Neutral and anionic silyl hydride derivatives of the tantalum imido fragment Cp*(DippN=)Ta (Cp* = η5-C5Me5; Dipp = 2,6-iPr2C6H3). Reactive σ-bonds and intramolecular C-H bond act

Article abstract of DOI:10.1021/om0203227

Reactive σ-bonds and intramolecular C-H bond activations which involves the silyl ligands were discussed. Nuclear magnetic resonance (NMR) and X-ray analysis were performed. Results showed that complexes of this type possess two different bonds which read

Full text of DOI:10.1021/om0203227

3108
Organometallics 2002, 21, 3108-3122
Neu tr a l a n d An ion ic Silyl Hyd r id e Der iva tives of th e
Ta n ta lu m Im id o F r a gm en t Cp *(Dip p Nd)Ta (Cp * )
η5-C5Me5; Dip p ) 2,6-iP r 2C6H3). Rea ctive σ-Bon d s a n d
In tr a m olecu la r C-H Bon d Activa tion s In volvin g th e
Silyl Liga n d s
Urs Burckhardt, Gary L. Casty, J ohn Gavenonis, and T. Don Tilley*
Department of Chemistry, University of California, Berkeley, Berkeley, California 94720-1460
Received April 22, 2002
The syntheses and reactivities of alkyl, silyl, and hydride complexes of the type
Cp*(DippNd)TaRR′ (Cp* ) η5-C5Me5; Dipp ) 2,6-iPr2C6H3; R ) silyl, alkyl; R′ ) alkyl,
hydride) are described. The compounds Cp*(DippNd)Ta(SiR3)Cl (SiR3 ) Si(SiMe3)3 (1), SiPh3
(2), SiHMes2 (3); Mes ) 2,4,6-Me3C6H2), prepared from Cp*(DippNd)TaCl2 and (THF)nLiSiR3,
react with H2 to yield the dimeric hydride [Cp*(DippNd)TaCl(µ-H)]2 (4). The reaction
of 4 with (THF)3LiSi(SiMe3)3 (2 equiv) gave the 16-electron silyl hydride complex
Cp*(DippNd)Ta[Si(SiMe3)3]H (6), which is Lewis acidic and reversibly binds halide ions to
afford anionic Ta(V) complexes of the type A+{Cp*(DippNd)Ta(X)[Si(SiMe3)3]H}- (A ) Li-
(THF)3, X ) Cl (5); A ) NBu4, X ) Br (7) and I (8)). For complexes 5, 7, and 8, NMR and
X-ray data suggest the presence of weak three-center interactions involving Ta, Si,
and H. Silyl hydride 6 reacts with xylyl isocyanide (xylyl ) 2,6-dimethylphenyl) and
acetonitrile to give the corresponding η2-silaimine and azomethine insertion products,
respectively, while treatment with acetone yields the stable isopropoxide product arising
from insertion into the Ta-H bond. When complex 6 is treated with ethylene or diphenyl-
acetylene, elimination of HSi(SiMe3)3 occurs, along with formation of a five-membered
tantalacycle resulting from coupling of the unsaturated substrate at Ta. Hydrogenolysis of
6 yields HSi(SiMe3)3 and a dimeric dihydride [Cp*(DippNd)TaH(µ-H)]2 (14). At room
temperature, 6 rearranges (t1/2 ) 8.5 h) to an unusual alkyl hydride species resulting from
C-H bond activation, Cp*(DippNd)Ta[CH2Si(SiMe3)2SiMe2H]H (15). This complex exhibits
a γ-agostic Si-H interaction with the metal center. An attempt to prepare Cp*(DippNd)-
Ta(SiHMes2)H produced the alkyl hydride product Cp*(DippNd)Ta[η2-CH2(2-SiH2Mes-3,5-
Me2C6H2)]H (20), which apparently results from decomposition of the expected silyl hydride.
The alkyl hydride complex [Cp*(DippNd)TaMe(µ-H)]2 (25) was prepared by the hydro-
genolysis of Cp*(DippNd)Ta[Si(SiMe3)3]Me (23), whereas Cp*(DippNd)Ta(CH2CMe3)H (26)
was obtained by treatment of 4 with NpMgCl. Complex 26 possesses an R-agostic C-H
interaction, and the corresponding deuteride 26-d slowly scrambles deuterium into the
methylene positions.
In tr od u ction
derivatives,6,9-11 since such complexes have proven to
be precursors to the best dehydropolymerization cata-
lysts. Given the proven potential for chemical transfor-
mations mediated by early transition metals, consider-
able interest has focused on the development of catalysts
featuring ancillary ligands other than cyclopentadienyl
groups.6,9,10 In this context the formal 1σ, 2π donor
analogy between cyclopentadienyl [η5-C5R5]- and imido
[RNd]2- ligands12-16 suggests that complexes based on
Early transition metal silicon chemistry is receiving
increasing attention, especially with the discovery of
processes for the catalytic hydrosilylation of olefins1-5
and the polymerization of hydrosilanes.6-8 Work in our
laboratory has centered on group 4 d0 metallocene
(1) Fu, P.-F.; Brard, L.; Li, Y.; Marks, T. J . J . Am. Chem. Soc. 1995,
117, 7157.
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Chem. Soc. 1994, 116, 11667.
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17, 3754.
(6) Tilley, T. D. Acc. Chem. Res. 1993, 26, 22.
(7) Gauvin, F.; Harrod, J . F.; Woo, H. G. Adv. Organomet. Chem.
1998, 42, 363.
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114, 7047.
(10) Imori, T.; Tilley, T. D. Polyhedron 1994, 13, 2231.
(11) Woo, H.-G.; Tilley, T. D. J . Am. Chem. Soc. 1989, 111, 3757.
(12) Williams, D. S.; Schofield, M. H.; Anhaus, J . T.; Schrock, R. R.
J . Am. Chem. Soc. 1990, 112, 6728.
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Davis, W. M. J . Am. Chem. Soc. 1991, 113, 5480.
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(8) Corey, J . In Advances in Silicon Chemistry; Larson, G., Ed.; J AI
Press: Greenwich, CT, 1991; Vol. 1, p 327.
10.1021/om0203227 CCC: $22.00 © 2002 American Chemical Society
Publication on Web 06/22/2002

Tantalum Imido Fragment Cp*(DippNd)Ta
Organometallics, Vol. 21, No. 15, 2002 3109
imido ligands might represent interesting candidates for
investigation. We have therefore targeted the synthesis
and study of d0 imido complexes containing reactive
M-R (R ) H, silyl) σ-bonds.17-20 These investigations
have produced d0 bis(imido) silyl complexes of molyb-
denum and tungsten, and attempts to generate hydride
derivatives in this system revealed that the silyl hy-
drides (DippNd)2M[Si(SiMe3)3]H (Dipp ) 2,6-iPr2C6H3,
M ) Mo, W) are highly unstable and degrade via an
interesting â-silyl elimination of HSiMe3.18
In this account we describe the syntheses, structures,
and reactivities of a number of alkyl, silyl, and hydride
complexes of tantalum(V), Cp*(DippNd)TaRR′ (Cp* )
η5-C5Me5; R ) silyl, alkyl; R′ ) alkyl, hydride), which
contain a mixed pentamethylcyclopentadienyl-(aryl)-
imido ligand set. Important synthetic targets have been
coordinatively unsaturated d0 silyl hydride complexes,
especially since such species have been proposed as
intermediates in metal-catalyzed silane dehydropolym-
erization. Complexes of this type possess two different
bonds which readily participate in σ-bond metathesis
processes (M-Si and M-H), and perhaps for this reason
only a few examples have been reported.21-26 Herein we
describe an isolable 16-electron tantalum(V) silyl hy-
dride complex and its reaction chemistry.
Reactions of Cp*(DippNd)TaCl2 with (THF)3LiSiPh3
and (THF)2LiSiHMes2 (Mes ) 2,4,6-Me3C6H2) provided
the analogous silyl chloride complexes Cp*(DippNd)-
Ta(SiPh3)Cl (2) and Cp*(DippNd)Ta(SiHMes2)Cl (3),
respectively. These compounds are difficult to purify
due to their high solubility in hydrocarbon solvents. The
29Si NMR resonances for 2 and 3 were found at 51.4
and 17.6 ppm, respectively, and the latter appears as a
1
doublet with J SiH ) 163 Hz.
Hydrogenolysis (80 psi, room temperature, 1 h) of
hexanes solutions of the silyl chloride complexes 1-3
immediately led to precipitation of the yellow hydrido
chloride complex [Cp*(DippNd)TaCl(µ-H)]2 (4), in nearly
quantitative yield (eq 1). Complex 4 has poor solubility
in most solvents and can be easily isolated from the
hydrosilane byproduct by washing with hexanes. The
1H NMR spectrum of 4 (in dichloromethane-d2) contains
a singlet for the bridging hydrides at 7.91 ppm and two
sets of isopropyl resonances. The IR spectrum contains
a strong, broad band for the bridging hydrides at 1606
cm-1 (4-d : νTaD ) 1151 cm-1).
Resu lts a n d Discu ssion
Syn th esis a n d Hyd r ogen olysis of Silyl Ch lor id e
Com p lexes. The dichloride complex Cp*(DippNd)TaCl2
(Cp* ) η5-C5Me5; Dipp ) 2,6-iPr2C6H3)27,28 provides a
convenient starting material for the synthesis of tan-
talum(V) alkyl and silyl complexes containing a mixed
pentamethylcyclopentadienyl-(aryl)imido ligand set.
Treatment of Cp*(DippNd)TaCl2 with (THF)3LiSi-
(SiMe3)3 (1 equiv) afforded the highly crystalline red
complex Cp*(DippNd)Ta[Si(SiMe3)3]Cl (1) in 80%
yield. In the 29Si NMR spectrum of 1, the Ta-bound
silicon appears at -47.8 ppm, and this value is inter-
mediate between those reported for CpCp*Hf[Si(SiMe3)3]-
Cl (-77.9 ppm)29 and (DippNd)2W[Si(SiMe3)3]Cl (-22.9
ppm).18
(15) Huber, S. R.; Baldwin, T. C.; Wigley, D. E. Organometallics
1993, 12, 91.
(16) Gibson, V. C. J . Chem. Soc., Dalton Trans. 1994, 1607.
(17) Gountchev, T. I.; Tilley, T. D. J . Am. Chem. Soc. 1997, 119,
12831.
(18) Casty, G. L.; Tilley, T. D.; Yap, G. P. A.; Rheingold, A. L.
Organometallics 1997, 16, 4746.
(19) Burckhardt, U.; Casty, G. L.; Tilley, T. D.; Woo, T. K.; Roth-
lisberger, U. Organometallics 2000, 19, 3830.
(20) Burckhardt, U.; Tilley, T. D. J . Am. Chem. Soc. 1999, 121,
6328.
(21) Casty, G. L.; Lugmair, C. G.; Radu, N. S.; Tilley, T. D.; Walzer,
J . F.; Zargarian, D. Organometallics 1997, 16, 8.
(22) Kreutzer, K. A.; Fisher, R. A.; Davis, W. M.; Spaltenstein, E.;
Buchwald, S. L. Organometallics 1991, 10, 4301.
(23) Spaltenstein, E.; Palma, P.; Kreutzer, K. A.; Willoughby, C. A.;
Davis, W. M.; Buchwald, S. L. J . Am. Chem. Soc. 1994, 116, 10308.
(24) Nikonov, G. I.; Mountford, P.; Green, J . C.; Cooke, P. A.; Leech,
M. A.; Blake, A. J .; Howard, J . A. K.; Lemenovskii, D. A. Eur. J . Inorg.
Chem. 2000, 1917.
Syn th esis a n d Str u ctu r a l Ch a r a cter iza tion of
Silyl Hyd r id e Com p lexes. The addition of 2 equiv of
(THF)3LiSi(SiMe3)3 to a THF slurry of 4 produced a
clear yellow solution from which lemon-yellow crystals
(5, 89% yield) were isolated by crystallization from THF/
pentane. The crystals lose solvent after prolonged
exposure to vacuum, as evidenced by a color change
from yellow to dark red (see below). NMR and elemental
analysis characterize complex 5 as an adduct of the
desired silyl hydride 6 with 1 equiv of LiCl and 3 equiv
of THF. The IR spectrum contains a weak Ta-H stretch
1
at 1805 cm-1 (5-d : νTaD ) 1282 cm-1). In the H NMR
spectrum, the hydride resonance at 12.83 ppm displays
distinct 29Si satellites, and the 29Si NMR spectrum
contains a doublet for the Ta-Si resonance at -98.4
ppm (J SiH ) 31 Hz). The magnitude of this J SiH coupling
constant suggests the possibility of a nonclassical
(25) Nikonov, G. I. J . Organomet. Chem. 2001, 635, 24.
(26) Nikonov, G. I.; Mountford, P.; Ignatov, S. K.; Green, J . C.; Leech,
M. A.; Kuzmina, L. G.; Razuvaev, A. G.; Rees, N. H.; Blake, A. J .;
Howard, J . A. K.; Lemenovskii, D. A. J . Chem. Soc., Dalton Trans.
2001, 2903.
(27) Williams, D. N.; Mitchell, J . P.; Poole, A. D.; Siemeling, U.;
Clegg, W.; Hockless, D. C. R.; O’Neill, P. A.; Gibson, V. C. J . Chem.
Soc., Dalton Trans. 1992, 739.
(28) Baldwin, T. C.; Huber, S. R.; Bruck, M. A.; Wigley, D. E. Inorg.
Chem. 1993, 32, 5682.
(29) Woo, H.-G.; Heyn, R.; Tilley, T. D. J . Am. Chem. Soc. 1992,
114, 5698.

