Preparation of ansa-niobocene and ansa-tantalocene olefin hydride complexes as transition state analogues in metallocene-catalyzed olefin polymerization

172

Organometallics 2003, 22, 172-187

P r ep a r a tion of a n sa -Niobocen e a n d a n sa -Ta n ta locen e

Olefin Hyd r id e Com p lexes a s Tr a n sition Sta te An a logu es

in Meta llocen e-Ca ta lyzed Olefin P olym er iza tion

Paul J . Chirik,¹Deanna L. Zubris,²Lily J . Ackerman, Lawrence M. Henling,

Michael W. Day, and J ohn E. Bercaw*

Arnold and Mabel Beckman Laboratories of Chemical Synthesis, California Institute of

Technology, Pasadena, California 91125

Received August 2, 2002

To examine the effects of cyclopentadienyl and olefin substitution on preferred stereo-

chemistry, a series of singly [SiMe₂]-bridged ansa-niobocene and -tantalocene olefin hydride

complexes has been prepared via reduction and alkylation of the corresponding dichloride

complexes. In this manner, [Me₂Si(η⁵-C₅H₄)(η⁵-C₅H₃-3-R)]M(CH₂dCHR′)H (M ) Nb, Ta; R

) CHMe₂, CMe₃; R′ ) H, C₆H₅; M ) Ta; R ) CHMe₂, CMe₃; R′ ) Me), rac- and meso-[Me₂-

Si(η⁵-C₅H₃-3-R)(η⁵-C₅H₃-3-R)]Nb(CH₂dCH₂)H (R ) CMe₃), and [Me₂Si(η⁵-C₅H₄)(η⁵-C₅H₂-2,4-

(CHMe₂)₂)]Ta(CH₂dCHR′)H (R′ ) H, C₆H₅) have been prepared and characterized by NMR

spectroscopy and, in some cases, X-ray diffraction. The doubly [SiMe₂]-bridged ansa-

tantalocene ethylene hydride complex [(1,2-SiMe₂)₂(η⁵-C₅H-3,5-(CHMe₂)₂)(η⁵-C₅H₂-4-CMe₃)]-

Ta(CH₂dCH₂)H has been prepared from thermolysis of the methylidene methyl complex

[(1,2-SiMe₂)₂(η⁵-C₅H-3,5-(CHMe₂)₂)(η⁵-C₅H₂-4-CMe₃)]Ta(CH₂)CH₃. Addition of an excess of

propylene or styrene to the tantalocene ethylene hydride results in olefin exchange and

formation of the olefin hydride complexes [(1,2-SiMe₂)₂(η⁵-C₅H-3,5-(CHMe₂)₂)(η⁵-C₅H₂-4-

CMe₃)]Ta(CH₂dCHR′)H (R′ ) CH₃, C₆H₅). These compounds serve as stable transition state

analogues for the much more kinetically labile group 4 metallocenium cationic intermediates

in metallocene-catalyzed olefin polymerization. Characterization of the thermodynamically

preferred isomers of metallocene olefin hydride complexes reveals that alkyl substitution

on the cyclopentadienyl ligand array may have a significant effect on the stereochemistry of

olefin coordination.

Ch a r t 1

In tr od u ction

Stereospecific olefin polymerization promoted by group

3 and 4 ansa-metallocene catalysts represents one of the

most enantioselective chemical transformations known.³

Elucidation of the steric and electronic factors that

control this remarkable selectivity may aid in the design

of new catalysts and also result in the development of

new asymmetric transformations. Considerable effort

has been devoted toward understanding the nature of

the transition state for the C-C bond forming step with

metallocene polymerization catalysts. The electronic

requirements for an active catalyst are fairly well

accepted: a metallocene alkyl with two vacant orbitals

(Chart 1), where one orbital is used to accommodate the

incoming olefin while the other allows for R-agostic

assistance⁴in the transition state for carbon-carbon

bond formation.⁵

Understanding the key steric interactions in the olefin

insertion transition state has also been the focus of

many experimental and theoretical investigations. Cal-

culations by Corradini⁶demonstrate that the enantio-

facial approach of the olefin is determined by the

orientation of the metal alkyl unit, such that the olefin

substituent is placed in a trans relationship with the

â-carbon of the metal polymeryl unit. The polymeryl is

believed to orient toward the most open portion of the

metallocene framework (Scheme 1; schematic view of

C₂-symmetric metallocene looking into the wedge). The

first experimental evidence in support of this model was

(1) Current address: Department of Chemistry and Chemical

Biology, Cornell University, Ithaca, NY 14853-1301.

(2) Current address: Department of Chemistry, Villanova Univer-

sity, Villanova, PA 19085.

(3) For recent reviews see: (a) Brintzinger, H. H.; Fischer, D.;

Mu¨lhaupt, R.; Reiger, B.; Waymouth, R. M. Angew. Chem., Int. Ed.

Engl. 1995, 34, 1143. (b) Bochmann, M. J . Chem. Soc., Dalton Trans.

1996, 255.

(4) (a) Piers, W. E.; Bercaw, J . E. J . Am. Chem. Soc. 1990, 112, 9406.

(b) Krauledat, H.; Brintzinger, H. H. Angew. Chem., Int. Ed. Engl.

1990, 29, 1412. (c) Grubbs, R. H.; Coates, G. W. Acc. Chem. Res. 1996,

29, 85.

(5) J ordan, R. F. Adv. Organomet. Chem. 1991, 32, 325.

(6) Guerra, G.; Cavallo, L.; Moscardi, G.; Vacetello, M.; Corradini,

P. J . Am. Chem. Soc. 1994, 116, 2988.

Publication on Web 12/07/2002

ansa-Niobocene and ansa-Tantalocene Complexes

Organometallics, Vol. 22, No. 1, 2003 173

Sch em e 1

Sch em e 3

Sch em e 2

Sch em e 4

provided by Pino.⁷Hydrooligomerization of R-olefins

with optically pure [ethylenebis(4,5,6,7-tetrahydroinde-

nyl)]zirconium dichloride (EBTHIZrCl₂) produced chiral

hydrotrimers and hydrotetramers with the predicted

absolute configurations.⁸In contrast to polymerizations/

oligomerizations, deuteriations of R-olefins such as

styrene and pentene produced lower ee’s with the

opposite enantiofacial selectivity.⁹Similarly, work from

our laboratories defined the diastereoselective transition

structures for 1-pentene addition to yttrium-hydride

and yttrium-pentyl bonds.¹⁰An optically pure, isoto-

pically chiral 1-pentene was prepared and used to

evaluate the stereoselectivity with an optically pure

yttrocene. The absolute diastereoselectivities were es-

tablished: insertion into the yttrium-hydride bond

proceeds with modest selectivity (34% ee); insertion into

the yttrium-pentyl bond proceeds via the other dia-

stereomeric transition state with very high levels (>95%

ee) of selectivity. Analogous transition state arguments

have been proposed for the titanocene-catalyzed asym-

metric hydrogenation of olefins reported by Buchwald

and co-workers.¹¹

approach from one side of the metallocene wedge and

then the other.¹³

Although these stereochemical models have been

quite successful in explaining the high levels of stereo-

control observed with C₂- and C_s-symmetric catalysts,

the stereospecificity of some metallocene catalysts can-

not be readily rationalized. For example, the C₁-sym-

metric, monosubstituted, singly silylene-bridged zir-

conocene catalyst [Me₂Si(η⁵-C₅H₄)(η⁵-C₅H₃-3-R)]ZrCl₂/

MAO (R ) CMe₃, CHMe₂), originally reported by

Miya,^14apolymerizes propylene with [mmmm] contents

exceeding 70% (Scheme 3). Obviously, the stereocontrol

mechanisms of Schemes 1 and 2 do not apply, and thus

one cannot readily explain such high isospecificity.

A possible approach to understanding the stereo-

chemistry of olefin insertion would be to model the

carbon-hydrogen or carbon-carbon bond-forming tran-

sition states of a group 4 metallocene catalyst using the

ground-state analog, a (stable) group 5 (M ) Nb, Ta)

metallocene olefin hydride or olefin alkyl complex. These

complexes have been used to investigate the steric and

electronic effects for olefin insertion into metal-hydride

bonds with bis(cyclopentadienyl) and related ansa-

niobocene and -tantalocene olefin hydride complexes.¹⁵

The niobocene and tantalocene complexes are formally

M(III), d²metal centers (Scheme 4) that stabilize the

olefin-metal bond through a strong π-back-bonding

interaction.

Conventional NMR and X-ray diffraction experiments

may be used to determine the structure of the metal-

locenes and thus to establish the direction and magni-

tude of steric effects (shown for one of the enantiomers

of {[Me₂Si(η⁵-C₅H₄)(η⁵-C₅H₃-3-R¹)]M} in Chart 2). There

are three stereoisomeric relationships between the olefin

hydride complexes: (1) the preference for the olefin to

be on the side of the metallocene wedge distal or

proximal to the cyclopentadienyl substituent (R¹), (2)

the preference for olefin coordination with its substitu-

ent (R³) syn or anti to the substituted cyclopentadienyl

The stereochemical model developed in the C₂-sym-

metric isospecific systems has since been extended to

include C_s-symmetric syndiospecific catalysts.¹²The

favored transition state geometry for syndiospecific

catalysts is shown in Scheme 2 (schematic view of a C_s-

symmetric metallocene looking into the wedge), where

again the dominant stereo-directing interaction is a

trans relationship between propylene methyl and the

C_R-C_âbond of the polymer chain. Whether on the left

or right side of the metallocene wedge, the growing

polymer chain extends up and away from the more

sterically demanding cyclopentadienyl moiety, thus

forcing the propylene methyl group down. In syndiospe-

cific catalysts, this lower cyclopentadienyl ligand con-

tains an open region to accommodate the methyl sub-

stituent on the incoming monomer. Essential to the

stereospecificity is a regular alternation of propylene

(7) Pino, P.; Cioni, P.; Wei, J . J . Am. Chem. Soc. 1987, 109, 6189.

(8) Pino, P.; Galimberti, M. J . Organomet. Chem. 1989, 370, 1.

(9) Waymouth, R.; Pino, P. J . Am. Chem. Soc. 1990, 112, 4911.

(10) Gilchrist, J . H.; Bercaw, J . E. J . Am. Chem. Soc. 1996, 118,

12021.

(11) (a) Broene, R. D.; Buchwald, S. L. J . Am. Chem. Soc. 1993, 115,

12569. (b) Troutman, M. V.; Appella, D. H.; Buchwald, S. L. J . Am.

Chem. Soc. 1999, 121, 4916.

(13) Veghini, D.; Henling, L. M.; Burkhardt, T. J .; Bercaw, J . E. J .

Am. Chem. Soc. 1999, 121, 564.

(14) (a) Mise, T.; Miya, S.; Yamazaki, H. Chem. Lett. 1989, 1853.

(b) Zhong, A. H.; Bercaw, J . E. Unpublished results.

(15) (a) Doherty, N. M.; Bercaw, J . E. J . Am. Chem. Soc. 1985, 107,

2670. (b) Burger, B. J .; Santarsiero, B. D.; Trimmer, M. S.; Bercaw, J .

E. J . Am. Chem. Soc. 1988, 110, 3134. (c) Ackerman, L. J .; Green, M.

L. H.; Green, J . C.; Bercaw, J . E. Organometallics 2003, 22, 188.

(12) Herzog, T. A.; Zubris, D. L.; Bercaw, J . E. J . Am. Chem. Soc.

1996, 118, 11988.

174 Organometallics, Vol. 22, No. 1, 2003

Chirik et al.

Ch a r t 2

Sch em e 5

ring, and (3) the preference for the olefin substituent

(R³) to position itself endo or exo relative to the metal-

locene hydride or metallocene alkyl fragment (R²).

Reports of group 5 ansa-metallocenes are limited and

generally restricted to C_2v-symmetric metallocene frame-

works,^{16,17a,b,18-20}except for one report of C₁-symmetric

ansa-niobocene imido complexes.^17cIn this report we

describe the synthesis and characterization of a series

of low-symmetry, singly and doubly [SiMe₂]-bridged

group 5 metallocene olefin hydride complexes. The

preferred structures of these complexes have been

examined regarding the important stereodirecting in-

teractions between the coordinated olefin and the cy-

clopentadienyl ligand substituents. These olefin hydride

complexes are potential precursors for the preparation

of olefin alkyl complexes. Using these structures as

transition state analogues, we hope to gain some insight

into the basis of stereoselectivity for propylene poly-

merizations with group 4 metallocene catalysts.

Preparation of ansa-niobocene and ansa-tantalocene

dichloride complexes is accomplished by extension of

previously reported synthetic protocols. For the nio-

bocene complexes, metalation of singly [SiMe₂]-bridged

cyclopentadienyl ligands proceeds via addition of the

dilithio salt of the ligand to a slurry of NbCl₄(THF)₂in

diethyl ether.^17aIn this manner, [Me₂Si(η⁵-C₅H₄)(η⁵-

C₅H₃-3-CHMe₂)]NbCl₂(iPrSpNbCl₂; 1), [Me₂Si(η⁵-C₅H₄)-

(η⁵-C₅H₃-3-CMe₃)]NbCl₂(tBuSpNbCl₂; 2), and [Me₂Si(η⁵-

C₅H₃-3-CMe₃)₂]NbCl₂(DpNbCl₂; 3) have been prepared

(eq 1). Each dichloride complex is first extracted into

Resu lts a n d Discu ssion

P r ep a r a tion of Sin gly [SiMe₂]-Br id ged Olefin

Hyd r id e Com p lexes. The synthetic strategy for the

preparation of group 5 ansa-metallocene olefin hydride

complexes is based upon the methodology developed for

unlinked metallocene complexes. Essential to the syn-

thesis is a convenient route to the corresponding group

5 metallocene dichloride complex, which in turn may

be simultaneously reduced and alkylated via addition

of an excess of the appropriate Grignard reagent (Scheme

5).

(16) (a) Bailey, N. J .; Cooper, J . A.; Gailus, H.; Green, M. L. H.;

J ames, J . T.; Leech, M. A. J . Chem. Soc., Dalton Trans. 1997, 3579.

