Reactions of a cyclopentadienyl-amidinate titanium benzimidamido complex

Article

pubs.acs.org/Organometallics

Reactions of a Cyclopentadienyl−Amidinate Titanium

Benzimidamido Complex

Laura R. Groom, Adam F. Russell, Andrew D. Schwarz, and Philip Mountford*

Chemistry Research Laboratory, Department of Chemistry, University of Oxford, Mansﬁeld Road, Oxford OX1 3TA, U.K.

S

* Supporting Information

ABSTRACT: We report the ﬁrst reactivity study of a transition-metal benzimidamido complex, namely Cp*Ti{PhC(NⁱPr)₂}-

t

{NC(Ar^F⁵)NO^tBu} (5, Ar^F⁵= C₆F₅). Reaction with CO₂and BuNCO gave the cycloaddition products Cp*Ti{PhC(NⁱPr)₂}-

{OC(O)N(C{Ar^F⁵}NO^tBu)} and Cp*Ti{PhC(NⁱPr)₂}{OC(N^tBu)N(C{Ar^F⁵}NO^tBu)} (10), respectively, whereas with CS₂

slow extrusion of Ar^F⁵CN from 5 occurred to ultimately form Cp*Ti{PhC(NⁱPr)₂}{SC(S)N(O^tBu)}. Reaction of 5 with

t

ArC(O)H (Ar = Ph, 4-C₆H₄Me, 4-C₆H₄Bu, 4-C₆H₄OMe, 4-C₆H₄NMe₂, 4-C₆H₄CF₃) also gave the isolable metallacyclic

complexes Cp*Ti{PhC(NⁱPr)₂}{N(C{Ar^F⁵}NO^tBu)C(Ar)(H)O} (13) via reversible [2 + 2] cycloaddition reactions. In contrast,

reaction with HC(O)NMe₂formed Me₂N{NC(Ar^F⁵)NO^tBu}H (16) within 1 h at room temperature. Upon heating, 10 and 13

t

also underwent retrocyclization, forming the organic products BuNCNC(Ar^F⁵)NO^tBu and ArC{NC(Ar^F⁵)NO^tBu}H (14),

respectively. Selected examples of 14 and 16 were studied by DFT and UV−visible spectroscopy. Addition of isonitriles ^tBuNC

F₂

and XylNC (Xyl = 2,6-C₆H₃Me₂) to Cp*Ti{PhC(NⁱPr)₂}{NC(Ar)NO^tBu} (Ar = Ar^F(5), 2,6-C₆H₃F₂(Ar )) aﬀorded the σ

5

adducts Cp*Ti{PhC(NⁱPr)₂}{NC(Ar)NO^tBu}(CNR) (Ar = Ar^F⁵, R = Bu, Xyl (19); Ar = Ar^F², R = Xyl). Subsequently, 19

t

formed Cp*Ti{PhC(NⁱPr)₂}{NC(NO^tBu)C₆F₄N(Xyl)C}(F) (20) via C−F bond activation. Reaction of 5 with 2 equiv of

B(Ar^F⁵)₃gave Cp*Ti{PhC(NⁱPr)₂}{ON(B{Ar^F⁵}₃)C(Ar^F⁵)N(H)(B{Ar^F⁵}₃)} with elimination of 2-methylpropene.

reactivity. Indeed, the reaction chemistry of alkoxyimides in

general is still virtually unexplored.⁸As part of this research

program,^7cwe recently synthesized the tert-butoxyimido

complexes Cp*Ti{RC(NⁱPr)₂}(NO^tBu) (1; R = Me, Ph)

(Figure 1), supported by the cyclopentadienyl−amidinate

ligand set. This robust ligand platform was chosen as it was

used very successfully in the past to study the reactivity of a

range of imido,⁹hydrazido,^2i,r,u,v,xand alkylidene hydrazido (i.e.,

(L)TiNNCRR′)^4gcomplexes, therefore allowing meaningful

comparisons with these related functional groups.

INTRODUCTION

■

During the past 20 years, the chemistry of group 4 imido

complexes¹(L)MNR (R = alkyl, aryl) and, more recently,

hydrazido complexes (L)MNNR₂(M = Ti,²Zr³in

particular) has been extensively developed. The structure and

bonding of these complexes is well-established. The unsatu-

rated MN multiple bonds (formally σ²π⁴triple bonds in

most instances^2l,t,4) normally act as the reactive site in imido

and hydrazido complexes and undergo a wide range of addition

reactions with saturated and, especially, unsaturated substrates.

In some instances, imides act only as supporting or spectator

ligands: for example, in the context of oleﬁn polymerization.⁵In

addition to MN_αbond reactivity, it has been shown that

group 4 hydrazido complexes can also undergo reductive

cleavage (e.g., with CO^3aand isonitriles^2s,z,3b) or reductive

insertion reactions of the N_α−N_βbonds (with alkynes^2i,q,z,3n).

The mechanism^2q,3j,nof alkyne insertion into the N_α−N_βbond

of certain titanium and zirconium hydrazides is related to that

for reductive cleavage of the peroxide ligand O_α−O_βbond in

Cp*₂Hf(R)(OO^tBu) to form Cp*₂Hf(OR)(O^tBu) (R = H,

alkyl).⁶Following on from these studies, we have therefore

started to explore the chemistry of titanium alkoxyimido

complexes, (L)TiNOR,⁷with the aim of developing both

new TiN_αmultiple-bond chemistry as well as N_α−O_βbond

The TiNO^tBu moiety of 1 undergoes a number of new

transformations for this type of functional group, including [2 +

2] cycloaddition reactions with CS₂and Ar′NCO (forming 2,

i

Figure 1; Ar′ = 2,6-C₆H₃Pr₂), a double-addition reaction with

TolNCO, coupling 2 equiv of substrate, net NO^tBu group

t

transfer with BuNCO and PhC(O)R (R = H, Me, Ph) via

CO/TiN_αmetathesis, reductive N_α−O_βbond cleavage

with Ar^F⁵CCH (Ar^F⁵= C₆F₅), and formation of an unusual

nitroxyl (HNO) complex upon reaction with B(Ar^F⁵)₃with

concomitant 2-methylpropene elimination.^7cOf particular

relevance to our current contribution are the reactions of 1

Received: December 5, 2013

Published: February 3, 2014

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Figure 1. Cp*Ti{PhC(NⁱPr)₂}(NO^tBu) (1) and its reaction products with Ar′NCO and benzonitriles. Atoms from the NO^tBu ligand are shown in

i

red, and those from the organic substrates are shown in blue. Ar′ = 2,6-C₆H₃Pr₂.

with benzonitriles to form the net TiN_αbond insertion

products Cp*Ti{PhC(NⁱPr)₂}{NC(Ar)NO^tBu} (Ar = Ph (3),

C₆H₃F₂(Ar^F², 4), Ar^F⁵(5)) containing new benzimidamido

functional groups, i.e. TiNC(Ar)NO^tBu (Figure 1), also

possessing a formal TiN σ²π⁴triple bond. The stability of

these complexes increases in the order Ar = Ph (3) < Ar^F²(4) <

solvents, 5 could only be isolated in less than 35% yield on a

preparative scale. Therefore, in the following chemistry 5 was

generally generated in situ for reasons of eﬃciency. When

reactions of formally isolated 5 were compared with those of

the in situ generated system, no diﬀerences were observed.

Reactions with Heterocumulenes. The inﬂuence of N_α

substituents, (L)TiN_α−X, on the reactivity of titanium imido

and hydrazido complexes with CO₂, CS₂, and isocyanates is

well established,^{1c−e,2b,k,n,v,z,4g,9c−e}and initial studies of

alkoxyimido complexes have shown behavior similar to that

of their hydrazido and imido counterparts.^4g,7cA [2 + 2]

cycloaddition product is the key intermediate in all reactions

with these substrates. However, there are relatively few

examples where these intermediates are observed or

(especially) isolated. Their unstable nature often leads to

extrusion (via retrocyclization) of an organic fragment, leaving

behind a titanium oxo species which, unless trapped, rapidly

dimerizes via μ-oxo bridge formation. We were therefore

interested to explore the reactions of Cp*Ti{PhC(NⁱPr)₂}-

{NC(Ar^F⁵)NO^tBu} (5) with heterocumulenes, in particular

since the electron-withdrawing nature of the C₆F₅substituent

should help to stabilize these intermediates.

Ar^F(5) as the aryl groups become better able to stabilize the

5

electron-rich multiple bond. Although the reactions of nitriles

with transition-metal nitrides,¹⁰alkylidenes,¹¹and alkylidynes¹²

are very well established, their reactions with transition-metal

imides and hydrazides are still relatively uncommon,^2q,s,z,4g,13

and the reaction chemistry of the benzimidamido ligand has

therefore remained unexplored. Encouragingly in this regard,

compound 5 reacted with an excess of Ar^F⁵CN to form 6

(Figure 1), containing two additional benzonitrile units. This

was the ﬁrst example of any reaction of an isolated

benzimidamido complex and only the second example of the

double addition of nitriles across a MNR bond. The ﬁrst

example was the very recently reported head-to-tail coupling of

2 equiv of ArCN across the TiNNCPh₂bond of the

alkylidene hydrazide Cp*Ti{MeC(NⁱPr)₂}(NNCPh₂), forming

Cp*Ti{MeC(NⁱPr)₂}{N(NCPh₂)C(Ar)NC(Ar)N} (Ar = Ph,

Ar^F⁵).^4gThe only related example of a similar double addition

of nitriles was reported for the transient oxo and sulﬁdo

complexes Cp*₂Zr(E) (E = O, S).¹⁴

When the reaction of 5 with an excess of CO₂(ca. 1.5 atm)

was followed by NMR spectroscopy, quantitative conversion to

compound 7 was observed within 50 min at room temperature

(Scheme 1). Compound 7 was isolated on the preparative scale

in 66% yield and characterized by NMR and IR spectroscopy

and elemental analysis. The NMR and other data are

characteristic of the C₁-symmetric [2 + 2] cycloaddition

product Cp*Ti{PhC(NⁱPr)₂}{OC(O)N(C{Ar^F⁵}NO^tBu)}

The reaction of 5 with further equivalents of Ar^F⁵CN

prompted us to develop this compound’s reaction chemistry

with other substrates. This would also, in eﬀect, result in

multiple substrate/cross-coupling reactions of the TiNO^tBu

functional group of 1. Interestingly, while multiple couplings of

CO₂,^2r,v,9a,disocyanates,^2v,4g,9dand alkynes,¹⁵and of an alkyne

and a nitrile^2sor isonitrile^2eacross a number of TiN_αR (R =

hydrocarbyl, NR₂, NCPh₂, NO^tBu) bonds are known, these

predominately involve insertion of the second substrate into the

single Ti−X bond (X = N, C) of an initial [2 + 2] cycloaddition

intermediate (for example of the type 2 in Figure 1). Multiple

substrate coupling starting from 1 and nitriles would follow a

diﬀerent mechanistic paradigm, with each stage involving

substrate addition to a metal−nitrogen multiple bond (i.e.,

TiNO^tBu followed by TiNC(Ar)NO^tBu). In this

contribution we report our studies of the reactions of

Cp*Ti{PhC(NⁱPr)₂}{NC(Ar^F⁵)NO^tBu} (5) with heterocumu-

lenes, aldehydes, dimethyl formamide, isonitriles, and B(Ar^F⁵)₃.

Scheme 1. Reaction of

Cp*Ti{PhC(NⁱPr)₂}{NC(Ar^F⁵)NO^tBu} (5) with CO₂

RESULTS AND DISCUSSION

■

The benzimidamido compounds Cp*Ti{PhC(NⁱPr)₂}{NC-

(Ar)NO^tBu} (Ar = Ar^F(4), Ar^F(5)) are readily formed

from Cp*Ti{PhC(NⁱPr)₂}(NO^tBu) (1) and the corresponding

nitrile, and when they are followed on the NMR tube scale the

conversions are quantitative and eﬀectively immediate in the

case of 5.^7cHowever, owing to its high solubility in aliphatic

2

5

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Scheme 2. Reaction of Cp*Ti{PhC(NⁱPr)₂}{NC(Ar^F⁵)NO^tBu} (5) with CS₂

t

Scheme 3. Reaction of Cp*Ti{PhC(NⁱPr)₂}{NC(Ar^F⁵)NO^tBu} (5) with BuNCO

(7), containing a N,O-bound carbamate-type ligand. For

example, two apparent septets are observed in the H NMR

related compounds Cp*Ti{MeC(NⁱPr)₂}{OC(O)N(NR₂)C-

(O)O} (R = Me, Ph) show corresponding CO resonances

at δ 152.8 and 152.7 ppm, respectively, which are also shifted to

higher ﬁeld in comparison to their respective cycloaddition

precursors.^2v

1

spectrum for the inequivalent isopropyl groups, along with four

doublets for the two pairs of diastereotopic methyl groups. The

¹⁹F NMR spectrum shows ﬁve new resonances (range δ −137.7

to −164.9 ppm), which indicates restricted rotation about the

C−C_ipsobond to the C₆F₅ring. The ¹³C NMR CO resonance

at δ 155.8 ppm is similar to the corresponding values for

Cp*Ti{MeC(NⁱPr)₂}{OC(O)N(NR₂)} (R = Ph, δ 160.6 ppm;

R = Me, δ 158.1 ppm).^2vThe IR spectra of 7 and these

compounds all show strong ν(CO) bands at 1690, 1684, and

1667 cm⁻¹, respectively, for the carbamate ligands.

Reaction of 5 with a small excess of CS₂in C₆D₆resulted in

the very slow formation of the known^7ccompound Cp*Ti-

{PhC(NⁱPr)₂}{SC(S)N(O^tBu)} (9; Scheme 2) and Ar^F⁵CN,

and after 15 days at room temperature the reaction had still

only reached ca. 65% conversion. At this stage signiﬁcant

1

amounts of unknown side products were present in the H

NMR spectrum. The amount of these increased upon heating

at 70 °C. The reaction probably proceeds via slow extrusion of

Ar^F⁵CN from 5, yielding the alkoxyimido compound 1 (not

observed) which, as previously reported, reacts relatively

quickly with CS₂to form the apparently more stable

cycloaddition product 9.^7cThe extrusion of ArCN from

benzimidamido complexes is precedented, having previously

been observed in the reactions of insertion complexes 3 and 4

with Ar^F⁵CN.^7cThe corresponding reactions of the dimethyl,

diphenyl, and alkylidene hydrazido analogues of 5 all form

stable [2 + 2] cycloaddition products with CS₂,^2v,4gwhereas the

imido-derived dithiocarbamates rapidly undergo retrocycliza-

tion and formation of isothiocyanates.^9d

Like its arylimido-^9dand hydrazido-^2vderived counterparts,

compound 7 does not undergo retro-cyclization to eliminate

^tBuONC(Ar^F⁵)NCO, and is stable for many days in solution at

room temperature. However, in the presence of an excess of

CO₂, 7 eventually forms the C_s-symmetric “double-insertion”

product Cp*Ti{PhC(NⁱPr)₂}{OC(O)N(C{Ar^F⁵}NO^tBu)C-

(O)O} (8, Scheme 1). The reaction is slow, and when

followed in C₆D₆reaches only 70% conversion after ca. 90 h, at

which stage no further reaction is observed. Upon heating the

reaction mixture or attempted isolation, compound 8 eliminates

CO₂reforming 7. The reversible insertion of a heterocumulene

into the Ti−N bond of a metallacyclic species such as 7 has

been observed previously.^2bCompound 8 was characterized in

t

Reaction of 5 with 1 equiv of BuNCO gave the N,O-bound

ureate-type product Cp*Ti{PhC(NⁱPr)₂}{N(C{Ar^F⁵}NO^tBu)-

C(N^tBu)O} (10; Scheme 3), analogous to 7, which was

isolated as a brown microcrystalline solid in 65% yield upon

scale-up. In contrast, reactions with aryl isocyanates (TolNCO

1

situ by H, ¹³C and ¹⁹F NMR spectroscopy. The H NMR

spectrum shows one septet corresponding to the equivalent iso-

propyl methine hydrogens, and two doublets for the methyl

groups of these substituents. The ¹³C NMR spectrum shows a

shift in the CO and NC(Ar^F⁵)N resonances from δ 155.8 to

152.8 ppm and from δ 146.2 to 137.3 ppm, respectively. The

i

and Ar′NCO (Ar′ = 2,6-C₆H₃Pr₂)) or isothiocyanates

(^tBuNCS and TolNCS) produced complex mixtures and

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were not investigated further. The NMR data for 10 are

consistent with the proposed structure, and the IR spectrum

shows a strong band at 1631 cm⁻¹attributed to ν(CN) of

the ureate ligand, similar to those found for Cp*Ti{MeC-

(NⁱPr)₂}{N(NPh₂)C(N^tBu)O}^2vand Cp*Ti{MeC(NⁱPr)₂}{N-

(Xyl)C(N^tBu)O}^9d(1653 and 1652 cm⁻¹, respectively).

