Syntheses of tantalum (V) complexes containing tetramethylpyrrolyl, pyrrolyl, and indolyl ligands

Article abstract of DOI:10.1021/ic9509544

The reaction of TaMe₃Cl₂ with the lithium salt of tetramethylpyirole (Li-TMP) led to the formation of (η⁵TMP)TaMe₃Cl (1). Reactions of 1 with a series of anionic ligands have been carried out to form products of the formula (η⁵-TMP)TaMe₃X, where X = SR, Me, pyrrolyl, or indolyl. Crystals of (η⁵-TMP)TaMe₃(mdolyl) (5), were isolated in space group P2₁/c with a = 8.957(2) A?, b = 28.540(6) A?, c = 14.695(3) A?, β= 99.40(3)°. V = 3706.1(14) A?³, and Z = 8. The structure confirmed the η⁵-bonding mode of the tetramethylpyrrolyl ligand and the η¹-N-coordination mode of the indolyl ligand.The derivatives (η⁵-TMP)TaMe₃X showed limited stability, and decomposition products which formed in toluene solutions at room temperature have been identified in some cases. The reaction of (η³-TMP)TaMe₃(pyrrolyl) with hydrogen (2-3 atm) in benzene-d₆ solution at room temperature was studied. The stoichiometric formation of cyclohexane-d₆ by hydrogenation of an equivalent of solvent was confirmed by ¹H and ¹³C NMR and gas chromatographic/mass spectroscopic data. The characteristics and scope of the room temperature arene hydrogenation process are discussed.

Full text of DOI:10.1021/ic9509544

3228
Inorg. Chem. 1996, 35, 3228-3234
Syntheses of Tantalum(V) Complexes Containing Tetramethylpyrrolyl, Pyrrolyl, and Indolyl
Ligands
Keith G. Parker, Bruce Noll, C. G. Pierpont, and M. Rakowski DuBois*
Department of Chemistry and Biochemistry, Univeristy of Colorado, Boulder, Colorado 80309
ReceiVed July 28, 1995X
The reaction of TaMe3Cl2 with the lithium salt of tetramethylpyrrole (Li-TMP) led to the formation of (η5-
TMP)TaMe3Cl (1). Reactions of 1 with a series of anionic ligands have been carried out to form products of the
formula (η5-TMP)TaMe3X, where X ) SR, Me, pyrrolyl, or indolyl. Crystals of (η5-TMP)TaMe3(indolyl) (5),
were isolated in space group P21/c with a ) 8.957(2) Å, b ) 28.540(6) Å, c ) 14.695(3) Å, â ) 99.40(3)°, V
) 3706.1(14) Å3, and Z ) 8. The structure confirmed the η5-bonding mode of the tetramethylpyrrolyl ligand
and the η1-N-coordination mode of the indolyl ligand.The derivatives (η5-TMP)TaMe3X showed limited stability,
and decomposition products which formed in toluene solutions at room temperature have been identified in some
cases. The reaction of (η5-TMP)TaMe3(pyrrolyl) with hydrogen (2-3 atm) in benzene-d6 solution at room
temperature was studied. The stoichiometric formation of cyclohexane-d6 by hydrogenation of an equivalent of
solvent was confirmed by 1H and 13C NMR and gas chromatographic/mass spectroscopic data. The characteristics
and scope of the room temperature arene hydrogenation process are discussed.
Introduction
has been found to be significantly enhanced. For example, the
pyrrolyl ligand in (C4H4N)Re(PR3)2HI was found to undergo
nucleophilic substitution reactions at the R carbons, eq 1,6b-d
The derivatization of pyrrole and indole rings is of interest
for the elaboration of the ring systems and in the synthesis of
natural products,1,2 while the reduction and denitrogenation of
such rings are problems inherent in the industrial hydrotreating
process.3,4 The role of transition metal ions in facilitating these
types of reactions has been recognized,2d,5,6 but considerable
work remains to establish a thorough understanding of metal
coordination effects on pyrrole and indole reactivity.
Many examples of transition metal complexes which contain
an η5-pyrrole or a deprotonated η5-pyrrolyl ring have been
reported.7 The majority of these complexes involve the platinum
group metals (Groups 8-10). Examples of η5-pyrrole deriva-
tives with early transition metals include (HNC4H4)Cr(CO)3,8
(C4H4N)Mn(CO)3 and related derivatives,9 and derivatives of
10
C5H5(C4H4N)TiCl2 and of (C4H4N)Re(PR3)2H2.6b In some
cases, the reactivity of the heterocyclic ligand in these complexes
X Abstract published in AdVance ACS Abstracts, May 1, 1996.
(1) Sundberg, R. J. ComprehensiVe Heterocyclic Chemistry; Katritzky,
A. R., Rees, C. W., Eds.; Pergamon Press: Oxford, 1984; Vol. 4, p
313.
(2) (a) Floss, H. G. Tetrahedron 1976, 32, 873. (b) Christophersen, C.
Marine Indoles in Marine Natural Products: Chemical and Biological
PerspectiVes; Scheuer, P. J., Ed.; Academic: New York, 1982; Vol.
V, p 259. (c) Repke, D. B.; Ferguson, W. J. J. Heterocycl. Chem.
1982, 19, 845. (d) Moriarty, R. M.; Ku, Y.-Y.; Gill, U. S. Organo-
metallics 1988, 7, 660 and references within.
while free pyrrole does not react with nucleophiles. The topic
of reactions of coordinated pyrrole ligands has been reviewed
recently.7b,11
Most of the previous examples of η5-pyrrole complexes of
the early transition metals involved relatively low valent
derivatives. We wished to explore the reactivity of η5-pyrrole
ligands in coordination complexes of high-valent early transition
metals. A high-valent metal ion has the potential to serve as
an effective Lewis acid, enhancing the activation of the
π-coordinated heterocycle toward reduction or nucleophilic
addition reactions. Previous examples of η1-(N)-pyrrolyl com-
plexes of Zr(IV) and Ti(IV) have been reported, Cp2M(NC4H4)2
and Zr(NC4H2Me2)4,12 and structural studies suggested that the
metal ions served as an effective Lewis acid for accepting π
(3) Ho, T. C. Catal. ReV.-Sci. Eng. 1988, 30, 117.
(4) (a) Stern, E. W. J. Catal. 1979, 57, 390. (b) Olive, J. L.; Biyoko, S.;
Moulinas, C.; Geneste, P. Appl. Catal. 1985, 19, 165. (c) Odebunmi,
E. O.; Ollis, D. F. J. Catal. 1983, 80, 76.
(5) See, for example, (a) Lomenzo, S. A.; Nolan, S. P.; Trudell, M. L.