(b) Bailey, N. J .; Green, M. L. H.; Leech, M. A.; Saunders, J . F.;

Tidswell, H. M. J . Organomet. Chem. 1997, 538, 111. (c) Conway, S.

L.; Doerrer, L. H.; Green, M. L. H.; Leech, M. A. Organometallics 2000,

19, 630.

(17) (a) Antin˜olo, A.; Martinez-Ripoll, M.; Mugnier, Y.; Otero, A.;

Prashar, S.; Rodr´ıguez, A. M. Organometallics 1996, 15, 3241. (b)

Garc´ıa-Yebra, C.; Carrero, F.; Lo´pez-Mardomingo, C.; Fajardo, M.;

Rodr´ıguez, A. M.; Antin˜olo, A.; Otero, A.; Lucas, D.; Mugnier, Y.

Organometallics 1999, 18, 1287. (c) Antin˜olo, A.; Lo´pez-Solera, I.; Orive,

I.; Otero, A.; Prashar, S.; Rodr´ıguez, A. M.; Villasen˜or, E. Organome-

tallics 2001, 20, 71.

(18) (a) Herrmann, W. A.; Baratta, W. J . Organomet. Chem. 1996,

506, 357. (b) Herrmann, W. A.; Baratta, W.; Herdtweck, E. Angew.

Chem., Int. Ed. Engl. 1996, 35, 1951.

CH₂Cl₂to remove LiCl. This purification is satisfactory

for 1, but further purification is necessary for 2 and 3.

Complex 2 is isolated after sublimation at 160 °C.

Complex 3 is obtained as a mixture of racemic and meso

isomers (vide infra), and this mixture is isolated after

(19) Chernega, A. N.; Green, M. L. H.; Sua´rez, A. G. Can. J . Chem.

1995, 73, 1157.

(20) Shin, J . H.; Parkin, G. Chem. Commun. 1999, 887.

ansa-Niobocene and ansa-Tantalocene Complexes

Organometallics, Vol. 22, No. 1, 2003 175

a second extraction into petroleum ether. Characteriza-

tion of the paramagnetic Nb(IV) dichloride complexes

has been accomplished by ambient-temperature EPR

spectroscopy and by elemental analysis. The EPR

spectra for 1-3 display 10-line patterns indicative of a

single electron localized on a Nb(IV) center (⁹³Nb )

9

100%, S ) /₂).

Synthesis of the requisite ansa-tantalocene complexes

follows a different route. Lacking a suitable Ta(IV)

precursor, metalation of singly [SiMe₂]-bridged cyclo-

pentadienyl ligands is accomplished via reaction of

TaCl₅with the corresponding stannylated cyclopenta-

diene as described previously.²⁰In this manner, [Me₂-

Si(η⁵-C₅H₄)(η⁵-C₅H₃-3-CHMe₂)]TaCl₃(iPrSpTaCl₃; 4),

[Me₂Si(η⁵-C₅H₄)(η⁵-C₅H₂-2,4-(CHMe₂)₂)]TaCl₃(iPr₂SpTa-

Cl₃; 5), and [Me₂Si(η⁵-C₅H₄)(η⁵-C₅H₃-3-CMe₃)]TaCl₃

(tBuSpTaCl₃; 6) have been prepared in modest yield (eq

2). The diamagnetic Ta(V) trichloride complexes display

resonances sharpen and are analogous to the (static) 25

°C H NMR spectrum for 11.

1

Structural assignment of the predominant ethylene

hydride isomer for both 10 and 11 has been ac-

complished with NOE difference NMR spectroscopy.²¹

Irradiation of the metal hydride affords a strong NOE

enhancement in the isopropyl methine, isopropyl methyl

groups, and the endo-ethylene protons. Likewise, ir-

radiation of the isopropyl methine results in enhance-

ment in the metal hydride resonance. No enhancement

in any ethylene peaks is observed. These data, taken

together with more subtle cyclopentadienyl and [SiMe₂]

NOE enhancements, allow the assignment of the major

isomers. In both cases, the ethylene is coordinated in

the more open portion of the metallocene wedge, away

from the isopropyl substituent (i.e., the distal isomer).

The formation of predominantly one ethylene hydride

isomer (95%) demonstrates the moderate stereodirecting

ability of a monosubstituted ansa-metallocene.

typical ligand resonances in their ¹H and ¹³C NMR

spectra (see Experimental Section for details).

Reduction of the orange ansa-tantalocene trichloride

complexes 4-6 with zinc dust in dimethoxyethane

affords green iPrSpTaCl₂(7), iPr₂SpTaCl₂(8), and

tBuSpTaCl₂(9) in approximately 70% yields (eq 3). The

Addition of CH₃CH₂MgBr to either 2 or 9 affords the

corresponding ansa-niobocene or -tantalocene ethylene

hydride complex [Me₂Si(η⁵-C₅H₄)(η⁵-C₅H₃-CMe₃)M(η²-

CH₂CH₂)H] (M ) Nb (12), Ta (13)) in modest yield (eq

5). In both cases only one isomer is observed by ¹H NMR

spectroscopy. Slow cooling of a petroleum ether solution

of 12 affords yellow crystals suitable for X-ray diffrac-

tion. The solid-state structure of 12 is shown in Figure

1 and reveals that the ethylene ligand is coordinated

in the open portion of the metallocene wedge away from

the tert-butyl substituent. The niobium hydride was

located in a difference map, and the Nb-H bond length

was refined to 1.68(2) Å. The C(17)-C(18) bond distance

of 1.411(2) Å is consistent with other group 5 olefin

adducts, indicative of substantial metallocyclopropane

character.²²

tantalocene dichloride complexes are isolated as forest

green solids that are soluble in dimethoxyethane and

dichloromethane. Ambient-temperature EPR spectros-

copy in dichloromethane solution affords characteristic

7

eight-line spectra (¹⁸¹Ta ) 99.99%, S ) /₂) consistent

with a Ta(IV) d¹species.

Addition of 2.2 equiv of CH₃CH₂MgBr to either 1 or

7 in diethyl ether results in formation of the ethylene

hydride complexes iPrSpM(η²-CH₂CH₂)H (M ) Nb (10),

1

Ta (11)) (eq 4). For both complexes, H NMR spectros-

copy reveals that, of the two possible ethylene hydride

isomers, one isomer is formed preferentially in a 95:5

ratio. Diagnostic upfield metal hydride resonances are

observed at -2.60 and -2.81 ppm for 10 and 11,

respectively. The ¹H NMR spectrum for 10 at 25 °C

contains broad resonances for the niobium hydride and

for the coordinated ethylene, indicative of rapid and

reversible olefin insertion and â-hydrogen elimination.¹⁵

When the temperature is lowered to -30 °C, these

(21) The NOE difference experiment for 10 was performed at -30

°C in order to obtain sharp NMR resonances and reliable structural

data.

(22) Guggenberger, L. J .; Meakin, P.; Tebbe, F. N. J . Am. Chem.

Soc. 1974, 96, 5420.

176 Organometallics, Vol. 22, No. 1, 2003

Chirik et al.

F igu r e 1. Molecular structure of 12 with 50% probability ellipsoids. Hydrogen atoms (other than hydride shown at arbitrary

scale) have been omitted.

Reaction of complex 3 with CH₃CH₂MgBr results in

a mixture of two isomers in a 50:50 ratio. The number

1

of cyclopentadienyl resonances in the H NMR indicate

that one isomer is of C_ssymmetry, meso-DpNb(η²-CH₂-

CH₂)H (15a ); the other isomer, rac-DpNb(η²-CH₂CH₂)H

(15b), is C₁symmetric. The isomers, 15a and 15b, can

be separated by fractional recrystallization from cold

petroleum ether. NOE difference experiments for the

meso isomer 15a indicate that the ethylene is coordi-

nated away from the tert-butyl substituents (eq 7).

Reaction of the disubstituted ansa-tantalocene 8 with

CH₃CH₂MgBr affords two isomers of the ethylene hy-

dride complex iPr₂SpTa(η²-CH₂CH₂)H (14) in a 78:22

ratio (eq 6). NOE difference ¹H NMR spectroscopy

Preparation of ansa-niobocene and -tantalocene com-

plexes with R-olefins is accomplished via addition of the

appropriate Grignard reagent to the metallocene dichlo-

ride complexes. Thus, addition of CH₃CH₂CH₂MgCl to

7 affords a mixture containing four (¹H NMR detectable)

reveals that the ethylene ligand is coordinated on the

side of the metallocene wedge proximal to the 4-isopro-

isomers of the eight possible for iPrSpTa(η²-CH₂-

CHCH₃)H (16) as a yellow oil with the following

percentages: 63.7% (16a ), 20.6% (16b), 13.0% (16c), and

2.6% (16d ). Identification of the major isomer has been

achieved using NOE difference NMR spectroscopy. The

propylene ligand is coordinated in an distal, endo

pyl substituent for the major isomer. Although the

energy difference between these two isomers is quite

small, there does appear to be a slight preference for

the olefin to coordinate away from the isopropyl sub-

stituent that is R to the linking [SiMe₂] group.

ansa-Niobocene and ansa-Tantalocene Complexes

Organometallics, Vol. 22, No. 1, 2003 177

fashion with the methyl group directed anti to the

cyclopentadienyl substituent.

The preparation of tBuSpTa(η²-CH₂CHCH₃)H (17)

has been accomplished in a manner analogous to that

for 16. However, for the more substituted tert-butyl

derivative, only two isomers of the olefin hydride

complex are observed by NMR spectroscopy where the

major isomer, 17a , constitutes 92% of the product

mixture. As with 16a , the structure of 17a has been

elucidated using NOE difference NMR spectroscopy and

shown to have the propylene coordinated in the endo

arrangement with the propylene directed anti to the

cyclopentadienyl substituent.²³

18 and 19 the major styrene hydride isomer is the one

for which the phenyl ring is directed away from the

isopropyl substituent, whereas in the minor isomer, the

phenyl ring is directed toward the isopropyl group, with

the diastereoselectivity for olefin coordination for 19

being approximately 18%. A small enantiofacial dif-

ferentiation was also noted for insertion of 1-pentene

into the Y-H bond of a C₂-symmetric yttrocene catalyst

and for R-olefin deuteriations.^9,10Although the enan-

tiofacial preference for olefin coordination is poor, the

site selectivity is quite good, where for both 18 and 19

approximately 90% of the styrene coordination occurs

with phenyl away from the isopropyl substituent, i.e.

“distally”.²⁴

Addition of 2.2 equiv of C₆H₅CH₂CH₂MgBr to an

ethereal solution of either 1 or 7 affords the styrene

hydride complexes iPrSpM(η²-CH₂CHC₆H₅)H (M ) Nb,

18; Ta, 19) (eq 8). Three isomers are formed in a 53:

Slow cooling of a petroleum ether solution of the

mixture of isomers of 19 affords yellow needles that are

suitable for X-ray diffraction analysis. The solid-state

structure for the minor tantalocene styrene hydride

complex 19b reveals the expected η²coordination of the

styrene ligand. Although the hydride ligand was not

located in the difference map, the styrene is bound to

the coordination site distal to the isopropyl group

(Figure 2). The isopropyl group orients itself such that

the methine hydrogen points toward the phenyl group

of the coordinated styrene ligand, thus minimizing

unfavorable steric interactions between the olefin sub-

stituent and the ancillary ligand substitution. The

C(16)-C(17) bond distance of 1.420(9) Å is typical for

group 5 olefin hydride complexes. The phenyl group on

the styrene ligand is twisted 6.2° from the olefin plane

(C(16)-C(17)-C(18)), which is reduced substantially

from the 32° observed for (η⁵-C₅Me₅)₂Nb(η²-CH₂CH-

C₆H₅)H.^15b

Dissolving the single crystals of 19b in benzene-d₆

reveals that only the minor isomer, 19b, is initially

present. However, over the course of minutes at 25 °C,

19b isomerizes to the previously observed mixture of

products (19a :19b:19c ) 54:36:10). The rapid equilibra-

tion of the styrene hydride isomers could take place by

three possible mechanisms: (a) olefin insertion/â-H

elimination, (b) styrene dissociation/recoordination, (c)

styrene rotation while coordinated. Because we have

independently established that path a is sufficiently

rapid to allow rates to be measured by dynamic NMR

techniques for niobium and tantalum olefin hydride

38:9 ratio for 18 and a 54:36:10 ratio for 19 (major:

minor:trace isomers).

Analysis of the product mixture by NOE difference

¹H NMR spectroscopy allows assignment of the major

and minor styrene hydride complexes. Irradiation of the

metal hydride resonances results in strong NOE en-

hancements in the isopropyl substituents, one olefinic

styrene resonance, and the ortho hydrogens of the

phenyl ring. Likewise, irradiation of the isopropyl

substituents results in no enhancement of the styrene

protons. These data indicate that for the major isomer

of each metal complex, the styrene is coordinated anti

to the isopropyl substituents and in endo fashion, where

the phenyl ring is directed toward the interior of the

metallocene wedge. Presumably unfavorable steric in-

teractions between the phenyl ring and the [Me₂Si]

linker discourage formation of exo isomers that were

observed with unlinked niobocene and tantalocene

styrene hydride complexes.^15a,b

Differentiation between the enantiofacial preference

of styrene hydride isomers has been obtained from more

subtle NOE enhancements between the cyclopentadi-

enyl, dimethylsilylene, and styrene hydrogens. For both

(23) The stereoisomer shown (with the S metallocene configuration)

is one of two mirror-plane-related structures with si olefin coordination;

the other distal, endo isomer has re olefin coordination to the R

metallocene.

(24) The structure of the isomer comprising 10% of the mixture has

not been established, and thus it could be one of the four proximal

isomers or one of the two exo distal isomers.

178 Organometallics, Vol. 22, No. 1, 2003

Chirik et al.

F igu r e 2. Molecular structure of 19b with 50% probability ellipsoids. Hydrogen atoms have been omitted.