3110 Organometallics, Vol. 21, No. 15, 2002
Burckhardt et al.
Ta ble 1. Selected Bon d Len gth s (Å) a n d An gles
(d eg) for {Li(THF )4}{Cp *(Dip p Nd)Ta (Cl)-
[Si(SiMe3)3]H}‚(THF ) (5‚(THF )2)a
Bond Lengths
Ta(1)-Cl(1)
Ta(1)-N(1)
Ta(1)-H(60)
Si(1)-Si(2)
2.483(2)
1.811(8)
1.67(8)
2.345(4)
Si(1)-Si(4)
Ta(1)-Si(1)
Ta(1)-Cp*cent
N(1)-C(11)
Si(1)-Si(3)
2.351(5)
2.722(3)
2.160
1.38(1)
2.352(5)
Bond Angles
79.38(9) Cl(1)-Ta(1)-N(1)
Cl(1)-Ta(1)-Si(1)
99.7(2)
107.7(2)
120.8(2)
117.4(1)
107.7(2)
65(3)
Cl(1)-Ta(1)-Cp*cent 106.28(6) Si(1)-Ta(1)-N(1)
Si(1)-Ta(1)-Cp*cent 128.63(7) N(1)-Ta(1)-Cp*cent
Ta(1)-N(1)-C(11)
Ta(1)-Si(1)-Si(3)
174.1(6) Ta(1)-Si(1)-Si(2)
121.8(1) Ta(1)-Si(1)-Si(4)
Cl(1)-Ta(1)-H(60) 144(3)
Si(1)-Ta(1)-H(60)
Cp*cent-Ta(1)-H(60) 95(3)
N(1)-Ta(1)-H(60)
93(3)
a
Cp*cent represents the average of the x, y, and z coordinates of
the η5-C5Me5 ring carbons.
F igu r e 1. ORTEP diagram of {Li(THF)4}{Cp*(DippNd)-
Ta(Cl)[Si(SiMe3)3]H}‚(THF) (5‚(THF )2, cation not shown).
complex is composed of separated cations and anions,
as indicated by a Ta-Cl bond length of 2.483(2) Å
and a closest Cl‚‚‚Li distance of 5.286 Å. The slight
elongation of the Ta-Si (2.722(3) Å) and Ta-N
(1.811(8) Å) bonds compared to those of the correspond-
ing 16-electron complex Cp*(DippNd)Ta[Si(SiMe3)3]H
(6, vide infra) can be rationalized by the higher coordi-
nation number for 5. The hydride ligand, H(60), was
located in the Fourier difference map and its position
was refined isotropically. The Ta-H distance appears
to be somewhat shorter than in other silyl hydride
examples, although uncertainties in the hydrogen atom
position preclude a meaningful comparison. Further
support for a three-center Ta‚‚‚Si‚‚‚H interaction is
found in the small values for both the H(60)‚‚‚Si(1)
distance (2.51 Å in 5‚(THF )2, vs 3.39 Å in 6) and the
Si(1)-Ta(1)-H(60) angle (65(3)° in 5‚(THF )2 vs 97(1)°
in 6).
Ch a r t 1. Bon d in g Sch em es for Tr a n sition Meta l
Silyl Hyd r id e Com p lexes
interaction between the hydride and the Ta-bound
silicon atom.23-25,30-35
Transition metal complexes containing silyl and hy-
dride ligands adopt different structure types, depending
on the degree to which the hydride and silyl groups
interact (A-C, Chart 1).31 In one limiting case (A), there
is no such interaction, as indicated by long H-Si
distances and a low J SiH coupling constant (e ca. 20
Hz).30,36 Another limiting case features strong Si-H
bonding, such that the complex is best described as
possessing an η2-silane ligand (C). Such cases often
exhibit relatively high J SiH coupling constants of g50
Hz.30-35,37 Other cases appear to be intermediate in
character (B), with J SiH values falling in the 20-50 Hz
range.24,25,30-35 For example, Buchwald’s Cp2Ti(H)-
(SiHPh2)(PMe3) exhibits a J SiH value of 28 Hz,23 and
Nikonov’s Cp(DippNd)Ta(H)(SiMe2Cl)(PMe3), described
as possessing a hydridefsilicon donor interaction, has
a J SiH value of 33 Hz.24-26 The observed J SiH coupling
constant of 31 Hz for 5 also appears to reflect “inter-
mediate character” of type B.
The addition of hexanes or toluene to 5 immediately
generated a deep red solution and a precipitate. After
cooling and repeatedly filtering the solution to re-
move LiCl, the 16-electron silyl hydride complex Cp*-
(DippNd)Ta[Si(SiMe3)3]H (6) was isolated as thermally
unstable red crystals in 58% yield (eq 2). Despite the
The molecular structure of 5‚(THF )2 was determined
by X-ray diffraction analysis (Figure 1). Selected bond
lengths and angles are listed in Table 1. This ate
(30) Schubert, U. Adv. Organomet. Chem. 1990, 30, 151.
(31) Schubert, U.; Scholz, G.; Mu¨ller, J .; Ackermann, K.; Wo¨rle, B.;
Stansfield, R. F. D. J . Organomet. Chem. 1986, 306, 303.
(32) Colomer, E.; Corriu, R. J . P.; Marzin, C.; Vioux, A. Inorg. Chem.
1982, 21, 368.
(33) Matarasso-Tchiroukhine, E.; J aouen, G. Can. J . Chem. 1988,
66, 2157.
(34) Schubert, U.; Wo¨rle, B.; J andik, P. Angew. Chem., Int. Ed. Engl.
1981, 20, 695.
(35) Schubert, U.; Mu¨ller, J .; Alt, H. G. Organometallics 1987, 6,
469.
(36) Tilley, T. D. In Chemistry of Organic Silicon Compounds; Patai,
S., Rappoport, Z., Eds.; Wiley: New York, 1989; p 1415.
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modest isolated yield for 6, the conversion was quanti-
1
tative as determined by H NMR spectroscopy. In the
1H NMR spectrum of 6, the hydride resonance appears

Tantalum Imido Fragment Cp*(DippNd)Ta
Organometallics, Vol. 21, No. 15, 2002 3111
reflects a slight steric influence on the degree of Si‚‚‚H
interaction.
Rea ction s of Cp *(Dip p Nd)Ta [Si(SiMe3)3]H (6).
The relatively rapid decomposition of 6 precludes its
large-scale synthesis and storage. However, the chloride
adduct 5 can be stored for several weeks, and 6 may be
freshly generated before use by dissolving 5 in hexanes,
filtering to remove LiCl, and then evaporating the
solvent. The reactivity of 6 toward various small mol-
ecules is summarized in Scheme 1. Treatment with xylyl
(xylyl ) 2,6-dimethylphenyl) isocyanide (toluene, room
temperature, 6 h) afforded the η2-silaimine inser-
tion product Cp*(DippNd)Ta[η2-C(N-2,6-Me2C6H3)Si-
(SiMe3)3]H (9) in 49% yield, although this conversion is
1
quantitative by H NMR spectroscopy. The structural
assignment is based upon comparing spectroscopic data
with those reported for other early transition metal η2-
silaimines.39 Similarly, treatment of 6 with acetonitrile
(benzene-d6, room temperature, 5 min, quantitative
conversion) gave a thermally unstable product which
appears to be the azomethine derivative Cp*-
(DippNd)Ta[CdN(Me)Si(SiMe3)3]H (10), on the basis of
comparisons with NMR data for known compounds.40,41
While both xylyl isocyanide and acetonitrile selectively
insert into the Ta-Si bond, acetone inserts into only
the Ta-H bond to yield the isopropoxide complex
Cp*(DippNd)Ta(OiPr)[Si(SiMe3)3] (11) (benzene-d6, room
temperature, 5 min, quantitative conversion; character-
ized by NMR spectroscopy).
Figu r e 2. ORTEP diagram of Cp*(DippNd)Ta[Si(SiMe3)3]H
(6).
Ta ble 2. Selected Bon d Len gth s (Å) a n d An gles
(d eg) for Cp *(Dip p Nd)Ta [Si(SiMe3)3]H (6)a
Bond Lengths
Ta(1)-Si(1)
Ta(1)-Cp*cent
Si(1)-Si(2)
Si(1)-Si(4)
2.689(1)
2.1217(2)
2.356(2)
2.376(2)
Ta(1)-N(1)
Ta(1)-H(1)
Si(1)-Si(3)
N(1)-C(11)
1.793(4)
1.77(5)
2.369(2)
1.407(6)
Bond Angles
106.2(1) Si(1)-Ta(1)-Cp*cent 122.82(3)
Si(1)-Ta(1)-N(1)
N(1)-Ta(1)-Cp*cent 122.6(1) Ta(1)-N(1)-C(11)
169.7(3)
116.31(6)
97(1)
Ta(1)-Si(1)-Si(2)
Ta(1)-Si(1)-Si(4)
N(1)-Ta(1)-H(1)
123.36(6) Ta(1)-Si(1)-Si(3)
100.83(6) Si(1)-Ta(1)-H(1)
96(1)
Cp*cent-Ta(1)-H(1) 105(1)
Treatment of complex 6 with ethylene (1 atm, <5 min)
or diphenylacetylene (2 equiv, 24 h) resulted in the
elimination of HSi(SiMe3)3 with formation of the stable,
a
Cp*cent represents the average of the x, y, and z coordinates of
the η5-C5Me5 ring carbons.
five-membered tantalacycles Cp*(DippNd)Ta[CH2CH2-
at very low field (21.49 ppm), reflecting the highly elec-
tron-deficient character of the metal center. This is
also true for the 29Si NMR shift of the central silicon
atom, which also appears at unusually low field (-22.9
CH2CH2] (12) and Cp*(DippNd)Ta[CPhCPhCPhCPh]
(13), respectively. The hydrogenolysis of 6 in hexanes
yielded the dimeric dihydride complex [Cp*(DippNd)-
TaH(µ-H)]2 (14) in 56% yield. Treatment of 6 with
dichloromethane gave a mixture of products, the major
ones being Cp*(DippNd)TaCl2, the silyl chloride 1, and
the hydrido chloride 4. In summary, the reactivity of 6
is similar to that observed for the group 4 silyl hydride
complexes CpCp*Hf[Si(SiMe3)3]H21 and Cp2Zr(SiPh3)-
(PMe3)H.22
ppm). A weak IR absorption at 1785 cm-1 (6-d : νTaD
)
1276 cm-1) is consistent with a terminal Ta-H bond.38
The molecular structure of 6 is shown in Figure 2, and
important bond distances and angles are given in
Table 2.
Solutions of 6 decompose at room temperature (t1/2
)
8.5 h in benzene-d6), but can be stored at -30 °C for
several days; as a solid, 6 has a longer lifetime. Unlike
Th er m a l Decom p osition of Cp *(Dip p Nd)Ta [Si-
(SiMe3)3]H (6) via C-H Bon d Activa tion . As men-
tioned above, solutions of 6 decompose at room temper-
ature (t1/2 ) 8.5 h), as evidenced by a gradual color
change from red to yellow. The 1H NMR spectra indicate
that a ca. 2:1 mixture of a new complex (15, ca. 60%)
and HSi(SiMe3)3 (from other decomposition pathways)
is formed in various solvents (benzene, hexanes, and
diethyl ether), along with small quantities of other
products. Attempts to crystallize 15 from the reaction
mixture failed. Thus, the structural assignment of 15
is based on spectroscopic data and in particular on
1H,29Si-HMQ(B)C spectra, which were essential to
establish the connectivity of atoms. Complex 15 was
determined to be a product of the rearrangement of 6,
its hafnocene counterpart CpCp*Hf[Si(SiMe3)3]H,21
6
does not appear to be light-sensitive, as decomposition
rates are the same in the presence or absence of ambient
light. Complex 6 is not stable when isolated, suggesting
that THF stabilizes this complex (see below for a
discussion of the thermal rearrangement).
Interestingly, the chloride dissociation from 5 is
reversible. Solutions of the free silyl hydride 6 in THF-
d8 reacted instantly with 1 equiv of LiCl to form 5 (by
1H NMR spectroscopy). In addition, complex 6 was
found to react cleanly with other halides. While THF
solutions of 6 decomposed upon addition of CsF, rapid,
clean reactions were observed with [NBu4]Br and
[NBu4]I in THF-d8, to produce the bromide (7) and
iodide (8) complexes, respectively (observed and char-
acterized by NMR spectroscopy). The trend in J SiH
coupling constants (5, 31 Hz; 7, 33 Hz; 8, 36 Hz) possibly
(39) Elsner, F. H.; Tilley, T. D.; Rheingold, A. L.; Geib, S. J . J .
Organomet. Chem. 1988, 358, 169.
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1986, 5, 443.
(41) Woo, H.-G.; Tilley, T. D. J . Organomet. Chem. 1990, 393, C6.
(38) J iang, Q.; Carroll, P. J .; Berry, D. H. Organometallics 1991,
10, 3648.

3112 Organometallics, Vol. 21, No. 15, 2002
Burckhardt et al.
Sch em e 1
5
with the former silyl ligand, -Si(SiMe3)3, being con-
hardly conceivable for a regular J HH coupling (assign-
ment of J -values confirmed by homonuclear decoupling
verted to the alkyl ligand -CH2Si(SiMe3)2SiMe2H (eq
1
3).
experiments). A phase-sensitive H NOESY experiment
revealed a strong NOE signal correlating Ta-H and
Si-H, but no chemical exchange.
The 29Si NMR spectrum of 15 provides further sup-
port for the above interpretations. While normal chemi-
cal shift values are observed for the 29Si NMR reso-
nances of the -Si(SiMe3)2- group (-10.5, -11.1, and
-88.3 ppm), the -SiMe2H resonance is located at -68.9
ppm, >30 ppm upfield from what is expected. The J SiH
coupling constant is moderately temperature-dependent
(78 Hz at -70 °C, 87 Hz at 25 °C, and 101 Hz at 80 °C)
and is about half the magnitude of a normal one-bond
J SiH coupling constant, once again suggesting a γ-agostic
interaction with the tantalum metal center. The IR
spectrum of 15 contains a νSiH stretch at 1726 cm-1, well
below typical values found for normal Si-H bonds
(around 2150 cm-1).43 The mass spectrum (EI+) of 15
contains a peak at 737 ([M]+ - 2), which is more intense
than the molecular peak at 739 (by a factor of 11),
indicating facile loss of H2, presumably from the Ta-H
and Si-H hydrogens, which are in close contact (as
1
determined by the H NOESY experiment, see above).
1
The H NMR spectrum of 15 reveals chemical shift
However, no hydrogen evolution was observed by 1H
NMR spectroscopy on heating the sample in a sealed
NMR tube (toluene-d8, 90 °C).
All of the above spectroscopic features are in good
agreement with those reported for the related zirconium
complexes Cp2Zr(X)(NtBuSiMe2H)44 and Cp2Zr(η2-RCt
CSiMe2H),45 both of which have been demonstrated to
values of 9.58 ppm (dd) for the tantalum hydride
resonance, 0.70 ppm (ddd) and -0.90 ppm (dd) for the
resonances of the diastereotopic methylene hydrogens,
and 2.57 ppm (dddq) for the Si-H resonance. The Si-H
resonance appears at ca. 2 ppm higher field than
expected for a typical Si-H group, suggesting a non-
classical (agostic)42 interaction of the Si-H unit with
the tantalum. Consistent with this hypothesis is the
exceptionally large J HH coupling constant of 9 Hz
involving the Si-H and Ta-H hydrogens, a value
(43) Corey, J . Y.; Braddock-Wilking, J . Chem. Rev. 1999, 99, 175.
(44) Procopio, L. J .; Carroll, P. J .; Berry, D. H. J . Am. Chem. Soc.
1994, 116, 177.
(45) Peulecke, N.; Ohff, A.; Kosse, P.; Tillack, A.; Spannenberg, A.;
Kempe, R.; Baumann, W.; Burlakov, V. V.; Rosenthal, U. Chem. Eur.
J . 1998, 4, 1852.
(42) Brookhart, M.; Green, M. L. H. J . Organomet. Chem. 1983, 250,
395.

