Reactions of neutral and cationic diamide-supported imido complexes with CO2 and other heterocumulenes: Issues of site selectivity

2368

Organometallics 2005, 24, 2368-2385

Reactions of Neutral and Cationic Diamide-Supported

Imido Complexes with CO₂and Other Heterocumulenes:

Issues of Site Selectivity

Benjamin D. Ward,^†Gavin Orde,^†Eric Clot,*^,‡Andrew R. Cowley,^†

Lutz H. Gade,*^,§and Philip Mountford*^,†

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

LSDSMS (UMR 5636), cc 14, Universite´ Montpellier 2, Place Euge`ne Bataillon, 34095

Montpellier Cedex 5, France, and Anorganisch-Chemisches Institut, Universita¨t Heidelberg,

Im Neuenheimer Feld 270, 69120 Heidelberg, Germany

Received January 13, 2005

Experimental and DFT computational studies of the reactions of the cationic diamide-

pyridine-supported methyl tungsten complex [W(NPh)(N₂N_py)Me]⁺(2⁺) with the heterocu-

mulenes CO₂, CS₂, RNCO, and RNCS (R ) tert-butyl or aryl) are described, together with

comparative studies of the group 5 compounds M(N^tBu)(N₂N_py)Cl(py) (M ) Nb (13) or Ta

(14)) and Ta(N^tBu)(N₂N_py)Me (N₂N_py) MeC(2-C₅H₄N)(CH₂NSiMe₃)₂). In all of the reactions

of 2⁺the heterocumulene inserted exclusively into a W-N_amidebond to give unstable

intermediates (isolated for certain isocyanate substrates), which subsequently underwent

rearrangement via a 1,3-migration of a SiMe₃group. DFT studies of the reaction of 2⁺with

CO₂showed that insertion into the W-Me bond is in fact thermodynamically preferred, but

that insertion into W-N_amideis kinetically more facile. Cycloaddition to the WdNPh bond

was neither kinetically nor thermodynamically viable. The reactions between Ta(N^tBu)-

(N₂N_py)Me and CO₂or RNCO in contrast gave complex mixtures, as did those between 13

and 14 and CO₂. DFT studies showed that in a neutral but otherwise identical tantalum

analogue of 2⁺the preference for substrate attack at the M-N_amidebond is much less

pronounced. Reactions of 13 and 14 with the more sterically discriminating RNCO (R ) ^tBu

or p-tolyl) did, however, exclusively afford M-N_amideinsertion/1,3-SiMe₃migration products,

and Nb(N^tBu){MeC(2-C₅H₄N)(CH₂NSiMe₃)(CH₂NC(OSiMe₃)N^tBu)}Cl was structurally char-

acterized. The previously reported reaction of Ti(N^tBu)(N₂N_py)(py) with ArNCO (Ar ) 2,6-

i

C₆H₃Pr₂) was re-evaluated and shown to involve an analogous Ti-N_amideinsertion/1,3-SiMe₃

migration reaction, ultimately forming Ti(N^tBu){MeC(2-C₅H₄N)(CH₂NSiMe₃)(CH₂NC(N(Ar)-

SiMe₃)O)}(py).

Introduction

We have been interested in the chemistry of imido

compounds supported by the diamido-pyridine ligand

MeC(2-C₅H₄N)(CH₂NSiMe₃)₂(N₂N_py)^11,18and have de-

scribed the synthesis and reactions of such derivatives

for group 4,^19-235,^24,25and 6^26,27d⁰transition metals.

Although transition metal imido complexes have been

known since the late 1950s,^1,2it was only during the

last two decades that a particularly intense effort was

made in exploring their syntheses, structures, and

reactivity. Several general reviews^3-5and a number of

more specific ones are available.^6-17

(14) Leung, W.-H. Eur. J. Inorg. Chem. 2003, 583.

(15) Mountford, P. In Perspectives in Organometallic Chemistry;

Screttas, C. G., Steele, B. R., Eds.; Royal Society of Chemistry, 2003.

(16) Radius, U. Z. Anorg. Allg. Chem. 2004, 630, 957.

(17) Giesbrecht, G.; Gordon, J. C. Dalton Trans. 2004, 2387.

(18) Friedrich, S.; Schubart, M.; Gade, L. H.; Scowen, I. J.; Edwards,

A. J.; McPartlin, M. Chem. Ber./Recl. 1997, 130, 1751.

(19) Tro¨sch, D. J. M.; Collier, P. E.; Bashall, A.; Gade, L. H.;

McPartlin, M.; Mountford, P.; Radojevic, S. Organometallics 2001, 20,

3308.

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

univ-montp2.fr.; lutz.gade@uni-hd.de; philip.mountford@chem.ox.ac.uk.

^†University of Oxford.

^‡Universite´ Montpellier 2.

^§Universita¨t Heidelberg.

(1) Clifford, A. F.; Kobayashi, C. S. Abstracts of the 130th National

Meeting, ACS, 50R, Atlantic City, NJ, 1956.

(2) Milas, N. A.; Ilipulos, M. I. J. Am. Chem. Soc. 1959, 81, 6089.

(3) Nugent, W. A.; Haymore, B. L. Coord. Chem. Rev. 1980, 31, 123.

(4) Nugent, W. A.; Mayer, J. M. Metal-Ligand Multiple Bonds;

Wiley-Interscience: New York, 1988.

(5) Wigley, D. E. Prog. Inorg. Chem. 1994, 42, 239.

(6) Mountford, P. Chem. Commun. 1997, 2127 (feature article).

(7) Romao, C. C.; Ku¨hn, F. E.; Herrmann, W. A. Chem. Rev. 1997,

97, 3197.

(8) Danopoulos, A. A.; Green, J. C.; Hursthouse, M. B. J. Organomet.

Chem. 1999, 591, 36.

(9) Cundari, T. R. Chem. Rev. 2000, 100, 807.

(10) Sharp, P. J. Chem. Soc., Dalton Trans. 2000, 2647.

(11) Gade, L. H.; Mountford, P. Coord. Chem. Rev. 2001, 216-217, 65.

(12) Duncan, A. P.; Bergman, R. G. Chem. Rec. 2002, 2, 431.

(13) Eikey, R. A.; Abu-Omar, M. M. Coord. Chem. Rev. 2003, 243, 83.

(20) Blake, A. J.; Collier, P. E.; Gade, L. H.; Mountford, P.; Pugh,

S. M.; Schubart, M.; Skinner, M. E. G.; Tro¨sch, D. J. M. Inorg. Chem.

2001, 40, 870.

(21) Pugh, S. M.; Tro¨sch, D. J. M.; Wilson, D. J.; Bashall, A.; Cloke,

F. G. N.; Gade, L. H.; Hitchcock, P. B.; McPartlin, M.; Nixon, J. F.;

Mountford, P. Organometallics 2000, 19, 3205.

(22) Bashall, A.; Collier, P. E.; Gade, L. H.; McPartlin, M.; Mount-

ford, P.; Pugh, S. M.; Radojevic, S.; Schubart, M.; Scowen, I. J.; Tro¨sch,

D. J. M. Organometallics 2000, 19, 4784.

(23) Ward, B. D.; Maisse-Franc¸ois, A.; Mountford, P.; Gade, L. H.

Chem. Commun. 2004, 704.

(24) Pugh, S. M.; Tro¨sch, D. J. M.; Skinner, M. E. G.; Gade, L. H.;

Mountford, P. Organometallics 2001, 20, 3531.

(25) Pugh, S. M.; Blake, A. J.; Gade, L. H.; Mountford, P. Inorg.

Chem. 2001, 40, 3992.

Publication on Web 04/02/2005

Diamide-Supported Imido Complexes

Organometallics, Vol. 24, No. 10, 2005 2369

Chart 1

In terms of reactivity of the MdNR bonds themselves

in complexes of the type M(NR)(N₂N_py)(L) (L ) neutral

or anionic co-ligand or none), only the group 4 systems

Ti(NR)(N₂N_py)(py) and certain homologues have so far

been exploited (with a good degree of success).^{11,19,21,23,28,29}

We very recently described the synthesis and DFT

bonding analysis of the cation [W(NPh)(N₂N_py)Me]⁺(2⁺,

valence isoelectronic with Ti(NR)(N₂N_py)(py)), which is

studies reported here since its rapid reactions with het-

erocumulenes are complete well before the onset of de-

composition. Unfortunately the reactions of 2-MeBAr^F

3

with substituted allenes gave ill-defined mixtures.

formed by reaction of W(NPh)(N₂N_py)Me₂(1) with BAr^F

3

or [CPh₃][BAr^F₄] (Ar^F) C₆F₅).²⁷Cation 2⁺undergoes

facile methyl group exchange reactions with ZnMe₂and

Cp₂ZrMe₂, as well as with 1 itself, and we were inter-

ested in exploring its reactions with unsaturated sub-

strates. In particular, the question of reactive site selec-

tivity was an interesting one since all three bonds

W-Me, W-N_amide, and WdN_imidemight react, depend-

ing on the chosen substrates. We note in particular in

this context previous studies by Legzdins et al. on some

pentamethylcyclopentadienyl-supported tungsten alkyl-

imido and -amido and related complexes and their reac-

tions with heterocumulenes,^30,31and work by Lam et

al.³²on the reactions of Cr(N^tBu)₂(NH^tBu)₂with isocy-

anates.

Reaction of 2-MeBAr^Fwith CO₂(1 atm) in CH₂Cl₂

3

at 0 °C affords [W(NPh)(MeC(2-C₅H₄N)(CH₂NSiMe₃)-

{CH₂NCO(SiMe₃)O}Me][MeBAr^F₃] (3-MeBAr^F₃, eq 1)

as a golden-brown solid. The formation of 3⁺is quan-

titative when the reaction is followed by NMR spectros-

copy in CD₂Cl₂. No intermediates were observed when

the reaction was carried out on an NMR tube scale in

CD₂Cl₂at -78 °C. We have been unable to grow

diffraction-quality crystals of 3-MeBAr^F₃or any of the

cationic insertion products formed from 2-MeBAr^F₃and

heterocumulenes. The structures of the analytically

pure and isolated tungsten products have been assigned

by careful NMR analysis (employing as appropriate ¹³C-

labeled isotopomers, namely, the previously reported²⁷

[W(NPh)(N₂N_py)(¹³CH₃)]⁺or/and ¹³CO₂) and by analogy

with a crystallographically characterized, valence iso-

electronic niobium analogue 15 (vide infra).

In this contribution we describe the reactions of 2⁺

with a number of heterocumulenes and employ DFT

calculations to rationalize the respective outcome and

gain additional insight into these systems. Together

with this work, we also report comparative studies of

some group 5 analogues and a re-evaluation of the

reaction of Ti(N^tBu)(N₂N_py)(py) with ArNCO (Ar ) 2,6-

i

C₆H₃Pr₂).

Cation 3⁺appears to be formed from the insertion of

CO₂into one of the W-N_amidebonds of 2⁺followed by a

N to O SiMe₃group 1,3-migration. The likely intermedi-

ate 3a⁺(Chart 1) was not observed at -78 °C, but DFT

evidence in support of its formation is presented later.

The metal-bound Me group shows ¹⁸³W satellites in the

¹³C NMR spectrum (¹⁸³W natural abundance ) 14.5%,

I ) ¹/₂; ¹J(WC) ) 92 Hz), which rules out an alternative

product 3b⁺(or one of its coordination isomers). Only

one of the chemically inequivalent SiMe₃groups of 3⁺

shows an NOE to the rest of the compound [careful

analysis of the ROESY (rotating frame nuclear Over-

hauser enhancement spectroscopy) spectrum estab-

lished this as the N-bound SiMe₃group]. Significantly

the HMBC (heteronuclear multiple bond correlation)

¹H-¹³C NMR spectrum clearly shows a correlation

between the quaternary COSiMe₃carbon (δ 166.6 ppm,

confirmed by use of ¹³CO₂) and two of the ligand “arm”

methylene H atoms. This unambiguously establishes

the formation of a N_amide-CO₂bond as opposed to

alternative coupling between CO₂and the WdN_imide

moiety (as in 3c⁺). This long-range correlation between

Results and Discussion

Reactions of [W(NPh)(N₂N_py)Me][MeBAr^F₃] (2-

MeBAr^F₃) with Heterocumulenes. We describe first

the reactions of heterocumulenes with the in-situ-gener-

ated tungsten methyl cation [W(NPh)(N₂N_py)Me]⁺(2⁺),

prepared as its [MeBAr^F₃]^-salt by reaction of W(NPh)-

(N₂N_py)Me₂(1) with B(Ar^F)₃as reported previously.²⁷

Compound 2-MeBAr^Fhas limited stability in CH₂Cl₂

3

at ambient temperature (half-life of ca. 2 h), but this is

not a problem from the point of view of the reactivity

(26) Ward, B. D.; Dubberley, S. R.; Gade, L. H.; Mountford, P. Inorg.

Chem. 2003, 42, 4961.

(27) Ward, B. D.; Orde, G.; Clot, E.; Cowley, A. R.; Gade, L. H.;

Mountford, P. Organometallics 2004, 23, 4444.

(28) Blake, A. J.; Collier, P. E.; Gade, L. H.; Mountford, P. Chem.

Commun. 1997, 1555.

(29) Bashall, A.; Collier, P. E.; Gade, L. H.; McPartlin, M.; Mount-

ford, P.; Troesch, D. J. M. Chem. Commun. 1998, 2555.

(30) Legzdins, P.; Phillips, E. C.; Rettig, S. J.; Trotter, J.; Veltheer,

J. E.; Yee, V. C. Organometallics 1992, 11, 3104.

(31) Legzdins, P.; Rettig, S. J.; Ross, K. J. Organometallics 1994,

13, 569.

(32) Lam, H. W.; Wilkinson, G.; Hussain-Bates, B.; Hursthouse, M.

B. J. Chem. Soc., Dalton Trans. 1993, 781.

2370 Organometallics, Vol. 24, No. 10, 2005

Ward et al.

Scheme 1

Scheme 2

Table 1. Rate Constants and Activation

Parameters for the Conversion of 11⁺into 12⁺

Rate Constants

temp (K)

rate constant (s^-1

)

293

298

303

308

313

1.94 × 10^-4

2.32 × 10^-4

3.64 × 10^-4

3.81 × 10^-4

7.20 × 10^-4

methylene H atoms and the quaternary carbon originat-

ing from the heterocumulene is a common feature of all

of the new M-N_amideinsertion products reported herein

(including the crystallographically characterized 15).

CS₂undergoes an analogous reaction with 2⁺to form 4⁺

(eq 1), this time involving an N to S SiMe₃1,3-migration.

Activation Parameters

∆H^q(kJ‚mol^-1

44.9 ( 2.0

)

∆S^q(J‚K^-1‚mol^-1

)

∆G^q₂₉₈(kJ‚mol^-1

93.8 ( 5

)

-164 ( 10

of the molecule; the ^tBu group shows a clear ¹H NOE to

the pyridyl H⁶.

The reaction of 2-MeBAr^F₃with tert-butyl, p-tolyl, or

phenyl isocyanates, or with tert-butyl isothiocyanate,

Reaction of 2-MeBAr^Fwith the sterically more

3

i

afforded products analogous to 3-MeBAr^Fand 4-Me-

demanding isocyanate ArNCO (Ar ) 2,6-C₆H₃Pr₂) af-

3

BAr^F₃, namely, [W(NPh)(MeC(2-C₅H₄N)(CH₂NSiMe₃)-

fords a longer-lived intermediate compound [W(NPh)-

(MeC(2-C₅H₄N)(CH₂NSiMe₃)(CH₂N(SiMe₃)C(NAr)O)-

Me][MeBAr^F₃] (11-MeBAr^F₃), in which the isocyanate

has undergone W-N_amidebond insertion with the iso-

cyanate O atom bound to W. The intermediate cation

11⁺transforms to the silyl-migrated species [W(NPh)-

(MeC(2-C₅H₄N)(CH₂NSiMe₃)(CH₂NC{N(SiMe₃)Ar}O]⁺

(12⁺) over ca. 4 h in CD₂Cl₂at ambient temperature

(Scheme 2). The NMR data for cations 11⁺and 12⁺are

consistent with isocyanate insertion into a W-N_amide

bond. However, the ROESY experiments show that the

isocyanate-derived aryl group is close to one of the N₂N_py

methylene groups and one SiMe₃group, but is not near

t

{CH₂NCO(SiMe₃)NR}Me][MeBAr^F₃] (R ) Bu (6-Me-

BAr^F₃), Tol (Tol ) 4-C₆H₄Me, 8-MeBAr^F₃), or Ph (9-

MeBAr^F₃)) or [W(NPh)(MeC(2-C₅H₄N)(CH₂NSiMe₃)-

{CH₂NCS(SiMe₃)N^tBu}Me][MeBAr^F] (10-MeBAr^F), re-

3

spectively (Scheme 1). The NMR data are consistent

with the heterocumulene having inserted into one of the

W-N_amidebonds of 2⁺with the subsequent migration

of a SiMe₃group, as proposed for the reaction with CO₂

and CS₂. The non-centrosymmetric heterocumulenes

can insert into a W-N_amidebond of 2⁺with either the

t

N or O (or S for BuNCS) bound to W. The ROESY

spectra for cations 6⁺and 8⁺-10⁺show a clear NOE

between the pyridyl H⁶atom (i.e., ortho to N) and H

atoms of the R group originating from the heterocumu-

lene, therefore confirming the geometries proposed in

Scheme 1. As for 3⁺and 4⁺only one of the two

inequivalent SiMe₃groups shows NOE interactions with

other parts of the cation.

the pyridyl moiety. The IR spectrum of 11-MeBAr^F

3

exhibits a band at 1690 cm^-1, which we attribute to ν-

(CdN), and this band is absent in the IR spectrum of

12-MeBAr^F₃. We propose that the O-bound isomer is

formed in this case due to the increased steric demand

i

imposed by the 2,6-C₆H₃Pr group. The slower isomer-

ization rate of cation 11⁺compared to 5⁺and 7⁺can be

attributed (at least in part) to the SiMe₃migrating to a

substituted acceptor atom in the case of 11⁺.