Organometallics 1994, 13, 676. (b) Semmelhack, M. F.; Wolff, W.;
Garcia, J. L. J. Organomet. Chem. 1982, 240, C5. (c) Ryan, W. J.;
Peterson, P. E.; Cao, Y.; Williard, P. G.; Sweigart, D. A.; Baer, C.
D.; Thompson, C. F.; Chung, Y. K.; Chung, T. M. Inorg. Chim. Acta
1993, 211, 1. (d) Kozikowski, A. P.; Isobe, K. J. Chem. Soc., Chem.
Commun. 1978, 1076.
(6) (a) Johnson, T. J.; Arif, A. M.; Gladysz, J. A. Organometallics 1993,
12, 4728. (b) Felkin, H.; Zakrzewski, J. J. Am. Chem. Soc. 1985, 107,
3374. (c) Zakrzewski, J. J. Organomet. Chem. 1987, 333, 71. (d)
Zakrzewski, J. J. Organomet. Chem. 1982, 362, C31. (e) Myers, W.
H.; Koontz, J. I.; Harman, W. D. J. Am. Chem. Soc. 1992, 114, 5684.
(7) See, for example, (a) Kvietok, F. A.; Allured, V.; Carperos, V.;
Rakowski DuBois, M. Organometallics 1994, 13, 60 and references
within. (b) Kuhn, N. Bull. Soc. Chem. Belg. 1990, 99, 707.
(8) Ofele, K.; Dotzauer, E. J. Organomet. Chem. 1971, 30, 211.
(9) (a) Kerschner, D. L.; Rheingold, A. L.; Basolo, F. Organometallics
1987, 6, 196. (b) Pyshnograeva, N. I.; Batsanov, A. S.; Struchkov, Y.
T.; Ginsberg, A. G.; Setkina, V. N. J. Organomet. Chem. 1985, 297,
69.
(10) Sodhi, G. S.; Sharma, A. K.; Kaushik, N. K. J. Organomet. Chem.
1982, 238, 177.
(11) (a) Zakrzewski, J. Heterocycles 1990, 31, 383. (b) Blackman, A. AdV.
Heterocycl. Chem. 1993, 58, 123. (c) Kerschner, D. L.; Basolo, F.
Coord. Chem. ReV. 1987, 78, 279.
S0020-1669(95)00954-2 CCC: $12.00 © 1996 American Chemical Society

Syntheses of Ta(V) Complexes
Inorganic Chemistry, Vol. 35, No. 11, 1996 3229
electron density from the pyrrolyl nitrogen. The bond distances
in the pyrrolyl ring of the Zr complex showed a significant de-
gree of localization of π electron density. Recently a macrocy-
clic tetrapyrrole ligand, meso-octaethylporphyrinogen, has been
shown to undergo both η5- and η1-coordination with Zr(IV).13
An unusual homologation of an η1-pyrrolyl ligand to pyridine
upon CO addition was characterized for this Zr macrocycle.
The reactions of tantalum complexes with other nitrogen
heterocycles have established unusual coordination geometries
and new reactivities. For example, examples of η2-pyridine
complexes of Ta(III) have been reported,14,15 and the nucleo-
philic cleavage of a C-N bond in an η2-pyridine ligand has
been studied mechanistically.16,17 In this paper we report the
synthesis and characterization of a series of η5-tetramethylpyr-
rolyl (TMP) complexes of Ta(V) of the formula (TMP)TaMe3X.
The series includes derivatives where X ) η1-bonded pyrrolyl
and indolyl ligands. These were synthesized in the hope that
the effects of different bonding modes on nitrogen heterocycle
reactivity might be compared. These complexes appear to be
the first examples of η5-tetramethylpyrrole coordination to a d0
metal ion.
Figure 1. Perspective drawing and numbering scheme for 2. Thermal
ellipsoids are shown at 50% probability.
Table 1. Bond Lengths (Å) and Bond Angles (deg) for
Ta(NC4Me4)Cl2Me2
Bond Lengths
Ta-Cl
2.358(5)
2.350(9)
2.178(15)
1.368(17)
1.520(18)
Ta-N
2.253(14)
2.536(11)
1.373(13)
1.527(13)
1.397(19)
Ta-C(1)
Ta-C(3)
C(1)-C(2)
C(2)-C(5)
Ta-C(2)
N-C(1)
Results and Discussion
C(1)-C(4)
C(2)-C(2A)
Syntheses of (η5-TMP)TaMe3Cl and (η5-TMP)TaMe2Cl2.
The reaction of TaMe3Cl2 with (tetramethylpyrrolyl)lithium in
diethyl ether proceeded at room temperature to form an orange
crystalline derivative with the formula (TMP)TaMe3Cl (1). We
tentatively assigned a structure with a π-bonded TMP ligand
by analogy to the reaction of TaMe3Cl2 with TlCp or LiCp* to
Bond Angles
Cl-Ta-N
96.5(3)
34.6(3)
56.2(4)
81.5(4)
93.3(4)
Cl-Ta-C(1)
92.3(3)
117.9(3)
32.2(4)
127.9(4)
78.8(4)
96.5(3)
146.0(3)
34.6(3)
53.7(3)
32.0(4)
79.9(8)
111.7(9)
N-Ta-C(1)
Cl-Ta-C(2)
N-Ta-C(2)
C(1)-Ta-C(2)
N-Ta-C(3)
C(2)-Ta-C(3)
N-Ta-ClA
Cl-Ta-C(3)
1
form η5-Cp′TaMe3Cl.18 The H NMR spectrum of 1 at room
C(1)-Ta-C(3)
Cl-Ta-ClA
81.9(2)
temperature showed three methyl singlets in a ratio of 2:2:3,
which corresponded to inequivalent pyrrolyl methyls and the
methyl ligands on tantalum. Below -30 °C, the single
tantalum-methyl resonance was split into two singlets assigned
to the two methyl groups cis to the chloride ligand and one
trans methyl ligand. A similar scrambling of methyl positions
at room temperature by a proposed pseudorotation process has
been observed previously for Cp*TaMe3Cl.18a The ∆Gq for the
scrambling process in the tetramethylpyrrole derivative was
determined from the variable temperature NMR data to be 12.3
( 0.3 kcal/mol, similar to the value of 13.6 ( 1.4 kcal/mol for
the Cp* analogue. The 13C NMR and mass spectral data for 1
were also consistent with the proposed formulation and structure.
The compositions of 1 and of the other tantalum derivatives
reported here were verified by high-resolution exact mass
determinations.