Tantalum Imido Fragment Cp*(DippNd)Ta
Organometallics, Vol. 21, No. 15, 2002 3113
exhibit agostic â-Si-H interactions. Consequently, we
postulate a cyclic structure for 15 in which the terminal
-Me2SiH group of the polysilyl ligand is coordinated to
tantalum (an agostic γ-Si-H interaction). To the best
of our knowledge, this coordination mode is unprec-
edented.37 To confirm the connectivity of the alkyl group,
complex 15 was hydrolyzed with H2O and D2O and the
silane products were analyzed by NMR spectroscopy and
GCMS. Spectroscopic data for the principal products are
consistent with the formula Me2HSi-Si(SiMe3)2CH2-
H(D) and match recently published data for this si-
lane.46
Evaporation of solutions of 15 produced a yellow oil,
as did attempts to crystallize the product from pentane
and hexamethyldisiloxane (at -30 °C). On standing at
room temperature, solutions of 15 completely decompose
to a complex mixture of species within ca. one week (1
day at 50 °C). From the complex product mixture, a
small amount of a yellow solid separates, which was
determined by IR and NMR spectroscopy to be an
asymmetric, hydride-bridged dimer, {Cp*(DippNd)Ta-
[CH2Si(SiMe3)2SiMe2H]}(µ-H)2{TaCp*(dNDipp)H} (18),
characterized by two bridging hydrides (νTa-(µ-H) ) 1508
and 1619 cm-1; δ(Ta-(µ-H)) ) 6.99 (m) and 6.63 (dd)),
a terminal hydride (νTaH ) 1794 cm-1; δ(Ta-H) ) 14.93
(dd)), and an Si-H group (νSiH ) 2086 cm-1; δ(Si-H) )
4.69 (m)).
The mechanism for the formation of 15 is not appar-
ent since no intermediate species could be detected by
monitoring the reaction progress by NMR spectroscopy.
The only direct information relates to the fate of the
hydride ligand. Rearrangement of the deuterium-labeled
compound Cp*(DippNd)Ta[Si(SiMe3)3]D (6-d ) results in
deuterium incorporation into only the agostic -Me2SiD
group, and no deuterium is found in the SiH position of
the HSi(SiMe3)3 byproduct. In the final stage of the
reaction, some hydrogen scrambling into the -Me2SiD
position is observed, probably due to reaction with the
tantalum hydride species. This finding rules out the
otherwise plausible possibility of a reductive silane
elimination from 6 followed by an oxidative addition of
an -SiMe3 group, as the competitive loss of silane would
produce DSi(SiMe3)3. On the basis of the observed
-SiMe3 migrations in isomerizations of the coordina-
tively unsaturated rhodium and iridium polysilyl com-
plexes (Me3P)3M-Si(SiMe3)3,47,48 a reaction sequence
like the one depicted in Scheme 2 seems possible for
the rearrangement of 6 to 15. Migration of the hydride
to the imido nitrogen would create a coordinatively
unsaturated Ta(III) amido species in which a series of
silylene-generating 1,2- and 1,3-migrations of SiMe3
could take place. Such reactions have been observed for
late transition metal silyl complexes.36,49-51 Oxidative
addition of a C-H bond of a methyl group, followed by
R-abstraction of the amido hydrogen by the Ta-bound
SiMe2 group, would then produce 15. However, a gradi-
Complex 15 readily inserts unsaturated molecules
into its Ta-C or Ta-H bonds. In these reactions, the
agostic-Me2SiH group dissociates from the metal center,
as indicated by new spectroscopic properties of the
-Me2SiH group. Addition of xylyl isocyanide (1 equiv)
to a solution of crude 15 (benzene-d6, room temperature,
1 h) produced the dark orange-brown η2-imine complex
16, and the structure shown in eq 4 is based mostly on
1
ent-enhanced H,15N-HMQC experiment performed to
detect possible tantalum amido species did not show any
signals arising from N-H bonds upon monitoring the
decomposition of 6 in benzene-d6 (room temperature).
In the absence of further experimental data, alternative
mechanisms have to be considered, in particular those
involving a concerted double migration at silicon (i.e.,
a “dyotropic shift”).52
Th er m a l Rea r r a n gem en t via C-H Bon d Activa -
tion in a Ta -SiHMes2 Com p lex. Starting from [Cp*-
(DippNd)TaCl(µ-H)]2 (4), several attempts were made
to access silyl hydride complexes with silyl groups other
than -Si(SiMe3)3. Treatment of 4 with (THF)3LiSiPh3
(2 equiv) in THF (room temperature, 1 h) produced a
clear orange solution, from which the chloride-stabilized
silyl hydride {Li(THF)3}{Cp*(DippNd)TaCl(SiPh3)H}
(19) was isolated by precipitation with pentane in 66%
yield. The 1H NMR spectrum of 19 contains broad
resonances for the phenyl and isopropyl groups due to
hindered rotation, and characteristic silicon satellites
were identified for the hydride resonance at 11.97 ppm
(J SiH ) 34 Hz). In contrast to its -Si(SiMe3)3 analogue
6, complex 19 does not convert to a free silyl hydride
complex when dissolved in hydrocarbons; the chloride
remains coordinated. Within 1 day, 19 decomposed to
a mixture of unidentified products, none of which
possess a Ta-Si bond (by 29Si NMR spectroscopy).
In contrast, treatment of 4 with (THF)2LiSiHMes2 (2
equiv) in diethyl ether (room temperature, 12 h) pro-
spectroscopic data (δ(Ta-H) ) 10.26 ppm; δ(Si-H) )
3.83 ppm; δ(29Si-H) ) -27.2 ppm, 1J SiH ) 174 Hz), MS
([M]+ ) 870 m/z). This conversion is quantitative (by
NMR spectroscopy), but isolation of the pure product
was unsuccessful due to its high solubility in hydrocar-
bon solvents. Similarly, 15 reacted quantitatively with
1 equiv of benzophenone (hexanes, room temperature,
10 s) to produce the alkoxide complex 17, as indicated
by a color change to bright yellow (eq 4). A new SiH
(47) Mitchell, G. P.; Tilley, T. D.; Yap, G. P. A.; Rheingold, A. L.
Organometallics 1995, 14, 5472.
1
resonance is observed in the H NMR spectrum at 6.74
ppm, and other NMR data indicate the formation of an
(48) Mitchell, G. P.; Tilley, T. D. Organometallics 1996, 15, 3477.
(49) Pannell, K. H.; Sharma, H. K. Chem. Rev. 1995, 95, 1351.
(50) Schubert, U. Angew. Chem., Int. Ed. Engl. 1994, 33, 419.
(51) Tilley, T. D. In The Silicon-Heteroatom Bond; Patai, S., Rap-
poport, Z., Eds.; Wiley: New York, 1991; pp 245, 309.
(52) Reetz, M. T. Adv. Organomet. Chem. 1977, 16, 33.
alkoxide ligand (for OCHPh2: δ (O13CHPh2) ) 89.0 ppm,
1
δ(29Si-H) ) -27.4 ppm, J SiH ) 183 Hz).
(46) Herzog, U.; Roewer, G. J . Organomet. Chem. 1997, 527, 117.

3114 Organometallics, Vol. 21, No. 15, 2002
Burckhardt et al.
Sch em e 2
duced the hydride complex Cp*(DippNd)Ta[η2-CH2(2-
SiH2Mes-3,5-Me2C6H2)]H (20), which was isolated from
hexanes as pale yellow crystals in 7% yield (eq 5). This
F igu r e 3. ORTEP diagram of Cp*(DippNd)Ta[η2-CH2(3,5-
Me2-2-SiH2Mes)C6H2)]H (20).
1
The H NMR spectrum of 20 contains a singlet at 8.62
ppm attributed to the Ta-H resonance and two doublets
for the geminal Si-H hydrogens at 5.14 and 5.03 ppm
(2J HH ) 4 Hz). The presence of inequivalent mesityl
methyl groups and a diastereotopic methylene group
2
(doublets at 3.60 and 1.90 ppm, J HH ) 7.3 Hz) are
consistent with activation of a methyl group on the
mesityl ring.
The structure of 20 was determined by X-ray crystal-
lography (Figure 3), and selected bond distances and
angles are listed in Table 3. A peak found in the
difference Fourier map is consistent with the position
of a tantalum hydride (Ta(1)-H(1) ) 1.795 Å), but the
refinement did not include this atom. The hydrogens
on C(29) and Si(1) were located in the difference Fourier
conversion is quantitative (by NMR spectroscopy), but
the high solubility of the product hindered attempts to
isolate large amounts of the complex by crystallization.

Tantalum Imido Fragment Cp*(DippNd)Ta
Organometallics, Vol. 21, No. 15, 2002 3115
Ta ble 3. Selected Bon d Len gth s (Å) a n d An gles
(d eg) for Cp *(Dip p Nd)Ta [η2-
CH2(2-SiH2Mes-3,5-Me2C6H2)]H (20)a
Bond Lengths
Ta(1)-N(1)
Ta(1)-C(29)
N(1)-C(1)
1.801(8)
2.244(9)
1.41(1)
1.88(1)
Ta(1)-C(24)
Ta(1)-Cp*cent
Ta(1)-H(1)
Si(1)-C(32)
2.426(8)
2.1303(4)
1.795
Si(1)-C(23)
1.87(1)
Bond Angles
N(1)-Ta(1)-C(24)
N(1)-Ta(1)-Cp*cent
111.3(3) N(1)-Ta(1)-C(29)
103.5(3)
35.8(3)
120.3(2) C(24)-Ta(1)-C(29)
C(24)-Ta(1)-Cp*cent 125.7(2) C(29)-Ta(1)-Cp*cent 112.0(2)
Ta(1)-N(1)-C(1)
N(1)-Ta(1)-H(1)
C(29)-Ta(1)-H(1)
164.4(6) Ta(1)-C(24)-C(29)
65.2(5)
79.26
92.37
114.72
C(24)-Ta(1)-H(1)
Cp*cent-Ta(1)-H(1) 112.51
a
Cp*cent represents the average of the x, y, and z coordinates of
the η5-C5Me5 ring carbons.
F igu r e 4.
ORTEP diagram of Cp*(DippNd)Ta-
(CH2CMe3)H (26).
map but placed in calculated positions. The alkyl ligand
is bound to the metal through asymmetric η2-coordina-
tion of C(24) and C(29) with tantalum-carbon bond
distances of 2.426(8) and 2.244(9) Å, respectively.
The silyl hydride complex generated upon loss of LiCl
(analogous to the formation of 6 from 5) is apparently
not stable, giving rise to 20. Monitoring the reaction of
4 and (THF)2LiSiHMes2 by 1H NMR spectroscopy as the
chilled (-30 °C) THF-d8 reaction mixture was slowly
warmed to room temperature in the NMR probe re-
vealed the initial formation of the silyl hydride complex
{Li(THF)n}{Cp*(DippNd)TaCl[SiHMes2]H} (21), with
δ(Ta-H) ) 12.26 ppm and diagnostic 29Si satellites (J SiH
) 33 Hz). Within 5 min, 21 is completely converted to
20 and several unidentified byproducts (the reaction is
not as clean as in diethyl ether). A likely mechanism
for the formation of 20 involves reductive elimination
of H2SiMes2, followed by oxidative addition of a C-H
bond of H2SiMes2 to the TaIII center of Cp*(DippNd)Ta
to produce the observed product. However, alternative
mechanisms involving an intramolecular σ-bond meta-
thesis pathway or an initial hydride shift to nitrogen
(vide supra) cannot be ruled out. Treatment of deuterium-
labeled 4-d with (THF)2LiSiHMes2 gave inconclusive
results regarding deuterium incorporation and did not
provide evidence to eliminate any of the above possibili-
ties.
The pairwise conversion of Ta-Si and C-H bonds to
Ta-C and Si-H bonds seems to be a generally favorable
process, given that both tantalum silyl hydrides 6 and
21 intramolecularly rearrange to tantalum alkyl hy-
drides. However, no evidence has been found for inter-
molecular C-H bond activation processes (e.g., the
activation of solvent). For example, use of mesitylene
as the solvent for the decomposition of 5 to 15 and HSi-
(SiMe3)3 did not result in any visible mesitylene activa-
tion or change in the product distribution.
Syn th esis a n d Rea ctivity of Alk yl Silyl a n d Alk yl
Hyd r id e Com p lexes. The Cp*(DippNd)Ta fragment
was also found to support alkyl hydride complexes of
the type Cp*(DippNd)Ta(R)H (R ) Me, CH2CMe3).
However, an initial attempt to prepare a methyl hydride
complex from 4 and MeMgBr (2 equiv) was unsuccess-
ful. Surprisingly, only the dimethyl derivative Cp*-
(DippNd)TaMe2 (22) was formed in this reaction,
instead of the expected methyl hydride complex. There-
fore, efforts were focused on synthesis of the methyl
hydride via hydrogenolysis of the methyl silyl complex
Cp*(DippNd)Ta[Si(SiMe3)3]Me (23), prepared by treat-
ing 1 with MeMgBr (1 equiv) in diethyl ether (55%
yield). At room temperature in benzene-d6 solution, 23
decomposed to Si(SiMe3)4 and 22 (over 36 h). Complex
23 reacted quantitatively with carbon monoxide (1 atm,
benzene-d6, 1 h) to give the stable η2-silaacyl complex
Cp*(DippNd)Ta[η2-COSi(SiMe3)3]Me (24), identified by
the characteristic low-field 13C NMR shift of the carbo-
nyl resonance (δ ) 408.6 ppm) and the very low carbonyl
stretching frequency (νCO ) 1421 cm-1).39 Hydrogenoly-
sis of 23 (1 atm of H2) afforded the desired methyl
hydride [Cp*(DippNd)TaMe(µ-H)]2 (25) in 94% yield
1
(quantitative conversion by H NMR spectroscopy); at
40 psi H2, only decomposition products were observed.
A strong νTa-(µ-H) stretch at 1595 cm-1 and poor solubil-
ity in hydrocarbon solvents support the formulation of
complex 25 as a dimer containing bridging hydride
ligands.38
The analogous neopentyl hydride complex Cp*-
(DippNd)Ta(CH2CMe3)H (26) was obtained directly
from 4 and neopentylmagnesium chloride (2 equivs) in
66% yield. Complex 26 is stable in solution at temper-
atures up to approximately 50 °C (benzene-d6), but
rapidly decomposes at higher temperatures to give
1
neopentane, H2, and many unidentified species (by H
NMR spectroscopy). The 1H NMR spectrum of 26
contains a doublet at 15.09 ppm for the hydride ligand,
and the two signals for the diastereotopic methylene
hydrogens are separated by more than 5 ppm (doublet
at 3.04 ppm, doublet of doublets at -2.21 ppm). The
latter, upfield chemical shift indicates that one of the
methylene hydrogens possesses some agostic character.
This assumption is further supported by the unusually
low-field methylene resonance (127.0 ppm) and the low
1J CH value associated with the hydrogen resonating at
-2.21 ppm (89 Hz), which is typical for an agostic
structure (1J CH ) 115 Hz for the other methylene
hydrogen).42
An ORTEP drawing of 26 is shown in Figure 4, and
selected bond distances and angles are given in Table
4. The hydride, H(1), was located in the difference
Fourier map but not included in the refinement. The
hydrogens bonded to the Ta-bound carbon, C(1), could
not be located from the difference Fourier map and
were refined in geometrically calculated positions. The
Ta(1)-C(1)-C(2) angle of 134.0(5)° is somewhat large,
as might be expected for an R-agostic C-H interaction.

