Synthetic, structural, and redox studies of arene alkyl complexes of tantalum(III) supported by aryloxide and arenethiolate ligands

Article abstract of DOI:10.1021/om970197c

A series of η⁶-hexamethylbenzene alkyl and aryl complexes of tantalum(III) supported by aryloxide and arenethiolate ligands have been prepared, characterized, and compared to their halide analogues. Thus, (η⁶-C₆Me₆)Ta(OAr)₂Cl (1, Ar = 2,6-C₆H₃ⁱPr₂) reacts with MeMgBr at low temperature to afford (η⁶-C₆Me₆)Ta(OAr)₂Me (3). Low-temperature alkylation of (η⁶-C₆Me₆)Ta(OAr)Cl₂ (2) with 2 equiv of RMgBr forms (η⁶-C₆Me₆)Ta(OAr)R₂ (4, R = Me; 5, R = Et) and with 2 equiv of RLi affords (η⁶-C₆Me₆)Ta(OAr)R₂ (6, R = CH₂SiMe₃; 7, R = Ph). Complexes 3-7 are more stable than their halide precursors; no products arising from α- or β-H elimination processes were identified upon thermolysis. In addition to NMR studies of these compounds, cyclic voltammetry experiments show two oxidation processes; the Ta(III) ? Ta(IV) couple is quasi-reversible, and the Ta(IV) → Ta(V) process is irreversible. Molecules of 5 exhibit a folded arene ligand with π-electron localization (diene-diyl structure) and normal ethyl ligands (no evidence for agostic interactions). Under the appropriate conditions, (η⁶-C₆Me₆)Ta(OAr)Cl₂ (2) can be monoalkylated using 1 equiv of LiCH₂SiMe₃ or LiPh to afford (η⁶-C₆Me₆)Ta(OAr)(CH₂SiMe ₃)Cl (8) and (η⁶-C₆Me₆)Ta(OAr)(Ph)Cl (9). However, attempts to monoalkylate (η⁶-C₆Me₆)Ta(OAr)Cl₂ with 1 equiv of either MeMgBr or EtMgBr provide the double-exchange products (η⁶-C₆Me₆)Ta(OAr)(Me)Br (10) and (η⁶-C₆-Me₆)Ta(OAr)(Et)Br (11), respectively. The metathesis product (η⁶-C₆Me₆)Ta(OAr)(Et)Cl (12) is isolated in good yield upon attempts to alkylate (η⁶-C₆Me₆)Ta(OAr)(CH₂SiMe ₃)Cl (8) with ZnEt₂. However, (η⁶-C₆Me₆)Ta(OAr)(CH₂SiMe ₃)Cl (8) reacts with PhLi to afford (η⁶-C₆-Me₆)Ta(OAr)(CH₂SiMe ₃)Ph (13). The halide alkyl complexes (η⁶-C₆Me₆)Ta(OAr)(Et)Br (11) and (η⁶-C₆Me₆)Ta(OAr)(CH₂SiMe ₃)Cl (8) react with LiBEt₃H to provide the hydrido complexes (η⁶-C₆Me₆)Ta(OAr)(Et)H (14) and (η⁶-C₆Me₆)Ta(OAr)(CH₂SiMe ₃)H (15), respectively. The arenethiolate complexes (η⁶-C₆Me₆)Ta(OAr)(SAr′)Cl (16) (Ar′ = 2,4,6-C₆H₂ⁱPr₃) and (η⁶-C₆-Me₆)Ta(OAr)(S(mes))Cl (17) (mes = 2,4,6-C₆H₂Me₃) are formed upon reacting (η⁶-C₆Me₆)-Ta(OAr)Cl₂ (2) with the appropriate lithium arenethiolate reagent, and the characterization of these species is discussed.

Full text of DOI:10.1021/om970197c

Organometallics 1997, 16, 3421-3430
3421
Syn th etic, Str u ctu r a l, a n d Red ox Stu d ies of Ar en e Alk yl
Com p lexes of Ta n ta lu m (III) Su p p or ted by Ar yloxid e a n d
Ar en eth iola te Liga n d s
David S. J . Arney,† Peter A. Fox,‡ Michael A. Bruck, and David E. Wigley*
Carl S. Marvel Laboratories of Chemistry, Department of Chemistry, University of Arizona,
Tucson, Arizona 85721-0041
Received March 10, 1997X
A series of η6-hexamethylbenzene alkyl and aryl complexes of tantalum(III) supported by
aryloxide and arenethiolate ligands have been prepared, characterized, and compared to
their halide analogues. Thus, (η6-C6Me6)Ta(OAr)2Cl (1, Ar ) 2,6-C6H3 Pr2) reacts with
i
MeMgBr at low temperature to afford (η6-C6Me6)Ta(OAr)2Me (3). Low-temperature alkyl-
ation of (η6-C6Me6)Ta(OAr)Cl2 (2) with 2 equiv of RMgBr forms (η6-C6Me6)Ta(OAr)R2 (4, R
) Me; 5, R ) Et) and with 2 equiv of RLi affords (η6-C6Me6)Ta(OAr)R2 (6, R ) CH2SiMe3; 7,
R ) Ph). Complexes 3-7 are more stable than their halide precursors; no products arising
from R- or â-H elimination processes were identified upon thermolysis. In addition to NMR
studies of these compounds, cyclic voltammetry experiments show two oxidation processes;
the Ta(III) a Ta(IV) couple is quasi-reversible, and the Ta(IV) f Ta(V) process is irreversible.
Molecules of 5 exhibit a folded arene ligand with π-electron localization (diene-diyl structure)
and normal ethyl ligands (no evidence for agostic interactions). Under the appropriate
conditions, (η6-C6Me6)Ta(OAr)Cl2 (2) can be monoalkylated using 1 equiv of LiCH2SiMe3 or
LiPh to afford (η6-C6Me6)Ta(OAr)(CH2SiMe3)Cl (8) and (η6-C6Me6)Ta(OAr)(Ph)Cl (9). How-
ever, attempts to monoalkylate (η6-C6Me6)Ta(OAr)Cl2 with 1 equiv of either MeMgBr or
EtMgBr provide the “double-exchange” products (η6-C6Me6)Ta(OAr)(Me)Br (10) and (η6-C6-
Me6)Ta(OAr)(Et)Br (11), respectively. The metathesis product (η6-C6Me6)Ta(OAr)(Et)Cl (12)
is isolated in good yield upon attempts to alkylate (η6-C6Me6)Ta(OAr)(CH2SiMe3)Cl (8) with
ZnEt2. However, (η6-C6Me6)Ta(OAr)(CH2SiMe3)Cl (8) reacts with PhLi to afford (η6-C6-
Me6)Ta(OAr)(CH2SiMe3)Ph (13). The halide alkyl complexes (η6-C6Me6)Ta(OAr)(Et)Br (11)
and (η6-C6Me6)Ta(OAr)(CH2SiMe3)Cl (8) react with LiBEt3H to provide the hydrido complexes
(η6-C6Me6)Ta(OAr)(Et)H (14) and (η6-C6Me6)Ta(OAr)(CH2SiMe3)H (15), respectively. The
arenethiolate complexes (η6-C6Me6)Ta(OAr)(SAr′)Cl (16) (Ar′ ) 2,4,6-C6H2 Pr3) and (η6-C6-
i
Me6)Ta(OAr)(S(mes))Cl (17) (mes ) 2,4,6-C6H2Me3) are formed upon reacting (η6-C6Me6)-
Ta(OAr)Cl2 (2) with the appropriate lithium arenethiolate reagent, and the characterization
of these species is discussed.
In tr od u ction
istry;13-19 or more recently, (5) by arene exchange reac-
tions.8 These synthetic strategies have permitted access
Several successful forays have been made into nio-
bium and tantalum arene chemistry in recent years as
a result of improved methods for introducing an arene
ligand to these metals.1 The initial coordination of an
η6-arene is typically accomplished by one of the following
approaches: (1) by reduction of a metal halide in the
presence of the arene,2-4 including Al/AlX3 reduction
under Fischer-Hafner conditions;5 (2) by substitution
reactions using a halide acceptor, in which reduction of
the metal does not occur;6-8 (3) by metal vapor synthesis
procedures;9-12 (4) by alkyne cyclotrimerization chem-
(6) Calderazzo, F.; Castellani, M.; Pampaloni, G.; Zanazzi, P. F. J .
Chem. Soc., Dalton Trans. 1985, 1989.
(7) Calderazzo, F.; Pampaloni, G. J . Organomet. Chem. 1992, 423,
307.
(8) Calderazzo, F.; Englert, U.; Pampaloni, G.; Rocchi, L. Angew.
Chem., Int. Ed. Engl. 1992, 31, 1235.
(9) Cloke, F. G. N.; Green, M. L. H. J . Chem. Soc., Dalton Trans.
1981, 1938.
(10) Cloke, F. G. N.; Courtney, K. A. E.; Sameh, A. A.; Swain, A. C.
Polyhedron 1989, 8, 1641.
(11) Green, M. L. H.; O’Hare, D.; Watkin, J . G. J . Chem. Soc, Chem.
Commun. 1989, 698.
(12) Green, M. L. H.; O’Hare, D.; Mountford, P.; Watkin, J . G. J .
Chem. Soc., Dalton Trans. 1991, 1705.
(13) Bruck, M. A.; Copenhaver, A. S.; Wigley, D. E. J . Am. Chem.
Soc. 1987, 109, 6525.
(14) Ballard, K. R.; Gardiner, I. M.; Wigley, D. E. J . Am. Chem. Soc.
1989, 111, 2159.
(15) Wexler, P. A.; Wigley, D. E. J . Chem. Soc., Chem. Commun.
1989, 664.
(16) Arney, D. J .; Wexler, P. A.; Wigley, D. E. Organometallics 1990,
9, 1282.
(17) Arney, D. J .; Wexler, P. A.; Wigley, D. E. Organometallics 1991,
10, 3947.
† Present address: 3M Ceramic Technology Center, St. Paul, MN,
55144-1000.
‡ Present address: Department of Chemistry, Chiang Mai Univer-
sity, Chiang Mai 50200 Thailand.
X Abstract published in Advance ACS Abstracts, J une 15, 1997.
(1) 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, pp 57-153.
(2) Calderazzo, F.; Pampaloni, G.; Rocchi, L.; Stra¨hle, J .; Wurst, K.