C(1)-Ta-ClA
C(3)-Ta-ClA
C(1)-Ta-C(1A)
C(3)-Ta-C(1A)
C(3)-Ta-C(2A)
C(1)-N-C(1A)
C(1)-C(2)-C(2A)
130.0(3)
133.9(4)
54.8(4)
130.5(5)
99.2(5)
104.0(13)
106.3(6)
C2-Ta-ClA
N-Ta-C(1A)
C(2)-Ta-C(1A)
C(2)-Ta-C(2A)
C(3)-Ta-C(3A)
N-C(1)-C(2)
led to decomposition with tetramethylpyrrole dissociation,
perhaps through intermediate tantalum hydride formation. The
facile ligand dissociation suggested that the interaction between
Ta(V) and the heterocycle was quite ionic in character.
Attempts were made to increase the stability of the TMP-TaV
interaction by changing the nature of the coligand X in the
complexes, and the syntheses of additional derivatives of (TMP)-
TaMe3X are described below.
A minor product (about 10%) formed in the above synthesis
of 1 was identified as (TMP)TaMe2Cl2 (2). It is likely that this
product arises from the reaction of TaMe2Cl3 with pyrrolyl-
lithium.19 TaMe2Cl3 is a potential side product which may be
formed during the synthesis of TaMe3Cl2,18b and it was detected
in the NMR spectrum of the latter starting reagent in a 1:10
ratio. In the 1H NMR spectrum of 2 three methyl singlets were
observed in a 1:1:1 ratio, shifted downfield slightly relative to
the resonances of 1. In attempted crystallizations of the product
mixture, single crystals of 2 were isolated from a saturated ether
solution at -40 °C. This product, 2, was characterized by an
X-ray diffraction study to confirm the bonding mode of the
tetramethylpyrrolyl ligand.
Complex 1 was extremely sensitive to trace amounts of water
or other proton sources which led to the dissociation of free
tetramethylpyrrole. Reactions of 1 with hydride reagents also
(12) (a) Bynum, R. V.; Hunter, W. E.; Rogers, R. D.; Atwood, J. L. Inorg.
Chem. 1980, 19, 2368. b) Bynum, R. V.; Zang, H. M.; Hunter. W. E.;
Atwood, J. L. Can. J. Chem. 1986, 64, 1304.
(13) (a) Jacoby, D.; Floriani, C.; Chiesi-Villa, A.; Rizzoli, C. J. Am. Chem.
Soc. 1993, 115, 3595. (b) Jacoby, D.; Floriani, C.; Chiesi-Villa, A.;
Rizzoli, C. J. Am. Chem. Soc. 1993, 115, 7025.
(14) (a) Neithamer, D. R.; Parkranyi, L.; Mitchell, J. F.; Wolczanski, P. T.
J. Am. Chem. Soc. 1988, 110, 4421-4423. (b) Covert, K. J.;
Neithamer, D. R.; Zonnevylle, M. C.; LaPointe, R. E.; Schaller, C.
P.; Wolczanski, P. T. Inorg. Chem. 1991, 30, 2494.
(15) Smith, D. P.; Strickler, J. R.; Gray, S. D.; Bruck, M. A.; Holmes, R.
S.; Wigley, D. E. Organometallics 1992, 11, 1275-1288.
(16) (a) Gray, S. D.; Weller, K. J.; Bruck, M. A.; Briggs, P. M.; Wigley,
D. E. J. Am. Chem. Soc. 1995, 117, 10678. (b) Wigley, D. E. J. Am.
Chem. Soc. 1992, 114, 5462.
A perspective drawing and numbering scheme for the
molecule are shown in Figure 1, and selected bond distances
and angles are presented in Table 1. The structure confirmed
a piano stool arrangement with an η5-TMP ligand and cis pairs
(17) Weller, K. J.; Gray, S. D.; Briggs, P. M.; Wigley, D. E. Organome-
tallics 1995, 14, 5588-5597.
(18) (a) McLain, S. J.; Wood, C. D.; Schrock, R. R. J. Am. Chem. Soc.
1979, 101, 4558. (b) Schrock, R. R.; Sharp, P. R. J. Am. Chem. Soc.
1978, 100, 2389.
(19) A second possible pathway for the formation of 2, suggested by a
referee, is a ligand methathesis reaction between 1 and TaMe2Cl3.

3230 Inorganic Chemistry, Vol. 35, No. 11, 1996
Parker et al.
of methyl and chloride ligands. An examination of the
interactions between the metal ion and the TMP ligand shows
that the Ta-N and Ta-C(1) distances are significantly shorter
than the Ta-C(2) distance, suggesting some degree of ring
slippage toward an η3-azaallyl bonding interaction. Similar
differences in M-C distances for a η5-pyrrolyl ligand have been
observed previously for the complex (η5-3,4-Me2-pyrrolyl)Mn-
(CO)3.9a However, in both the Ta and Mn structures, compari-
sons of C-C bonding distances within the TMP ligand do not
provide clear evidence for an allyl-ene localization of electron
density.
Syntheses of (η5-TMP)TaMe3X Derivatives. The reactions
of 1 with a series of anionic ligands, LiX, were carried out in
diethyl ether at room temperature, eq 2, and the metathesis
products (η5-TMP)TaMe3X, where X ) Me, pyrrolyl, or indolyl,
were isolated and characterized.
Figure 2. Perspective drawing and numbering scheme for 5. Thermal
ellipsoids are shown at 50% probability.
Experimental Section. In the 1H NMR spectrum of (η5-TMP)-
TaMe3(NC4H4) (4) in benzene-d6, resonances for the pyrrolyl
hydrogens were observed at 7.48 and 6.60 ppm, downfield from
the resonances for free pyrrole in this solvent which occur at
6.35 and 6.32 ppm. Downfield shifts for metal N-bonded
derivatives of related heterocycles have been reported previ-
ously.23 At room temperature a single broad resonance was
observed for the Ta-Me groups, but in the 13C NMR spectrum
two resonances were observed as expected for methyls cis and
trans to the pyrrolyl ligand at 73.3 and 66.2 ppm. Variable
temperature 1H NMR experiments showed two resolved methyl
singlets at T < 5 °C, and a ∆Gq ) 14.5 ( 0.3 kcal/mol was
determined for the fluxional process.
Reactions of 1 with potential π donor ligands such as
thiolates, alkoxides, and dialkylamides were also attempted, but
products appeared to be significantly less stable in these cases
and were not successfully isolated. For example, in an NMR
tube reaction monitored spectroscopically, the derivative (η5-
TMP)TaMe3(StBu) appeared to be formed quantitatively from
the reaction of 1 equiv of lithium tert-butyl thiolate with 1. NMR
data for the proposed product are given in the Experimental
Section. However, the product decomposed in the benzene-d6
solution over a period of about 1 h with the loss of isobutene
and methane, which were identified by NMR spectroscopy. Free
tetramethylpyrrole was also observed in the NMR spectrum of
the decomposition products. The elimination of isobutene from
a tert-butyl thiolate ligand is a common decomposition pathway
which has been used in some cases for the preparation of metal
sulfido complexes.20 Some TaVdS complexes have been
reported previously,21,22 but attempts to trap a TadS intermediate
in this system with phosphine or with nitrogen donor ligands
were unsuccessful. The reaction of 1 with a less reactive thiolate
ligand, 2,6-dimethylbenzenethiolate, also led to a thiolate
complex of limited stability. The product decomposed in
solution by an uncharacterized pathway.