3116 Organometallics, Vol. 21, No. 15, 2002
Burckhardt et al.
Ta ble 4. Selected Bon d Len gth s (Å) a n d An gles
or silyl ligands, possible alternate reaction pathways
include migration of a group to the imido nitrogen atom
and concerted addition of a substrate (e.g., H2 or a
hydrosilane) across the MdN double bond. We have
recently reported the reaction of Cp*(DippNd)Ta[Si-
(SiMe3)3]H (6) with PhSiH3 by way of Si-N coupling to
form two dimeric products with bridging silanimine
ligands.19 The major product, Cp*2Ta2H2(µ-DippNSi-
HPh)2, is a dimer resulting from the net addition of
PhSiH3 across the TadN double bond. While the mech-
anism for the formation of this product is uncertain, one
possible pathway involves the introduction of a -SiH2-
Ph ligand, which can then migrate to the imido nitrogen
atom. For chelating imido-amido complexes of the type
(d eg) for Cp *(Dip p Nd)Ta (CH2CMe3)H (26)a
Bond Lengths
Ta(1)-N(1)
Ta(1)-Cp*cent
N(1)-C(6)
1.791(4)
2.1215(2)
1.403(7)
Ta(1)-C(1)
Ta(1)-H(1)
C(1)-C(2)
2.121(7)
1.660
1.539(9)
Bond Angles
N(1)-Ta(1)-C(1)
107.9(2) N(1)-Ta(1)-Cp*cent 124.0(1)
C(1)-Ta(1)-Cp*cent 116.2(2) Ta(1)-N(1)-C(6)
168.6(4)
95.61
Ta(1)-C(1)-C(2)
C(1)-Ta(1)-H(1)
134.0(5) N(1)-Ta(1)-H(1)
101.55
Cp*cent-Ta(1)-H(1) 106.92
a
Cp*cent represents the average of the x, y, and z coordinates of
the η5-C5Me5 ring carbons.
For comparison, the neopentyl molybdenum complex
(DippNd)2Mo[Si(SiMe3)3]CH2CMe3, characterized as
possessing an R-H agostic interaction, exhibits a
Mo(1)-C(1)-C(2) bond angle of 136.6(2)°.18
Cp*Ta[dN(C6H3Me)2NSiMe3]Me, silanes were observed
to add across the TadN double bond via an intermediate
pentacoordinate silicon species.17
To investigate the possibility of reductive elimination/
oxidative addition equilbria involving the reversible
formation of neopentane, the deuteride Cp*(DippNd)-
Ta(CH2CMe3)D (26-d ) was prepared and studied. Ben-
zene-d6 solutions of 26-d did not exhibit deuterium
scrambling after 7 days at room temperature. At higher
temperatures, decomposition was fast with only slight
(<10%) deuterium incorporation into liberated neopen-
tane and the methylene positions of the intact complex
(by 1H and 2H NMR spectroscopy). However, NMR
spectra of 26-d which had been stored as a crystalline
material for four months at room temperature exhibited
ca. 50% hydrogen incorporation into the deuteride
position and deuterium scrambling mainly into the
The silyl hydride complex 6 is remarkably electro-
philic, as indicated by its reversible binding of halides
(Cl, Br, I) to form ate complexes of the type {Cp*-
(DippNd)Ta(X)[Si(SiMe3)3]H}- (5, 7, and 8, respec-
tively). Interestingly, the formation of these species is
accompanied by intramolecular couplings of the silyl
and hydride ligands to give σ-complexes with significant
Si‚‚‚H interactions. This halide binding provides an
unusual example of an observed, reversible Si-H bond-
making/bond-breaking reaction at a metal center. These
Si‚‚‚H interactions are characterized by J SiH coupling
constants (31-36 Hz) that are intermediate between
J SiH values reported for silyl hydride complexes and
silane complexes with the strongest Si-H inter-
actions.24-26,30-35 It is becoming increasingly clear that
interactions of this type are common in metal silicon
chemistry, in particular for the early transition metals.25
i
CMe3 and Pr (methyl and methine) positions of the
ligands. This indicates that various C-H addition and
elimination reactions indeed take place, but they are
slow and not very selective.
Thermal decomposition of 6 at room temperature
(t1/2
) 8.5 h) gives the rearrangement product
Con clu sion s
Cp*(DippNd)Ta[CH2Si(SiMe3)2SiMe2H]H (15), which
contains an unusual γ-agostic Si-H interaction with the
metal center. This rearrangement involves an interest-
ing series of Si-Si and Si-C bond-breaking/bond-
making steps and C-H bond activation. This process
may be compared to the thermal degradation of
(DippNd)2Mo[Si(SiMe3)3]H, generated by the reac-
tion of (DippNd)2Mo[Si(SiMe3)3]CH2CMe3 with H2,
which decomposes by the elimination of HSiMe3 (1.8
equiv), presumably via molybdenum silylene complexes
such as (DippNd)2ModSi(SiMe3)2.18 Thus, despite the
apparent potential for such 1,2-eliminations to occur
in d0 imido silyl hydride complexes of this type, this
is not observed in the Ta system described here,
which in contrast features Si-C and C-H bond cleav-
ages. A C-H bond cleavage is also observed in the
room-temperature decomposition of {Li(THF)n}-
{Cp*(DippNd)TaCl[SiH(2,4,6-Me3C6H2)2]H} (21) to
Cp*(DippNd)Ta[η2-CH2(2-SiH2Mes-3,5-Me2C6H2)]H (20),
presumably via the silyl hydride intermediate
Cp*(DippNd)Ta[SiH(2,4,6-Me3C6H2)2]H.
The results described here demonstrate that the Cp*-
(DippNd)Ta fragment supports stable complexes con-
taining reactive M-R (R ) H, silyl) σ-bonds. This
system represents some of the few complexes that
contain both imido ligands and highly reactive σ-bonds
and provides further examples of novel σ-bond meta-
thesis reactivity for d0 imido complexes. One of the first
tantalum complexes of this type was Bercaw’s imido
hydride Cp*Ta(dNPh)H, which is formed by an R-mi-
gration in the amido complex Cp*2TaNHPh.53-55 Xue
and co-workers have reported imido silyl complexes of
the type (Me3SiNd)[(Me3Si)2N]Ta[Si(tBu)Ph2]X (X )
Si(tBu)Ph2, NMe2), which are being studied as precur-
sors for the preparation of Ta-Si-N ternary materi-
als.56 More recently, Nikonov has described the silyl
hydride Cp(DippNd)Ta(H)(SiMe2Cl)(PMe3), which con-
tains a three-center Si‚‚‚Ta‚‚‚H interaction.24-26
We are interested in possible σ-bond metathesis
reactions of d0 imido complexes. Of course, for early
metal imido complexes possessing reactive hydride and/
(53) Antonelli, D. M.; Schaefer, W. P.; Parkin, G.; Bercaw, J . E. J .
Organomet. Chem. 1993, 462, 213.
(54) Blake, R. E.; Antonelli, D. M.; Henling, L. M.; Schaefer, W. P.;
Hardcastle, K. I.; Bercaw, J . E. Organometallics 1998, 17, 718.
(55) Parkin, G.; van Asselt, A.; Leahy, D. J .; Whinnery, L.; Hua, N.
G.; Quan, R. W.; Henling, L. M.; Schaefer, W. P.; Santarsiero, B. D.;
Bercaw, J . E. Inorg. Chem. 1992, 31, 82.
In summary, the observed reaction chemistry sug-
gests that new stoichiometric and catalytic processes
involving bond-making/bond-breaking steps might be
based on silyl and/or hydride complexes containing d0
imido fragments. Current research efforts are directed
toward the development of such processes with sterically
(56) Wu, Z.; Xue, Z. Organometallics 2000, 19, 4191.

Tantalum Imido Fragment Cp*(DippNd)Ta
Organometallics, Vol. 21, No. 15, 2002 3117
acquired as 2048 × 128 files with 16 transients accumulated
per T1 increment and were optimized for an assumed 4 Hz
(125 ms) long-range J SiH. The 1H,15N-HMQC experiment for
compound 15 was carried out with a gradient (TBI) probe
operating at 500.13 MHz (1H) and 50.69 MHz (15N), using the
inv4gp pulse sequence with a gradient pulse ratio of 70:30:
50, a gradient pulse length of 1 ms, and a gradient pulse
recovery delay of 200 µs; the spectrum was optimized for an
demanding imido ligands that support monomeric silyl
hydride derivatives.
Exp er im en ta l P r oced u r es
Gen er a l P r oced u r es. All experiments were conducted
under a nitrogen atmosphere using standard Schlenk tech-
niques or in a Vacuum Atmospheres drybox unless otherwise
noted. Dry, oxygen-free solvents were used unless otherwise
indicated. Olefin impurities were removed from pentane by
treatment with concentrated H2SO4, 0.5 N KMnO4 in 3 M
H2SO4, and saturated NaHCO3. Pentane was then dried over
MgSO4, stored over activated 4 Å molecular sieves, and
distilled from potassium benzophenone ketyl under a nitrogen
atmosphere. Thiophene impurities were removed from benzene
and toluene by treatment with H2SO4 and saturated NaHCO3.
Benzene and toluene were then dried over MgSO4 and distilled
from potassium under a nitrogen atmosphere. Diethyl ether,
THF, and hexanes were distilled from sodium benzophenone
ketyl under a nitrogen atmosphere, and dichloromethane was
distilled from CaH2 under a nitrogen atmosphere. Benzene-
d6, toluene-d8, and THF-d8 were purified and dried by vacuum
distillation from sodium/potassium alloy.
1
assumed 80 Hz (6.3 ms) J NH. Bruker XWINNMR software
(ver. 2.1) was used for all NMR data processing.
Cp *(Dip p Nd)Ta [Si(SiMe3)3]Cl (1). A solution of Cp*-
(DippNd)TaCl2 (3.92 g, 6.97 mmol) and (THF)3LiSi(SiMe3)3
(3.78 g, 8.03 mmol) in cold (-50 °C) diethyl ether (60 mL) was
allowed to warm to room temperature and was then stirred
for 12 h. The ether was removed in vacuo, the crystalline
residue was extracted four times with a total of 80 mL of
hexanes, and the combined extracts were filtered. The result-
ing solution was concentrated to ca. 50 mL and cooled to -25
°C to produce three crops of dark red crystals of 1 (4.33 g, 80%).
1H NMR (400 MHz, benzene-d6): δ 7.18 (d, 2 H, m-Ar-H, J )
8), 6.90 (t, 1 H, p-Ar-H, J ) 8), 3.89 (septet, 1 H, iPr-H, J ) 7),
i
3.55 (septet, 1 H, Pr-H, J ) 7), 1.96 (s, 15 H, C5Me5), 1.41,
i
1.31 (2 × d, 2 × 3 H, Pr-Me, J ) 7), 1.40, 1.21 (2 × d, 2 × 3
All chemicals were purchased from Aldrich or Fluka and
used without further purification unless otherwise indicated.
Commercial silanes were dried over 4 Å molecular sieves and
distilled prior to use. Carbon monoxide was purchased from
Scott Specialty Gases, hydrogen was purchased from Praxair,
and deuterium was purchased from Airgas. The compounds
Cp*TaCl4,57-60 DippNH(SiMe3) (Dipp ) 2,6-iPr2C6H3),61 Cp*-
(DippNd)TaCl2,27,28 (THF)3LiSi(SiMe3)3,62 (THF)3LiSiPh3,63,64
i
H, Pr-Me, J ) 7), 0.46 (s, 27 H, Si(SiMe3)3). 13C{1H} NMR
(100 MHz, benzene-d6): δ 150.1, 148.0, 145.2 (Ar C), 125.3,
123.8, 123.4 (Ar CH), 120.5 (C5Me5), 28.2, 27.3, 27.1, 26.4, 25.7,
24.5 (iPr), 12.9 (C5Me5), 6.1 (Si(SiMe3)3). 29Si{1H} NMR (99
MHz, benzene-d6): δ -3.4 (Si(SiMe3)3), -47.8 (Si(SiMe3)3).
Anal. Calcd for C31H59ClNSi4Ta (774.56): C, 48.07; H, 7.68;
N, 1.81. Found: C, 48.01; H, 7.84; N, 1.76.
Cp *(Dip p Nd)Ta (SiP h 3)Cl (2).
A
solution of Cp*-
65
and (THF)2LiSiHMes2 were prepared as reported in the
(DippNd)TaCl2 (1.34 g, 2.38 mmol) and (THF)3LiSiPh3 (1.7 g,
0.38 mmol) in diethyl ether (40 mL) was stirred at room
temperature for 1.5 h. The diethyl ether was removed in vacuo,
and the oily residue was then extracted into pentane (4 × 20
mL). The combined extracts were concentrated to ca. 20 mL
and cooled to -25 °C to produce 2 in two crops as a dark red
foam (1.49 g), which could not be completely separated from
silane impurities. 1H NMR (500 MHz, benzene-d6): δ 7.80 (br
m, 6 H, o-Ph-H), 7.22-6.99 (m, 11 Ar-H), 6.91 (m, 1 H, p-Ar-
literature. Elemental analyses were performed by the College
of Chemistry Microanalytical Laboratory at the University of
California, Berkeley. Mass spectra (EI+) were recorded on a
Micromass VG ProSpec instrument (ionization energy: 70 eV).
GCMS spectra were measured on a HP 6890 GC instrument
equipped with a HP 5973 MS detector. Infrared spectra were
recorded on a Mattson FTIR 3000 instrument, as KBr pellets
(solids) or as a thin film between KBr plates (oils).
NMR Mea su r em en ts. Routine 1H, 2H, and 13C NMR
spectra were recorded at 298 K on Bruker AMX-300 and AMX-
400 spectrometers (fields and solvents specified). All other
spectra were measured at 298 K on a Bruker DRX-500
instrument equipped with a 5 mm broad band probe and
operating at 500.13 MHz (1H), 194.37 MHz (7Li), 125.77 MHz
(13C), 99.36 MHz (29Si), or 202.45 MHz (31P). Chemical shifts
are reported in ppm downfield from internal SiMe4 (1H/2H, 13C,
29Si) and external 10% aqueous LiCl (7Li) and 85% H3PO4 (31P).
Coupling constants are given in Hz. For 1H NOESY spectra,
a mixing time of 1.0 s was applied. 1H,13C-HMQC spectra were
recorded as 2048 × 256 files using a BIRD sequence [π/2(1H)
) 14.0 µs; π/2(13C) ) 6.4 µs]. 29Si NMR spectra were obtained
using a refocused INEPT pulse sequence with or without 1H
decoupling; delays were optimized for J SiH ) 6.4 Hz [π/2(1H)
i
H), 3.55 (septet, 2 H, Pr-H, J ) 7), 1.83 (s, 15 H, C5Me5),
i
i
1.27 (d, 6 H, Pr-Me, J ) 7), 1.04 (br d, 6 H, Pr-Me, J ) 7).
13C{1H} NMR (126 MHz, benzene-d6): δ 150.1, 145.7 (Ar C),
139.2 (o-Ph CH), 128.8, 128.3, 125.1, 123.1 (Ar CH), 120.6 (C5-
i
i
Me5), 28.6 (br, Pr CH), 25.4, 24.1 (br, Pr Me), 12.1 (C5Me5).
29Si{1H} NMR (99 MHz, benzene-d6): δ 51.4. MS: m/z 785 (24,
[M]+), 561 (7), 259 (100, [SiPh3]+), 182 (32, [SiPh2]+), 105 (7,
[SiPh]+).
Cp *(Dip p Nd)Ta [SiH(2,4,6-Me3C6H2)2]Cl (3). A solution
of Cp*(DippNd)TaCl2 (1.00 g, 1.78 mmol) and (THF)2LiSiHMes2
(0.93 g, 2.2 mmol) in diethyl ether (30 mL) was stirred at room
temperature for 4 h. The diethyl ether was removed in vacuo,
and the resulting oily residue was extracted into pentane (3
× 15 mL). The extracts were concentrated and cooled to -30
°C to afford two crops of 3 as orange-red crystals (1.09 g, 77%).
1H NMR (500 MHz, benzene-d6): δ 7.10 (d, 2 H, m-Ar-H, J )
10), 6.83 (t, 1 H, p-Ar-H, J ) 10), 6.78, 6.66 (2 × s, 2 × 2
Mes-H), 6.66 (s, 1 H, Si-H), 3.72 (sept., 2 H, iPr-H, J ) 7), 2.89,
2.68 (2 × s, 2 × 6 H, o-Mes-Me), 2.17, 2.01 (2 × s, 2 × 3 H,
1
) 14.0 µs; π/2(29Si) ) 6.6 µs]. H,29Si-HMQ(B)C spectra were
(57) Sanner, R. D.; Carter, S. T.; Bruton, W. J . J . Organomet. Chem.
1982, 240, 157.
(58) Gibson, V. C.; Bercaw, J . E.; Bruton, W. J .; Sanner, R. D.
Organometallics 1986, 5, 976.
(59) Yasuda, H.; Okamoto, T.; Nakamura, A. In Organometallic
Syntheses; King, R. B., Eisch, J . J ., Eds.; Elsevier: Amsterdam, 1988;
Vol. 4, p 20.
(60) Okamoto, T.; Yasuda, H.; Nakamura, A.; Kai, Y.; Kaneshida,
N.; Kasai, N. Organometallics 1988, 7, 2266.
(61) Chao, Y.-W.; Wexler, P. A.; Wigley, D. E. Inorg. Chem. 1989,
28, 3860.
(62) Gutekunst, G.; Brook, A. G. J . Organomet. Chem. 1982, 225,
1.
(63) Olah, G. A.; Hunadi, R. J . J . Am. Chem. Soc. 1980, 102, 6989.
(64) Dias, H. V. R.; Olmstead, M. M.; Ruhlandt-Senge, K.; Power,
P. P. J . Organomet. Chem. 1993, 462, 1.
(65) Weidenbruch, M.; Kramer, K.; Pohl, S.; Saak, W. J . Organomet.
Chem. 1986, 316, C13.
i
p-Mes-Me), 1.85 (s, 15 H, C5Me5), 1.35 (d, 6 H, Pr-Me, J ) 7),
i
1.04 (br d, 6 H, Pr-Me, J ) 7). 13C{1H} NMR (126 MHz,
benzene-d6): δ 149.3, 146.4, 146.0, 144.9, 139.8, 139.4, 138.8,
129.5, 128.0 (Ar C), 129.7, 129.3 (Mes CH), 125.3, 123.0 (Ar
CH), 120.7 (C5Me5), 28.2, 25.9 (o-Mes Me), 28.0 (iPr CH), 25.7,
24.7 (iPr Me), 21.4, 21.3 (p-Mes Me), 11.9 (C5Me5). 29Si NMR
(99 MHz, benzene-d6): δ 17.6 (d, 1J SiH ) 163). IR (KBr, cm-1):
2097 (m, νSiH). MS: m/z 793 (2, [M]+), 702 (7), 561 (40), 546
(9), 268 (73, [H2SiMes2]+), 253 (12), 177 (8), 162 (14), 148 (100,
[HSiMes]+), 133 (30), 120 (31), 105 (11). Anal. Calcd for C40H55
-
ClNSiTa (794.37): C, 60.48; H, 6.98; N, 1.76. Found: C, 60.40;
H, 7.28; N, 1.83.