Angew. Chem., Int. Ed. Engl. 1991, 30, 102.
(3) Calderazzo, F.; Pampaloni, G.; Rocchi, L.; Stra¨hle, J .; Wurst, K.
J . Organomet. Chem. 1991, 413, 91.
(4) Stollmaier, F.; Thewalt, U. J . Organomet. Chem. 1981, 222, 227.
(5) Fischer, E. O.; Ro¨hrscheid, F. J . Organomet. Chem. 1966, 6, 53.
(18) Wexler, P. A.; Wigley, D. E.; Koerner, J . B.; Albright, T. A.
Organometallics 1991, 10, 2319.
(19) Smith, D. P.; Strickler, J . R.; Gray, S. D.; Bruck, M. A.; Holmes,
R. S.; Wigley, D. E. Organometallics 1992, 11, 1275.
S0276-7333(97)00197-0 CCC: $14.00 © 1997 American Chemical Society

3422 Organometallics, Vol. 16, No. 15, 1997
Arney et al.
Sch em e 1
to a range of oxidation states in niobium and tantalum
arenes, from d6 M(-I) to d1 M(IV).1
In contrast to complexes of niobium, the majority of
d2 Ta(III) arene species have been prepared by alkyne
cycloaddition methods and contain a metal supported
by aryloxide ligation.1 These complexes have proven
especially valuable since rare Ta(II) and Ta(IV) arenes
are both accessible from redox reactions of their Ta(III)
counterparts. Thus, stable Ta(II) complexes (η6-C6R6)-
i
Ta(OAr)2 (R ) Me, Et; Ar ) 2,6-C6H3 Pr2) are prepared
from the one-electron reduction of (η6-C6R6)Ta(OAr)2-
Cl,15,18 and electrochemical evidence has been presented
for the existence of labile [(η6-C6Me6)Ta(OAr)R2]+ com-
plexes,17 thereby affording the first evidence for d1 arene
species.
Arene complexes of Ta(III) are also of interest for the
unusual structural properties they may exhibit. For
example, Wolczanski and co-workers have prepared {µ-
η2(1,2):η2(4,5)-C6H6}[Ta(silox)3]2 (silox ) OSitBu3) in
which two d2 tantalum species bind opposite faces of
benzene in an η2-fashion.20 More relevant to this study
are the structural distortions that often occur when an
arene coordinates η6 to Ta(III), viz. significant ligand
folding and π-electron localization are typically ob-
served.16
described.13,14 Turquoise (η6-C6Me6)Ta(OAr)Cl2 (2) is
subsequently available in near quantitative yield from
the metathesis of (η6-C6Me6)Ta(OAr)2Cl (1) with Ta-
(OAr)2Cl3(OEt2), a reaction that is driven by the stability
of byproduct Ta(OAr)3Cl2(OEt2).16 Alkylation studies of
both 1 and 2 have been carried out. (η6-C6Me6)Ta(OAr)2-
Cl (1) can be smoothly alkylated with 1 equiv of
MeMgBr (Et2O, -60 °C) to afford (η6-C6Me6)Ta(OAr)2-
Me (3) as purple crystals in moderate yield, eq 1.
Attempts to alkylate 1 with larger alkyls (Et, CH2SiMe3,
or Ph), using either Grignards or alkyllithium reagents,
yielded only intractable oils from which no organome-
tallic products could be isolated.
Finally, the hexamethylbenzene complex (η6-C6Me6)-
i
Ta(OAr)2Cl (1, Ar ) 2,6-C6H3 Pr2) has been shown to
engage in an intramolecular C-H bond activation to
provide an unstable, transient “tuck-in” complex (η6,η1-
C6Me5CH2)Ta(OAr)2(H)Cl.14 This oxidation reaction
generates a d0 metal center that labilizes the η6 portion
of a nascent η6,η1-C6Me5CH2 ligand and results in the
isolation of a d0 η1-C6Me5CH2 complex.16 This process
is related to the intramolecular metalation in pentam-
ethylcyclopentadienyl ligands (η5-C5Me5) that provide
stable complexes containing the η5,η1-C5Me4CH2 moi-
ety.21 The prospect of preparing isolable η6,η1-C6Me5-
CH2 complexes led us to identify η6-C6Me6 Ta(III)
species containing alkyl or hydride ligands as synthetic
targets, in an effort to induce C-H addition of the η6-
C6Me6 moiety to a Ta-R or Ta-H bond. In this report,
the preparation and properties of a series of arene alkyl
complexes of Ta(III) are described in which the metal
is supported by aryloxide or arenethiolate ligands. Nio-
bium and tantalum η6-arenes have recently been re-
viewed,1,7 and a portion of these results have been
communicated.17
(η6-C6Me6)Ta(OAr)Cl2 (2) reacts rapidly with 2 equiv
of MeMgBr at low temperature to provide, after ap-
propriate workup, blue-violet crystals of (η6-C6Me6)Ta-
(OAr)Me2 (4) in ca. 75% yield, Scheme 1. The analogous
reactions using 2 equiv of EtMgBr, Me3SiCH2Li, or PhLi
afford blue-violet (η6-C6Me6)Ta(OAr)Et2 (5), dark violet
(η6-C6Me6)Ta(OAr)(CH2SiMe3)2 (6), and purple (η6-C6-
Me6)Ta(OAr)Ph2 (7) in high yield. Molecular ion peaks
were obtained for (η6-C6Me6)Ta(OAr)Me2 (4, m/ z ) 550)
and (η6-C6Me6)Ta(OAr)Et2 (5, m/ z ) 578) in low resolu-
tion CI mass spectrometry studies. In contrast to the
precursor dichloride 2 and monochloride 1, complexes
3-7 are more stable thermally (qualitatively, 7 > 3 ≈
4 > 6 > 5). For example, while (η6-C6Me6)Ta(OAr)n-
Cl3-n (n ) 1, 2) are completely decomposed in <1 min
in refluxing toluene-d8,16 (η6-C6Me6)Ta(OAr)Me2 (4) and
(η6-C6Me6)Ta(OAr)Ph2 (7) are only ca. 40-50% decom-
posed after 5 days under these conditions. The only
identifiable product formed upon thermolyzing 3-7 is
C6Me6; the metal-containing species reduce to an in-
tractable, insoluble material. Even the â-H-containing
complex (η6-C6Me6)Ta(OAr)Et2 (5) thermally decom-
poses more slowly than the chloride complexes, and the
expected elimination products (e.g., C2H6 or C2H4) were
not observed spectroscopically. Exhaustive photolysis
of these compounds achieved the same results over a
shorter period; for example, solutions of 5 and 7 were
completely decomposed over a period of 2-3 h, and all
the Ta(III) arene complexes were decomposed in <48 h
under photolytic conditions (1H NMR, C6D6).
Resu lts
Alk yla tion Stu d ies of Ta n ta lu m (III) Ar en e Sp e-
cies: Dia lk yl a n d Alk yl Ha lid e Com p lexes. Blue
i
crystals of (η6-C6Me6)Ta(OAr)2Cl (1, Ar ) 2,6-C6H3 Pr2)
Because facile elimination or tuck-in reactions were
not observed in the dialkyl complexes,22 we sought to
prepare monoalkyl halide species from which we might
access the corresponding alkyl hydrides. However,
are prepared by reducing Ta(OAr)2Cl3(OEt2) by two
electrons in the presence of MeCtCMe as previously
(20) Neithamer, D. R.; Pa´rka´nyi, L.; Mitchell, J . F.; Wolczanski, P.
T. J . Am. Chem. Soc. 1988, 110, 4421.
(21) Rothwell, I. P. Polyhedron 1985, 4, 177.
(22) McDade, C.; Green, J . C.; Bercaw, J . E. Organometallics 1982,
1, 1629.

Arene Alkyl Complexes of Tantalum(III)
Organometallics, Vol. 16, No. 15, 1997 3423
Sch em e 2
reaction may provide some insight into the relative bond
energetics of early transition metal verses zinc alkyl
since qualitatively the bond energies follow [(TaIII-Et)
+ (Zn-CH2SiMe3)] > [(TaIII-CH2SiMe3) + (Zn-Et)].
The utility of the monohalide compounds as precur-
sors to mixed ligand dialkyl species was examined using
(η6-C6Me6)Ta(OAr)(CH2SiMe3)Cl (8). The reaction of
(η6-C6Me6)Ta(OAr)(CH2SiMe3)Cl (8) with 1 equiv of
PhLi gives (η6-C6Me6)Ta(OAr)(CH2SiMe3)Ph (13) in high
yield as a purple solid, eq 3. Compound 13 is formed
attempts to prepare pure (η6-C6Me6)Ta(OAr)(Me)Cl by
the reaction of (η6-C6Me6)Ta(OAr)Cl2 with 1 equiv of
MeMgCl or MeLi under the conditions outlined above
provides a low yield of (η6-C6Me6)Ta(OAr)Me2, along
with intractable decomposition products. However, the
mono(alkyl) complex (η6-C6Me6)Ta(OAr)(CH2SiMe3)Cl
(8) is readily prepared from reacting (η6-C6Me6)Ta-
(OAr)Cl2 (2) with 1 equiv of LiCH2SiMe3 under these
conditions (toluene/Et2O, -60 °C), Scheme 2. Our
attempts to prepare (η6-C6Me6)Ta(OAr)(Ph)Cl from (η6-
C6Me6)Ta(OAr)Cl2 and 1 equiv of LiPh under similar
conditions were also problematic. However, we have
found that this reaction proceeds smoothly using Ph-
MgCl in benzene/THF, even at room temperature, to
afford blue (η6-C6Me6)Ta(OAr)(Ph)Cl (9) in moderate to
high yields, Scheme 2. Therefore, the mixed alkyl
halide complexes such as 8 and 9 are clearly stable, but
the synthetic approaches to these species are critical to
ensure that further reaction or degradation is halted.