1
In the H NMR spectrum of (η5-TMP)TaMe3(indolyl), 5,
resonances for the hydrogens on the indolyl ligand were also
shifted downfield compared to those of free indole in the same
solvent. At ambient temperature the spectrum showed two
broadened Ta-Me resonances in a 2:1 ratio. The resonance
for the Me group trans to the indolyl ligand at 0.95 ppm was
shifted slightly downfield relative to the cis Me resonance at
0.86 ppm. Variable temperature NMR data established that a
fluxional process that exchanged these methyl resonances
occurred at elevated temperatures (methyl peaks coalesced at T
> 35 °C), and the ∆Gq for this process was found to be 15.5 (
0.4 kcal/mol. The ∆Gq values for the pseudorotation process
in the series of derivatives (η5-TMP)TaMe3X (1, 4, and 5)
increase as the steric bulk of the unique ligand X increases.
X-ray Diffraction Study of 5. Single crystals of 5 were
obtained from a saturated Et2O solution. The complex crystal-
lized in space group P21/c with two independent molecules per
asymmetric unit. The structural features of the two molecules
are very similar, and a perspective drawing of one of these is
shown in Figure 2. Selected bond distances and angles are given
in Table 2. The arrangement of η5-TMP and η1-indolyl and
methyl ligands gives a normal piano stool structure. The
bonding distances between tantalum and the η5-TMP ligand in
5 are similar to those found for 2, and the same pattern of shorter
distances for the bonds to the R carbons than for those to the â
carbons of the ring is observed.
Complexes 3-5 were isolated as orange crystalline materials
and were characterized by spectroscopic techniques. Spectro-
scopic data for 3 are unremarkable and are presented in the
(20) (a) Birnbaum, J.; Haltiwanger, R. C.; Bernatis, P.; Teachout, C.; Parker,
K.; Rakowski DuBois, M. Organometallics 1991, 10, 1779. (b)
Birnbaum, J.; Laurie, J. C. V.; Rakowski DuBois, M. Organometallics
1990, 9, 156. (c) Coucouvanis, D.; Al-Ahmad, S.; Kim, C. G.; Koo,
S. M. Inorg. Chem. 1992, 31, 2996. (d) Coucouvanis, D.; Chen, S. J.;
Mandimutsira, B. S.; Kim, C. G. Inorg. Chem. 1994, 33, 4429. (e)
Piers, W. E.; Koch, L.; Ridge, D. S.; MacGillvray, L. R.; Zaworotko,
M. Organometallics 1992, 11, 3148. (f) Blower, P. J.; Dilworth, J. R.
Coord. Chem. ReV. 1987, 76, 121. (g) Kamata, M.; Yoshida, T.;
Otsuka, S.; Hirotsu, K.; Higuchi, T. J. Am. Chem. Soc. 1981, 103,
3572.
The η1-N-bonded indolyl ligand maintains its planar character.
The Ta-N(1) bond distance (2.070(11) Å) is relatively long,
but still within the range of Ta-N distances in other Ta(V)
amide complexes.24-26 Using a criterion developed by Chish-
(21) (a) Nelson, J. E.; Parkin, G.; Bercaw, J. E. Organometallics 1992,
11, 2181-2189. (b) Proulx, G.; Bergman, R. G. J. Am. Chem. Soc.
1994, 116, 7953; Organometallics 1996, 15, 133.
(22) Bach, H. J.; Brunner, H.; Wachter, J.; Kubicki, M. M.; Leblanc, J.
C.; Moise, C.; Volpato, F.; Nuber, B.; Ziegler, M. L. Organometallics
1992, 11, 1403-1407.
(23) Fish, R. H.; Baralt, E.; Kim, H. S. Organometallics 1991, 10, 1965.
(24) Galakhov, M. V.; Gomez, M.; Jimenez, G.; Royo, P. Pellinghelli, M.
A.; Tiripicchio, A. Organometallics 1995, 14, 1901-1910.

Syntheses of Ta(V) Complexes
Inorganic Chemistry, Vol. 35, No. 11, 1996 3231
involving the elimination of methane and the formation of (η5-
TMP)TaMe2(NC4H4)2 in this system.
Table 2. Selected Bond Lengths (Å) and Bond Angles (deg) for
Ta(NC4Me4)Me3(indolyl) (5)
The thermal stabilities of 4 and 5 were also investigated.
Previous well-defined decomposition pathways for high-valent
Cp*metal-L complexes (L ) alkyl, amide, aryl, or pyridine)
include â hydrogen elimination to produce an η2-coordination
mode of the ligand L-H,27-30 or hydrogen migration from a
methyl group of the Cp* ligand to the metal ion, followed by
further alkane elimination.31,32 We were interested in determin-
ing whether the thermal decomposition of 4 and 5 might lead
to derivatives with η2-coordinated pyrrolyl and indolyl ligands
or with a modified tetramethylpyrrolyl ligand. Further reactivity
of such bonding modes would be of interest. However, these
decomposition pathways were not identified for the present
systems.
Bond Lengths
Ta(1)-N(1)
Ta(1)-C(9)
Ta(1)-C(11)
Ta(1)-C(17)
Ta(1)-C(19)
N(1)-C(8)
2.070(11)
2.373(11)
2.572(13)
2.226(13)
2.197(13)
1.41(2)
1.46(2)
1.37(2)
1.40(2)
1.40(2)
Ta(1)-N(2)
Ta(1)-C(10)
Ta(1)-C(12)
Ta(1)-C(18)
N(1)-C(1)
2.238(11)
2.552(13)
2.347(13)
2.19(2)
1.42(2)
1.35(2)
1.39(2)
1.40(2)
1.35(2)
C(1)-C(2)
C(2)-C(3)
C(3)-C(8)
C(9)-C(10)
C(11)-C(12)
N(2)-C(9)
C(10)-C(11)
N(2)-C(12)
Bond Angles
N(1)-Ta(1)-C(17)
N(1)-Ta(1)-C(19)
86.3(5) N(1)-Ta(1)-C(18)
131.5(5)
76.1(6)
84.4(6) C(17)-Ta(1)-C(18)
C(18)-Ta(1)-C(19) 78.0(6) C(17)-Ta(1)-C(19) 135.3(6)
Benzene solutions of both 4 and 5 under an inert atmosphere
or under vacuum underwent slow decomposition at room
temperature. For example, in the NMR spectrum of 4, the
resonances of the starting reagent slowly decreased over a period
of several days and a single new set of resonances grew in.