Compound 10 is relatively stable in solution at room

temperature. Thus, after 16 h, solutions in C₆D₆undergo less

than 10% conversion to the μ-oxo dimer [Cp*Ti{PhC(NⁱPr)₂}-

(μ-O)]₂(11)^7cand the new imidoylcarbodiimide BuNCNC-

t

C₅Me₅ligand and PhCH of the metallacycle, consistent with

the structure shown in eq 1: namely, with the phenyl group

oriented away from the η-C₅Me₅ligand in the sterically least

congested arrangement. This geometry was conﬁrmed in the

solid state by X-ray diﬀraction (vide infra). Interestingly, both

the PhCH and PhCH signals of 13a appar as doublets (J = 2.1

and 1.4 Hz, respectively) due to C−H···F interactions. This is

either a consequence of scalar coupling or cross-correlation

interactions¹⁸between PhCH and Ar^F⁵, as conﬁrmed by a

¹H{¹⁹F} NMR experiment in which PhCH appared as a singlet.

The ¹⁹F NMR spectrum showed ﬁve resonances at room

temperature, consistent with restricted rotation about the C−

C_ipsobond to the C₆F₅ring.

The PhCH “up” arrangement for 13a is the same as that

found in Cp*Ti{MeC(NⁱPr)₂}{N(^tBu)C(Ph)(H)O}, formed

from Cp*Ti{MeC(NⁱPr)₂}(N^tBu) and PhC(O)H. In contrast,

for the reaction of the homologous tolylimido compound and

PhC(O)H both isomers were observed,^9das was also the case

for the reaction of Cp*Ti{PhC(NⁱPr)₂}(NO^tBu) (1) with

PhC(O)H and PhC(O)Me.^7cInterestingly, whereas 13a is

stable in solution for several hours, all of these previously

reported metallacycles rapidly underwent extrusion of the

corresponding imines below room temperature, thus prevent-

ing their isolation on a preparative scale. To take advantage of

this opportunity to study further these types of metallacycles,

we prepared a series of homologues,: namely, Cp*Ti{PhC-

(NⁱPr)₂}{N(C{Ar^F⁵}NO^tBu)C(Ar)(H)O} (Ar = 4-C₆H₄X; X =

Me (13b), ^tBu (13c), OMe (13d), NMe₂(13e), CF₃(13f); eq

1). The new compounds were obtained in near-quantitative

yields, and their NMR and other data were analogous to those

of 13a, indicating the presence of a single ArCH “up” isomer in

each case.

(Ar^F⁵)NO^tBu (12; Scheme 3), and even after 1 week only ca.

20% conversion occurs. Under analogous conditions, the

t

reaction of Cp*Ti{PhC(NⁱPr)₂}(NO^tBu) (1) with BuNCO

t

slowly forms the carbodiimide BuNCNO^tBu and 11 following

extrusion from the proposed intermediate Cp*Ti{PhC-

(NⁱPr)₂}{N(O^tBu)C(N^tBu)O}, which was not observed in

this case.^7cTherefore, the benzimidamide functional group

appears to help stabilize this type of intermediate (i.e., 10). The

t

behavior of 1 toward BuNCO is comparable to that of its

imido and dimethyl- and alkylidenehydrazido homologues,

whereas the diphenylhydrazide Cp*Ti{MeC(NⁱPr)₂}(NNPh₂)

t

forms a [2 + 2] cycloaddition product with BuNCO without

carbodiimide elimination. DFT studies have shown that both

electronic and steric factors are important in such cyclo-

addition−elimination processes.^2v,7c,9d

Heating a solution of 10 to 65 °C for 15 h nonetheless gave

near-quantitative conversion to 12, along with 11. Compound

12 was isolated as an analytically pure, yellow oil in 71% yield

1

by distillation (100 °C, 8 × 10⁻²mbar). The H, ¹³C, and ¹⁹F

NMR spectra indicate a single geometric isomer which, by

analogy to 13a (vide infra) and taking into account the steric

constraints present in 10, is assigned as having a cis

arrangement of the −O^tBu and −Ar^Fgroups across (^tBuO)-

5

NC(Ar^F⁵) and a trans arrangement of groups across NC

N, as shown in Scheme 3. Imidoylcarbodiimides RNC(R¹)-

NCNR²can in general be synthesized from the corresponding

imidoylthioureas, RNC(R¹)N(H)C(S)NHR², and have been

shown to undergo a rearrangement, forming synthetically useful

aminoquinazolines (R = Ph) or dihydrotriazines (R =

CHR′R″).¹⁶The transformation of 5 to 12 shown in Scheme

3 represents a new synthetic method for this type of

compound, albeit rather limited in scope and presently without

opimization.

Diﬀraction-quality crystals of 13a were grown from a

saturated n-hexane solution at 4 °C. The molecular structure

is shown in Figure 2, and selected distances and angles are

given in Table 1. The structure conﬁrms the incorporation of

PhC(O)H to form a new N{C(Ar^F⁵)NO^tBu}C(H)(Ph)O

ligand binding to titanium in a κ²N,O-coordination mode.

The stereochemistry around C(1) is consistent with the

solution NOE measurements with H(1) directed toward the

η-C₅Me₅ligand. The metric parameters associated with both

Cp*Ti{PhC(NⁱPr)₂} and NC(Ar^F⁵)NO^tBu fragments lie within

the expected values.¹⁹The Ti(1)−N(1) bond length is

increased relative to 5 (2.0070(16) vs 1.747(4) Å) and is

similar to the Ti−N_amidinatesingle bonds (2.1160(18) and

2.0972(17) Å). The H(1)···F(1) distance of 2.59 Å is

consistent with the observed doublet coupling in the ¹H

NMR spectra of 13a−f. Despite the apparent close proximity of

Reactions with Benzaldehydes and HC(O)NMe₂.

Previous reports of the reactions of group 4 imides,^{1b−e,9d,e,17}

hydrazides,^2k,v,4gand alkoxyimides^7cwith aldehydes and

ketones have shown their tendency to form unstable [2 + 2]

cycloaddition products which rapidly extrude the correspond-

ing imine, hydrazone, or oxime ether, respectively. Given the

relative stability of 10, it was hoped that the reaction of 5 with

aldehydes and related substrates would likewise form more

stable cycloaddition products of the type Cp*Ti{PhC(NⁱPr)₂}-

{N(C{Ar^F⁵}NO^tBu)C(R)(R′)O}.

Reactions with Aldehydes. As shown in eq 1, reaction of

PhC(O)H with 5 rapidly formed the [2 + 2] cycloaddition

product Cp*Ti{PhC(NⁱPr)₂}{N(C{Ar^F⁵}NO^tBu)C(Ph)(H)O}

(13a). Compound 13a formed in quantitative yield on the

NMR tube scale and on scale-up was isolated as a dark brown,

microcrystalline solid in 82% yield. Only one set of signals were

observed in the NMR spectra, consistent with formation of a

single diastereomer (both the titanium and PhCH carbon being

stereogenic centers). A nuclear Overhauser eﬀect (NOE)

experiment showed a through-space interaction between the η-

the Ph and Ar^Frings and the potential for mixed arene−

5

perﬂuoroarene π−π interactions,²⁰only two close interactions

are observed: namely, C(2)···C(9) = 3.054(3) Å and C(2)···

C(14) = 3.346(3) Å.

Although the ¹H NMR signals for 13a are not broad at room

temperature, spin saturation transfer measurements showed

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formed from imides (and related systems) and unsaturated

substrates.^2q,sThe relative stability of the complexes Cp*Ti-

{PhC(NⁱPr)₂}{N(C{Ar^F⁵}NO^tBu)C(Ar)(H)O} (13a−f) and

the reversible addition of PhC(O)H to the benzimidamido

ligand of 5 prompted us to investigate this further. Thus, ca. 1

equiv of ArC(O)H (Ar = 4-C₆H₄X; X = Me, ^tBu, OMe, NMe₂,

CF₃) was added to solutions of 13a in C₆D₆and the reactions

1

monitored by H NMR spectroscopy (eq 3). Competition

equilibria were established within 10 min.

Figure 2. Displacement ellipsoid plot (20% probability) of Cp*Ti-

{PhC(NⁱPr)₂}{N(C{Ar^F⁵}NO^tBu)C(Ph)(H)O} (13a). H(1) is drawn

as a sphere of an arbitrary radius. All other H atoms are omitted for

clarity.

Table 1. Selected Bond Lengths (Å) and Angles (deg) for

Cp*Ti{PhC(NⁱPr)₂}{N(C{Ar^F⁵}NO^tBu)C(Ph)(H)O}

a

(13a)

Ti(1)−Cp_cent

Ti(1)−N(1)

Ti(1)−N(3)

Ti(1)−N(4)

Ti(1)−O(1)

Cp_cent−Ti(1)−N(1)

Cp_cent−Ti(1)−N(3)

N(3)−Ti(1)−N(4)

Ti(1)−N(1)−C(1)

O(1)−C(1)−N(1)

2.055

N(1)−C(1)

N(1)−C(8)

O(1)−C(1)

C(1)−C(2)

1.469(2)

1.358(2)

1.419(2)

1.519(3)

2.0070(16)

2.1160(18)

2.0972(17)

1.8980(14)

118.3

The equilibrium constants (K_eq) are given in Table S1 of the

Supporting Information and are depicted graphically in

semilogarithmic form in Figure 3 against the corresponding

Cp_cent−Ti(1)−O(1)

Cp_cent−Ti(1)−N(4)

O(1)−Ti(1)−N(1)

Ti(1)−O(1)−C(1)

118.9

112.6

113.7

62.63(7)

90.95(11)

101.85(14)

70.05(6)

97.13(10)

a

Cp_centis the computed Cp* ring carbon centroid.

exchange on the NMR time scale between free (added)

benzaldehyde and the PhC(O)H incorporated in 13a. This is

proposed to occur via a dissociative mechanism, transiently

forming 5, as has been observed previously for related imido

compounds and carbonyl complexes.^9dAs a test of this, bearing

in mind the known irreversible reaction of 1 with PhC(O)H to

form 11 and the oxime ether PhC(NO^tBu)H,^7can NMR tube

scale crossover experiment between 13a and 1 in C₆D₆was

carried out, which showed quantitative formation of the

products shown in eq 2.

Figure 3. Plot of log K_eqvs σ_pfor the reactions of Cp*Ti{PhC-

(NⁱPr)₂}{N(C{Ar^F⁵}NO^tBu)C(Ph)(H)O} (13a) with ArC(O)H (Ar

t

= 4-C₆H₄X; X = Me, Bu, OMe, NMe₂, CF₃) at 293 K in C₆D₆. The

best-ﬁt line shown has R²= 0.962.

21

Hammett substituent constants, σ_p.The K_eqdata clearly

indicate a thermodynamic preference for cycloaddition

products with electron-withdrawing para substituents.

Compound 13a is not indeﬁnitely stable in solution, and on

heating a C₆D₆solution at 70 °C, a color change from brown to

yellow, characteristic of the μ-oxo dimer 11, is observed. The

¹H NMR spectrum showed quantitative formation of 11 and

the 1,3-diazabutadiene derivative PhC{NC(Ar^F⁵)NO^tBu}H

(14a; eq 4). Analogous results were obtained for 13b−f, and

the corresponding organic products 14a−f were isolated by

sublimation on scale-up in 62−93% yield. Compounds 14a−f

exist predominantly as the single geometric isomer depicted in

eq 4 on the basis of the X-ray structure of 14e and DFT

calculations (vide infra). It is interesting to note the trans

arrangement of the Ar− and −C(Ar^F⁵)NO^tBu groups with

respect to the Ar(H)CNC(Ar^F⁵)NO^tBu double bond. This is

consistent with previous DFT studies of the cycloaddition−

extrusion reactions of Cp*Ti{PhC(NⁱPr)₂}(NO^tBu) (1) with

PhC(O)R (R = H, Me), in which the metallacycle

Previous studies have suggested that electron-withdrawing

substituents can help to stabilize metallacyclic complexes

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The increased k_obsvalues (range [8.09(3)−11.80(8)] × 10⁻⁵

s⁻¹) in comparison to that for 13a at this temperature (6.79(3)

× 10⁻⁵s⁻¹) show that electron-releasing para substituents

reduce the activation energy for extrusion. Attempts to measure

k

_obsat 70 °C for 13e, possessing the most electron-releasing p-

NMe₂substituent, were unsuccessful. The reaction reached

greater than 50% completion within 5 min, and the broad H

resonances hindered accurate H NMR integration. Therefore

1

k

_obsfor 13e was measured at 32 °C and the corresponding k_obs

value for 13a at 32 °C was calculated from the Eyring plot in

Figure 4. Comparison of the k_obsvalues at 32 °C (1.22(1) ×

10⁻⁵s⁻¹for 13e vs 0.04 × 10⁻⁵s⁻¹for 13a) shows that

extrusion from 13e is ca. 30 times faster than that from 13a.