3118 Organometallics, Vol. 21, No. 15, 2002
Burckhardt et al.
[Cp *(Dip p Nd)Ta Cl(µ-H)]2 (4). A hexanes solution (50 mL)
of 1 (4.80 g, 6.20 mmol) was pressurized (80 psi) with hydrogen
at room temperature. After 45 min the color of the solution
had changed from deep red to light orange, and a yellow
precipitate had formed. The precipitate was collected on a
fritted glass filter and was washed twice with hexanes to give
pure 4 (2.56 g, 78%). Concentrating and cooling the combined
filtrate to -25 °C afforded a second crop of 4 (0.57 g, 17%).
Total yield: 3.13 g (96%). 1H NMR (500 MHz, CD2Cl2): δ 7.91
(s, 2 H, Ta-H), 7.09 (m, 4 H, m-Ar-H), 6.92 (t, 2 H, p-Ar-H, J
fitted with a J . Young Teflon stopper (containing a sealed
capillary filled with D2O), and the mixture was shaken. A color
change from red to light yellow occurred within 1 min. The
resulting bromide adduct 7 was identified by NMR spectros-
copy (quantitative conversion). Selected data: 1H NMR (500
MHz, THF-d8): δ 12.01 (s, 1 H, Ta-H; J SiH ) 33), 2.04 (s, 15
H, C5Me5), 0.16 (s, 27 H, Si(SiMe3)3). 29Si{H} NMR (99 MHz,
THF-d8): δ -4.8 (Si(SiMe3)3), -105.9 (Si(SiMe3)3).
{NBu 4}{Cp *(Dip p Nd)Ta (I)[Si(SiMe3)3]H} (8). An es-
sentially identical procedure to that used to prepare 7 (sub-
stituting [NBu4]I for [NBu4]Br) yielded 8 as a light yellow
solution (quantitative conversion). Selected data: 1H NMR
(500 MHz, THF-d8): δ 10.71 (s, 1 H, Ta-H; J SiH ) 36), 2.09 (s,
15 H, C5Me5), 0.18 (s, 27 H, Si(SiMe3)3). 29Si{1H} NMR (99
MHz, THF-d8): δ -3.8 (Si(SiMe3)3), -112.4 (Si(SiMe3)3).
i
) 8), 3.99, 3.27 (2 × septet, 2 × 2 H, Pr-H, J ) 7), 2.05 (s, 30
i
H, C5Me5), 1.31, 1.21, 1.20, 1.14 (4 × d, 4 × 6 H, Pr-Me, J )
7). 13C{1H} NMR (100 MHz, CD2Cl2): δ 149.1, 148.4, 145.1
(Ar C), 124.9, 122.9, 122.4, (Ar CH), 119.0 (C5Me5), 27.3, 27.2,
26.9, 24.8 24.2, 24.2 (iPr), 11.9 (C5Me5). IR (KBr, cm-1): 1606
(vs, br, νTaH). Anal. Calcd for C44H66Cl2N2Ta2 (1055.82): C,
50.05; H, 6.30; N, 2.65. Found: C, 50.45; H, 6.25; N, 2.49.
[Cp *(Dip p Nd)Ta Cl(µ-D)]2 (4-d ). Selected data: IR (KBr,
cm-1): 1151 (vs, br, νTaD).
Cp *(Dip p Nd)Ta [η2-C(N-2,6-Me2C6H3)Si(SiMe3)3]H (9).
A toluene solution (15 mL) of complex 6 (450 mg, 0.608 mmol)
and xylyl isocyanide (94 mg, 0.72 mmol) was stirred at room
temperature for 6 h. The initial red solution gradually turned
brown-orange. The solvent was removed in vacuo, and the
residue was extracted with pentane (20 mL). The orange
solution was filtered and cooled to -25 °C to afford light orange
{Li(THF )3}{Cp *(Dip p Nd)Ta (Cl)[Si(SiMe3)3]H} (5). A
THF suspension (20 mL) of 4 (650 mg, 0.62 mmol) and
(THF)3LiSi(SiMe3)3 (580 mg, 1.23 mmol) was stirred at room
temperature until a clear, yellow solution was obtained (2 h).
The solution was filtered and concentrated in vacuo to ca. 10
mL. After the solution was cooled to -30 °C and layered with
pentane (20 mL), yellow crystals formed instantly and were
allowed to settle. The supernatant liquid was removed by
filtration, and the crystals were washed with a few milliliters
of cold (-30 °C) pentane and briefly dried in vacuo to isolate
5 (1.09 g, 89%). The bright yellow crystals of 5 can be stored
in a closed vial for several weeks with only slight decomposi-
tion. Further drying results in loss of coordinated THF and
subsequent separation of LiCl, as evidenced by a color change
1
crystals of 9 (257 mg, 49%). H NMR (300 MHz, benzene-d6):
δ 10.14 (s, 1 H, Ta-H), 7.20, 7.10 (2 × m, 2 × 1 Ar-H), 6.97-
6.82 (m, 4 Ar-H), 4.44, 3.75 (2 × septet, 2 × 1 H, iPr-H, J ) 7),
2.28 (s, 3 H, Xylyl-Me), 1.91 (s, 15 H, C5Me5), 1.78 (s, 3 H,
i
Xylyl-Me), 1.53, 1.49, 1.36, 1.15 (4 × d, 4 × 3 H, Pr-Me, J )
7), 0.20 (s, 27 H, Si(SiMe3)3). 13C{1H} NMR (100 MHz, benzene-
d6): δ 280.2 (CNXylyl), 152.8, 149.4, 142.1, 141.6, 131.0, 129.8,
129.5, 129.1, 127.0, 123.0, 122.5, 121.4 (Ar C, CH), 113.5
(C5Me5), 27.1, 26.7, 26.1, 25.9 (br), 21.3, 19.3 (iPr, Xylyl-Me),
12.0 (C5Me5), 2.9 (Si(SiMe3)3). 29Si{1H} NMR (60 MHz, benzene-
d6): δ -10.7 (Si(SiMe3)3), -80.6 (Si(SiMe3)3). IR (KBr, cm-1):
1782 (s, νTaH), 1587 (w, νCtN), 1531 (m, νCtN). Anal. Calcd for
1
to dark red 6. Addition of THF reverses the process. H NMR
(400 MHz, THF-d8): δ 12.83 (s, 1 H, Ta-H; J SiH ) 31), 6.73
(t, 2 Ar-H), 6.47 (t, 1 Ar-H), 4.24, 4.18 (2 × septet., 2 × 1 H,
iPr-H, J ) 7), 2.01 (s, 15 H, C5Me5), 1.14, 1.12, 1.06, 1.00 (4 ×
d, 4 × 3 H, iPr-Me, J ) 7), 0.13 (s, 27 H, Si(SiMe3)3). 7Li NMR
(194 MHz, 0.06 M in THF-d8): δ -0.33. 13C{1H} NMR (126
MHz, THF-d8): δ 152.7, 145.1, 141.1 (Ar C), 121.2, 121.0, 118.5
(Ar CH), 111.5 (C5Me5), 26.0, 25.6, 25.4, 25.0, 24.8, 24.2 (iPr),
12.0 (C5Me5), 6.1 (SiMe3). 29Si NMR (99 MHz, THF-d8): δ -0.1
(Si(SiMe3)3), -98.4 (Si(SiMe3)3), J SiH ) 31). IR (KBr, cm-1):
1805 (m, br, νTaH). Anal. Calcd for C43H84ClLiNO3Si4Ta
(998.83): C, 51.71; H, 8.48; N, 1.40; Cl, 3.55. Found: C, 51.61;
H, 8.68; N, 1.65; Cl, 3.33.
C
40H69N2Si4Ta (871.29): C, 55.14; H, 7.98; N, 3.22. Found: C,
55.08; H, 8.31; N, 3.15.
Cp *(Dip p Nd)Ta [CdN(Me)Si(SiMe3)3]H (10). A benzene-
d6 solution (∼0.7 mL) of complex 6 (34.0 mg, 0.046 mmol) was
transferred to an NMR tube capped with a septum, and
acetonitrile (1.9 mg, 0.046 mmol) was added to the NMR tube
via syringe. Upon addition, the reaction mixture changed from
dark red to bright yellow. The conversion to 10 was determined
to be >95% (by integration of the Cp* resonances in the 1H
NMR spectrum of the reaction mixture). After 18 h at room
temperature, most of 10 had decomposed to unidentified
species. 1H NMR (500 MHz, benzene-d6): δ 12.60 (s, 1 H,
Ta-H), 7.17-6.96 (m, 3 H, Ar-H), 4.23, 3.60 (2 × septet, 2 × 1
H, iPr-H, J ) 7), 1.92 (s, 15 H, C5Me5), 1.43, 1.30, 1.30, 1.12 (4
× d, 4 × 3 H, iPr-Me, J ) 7), 1.39 (s, 3 H, NdCMe), 0.44 (s, 27
H, Si(SiMe3)3).
Cp *(Dip p Nd)Ta (OiP r )[Si(SiMe3)3] (11). An essentially
identical procedure to that used to prepare 10 (substituting
acetone for acetonitrile) yielded a bright yellow solution of 11
in >95% conversion. No change was observed in the spectrum
after 2 days at room temperature. 1H NMR (300 MHz,
benzene-d6): δ 7.15 (m, 2 Ar-H), 6.91 (m, 1 Ar-H), 4.88 (septet,
{Li(THF)3}{Cp*(Dip p Nd)Ta (Cl)[Si(SiMe3)3]D} (5-d). Se-
lected data: IR (KBr, cm-1): 1282 (w, νTaD).
Cp *(Dip p Nd)Ta [Si(SiMe3)3]H (6). A THF (80 mL) solu-
tion of complex 4 (2.00 g, 1.89 mmol) and (THF)3LiSi(SiMe3)3
(1.79 g, 3.80 mmol) was stirred at room temperature for 6 h.
The THF was removed in vacuo, and the yellow residue was
extracted with hexanes (4 × 20 mL). The combined red extracts
were filtered, concentrated to ca. 30 mL, and cooled to -30 °C
1
to afford two crops of dark red crystals of 6 (1.63 g, 58%). H
NMR (300 MHz, benzene-d6): δ 21.49 (s, 1 H, Ta-H), 7.21 (t,
2 H, m-Ar-H, J ) 8), 6.95 (t, 1 H, p-Ar-H, J ) 8), 4.30, 3.68
(2 × septet, 2 × 1 H, iPr-H, J ) 7), 1.98 (s, 15 H, C5Me5), 1.44,
i
1 H, OCHMe2, J ) 6), 3.84, 3.52 (2 × septet, 2 × 1 H, Pr-H,
J ) 7), 1.97 (s, 15 H, C5Me5), 1.39, 1.35, 1.29, 1.27 (4 × d, 4 ×
i
1.43, 1.27, 1.18 (4 × d, 4 × 3 H, Pr-Me, J ) 7), 0.45 (s, 27 H,
i
3 H, Pr-Me, J ) 7), 1.25, 1.18 (2 × d, 2 × 3 H, OCHMe2, J )
Si(SiMe3)3). 13C{1H} NMR (100 MHz, benzene-d6): δ 150.7,
146.2, 144.5 (Ar C), 124.7, 123.0, 123.0 (Ar CH), 118.6
(C5Me5), 27.9, 27.4, 26.4, 25.8, 25.3, 25.1 (iPr), 12.5 (C5Me5),
7.0 (Si(SiMe3)3). 29Si{1H} NMR (99 MHz, benzene-d6): δ -1.1
(Si(SiMe3)3), -22.9 (Si(SiMe3)3). IR (KBr, cm-1): 1785 (w, νTaH).
Anal. Calcd for C31H60NSi4Ta (740.11): C, 50.31; H, 8.17; N,
1.89. Found: C, 50.52; H, 8.08; N, 1.92.
6), 0.47 (s, 27 H, Si(SiMe3)3). 13C{1H} NMR (126 MHz, benzene-
d6): δ 151.3, 145.3, 144.2 (Ar C), 124.1, 123.7, 123.0 (Ar CH),
117.9 (C5Me5), 78.3 (OCHMe2), 28.4, 27.7, 27.3, 27.0, 25.7, 25.4,
25.3, 22.9 (iPr), 13.0 (C5Me5), 7.0 (SiMe3).
Cp *(Dip p Nd)Ta [CH2CH2CH2CH2] (12). Exposure of a
stirred suspension of 6 in pentane (10 mL) to ethylene (1 atm)
for a few seconds gave a clear yellow solution. After stirring
for an additional 5 min, the solution was filtered, concentrated
in vacuo to a few milliliters, and cooled to -30 °C to afford
three crops of yellow crystals, which were isolated, washed
with a little cold pentane, and dried in vacuo to give pure 12
Cp *(Dip p Nd)Ta [Si(SiMe3)3]D (6-d ). Selected data: IR
(KBr, cm-1): 1276 (w, νTaD).
{NBu 4}{Cp *(Dip p Nd)Ta (Br )[Si(SiMe3)3]H} (7). [NBu4]-
Br (ca. 10 mg, 31 µmol) was added to a THF-d8 solution of 6
(freshly generated from 5 (13 mg, 13 µmol)) in an NMR tube