The spectrum of the product showed two new pyrrolyl multiplets
in the aromatic region and three new singlets in the upfield
region with integrations consistent with the formulation (η5-
TMP)TaMe2(NC4H4)2 (6), eq 3. The 13C NMR data and mass
Table 3. Crystal Data for Compounds 2 and 5
2
5
formula
fw
C10H18NCl2Ta
404.1
C19H27N2Ta
464.39
monoclinic
crystal system
orthorhombic
unit cell dimensions
a, Å
b, Å
c, Å
13.102(4)
14.167(3)
6.946(2)
8.957(2)
28.540(6)
14.695(3)
99.40(3)
3706.1(14)
P21/c
â, deg
volume, Å3
1289.3(6)
Pnma
space group
Z
4
8
density, calc, g/cm3
2.082
1.665
λ(Mo KR, Å)
0.71073
-55 to -60
2θ-θ
3.0 to 50.0
1187
0.71073
25(2)
2θ-θ
1.58-22.55
4875
temp, °C
scan type
θ range
independent reflections
spectral data for this product also supported the proposed
formulation. Only 25-30% of 4 was accounted for by the
formation of 6, but other (TMP)Ta-containing products were
not observed in the spectrum. For example, our failure to detect
any (η5-TMP)TaMe4 (3) in the decomposition solution appears
to rule out the formation of 6 by a bimolecular ligand exchange,
since 3 is expected to be stable under these reaction conditions.33
Complex 5 also underwent a similar slow decomposition in
toluene solution at room temperature. Spectroscopic data for
the resulting product, formed in ca. 25% yield, supported the
formulation (TMP)TaMe2(indolyl)2.
Attempts were made to prepare cationic pentamethylpyrrole
derivatives by reacting 2-4 with methylating agents such as
Me3OBF4 or MeOTf. However, these reactions led only to
decomposition of the tantalum(V) derivatives. Neither the η5-
tetramethylpyrrole ligand nor the η1-heterocycles appeared to
reflections observed
1034
3315
absorption corr
semiempirical
0.0630
0.0890b
2.00
XABS2
0.0477
0.0915c
0.968
Ra
Rw
GOF
largest peak in final diff map, e/Å3
4.84
1.075
a R ) R1 ) ∑||Fo| - |Fc||/∑|Fo|. b Rw ) [∑w(|Fo| - |Fc|)2/
∑w(Fo)2]1/2 c Rw ) [∑[w(Fo - Fc2)2]/∑[w(Fo )2]]1/2
. .
2
2
olm and co-workers,26 in which Ta-N and Ta-C bond lengths
within the same molecule are compared, there appears to be
relatively little contribution to this bond from a N-Ta π
interaction. On the basis of covalent radii, the σ-only Ta-
N(sp2) bond is expected to be about 0.1 Å shorter than the Ta-
C(sp3) bonds. In other Ta(V) amide complexes in which
significant π bonding was proposed (e.g., Ta(NMe2)3(p-tolyl)-
(Br)), the Ta-N and Ta-C distances differed by 0.2 Å or more.
However, in 5 a difference of only 0.13-0.14 Å is observed
between these two bond types. The expected delocalization of
the nitrogen lone pair in the aromatic ring system is consistent
with the relatively long Ta-N bond distance.
(27) (a) Mashima, K.; Tanaka, Y.; Nakamura, A. Organometallics 1995,
14, 5642. (b) Houseknecht, K. L.; Stockman, K. E.; Sabat, M.; Finn,
M. G.; Grimes, R. N. J. Am. Chem. Soc. 1995, 117, 1163.
(28) Thompson, M. E.; Baxter, S. M.; Bulls, A. R.; Burger, B. J.; Nolan,
M. C.; Santarsiero, B. D.; Schaefer, W. P.; Bercaw, J. E. J. Am. Chem.
Soc. 1987, 109, 203.
Reactivity Studies of (TMP)TaMe3X Derivatives. The
reaction of (η5-TMP)TaMe3(NC4H4) (4) with an equivalent of
free pyrrole in C6D6 was monitored by NMR spectroscopy at
room temperature. Sharp distinct resonances for free pyrrole
and coordinated pyrrolyl were observed, indicating that ligand
exchange was slow on the NMR time scale under these
conditions. We did not observe evidence for a further reaction
(29) Mayer, J. M.; Curtis, C. J.; Bercaw, J. E. J. Am. Chem. Soc. 1983,
105, 2651.
(30) McLain, S. J.; Schrock, R. R.; Sharp, P. R.; Churchill, M. R.; Youngs,
W. J. J. Am. Chem. Soc. 1979, 101, 263.
(31) (a) Luinstra, G. A.; Teuben, J. H. J. Am. Chem. Soc. 1992, 114, 3361-
3367 and references within. (b) Horton, A. D. Organometallics 1992,
11, 3271-3275. (c) Miller, F. D.; Sanner, R. D. Organometallics 1988,
7, 818-825.
(32) (a) McAlister, D. R.; Erwin, D. K.; Bercaw, J. E. J. Am. Chem. Soc.
1978, 100, 5966. (b) Bercaw, J. E. J. Am. Chem. Soc. 1974, 96, 5087.
(c) McDade, C.; Green, J. C.; Bercaw, J. E. Organometallics 1982, 1,
1629. (d) Schock, L. E.; Brock, C. P.; Marks, T. J. Organometallics
1987, 6, 232.
(25) Profilet, R. D.; Fanwick, P. E.; Rothwell, I. P. Polyhedron 1992, 12,
1559.
(26) (a) Chisholm, M. H.; Huffman, J. C.; Tan, L. S. Inorg. Chem. 1981,
20, 1859-1866. (b) Chisholm, M. H.; Tan, L. S.; Huffman, J. C. J.
Am. Chem. Soc. 1982, 104, 4879-4884.
(33) Complex 6 has been found to be stable in benzene solution both at
room temperature and at 50 °C over a period of 8 days.

3232 Inorganic Chemistry, Vol. 35, No. 11, 1996
Parker et al.
be activated to nucleophilic attack, and reactions with reagents
such as triethylborohydride, methyl lithium, or diethyl malonate
also led to decomposition of the complexes.
identified, but no evidence for cyclohexane formation was
observed by NMR spectroscopy or by GC/mass spectral
analysis. Naphthalene was partially reduced under similar
conditions to form tetrahydronaphthalene. In contrast no
hydrogenation activity was observed under these conditions for
an arene with an electron-withdrawing group, such as benzoni-
trile, or for heteroaromatics, such as thiophene.