Unlike the reactions of 2-MeBAr^F₃with CO₂and CS₂,

reaction intermediates were observed in the heterocumul-

ene reactions, and these have been analyzed in detail

for tert-butyl and tolyl isocyanate (Scheme 1). The cat-

ions [W(NPh)(MeC(2-C₅H₄N)(CH₂NSiMe₃){CH₂N(SiMe₃)-

The transformation of cation 11⁺into 12⁺was moni-

tored by ¹H NMR spectroscopy at five temperatures

between 293 and 313 K. The decay of 11⁺and evolution

of 12⁺followed first-order kinetics (see the SI for further

details), and the rate constants are given in Table 1,

together with associated activation parameters. The

very negative ∆S^qand first-order kinetics strongly

indicate a highly concerted, intramolecular process,

most likey involving a transition state featuring a

hypervalent Si atom bridging the originating N_amide

atom and the acceptor N atom (or O or S for the

reactions in Scheme 1 and eq 1). 1,3-SiMe₃shifts

between various types of donor and acceptor atoms have

been well characterized in a number of organic³³and

inorganic^34-36systems.

t

C(O)NR}Me]⁺(R ) Bu (5⁺) or Tol (7⁺)) were readily

observed when the reactions were carried out in an

NMR tube at -78 °C (CD₂Cl₂). Compound 7-MeBAr^F

3

was insufficiently stable to be isolated in the absence

of significant amounts of the migration product 8-Me-

BAr^Fand was analyzed only by NMR. However,

3

compound 5-MeBAr^F₃could be isolated by carrying out

the synthesis and workup at temperatures below -30

°C. The NMR data show that these compounds are the

product of insertion into one of the W-N_amidebonds of

2⁺. For example, for 5⁺long-range coupling was ob-

served between two methylene H atoms and the qua-

ternary carbon (δ 157.5 ppm) originating from ^tBuNCO;

both SiMe₃groups showed NOE interactions to the rest

(33) Kwart, H.; Barnette, W. E. J. Am. Chem. Soc. 1977, 99, 614.

Diamide-Supported Imido Complexes

Organometallics, Vol. 24, No. 10, 2005 2371

Reactions of M(N^tBu)(N₂N_py)Cl(py) (M ) Nb or

Ta) with Heterocumulenes. The methyl tungsten

cation [W(NPh)(N₂N_py)Me]⁺(2⁺) evidently shows a

strong preference (at least kinetically) for insertion of

heterocumulenes into a W-N_amidebond as opposed to

cycloaddition to WdNPh or insertion into W-Me. We

were therefore interested in examining the correspond-

ing reactions of neutral group 5 analogues. We have

previously reported the chloride compounds M(N^tBu)-

(N₂N_py)Cl(py) (M ) Nb (13) or Ta (14)).²⁵These are in

dynamic equilibrium with the five-coordinate M(N^tBu)-

(N₂N_py)Cl (valence isoelectronic with 2⁺) and pyridine

on the NMR time scale.²⁵The pyridine-free niobium

compound Nb(N^tBu)(N₂N_py)Cl has been isolated and

characterized but is much less stable than the six-

coordinate 13. The tantalum compound 14 is cleanly

methylated to form Ta(N^tBu)(N₂N_py)Me,²⁷again valence

isoelectronic with 2⁺, whereas the congeneric 13 cannot

be successfully methylated.²⁴Although group 5 tert-

butyl imido compounds 13 and 14 could be readily made,

phenyl imido homologues were not accessible.^27,37Our

group 5 work has therefore concentrated on reactions

of M(N^tBu)(N₂N_py)Cl(py) and Ta(N^tBu)(N₂N_py)Me.

The reactions of M(N^tBu)(N₂N_py)Cl(py) and Ta(N^tBu)-

(N₂N_py)Me are much less well-behaved than those of

2-MeBAr^F₃. Briefly, reactions of Ta(N^tBu)(N₂N_py)Me

Figure 1. Displacement ellipsoid plot (20%) of one of the

four crystallographically independent molecules of Nb(N^t-

Bu){MeC(2-C₅H₄N)(CH₂NSiMe₃)(CH₂NC(OSiMe₃)N^tBu)}-

Cl (15). H atoms are omitted for clarity.

molecules is shown in Figure 1, and selected bond

distances and angles are listed in Table 2.

t

The structure of 15 confirms the presence of a 1,3-

migrated SiMe₃group and the presence of a new bond

formed between Nb-N_amideand the carbon of ^tBuNCO.

In general the distances and angles are in agreement

with previously reported values.^39,40The starting com-

pound M(N^tBu)(N₂N_py)Cl(py) (13) has been structurally

characterized previously.²⁵The Nb(1)-N(1), Nb(1)-

N(4), and Nb(1)-N(5) distances are similar in both

molecules, whereas Nb(1)-Cl(1) is somewhat longer in

13 (2.548(2) Å) than in 15 (range 2.4488(13)-2.4575-

(13) Å). This reflects the different nature of the N atom

trans to the Nb-Cl bond (N_amideof N₂N_pyin 13 vs N(3)

in 15) and that the Nb(1)-N(3) distance in 15 (range

2.148(4)-2.150(4) Å) is ca. 0.01 Å longer than the

equivalent bond in 13, clearly refecting the modification

that this part of the molecule has undergone. The {O-

(1)C(5)N(3)N(2)} moiety in 15 posesses a trigonal planar

geometry at C(5) and forms the core of a formally

monoanionic O-silylated ureate fragment. Comparison

of the C(5)-N(2) and C(5)-N(3) distances suggests

there is greater multiple bond character in the former

bond. Consistent with the greater implied imino char-

acter of N(2), the Nb(1)-N(2) distances are ca. 0.12-

0.14 Å longer than the Nb(1)-N(3) values (thus N(3)

appears to have more residual N_amidecharacter).

with CO₂, TolNCO, BuNCO, or ArNCO all gave com-

plex mixtures of products. Reaction of 13 or 14 with CO₂

also gave complex mixtures, although in these instances

^tBuNCO was apparent in NMR tube scale reactions

(formed in 30-40% yield based on 13 and 14 starting

complexes). The ^tBuNCO is most likely formed by

cycloaddition to MdN^tBu followed by extrusion (cyclo-

reversion), which is a well-established reaction sequence

in transition metal imido chemistry.³⁸However, the low

yield of ^tBuNCO and complex mixture formed suggests

that other modes of reaction are easily accessible.

However, despite these discouraging results, com-

pounds 13 and 14 do undergo very clean reactions with

^tBuNCO and TolNCO to form the corresponding

products M(N^tBu){MeC(2-C₅H₄N)(CH₂NSiMe₃)(CH₂-

NC(OSiMe₃)NR)}Cl) (M ) Nb, R ) ^tBu (15) or Tol (17);

Re-evaluation of the Reaction of Ti(N^tBu)(N₂N_py)-

(py) with ArNCO. The TidN^tBu bond in Ti(N^tBu)-

(N₂N_py)(py) undergoes a wide range of cycloaddition and

other coupling reactions with unsaturated substrates.^19-23

The reaction with allene itself affords crystallographi-

cally characterized 19 (Chart 2).¹⁹Reactions with a

range of isocyanates and CO₂all gave multiple products

t

M ) Ta, R ) Bu (16) or Tol (18)) (eq 2). These are

neutral analogues of the cations 6⁺and 8⁺(eq 1) and

again possess 1,3-migrated SiMe₃groups. The isocyan-

ates have reacted in such a way that the N atoms are

bound to the metal. The NMR data are consistent with

the proposed structures in the same manner as de-

scribed above for the cationic tungsten systems. Fur-

thermore the niobium compound 15 has been structur-

ally characterized. Crystals of 15 contain four indepen-

dent molecules in the asymmetric unit. There are no

substantial differences between them. The molecular

structure of one of the crystallographically independent

i

except in the case of the bulky ArNCO (Ar ) 2,6-C₆H₃-

Pr₂), which formed a rather thermally unstable product,

which was assigned the structure 20a (Chart 2) on the

1

basis of available NMR data (300 MHz H frequency)

at the time. In light of the new results described above

for Nb, Ta, and W imido compounds supported by N₂N_py

the structure 20a appeared questionable, and we have

2372 Organometallics, Vol. 24, No. 10, 2005

Table 2. Selected Bond Lengths (Å) and Angles (deg) for

Ward et al.

Nb(N^tBu){MeC(2-C₅H₄N)(CH₂NSiMe₃)(CH₂NC(OSiMe₃)N^tBu)}Cl (15)^a

Nb(1)-N(1)

Nb(1)-N(2)

Nb(1)-N(3)

Nb(1)-N(4)

Nb(1)-N(5)

Nb(1)-Cl(1)

C(5)-N(2)

1.771(4)

2.267(4)

2.148(4)

2.468(4)

2.006(4)

2.4488(13)

1.302(6)

1.335(6)

[1.774(4)]

[2.280(4)]

[2.150(4)]

[2.449(4)]

[2.006(4)]

[2.4531(14)]

[1.296(6)]

[1.334(6)]

[1.776(4)]

[2.281(4)]

[2.149(4)]

[2.453(4)]

[2.010(4)]

[2.4492(14)]

[1.299(7)]

[1.344(7)]

[1.776(4)]

[2.287(4)]

[2.148(4)]

[2.447(4)]

[2.008(4)]

[2.4575(13)]

[1.308(6)]

[1.330(6)]

C(5)-N(3)

Cl(1)-Nb(1)-N(1)

Cl(1)-Nb(1)-N(2)

N(1)-Nb(1)-N(2)

Cl(1)-Nb(1)-N(3)

N(1)-Nb(1)-N(3)

N(2)-Nb(1)-N(3)

Cl(1)-Nb(1)-N(4)

N(1)-Nb(1)-N(4)

N(2)-Nb(1)-N(4)

N(3)-Nb(1)-N(4)

Cl(1)-Nb(1)-N(5)

N(1)-Nb(1)-N(5)

N(2)-Nb(1)-N(5)

N(3)-Nb(1)-N(5)

N(4)-Nb(1)-N(5)

Nb(1)-N(1)-C(10

97.09(14)

[95.83(15)]

[96.78(15)]

[100.43(11)]

[95.59(18)]

[153.76(12)]

[101.95(18)]

[59.82(16)]

[84.2(1)]

[172.80(17)]

[77.23(14)]

[75.03(15)]

[105.80(13)]

[105.36(19)]

[143.88(17)]

[86.86(18)]

[81.16(15)]

[168.1(4)]

[93.84(13)]

[101.84(11)]

[97.43(17)]

[154.66(12)]

[104.87(17)]

[59.43(15)]

[83.15(11)]

[173.42(17)]

[77.56(14)]

[76.40(15)]

[107.38(13)]

[104.17(18)]

[142.02(16)]

[84.76(16)]

[82.34(15)]

[166.7(4)]

100.29(11)

97.00(18)

152.10(11)

104.12(18)

59.66(15)

83.13(9)

[101.61(11)]

[97.02(17)]

[153.66(12)]

[104.02(18)]

[59.31(15)]

[82.97(11)]

[173.40(18)]

[76.93(15)]

[75.32(15)]

[106.58(13)]

[104.00(19)]

[142.55(16)]

[85.43(16)]

[82.54(16)]

[168.6(4)]

173.87(17)

76.96(14)

73.86(14)

106.03(13)

103.95(19)

143.71(16)

86.39(16)

81.81(16)

168.0(4)

^aThe values in brackets refer to the other crystallographically independent molecules in the asymmetric unit.

Chart 2

therefore re-evaluated the reaction of Ti(N^tBu)(N₂N_py)-

(py) with ArNCO. Careful re-examination of the NMR

data (in particular the high-field ROESY and HMBC

spectra which were not available to us at the time of

the previous studies) have allowed us to reassign the

structure as 20b (Chart 2). Thus d⁰20b and 12⁺are

valence isoelectronic with two structural differences:

the imido ligand in 12⁺is trans to the pyridyl donor,

whereas in 20b it is cis; in 12⁺the migrated SiMe₃

group lies toward the “front” of the complex (i.e.,

proximal to the pyridyl H⁶atom), whereas in 20b the

SiMe₃lies more toward the “rear” according to the

ROESY spectra.

Computed Reaction Mechanism with CO₂. In the

next part of this contribution we describe a detailed DFT

study of the reactions of the group 5 and 6 diamide-

supported imido compounds with CO₂and isocyanates.

The bonding in imido compounds in general is well

understood, especially with regard to aspects of their

ground state electronic structures.^9,27However, there

have been relatively few computational (and in particu-

lar DFT) studies of the reactions of imido complexes

described to date in which reaction site selectivity has

been explored.⁴¹

The reaction between the cation [W(NPh)(N₂N_py)Me]⁺

(2⁺) and CO₂can potentially occur at three different

sites: the WdN_imide, the W-Me, and the W-N_amide

bonds (Chart 1, Scheme 3). As CO₂is a poor ligand, the

main interaction expected upon reaction of CO₂with 2⁺

is a donation from an occupied MO of the complex into

the LUMO of CO₂. The shape and energy of the occupied

frontier orbitals of 2⁺are thus indicative of the potential

reactivity of the latter with CO₂. We have previously

shown that the two highest occupied MOs of 2⁺are

essentially nonbonding combinations of the N_amidelone

pairs lying in the W-Me bonding equatorial plane.²⁷

The π-bonding MOs of the WdN_imidelinkage are at

significantly lower energy. It is thus expected that the

preferred kinetic reaction between 2⁺and CO₂is the

insertion at a W-N_amideor W-Me bond. In reality,

insertion into the W-N_amideoccurs exclusively, followed

by a 1,3-SiMe₃migration.

(37) Orde, G. Part II Thesis: Honours School of Natural Science:

Chemistry; University of Oxford, 2003.

(38) See for example: Guiducci, A. E.; Cowley, A. R.; Skinner, M.

E. G.; Mountford, P. J. Chem. Soc., Dalton Trans. 2001, 1392.

(39) Allen, F. H.; Kennard, O. Chem. Des. Autom. News 1993, 8, 1

& 31.

(40) Fletcher, D. A.; McMeeking, R. F.; Parkin, D. J. Chem. Inf.

Comput. Sci. 1996, 36, 746.

(41) (a) Jensen, V. R.; Borve, K. J. Chem. Commun. 2002, 542. (b)

Jensen, V. R.; Borve, K. J. Organometallics 2001, 20, 616. (c) Monteyne,

K.; Ziegler, T. Organometallics 1998, 17, 5901.

(34) Campion, B. K.; Heyn, R. H.; Tilley, T. D. Organometallics 1993,

12, 2584.

(35) Gerlach, C. P.; Arnold, J. Inorg. Chem. 1996, 35, 5770.

(36) Cole, M. L.; Junk, P. C. Dalton Trans. 2003, 2109.

Diamide-Supported Imido Complexes

Organometallics, Vol. 24, No. 10, 2005 2373

Scheme 3. Schematic Representation of the

Three Different Insertion Reactions Possible

between 2⁺and CO₂: Insertion into the W-N_amide

Bond (AmInt), Insertion into the W-Me Bond

(MeProd), and Cycloaddition to the WdN_imide

π-Bond (ImProd)^a

Figure 2. Optimized geometry (B3PW91) for the three

transition states associated with the three different inser-

tion/cycloaddition reactions of CO₂with 2_qm. Energy values

(Gibbs free energy values in parentheses) are given relative

to separated reactants 2_qmand CO₂in kJ‚mol^-1

.

in AmTS1_qm), but the Me group is not strictly trans to

N1 and the W-C1 bond only slightly lengthens (2.161

Å in 2_qmvs 2.180 Å in AmTS1_qm). The WdN_imidelinkage

is unaffected, and so the stabilizing influence through

π-donation from N3 to W is maintained in AmTS1_qm

.

Furthermore, the transition state structure is relatively

early on the pathway to the product, as illustrated by

the long W‚‚‚O1 (2.347 Å) and C2‚‚‚N1 (2.226 Å)

^aThe numbering scheme shown will be used systematically

in the theoretical study. For the clarity of the representation,

the group N3-Ph has been omitted in AmInt.

distances. This is due to the shape of the HOMO of 2_qm

,

effectively a nonbonding lone pair on N1 ready to

interact with CO₂. Because of the nonbonding character

of this lone pair, no large destabilization occurs when

the W-N1 bond is elongated from 1.928 Å in 2_qmto

2.075 Å in AmTS1_qmto accommodate the CO₂insertion.

The situation is rather different for the insertion into

the W-Me bond (MeTS_qmin Figure 2). Here the W-C1

bond is significantly elongated (2.161 Å in 2_qmvs 2.343

Å in MeTS_qm), and to achieve better overlap with the

LUMO of CO₂, the C1‚‚‚C2 distance has to be short and

the CH₃group must be tilted to put the “sp³lone pair”

in the proper direction for interaction. The π-donation

from the imido and amido ligands is preserved in

MeTS_qm(see geometric parameters in Table S1), and

the destabilization is mainly associated with the stretch-

ing of the W-Me bond to achieve insertion.

Finally, the least preferred pathway corresponds to

cycloaddition to the WdNPh bond with a higher energy

barrier to overcome (Figure 2). We have shown²⁷that

the geometry of 2⁺results from a subtle interplay be-

tween π-donating properties of the ligands and steric

repulsion between the ligands. The electronic preference

is to have the good π-donor imido ligand trans to pyridyl,

and therefore the associated π-bond bonding MOs are

rather low-lying and less available for reaction. This ex-

plains the high energy barrier associated with the cyclo-

addition and is reflected by some important geometric

changes in ImTS_qm(see Table S1). Most noticeably, one

π-bond to W is lost, as illustrated by the lengthening of

W-N3 (1.747 Å in 2_qmvs 1.823 Å in ImTS_qm) and the

tilting of the imido substituent (W-N3-Ph ) 150.7° in

ImTS_qm). Also the C2‚‚‚N3 distance in ImTS_qmis much

shorter than the corresponding one (C2‚‚‚N1) in

AmTS1_qm(1.892 vs 2.226 Å) to achieve better overlap.

The products for the three reactions are shown in

Figure 3 along with their relative energies with respect

To study in more detail these competitive pathways,

DFT(B3PW91) and ONIOM(B3PW91:UFF) calculations

have been carried out. In the DFT calculations the

ligand N₂N_pyhas been modeled by HC(2-C₅H₄N)(CH₂-

NSiH₃)₂(abbreviated as N₂N_qm), and the missing Me

groups have been treated at the MM level (UFF) in the

ONIOM calculations. The QM part of the ONIOM

calculations is the same as that for the DFT-only

calculations. CO₂has been explicitly treated at the DFT

level in all calculations.

Electronic Effects: B3PW91 Calculations. The

cation [W(NPh)(N₂N_qm)(Me)]⁺(2_qm) can react at three

different sites with CO₂(Scheme 3), and the corre-

sponding transition states have been located on the

potential energy surface (PES) and are shown in Figure

2. The energies (with ∆_rG values in parentheses) are

expressed (kJ‚mol^-1) with respect to the separated

reactants 2_qmand CO₂. Table S1 in the Supporting

Information lists some selected geometric parameters

for the three TS structures and also for 2_qmas a

reference.

The transition state (TS) corresponding to the inser-

tion into the W-N_amidebond, AmTS1_qm, is at a signifi-

cantly lower energy than the two other TS structures

(31.5 kJ‚mol^-1for AmTS1_qmvs 88.8 and 106.6 kJ‚mol^-1

for MeTS_qmand ImTS_qm, respectively) and is therefore

associated with the preferred pathway, in excellent

agreement with the experimental results and the quali-

tative frontier MO analysis.