Quite different results were obtained when alkylating
(η6-C6Me6)Ta(OAr)Cl2 with the bromide Grignards. (η6-
C6Me6)Ta(OAr)Cl2 (2) reacts with 1 equiv of MeMgBr
or EtMgBr to provide the “double-exchange” products
(η6-C6Me6)Ta(OAr)(Me)Br (10) and (η6-C6Me6)Ta(OAr)-
(Et)Br (11), respectively, in >95% yield (1H NMR),
Scheme 2. The complexes were identified by, inter alia,
their CI mass spectra in which bromine isotopes were
obvious: (η6-C6Me6)Ta(OAr)(Me)Br (10) m/ z ) 614
(79Br), 616 (81Br); (η6-C6Me6)Ta(OAr)(Et)Br (11) m/ z )
628 (79Br), 630 (81Br). These reactions are no doubt
driven by the lattice energy of MgCl2, as the expected
product in the reaction 2 and EtMgBr, (η6-C6Me6)Ta-
(OAr)(Et)Cl (vide infra), is observed in <5% yield. This
species is presumed to be a kinetic product that reacts
further with byproduct “MgClBr” to form 11 and MgCl2.
(η6-C6Me6)Ta(OAr)(Et)Cl (12) can be prepared by an
independent and equally interesting metathesis reac-
tion. Thus, (η6-C6Me6)Ta(OAr)(CH2SiMe3)Cl reacts with
0.5 equiv of ZnEt2 (Et2O, -30 °C) in an unusual
exchange reaction to form the unexpected product (η6-
C6Me6)Ta(OAr)(Et)Cl (12) in moderate yield, along with
Zn(CH2SiMe3)2, identified by 1H NMR, eq 2. This
in near quantitative yield but isolated in ca. 70% yield
due to its high solubility. This species exhibits a
thermal stability that is intermediate between the
diphenyl derivative 7 and the bis(CH2SiMe3) complex
6.
Ta n ta lu m (III) Ar en e Alk yl Hyd r id e Com p lexes.
With the required alkyl halide species in hand, we set
out to prepare alkyl hydride complexes. (η6-C6Me6)Ta-
(OAr)EtBr (11) reacts with LiBEt3H (Et2O, -60 °C) to
provide (η6-C6Me6)Ta(OAr)Et(H) (14) as an extremely
soluble, magenta solid in high yield, eq 4. Similarly,
(η6-C6Me6)Ta(OAr)(CH2SiMe3)Cl (8) reacts smoothly
with LiBEt3H under identical conditions to afford (η6-
C6Me6)Ta(OAr)(CH2SiMe3)(H) (15) as a violet solid, eq
5. Both 14 and 15 exhibit a TaH hydride resonance at
ca. δ 5.87 (C6D6) and a ν(Ta-H) stretch between 1760
and 1780 cm-1. While 15 is stable for months at room
temperature in Et2O (and for days in THF), 14 decom-
poses slowly (to C6Me6 and unidentified products) over
a period of hours.
Thermolytic decomposition studies of 14 and 15
provided no evidence for the generation of a tuck-in
complex (see Discussion section). However, preliminary
evidence was uncovered for the occurrence of a hydride
exchange process. Thus, the reaction of (η6-C6Me6)Ta-
(OAr)(CH2SiMe3)Cl (10) with LiBEt3D (Et2O, -60 °C)
1
provides a violet solid in high yield. A H NMR study
of this complex reveals a spectrum identical to that of
(η6-C6Me6)Ta(OAr)(CH2SiMe3)(H) (15) with the hydride
resonance at δ 5.87 (C6D6) integrating to ca. 0.67
protons. Although evidence as to the nature of this
supposed exchange process has not been obtained,

3424 Organometallics, Vol. 16, No. 15, 1997
Arney et al.
Ta ble 1. Deta ils of th e X-r a y Diffr a ction Stu d y of
Ta ble 2. Selected Bon d Dista n ces (Å) a n d Bon d
(η6-C6Me6)Ta (OAr )Et2 (5)
An gles (d eg) in (η6-C6Me6)Ta (OAr )Et2 (5)a ,b
Crystal Parameters
Bond Distances
molecular formula
mol wt
F(000)
C
28H45OTa
Ta-C(1)
Ta-C(2)
Ta-C(3)
Ta-C(4)
C(1)-C(2)
C(2)-C(3)
2.302(5)
2.513(3)
2.448(4)
2.187(5)
1.457(4)
1.372(5)
Ta-C(10)
2.207(4)
1.510(7)
1.912(3)
1.363(6)
1.469(5)
578.62
1176
red
C(10)-C(11)
Ta-O(20)
cryst color
O(20)-C(21)
C(3)-C(4)
space group
unit cell volume, Å3
a, Å
b, Å
c, Å
orthorhombic, Pnma (No. 62)
2671.8(11)
18.717(4)
12.487(4)
11.431(3)
4
Bond Angles
109.38(9)
151.0(1)
103.1(1)
88.0(1)
Bz*-Ta-C(10)
Bz*-Ta-O(20)
C(10)-Ta-C(10′)
O(20)-Ta-C(10)
Ta-O(20)-C(21)
Ta-C(10)-C(11)
C(1)-Ta-O(20)
C(1)-Ta-C(10)
C(1)-Ta-C(4)
C(4)-Ta-O(20)
C(4)-Ta-C(10)
170.9(2)
86.4(1)
78.2(2)
110.9 (2)
126.3(1)
Z
D(calcd), g cm-3
cryst dimens, mm
ω width, deg
abs coeff, cm-1
data collection temp, °C
1.44
0.50 × 0.30 × 0.62
177.4(3)
113.1(3)
0.25
40.8
23 ( 1
a
Numbers in parentheses are estimated standard deviations
b
in the least significant digits. Bz* ) C6Me6 centroid.
Data Collection
diffractometer
monochromator
Enraf-Nonius CAD4
graphite crystal,
incident beam
0.710 73
Mo KR radiation, λ, Å
2θ range, deg
2-50
octants collected
scan type
+h,+k,(l
ω-2θ
scan speed, deg min-1
scan width, deg
total no. of reflns measd
corrections
1-7
2.0 + (2θKR2-2θKR1)
2811 (2456 unique)
Lorentz-polarization; ψ-scan
absorption (min 0.585,
max 0.999, avg 0.789)
Solution and Refinement
solution
direct methods
refinement
minimization function
no. of reflns used in refinement; 2005
full-matrix least-squares
∑w(|Fo| - |Fc|)2
I > 3σ(I)
no. of params refined
R (∑|Fo| - |Fc|/∑|Fo|)
Rw ([∑w(|Fo| - |Fc|)2/∑w(Fo)2]1/2
esd of obs of unit weight
convergence, largest shift
∆/σ(max), e-1/Å3
228
0.020
0.027
1.18
0.02σ
0.42 (6)
-0.58 (0)
VAX
)
∆/σ(min), e-1/Å3
computer hardware
computer software
SDP/VAX (Enraf-Nonius)
experiments to determine whether this is an exchange
process into the ring or other ancillary ligand are being
examined.
X-r a y Str u ctu r a l Stu d y of (η6-C6Me6)Ta (OAr )Et2
(5). Dark blue-violet, single crystals of (η6-C6Me6)Ta-
(OAr)Et2 (5) suitable for an X-ray structural study were
grown from pentane at -35 °C. A summary of the
crystal data and structural analysis are given in Table
1, and relevant bond distances and bond angles are
provided in Table 2. Figure 1 presents two views of (η6-
C6Me6)Ta(OAr)Et2 in which the crystallographic mirror
symmetry is evident. Altogether, 14 of the 30 non-
hydrogen atoms lie along this crystallographically im-
posed mirror plane. The plane is perpendicular to the
C6Me6 ligand and contains the OAr phenyl ring and the
Ta atom, as well as C(1), C(1a), C(4), and C(4a) of the
C6Me6 ligand.
The molecular structure of (η6-C6Me6)Ta(OAr)Et2 (5)
reveals an interesting comparison to the parent dichlo-
ride complex 2.16 As seen in Figure 1, the η6-C6Me6
ligand in (η6-C6Me6)Ta(OAr)Et2 displays the following
structural features: (1) substantial folding; the dihedral
angle between the C(1)-C(2)-C(3)-C(4) and C(1)-
C(2′)-C(3′)-C(4) planes in 5 is 27.3(0.2)°, which com-
pares with 26.8(0.3)° for (η6-C6Me6)Ta(OAr)Cl2 (2); (2)
F igu r e 1. ORTEP drawings of (η6-C6Me6)Ta(OAr)Et2 (5),
with atoms shown as 50% probability ellipsoids.
an interruption of aromaticity within the C6Me6 ring,
viz. a diene-diyl (or 1,4-diene) type π localization; and
(3) the close approach of C(1) and C(4) to the metal, viz.
2.302(5) and 2.187(5) Å, respectively, compared to an
average 2.481(4) Å for the other arene carbons. These
structural features are duplicated in all other structur-
ally characterized Ta(III) arene compounds and may be
attributed to a selective interaction between a filled
2
2
metal δ function (dx -y , where the mirror plane consti-
tutes the xz plane) with one arene π* LUMO of the E2u
set (B2 symmetry), as previously described.16,18 In
addition, the longer Ta-C(1) vs Ta-C(4) metal-carbon
distances may reflect a structural trans effect since C(1)
is trans to the strong π donor aryloxide ligand. The

Arene Alkyl Complexes of Tantalum(III)
Organometallics, Vol. 16, No. 15, 1997 3425
Sch em e 3
rotational conformation of the arene ring in 5 is such
that the ethyl Ta-CR bonds and alkoxide Ta-O-Cipso
linkage perfectly eclipse the arene carbon atoms rather
than stagger them as the Ta-ligand bonds do in (η6-
C6Me6)Ta(OAr)Cl2 (2).16 Finally, the plane of the OAr
ligand in 5 is oriented perpendicular to the arene ligand
and 90° from the analogous OAr plane in 2. The
orientation of the ethyl ligands about the metal center
can be rationalized by a space-filling model of the
molecule in which the ethyl ligands are shown to fit
nicely into a steric pocket formed by the hexamethyl-
benzene ring and the isopropyl groups on the OAr
ligand. No evidence for agostic interactions in the
structure of 5 are evident.
Ta n ta lu m (III) Ar en e Com p lexes Su p p or ted by
Ar en eth iola te Liga n d s. We recently described the η2-
t
(N,C)-pyridine complexes [η2(N,C)-2,4,6-NC5 Bu3H2]Ta-
(OAr)2X (X ) Cl,23,24 alkyl,24 or aryl25 ) and related
[η2(N,C)-quinoline]Ta(OAr)nCl3-nL (n ) 2, 3)26 as models
for the catalyst-substrate complex in hydrodenitroge-
nation (HDN) catalysis.27,28 The importance of the
reducing ability of Ta(III) in binding and disrupting the
aromaticity of the nitrogen heterocycle has been dem-
onstrated,20,29 although the nitrogen heterocycles are π
localized in a 1,3-diene fashion rather than the 1,4-diene
localization described above for the η6-C6Me6 ligands.