Tantalum Imido Fragment Cp*(DippNd)Ta
Organometallics, Vol. 21, No. 15, 2002 3119
(82 mg, 75%). 1H NMR (400 MHz, benzene-d6): δ 7.25 (d, 2
Ar-H), 6.99 (t, 1 Ar-H), 3.72 (septet, 2 H, Pr-H, J ) 7), 2.47,
(br, SiH), -87.7 (SiMe(SiMe3)2). GCMS: m/z 248 (3, [M]+), 233
(9, [M - Me]+), 189 (5), 174 (100, [M - HSiMe3]+), 159 (51),
145 (11), 131 (22), 129 (23), 115 (21), 101 (12), 85 (5), 73 (86),
59 (15), 45 (16).
i
2.23 (2 × m, 2 × 2 H, R-CH2), 1.73 (s, 15 H, C5Me5), 1.68 (m,
2 H, â-CH2), 1.38 (d, 12 H, iPr-Me, J ) 7), 0.96 (m, 2 H, â-CH2).
13C{1H} NMR (101 MHz, benzene-d6): δ 152.1, 144.4 (Ar C),
122.6 (Ar CH), 116.3 (C5Me5), 53.9 (R-CH2), 28.6 (CHMe2), 24.8
(CHMe2), 16.6 (â-CH2), 11.0 (C5Me5). MS: m/z 547 (24, [M]+),
519 (28), 489 (100), 487 (88), 445 (16), 244 (10). Anal. Calcd
for C26H40NTa (547.56): C, 57.03; H, 7.36; N, 2.56. Found: C,
57.00; H, 7.11; N, 2.61.
Cp *(Dip p Nd)Ta [η2-C(N-2,6-Me 2C6H 3)CH 2Si(SiMe 3)2-
SiMe2H]H (16). Xylyl isocyanide (9.1 mg, 0.069 mmol) was
added to a solution of 15 and HSi(SiMe3)3 (freshly prepared
from 6 (70 mg, 0.070 mmol) in benzene-d6 (2 mL), 36 h reaction
time). A dark brown-orange solution formed within 15 min,
and the reaction mixture was stirred for an additional 1 h.
The solvents were removed in vacuo to yield a dark brown oil,
which resisted all crystallization attempts (the conversion of
15 to 16 was quantitative by 1H NMR). 1H NMR (500
MHz, benzene-d6): δ 10.26 (d, 1 H, Ta-H, J ) 1), 7.11 (d, 2 H,
m-Ar-H, J ) 8), 6.89-6.81 (m, 4 Ar-H), 3.83 (ddq, 1 H, Si-H,
Cp *(Dip p Nd)Ta [CP h CP h CP h CP h ] (13). A benzene-d6
(∼0.7 mL) solution of complex 6 (20.0 mg, 0.0270 mmol) and
diphenylacetylene (9.6 mg, 0.054 mmol) was transferred to an
NMR tube fitted with a J . Young Teflon stopper. After 24 h
(room temperature), the conversion to 13 was determined to
i
J ) 10, 5, 2), 3.75 (septet, 2 H, Pr-H, J ) 7), 1.95 (br s, 6 H,
1
be 52% (by integration of the Cp* resonances in the H NMR
i
Xylyl-Me), 1.84 (s, 15 H, C5Me5), 1.38 (m, 12 H, Pr-Me), 0.64
spectrum of the reaction mixture). 1H NMR (500 MHz, ben-
(ddd, 1 H, TaCH2Si, J ) 13, 9, 1), 0.32 (s, 9 H, SiMe3), 0.29 (s,
3 H, SiMe), 0.22 (s, 9 H, SiMe3), 0.05 (d, 3 H, SiMe, J ) 5),
-0.07 (dd, 1 H, TaCH2Si, J ) 13, 2). 13C{1H} NMR (126 MHz,
benzene-d6): δ 242.0 (CNXylyl), 151.6, 146.9 (Ar C), 129.3 (br,
m-Ar CH), 126.8 (p-Ar CH), 122.6 (br, m-Ar CH), 122.1 (p-Ar
CH), 114.3 (C5Me5), 27.2 (iPr CH), 26.5, 24.7 (br, iPr Me), 19.3
(br, Xylyl-Me), 18.9 (TaCH2Si), 11.6 (C5Me5), 1.1, 1.0 (SiMe3),
-2.0, -11.8 (SiMe). 29Si NMR (99 MHz, benzene-d6): δ -11.6,
zene-d6): δ 7.37-6.63 (m, 23 Ar-H), 5.04 (septet, 2 H, iPr-H, J
i
) 6), 1.86 (s, 15 H, C5Me5), 1.52 (d, 12 H, Pr-Me, J ) 6).
[Cp *(Dip p Nd)Ta H(µ-H)]2 (14). A hexanes (20 mL) solu-
tion of 6 (0.51 g, 0.69 mmol) was pressurized (60 psi) with
hydrogen at room temperature. After 15 min, the color of the
solution had changed from red to yellow, and a fine yellow
precipitate had settled. The precipitate was collected on a
fritted glass filter and was washed with hexanes to yield pure
1
-11.7 (SiMe(SiMe3)2), -27.2 (HSiMe2, J SiH ) 174), -84.3
1
(Si(SiMe3)2). MS: m/z 870 (6, [M]+), 855 (2), 681 (100), 248
(8), 233 (6), 174 (12), 162 (14), 73 (18).
14 (0.19 g, 56%). H NMR (400 MHz, benzene-d6): δ 15.54 (t,
2 H, Ta-H, J ) 8), 7.17 (m, 4 H, m-Ar-H), 6.96 (t, 2 H, p-Ar-H,
J ) 8), 6.48 (t, 2 H, Ta-(µ-H), J ) 8), 4.67, 3.83 (2 × septet, 2
Cp *(Dip p Nd)Ta (OCHP h 2)[CH2Si(SiMe3)2SiMe2H] (17).
A hexanes (15 mL) solution of benzophenone (36 mg, 0.20
mmol) was added to a solution of 15 and HSi(SiMe3)3 (freshly
prepared from 6 (196 mg, 0.200 mmol in hexanes (15 mL), 72
h reaction time). A color change from orange to bright yellow
occurred instantly. After stirring the reaction mixture at room
temperature for 1 h, the solvent was removed in vacuo to yield
an orange oil, which resisted all crystallization attempts (the
conversion of 15 to 17 was quantitative by 1H NMR). 1H
NMR (500 MHz, benzene-d6): δ 7.47 (m, 2 o-Ph-H), 7.43 (m, 2
o-Ph-H), 7.19-6.99 (m, 8 Ar-H), 6.90 (m, 1 H, p-Ar-H), 6.74
(s, 1 H, OCHPh2), 3.99 (dq, 1 H, Si-H, J ) 10, 4), 3.71 (septet,
2 H, iPr-H, J ) 7), 1.82 (s, 15 H, C5Me5), 1.30 (t, 12 H, iPr-Me,
J ) 7), 0.90 (dd, 1 H, TaCH2Si, J ) 13, 10), 0.47 (d, 1 H, TaCH2-
Si, J ) 13), 0.28 (s, 9 H, SiMe3), 0.24 (s, 3 H, SiMe), 0.22 (s, 9
H, SiMe3), 0.19 (d, 3 H, SiMe, J ) 4). 13C{1H} NMR (126 MHz,
benzene-d6): δ 151.0, 145.9, 145.8, 144.1 (Ar C), 128.9-122.9
(Ar CH), 117.3 (C5Me5), 89.0 (OCHPh2), 30.6 (TaCH2), 27.9,
26.5, 26.0, 24.8 (iPr), 11.4 (C5Me5), 1.0 (SiMe3), -2.3 (SiMe),
-12.2 (SiMe). 29Si NMR (99 MHz, benzene-d6): δ -11.6, -11.7
i
× 2 H, Pr-H, J ) 7), 2.05 (s, 30 H, C5Me5), 1.43, 1.37, 1.35,
i
1.32 (4 × d, 4 × 6 H, Pr-Me, J ) 7). 13C{1H} NMR (100 MHz,
benzene-d6): δ 151.2, 144.9, 144.1 (Ar C), 123.9, 122.8, 122.7,
(Ar CH), 115.4 (C5Me5), 27.5, 27.3, 25.8, 25.4, 24.2, 24.0 (iPr),
12.7 (C5Me5). IR (KBr, cm-1): 1771 (m, νTaH), 1529 (s, br,
νTa-(µ-H)). Anal. Calcd for C44H68N2Ta2 (986.93): C, 53.55; H,
6.94; N, 2.84. Found: C, 53.54; H, 6.98; N, 2.90.
Cp *(Dip p Nd)Ta [CH2Si(SiMe3)2SiMe2H]H (15). A solu-
tion of 6 (freshly generated from 5 (88 mg, 0.088 mmol)) in
benzene (5 mL) was stirred at room temperature for 48 h. The
resulting clear yellow solution was filtered, and the solvent
and volatile byproducts were removed in vacuo to give an
orange oil, which consisted mainly of a 2:1 mixture of 15 (ca.
60%) and HSi(SiMe3)3, along with minor impurities (one of
which is 18, see below). Numerous attempts to separate 15
by crystallization from a variety of solvents failed. Decomposi-
tion rate: t1/2 ) 8.5 h (by NMR). 1H NMR (500 MHz, benzene-
d6): δ 9.58 (dd, 1 H, Ta-H, J ) 9, 4), 7.07 (d, 2 H, m-Ar-H, J
i
) 8), 6.88 (t, 1 H, p-Ar-H, J ) 8), 4.10 (septet, 2 H, Pr-H, J )
1
7), 2.57 (dddq, 1 H, SiH, J ) 9, 4, 4, 1), 1.96 (s, 15 H, C5Me5),
(SiMe(SiMe3)2), -27.4 (HSiMe2, J SiH ) 183), -83.6 (Si-
i
(SiMe3)2). MS: m/z 921 (1, [M]+), 906 (2), 746 (5), 732 (62),
689 (10), 636 (7), 579 (9), 565 (47), 522 (6), 248 (7), 233 (4),
182 (11), 174 (12), 167 (100, [CHPh2]+), 105 (14), 73 (14).
{Cp*(DippNd)Ta[CH2Si(SiMe3)2SiMe2H]}(µ-H)2{TaCp*-
(dNDip p )H} (18). Complex 18 was isolated as orange crystals
from cold pentane solutions of the reaction mixture resulting
from decomposition of 6 (ca. 5% yield). 1H NMR (500 MHz,
benzene-d6): δ 14.93 (dd, 1 H, Ta-H, J ) 9, 6), 7.17 (dd, 1 m-Ar-
H, J ) 8, 2), 7.12 (m, 3 m-Ar-H), 6.99 (m, 1 H, Ta-(µ-H)), 6.96-
6.90 (m, 2 p-Ar-H), 6.63 (dd, 1 H, Ta-(µ-H), J ) 6, 4), 4.69 (m,
1.35, 1.29 (2 × d, 2 × 6 H, Pr-Me, J ) 7), 0.70 (ddd, 1 H,
TaCH2Si, J ) 13, 4, 4; J CH ) 129), 0.43 (d, 3 H, SiMe, J ) 4),
0.24, 0.22 (2 × s, 2 × 9 H, Si(SiMe3)2), -0.09 (s, 3 H, SiMe),
-0.90 (dd, 1 H, TaCH2Si, J ) 13, 1; J CH ) 129). 13C{1H} NMR
(126 MHz, benzene-d6): δ 151.2, 144.1 (Ar C), 123.4 (p-Ar CH),
122.4 (m-Ar CH), 113.2 (C5Me5), 27.6 (iPr CH), 24.9, 24.4 (iPr
Me), 11.9 (C5Me5), 10.8 (TaCH2Si), 0.7, 0.6 (SiMe3), -1.8, -11.8
(SiMe2H). 29Si NMR (99 MHz, benzene-d6): δ -10.5, -11.1 (Si-
1
(SiMe3)2), -68.9 (HSiMe2, J SiH ) 87), -88.3 (Si(SiMe3)2).
1
Temperature-dependence of J SiH: 78 Hz at -70 °C; 101 Hz
at 80 °C. MS: m/z 739 (5, [M]+), 737 (56, [M - H2]+), 693 (19),
664 (84), 622 (33), 562 (20), 546 (15), 489 (16), 248 (41), 233
(31), 174 (97), 160 (50), 73 (100). IR (neat decomp mixture,
film on KBr, cm-1): 2052 (m, νSiH of HSi(SiMe3)3), 1726 (m,
i
1 H, Si-H), 4.39, 4.15, 3.90, 3.69 (4 × septet, 4 × 1 H, Pr-H, J
) 7), 2.05, 1.98 (2 × s, 2 × 15 H, C5Me5), 1.482, 1.478, 1.45,
i
1.42, 1.37, 1.36, 1.32, 1.29 (8 × d, 8 × 3 H, Pr-Me, J ) 7),
0.96 (dd, 1 H, TaCH2Si, J ) 12, 9), 0.68 (d, 3 H, SiMe, J ) 4),
0.44 (s, 3 H, SiMe), 0.41, 0.38 (2 × s, 2 × 9 H, SiMe(SiMe3)2),
-0.57 (dd, 1 H, TaCH2Si, J ) 12, 3). 13C{1H} NMR (126 MHz,
benzene-d6): δ 151.8, 147.8, 144.6 (Ar C), 124.4, 123.9, 123.7,
123.0, 122.9, 122.7 (Ar CH), 117.3, 114.6 (C5Me5), 34.5 (TaCH2-
Si), 28.2, 27.5, 27.2, 26.3 (iPr CH), 26.7, 26.1, 25.9, 25.8, 25.1,
24.1, 23.4 (iPr Me), 12.8, 12.4 (C5Me5), 1.3, 0.9 (SiMe3), 0.0,
-11.2 (SiMe2H). 29Si{1H} NMR (99 MHz, benzene-d6): δ -11.2,
-11.7 (SiMe(SiMe3)2), -29.4 (HSiMe2), -84.1 (SiMe(SiMe3)2).
ν
SiH).
Me2HSi-Si(SiMe3)2Me.46 This compound was obtained by
hydrolysis of 15 with H2O. 1H NMR (500 MHz, benzene-d6):
δ 4.20 (septet, 1 H, SiH, J ) 4.5, J SiH ) 175), 0.23 (d, 6 H,
HSiMe2, J ) 4), 0.19 (s, 18 H, SiMe(SiMe3)2), 0.12 (s, 3 H,
SiMe(SiMe3)2). 13C{1H} NMR (126 MHz, benzene-d6): δ 0.6
(SiMe3), -3.7 (br, HSiMe2), -12.9 (SiMe(SiMe3)2). 29Si{1H}
NMR (99 MHz, benzene-d6): δ -11.9 (SiMe(SiMe3)2), -34.2