F igu r e 3. Molecular structure of 20a with 50% probability ellipsoids. Hydrogen atoms have been omitted except for the

hydride, which was located in a difference map but not refined.

hydride complexes 19a -c results in formation of 11 over

the course of 1 week at 23 °C. Thus, a combination of

(a) and (c) (Scheme 6) would allow interconversion of

all eight possible mirror-plane-related pairs of diaster-

eomers.

complexes, olefin insertion/â-H elimination would ap-

pear to be the logical pathway for interconverting 18a

with 18b and 19a with 19b. Styrene dissociation/

recoordination, known to be slow for other group 5 olefin

hydride complexes,^15a,bneed not be invoked. Indeed,

addition of ¹³CH₂d¹³CH₂to a benzene-d₆solution of 11

results in very slow incorporation of the labeled olefin,

requiring 3 days at 25 °C to equilibrate (eq 9). Similarly,

addition of ethylene to the isomeric mixture of styrene

Preparation of the styrene hydride complex iPr₂SpTa-

(η²-CH₂CHPh)H (20) has been accomplished via addi-

tion of phenethylmagnesium chloride to a diethyl ether

solution of 8 (eq 10). Analysis of the product mixture

by ¹H NMR spectroscopy indicates that four isomers are

formed. The product distribution consists of major and

minor isomers formed in a 48:39 ratio constituting 87%

of the styrene hydrides. Two trace isomers composing

13% of the product mixture are formed in roughly equal

amounts. Recrystallization of the reaction mixture from

cold petroleum ether affords yellow needles that are

suitable for X-ray diffraction analysis, identifying one

isomer (Figure 3). Dissolving the crystals in benzene-

d₆reveals that the isomer that crystallizes is the major

isomer formed (20a ). Monitoring the solution over time

1

by H NMR spectroscopy demonstrates that pure 20a

ansa-Niobocene and ansa-Tantalocene Complexes

Organometallics, Vol. 22, No. 1, 2003 179

Sch em e 6

portion of the metallocene wedge with the phenyl group

directed anti to the tert-butyl substituent. The Nb-H

was located in a difference map; the distance was

refined to 1.70(2) Å. The phenyl ring exhibits a modest

3.2° twist with respect to the olefinic plane. The C(17)-

C(18) bond distance of 1.418(3) Å is similar to that of

the complexes described herein and elsewhere.

Slow cooling of a petroleum ether solution of 22

deposits yellow crystals that are suitable for X-ray

diffraction analysis. The solid-state structure of 22,

shown in Figure 5, reveals that 22 adopts the geometry

of the favored niobium isomer 21a (the two are iso-

structural). The hydride ligand was located in the

difference map (but not refined) and displays a Ta-H

bond length of 1.87 Å. The C(17)-C(18) bond length of

1.445(9) Å and the 4.2° twist to the phenyl ring with

respect to the plane of the olefin is consistent with the

other ansa-tantalocene styrene hydride complexes.

P r ep a r a tion of Dou bly Silylen e-Br id ged a n sa -

Ta n ta locen e Olefin Hyd r id e Com p lexes. Reaction

of the dipotassio ligand salt K₂[(1,2-SiMe₂)₂(η⁵-C₅H-3,5-

(CHMe₂)₂)(η⁵-C₅H₂-4-CMe₃)] (K₂tBuThp) with TaCl₂-

Me₃in benzene-d₆provides a mixture of two products:

[(1,2-SiMe₂)₂(η²-C₅H-3,5-(CHMe₂)₂)(η⁵-C₅H₂-4-CMe₃)]Ta-

(CH₃)₃(tBuThpTa(CH₃)₃; 23) and [(1,2-SiMe₂)₂(η⁵-C₅H-

3,5-(CHMe₂)₂)(η⁵-C₅H₂-4-CMe₃)]Ta(CH₂)CH₃(tBuThpTa-

(CH₂)CH₃; 24) (but in only 24% and 18% yields; eq 12).

undergoes isomerization (∼30 min at 23 °C) to the

thermodynamic mixture of styrene hydride products.

The major isomer, 20a , is the one for which the phenyl

group of the styrene ligand is directed anti to the

isopropyl groups and proximal to the 4-isopropyl sub-

stituent, consistent with the major isomer formed in the

ethylene hydride complex, 14a (vide supra), although

for 20 the preference is even less.

Alkylation of either 2 or 9 with C₆H₅CH₂CH₂MgCl in

diethyl ether affords two isomers of tBuSpNb(η²-CH₂-

CHPh)H (21) in a 90:10 ratio and only one detectable

isomer of tBuSpTa(η²-CH₂CHPh)H, 22 (eq 11). Slow

cooling of a petroleum ether solution of 21 provides

yellow crystals (major isomer, 21a ) suitable for X-ray

diffraction analysis, as shown in Figure 4. The solid-

state structure reveals that the preferred isomer has

the styrene coordinated in an endo fashion in the open

Related ansa-tantalocenes with cyclopentadienyl coor-

dination modes analogous to 23 are described else-

where.²⁵

180 Organometallics, Vol. 22, No. 1, 2003

Chirik et al.

F igu r e 4. Molecular structure of 21a with 50% probability ellipsoids. Hydrogen atoms (other than hydride shown at

arbitrary scale) have been omitted.

F igu r e 5. Molecular structure of 22 with 50% probability ellipsoids. Hydrogen atoms have been omitted except for the

hydride, which was located in the difference map but not refined.

The mechanism for the formation of 24 is not under-

stood. Interestingly, complex 23 does not appear to be

a precursor to 24; benzene-d₆solutions of isolated 23

do not provide detectable amounts of 24 over the course

of months at 25 °C, whereas heating these solutions

leads only to decomposition to other unidentified ma-

terials. Moreover, addition of the dipotassio ligand salt

K₂tBuThp to solutions of 23 does not lead to 24,

implying that there are two independent pathways for

formation of 23 and 24 from TaCl₂(CH₃)₃.

The NMR spectral features of the methylene ligand

for 24 are most diagnostic of a tantalum alkylidene

complex (¹H NMR (benzene-d₆), two doublets at δ 9.30

and 9.58; ¹³C NMR (benzene-d₆), δ 211.3).²⁶Slow cooling

of a diethyl ether/pentane solution of 24 affords trans-

parent crystals that are suitable for X-ray diffraction.

The solid-state structure of 24, shown in Figure 6,

reveals η⁵:η⁵hapticity of the cyclopentadienyl rings. The

tantalocene unit sits on a crystallographic mirror plane,

and as a consequence the methylidene and methyl

substituents are disordered, so that the Ta-C(15) bond

distance is 2.154 Å, representing an average of Ta-CH₃

and TadCH₂tantalum-carbon bond lengths.

Much like the (η⁵-C₅Me₅)₂Ta(CH₂)(CH₃) analogue,²⁶

thermolysis of 24 at 100 °C in benzene-d₆over the

course of days affords the ansa-tantalocene ethylene

hydride complex tBuThpTa(η²-CH₂CH₂)H (25), arising

from R-migratory insertion of the methyl group into the

carbene ligand, followed by â-H elimination (eq 13).

(25) Chirik, P. J . Ph.D. Thesis, California Institute of Technology,

2000. Zubris, D. L. Ph.D. Thesis, California Institute of Technology,

2001. A drawing and structural details for 23 are included in the

Supporting Information.

(26) Parkin, G.; Bunel, E.; Burger, B. J .; Trimmer, M. S.; Asselt, A.

V.; Bercaw, J . E. J . Mol. Catal. 1987, 41, 21.

ansa-Niobocene and ansa-Tantalocene Complexes

Organometallics, Vol. 22, No. 1, 2003 181

ligand array in the syndiospecific polymerization of

propylene.^12,13

In the case of the styrene hydride complex 27, three

distinct ortho, meta, and para styrene resonances are

detected in the ¹H NMR spectrum at room temperature,

suggesting free rotation about the C-C_ipsobond. NOE

difference NMR spectroscopy indicates that the pre-

ferred isomer is analogous to that for the propylene

hydride analogue, i.e. where the olefin is coordinated

in an endo arrangement with the phenyl substituent

directed between the two isopropyl groups on the

cyclopentadienyl ring. It is notable that complexes 26

and 27 exhibit the same thermodynamic preferences for

olefin binding despite the increased sterics of bound

styrene.

Con clu sion s

A series of singly [SiMe₂]-bridged ansa-niobocene and

-tantalocene olefin hydride complexes have been pre-

pared via reduction and alkylation of the corresponding

niobocene and tantalocene dichloride complexes. Their

geometries have been determined in solution by NOE

difference NMR spectroscopy, and in several cases the

solid-state structures have been established by X-ray

diffraction. Monosubstitution of the cyclopentadienyl

framework with an isopropyl or tert-butyl group ([Me₂-

Si(η⁵-C₅H₄)(η⁵-C₅H₃-3-R)]M(olefin)H; R ) CHMe₂, CMe₃)

directs ethylene coordination such that the olefin is

distal from R, ca. 20:1 for isopropyl and >50:1 for tert-

butyl. The methyl groups of the [SiMe₂] linker force

R-olefins such as propylene and styrene to coordinate

with the olefin substituent directed toward the hydride

ligand (endo), unlike the “parent” [(η⁵-C₅H₅)₂M] olefin

hydride complexes, for which approximately equal

amounts of endo and exo isomers are obtained for the

propylene and styrene hydrides. As for the ethylene

hydride complexes, distal coordination of R-olefins is

preferred; however, neither isopropyl nor tert-butyl

substitution of one cyclopentadienyl ligand enforces a

strong enantiofacial preference for olefin coordination

for the niobium olefin hydrides, although only one

mirror-plane-related pair of diastereomers is observed

for [Me₂Si(η⁵-C₅H₄)(η⁵-C₅H₃-3-CMe₃)]Ta(CH₂CHPh)H.

For singly [SiMe₂]-bridged ansa-tantalocene olefin

hydride complexes having 2,4-diisopropyl substitution

on one cyclopentadienyl ring ([Me₂Si(η⁵-C₅H₄)(η⁵-C₅H₂-

2,4-(CHMe₂)₂)]Ta(olefin)H), generally lower selectiv-

ity for one coordination site over the other is exhibited

for ethylene or styrene. Thus, the isopropyl groups

appear to work at odds to each other. It is interesting

to note that Miya noted lower isospecificity for propyl-

ene polymerizations with [Me₂Si(η⁵-C₅H₄)(η⁵-C₅H₂-2,4-

(CHMe₂)₂)]ZrCl₂/MAO as compared with [Me₂Si(η⁵-

C₅H₄)(η⁵-C₅H₃-3-CHMe₂)]ZrCl₂/MAO.^14aA doubly 1,2-

[SiMe₂]-bridged ansa-tantalocene ethylene hydride

complex has been prepared from thermolysis of the

corresponding methylidene methyl complex. Addition of

propylene or styrene to the ethylene hydride complex

results in olefin exchange, affording only one isomer of

the R-olefin hydride complex, that with the methyl or

phenyl group directed away from the tert-butyl sub-

stituent and between the isopropyl groups of the other

cyclopentadienyl ligand.

F igu r e 6. Molecular structure of 24 with 50% probability

ellipsoids. Hydrogen atoms have been omitted.

Monitoring the progress of the reaction by ¹H NMR

spectroscopy reveals the reaction is quantitative and

may be carried out on a preparative scale in toluene

with only slightly diminished yields.

Addition of excess propylene or styrene to 25 (in

separate experiments) results in olefin exchange and

formation of the tantalocene propylene hydride or

styrene hydride complex tBuThpTa(η²-CH₂CHR)H (R )

CH₃(26), C₆H₅(27)) (eq 14). Although several stereo-

isomers are possible, only two mirror-plane-related

diastereomeric tantalocene R-olefin hydride complexes

form in NMR-detectable amounts (one is shown in eq

14; syn indicates the relative orientation of the olefin

substituents and the isopropyl groups on the cyclopen-

tadienyl ring). NOE difference NMR spectroscopy indi-

cates that the preferred diastereomer has the propylene

coordinated in an endo fashion with the propylene

methyl group directed away from the bulky tert-butyl

substituent toward the open region between the isopro-

pyl groups. Unfavorable steric interactions between the

propylene methyl group and the dimethylsilylene link-

ers appear to restrict coordination of the olefin to an

endo arrangement. Formation of this diastereomer is

consistent with the model previously proposed for this

Thus, the placement of a single isopropyl or tert-butyl

substituent on a cyclopentadienyl ligand of [Me₂Si(η⁵-

182 Organometallics, Vol. 22, No. 1, 2003

Chirik et al.

C₅H₄)(η⁵-C₅H₃-3-R)]M(olefin)H has a modest effect in

directing the olefin coordination geometry. It would, of

course, be of interest to establish the stereodirecting

effects of these substituents on the conformations of the

M-R′ group for the corresponding [Me₂Si(η⁵-C₅H₄)(η⁵-

C₅H₃-3-R)]M(olefin)R′ complexes, particularly for those

metal alkyls that mimic the polymeryl groups during

propylene polymerization, e.g. [M-CH₂CH(CH₃)CH₂-

CHMe₂], since it has been established that the interac-

tions of the olefin substituents with the polymeryl group

are greater than with the ligand substituents (vide

supra). Unfortunately, all attempts to induce olefin

insertion and olefin coordinative trapping of the result-

ant alkyls using these ansa-olefin hydrides were unsuc-

cessful. Presumably, the equilibrium needed to provide

such olefin alkyls (eq 15) lies too far to the left, even for

R-olefins.

overnight. The Et₂O was removed in vacuo leaving a light

brown powder. The product was isolated by dissolving the

crude mixture in 100 mL of CH₂Cl₂followed by filtration of

the LiCl. The CH₂Cl₂was removed in vacuo, leaving 5.63 g

(87.2%) of a dark brown solid identified as 1. Anal. Calcd for

NbSiC₁₅H₂₀Cl₂: C, 46.05; H, 4.90. Found: C, 46.04; H, 4.97.

EPR (CH₂Cl₂): g_iso) 2.01; a_iso) 99.0 G.

[Me₂Si(η⁵-C₅H₄)(η⁵-C₅H₃-3-CMe₃)]NbCl₂(2). This com-

pound was prepared in a manner analogous to that for 1,

employing 1.00 g (3.91 mmol) of Li₂[Me₂Si(η⁵-C₅H₄)(η⁵-C₅H₃-

3-CMe₃)] and 1.48 g of NbCl₄(THF)₂(3.91 mmol), to afford 1.20

g (75.9%) of a dark brown solid identified as 2. The isolated

solid was sublimed, leaving 0.276 g (17.5%) of product. Anal.

