Tantalum alkyl and silyl complexes of the bulky (terphenyl)imido ligand

Article abstract of DOI:10.1021/om020509y

Tantalum alkyl and silyl complexes of the bulky (terphenyl)imido ligand were investigated. Terphenyl ligands were employed extensively for the stabilization of highly reactive main group species. The TaMe₂ group was identified by a ¹H nuclear magnetic resonance (NMR) at -0.41 ppm and a ¹³C NMR resonance at 57.5 ppm.

Full text of DOI:10.1021/om020509y

Organometallics 2002, 21, 5549-5563
5549
Ta n ta lu m Alk yl a n d Silyl Com p lexes of th e Bu lk y
(Ter p h en yl)im id o Liga n d [2,6-(2,4,6-Me3C6H2)2C6H3Nd]2-
([Ar *Nd]2-). Gen er a tion a n d Rea ctivity of
[(Ar *Nd)(Ar *NH)Ta (H)(OSO2CF 3)], Wh ich Rever sibly
Tr a n sfer s Hyd r id e to a n Ar om a tic Rin g of th e Ar yla m id e
Liga n d
J ohn Gavenonis and T. Don Tilley*
Department of Chemistry, University of California, Berkeley, Berkeley, California 94720-1460
Received J une 26, 2002
Tantalum complexes containing the sterically demanding imido ligand [2,6-(2,4,6-
Me3C6H2)2C6H3Nd]2- ([Ar*Nd]2-) are reported. The dimethyl complex (Ar*Nd)(Ar*NH)-
TaMe2 (4), prepared by the reaction of 2 equiv of Ar*NHLi (3) with TaMe3Cl2, was structurally
characterized. Compound 4 forms adducts with donor ligands such as trimethylphosphine
and pyridine and readily reacts with CO to give a monoinsertion product, the η2-acyl complex
5. The dichloride complex (Ar*Nd)(Ar*NH)TaCl2 (7), formed by the reaction of 2 equiv of
Ar*NH2 (2) with TaMe3Cl2, reacted cleanly with NpLi (Np ) Me3CCH2) to give the
dineopentyl species (Ar*Nd)(Ar*NH)TaNp2 (8), which was structurally characterized. Unlike
4, complex 8 does not form adducts with donor ligands. The cationic complex [(Ar*Nd)-
(Ar*NH)TaMe][MeB(C6F5)3] (9), prepared by the reaction of 4 with B(C6F5)3, slowly converted
to [(Ar*Nd)(Ar*NH)TaMe][HBC6F5)3] in the presence of PhSiH3. Treatment of 4 with AgOTf
(OTf ) OSO2CF3) provided the methyl triflate complex (Ar*Nd)(Ar*NH)TaMeOTf (10), which
reacts cleanly with H2 in bromobenzene to provide red-orange crystals of the η5-cyclohexa-
dienyl complex (Ar*Nd)[2-(η5-2,4,6-Me3C6H3)-6-MesC6H3NH]Ta(OTf) (11, Mes ) 2,4,6-
Me3C6H2). Complex 11, which was structurally characterized, represents a previously
unobserved arene hydrogenation intermediate formed by endo transfer of a single hydride
from the metal center to an arene ring. The hydride intermediate [(Ar*Nd)(Ar*NH)Ta(H)-
OTf] (A) was not observed spectroscopically, but was trapped by exposure of 10 to H2 in the
presence of excess 1-hexene to give (Ar*Nd)(Ar*NH)Ta(Hex)OTf (12). Complex 11 is in
equilibrium with A, which is the more reactive isomer, as indicated by its ability to catalyze
olefin hydrogenation, diene cyclization, and silane deuteration. Complex 7 reacts cleanly
with (THF)3LiSi(SiMe3)3, (THF)2LiSi(tBu)Ph2, and (THF)2LiSiHMes2 to provide the silyl
chloride complexes (Ar*Nd)(Ar*NH)Ta(SiR3)Cl (SiR3 ) Si(SiMe3)3, (tBu)Ph2, SiHMes2; 13a -
c). Hydrogenolysis of 13a -c yields silane and a species with spectroscopic features similar
to those of 11.
In tr od u ction
hydrogenation,12,13 and alkane activation.14-20 Because
much of the previous research in this area is based on
metallocene derivatives, recent efforts have sought to
expand the utility of this chemistry with the develop-
In recent years, early transition metal complexes have
received increased attention, as such species have been
found to mediate a number of stoichiometric and
catalytic processes including olefin polymerization,1-3
dehydropolymerizations,4-6 hydrosilylation,7-11 arene
(10) Molander, G. A.; J ulius, M. J . Org. Chem. 1992, 57, 6347.
(11) Molander, G. A.; Dowdy, D. E.; Noll, B. C. Organometallics
1998, 17, 3754.
(12) Visciglio, V. M.; Clark, J . R.; Nguyen, M. T.; Mulford, D. R.;
Fanwick, P. E.; Rothwell, I. P. J . Am. Chem. Soc. 1997, 119, 3490.
(13) Rothwell, I. P. J . Chem. Soc., Chem. Commun. 1997, 1331.
(14) Rothwell, I. P. In Selective Hydrocarbon Activation; Davis, J .
A., Watson, P. L., Liebman, J . F., Greenberg, A., Eds.; Wiley: New
York, 1990; p 43.
(1) Britovsek, G. J . P.; Gibson, V. C.; Wass, D. F. Angew. Chem.,
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Waymouth, R. M. Angew. Chem., Int. Ed. Engl. 1995, 34, 1143.
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1998, 42, 363.
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Press: Greenwich, CT, 1991; Vol. 1, p 327.
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Nolan, M. C.; Santarsiero, B. D.; Schaefer, W. P.; Bercaw, J . E. J . Am.
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10.1021/om020509y CCC: $22.00 © 2002 American Chemical Society
Publication on Web 11/06/2002

5550 Organometallics, Vol. 21, No. 25, 2002
Gavenonis and Tilley
Ch a r t 1. Exa m p les of Im id o Com p lexes Con ta in in g Silyl a n d /or Hyd r id e Liga n d s
ment of new ancillary (non-cyclopentadienyl) ligand
sets.5,21,22 One strategy for ligand design targets steri-
cally demanding ligands that enforce a low coordination
number at the metal center for enhancement of metal-
centered electrophilicity.
developed aryl oxide analogues as ancillary ligands for
early metal hydrides.13,34-37 Sterically encumbering
terphenyl imido ligands can potentially prevent hydride
dimerization while enforcing coordinative unsaturation
at a highly electron-deficient metal center. Recently, we
have reported preliminary results describing a “masked
hydride” complex based upon this imido-amido ligand
set.38 Here, we describe our further investigations into
this system which have resulted in the isolation of alkyl,
hydride, and silyl derivatives.
Recent efforts in our laboratory have focused on
reactive early metal centers containing imido ancillary
ligands. This work resulted in the isolation of molyb-
denum and tungsten complexes of the type (2,6-
iPr2C6H3Nd)2M[Si(SiMe3)3]CH2CMe3, which possess
highly reactive M-C and M-Si σ-bonds.23 In addition,
we have prepared the stable silyl hydride complex Cp*-
(2,6-iPr2C6H3Nd)Ta[Si(SiMe3)3](H),24 which reacts with
PhSiH3 by way of Si-N coupling to form two dimeric
products with bridging silanimine ligands25,26 and ther-
mally rearranges via an interesting series of Si-Si and
Si-C bond-breaking/bond-making and C-H bond acti-
vation steps.24 Generally, however, d0 tantalum imido
complexes have not featured prominently in σ-bond
metathesis reactivity, and complexes containing both
imido ligands and reactive σ-bonds (M-H, M-Si) are
exceedingly rare.27-32 Of the few examples known
(Chart 1), most are supported by cyclopentadienyl
ligands. Replacing the cyclopentadienyl substituents
with less electron-rich ligands may lead to a more
electrophilic metal center capable of enhanced reactiv-
ity.
Resu lts a n d Discu ssion
Liga n d Syn th esis. The terphenyl aniline 2 was
prepared according to the literature procedure reported
by Yoshifuji and co-workers for a similar compound.39
2,6-Dimesitylphenyliodide reacted with nBuLi at 0 °C
in hexanes,40,41 and the resulting lithium salt was
treated with p-toluenesulfonyl azide to provide 2,6-
Mes2C6H3N3 (1, Mes ) 2,4,6-Me3C6H2) as red-orange
crystals in 96% yield (eq 1). Reduction of compound 1
using LiAlH4 in diethyl ether afforded clear, diamond-
shaped crystals of aniline 2 in 86% yield. Lithium
anilide 3 was obtained as a yellow powder in 99% yield
by warming a chilled pentane solution of compound 2
to room temperature in the presence of nBuLi and
removing the solvent and volatile byproducts in vacuo
(eq 1).
In this report, we describe the preparation of non-
cyclopentadienyl tantalum complexes containing the
new terphenyl imido ligand [2,6-(2,4,6-Me3C6H2)2C6H3-
Nd]2- ([Ar*Nd]2-). Terphenyl ligands have been em-
ployed extensively for the stabilization of highly reactive
main group species,33 and Rothwell and co-workers have
(21) Imori, T.; Tilley, T. D. Polyhedron 1994, 13, 2231.
(22) Woo, H.-G.; Walzer, J . F.; Tilley, T. D. J . Am. Chem. Soc. 1992,
114, 7047.
(23) Casty, G. L.; Tilley, T. D.; Yap, G. P. A.; Rheingold, A. L.
Organometallics 1997, 16, 4746.
(24) Burckhardt, U.; Casty, G. L.; Gavenonis, J .; Tilley, T. D.
Organometallics 2002, 21, 3108.
(25) Burckhardt, U.; Tilley, T. D. J . Am. Chem. Soc. 1999, 121, 6328.
(26) Burckhardt, U.; Casty, G. L.; Tilley, T. D.; Woo, T. K.; Roth-
lisberger, U. Organometallics 2000, 19, 3830.
Syn th esis a n d Rea ctivity of Im id o-Am id o Di-
a lk yl Ta n ta lu m Com p lexes. Lithium anilide 3 (1 or
(27) 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.
(28) 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.
(29) Wu, Z.; Xue, Z. Organometallics 2000, 19, 4191.
(30) Mayer, J . M.; Curtis, C. J .; Bercaw, J . E. J . Am. Chem. Soc.
1983, 105, 2651.
(34) Parkin, B. C.; Clark, J . R.; Visciglio, V. M.; Fanwick, P. E.;
Rothwell, I. P. Organometallics 1995, 14, 3002.
(35) Steffey, B. D.; Chestnut, R. W.; Kerschner, J . L.; Pellechia, P.
J .; Fanwick, P. E.; Rothwell, I. P. J . Am. Chem. Soc. 1989, 111, 378.
(36) Lockwood, M. A.; Potyen, M. C.; Steffey, B. D.; Fanwick, P. E.;
Rothwell, I. P. Polyhedron 1995, 14, 3293.
(37) Lockwood, M. A.; Fanwick, P. E.; Rothwell, I. P. Polyhedron
1995, 14, 3363.
(31) Blake, R. E.; Antonelli, D. M.; Henling, L. M.; Schaefer, W. P.;
Hardcastle, K. I.; Bercaw, J . E. Organometallics 1998, 17, 718.
(32) Antonelli, D. M.; Schaefer, W. P.; Parkin, G.; Bercaw, J . E. J .
Organomet. Chem. 1993, 462, 213.
(33) Twamley, B.; Haubrich, S. T.; Power, P. P. Adv. Organomet.
Chem. 1999, 44, 1.
(38) Gavenonis, J .; Tilley, T. D. J . Am. Chem. Soc. 2002, 124, 8536.
(39) Shigeru, S.; Hatsushiba, H.; Yoshifuji, M. J . Chem. Soc., Chem.
Commun. 1998, 2221.
(40) Saednya, A.; Hart, H. Synthesis 1996, 1455.
(41) Ruhlandt-Senge, K.; Ellison, J . J .; Wehmschulte, R. J .; Pauer,
F.; Power, P. P. J . Am. Chem. Soc. 1993, 115, 5.