The k_obsvalue for 13e at 70 °C was estimated as 207 × 10⁻⁵

s⁻¹, assuming that k_obs(70 °C)/k_obs(32 °C) is the same for both

13a and 13e (i.e., that ΔS^⧧is negligible in each case, as found

experimentally for 13a, and that the ΔG^⧧values for extrusion

are primarily controlled by the ΔH^⧧terms).

intermediates with the PhCR substituent oriented “up” toward

Cp* gave rise to the product oxime ether PhC(NO^tBu)R with a

trans arrangement of the Ph− and −O^tBu groups.^7c

Figure 5 shows a plot of log(k_obs/k_H) vs σ_pfor 13a−e based

on the experimental or estimated k_obsvalues (k_H= k_obsfor 13a;

The molecular structure of 14e is shown in Figure S1 of the

Supporting Information, and selected bond distances and

angles can be found in Table S2. The structure establishes both

the connectivity and the geometric isomer isolated. The metric

parameters lie within the expected ranges and indicate discrete

single and double bonds. The electronic and molecular

structure of 14e and its homologues are discussed in further

detail below.¹⁹

The extrusion reaction of Cp*Ti{PhC(NⁱPr)₂}{N(C{Ar^F⁵}-

NO^tBu)C(Ph)(H)O} (13a) in C₆D₆was monitored in the

temperature range 54−76 °C. The reactions followed ﬁrst-

order kinetics, as judged by linear semilogarithmic plots of

ln([13a]_t/[13a]₀) vs time. An Eyring analysis of the rate

constant k_obswas carried out (Figure 4), giving the activation

Figure 5. Plot of log(k_obs/k_H) vs σ_pfor the extrusion reaction of

Cp*Ti{PhC(NⁱPr)₂}{N(C{Ar^F⁵}NO^tBu)C(4-C₆H₄X)(H)O} (13a−

e) at 70 °C in C₆D₆. The best-ﬁt line shown has R²= 0.965, giving

ρ = −1.9(2). k_His k_obsfor X = H, and k_obsfor 13e was estimated from

data collected at 32 °C (see Table S3 and Figure S2 (representative

individual ﬁrst-order semilogarithmic plots) of the Supporting

Information).

σ_pis the Hammett substituent constant), which gave a reaction

constant (ρ) of −1.9(2). This is consistent with a buildup of

positive charge on the ArC(H) carbon in the rate-determining

step, indicating a transition state consisting more of C−O

bond-breaking than of C−N bond-forming character. Un-

expectedly, the k_obsvalue of 14.5 × 10⁻⁵s⁻¹(average of two

independent experiments) for compound 13f was signiﬁcantly

higher than that expected (0.42 × 10⁻⁵s⁻¹) on the basis of the

Hammett plot and the data for 13a−e. We are unable to

explain this diﬀerence, which may reﬂect a switch to a diﬀerent

mechanism for this most stable of the metallacycles (cf. the K_eq

data in Table S1 of the Supporting Information and Figure 3).

Figures 3 and 5 show that, while PhC(O)H forms a more

thermodynamically stable metallacycle (13a) than (4-

C₆H₄NMe₂)C(O)H, the latter (13e) is signiﬁcantly more

reactive toward extrusion, forming 14e. Scheme 4 summarizes a

competition experiment that illustrates this interplay. Addition

of an equimolar solution of PhC(O)H and (4-C₆H₄NMe₂)C-

(O)H to 5 (1:1:1 initial ratio) in C₆D₆at room temperature

Figure 4. Eyring plot for the extrusion reaction of Cp*Ti{PhC-

(NⁱPr)₂}{N(C{Ar^F⁵}NO^tBu)C(Ph)(H)O} (13a) in C₆D₆. Activation

parameters are derived from a linear regression analysis (R²= 0.963):

ΔH^⧧= 27(3) kcal mol⁻¹; ΔS^⧧= 1(9) cal mol⁻¹K⁻¹; ΔG₃₄₃^⧧= 27(4)

kcal mol⁻¹.

parameters ΔH^⧧= 27(3) kcal mol⁻¹and ΔS^⧧= 1(9) cal mol⁻¹

K⁻¹(ΔG₃₄₃= 27(4) kcal mol⁻¹). The negligible value of ΔS^⧧

⧧

is consistent with an early transition state for these dissociative

reactions.²²

The corresponding ﬁrst-order rate constants (k_obs) were also

determined for the extrusion reactions of Cp*Ti{PhC(NⁱPr)₂}-

t

{N(C{Ar^F⁵}NO^tBu)C(4-C₆H₄X)(H)O} (X = Me (13b), Bu

(13c), OMe (13d)) at 70 °C and are given in Table S3 of the

Supporting Information, along with representative examples of

the corresponding ﬁrst-order semilogarithmic plots (Figure S2).

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Scheme 4. Reaction of Cp*Ti{PhC(NⁱPr)₂}{NC(Ar^F⁵)NO^tBu} (5) with PhC(O)H and ArC(O)H (Ar = 4-C₆H₄NMe₂; 1:1

a

Ratio)

a

13a and 13e are formed in a 10:1 ratio and 14a and 14e in a 2:1 ratio.

gave predominantly 13a and unreacted (4-C₆H₄NMe₂)C(O)H

(13a:13e ≈ 10:1 in accordance with Figure 3). After the

1

mixture was heated to 65 °C for 21 h, the H NMR spectrum

showed formation of the organic products 14a,e in a ca. 2:1

ratio, in accordance with the Curtin−Hammett principle.²³

The organic products 14a−d were isolated as white/cream

solids, and 14f was a pale yellow oil. In contrast, 14e was

obtained as an orange solid, suggesting a signiﬁcant eﬀect of the

p-NMe₂group on the electronic structure. Table 2 gives the

Table 2. Experimental λ_maxValues (nm) and Absorption

Coeﬃcients (ε) for (4-C₆H₄X)C{NC(Ar^F⁵)NO^tBu}H (14a−

f), Together with HOMO−LUMO Gaps (eV) Determined

a

by DFT

λ_max

(nm)

ε × 10⁻⁴

HOMO−LUMO

compound

p-X

(M⁻¹cm⁻¹

)

separation (eV)

14a

14b

14c

14d

14e

14f

H

271

275

281

360

274

3.0

5.4

3.7

5.8

5.3

1.4

2.64

2.60

2.61

2.53

2.28

2.55

Me

^tBu

Figure 6. Isosurfaces and energies of the HOMO and LUMO of

PhC{NC(Ar^F⁵)NO^tBu}H (14a, top) and (4-C₆H₄NMe₂)C{NC(Ar^F⁵)-

NO^tBu}H (14e, bottom).

OMe

NMe₂

CF₃

a

The spectra were measured as n-hexane solutions.

further details for the other derivatives are given in Figure S3 of

the Supporting Information. The computed HOMO−LUMO

separations are given for all of the compounds in Table 2. For

14a−d,f the values fall in the range 2.53−2.64 eV. For 14e the

value is 2.28 eV and hence the trends in HOMO−LUMO

experimental λ_maxvalues and absorption coeﬃcients for the

lowest energy bands in the UV−visible spectra of 14a−f

measured in n-hexane. The absorption coeﬃcients lie in the

range (1.4−5.8) × 10⁴M⁻¹cm⁻¹and do not vary in a

systematic manner. However, whereas the λ_maxvalues for 14a−

d,f appear between ca. 271 and 281 nm, that for 14e is at 360

nm, tailing into the blue end of the visible spectrum, accounting

for the orange color. UV−visible spectra were also measured for

14a,e in MeOH, the higher dielectric constant (ε = 32.6)²⁴

resulting in a small bathochromic shift in the λ_maxvalue in each

case (Δλ_max= 15 and 23 nm, respectively) but having no

signiﬁcant eﬀect on the extinction coeﬃcients.

separations qualitatively correspond well with the trends in

25

λ_max

.

The destabilizing eﬀect of the p-NMe₂substituent on

both the HOMO and LUMO of 14e is evident from Figure 6

(bottom), which clearly shows the antibonding aryl(π)−

NMe₂(2p_π) interaction in each orbital. This is responsible for

the general destabilization of these frontier MOs relative to

those of 14a (Figure 6, top). However, while the HOMO of

14e is destabilized by 0.87 eV, the LUMO is only destabilized

by 0.51 eV, explaining the decrease in HOMO−LUMO

separation from 14a to 14e. Mulliken population analyses

found that the NMe₂(2p_π) atomic orbital contributions to the

HOMO and LUMO of 14e are 18% and 3.4%, respectively,

consistent with the diﬀerent changes in energy. Similarly, the

DFT calculations were carried out at the GGA:BP/TZP level

on minimized geometries of compounds 14a−f and, as

expected, revealed generally similar electronic structures. Figure

6 depicts isosurfaces for the HOMO and LUMO of 14a,e;

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Scheme 5. Reaction of Cp*Ti{PhC(NⁱPr)₂}{NC(Ar^F⁵)NO^tBu} (5) with HC(O)NMe₂

next smallest HOMO−LUMO separation (and next longest

λ_max) was found for 14d (p-OMe group), although the higher

electronegativity of oxygen in comparison with nitrogen leads

to an overall decrease in contribution of the OMe(2p_π) lone

pair to both the HOMO and LUMO (8.6% and 2.2%

contributions, respectively).

NMR timescale with the enantiomer 15′, as implied by the

apparent molecular C_ssymmetry. Compound 16 is proposed to

form by a cycloaddition−extrusion mechanism similar to that

established for 14a−f. The fast reaction and lack of observation

of any intermediates (i.e., analogues of 13a−f) are consistent

with the strong π-donor ability of the −NMe₂group which, by

analogy with the results for the reactions with ArC(O)H, would

both destabilize any intermediate metallacycle and accelerate

the rate of extrusion of 16 from it.

Reactions with Ketones and HC(O)NMe₂. Reaction of 5

with either PhC(O)Me or PhC(O)Ph gave disappointing

results. The reaction with PhC(O)Me in C₆D₆at room

temperature generated a complex mixture of products within 15

min.^9dAnalogous results were found for Cp*Ti{MeC(NⁱPr)₂}-

(NTol) with ketones of the type RC(O)Me, and it was

proposed that concomitant side reactions stemming from the

enol tautomers (RC(OH)CH₂) may be responsible.²⁶Heating

to 70 °C was required to eﬀect a reaction between 5 and

PhC(O)Ph, and even after 9 days it had only reached ca. 45%

The solid-state structures of (4-C₆H₄NMe₂)C{NC(Ar^F⁵)-

NO^tBu}H (14e) and 16 are shown in Figure S1 and selected

bond distances and angles are given in Tables S2 and S4 of the

Supporting Information. In general terms the metric parameters

lie within the expected ranges for the various bond types and

functional groups.^19,27In particular, the N(1)−C(5) and

N(2)−C(12) distances (range 1.281(5)−1.2990(16) Å) and

N(2)−C(5) distances (1.394(5) and 1.3826(15) Å) are

consistent with double and single bonds and the 1,3-

diazabutadiene structures shown in eq 4 and Scheme 5. The

Ar^F⁵ring is appoximately orthogonal to the N(1)−C(5)−N(2)

moiety in each compound (angles between the least-squares

planes of 76.8 and 68.8°), whereas the −C₆H₄NMe₂and

−NMe₂groups are appoximately coplanar with the C(12)−

N(2)−C(5) linkage, as shown by the dihedral angles N(2)−

C(12)−C(13)−C(14,18) = 3.1° (average) and N(2)−C(12)−

N(3)−C(13,14) = 3.9° (average). The most signiﬁcant

diﬀerence between 14e and 16 is the geometry about the

N(2)−C(5) bond. Compound 14e adopts a transoid

conformation of the CN double bonds, whereas 16 has a

cisoid form (cf. Figure 7), as quantiﬁed by the N(1)−C(5)−

N(2)−C(12) dihedral angles of 174.8 and 35.3°, respectively.

For 14e this dihedral angle is close to the ideal value of 180°,

whereas for 16 it deviates signiﬁcantly from 0° due to repulsion

1

conversion. At this point the H NMR spectrum also showed

signiﬁcant impurities and was not investigated further. In

contrast, reaction between 5 and HC(O)NMe₂in C₆D₆formed

the ﬂuxional intermediate Cp*Ti{PhC(NⁱPr)₂}{NC(Ar^F⁵)-

NO^tBu}{OC(NMe₂)H} (15) within 5 min. This, in turn,

converted quantitatively to the μ-oxo dimer 11 and the 1,3-

diazabutadiene Me₂N{NC(Ar^F⁵)NO^tBu}H (16) after 1 h

(Scheme 5; 10% conversion after 5 min). On scale-up 16 was

isolated by sublimation as a white solid in 64% yield and its

identity conﬁrmed by X-ray diﬀraction (vide infra).

Compound 15 was too unstable to be isolated, and the

resonances of the Cp*, PhC(NⁱPr)₂, and HC(O)NMe₂group

were only slightly shifted from those of 5 and the free

formamide. Accordingly, by analogy to the crystallographically

characterized σ adduct Cp*Ti{PhC(NⁱPr)₂}{NC(Ar^F⁵)-

NO^tBu}(CN^tBu) (18; vide infra) it was assigned the structure

shown in Scheme 5, existing in dynamic equilibrium on the

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present in solution but be in fast dynamic equilibrium on the

NMR time scale. To determine whether cisoid ↔ transoid

interconversions might be energetically feasible, the corre-

sponding transition states were determined by DFT for the

transoid to cisoid isomerization of 14a,e (ΔH^⧧= 4.71 and 4.36

kcal mol⁻¹, respectively) and for the cisoid to transoid

isomerization of 16 (ΔH^⧧= 3.78 kcal mol⁻¹). The computed

ΔS^⧧values were small and were in the range −3 to −5 cal

mol⁻¹K⁻¹. Taken together, the DFT calculations estimate that

the maximum ΔG^⧧value for the cisoid ↔ transoid trans-

formations is less than ca. 6.5 kcal mol⁻¹at 298 K. This readily

accessible barrier is consistent with the observation of a single

time-averaged set of NMR resonances in all cases.

Figure 7. transoid and cisoid isomers of RC{NC(Ar^F⁵)NO^tBu}H (R =

4-C₆H₄X (14a−f), NMe₂(16), H (17)).

between H(1) and the N(1) lone pair. The relatively poor

precision of the structure for 14e (due to the poorly diﬀracting

nature of the crystals) prevents a more detailed comparison of

its individual bond distances and angles with those of 16, but

analysis of the DFT computed structures for each compound

conﬁrmed the general structural features.