The promotion of arene hydrogenations by metal complexes
has been reported previously.36,37 Many of these examples
involve platinum group metals, but recently, high-valent niobium
and tantalum derivatives have been found to catalyze the
hydrogenation of arenes under conditions of high temperature
and pressure.36a-c The arene hydrogenations reported here are
of limited practical application because of the stoichiometric
nature of the reaction, but the composition and structure of the
reduction product of 4 which promotes this arene reduction
under such mild conditions are of interest. Although the closely
related complex 5 showed similar NMR broadening under
hydrogen pressure, no arene hydrogenation products were
detected in this system. Further work to identify reduced
tantalum products or to trap or stabilize such products in the
presence of additional donor ligands is planned.
Conclusions. A series of new Ta(V) complexes containing
the η5-tetramethylpyrrolyl anion have been synthesized and
characterized, including derivatives that also contained η1-
pyrrolyl or indolyl ligands. Although the coordination of the
nitrogen heterocycles to the Ta(V) ion has the potential for
activating the rings toward further reactions, we found that the
highly reactive nature of the complexes, which includes slow
ligand dissociations, facile redox processes, and extreme
sensitivity toward electrophiles, does not permit a detailed
investigation of ligand-based reactivity in these systems.
A brief investigation of the insertion chemistry of the (TMP)-
TaV derivatives also provided evidence that these complexes
are relatively unstable relative to the Cp* analogues. Facile
insertions of carbon monoxide and isocyanides into the Ta-
alkyl ligands of many Cp*TaV derivatives have been character-
ized.34 For example, one of the first examples of such an
insertion was the reaction of Cp*TaMe4 with carbon monoxide
to form an η2-acetone derivative Cp*TaMe2(η2-Me2CO), which
was isolated and characterized.34a A similar insertion was
tentatively identified for the reaction of CO (1 equiv) with (η5-
1
TMP)TaMe3(NC4H4) (4), and H NMR data for (η5- TMP)-
TaMe(NC4H4)(η2-Me2CO) are included in the Experimental
Section. However, this product was not successfully isolated
because it underwent decomposition in solution in the sealed
NMR tube over a period of 30 min.
Attempted Reductions of (η5-TMP)TaMe3X Derivatives.
Certain Cp*TaV derivatives have been found to react with
hydrogen to form tantalum hydride complexes.29b Both (η5-
TMP)TaMe3(NC4H4) (4) and (η5-TMP)TaMe3(indolyl) (5)
reacted with hydrogen (ca. 3.5 atm) in benzene solution at room
temperature. The reactions in sealed tubes were followed by
1
NMR spectroscopy. An interesting feature of the H NMR
spectrum for the reaction of 4 with hydrogen in C6D6 was the
appearance of a strong singlet at 1.36 ppm. This resonance
was assigned to deuterated cyclohexane, C6H6D6, which resulted
from the hydrogenation of the benzene-d6 solvent. In the
{1H}13C NMR spectrum, a three-line pattern centered at 26.5
ppm with JC-D ) 19 Hz supported the formation of the reduced
arene. The GC/mass spectrum of the volatile component of
this reaction also confirmed that cyclohexane-d6 was present.
Approximately 1 equiv of cyclohexane per mole of starting
tantalum complex was produced. Intermediate hydrogenation
products such as cyclohexadiene-d2 or cyclohexene-d4 were not
observed. No precipitate was visible in the dark brown reaction
mixtures, and addition of mercury to the reaction did not affect
the product formation, suggesting that the aromatic hydrogena-
tion is a homogeneous process.35
The nature of the active species in the arene reduction has
not been identified. The resonances for the starting tantalum
complex slowly disappeared, but no signals corresponding to
new tantalum derivatives were observed in the normal diamag-
netic region of the spectrum. A small amount of free tetrameth-
ylpyrrole (15%) was initially formed, but resonances for this
compound did not increase in intensity as the starting material
disappeared. Methane was also observed to form during the
reaction and was identified by NMR spectroscopy; this product
was not quantified by the NMR data because of its volatility.
The scope of the arene hydrogenation activity of 4 has been
explored briefly. Toluene-d8 was hydrogenated under similar
reaction conditions as benzene, and the formation of methyl-
cyclohexane-d8 was confirmed by GC/MS data. A competition
reaction of excess hydrogen with equimolar amounts of deu-
terated benzene and toluene in the presence of 4 resulted in the
selective hydrogenation of toluene. Methylcyclohexane-d8 was
Experimental Section
TaMe3Cl218b and tetramethylpyrrole38 were synthesized according to
literature procedures. Pyrrole was distilled from CaH2 and stored over
Linde 4A molecular sieves. Lithiated pyrroles, indoles, and thiolates
were prepared by combining the appropriate compound with an
equimolar amount of n-butyllithium in hexane and filtering off the
insoluble product. Diethyl ether and benzene-d6 were distilled from
sodium benzophenone ketyl and stored over 4 Å molecular sieves before
use. Hexane, pentane, and deuterated chloroform were distilled from
CaH2 and stored over 4 Å molecular sieves before use. All reactions
were carried out under a nitrogen atmosphere using standard Schlenk
techniques.
1H NMR spectra were recorded at 300 MHz, and 13C NMR spectra
were recorded at 75.4 MHz on a Varian VXR-300 NMR spectrometer.
Chemical shifts were referenced to tetramethylsilane by using the
(36) (a) Potyen, M. C.; Rothwell, I. P. Abstract 79, 208th ACS National
Meeting, Washington, DC, August, 1994. (b) Yu, J. S.; Ankianiec, B.
C.; Nguyen, M. T.; Rothwell, I. P. J. Am. Chem. Soc. 1992, 114,
1927-1929. (c) Ankianiec, B. D.; Fanwick, P. E.; Rothwell, I. P. J.
Am. Chem. Soc. 1991, 113, 4710. (d) Steffey, B. D.; Chesnut, R. W.;
Kerschner J. L.; Pellechia, P. J.; Fanwick, P. E.; Rothwell, I. P. J.
Am. Chem. Soc. 1989, 111, 378.
(37) (a) Linn, D. E., Jr.; Halpern, J. J. Am. Chem. Soc. 1987, 109, 2969.
(b) Wilczynski, R.; Fordyce, W. A.; Halpern, J. J. Am. Chem. Soc.
1983, 105, 2066. (c) Amer, I.; Amer, H.; Ascher, R.; Blum, J.; Saasson,
Y.; Vollhardt, K. C. P. J. Mol. Catal. 1987, 39, 185. (d) DeWit, D.