Because most hydrotreating catalysts are metal sulfide
based,27,28,30,31 we set out to compare these aryloxide-
supported species to more sulfur-rich Ta(III) com-
plexes.32
Reacting (η6-C6Me6)Ta(OAr)Cl2 (2) with 1 equiv of
i
LiSAr′ (Ar′ ) 2,4,6-C6H2 Pr3, Et2O, room temperature)
afforded blue (η6-C6Me6)Ta(OAr)(SAr′)Cl (16) in high
yield, Scheme 3. This compound exhibits improved
thermal stability over its aryloxide analogue (η6-C6-
Me6)Ta(OAr)2Cl (1) as well as its precursor 2. Light
blue crystals of (η6-C6Me6)Ta(OAr)(S(mes))Cl (17, mes
) 2,4,6-C6H2Me3) are also available in high yield from
2 and LiS(mes)(dme) under similar conditions, Scheme
3. The greater thermal stability of this complex as
compared to 1 or 2 is also noted.
Attempts to alkylate (η6-C6Me6)Ta(OAr)(SAr′)Cl (16)
using a variety of alkylating agents did not afford the
alkyl (η6-C6Me6)Ta(OAr)(SAr′)R species cleanly; consid-
erable decomposition of the starting complex is observed
in these reactions. Since (η6-C6Me6)Ta(OAr)Cl2 (2) is
prepared from (η6-C6Me6)Ta(OAr)2Cl (1) and Ta(OAr)2-
Cl3(OEt2),16 we attempted the reaction of (η6-C6Me6)-
F igu r e 2. Cyclic voltammograms of (a) (η6-C6Me6)Ta-
(OAr)Ph2 (7) in CH2Cl2, (b) (η6-C6Me6)Ta(OAr)Me2 (4) in
THF, and (c) (η6-C6Me6)Ta(OAr)(CH2SiMe3)Cl (8) in THF.
All solutions are 0.1 M in nBu4NPF6 and voltammograms
are taken at a Pt disk electrode (vs Ag/AgCl) at a scan rate
of 150 mV/s.
Ta(OAr)(S(mes))Cl (17) with Ta(OAr)2Cl3(OEt2) in an
attempt to prepare (η6-C6Me6)Ta(S(mes))Cl2. However,
under a variety of conditions, only starting material was
recovered from this reaction. Finally, we note that some
of the HDN substrate-catalyst models such as [η2(N,C)-
6-methylquinoline]Ta(OAr)2Cl(OEt2)26 are best prepared
by arene displacement in (η6-C6Me6)Ta(OAr)2Cl (1) with
the appropriate N-heterocycle. In preliminary studies,
however, neither 16 nor 17 have, in our hands, reacted
smoothly with 6-methylquinoline to afford the corre-
sponding [η2(N,C)-6-methylquinoline]Ta(SR)(OAr)Cl spe-
cies.
Electr och em ica l Stu d ies of Ta n ta lu m (III) Ar en e
Alk yls: Evid en ce for th e F or m a tion of Ta n ta lu m -
(IV) Ar en e Sp ecies. The stability of some of these
arene alkyl complexes has permitted their electrochemi-
cal characterization. Cyclic voltammetry (CV) experi-
ments on (η6-C6Me6)Ta(OAr)Ph2 (7) (CH2Cl2, 0.1 M in
nBu4NPF6) reveal two, one-electron oxidation processes,
Figure 2 and Table 3. A quasi-reversible oxidation (the
Ta(III) a Ta(IV) couple) occurs at E1/2 ) -0.22 V vs Ag/
AgCl (Epa - Epc ) 120 mV) while a second, irreversible
oxidation (Ta(IV) f Ta(V)) comes at Epa ) +0.53 V vs
(23) Gray, S. D.; Smith, D. P.; Bruck, M. A.; Wigley, D. E. J . Am.
Chem. Soc. 1992, 114, 5462.
(24) Gray, S. D.; Weller, K. J .; Bruck, M. A.; Briggs, P. M.; Wigley,
D. E. J . Am. Chem. Soc. 1995, 117, 10678.
(25) Weller, K. J .; Gray, S. D.; Briggs, P. M.; Wigley, D. E.
Organometallics 1995, 14, 5588.
(26) Allen, K. D.; Bruck, M. A.; Gray, S. D.; Kingsborough, R. P.;
Smith, D. P.; Weller, K. J .; Wigley, D. E. Polyhedron 1995, 14, 3315.
(27) Angelici, R. J . In Encyclopedia of Inorganic Chemistry; King,
R. B., Ed.; J ohn Wiley and Sons: New York, 1994; Vol. 3; pp 1433-
1443.
(28) Satterfield, C. N. Heterogeneous Catalysis in Industrial Practice,
2nd ed.; McGraw-Hill, Inc.: New York, 1991.
(29) Covert, K. J .; Neithamer, D. R.; Zonnevylle, M. C.; LaPointe,
R. E.; Schaller, C. P.; Wolczanski, P. T. Inorg. Chem. 1991, 30, 2494.
(30) Ho, T. C. Catal. Rev.-Sci. Eng. 1988, 30, 117.
(31) Ledoux, M. J . Catalysis; The Chemical Society: London, 1988;
Vol. 7, pp 125-148.
(32) Industrial hydrodenitrogenation catalysis is typically effected
over sulfided CoMo/Al2O3 or NiMo/Al2O3 in which the most active site
consists of crystallites of MoS2 dotted with cobalt (or nickel) atoms;
therefore, the active site is very sulfur-rich. See refs 27, 28, 30, and
31.

3426 Organometallics, Vol. 16, No. 15, 1997
Arney et al.
Ta ble 3. Red ox P oten tia ls (V vs Ag/AgCl) for
Ar en e Alk yl Com p lexes of Ta n ta lu m a
activity. Finally, we note that the cyclic voltammetry
scans shown in Figure 2 represent the best data
obtained with these compounds. Most of the complexes
reported here (including the arenethiolate species)
exhibit limited stability in polar electrochemical sol-
vents, and the irreversible processes that typically follow
the initial oxidation suggest an even greater instability
of a resulting cation.
complex
E1
E2
Epc
pa
pa
THF Solution
(η6-C6Me6)Ta(OAr)Me2 (4)
(η6-C6Me6)Ta(OAr)Et2 (5)b
(η6-C6Me6)Ta(OAr)(Ph)2 (7)
+0.07
+0.50
+0.39
-0.02
+0.12
(η6-C6Me6)Ta(OAr)(CH2SiMe3)Cl (8) +0.18
+0.64 -1.91
+0.70 -2.03
(η6-C6Me6)Ta(OAr)(Et)Br (11)
+0.22
(η6-C6Me6)Ta(OAr)(CH2SiMe3)H (15) +0.12
CH2Cl2 Solution
Discu ssion
(η6-C6Me6)Ta(OAr)(Ph)2 (7)
E1/2 ) -0.22 +0.53
a
The arene alkyl complexes reported here all contain
the η6-C6Me6 ligand and were of interest to us for,
among other things, their potential ability to tuck-in or
C-H activate a C6Me6 methyl group to afford an η6,η1-
C6Me5CH2 ligand.14 Intramolecular metalation of a
C-H bond21 in pentamethylcyclopentadienyl ligands
(η5-C5Me5) can provide isolable complexes containing the
tucked-in η5,η1-C5Me4CH2 moiety, and various mecha-
nistic pathways may lead to such compounds. For
example, C-H addition to an alkyl,33,34 hydride,35 or
benzyne36 ligand, C-H addition across a metal-carbon
double bond,22,37 or C-H oxidative addition to a d2 or
d4 metal38,39 are all known to afford tucked-in complexes.
The hexamethylbenzene ligand (η6-C6Me6) has also been
observed to tuck-in, but only by oxidative addition to a
d2 metal.14 This process renders the η6 portion of a
nascent η6,η1-C6Me5CH2 ligand labile and results in the
isolation of a d0 η1-C6Me5CH2 complex.16 Apparently,
when δ symmetry back-bonding between a metal orbital
and arene LUMO levels is lost, i.e., upon oxidizing the
metal to a neutral d0 complex, simple L f M donation
is not sufficient to maintain arene coordination in a
neutral complex. (This does not appear to be the case
when the metal center is cationic, as [(η6-C6Me6)TiCl3]+
All solutions were 0.1 M in nBu4NPF6, and all potentials are
reported in V vs Ag/AgCl. The redox potentials for (η6-C6Me6)-
Ta(OAr)Et2 (5) in THF solution are incorrectly reported in ref
17.
b
Ag/AgCl. A plot of ipa vs the square root of the sweep
rate (ν1/2) is linear (R ) 0.993) for the -0.22 V oxidation,
and ipa/ipc becomes 1.0 at scan rates >300 mV/s, so this
process is best described as quasi-reversible on the CV
time scale. Bulk electrolysis of a solution of 7 reveals
that 1.0 ( 0.1 electron is transferred in the -0.22 V
oxidation, although the resulting solution is devoid of
electrochemically active species. Dilute samples of 7
may be chemically oxidized in toluene ([Cp2Fe][BPh4],
-78 °C) to afford highly reactive solutions that exhibit
an ESR signal (X-band, <g> ) 1.933, peak-to-peak
separation ) 145 G). However, 181Ta hyperfine is not
observed (at room temperature or -196 °C) nor neces-
sarily expected in these spectra.16,18 Cyclic voltammetry
experiments on THF solutions of (η6-C6Me6)Ta(OAr)Ph2
(7) were less informative, since these solutions were less
stable than the CH2Cl2 solutions described above. An
irreversible oxidation of 7 appears at +0.12 V vs Ag/
AgCl and is followed by ill-defined, irreversible electro-
chemical processes.
is a stable species.40,41
)
Cyclic voltammetry experiments on the remaining
arene complexes, however, were conducted in THF since
their CH2Cl2 and NtCMe solutions are unstable. Com-
pounds 4 and 5 also exhibit two electrochemical oxida-
tions in THF but both are irreversible, Table 3. Con-
sistent with the ease of oxidation of (η6-
C6Me6)Ta(OAr)Me2 (4) is the very low energy ionization
band (5.91 eV) observed in its He I photoelectron
spectrum. Although (η6-C6Me6)Ta(OAr)2Cl (1) can be
reduced by one electron to afford isolable (η6-C6Me6)-
Ta(OAr)2,18 4 or 5 do not undergo electrochemical
reduction to ca. -2 V vs Ag/AgCl in THF. All of these
data support the metal center in these complexes being
more electron rich than in their chloride precursor and
suggest that their improved thermal stability may arise
through enhanced back-bonding to the arene.