Calcd for NbSiC₁₆H₂₂Cl₂: C, 47.30; H, 5.35. Found: C, 46.88;

H, 5.46. EPR (CH₂Cl₂): g_iso) 2.00; a_iso) 103.8 G.

[Me₂Si(η⁵-C₅H₃-3-CMe₃)₂]NbCl₂(3). This compound was

prepared in a manner analogous to that for 1, employing 3.00

g (9.60 mmol) of Li₂[Me₂Si(η⁵-C₅H₃-3-CMe₃)₂] and 3.64 g of

NbCl₄(THF)₂(9.60 mmol), to afford a dark brown solid identi-

fied as 3. The isolated solid was further purified by extraction

into petroleum ether. The solvent was removed, leaving 1.26

g (28.4%) of product. Anal. Calcd for NbSiC₂₀H₃₀Cl₂: C, 52.00;

H, 6.54. Found: C, 52.42; H, 6.79. EPR (CH₂Cl₂): g_iso) 2.00;

a_iso) 103.8 G.

[Me₂Si(C₅H₄)(C₅H₃-3-CHMe₂)][Sn (n -Bu )₃]₂. In the dry-

box, a 100 mL round-bottom flask was charged with 3.50 g

(12.3 mmol) of Li₂[Me₂Si(η⁵-C₅H₄)(η⁵-C₅H₃-3-CHMe₂)]. The

white solid was slurried in 30 mL of Et₂O. Via pipet, 8.32 g

(25.2 mmol) of n-Bu₃SnCl was added over the course of 15 min.

The reaction flask was then attached to a swivel frit assembly,

and the contents were stirred for 3 days at room temperature.

The yellow Et₂O solution was filtered and the white precipitate

washed with recycled Et₂O. The Et₂O was removed in vacuo,

leaving 15.5 g (76.1%) of a yellow oil identified as [Me₂Si(C₅H₄)-

Exp er im en ta l Section

Gen er a l Con sid er a tion s. All air- and moisture-sensitive

compounds were manipulated using standard vacuum line,

Schlenk, or cannula techniques or in a drybox under a nitrogen

atmosphere as described previously.²⁷Argon, dinitrogen, and

dihydrogen gases were purified by passage over columns of

MnO on vermiculite and activated molecular sieves. Toluene

and petroleum ether were distilled from sodium and stored

under vacuum over titanocene.²⁸Tetrahydrofuran, dimethoxy-

ethane, and ether were distilled from sodium benzophenone

ketyl. Tetrahydrofuran-d₈was distilled from sodium ben-

zophenone ketyl and stored over 4 Å molecular sieves. Dichlo-

romethane-d₂was distilled from CaH₂. NbCl₄(THF)₂, TaCl₅, (n-

Bu)₃SnCl, 3.0 M CH₃CH₂MgBr in diethyl ether, 2.0 M

CH₃CH₂CH₂MgCl in diethyl ether, and 1.0 M PhCH₂CH₂MgCl

in THF were purchased from Aldrich and used as received.

Propylene was purchased from Aldrich and dried over Al(iBu)₃.

Styrene was purchased from Aldrich and distilled from CaH₂

before use. Zinc dust was purchased from Strem Chemical and

dried at 100 °C under high vacuum. Ethylene was purchased

from Matheson and passed through a dry ice trap immediately

before use. All dilithio salts of ligands were prepared according

to standard procedures,²⁹while TaCl₂Me₃was prepared ac-

cording to the literature procedure.³⁰

1

(C₅H₃-3-CHMe₂)][Sn(n-Bu)₃]₂. H NMR (benzene-d₆): δ 0.162

(s, 6H, SiMe₂); 0.760 (t, 7 Hz, 18H, SnCH₂CH₂CH₂CH₃); 1.10

(m, 12H, SnBu₃); 1.13 (m, 12H, SnBu₃); 1.41 (m, 12H, SnBu₃);

2.03 (sept, 1H, CHMe₂); not located (CHMe₂); 5.13, 5.24, 5.67,

5.96, 6.02, 6.58 (m, 1H, Cp). ¹³C NMR (benzene-d₆): δ 0.595

(SiMe₂); 12.23 (SnBu₃); 14.37 (SnBu₃); 17.92 (SnBu₃); 28.32

(SnBu₃); 27.68, 29.89 (CHMe₂); 29.78 (CHMe₂); 109.96, 114.67,

116.78, 121.59, 124.46, 133.24, 133.46, 3 not located (Cp).

[Me₂Si(η⁵-C₅H₄)(η⁵-C₅H₃-3-CHMe₂)]Ta Cl₃(4). In the dry-

box, 4.90 g (6.06 mmol) of [Me₂Si(C₅H₄)(C₅H₃-3-CHMe₂)][Sn-

(n-Bu)₃]₂was charged into a 100 mL round-bottom flask and

dissolved in approximately 50 mL of Et₂O, forming a clear

solution. Small portions of TaCl₅(2.17 g, 6.05 mmol) were then

added slowly to the reaction mixture. A red solid forms upon

addition of TaCl₅. The flask was attached to a swivel frit

assembly and the reaction mixture stirred for 12 h. The solid

was collected by filtration and was washed with small portions

of Et₂O. The red solid was dried in vacuo, yielding 2.00 g (64%)

of 4. ¹H NMR (THF-d₈): 0.895 (s, 3H, SiMe₂); 0.961 (s, 3H,

SiMe₂); 1.29 (d, 6.5 Hz, 3H, CHMe₂); 1.37 (d, 6.5 Hz, 3H,

CHMe₂); 2.87 (sept, 7 Hz, 1H, CHMe₂); 6.28, 6.56, 6.97, 7.28,

7.35, 7.61, 7.79 (m, 1H, Cp). ¹³C NMR: (THF-d₈): δ -5.20

(SiMe₂); -5.48 (SiMe₂); 20.77 (CHMe₂); 24.44 (CHMe₂); 30.52

(CHMe₂); 115.34, 116.56, 116.78, 135.42, 138.29, 138.47 (Cp).

[Me₂Si(C₅H₄)(C₅H_2-2,4-(CHMe₂)₂)][Sn (n -Bu )₃]₂. This com-

plex was prepared in a manner analogous to that for [Me₂Si-

(C₅H₄)(C₅H₃-3-CHMe₂)][Sn(n-Bu)₃]₂with 3.80 g (13.6 mmol) of

Li₂[Me₂Si(C₅H₄)(C₅H₂-2,4-(CHMe₂)₂)] and 9.70 g (29.4 mmol)

of n-Bu₃SnCl, affording 10.5 g (90.5%) of [Me₂Si(C₅H₄)(C₅H₂-

2,4-(CHMe₂)₂)][n-Bu₃Sn]₂. ¹H NMR (CD₂Cl₂): δ 0.02 (s, 3H,

SiMe₂); 0.30 (s, 3H, SiMe₂); 0.81 (t, 7 Hz, 18H, SnBu₃); 0.89 (t,

7 Hz, 18H, SnBu₃); 1.1-1.6 (m, SnBu₃); 2.60 (sept, 7 Hz, 1H;

CHMe₂); 2.69 (sept, 7 Hz, 1H; CHMe₂); 5.30, 5.55, 5.91, 6.02,

6.10, 1 not located (m, 1H, Cp).

NMR spectra were recorded on a J EOL GX-400 (¹H, 399.78

MHz; ¹³C, 100.53 MHz) or a Varian Inova 500 instrument (¹H,

500.13 MHz; ¹³C, 125.77 MHz). All chemical shifts are relative

to TMS for ¹H (residual) and ¹³C (solvent as a secondary

standard). Nuclear Overhauser difference experiments were

carried out on a Varian Inova 500 MHz spectrometer. Elemen-

tal analyses were carried out at the Caltech Analytical Facility

by Fenton Harvey or by Midwest Microlab, Indianapolis, IN.

[Me₂Si(η⁵-C₅H₄)(η⁵-C₅H₃-3-CHMe₂)]NbCl₂(1). In the dry-

box, 6.23 g (16.4 mmol) NbCl₄(THF)₂and 4.00 g (16.4 mmol)

of Li₂[Me₂Si(η⁵-C₅H₄)(η⁵-C₅H₃-3-CHMe₂)] were combined in a

300 mL round-bottom flask. On the vacuum line, 175 mL Et₂O

was added by vacuum transfer. The reaction was stirred

(27) Burger, B. J .; Bercaw, J . E. In Experimental Organometallic

Chemistry; Wayda, A. L., Darensbourg, M. Y., Eds.; ACS Symposium

Series 357; American Chemical Society: Washington, DC, 1987;

Chapter 4.

(28) Marvich, R. H.; Brintzinger, H. H. J . Am. Chem. Soc. 1971,

93, 2046.

(29) Herzog, T. A. Ph.D. Thesis, California Institute of Technology,

1997.

(30) Schrock, R. R.; Sharp, P. R. J . Am. Chem. Soc. 1978, 100, 2389.

[Me₂Si(η⁵-C₅H₄)(η⁵-C₅H₂-2,4-(CHMe₂)₂)]Ta Cl₃(5). This

complex was prepared in a manner analogous to that for 4

with 6.50 g

(7.58 mmol) of [Me₂Si(η⁵-C₅H₄)(η⁵-C₅H₂-2,4-

ansa-Niobocene and ansa-Tantalocene Complexes

Organometallics, Vol. 22, No. 1, 2003 183

1

(CHMe₂)₂)][Sn(n-Bu)₃]₂and 2.70 g (7.54 mmol) of TaCl₅,

affording 2.54 g (60%) of 5. H NMR (CD₂Cl₂): δ 0.05 (s, 3H,

SiMe₂); 0.30 (s, 3H, SiMe₂); 2.64 (sept; 7 Hz, 1H, CHMe₂); 2.72

(sept; 7 Hz, 1H, CHMe₂); 5.31, 5.57, 5.92, 6.42, 6.49, 6.58 (m,

1H, Cp). Anal. Calcd for TaSiC₁₈H₂₆Cl₃: C, 38.76; H, 4.70.

Found: C, 37.96; H, 4.55.

Min or Isom er 10b. Yield: 5%. H NMR (-30 °C, toluene-

1

d₈): δ -2.69 (s, 1H, Nb-H); 0.13 (s, 3H, SiMe₂); not located

(s, 3H, SiMe₂); 0.96 (m, 2H, CH₂dCH₂, exo); 1.57 (m, 2H, CH₂d

CH₂, endo); 2 not located (CHMe₂); 2.67 (m, 1H, CHMe₂); 3.43,

5.03 (2H), 5.53, 5.76, 2 not located (m, 1H, Cp). ¹³C NMR

(benzene-d₆): δ -6.53 (SiMe₂); -4.49 (SiMe₂); 12.88 (CH₂dCH₂,

endo); 18.27 (CH₂dCH₂, exo); 21.58 (CHMe₂); 25.77 (CHMe₂);

31.72 (CHMe₂); 85.01, 85.22, 93.29, 100.13, 102.26, 105.70, 4

not located (Cp).

[Me₂Si(η⁵-C₅H₄)(η⁵-C₅H₃-3-CHMe₂)]Ta(η²-CH₂CH₂)H (11).

In the drybox, a fine swivel frit assembly was charged with

0.300 g (0.625 mmol) of 7. On the vacuum line, approximately

25 mL of Et₂O was added by vacuum transfer. At -80 °C,

against an Ar counterflow, 1.00 mL (3.0 mmol) of a 3.0 M CH₃-

CH₂MgBr solution was added by syringe. The reaction mixture

was stirred and slowly warmed to room temperature. After 2

h, an orange solution and brown precipitate form. The reaction

mixture was stirred for 16 h, after which time the Et₂O was

removed and replaced with 10 mL of petroleum ether. The

product was extracted several times with petroleum ether. The

solvent was removed in vacuo, leaving a yellow oil identified

[Me₂Si(C₅H₄)(C₅H_3-3-CMe₃)][n -Bu ₃Sn ]₂. This complex was

prepared in a manner analogous to that for [Me₂Si(C₅H₄)(C₅H₃-

3-CHMe₂)][Sn(n-Bu)₃]₂with 3.00 g (11.7 mmol) of Li₂[Me₂Si-

(C₅H₄)(C₅H₃-3-CMe₃)] and 8.10 g (24.5 mmol) of n-Bu₃SnCl,

affording 8.73 g (90%) of [Me₂Si(C₅H₄)(C₅H₃-3-CMe₃)][Sn(n-

Bu)₃]₂. ¹H NMR (benzene-d₆): δ 0.42 (s, 3H, SiMe₂); 0.45 (s,

3H, SiMe₂); 1.45 (s, 9H, CMe₃); 0.90-1.10, 1.30-1.65 (m,

SnBu₃); 5.75, 5.81, 6.24, 6.56, 6.73, 6.88, 6.92 (m, 1H, Cp). ¹³

C

NMR (benzene-d₆): δ 11.72, 12.17 (SiMe₂); 30.11 (CMe₃);

154.45 (CMe₃); 14.26, 17.71, 27.45, 28.00, 28.22, 28.55, 29.85,

32.00 (SnBu₃); 97.54, 98.67, 103.59, 115.63, 121.51, 126.86,

132.29, 132.98, 2 not located (Cp).

[Me₂Si(η⁵-C₅H₄)(η⁵-C₅H₃-3-CMe₃)]Ta Cl₃(6). This complex

was prepared in a manner analogous to that for 4 with 4.55 g

(5.47 mmol) of [Me₂Si(C₅H₄)(C₅H₃-3-CMe₃)][Sn(n-Bu)₃]₂and

1

1.96 g (5.47 mmol) of TaCl₅, affording 1.50 g (51.7%) of 6. H

as 11. H NMR (benzene-d₆): δ -2.81 (s, 1H, Ta-H); 0.16 (s,

NMR (THF-d₈): δ 1.113 (s, 3H, SiMe₂); 1.17 (s, 3H, SiMe₂);

1.40 (s, 9H, CMe₃); 6.23, 6.38, 6.69, 7.27, 7.64, 7.80, 1 not

located (s, 1H, Cp). ¹³C NMR (THF-d₈): -1.68, -0.27 (SiMe₃);

33.34 (CMe₃); 151.71 (CMe₃); 109.52, 112.83, 118.90, 120.72,

124.61, 131.82, 144.62, 3 not located (Cp). Anal. Calcd for

TaSiC₁₆H₂₂Cl₃: C, 36.28; H, 4.19. Found: C, 35.75; H, 3.55.