G.; Folting, K.; Streib, W. E.; Caulton, K. G. Organometallics 1985,
4, 1149. (e) Yalpani, M.; Koster, R. Chem. Ber. 1990, 123, 719. (f)
Stobart, S. R.; Zaworotko, M. J. J. Chem. Soc., Chem. Commun. 1984,
1700. (g) Grey, R. A.; Pez, G. P.; Wallo, A. J. Am. Chem. Soc. 1980,
102, 5948. (h) Bleeke, J. R.; Muetterties, E. L. J. Am. Chem. Soc.
1981, 103, 556. (i) Muetterties, E. L.; Rakowski, M. C.; Hirsekorn,
F. J.; Larson, W. D.; Basus, V. J.; Anet, F. A. L. J. Am. Chem. Soc.
1975, 97, 1266. (j) Russell, M. J.; White, C.; Maitlis, P. M. J. Chem.
Soc., Chem. Commun. 1977, 427. (k) Bennett, M. A.; Huang, T. N.;
Turney, T. W. J. Chem. Soc., Chem. Commun. 1979, 312.
(34) (a) Wood, C. D.; Schrock, R. R. J. Am. Chem. Soc. 1979, 101, 5421.
(b) Meyer, T. Y.; Garner, L. R.; Baenziger, N. C.; Messerle, L. Inorg.
Chem. 1990, 29, 4045-4050. (c) Arnold, J.; Tilley, T. D.; Rheingold,
A. L.; Geib, S. J.; Arif, A. M. J. Am. Chem. Soc. 1989, 111, 149-
164. (d) Galakhov, M. V.; Gomez, M.; Jimenez, G.; Royo, K. P.;
Pellinghelli, M. A.; Tiripicchio, A. Organometallics 1995, 14, 2843-
2854. (e) Bazan, G. C.; Donnelly, S. J.; Rodriguez, G. J. Am. Chem.
Soc. 1995, 117, 2671.
(35) Foley, P.; Di Cosimo, R.; Whitesides, G. M. J. Am. Chem. Soc. 1980,
102, 6713.
(38) Johnson, A. W.; Markham, E.; Price, R.; Shaw, K. B. J. Chem. Soc.
1958, 4254.

Syntheses of Ta(V) Complexes
Inorganic Chemistry, Vol. 35, No. 11, 1996 3233
deuterated solvent signal as a secondary reference. 1H NMR chemical
shifts are reported at ambient probe temperature, 18 °C, unless otherwise
indicated. Mass spectra were obtained on a VG Analytical 7070 EQ-
HF mass spectrometer.
ether with vigorous stirring. An immediate color change from orange
to brown with the formation of a pale precipitate (LiCl) occurred. The
solution was stirred for 2.5 h and filtered through Celite. The brown
filtrate was concentrated in vacuo and cooled at -20 °C for 36 h giving
brown-green crystals of 5 in 29% yield. 1H NMR with tentative
assignments (C6D6): δ 8.20, 7.71 (2d, each 1H, J ) 8 Hz, ind); 8.08,
6.71 (2d, each 1H, J ) 3 Hz, ind); 7.30 (m, 2H, ind); 1.85, 1.33 (2s,
each 6H, TMP); 0.95 (br s, 3H, TaMe); 0.86 (br s, 6H, TaMe2). At
-19 °C, two sharp Ta-Me resonances were observed at 0.92 and 0.79
ppm. Their coalescence temperature was 38 °C. 13C NMR (C6D6): δ
143.8, 134.9, 134.3, 132.6, 130.7, 122.5, 121.9, 120.6, 116.7 (TMP
and ind aromatics); 73.2 (s, TaMe); 64.6 (s, TaMe2); 14.3, 9.8 (2s,
TMP-Me). MS (CI-): m/z 464 (P). (EI+): m/z 449 (P-Me). HR
MS (CI-): Calcd m/z 464.1648. Found: m/z 464.1609.
X-ray Diffraction Study of (η5-NC4Me4)TaMe3(indolyl) (5).
Crystals suitable for diffraction were grown from a saturated Et2O
solution of 5 cooled at 253 K for 72 h. A sample of crystals was placed
under Apiezon T grease. The datum crystal was mounted on a glass
fiber and quickly placed in the 233 K N2 stream of a locally-modified
Nicolet LT-2 apparatus on a Nicolet R3 diffractometer. Data were
collected to a Data General Nova 4/C minicomputer.
Structure solution and refinement were performed on the DEC
microVAX 3200 in the X-ray laboratory at the University of California
at Davis using the SHELX suite of programs. All non-hydrogen atoms
were refined with anisotropic thermal parameters. Data were corrected
for absorption. Largest residuals in the final difference map were
e1.075 e‚Å-3 and were located unreasonably close to the two Ta
centers. These were dismissed as absoprtion artifacts.
Hydrogen atoms were placed at calculated distances and allowed to
ride on the position of the parent atom. The isotropic themal parameter
was modeled to ride as 1.2 times the equivalent isotropic thermal
parameter of the parent atom. No notable intermolecular contacts were
observed.
Synthesis of (η5-NC4Me4)TaMe3Cl (1). TaMe3Cl2 (0.945 g, 3.18
mmol) with a small impurity of TaMe2Cl3 (ca. 10%) was dissolved in
100 mL of diethyl ether. This solution was transferred into a suspension
of (tetramethylpyrrolyl)lithium (0.406 g, 3.15 mmol) in 30 mL of diethyl
ether with vigorous stirring. A rapid color change from pale yellow
to orange occurred with the formation of a pale precipitate (LiCl). The
solution was stirred for 1 h and filtered through a bed of Celite. The
orange filtrate was then concentrated in vacuo and kept at -20 °C for
48 h giving orange crystals of 1 in 42% yield. 1H NMR (C6D6): δ
2.03, 1.30 (s, 6H, TMP); 1.00 (s, 9H, TaMe). At -43 °C, two sharp
Ta-Me resonances were resolved at 0.77 and 1.03 ppm. Their
coalescence temperature was -17.5 °C. 13C NMR (C6D6): δ 137.3,
132.9 (s, TMP); 71.6 (br s, TaMe); 15.0, 9.9 (TMP-Me). MS (CI-):
m/z 383 (P). (EI+): m/z 368 (P-Me), 353 (P-2Me). HR MS (CI-):
Calcd m/z 383.0843. Found: m/z 383.0848.
A minor impurity in the product (ca. 10%) was identified as (η5-
NC4Me4)TaMe2Cl2 (2). This complex probably arises from the reaction
of TaMe2Cl3 (vida supra) with the Li-pyrrolyl reagent. The same
complex can also be prepared in about 30% yield from the reaction of
TaMe3Cl2 with neutral tetramethylpyrrole. 1H NMR (C6D6): δ 2.12,
1.36 (2 s, 6H, TMP); 1.09 (s, 6H TaMe).