The insertion into the W-N_amidebond is easier than

insertion into the W-Me bond or cycloaddition at the

WdN_imidebond because less structural reorganization

is necessary to accommodate the incoming ligand. The

N1-W-C1 angle (numbering for the atoms is shown

in Scheme 3) opens up by ca. 30° (121.9° in 2_qmvs 149.2°

2374 Organometallics, Vol. 24, No. 10, 2005

Ward et al.

Figure 3. Optimized geometry (B3PW91) for the three

products associated with the three different insertion

reactions of CO₂with 2_qm. Energy values (Gibbs free energy

values in parentheses) are given relative to separated

reactants 2_qmand CO₂in kJ‚mol^-1

.

Figure 4. Optimized geometry (B3PW91) for the transi-

tion state and the product associated with the migration

of SiH₃from N to O. Energy values (Gibbs free energy

values in parentheses) are given relative to separated

to the reactants. A comprehensive listing of geometric

parameters is given in Table S1 in the SI. The cycload-

dition reaction giving ImProd_qmis very endothermic

(∆_rG ) +123.7 kJ‚mol^-1), and this product is very

unlikely to be formed. The high energy of ImProd_qm

results from the loss of π-donation from N3, as this atom

is now involved in the metallacycle. This is illustrated

by the lengthening of W-N3 (1.747 Å in 2_qmvs 2.002 Å

in ImProd_qm) and the tilting of the phenyl group

attached to N3 (168.7° in 2_qmvs 140.9° in ImProd_qm).

The Me ligand is also significantly out of the equatorial

plane with a slightly longer W-C1 bond, which could

reactants 2_qmand CO₂in kJ‚mol^-1

.

the principal contribution to the -T∆_rS value stems

from the loss of translational degrees of freedom and

amounts to ca. 40 kJ‚mol^-1at 298 K.⁴³In the present

case, upon formation of the four-membered ring, some

rotational degrees of freedom of CO₂are lost and these

modes have large entropy contributions. The same

observation holds for ImProd_qm, where the -T∆_rS

value is also close to +60 kJ‚mol^-1at 298 K (Figure 3).

The complex AmInt_qmis less stable than the reac-

tants by ca. 10 kJ‚mol^-1and would therefore be present

in trace amounts (less than 2%), explaining why the

proposed real intermediate 3a⁺(Chart 1) is not observed

experimentally. However, this complex can rearrange

to a more stable complex, AmProd_qm, via a 1,3-

migration of the SiH₃group from N1 to O2, as il-

lustrated by the transition state AmTS2_qm(Figure 4).

The activation barrier of 105.6 kJ‚mol^-1(∆_rG^#) 102.2

kJ‚mol^-1) results from the bridging position of SiH₃in

AmTS2_qmwith two bond distances of intermediate

values (Si1-N1 ) 2.182 Å and Si1-O2 ) 2.059 Å; for

further details see Table S2 in the SI). The O2 atom

slightly moves out of the plane of the four-membered

ring (W-N1-C2-O1) to achieve better interaction with

Si1 in the transition state (dihedral angle N1-C2-O1-

O2 ) 164°). In terms of formal hybridization, N1

changes from an sp³amine function in AmInt_qmto an

sp²imine/amide one in AmProd_qm. Consequently, the

N1-C2 bond distance shortens from the value for a

single bond in AmInt_qm(1.483 Å) to that with an

increased double-bond character in AmProd_qm(1.318

Å). We have also searched for a concerted pathway

leading directly from 2_qmto AmProd_qmthrough a

transition state structure with a semichair conforma-

tion, as represented schematically below, but all our

attempts have failed and, in every case, the TS obtained

also contribute to the high energy of ImProd_qm

.

The insertion reaction into the W-Me bond is very

exothermic (∆_rG ) -94.8 kJ‚mol^-1) and leads to the η²-

acetato complex MeProd_qm(Figure 3). From MeTS_qm

we were not able to locate any intermediate (e.g., an

η¹-acetato) and MeProd_qmwas the only structure

obtained. The thermodynamic stability of MeProd_qm

is due to the ligand environment with six hard π-donor

ligands attached to the hard d⁰W^VIcenter. However,

the kinetic barrier to form MeProd_qmis too high (∆_rG^#

) +137.5 kJ‚mol^-1) and, also, another lower energy

pathway is available. This explains why, despite its

thermodynamic stability, MeProd_qmis not observed

experimentally.

CO₂insertion into the W-N_amidebond is slightly

endothermic (∆_rG ) +9.6 kJ‚mol^-1) and leads to the

complex AmInt_qm(Figure 3). In the absence of unfavor-

able entropy contributions, the reaction is actually

favored (∆E ) -51.8 kJ‚mol^-1). However, the steric

strain introduced in the modified N₂N_qmligand by the

formation of the four-membered ring and also the

lengthening of W-N1 associated with the new sp³

hybridization of the nitrogen atom N1 mitigate against

as large a stabilization as in MeProd_qm. Consequently,

the thermodynamic instability of AmInt_qmis entropi-

cally driven with a -T∆_rS value of ca. +60 kJ‚mol^-1at

298 K (we assume that the E and H values are close).⁴²

Usually, for a coupling reaction between two molecules,

(42) The computed ∆H^#and ∆H values were 31.8 and -45.1 kJ mol

for AmTS1_qmand AmInt_qm, respectively. See http://www.gaussian-

.com/g_whitepap/thermo/thermo.pdf for details about thermochemistry

calculations within Gaussian.

was AmTS1_qm

.

(43) Watson, L. A.; Eisenstein, O. J. Chem. Educ. 2002, 79, 1269.

Diamide-Supported Imido Complexes

Organometallics, Vol. 24, No. 10, 2005 2375

Steric Effects: ONIOM(B3PW91:UFF) Calcula-

tions. The DFT study has allowed the identification of

the preferred reaction pathway as the two-step process

of CO₂insertion into the W-N_amidebond, followed by N

to O SiMe₃migration (Figure 5). We have already shown

that the electronic structure of 2⁺and related systems

is influenced by the steric demand of the ligands in a

very subtle way.²⁷It was thus important to check if the

steric interactions in the real system could alter the

relative positions of the various reaction pathways. To

this end we have performed ONIOM(B3PW91:UFF)

calculations on the experimental system where the QM

part was the same as in the DFT study and the

“missing” Me groups (SiMe₃and Me attached to bridging

C in N₂N_py) were treated at the MM level (UFF). The

ONIOM energies (kJ‚mol^-1, evaluated from separated

reactants) are given in brackets in Figure 5 for a direct

comparison with the results of the B3PW91 study.

Interestingly, despite many attempts, we were not able

to locate at the ONIOM level a transition state structure

for the CO₂cycloaddition to WdNPh. We always ob-

served dissociation of CO₂. The reaction at the WdN_imide

linkage is thus highly disfavored on both electronic and

steric grounds.

Figure 5. Comparison between the three different path-

ways computed at the DFT(B3PW91) (energies in kJ‚mol^-1

)

and ONIOM(B3PW91:UFF) (values in brackets) levels for

the reaction of 2_qm(2_oniomin the ONIOM study) with CO₂.

Overall, the picture provided by the ONIOM calcula-

tions is the same as that obtained with the DFT study.

The preferred pathway remains the insertion into the

W-N_amidebond followed by a 1,3-SiMe₃migration. Due

to the steric repulsions, all the extrema located at the

ONIOM level are destabilized with respect to separated

reactants when compared to the extrema at the DFT

level. The insertion products MeProd_oniomand Am-

Table 3. Energies (kJ‚mol^-1, with Respect to

Separated Reactants) for the Various Extrema

along the Preferred Pathway for CO₂Reaction

with 2^+a

AmTS1

AmInt

AmTS2

AmProd

qm

oniom

ONIOM

31.5

37.0

43.2

-51.8

-37.2

-49.0

53.8

80.0

56.1

-109.1

-104.9

-129.5

Prod_oniomare only slightly destabilized (ca. 5 kJ‚mol^-1

,

^aDFT(B3PW91) results are expressed as qm, while for the

ONIOM(B3PW91:UFF) results, oniom corresponds to the situation

where the migrating SiMe₃group is modeled by SiH₃in the QM

part and ONIOM to the situation where it is fully considered in

the QM part.

Figure 5) because the bulky ligands are either far from

the site of reaction (MeProd_oniom) or pushed away from

the metal center (AmProd_oniom). When CO₂is closer

to the SiMe₃groups, then the destabilization is larger,

as illustrated by the 15-25 kJ‚mol^-1energy increase

for AmInt_oniom

AmTS2_oniom

,

ImProd_oniom

,

MeTS_oniom

,

and

SiMe₃are shown in Table S2 of the SI for a comparison

with the DFT values (AmTS2_oniom, SiH₃in QM part;

AmTS2_ONIOM, SiMe₃in QM part). A direct comparison

of the energies associated with the preferred pathway

obtained with the different methods is given in Table

3. When no significant Si‚‚‚O interaction is present (i.e.,

in AmTS1 and AmInt), the results are similar what-

ever the method used. In the transition state associated

with 1,3-silyl migration, one Si-N bond is broken while

one Si-O bond is made at the same time. Hydrogen

atoms are not efficient at stabilizing a SiR₃⁺group; thus

distances to Si have to remain substantially short in

.

The transition state associated with the SiMe₃migra-

tion, AmTS2_oniom, is destabilized by more than 25

kJ‚mol^-1(with respect to the DFT value) when the steric

bulk is introduced. We suspected that the electronic

influence of the migrating SiMe₃group could be a factor

to consider more carefully in the reaction pathway

leading to the observed product. In fact, in all calcula-

tions (DFT and ONIOM) the SiR₃groups (DFT, R ) H;

ONIOM R ) Me) were modeled by SiH₃in the QM

calculations. To test the electronic influence of the

precise nature of R, the reaction pathway leading to

AmProd was computed at the ONIOM(B3PW91:UFF)

level but with the migrating SiMe₃entirely considered

in the QM part (the other SiMe₃being kept as SiH₃in

the QM part).

the TS (Si‚‚‚N ) 2.182 Å, Si‚‚‚O ) 2.059 Å for AmTS2_qm

;

Si‚‚‚N ) 2.306 Å, Si‚‚‚O ) 2.114 Å for AmTS2_oniom).

When the migrating SiMe₃group is entirely treated at

the DFT level, the stabilizing electronic inductive effect

of the methyl groups is included. In the transition state,

the Si‚‚‚N distance can thus be significantly longer

(2.422 Å) without introducing severe destabilization.

Key geometric parameters for the transition states

optimized at the ONIOM level for the migration of

2376 Organometallics, Vol. 24, No. 10, 2005

Ward et al.

Scheme 4. Schematic Representation of the Two

Different Insertion Reactions Studied between 2⁺

and RNCO: Insertion into the W-N_amideBond with

Formation of a W-N Bond (N-AmInt); Insertion

into the W-N_AmideBond with Formation of a W-O

Bond (O-AmInt)^a

Figure 6. Comparison between the two insertion-SiMe₃

migration pathways computed at the DFT(B3PW91) level

(energies in kJ‚mol^-1) and at the ONIOM(B3PW91:UFF)

(values in brackets) for the reaction of 2_qm(2_oniomin the

ONIOM study) with RNCO (R ) Me in the DFT study and

t

R ) Bu in the ONIOM study).

tronic features of the reaction. Then the ONIOM-

(B3PW91:UFF) method was used to estimate the effect

of the bulkiness of the isocyanate R group (R ) Bu

only).

^aThe numbering scheme shown will be used systematically

in the theoretical study. For the clarity of the representation,

the group N3-Ph has been omitted in the insertion products

N-AmInt and O-AmInt.

t

Electronic Effects: 2_qm+ MeNCO. The two path-

ways, computed at the B3PW91 level, are shown

schematically in Figure 6. The main conclusion is that

the path associated with the formation of a W-N bond

(N-AmInt) is electronically favored over the other one

(O-AmInt). In particular, the existence of a precomplex

(N-Adduct) indicates that an initial stabilizing W‚‚‚N5

interaction orientates the regiochemistry of the insertion

(numbering of the atoms is shown in Scheme 4). RNCO

are not as poor ligands as CO₂, and the HOMO is higher

in energy and heavily weighted on N (although the

steric effects of the N substituent should not be ne-

glected). Also, the thermodynamic stability of the inser-

tion products N-AmInt_qm(∆_rG ) -52.8 kJ‚mol^-1) or

O-AmInt_qm(∆_rG ) -21.1 kJ‚mol^-1) is significantly

larger than the corresponding intermediate for CO₂

(AmInt_qm, ∆_rG ) 9.6 kJ‚mol^-1), thus explaining why

they are observed experimentally (real systems 5⁺, 7⁺,

and 11⁺) while no intermediate was seen with CO₂.

Views of the extrema located along both pathways are

shown in Figures 7 and 8, and selected geometric

parameters are given in the SI (Tables S3 and S4). The

precomplex N-Adduct_qmis not thermodynamically

stable (∆_rG ) 38.6 kJ‚mol^-1), but it lowers the activation

barrier for insertion by creating a stabilizing W‚‚‚N

interaction. In the transition states for insertion

N-AmTS1_qmand O-AmTS1_qm, the bond to W (W-N5

) 2.278 Å and W-O1 ) 2.208 Å, respectively) is

elongated by 10% with respect to that of the corre-

sponding insertion products (W-N5 ) 2.071 Å and

W-O1 ) 2.016 Å, respectively). The corresponding

value was 15% for the reaction with CO₂. The develop-

ing N1‚‚‚C2 bond in the TS for insertion (N-AmTS1_qm

and O-AmTS1_qm) is 61.5% and 46.5% longer than in

the corresponding products (N-AmInt_qmand

O-AmInt_qm), respectively. The corresponding value for

Similarly, the Si‚‚‚O distance is not necessarily short

in the TS (2.252 Å). This rather “remote” position of the

migrating SiMe₃group in the TS originates from the

stabilizing influence of the Me substituents, which is

only fully accounted for when SiMe₃is in the QM part.

The other consequence of this geometry is a diminution

of the steric repulsions. All these aspects explain the

value of 56.1 kJ‚mol^-1(Table 3) obtained for the energy

of AmTS2_ONIOM. The electronic influence of SiMe₃vs

SiH₃is clearly illustrated by the energy of the product

of migration with an increase by 25 kJ‚mol^-1in the

exothermicity of the overall reaction leading to Am-

Prod_ONIOM(Table 3).

In conclusion, in the case of the cationic tungsten

complex 2⁺, upon reaction with CO₂, there is one

reaction pathway exclusively followed. It consists of the

insertion of CO₂into the W-N_amidebond, followed by

SiMe₃migration from N to O. The intermediate is not

expected to be observed, fully in accord with the

experimental observations.

Computed Reaction Mechanism for RNCO. Re-

action of 2⁺with RNCO follows the same general

mechanism as for CO₂: insertion into the W-N_amide

bond, followed by SiMe₃migration. However, in the case

of RNCO, the heterocumulene is not symmetric and the

reaction may proceed with two different regiochemis-

tries of insertion (Scheme 4). Experimentally, when R

t

) Bu, p-tolyl, or phenyl, the insertion proceeds with

formation of a W-N bond, whereas a W-O bond is

i

formed when R ) 2,6-C₆H₃Pr₂. The insertion reaction

to form N-AmInt (Scheme 4) therefore seems to be the

preferred pathway unless the isocyanate substituent is

very bulky. A computational study of the two pathways

was carried out at the DFT(B3PW91) level for the model

systems 2_qmand MeNCO to determine the main elec-

Diamide-Supported Imido Complexes

Organometallics, Vol. 24, No. 10, 2005 2377

Figure 7. Optimized geometry (B3PW91) for the extrema along the pathway for insertion of MeNCO into the W-N_amide

bond of 2_qmwith formation of a W-N bond. Energy values (Gibbs free energy values in parentheses) are given relative to

separated reactants 2_qmand MeNCO in kJ‚mol^-1

.

Figure 8. Optimized geometry (B3PW91) for the extrema along the pathway for insertion of MeNCO into the W-N_amide

bond of 2_qmwith formation of a W-O bond. Energy values (Gibbs free energy values in parentheses) are given relative to

separated reactants 2_qmand MeNCO in kJ‚mol^-1

.

insertion of CO₂is 50%. This indicates that N-AmTS1_qm

is an earlier TS than O-AmTS1_qmand is thus associ-

ated with a lower activation barrier (49.4 vs 76.5

kJ‚mol^-1), as both reactions are exothermic. Inter-

estingly, the activation barrier corresponding to

O-AmTS1_qmis very similar to that for AmTS1_qmin the

CO₂insertion (76.5 vs 81.5 kJ‚mol^-1). In both cases the

N1‚‚‚C2 bond is longer than the value in the product

by 50%, but the W‚‚‚O distance is 10% and 15% longer,

respectively. This seems to indicate that W‚‚‚O interac-

tion in the TS is not a determining factor because O is

not a good enough donor. Thus the regiochemistry of

the insertion is essentially driven by the initially

stabilizing W‚‚‚N interaction resulting from donation

from the MeNCO HOMO to W. However, the activation

barrier corresponds to the bond-forming process be-

tween N1 and C2.

The second step of the insertion is SiMe₃migration

from N1 to either O1 (N-AmTS2_qm) or N5 (O-AmTS2_qm),

very similar to the reaction in the case of CO₂. The

migration from N1 to N5 (O-AmTS2_qm) is easier than

that from N1 to O1 (N-AmTS2_qm) with an activation

barrier ∆_rG^#(estimated from the intermediate) of 62.2

and 89.4 kJ‚mol^-1, respectively. The corresponding ∆_rE^#

values (Figure 6) are very close to the ∆_rG^#values (66.9

and 90.2 kJ‚mol^-1, respectively). Experimentally, the

activation barrier for N1 to N5 1,3-SiMe₃migration

associated with the formation of 12⁺from 11⁺was

evaluated to be ∆_rG^#) 93.8 ( 5 kJ‚mol^-1at 298 K. Thus

the calculated value of 62.2 kJ‚mol^-1is only in qualita-

tive agreement with the experimental results. However,

the experimental system corresponds to a bulky group

i

(2,6-C₆H₃Pr₂), whereas the calculated system was MeN-

CO reacting with 2_qm, in which the actual steric bulk

of both the isocyanate and the SiMe₃group is missing.

It is therefore important to take into account the sterics

of the system (and probably also solvent reorganization

effects for these ionic compounds). The DFT study has,

however, shown that there is indeed an electronic origin

in the regiochemistry of the insertion and that both the

insertion intermediate and 1,3-migrated products are

more stable than the reactants, accounting for their

experimental observation.

Steric Effects: 2_oniom+ ^tBuNCO. The two reaction

pathways were computed at the ONIOM(B3PW91:UFF)

level, and only ^tBuNCO was considered as a substrate.