[Me₂Si(η⁵-C₅H₄)(η⁵-C₅H₃-3-CHMe₂)]Ta Cl₂(7). This com-

pound was prepared in a manner analogous to that for 8,

employing 0.960 g (1.86 mmol) of 4 and 0.121 g (1.86 mmol)

of Zn dust, to afford 0.600 g (67.1%) of a green solid identified

as 7. EPR (CH₂Cl₂): g_iso) 1.94; a_iso) 101.3 G.

3H, SiMe₂); 0.29 (s, 3H, SiMe₂); 0.90 (m, 2H, CH₂dCH₂, endo);

0.93 (m, 2H, CH₂dCH₂, exo); 1.04 (s, 9H, CMe₃); 3.21, 3.23,

4.14, 4.22, 5.38 (2H), 5.97 (m, 1H, Cp). ¹³C NMR (benzene-d₆):

δ -6.23 (SiMe₂); -3.14 (SiMe₂); 2.44 (CH₂dCH₂, endo); 14.33

(CH₂dCH₂, exo); 32.36 (CMe₃); 143.78 (CMe₃); 79.54, 82.27,

92.55, 94.03, 100.94, 103.79, 105.28, 3 not located (Cp).

[Me₂Si(η⁵-C₅H₄)(η⁵-C₅H₃-3-CMe₃)]Nb(η²-CH₂CH₂)H (12).

This compound was prepared in a manner similar to that for

11, employing 0.180 g (0.444 mmol) of 2 and 326 µL (0.98

mmol) of a 3.0 M CH₃CH₂MgBr solution in Et₂O. Extraction

with petroleum ether followed by slow cooling to -40 °C

afforded 0.044 g (27%) of a yellow crystalline solid identified

[Me₂Si(η⁵-C₅H₄)(η⁵-C₅H₂-2,4-(CHMe₂)₂)]Ta Cl₂(8). In the

drybox, a fine swivel frit assembly was charged with 2.47 g

(4.43 mmol) of 5 and 0.288 g (4.43 mmol) of Zn dust. On the

vacuum line, the frit assembly was evacuated and approxi-

mately 50 mL of DME was added by vacuum transfer. After 4

h, a forest green reaction mixture formed. After it was stirred

for 16 h, the green DME solution was filtered and the

remaining solid was washed three times with recycled solvent.

The DME was removed in vacuo, leaving an oily green solid.

Addition of Et₂O afforded a green precipitate that was collected

by filtration and dried in vacuo, leaving 2.20 g (95.1%) of 8.

Sublimation at 140 °C and 10^-4Torr affords analytically pure

material. Anal. Calcd for TaSiC₁₅H₂₀Cl₂: C, 37.51; H, 4.20.

1

as 12. H NMR (benzene-d₆): δ -2.56 (s, 1H, Nb-H); 0.10 (s,

3H, SiMe₂); 0.19 (s, 3H, SiMe₂); 0.91 (m, 2H, CH₂dCH₂, exo);

1.33 (s, 9H, CMe₃); 1.42 (m, 2H, CH₂dCH₂, endo); 3.16, 3.19,

4.19, 4.25, 5.27 (2H), 5.86 (m, 1H, Cp). ¹³C NMR (benzene-d₆):

δ -6.94 (SiMe₂); -4.04 (SiMe₂); 11.17 (CH₂dCH₂, endo); 21.51

(CH₂dCH₂, exo); 31.85 (CMe₃); 32.00 (CMe₃); 70.18, 71.62,

81.22, 83.97, 92.09, 93.60, 102.55, 103.58, 105.46, 142.81 (Cp).

[Me₂Si(η⁵-C₅H₄)(η⁵-C₅H₃-3-CMe₃)]Ta (η²-CH₂CH₂)H (13).

This compound was prepared in a manner similar to that for

11, employing 0.400 g (0.833 mmol) of 9 and 0.610 mL (1.8

mmol) of a 3.0 M CH₃CH₂MgBr solution in diethyl ether.

Extraction with petroleum ether afforded 0.245 g (69.5%) of a

Found: C, 37.28; H, 4.23. EPR (CH₂Cl₂): g_iso) 1.94; a_iso

92.9 G.

)

1

yellow oil identified as 13. H NMR (benzene-d₆): δ -2.72 (s,

1H, Ta-H); 0.19 (s, 3H, SiMe₂); 0.30 (s, 3H, SiMe₂); 0.51 (m,

2H, CH₂dCH₂, endo); 1.10 (m, 2H, CH₂dCH₂, exo); 1.23 (d, 7

Hz, 3H, CHMe₂); 1.29 (d, 7 Hz, 3H, CHMe₂); 2.90 (sept, 7 Hz,

1H, CHMe₂); 3.40, 4.32, 4.42, 4.47, 5.35 (2H), 5.94 (m, 1H, Cp).

¹³C NMR (benzene-d₆): δ -6.92 (SiMe₂); -4.10 (SiMe₂); 11.87

(CH₂dCH₂, endo); 30.10 (CH₂dCH₂, exo); 23.65 (CHMe₂); 25.73

(CHMe₂); 28.45 (CHMe₂); 80.62, 82.38, 91.90, 93.26 102.10,

104.05, 105.49, 3 not located (Cp).

[Me ₂S i(η⁵-C ₅H ₄)(η⁵-C ₅H _2-2,4-(C H Me ₂)₂)]T a (η²-C H ₂-

CH₂)H (14a ,b). This compound was prepared in a manner

similar to that for 11, employing 0.340 g (0.651 mmol) of 8

and 0.477 mL (1.4 mmol) of a 3.0 M CH₃CH₂MgBr solution in

diethyl ether. Extraction with petroleum ether afforded 14 as

a yellow oil.

[Me₂Si(η⁵-C₅H₄)(η⁵-C₅H₃-3-CMe₃)]Ta Cl₂(9). This com-

pound was prepared in a manner analogous to that for 8,

employing 1.00 g (1.73 mmol) of 6 and 0.115 g (1.73 mmol) of

Zn dust, to afford 0.726 g (78.5%) of a green solid identified as

9. Anal. Calcd for TaSiC₁₆H₂₂Cl₂: C, 38.88; H, 4.49. Found:

C, 37.55; H, 4.36. EPR (CH₂Cl₂): g_iso) 1.94; a_iso) 101.2 G.

[Me ₂Si(η⁵-C₅H ₄)(η⁵-C₅H ₂-3-CH Me ₂)]Nb (η²-CH ₂CH ₂)H

(10a ,b). This compound was prepared in a manner analo-

gous to that for 11, employing 1.50 g (3.824 mmol) of 1 and

3.00 mL (9.0 mmol) of a 3.0 M CH₃CH₂MgBr solution in Et₂O

with dimethoxyethane as the solvent. Extraction with petro-

leum ether followed by slow cooling to -78 °C afforded 0.205

g (15.3%) of an oily yellow solid identified as 10.

Ma jor Isom er 10a . Yield: 95%. ¹H NMR (-30 °C, toluene-

d₈): δ -2.60 (s, 1H, Nb-H); 0.16 (s, 3H, SiMe₂); 0.08 (s, 3H,

SiMe₂); 0.85 (m, 2H, CH₂dCH₂, exo); 1.22 (d, 7 Hz, 3H,

CHMe₂); 1.26 (d, 7 Hz, 3H, CHMe₂); 1.37 (m, 1H, CH₂dCH₂,

endo); 1.48 (m, 1H, CH₂dCH₂, endo); 2.81 (sept, 7 Hz, 1H,

CHMe₂); 3.18, 3.19, 4.24, 4.28, 5.24, 5.26, 5.80 (m, 1H, Cp).

¹³C NMR (benzene-d₆): δ -6.79 (SiMe₂); -4.19 (SiMe₂); 11.25

(CH₂dCH₂, endo); 20.54 (CH₂dCH₂, exo); 23.93 (CHMe₂); 24.52

(CHMe₂); 28.96 (CHMe₂); 70.56, 71.97, 82.94, 84.66, 91.71,

93.00, 103.30, 104.65, 106.23, 136.86 (Cp).

1

Ma jor Isom er 14a . Yield: 78%. H NMR (benzene-d₆): δ

-2.69 (s, 1H, Ta-H); 0.15 (s, 3H, SiMe₂); 0.29 (s, 3H, SiMe₂);

0.42 (m, 2H, CH₂dCH_2,endo); 0.89 (m, 2H, CH₂dCH₂, exo);

0.95 (d, 7 Hz, 3H, CHMe₂); 0.92 (d, 7 Hz, 3H, CHMe₂); 1.26 (d,

7 Hz, 3H, CHMe₂); 1.36 (d, 7 Hz, 3H, CHMe₂); 2.78 (sept, 7

Hz, 1H, CHMe₂); 2.47 (sept, 7 Hz, 1H, CHMe₂); 3.88, 4.45, 4.97,

5.02, 5.26, 5.41. ¹³C NMR: (benzene-d₆): δ -1.86 (SiMe₂); 1.36

(SiMe₂); 30.55 (CH₂dCH₂, exo); 8.80 (CH₂dCH₂, endo); 20.88

(CMe₂); 22.72 (CMe₂); 27.71 (CMe₂); 13.90 (CMe₂); 29.70 (CMe₂);

184 Organometallics, Vol. 22, No. 1, 2003

Chirik et al.

30.18 (CMe₂); 89.57, 92.60, 96.61, 102.03, 104.22, 123.89,

125.99, 3 not located (Cp).

(CHMe₂); 79.50, 84.81, 91.02, 94.26, 98.10, 99.34, 107.38, 3 not

located (Cp).

[Me₂Si(η⁵-C₅H ₄)(η⁵-C₅H ₃-3-CMe₃)]Ta (η²-CH ₂CH CH ₃)H

(17a ). This compound was prepared in a manner similar to

that for 16, employing 0.500 g (0.935 mmol) of 9 and 1.05 mL

(2.1 mmol) of 2.0 M CH₃CH₂CH₂MgCl solution, to yield 0.220

1

Min or Isom er 14b. Yield: 22%. H NMR (benzene-d₆): δ

-2.78 (s, 1H, Ta-H); 0.17 (s, 3H, SiMe₂); 0.30 (s, 3H, SiMe₂);

0.25 (m, 2H, CH₂dCH₂, endo); 0.76 (m, 2H, CH₂dCH₂, exo);

0.97 (d, 7 Hz, 3H, CHMe₂); 1.30 (d, 7 Hz, 3H, CHMe₂); 2 not

located (CHMe₂); 2.04 (sept, 7 Hz, 1H, CHMe₂); 2.23 (sept, 7

Hz, 1H, CHMe₂); 4.26, 4.33, 4.78, 5.80, 5.10, 5.21. ¹³C NMR

(benzene-d₆): δ -4.63 (SiMe₂); 1.14 (SiMe₂); 30.61 (CH₂dCH₂,

exo); 10.72 (CH₂dCH₂, endo); 11.31 (CMe₂); 16.52 (CMe₂); 18.62

(CMe₂); not located (CMe₂); 26.29 (CMe₂); not located (CMe₂);

91.12, 94.08, 95.16, 100.82, 100.91, 123.60, 125.73, 3 not

located (Cp).

[Me₂Si(η⁵-C₅H₃-3-CMe₃)₂]Nb(η²-CH₂CH₂)H (15a ,b). This

compound was prepared in a manner similar to 11 by employ-

ing 0.286 g (0.619 mmol) of 3 and 0.480 mL (1.4 mmol) of a

3.0 M CH₃CH₂MgBr solution in Et₂O. Extraction with petro-

leum ether followed by slow cooling to -40 °C afforded a yellow

crystalline solid identified as the meso isomer. Subsequent

fractional recrystallization allows separation of rac and meso

isomers (0.030 g meso isomer isolated; 12% of total yield). The

rac isomer is isolated as an orange oil.

1

g (46.5%) of a yellow oil identified as 17. H NMR (benzene-

d₆): δ -2.88 (s, 1H, Ta-H); 0.02 (s, 3H, SiMe₂); 0.09 (s, 3H,

SiMe₂); 1.30 (s, 9H, CMe₃); 0.44 (dd, 10 Hz, 6 Hz, 1H, CH₂d

CHCH₃); 0.89 (dd, 10 Hz, 6 Hz, 1H, CH₂dCHCH₃); 1.02 (m,

1H, CH₂dCHCH₃); 2.17 (d, 6.5 Hz, 3H, CH₂dCHCH₃); 3.24,

3.63, 4.23, 4.56, 5.11, 5.59, 5.62 (m, 1H, Cp). ¹³C NMR

(benzene-d₆): δ -6.32 (SiMe₂); -4.01 (SiMe₂); 28.18 (CH₂d

CHCH₃); 28.98 (CH₂dCHCH₃); 23.19 (CH₂dCHCH₃); 32.23

(CMe₃); 142.46 (CMe₃); 80.37, 82.53, 92.42, 92.94, 103.91,

105.23, 105.58, 3 not located (Cp).

[Me₂Si(η⁵-C₅H₄)(η⁵-C₅H₃-3-CHMe₂)]Nb(η²-CH₂CHP h )H

(18a -c). This compound was prepared in a manner similar

to that for 11, employing 2.00 g (5.09 mmol) of 1 and 12 mL

(12 mmol) of a 1.0 M PhCH₂CH₂MgCl solution in THF.

Extraction with petroleum ether followed by removal of the

solvent in vacuo afforded 0.325 g (15.0%) of an oily yellow solid

identified as 18.

m eso Isom er 15a . Yield: 50%. ¹H NMR (500 MHz, toluene-

d₈): δ -2.70 (s, 1H, Nb-H); 0.15 (s, 3H, SiMe₂); 0.21 (s, 3H,

SiMe₂); 0.87 (m, 2H, CH₂dCH₂, exo); 1.28 (s, 18H, CMe₃); 1.37

(m, 2H, CH₂dCH₂, endo); 3.16, 4.28, 5.26 (m, 2H, Cp). ¹³C NMR

(300 MHz, toluene-d₈): δ -6.91 (SiMe₂); -3.73 (SiMe₂); 12.57

(CH₂dCH₂, endo); 22.60 (CH₂dCH₂, exo); 26.16 (CMe₃); 32.14

(CMe₃); 69.64, 81.19, 92.99, 102.13, 143.10 (Cp).