Reactions with Isonitriles. There have been a number of

reports of the reactions of transition-metal imido and hydrazido

complexes with organic isonitriles, with the commercially

available ^tBuNC and XylNC derivatives being the most

commonly studied.^1a,b,e[1 + 2] addition of RNC to the

MNR′ bond of imides or hydrazides forms η²-carbodiimide

complexes of the type (L)M(η²-RNCNR′), which have been

isolated in several cases for R′ = hydrocarbyl.^15,29In other

instances the ﬁrst-formed carbodiimides are not observed but

undergo further reactions. These include reactions with further

equivalent(s) of isonitrile, giving new heterocyclic complexes,³⁰

or in the case of certain group 4 diphenylhydrazido complexes,

N_α−N_βbond cleavage resulting in mixed amide-metalated

carbodiimide complexes (L)M(NPh₂)(NCNR).^2s,z,3bIn some

instances, reaction with the MNR′ bond does not occur and

RNC σ adducts can be isolated.^4g,31We have previously

reported on the reactions of Cp*Ti{RC(NⁱPr)₂}(NR′) (R =

Me, Ph; R′ = ^tBu, aryl, NPh₂, NMe₂, NNCPh₂, O^tBu (1)) with

isonitriles. In the case of the imido complexes no reaction was

observed, whereas unknown mixtures were formed with the

diphenylhydrazido compound or 1.^7c,31cThe other complexes

formed labile σ adducts, one of which was structurally

characterized.^4g,31c

To explore further these diﬀerences in apparent conforma-

tional preference, DFT calculations were carried out on

PhC{NC(Ar^F⁵)NO^tBu}H (14a), 14e, and 16 in their transoid

and cisoid conformations. Corresponding calculations were also

carried out on the hypothetical HC{NC(Ar^F⁵)NO^tBu}H (17)

with H in place of the 4-C₆H₄X or NMe₂groups of 14a,e and

16. In each case the calculated geometries and metric

parameters compared well with the experimental values:

namely, a well-deﬁned 1,3-diazabutadiene structure with an

approximately coplanar −N(1)C−N(2)C− linkage in the

transoid conformations and a somewhat twisted arrangement

for the cisoid forms. Since the calculations found no signiﬁcant

or systematic diﬀerence in entropy between each pair of

conformers (ΔS typically varied between ca. −0.5 and 1.5 cal

mol⁻¹K⁻¹), we focus only on the diﬀerences in enthalpy

(ΔH).²⁸

In agreement with the experimental structures, the transoid

isomer of 14e was 0.67 kcal mol⁻¹more stable than the cisoid

alternative, whereas for 16 the cisoid conformer was the more

stable isomer by a similar amount (ΔH = 0.65 kcal mol⁻¹). The

enthalpy diﬀerence (ΔH = 0.80 kcal mol⁻¹) in favor of the

transoid form for 14a was slightly larger than for 14e, suggesting

that the p-NMe₂group in the latter slightly destabilizes the

transoid conformer relative to the cisoid form. For the

hypothetical HC{NC(Ar^F⁵)NO^tBu}H (17), having only a H

atom in place of 4-C₆H₄X or NMe₂, the transoid form was 1.23

kcal mol⁻¹more stable, showing that this is the intrinsically

more stable conformer in this type of system, although the

energy diﬀerences are all rather small. Examination of the

Mulliken atomic charges (q) for N(2) revealed that changing

the R group in transoid-RC{NC(Ar^F⁵)NO^tBu}H from H to

progressively better π-donor substituents gave a build-up of

charge on this atom: R = H, q(N(2)) = −0.2291; R = Ph,

q(N(2)) = −0.2578; R = 4-C₆H₄NMe₂, q(N(2)) = −0.2751; R

= NMe₂, q(N(2)) = −0.3286. Therefore, it seems that

increased lone pair repulsion between N(2) and N(1) in the

transoid conformer with increasing R group π donation may be

an important factor leading to a switch toward the cisoid

alternative.

t

Reaction of 5 with BuNC in Et₂O at room temperature

formed the σ adduct Cp*Ti{PhC(NⁱPr)₂}{NC(Ar^F⁵)NO^tBu}-

(CN^tBu) (18, eq 5), which was isolated in 61% yield after

crystallization from pentane. When the reaction was followed

on the NMR tube scale in C₆D₆, the conversion was

quantitative. The NMR spectrum at room temperature

indicated a C_s-symmetric product on the NMR time scale.

This is attributed to the rapid interconversion between the two

enantiomers 18 and 18′ (eq 5). Cooling a solution of 18 in

toluene-d₈showed the isopropyl groups as two apparent septets

and four doublets, although the resonances remained broad,

even at −80 °C. At this temperature the o-and m-C₆F₅¹⁹F

resonances were inequivalent, indicative of restricted rotation

The small ΔH values suggest that both the transoid and cisoid

forms of 14a−f and 16 may exist in solution, since ΔH values

of ca.

0.70 kcal mol⁻¹(assuming ΔS ≈ 0 as discussed)

correspond to ca. 1:( 4) ratios of conformers. Although only

one set of NMR resonances is seen experimentally in all cases,

we considered it possible that both conformers could be

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about the N₂C−C_ipsobond of the benzimidamido ligand. The

IR spectrum of 18 shows a strong ν(CN) band at 2186 cm⁻¹,

No further reaction was found for 18 at room temperature,

and after 18 h at 70 °C in C₆D₆only new low-intensity

resonances were observed. After 5 days complicated mixtures

were formed and this reaction was not scaled up. The poor

reactivity of 18 is probably due to the steric demands of the

^tBuNC ligand.

Reaction of 5 with XylNC on the NMR tube scale in C₆D₆

resulted in the immediate and quantitative formation of

Cp*Ti{PhC(NⁱPr)₂}{NC(Ar^F⁵)NO^tBu}(CNXyl) (19; Scheme

6), which is unstable at room temperature and was

t

at a frequency higher than that for free BuNC (2138 cm⁻¹),

consistent with coordination to the electron-deﬁcient tita-

nium.³²The ν(CN) band is at a higher frequency than those

for Cp*Ti{MeC(NⁱPr)₂}(NNCPh₂)(CN^tBu) (2164 cm⁻¹)^4g

and the comparatively labile σ adduct Cp*Ti{MeC(NⁱPr)₂}-

(NNMe₂)(CN^tBu)^31c(2152 cm⁻¹). Other examples of

titanium d⁰tert-butyl isocyanide adducts include (MPB)-

TiCl₂(CN^tBu) (MPB²⁻= 2,2′-methylenbis(6-tert-butyl-4-meth-

ylphenolate) dianion)³³and TiCl₄(CN^tBu)₂.^32b

Diﬀraction-quality crystals of 18 were grown from a saturated

pentane solution at −30 °C. The molecular structure is shown

in Figure 8, and selected distances and angles are given in Table

Scheme 6. Reaction of

Cp*Ti{PhC(NⁱPr)₂}{NC(Ar^F⁵)NO^tBu} (5) with XylNC

Figure 8. Displacement ellipsoid plot (20% probability) of Cp*Ti-

{PhC(NⁱPr)₂}{NC(Ar^F⁵)NO^tBu}(CN^tBu) (18). H atoms are omitted

for clarity.

characterized in situ. The dynamic NMR behavior is analogous

to that for 18: the room-temperature NMR spectra indicate a

C_s-symmetric compound, while at −70 °C the resonances are

reminiscent of those for 18. Over a period of ca. 16 h 19

converted quantitatively to a new compound, Cp*Ti{PhC-

(NⁱPr)₂}{NC(NO^tBu)C₆F₄N(Xyl)C}(F) (20), which was

isolated as an orange powder in 85% yield after scale-up in

diethyl ether. Diﬀraction-quality crystals were grown from n-

hexane. The molecular structure is shown in Figure 9, and

selected bond distances and angles are given in Table 4.

Table 3. Selected Bond Lengths (Å) and Angles (deg) for

a

Cp*Ti{PhC(NⁱPr)₂}{NC(Ar^F⁵)NO^tBu}(CN^tBu) (18)

Ti(1)−Cp_cent

Ti(1)−N(1)

Ti(1)−N(4)

2.091

Ti(1)−C(12)

N(3)−C(12)

N(3)−C(13)

2.191(2)

1.153(3)

1.462(3)

1.7739(17)

2.1459(17)

2.1957(17)

119.37

Ti(1)−N(5)

Cp_cent−Ti(1)−N(1)

Cp_cent−Ti(1)−N(4)

N(4)−Ti(1)−N(5)

Ti(1)−N(1)−C(1)

Cp_cent−Ti(1)−N(5)

Cp_cent−Ti(1)−C(12)

Ti(1)−C(12)−N(3)

C(12)−N(3)−C(13)

115.62

121.35

104.69

62.09(6)

169.81(15)

167.35(19)

179.5(2)

a

Cp_centis the computed Cp* ring carbon centroid.

3. The structure conﬁrms 18 as a half-sandwich complex with a

four-legged piano-stool geometry around the metal with η⁵-

C₅Me₅and κ²N,N′-amidinate ligands and is similar to that

reported recently for Cp*Ti{MeC(NⁱPr)₂}(NNCPh₂)-

(CNXyl).^4gThe Ti(1)−C(12) bond distance of 2.191(2) Å

in 18 is relatively short in comparison to those of other

titanium(+4) isonitrile adducts (2.232(2)−2.256(6) Å),

consistent with the relatively high frequency observed for

ν(CN).^4g,32b−dThe σ-only nature of the Ti−CN^tBu bond is

also indicated by the short C(12)−N(3) bond distance of

1.153(3) Å. The Ti(1)−N(1) bond is slightly longer than that

in 5 (1.7739(17) in 18 vs 1.747(4) Å), which is attributed to

the increased coordination number around titanium.

Figure 9. Displacement ellipsoid plot (20% probability) of Cp*Ti-

{PhC(NⁱPr)₂}{NC(NO^tBu)C₆F₄N(Xyl)C}(F) (20). H atoms are

omitted for clarity.

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methylamines with amides in the presence of air, CuI,

K₂CO₃, and i-PrOH,³⁸and using imidoylcarbodiimides.¹⁶

Table 4. Selected Bond Lengths (Å) and Angles (deg) for

Cp*Ti{PhC(NⁱPr)₂}{NC(NO^tBu)C₆F₄N(Xyl)C}(F) (Xyl =

2,6-C₆H₃Me₂, 20)

Ti(1)−Cp_cent

Ti(1)−C(20)

Ti(1)−N(1)

Ti(1)−N(4)

Ti(1)−N(5)

2.115

N(1)−C(1)

N(2)−C(1)

N(3)−C(11)

N(3)−C(12)

N(3)−C(20)

1.414(2)

1.283(2)

1.417(2)

1.456(2)

1.367(2)

1.472(3)

1.416(3)

109.23

2.1407(18)

2.0773(16)

2.1089(17)

2.2471(16)

1.8688(11)

1.305(2)

113.3

Ti(1)−F(1)

N(1)−C(20)

C(6)−C(1)

C(11)−C(6)

Cp_cent−Ti(1)−N(1)

Cp_cent−Ti(1)−N(4)

Cp_cent−Ti(1)−F(1)

Ti(1)−N(1)−C(20)

C(1)−N(2)−O(1)

Cp_cent−Ti(1)−N(5)

Cp_cent−Ti(1)−C(20)

N(4)−Ti(1)−N(5)

Ti(1)−C(20)−N(1)

N(1)−C(1)−N(2)

110.17

148.90

In an attempt to extend this reactivity, XylNC was added to a

C₆D₆solution of Cp*Ti{PhC(NⁱPr)₂}{NC(Ar^F²)NO^tBu} (4)

and the reaction monitored by ¹H and ¹⁹F NMR spectroscopy.

Within 5 min a σ adduct, Cp*Ti{PhC(NⁱPr)₂}{NC(Ar^F²)-

NO^tBu}(CNXyl) (21), analogous to 18 and 19 was formed

and characterized by NMR in situ. After 16 h at room

101.81

60.31(6)

69.35(10)

116.64(16)

74.65(10)

111.76(15)

1

Compound 20 is a half-sandwich complex with a ﬁve-legged

piano-stool geometry. In addition to the expected η⁵-C₅Me₅

and κ²N,N′-amidinate ligands, the titanium is coordinated to a

ﬂuoride ligand and a new heterocyclic fragment derived from

the XylNC (i.e., C(20), N(3), and the xylyl group) and

benzimidamido ligands of 19. The metric parameters for the

Cp*Ti{PhC(NⁱPr)₂} fragment lie within the expected values

and are similar to those for complexes supported by this ligand

set. The Ti(1)−F(1) bond length of 1.8688(11) Å lies within

the range reported for other cyclopentadienyl-supported

titanium complexes reported in the literature (range 1.812−

1.937 Å).¹⁹The heterocyclic fragment may be viewed as a 2-

metalated 1-arylquinazolin-4(1H)-one oxime ether. The

Ti(1)−N(1) and Ti(1)−C(20) bond distances of 2.0773(16)

and 2.1407(18) Å, respectively, are consistent with this

description. The C(20)−N(1) and C(1)−N(2) bond lengths

are indicative of CN double bonds (1.305(2) and 1.283(2)

Å, respectively). The intermediate distances for N(3)−C(20)

(1.367(2) Å) and N(1)−C(1) (1.414(2) Å) indicate slightly

diﬀering extents of delocalization. The NMR data are consistent

with this C₁-symmetric complex existing also in solution. In

particular, one singlet and four multiplets were observed in the

¹⁹F NMR spectrum at −20 °C, the singlet being at δ −101.8

ppm, which suggested that it may no longer be bonded to

carbon. A ¹⁹F COSY NMR experiment showed scalar coupling

between the other four ¹⁹F atoms (range δ −118.7 to −162.7

ppm) but no apparent coupling to the singlet resonance. In the

¹³C NMR spectrum the quaternary signal corresponding to the

metal-bound C(20) was observed at δ 216.7 ppm.

temperature a second set of signals were observed in the H

and ¹⁹F NMR spectra, attributed to the new compound

Cp*Ti{PhC(NⁱPr)₂}{NC(NO^tBu)C₆FH₃N(Xyl)C}(F) (22),

the analogue of 20. However, the rate of the reaction was

slow and after 5 days had only reached 70% completion. At this

stage signiﬁcant amounts of other (unknown) products had

also formed and it was decided not to scale up this reaction.

The slower rate of reaction of 21 in comparison to that of 19 is

attributed to the decreased electrophilicity of the 2,6-C₆H₃F₂

ring in the former in comparison to that of C₆F₅.

Reaction with B(Ar^F⁵)₃. Group 4 hydrazides M(N₂N^py)-

R

(NNPh₂)(py) readily form zwitterionic N_αadducts of the type

M(N₂N^py){η²-N(NPh₂)B(Ar^F⁵)₃} (23; M = Ti, Zr, Hf, R =

R

2s,3c

t

SiMe₃, SiMe₂Bu) with B(Ar^F⁵)₃.

In an extension of this

chemistry we recently found that Cp*Ti{PhC(NⁱPr)₂}(NO^tBu)

(1) reacts with an excess of B(Ar^F⁵)₃, but in this case the

reaction is accompanied by 2-methylpropene elimination from

the O-tert-butyl group and formation of a HNO (nitroxyl)

ligand with B(Ar^F⁵)₃stabilization (24).^7c2-Methylpropene

elimination from cationic group 4 complexes containing either

tert-butoxy ligands³⁹or ligands with −O^tBu substituents such as

Carpentier’s cationic tert-butyl enolate systems is not

uncommon.⁴⁰We were therefore interested in examining the

corresponding reaction of 5.

Following the initial formation of 19, the mechanism for

formation of 20 is proposed to proceed via [1 + 2] addition of

XylNC to TiNC(Ar^F⁵)NO^tBu, forming 20_int containing an

η²-carbodiimide ligand. Related compounds have been reported

previously: for example, in the reactions of Cp*Ir(N^tBu)¹⁵and

29

t

Cp₂Zr(N^tBu)(THF) with BuNC. Subsequent nucleophilic

attack by XylNC on the ortho position of the Ar^Fgroup

5

displaces the ﬂuorine, which forms a new bond to titanium.

Other examples of C−F activation using d⁰group 4 metal

species have been reported,³⁴and titanium ﬂuoride compounds

are not uncommon.^34,35Quinazoline-type compounds have

been used in the synthesis of many pharmaceutical agents,

including antifungal, anticancer, and anti-HIV drugs.³⁶Conven-

tional synthetic routes to quinazolines include reaction of 2-

aminobenzophenones and benzylic amines in the presence of I₂

and tert-butyl hydroperoxide,³⁷using (2-bromophenyl)-

Addition of B(Ar^F⁵)₃to 5 in C₆D₆at room temperature led to

consumption of ca. 1 equiv. of borane after 7 h and formation

of two new compounds, 25 (major, 86%) and 26 (minor, 14%),

together with a small amount of 2-methylpropene (Scheme 7).