Tantalum Alkyl and Silyl Complexes
Organometallics, Vol. 21, No. 25, 2002 5551
Ta ble 1. Selected Bon d Len gth s (Å) a n d An gles
(d eg) for (Ar *Nd)(Ar *NH)Ta Me2 (4)
Bond Lengths
Ta(1)-N(1)
Ta(1)-N(2)
N(1)-C(3)
1.743(6)
1.994(6)
1.405(9)
Ta(1)-C(1)
Ta(1)-C(2)
N(2)-C(27)
2.136(7)
2.132(7)
1.418(9)
Bond Angles
Ta(1)-N(1)-C(3)
N(1)-Ta(1)-N(2)
N(1)-Ta(1)-C(1)
N(1)-Ta(1)-C(2)
169.2(5)
102.4(3)
104.2(3)
99.7(3)
Ta(1)-N(2)-C(27)
C(1)-Ta(1)-C(2)
N(2)-Ta(1)-C(1)
N(2)-Ta(1)-C(2)
132.8(5)
109.7(3)
115.0(3)
122.5(3)
within the expected ranges, including a TadN bond
length of 1.743(6) Å and a Ta-N bond length of 1.994-
(6) Å.43-46
Initial experiments with 4 focused on replacing the
amido ligand to obtain other (Ar*Nd)TaMe2X complexes
as possible precursors to a trihydride derivative. How-
ever, complex 4 did not react with Me3SiCl or MeI
(benzene-d6, 85 °C, 2 days), and treatment with PCl5
(toluene, room temperature, 21 h) provided an intrac-
table mixture of products. Since the amido ligand could
not be substituted, several attempts were made to
prepare imido-amido dihydride derivatives. Treatment
of benzene-d6 solutions of 4 with H2 under a variety of
temperatures and pressures resulted in complex reac-
tion mixtures, which contained free amine. Heating 4
in the presence of excess PhSiH3 (6 equiv) to 120 °C
(benzene-d6) over 2 days resulted in the formation of
methane and Ph2SiH2 (1 equiv), as well as several
unidentified products.47-51 Analysis of the reaction
mixture by GC/MS revealed a trace amount of Ph-
MeSiH2, which is consistent with σ-bond metathesis
reactivity.48,52
F igu r e 1. ORTEP diagram of (Ar*Nd)(Ar*NH)TaMe2 (4).
2 equiv) reacted with TaMe3Cl2 in hexanes to provide
(Ar*Nd)(Ar*NH)TaMe2 (4, Ar* ) 2,6-Mes2C6H3) as a
yellow, crystalline solid in 71% yield (eq 2). No com-
Although compound 4 could not be converted to a
hydride derivative, other reactivity modes (insertion
chemistry and adduct formation) were observed (Scheme
1). Placing a benzene-d6 solution of 4 under an atmos-
phere of CO led to the monoinsertion product (Ar*Nd)-
(Ar*NH)Ta(η2-COMe)Me (5), which was characterized
1
by a COMe H NMR resonance at 2.26 ppm, a COMe
13C NMR resonance at 317.1 ppm, and a νCO infrared
stretch at 1421 cm-1. The downfield 13C NMR acyl
resonance and low-energy IR absorbance are indicative
of an η2-acyl bonding mode.53,54 Complex 5 was charac-
terized in solution and could not be obtained on a
preparative scale since exposing solutions of 5 to
vacuum led to decarbonylation and the subsequent
isolation of 4.
plexes containing only one nitrogen-donor ligand were
isolated or observed. Compound 4 displays distinct
imido and amido ligand environments and Cs symmetry
by NMR spectroscopy (22 °C). The TaMe2 group was
1
identified by a H NMR resonance at -0.41 ppm and a
13C NMR resonance at 57.5 ppm, while the amido NH
(43) Wigley, D. E. Prog. Inorg. Chem. 1994, 42, 239.
(44) Nugent, W. A.; Haymore, B. L. Coord. Chem. Rev. 1980, 31,
123.
(45) Chisholm, M. H.; Rothwell, I. P. In Comprehensive Coordination
Chemistry; Wilkinson, G., Gillard, R. D., McCleverty, J . A., Eds.;
Pergamon: Oxford, 1987; Vol. 2, p 161.
1
group was observed as a H NMR resonance at 7.34 ppm
and a νNH infrared stretch at 3303 cm-1. The observed
spectroscopic data are consistent with those reported
for the only other known imido-amido dialkyl tantalum
compound, (tBu3SiNd)(tBu3NH)TaMe2.42
(46) Nugent, W. A.; Mayer, J . M. Metal-Ligand Multiple Bonds;
J ohn Wiley & Sons: New York, 1988.
(47) Radu, N. S.; Hollander, F. J .; Tilley, T. D.; Rheingold, A. L. J .
Chem. Soc., Chem. Commun. 1996, 2459.
X-ray quality crystals were obtained by cooling a
concentrated pentane solution of complex 4 to -35 °C
over several days. The molecular structure is shown in
Figure 1, and important bond distances and angles are
listed in Table 1. The structure reveals a pseudo-
tetrahedral geometry with bond lengths and angles well
(48) Sadow, A. D.; Tilley, T. D. Organometallics 2001, 20, 4457.
(49) Castillo, I.; Tilley, T. D. Organometallics 2000, 19, 4733.
(50) Castillo, I.; Tilley, T. D. Organometallics 2001, 20, 5598.
(51) Castillo, I.; Tilley, T. D. J . Am. Chem. Soc. 2001, 123, 10526.
(52) Radu, N. S.; Tilley, T. D.; Rheingold, A. L. J . Organomet. Chem.
1996, 516, 41.
(53) Castro, A.; Galakhov, M. V.; Go´mez, M.; Go´mez-Sal, P.; Mart´ın,
A.; Sa´nchez, F. J . Organomet. Chem. 2000, 595, 36.
(54) Tatsumi, K.; Nakamura, A.; Hofmann, P.; Stauffert, P.; Hoff-
mann, R. J . Am. Chem. Soc. 1985, 107, 4440.
(42) Schaller, C. P.; Wolczanski, P. T. J . Am. Chem. Soc. 1993, 32,
131.

5552 Organometallics, Vol. 21, No. 25, 2002
Gavenonis and Tilley
Sch em e 1
Adduct formation was observed upon treating com-
pound 4 with donor ligands such as trimethylphosphine
and pyridine. Compound 4 reacted cleanly with PMe3
(1 equiv) in benzene-d6 at room temperature to provide
(Ar*Nd)(Ar*NH)TaMe2(PMe3) (6) in 99% yield (relative
to internal standard, Scheme 1). Complex 6 was spec-
troscopically characterized in solution. The 1H NMR
spectrum of 6 contained two sets of resonances for the
mesityl methyl groups, indicating that the complex has
Cs symmetry. The PMe3 group appeared in the 1H NMR
spectrum as a sharp doublet at 0.56 ppm (2J PH ) 5 Hz),
but a very broad resonance was observed in the 31P-
{1H} NMR spectrum at -19.1 ppm, suggesting that
rapid exchange between free and bound PMe3 occurs
at room temperature. Upon treatment of 4 with excess
PMe3 (3 equiv, toluene-d8), all of the resulting 1H NMR
resonances are identical to those of 6, with the exception
of the PMe3 signal, which appears as a broad singlet at
0.70 ppm (the chemical shift for free PMe3). No reso-
nance was observed for the bound phosphine. However,
cooling the toluene-d8 solution to -40 °C revealed a
sharp doublet at 0.44 ppm (2J PH ) 10 Hz) in the 1H
NMR spectrum and a singlet at -12.59 ppm in the 31P-
{1H} NMR spectrum, along with separate signals due
to free PMe3. Unlike Wolczanski’s (tBu3SiNd)(tBu3NH)-
TaMe2, which reacts cleanly with Lewis bases to liberate
methane and form a bis(imido) complex,42 compound 4
decomposes to a mixture of products when heated (120
°C) in benzene-d6 in the presence of excess PMe3 or
pyridine. Furthermore, heating a benzene-d6 solution
of 6 to 85 °C in the presence of H2 (1 atm) provided a
complex reaction mixture.
F igu r e 2. ORTEP diagram of (Ar*Nd)(Ar*NH)TaNp2 (8).
Complex 7 contains distinct imido and amido ligand
environments by NMR spectroscopy, which also indi-
cates Cs symmetry for 7 (22 °C). On an NMR tube scale,
1 equiv of aniline 2 was observed to react with TaMe3-
Cl2 to give complex 7 along with unreacted TaMe3Cl2
(in benzene-d6, 80 °C).
Complex 7 reacted cleanly with excess NpLi (3 equiv,
Np ) Me3CCH2) in toluene over 5.5 h to provide yellow
crystals of (Ar*Nd)(Ar*NH)TaNp2 (8) in 72% yield (eq
4). Diastereotopic methylene resonances were observed
in the 1H NMR spectrum (dichloromethane-d2) at -0.02
and -0.36 ppm (J ) 15 Hz), but one of the mesityl
methyl resonances was not observed. A VT NMR study
revealed that the missing signal appears as a sharp
singlet (integrating to 12 H) upon heating a toluene-d8
sample to 90 °C. The broad mesityl resonance observed
at room temperature is likely due to restricted rotation
resulting from introduction of the bulky neopentyl
ligands.
Aniline 2 (2 equiv) reacts cleanly with TaMe3Cl2 in
toluene at 75 °C over 21 h to yield (Ar*Nd)(Ar*NH)-
TaCl2 (7) as light brown crystals in 68% yield (eq 3).
An X-ray crystallographic study revealed a pseudo-
tetrahedral geometry about the tantalum metal center
of 8. The molecular structure is shown in Figure 2, and
important bond lengths and angles are listed in Table
2. The data were collected for a twinned crystal which
contained two molecules of 8 and one pentane molecule
per unit cell. The complex has TadN and Ta-N bond
lengths of 1.777(9) and 1.998(9) Å for Ta(1) and 1.774-