Therefore, trapping a neutral η6,η1-C6Me5CH2 com-
plex will most likely require a d2 metal, a possibility
that prompted us to prepare the η6-C6Me6 complexes of
Ta(III) containing alkyl, phenyl, and hydride ligands
reported here. However, thermolytic decomposition
studies of all these compounds provided no evidence for
the generation of a tuck-in complex; only free C6Me6 and
uncharacterizable, intractable decomposition products
were obtained.
The most striking feature of these Ta(III) arene alkyl
complexes is their resistance to elimination processes.
In contrast to the precursor chloride compounds (η6-C6-
Me6)Ta(OAr)2Cl (1) and (η6-C6Me6)Ta(OAr)Cl2 (2), alkyl
and dialkyl complexes are comparatively stable ther-
mally, although they are extremely air sensitive. Also
significant is the electrochemical evidence for the for-
The alkyl halide complexes (η6-C6Me6)Ta(OAr)(CH2-
SiMe3)Cl (8) and (η6-C6Me6)Ta(OAr)(Et)Br (11) were
examined in THF solution and, like their dialkyl
analogues, exhibit two irreversible electrochemical oxi-
dations. However, unlike 4 and 5, compounds 8 and
11 now reveal an irreversible electrochemical reduction
near -2 V vs Ag/AgCl in THF, Figure 2 and Table 3.
This reduction does not constitute a product wave that
arises as a result of the oxidations, since this feature is
also observed with initial negative potential sweeps. The
alkyl hydrides were found to be very unstable in these
experiments. Thus, (η6-C6Me6)Ta(OAr)(CH2SiMe3)H
(15) reveals a single, completely irreversible oxidation
process that produces a species without electrochemical
(33) Bruno, J . W.; Smith, G. M.; Marks, T. J .; Fair, C. K.; Schultz,
A. J .; Williams, J . M. J . Am. Chem. Soc. 1986, 108, 40.
(34) Watson, P. L.; Parshall, G. W. Acc. Chem. Res. 1985, 18, 51.
(35) Cloke, F. G. N.; Green, J . C.; Green, M. L. H.; Morley, C. P. J .
Chem. Soc., Chem. Commun. 1985, 945.
(36) Schock, L. E.; Brock, C. P.; Marks, T. J . Organometallics 1987,
6, 232.
(37) Bulls, A. R.; Schaefer, W. P.; Serfas, M.; Bercaw, J . E. J . Am.
Chem. Soc. 1987, 6, 1219.
(38) Parkin, G.; Bunel, E.; Burger, B. J .; Trimmer, M. S.; Van Asselt,
A.; Bercaw, J . E. J . Mol. Catal. 1987, 41, 21.
(39) Some tuck-in processes are proposed as oxidative additions to
a d4 metal, see: Parkin, G.; Bercaw, J . E. Polyhedron 1988, 7, 2053.
(40) Solari, E.; Floriani, C.; Chiesi-Villa, A.; Guastini, C. J . Chem.
Soc., Chem. Commun. 1989, 1747.
(41) Bochmann, M.; Karger, G.; J aggar, A. J . J . Chem. Soc., Chem.
Commun. 1990, 1038.

Arene Alkyl Complexes of Tantalum(III)
Organometallics, Vol. 16, No. 15, 1997 3427
cell and conditions previously described.46 The gas-phase
mation and limited stability of a d1 arene complex, an
observation that supports their proposed participation
in the deoxygenative coupling of an acyl and a cyclo-
pentadienyl ligand as suggested by Meyer and Mes-
serle.42
sample was generated at ca. 110 °C and 2 × 10-5 Torr, and
2
the operating resolution for the argon P3/2 ionization (15.76
eV) was maintained at 0.016-0.020 eV throughout data
collection. This argon ionization was used as an internal “lock”
during the high-resolution He I signal averaging (7-12 eV
collections) to maintain drift from the absolute kinetic energy
at <0.005 eV. X-band ESR spectra were recorded on toluene
solutions or glasses using a Varian E-3 spectrometer. Mi-
croanalytical samples were handled under nitrogen and were
combusted with WO3 (Desert Analytics, Tucson, AZ).
Exp er im en ta l Section
Gen er a l Deta ils. All experiments were performed under
a nitrogen atmosphere either by standard Schlenk techniques43
or in a Vacuum Atmospheres HE-493 drybox at room temper-
ature (unless otherwise indicated). Solvents were distilled
under N2 from an appropriate drying agent44 and were
transferred to the drybox without exposure to air. NMR
solvents were passed down a short (5-6 cm) column of
activated alumina prior to use. The “cold” solvents used to
wash isolated products were cooled to ca. -35 °C before use.
Because compound colors are quite similar, yet exhibit subtle
differences, the reported colors are based upon close compari-
sons with the nominal crayon colors of the Crayola 64
Sta r tin g Ma ter ia ls. TaCl5 was obtained from Cerac and
used as received. [Ta(OAr)Cl4]2,16 LiOAr‚OEt2,47 (η6-C6Me6)-
Ta(OAr)2Cl (1),14 (η6-C6Me6)Ta(OAr)Cl2 (2),16 LiCH2SiMe3,48
2,4,6-triisopropylthiophenol (HSAr′),49 and 2,4,6-trimethylth-
iophenol (HS(mes))49 were prepared as previously described.
The lithium salts LiSAr′ and LiS(mes) were prepared by
reacting HSAr′ and HS(mes), respectively, with n-butyllithium
in pentane. The more soluble dimethoxyethane (dme) adduct
LiS(mes)(dme) was prepared by dissolving LiS(mes) in dme
and removing the excess solvent in vacuo. The reagents
MeMgBr (3 M in Et2O), EtMgBr (3 M in Et2O), LiBEt3H (1 M
in THF), PhLi (1.8 M in Et2O), PhMgCl (2 M in THF), and
n-butyllithium (1.6 M in hexanes) were obtained from Aldrich
and were used as received. ZnMe2 (Alfa), ZnEt2 (Aldrich), and
AlMe3 (Aldrich) were obtained from commercial sources and
prepared as either 1.0 or 2.0 M solutions in heptane.
Com p ou n d P r ep a r a tion s. (η6-C6Me6)Ta (OAr )2Me (3).
(η6-C6Me6)Ta(OAr)2Cl (0.30 g, 0.41 mmol) was dissolved in 30
mL of Et2O and cooled to -60 °C (CO2/isopropyl alcohol bath).
This navy blue solution was rapidly stirred while 0.13 mL (0.39
mmol) of a MeMgBr solution (3 M in Et2O) was added dropwise
over 15 min. The solution color immediately changed to dark
purple with the precipitation of a white solid. The reaction
was allowed to warm to room temperature over a period of 4
h, over which it became darker purple in color. After the
reaction mixture was stirred for an additional 12 h, the solvent
was removed in vacuo yielding a purple cake. The product was
extracted from the cake with ca. 50 mL of pentane, the extract
was filtered through Celite, and the solvent was removed in
vacuo yielding a purple, oily solid. This solid was dissolved
in minimal pentane and cooled to -35 °C for 48 h to provide
0.19 g (0.26 mmol, 64%) of dark purple crystals, which were
filtered off and dried in vacuo. 1H NMR (C6D6): δ 7.09-6.91
(A2B m, 6 H, Haryl), 3.16 (sept, 4 H, CHMe2), 1.87 (s, 18 H,
C6Me6), 1.19, 1.15 (d, 12 H each, J HH′ ) 6.9 Hz, CHMe2), 0.22
(s, 3 H, TaCH3). The solution was cooled to 185 K in toluene-
d8 with no observed change in its spectrum other than
broadening due to increased solvent viscosity. 13C NMR
(C6D6): δ 156.9 (Cipso), 136.7 (Co), 123.6 (Cm), 121.5 (Cp), 113.5
(C6Me6), 26.1 (CHMe2), 25.0, 24.5 (CHMe2), 24.00 (TaCH3), 15.9
(C6Me6). Anal. Calcd for C37H55O2Ta: C, 62.35; H, 7.72.
Found: C, 62.07; H, 8.00.
i
collection. In all preparations, Ar ) 2,6-C6H3 Pr2, Ar′ ) 2,4,6-
i
C6H2 Pr3, and mes ) 2,4,6-C6H2Me3.
P h ysica l Mea su r em en ts. 1H and 13C NMR spectra were
recorded at probe temperature (unless otherwise specified) on
a Bruker AM-250, Varian Gemini 200, or Varian Unity 300
spectrometer in C6D6 or toluene-d8 solvent. Chemical shifts
are referenced to protio impurities (δ 7.15, C6D6; 2.09, toluene-
d8) or solvent 13C resonances (δ 128.0, C6D6; 20.4, toluene-d8)
and are reported downfield of SiMe4. Routine coupling con-
stants are not reported. NMR assignments were assisted by
COSY, APT, or gated 13C{1H}-decoupled spectra. Routine
coupling constants are not reported. Electron ionization mass
spectra (70 eV) were recorded to m/ z ) 999 on a Hewlett-
Packard 5970 mass selective detector and RTE-6/VM data
system. For GC-mass spectra, the sample was introduced into
the mass spectrometer by a Hewlett-Packard Model 5890 gas
chromatograph equipped with an HP-5 column. Cyclic volta-
mmetry experiments were performed in a drybox under a
nitrogen atmosphere using a BioAnalytical Systems CV-27
voltammograph and recorded on a Houston Instruments Model
100 X-Y recorder. Measurements were taken on a Pt disk
electrode in CH2Cl2 or THF solutions containing 0.1 M nBu4-
NPF6 as supporting electrolyte in
a single-chamber cell.