In this ONIOM study, the QM parts were the same as

those in the DFT cases, and the “missing” Me groups

were added and treated at the MM level (UFF). The

ONIOM energy values are given in brackets in Figure

6. Some geometric parameters are given in the SI

(Tables S3 and S4).

The first important difference with respect to the DFT

study is the absence of a precomplex similar to N-Ad-

duct_qm. In fact the W‚‚‚N5 distance in N-Adduct_qm

(2.411 Å) has the same value in the TS optimized at

the ONIOM level, N-AmTS1_oniom. This TS for insertion

t

brings the Bu group close to the methyl ligand, and

2378 Organometallics, Vol. 24, No. 10, 2005

Ward et al.

Table 4. DFT(B3PW91) Gibbs Free Energies (kJ‚mol^-1, with Respect to Separated Reactants) for the

Various Extrema along the Three Pathways Studied for the Reaction of CO₂with M(NPh)(N₂N_qm)(Me)

(M ) W⁺, Ta)

AmTS1

AmInt

AmTS2

AmProd

MeTS

MeProd

ImTS

ImProd

W⁺

Ta

81.5

61.0

9.6

-19.5

111.8

77.2

-52.6

-50.5

137.9

92.5

-94.8

-131.3

157.4

83.5

123.7

10.4

Table 5. Selected NBO Results Obtained on the

DFT(B3PW91) Optimized Geometries for

M(NPh)(N₂N_py)(Me) (M ) W⁺, Ta)^a

steric repulsions develop, preventing the formation of

a stable adduct. The electronic preference for the

regiochemistry of the insertion is destroyed by the steric

repulsions, and the two transition states are now

isoenergetic (7.2 kJ‚mol^-1for N-AmTS1_oniom; 7.4

kJ‚mol^-1for O-AmTS1_oniom). These results qualita-

tively show that the regiochemistry of the insertion

depends on the bulk of the isocyanate substituent, even

though the electronic preference is to make a W-N bond

in the insertion product. However, a quantitative pre-

diction of which pathway is followed for a given R group

is difficult because the two possible cases are associated

with activation barriers of similar energy values. Nev-

ertheless, the fact that with a bulky ^tBu group both TSs

are of similar energy tends to indicate that the size of

the substituent is not a critical parameter and that its

size has to be considerable to alter the regiochemistry.

Hence with isocyanate N-substituents tert-butyl and

p-tolyl the RNCO insertion leading to N-AmInt is

observed (5⁺and 7⁺experimentally), whereas only with

π(M-N1)

π₁(M-N3)

π₂(M-N3)

%N

ꢀ

%N

ꢀ

%N

ꢀ

W⁺

Ta

89.2

92.7

-0.091

-0.145

73.3

78.0

-0.297

-0.194

88.6

81.7

-0.287

-0.166

^aThe π-bonds correspond to the results of the NBO search for

the best Lewis structure. %N gives in each π-bond the contribution

of the hybrid localized on N (100% p character) to the bond orbital

and ꢀ gives the energy (au) as estimated by the NBO procedure.

N1 corresponds to the amido group involved in the insertion

reaction, and N3 corresponds to the imido group.

pathway. We have therefore carried out a DFT(B3PW91)

study of the reaction of CO₂with the strictly isoelec-

tronic phenylimido compound Ta(NPh)(N₂N_qm)(Me),

where only the nature of the metal center has been

changed.

In Table 4, the Gibbs free energy values of the various

extrema located on the potential energy surface are

given. With Ta, the amide insertion/1,3-SiMe₃migration

pathway is still the kinetically preferred one, and it is

associated with lower activation barriers than with W⁺.

However, the activation barrier to cycloaddition at Md

N_imidedrops from a value of 157.4 kJ‚mol^-1(W⁺) to a

competitive value of 83.5 kJ‚mol^-1(Ta). Therefore, for

Ta, the situation is strikingly different and no single

pathway is very strongly preferred. This explains the

observation of a mixture of products. Also, the fact that

the cycloaddition is possible is in agreement with the

experimental observation of ^tBuNCO. In the case of Ta

the intermediate AmInt is thermodynamically more

stable that the separated reactants, thus adding one

possible component to the reaction mixture.

i

a very bulky group (R ) 2,6-C₆H₃Pr₂) is the other

isomer obtained (11⁺).

The size of the substituent also plays an important

role in the SiMe₃migration step. As expected the

inclusion of the Me groups on Si and in R ) ^tBu

increases the activation barrier for SiMe₃migration

from N1 to N5 in O-AmInt, as both bulky groups are

brought in close contact in the TS. There is a 30

kJ‚mol^-1increase in the value of ∆_rE^#(energy difference

between O-AmTS2_oniomand O-AmInt_oniom) with a

value computed to be 95.7 kJ‚mol^-1at the ONIOM level

(66.9 kJ‚mol^-1at the DFT level, Figure 6). For the other

regioisomer, N-AmInt, the activation barrier is slightly

lower at the ONIOM level but still of comparable value

with respect to the DFT case (83.9 kJ‚mol^-1with

ONIOM, 90.2 kJ‚mol^-1with DFT), showing that the

bulk of the R group has little influence on the SiMe₃

migration within N-AmInt.

The difference in behavior between the W⁺and Ta

complexes finds its origin in the energies of the orbitals

that play an important role in the reaction with CO₂. A

comparative natural bonding orbital (NBO)⁴⁴study of

the complexes [W(NPh)(N₂N_qm)(Me)]⁺and Ta(NPh)-

(N₂N_qm)(Me) yielded the results reported in Table 5. Due

to the cationic nature of the W complex, the bond

distances from N to the metal center are shorter (W-

N1 ) 1.928 Å, W-N3 ) 1.747 Å; Ta-N1 ) 2.015 Å,

Ta-N3 ) 1.802 Å; the numbering scheme is shown in

Scheme 3). The two π-bonds of the imido linkage are at

a significantly lower energy for the W system (Table 5)

and are thus less available for reaction with CO₂,

consistent with experimental observations for 2⁺. The

M-N_amideπ-bond (essentially a N lone pair, see Table

5) is constructed with a metal d AO also used in the

M-C bond (see below).

In conclusion of the ONIOM study, the precise bulk

of the isocyanate R group has an important influence

on the regiochemistry of the insertion reaction. How-

ever, the electronic preference for making a W‚‚‚N

interaction is strong and the R group has to be very

bulky to prevent such an approach. The isocyanate-

originated R group also has an influence on the 1,3-

SiMe₃migration in the case where the alternate regio-

isomer O-AmInt has been obtained. The repulsion

between migrating SiMe₃and the isocyanate R group

increases the activation barrier, leading to a longer lived

intermediate, as observed experimentally.

Role of the Metal: DFT Calculations with Ta.

Experimentally, the reactions of CO₂with the group 5

tert-butyl imido compounds M(N^tBu)(N₂N_py)Cl(py) (M

) Nb (13) or Ta (14)) or Ta(N^tBu)(N₂N_py)(Me) are not

as well-controlled as with [W(NPh)(N₂N_py)(Me)]⁺, and

mixtures of products are obtained. For the Cl systems

t

13 and 14, observation of BuNCO as a side product

Consequently, depending on the effective electrone-

gativity of the metal, the combinations of these three

suggests that cycloaddition to TadN^tBu is a competitive

Diamide-Supported Imido Complexes

Organometallics, Vol. 24, No. 10, 2005 2379

t

expect that for BuNCO strong steric repulsions are

likely to develop in the TS for cycloaddition, rendering

this pathway energetically prohibitive. As a matter of

fact, our attempts to optimize a TS analogous to TaN-

t

AmTS1 at the ONIOM level failed, as the BuNCO

dissociated during the optimization procedure.

Conclusions

The complexes [W(NPh)(N₂N_py)Me]⁺(2⁺), Ta(N^tBu)-

(N₂N_py)Me, M(N^tBu)(N₂N_py)Cl(py) (M ) Nb (13) or Ta

(14)), and Ti(N^tBu)(N₂N_py)(py) (19) have several poten-

tial sites of reactivity with heterocumulenes. In all cases

with isocyanates, and for 2⁺with CO₂, the reactions

exclusively proceed via insertion in M-N_amidebonds

followed by 1,3-SiMe₃migrations. For 2⁺the intermedi-

ates can be observed only in the reactions with isocy-

anates due to stronger W-N vs W-O bonding. Despite

thermodynamic preferences in the tungsten systems,

the reactions are governed by frontier orbital control and

kinetic considerations. Steric preferences (with regard

to the orientation of isocyanate substrate insertion) play

a secondary role in the reaction outcomes. In the

isoelectronic group 5 model system Ta(NPh)(N₂N_py)Me

there is a less pronounced stability of the imido bond,

which leads to less well-defined reaction pathways with

CO₂since attack at M-N_amideand MdN_imideis kineti-

cally more equally favored. The better defined reactions

of 13 and 14 with RNCO may be attributed to steric

hindrance disfavoring attack at MdN^tBu in these cases,

and this is probably also the case for the titanium

system 19.

Figure 9. Optimized geometry (B3PW91) for the transi-

tion states associated with the insertion of MeNCO into

the Ta-N_amidebond (TaN-AmTS1) or with the cycloaddi-

tion to the Ta-N_imidebond (TaN-ImTS) of Ta(N₂N_qm)-

(NPh)(Me). Energy values (Gibbs free energy values in

parentheses) are given relative to separated reactants Ta-

(N₂N_qm)(NPh)(Me) and MeNCO in kJ‚mol^-1

.

AOs may yield different results. In the case of W the

lone pair centered on N1 is at a higher energy than in

the case of Ta. Overall the more efficient π-donation in

2

_qmresults in deeper π-bonds for the imido linkage and

higher lone pair energy for the amido group in the case

of W⁺with respect to Ta. This situation selects one

pathway among the possible ones in the case of W⁺. For

Ta, the discrimination is less efficient, as the imido

π-bonds and the amido lone pairs are of comparable

energy.

Experimental Section

t

Interestingly, the reaction of 13 or 14 with BuNCO

General Methods and Instrumentation. All manipula-

tions were carried out using standard Schlenk line or drybox

techniques under an atmosphere of argon or dinitrogen.

Solvents were predried over activated 4 Å molecular sieves

and were refluxed over potassium (benzene), sodium/potassium

alloy (pentane), or calcium hydride (dichloromethane) under

a dinitrogen atmosphere and collected by distillation. Deuter-

ated solvents were dried over potassium (C₆D₆) or phosphorus

pentoxide (CD₂Cl₂), distilled under reduced pressure, and

stored under dinitrogen in Teflon valve ampules. NMR samples

were prepared under dinitrogen or Ar in 5 mm J. Young NMR

tubes.

is selective, and only insertion into the M-N_amidebond

followed by 1,3 SiMe₃migration is observed. We there-

fore studied computationally some aspects of the reac-

tion of Ta(N₂N_qm)(NPh)(Me) with MeNCO. Only DFT

calculations have been performed, and the TS for

insertion into the Ta-N_amidebond, TaN-AmTS1, and

the TS for cycloaddition, TaN-ImTS, have been located

(Figure 9). With respect to separated reactants, the

Gibbs free energies of both TS are lower than for the

reaction with CO₂(see Table 4). This is due to the

stabilizing interaction between the isocyanate nitrogen

atom and the metal center in both systems. The

electronic energy of TaN-AmTS1 is even lower than

that of the separated reactants, clearly showing the

electronic stabilization.

However, the energy difference between the two TS

structures is of the same magnitude as for the CO₂

analogue (Table 4). Therefore the selectivity in reactions

with RNCO is not related to a particular electronic effect

associated with a given site of reaction, and thus is not

accounted for by the DFT calculations. Nevertheless,

reactions with RNCO are more sensitive to the actual

bulk of the system. In the transition states AmTS1 and

ImTS associated with reactions with CO₂, the Ta‚‚‚O

bond distances are 2.514 and 2.315 Å, respectively. For

the reactions with MeNCO, the Ta‚‚‚N bond distance

in the corresponding TS, TaN-AmTS1 and TaN-ImTS,

are 2.363 and 2.238 Å, respectively. We may therefore

¹H, ¹³C{¹H}, ¹³C, ¹⁹F, and ¹¹B{¹H} NMR spectra were

recorded on Varian Mercury-VX 300 and Varian Unity Plus

500 spectrometers. ¹H and ¹³C assignments were confirmed

when necessary with the use of DEPT-135, DEPT-90, and two-

dimensional ¹H-¹H and ¹³C-¹H NMR experiments. ¹H and

¹³C spectra were 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(¹⁹F) and BF₃‚Et₂O (¹¹B).

Chemical shifts are quoted in δ (ppm) and coupling constants

in hertz.

Infrared spectra were prepared as KBr pellets or Nujol

mulls between KBr or NaCl plates and were recorded on

Perkin-Elmer 1600 and 1710 series FTIR spectrometers.

Infrared data are quoted in wavenumbers (cm^-1). Mass spectra

were recorded by the mass spectrometry service of the Uni-

versity of Oxford Chemistry Department, and elemental

analyses by the analytical services of the University of Oxford

Inorganic Chemistry Laboratory.

Literature Preparations. The compounds Nb(N^tBu)-

(N₂N_py)Cl(py) (13), Ta(N^tBu)(N₂N_py)Cl(py) (14),²⁵Ta(N^tBu)-

(N₂N_py)Me,²⁷W(NPh)(N₂N_py)Me₂(1),²⁶and [W(NPh)(N₂N_py)-

(44) Reed, A. E.; Curtiss, L. A.; Weinhold, F. Chem. Rev. 1988, 88,

899.

2380 Organometallics, Vol. 24, No. 10, 2005

Ward et al.

Me][MeBAr^F₃] (2-MeBAr^F₃),²⁷and their corresponding ¹³C- and

d-isotopomers, were prepared according to published methods.

The compound BAr^F₃was provided by DSM Research. Liquid

isocyanates and CS₂were degassed by three freeze-pump-

thaw cycles. All other compounds and reagents were purchased

from commercial suppliers and used without further purifica-

tion.

(CS), 129.0 (o-C₆H₅, ¹J(CH) ) 157 Hz), 128.9 (m-C₆H₅, ¹J(CH)

) 154 Hz), 128.2 (p-C₆H₅, ¹J(CH) ) 159 Hz), 124.7 (C,^{5 1}J(CH)

) 160 Hz), 122.4 (C³, ¹J(CH) ) 166 Hz), 65.6 (CH₂NSiMe₃,

¹J(CH) ) 137 Hz), 58.5 (C(CH₂NSiMe₃)₂), 53.7 (CH₂NCOSiMe₃,

¹J(CH) ) 138 Hz), 47.4 (WMe, ¹J(CH) ) 124 Hz), 21.6

(MeCCH₂, ¹J(CH) ) 128 Hz), 10.5 (BMe), 1.9 (NSiMe₃, ¹J(CH)

) 121 Hz), 1.3 (OSiMe₃, ¹J(CH) ) 124 Hz) ppm. ¹⁹F NMR (CD₂-

3

Cl₂, 282.2 MHz, 293 K): -133.4 (6 F, d, o-Ar^F, J ) 19.8 Hz),

[W(NPh)(MeC(2-C₅H₄N)(CH₂NSiMe₃)(CH₂NC(OSiMe₃)-

O)Me][MeBAr^F₃] (3-MeBAr^F₃). To a stirred solution of

[W(NPh)(N₂N_py)Me₂] (1) (150 mg, 0.24 mmol) in CH₂Cl₂(20

3

-165.6 (6 F, app. t, p-Ar^F, app. J ) 19.8 Hz), -168.1 (3 F, t,

m-Ar^F, ³J ) 22.7 Hz) ppm. ¹¹B{¹H} NMR (CD₂Cl₂, 160.4 MHz,

293 K): -14.7 (br s, MeBAr^F₃) ppm. IR (KBr pellet): 3064 (w),

2958 (s), 2904 (m), 2848 (w), 1640 (s), 1608 (s), 1510 (s), 1448

(s), 1384 (m), 1356 (m), 1258 (s), 1180 (w), 1170 (w), 1086 (s),

1048 (m), 1020 (m), 968 (s), 952 (s), 934 (s), 838 (s), 806 (s),

784 (m), 734 (w), 686 (m), 660 (w), 646 (w), 628 (w), 604 (w),

570 (w), 556 (w), 512 (w), 438 (w), 406 (w) cm^-1. Anal. Calcd

for C₄₂H₄₀BF₁₅N₄S₂Si₂W: C, 42.0; H, 3.4; N, 4.7. Found: C,

42.6; H, 3.4; N, 5.0.

mL) cooled to 0 °C was added a solution of BAr^F(125 mg,

3

0.24 mmol) in CH₂Cl₂(20 mL). The solution was degassed by

three freeze-pump-thaw cycles before adding CO₂(1 atm).