Tantalum Alkyl and Silyl Complexes
Organometallics, Vol. 21, No. 25, 2002 5553
yl group is associated with a 13C NMR resonance at ∼10
ppm, even though no signal was observed in the 13C-
{1H} NMR spectrum at room temperature.
Ta ble 2. Selected Bon d Len gth s (Å) a n d An gles
(d eg) for (Ar *Nd)(Ar *NH)Ta Np 2 (8)
Bond Lengths
Ta(2)-N(3)
Ta(2)-N(4)
N(3)-C(59)
1.774(9)
1.986(8)
1.42(1)
Ta(2)-C(107)
Ta(2)-C(112)
N(4)-C(83)
2.12(1)
2.13(1)
1.42(1)
An unusual spectroscopic feature of complex 9 is a
small, slightly broad peak integrating to 1 H at 5.19
ppm in the H NMR spectrum. This resonance does not
1
Bond Angles
162.9(8) Ta(2)-N(4)-C(83)
106.6(4) C(107)-Ta(2)-C(112) 108.0(5)
exhibit coupling to other nuclei over the temperature
Ta(2)-N(3)-C(59)
N(3)-Ta(2)-N(4)
N(3)-Ta(2)-C(107) 116.9(4) N(4)-Ta(2)-C(107)
N(3)-Ta(2)-C(112) 104.4(4) N(4)-Ta(2)-C(112)
135.1(8)
range of -80 to 100 °C in toluene-d8 solution (by 1H
1
NMR spectroscopy). However, the H,1H-TOCSY spec-
108.0(5)
112.9(4)
trum revealed that this resonance is coupled to another
1
one integrating to 1 H at 6.68 ppm, and a H,13C-HMQC
experiment indicated that the signals at 5.19 and 6.68
ppm are due to hydrogens bonded to aromatic carbons
which give rise to 13C NMR shifts at 119.6 and 136.1
ppm, respectively. These data suggest that the reso-
nance at 5.19 ppm is actually due to an upfield-shifted
proton bonded to an aromatic ring. A possible explana-
tion for this unusual chemical shift is the presence of
(9) Å and 1.986(8) Å for Ta(2). The structure does not
differ significantly from that of complex 4, although the
neopentyl ligands provide much more steric crowding
about the metal center.
Attempts to observe clean σ-bond metathesis reac-
tions of the Ta-C bonds of 8 were not successful. No
reaction was observed with H2 (1 atm) in benzene-d6 at
room temperature, and heating the reaction mixture to
50 °C over 4 days resulted in conversion to an intrac-
table mixture of products (which contained neopentane).
Similarly, treating a benzene-d6 solution of 8 with
PhSiH3 (2 equiv) resulted in no reaction at room
temperature, and a complex reaction mixture after
heating to 80 °C for 8 days. In addition, 8 does not form
observable adducts with PMe3 or pyridine (benzene-d6,
room temperature), presumably due to the increased
steric bulk of the neopentyl ligands (relative to 4).
Syn t h esis a n d R ea ct ivit y of [(Ar *Nd)(Ar *NH)-
Ta Me][MeB(C6F 5)3] (9). Since the dialkyl complexes
4, 6, and 8 could not be converted to hydride derivatives
by reactions with H2 or PhSiH3, investigations focused
on the preparation of cationic derivatives which might
feature enhanced electrophilicity and higher reactivities.
Complex 4 was found to react rapidly with B(C6F5)3 at
room temperature in toluene to yield the cationic
complex [(Ar*Nd)(Ar*NH)TaMe][MeB(C6F5)3] (9) as a
red-orange solid in 81% yield (eq 5). The terminal TaMe
1
an agostic C‚‚‚H‚‚‚Ta interaction. However, the J CH
coupling constant associated with this resonance is 180
1
Hz, whereas the other aromatic carbons exhibit J CH
values ranging from 160 to 165 Hz. Typically, for agostic
interactions, 1J CH values are significantly lower relative
to those of other C-H bonds in the molecule.55,56 A more
likely explanation for this unusual chemical shift is that
steric restrictions lead to an interaction between this
aromatic hydrogen and the π-system of another aro-
matic ring (see below).
According to Horton and co-workers, the chemical
shift difference between the meta and para fluorines in
the 19F{1H} NMR spectrum indicates the degree of
[MeB(C6F5)3]- coordination to a cationic metal center.57
Differences of fewer than 3 ppm appear to indicate a
noncoordinated [MeB(C6F5)3]- anion, whereas values of
3-6 ppm suggest coordination. In the case of complex
9, ∆(m,p-F) ) 2.6 ppm, indicating noncoordination.
Further insight into the degree of anion coordination
can be obtained from the chemical shift change of the
[MeB(C6F5)3]- 1H NMR resonance upon addition of a
Lewis base. Significant changes suggest a more coordi-
nated anion. Treating a bright orange benzene-d6 solu-
tion of 9 with pyridine (2 equiv) instantaneously gave
rise to a bright yellow solution. The [MeB(C6F5)3]- 1
H
NMR resonance (determined by 1H,11B-HMQC spec-
troscopy) shifted only slightly, from 1.16 to 1.25 ppm,
suggesting a noncoordinated anion.
Attempts to prepare hydride derivatives of com-
pound 9 were unsuccessful. Complex 9 did not react
with H2 (1 atm), even upon heating a benzene-d6
solution as high as 120 °C for 3 days, or a bromoben-
zene-d5 solution as high as 105 °C for 3 days. Treating
complex 9 with PhSiH3 (1 equiv) in benzene-d6 at room
temperature resulted in the slow formation (>1 week)
of PhMeSiH2 and a new tantalum-containing product.
However, 1H,11B-HMQC spectroscopy and labeling
experiments with PhSiD3 (1 equiv) revealed that a
hydride was transferred to boron to give a hydrido-
borate functionality.48 Complex 9 was found to slowly
polymerize ethylene (1 atm) at room temperature in
group in complex 9 was identified by a 1H NMR
resonance at -0.19 ppm and a 13C NMR signal at 29.8
1
ppm, which was confirmed by a H,13C-HMQC experi-
ment. The 11B NMR spectrum contained a singlet at
-14.93 ppm, suggesting a tetrahedral environment
about the boron atom, while 1H,11B-HMQC spectroscopy
confirmed that a singlet at 1.16 ppm in the 1H NMR
spectrum is due to [MeB(C6F5)3]-. The 1H,13C-HMQC
experiment also indicated that the [MeB(C6F5)3]- meth-
(55) Doerrer, L. H.; Green, M. L. H.; Ha¨ussinger, D.; Sassmann-
shausen, J . J . Chem. Soc., Dalton Trans. 1999, 2111.
(56) Brookhart, M.; Green, M. L. H.; Wong, L.-L. Prog. Inorg. Chem.
1988, 36, 1.
(57) Horton, A. D.; de With, J .; van der Linden, A. J .; van de Weg,
H. Organometallics 1996, 15, 2672.

5554 Organometallics, Vol. 21, No. 25, 2002
Gavenonis and Tilley
Sch em e 2
benzene-d6, but it did not react with excess 1-hexene
under identical conditions.
converted to 0.5 equiv of Ph2SiH2 as complex 10 gave
rise to several unidentified products. Trace quantities
of PhMeSiH2, (Ph2SiH)2, and Ph3SiH were also observed
in the reaction mixture (by GC/MS). The observed
silicon-containing products are consistent with σ-bond
metathesis reactivity leading to redistribution of sub-
stituents at silicon.47-51 Similar results were observed
for the reaction at 95 °C in bromobenzene-d5.
Th e Ma sk ed Hyd r id e [(Ar *Nd)(Ar *NH)Ta (H)-
(OSO2CF 3)] (A). Complex 4 reacted cleanly with AgOTf
(1 equiv, OTf ) OSO2CF3) in toluene at room temper-
ature over 27 h to provide (Ar*Nd)(Ar*NH)TaMeOTf
(10) as yellow-orange crystals in 78% yield (eq 6).
Resonances were observed for the TaMe group at 0.22
As described above, attempts to prepare a hydride
derivative by the reactions of 4, 6, 8, and 9 with H2 or
PhSiH3 failed due to the formation of complex product
mixtures. However, heating a bromobenzene solution
of complex 10 to 95 °C over 2 days in the presence of
H2 (1 atm) provided a red-orange crystalline solid (11),
which was purified by recrystallization from toluene at
-35 °C (Scheme 2). The 1H NMR spectrum of 11
contains two coupled doublets at 3.53 and 4.54 ppm (J
) 16 Hz), consistent with the presence of a methylene
group with diastereotopic protons. A small broad singlet
at 4.26 ppm is correlated to a 13C NMR resonance at
118.8 ppm according to 1H,13C-HMQC spectroscopy.
Thus, this complex also appears to possess an aromatic
hydrogen atom that exhibits an unusual upfield shift
due to interaction with an aromatic ring. In addition,
this spectrum appears to be devoid of a resonance
attributable to a Ta hydride ligand.
1
ppm in the H NMR spectrum and at 45.0 ppm in the
13C{1H} NMR spectrum. Monitoring an NMR tube scale
reaction of complex 4 with AgOTf (benzene-d6, room
temperature, 22 h) by H NMR spectroscopy revealed
The identity of 11 was established by X-ray crystal-
lography, which revealed a structure containing an η5-
1
complete conversion to 10 along with ethane formation,
consistent with the oxidative cleavage reactions reported
by J ordan and co-workers58-61 and Tilley and co-
workers.62
Complex 10 did not react with PhSiH3 (5 equiv) in
benzene-d6 at temperatures up to 100 °C, but upon
heating to 120 °C for 4 days, the PhSiH3 was completely
(58) J ordan, R. F.; Dasher, W. E.; Echols, S. F. J . Am. Chem. Soc.
1986, 108, 1718.
(59) J ordan, R. F.; LaPointe, R. E.; Bajgur, C. S.; Echols, S. F.;
Willett, R. J . Am. Chem. Soc. 1987, 109, 4111.
(60) J ordan, R. F.; Bajgur, C. S.; Dasher, W. E.; Rheingold, A. L.
Organometallics 1987, 6, 1041.
(61) J ordan, R. F.; Echols, S. F. Inorg. Chem. 1987, 26, 383.
(62) Roddick, D. M.; Heyn, R. H.; Tilley, T. D. Organometallics 1989,
8, 324.

Tantalum Alkyl and Silyl Complexes
Organometallics, Vol. 21, No. 25, 2002 5555
Complex 11 presumably forms via the tantalum
hydride intermediate species [(Ar*Nd)(Ar*NH)Ta(H)-
OTf] (A, Scheme 2), and evidence for the existence of
this species was obtained by trapping experiments.
Heating a bromobenzene-d5 solution of complex 10 to
95 °C under H2 (1 atm) in the presence of MeCCMe,
tBuCCMe, or PhCCPh resulted in complicated reaction
mixtures containing complex 11. When 10 was treated
with H2 in the presence of Me3SiCCSiMe3 under similar
conditions, complete conversion of 10 to 11 was ob-
served, while the alkyne remained unchanged. However,
when a bromobenzene-d5 solution of 10 was treated with
H2 in the presence of 1-hexene (1 equiv), methane was
produced along with quantitative amounts of n-hexane
and complex 11. This observation suggests that A
reacted with 1-hexene to give a hexyl triflate complex
(12), which then reacted with H2 to give n-hexane and
complex 11 (Scheme 2). In fact, the reaction of 10 with
H2 (1 atm) in the presence of 10 equiv of 1-hexene
(bromobenzene, 95 °C, 2 days) produced the hexyl
derivative (Ar*Nd)(Ar*NH)Ta(Hex)OTf (12) as yellow
crystals in 66% yield. The diastereotopic TaCH2 hydro-
gens are clearly identified by two triplets of doublets
appearing at 0.58 and 1.00 ppm. The 13C NMR signal
for this methylene group occurs at 66.9 ppm, in ap-
proximately the same region as those for the related
carbon atoms in 4 and 10.
Figu r e 3. ORTEP diagram of(Ar*Nd)[2-(η5-2,4,6-Me3C6H3)-
6-MesC6H3NH]TaOTf (11).
Ta ble 3. Selected Bon d Len gth s (Å) a n d An gles
(d eg) for
(Ar *Nd)[2-(η5-2,4,6-Me3C6H3)-6-MesC6H3NH]Ta (OTf)
(11)a
Experimental data suggest that in solution complexes
11 and A exist in equilibrium. Thus, 11 reacts with
1-hexene (1 equiv, in the absence of H2) in bromoben-
zene-d5 at 95 °C to form the n-hexyl derivative 12
(Scheme 2). However, attempts to observe A by moni-
toring the reaction of complex 10 with H2 (1 atm) by 1H
NMR spectroscopy in bromobenzene-d5 at 95 °C were
unsuccessful; only resonances due to complexes 10 and
11 were observed. Thus, 11 is highly favored in its
equilibrium with A.
Bond Lengths
Ta(1)-N(1)
Ta(1)-N(2)
Ta(1)‚‚‚C(1)
Ta(1)-C(2)
Ta(1)-C(3)
Ta(1)-C(4)
Ta(1)-C(5)
Ta(1)-C(6)
1.990(6)
1.788(5)
3.141(7)
2.466(6)
2.369(6)
2.349(6)
2.530(6)
2.830(6)
Ta(1)-O(1)
Ta(1)-C(100)
C(1)-C(2)
C(2)-C(3)
C(3)-C(4)
C(4)-C(5)
C(5)-C(6)
C(1)-C(6)
2.060(4)
2.0990(3)
1.526(9)
1.409(9)
1.404(9)
1.475(9)
1.352(9)
1.484(9)
Bond Angles
129.0(4) Ta(1)-N(2)-C(25) 175.6(5)
N(1)-Ta(1)-C(100) 100.9(1) C(6)-C(1)-C(2)
N(2)-Ta(1)-C(100) 128.5(2) C(1)-C(2)-C(3)
O(1)-Ta(1)-C(100) 108.5(1) C(2)-C(3)-C(4)
Ta(1)-N(1)-C(15)
109.6(6)
116.2(6)
119.3(6)
118.4(6)
121.0(6)
120.1(6)
Further insight into the mechanism of formation of
11 was gained by a deuterium-labeling experiment.
Treatment of 10 with D2 (1 atm) in bromobenzene at
N(1)-Ta(1)-N(2)
N(1)-Ta(1)-O(1)
N(2)-Ta(1)-O(1)
105.6(2) C(3)-C(4)-C(5)
102.9(2) C(4)-C(5)-C(6)
107.4(2) C(5)-C(6)-C(1)
95 °C over
3
days provided (Ar*Nd)[2-(η5-2,4,6-
Me3C6H2D)-6-MesC6H3NH]TaOTf (11-d ) as red-orange
crystals from toluene (Scheme 2). The 1H NMR spec-
trum of 11-d has no signal at ∼3.5 ppm (unlike 11), but
contains a broad singlet at 4.52 ppm integrating to 1
H. The 2H NMR spectrum, as expected, contains a
single, broad resonance at 3.48 ppm, consistent with
deuterium incorporation into only one position of the
molecule. The 13C{1H} NMR spectrum contains a 1:1:1
triplet at 34.5 ppm due to C-D coupling (1J CD ) 20 Hz).
No further deuterium incorporation was observed after
heating a bromobenzene-d5 solution of complex 11-d
with D2 (1 atm) to 95 °C for 3 days. Furthermore,
complex 11-d was found to undergo H/D exchange to
yield 11 upon exposure to H2 (1 atm) in bromobenzene-
d5 at 95 °C (24 h, Scheme 3). This is believed to proceed
via a σ-bond metathesis pathway involving the postu-
lated intermediate A (or A-d ).
a
C(100) represents the average of the x, y, and z coordinates of
the η5-cyclohexadienyl ring carbons C(2)-C(6).
cyclohexadienyl ligand (Figure 3). Important bond lengths
and angles are listed in Table 3. This complex contains
a normal imido ligand, as indicated by the Ta(1)-N(2)-
C(25) bond angle of 175.6(5)° and the TadN bond length
of 1.788(5) Å. The amido ligand in 11 is metalated via
the transfer of a hydrogen atom to one mesityl group,
to give rise to an η5-cyclohexadienyl ligand. The reduced
mesityl ring is puckered at C(1), such that the Ta(1)‚‚
‚C(1) distance is 3.141(7) Å, while the bond lengths from
Ta(1) to C(2)-C(6) range from 2.349(6) to 2.830(6) Å.
The internal angles of the cyclohexadienyl ligand in-
volving C(2)-C(6) range from 116.2(6)° to 121.0(6)°,
suggesting sp2 hybridization, whereas the C(6)-C(1)-
C(2) angle (109.6(6)°) is consistent with sp3 hybridiza-
tion. In addition, the remaining aromatic hydrogen on
the η5-cyclohexadienyl ring is sterically positioned near
the π-system of one of the imido mesityl substituents,
To determine the fate of the hydrogen atom that is
introduced in the formation of 11, the through-space
couplings involving the reduced mesityl ring in 11 were
determined. A 1H-ROESY NMR experiment (mixing
time ) 1 s) was used to observe an ROE (rotating frame
Overhauser effect) between the singlet at 4.26 ppm
1
likely causing the unusual upfield aromatic H NMR
resonance observed for both complexes 9 and 11.

