinement confirmed this choice.

A disordered solvent (possibly hexane) was present, lying at

a center of inversion in the cell. All non-hydrogen atoms were

refined with anisotropic displacement parameters. All hydro-

gen atoms were located in subsequent Fourier maps and

included as isotropic contributors in the final cycles of refine-

ment. The remaining electron density is located on bonds.

Supporting Information Available: Complete crystal-

lographic data for compounds [2][B(C₆F₅)₄], 2-F, 3-I, 3-Br,

3-BH₄, 4, 5-I, 5-BH₄, 7-OTf, 7-BH₄, 9-15, and 17-19 (CIF),

and complete geometrical parameters for the optimized ge-

ometries of ([2,6-Me₂C₆H₃]NC(H)CHC(H)N[2,6-Me₂C₆H₃])-

Ti(CH₃)₂and ([2,6-Me₂C₆H₃]NC(H)CHC(H)N[2,6-Me₂C₆H₃])-

TidCHMe(Cl) (PDF). This material is available free of charge

via the Internet at http://pubs.acs.org.

Acknowledgment. We thank Indiana University-

Bloomington, the Camille and Henry Dreyfus Founda-

tion, the Ford Foundation, and the National Science

OM049400B

Article Doi

Four-coordinate titanium alkylidene complexes: Synthesis, reactivity, and kinetic studies involving the terminal neopentylidene functionality

DOI: 10.1021/om049400b

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Article abstract of DOI:10.1021/om049400b

Full text of DOI:10.1021/om049400b

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