Voltammograms were recorded at room temperature and at a
sweep rate of 150 mV/s. E1/2 values (taken as (Ep,a + Ep,c)/2)
are referenced to Ag/AgCl and are uncorrected for junction
potentials. Values of n, where n is the number of equivalents
of electrons transferred in an exhaustive electrolysis at a
constant potential, were determined by measuring the total
area under the current vs time curves for the complete
reaction. Redox reactions were considered one-electron pro-
cesses if n ) 1.0 ( 0.1. The n-value determinations were
obtained in a three-chamber cell configuration (with solution
contact through fritted disks) to separate reference and
auxiliary electrodes from the Pt flag working electrode. Re-
versibility criteria have been stated.45 Chemical ionization
mass spectra were recorded to m/ z ) 999 on a Hewlett-
Packard 5988A in both positive ion (isobutane reagent) and
negative ion (methane reagent) modes. The He I valence PES
spectrum of 4 was measured using a McPherson ESCA 36
spectrometer modified with a temperature-controlled sample
(η6-C6Me6)Ta (OAr )Me2 (4). (η6-C6Me6)Ta(OAr)Cl2 (1.00 g,
1.69 mmol) was dissolved in 30 mL of toluene and cooled to
-60 °C (CO2/isopropyl alcohol bath). This turquoise solution
was stirred rapidly while a MeMgBr solution (1.07 mL, 3.21
mmol, 3 M in Et2O) was added dropwise over 15 min. The
reaction color quickly changed to blue violet with the formation
of a white precipitate. This solution was slowly warmed to
room temperature (over 4 h), during which time it became a
darker blue violet. After an additional 12 h, the solvent was
removed in vacuo yielding a blue violet cake. The product was
extracted from the cake with pentane (ca. 50 mL), the extract
(42) A transient Ta(IV) arene complex, (arene)Ta(O)Cl2, is a possible
intermediate in this reaction, see: Meyer, T. Y.; Messerle, L. J . Am.
Chem. Soc. 1990, 112, 4564.
(43) Shriver, D. F.; Drezdzon, M. A. The Manipulation of Air-
Sensitive Compounds, 2nd ed; J ohn Wiley and Sons: New York, 1986.
(44) Perrin, D. D.; Armarego, W. L. F. Purification of Laboratory
Chemicals, 3rd ed; Pergamon Press: Oxford, 1988.
(46) Calabro, D. C.; Hubbard, J . L.; Blevens, C. H., II; Campbell, A.
C.; Lichtenberger, D. L. J . Am. Chem. Soc. 1981, 103, 6839.
(47) Chao, Y.-W.; Wexler, P. A.; Wigley, D. E. Inorg. Chem. 1989,
28, 3860.
(48) Schrock, R. R.; Fellmann, J . D. J . Am. Chem. Soc. 1979, 100,
3359.
(45) Bard, A. J .; Faulkner, L. R. Electrochemical Methods; J ohn
Wiley & Sons, Inc.: New York, (1980).
(49) Blower, P. J .; Dilworth, J . R.; Hutchinson, J . P.; Zubieta, J . A.
J . Chem. Soc., Dalton Trans. 1985, 1533.

3428 Organometallics, Vol. 16, No. 15, 1997
Arney et al.
was filtered through Celite, and the solvent was removed in
vacuo yielding an oily, blue violet solid. This solid was
dissolved in minimal pentane and cooled to -35 °C for 48 h to
afford 0.74 g (1.34 mmol, 79%) of dark blue violet crystals,
which were filtered off and dried in vacuo. 1H NMR (C6D6):
δ 7.04-6.91 (A2B m, 3 H, Haryl), 3.33 (sept, 2 H, CHMe2), 1.69
(s, 18 H, C6Me6), 1.24 (d, 12 H, CHMe2), 0.82 (s, 6 H, TaCH3).
13C NMR (C6D6): δ 157.1 (Cipso), 136.2 (Co), 123.4 (Cm), 122.5
(Cp), 107.2 (C6Me6), 27.2 (CHMe2), 23.7 (CHMe2), 22.7 (TaCH3),
15.7 (C6Me6). MS: [CI-], m/ z 550, [(η6-C6Me6)Ta(OAr)-
(CH3)2]-; [CI+], m/ z 535, [(η6-C6Me6)Ta(OAr)(CH3)]+ ([M -
CH3]+). Anal. Calcd for C26H41OTa: C, 56.72; H, 7.51.
Found: C, 56.74; H, 7.72.
0.16 g (1.68 mmol) of LiCH2SiMe3 in ca. 10 mL of Et2O was
carried out at -60 °C in the manner described above for
preparing (η6-C6Me6)Ta(OAr)Me2 (4). The resulting navy blue
reaction solution was filtered through Celite, and the solvent
was removed in vacuo yielding a dark blue oily crust. Tritu-
ration with cold pentane yielded 0.50 g (0.78 mmol, 92%) of
product as a dark blue powder sufficiently pure for further
reactions. Analytically pure samples were obtained by recrys-
tallization at -35 °C from concentrated solutions of Et2O/
pentane (ca. 50:50 v/v). 1H NMR (C6D6): δ 6.98-6.83 (A2B
m, 3 H, Haryl), 3.28 (sept, 2 H, CHMe2), 1.75 (s, 18 H, C6Me6),
1.29 and 1.27 (overlapping d, 12 H total, CHMe2), 1.12 (d, 1
H, J HH′ ) 12 Hz, CHH′SiMe3), 0.51 (s, 9 H, CHH′SiMe3), 0.32
(d, 1 H, J HH′ ) 12 Hz, CHH′SiMe3). 13C NMR (C6D6): δ 156.4
(Cipso), 136.2 (Co), 123.6 (Cm), 123.5 (Cp), 114.7 (C6Me6), 29.5
(CH2SiMe3), 26.9 (CHMe2), 24.4 and 23.9 (CHMe2), 16.0
(C6Me6), 4.5 (CH2SiMe3). Anal. Calcd for C28H46OClSiTa: C,
52.29; H, 7.21. Found: C, 52.52; H, 7.52.
(η6-C6Me6)Ta (OAr )(P h )Cl (9). To a solution of 0.462 g
(0.782 mmol) of (η6-C6Me6)Ta(OAr)Cl2 (2) in 20 mL of benzene
was added PhMgCl (0.400 mL, 2 M in THF, 0.800 mmol) at
room temperature. This mixture was stirred for 20 h and
filtered through Celite, and the solvent was removed from the
filtrate in vacuo yielding a blue oil. The product was extracted
from this oil with pentane (ca. 20 mL), the extract was filtered
through Celite, and the filtrate was concentrated to ca. 10 mL
in vacuo and cooled to -35 °C. This procedure afforded 0.38
g (0.60 mmol, 77%) of (η6-C6Me6)Ta(OAr)(Ph)Cl as a blue,
crystalline solid that was filtered off and dried in vacuo.
Analytically pure samples were obtained by recrystallization
from pentane at -35 °C. 1H NMR (C6D6): δ 7.86 (d, 2 H, Ho,
C6H5), 7.31 (t, 2 H, Hm, C6H5), 7.13 (t, 1 H, Hp, C6H5), 7.00-
6.80 (A2B m, 3 H, Haryl, OAr), 3.35 (sept, 2 H, CHMe2), 1.72 (s,
18 H, C6Me6), 1.23, 1.19 (d, 6 H each, CHMe2). 13C NMR
(C6D6): δ 174.82 (Cipso, C6H5), 157.15 (Cipso, OAr), 141.63 (Co,
C6H5), 136.74 (Co, OAr), 126.85 (Cm, C6H5), 126.07 (Cp, Ph),
123.65 (Cm, OAr), 123.52 (Cp, OAr), 114.93 (C6Me6), 27.39
(CHMe2), 24.46, 24.11 (CHMe2), 16.67 (C6Me6). Anal. Calcd
for C30H40OClTa: C, 56.92; H, 6.36. Found: C, 55.91; H, 6.57.
(η6-C6Me6)Ta (OAr )(Me)Br (10). The reaction of 0.50 g
(0.85 mmol) of (η6-C6Me6)Ta(OAr)Cl2 in 30 mL of toluene with
MeMgBr (0.25 mL, 3 M in Et2O, 0.76 mmol) was carried out
at -60 °C in the manner described above for preparing (η6-
C6Me6)Ta(OAr)Me2 (4). Workup in an analogous manner
afforded 0.40 g (0.65 mmol, 77%) of product as dark purple,
analytically pure crystals. 1H NMR (C6D6): δ 6.97-6.88 (A2B
m, 3 H, Haryl), 3.34 (sept, 2 H, CHMe2), 1.78 (s, 18 H, C6Me6),
1.28, 1.20 (d, 6 H each, J HH′ ) 6.8 Hz, CHMe2), 1.23 (s, 3 H,
CH3). 13C NMR (C6D6): δ 157.15 (Cipso), 136.18 (Co), 123.36
(Cm), 122.47 (Cp), 107.24 (C6Me6), 27.22 (CHMe2), 23.73
(CHMe2 coincident), 22.72 (CH3), 15.69 (C6Me6). MS: [CI-],
m/ z 616, [(η6-C6Me6)Ta(OAr)(CH3)81Br]-; [CI-], m/ z 614, [(η6-
C6Me6)Ta(OAr)(CH3)79Br]-; [CI-], m/ z 572, <3% of total
product, [(η6-C6Me6)Ta(OAr)(CH3)37Cl]-; [CI-], m/z 570, <3%
of total product, (η6-C6Me6)Ta(OAr)(CH3)35Cl]-; [CI+], m/ z 535,
[(η6-C6Me6)Ta(OAr)(CH3)]+ ([M - X]+). Anal. Calcd for
(η6-C6Me6)Ta (OAr )Et2 (5). The reaction of 0.30 g (0.51
mmol) of (η6-C6Me6)Ta(OAr)Cl2 in 30 mL of toluene and 0.32
mL (0.963 mmol) of a EtMgBr solution (3 M in Et2O) at -60
°C was carried out in the manner described above for preparing
(η6-C6Me6)Ta(OAr)Me2 (4). Workup of the reaction by an
identical procedure afforded 0.24 g (0.65 mmol, 82%) of product
as dark blue violet crystals. 1H NMR (C6D6): δ 7.11-6.96 (A2B
m, 3 H, Haryl), 3.48 (sept, 2 H, CHMe2), 1.79 (s, 18 H, C6Me6),
1.65 (m, 4 H, CH2CH3), 1.50 (t, 6 H, CH2CH3), 1.27 (d, 6 H,
CHMe2). 13C NMR (C6D6): δ 157.2 (Cipso), 137.0 (Co), 123.8
(Cm), 122.4 (Cp), 107.9 (C6Me6), 34.8 (CH2CH3), 26.5 (CHMe2),
24.4 (CHMe2), 15.8 (C6Me6), 15.4 (CH2CH3). MS: [CI-], m/ z
578, [(η6-C6Me6)Ta(OAr)(CH2CH3)2]-; [CI+], m/ z 549, [(η6-
C6Me6)Ta(OAr)(CH2CH3)]+([M
28H45OTa: C, 58.12; H, 7.84. Found: C, 58.37; H, 8.09.