5556 Organometallics, Vol. 21, No. 25, 2002
Gavenonis and Tilley
Sch em e 3
(HMes, Figure 4) and the doublet at 4.54 ppm (Hexo).
F igu r e 4. 1H NMR assignments of 11 based upon 1H-
ROESY data.
However, no ROE was observed between HMes and Hendo
.
In the structure of complex 11, the η5-cyclohexadienyl
ring places HMes closer to Hexo than Hendo (3.53 vs 4.06
evidence been observed for transfer of the hydride ligand
to the coordinated arene. However, Wigley and co-
workers have prepared tantalum chlorides containing
η2-pyridine ligands, which react with LiBEt3H to give
an intermediate tantalum hydride, which then transfers
the hydride to the coordinated pyridine.72,73 In this
instance, though, the product is a metallacycle resulting
from the cleavage of a C-N bond. Thus, the isolation
and structural characterization of complex 11 appears
to be the first example of an intramolecular transfer of
a single hydride to give a stable, η5-cyclohexadienyl
complex and lends support to one of the mechanisms
postulated by Rothwell and co-workers for arene hy-
drogenation by early metal catalysts.36
A pronounced solvent effect was observed for the
hydrogenolysis of complex 10 to 11. With benzene-d6 as
the solvent, complete conversion of complex 10 to 11
requires heating at 95 °C for 5 days in the presence of
H2 (1 atm). However, with the more polar solvent
bromobenzene-d5, the reaction proceeds significantly
faster such that complete conversion is observed after
heating at 95 °C for 18 h. The source of this solvent
effect is not understood. Triflate dissociation from 10
to produce the transient, cationic species [(Ar*Nd)-
(Ar*NH)TaMe]+[OTf]- seems unlikely, since the cationic
complex 9 is unreactive toward H2.
Hyd r ogen a tion Ca ta lysis w ith th e Ma sk ed Hy-
d r id e 11. The observed stoichiometric hydrogenation
of 1-hexene with 11 suggested the use of this complex
as a hydrogenation catalyst. Such catalytic hydrogena-
tions were observed for a number of substrates as shown
in Table 4. In a typical reaction, complex 10 was treated
with the olefin (or silane) substrate and H2 (or D2, 1
atm) in bromobenzene-d5 at 95 °C. In all cases, the
hydrogenation reactions were extremely slow, as might
be expected for a sterically encumbered hydride catalyst.
More sterically crowded substrates such as cyclohexene
and R-methylstyrene reacted much more slowly than
Å). Therefore, the doublet at 4.54 ppm is due to Hexo
,
and the doublet at 3.53 ppm is due to Hendo. Further-
more, since the deuterium labeling experiment incor-
porates deuterium into only one position of the molecule
(the Hendo position), the hydride transfer proceeds in an
endo fashion.
Complex 11 results from the insertion of an arene ring
into a M-H bond to give a stable η5-cyclohexadienyl
complex. Complexes of this type have been postulated
by Rothwell and co-workers as intermediates in the
intramolecular hydrogenation of aryl oxide phenyl sub-
stituents to cyclohexyl groups.36 Previous work aimed
at the characterization of potential arene hydrogenation
intermediates resulted in the isolation of a niobium
species in which two hydrogens were transferred to an
aromatic ring to give an η4-cyclohexadiene ligand,35,63
and a tungsten complex resulting from transfer of four
hydrogens to an aromatic ring to yield an η2-cyclohexene
ligand.37 Complex 11 appears to represent the first
example of the product of intramolecular transfer of a
single hydride to an aromatic ring to give a stable, η5-
cyclohexadienyl complex.13 Chromium and manganese
complexes containing η5-cyclohexadienyl ligands result-
ing from intermolecular hydride transfer to a coordi-
nated arene are known,64,65 but to the best of our
knowledge, no examples of intermolecular or intramo-
lecular hydride transfer to give a stable, η5-cyclohexa-
dienyl ligand exist among group V arene complexes.66,67
Green and co-workers have reported low-valent niobium
complexes containing both η6-arenes and hydride
ligands,68,69 and Wigley and co-workers have described
similar tantalum species,70,71 but in neither case has any
(63) Steffey, B. D.; Rothwell, I. P. J . Chem. Soc., Chem. Commun.
1990, 213.
(64) Rose-Munch, F.; Gagliardini, V.; Renard, C.; Rose, E. Coord.
Chem. Rev. 1998, 180, 249.
(65) Djukic, J .-P.; Rose-Munch, F.; Rose, E. J . Am. Chem. Soc. 1993,
115, 6434.
(66) Wigley, D. E.; Gray, S. D. In Comprehensive Organometallic
Chemistry II; Abel, E. W., Stone, F. G. A., Wilkinson, G., Eds.;
Pergamon Press: Oxford, 1995; Vol. 5, p 57.
(67) Calderazzo, F.; Pampaloni, G. J . Organomet. Chem. 1992, 423,
307.
(68) Green, M. L. H.; O’Hare, D.; Watkin, J . G. J . Chem. Soc., Chem.
Commun. 1989, 698.
(70) Arney, D. S. J .; Fox, P. A.; Bruck, M. A.; Wigley, D. E.
Organometallics 1997, 16, 3421.
(71) Arney, D. J .; Bruck, M. A.; Wigley, D. E. Organometallics 1991,
10, 3947.
(72) Gray, S. D.; Smith, D. P.; Bruck, M. A.; Wigley, D. E. J . Am.
Chem. Soc. 1992, 114, 5462.
(69) Green, M. L. H.; O’Hare, D.; Mountford, P.; Watkin, J . G. J .
Chem. Soc., Dalton Trans. 1991, 1991.
(73) Gray, S. D.; Weller, K. J .; Bruck, M. A.; Briggs, P. M.; Wigley,
D. E. J . Am. Chem. Soc. 1995, 117, 10678.

Tantalum Alkyl and Silyl Complexes
Organometallics, Vol. 21, No. 25, 2002 5557
Ta ble 4. Resu lts for Rea ction s of th e Ca ta lytic P r ecu r sor 10
substrate
1-hexene
equiv
10
conversion (%)
100
product(s)
n-hexane
2-hexene, 3-hexene
cyclohexane
methylcyclopentane
methylenecyclopentane
n-hexane
yield (%)
time (days)
7
90
10
97
50
20
20
90
99
cyclohexene
1,5-hexadiene
5
14
80
100
12
9
R-methylstyrene
phenylsilane
1
5
60
100
cumene
phenylsilane-d3
4
2
Sch em e 4
less substituted alkenes (e.g., 1-hexene and 1,5-hexa-
diene). Attempts to hydrogenate ethylene were unsuc-
cessful since complex 10 was found to slowly polymerize
ethylene (1 atm) at room temperature to yield polyeth-
ylene (melting point 132 °C by differential scanning
calorimetry). No hydrogenation was observed when
benzene was used as the substrate (bromobenzene-d5,
120 °C, 2 days).
The observed products for the reaction with 1,5-
hexadiene are methylcyclopentane (50%), methylenecy-
clopentane (20%), and n-hexane (20%). Methylcyclopen-
tane could arise via olefin insertion into the tantalum
hydride, followed by intramolecular olefin insertion into
the resulting Ta-C bond, and hydrogenolysis of the
resulting tantalum alkyl to yield the alkane and regen-
erate the catalyst (Scheme 4). However, â-hydride
elimination appears to be a competing process in this
reaction, given the formation of methylenecyclopentane
(and the formation of 2-hexene and 3-hexene observed
during the hydrogenation of 1-hexene). The reductive
cyclization of 1,5-hexadiene to methylcyclopentane has
previously been observed by Bercaw and co-workers for
scandocene hydride catalysts,74,75 and Marks has re-
ported efficient catalysts for this transformation based
on yttrium and lutetium.76 The decreased reactivity of
[(Ar*Nd)(Ar*NH)Ta(H)OTf] (A) relative to the Bercaw
and Marks systems can be attributed to the steric bulk
of the terphenyl ligands, which should decrease the
rates of insertion and hydrogenolysis.
In addition to olefin hydrogenation and diene cycliza-
tion, this system was found to effect the catalytic
deuteration of the Si-H positions of PhSiH3 by D2
(Table 4). This process may proceed in a concerted
fashion through a σ-bond metathesis mechanism which
places silicon in the â-position of a four-center transition
state.5 An alternative pathway involves possible silyl
intermediates of the form [(Ar*Nd)(Ar*NH)Ta(Si-
HxD(2-x)Ph)OTf], though attempts to observe such spe-
cies by 1H NMR spectroscopy were unsuccessful. No
reaction was observed upon treating complex 11 with
PhSiH3 (1 equiv) in bromobenzene-d5 at room temper-
ature, and an intractable mixture of products was
observed upon heating the reaction mixture to 105 °C
for 5 days. Similarly, no reaction was observed between
complex 11 and CH4 under identical conditions.
Syn th esis of Ta n ta lu m Silyl Com p lexes. To de-
termine if this system would support reactive Ta-Si
σ-bonds, we sought the preparation of silyl derivatives
of 7 via salt-metathesis reactions. Complex 7 reacts
cleanly with the lithium silyl reagents (THF)3LiSi-
(SiMe3)3, (THF)2LiSi(tBu)Ph2, and (THF)2LiSiHMes2 to
provide the corresponding silyl chloride complexes
(Ar*Nd)(Ar*NH)Ta(SiR3)Cl (SiR3 ) Si(SiMe3)3 (13a ),
(74) Piers, W. E.; Shapiro, P. J .; Bunel, E. E.; Bercaw, J . E. Synlett
1990, 74.
(75) Bunel, E.; Burger, B. J .; Bercaw, J . E. J . Am. Chem. Soc. 1988,
110, 976.
(76) Haar, C. M.; Stern, C. L.; Marks, T. J . Organometallics 1996,
15, 1765.