1

Ma jor Isom er 18a . Yield: 53%. H NMR (benzene-d₆): δ

-1.96 (s, 1H, Nb-H); -0.04 (s, 3H, SiMe₂); 0.08 (s, 3H, SiMe₂);

1.07 (dd, 10 Hz, 5 Hz, 1H, CH₂dCHPh, trans); 1.21 (d, 7 Hz,

3H, CHMe₂); 1.32 (d, 7 Hz, 3H, CHMe₂); 1.46 (dd, 13 Hz, 5

Hz, 1H, CH₂dCHPh, cis); 2.80 (sept, 7 Hz, 1H, CHMe₂); 3.50

(m, 1H, CH₂dCHPh); 3.58, 3.60, 4.35, 4.38, 4.96, 5.14, 5.49

(m, 1H, Cp). ¹³C NMR (benzene-d₆): δ -6.04 (SiMe₂); -3.94

(SiMe₂); 19.96 (CH₂dCHPh); 24.77 (CHMe₂); 25.15 (CHMe₂);

29.67 (CHMe₂); 39.43 (CH₂dCHPh); 73.63, 75.06, 85.55, 88.63,

94.21, 96.06, 105.15, 108.73, 113.36, 136.53 (Cp); 122.49 (C₆H₅,

para); not located (C₆H₅, ortho); 128.20 (C₆H₅, meta); 153.23

(C₆H₅, ipso).

1

r a c Isom er 15b. Yield: 50%. H NMR (300 MHz, toluene-

d₈): δ -2.39 (s, 1H, Nb-H); 0.18 (s, 3H, SiMe₂); 0.25 (s, 3H,

SiMe₂); not located (CH₂dCH₂, exo); 0.89 (s, 9H, CMe₃); not

located (CH₂dCH₂, endo); 1.29 (s, 9H, CMe₃); 3.15, 4.24, 4.27,

5.25, 5.34, 5.89 (m, 1H, Cp). ¹³C NMR (500 MHz, toluene-d₈):

δ -6.84 (SiMe₂); -3.33 (SiMe₂); 11.10 (CH₂dCH₂, endo); 17.85

(CH₂dCH₂, exo); 30.60, 32.07 (CMe₃); not located (CMe₃); 69.97,

72.42, 81.38, 85.96, 91.70, 95.65, 102.06, 102.43, 130.65, 142.87

(Cp).

[Me ₂S i (η⁵-C ₅H ₄)(η⁵-C ₅H ₃-3-C H Me ₂)]T a (η²-C H ₂C H -

CH₃)H (16a ,b). In the drybox, a fine swivel frit assembly was

charged with 0.225 g (0.514 mmol) of 7. On the vacuum line,

approximately 25 mL of Et₂O was added by vacuum transfer.

At -80 °C, against an Ar counterflow, 0.56 mL (1.1 mmol) of

a 2.0 M CH₃CH₂CH₂MgCl solution in diethyl ether was added

via syringe. The reaction mixture was stirred and slowly

warmed to room temperature. With warming an orange

solution and brown precipitate form. The reaction mixture was

stirred for 24 h, and the Et₂O was removed in vacuo and

replaced with petroleum ether. The product was extracted

several times with petroleum ether. The solvent was removed

in vacuo, leaving a thick orange oil identified as 16.

1

Min or Isom er 18b. Yield: 38%. H NMR (benzene-d₆): δ

-2.15 (s, 1H, Nb-H); -0.02 (s, 3H, SiMe₂); 0.09 (s, 3H, SiMe₂);

0.74 (d, 7 Hz, 3H, CHMe₂); 1.12 (dd, 10 Hz, 5 Hz, 1H, CH₂d

CHPh, trans); 1.23 (d, 7 Hz, 3H, CHMe₂); 1.36 (dd, 13 Hz, 5

Hz, 1H, CH₂dCHPh, cis); 1.52 (sept, 7 Hz, 1H, CHMe₂); 3.50

(m, 1H, CH₂dCHPh); 3.42, 3.45, 4.27, 4.30, 5.19, 5.55, 5.95

(m, 1H, Cp). ¹³C NMR (benzene-d₆): δ -6.24 (SiMe₂); -3.67

(SiMe₂); 20.63 (CH₂dCHPh); 22.53 (CHMe₂); 26.43 (CHMe₂);

26.79 (CHMe₂); 37.93 (CH₂dCHPh); 73.83, 75.04, 82.96, 86.81,

95.31, 96.15, 104.16, 110.15, 113.34, 138.34 (Cp); 122.15 (C₆H₅,

para); 127.45 (C₆H₅, ortho); not located (C₆H₅, meta); 152.95

(C₆H₅, ipso).

Tr a ce Isom er 18c. Yield: 9%. ¹H NMR (benzene-d₆): δ

-2.09 (s, 1H, Nb-H); -0.05 (s, 3H, SiMe₂); 0.06 (s, 3H, SiMe₂);

0.38 (dd, 10 Hz, 5 Hz, 1H, CH₂dCHPh, trans); 0.94 (d, 7 Hz,

3H, CHMe₂); not located (CHMe₂); not located (CH₂dCHPh,

cis); not located (CHMe₂); not located (CH₂dCHPh); 4.15, 4.46,

4.67, 4.76, 4.92, 5.21, 5.32 (m, 1H, Cp). ¹³C NMR (benzene-

d₆): δ -5.40 (SiMe₂); -4.71 (SiMe₂); 18.77 (CH₂dCHPh); 23.32

(CHMe₂); 24.36 (CHMe₂); 27.03 (CHMe₂); 39.21 (CH₂dCHPh);

89.41, 90.51, 92.66, 95.20, 100.53, 105.09, 112.02, 137.66 (Cp);

122.34 (C₆H₅, para); not located (C₆H₅, ortho); not located

(C₆H₅, meta); 154.24 (C₆H₅, ipso).

1

Ma jor Isom er 16a . Yield: 64%. H NMR (benzene-d₆): δ

-2.71 (s, 1H, Ta-H); 0.04 (s, 3H, SiMe₂); 0.12 (s, 3H, SiMe₂);

1.23 (d, 7 Hz, 3H; CHMe₂); 1.30 (d, 7 Hz, 3H, CHMe₂); 2.89

(sept, 6.5 Hz, 1H, CHMe₂); 0.44 (dd, 10 Hz, 6 Hz, 1H, CH₂d

CHCH₃); 0.87 (dd, 10 Hz, 6 Hz, 1H CH₂dCHCH₃); 1.01 (m,

1H, CH₂dCHCH₃); 2.21 (d, 6.5 Hz, 3H, CH₂dCHCH₃); 3.37,

3.79, 4.40, 4.56, 5.15, 5.57, 5.59 (m, 1H, Cp). ¹³C NMR

(benzene-d₆): δ -6.63 (SiMe₂); -4.27 (SiMe₂); 30.17 (CH₂d

CHCH₃); 28.77 (CH₂dCHCH₃); 24.23 (CH₂dCHCH₃); 21.63

(CHMe₂); 21.76 (CHMe₂); 29.03 (CHMe₂); 81.62, 83.10, 81.31,

91.39, 104.37, 106.70, 106.81, 3 not located (Cp).

[Me₂Si(η⁵-C₅H ₄)(η⁵-C₅H ₃-3-CH Me₂)]Ta (η²-CH ₂CH P h )H

(19a -c). This compound was prepared in a manner similar

to that for 11, employing 0.400 g (0.914 mmol) of 7 and 2.28

mL (2.3 mmol) of 1.0 M PhCH₂CH₂MgCl in THF solution. A

golden yellow solid identified as 19 was obtained by slow

cooling of a petroleum ether solution followed by filtration

(0.120 g, 25.6%). Anal. Calcd for TaSiC₂₃H₂₉: C, 53.69; H, 5.68.

Found: C, 53.12; H, 5.82%.

1

Min or Isom er 16b. Yield: 21%. H NMR (benzene-d₆): δ

-2.65 (s, 1H, Ta-H); 0.10 (s, 3H, SiMe₂); 0.15 (s, 3H, SiMe₂);

1.25 (d, 7 Hz, 3H, CHMe₂); 1.27 (d, 7 Hz, 3H, CHMe₂); 2.80

(sept, 6.5 Hz, 1H, CHMe₂); 3 propylene resonances not located;

2.25 (d, 6.5 Hz, 3H, CH₂dCHCH₃); 3.30, 3.65, 4.36, 4.37, 5.18,

5.36, 5.91 (m, 1H, Cp). ¹³C NMR (benzene-d₆): δ -7.01 (SiMe₂);

-3.44 (SiMe₂); 29.69 (CH₂dCHCH₃); 23.85 (CH₂dCHCH₃);

25.98 (CH₂dCHCH₃); 21.28 (CHMe₂); 22.52 (CHMe₂); 27.76

1

Ma jor Isom er 19a . Yield: 54%. H NMR (benzene-d₆): δ

-1.53 (s, 1H, Ta-H); -0.03 (s, 3H, SiMe₂); 0.09 (s, 3H, SiMe₂);

1.26 (d, 7 Hz, 3H, CHMe₂); 1.34 (d, 7 Hz, 3H, CHMe₂); 2.92

(sept, 6.5 Hz, 1H, CHMe₂); 3.56 (m, 1H, CH₂dCHPh, cis); 3.59

(m, 1H, CH₂dCHPh, trans); 3.10 (t, 10 Hz, 1H, CH₂dCHPh);

ansa-Niobocene and ansa-Tantalocene Complexes

Organometallics, Vol. 22, No. 1, 2003 185

4.29, 4.33, 5.17, 5.23, 5.28, 5.56, 5.61 (m, 1H, Cp). ¹³C NMR

(benzene-d₆): δ -6.75 (SiMe₂); -4.36 (SiMe₂); 13.77 (CH₂d

CHPh); 30.11 (CH₂dCHPh); 21.70 (CMe₂); 24.26 (CHMe₂);

29.09 (CHMe₂); 83.12, 86.05, 93.46, 95.19, 105.34, 110.55, 4

not located (Cp); 121.90 (C₆H₅, meta); 136.79 (C₆H₅, ortho);

154.81 (C₆H₅, para); not located (C₆H₅, ipso).

0.99 (dd, 10 Hz, 5 Hz, 1H, CH₂dCHPh, trans); 1.34 (s, 9H,

CMe₃); 1.42 (dd, 13 Hz, 5 Hz, 1H, CH₂dCHPh, cis); 3.53 (dd,

13 Hz, 10 Hz, 1H, CH₂dCHPh); 3.25, 3.44, 4.13, 4.18, 5.16,

5.25, 5.49 (m, 1H, Cp). ¹³C NMR (toluene-d₈): δ -6.94 (SiMe₂);

-4.13 (SiMe₂); 20.96 (CH₂dCHPh); 32.13 (CMe₃); 32.21 (CMe₃);

37.81 (CH₂dCHPh); 73.00, 73.94, 82.70, 86.73, 96.48, 94.62,

104.20, 104.62, 111.90, 143.06 (Cp); 121.97 (C₆H₅, para); 127.28

(C₆H₅, ortho); 127.53 (C₆H₅, meta); 153.23 (C₆H₅, ipso).

Min or Isom er 21b. Yield: 10%. ¹H NMR (toluene-d₈): δ

-1.98 (s, 1H, Nb-H); 0.05 (s, 3H, SiMe₂); 0.15 (s, 3H, SiMe₂);

0.83 (s, 9H, CMe₃); not located (1H, CH₂dCHPh, trans); not

located (1H, CH₂dCHPh, cis); 3.18 (dd, 13 Hz, 10 Hz, 1H,

CH₂dCHPh); 3.07, 3.41, 4.00, 4.07, 5.09, 5.55, 6.13 (m, 1H,

Cp). ¹³C NMR (toluene-d₈): δ -7.09 (SiMe₂); -3.51 (SiMe₂);

not located (CH₂dCHPh); not located (CMe₃); 32.83 (CMe₃);

35.29 (CH₂dCHPh); 83.45, 85.24, 96.00, 100.23, 103.88, 106.11,

107.25, 121.59; 2 not located (Cp); not located (C₆H₅, ortho);

not located (C₆H₅, meta); not located (C₆H₅, para); not located

(C₆H₅, ipso).

[Me₂Si(η⁵-C₅H₄)(η⁵-C₅H₃-3-CMe₃)]Ta(η²-CH₂CHP h )H (22).

This complex was prepared in a manner analogous to that for

11, employing 0.500 g (0.935 mmol) of 9 and 2.10 mL (2.1

mmol) of 1.0 M PhCH₂CH₂MgCl solution in THF. Cooling a

concentrated petroleum ether solution affords 0.120 g (22.5%)

of a yellow solid identified as 22. ¹H NMR (benzene-d₆): δ

-1.602 (s, 1H, Ta-H); -0.0375 (s, 3H, SiMe₃); 0.0785 (s, 3H,

SiMe₃); 1.41 (s, 9H, CMe₃); 0.928 (m, 1H, CH₂dCHPh); 1.54

(m,1H, CH₂dCHPh); 3.142 (t, 12.2 Hz, 1H, CH₂dCHPh); 3.33,

3.46, 4.06, 4.17, 5.27, 5.46, 5.60 (m, 1H, Cp). ¹³C NMR

(benzene-d₆): δ -7.10 (SiMe₂); -4.15 (SiMe₂); 14.90 (CH₂d

CHPh); 28.68 (CH₂dCHPh); 31.73 (CMe₃); 143.59 (CMe₃);

72.39, 73.32, 80.52, 84.60, 94.59, 96.27, 102.40, 105.11, 2 not

located (Cp); 109.73 (C₆H₅, para); 121.96 (C₆H₅, ortho), 154.63

(C₆H₅, meta); not located (C₆H₅, ipso).