3120 Organometallics, Vol. 21, No. 15, 2002
Burckhardt et al.
IR (KBr, cm-1): 2086 (m, νSiH), 1794 (w, νTaH), 1619 (m, br,
cold (-30 °C) THF-d8 in an NMR tube fitted with a J . Young
Teflon stopper. The reaction mixture instantly became deep
purple in color. The NMR tube was transferred to the precooled
NMR probe, and the reaction progress was followed by 1H
NMR spectroscopy while the probe was slowly warmed to room
temperature in order to observe the formation of 21. After ca.
5 min, 21 was completely converted to 20 and several
unidentified decomposition products. 1H NMR (500 MHz, THF-
d8): δ 12.26 (s, 1 H, Ta-H; J SiH ) 33), 6.77 (d, 2 Ar-H), 6.61 (t,
1 Ar-H), 6.48, 6.42 (2 × s, 2 × 2 Ar-H), 5.38 (s, 1 H, Si-H, 1J SiH
) 146), 4.24 (septet, 2 H, iPr-H, J ) 7), 2.40, 2.37, 2.30 (3 × s,
3 × 6 H, Mes-Me), 1.88 (s, 15 H, C5Me5), 1.06 (d, 12 H,
iPr-Me, J ) 7).
ν
Ta-(µ-H)), 1508 (m, br, νTa-(µ-H)). Anal. Calcd for C53H94N2Si4-
Ta2 (1233.58): C, 51.60; H, 7.68; N, 2.27. Found: C, 51.34; H,
7.81; N, 2.10.
{Li(THF )3}{Cp *(Dip p Nd)Ta Cl(SiP h 3)H} (19). A THF (15
mL) suspension of complex 4 (200 mg, 0.189 mmol) and
(THF)3LiSiPh3 (170 mg, 0.352 mmol) was stirred at room
temperature until a clear orange solution was obtained (1 h).
The solution was concentrated in vacuo to ca. 7 mL and then
layered with pentane (30 mL) and cooled to -78 °C. After 4 h,
the dark yellow oil that formed was separated from the
supernatant liquid, dried in vacuo, and then extracted into
pentane (30 mL). The extracts were filtered, concentrated to
half volume (ca. 15 mL), and cooled to -78 °C for 24 h. The
resulting precipitate was isolated by filtration and dried in
vacuo to give 19 (225 mg, 66%) as a bright yellow powder.
Complex 19 can be stored at -40 °C for a few days. 1H
NMR (500 MHz, THF-d8): δ 11.97 (s, 1 H, Ta-H, J SiH ) 34),
7.57 (m, 6 H, o-Ph-H), 6.94 (m, 6 H, m-Ph-H), 6.88 (m, 3 H,
p-Ph-H), 6.80-6.64 (br m, 2 H, m-Ar-H), 6.49 (t, 1 H, p-Ar-H),
Cp *(Dip p Nd)Ta Me2 (22). A solution of MeMgBr in diethyl
ether (0.25 mL, 1.4 M, 0.35 mmol) was added dropwise via
syringe to a stirred suspension of 4 (186 mg, 0.176 mmol) in
diethyl ether (30 mL). After 30 min, a clear yellow solution
had formed. The solvent was removed in vacuo, and the
residue was extracted with hexanes (2 × 10 mL). The combined
extracts were filtered, concentrated, and cooled to -25 °C to
afford 22 (88 mg, 48%) as a yellow powder. 1H NMR (300
MHz, benzene-d6): δ 7.22 (d, 2 H, m-Ar-H, J ) 8), 6.96 (t, 1
i
4.11 (br s, 2 H, Pr-H), 1.80 (s, 15 H, C5Me5), 1.19, 1.08, 1.02,
0.38 (4 × br s, 4 × 3 H, iPr-Me). 13C{1H} NMR (126 MHz, THF-
d8): δ 154.2 (Ar C), 153.3 (Ph C), 139.1 (br, o-Ph CH), 127.2
(br, m-Ph CH), 126.4 (br, p-Ph CH), 122.3 (br, m-Ar CH), 120.2
i
H, p-Ar-H, J ) 8), 3.69 (septet, 2 H, Pr-H, J ) 7), 1.71 (s, 15
i
H, C5Me5), 1.36 (d, 12 H, Pr-Me, J ) 7), 0.30 (s, 6 H, Ta-Me).
i
(p-Ar CH), 111.9 (C5Me5), 28.4 (br, Pr CH), 27.2, 25.3, 24.7
Cp *(Dip p Nd)Ta [Si(SiMe3)3]Me (23). A solution of MeMg-
Br in diethyl ether (0.62 mL, 3.0 M, 1.8 mmol) was added
dropwise via syringe to a stirred solution of 1 (1.30 g, 1.68
mmol) in diethyl ether (30 mL) at -50 °C. The reaction mixture
was stirred at -50 °C for 30 min and then slowly warmed to
room temperature. After 2 days, the solvent was removed in
vacuo, and the residue was extracted with pentane (2 × 15
mL). The combined extracts were filtered, concentrated, and
cooled to -30 °C to yield large red-orange crystals of 23 (702
i
(br, Pr Me), 12.8 (C5Me5). 29Si{1H} NMR (99 MHz, THF-d8):
δ 47.6. 1H NMR (400 MHz, benzene-d6): δ 13.11 (s, 1 H,
Ta-H, J SiH ) 32), 8.02 (m, 6 H, o-Ph-H), 7.18 (m, 6 H,
m-Ph-H), 7.04 (m, 3 H, p-Ph-H), 7.01 (br m, 2 H, m-Ar-H), 6.85
(t, 1 H, p-Ar-H), 4.36, 4.00 (2 × br m, 2 × 1 H, iPr-H), 3.08 (m,
12 H, THF), 2.02 (s, 15 H, C5Me5), 1.39, 1.30 (2 × br d, 2 × 3
i
H, Pr-Me, J ) 7), 1.18 (m, 12 H, THF), 1.14, 1.06 (2 × br d, 2
i
7
× 3 H, Pr-Me, J ) 7). Li NMR (194 MHz, 0.06 M in THF):
δ -0.264. 13C{1H} NMR (101 MHz, benzene-d6): δ 152.5 (Ar
C), 150.0 (Ph C), 138.1 (o-Ph CH), 127.8 (m-Ph CH), 127.4
(p-Ph CH), 122.4 (br, m-Ar CH), 122.4 (p-Ar CH), 112.5
1
mg, 55%). H NMR (500 MHz, benzene-d6): δ 7.20, 7.17 (2 ×
dd, 2 × 1 H, m-Ar-H, J ) 8, 2), 6.94 (t, 1 H, p-Ar-H, J ) 8),
i
i
3.91 (septet, 1 H, Pr-H, J ) 7), 3.67 (septet, 1 H, Pr-H, J )
i
i
i
(C5Me5), 68.1 (THF), 27.5 (br, Pr CH), 26.3, 25.9 (br, Pr Me),
25.7 (THF), 25.3, 23.4 (br, iPr Me), 12.0 (C5Me5). 29Si{1H} NMR
(99 MHz, benzene-d6): δ 47.2 (br). IR (KBr, cm-1): 1820 (m,
br, νTa-H). Anal. Calcd for C52H72ClLiNO3SiTa (1010.58): C,
61.80; H, 7.18; N, 1.39. Found: C, 61.79; H, 6.85; N, 1.53.
Cp*(DippNd)Ta[η2-CH2(2-SiH2Mes-3,5-Me2C6H2)]H (20).
Complex 4 (500 mg, 0.474 mmol) and (THF)2LiSiHMes2 (396
mg, 0.946 mmol) were dissolved in cold (-80 °C) Et2O (150
mL). The stirred reaction mixture was then allowed to warm
to room temperature, and stirring was continued for a total
reaction time of 12 h. The solvent was removed in vacuo from
the clear orange solution, and the resulting residue was
extracted with hexanes (50 mL). The extracts were filtered,
concentrated, and cooled to -25 °C to afford bright yellow
crystals of 20 (50 mg, 7%). Removal of the solvent from the
supernatant in vacuo gave a yellow foam which consisted of a
4:1 mixture of 20 and H2SiMes2. 1H NMR (300 MHz, benzene-
d6): δ 8.62 (s, 1 H, Ta-H), 7.17 (d, 2 H, m-Ar-H, J ) 8), 6.97 (t,
1 H, p-Ar-H, J ) 8), 6.60 (s, 2 H, Mes-H), 6.34, 5.51 (2 × s, 2
7), 1.87 (s, 15 H, C5Me5), 1.46, 1.30 (2 × d, 2 × 3 H, Pr-Me, J
i
) 7), 1.42, 1.17 (2 × d, 2 × 3 H, Pr-Me, J ) 7), 0.43 (s, 27 H,
Si(SiMe3)3), 0.28 (s, 3 H, Ta-Me). 13C{1H} NMR (126 MHz,
benzene-d6): δ 151.2, 147.5, 145.2 (Ar C), 124.6, 123.6, 123.1
(Ar CH), 118.0 (C5Me5), 72.7 (Ta-Me), 27.6, 27.5, 27.2, 26.2,
25.3, 24.7 (iPr), 12.3 (C5Me5), 6.1 (Si(SiMe3)3). 29Si{1H} NMR
(99 MHz, benzene-d6): δ -3.6 (Si(SiMe3)3), -54.1 (Si(SiMe3)3).
MS: m/z 753 (18, [M]+), 738 (10, [M - Me]+), 606 (11), 562
(84), 489 (100). Anal. Calcd for C32H62NSi4Ta (754.14): C,
50.97; H, 8.29; N, 1.86. Found: C, 51.22; H, 8.60; N, 1.92.
Cp *(Dip p Nd)Ta [η2-COSi(SiMe3)3]Me (24). A benzene-d6
solution (∼0.7 mL) of compound 23 (13 mg, 0.017 mmol) was
transferred to an NMR tube fitted with a J . Young Teflon
stopper. The NMR tube was then sealed under CO (1 atm)
and was shaken. Within 1 min, the color of the solution had
changed from red-orange to red. After 1 h, the solvent and
volatile byproducts were removed in vacuo and the resulting
residue was extracted with pentane. The extracts were filtered
and cooled to -30 °C to afford two crops of dark red needles
1
1
× 1 Mes-H), 5.14 (d, 1 H, Si-H, J ) 4, J SiH ) 205), 5.03
of 24 (11.6 mg, 86%). H NMR (500 MHz, benzene-d6): δ 7.10
1
i
(d, 1 H, Si-H, J ) 4, J SiH ) 213), 4.24 (br s, 2 H, Pr-H), 3.60
(d, 1 H, Ta-CH2, J ) 7), 2.30 (s, 3 H, Mes-Me), 2.28 (s, 6 H,
Mes-Me), 2.07, 2.02 (2 × s, 2 × 3 H, Mes-Me), 1.91 (s, 15 H,
(d, 2 H, m-Ar-H, J ) 8), 6.88 (t, 1 H, p-Ar-H, J ) 8), 3.63 (br
s, 2 H, iPr-H), 1.82 (s, 15 H, C5Me5), 1.53, 1.36, 1.32, 1.13 (4 ×
i
br s, 4 × 3 H, Pr-Me), 0.66 (s, 3 H, Ta-Me), 0.27 (s, 27 H, Si-
i
1
C5Me5), 1.90 (hidden d, 1 H, Ta-CH2), 1.44 (d, 6 H, Pr-Me, J
(SiMe3)3). H NMR (500 MHz, toluene-d8, -40 °C): δ 7.09 (d,
) 7), 1.41 (br s, 6 H, iPr-Me). 13C{1H} NMR (100 MHz, benzene-
d6): δ 152.1, 149.2, 145.2, 144.1, 141.5, 139.3, 130.2, 129.7,
129.3, 125.3, 122.9 (br) (Ar C, CH), 113.1 (C5Me5), 53.1
(Ta-CH2), 32.3, 27.4, 26.0, 25.4 (br), 24.0, 23.9, 22.2, 21.5 (iPr,
Me), 12.3 (C5Me5). 29Si{1H} NMR (99 MHz, benzene-d6): δ
-56.2. IR (KBr, cm-1): 2194 (m, νSiH), 2183 (m, νSiH), 1778 (m,
2 H, m-Ar-H, J ) 8), 6.88 (t, 1 H, p-Ar-H, J ) 8), 3.64, 3.57 (2
i
× septet, 2 × 1 H, Pr-H, J ) 7), 1.81 (s, 15 H, C5Me5), 1.58,
i
1.41, 1.36, 1.13 (4 × d, 4 × 3 H, Pr-Me, J ) 7), 0.68 (s, 3 H,
Ta-Me), 0.29 (s, 27 H, Si(SiMe3)3). 13C{1H} NMR (126 MHz,
benzene-d6): δ 408.6 (COSi), 152.0 (Ar C), 122.6 (Ar CH), 114.1
i
(C5Me5), 27.9, 26.0, 23.7 (br, Pr), 27.5, 27.2, 26.2, 25.3, 24.7
(iPr), 19.0 (Ta-Me), 10.8 (C5Me5), 1.9 (Si(SiMe3)3). 29Si{1H}
NMR (99 MHz, benzene-d6): δ -10.0 (Si(SiMe3)3), -69.0
(Si(SiMe3)3). IR (KBr, cm-1): 1421 (w, νCO, from difference
spectrum). MS: m/z 809 (18, [M + CO]+), 781 (26, [M]+), 766
(8, [M - Me]+), 753 (13, [M - CO]+), 738 (12, [M - CO - Me]+),
νTaH). Anal. Calcd for C40H56NSiTa (759.93): C, 63.22; H, 7.43;
N, 1.84. Found: C, 63.59; H, 7.88; N, 1.91.
{ L i ( T H F ) n } { C p * ( D i p p N d ) T a C l [ S i H ( 2 , 4 , 6 -
Me3C6H2)2]H} (21). Complex 4 (24.5 mg, 0.0232 mmol) and
(THF)2LiSiHMes2 (19.4 mg, 0.0463 mmol) were dissolved in