5558 Organometallics, Vol. 21, No. 25, 2002
Gavenonis and Tilley
Si(tBu)Ph2 (13b), SiHMes2 (13c)) in 42%, 69%, and 80%
yields, respectively (eq 7). The only other examples of
which is then transferred to one of the mesityl rings to
produce the η5-cyclohexadienyl complex (Ar*Nd)[2-(η5-
2,4,6-Me3C6H3)-6-MesC6H3NH]TaCl. Attempts to isolate
this complex were complicated by persistent quantities
of unidentified impurities.
Treating complex 13a with silanes such as PhSiH3
or Ph2SiH2 (1 equiv) in benzene-d6 at 85 °C over several
days in the dark led to the formation of HSi(SiMe3)3,
but no metal-containing products could be identified and
most of the silane remained unchanged (70% PhSiH3
remained after 6 days; 90% Ph2SiH2 remained after 3
days). Heating complex 13a in the absence of silanes
also yields (Me3Si)3SiH, which appears to be a kinetic
product of the thermal decomposition of 13a .
Con clu sion s
The results described here provide further evidence
that imido hydride complexes are accessible, potentially
reactive species that can give rise to novel chemistry.
Complex 11, with an imido-amido ligand set featuring
the sterically demanding 2,6-Mes2C6H3 aryl group,
represents an arene hydrogenation intermediate result-
ing from the transfer of hydride from tantalum to one
of the mesityl rings. In previous work by Rothwell and
co-workers, intramolecular transfers of two and four
hydrides to aryl oxide phenyl substituents were ob-
served.35,37,63 Complex 11 reacts with small molecules
via its more reactive isomer A, with which it is in
equilibrium, and exhibits many of the reactions expected
for a d0 metal hydride. Thus, it is a catalyst for olefin
hydrogenation, diene cyclization, and H/D exchange at
silicon, although the sterically encumbering nature of
the terphenyl ligands leads to relatively low turnover
rates.
Complexes 13a -c represent rare examples of imido
silyl tantalum species and confirm that the (Ar*Nd)-
(Ar*NH) ligand set can be used to support reactive Ta-
Si σ-bonds. These silyl species react with H2 to yield
the corresponding silane and a complex that has spec-
troscopic features similar to those for the η5-cyclohexa-
dienyl complex 11. As shown in other d0 systems, silyl
ligands appear to provide facile routes to the corre-
sponding hydride derivative via hydrogenolysis.24 Con-
tinuing studies focus on the development of imido
hydrides and silyls that are highly reactive in σ-bond
metathesis processes.
imido silyl complexes of tantalum are (Me3SiNd)[(Me3-
Si)2N]Ta[Si(tBu)Ph2]2,29 (Me3SiNd)[(Me3Si)2N](Me2N)-
Ta[Si(tBu)Ph2],29 Cp(2,6-iPr2C6H3Nd)Ta(H)(SiMe2Cl)-
(PMe3),28 and Cp*(2,6-iPr2C6H3Nd)Ta(SiR3)X (SiR3
)
Si(SiMe3)3, SiPh3, SiHMes2; X ) Cl, alkyl, hydride).25,26
Silyl complexes 13a -c are extremely light sensitive,
necessitating their preparation and storage under con-
ditions of low ambient lighting. Monitoring a benzene-
d6 solution of complex 13a exposed to ambient lab light
over 2 days by 1H NMR spectroscopy revealed the
formation of HSi(SiMe3)3, but no tantalum-containing
products could be identified.
All three silyl complexes contain broad mesityl reso-
nances in their 1H NMR spectra, presumably due to
restricted rotation caused by introduction of the steri-
cally bulky silyl ligands. Although the structure and
connectivity of complex 13a were confirmed by X-ray
crystallography, the refinement of accurate metric
parameters was not possible due to severe positional
disorder of the tantalum and nitrogen atoms and
rotational disorder of the -Si(SiMe3)3 group.
Complexes 13a -c were found to react cleanly with
H2 (1 atm) in benzene-d6 at room temperature over 3
days in the dark to yield HSiR3 and a complex that
exhibits spectroscopic features similar to those of 11,
including a pair of doublets at 3.12 and 4.53 ppm (J )
15 Hz) and a broad, aromatic singlet integrating to 1 H
at 4.01 ppm (eq 8). The formation of HSiR3 (1 equiv)
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 H2-
SO4, 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 toluene
by treatment with H2SO4 and saturated NaHCO3. Toluene was
then dried over MgSO4 and distilled from potassium under a
nitrogen atmosphere. Diethyl ether and hexanes were distilled
from sodium benzophenone ketyl under a nitrogen atmosphere,
and bromobenzene was distilled from CaH2 under a nitrogen
atmosphere. Benezene-d6 and toluene-d8 were purified and
dried by vacuum distillation from sodium/potassium alloy.
suggests that this reaction proceeds through a σ-bond
metathesis pathway to give an intermediate hydride,

Tantalum Alkyl and Silyl Complexes
Organometallics, Vol. 21, No. 25, 2002 5559
Bromobenzene-d5 was degassed and then dried over 4 Å
molecular sieves. Dichloromethane-d2 was purified and dried
by vacuum distillation from CaH2.
orange crystals of compound 1 (13.8 g, 96%). The analytically
pure compound was obtained by recrystallization from diethyl
1
ether at -35 °C. Mp: 144-152 °C. H NMR: δ 2.10 (s, 12 H,
NMR spectra were recorded at 500.132 MHz (1H), 61.423
MHz (2H), 125.759 MHz (13C), 376.503 MHz (19F), 99.376 MHz
(29Si), 160.462 MHz (11B), or 202.457 MHz (31P) using a Bruker
DRX-500 (1H, 13C, 29Si, 11B, 31P) or AMX-400 (2H, 19F) spec-
o-Me), 2.17 (s, 6 H, p-Me), 6.85 (s, 4 H, Mes-H), 6.88 (d, 2 H,
m-H), 6.94 (t, 1 H, p-H). 13C{1H} NMR: δ 20.9 (o-Me), 21.5
(p-Me), 126.2, 128.9, 130.6, 135.7, 135.7, 136.7, 136.8, 137.9
(aromatic C’s). IR (Nujol, cm-1): 2137 (m), 2098 (s), 2083 (s),
1414 (m), 1313 (m), 847 (w), 802 (w), 757 (w). Anal. Calcd for
1
2
trometer. H and H NMR spectra were referenced internally
by the residual solvent signal relative to tetramethylsilane.
13C{1H} NMR spectra were referenced internally by the 13C
NMR signal of the NMR solvent relative to tetramethylsilane.
19F{1H} NMR spectra were referenced relative to an R,R,R-
trifluorotoluene external standard. 29Si NMR spectra were
referenced using a tetramethylsilane external standard. 11B
NMR spectra were referenced using a BF3‚OEt2 external
standard. 31P{1H} NMR spectra were referenced relative to
an 85% aqueous H3PO4 external standard. In some cases,
distortionless enhancement by polarization transfer (DEPT)
was used to assign the 13C NMR resonances as CH3, CH2, CH,
or C, and 1H-coupled and decoupled insensitive nuclei en-
hanced by polarization transfer (INEPT) were used to identify
C
24H25N3: C, 81.09; H, 7.09; N, 11.82. Found: C, 81.03; H,
7.30; N, 11.59.
2,6-Mes2C6H3NH2 (2). A solution of compound 1 (5.09 g,
14.3 mmol) in diethyl ether (100 mL) was added dropwise over
30 min via addition funnel to a stirred solution of lithium
aluminum hydride (0.561 g, 14.8 mmol) in diethyl ether (100
mL). After the addition was completed, the reaction mixture
was heated at reflux for 4.5 h (55 °C bath). The reaction
mixture was cooled to room temperature and quenched with
reagent grade diethyl ether (50 mL) and water (50 mL).
Bu¨chner funnel filtration followed by separation, drying of the
organic phase over MgSO4, and solvent reduction in vacuo
afforded colorless, diamond-shaped crystals of compound 2
(4.06 g, 86%) from diethyl ether at -35 °C. Mp: 142-143 °C.
1H NMR: δ 2.12 (s, 12 H, o-Me), 2.20 (s, 6 H, p-Me), 3.02 (br
s, 2 H, NH2), 6.86 (t, 1 H, p-H), 6.87 (s, 4 H, Mes-H), 6.90 (d,
2 H, m-H). 13C{1H} NMR: δ 20.7 (o-Me), 21.5 (p-Me), 118.9,
126.6, 129.2, 129.4, 136.3, 137.2, 137.4, 141.6 (aromatic C’s).
IR (Nujol, cm-1): 3475 (m, νNH2), 3380 (m, νNH2). Anal. Calcd
for C24H27N: C, 87.49; H, 8.26; N, 4.25. Found: C, 87.44; H,
8.48; N, 4.13.
2,6-Mes2C6H3NHLi (3). A solution of nBuLi in hexanes (2.8
M, 22.4 mmol) was added via syringe to a stirred solution of
compound 2 (7.19 g, 21.8 mmol) in pentane (200 mL) at 0 °C.
After warming the reaction mixture to room temperature over
1.5 h, the solvent and volatile byproducts were removed in
vacuo to yield 3 (7.24 g, 99%) as a light yellow powder. 1H
NMR: δ 1.41 (br s, 1 H, NH), 1.97 (s, 12 H, o-Me), 2.20 (s, 6
H, p-Me), 6.66 (t, 1 H, J ) 7 Hz, p-H), 6.77 (s, 4 H, Mes-H),
6.84 (d, 2 H, J ) 7 Hz, m-H). 13C{1H} NMR: δ 20.6 (o-Me),
21.5 (p-Me), 113.2 (CH), 127.2 (aromatic C), 129.0, 129.8
(CH’s), 136.8, 136.8, 140.2, 156.7 (aromatic C’s). IR (Nujol,
cm-1): 3284 (w, νNH). Anal. Calcd for C24H26LiN: C, 85.94; H,
7.81; N, 4.18. Found: C, 85.92; H, 7.66; N, 4.44.
1
1
29Si resonances, J SiH values, and J CH values. Heteronuclear
multiple quantum coherence (HMQC) was used to identify
1
1H,13C and H,11B coupling and total correlation spectroscopy
1
(TOCSY) was used to identify some coupled H NMR systems.
Nuclear Overhauser enhancement spectroscopy (NOESY) was
used to identify the η5-cyclohexadienyl methyl groups in
complexes 11 and 11-d , and rotating frame Overhauser
enhancement spectroscopy (ROESY) was used to identify the
Hendo and Hexo hydrogens in complexes 11 and 11-d (mixing
time ) 1 s). All spectra were recorded at room temperature
(∼22 °C) unless otherwise indicated. Infrared spectra were
recorded as thin film Nujol mulls on NaCl plates, as KBr
pellets, or in solution using a Mattson FTIR spectrometer at
a resolution of 4 cm-1. Elemental analyses were performed by
the College of Chemistry Microanalytical Laboratory at the
University of California, Berkeley.
All chemicals were purchased from Aldrich or Fluka and
used without further purification. Lithium aluminum hydride
was purified by diethyl ether extraction. Carbon monoxide was
purchased from Scott Specialty Gases, hydrogen was pur-
chased from Praxair, and deuterium was purchased from
Airgas. The compounds 2,6-Mes2C6H3I,77 p-toluenesulfonyl
azide,78 TaMe3Cl2,79 NpLi,80 B(C6F5)3,81 (THF)3LiSi(SiMe3)3,82
(2,6-Mes2C6H3Nd)(2,6-Mes2C6H3NH)Ta Me2 (4). A solu-
tion of TaMe3Cl2 (0.880 g, 2.96 mmol) in hexanes (20 mL) was
added to a stirred slurry of compound 3 (1.985 g, 5.92 mmol)
in hexanes (120 mL) at -35 °C. Upon addition, the reaction
mixture changed from yellow to orange. After warming to room
temperature over 2 h, the reaction mixture was filtered and
the remaining solid was extracted with hexanes (4 × 120 mL).
Concentrating and cooling the combined extracts to -35 °C
afforded several crops of yellow, blocklike crystals of 4 (1.834
62
(THF)2LiSi(tBu)Ph2,83 and (THF)2LiSiHMes2 were prepared
as reported in the literature.
2,6-Mes2C6H3N3 (1). A solution of nBuLi in hexanes (1.6
M, 40.8 mmol) was added dropwise over 20 min via addition
funnel to a stirred slurry of 2,6-Mes2C6H3I (18.00 g, 40.87
mmol) in hexanes (400 mL) at 0 °C. The resulting yellow slurry
was warmed to room temperature and stirred for an additional
16 h. The reaction mixture was then cooled to 0 °C, and a
solution of p-toluenesulfonyl azide (8.26 g, 41.9 mmol) in
diethyl ether (125 mL) was added via cannula. After stirring
for 2 h at 0 °C, the reaction mixture was quenched with water
(250 mL). The aqueous phase was separated and extracted
with diethyl ether (3 × 200 mL). The organic phases were
combined, washed with water (200 mL), and dried over MgSO4.
Filtration followed by solvent removal in vacuo resulted in red-
1
g, 71%). Mp: 208-211 °C. H NMR: δ -0.41 (s, 6 H, TaMe2),
2.02 (s, 12 H, o-Me), 2.06 (s, 12 H, o-Me), 2.09 (s, 6 H, p-Me),
2.22 (s, 6 H, p-Me), 6.74 (m, 3 H, m-H, p-H), 6.85 (s, 4 H, Mes-
H), 6.87 (s, 4 H, Mes-H), 6.88 (t, 1 H, J ) 10 Hz, p-H), 6.99 (d,
2 H, J ) 5 Hz, m-H), 7.34 (br s, 1 H, NH). 13C{1H} NMR
(dichloromethane-d2): δ 20.7 (o-Me), 20.9 (o-Me), 21.3 (p-Me),
21.4 (p-Me), 57.5 (TaMe2), 120.9, 122.8, 128.0, 128.2, 129.7,
130.1, 130.2, 136.3, 136.4, 137.0, 137.5, 138.5, 138.8, 139.0,
148.3, 154.0 (aromatic C’s). IR (Nujol, cm-1): 3303 (m, νNH).
Anal. Calcd for C50H57N2Ta: C, 69.27; H, 6.63; N, 3.23.
Found: C, 68.93; H, 6.87; N, 2.99.
(77) Du, C. F.; Hart, H.; Ng, K. D. J . Org. Chem. 1986, 51, 3162.
(78) Regitz, M.; Hocker, J .; Leidhegener, A. Org. Synthesis 1973, 5,
179.
(2,6-Mes2C6H3Nd)(2,6-Mes2C6H3NH)TaMe(η2-COMe) (5).
Compound 4 (25.7 mg, 29.6 µmol) was dissolved in benzene-
d6 (∼0.7 mL) and transferred to an NMR tube fitted with a J .
Young Teflon stopper. The solution was degassed via three
cycles of freeze-pump-thaw and closed under CO (1 atm).
Upon exposure to CO (<1 min), the reaction mixture changed
from pale yellow to bright yellow. Compound 5 was observed
(79) Schrock, R. R.; Sharp, P. R. J . Am. Chem. Soc. 1978, 100, 2389.
(80) Schrock, R. R.; Fellmann, J . D. J . Am. Chem. Soc. 1978, 100,
3359.
(81) Chernega, A. N.; Graham, A. J .; Green, M. L. H.; Haggitt, J .;
Lloyd, J .; Mehnert, C. P.; Metzler, N.; Souter, J . J . Chem. Soc., Dalton
Trans. 1997, 2293.
(82) Gutekunst, G.; Broook, A. G. J . Organomet. Chem. 1982, 225,
1.
1
as the only product by H NMR spectroscopy (99% relative to
(83) Campion, B. K.; Heyn, R. H.; Tilley, T. D. Organometallics 1993,
12, 2584.
1
internal standard). H NMR: δ 0.04 (s, 3 H, TaMe), 1.83 (s, 6