After stirring at room temperature for 1 h the solvent was

removed under reduced pressure to afford 3-MeBAr^Fas a

3

golden-brown solid. Yield: 96 mg (34%). ¹H NMR (CD₂Cl₂,

500.0 MHz, 293 K): 8.80 (1 H, dd, H⁶, ³J(H⁵H⁶) ) 5.9 Hz,

⁴J(H⁴H⁶) ) 1.5 Hz), 8.21 (1 H, td, H⁴, ³J(H³H⁴H⁵) ) 7.8 Hz, ⁴J

(H⁴H⁶) ) 1.7 Hz), 7.83 (1 H, d, H³, ³J(H³H⁴) ) 8.3 Hz), 7.66 (1

H, td, H⁵, ³J(H⁴H⁵H⁶) ) 5.6 Hz, ⁴J(H³H⁵) ) 1.3 Hz), 7.57 (2 H,

t, m-C₆H₅, ³J ) 7.6 Hz), 7.33 (2 H, d, o-C₆H₅, ³J ) 7.6 Hz),

[W(NPh){MeC(2-C₅H₄N)(CH₂NSiMe₃)(CH₂N(SiMe₃)C-

(O)N^tBu}Me][MeBAr^F₃] (5-MeBAr^F₃). To a stirred solution

of [W(NPh)(N₂N_py)Me₂] (1) (200 mg, 0.324 mmol) in CH₂Cl₂

3

7.26 (1 H, t, p-C₆H₅, J ) 7.6 Hz), 4.31 (1 H, d, CHHNSiMe₃,

was added a solution of BAr^F(166 mg, 0.324 mmol) in CH₂-

3

2

²J ) 16.1 Hz), 4.10 (1 H, d, CHHNSiMe₃, J ) 16.4 Hz), 3.80

Cl₂(10 mL). The resulting brown solution was stirred for 1

min before cooling to -78 °C. The reaction temperature was

maintained at -78 °C for the addition of ^tBuNCO (37 µL, 32.1

mg, 0.324 mmol) and the subsequent workup. The reaction

was stirred for 1 h before concentrating to ca. 10 mL under

reduced pressure. Cold (-78 °C) pentane (30 mL) was added,

which caused a brown oil to separate from the solution. The

supernatant was decanted, and the oil was washed with

pentane (3 × 5 mL) and dried in vacuo to yield [W(NPh){MeC-

(2-C₅H₄N)(CH₂NSiMe₃)(CH₂N(SiMe₃)C(O)N^tBu}Me][Me-

BAr^F₃] as a brown solid. Yield: 202 mg (51%). ¹H NMR

(CD₂Cl₂, 500.0 MHz, 193 K): 8.72 (1 H, d, H⁶, ³J(H⁵H⁶) ) 5.4

(1 H, d, CHHNCOSiMe₃, ²J ) 12.4 Hz), 3.08 (1 H, d,

2

CHHNCOSiMe₃, J ) 12.7 Hz), 1.68 (3 H, s, MeCCH₂), 1.40

(3 H, s, WMe, ²J(WH) ) 6.8 Hz (15%)), 0.45 (3 H, br s, BMe),

0.40 (9 H, s, NSiMe₃), 0.04 (9 H, s, OSiMe₃) ppm. ¹³C NMR

(CD₂Cl₂, 125.7 MHz, 293 K): 166.6 (CO), 162.6 (C²), 155.0

1

(ipso-C₆H₅), 151.2 (C,^{6 1}J(CH) ) 166 Hz), 148.7 (C₆F₅, J(CF)

) 239 Hz), 144.1 (o-C₆H₅, ¹J(CH) ) 160 Hz), 137.8 (C₆F₅,

1

¹J(CF) ) 251 Hz), 136.7 (C₆F₅, J(CF) ) 251 Hz), 130.9 (C,⁴

¹J(CH) ) 166 Hz), 130.2 (m-C₆H₅, ¹J(CH) ) 157 Hz), 128.7

(p-C₆H₅, ¹J(CH) ) 166 Hz), 126.4 (C,^{5 1}J(CH) ) 164 Hz), 123.4

(C³, ¹J(CH) ) 167 Hz), 66.0 (2 × MeCCH₂, ¹J(CH) ) 131 Hz),

51.3 (C(CH₂NSiMe₃)), 44.5 (WMe, ¹J(CH) ) 125.0 Hz, ¹J(WC)

) 92 Hz (15%)), 20.4 (MeCCH₂, ¹J(CH) ) 129 Hz), 10.5 (BMe,

¹J(CH) ) 117.4 Hz), 0.8 (NSiMe₃, ¹J(CH) ) 122 Hz), -1.2

(OSiMe₃, ¹J(CH) ) 120 Hz) ppm. ¹⁹F NMR (CD₂Cl₂, 282.2 MHz,

293 K): -133.4 (6 F, d, o-Ar^F, ³J ) 19.8 Hz), -165.6 (6 F, app.

3

Hz), 8.14 (1 H, t, H⁴, J(H³H⁴H⁵) ) 7.8 Hz), 7.74 (1 H, d, H³,

³J(H³H⁴) ) 8.3 Hz), 7.60 (1 H, t, H⁵, ³J(H⁴H⁵H⁶) ) 6.8 Hz),

3

7.56 (2 H, t, m-C₆H₅, J ) 7.8 Hz), 7.26 (2 H, d, o-C₆H₅, J )

7.8 Hz), 7.24 (1 H, t, p-C₆H₅, ³J ) 7.8 Hz), 4.11 (1 H, d,

CHHNSiMe₃“out”, J ) 17.1 Hz), 3.90 (1 H, d, CHHNSiMe₃

2

3

t, p-Ar^F, app. J ) 19.8 Hz), -168.1 (3 F, t, m-Ar^F, J ) 22.7

Hz) ppm. ¹¹B{¹H} NMR (CD₂Cl₂, 160.4 MHz, 293 K): -14.7

(br s, MeBAr^F₃) ppm. IR (KBr pellet): 3066 (w), 2958 (m), 2908

(m), 2852 (w), 1640 (m), 1608 (m), 1552 (s), 1510 (s), 1482 (s),

1450 (s), 1384 (m), 1356 (m), 1258 (s), 1230 (w), 1170 (w), 1146

(w), 1086 (s), 1032 (m), 1020 (m), 966 (s), 952 (s), 934 (m), 838

(s), 816 (m), 786 (m), 762 (m), 732 (w), 688 (m), 660 (w), 646

(w), 594 (w), 582 (w), 570 (w), 542 (w), 512 (w), 420 (w), 408

(w) cm^-1. Anal. Calcd for C₄₂H₄₀BF₁₅N₄O₂Si₂W: C, 43.2; H, 3.5;

N, 4.8. Found: C, 43.0; H, 3.8; N, 4.9.

“in”, ²J ) 17.1 Hz), 3.81 (1 H, d, CHHNCN^tBu “out”, ²J ) 14.2

Hz), 2.65 (1 H, d, CHHNCN^tBu “in”, ²J ) 14.2 Hz), 1.65 (3 H,

s, MeCCH₂), 1.46 (3 H, s, WMe), 1.15 (9 H, s, CMe₃), 0.30 (12

H, br s, NC(O)NSiMe₃, BMe), 0.14 (18 H, s, CH₂NSiMe₃) ppm.

¹³C{¹H} NMR (CD₂Cl₂, 125.7 MHz, 193 K): 157.9 (C²), 157.5

(CdO), 152.6 (ipso-C₆H₅), 149.9 (C⁶), 147.2 (C₆F₅, ¹J(CF) ) 239

Hz), 141.7 (C⁴), 136.6 (C₆F₅, ¹J(CF) ) 251 Hz), 135.8 (C₆F₅,

¹J(CF) ) 251 Hz), 128.5 (m-C₆H₅), 128.1 (p-C₆H₅), 125.3 (o-

C₆H₅), 124.3 (C⁵), 121.0 (C³), 63.6 (CH₂N(SiMe₃)CO), 60.9

(C(CH₂NSiMe₃)₂), 57.6 (CMe₃), 50.4 (CH₂NSiMe₃), 41.2 (WMe,

1

¹J(CH) ) 130 Hz, J(WC) ) 106 Hz (15%)), 29.3 (CMe₃), 22.2

[W(NPh)(MeC(2-C₅H₄N)(CH₂NSiMe₃)(CH₂NC(SSiMe₃)S)-

Me][MeBAr^F₃] (4-MeBAr^F₃). To a stirred solution of [W(NPh)-

(N₂N_py)Me₂] (1) (150 mg, 0.24 mmol) in CH₂Cl₂(20 mL) cooled

to 0 °C was added a solution of BAr^F₃(125 mg, 0.24 mmol) in

CH₂Cl₂(20 mL). To the stirred reaction was added CS₂(19

mg, 14.6 µL, 0.24 mmol). After stirring at room temperature

for 18 h the solvent was removed under reduced pressure to

(MeCCH₂), 9.1 (BMe), 0.9 (SiMe₃) ppm. ¹⁹F NMR (CD₂Cl₂, 282.2

3

MHz, 193 K): -135.5 (6 F, d, o-Ar^F, J ) 19.8 Hz), -166.8 (6

3

F, app. t, p-Ar^F, app. J ) 19.8 Hz), -169.7 (3 F, t, m-Ar^F, J

) 22.7 Hz) ppm. ¹¹B{¹H} NMR (CD₂Cl₂, 160.4 MHz, 193 K):

-15.0 (br s, MeBAr^F₃) ppm. IR (KBr pellet): 2964 (m), 2906

(w), 2856 (w), 1642 (m), 1608 (m), 1590 (m), 1510 (s), 1460 (s),

1384 (m), 1364 (m), 1260 (s), 1020 (w), 966 (m), 952 (m), 934

(m), 844 (s), 800 (m), 762 (m), 688 (w), 660 (w), 644 (w), 620

(w), 598 (w), 566 (w), 436 (w) cm^-1. Anal. Calcd for C₄₆H₄₉-

BF₁₅N₅OSi₂W: C, 45.2; H, 4.2; N, 5.7. Found: C, 45.1; H, 4.1;

N, 5.7.

afford 4-MeBAr^Fas a golden-brown solid. Yield: 192 mg

3

1

(66%). H NMR (CD₂Cl₂, 500.0 MHz, 293 K): 8.99 (1 H, dd,

H⁶, J(H⁵H⁶) ) 5.9 Hz, J(H⁴H⁶) ) 1.5 Hz), 8.20 (1 H, td, H⁴,

³J(H³H⁴H⁵) ) 7.8 Hz, ⁴J(H⁴H⁶) ) 1.5 Hz), 7.84 (1 H, d, H³,

³J(H³H⁴) ) 7.8 Hz), 7.65-7.58 (3 H, m, H⁵, m-C₆H₅), 7.38 (2

H, dd, o-C₆H₅, ³J ) 8.8 Hz, ⁴J ) 1.5 Hz), 7.30 (1 H, t, p-C₆H₅,

³J ) 7.3 Hz), 4.30 (1 H, d, CHHNCOSiMe₃, ²J ) 12.2 Hz), 4.23

(1 H, d, CHHNSiMe₃, ²J ) 16.1 Hz), 4.08 (1 H, d, CHHNSiMe₃,

3

4

[W(NPh)(MeC(2-C₅H₄N)(CH₂NSiMe₃)(CH₂NC(OSiMe₃)-

N^tBu)Me][MeBAr^F₃] (6-MeBAr^F₃). To a stirred solution of

[W(NPh)(N₂N_py)Me₂] (1) (150 mg, 0.24 mmol) in CH₂Cl₂(20

²J ) 16.1 Hz), 3.36 (1 H, d, CHHNCOSiMe₃, J ) 12.2 Hz),

mL) cooled to 0 °C was added a solution of BAr^F(125 mg,

2

3

2

1.75 (3 H, s, MeCCH₂), 1.30 (3 H, s, WMe, J(WH) ) 7.8 Hz

0.24 mmol) in CH₂Cl₂(20 mL). To the stirred reaction was

t

(15%)), 0.43 (3 H, br s, BMe), 0.41 (9 H, s, NSiMe₃), 0.28 (9 H,

s, OSiMe₃) ppm. ¹³C NMR (CD₂Cl₂, 125.7 MHz, 293 K): 160.3

(C²), 153.4 (ipso-C₆H₅), 150.9 (C⁶, ¹J(CH) ) 168 Hz), 148.7

added BuNCO (24 mg, 28 µL, 0.24 mmol). After stirring at

room temperature for 1 h the solvent was removed under

reduced pressure to afford 6-MeBAr^F₃as a golden-brown solid.

1

(C₆F₅, J(CF) ) 239 Hz), 142.3 (C⁴, J(CH) ) 163 Hz), 137.8

Yield: 155 mg (52%). H NMR (CD₂Cl₂, 500.0 MHz, 293 K):

(C₆F₅, ¹J(CF) ) 251 Hz), 136.7 (C₆F₅, ¹J(CF) ) 251 Hz), 130.2

8.79 (1 H, dd, H⁶, ³J(H⁵H⁶) ) 7.8 Hz, ⁴J(H⁴H⁶) ) 1.2 Hz), 8.15

Diamide-Supported Imido Complexes

Organometallics, Vol. 24, No. 10, 2005 2381

1

(1 H, td, H⁴, ³J(H³H⁴H⁵) ) 7.5 Hz, ⁴J(H⁴H⁶) ) 1.7 Hz), 7.78 (1

in 260 mg (64%) yield. H NMR (CD₂Cl₂, 500.0 MHz, 293 K):

3

H, d, H³, J(H³H⁴) ) 7.7 Hz), 7.60-7.53 (3 H, overlapping m,

8.65 (1 H, dd, H⁶, ³J(H⁵H⁶) ) 5.9 Hz, ⁴J(H³H⁶) ) 1.5 Hz), 8.18

(1 H, td, H⁴, ³J(H³H⁴H⁵) ) 7.8 Hz, ⁴J(H⁴H⁶) ) 1.5 Hz), 7.83 (1

H⁵, m-C₆H₅), 7.30 (2 H, dd, o-C₆H₅, ³J ) 8.7 Hz, ⁴J ) 1.2 Hz),

3

7.24 (1 H, t, p-C₆H₅, J ) 7.5 Hz), 4.13 (1 H, d, CHHNSiMe₃,

H, d, H³, J(H³H⁴) ) 8.3 Hz), 7.57-7.52 (3 H, overlapping m,

2

²J ) 16.1 Hz), 3.95 (1 H, d, CHHNSiMe₃, J ) 16.3 Hz), 3.38

H⁵, m-C₆H₅), 7.29 (2 H, dd, o-C₆H₅, ³J ) 8.8 Hz, ⁴J ) 1.5 Hz),

(1 H, d, CHHNCOSiMe₃, ²J ) 11.7 Hz), 3.05 (1 H, d,

7.24 (1 H, t, p-C₆H₅, J ) 7.8 Hz), 7.02 (2 H, d, m-4-C₆H₄Me,

3

2

3

CHHNCOSiMe₃, J ) 11.7 Hz), 1.66 (3 H, s, MeCCH₂), 1.20

³J ) 7.8 Hz), 6.89 (2 H, d, o-4-C₆H₄Me, J ) 8.3 Hz), 4.21 (1

2

(3 H, s, WMe, J(WH) ) 6.3 Hz (15%)), 1.08 (9 H, s, CMe₃),

H, d, CHHNSiMe₃, J ) 16.1 Hz), 4.03 (1 H, d, CHHNSiMe₃,

²J ) 16.6 Hz), 3.62 (1 H, d, CHHNCOSiMe₃, J ) 12.2 Hz),

2

0.45 (3 H, br s, BMe), 0.33 (9 H, s, NSiMe₃), 0.27 (9 H, s,

OSiMe₃) ppm. ¹³C NMR (CD₂Cl₂, 125.7 MHz, 293 K): 161.6

3.12 (1 H, d, CHHNCOSiMe₃, ²J ) 12.2 Hz), 2.25 (3 H, s,

4-C₆H₄Me), 1.69 (3 H, s, MeCCH₂), 1.21 (3 H, s, WMe, ²J(WH)

) 6.8 Hz (15%)), 0.45 (3 H, br s, BMe), 0.40 (9 H, s, NSiMe₃),

0.05 (9 H, s, OSiMe₃) ppm. ¹³C{¹H} NMR (CD₂Cl₂, 125.7 MHz,

293 K): 161.2 (CO), 160.7 (C²), 153.8 (ipso-C₆H₅), 149.2 (o-

C₆H₅, ¹J(CH) ) 168 Hz), 148.7 (C₆F₅, ¹J(CF) ) 239 Hz), 142.2

(C⁶, ¹J(CH) ) 161 Hz), 138.5 (ipso-4-C₆H₄Me), 137.8 (C₆F₅,

1

(CN^tBu), 160.6 (C²), 154.1 (ipso-C₆H₅), 148.7 (C₆F₅, J(CF) )

1

239 Hz), 142.1 (C⁶, J(CH) ) 166 Hz), 137.8 (C₆F₅, J(CF) )

251 Hz), 136.7 (C₆F₅, ¹J(CF) ) 251 Hz), 128.9 (o-C₆H₅, ¹J(CH)

1

) 159 Hz), 128.8 (m-C₆H₅, J(CH) ) 158 Hz), 127.6 (p-C₆H₅,

¹J(CH) ) 156 Hz), 127.5 (C,^{4 1}J(CH) ) 165 Hz), 124.5 (C,⁵

¹J(CH) ) 163 Hz), 121.6 (C³, ¹J(CH) ) 167 Hz), 64.4 (CH₂-

NSiMe₃, ¹J(CH) ) 135 Hz), 55.6 (CMe₃), 54.5 (C(CH₂NSiMe₃)₂),

¹J(CF) ) 251 Hz), 136.7 (C₆F₅, J(CF) ) 251 Hz), 136.1 (p-4-

1

C₆H₄Me), 130.4 (o-4-C₆H₄Me, ¹J(CH) ) 172 Hz), 130.0 (m-C₆H₅,

¹J(CH) ) 160 Hz), 129.0 (p-C₆H₅, ¹J(CH) )162 Hz), 127.1 (C,⁴

¹J(CH) ) 159 Hz), 124.7 (m-4-C₆H₄Me, ¹J(CH) ) 165 Hz),

123.8 (C⁵, ¹J(CH) ) 160 Hz), 121.9 (C³, ¹J(CH) ) 164 Hz), 64.6

(CH₂NSiMe₃, ¹J(CH) ) 138 Hz), 54.6 (CH₂NCOSiMe₃, ¹J(CH)

51.7 (CH₂NCOSiMe₃, J(CH) ) 139 Hz), 42.1 (WMe, J(CH)

1

) 123 Hz, J(WC) ) 97 Hz (15%)), 31.4 (CMe₃, J(CH) ) 126

Hz), 21.7 (MeCCH₂, ¹J(CH) ) 128 Hz), 1.9 (NSiMe₃, ¹J(CH) )

121 Hz), 0.6 (OSiMe₃, ¹J(CH) ) 122 Hz) ppm. ¹⁹F NMR (CD₂-

Cl₂, 282.2 MHz, 293 K): -133.4 (6 F, d, o-Ar^F, J ) 19.8 Hz),

3

-165.6 (6 F, app. t, p-Ar^F, app. J ) 19.8 Hz), -168.1 (3 F, t,

3

1

) 134 Hz), 51.2 ((C(CH₂NSiMe₃)₂), 43.1 (WMe, J(CH) ) 125

m-Ar^F, ³J ) 22.7 Hz) ppm. ¹¹B{¹H} NMR (CD₂Cl₂, 160.4 MHz,

293 K): -14.7 (br s, MeBAr^F₃) ppm. IR (KBr pellet): 3066 (w),

2972 (s), 2906 (s), 2854 (m), 2258 (w), 1640 (m), 1608 (m), 1584

(w), 1574 (w), 1514 (s), 1446 (s), 1394 (m), 1384 (m), 1330 (w),

1296 (w), 1260 (s), 1222 (w), 1172 (m), 1084 (s), 1046 (m), 1020

(m), 966 (m), 952 (m), 932 (m), 812 (m), 764 (m), 738 (m), 688

(m), 648 (w), 620 (w), 598 (w), 570 (w), 546 (w), 504 (w), 442

(w) cm^-1. Anal. Calcd for C₄₆H₄₉BF₁₅N₅OSi₂W‚0.3CH₂Cl₂: C,

44.5; H, 4.0; N, 5.6. Found: C, 44.5; H, 4.3; N, 5.6.

Hz, ¹J(WC) ) 93 Hz (15%)), 21.4 (MeCCH₂, ¹J(CH) ) 128 Hz),

1

20.9 (4-C₆H₄Me, J(CH) ) 128 Hz), 10.5 (BMe, J(CH) ) 116

Hz), 1.9 (NSiMe₃, ¹J(CH) ) 125 Hz), -0.2 (OSiMe₃, ¹J(CH) )

118 Hz) ppm. ¹⁹F NMR (CD₂Cl₂, 282.2 MHz, 293 K): -133.4

(6 F, d, o-Ar^F, J ) 19.8 Hz), -165.6 (6 F, app. t, p-Ar^F, app.