1

Min or Isom er 19b. Yield: 36%. H NMR (benzene-d₆): δ

-1.69 (s, 1H, Ta-H); -0.01 (s, 3H, SiMe₂); 0.093 (s, 3H, SiMe₂);

1.25 (d, 7 Hz, 3H, CHMe₂); 0.76 (d, 7 Hz, 3H, CHMe₂); 1.76

(sept, 6.5 Hz, 1H, CHMe₂); 3.48 (m, 1H, CH₂dCHPh, cis); 3.48

(m, 1H, CH₂dCHPh); 3.10 (t, 10 Hz, 1H, CH₂dCHPh); 4.24,

4.29, 4.32, 5.21, 5.27, 5.62, 6.04 (m, 1H, Cp). ¹³C NMR

(benzene-d₆): δ -6.92 (SiMe₂); -4.10 (SiMe₂); 14.26 (CH₂d

CHPh); 28.65 (CH₂dCHPh); 24.22 (CMe₂); 25.48 (CHMe₂);

26.31 (CHMe₂); 80.72, 84.27, 94.67, 95.66, 104.79, 106.37,

107.73, 110.34 (Cp); 121.54 (C₆H₅, meta); 138.88 (C₆H₅, ortho);

153.84 (C₆H₅, para); not located (C₆H₅, ipso).

Tr a ce Isom er 19c. Yield: 10%. ¹H NMR (benzene-d₆): δ

-1.74 (s, 1H, Ta-H); -0.04 (s, 3H, SiMe₂); 0.06 (s, 3H, SiMe₂);

0.92 (d, 7 Hz, 3H, CHMe₂); 1.13 (d, 7 Hz, 3H, CHMe₂); 2.40

(sept, 6.5 Hz, 1H, CHMe₂); not located (m, 2H, CH₂dCHPh);

4.12 (t, 10 Hz, 1H, CH₂dCHPh); 4.67, 4.69, 4.99, 5.34, 5.36, 2

not located (m, 1H, Cp). ¹³C NMR (benzene-d₆): δ -5.20

(SiMe₂); -6.02 (SiMe₂); 11.90 (CH₂dCHPh); 30.61 (CH₂d

CHPh); 20.28 (CMe₂); 22.50 (CHMe₂); not located (CHMe₂);

87.25, 88.31, 91.15, 91.59, 93.95, 100.24, 104.60, 110.07, 2 not

located (Cp); 123.99 (C₆H₅, meta); not located (C₆H₅, ortho);

not located (C₆H₅, para); not located (C₆H₅, ipso).

[Me₂Si(η⁵-C₅H ₄)(η⁵-C₅H ₂-2,4-(CH Me₂)₂)]Ta (η²-CH ₂CH -

P h )H (20a ,b). This compound was prepared in a manner

similar to that for 11, employing 0.385 g (0.737 mmol) of 8

and 1.62 mL (1.6 mmol) of 1.0 M PhCH₂CH₂MgCl in THF, to

yield 0.145 g (35.4%) of 20 as a yellow solid. Anal. Calcd for

TaSiC₂₆H₃₅: C, 56.11; H, 6.34. Found: C, 55.71; H, 6.54.

1

[(1,2-SiMe₂)₂(η²-C₅H-3,5-(CHMe₂)₂)(η⁵-C₅H₂-4-CMe₃)]Ta -

(CH₃)₃(23). A 250 mL round-bottom flask equipped with stir

bar was charged with K₂tBuThp (1.05 g, 2.27 mmol), and a

medium swivel frit assembly was attached. A 50 mL round-

bottom flask equipped with stir bar was charged with TaCl₂-

Me₃(0.674 g, 2.27 mmol), and a 180° needle valve was

attached. On the vacuum line, Et₂O (250 mL) was added to

the swivel frit assembly by vacuum transfer to provide an off-

white slurry, and Et₂O (25 mL) was added to the TaCl₂Me₃-

containing assembly by vacuum transfer to provide a bright

yellow solution. The K₂tBuThp slurry was cooled to -78 °C,

and the solution of TaCl₂Me₃was slowly added via cannula

transfer. After 15 min of stirring at -78 °C, the dry ice/acetone

bath was replaced with a 0 °C ice/water bath and the reaction

apparatus was covered in foil to exclude light. The reaction

mixture was stirred for 3 h, and then an orange solution was

filtered away from an off-white solid. The solid was washed

with Et₂O (3 × 25 mL), and solvent was removed in vacuo to

Ma jor Isom er 20a . Yield: 48%. H NMR (benzene-d₆): δ

-1.760 (s, 1H, Ta-H); 0.172 (s, 3H, SiMe₂); 0.094 (s, 3H,

SiMe₂); 0.940 (d, 6.5 Hz, 3H, CHMe₂); 1.06 (d, 6.5 Hz, 3H,

CHMe₂); 1.08 (d, 6.5 Hz, 3H, CHMe₂); 1.54 (d, 6.5 Hz, 3H,

CHMe₂); 2.61 (sept, 6.5 Hz, 1H, CHMe₂); 2.68 (sept, 6.5 Hz,

1H, CHMe₂); styrene resonances not located; 4.46, 4.51, 4.75,

4.85, 4.99, 5.23 (m, 1H, Cp); 6.90 (t, 5 Hz, 1H, C₆H₅, para);

7.47 (d, 7 Hz, 2H, C₆H₅, ortho); 7.28 (m, 2H, C₆H₅, meta). ¹³C

NMR (benzene-d₆): δ -3.82 (SiMe₂); -3.49 (SiMe₂); 12.48

(CH₂dCHPh); 30.73 (CH₂dCHPh); 21.47 (CHMe₂); 22.79

(CHMe₂); 24.24 (CHMe₂); 27.28 (CHMe₂); 29.28 (CHMe₂); 29.64

(CHMe₂); 88.69, 92.12, 96.08, 104.54, 109.03, 111.11, 113.58,

3 not located (Cp); 126.33 (C₆H₅, para); 136.96 (C₆H₅, ortho);

155.99 (C₆H₅, meta); not located (C₆H₅, ipso).

1

Min or Isom er 20b. Yield: 39%. H NMR (benzene-d₆): δ

-1.719 (s, 1H, Ta-H); -0.071 (s, 3H, SiMe₂); 0.257 (s, 3H,

SiMe₂); 1.01 (d, 6.5 Hz, 3H, CHMe₂); 1.11 (d, 6.5 Hz, 3H,

CHMe₂); 1.34 (d, 6.5 Hz, 3H, CHMe₂); 1.44 (d, 6.5 Hz, 3H,

CHMe₂); 2.42 (sept, 7 Hz, 1H, CHMe₂); 2.91 (sept, 1H, CHMe₂);

styrene resonances not located; 4.68, 4.82, 4.92, 5.07, 5.17, 5.89

(m, 1H, Cp); 6.87 (t, 5 Hz, 1H, C₆H₅, para); 7.40 (d, 7 Hz, 2H,

C₆H₅, ortho); 7.27 (m, 2H, C₆H₅, meta). ¹³C NMR (benzene-

d₆): δ -1.94 (SiMe₂); -5.45 (SiMe₂); 7.70 (CH₂dCHPh); 34.05

(CH₂dCHPh); 18.37 (CHMe₂); 21.97 (CHMe₂); 25.78 (CHMe₂);

28.24 (CHMe₂); not located (CHMe₂); not located (CHMe₂);

86.32, 88.06, 92.94, 94.23, 103.48, 108.65, 112.78, 3 not located

(Cp); 125.52 (C₆H₅, para); 140.03 (C₆H₅, ortho); not located

(C₆H₅, meta); not located (C₆H₅, ipso).

1

provide an orange residue in 25.6% yield (0.354 g). H NMR

(benzene-d₆): δ 0.38 (s, 6H, (CH₃)₂Si); 0.48 (s, 6H, (CH₃)₂Si);

1.12 (s, 9H, (CH₃)₃C); 1.16 (d, 6H, (CH₃)₂CH, 6.6 Hz); 1.38 (d,

6H, (CH₃)₂CH, 6.9 Hz); 1.49 (s, 9H, Ta-CH₃); 2.72 (m, 2H,

(CH₃)₂CH, 6.6 Hz); 6.51 (s, 1H, Cp); 6.59 (s, 1H, Cp). ¹³C NMR

(benzene-d₆): δ -28.52 (Ta-CH₃); -0.60, 5.24 ((CH₃)₂Si);

21.61, 25.39, 31.63 ((CH₃)₂CH); 31.93 ((CH₃)₃C); 32.67 ((CH₃)₃C);

73.99, 74.05, 75.83, 75.88 (C₅H₁); 102.36, 118.32, 123.41, 129.63

(C₅H₂).

[(1,2-SiMe₂)₂(η⁵-C₅H-3,5-(CHMe₂)₂)(η⁵-C₅H₂-4-CMe₃)]Ta -

(CH₂)CH₃(24). A 250 mL round-bottom flask equipped with

stir bar was charged with K₂tBuThp (2.68 g, 5.82 mmol) and

TaCl₂Me₃(1.73 g, 5.82 mmol), and a 180° needle valve was

attached. On the vacuum line, 125 mL of toluene was added

to the reaction flask by vacuum transfer at -78 °C. The dry

ice/acetone cooling bath was removed, and the reaction mixture

was warmed slowly to 25 °C. The reaction mixture was stirred

for 72 h, and then the solvent was removed in vacuo. In the

drybox, Et₂O (150 mL) was added to the reaction mixture and

[Me₂Si(η⁵-C₅H ₄)(η⁵-C₅H ₃-3-CMe₃)]Nb (η²-CH ₂CH P h )H

(21a ,b). This compound was prepared in a manner similar

to that for 11, employing 0.205 g (0.506 mmol) of 2 and 1.11

mL (1.1 mmol) of a 1.0 M PhCH₂CH₂MgCl solution in THF.

Extraction with petroleum ether followed by cooling to -40

°C afforded 0.054 g (24%) of a crystalline yellow solid identified

as 21.

Ma jor Isom er 21a . Yield: 90%. ¹H NMR (toluene-d₈): δ

-2.07 (s, 1H, Nb-H); -0.01 (s, 3H, SiMe₂); 0.12 (s, 3H, SiMe₂);

186 Organometallics, Vol. 22, No. 1, 2003

Ta ble 1. X-r a y Exp er im en ta l Da ta ^a

Chirik et al.

12

19b

20a

21a

22

24

formula

fw

cryst syst

space group

a, Å

b, Å

c, Å

R, deg

â, deg

γ, deg

V, Å³

C

₁₈H₂₇NbSi

C₂₃H₂₉SiTa

514.50

C₂₆H₃₅SiTa

556.58

C₂₄H₃₁NbSi

440.49

C₂₄H₃₁SiTa

528.53

C₂₆H₄₃Si₂Ta

592.73

364.40

monoclinic

P2₁/c (No. 14)

9.2022(6)

21.1677(14)

9.2632(6)

90

108.940(1)

90

1706.68(19)

4

monoclinic

P2₁/n (No. 14)

14.5277(9)

6.6809(4)

21.0147(12)

90

99.331(1)

90

2012.7(2)

4

triclinic

P1h (No. 2)

9.2305(8)

10.1700(9)

12.9218(12)

90.336(1)

104.187(1)

100.414(1)

1155.05(18)

2

triclinic

P1h (No. 2)

10.3001(11)

10.5568(11)

11.2207(12)

74.243(2)

84.333(2)

66.971(2)

1080.6(2)

2

triclinic

P1h (No. 2)

10.2246(18)

10.5009(18)

11.1345(19)

74.140(3)

84.118(3)

66.993(2)

1058.5(3)

2

monoclinic

P2₁/m (No. 11)

8.508(3)

16.123(6)

9.984(3)

90

112.39(3)

90

1266.3(8)

2

Z

F

_calcd, g/cm³

1.418

0.76

760

1.698

5.52

1016

1.600

4.82

556

1.354

0.62

460

1.658

5.25

524

1.555

4.45

600

µ, mm^-1

F₀₀₀

cryst shape

cryst color

cryst size, mm

lozenge

yellow

needle

yellow

thick blade

yellow

lozenge

yellow

stacked plates

yellow

twinned plate

golden orange

0.07 × 0.18 ×

0.19

0.10 × 0.26 ×

0.04 × 0.08 ×

0.14 × 0.18 ×

0.14 × 0.20 ×

0.03 × 0.15 ×

0.26

0.12

0.33

0.31

0.26

T, K

98

95

98

93

84

type of diffractometer

θ range, deg

h,k,l limits

SMART 1000 ccd

2.0-28.2

-11 to +12;

-13 to +13;

-17 to +16

10 310

CAD-4

2.2-25

-10 to +10;

-19 to +19;

-11 to +11

11 978

1.9-28.5

-12 to +12;

-28 to +28;

-12 to +12

25 466

1.6-27.5

-18 to +18;

-8 to +8;

-27 to +27

34 421

1.9-28.5

-13 to +13;

-13 to +14;

-14 to +14

22 478

1.9-28.2

-13 to +13;

-14 to +14

19 961

no. of data measd

no. of unique data

no. of data, F_o> 4σ(F_o)

no. of params/restraints

R1,^bwR2^c(all data

R1,^bwR2^c(F_o> 4σ(F_o))

GOF^don F²

4105

3597

289/0

0.028, 0.048

0.023, 0.048

1.84

4617

3186

226/0

0.084, 0.072

0.042, 0.066

1.58

5071

4848

389/0

0.027, 0.050

0.026, 0.050

2.19

5072

4407

359/0

0.037, 0.051

0.031, 0.050

1.53

4860

4347

257/0

0.052, 0.114

0.047, 0.112

1.29

1651

1576

140/0

0.030, 0.066

0.027, 0.066

2.38

∆F_max, ∆F_min, e Å^-3

0.51, -0.37

1.92, -1.84

2.33, -1.76

0.60, -0.43

3.62, -3.29

1.58, -0.82

a

All data were collected with graphite-monochromated Mo KR radiation (λ ) 0.710 73 Å). The structure of 23 is included in the Supporting

Information. R1 ) ∑||F_o| - |F_c||/∑|F_o|. ^cwR2 ) {∑[w(F_o- F_c²)²]/∑[w(F_o²)²]}^1/2

.