In addition to the expected new signals in the ¹H and ¹⁹F NMR

spectra for 25, a new ¹¹B NMR resonance was observed at −7

ppm, indicative of four-coordinate boron⁴¹and N coordination

of B(Ar^F⁵)₃. Compound 25 is tentatively assigned as Cp*Ti-

{PhC(NⁱPr)₂}{N(B{Ar^F⁵}₃)C(Ar^F⁵)N(O^tBu)}, as shown in

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corresponding metallacycles as a function of the ring

Scheme 7. Reactions of

t

Cp*Ti{PhC(NⁱPr)₂}{NC(Ar^F⁵)NO^tBu} (5) with B(Ar^F⁵)₃

substituents. Upon heating, 10 and 13a−f gave BuNCNC-

(Ar^F⁵)NO^tBu (8) and (4-C₆H₄X)C{NC(Ar^F⁵)NO^tBu}H (14a−

f), respectively, via a stereospeciﬁc extrusion reaction. Reaction

of 5 with HC(O)NMe₂was more facile, yielding Me₂NC{NC-

(Ar^F⁵)NO^tBu}H (16) within 1 h at room temperature. DFT

calculations on 14a−f and 16 probed the electronic structures

of these compounds and the relative energies of their

t

conformers. The reaction of 5 with BuNC and XylNC gave

σ adducts, Cp*Ti{PhC(NⁱPr)₂}{NC(Ar^F⁵)NO^tBu}(CNR) (R =

^tBu (18), Xyl (19)), and the latter underwent further reaction,

yielding Cp*Ti{PhC(NⁱPr)₂}{NC(NO^tBu)C₆F₄N(Xyl)C}(F)

(20) following a C−F activation mechanism. Reaction of 5

with 2 equiv of B(Ar^F⁵)₃resulted in Cp*Ti{PhC(NⁱPr)₂}{ON-

(B{Ar^F⁵}₃)C(Ar^F⁵)N(H)(B{Ar^F⁵}₃)} (26) following loss of 2-

methylpropene. In all of these reactions, the new organo-

metallic or organic products contain the benzimidamide

moiety, NC(Ar^F⁵)NO^tBu, derived initially from the tert-

butoxyimido ligand of Cp*Ti{PhC(NⁱPr)₂}(NO^tBu) (1) and

Ar^F⁵CN, and thus these represent new multicomponent

coupling reactions.

Scheme 7 with a labile κ²N,N′-borataamidinate-type ligand,

formed by addition of the borane to the N_αatom of 5 (cf. the

corresponding N_αadducts 23 and 24, which have similar ¹¹B

NMR shifts). Further characterization and isolation of 25 was

complicated by its instability toward elimination of 2-

methylpropene and emergence of Cp*Ti{PhC(NⁱPr)₂}{ON-

(B{Ar^F⁵}₃)C(Ar^F⁵)N(H)(B{Ar^F⁵}₃)} (26). Formation of 26 was

relatively slow at room temperature, but the compound could

nonetheless be isolated in analytically pure form, albeit only in

26% yield, after heating a mixture of 5 and 2 equiv of the

borane in C₆H₆for 4 days at 70 °C. We were not able to obtain

diﬀraction-quality crystals of 26, and the structure is assigned

by analogy to those of 23 and 24 on the basis of the

spectroscopic and other analytical data.

EXPERIMENTAL SECTION

■

General Methods and Instrumentation. All manipulations were

carried out using standard Schlenk line or drybox techniques under an

atmosphere of argon or dinitrogen. Solvents were either degassed by

sparging with dinitrogen and dried by passing through a column of the

appopriate drying agent or reﬂuxed over sodium (toluene), potassium

(THF), or Na/K alloy (Et₂O) and distilled. Deuterated solvents were

dried over sodium (C₆D₆and toluene-d₈), distilled under reduced

pressure, and stored under argon in Teﬂon valve ampules. Unless

stated, NMR samples were prepared under dinitrogen in 5 mm

Wilmad 507-PP tubes ﬁtted with J. Young Teﬂon valves. ¹H, ¹³C{¹H},

¹³C{¹⁹F}, ¹¹B, and ¹⁹F spectra were recorded on Varian Mercury-VX

300 or Bruker Avance III 500 spectrometers or on a Bruker AVC 500

spectrometer ﬁtted with a ¹³C cryoprobe. H, ¹³C{¹H}, and ¹³C{¹⁹F}

1

The room-temperature ¹H NMR spectrum of 26 is

consistent with a C₁-symmetric complex. In addition to

resonances for the isopropyl, Cp*, and phenyl groups, a

slightly broad singlet at δ 6.67 ppm (integral 1 H), which

couples to the C_ipsoof (H)NCAr^F⁵NO and also to (H)-

NCAr^F⁵N, was assigned to NH (the corresponding ν(N−H) is

found at 3393 cm⁻¹in the IR spectrum). The room-

temperature ¹⁹F NMR spectrum was very broad at room

temperature, but when the temperature was lowered to −60 °C

the appearance of at least 22 multiplets (some overlapping)

qualitatively conﬁrmed the presence of 2 equiv of B(Ar^F⁵)₃in

26. The room-temperature ¹¹B NMR spectrum showed a single

resonance δ −7 ppm, which broadened into the baseline on

cooling to −80 °C. Taken together, the ¹⁹F and ¹¹B NMR

variable-temperature spectra suggest that at room temperature

the two B(Ar^F⁵)₃groups are in dynamic exchange on the NMR

time scale, but unfortunately low-temperature limiting spectra

could not be obtained. The loss of 2-methylpropene from 25 to

form 26 is reminiscent of the reactions of 1 to form 24.

spectra are referenced internally to residual protio solvent (¹H) or

solvent (¹³C) resonances and are reported relative to tetramethylsilane

(δ 0 ppm). ¹⁹F and ¹¹B spectra were referenced externally to CFCl₃

and Et₂O·BF₃, respectively. Assignments were conﬁrmed as necessary

1

with the use of two-dimensional H−¹H, ¹³C−¹H, and ¹³C−¹⁹F NMR

correlation experiments. IR spectra were recorded on a Perkin-Elmer

Paragon 1000 FTIR or a Thermo Scientiﬁc Nicolet iS5 FTIR

spectrometer and samples prepared in a drybox using NaCl plates as a

Nujol mull or as a thin ﬁlm. UV−visible spectra were recorded on a

T60U spectrometer at 298 K in 1 cm path length cuvettes with [14 or

16] = 5.17 μmol dm⁻³in either n-hexane or MeOH. Mass spectra

were recorded by the mass spectrometry service of Oxford University’s

Department of Chemistry. Elemental analyses were carried out by the

Elemental Analysis Service at the London Metropolitan University.

Starting Materials. Cp*Ti{PhC(NⁱPr)₂}(NO^tBu) (1) and

Cp*Ti{PhC(NⁱPr)₂}{NC(Ar^F⁵)NO^tBu} (5) were synthesized accord-

ing to published procedures.^7cB(Ar^F⁵)₃was provided by LANXESS

Elastomers BV. All other reagents were purchased from commercial

suppliers and (for liquid reagents) degassed before use, unless

i

speciﬁed otherwise. Ar^F⁵CN and Ar′NCO (Ar′ = 2,6-C₆H₃Pr₂) were

dried over CaH₂and distilled before use. HC(O)NMe₂was degassed

by sparging with dinitrogen and dried by passing through a column of

activated alumina.

CONCLUSIONS

■

In Situ Synthesis of Cp*Ti{PhC(NⁱPr)₂}{NC(Ar^F⁵)NO^tBu} (5). In

all reactions Cp*Ti{PhC(NⁱPr)₂}{NC(Ar^F⁵)NO^tBu} (5) was synthe-

sized in situ for convenience. An example of the method is as follows.

To a stirred solution of Cp*Ti{PhC(NⁱPr)₂}(NO^tBu) (1; 0.350 g,

0.739 mmol) in benzene (5 mL) was added Ar^F⁵CN (93.2 μL, 0.739

mmol), all at room temperature. An immediate color change from dark

green to lime green was observed, and the solution was stirred for 15

min, quantitatively forming 5.

The ﬁrst reactivity study of the benzimidamido ligand in

Cp*Ti{PhC(NⁱPr)₂}{NC(Ar^F⁵)NO^tBu} (5) has allowed access

to a range of stable [2 + 2] cycloaddition intermediates which

were isolated in good yields with CO₂(forming 7), isocyanates

(forming 10), and a series of aryl aldehydes (forming 13a−f).

The reactions with aryl aldehydes were reversible and allowed

quantitative assessment of the relative stability of the

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t

Cp*Ti{PhC(NⁱPr)₂}{OC(O)N(C{Ar^F⁵}NO^tBu)} (7). A solution of

Cp*Ti{PhC(NⁱPr)₂}{NC(Ar^F⁵)NO^tBu} (5; 0.422 g, 0.634 mmol) in

benzene (5 mL) was freeze−pump−thawed three times. The solution

was then exposed to CO₂at a pressure of ca. 1.5 atm at room

temperature. After 1 h the volatiles were removed under reduced

pressure to aﬀord 7 as a dark green solid which was washed with cold

pentane (3 × 5 mL, −78 °C), ﬁltered, and dried in vacuo. Yield: 0.296

g (66%). ¹H NMR (C₆D₆, 499.9 MHz, 293 K): δ 7.23 (1 H, d, ³J = 7.5

Hz, o_a-C₆H₅), 7.05 - 6.92 (4 H, m, o_b-C₆H₅, m-C₆H₅, and p-C₆H₅),

mmol) in diethyl ether (5 mL) was added BuNCO (60.3 μL, 0.528

mmol), all at room temperature. A gradual color change from lime

green to dark yellow was observed, and the solution was stirred for 6.5

h. Volatiles were then removed under reduced pressure to aﬀord 11 as

1

a brown powder which was dried in vacuo. Yield: 0.261 g (65%). H

NMR (C₆D₆, 499.9 MHz, 293 K): δ 7.37 (1 H, d, J = 7.5 Hz, o_a-

C₆H₅), 7.00 (4 H, m, overlapping o_b-C₆H₅, m-C₆H₅, and p-C₆H₅), 4.20

(1 H, app sept, app J = 7.0 Hz, NCH_aMeMe), 3.24 (1 H, app sept,

3

app J = 6.5 Hz, NCH_bMeMe), 2.17 (15 H, s, C₅Me₅), 1.33 (9 H, s,

NCMe₃), 1.33 (9 H, s, NOCMe₃), 1.15 (3 H, d, ³J = 7.0 Hz,

NCH_aMeMe), 0.97 (9 H, m, overlapping NCH_aMeMe, NCH_bMeMe,

NCH_bMeMe) ppm. ¹³C{¹H} NMR (C₆D₆, 125.7 MHz, 293 K): δ

170.6 (PhCN₂), 148.2, 146.7 (OCN and NC(Ar^F⁵)N), 133.9 (i-C₆H₅),

130.3 (C₅Me₅), 129.3 (m-C₆H₅), 128.7 (o_a-C₆H₅), 128.7 (o_b-C₆H₅),

3

4.20 (1 H, app sept, app J = 6.5 Hz, NCH_aMeMe), 3.20 (1 H, app

sept, app ³J = 6.5 Hz, NCH_bMeMe), 2.16 (15 H, s, C₅Me₅), 1.28 (9 H,

s, NOCMe₃), 1.07 (3 H, d, ³J = 6.5 Hz, NCH_aMeMe), 1.02 (3 H, d, ³J

= 6.5 Hz, NCH_bMeMe) 1.00 (3 H, d, ³J = 7.0 Hz, NCH_aMeMe), 0.89

(3 H, d, ³J = 6.5 Hz, NCH_bMeMe) ppm. ¹³C{¹H} NMR (C₆D₆, 125.7

MHz, 293 K): δ 171.6 (PhCN₂), 155.8 (OC(O)N), 146.2

2

128.6 (p-C₆H₅) 113.9 (1 C, app t, app J_C−F= 21.3 Hz, i-C₆F₅), 77.8

1

(NOCMe₃), 51.0 (NCMe₃), 50.8 (NCH_bMeMe), 50.1 (NCH_aMeMe),

32.2 (NCMe₃), 27.8 (NOCMe₃), 25.5 (NCH_aMeMe), 25.6, 24.6, 24.5

(NCH_aMeMe NCH_bMeMe, NCH_bMeMe), 12.9 (C₅Me₅) ppm. ¹³C

NMR (HMQC ¹⁹F-observed, C₆D₆, 282.1 MHz, 293 K): δ 144.2 (o_a-

C₆F₅), 142.8 (o_b-C₆F₅), 140.6 (p-C₆F₅), 137.6 (m-C₆F₅), 137.3 (m-

C₆F₅) ppm. ¹⁹F{¹H} NMR (C₆D₆, 282.1 MHz, 293 K): δ −135.3 (1

H, m, o_a-C₆F₅), −143.6 (1 H, m, o_b-C₆F₅), −159.7 (1 H, m, p-C₆F₅),

−166.9 (1 H, m, m_a-C₆F₅), −167.2 (1 H, m, m_b-C₆F₅) ppm. IR (NaCl

plates, Nujol mull, cm⁻¹): 1631 (s), 1579 (m), 1516 (s), 1494 (s),

1483 (m), 1453 (s), 1423 (m), 1401 (m), 1360 (s), 1335 (m), 1289

(w), 1261 (m), 1221 (m), 1196 (w), 1174 (m), 1148 (w), 1101 (m),

1062 (m), 1020 (m), 990 (m), 967 (m), 943 (w), 899 (w), 861 (w),

797 (br w), 732 (w), 704 (w). EI-MS: m/z 666 [Cp*Ti{PhC-

(NⁱPr)₂}{NC(Ar^F⁵)NO^tBu}]⁺(5%). Anal. Found (calcd for

C₃₉H₅₂F₅N₅O₂Ti): C, 60.99 (61.17); H, 6.72 (6.84); N, 8.98 (9.15).

^tBuNCNC(Ar^F⁵)NO^tBu (12). To a solution of Cp*Ti{PhC(NⁱPr)₂}-

{NC(Ar^F⁵)NO^tBu} (5; 0.493 g, 0.739 mmol) in benzene (5 mL) was

(NC(C₆F₅)N), 144.0 (1 C, d, J_C−F= 246.2 Hz, o_a-C₆F₅), 143.5 (1

1

C, d, J_C−F= 242.8 Hz, o_b-C₆F₅), 141.2 (1 C, d, J_C−F= 251.4 Hz, p-

C₆F₅), 138.0 (1 C, d, ¹J_C−F= 248.8 Hz, m_a-C₆F₅), 137.8 (1 C, d, ¹J_C−F

= 248.9 Hz, m_b-C₆F₅), 133.0 (i-C₆H₅), 132.2 (C₅Me₅), 129.5 (p-

C₆H₅), 128.6 (o_b-C₆H₅), 128.5 (o_a-C₆H₅), 128.1 (overlapping with

2

3

solvent m-C₆H₅), 112.3 (1 C, appt of t, J_C−F= 19.8 Hz, J_C−F= 3.8

Hz, i-C₆F₅), 78.0 (NOCMe₃), 51.4 (NCH_bMeMe), 50.5

(NCH_aMeMe), 27.8 (NOCMe₃), 25.3 (NCH_aMeMe), 25.1

(NCH_aMeMe), 24.8 (NCH_bMeMe), 24.0 (NCH_bMeMe), 13.0

(C₅Me₅) ppm. ¹⁹F{¹H} NMR (C₆D₆, 282.1 MHz, 293 K): δ −137.7

3

(1 F, app d, app J = 23.1 Hz, o_a-C₆F₅), −143.3 (1 F, app d, app J =

23.1 Hz, o_b-C₆F₅), −156.2 (1 F, t, ³J = 21.4 Hz, p-C₆F₅), −164.6 (1 F,

m, m_a-C₆F₅), −164.9 (1 F, m, m_b-C₆F₅) ppm. IR (NaCl plates, Nujol

mull, cm⁻¹): 1690 (s), 1655 (w), 1578 (w), 1521 (m), 1496 (s), 1424

(m), 1362 (s), 1337 (m), 1293 (w), 1259 (w), 1218 (m), 1176 (m),

1143 (w), 1110 (m), 1070 (w), 1044 (w), 1023 (m), 990 (m), 954

(m), 921 (m), 901 (w), 786 (w), 707 (w). Anal. Found (calcd for

C₃₅H₄₃F₅N₄O₃Ti): C, 58.89 (59.16); H, 6.29 (6.10); N, 7.76 (7.88).