(η6-C6Me6)Ta (OAr )(CH2SiMe3)2 (6). The reaction of 0.50
-
Et]+). Anal. Calcd for
C
g (0.85 mmol) of (η6-C6Me6)Ta(OAr)Cl2 in 30 mL of toluene and
0.16 g (1.68 mmol) of LiCH2SiMe3 in ca. 10 mL of Et2O was
carried out at -60 °C in the manner described above for
preparing (η6-C6Me6)Ta(OAr)Me2 (4). Workup by an analogous
procedure provided 0.40 g (0.57 mmol, 69%) of product as dark
violet crystals. 1H NMR (C6D6): δ 6.97-6.83 (A2B m, 3 H,
H
aryl), 3.27 (sept, 2 H, CHMe2), 1.78 (s, 18 H, C6Me6), 1.37 (d,
J HH′ ) 11.5 Hz, 2 H, CH2SiMe3), 1.21 (d, 12 H, CHMe2), 0.35
(s, 18 H, CH2SiMe3), 0.28 (d, J HH′ ) 11.5 Hz, 2 H, CH2SiMe3).
The sample was cooled to 185 K in toluene-d8 with no observed
change its 1H in spectrum other than broadening due to
increased solvent viscosity. 13C NMR (C6D6): δ 156.59 (Cipso),
136.47 (Co), 123.78 (Cm), 122.64 (Cp), 109.27 (C6Me6), 34.34
(CH2SiMe3), 26.68 (CHMe2), 24.82 (CHMe2), 16.49 (C6Me6),
4.43 (CH2SiMe3). Anal. Calcd for C32H57OSi2Ta: C, 55.31; H,
8.27. Found: C, 55.58; H, 8.34.
(η6-C6Me6)Ta (OAr )P h 2 (7). (η6-C6Me6)Ta(OAr)Cl2 (1.00 g,
1.69 mmol) was dissolved in 30 mL of toluene and cooled to
-60 °C (CO2/isopropyl alcohol bath). This turquoise solution
was rapidly stirred while a PhLi solution (1.83 mL, 3.29 mmol,
1.8 M in Et2O) was added dropwise over 15 min. The solution
color changed to dark purple with the formation of a white
precipitate. This solution was allowed to warm to room
temperature slowly (over 4 h), during which time it became a
darker purple. The reaction was stirred for an additional 12
h, the mixture was filtered through Celite, and the solvent
was removed in vacuo to yield the product as a purple powder.
Washing this powder with minimal cold pentane yielded 1.00
g (1.48 mmol, 88%) of product as a brilliant purple powder
suitable for further reactions. Analytically pure samples were
obtained by recrystallization from Et2O at -35 °C. 1H NMR
(C6D6): δ 7.51-6.87 (m, 13 H, Haryl (OAr and C6H5)), 3.47 (sept,
2 H, CHMe2), 1.68 (s, 18 H, C6Me6), 0.91 (d, 12 H, CHMe2).
The solution was cooled to 185 K in toluene-d8 with no
observed change in its 1NMR other than broadening due to
C
25H38BrOTa: C, 48.79; H, 6.22. Found: C, 49.38; H, 6.47.
(η6-C6Me6)Ta (OAr )(Et)Br (11). The reaction of 0.50 g
(0.85 mmol) of (η6-C6Me6)Ta(OAr)Cl2 in 30 mL of toluene and
EtMgBr (0.25 mL, 3 M in Et2O, 0.76 mmol) was carried out at
-60 °C in the manner described above for preparing (η6-C6-
Me6)Ta(OAr)Me2 (4). Workup in an analogous manner af-
forded 0.39 g (0.61 mmol, 72%) of product as dark purple
crystals that were filtered off and dried in vacuo. 1H NMR
(C6D6): δ 7.04-6.88 (A2B m, 3 H, Haryl), 3.36 (sept, 2 H,
CHMe2), 2.11 (t, 3 H, CH2CH3), 1.95 (m, 2 H, CH2CH3), 1.80
(s, 18 H, C6Me6), 1.28 and 1.21 (d, 6 H each, J HH′ ) 6.8 Hz,
CHMe2). 13C NMR (C6D6): δ 157.4 (Cipso), 136.3 (Co), 123.6
(Cm), 123.3 (Cp), 113.4 (C6Me6), 36.5 (CH2CH3) 26.9 (CHMe2),
24.4 (CHMe2 coincident), 19.5 (CH2CH3), 16.3 (C6Me6). MS:
[CI-], m/ z 630, [(η6-C6Me6)Ta(OAr)(CH2CH3)81Br]-; [CI-], m/ z
628, [(η6-C6Me6)Ta(OAr)(CH2CH3)79Br]-; [CI+], m/ z 601, [(η6-
increased solvent viscosity. 13C NMR (C6D6): δ 173.3 (Cipso
,
C6H5), 157.8 (Cipso, OAr), 138.0 (Co, OAr), 135.2 (Co, C6H5),
126.8 (Cm, C6H5), 125.8 (Cp, C6H5), 123.5 (Cm, OAr), 122.9 (Cp,
OAr), 111.1 (C6Me6), 27.0 (CHMe2), 23.9 (CHMe2), 16.2 (C6Me6).
Anal. Calcd for C36H45OTa: C, 64.09; H, 6.72. Found: C,
63.88; H, 6.77.
(η6-C6Me6)Ta (OAr )(CH2SiMe3)Cl (8). The reaction of 1.00
g (1.69 mmol) of (η6-C6Me6)Ta(OAr)Cl2 in 30 mL of toluene with

Arene Alkyl Complexes of Tantalum(III)
Organometallics, Vol. 16, No. 15, 1997 3429
C6Me6)Ta(OAr)81Br]+([M - Et]+); [CI+], m/ z 599, [(η6-C6-
Me6)Ta(OAr)79Br]+([M - Et]+). An elemental analysis on this
compound was not obtained due to the <5% impurity of (η6-
C6Me6)Ta(OAr)(Et)Cl.
(η6-C6Me6)Ta (OAr )(CH2SiMe3)H (15). A solution of 1.00
g (1.55 mmol) of (η6-C6Me6)Ta(OAr)(CH2SiMe3)Cl in 20 mL of
Et2O was prepared, cooled to -60 °C (CO2/isopropyl alcohol
bath), and rapidly stirred while LiBEt3H (1.90 mL, 1 M in
THF, 1.90 mmol) was added dropwise over a period of 15 min.
The reaction was allowed to warm to room temperature over
4 h and stirred for an additional 48 h, over which time the
solution developed an inky violet color and formed a white
precipitate. The solution was filtered through Celite, the
solvent was removed in vacuo, and the resulting oil was
dissolved in minimal pentane and cooled to -35 °C for 48 h.
This procedure yielded 0.65 g (1.17 mmol, 75%) of product as
purple rocks. Analytically pure samples were obtained by
recrystallization from pentane at -35 °C. 1H NMR (C6D6): δ
7.05-6.87 (A2B m, 3 H, Haryl), 5.87 (s, 1 H, TaH), 3.47 (sept, 2
H, CHMe2), 1.88 (s, 18 H, C6Me6), 1.29, 1.26 (overlapping d,
12 H total, CHMe2), 1.01 (d, 1 H, J HH′ ) 12 Hz, CHH′SiMe3),
0.33 (s, 9 H, CHH′SiMe3), -0.27 (d, 1 H, J HH′ ) 12 Hz,
CHH′SiMe3). 13C NMR (C6D6): δ 157.2 (Cipso), 136.1 (Co), 123.2
(Cm), 122.5 (Cp), 106.4 (C6Me6), 30.0 (CH2SiMe3), 27.0 (CHMe2),
23.5 (CHMeMe′, coincident), 16.6 (C6Me6), 3.3 (CH2SiMe3).
Anal. Calcd for C28H47OSiTa: C, 55.25; H, 7.78. Found: C,
55.12; H, 7.85.
(η6-C6Me6)Ta (OAr )(Et)Cl (12). A 0.34 g (0.53 mmol)
sample of (η6-C6Me6)Ta(OAr)(CH2SiMe3)Cl was dissolved in 20
mL of Et2O and cooled to -30 °C. To this rapidly stirred, navy
blue solution was added dropwise 0.03 g (0.258 mmol) of ZnEt2
in ca. 10 mL of Et2O over a period of 1 min. The color of the
solution did not change over the course of the reaction. This
solution was allowed to warm to room temperature over 40
min and stirred an additional 48 h. The resultant navy blue
solution was filtered, and the solvent was removed in vacuo
yielding a dark blue oil. Dissolving this oil in minimal pentane
and cooling the solution to -35 °C for 48 h yielded 0.18 g (0.33
mmol, 62%) of product as dark purple rocks suitable for further
reactions. Analytically pure samples were obtained by recrys-
tallization from pentane at -35 °C. 1H NMR (C6D6): δ 7.02-
6.87 (A2B m, 3 H, Haryl), 3.36 (sept, 2 H, CHMe2), 2.04 (t, 3 H,
CH2CH3), 1.90 (m, 3 H, CH2CH3), 1.78 (s, 18 H, C6Me6), 1.29,
1.23 (d, 6 H each, J HH′ ) 6.7 Hz, CHMe2). 13C NMR (C6D6): δ
156.4 (Cipso), 136.2 (Co), 123.5 (Cm), 123.1 (Cp), 113.5 (C6Me6),
36.3 (CH2CH3), 26.9 (CHMe2), 24.2, 23.9 (CHMe2), 18.22
(CH2CH3), 15.9 (C6Me6). Anal. Calcd for C26H40OClTa: C,
53.38; H, 6.89. Found: C, 53.22; H, 7.17.