3

³J ) 19.8 Hz), -168.1 (3 F, t, m-Ar^F, J ) 22.7 Hz) ppm. ¹¹B-

3

{¹H} NMR (CD₂Cl₂, 160.4 MHz, 293 K): -14.7 (br s, MeBAr^F

)

3

ppm. IR (KBr pellet): 2960 (m), 2906 (m), 2856 (w), 2274 (w),

1640 (m), 1608 (m), 1568 (w), 1510 (s), 1484 (s), 1454 (s), 1384

(w), 1356 (m), 1296 (s), 1260 (s), 1170 (w), 1132 (w), 1086 (s),

1044 (w), 1030 (w), 1018 (w), 980 (m), 952 (m), 932 (w), 840

(s), 810 (m), 762 (m), 738 (w), 688 (w), 660 (w), 646 (w), 622

(w), 604 (w), 572 (w), 512 (w) cm^-1. Anal. Calcd for C₄₉H₄₇-

BF₁₅N₅OSi₂W: C, 46.8; H, 3.8; N, 5.6. Found: C, 46.4; H, 3.4;

N, 5.4.

NMR Tube Scale Synthesis of [W(NPh){MeC(2-C₅H₄N)-

(CH₂NSiMe₃)(CH₂N(SiMe₃)C(O)NTol}-Me][MeBAr^F₃] (7-

MeBAr^F₃). [W(NPh)(N₂N_py)Me₂] (1) (23.3 mg, 0.038 mmol) was

dissolved in CD₂Cl₂(0.5 mL), and this solution was added to

solid BAr^F(19.3 mg, 0.038 mmol). The resulting brown

3

solution was added to a J. Young NMR tube and cooled to -78

°C, and TolNCO (4.9 µL, 0.038 mmol) was added. The NMR

tube was sealed and placed in a precooled NMR probe and

analyzed in situ at -50 °C. ¹H NMR (CD₂Cl₂, 500.0 MHz, 223

[W(NPh)(MeC(2-C₅H₄N)(CH₂NSiMe₃)(CH₂NC(OSiMe₃)-

NPh)Me][MeBAr^F₃] (9-MeBAr^F₃). To a stirred solution of

TMS

[W(NPh)(N₂N_py)Me₂] (1) (200 mg, 0.32 mmol) in CH₂Cl₂(20

3

K): 8.21 (1 H, t, H⁴, J(H³H⁴H⁵) ) 8.1 Hz), 8.13 (1 H, d, H⁶,

mL) cooled to 0 °C was added a solution of BAr^F(166 mg,

3

³J(H⁵H⁶) ) 5.6 Hz), 7.82 (1 H, d, H³, ³J(H³H⁴) ) 8.1 Hz), 7.68

0.32 mmol) in CH₂Cl₂(20 mL) followed by PhNCO (38 mg, 35

µL, 0.324 mmol). After stirring at room temperature for 1 h

the solvent was removed under reduced pressure to afford

9-MeBAr^F₃as a golden-brown solid. Yield: 280 mg (69%). ¹H

NMR (CD₂Cl₂, 500.0 MHz, 293 K): 8.66 (1 H, dd, H⁶, ³J(H⁵H⁶)

) 5.6 Hz, ⁴J(H³H⁶) ) 1.2 Hz), 8.19 (1 H, td, H⁴, ³J(H³H⁴H⁵) )

3

(2 H, t, m-C₆H₅, J ) 7.9 Hz), 7.57 (1 H, t, H⁵, J(H⁴H⁵H⁶) )

6.7 Hz), 7.42 (2 H, d, o-C₆H₅, ³J ) 7.9 Hz), 7.34 (1 H, t, p-C₆H₅,

³J ) 7.6 Hz), 6.99 (2 H, d, m-4-C₆H₄Me, J ) 8.1 Hz), 6.94 (2

3

H, d, o-4-C₆H₄Me, ³J ) 8.1 Hz), 4.25 (1 H, d, CHHNSiMe₃, ²J

2

) 17.1 Hz), 4.03 (1 H, d, CHHNSiMe₃, J ) 17.1 Hz), 3.91 (1

2

8.0 Hz, J(H⁴H⁶) ) 1.7 Hz), 7.84 (1 H, d, H³, J(H³H⁴) ) 8.0

4

3

H, d, CHHNCO, J ) 13.9 Hz), 2.71 (1 H, d, CHHNCO, J )

13.9 Hz), 2.27 (3 H, s, 4-C₆H₄Me), 1.74 (3 H, s, MeCCH₂), 1.33

(3 H, s, WMe), 0.36 (12 H, br s, BMe and CH₂N(SiMe₃)CO),

0.28 (9 H, s, CH₂NSiMe₃) ppm. ¹³C{¹H} NMR (CD₂Cl₂, 125.7

MHz, 223 K): 157.6 (C²), 156.0 (CO), 148.6 (C⁶), 147.7 (Ar^F,

¹J ) 237 Hz), 141.6 (C⁴), 137.9 (Ar^F, ¹J ) 242 Hz), 136.9 (Ar^F,

¹J ) 243 Hz), 135.6 (ipso-4-C₆H₄Me), 135.0 (p-4-C₆H₄Me), 128.5

(m-C₆H₅), 128.3 (p-C₆H₅), 126.5 (ipso-C₆H₅), 125.2 (o-C₆H₅),

125.0 (C⁵), 124.0 (m-4-C₆H₄Me), 122.9 (o-4-C₆H₄Me), 121.3 (C³),

63.7 (CH₂SiMe₃), 51.4 (C(CH₂NSiMe₃)), 50.4 (CH₂NCO), 45.1

(WMe), 22.0 (MeCCH₂), 20.9 (4-C₆H₄Me), 9.5 (br, BMe), 1.5

(SiMe₃) ppm. ¹⁹F NMR (CD₂Cl₂, 470.4 MHz, 223 K): -135.4 (6

F, d, o-Ar^F, ³J ) 23.0 Hz), -167.4 (3 F, t, p-Ar^F, ³J ) 21.7 Hz),

Hz), 7.58-7.52 (3 H, overlapping m, H⁵, m-C₆H_5,imide), 7.29 (2

3

4

H, dd, o-C₆H_5,imide, J ) 8.3 Hz, J ) 1.2 Hz), 7.25-7.20 (3 H,

overlapping m, m-Me₃SiOCNC₆H₅, p-C₆H_5,imide), 7.10 (1 H, t,

p-Me₃SiOCNC₆H₅, ³J ) 7.6 Hz), 6.81 (2 H, d, o-Me₃SiOCNC₆H₅,

³J ) 8.3 Hz), 4.22 (1 H, d, CHHNSiMe₃, ²J ) 16.1 Hz), 4.04 (1

H, d, CHHNSiMe₃, ²J ) 16.3 Hz), 3.63 (1 H, d, CHHNCO-

SiMe₃, ²J ) 12.0 Hz), 3.14 (1 H, d, CHHNCOSiMe₃, ²J ) 12.2

2

Hz), 1.70 (3 H, s, MeCCH₂), 1.23 (3 H, s, WMe, J(WH) ) 6.5

Hz (15%)), 0.45 (3 H, br s, BMe), 0.40 (9 H, s, NSiMe₃), -0.09

(9 H, s, OSiMe₃) ppm. ¹³C NMR (CD₂Cl₂, 125.7 MHz, 293 K):

161.2 (CO), 160.7 (C²), 153.8 (ipso-C₆H_5,imide), 149.2 (o-C₆H_5,imide

,

¹J(CH) ) 161 Hz), 148.7 (C₆F₅, J(CF) ) 239 Hz), 142.2 (C⁶,

¹J(CH) ) 166 Hz), 141.1 (ipso-Me₃SiOCNC₆H₅), 137.8 (C₆F₅,

¹J(CF) ) 251 Hz), 136.7 (C₆F₅, ¹J(CF) ) 251 Hz), 129.6 (o-

Me₃SiOCNC₆H₅, ¹J(CH) ) 159 Hz), 129.5 (m-Me₃SiOCNC₆H₅,

¹J(CH) ) 160 Hz), 128.9 (o-C₆H_5,imide, ¹J(CH) ) 161 Hz), 127.1

(C,^{4 1}J(CH) ) 163 Hz), 126.0 (m-C₆H_5,imide, ¹J(CH) ) 161 Hz),

124.8 (p-Me₃SiOCNC₆H₅, ¹J(CH) ) 162 Hz), 123.9 (C,^{5 1}J(CH)

) 159 Hz), 122.0 (C³, ¹J(CH) ) 165 Hz), 64.6 (CH₂NSiMe₃,

¹J(CH) ) 137 Hz), 54.6 (CH₂NCOSiMe₃, ¹J(CH) ) 136 Hz),

51.2 (C(CH₂NSiMe₃)₂), 43.1 (WMe, ¹J(CH) ) 123 Hz), 21.4

(MeCCH₂, ¹J(CH) ) 128 Hz), 10.5 (BMe), 1.9 (NSiMe₃, ¹J(CH)

1

3

-170.2 (6 F, app. t, m-Ar^F, app. J ) 21.3 Hz) ppm. ¹¹B{¹H}

NMR (CD₂Cl₂, 160.4 MHz, 223 K): -15.0 (BMe) ppm.

[W(NPh)(MeC(2-C₅H₄N)(CH₂NSiMe₃)(CH₂NC(OSiMe₃)-

NTol)Me][MeBAr^F₃] (8-MeBAr^F₃). To a stirred solution of

[W(NPh)(N₂N_py)Me₂] (1) (200 mg, 0.32 mmol) in CH₂Cl₂(20

mL) cooled to 0 °C was added a solution of BAr^F(166 mg,

3

0.32 mmol) in CH₂Cl₂(20 mL). To the stirred solution was

added TolNCO (43 mg, 40 µL, 0.324 mmol). After stirring at

room temperature for 1 h the solvent was removed under

reduced pressure to afford 8-MeBAr^F₃as a golden-brown solid

2382 Organometallics, Vol. 24, No. 10, 2005

Ward et al.

1

) 120 Hz), -0.2 (OSiMe₃, J(CH) ) 122 Hz) ppm. ¹⁹F NMR

in a precooled NMR probe at -30 °C. On warming to room

(CD₂Cl₂, 282.2 MHz, 293 K): -133.4 (6 F, d, o-Ar^F, J ) 19.8

temperature, quantitative conversion to 12-MeBAr^Fwas

3

Hz), -165.6 (6 F, app. t, p-Ar^F, app. J ) 19.8 Hz), -168.1 (3

observed after 4 h. ¹H NMR (CD₂Cl₂, 500.0 MHz, 293 K): 8.56

(1 H, dd, H⁶, ³J(H⁵H⁶) ) 5.8 Hz, ⁴J(H⁴H⁶) ) 1.1 Hz), 8.27 (1 H,

td, H⁴, ³J(H³H⁴H⁵) ) 7.8 Hz, ⁴J(H⁴H⁶) ) 1.4 Hz), 7.89 (1 H, d,

H³, ³J(H³H⁴) ) 8.1 Hz), 7.65 (2 H, t, m-C₆H₅, ³J ) 8.0 Hz),

7.56 (1 H, t, H⁵, ³J(H⁴H⁵H⁶) ) 7.1 Hz), 7.34 (2 H, d, o-C₆H₅, ³J

3

F, t, m-Ar^F, J ) 22.7 Hz) ppm. ¹¹B{¹H} NMR (CD₂Cl₂, 160.4

3

MHz, 293 K): -14.7 (br s, MeBAr^F₃) ppm. IR (KBr pellet): 3064

(w), 2958 (m), 2908 (m), 2856 (m), 2262 (w), 1948 (w), 1642

(s), 1608 (s), 1596 (m), 1584 (m), 1510 (s), 1454 (w), 1382 (w),

1358 (m), 1296 (m), 1258 (s), 1086 (s), 1046 (m), 1020 (m), 980

(s), 834 (m), 788 (m), 688 (w), 660 (w), 646 (w), 604 (w), 594

(w), 538 (w), 474 (w), 444 (w), 420 (w), 406 (w) cm^-1. Anal.

Calcd for C₄₈H₄₅BF₁₅N₅OSi₂W: C, 45.8; H, 3.7; N, 5.6. Found:

C, 46.3; H, 3.7; N, 5.4.

3

) 7.6 Hz), 7.19 (1 H, t, p-C₆H₅, J ) 7.5 Hz), 7.00-6.94 (3 H,

i

overlapping m, 2,6-C₆H₃Pr₂), 4.39 (1 H, d, CHHNSiMe₃“out”,

2

²J ) 16.7 Hz), 4.18 (1 H, d, CHHNCO “out”, J ) 14.3 Hz),

4.13 (1 H, d, CHHNSiMe₃“in”, ²J ) 16.7 Hz), 3.08 (1 H, d,

2

CHHNCO “in”, J ) 14.3 Hz), 1.75 (3 H, s, MeCCH₂), 1.69 (2

H, br s, CHMe₂), 1.37 (3 H, s, WMe, ²J(WH) ) 8.0 Hz), 1.01 (6

H, d, CHMe₂, ³J ) 6.8 Hz), 0.85 (6 H, d, CHMe₂, ³J ) 6.8 Hz),

0.47 (9 H, s, CH₂N(SiMe₃)CO), 0.45 (3 H, br s, BMe), 0.41 (9

H, s, CH₂NSiMe₃) ppm. ¹³C{¹H} NMR (CD₂Cl₂, 125.7 MHz,

243 K): 159.2 (C², CdO), 152.0 (ipso-C₆H₅), 149.6 (C⁶), 148.0

Preparation of [W(NPh)(MeC(2-C₅H₄N)(CH₂NSiMe₃)-

(CH₂NC(SSiMe₃)N^tBu)Me][MeBAr^F₃] (10-MeBAr^F₃). To a

stirred solution of [W(NPh)(N₂N_py)Me₂] (1) (150 mg, 0.24 mmol)

in CH₂Cl₂(20 mL) cooled to 0 °C was added a solution of BAr^F

3

t

(125 mg, 0.24 mmol) in CH₂Cl₂(20 mL) followed by BuNCS

(28 mg, 31 µL, 0.243 mmol). After stirring at room temperature

(Ar^F, ¹J(CF) ) 238 Hz), 143.0 (C⁴), 138.0 (o-2,6-C₆H₃Pr₂), 137.9

i

1

(Ar^F, J(CF) ) 242 Hz), 136.9 (Ar^F, J(CF) ) 244 Hz), 130.1

for 1 h the volatiles were removed under reduced pressure to

i

afford 10-MeBAr^Fas a golden-brown solid. Yield: 157 mg

(o-C₆H₅), 129.3 (ipso-2,6-C₆H₃Pr₂), 129.2 (m-C₆H₅), 126.1 (p-

3

i

C₆H₅), 125.7 (C⁵), 124.2 (p-2,6-C₆H₃Pr₂), 122.7 (br, m-C₆H₃-

Pr₂), 120.8 (C³), 64.3 (CH₂NSiMe₃), 56.9 (CH₂NCO), 51.9

(C(CH₂NSiMe₃)), 46.9 (WMe), 27.9 (CHMe₂), 22.5 (CHMe₂),

22.3 (CHMe₂), 1.3 (CH₂NSiMe₃), -0.9 (CH₂N(SiMe₃)CO) ppm.

¹⁹F NMR (CD₂Cl₂, 470.4 MHz, 243 K): -135.2 (6 F, d, o-Ar^F,

1

(52%). H NMR (CD₂Cl₂, 500.0 MHz, 293 K): 9.01 (1 H, dd,

H⁶, J(H⁵H⁶) ) 5.8 Hz, J(H⁴H⁶) ) 1.2 Hz), 8.12 (1 H, td, H⁴,

³J(H³H⁴H⁵) ) 7.8 Hz, ⁴J(H⁴H⁶) ) 1.5 Hz), 7.74 (1 H, d, H³,

³J(H³H⁴) ) 8.1 Hz), 7.66-7.56 (3 H, overlapping m, H⁵,

m-C₆H₅), 7.40-7.33 (3 H, overlapping m, o-C₆H₅, p-C₆H₅), 4.70

3

4

³J ) 21.5 Hz), -167.7 (3 F, t, p-Ar^F, J ) 21.8 Hz), -170.4 (6

3

2

(1 H, d, CHHNSiMe₃, J ) 13.5 Hz), 4.06 (1 H, d, CHHNCO-

F, app. t, m-Ar^F, app. ³J ) 20.2 Hz) ppm. ¹¹B{¹H} NMR (CD₂-

Cl₂, 160.4 MHz, 243 K): -14.9 (BMe) ppm. IR (KBr pellet):

2962 (s), 2872 (w), 1690 (m, ν_CdN), 1640 (m), 1608 (m), 1574

(m), 1570 (s), 1454 (s), 1384 (m), 1356 (m), 1326 (w), 1298 (w),

1260 (w), 1212 (w), 1084 (s), 1022 (w), 966 (s), 952 (s), 934

(m), 840 (s), 804 (s), 804 (m), 762 (m), 736 (w), 686 (w), 660

(w), 644 (w), 594 (w), 570 (w), 556 (w), 512 (w), 464 (w), 422

(w) cm^-1. Anal. Calcd for C₅₄H₅₇BF₁₅N₅OSi₂W‚0.5CH₂Cl₂: C,

47.8; H, 4.3; N, 5.1. Found: C, 47.7; H, 4.3; N, 5.3.