5560 Organometallics, Vol. 21, No. 25, 2002
Gavenonis and Tilley
H, Mes-Me), 2.00 (s, 6 H, Mes-Me), 2.04 (s, 6 H, Mes-Me), 2.07
(s, 6 H, Mes-Me), 2.08 (s, 6 H, Mes-Me), 2.26 (s, 3 H, COMe),
2.29 (s, 6 H, Mes-Me), 6.54 (br s, 2 H, Ar*-H), 6.63 (br d, 2 H,
J ) 7 Hz, Ar*-H), 6.69 (t, 1 H, J ) 8 Hz, p-H), 6.78 (dd, 1 H,
J ) 7 Hz, J ) 8 Hz, p-H), 6.84 (br s, 2 H, Ar*-H), 6.88 (d, 2 H,
J ) 8 Hz, m-H), 6.90 (s, 2 H, Ar*-H), 6.94 (s, 2 H, Ar*-H), 7.91
(s, 1 H, NH). 13C{1H} NMR: δ 20.2, 20.5, 21.4, 21.4, 21.7, 22.2,
31.3, 33.6 (aliphatic C’s), 121.2, 123.0, 129.7, 130.0, 130.5,
130.6, 135.9, 136.1, 136.5, 137.2, 137.4, 138.8, 138.8, 139.1,
148.8, 153.7 (aromatic C’s), 317.1 (COMe). IR (benzene-d6,
cm-1): 3308 (w, νNH), 1421 (s, νCO).
(2,6-Mes2C6H 3Nd)(2,6-Mes2C6H 3NH )Ta Me2(P Me3) (6).
Compound 4 (31.1 mg, 35.9 µmol) was dissolved in benzene-
d6 (∼0.7 mL) and transferred to an NMR tube fitted with a J .
Young Teflon stopper. Trimethylphosphine (3.7 µL, 35.7 µmol)
was then added to the NMR tube via syringe. Upon addition,
the reaction mixture became pale yellow. Compound 6 was
observed as the only product by 1H NMR spectroscopy (99%
relative to internal standard). 1H NMR: δ -0.61 (s, 6 H,
TaMe2), 0.56 (d, 9 H, J PH ) 5 Hz, PMe3), 2.04 (s, 12 H, o-Me),
2.09 (s, 6 H, p-Me), 2.21 (s, 12 H, o-Me), 2.31 (s, 6 H, p-Me),
6.81 (br m, 8 H, Ar*-H), 6.94 (d, 2 H, J ) 8 Hz, m-H), 6.94 (s,
4 H, Mes-H), 7.23 (s, 1 H, NH). 13C{1H} NMR: δ 14.1 (d, J PC
) 13 Hz, PMe3), 21.4 (p-Me), 21.6 (p-Me), 21.7 (o-Me), 21.7
(o-Me), 41.0 (br s, TaMe2), 121.0, 122.4, 128.8, 129.4, 129.9,
130.7 (CH’s), 131.4, 136.1, 137.2, 137.3, 137.3, 137.7, 138.5,
139.2, 150.3, 155.4 (aromatic C’s). 31P{1H} NMR: δ -19.1 (br
s, PMe3). IR (benzene-d6, cm-1): 3281 (w, νNH).
give a bright orange solution. After stirring at room temper-
ature for 1.5 h, the solution was filtered. The filtrate was
concentrated to ca. 10 mL, layered with an equal volume of
pentane, and cooled to -35 °C to afford two crops of a red-
orange oil. Decanting the solvent and drying the oil in vacuo
1
provided compound 9 (0.277 g, 81%) as a red-orange foam. H
NMR: δ -0.19 (s, 3 H, TaMe), 1.16 (br s, 3 H, [MeB(C6F5)3]-),
1.61 (br s, 6 H, Mes-Me), 1.67 (s, 3 H, Mes-Me), 1.68 (s, 3 H,
Mes-Me), 1.76 (s, 3 H, Mes-Me), 1.80 (s, 3 H, Mes-Me), 1.82
(s, 3 H, Mes-Me), 1.89 (br s, 6 H, Mes-Me), 2.20 (s, 3 H, Mes-
Me), 2.26 (s, 6 H, Mes-Me), 5.19 (s, 1 H, Ar*-H), 6.60 (br s, 2
H, Ar*-H), 6.68 (s, 1 H, Ar*-H), 6.71 (m, 3 H, Ar*-H), 6.78 (s,
2 H, Ar*-H), 6.83 (s, 2 H, Ar*-H), 6.92 (t, 1 H, J ) 8 Hz, Mes-
H), 6.96 (s, 1 H, Ar*-H), 6.99 (d, 1 H, J ) 8 Hz, Mes-H). 13C-
{1H} NMR: δ 19.6, 19.8, 20.2, 20.3, 20.5, 20.5, 20.7, 21.2, 21.3,
21.8 (Mes-Me’s), 29.8 (TaMe), 119.6 (CH), 125.0 (aromatic C),
126.0, 126.1, 126.2, 128.9, 128.9, 129.0, 129.7, 129.7, 130.1
(CH’s), 131.8, 132.5 (aromatic C’s), 132.8, 136.1 (CH’s), 136.4
(C6F5), 136.5, 136.8, 137.1, 137.6, 137.9, 138.2 (aromatic C’s),
138.6 (C6F5), 139.3, 140.3 (aromatic C’s), 148.6 (C6F5), 149.8,
150.0 (aromatic C’s), 150.5 (C6F5), 151.7, 152.0, 155.1 (aromatic
C’s). 11B NMR: δ -14.93. 19F{1H} NMR: δ -132.3 (d, J ) 19
Hz), -164.4 (t, J ) 19 Hz), -167.0 (t, J ) 19 Hz). IR (Nujol,
cm-1): 3304 (m, νNH). Anal. Calcd for C68H57BF15N2Ta: C,
59.23; H, 4.17; N, 2.03. Found: C, 59.61; H, 4.08; N, 1.94.
(2,6-Mes2C6H 3Nd)(2,6-Mes2C6H 3NH )Ta Me(OSO2CF 3)
(10). Compound 4 (0.566 g, 0.653 mmol) and AgOSO2CF3
(0.169 g, 0.659 mmol) were dissolved in toluene (50 mL) to
give a dark brown reaction mixture. After stirring the reaction
mixture for 27 h at room temperature, the solvent and volatile
byproducts were removed in vacuo to leave behind a yellow-
brown oily solid. The solid was extracted with pentane (3 ×
30 mL), and the combined extracts were filtered to give a gold-
colored solution. The solution was concentrated to ca. 15 mL
and cooled to -35 °C overnight to afford two crops of yellow-
orange crystals of compound 10 (0.508 g, 78%). 1H NMR: δ
0.22 (s, 3 H, TaMe), 1.88 (s, 6 H, Mes-Me), 2.02 (s, 6 H, Mes-
Me), 2.04 (s, 6 H, Mes-Me), 2.15 (s, 6 H, Mes-Me), 2.21 (s, 6 H,
Mes-Me), 2.25 (s, 6 H, Mes-Me), 6.62 (d, 2 H, J ) 7 Hz, p-H),
6.73 (t, 3 H, J ) 7 Hz, m-H), 6.79 (dd, 1 H, J ) 7 Hz, J ) 8
Hz, p-H), 6.84 (s, 2 H, Ar*-H), 6.88 (s, 2 H, Ar*-H), 6.90 (s, 2
H, Ar*-H), 6.93 (s, 4 H, Mes-H), 8.69 (br s, 1 H, NH). 13C{1H}
NMR: δ 20.2, 20.7, 21.0, 21.3, 21.6, 21.7, 21.8, 23.1 (Mes-Me’s),
45.0 (TaMe), 119.4, 122.0 (aromatic C’s), 123.2, 125.5, 128.7,
128.8 (CH’s), 129.2 (aromatic C), 130.3, 132.0 (CH’s), 134.9,
136.5, 137.3, 137.4, 137.8, 137.9, 139.2, 139.7, 141.8, 147.5,
152.6 (aromatic C’s). 19F{1H} NMR: δ -75.3. IR (Nujol, cm-1):
(2,6-Mes2C6H3Nd)(2,6-Mes2C6H3NH)Ta Cl2 (7). TaMe3Cl2
(1.157 g, 3.90 mmol) and compound 2 (2.528 g, 7.67 mmol) were
dissolved in toluene (80 mL) and heated to 75 °C for 21 h. After
cooling the reaction mixture to room temperature, the solvent
and volatile byproducts were removed in vacuo to give a brown
solid. The solid was extracted with toluene (3 × 30 mL) and
filtered to give a red-brown solution. The combined extracts
were concentrated to ca. 30 mL, layered with an equal volume
of pentane, and cooled to -35 °C to give several crops of light
brown crystals. Each crop of crystals was washed with cold
pentane (2 × 15 mL) and dried in vacuo to afford analytically
1
pure 7 (2.369 g, 68%). Mp: 177-180 °C. H NMR: δ 2.01 (s,
12 H, o-Me), 2.05 (s, 12 H, o-Me), 2.13 (s, 6 H, p-Me), 2.22 (s,
6 H, p-Me), 6.70 (m, 3 H, m-H, p-H), 6.80 (t, 1 H, J ) 7 Hz,
p-H), 6.87 (s, 4 H, Mes-H), 6.88 (s, 4 H, Mes-H), 6.91 (d, 2 H,
J ) 7 Hz, m-H), 8.93 (br s, 1 H, NH). 13C{1H} NMR: δ 21.1
(o-Me), 21.4 (o-Me), 21.5 (p-Me), 21.6 (p-Me), 123.8, 126.1,
128.3, 128.7, 130.5, 130.7, 130.9, 134.8, 136.8, 137.0, 137.2,
139.2, 139.2, 139.8, 146.1, 152.9 (aromatic C’s). IR (Nujol,
cm-1): 3305 (m, νNH). Anal. Calcd for C48H51Cl2N2Ta: C, 63.51;
H, 5.66; N, 3.09. Found: C, 63.64; H, 5.52; N, 2.97.
3321 (w, νNH). Anal. Calcd for (C50H54F3N2O3STa)‚(C5H12
) :
0.5
C, 60.80; H, 5.83; N, 2.70. Found: C, 60.50; H, 6.09; N, 2.68.
(2,6-Mes2C6H3Nd)[2-(η5-2,4,6-Me3C6H3)-6-MesC6H3NH]-
Ta (OSO2CF 3) (11). Compound 10 (0.252 g, 0.252 mmol) was
dissolved in bromobenzene (10 mL), and the resulting solution
was transferred to a 50 mL reaction vessel. The solution was
degassed, H2 (1 atm) was admitted, and the reaction solution
was heated to 95 °C. After 2 days, the reaction mixture was
cooled to room temperature and diluted with toluene (10 mL).
The solvent was removed in vacuo to leave behind a red-orange
solid, which was extracted with toluene (2 × 15 mL). The
combined extracts were concentrated to ca. 1 mL and cooled
to -35 °C to afford two crops of red-orange crystals of
compound 11 (0.187 g, 75%). 1H NMR: δ 1.60 (s, 3 H,
η5-cyclohexadienyl Me), 1.89 (s, 3 H, η5-cyclohexadienyl Me),
1.92 (s, 3 H, Mes-Me), 1.94 (s, 6 H, Mes-Me), 1.97 (s, 3 H, Mes-
Me), 2.07 (s, 3 H, η5-cyclohexadienyl Me), 2.20 (s, 6 H, Mes-
Me), 2.26 (s, 6 H, Mes-Me), 2.27 (s, 3 H, Mes-Me), 3.53 (d, 1
H, J ) 16 Hz, Hendo), 4.26 (s, 1 H, Ar*-H), 4.54 (d, 1 H, J ) 16
Hz, Hexo), 6.46 (s, 2 H, Ar*-H), 6.69 (m, 1 H, Ar*-H), 6.74 (d, 1
H, Ar*-H), 6.75 (s, 1 H, Ar*-H), 6.80 (m, 1 H, Ar*-H), 6.82 (s,
1 H, Ar*-H), 6.83 (d, 1 H, Ar*-H), 6.89 (br s, 1 H, Ar*-H), 7.00
(br s, 2 H, Ar*-H), 7.06 (br s, 1 H, Ar*-H), 7.34 (br s, 1 H,
NH). 13C{1H} NMR: δ 17.2, 19.8, 19.8, 19.9, 20.9, 21.1, 21.5,
(2,6-Mes2C6H3Nd)(2,6-Mes2C6H3NH)Ta (CH2CMe3)2 (8).
Compound 7 (0.301 g, 0.331 mmol) and Me3CCH2Li (0.083 g,
1.07 mmol) were dissolved in toluene (25 mL) and stirred at
room temperature for 5.5 h. The solvent was removed in vacuo,
and the light brown solid was extracted with pentane (3 × 20
mL) and filtered. The combined extracts were concentrated to
ca. 50 mL and cooled to -35 °C to afford two crops of yellow
1
crystals of 8 (0.234 g, 72%). H NMR (dichloromethane-d2): δ
-0.36 (d, 2 H, J ) 15 Hz, CH2), -0.02 (d, 2 H, J ) 15 Hz,
CH2), 0.54 (s, 18 H, tBu), 1.88 (br s, 12 H, o-Me), 2.03 (s, 12 H,
o-Me), 2.24 (s, 6 H, p-Me), 2.27 (s, 6 H, p-Me), 6.66 (br s, 1 H,
NH), 6.77 (br s, 3 H, Ar*-H), 6.8-6.9 (complex, 7 H, Ar*-H),
6.90 (s, 4 H, Mes-H). 13C{1H} NMR (dichloromethane-d2): δ
21 (br, o-Me), 21.4 (p-Me), 21.4 (p-Me), 21.9 (o-Me), 34.9 (CMe3),
35.5 (CMe3), 105.8 (TaCH2), 121.6, 122.1, 128.8 (br), 129.7,
129.8, 131.4, 131.7, 136.3, 136.8, 137.2, 137.4, 138.0, 138.3,
139.0, 149.0, 155.5 (aromatic C’s). IR (Nujol, cm-1): 3264 (w,
νNH). Anal. Calcd for C58H73N2Ta: C, 71.14; H, 7.51; N, 2.86.
Found: C, 71.09; H, 7.16; N, 3.10.
[(2,6-Me s 2 C 6 H 3 N d)(2,6-Me s 2 C 6 H 3 N H )T a Me ][Me B -
(C6F 5)3] (9). Compound 4 (0.215 g, 0.248 mmol) and B(C6F5)3
(0.140 g, 0.274 mmol) were dissolved in toluene (40 mL) to