(η6-C6Me6)Ta (OAr )(SAr ′)Cl (16). A solution of 0.205 g
i
(0.846 mmol) of LiSAr′ (Ar′ ) 2,4,6-C6H2 Pr3) in 15 mL of Et2O
(η6-C6Me6)Ta (OAr )(CH2SiMe3)P h (13). A 0.19 g (0.29
mmol) sample of (η6-C6Me6)Ta(OAr)(CH2SiMe3)Cl was dis-
solved in 20 mL of Et2O and cooled to -60 °C (CO2/isopropyl
alcohol bath). To this rapidly stirred, navy blue solution was
added 0.16 mL (0.29 mmol) of a PhLi solution dropwise (1.8
M in Et2O, diluted to ca. 10 mL in Et2O) over the course of ca.
10 min. The solution developed an inky purple color and
formed a white precipitate as the solution was allowed to warm
to room temperature over 4 h. After the reaction was stirred
for another 18 h, the mixture was filtered through Celite and
the solvent was removed in vacuo yielding a dark purple, oily
solid. Dissolving this solid in minimal Et2O and cooling to -35
°C for 48 h yielded 0.14 g (0.21 mmol, 72%) of product as purple
rocks. Analytically pure samples were obtained by recrystal-
lization from pentane at -35 °C. 1H NMR (C6D6): δ 7.60-
6.87 (A2B m, 8 H, Haryl), 3.37 (sept, 2 H, CHMe2), 1.67 (s, 18
H, C6Me6), 1.34 (d, 1 H, J HH′ ) 12.4 Hz, CHH′SiMe3), 1.31,
was prepared and added dropwise to a rapidly stirred solution
of (η6-C6Me6)Ta(OAr)Cl2 (0.500 g, 0.846 mmol) in 5 mL of Et2O.
This mixture was stirred for 18 h, after which it was filtered
through Celite, and the solvent was removed from the filtrate
in vacuo to yield a blue oil. This oil dissolved in pentane (ca.
5 mL), whereupon the product precipitated as blue microcrys-
tals. These crystals were filtered off, washed with cold
pentane, and dried in vacuo to afford 0.535 g (0.676 mmol,
80%) of (η6-C6Me6)Ta(OAr)(SAr′)Cl as an analytically pure,
microcrystalline, blue solid. 1H NMR (C6D6): δ 7.68-7.00 (A2B
m, 3 H, Haryl, OAr), 7.21 (s, 2 H, Hm, SAr′), 3.40 (br, 3 H,
CHMe2, SAr′), 2.87 (sept, 2 H, CHMe2, OAr), 1.94 (s, 18 H,
C6Me6), 1.34, 1.25, 1.23 (d, 6 H each, CHMe2), 1.24 (d, 12 H,
CHMe2). 13C NMR (C6D6): δ 156.1 (Cipso, OAr), 146.2 (Cipso
,
SAr′), 137.2 (Co, OAr), 123.8 (Cp, OAr), 123.6 (Cm, OAr), 120.59
(Cp, SAr′), 120.56 (Cm, SAr′), 117.4 (C6Me6), 34.5 (p-CHMe2,
SAr′), 31.4 (o-CHMe2, SAr′), 26.6 (CHMe2, OAr), 24.71, 24.68,
24.42, 24.38 (CHMe2), 16.6 (C6Me6). One resonance (Cp, SAr′)
was not observed and is presumed to be coincident with
another signal or the solvent resonance. Anal. Calcd for
1.27 (d, 6 H each, J HH′ ) 6.8 Hz, CHMe2), 0.55 (d, 1 H, J HH′
)
12.4 Hz, CHH′SiMe3), 0.11 (s, 9 H, CH2SiMe3). 13C NMR
(C6D6): δ 179.5 (Cipso, C6H5), 157.2 (Cipso), 140.1 (Co, C6H5),
136.6 (Co), 126.9 (Cm, C6H5), 125.2 (Cp, C6H5), 123.7 (Cm), 122.9
(Cp), 110.1 (C6Me6), 37.4 (CH2SiMe3), 27.1 (CHMe2), 24.7, 24.4
(CHMe2), 16.6 (C6Me6), 4.5 (CH2SiMe3). Anal. Calcd for
C
39H58ClOSTa: C, 59.20; H, 7.38. Found: C, 59.56; H, 7.45.
(η6-C6Me6)Ta (OAr )(S(m es))Cl (17). A solution of 0.105 g
(0.423 mmol) of LiS(mes)(dme) (mes ) 2,4,6-C6H2Me3) in 20
mL of Et2O was slowly added to a rapidly stirred solution of
(η6-C6Me6)Ta(OAr)Cl2 (0.250 g, 0.423 mmol) in 5 mL of Et2O.
This mixture was stirred for 20 h, after which it was filtered
through Celite, and the filtrate was stripped of solvent under
reduced pressure to afford a blue oil. Upon dissolving this oil
in pentane (ca. 2-3 mL), the product precipitated as blue
microcrystals. These crystals were filtered off, washed with
cold pentane, and dried in vacuo to afford 0.230 g (0.320 mmol,
76%) of (η6-C6Me6)Ta(OAr)(S(mes))Cl as a microcrystalline,
blue solid. This compound can be recrystallized from pentane
at -35 °C to afford analytically pure samples. 1H NMR
(C6D6): δ 7.00-6.94 (m, 4 H, Hm, OAr and S(mes)), 6.91 (dd,
1 H, Hp, OAr), 3.37 (sept, 2 H, CHMe2), 2.45 (br, 6 H, o-Me,
S(mes)), 1.27 (s, 3 H, p-Me, S(mes)), 1.91 (s, 18 H, C6Me6), 1.31,
C
34H51OSiTa: C, 59.63; H, 7.51. Found: C, 60.00; H, 7.87.
(η6-C6Me6)Ta (OAr )(Et)H (14). A solution of (η6-C6Me6)-
Ta(OAr)(Et)Br (1.00 g, 1.59 mmol) in 20 mL of Et2O was
prepared, cooled to -60 °C (CO2/isopropyl alcohol bath), and
rapidly stirred while LiBEt3H (1.90 mL, 1 M in THF, 1.90
mmol) was added dropwise over a period of 15 min. The
solution developed an inky violet color and formed a white
precipitate as the solution was allowed to warm to room
temperature over 4 h. After the reaction was stirred for an
additional 1 h, the solvent was removed in vacuo to afford a
violet cake. The compound was extracted from the cake with
pentane (ca. 50 mL), the extract was filtered through Celite,
and the solvent was removed in vacuo to provide a dark violet
oil. Dissolving this oil in minimal heptane and cooling to -35
°C for 48 h yielded 0.65 g (1.18 mmol, 76%) of product as violet
rocks. Analytically pure samples were obtained by recrystal-
lization from heptane at -35 °C. 1H NMR (C6D6): δ 7.09-
6.93 (A2B m, 3 H, Haryl), 5.87 (s, 1 H, TaH), 3.56 (sept, 2 H,
CHMe2), 1.91 (overlapping s and t, 21 H total, C6Me6, CH2CH3),
1.62 (m, 2 H, CH2CH3), 1.29, 1.27 (d, 6 H each, J HH′ ) 3.7 Hz,
CHMe2). 13C NMR (C6D6): δ 157.4 (Cipso), 136.4 (Co), 123.3 (Cm),
123.4 (Cp), 105.8 (C6Me6), 31.6 (CH2CH3), 27.2 (CHMe2), 23.5
(CHMe2 coincident), 18.3 (CH2CH3), 16.4 (C6Me6). Anal. Calcd
for C26H41OTa: C, 56.72; H, 7.51. Found: C, 57.12; H, 7.66.
1.26 (d, 6 H each, CHMe2). 13C NMR (C6D6): δ 155.6 (Cipso
,
OAr), 139.9 (Cipso, S(mes)), 136.8 (Co, OAr), 134.2 (Co, S(mes)),
128.5 (Cm, S(mes)), 123.8 (Cp, OAr), 123.6 (Cm, OAr), 117.1 (C6-
Me6), 26.6 (CHMe2), 24.8 (o-Me, S(mes)), 24.1 (CHMe2), 20.9
(p-Me, S(mes)), 16.5 (C6Me6). Anal. Calcd for C33H46ClOSTa:
C, 56.05; H, 6.56. Found: C, 55.61; H, 6.19.
X-r a y Str u ctu r a l Deter m in a tion of (η6-C6Me6)Ta (OAr )-
Et2 (5). A red, irregular block crystal of C28H45OTa was
crystallized from pentane solution (at -35 °C) and mounted
in a glass capillary in a random orientation. From the

3430 Organometallics, Vol. 16, No. 15, 1997
Arney et al.
systematic absences of hk0, h ) 2n + 1; 0kl, k + l ) 2n + 1
and from subsequent least-squares refinement the space group
was determined to be Pnma (No. 62). Hydrogen atoms were
located and their positions were refined in least-squares; their
isotropic thermal parameters were held fixed at 5.0 Å2.
Scattering factors were taken from Cromer and Waber.50
Anomalous dispersion effects were included in Fc;51 the values
for ∆f′ and ∆f′′ were those of Cromer.52 All calculations were
performed on a VAX computer using SDP/VAX.53 Details of
the structural determination and refinement are reported in
Table 1.
Ackn owledgem en t is made to the Division of Chemi-
cal Sciences, Office of Basic Energy Sciences, Office of
Energy Research, U.S. Department of Energy (Grant
No. DE-FG03-93ER14349) for support of this research.
We also thank Dr. Carina Grittini for her assistance.
(50) Cromer, D. T.; Waber, J . T. International Tables for X-ray
Crystallography; The Kynoch Press: Birmingham, England, 1974. Vol.
IV, Table 2.2B.
Su p p or tin g In for m a tion Ava ila ble: Text giving com-
plete crystallographic details, tables of atomic positional and
thermal parameters, bond distances and angles, least-squares
planes, and dihedral angles, and ORTEP diagrams for (η6-C6-
Me6)Ta(OAr)Et2 (13 pages). Ordering information is given on
any current masthead page.
(51) Ibers, J . A.; Hamilton, W. C. Acta Crystallogr. 1964, 7, 781.
(52) Cromer, D. T. International Tables for X-ray Crystallography;
The Kynoch Press: Birmingham, England, 1974. Vol. IV, Table 2.3.1.
(53) Frenz, B. A. The Enraf-Nonius CAD 4 SDPsA Real-time
System for Concurrent X-ray Data Collection and Crystal Structure
Determination. In Computing in Crystallography; Schenk, H., Olthof-
Hazelkamp, R., vanKonigsveld, H., Bassi, G. C., Eds.; Delft University
Press: Delft, Holland, 1978; pp 64-71.
OM970197C