SiMe₃, ²J ) 16.8 Hz), 3.90 (1 H, d, CHHNCOSiMe₃, ²J ) 16.8

Hz), 2.55 (1 H, d, CHHNSiMe₃, ²J ) 13.5 Hz), 1.70 (3 H, s,

2

MeCCH₂), 1.40 (9 H, s, CMe₃), 1.36 (3 H, s, WMe, J(WH) )

7.3 Hz (15%)), 0.43 (3 H, br s, BMe), 0.37 (9 H, s, OSiMe₃),

0.32 (9 H, s, NSiMe₃) ppm. ¹³C NMR (CD₂Cl₂, 125.7 MHz, 293

K): 160.1 (C²), 153.1 (CS), 152.9 (ipso-C₆H₅), 148.7 (C₆F₅,

¹J(CF) ) 239 Hz), 141.9 (C⁶, J(CH) ) 166 Hz), 137.8 (C₆F₅,

1

¹J(CF) ) 251 Hz), 136.7 (C₆F₅, ¹J(CF) ) 251 Hz), 130.1 (o-

C₆H₅, ¹J(CH) ) 156 Hz), 129.4 (m-C₆H₅, ¹J(CH) ) 162 Hz),

127.5 (p-C₆H₅, ¹J(CH) ) 157 Hz), 125.3 (C,^{4 1}J(CH) ) 160 Hz),

121.1 (C,^{5 1}J(CH) ) 165 Hz), 119.7 (C³, ¹J(CH) ) 167 Hz), 64.7

(CH₂NSiMe₃, ¹J(CH) ) 140 Hz), 55.5 (CH₂NCOSiMe₃, ¹J(CH)

) 137 Hz), 54.8 (CMe₃), 51.7 (C(CH₂NSiMe₃)₂), 43.4 (WMe,

¹J(CH) ) 126 Hz), 30.7 (CMe₃, ¹J(CH) ) 128 Hz), 22.9

(MeCCH₂, ¹J(CH) ) 126 Hz), 10.5 (BMe), 1.4 (NSiMe₃, ¹J(CH)

) 120 Hz), 0.0 (OSiMe₃, ¹J(CH) ) 123 Hz) ppm. ¹⁹F NMR (CD₂-

[W(NPh){MeC(2-C₅H₄N)(CH₂NSiMe₃)(CH₂NC{N(SiMe₃)-

Ar}O}Me] [MeBAr^F₃] (12-MeBAr^F₃). To a stirred solution

of [W(NPh)(N₂N_py)Me₂] (1) (200 mg, 0.324 mmol) in CH₂Cl₂

(10 mL) was added a solution of BAr^F(166 mg, 0.324 mmol)

3

in CH₂Cl₂(10 mL). The resulting brown solution was allowed

to stir for 1 min before adding a solution of 2,6-diisopropy-

lphenyl isocyanate (66 mg) in CH₂Cl₂(5 mL). The solution was

allowed to stir for 18 h before concentrating to ca. 5 mL. The

product was precipitated from the solution by the addition of

pentane (30 mL), which afforded a brown oil. The supernatant

Cl₂, 282.2 MHz, 293 K): -133.4 (6 F, d, o-Ar^F, J ) 19.8 Hz),

3

-165.6 (6 F, app. t, p-Ar^F, app. J ) 19.8 Hz), -168.1 (3 F, t,

3

m-Ar^F, ³J ) 22.7 Hz) ppm. ¹¹B{¹H} NMR (CD₂Cl₂, 160.4 MHz,

293 K): -14.7 (br s, MeBAr^F₃) ppm. IR (KBr pellet): 3396 (w),

3066 (w), 2970 (m), 2904 (m), 2850 (w), 2100 (br m), 1966 (w),

1644 (s), 1608 (m), 1552 (m), 1510 (s), 1482 (s), 1454 (s), 1382

(w), 1354 (w), 1298 (w), 1258 (s), 1200 (s), 1172 (w), 1088 (s),

1044 (w),1020 (w), 966 (s), 952 (m), 934 (m), 836 (s), 812 (w),

782 (w), 722 (w), 686 (w), 660 (w), 644 (w), 570 (w), 484 (w),

442 (w), 428 (w), 402 (w) cm^-1. Anal. Calcd for C₄₆H₄₉BF₁₅N₅-

SSi₂W: C, 44.5; H, 4.0; N, 5.7. Found: C, 44.2; H, 4.1; N, 5.6.

was decanted and the oil dried in vacuo to afford 12-MeBAr^F

3

as a brown solid. Yield: 185 mg (43%). ¹H NMR (CD₂Cl₂, 500.0

MHz, 293 K): 8.89 (1 H, d, H⁶, ³J(H⁵H⁶) ) 5.9 Hz), 8.18 (1 H,

3

4

td, H⁴, J(H³H⁴H⁵) ) 7.8 Hz, J(H⁴H⁶) ) 1.5 Hz), 7.71 (2 H,

overlapping m, H³and H⁵), 7.61 (2 H, t, m-C₆H₅, ³J ) 8.3 Hz),

7.34 (3 H, overlapping m, o-C₆H₅and p-2,6-C₆H₃Pr₂), 7.26 (2

i

H, overlapping m, p-C₆H₅and m-2,6-C₆H₃Pr₂“in”), 7.10 (1 H,

i

dd, m-2,6-C₆H₃Pr₂“out”, ³J ) 7.8 Hz, ⁴J ) 1.5 Hz), 4.20 (1 H,

d, CHHNSiMe₃“out”, ²J ) 15.6 Hz), 3.85 (1 H, d, CHHNSiMe₃

“in”, ²J ) 15.6 Hz), 2.80 (1 H, sept, CHMe₂“in”, ³J ) 6.8 Hz),

2.75 (1 H, d, CHHNCO “out”, ²J ) 13.2 Hz), 2.45 (1 H, d,

[W(NPh){MeC(2-C₅H₄N)(CH₂NSiMe₃)(CH₂N(SiMe₃)C-

(NAr)O}Me] [MeBAr^F₃] (11-MeBAr^F₃). To a stirred solution

of [W(NPh)(N₂N_py)Me₂] (1) (200 mg, 0.324 mmol) in CH₂Cl₂

2

was added a solution of BAr^F(166 mg, 0.324 mmol) in CH₂-

CHHNCO “in”, J ) 13.2 Hz), 2.11 (1 H, sept, CHMe₂“out”,

3

Cl₂(10 mL). The resulting brown solution was stirred for 1

min before cooling to -78 °C. To the cold reaction was added

2,6-diisopropylphenyl isocyanate (69.3 µL, 65.9 mg, 0.324

mmol). The mixture was again stirred for 1 h before warming

to -30 °C and concentrating to ca. 10 mL under reduced

pressure. Cold (-30 °C) pentane (30 mL) was added, which

caused a brown oil to separate from the solution, which was

washed with pentane (3 × 5 mL) and dried in vacuo to yield

³J ) 6.8 Hz), 1.36 (6 H, s, WMe and MeC(CH₂NSiMe₃), ²J(WH)

3

) 7.2 Hz), 1.30 (3 H, d, CHMe₂“in, back”, J ) 6.8 Hz), 1.09

(3 H, d, CHMe₂“in, forward”, ³J ) 6.8 Hz), 1.04 (3 H, d, CHMe₂

3

“out, forward”, J ) 6.8 Hz), 0.48 (3 H, d, CHMe₂“out, back”,

³J ) 6.8 Hz), 0.45 (3 H, br s, BMe), 0.36 (9 H, s, CH₂NSiMe₃),

0.06 (9 H, s, OCNSiMe₃) ppm. ¹³C{¹H} NMR (CD₂Cl₂, 125.7

MHz, 293 K): 169.2 (OCN-2,6-C₆H₃Pr₂), 159.8 (C²), 153.6

i

(ipso-C₆H₅), 148.8 (Ar^F, ¹J(CF) ) 240 Hz), 148.5 (C⁶), 145.7

i

11-MeBAr^Fas a brown solid. Yield: 207 mg (48%). The

(o-2,6-C₆H₃Pr₂“in”), 144.7 (o-2,6-C₆H₃Pr₂“out”), 141.9 (C⁴),

3

1

compound was also prepared and analyzed in situ by reaction

with [W(NPh)(N₂N_py)Me][MeBAr^F₃] prepared in CD₂Cl₂in a

J. Young NMR tube and cooled to -78 °C. To this was added

2,6-diisopropylphenyl isocyanate (3 mg, 3.2 µL, 0.03 mmol) also

at -78 °C. The mixture was analyzed by NMR spectroscopy

137.9 (Ar^F, J(CF) ) 242 Hz), 136.9 (Ar^F, J(CF) ) 244 Hz),

i

133.0 (ipso-C₆H₃Pr₂), 129.6 (o-C₆H₅), 129.3 (p-C₆H₅), 128.9 (m-

i

C₆H₅), 127.0 (p-2,6-C₆H₅Pr₂), 125.9 (m-2,6-C₆H₃Pr₂“in”), 125.1

i

(C⁵), 124.9 (m-2,6-C₆H₃Pr₂“out”), 122.9 (C³), 65.2 (CH₂-

NSiMe₃), 53.8 (C(CH₂NSiMe₃)), 50.6 (CH₂NCO), 45.2 (WMe,

Diamide-Supported Imido Complexes

Organometallics, Vol. 24, No. 10, 2005 2383

¹J(WC) ) 93 Hz), 28.9 (CHMe₂), 28.8 (CHMe₂), 25.3 (CHMe₂

“in, forward”), 24.7 (CHMe₂“out, back”), 24.1 (CHMe₂“in,

back”), 23.3 (CHMe₂“out, forward”), 21.1 (MeC(CH₂NSiMe₃)),

10.4 (br, BMe), 1.7 (CH₂NSiMe₃), 0.0 (OCNSiMe₃) ppm. ¹⁹F

NMR (CD₂Cl₂, 470.4 MHz, 293 K): -135.0 (6 F, d, o-Ar^F, ³J )

21.1 Hz), -168.5 (3 F, t, p-Ar^F, ³J ) 21.1 Hz), -171.2 (6 F,

app. t, m-Ar^F, app. ³J ) 21.1 Hz) ppm. ¹¹B{¹H} NMR (CD₂Cl₂,

160.4 MHz, 293 K): -14.7 (BMe) ppm. IR (KBr pellet): 2966

(m), 2906 (w), 2874 (w), 1640 (m), 1608 (m), 1590 (w), 1510

(s), 1482 (m), 1456 (s), 1404 (m), 1384 (m), 1356 (m), 1326 (w),

1296 (m), 1258 (s), 1086 (s), 1022 (w), 992 (m), 966 (m), 952

(m), 934 (m), 894 (w), 838 (s), 804 (m), 784 (w), 764 (m), 738

(w), 708 (w), 688 (w), 594 (w), 570 (w), 512 (w), 418 (w), 408

(w) cm^-1. Anal. Calcd for C₅₄H₅₇BF₁₅N₅OSi₂W‚CH₂Cl₂: C, 46.8;

H, 4.2; N, 5.0. Found: C, 46.6; H, 4.3; N, 5.3.

51.6 (CMe_3,amide), 51.5 (MeC(CH₂N)₂), 34.1 (CMe_3,imide), 31.6

(CMe_3,amide), 22.6 (MeC(CH₂N)₂), 2.5 (NSiMe₃), 0.9 (OSiMe₃)

ppm. IR (KBr pellet): 3303 (m), 2966 (s), 2898 (m), 2744 (w),

2719 (w), 2106 (w), 1868 (w), 1651 (m), 1603 (m), 1573 (m),

1520 (s), 1472 (s), 1455 (m), 1432 (s), 1391 (m), 1375 (w), 1355

(m), 1337 (w), 1326 (w), 1272 (s), 1244 (s), 1214 (w), 1168 (m),

1140 (w), 1092 (w), 1063 (s), 1022 (m), 1012 (m), 978 (m), 927

(s), 844 (s), 797 (w), 786 (w), 756 (m), 718 (w), 670 (m), 640

(w), 618 (w), 595 (w), 564 (m), 545 (w) cm^-1. High-resolution

EI mass spectrum: Found (calculated for C₂₄H₄₆ClN₅OSi₂Ta-

H): m/z ) 692.2430 (692.2410). Anal. Calcd for C₂₄H₄₇ClN₅-

OSi₂Ta: C, 41.5; H, 6.8; N, 10.1. Found: C, 41.5; H, 6.6; N,

10.0.

Nb(N^tBu){MeC(2-C₅H₄N)(CH₂NSiMe₃)(CH₂NC(OSiMe₃)-

NTol)}Cl (17). To a stirred solution of [Nb(N^tBu)(N₂N_py)Cl-

(py)] (200 mg, 0.341 mmol) in CH₂Cl₂(20 mL) was added

TolNCO (43 µL, 0.341 mmol). After stirring for 3 h at room

temperature the volatiles were removed under reduced pres-

sure to afford 17 as a yellow powder. Yield: 115 mg (52%). ¹H

NMR (CD₂Cl₂, 500.0 MHz, 293 K): 9.36 (1 H, d, H⁶, ³J(H⁵H⁶)

Nb(N^tBu){MeC(2-C₅H₄N)(CH₂NSiMe₃)(CH₂NC(OSiMe₃)-

N^tBu)}Cl (15). To a stirred solution of [Nb(N^tBu)(N₂N_py)Cl-

(py)] (260 mg, 0.444 mmol) in CH₂Cl₂(20 mL) was added

^tBuNCO (51 µL, 0.444 mmol). The yellow solution was stirred

for 3 h at room temperature. The volatiles were removed under

reduced pressure to afford a yellow powder, which was

extracted into pentanes and filtered. The volatiles were

removed under reduced pressure to afford 15 as a pale yellow

powder. Yield: 150 mg (55%). Diffraction quality crystals were

grown by slow evaporation of a pentane solution at room

3

) 5.4 Hz), 7.83 (1 H, m, H⁴), 7.52 (1 H, d, H³, J(H³H⁴) ) 8.3

Hz), 7.29 (1 H, m, H⁵), 7.05 (4 H, m, NC₆H₄Me), 3.60 (1 H, d,

CHHNSiMe₃, ²J ) 13.7 Hz), 3.47 (1 H, d, CHHNCOSiMe₃, ²J

2

) 11.7 Hz), 3.30 (1 H, d, CHHNSiMe₃, J ) 13.7), 3.02 (1 H,

d, CHHNCOSiMe₃, ²J ) 11.7 Hz), 2.25 (3 H, s, NC₆H₄Me), 1.46

(3 H, s, MeC(CH₂N)₂), 1.09 (9 H, s, CMe₃), 0.33 (9 H, s,

NSiMe₃), -0.28 (9 H, s, OSiMe₃) ppm. ¹³C{¹H} NMR (CD₂Cl₂,

125.7 MHz, 293 K): 163.5 (C²), 155.7 (CO), 150.3 (C⁶), 143.5

(p-C₆H₄Me), 139.2 (C⁴), 131.4 (i-C₆H₄Me), 129.3 (m-C₆H₄Me),

123.2 (o-C₆H₄Me), 122.5 (C⁵), 121.1 (C³), 66.8 (CMe₃), 63.3 (CH₂-

NSiMe₃), 53.2 (CH₂NCOSiMe₃), 51.2 (MeC(CH₂N)₂), 31.8

(CMe₃), 22.9 (MeC(CH₂N)₂), 20.9 (C₆H₄Me), 2.1 (NSiMe₃), 0.1

(OSiMe₃) ppm. IR (KBr pellet): 3316 (w), 3109 (w), 3072 (w),

2968 (s), 2921 (m), 2894 (m), 2854 (m), 2752 (w), 2655 (w),

2592 (w), 2332 (w), 2161 (w), 2095 (w), 2002 (w), 1968 (w),

1923 (w), 1891 (w), 1868 (w), 1830 (w), 1792 (w), 1772 (w),

1749 (w), 1733 (w), 1716 (w), 1696 (w), 1684 (w), 1652 (w),

1636 (w), 1601 (s), 1572 (m), 1558 (w), 1510 (s), 1505 (s), 1468

(s), 1447 (m), 1438 (m), 1407 (m), 1379 (w), 1356 (m), 1333

(m), 1312 (w), 1284 (s), 1245 (s), 1216 (m), 1179 (w), 1165 (w),

1141 (w), 1122 (s), 1089 (m), 1061 (s), 1053 (s), 1023 (s), 1012

(m), 972 (m), 926 (s), 879 (m), 844 (s), 790 (w), 761 (m), 754

(m), 739 (w), 688 (w), 666 (m), 642 (w), 622 (w), 592 (w), 572

(m), 555 (w), 549 (w), 529 (w), 515 (w) cm^-1. EI mass

spectrum: m/z ) 624 (5%) [M - Me]⁺. Anal. Calcd for C₂₇H₄₅-

ClN₅NbOSi₂: C, 50.6; H, 7.1; N, 10.9. Found: C, 50.3; H, 7.0;

N, 10.8.

1

temperature over 2 days. H NMR (CD₂Cl₂, 500.0 MHz, 293

K): 9.29 (1 H, dd, H⁶, ³J(H⁵H⁶) ) 5.4 Hz, ⁴J(H⁴H⁶) ) 1.9 Hz),

3

4

7.81 (1 H, td, H⁴, J(H⁴H⁵H⁶) ) 7.8 Hz, J(H⁴H⁶) ) 1.9 Hz),

7.44 (1 H, d, H³, ³J(H³H⁴) ) 7.8 Hz), 7.27 (1 H, m, H⁵), 3.52 (1

H, d, CHHNSiMe₃, ²J ) 13.7 Hz), 3.29 (1 H, d, CHHNCO-

SiMe₃, ²J ) 11.7 Hz), 3.19 (1 H, d, CHHNSiMe₃, ²J ) 13.7

Hz), 2.89 (1 H, d, CHHNCOSiMe₃, ²J ) 11.7 Hz), 1.39 (3 H, s,

MeC(CH₂N)₂), 1.31 (9 H, s, CMe_3,imide), 1.17 (9 H, s, CMe_3,amide),

0.31 (9 H, s, NSiMe₃), 0.20 (9 H, s, OSiMe₃) ppm. ¹³C{¹H} NMR

(CD₂Cl₂, 125.7 MHz, 293 K): 163.8 (C²), 156.8 (CO), 150.6 (C⁶),

138.9 (C⁴), 121.6 (C⁵), 120.6 (C³), 65.9 (CMe_3,imide), 62.9

(CH₂NSiMe₃), 54.7 (CH₂NCOSiMe₃), 52.0 (CMe_3,amide), 51.3

(MeC(CH₂N)₂), 32.4 (CMe_3,imide), 31.6 (CMe_3,amide), 22.7 (MeC-

(CH₂N)₂), 2.4 (NSiMe₃), 1.0 (OSiMe₃) ppm. IR (KBr pellet):

2968 (s), 2356 (w), 1867 (w), 1842 (w), 1824 (w), 1651 (m), 1601

(m), 1573 (m), 1524 (s), 1479 (w), 1470 (m), 1455 (w), 1427 (s),

1391 (m), 1377 (w), 1356 (m), 1338 (w), 1321 (w), 1296 (w),

1245 (s), 1225 (m), 1160 (m), 1134 (w), 1127 (w), 1090 (w), 1060

(m), 1051 (w), 1020 (m), 1012 (w), 976 (m), 924 (m), 900 (w),

844 (s), 796 (w), 786 (w), 758 (m), 738 (w), 717 (w), 689 (w),

673 (m), 640 (w), 618 (w), 598 (w), 565 (m), 485 (w), 470 (w),

451 (w), 431 (w), 408 (w) cm^-1. High-resolution EI mass

spectrum for [Nb(N^tBu){MeC(2-C₅H₄N)(CH₂NSiMe₃)(CH₂-

NC(OSiMe₃)N^tBu)}Cl]⁺: Found (calculated for C₂₄H₄₇ClN₅-

NbOSi₂): m/z ) 605.2083 (605.2071). Anal. Calcd for C₂₄H₄₇-

ClN₅NbOSi₂: C, 47.5; H, 7.8; N, 11.6. Found: C, 47.0; H, 8.1;

N, 11.6.