Tantalum Alkyl and Silyl Complexes
Organometallics, Vol. 21, No. 25, 2002 5561
21.6, 22.8 (Mes-Me’s), 34.9 (CH2), 118.8 (CH), 119.0, 121.6
(aromatic C’s), 122.4, 124.3, 127.3, 128.4 (CH’s), 128.9 (aro-
matic C), 129.0, 129.3, 129.4, 129.4 (CH’s), 129.7 (aromatic C),
129.9, 130.3 (CH’s), 130.5 (aromatic C), 130.7 (CH), 133.1,
135.0, 135.3, 136.2, 136.9, 137.2, 137.3, 137.4, 138.2, 138.5,
combined extracts were concentrated to ca. 10 mL and cooled
to -35 °C to afford two crops of yellow crystals of 13a (0.309
1
g, 42%). H NMR: δ 0.10 (s, 27 H, SiMe3), 2.13 (s, 6 H, Mes-
Me), 2.17 (s, 3 H, Mes-Me), 2.18 (s, 3 H, Mes-Me), 2.27 (br s,
3 H, Mes-Me), 2.29 (s, 3 H, Mes-Me), 2.33 (br s, 3 H, Mes-Me),
2.34 (s, 3 H, Mes-Me), 2.36 (br s, 6 H, Mes-Me), 2.39 (s, 6 H,
Mes-Me), 6.65 (dd, 2 H, J ) 3 Hz, J ) 7 Hz, Ar*-H), 6.68 (s, 1
H, Ar*-H), 6.74-6.85 (complex, 8 H, Ar*-H), 6.89 (s, 1 H, Ar*-
H), 6.92 (br s, 1 H, Ar*-H), 6.96 (s, 1 H, Ar*-H), 8.23 (br s, 1
H, NH). 13C{1H} NMR: δ 5.3 (SiMe3), 21.6, 21.8, 22.0 (br), 22.1,
22.1, 22.3 (br), 23.1, 23.3 (br), 23.8 (Mes-Me’s), 123.8, 125.0
(CH’s), 126.0, 128.9 (aromatic C’s), 129.2 (CH), 129.4 (br, CH’s),
129.7 (aromatic C), 129.9, 130.3, 130.5 (br), 131.3 (br), 131.8,
131.9, 132.0, 132.9 (CH’s), 133.0, 135.9, 136.5 (br), 136.8, 137.0,
137.6, 137.7 (br), 138.5, 138.5, 139.3 (br), 139.5, 139.6 (br),
140.0, 142.8 (br), 150.0, 156.0 (aromatic C’s). 29Si NMR: δ
-5.68 (Si(SiMe3)3), -37.09 (Si(SiMe3)3). IR (Nujol, cm-1): 3254
(w, νNH). Anal. Calcd for C57H78ClN2Si4Ta‚C5H12: C, 62.46; H,
7.61; N, 2.35. Found: C, 62.83; H, 7.34; N, 2.17.
138.8, 138.9, 151.6, 152.7 (aromatic C’s). 19F{1H} NMR:
-75.5. IR (KBr, cm-1): 3308 (m, νNH). Anal. Calcd for
49H52F3N2O3STa: C, 59.63; H, 5.31; N, 2.84; S, 3.25. Found:
δ
C
C, 59.73; H, 5.08; N, 2.81, S, 3.49.
(2,6-Mes2C6H3Nd)[2-(η5-2,4,6-Me3C6H2D)-6-MesC6H3NH]-
Ta (OSO2CF 3) (11-d ). An essentially identical procedure to
that used to prepare 11 (substituting D2 for H2, heating for 3
days) yielded red-orange crystals of compound 11-d (0.187 g,
79%). Selected data: 1H NMR: δ 4.52 (br s, 1 H, CHD). 2H
1
NMR: δ 3.48 (br s, CHD). 13C{1H} NMR: δ 34.5 (1:1:1 t, J CD
) 20 Hz, CHD). Anal. Calcd for C49H51DF3N2O3STa: C, 59.57;
H, 5.20; N, 2.84; S, 3.25. Found: C, 59.67; H, 5.41; N, 2.87; S,
3.40.
( 2 , 6 -M e s 2 C 6 H 3 N d) ( 2 , 6 -M e s 2 C 6 H 3 N H ) T a ( H e x ) -
(OSO2CF 3) (12). 1-Hexene (0.32 mL, 2.6 mmol) was added
via syringe to a 50 mL reaction vessel containing compound
10 (0.254 g, 0.254 mmol) dissolved in bromobenzene (10 mL).
The reaction mixture was degassed via three freeze-pump-
thaw cycles, sealed under H2 (1 atm), and then heated to 95
°C. After 2 days, the reaction mixture was cooled to room
temperature and diluted with toluene (10 mL). The solvent
and excess 1-hexene were removed in vacuo to leave behind a
yellow-brown crystalline solid. The solid was extracted into
toluene (2 × 10 mL), and the combined extracts were concen-
trated to ca. 2 mL and cooled to -35 °C to afford two crops of
yellow crystals of compound 12 (0.179 g, 66%). Analytically
pure material was obtained by recrystallization from pentane
at -35 °C. 1H NMR: δ 0.58 (td, 1 H, J ) 13 Hz, J ) 3 Hz,
TaCH2), 0.91 (t, 3 H, J ) 7 Hz, Ta(CH2)5CH3), 1.00 (td, 1 H, J
) 13 Hz, J ) 4 Hz, TaCH2), 1.01 (m, 4 H, TaCH2(CH2)3CH2-
CH3), 1.15 (m, 2 H, TaCH2(CH2)3CH2CH3), 1.28 (sextet, 2 H,
J ) 7 Hz, Ta(CH2)4CH2CH3), 1.95 (s, 6 H, Mes-Me), 2.12 (s, 6
H, Mes-Me), 2.27 (s, 6 H, Mes-Me), 2.29 (s, 6 H, Mes-Me), 1.4-
2.7 (br s, 12 H, Mes-Me), 6.68 (br s, 2 H, Ar*-H), 6.74 (d, 1 H,
J ) 7 Hz, Ar*-H), 6.79 (m, 2 H, Ar*-H), 6.83 (br s, 2 H, Ar*-
H), 6.85 (s, 1 H, Ar*-H), 6.87 (br m, 3 H, Ar*-H), 6.93 (d, 1 H,
J ) 7 Hz, Ar*-H), 6.96 (br s, 2 H, Ar*-H), 8.43 (br s, 1 H, NH).
13C{1H} NMR: δ 14.7 (Ta(CH2)5CH3), 20.6, 21.2, 21.5, 21.7
(Mes-Me’s), 23.6 (Ta(CH2)4CH2CH3), 28.1 (TaCH2(CH2)3CH2-
CH3), 32.4 (TaCH2(CH2)3CH2CH3), 36.0 (TaCH2(CH2)3CH2-
CH3), 66.9 (TaCH2), 119.5, 122.1 (aromatic C’s), 122.8, 125.3
(CH’s), 128.9 (aromatic C), 129.0, 129.3, 130.6 (CH’s), 134.9,
136.3, 137.4, 138.5, 139.4, 148.0, 152.8 (aromatic C’s). 19F{1H}
NMR: δ -75.6. IR (KBr, cm-1): 3327 (m, νNH). Anal. Calcd
for C55H64F3N2O3STa: C, 61.67; H, 6.02; N, 2.62; S, 2.99.
Found: C, 61.39; H, 5.97; N, 2.50; S, 3.19.
Hyd r ogen a tion Ca ta lysis w ith Ca ta lyst P r ecu r sor 10.
In a typical reaction, compound 10 (∼2 mg, ∼2 µmol) and Cp2-
Fe (∼1 mg, ∼5.4 µmol) were dissolved in ∼0.7 mL of bro-
mobenzene-d5, and the resulting solution was transferred to
an NMR tube fitted with a J . Young Teflon stopper. The olefin
or silane substrate (∼14 µmol) was added via syringe, and the
reaction mixture was degassed via three freeze-pump-thaw
cycles. The NMR tube was then closed under H2 (or D2, 1 atm)
and heated to 95 °C for several days. Reaction progress and
yields were determined by 1H NMR spectroscopy relative to
the Cp2Fe internal standard, and product identities were
confirmed by GC/MS analysis and spectroscopic comparison
to authentic samples.
(2,6-Mes2C6H 3Nd)(2,6-Mes2C6H 3NH )Ta [Si(SiMe3)3]Cl
(13a ). Compound 7 (0.595 g, 0.655 mmol) and (THF)3LiSi-
(SiMe3)3 (0.372 g, 0.790 mmol) were dissolved in toluene (50
mL), and the resulting solution was stirred at room temper-
ature for 23 h in the dark. The solvent was removed in vacuo,
and the yellow-brown, oily solid was extracted into pentane
(3 × 20 mL) and filtered to give a yellow-brown solution. The
(2,6-Mes2C6H 3Nd)(2,6-Mes2C6H 3NH )Ta [Si(t Bu )P h 2]Cl
(13b). An essentially identical procedure to that used to
prepare 13a (substituting (THF)2LiSi(tBu)Ph2 for (THF)3LiSi-
(SiMe3)3) yielded bright, yellow-orange crystals of 13b (0.428
1
t
g, 69%). H NMR: δ 1.07 (s, 9 H, Bu), 1.90 (br s, 6 H, Mes-
Me), 1.96 (br s, 6 H, Mes-Me), 2.11 (s, 6 H, Mes-Me), 2.32 (s,
6 H, Mes-Me), 2.35 (s, 12 H, Mes-Me), 6.59 (d, 2 H, J ) 7 Hz,
Ar*-H), 6.70 (br s, 4 H, Ar*-H), 6.73 (t, 2 H, J ) 7 Hz, Ar*-H),
6.77 (s, 2 H, Ar*-H), 6.82 (s, 1 H, Ar*-H), 6.83 (s, 1 H, Ar*-H),
6.85 (s, 2 H, Ar*-H), 6.88 (s, 2 H, Ar*-H), 6.99 (d, 2 H, J ) 7
Hz, Ar*-H), 7.02 (m, 1 H, Ar*-H), 7.08 (t, 1 H, J ) 7.5 Hz,
Ar*-H), 7.11 (d, 1 H, J ) 7 Hz, Ar*-H), 7.17 (d, 1 H, J ) 3 Hz,
Ar*-H), 7.31 (br s, 2 H, Ar*-H), 8.90 (s, 1 H, NH). 13C{1H}
NMR: δ 21.1, 21.4, 22.0, 22.1, 22.2 (Mes-Me’s), 23.6 (C(CH3)3),
30.9 (C(CH3)3), 123.9, 125.0 (CH’s), 126.0 (aromatic C), 127.9,
128.0, 128.1, 128.8 (CH’s), 128.9, 129.7 (aromatic C’s), 129.9
(CH), 130.1 (aromatic C), 130.7, 131.3, 131.8 (CH’s), 131.9,
136.2, 136.4, 136.5, 137.3 (aromatic C’s), 137.6, 138.4 (CH’s),
139.1, 144.2, 149.7, 155.4 (aromatic C’s). 29Si NMR: δ 68.86
(Si(tBu)Ph2). IR (KBr, cm-1): 3270 (w, νNH). Anal. Calcd for
(C64H70ClN2SiTa)‚(C5H12
) : C, 69.59; H, 6.67; N, 2.44.
Found: C, 69.68; H, 6.69; N, 2.43.
0.5
(2,6-Mes2C6H3Nd)(2,6-Mes2C6H3NH)Ta(SiHMes2)Cl (13c).
An essentially identical procedure to that used to prepare 13a
(substituting (THF)2LiSiHMes2 for (THF)3LiSi(SiMe3)3) yielded
1
yellow crystals of 13c (0.461 g, 80%). H NMR: δ 1.83 (vbr s,
6 H, Mes-Me), 1.95 (vbr s, 6 H, Mes-Me), 2.08 (s, 3 H, Mes-
Me), 2.12 (s, 3 H, Mes-Me), 2.14 (br s, 6 H, Mes-Me), 2.22 (s,
6 H, Mes-Me), 2.36 (s, 6 H, Mes-Me), 1.5-2.5 (vbr s, 18 H,
Mes-Me), 5.32 (s, 1 H, J SiH ) 183 Hz, SiH), 6.59 (s, 2 H, Ar*-
H), 6.60 (s, 2 H, Ar*-H), 6.64 (vbr s, 4 H, Ar*-H), 6.76 (br s, 2
H, Ar*-H), 6.77 (d, 1 H, J ) 8 Hz, Mes-H), 6.79 (d, 1 H, J ) 5
Hz, Mes-H), 6.82 (d, 2 H, J ) 8 Hz, Mes-H), 6.84 (br s, 2 H,
Ar*-H), 6.94 (d, 2 H, J ) 8 Hz, Ar*-H), 9.20 (s, 1 H, NH). 13C-
{1H} NMR: δ 20.7, 20.8 (br), 21.2, 21.3, 21.4, 21.5, 21.9 (Mes-
Me’s), 24.8 (br, Mes-Me), 123.9, 124.4, 128.6, 128.7 (CH’s),
128.9 (aromatic C), 129.1, 129.3, 129.7, 131.1 (CH’s), 136.2,
136.5, 137.6, 137.8 (br) 138.2, 138.5, 139.0, 143.6, 145.8 (br
m), 145.8, 151.1, 154.6 (aromatic C’s). 29Si NMR: δ 6.92 (d,
J SiH ) 181 Hz, SiHMes2). IR (KBr, cm-1): 3274 (m, νNH), 2120
(w, νSiH). Anal. Calcd for C66H74ClN2SiTa: C, 69.55; H, 6.54;
N, 2.46. Found: C, 69.26; H, 6.84; N, 2.21.
X-r a y Str u ctu r e Deter m in a tion s. X-ray diffraction mea-
surements were made on a Siemens SMART diffractometer
with a CCD area detector, using graphite-monochromated Mo
KR radiation. The crystal was mounted on a glass fiber using
Paratone N hydrocarbon oil. A hemisphere of data was
collected using ω scans of 0.3°. Cell constants and an orienta-
tion matrix for data collection were obtained from a least-
squares refinement using the measured positions of reflections
in the range 4° (or 3.5°) < 2θ < 45°. The frame data were
integrated using the program SAINT (SAX Area-Detector