NMR Tube Scale Synthesis of Cp*Ti{PhC(NⁱPr)₂}{OC(O)N(C-

{Ar^F⁵}NO^tBu)C(O)O} (8). A solution of Cp*Ti{PhC(NⁱPr)₂}{OC(O)-

N(C{Ar^F⁵}NO^tBu)} (7; 25.0 mg, 0.0351 mmol) in C₆D₆(0.35 mL) in

a 0.77 mm New Era NE-HP5-M NMR tube equipped with a

controlled-atmosphere valve was freeze−pump−thawed three times.

The solution was then exposed to CO₂at a pressure of ca. 1.5 atm at

t

added BuNCO (84.4 μL, 0.739 mmol), all at room temperature. A

gradual color change from lime green to dark yellow was observed, and

the solution was stirred for 3 h before being heated to 65 °C and

stirred for a further 16 h. A further color change to yellow was

observed. Volatiles were then removed under reduced pressure to

aﬀord [Cp*Ti{PhC(NⁱPr)₂}(μ-O)]₂(11) and 12 as a brown oily

solid. The resultant solid was extracted into diethyl ether (3 × 5 mL,

−78 °C), and the volatiles were removed under reduced pressure. The

resultant brown oil was then distilled (100 °C, 8 × 10⁻²mbar, 4.5 h)

onto a dry ice/acetone cold ﬁnger, aﬀording 12 as a yellow oil. Yield:

1

room temperature. The reaction was monitored by H and ¹⁹F NMR

1

spectroscopy. The H NMR spectrum recorded after 5 days at room

temperature indicated that the reaction had reached 63% conversion to

1

8. 8 was characterized by H, ¹³C, and ¹⁹F NMR spectroscopy. H

NMR (C₆D₆, 499.9 MHz, 293 K): δ 7.03−6.92 (3 H, m, m-C₆H₅and

p-C₆H₅), 6.82 (1 H, m, o_a-C₆H₅), 6.65 (1 H, d, ³J = 7.0 Hz, o_b-C₆H₅),

0.190 g (71%). H NMR (C₆D₆, 499.9 MHz, 293 K): δ 1.25 (9 H, s,

NOCMe₃), 1.00 (9 H, s, NCMe₃) ppm. ¹³C{¹H} NMR (C₆D₆, 125.7

MHz, 293 K): δ 143.6 (2 C, d, ¹J_C−F= 249.3 Hz, o-C₆F₅), 141.9 (1 C,

d, ¹J_C−F= 253.8 Hz, p-C₆F₅), 139.8 (NCN or NC(Ar^F⁵)N), 137.8 (2 C,

3

3.41 (2 H, app sept, app J = 6.5 Hz, NCHMeMe), 1.99 (15 H, s,

d, J_C−F= 251.1 Hz, m-C₆F₅), 133.0 (NC(Ar^F⁵)N or NCN), 108.6 (1

1

C₅Me₅), 1.37 (9 H, s, NOCMe₃), 0.95 (6 H, d, ³J = 6.5 Hz,

2

3

NCHMeMe), 0.93 (6 H, d, J = 6.5 Hz, NCHMeMe) ppm. ¹³C{¹H}

3

C, t of d, J_C−F= 20.1 Hz, J_C−F= 3.9 Hz, i-C₆F₅), 80.6 (NOCMe₃),

58.3 (NCMe₃), 31.0 (NCMe₃), 27.4 (NOCMe₃) ppm. ¹⁹F{¹H} NMR

(C₆D₆, 282.1 MHz, 293 K): δ −138.2 (2 H, app d, app ³J = 13.0 Hz, o-

NMR (C₆D₆, 125.7 MHz, 293 K): δ 171.7 (PhCN₂), 152.8

1

(OC(O)N), 145.0 (2 C, d, J_C−F= 249.5 Hz, o-C₆F₅), 141.9 (1 C,

3

1

C₆F₅), −152.7 (1 H, t, J = 21.7 Hz, p-C₆F₅), −161.9 (2 H, m, m-

d, J_C−F= 254.6 Hz, p-C₆F₅), 137.9 (2 C, d, J_C−F= 250.8 Hz, m-

C₆F₅), 137.3 (NC(C₆F₅)N), 132.4 (C₅Me₅), 132.3 (i-C₆H₅), 129.4 (p-

C₆H₅), 128.1 (overlapping with solvent o_b-C₆H₅, m-C₆H₅), 127.7 (o_a-

C₆H₅), 109.2 (1 C, app t of t, ²J_C−F= 17.6 Hz, ³J_C−F= 3.3 Hz, i-C₆F₅),

81.5 (NOCMe₃), 51.6 (NCHMeMe), 27.7 (NOCMe₃), 24.7, 24.6

(NCHMeMe), 12.8 (C₅Me₅) ppm. ¹⁹F{¹H} NMR (C₆D₆, 282.1 MHz,

293 K): δ −134.1 (2 F, app d, app ³J = 18.9 Hz, o-C₆F₅), −154.2 (1 F,

app t of t, app ³J = 21.4 Hz, ⁴J = 3.1 Hz, p-C₆F₅), −164.1 (2 F, m, m-

C₆F₅) ppm.

C₆F₅) ppm. IR (NaCl plates, thin ﬁlm, cm⁻¹): 2977 (m), 2933 (w),

2872 (w), 2141 (s), 1653 (m), 1594 (br, m), 1521 (s), 1499 (s), 1457

(m), 1437 (m), 1389 (w), 1366 (m), 1317 (m), 1261 (w), 1238 (m),

1204 (s), 1173 (m), 1116 (w), 1103 (w), 1088 (m), 1071 (w), 1034

(w), 991 (s), 905 (m), 880 (m), 837 (w), 795 (w), 745 (w), 727 (w),

692 (w), 668 (w), 636 (w), 586 (w), 567 (w). FI-HRMS found (calcd

for [C₁₆H₁₈F₅N₃O]⁺): 363.1380 (363.1370).

Cp*Ti{PhC(NⁱPr)₂}{N(C{Ar^F⁵}NO^tBu)C(Ph)(H)O} (13a). To a

solution of Cp*Ti{PhC(NⁱPr)₂}{NC(Ar^F⁵)NO^tBu} (5; 0.422 g,

0.634 mmol) in diethyl ether (5 mL) was added PhC(O)H (64.4

μL, 0.634 mmol), all at room temperature. An immediate color change

from lime green to brown was observed, and the solution was stirred

for 15 min. Volatiles were then removed under reduced pressure to

aﬀord 13a as a brown powder which was dried in vacuo. Yield: 0.403 g

(82%). Diﬀraction-quality crystals were grown from a saturated hexane

solution at 4 °C. ¹H NMR (C₆D₆, 299.9 MHz, 293 K): δ 7.47 (1 H, d,

NMR Tube Scale Reaction of Cp*Ti{PhC(NⁱPr)₂}{NC(Ar^F⁵)-

NO^tBu} (5) with CS₂. To a solution of Cp*Ti{PhC(NⁱPr)₂}{NC-

(Ar^F⁵)NO^tBu} (5; 14.4 mg, 0.0215 mmol) in C₆D₆(0.5 mL) in an

NMR tube equipped with a J. Young Teﬂon valve was added CS₂

(1.55 μL, 0.0258 mmol) at room temperature. The reaction was

monitored by ¹H and ¹⁹F NMR spectroscopy. After 15 days

Cp*Ti{PhC(NⁱPr)₂}{N(O^tBu)C(S)S} (9) and Ar^F⁵CN were observed

in 70% yield. Resonances attributed to 9 were consistent with values

reported in the literature.^7c

³J = 7.2 Hz, o_a-C₆H₅CN₂), 7.22 (1 H, d, J = 6.9 Hz, o_b-C₆H₅CN₂),

7.11 (3 H, m, overlapping m-C₆H₅CN₂and p-C₆H₅), 6.93 (5 H, m,

overlapping p-C₆H₅CN₂, o-C₆H₅, and m-C₆H₅), 5.99 (1 H, d, J_H−F

3

Cp*Ti{PhC(NⁱPr)₂}{N(C{Ar^F⁵}NO^tBu)C(N^tBu)O} (10). To a sol-

ution of Cp*Ti{PhC(NⁱPr)₂}{NC(Ar^F⁵)NO^tBu} (5; 0.352 g, 0.528

=

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1

2.1 Hz, PhC(H)), 4.50 (1 H, app sept, app ³J = 6.6 Hz, NCH_aMeMe),

white solid. Yield: 0.104 g (62%). H NMR (C₆D₆, 499.9 MHz, 293

K): δ 8.50 (1 H, s, PhC(N)H), 7.69 (2 H, m, o-C₆H₅), 7.05−6.97 (3

H, m, overlapping m-C₆H₅and p-C₆H₅), 1.30 (9 H, s, NOCMe₃).

¹³C{¹H} NMR (C₆D₆, 125.7 MHz, 293 K): δ 162.0 (PhC(N)H),

3

3.44 (1 H, app sept, app J = 6.6 Hz, NCH_bMeMe), 2.28 (15 H, s,

C₅Me₅), 1.32 (3 H, d, ³J = 6.6 Hz, NCH_aMeMe), 1.27 (9 H, s,

3

OCMe₃), 1.19 (3 H, d, J = 6.9 Hz, NCH_aMeMe), 0.90 (3 H, d, J =

3

149.2 (NC(Ar^F⁵)NO^tBu), 143.9 (2 C, d, J_C−F= 251.3 Hz, o-C₆F₅),

141.9 (1 C, d overlapping with o-C₆F₅, J_C−F= 253.8 Hz, p-C₆F₅),

138.0 (2 C, d, J_C−F= 252.5 Hz, m-C₆F₅), 135.6 (i-C₆H₅), 132.5 (p-

C₆H₅), 129.9 (o-C₆H₅), 129.0 (m-C₆H₅), 108.0 (1 C, t of d, J_C−F

1

6.9 Hz, NCH_bMeMe), 0.83 (3 H, d, J = 6.6 Hz, NCH_bMeMe) ppm.

¹³C{¹H} NMR (C₆D₆, 75.4 MHz, 293 K): δ 167.9 (PhCN₂), 148.7

1

(NC(Ar^F⁵)NO^tBu), 145.6 (i-C₆H₅), 135.6 (i-C₆H₅CN₂), 129.7 (o_a-

C₆H₅CN₂), 128.9 (p-C₆H₅), 128.5 (o_b-C₆H₅CN₂), 127.9 (m-

C₆H₅CN₂), 127.8 (m-C₆H₅), 127.7 (overlapping with solvent p-

C₆H₅CN₂), 127.5 (C₅Me₅), 126.9 (o-C₆H₅), 112.6 (1 C, m, i-C₆F₅),

83.4 (1 C, d, J_C−F= 1.2 Hz, PhC(H)), 76.9 (OCMe₃), 49.9

(NCH_bMeMe), 48.9 (NCH_aMeMe), 28.2 (OCMe₃), 25.7

(NCH_bMeMe), 25.4 (NCH_aMeMe), 24.8 (NCH_aMeMe), 24.6

(NCH_bMeMe), 12.4 (C₅Me₅) ppm. ¹³C NMR (HMQC ¹⁹F-observed,

C₆D₆, 282.1 MHz, 293 K): δ 142.5 (o_a-C₆F₅), 141.9 (o_b-C₆F₅), 140.7

(p-C₆F₅), 137.3 (m_a-C₆F₅), 137.2 (m_b-C₆F₅) ppm. ¹⁹F{¹H} NMR

(C₆D₆, 282.1 MHz, 293 K): δ −134.8 (1 F, m, o_a-C₆F₅), −141.7 (1 F,

m, o_b-C₆F₅), −156.6 (1 F, m, p-C₆F₅), −164.1 (1 F, m, m_a-C₆F₅),

−164.8 (1 F, m, m_b-C₆F₅) ppm. IR (NaCl plates, Nujol mull, cm⁻¹):

1653 (w), 1550 (m), 1519 (m), 1495 (s), 1363 (m), 1358 (m), 1346

(m), 1289 (w), 1267 (w), 1260 (w), 1230 (w), 1221 (w), 1196 (m),

1169 (w), 1151 (w), 1133 (w), 1112 (m), 1085 (w), 1059 (m), 1021

(m), 994 (m), 987 (m), 959 (m), 922 (w), 905 (w), 893 (w), 795 (w),

781 (w), 730 (w), 721 (w), 711 (w), 704 (m), 696 (w), 642 (m), 615

(w), 590 (w), 576 (w), 537 (w). Anal. Found (calcd for

C₄₁H₄₉F₅N₄O₂Ti): C, 63.60 (63.73); H, 6.28 (6.39); N, 7.18 (7.25).

Equilibrium Constants. A total of 0.5 mL of C₆D₆was used in

each experiment. The general procedure is as follows. To a solution of

Cp*Ti{PhC(NⁱPr)₂}{N(C{Ar^F⁵}NO^tBu)C(Ph)(H)O} (13a; 10.0 mg,

0.0129 mmol) in C₆D₆in an NMR tube equipped with a J. Young

Teﬂon valve was added ArC(O)H (0.0129 mmol), in C₆D₆where

necessary, all at room temperature. The reaction was monitored by ¹H

(relaxtion delay 100 s) and ¹⁹F NMR spectroscopy. The ratios of

reagents and products were calculated by measuring the integrals of

the free and bound ArCH hydrogens, giving the required equilibrium

constants (in the case of 4-C₆H₄Me and 4-C₆H₄OMe it was necessary

to integrate the methyl groups of the isopropyl substituents instead).

Kinetic Measurements. The general procedure is as follows.

Cp*Ti{PhC(NⁱPr)₂}{N(C{Ar^F⁵}NO^tBu)C(Ar)(H)O} (13a−f;

0.0129 mmol) was dissolved in C₆D₆(0.6 mL) and transferred to a

NMR tube equipped with a J. Young Teﬂon valve. The NMR probe

was warmed to 70 °C and the sample added before being locked and

shimmed. After the sample was allowed to thermally equilibrate (ca. 10

min), the shimming was checked and an array set up recording a

spectrum of four scans every 120 s. The ratios of the species were

calculated by measuring the integrals of the ArCH protons. First-order

rate contants were obtained from linear plots of ln([13a−f]_t/[13a−

f]₀) vs time.