Ta(N^tBu){MeC(2-C₅H₄N)(CH₂NSiMe₃)(CH₂NC(OSiMe₃)-

NTol)}Cl (18). To a stirred solution of [Ta(N^tBu)(N₂N_py)Cl-

(py)] (230 mg, 0.341 mmol) in CH₂Cl₂(20 mL) was added

p-tolyl isocyanate (43 µL, 0.341 mmol). After stirring for 23 h

at room temperature the volatiles were removed under reduced

pressure to afford a yellow powder. Crystallization from CH₂-

Cl₂/hexanes at -30 °C afforded 18 as a pale yellow powder.

Ta(N^tBu){MeC(2-C₅H₄N)(CH₂NSiMe₃)(CH₂NC(OSiMe₃)-

N^tBu)}Cl (16). To a stirred solution of [Ta(N^tBu)(N₂N_py)Cl-

(py)] (230 mg, 0.341 mmol) in CH₂Cl₂(20 mL) was added

^tBuNCO (39 µL, 0.341 mmol). The yellow solution was stirred

for 3 h at room temperature. The volatiles were removed under

reduced pressure to afford a yellow powder, which was

extracted into pentanes and filtered. The volatiles were

removed under reduced pressure to afford 16 as a pale yellow

1

Yield: 120 mg (48%). H NMR (CD₂Cl₂, 500.0 MHz, 293 K):

9.36 (1 H, d, H⁶, ³J(H⁵H⁶) ) 5.4 Hz), 7.83 (1 H, m, H⁴), 7.52 (1

3

H, d, H³, J(H³H⁴) ) 8.3 Hz), 7.29 (1 H, m, H⁵), 7.05 (4 H, m,

2

NC₆H₄Me), 3.60 (1 H, d, CHHNSiMe₃, J ) 13.7 Hz), 3.47 (1

H, d, CHHNCOSiMe₃, ²J ) 11.7 Hz), 3.30 (1 H, d, CHHNSiMe₃,

2

²J ) 13.7), 3.02 (1 H, d, CHHNCOSiMe₃, J ) 11.7 Hz), 2.25

1

powder. Yield: 110 mg (46%). H NMR (CD₂Cl₂, 300.0 MHz,

(3 H, s, NC₆H₄Me), 1.46 (3 H, s, MeC(CH₂N)₂), 1.09 (9 H, s,

CMe₃), 0.33 (9 H, s, NSiMe₃), -0.28 (9 H, s, OSiMe₃) ppm. ¹³C-

{¹H} NMR (CD₂Cl₂, 125.7 MHz, 293 K): 163.5 (C²), 155.7 (CO),

150.3 (C⁶), 143.5 (p-C₆H₄Me), 139.2 (C⁴), 131.4 (i-C₆H₄Me),

129.3 (m-C₆H₄Me), 123.2 (o-C₆H₄Me), 122.5 (C⁵), 121.1 (C³),

66.8 (CMe₃), 63.3 (CH₂NSiMe₃), 53.2 (CH₂NCOSiMe₃), 51.2

(MeC(CH₂N)₂), 31.8 (CMe₃), 22.9 (MeC(CH₂N)₂), 20.9 (C₆H₄Me),

2.1 (NSiMe₃), 0.1 (OSiMe₃) ppm. IR (KBr pellet): 3316 (w),

3109 (w), 3072 (w), 2968 (s), 2921 (m), 2894 (m), 2854 (m),

2752 (w), 2655 (w), 2592 (w), 2332 (w), 2161 (w), 2095 (w),

2002 (w), 1968 (w), 1923 (w), 1891 (w), 1868 (w), 1830 (w),

3

4

293 K): 9.33 (1 H, dd, H⁶, J(H⁵H⁶) ) 5.3 Hz, J(H⁴H⁶) ) 1.8

Hz), 7.80 (1 H, m, H⁴), 7.45 (1 H, d, H³, J(H³H⁴) ) 7.6 Hz),

3

7.28 (1 H, m, H⁵), 3.63 (1 H, d, CHHNSiMe₃, J ) 13.5 Hz),

2

3.43 (1 H, d, CHHNCOSiMe₃, ²J ) 11.7 Hz), 3.40 (1 H, d,

CHHNSiMe₃, ²J ) 13.5 Hz), 2.96 (1 H, d, CHHNCOSiMe₃, ²J

) 11.7 Hz), 1.38 (3 H, s, MeC(CH₂N)₂), 1.30 (9 H, s, CMe_3,imide),

1.21 (9 H, s, CMe_3,amide), 0.32 (9 H, s, NSiMe₃), 0.24 (9 H, s,

OSiMe₃) ppm. ¹³C{¹H} NMR (CD₂Cl₂, 75.5 MHz, 293 K): 163.8

(C²), 156.7 (CO), 150.6 (C⁶), 139.0 (C⁴), 121.8 (C⁵), 120.5 (C³),

64.1 (CMe_3,imide), 61.8 (CH₂NSiMe₃), 54.2 (CH₂NCOSiMe₃),

2384 Organometallics, Vol. 24, No. 10, 2005

Ward et al.

Table 6. X-ray Data Collection and Processing

Parameters for

1792 (w), 1772 (w), 1749 (w), 1733 (w), 1716 (w), 1696 (w),

1684 (w), 1652 (w), 1636 (w), 1601 (s), 1572 (m), 1558 (w), 1510

(s), 1505 (s), 1468 (s), 1447 (m), 1438 (m), 1407 (m), 1379 (w),

1356 (m), 1333 (m), 1312 (w), 1284 (s), 1245 (s), 1216 (m), 1179

(w), 1165 (w), 1141 (w), 1122 (s), 1089 (m), 1061 (s), 1053 (s),

1023 (s), 1012 (m), 972 (m), 926 (s), 879 (m), 844 (s), 790 (w),

761 (m), 754 (m), 739 (w), 688 (w), 666 (m), 642 (w), 622 (w),

592 (w), 572 (m), 555 (w), 549 (w), 529 (w), 515 (w) cm^-1. EI

mass spectrum: m/z ) 624 (5%) [M - Me]⁺. Anal. Calcd for

C₂₇H₄₅ClN₅OSi₂Ta: C, 44.5; H, 6.2; N, 9.6. Found: C, 44.6; H,

6.2; N, 9.6.

Nb(N^tBu){MeC(2-C₅H₄N)(CH₂NSiMe₃)-

(CH₂NC(OSiMe₃)N^tBu)}Cl (15)

empirical formula

C₂₀H₄₁N₄Si₂Ta

fw

606.21

temp/K

150

wavelength/Å

0.71073

space group

P2₁/n

a/Å

16.8425(3)

b/Å

21.5100(4)

c/Å

36.4135(7)

Reactions of M(N^tBu)(N₂N_py)Cl(py) (M ) Nb, Ta) with

CO₂. A solution of M(N^tBu)(N₂N_py)Cl(py) (0.03 mmol) in CD₂-

Cl₂(0.6 mL) was transferred to a J. Young NMR tube. The

solution was degassed by three freeze-pump-thaw cycles

before adding CO₂(1 atm). The NMR tube was sealed and the

reaction analyzed by NMR spectroscopy, which revealed a

complex mixture of products.

R/deg

90

â/deg

100.7414(6)

γ/deg

90

V/Å³

12960.8

Z

16

d(calcd)/Mg‚m^-3

abs coeff/mm^-1

R indices R₁, R_w[I > 3σ(I)]^a

1.243

0.551

R₁) 0.0433, R_w) 0.0501

Kinetic Analysis of SiMe₃Migration in [W(NPh)-

{MeC(2-C₅H₄N)(CH₂NSiMe₃)(CH₂N(SiMe₃)C(NAr)O}Me]-

2

^aR₁) ∑||F_o| - |F_c||/∑|F_o|; R_w) {∑w(|F_o| - |F_c|)²/∑(w|F_o| }^-1/2

.

[MeBAr^F₃] (11-MeBAr^F₃). To [W(NPh)(N₂N_py)Me][MeBAr^F

]

3

(2) generated in situ in CD₂Cl₂(0.6 mL) from W(NPh)(N₂N_py)-

TREAM unit. Diffraction data were measured using an Enraf-

Nonius KappaCCD diffractometer. Intensity data were pro-

cessed using the DENZO-SMN package.⁴⁵The structure was

solved in the space group P2₁/n using the direct-methods

program SIR92,⁴⁶which located all non-hydrogen atoms for

the four crystallographically independent molecules of 15 in

the asymmetric unit. Subsequent full-matrix least-squares

refinement was carried out using the CRYSTALS program

suite.⁴⁷Coordinates and anisotropic thermal parameters of all

non-hydrogen atoms were refined. Hydrogen atoms were

positioned geometrically after each cycle of refinement. A

three-term Chebychev polynomial weighting scheme was

applied. The crystal was of poor quality, causing considerable

expansion of the diffraction peaks. This together with the

relatively long c axis resulted in a substantial degree of peak

overlap. Rejection of the overlapping peaks is responsible for

the relatively poor completeness of the data set. It should be

noted that there are still over 10 observed independent

measurements per refined parameter. The thermal parameters

of some of the tert-butyl and trimethylsilyl C atoms were

relatively large. However, attempts to model these groups as

disordered did not significantly improve the agreement with

the X-ray data so were abandoned.

Me₂1 (17.5 mg, 0.03 mmol) and BAr^F(14.5 mg, 0.03 mmol)

3

in the presence of 1,4-dimethoxybenzene (5.2 mg, internal

standard) was added 2,6-diisopropylphenyl isocyanate (6.1 µL,

0.03 mmol). The mixture was transferred to a J. Young NMR

tube and then to an NMR spectrometer probe that had been

preheated to the desired temperature (one of 6 temperatures

1

between 20 and 45 °C). After thermal equilibration H NMR

spectra were recorded every 5 or 10 min for a period of up to

4 h. First-order rate contants were obtained from linear plots

of -ln(I/I₀) (I ) pyridyl H⁶intensity for 11⁺at a reaction time

t; I₀) estimated pyridyl H⁶intensity for 11⁺at t ) 0) vs t.

Re-evaluation of the Reaction of Ti(N^tBu)(N₂N_py)(py)

with ArNCO. To a solution of Ti(N^tBu)(N₂N_py)(py) (21 mg,

0.04 mmol) in C₆D₆(0.6 mL) was added 2,6-diisopropylphenyl

isocyanate (9 µL, 0.04 mmol) and the reaction product

Ti(N^tBu){MeC(2-C₅H₄N)(CH₂NSiMe₃)(CH₂NC{N(SiMe₃)Ar}

O}(py) (20b) analyzed by NMR spectroscopy (the chemical shift

data were consistent with those previously reported¹⁹). ¹H

NMR (C₆D₆, 300.1 MHz, 298 K): 9.66 (1 H, d, H⁶, ³J(H⁵H⁶) )

6.4 Hz), 9.35 (2 H, d, o-NC₅H₅, ³J(H⁵H⁶) ) 4.3 Hz), 7.07 (1 H,

apparent td, H⁴, ³J(H⁴H⁵) ) 8.0 Hz, ³J(H⁴H³) ) 8.0 Hz,

⁴J(H⁴H⁶) ) 1.9 Hz), 7.01 (1 H, d, p-C₆H₃Pr₂, ³J ) 7.6 Hz), 6.98

i

(1 H, m, p-C₅H₅N), 6.94-6.83 (3 H, overlapping m, m-C₆H₃-

A full listing of atomic coordinates, bond lengths and angles,

and displacement parameters for 15 have been deposited at

the Cambridge Crystallographic Data Center. See Notice to

Authors, Issue No. 1.

Computational Details. All calculations were performed

with the Gaussian98 set of programs⁴⁸within the framework

of hybrid DFT (B3PW91)^49,50and ONIOM(B3PW91/UFF),⁵¹

3

Pr₂and H³), 6.74 (2 H, t, m-C₅H₅N, J(H⁶H⁵) ) 6.3 Hz), 6.69

(1 H, m, H⁵), 3.72 (1 H, d, CHHC(NAr)OSiMe₃, ²J ) 12.9 Hz),

3.33 (1 H, d, CHHC(NAr)OSiMe₃, ²J ) 12.9 Hz), 2.77 (1 H, q,

CHMe₂, ³J ) 6.8 Hz), 2.67 (1 H, q, CHMe₂, ³J ) 6.9 Hz), 2.31

2

(1 H, d, CHHNSi, J ) 12.9 Hz), 2.21 (1 H, d, CHHNSi, J )

12.0 Hz), 1.58 (9 H, s, CMe₃), 0.98 (3 H, d, CHMeMe, ³J ) 6.9

Hz), 0.82 (3 H, d, CHMeMe, ³J ) 6.9 Hz), 0.78 (3 H, s,

3

MeCCH₂), 0.69 (3 H, d, CHMeMe, J ) 6.8 Hz), 0.45 (3 H, d,

(45) Otwinowski, Z.; Minor, W. Processing of X-ray Diffraction Data

Collected in Oscillation Mode, Methods in Enzymology; Academic

Press: New York, 1997.

(46) Altomare, A.; Cascarano, G.; Giacovazzo, G.; Guagliardi, A.;

Burla, M. C.; Polidori, G.; Camalli, M. J. Appl. Crystallogr. 1994, 27,

435.

(47) Watkin, D. J.; Prout, C. K.; Carruthers, J. R.; Betteridge, P.

W.; Cooper, R. I. CRYSTALS, issue 11; Chemical Crystallography

Laboratory: Oxford, UK, 2001.

CHMeMe, J ) 6.7 Hz), 0.40 (18 H, s, 2 × SiMe₃) ppm. ¹³C-

3

{¹H} NMR (C₆D₆, 75.5 MHz, 298 K): 167.1 (C(NAr)OSiMe₃),

i

163.2 (C²), 151.8 (C⁶), 151.7 (o-NC₅H₅), 146.9 (o-C₆H₃Pr₂), 145.7

i

(o-C₆H₃Pr₂), 137.4 (ipso-C₆H₃Pr₂), 136.9 (p-NC₅H₅), 136.8 (C⁴),

i

127.1 (p-C₆H₃Pr₂), 124.2 (C³), 123.3 (m-NC₅H₅), 123.0 (m-C₆H₃-

i

Pr₂), 122.1 (m-C₆H₃Pr₂), 119.8 (C⁵), 64.6 (NCMe₃), 64.4 (CH₂-

(NAr)OSiMe₃), 56.3 (CH₂NSiMe₃), 49.1 (MeCCH₂NSiMe₃), 33.5

(NCMe₃), 28.4 (overlapping 2 × CHMe₂), 25.1 (CHMeMe), 24.0

(CHMeMe), 23.7 (CHMeMe), 23.6 (CHMeMe), 22.1 (MeCCH₂),

3.1 (SiMe₃), 1.9 (SiMe₃) ppm.

(48) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;

Robb, M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. J.

A.; Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.;

Daniels, A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.;

Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo,

C.; Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.;

Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.;

Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Baboul, A. G.; Stefanov,

B. B.; Liu, G.; Liashenko, A.; Piskorz, Y.; Nanayakkara, A.; Gonzalez,

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Crystal Structure Determination of Nb(N^tBu){MeC-

(2-C₅H₄N)(CH₂NSiMe₃)(CH₂NC(OSiMe₃)N^tBu)}Cl (15).

Crystal data collection and processing parameters are given

in Table 6. Crystals of 15 were grown by slow evaporation of

a pentane solution. A single crystal having dimensions of

approximately 0.08 × 0.20 × 0.30 mm was mounted on a glass

fiber using perfluoropolyether oil and cooled rapidly to 150 K

in a stream of cold N₂using an Oxford Cryosystems CRYOS-

(49) Becke, A. D. J. J. Chem. Phys. 1993, 98, 5648.

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Diamide-Supported Imido Complexes

Organometallics, Vol. 24, No. 10, 2005 2385

where the QM part was treated at the B3PW91 level and the

MM calculations were performed with the Universal Force

Field (UFF) of Rappe´ et al.⁵²In the DFT calculations the ligand

N₂N_pyhas been modeled by HC(2-C₅H₄N)(CH₂NSiH₃)₂(ab-

breviated as N₂N_qm), and the missing Me groups have been

treated at the MM level (UFF) in the ONIOM calculations.

The QM part of the ONIOM calculations is the same as that

for the DFT-only calculations. CO₂has been explicitly treated

at the DFT level in all calculations. The isocyanate RNCO are

were followed by analytical calculations of the Hessian matrix

to check the nature (minimum or transition state) of the

located extrema. Connection between two local minima through

a particular TS was checked by slightly altering the TS

geometry along the TS vector in both directions followed by

optimization as a local minimum of the resulting guess

geometries. The NBO procedure⁴⁴used was that implemented

in Gaussian98. Gibbs free energy values were obtained from

the frequency calculations with Gaussian98 and are given at

T ) 298 K unless otherwise stated.

t

modeled by MeNCO in the DFT calculations and by BuNCO

in the ONIOM calculations, where the three methyl groups

on the tert-butyl are treated at the UFF level. In all calcula-

tions, the metal atom (W, Ta)⁵³and Si⁵⁴have been treated with

effective core pseudo-potentials from Dolg’s group and the

associated basis sets, augmented by polarization functions.^55,56

The atoms directly bonded to the metal (N and CH₃) as well

as CO₂and the NCO atoms in RNCO were treated with a

6-31G(d,p) basis set.⁵⁷All the other atoms were represented

with 6-31G basis sets in the calculations.⁵⁸Geometry optimi-

zations were performed without any symmetry constraint and

Acknowledgment. We thank the EPSRC, Lever-

hulme Trust, and Royal Society (P.M.), British Council

(P.M., L.H.G. and E.C.), the CNRS (E.C.), and the DFG

(L.H.G.). E.C. and P.M. thank the Royal Society of

Chemistry for financial support (International Authors

Travelling Grant). We acknowledge the use of the

EPSRC National Service for Computational Chemistry

Software and the UK Computational Chemistry Facilty.

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Sieber, S.; Morokuma, K. J. Phys. Chem. 1996, 100, 19357.

(52) Rappe, A. K.; Casewit, C. J.; Colwell, K. S.; Goddard, W. A.;

Skiff, W. M. J. Am. Chem. Soc. 1992, 114, 10024.

Supporting Information Available: X-ray crystallo-

graphic file in CIF format for the structure determination

of [Nb(N^tBu){MeC(2-C₅H₄N)(CH₂NSiMe₃)(CH₂NC(OSiMe₃)-

N^tBu)}Cl)] (15); rate plots for the conversion of 11⁺to 12⁺and

further details of the DFT-computed distances and angles. This

material is available free of charge via the Internet at

http://pubs.acs.org.

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Theor. Chim. Acta 1990, 77, 123.

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G. Chem. Phys. Lett. 1993, 208, 111.

OM0500265

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Article Doi

Reactions of neutral and cationic diamide-supported imido complexes with CO2 and other heterocumulenes: Issues of site selectivity

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