5562 Organometallics, Vol. 21, No. 25, 2002
Ta ble 5. Cr ysta llogr a p h ic Da ta for Com p ou n d s 4, 8, a n d 11
Gavenonis and Tilley
4
8
11
empirical formula
fw
C
50H57N2Ta
C58H73N2Ta
979.16
C49H50F3N2O3STa
984.95
866.96
cryst color, habit
cryst size (mm)
cryst syst
space group
a (Å)
yellow plate
yellow plate
red-orange tablet
0.32 × 0.19 × 0.05
monoclinic
P21/n (No. 14)
10.9651(4)
0.30 × 0.13 × 0.05
0.19 × 0.16 × 0.07
monoclinic
monoclinic
P21/c (No. 14)
18.2430(7)
P21 (No. 4)
10.9435(9)
b (Å)
11.1702(5)
43.319(4)
15.8166(5)
c (Å)
21.552(1)
104.766(2)
4246.7(3)
12.4363(10)
115.977(2)
5299.9(8)
27.5663(9)
95.869(1)
4755.8(2)
â (deg)
V (Å3)
no. of orientation reflns
(2θ range)
Z value
2701 (4.0-45.0°)
5338 (3.5-45.0°)
7126 (3.5-45.0°)
4
4
4
D
calc (g/cm3)
1.356
1.272
1.376
F000
1776.00
26.19
SMART
Mo KR
2116
2.11
SMART
Mo KR
1992.00
24.04
SMART
Mo KR
µ(Mo KR) (cm-1
diffractometer
radiation
)
(λ ) 0.71069 Å)
graphite monochromated
-112
ω (0.3° per frame)
10.0 s per frame
51.3
total: 19234
unique: 8557
0.062
(λ ) 0.71069 Å)
graphite monochromated
-114(2)
ω (0.3° per frame)
10.0 s per frame
46.5
total: 24124
unique: 16564
0.0457
(λ ) 0.71069 Å)
graphite monochromated
-115.0
ω (0.3° per frame)
10.0 s per frame
51.3
total: 21548
unique: 8872
0.058
temperature (°C)
scan type
scan rate
2θmax (deg)
no. of reflns measd
Rint
transmn factors
Tmax ) 0.98
Tmin ) 0.69
direct methods (SIR92)
Tmax ) 0.83
Tmin ) 0.61
direct methods
(SHELXS-86)
13184
Tmax ) 0.89
Tmin ) 0.46
direct methods (SIR92)
structure solution
no. of observations
3140
5137
(I > 3.00σ(I))
481
(I > 2.00σ(I))
1117
(I > 3.00σ(I))
557
no. of variables
reflns/param ratio
6.53
11.80
9.22
residuals: R; Rw; Rall
goodness of fit
0.027; 0.024; 0.101
0.65
0.0506; 0.0791; 0.0793
0.942
0.034; 0.036; 0.071
1.05
max. shift/error in final cycle
max. and min. peaks in final
diff map (e-/Å3)
0.00
0.83; -0.51
0.018
0.662; -0.542
0.01
0.64; -1.58
Integration Program; V4.024; Siemens Industrial Automation,
Inc.: Madison, WI, 1995). An empirical absorption correction
based on measurements of multiply redundant data was
performed using the programs XPREP (part of the SHELXTL
Crystal Structure Determination Package; Siemens Industrial
Automation, Inc.: Madison, WI, 1995) or SADABS. Equivalent
reflections were merged. The data were corrected for Lorentz
and polarization effects. A secondary extinction correction was
applied if appropriate. The structures were solved using the
teXsan crystallographic software package of the Molecular
Structure Corp. (compounds 4 and 11) or the SHELXTL
Crystal Structure Determination Package (compound 8), using
direct methods, and expanded with Fourier techniques. All
non-hydrogen atoms were refined anisotropically, and the
hydrogen atoms were included in calculated positions but not
refined unless otherwise noted. The function minimized in the
full-matrix least-squares refinement was ∑w(|Fo| - |Fo|)2. The
weighting scheme was based on counting statistics and
included a p-factor to downweight the intense reflections
(compounds 4 and 11). Crystallographic data are summarized
in Table 5.
F or Com p ou n d 4. Crystals were grown by slowly cooling
a concentrated pentane solution of compound 4 (68.6 mg, 10
mL) to -35 °C. The structure was found to contain one
molecule of complex 4 per asymmetric unit. All non-hydrogen
atoms were refined anisotropically. All hydrogen atoms were
refined isotropically in geometrically calculated positions
except for H(1), the amido NH, which was located from the
difference Fourier map and its positional coordinates refined.
F or Com p ou n d 8. Crystals were grown by cooling a
concentrated pentane solution of compound 8 (104 mg, 20 mL)
to -35 °C. The structure was found to contain two molecules
of complex 8 and one molecule of pentane per asymmetric unit.
The crystal was twinned with a twin law given by [-1 0 0
0-1 0 1 0 1], and the batch scale factor refined to 27.16(2)%
twin. The enantiomorph was tested by refinement of inversion
twin coefficients. The correct enantiomorph is as given with
no evidence of inversion twinning in the refinement. All non-
hydrogen atoms in the tantalum complexes were refined
anisotropically, except for the methyl carbons attached to
C(50), which were disordered rotationally over two positions
with C(51A), C(51B), C(52A), C(52B), C(53A), and C(53B)
refined isotropically in observed positions at 50% occupancy.
The pentane appears as a normal aliphatic hydrocarbon with
C(117)-C(121) refined isotropically. All hydrogen atoms were
refined isotropically in geometrically calculated positions.
F or Com p ou n d 11. Crystals were grown by vapor diffusion
of pentane (∼3 mL) into a concentrated benzene-d6 solution
of compound 11 (30 mg, ∼0.7 mL) at room temperature
followed by cooling to -35 °C. The structure was found to
contain one molecule of complex 11 and one-half molecule of
pentane per asymmetric unit. All non-hydrogen atoms in the
tantalum complex were refined anisotropically. C(100) was
added as the centroid of the η5-cyclohexadienyl ligand, defined
as the average of the x, y, and z coordinates of carbons C(2)-
C(6). All hydrogen atoms for the tantalum complex were
refined isotropically in geometrically calculated positions
except for H(59), the amido NH, which was located from the
difference Fourier map and its positional coordinates refined.
The pentane appears as a normal aliphatic hydrocarbon with
C(51) located on an inversion center, but is disordered over
six positions with the internal carbons (C(51), C(52)) refined

Tantalum Alkyl and Silyl Complexes
Organometallics, Vol. 21, No. 25, 2002 5563
anisotropically at full occupancy, and the terminal carbons
(C(50)) refined isotropically at 50% occupancy.
well (Purdue University) for insightful discussions.
Su p p or tin g In for m a tion Ava ila ble: Complete IR data
1
and H-ROESY NMR spectra; tables of crystal, data collection,
Ack n ow led gm en t is made to the National Science
Foundation for their generous support of this work. We
thank Dr. Frederick J . Hollander and Dr. Allen G.
Oliver for assistance with the X-ray structure determi-
nations, Dr. Rudi Nunlist for assistance with the 1H-
ROESY NMR experiment, and Professor Ian P. Roth-
and refinement parameters, atomic coordinates, bond dis-
tances, bond angles, and anisotropic displacement parameters
for complexes 4, 8, and 11. This material is available free of
charge via the Internet at http://pubs.acs.org.
OM020509Y