2

=

3

20.0 Hz, J_C−F= 3.8 Hz, i-C₆F₅), 81.2 (NOCMe₃), 27.5 (NOCMe₃)

ppm. ¹⁹F{¹H} NMR (C₆D₆, 282.1 MHz, 293 K): δ −138.2 (2 F, m, o-

C₆F₅), −152.7 (1 F, t of t, J = 21.2 Hz, ⁴J = 2.3 Hz, p-C₆F₅), −161.8

3

(2 F, m, m-C₆F₅) ppm. IR (NaCl plates, Nujol mull, cm⁻¹): 1657 (m),

1623 (m), 1602 (w), 1592 (w), 1579 (m), 1520 (s), 1497 (s), 1454

(m), 1434 (m), 1364 (m), 1323 (w), 1313 (w), 1296 (w), 1261 (w),

1243 (w), 1205 (m), 1187 (m), 1101 (m), 1073 (w), 998 (m), 985

(s), 970 (s), 905 (m), 892 (w), 858 (m), 798 (w), 766 (m), 707 (m),

695 (m), 668 (w), 615 (w), 597 (m), 582 (m). ESI⁺-HRMS found

(calcd for [C₁₈H₁₅F₅N₂NaO, M + Na⁺]⁺): 393.0989 (393.0997). UV−

vis: λ_max(n-hexane) 271 nm (ε = 3.0 × 10⁴mol⁻¹dm³cm⁻¹);

λ_max(MeOH) 284 nm (ε = 2.4 × 10⁴mol⁻¹dm³cm⁻¹).

NMR Tube Scale Synthesis of Cp*Ti{PhC(NⁱPr)₂}{NC(Ar^F⁵)-

NO^tBu}{OC(NMe₂)H} (15). To a solution of Cp*Ti{PhC(NⁱPr)₂}-

{N(C{Ar^F⁵}NO^tBu)} (5; 17.0 mg, 0.0256 mmol) in C₆D₆(0.3 mL) in

an NMR tube equipped with a J. Young Teﬂon valve was added

Me₂NC(O)H (1.87 mg, 0.0256 mmol) in C₆D₆(0.3 mL) at room

temperature. A color change from lime green to red-brown was

observed. The reaction was monitored by ¹H and ¹⁹F NMR

spectroscopy. Within 5 min 15 was observed alongside Me₂NC{NC-

(Ar^F⁵)NO^tBu}H (16) and [Cp*Ti{PhC(NⁱPr)₂}(μ-O)]₂(11) in a

92:8 ratio. 15 was characterized by ¹H and ¹⁹F NMR spectroscopy. ¹H

NMR (C₆D₆, 299.9 MHz, 293 K): δ 7.92 (1 H, br s, Me₂NC(H)), 7.32

(1 H, d, ³J = 8.1 Hz, o_a-C₆H₅), 7.21−6.98 (4 H, m, o_b-C₆H₅, m-C₆H₅,

and p-C₆H₅), 3.40 (2 H, sept, ³J = 6.6 Hz, NCHMeMe), 2.36 (3 H, s,

NMeMe), 2.22 (15 H, s, C₅Me₅), 1.95 (3 H, s, NMeMe), 1.40 (9 H, s,

NOCMe₃), 0.86 (6 H, d, ³J = 6.6 Hz, NCHMeMe), 0.81 (6 H, d, ³J =

6.6 Hz, NCHMeMe) ppm. ¹⁹F{¹H} NMR (C₆D₆, 282.1 MHz, 293 K):

3

δ −140.0 (2 F, m, o-C₆F₅), −158.2 (1 F, t, J = 21.2 Hz, p-C₆F₅),

−164.0 (2 F, m, m-C₆F₅) ppm.

Me₂NC{NC(Ar^F⁵)NO^tBu}H (16). To a solution of Cp*Ti{PhC-

(NⁱPr)₂}{NC(Ar^F⁵)NO^tBu} (5; 0.351 g, 0.528 mmol) in benzene (5

mL) was added HC(O)NMe₂(40.9 μL, 0.528 mmol), all at room

temperature. An immediate color change from lime green to red-

brown and then to yellow was observed, and the solution was stirred

for 3 h. The solution was then ﬁltered, and the volatiles were removed

under reduced pressure to aﬀord [Cp*Ti{PhC(NⁱPr)₂}(μ-O)]₂(11)

and 16 as a yellow-brown oily solid. The resultant solid was sublimed

(80 °C, 8 × 10⁻²mbar, 3 h) onto a dry ice/acetone cold ﬁnger,

aﬀording 16 as a white solid. Yield: 0.114 g (64%). Diﬀraction-quality

crystals were grown from the slow evaporation of a pentane solution at

room temperature. ¹H NMR (C₆D₆, 499.9 MHz, 293 K): δ 7.79 (1 H,

s, HC(N)NMe₂), 2.42 (3 H, s, HC(N)NMeMe), 1.90 (3 H, s,

HC(N)NMeMe), 1.37 (9 H, s, NOCMe₃) ppm.¹³C{¹H} NMR (C₆D₆,

125.7 MHz, 293 K): δ 152.8 (HC(N)NMe₂), 148.8 (NC(Ar^F⁵)N),

143.9 (2 C, d, ¹J_C−F= 247.5 Hz, o-C₆F₅), 141.2 (1 C, d, ¹J_C−F= 255.0

Hz, p-C₆F₅), 137.9 (2 C, d, ¹J_C−F= 250.0 Hz, m-C₆F₅), 112.1 (1 C, t of

NMR Tube Scale Reaction of Cp*Ti{PhC(NⁱPr)₂}{NC(Ar^F⁵)-

NO^tBu} (5) with PhC(O)H and (4-C₆H₄NMe₂)C(O)H (1:1 Ratio).

To a solution of Cp*Ti{PhC(NⁱPr)₂}{NC(Ar^F⁵)NO^tBu} (5; 21.1 mg,

0.0317 mmol) in C₆D₆(0.3 mL) in an NMR tube equipped with a J.

Young Teﬂon valve was added a premixed solution of PhC(O)H (3.22

μL, 0.0317 mmol) and (4-C₆H₄NMe₂)C(O)H (4.73 mg, 0.0317

mmol) in C₆D₆(0.3 mL) at room temperature. A color change from

lime green to brown was observed. After 1 h 45 min at room

temperature, the solution was heated to 65 °C. After 20.5 h

PhC{NC(Ar^F⁵)NO^tBu}H (14a) and (4-C₆H₄NMe₂)C{NC(Ar^F⁵)-

NO^tBu}H (14e) were observed, alongside [Cp*Ti{PhC(NⁱPr)₂}(μ-

2

3

d, J_C−F= 20.0 Hz, J_C−F= 3.8 Hz, i-C₆F₅), 78.8 (NOCMe₃), 39.2

(HC(N)NMeMe), 33.9 (HC(N)NMeMe), 27.7 (NOCMe₃) ppm.

¹⁹F{¹H} NMR (C₆D₆, 282.1 MHz, 293 K): δ −140.2 (2 F, m, o-C₆F₅),

−155.8 (1 F, t of t, J = 21.2 Hz, ⁴J = 1.41 Hz, p-C₆F₅), −163.3 (2 F,

3

1

O)]₂(11), in the H NMR spectrum in a 64:36 ratio, respectively.

m, m-C₆F₅) ppm. IR (NaCl plates, Nujol mull, cm⁻¹): 1626 (s), 1565

(m), 1520 (m), 1497 (s), 1441 (m), 1419 (m), 1410 (w), 1397 (w),

1364 (s), 1317 (w), 1256 (w), 1232 (w), 1191 (m), 1113 (s), 1095

(m), 1064 (w), 1036 (w), 995 (s), 985 (m), 960 (m), 927 (w), 910

(m), 900 (w), 866 (w), 800 (w), 700 (m), 681 (w), 610 (w), 581 (w).

ESI⁺-HRMS found (calcd for [C₁₄H₁₆F₅N₃NaO, M + Na⁺]⁺):

360.1102 (360.1106). UV−vis: dm³cm⁻¹).

PhC{NC(Ar^F⁵)NO^tBu}H (14a). To a solution of Cp*Ti{PhC-

(NⁱPr)₂}{NC(Ar^F⁵)NO^tBu} (5; 0.301 g, 0.452 mmol) in benzene (5

mL) was added PhC(O)H (45.9 μL, 0.452 mmol), all at room

temperature. An immediate color change from lime green to brown

was observed, and the solution was stirred for 15 min before being

heated to 70 °C and stirred for a further 19 h. A further color change

to yellow was observed. Volatiles were then removed under reduced

pressure to aﬀord [Cp*Ti{PhC(NⁱPr)₂}(μ-O)]₂(11) and 14a as a

yellow-brown oily solid. The resultant solid was sublimed (80 °C, 8 ×

10⁻²mbar, 2 h) onto a dry ice/acetone cold ﬁnger, aﬀording 14a as a

Cp*Ti{PhC(NⁱPr)₂}{NC(Ar^F⁵)NO^tBu}(CN^tBu) (18). To a solution

of Cp*Ti{PhC(NⁱPr)₂}{NC(Ar^F⁵)NO^tBu} (5; 0.422 g, 0.634 mmol)

t

in diethyl ether (5 mL) was added BuNC (71.7 μL, 0.634 mmol), all

at room temperature. A color change from lime green to orange was

1015

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observed, and the solution was stirred for 15 min. Volatiles were then

removed under reduced pressure to aﬀord 18 as an orange powder.

The resultant orange solid was recrystallized from a saturated pentane

(5 mL) solution at −30 °C, ﬁltered, and dried in vacuo. Yield: 0.292 g

(61%). Diﬀraction-quality crystals were grown from a saturated

(m), 1190 (m), 1166 (w), 1146 (m), 1121 (w), 1100 (m), 1090 (m),

1038 (m), 1018 (m), 981 (s), 919 (w), 908 (m), 887 (m), 804 (w),

787 (w), 771 (m), 722 (w), 711 (m), 547 (w). EI-MS: m/z (calcd for

[C₄₃H₅₂F₅N₅OTi]⁺): 797.3562 (797.3576) (2%). Anal. Found (calcd

for C₄₃H₅₂F₅N₅OTi): C, 64.86 (64.74); H, 6.45 (6.57); N, 8.81 (8.78).

NMR Tube Scale Synthesis of Cp*Ti{PhC(NⁱPr)₂}{N(B{Ar^F⁵}₃)-

C(Ar^F⁵)N(O^tBu)} (25). To a solution of Cp*Ti{PhC(NⁱPr)₂}{N(C-

{Ar^F⁵}NO^tBu)} (5; 14.1 mg, 0.0211 mmol) in C₆D₆(0.3 mL) in an

NMR tube equipped with a J. Young Teﬂon valve was added B(Ar^F⁵)₃

(12.5 mg, 0.0422 mmol) in C₆D₆(0.3 mL) at room temperature. The

1

pentane solution at −30 °C. H NMR (toluene-d₈, 299.9 MHz, 208

K): δ 7.14−6.89 (5 H, m, overlapping o-C₆H_5,m-C₆H₅and p-C₆H₅),

3

3.31 (1 H, app sept, app J = 6.3 Hz, NCH_aMeMe), 3.17 (1 H, app

sept, app ³J = 6.3 Hz, NCH_bMeMe), 2.30 (15 H, s, C₅Me₅), 1.52 (9 H,

s, NOCMe₃), 1.07 (6 H, d, overlapping NCH_aMeMe and

1

reaction was monitored by H, ¹⁹F, and ¹¹B NMR spectroscopy. After

3

NCH_bMeMe), 1.04 (9 H, s, NCMe₃), 0.96 (3 H, d, J = 6.3 Hz,

ca. 7 h Cp*Ti{PhC(NⁱPr)₂}{N(B{Ar^F⁵}₃)C(Ar^F⁵)N(O^tBu)} (25) was

formed alongside Cp*Ti{PhC(NⁱPr)₂}{ON(B{Ar^F⁵}₃)C(Ar^F⁵)N(H)-

(B{Ar^F⁵}₃)} (26) in a 86:14 ratio. 25 was characterized by ¹H, ¹⁹F, and

NCH_aMeMe), 0.91 (3 H, d, ³J = 6.3 Hz, NCH_bMeMe) ppm. ¹³C{¹H}

NMR (toluene-d₈, 75.4 MHz, 208 K): δ 172.3 (PhCN₂), 153.8

(TiCN^tBu), 149.4 (NC(Ar^F⁵)N), 135.1 (i-C₆H₅), 129.1−127.0 (over-

lapping with solvent, p-C₆H₅, m-C₆H₅, and o-C₆H₅), 118.3 (C₅Me₅),

1

¹¹B NMR spectroscopy. H NMR (C₆D₆, 299.9 MHz, 293 K): δ 7.40

(1 H, d, ³J = 7.8 Hz, o_a-C₆H₅), 7.12−6.91 (4 H, m, o_a-C₆H₅, m-C₆H₅,

and p-C₆H₅), 3.38 (2 H, sept, ³J = 6.6 Hz, NCHMeMe), 2.19 (15 H, s,

C₅Me₅), 1.46 (9 H, s, NOCMe₃), 0.80 (6 H, d, ³J = 6.6 Hz,

2

114.1 (1 C, t, J_C−F= 23.0 Hz, i-C₆F₅), 77.4 (NOCMe₃), 55.9

(CNMe₃), 49.3 (NCH_bMeMe), 48.8 (NCH_aMeMe), 29.1 (CNMe₃),

27.9 (NOCMe₃), 26.3 (NCH_bMeMe), 25.7 (NCH_aMeMe), 25.2, 24.9

(NCH_aMeMe and NCH_bMeMe), 12.7 (C₅Me₅) ppm.¹³C NMR

(HMQC ¹⁹F-observed, toluene-d₈, 282.1 MHz, 208 K): δ 143.2 (o_b-

C₆F₅), 142.9 (o_a-C₆F₅), 139.9 (p-C₆F₅), 137.5 (m_a-C₆F₅), 137.2 (m_b-

C₆F₅) ppm.¹⁹F{¹H} NMR (toluene-d₈, 282.1 MHz, 208 K): δ −138.5

(1 F, m, o_a-C₆F₅), −141.4 (1 F, m, o_b-C₆F₅), −158.1 (1 F, t, ³J = 21.4

Hz, p-C₆F₅), −163.8 (1 F, m, m_b-C₆F₅), −164.1 (1 F, m, m_a-C₆F₅)

ppm. IR (NaCl plates, Nujol mull, cm⁻¹): 2186 (s), 1648 (m), 1576

(w), 1519 (s), 1499 (s), 1486 (s), 1368 (s), 1357 (s), 1336 (s), 1309

(m), 1292 (m), 1257 (m), 1239 (w), 1231 (w), 1194 (s), 1169 (m),

1133 (m), 1106 (m), 1075 (w), 1030 (m), 1008 (m), 990 (s), 981 (s),

973 (s), 937 (s), 917 (m), 892 (m), 860 (w), 810 (w), 792 (w), 781

(m), 739 (w), 719 (m), 706 (m), 670 (m), 607 (m), 595 (w), 580

3

NCHMeMe), 0.76 (6 H, d, J = 6.3 Hz, NCHMeMe) ppm. ¹⁹F{¹H}

NMR (C₆D₆, 282.1 MHz, 293 K): δ −126.7 (m), −130.6 (6 F, br s,

B(Ar^F⁵)₃), −132.2 (m), −142.2 (3 F, m, B(Ar^F⁵)₃), −145.6 (br s),

−154.6 (m), −158.4 (t), −160.8 (6 F, br s, B(Ar^F⁵)₃), −163.9 (m)

11

ppm. B NMR (C₆D₆, 96.2 MHz, 293 K): δ −7 (B(Ar^F⁵)₃) ppm.