GOF ) S ) {∑[w(F_o- F_c²)²]/(n - p)}^1/2

.

b

2

d

2

then the contents were filtered through a bed of Celite and

washed with Et₂O (5 × 25 mL). A 250 mL round-bottom flask

equipped with stir bar was charged with the brown mother

liquor, and a fine swivel frit assembly was attached. On the

vacuum line the Et₂O was removed in vacuo and then 50 mL

of pentane was added by vacuum transfer. A brown superna-

tant was filtered away from a yellow precipitate, and the solid

was washed with pentane (2 × 25 mL). Solvent was removed

in vacuo to provide 24 as a mustard yellow solid in 20.9% yield

(0.723 g). ¹H NMR (benzene-d₆): δ 0.24 (s, 3H, SiMe₂); 0.36

(s, 3H, SiMe₂); 0.44 (s, 3H, SiMe₂); 0.45 (s, 3H, Ta-CH₃); 0.51

(s, 3H, SiMe₂); 1.10 (d, 7.0 Hz, 3H, CHMe₂); 1.12 (d, 5.9 Hz,

3H, CHMe₂); 1.16 (d, 6.6 Hz, 3H, CHMe₂); 1.39 (d, 6.6 Hz, 3H,

CHMe₂); 1.49 (s, 9H, CMe₃); 2.55 (m, 5.4 Hz, 1H, CHMe₂); 3.38

(m, 5.7 Hz, 1H, CHMe₂); 5.91 (s, 2H, Cp); 6.57 (s, 1H, Cp);

9.30 (d, 9.8 Hz, 1H, TadCH₂); 9.58 (d, 9.8 Hz, 1H, TadCH₂).

¹³C NMR (benzene-d₆): δ -2.01 (Ta-CH₃); -1.82, -1.40, 3.78,

4.00 (SiMe₂); 21.61, 22.20, 28.04, 28.90, 30.18, 30.64 (CHMe₂);

31.16 (CMe₃), 33.05 (CMe₃); 102.71, 118.67, 123.74 (Cp); 211.26

(TadCH₂).

[(1,2-SiMe₂)₂(η⁵-C₅H-3,5-(CHMe₂)₂)(η⁵-C₅H₂-4-CMe₃)]Ta -

(η²-CH₂CH₂)H (25). A 25 mL round-bottom flask equipped

with stir bar was charged with 24 (0.121 g, 0.203 mmol), and

a reflux condenser and 180° needle valve were attached. On

the vacuum line, 15 mL of toluene was added to the reaction

flask by vacuum transfer, providing a yellow-brown solution.

The apparatus was back-filled with Ar and then heated to

reflux. After 72 h at reflux, the toluene was removed in vacuo

and pentane (10 mL) was added by vacuum transfer and then

removed in vacuo to provide a brown solid in 96.5% yield (0.116

g). ¹H NMR (benzene-d₆): δ -2.06 (s, 1 H, Ta-H); 0.43 (s, 3H,

SiMe₂); 0.55 (s, 3H, SiMe₂); 0.63 (s, 3H, SiMe₂); 0.68 (s, 3H,

SiMe₂); 0.78 (d, 6.6 Hz, 3H, CHMe₂); 0.97 (s, 9H, CMe₃), 1.10

(d, 7.3 Hz, 3H, CHMe₂); 1.11 (d, 7.2 Hz, 3H, CHMe₂); 1.37 (d,

6.6 Hz, 3H, CHMe₂); 2.97 (m, 6.9 Hz, 1H, CHMe₂); 3.40 (m,

6.5 Hz, 1H, CHMe₂); 4.13, 5.81, 6.35 (s, 1H, Cp); CH₂dCH₂

not located. ¹³C NMR (benzene-d₆): δ -1.53, -0.78, 5.00, 6.14

SiMe₂); 21.75, 23.08 (CH₂dCH₂); 28.67, 29.18, 29.63, 30.44,

31.93, 33.04, 30.83 (CHMe_2,CMe₃); 98.36, 110.66, 111.18,

137.72 (Cp).

[(1,2-SiMe₂)₂(η⁵-C₅H-3,5-(CHMe₂)₂)(η⁵-C₅H₂-4-CMe₃)]Ta -

(η²-CH₂CHCH₃)H (26). A J . Young NMR tube was charged

with 25 (10.8 mg, 0.0182 mmol), and this compound was

dissolved in benzene-d₆to provide a red-brown solution. On

the vacuum line, the solution was frozen at -196 °C and the

NMR tube was degassed. Propylene (0.182 mmol) was con-

densed into the NMR tube at this temperature, and the tube

was closed and warmed to 25 °C. After 19 days at 25 °C, a

10:1 ratio of 26 to 25 is observed. ¹H NMR (benzene-d₆): δ

-1.86 (s, 1 H, Ta-H); -0.27 (br s, 1H, CH₂dCH(CH₃)); -0.19

(m, 2H, CH₂dCH(CH₃)); 0.42 (s, 3H, SiMe₂); 0.54 (s, 3H,

SiMe₂); 0.61 (s, 3H, SiMe₂); 0.66 (s, 3H, SiMe₂); 0.97 (d, 6.8

Hz, 3H, CHMe₂); 1.01 (s, 9H, CMe₃); 1.11 (d, 6.9 Hz, 3H,

CHMe₂); 1.16 (d, 7.1 Hz, 3H, CHMe₂); 1.35 (d, 6.6 Hz, 3H,

CHMe₂); 2.32 (d, 6.6 Hz, 3H, CH₂dCH(CH₃)); 3.10 (m, 6.3 Hz,

1H, CHMe₂); 3.38 (m, 7.0 Hz, 1H, CHMe₂); 4.19, 5.55, 6.32 (s,

1H, Cp). ¹³C NMR (benzene-d₆): δ -1.56, -0.91, 3.94, 4.17

(SiMe₂); 13.95, 21.77, 21.91 (CH₂dCH(CH₃)); 23.10, 28.68,

29.19, 29.40, 29.51, 30.67, 30.83, 32.03 (CHMe₂, CMe₃); 98.32,

110.14, 112.65, 126.66, 139.34, 147.42 (Cp).

[(1,2-SiMe₂)₂(η⁵-C₅H-3,5-(CHMe₂)₂)(η⁵-C₅H₂-4-CMe₃)]Ta -

(η²-CH₂CHP h )H (27). A J . Young NMR tube was charged

with 25 (20.0 mg, 0.0337 mmol), and this compound was

dissolved in benzene-d₆to provide a red-brown solution. A

small amount of a styrene/benzene-d₆solution was added

(0.4044 mmol, 12 equiv), and the reaction was monitored at

1

25 °C by H NMR spectroscopy. After 79 days at 25 °C, a 10:1

1

ratio of 27 to 25 is observed. H NMR (benzene-d₆): δ -1.10

(s, 1 H, Ta-H); 0.35 (s, 3H, SiMe₂); 0.48 (s, 3H, SiMe₂); 0.56

(s, 3H, SiMe₂); 0.58 (s, 3H, SiMe₂); 0.60 (d, 6.7 Hz, 3H, CHMe₂);

ansa-Niobocene and ansa-Tantalocene Complexes

Organometallics, Vol. 22, No. 1, 2003 187

0.95 (d, 7.0 Hz, 3H, CHMe₂); 1.08 (s, 9H, CMe₃); 1.14 (d, 7.1

Hz, 3H, CHMe₂); 1.18 (d, 6.7 Hz, 3H, CHMe₂); 1.29 (m, 2H

overlapping, CH₂dCHPh); 2.80 (t, 12.5 Hz, 1H, CH₂dCHPh);

3.19 (m, 7.2 Hz, 2H overlapping, CHMe₂); 3.63, 5.62, 6.43 (s,

1H, Cp); 6.82 (t, 7.1 Hz, 1H, C₆H₅, para); 7.23 (t, 8.0 Hz, 2H,

C₆H₅, meta); 7.47 (d, 7.9 Hz, 2H, C₆H₅, ortho). ¹³C NMR

(benzene-d₆): δ -1.84, -0.71, 3.89, 8.82 (SiMe₂); 21.29, 22.40

(CH₂dCHPh); 28.91, 29.42, 29.80, 31.26, 32.26, 33.13 (CHMe₂);

30.67 (CMe₃); 78.67, 81.34, 87.07, 102.93, 110.45, 112.08,

121.52, 126.31, 2 not located (Cp); not located (CMe₃); 142.41,

148.40, 156.48, 1 not located (C₆H₅).

tion, some of the largest peaks in the final difference map were

not next to the Ta atom. For 20a , there is one molecule in the

asymmetric unit. A satisfactory absorption correction was

obtained using SADABS. The coordinates and the temperature

factor of the hydride on the metal center were fixed during

the final least squares. All other hydrogen atoms were freely

refined. For 21a , there is one molecule in the asymmetric unit.

All hydrogen atoms, including the hydride, were refined

isotropically. Compound 22 (Ta) is isostructural with 21a (Nb);

therefore, there is one molecule in the asymmetric unit. The

hydrogen atoms were placed at calculated positions and

constrained to a reasonable geometry; the hydride parameters

were not refined. For 24, there is a half-molecule in the

asymmetric unit. The molecule lies on a mirror plane contain-

ing the Ta atom and bisecting the two Cp rings. This crystal-

lographically imposed symmetry has the unfortunate conse-

quence of confusing the chemically inequivalent dCH₂and

-CH₃ligands. All hydrogen atoms were placed at calculated

positions with U_isovalues fixed at 120% of the U_eqvalue of the

attached atom. The tert-butyl group is also disordered about

the mirror plane, as is seen in similar compounds. The first

attempts at refinement included all measured reflections; those

reflections which presumably included overlap between the

two twin components were given two sets of indices corre-

sponding to the two twins. Various criteria were used to

identify overlapping reflections. However, none of these ap-

proaches were particularly successful, presumably since most

reflections do not perfectly coincide. Finally, all reflections with

l ) 3, 6, 9, and 11 were removed from the data set; these

reflections transform with h′ deviating from integral values

by ∼0.05, 0.10, 0.15, and 0.15, respectively. Therefore, only

the hk0 reflections, which overlap perfectly with the h-k0

reflections of the twin, contain any twin contribution. The

h-k0 reflections were added as the symmetry-equivalent hk0

form. A twin parameter relating the relative contributions of

the two components was included in the least-squares matrix;

the fractional amount of the minor component was refined to

0.168(4).

Cr ysta llogr a p h y. Crystal data and intensity collection and

refinement details are presented in Table 1 for compounds 12,

19b, 20a , 21a , 22, and 24.

Da ta Collection a n d P r ocessin g. Data for compound 24

were collected on a Nonius CAD 4 serial diffractometer; data

for all other compounds were collected on a Bruker SMART

1000 area detector running SMART.³¹Both diffractometers

were equipped with Crystal Logic CL24 low-temperature

devices, and all data sets were collected at low temperature.

Both diffractometers used graphite-monochromated Mo KR

radiation with λ ) 0.710 73 Å.

CCD samples (12, 19b, 20a , 21a , 22): The crystals were

mounted on glass fibers with Paratone-N oil. Data were

collected as ω-scans at three to six values (depending on the

sample) of æ. For all crystals, the detector was 5 cm (nominal)

distant at a θ angle of -28°. The data were processed with

SAINT.³¹

CAD4 sample (24): The crystals were mounted on glass

fibers with Paratone-N oil. Indexing problems with several

crystals suggested twinning. Finally a crystal was found for

which a unit cell could be determined. Although this crystal

was also twinned, orientation matrices were obtained for both

twin components. The twin can be described as a reflection

twin across the (100) plane. The unit cell was calculated from

25 centered reflections. Approximately four sets of data were

collected with 1.0 ω-scans. CRYM³²programs were used to

apply Lorentz and polarization factors to the data set and to

merge the multiples in point group 2/m. Data which agreed

poorly in a preliminary merging were recollected. The indi-

vidual backgrounds were replaced by a background function

of 2θ derived from those reflections with I < 3σ(I). Weights w

were calculated as 1/σ²(F_o²); variances (σ²(F_o²)) were derived

from counting statistics plus an additional term, (0.014I)²;

variances of the merged data were obtained by propagation of

error plus another additional term, (0.014 I )².

Ack n ow led gm en t. This work has been supported

by the USDOE Office of Basic Energy Sciences (Grant

No. DE-FG03-85ER13431) and Exxon Chemicals

America. L.J .A. wishes to thank the National Science

Foundation for a Graduate Research Fellowship.

Str u ctu r e An a lysis a n d Refin em en t. SHELXTL v5.1³¹

was used to solve, via direct methods or by the Patterson

method, and to refine all structures using full-matrix least

squares. All non-hydrogen atoms were refined anisotropically.

For 12, there is one molecule in the asymmetric unit. All

hydrogen atoms, including the hydride, were refined isotro-

pically. For 19b, there is one molecule in the asymmetric unit.

SADABS was used to correct for absorption. Hydrogen atoms

were placed at calculated positions with U_isovalues of 120%

of U_eqof the attached atom. The hydride could not be located

in a difference map and was not included in the refinement.

There were unindexed peaks in the data frames, and this was

probably not a single crystal; consistent with this interpreta-

Su p p or tin g In for m a tion Ava ila ble: Text giving details

of the structure determination and tables of atomic coordi-

nates, all bond distances and angles, and anisotropic displace-

ment parameters for all structurally characterized complexes,

and a labeled drawing for complex 23. This material is

available free of charge via the Internet at http://pubs.acs.org.³³

OM020628D

(33) Crystallographic data for the structures of 12 (CCDC 147070),

19b (CCDC 162232), 20a (CCDC 140682), 21a (CCDC 147250), 22

(CCDC 142045), 23 (CCDC 147415), and 24 (CCDC 147413) have been

deposited with the Cambridge Crystallographic Data Centre as

supplementary publications. These data can be obtained free of charge

via http://www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cam-

bridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2

1EZ, U.K.; fax +44 1223 336033; email deposit@ccdc.cam.ac.uk).

Structure factors are available electronically by e-mail: xray@

caltech.edu.

(31) SMART, SAINT, and SHELXTL; Bruker AXS Inc., Madison,

WI, 1999.

(32) Duchamp, D. J . American Crystallographic Association Meet-

ing, Bozeman, MT, 1964; Paper B14, p 29.

Article Doi

Preparation of ansa-niobocene and ansa-tantalocene olefin hydride complexes as transition state analogues in metallocene-catalyzed olefin polymerization

DOI: 10.1021/om020628d

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Article abstract of DOI:10.1021/om020628d

Full text of DOI:10.1021/om020628d

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