Tantalum Imido Fragment Cp*(DippNd)Ta
Organometallics, Vol. 21, No. 15, 2002 3121
Ta ble 5. Cr ysta llogr a p h ic Da ta for Com p ou n d s 5‚(THF )2, 6, 20, a n d 26
5‚(THF )2
51H100ClLiNO5Si4Ta
1143.04
6
20
26
empirical formula
fw
C
C31H60NSi4Ta
740.11
C40H56NSiTa
759.91
C27H44NTa
563.60
cryst color, habit
cryst size (mm)
cryst syst
space group
a (Å)
b (Å)
c (Å)
yellow, polyhedral
0.20 × 0.20 × 0.15
monoclinic
Pn (No. 7)
12.9227(1)
13.3346(2)
18.4519(1)
90
105.745(1)
90
3060.31(5)
7706, 3.0-46.0
red, bladelike
0.30 × 0.10 × 0.04
triclinic
P1h (No. 2)
9.7681(5)
12.0615(6)
16.9644(9)
97.956(1)
102.623(1)
104.770(1)
1845.6(2)
clear, tablet
0.15 × 0.08 × 0.20
monoclinic
P21/n (No. 14)
17.0897(2)
11.8106(1)
18.9848(2)
90
107.403(1)
90
3656.48(7)
7165, 3.0-45.0
yellow, block
0.30 × 0.35 × 0.25
monoclinic
P21/n (No. 14)
9.2331(2)
17.3250(4)
16.8832(3)
90
99.062(1)
90
2666.99(9)
7759, 3.0-45.0
R (deg)
â (deg)
γ (deg)
V (Å3)
orientation reflns: number,
7800, 3.0-45.0
2θ range (deg)
Z
D
2
2
4
4
calc (g/cm3)
1.240
1204.00
19.55
1.332
764.00
31.22
1.380
1552.00
30.61
1.404
1140.00
41.27
F000
µ(Mo KR) (cm-1
diffractometer
radiation
)
SMART
Mo KR
(λ ) 0.71069 Å)
graphite-monochromated
134(1)
temperature (K)
scan type
scan rate
data collected, 2θmax (deg)
no. of reflns measd
173(1)
115(1)
183(1)
ω (0.3° per frame)
10.0 s per frame
49.4
total: 14574
unique: 9150
0.071
ω (0.3° per frame)
10.0 s per frame
52.1
ω (0.3° per frame)
10.0 s per frame
52.2
total: 17517
unique: 6792
0.076
ω (0.3° per frame)
10.0 s per frame
52.1
total: 12824
unique: 4834
0.037
total: 9896
unique: 6259
0.022
Rint
transmn factors
Tmax ) 0.35
Tmin ) 0.26
Tmax ) 0.93
Tmin ) 0.67
Tmax ) 0.907
Tmax ) 0.869
Tmin ) 0.684
Tmin ) 0.652
structure solution
direct methods
(SIR92)
no. of obsd data [I > 3σ(I)]
no. of params refined
reflns/param ratio
final residuals: R; Rw; Rall
goodness of fit indicatorb
max. shift/error final cycle
max. and min. peaks,
final diff map (e-/Å3)
7310
573
12.76
0.045; 0.046; 0.066
1.53
0.01
0.73, -1.31
5097
337
15.12
0.029; 0.032; 0.038
1.13
0.21
3787
388
9.76
0.044; 0.049; 0.092
1.32
0.01
3392
262
12.95
0.028; 0.036; 0.047
1.28
0.04
0.91, -0.97
a
1.21, -1.49
1.20, -1.21
a
2 1/2 b
R ) ∑||Fo| - |Fc||/∑|Fo|; Rw ) [∑w(|Fo| - |Fc|)2/∑wFo
]
.
Goodness of fit ) [∑w(|Fo| - |Fc|)2/(Nobs - Nparameters)]1/2
.
708 (10, [M - SiMe3]+), 562 (79), 489 (100). Anal. Calcd for
C33H62NOSi4Ta (782.15): C, 50.68; H, 7.99; N, 1.79. Found:
C, 50.66; H, 8.31; N, 1.75.
mmol) was added via syringe to a stirred diethyl ether (30 mL)
solution of complex 4 (500 mg, 0.47 mmol). After stirring the
reaction mixture for 12 h at room temperature, the solvent
was removed in vacuo to give a yellow solid, which was
extracted with hexanes (30 mL). The extracts were filtered,
concentrated to ca. 10 mL, and cooled to -25 °C to afford two
crops of yellow cubes of 26 (350 mg, 66%). 1H NMR (300
MHz, benzene-d6): δ 15.09 (d, 1 H, Ta-H, J ) 3), 7.21 (t, 2 H,
m-Ar-H, J ) 8), 6.96 (t, 1 H, p-Ar-H, J ) 8), 4.15 (br septet, 2
H, iPr-H), 3.04 (d, 1 H, Ta-CH2, J ) 12; J CH ) 115), 1.89 (s, 15
Cp *(Dip p Nd)Ta [η2-13CCOSi(SiMe3)3]Me (24-13C). Se-
lected data: 13C NMR (126 MHz, benzene-d6): δ 408.6 (q,
COSi, J ) 4). 29Si NMR (99 MHz, benzene-d6): δ -10.0 (d,
Si(SiMe3)3), J ) 2), -69.0 (s, Si(SiMe3)3). IR (KBr, cm-1): 1391-
13
(w, ν CO, from difference spectrum).
[Cp *(Dip p Nd)Ta Me(µ-H)]2 (25). A benzene-d6 (0.5 mL)
solution of compound 23 (15 mg, 0.02 mmol) was transferred
to an NMR tube fitted with a J . Young Teflon stopper. The
NMR tube was then sealed under H2 (1 atm) and shaken.
Within 10 s, the color of the solution had changed from red-
orange to light yellow, and a pale yellow precipitate had
settled. After 30 min, the solvent and volatile byproducts were
removed in vacuo, and the resulting residue was washed with
cold (-30 °C) pentane to afford pure 25 (9.5 mg, 94%). 1H
NMR (500 MHz, benzene-d6): δ 7.08, 6.98 (2 × dd, 2 × 2 H,
m-Ar-H, J ) 8, 2), 6.90 (br s, 2 H, Ta-H), 6.84 (t, 2 H, p-Ar-H,
i
H, C5Me5), 1.41, 1.37 (2 × d, 2 × 6 H, Pr-Me, J ) 7), 1.22 (s,
9 H, CMe3), -2.21 (dd, 1 H, Ta-CH2, J ) 12, 3; J CH ) 89).
13C{1H} NMR (100 MHz, benzene-d6): δ 151.3, 144.5 (Ar C),
127.0 (Ta-CH2), 123.3, 122.7 (Ar CH), 116.6 (C5Me5), 38.5
(CMe3), 35.1 (CMe3), 27.9, 25.2, 25.1 (br, iPr), 11.7 (C5Me5). IR
(KBr, cm-1): 1753 (m, νTaH). Anal. Calcd for C27H44NTa
(563.60): C, 57.45; H, 7.87. Found: C, 57.45; H, 8.15.
Cp *(Dip p Nd)Ta (CH 2CMe3)D (26-d ). Selected data: IR
(KBr, cm-1): 1254 (w, νTaD).
i
J ) 8), 4.18, 3.47 (2 × septet, 2 × 2 H, Pr-H, J ) 7), 1.77 (s,
X-r a y Str u ctu r e Deter m in a tion s. Single-crystal analysis
of all compounds was carried out by Dr. Fred Hollander and
Dr. Dana L. Caulder at the College of Chemistry CHEXRAY
crystallographic facility at the University of California, Ber-
keley. Measurements were made on a Bruker SMART CCD
area detector with graphite-monochromated Mo KR radiation
(λ ) 0.71069 Å). Data were integrated by the program SAINT,
were corrected for Lorentz and polarization effects, and were
analyzed for agreement and possible absorption using XPREP.
Empirical absorption corrections were made using SADABS.
i
30 H, C5Me5), 1.42, 1.36, 1.31, 0.81 (4 × d, 4 × 6 H, Pr-Me, J
) 7), 0.43 (s, 6 H, Ta-Me). 13C{1H} NMR (126 MHz, benzene-
d6): δ 151.6, 145.0, 144.7 (Ar C), 123.6, 122.9, 122.6 (Ar CH),
114.5 (C5Me5), 34.1 (Ta-Me), 27.7, 27.4, 26.9, 26.1, 24.1, 23.6
(iPr), 11.4 (C5Me5). IR (KBr, cm-1): 1595 (s, br, νTaH). Anal.
Calcd for C46H72N2Ta2 (1014.99): C, 54.44; H, 7.15; N, 2.76.
Found: C, 54.13; H, 6.84; N, 2.84.
Cp *(Dip p Nd)Ta (CH2CMe3)H (26). A freshly prepared
solution of Me3CCH2MgCl in diethyl ether (1.0 mL, 1.0 M, 1.0

3122 Organometallics, Vol. 21, No. 15, 2002
Burckhardt et al.
Structures were solved by direct methods and were expanded
using Fourier techniques. All calculations were performed
using the teXsan crystallographic software package.
data are summarized in Table 5. Non-hydrogen atoms were
refined anisotropically. Hydrogen atoms were included but
were not refined. All of the hydrogen atoms were placed in
calculated positions except for the hydride (H(1)), which was
located in the difference Fourier map and was included in its
found position. The final refinement cycle converged at R )
0.044 and Rw ) 0.049.
F or Com p ou n d 26. Suitable crystals were grown from
pentane solutions at -30 °C. A fragment (0.30 × 0.35 × 0.25
mm) was cut from a yellow blocky crystal, mounted on a quartz
fiber using Paratone N hydrocarbon oil, and cooled under a
nitrogen stream in the diffractometer. Selected crystal and
structure refinement data are summarized in Table 5. Non-
hydrogen atoms were refined anisotropically. The hydride
(H(1)) was located in the difference Fourier map and was
included in that position. All other hydrogen atoms were
included in calculated idealized positions but were not refined.
F or Com p ou n d 5‚(THF )2. Suitable crystals were grown
by slow vapor diffusion of pentane into a THF solution of 5 at
-30 °C. A yellow polyhedral crystal (0.20 × 0.20 × 0.15 mm)
was mounted on a quartz fiber using Paratone N hydrocarbon
oil and was cooled under a nitrogen stream in the diffracto-
meter. Selected crystal and structure refinement data are
summarized in Table 5. The correct enantiomorph (non-
centrosymmetric space group) was chosen by Friedel pair
analysis. Non-hydrogen atoms were refined anisotropically,
except for Li, which was refined isotropically. The hydride
(H(60)) was located in the difference Fourier map and was
refined isotropically. All other hydrogen atoms were included
in calculated idealized positions but were not refined. Hydro-
gen atoms were not included for the five THF molecules
because of the high thermal motion of their carbon atoms. The
final refinement cycle converged at R ) 0.045 and Rw ) 0.046.
F or Com p ou n d 6. Suitable crystals were grown from
hexane solutions at -30 °C. A red bladelike crystal (0.30 ×
0.10 × 0.04 mm) was mounted on a quartz fiber using Paratone
N hydrocarbon oil and was cooled under a nitrogen stream in
the diffractometer. Selected crystal and structure refinement
data are summarized in Table 5. Non-hydrogen atoms were
refined anisotropically. The hydride (H(1)) was located in the
difference Fourier map and was refined isotropically. All other
hydrogen atoms were included in calculated idealized positions
but were not refined. The final refinement cycle converged at
R ) 0.029 and Rw ) 0.032.
The final refinement cycle converged at R ) 0.028 and Rw
0.036.
)
Ack n ow led gm en t is made to the National Science
Foundation for their generous support of this work. U.B.
thanks the Schweiz. Nationalfonds and the Novartis
Stiftung for postdoctoral fellowships. We thank Dr.
Frederick J . Hollander and Dr. Dana L. Caulder for
determination of the crystal structures.
Su p p or tin g In for m a tion Ava ila ble: Complete IR data;
tables of crystal, data collection, and refinement parameters,
atomic coordinates, bond distances, bond angles, and aniso-
tropic displacement parameters for compounds 5‚(THF )2, 6,
20, and 26. This material is available free of charge via the
Internet at http://pubs.acs.org.
F or Com p ou n d 20. Suitable crystals were grown from
hexane solutions at -30 °C. A clear tabular crystal (0.15 ×
0.08 × 0.20 mm) was mounted on a quartz fiber using Paratone
N hydrocarbon oil and was cooled under a nitrogen stream in
the diffractometer. Selected crystal and structure refinement
OM0203227