Cp*Ti{PhC(NⁱPr)₂}{ON(B{Ar^F⁵}₃)C(Ar^F⁵)N(H)(B{Ar^F⁵}₃)} (26). To a

solution of Cp*Ti{PhC(NⁱPr)₂}{NC(Ar^F⁵)NO^tBu} (5; 0.282 g, 0.422

mmol) in benzene (5 mL) was added a solution of B(Ar^F⁵)₃(0.250 g,

0.845 mmol) in benzene (5 mL), all at room temperature. A color

change from lime green to dark green was observed, and the solution

was heated to 70 °C and stirred for 4 days. Volatiles were then

removed under reduced pressure to aﬀord 26 as a dark green oily solid.

The resultant green solid was washed with pentane (3 × 5 mL, room

temperature), ﬁltered, and dried in vacuo. Yield: 0.362 g (76%). The

resultant green powder was subsequently washed with diethyl ether (1

× 5 mL, room temperature), ﬁltered, and dried in vacuo to give an

t

(m). EI-MS: m/z 666 [M − BuNC]⁺(70%). Anal. Found (calcd for

C₃₉H₅₂F₅N₅OTi): C, 62.57 (62.48); H, 7.13 (6.99); N, 9.24 (9.34).

Cp*Ti{PhC(NⁱPr)₂}{NC(NO^tBu)C₆F₄N(C₆H₃Me₂)C}(F) (20). To a

solution of Cp*Ti{PhC(NⁱPr)₂}{NC(Ar^F⁵)NO^tBu} (5; 0.493 g, 0.739

mmol) in diethyl ether (2 mL) was added XylNC (97.0 mg, 0.739

mmol) in diethyl ether (2 mL), all at room temperature. A color

change from lime green to red-orange was observed, and the solution

was stirred for 16 h. Volatiles were then removed under reduced

pressure to aﬀord 20 as an orange powder which was dried in vacuo.

Yield: 0.501 g (85%). Diﬀraction-quality crystals were grown from a

1

analytically pure sample. Yield: 0.124 g (26%). H NMR (CD₂Cl₂,

499.9 MHz, 293 K): δ 7.52−7.47 (4 H, m, overlapping o_a-C₆H₅, m-

C₆H₅, and p-C₆H₅), 7.33 (1 H, m, o_b-C₆H₅), 6.67 (1 H, s, NH), 3.46

3

(2 H, 2 × app sept, app J = 6.5 Hz, overlapping NCH_aMeMe and

3

NCH_bMeMe), 2.02 (15 H, s, C₅Me₅), 0.98 (3 H, d, J = 6.5 Hz,

NCH_aMeMe), 0.93 (3 H, d, ³J = 7.0 Hz, NCH_aMeMe), 0.83 (3 H, d, ³J

= 6.5 Hz, NCH_bMeMe), 0.82 (3 H, d, ³J = 7.0 Hz, NCH_bMeMe) ppm.

¹³C{¹H} NMR (CD₂Cl₂, 125.7 MHz, 293 K): δ 167.1 (PhCN₂), 153.0

(NC(Ar^F⁵)N), 148.9 (2 C, br d, ¹J_C−F= 235.0 Hz, o-C₆F₅), 145.7 (2 C,

d, ¹J_C−F= 245.9 Hz, o-C₆F₅), 144.5 (1 C, d, ¹J_C−F= 251.2 Hz, p-C₆F₅),

143.1 (2 C, d, ¹J_C−F= 257.1 Hz, m-C₆F₅), 139.9 (1 C, d, ¹J_C−F= 248.0

Hz, p-C₆F₅), 137.2 (2 C, d, ¹J_C−F= 246.8 Hz, m-C₆F₅), 132.4 (i-C₆H₅),

131.1 (C₅Me₅), 129.9, 129.1, 129.0, 128.9 (o_a-C₆H₅, o_b-C₆H₅, m-C₆H₅,

and p-C₆H₅), 128.5 (1 C, br s, i-C₆F₅), 108.9 (1 C, app t, ²J_C−F= 19.6

Hz, i-C₆F₅), 52.1 (NCH_aMeMe), 51.1 (NCH_bMeMe), 26.1

(NCH_bMeMe), 25.4 (NCH_aMeMe), 25.4 (NCH_aMeMe), 24.2

(NCH_bMeMe), 13.1 (C₅Me₅) ppm. ¹⁹F{¹H} NMR (CD₂Cl₂, 282.1

MHz, 293 K): δ −132.5 (br s), −133.2 (2 F, br s, o-C₆F₅), −134.2 (br

s), −135.7 (2 F, br m, o-C₆F₅), −135.8 (br m), −136.6 (br s), 150.1 (1

1

saturated n-hexane solution at −4 °C. H NMR (toluene-d₈, 299.9

MHz, 253 K): δ 7.12−6.87 (7 H, m, overlapping o_a-C₆H₅, m-C₆H₅, p-

3

C₆H₅, m-C₆H₃Me₂,and p-C₆H₃Me₂), 6.07 (1 H, d, J = 7.5 Hz, o_b-

C₆H₅), 3.69 (1 H, app sept, app ³J = 6.6 Hz, NCH_aMeMe), 3.14 (1 H,

app sept, app ³J = 6.6 Hz, NCH_bMeMe), 2.46 (3 H, s, 2,6-

C₆H₃Me_aMe_b), 2.36 (15 H, s, C₅Me₅), 1.98 (3 H, s, 2,6-C₆H₃Me_aMe_b),

1.48 (9 H, s, NOCMe₃), 1.05 (3 H, d, ³J = 6.3 Hz, NCH_aMeMe), 1.00

(3 H, d, ³J = 6.6 Hz NCH_aMeMe), 0.88 (3 H, d, ³J = 6.9 Hz,

NCH_bMeMe), 0.68 (3 H, d, ³J = 6.6 Hz, NCH_bMeMe) ppm. ¹³C{¹H}

NMR (toluene-d₈, 75.4 MHz, 253 K): δ 216.7 (Ti-NC), 170.4

(PhCN₂), 141.7 (1 C, d, J = 3.1 Hz, i-C₆H₃Me₂), 139.6 (br s,

NC(Ar^F⁵)N), 138.8 (1 C, d, J = 4.2 Hz, o_a-C₆H₃Me₂), 136.1 (i-C₆H₅),

135.2 (1 C, d, J = 4.1 Hz, o_b-C₆H₃Me₂), 129.6 (m_a-C₆H₃Me₂), 129.1−

127.3 (overlapping with toluene-d₈, o_a-C₆H₅, o_b-C₆H₅, m-C₆H₅, p-

C₆H₅, and p-C₆H₃Me₂), 127.7 (m_b-C₆H₃Me₂), 126.4 (C₅Me₅), 103.0

4

F, t of t, ³J = 20.6, J = 3.7 Hz, p-C₆F₅), - 158.1 (br s), −160.7 (br s),

−161.5 (1 F, t of d, ³J = 21.6, ⁴J = 8.2 Hz, m-C₆F₅), −162.5 (1 F, t of d,

4

³J = 21.6, J = 7.3 Hz, m-C₆F₅), −166.0 (br m, m-C₆F₅), 166.8 (br s)

2

3

(1 C, dd, J_C−F= 18.7 Hz, J_C−F= 3.2 Hz, Ti{NC(NO^tBu)C}), 79.6

(NOCMe₃), 49.4 (NCH_aMeMe), 48.7 (NCH_bMeMe), 27.7 (over-

lapping NOCMe₃and NCH_aMeMe), 25.2 (NCH_aMeMe), 25.0

(NCH_bMeMe), 25.0 (NCH_bMeMe), 19.0 (1 C, br d, 2,6-

C₆H₃Me_aMe_b), 18.2 (2,6-C₆H₃Me_aMe_b), 12.7 (C₅Me₅) ppm. The

resonance attributed to Ti{CN(Xyl)C} could not be observed. ¹³C

NMR (HMQC ¹⁹F-observed, toluene-d₈, 282.1 MHz, 253 K): δ 144.2

(inner₁-C₆F₄N), 142.5 (inner₂-C₆F₄N), 137.6 (outer₄-C₆F₄N), 135.9

(outer₃-C₆F₄N) ppm. ¹⁹F{¹H} NMR (toluene-d₈, 282.1 MHz, 253 K):

δ −101.8 (1 F, br s, Ti−F), −118.7 (1 F, dt, J =24.8 Hz, J = 8.7 Hz,

inner₁-C₆F₄N), −152.4 (1 F, td, J = 22.0, J = 7.6 Hz, inner₂-C₆F₄N),

−155.2 (1 F, dd, J = 22.3, J = 22.0, outer₃-C₆F₄N), −162.7 (1 F, dd, J =

21.7, J = 21.7 Hz, outer₄-C₆F₄N) ppm. IR (NaCl plates, Nujol mull,

cm⁻¹): 2118 (w), 1638 (w), 1586 (w), 1511 (s), 1488 (s), 1366 (s),

1340 (m), 1328 (m), 1276 (w), 1258 (m), 1231 (w), 1218 (m), 1199

ppm. ¹¹B NMR (CD₂Cl₂, 96.2 MHz, 293 K): δ −7.1 (B(Ar^F⁵)₃) ppm.

¹H NMR (CD₂Cl₂, 299.9 MHz, 213 K): δ 7.46 (4 H, br s, overlapping

o_a-C₆H₅, m-C₆H₅, and p-C₆H₅), 7.30 (1 H, br m, o_b-C₆H₅), 6.65 (1 H,

s, NH), 3.35 (2 H, 2 × br m, overlapping NCH_aMeMe and

NCH_bMeMe), 1.90 (15 H, s, C₅Me₅), 0.91 (3 H, br d, NCH_aMeMe),

0.84 (3 H, d, ³J = 7.0 Hz, NCH_aMeMe), 0.73 (6 H, br m, overlapping

NCH_bMeMe and NCH_bMeMe) ppm. ¹⁹F{¹H} NMR (CD₂Cl₂, 282.1

MHz, 213 K): δ −121.5 (br s), −126.2 (br, t), −126.8 (br, t), −131.6

(br t), −132.6 (br m), −132.9 (br d), −133.7 (br m), −134.3 (br d),

3

−135.9 (br d), −139.6 (br s), −149.4 (1 F, t, J = 21.2, p₁-C₆F₅),

−156.9 (1 F, t, ³J = 21.7, p₂-C₆F₅), −159.8 (1 F, t, ³J = 21.4, p₃-C₆F₅),

3

−160.5 (1 F, t, J = 21.4, p₄-C₆F₅), −161.4 (1 F, app t, m_1a-C₆F₅),

−161.8 (1 F, app t, m_1b-C₆F₅), −162.8 (1 F, app t, m_2a-C₆F₅), −165.0

(1 F, app t, m_4a-C₆F₅), −166.1 (1 F, app t, m_3a-C₆F₅), −166.3 (1 F, app

1016

dx.doi.org/10.1021/om4011752 | Organometallics 2014, 33, 1002−1019

Organometallics

Article

t, m_4b-C₆F₅), −166.6 (1 F, app t, m_2b-C₆F₅), −167.0 (1 F, app t, m_3b-

C₆F₅) ppm. At this temperature the resonance attributed to B(Ar^F⁵)₃

in the ¹¹B NMR spectrum could not be identiﬁed. IR (NaCl plates,

Nujol mull, cm⁻¹): 3393 (w), 1642 (m), 1517 (s), 1504 (m), 1406

(m), 1334 (m), 1278 (w), 1261 (w), 1221 (w), 1096 (m), 1018 (m),

1000 (m), 979 (m), 864 (w), 812 (m), 782 (m), 766 (w), 749 (w),

707 (w), 695 (w), 687 (w), 679 (w), 659 (w), 615 (m), 596 (m). EI-

MS: m/z 610 [Cp*Ti{PhC(NⁱPr)₂}{ONC(Ar^F⁵)N(H)}, M − 2B-

(Ar^F⁵)₃]⁺(5%). Anal. Found (calcd for C₆₆H₃₅B₂F₃₅N₄OTi): C, 48.59

(48.50); H, 2.23 (2.16); N, 3.48 (3.43).

Crystal Structure Determinations. Crystal data collection and

processing parameters for Cp*Ti{PhC(NⁱPr)₂}{NC(Ar^F⁵)NO^tBu)C-

(Ph)(H)O} (13a), (4-C₆H₄NMe₂)C{NC(Ar^F⁵)NO^tBu}H (14e),

Me₂NC{NC(Ar^F⁵)NO^tBu}H (16), Cp*Ti{PhC(NⁱPr)₂}{NC(Ar^F⁵)-

NO^tBu}(^tBuNC) (18), and Cp*Ti{PhC(NⁱPr)₂}{NC(NO^tBu)-

C₆F₄N(C₆H₃Me₂)C}(F) (20) are given in Table S5 of the Supporting

Information. Crystals were mounted on glass ﬁbers using perﬂuor-

opolyether oil and cooled rapidly under a stream of cold N₂using an

Oxford Cryosystems Cryostream unit. Diﬀraction data were measured

using either an Enraf-Nonius KappaCCD or Agilent Technologies

Supernova diﬀractometer with Mo Kα or Cu Kα radiation,

respectively. As appropriate, absorption and decay corrections were

applied to the data and equivalent reﬂections merged.⁴²The structures

were solved with SIR92⁴³or Superﬂip,⁴⁴and further reﬁnements and

all other crystallographic calculations were performed using the

CRYSTALS program suite.⁴⁵Other details of the structure solution

and reﬁnements are given in the Supporting Information. A full listing

of atomic coordinates, bond lengths and angles, and displacement

parameters for all of the structures have been deposited at the

Cambridge Crystallographic Data Centre.

Computational Details. DFT calculations were carried out using

the Amsterdam Density Functional program suite ADF 2013.01.⁴⁶The

generalized gradient approximation was employed, using the local

density approximation of Vosko, Wilk, and Nusair⁴⁷together with

nonlocal exchange corrections by Becke⁴⁸and nonlocal correlation

corrections by Perdew.⁴⁹TZP basis sets were used with triple-ζ

accuracy sets of Slater-type orbitals and a polarization function added

to the main-group atoms. The default SCF convergence criteria were

used, together with a “good” Becke integration grid. The cores of the

atoms were frozen up to 1s for C, N, O, and F. Full optimization of

geometry was performed without any symmetry constraint, followed

by analytical computation of the Hessian matrix to identify the nature

of the located extrema as minima or transition states.

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ASSOCIATED CONTENT

■

S

* Supporting Information

Text, tables, ﬁgures, and CIF ﬁles giving general experimental

procedures and details of starting materials, remaining details of

the synthesis and characterization data for new compounds,

further details of the DFT calculations and crystal structure

determinations, including X-ray data collection and processing

parameters and further data. This material is available free of

charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION

■

Corresponding Author

*E-mail for P.M.: philip.mountford@chem.ox.ac.uk.

Notes

The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENTS

■

We thank Wolfson College, Oxford, U.K., for a Junior Research

Fellowship to A.D.S., the Department of Chemistry, University

of Oxford, Oxford, U.K., for a scholarship to L.R.G., and

LANXESS Elastomers for a studentship to A.F.R.

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