X-ray Diffraction Study of (η5-NC4Me4)TaMe2Cl2. Orange
crystals of 2 were obtained from a saturated Et2O solution of a mixture
of 1 and 2 cooled at 253 K for 48 h. A suitable crystal was coated
with Apiezon T grease, and data were collected at 193 K on a Siemens
P3/F diffractometer. Axial photographs indicated orthorhombic sym-
metry, and the centered settings of 25 intense reflections with 2Θ values
between 24.0° and 50.0° gave the unit cell dimensions listed in Table
5. A Patterson map gave the location of the Ta atom at a site of
crystallographic mirror symmetry. Phases derived from the location
of the Ta gave positions of other atoms of the structure. From
differences in electron density and bond length to the Ta, it was possible
to clearly differentiate between the coordinated chloro and methyl
carbon atoms. ψ scans recorded at the conclusion of data collection
were used to obtain an absorption profile, and an empirical correction
for absorption was applied to the data. Crystal data, data collection
parameters, and results of the analysis are given in Table 3.
Formation of (η5-NC4Me4)TaMe3(StBu). Complex 1 (12 mg, 3.1
× 10-2 mmol) and LiStBu (3 mg, 3 × 10-2 mmol) were placed in an
NMR tube connected to a Schlenk adapter. C6D6 (0.8 mL) was added
to the reaction vessel, the mixture was frozen at -196 °C, and the
vessel was evacuated. The NMR tube was then flame sealed and
thawed to room temperature. An immediate reaction occurred, and a
pale precipitate (LiCl) formed. The yellow benzene-soluble product
1
formed in quantitative yield by H NMR, but all attempts to isolate
Synthesis of (η5-NC4Me4)TaMe4 (3). Complex 1 (108 mg, 0.28
mmol) was dissolved in 10 mLof diethyl ether. 1.2 M MeLi in diethyl
ether (0.235 mL, 0.28 mmol) was added dropwise with stirring. An
immediate color change from orange to green with the formation of a
gray precipitate (LiCl) occurred. The solution was stirred for 30 min
and filtered through Celite. The dark green filtrate was concentrated
in vacuo and cooled at -20 °C for 18 h giving dark green, crystalline
3 in 49% yield. 1H NMR (C6D6): δ 2.00, 1.31 (s, 6H, TMP); 0.83 (s,
12H, TaMe). 13C NMR (CDCl3): δ 134.9, 132.2 (s, TMP); 73.1 (s,
TaMe); 14.7, 10.2 (TMP-Me). MS(CI-): m/z 363 (P). (EI+): m/z
348 (P-Me), 332 (P-2Me). HR MS (CI-): Calcd m/z 363.1389.
Found: m/z 363.1385.
Synthesis of (η5-NC4Me4)TaMe3(NC4H4) (4). Complex 1 (470 mg,
1.23 mmol) was dissolved in 100 mL of diethyl ether and added to a
suspension of pyrrolyllithium (90 mg, 1.23 mmol) in 50 mL of diethyl
ether with vigorous stirring. The solution was stirred at room
temperature for 3.5 h and filtered through a bed of Celite. The solvent
was removed from the resultant yellow filtrate in vacuo leaving a yellow
oil. The oil was extracted with hexanes, and the extract was filtered
and concentrated in vacuo. The solution was cooled at -78 °C for 18
h giving the yellow crystalline product 4 in 65% yield. 1H NMR
(C6D6): δ 7.48, 6.60 (2 m, each 2Η, pyrr); 1.87, 1.32 (2 s, each 6H,
TMP); 0.82 (br s, 9H, TaMe). At -40 °C, two Ta-Me resonances
were resolved at 0.79 and 0.67 ppm. Their coalescence temperature
was 18.5 °C. 13C NMR (C6D6): δ 135.0, 130.6 (s, TMP); 127.3, 111.6
(s, pyrr); 73.3 (s, TaMe); 66.2 (s, TaMe2); 14.6, 9.8 (TMP). MS (CI-):
m/z 414 (P). (EI+): m/z 399 (P-Me), 348 (P-pyrr). HR MS (CI-):
Calcd m/z 414.1498. Found: m/z 414.1507.
and purify the compound on a larger scale led to decomposition of the
product. 1H NMR (C6D6): δ 2.15, 1.34 (2s, each 6H, TMP); 1.60 (s,
9H, StBu); 0.95 (s, 9H, TaMe).
Formation of (η5-NC4Me4)TaMe3(dmpt) (dmpt ) 2,6-dimethyl-
benzenethiolate). Complex 1 (10 mg, 3 × 10-2 mmol) and lithium
2,6-dimethylbenzenethiolate (4 mg, 3 × 10-2 mmol) were placed in
an NMR tube connected to a Schlenk adapter. 0.8 mL of C6D6 was
added to the reaction vessel, the mixture was frozen at -196 °C, and
the vessel was evacuated. The NMR tube was then flame sealed and
thawed to room temperature. An immediate color change from pale
orange to red-orange occurred with the formation of a pale precipitate
(LiCl). The initial conversion to (η5-NC4Me4)TaMe3(dmpt) was 71%
by 1H NMR, but the compound showed significant decomposition after
1 day in solution at room temperature. The complex decomposed when
isolation and purification were attempted. 1H NMR (C6D6): δ 7.08
(d, 2H, SPhMe2); 6.90 (t, 1H, SPhMe2); 2.85 (s, 6H, SPhMe2); 1.91,
1.24 (2s, each 6H, TMP); 0.72 (s, 9H, TaMe).
Thermal Decomposition of (η5-NC4Me4)TaMe3(NC4H4) (4). In
a typical experiment complex 4 (15 mg, 0.036 mmol) was dissolved
in 0.6 mL of C6D6, the solution was degassed in two freeze-pump-
thaw cycles, and the NMR tube was flame sealed under vacuum. The
reaction was monitored by NMR at room temperature. The decomposi-
tion product (η5-NC4Me4)TaMe2(NC4H4)2 (6) grew in slowly over the
1
course of 44 days, and at this time the yield of 6 was 27% (by H
NMR). Resonances for other tantalum products were not observed.
1H NMR (C6D6): δ 7.00, 6.43 (2m, each 4H, pyrr); 1.73, 1.35 (2s,
each 6H, TMP); 0.91 (s, 6H, TaMe). 13C NMR (C6D6): δ 138.0, 133.5
(TMP); 130.2, 113.5 (pyrr); 67.3 (TaMe); 13.4, 10.0 (TMP-Me). MS
(CI-): m/z 465 (M).
Thermal Decomposition of (η5-NC4Me4)TaMe3(indolyl) (5). Com-
plex 5 (15 mg, 0.032 mmol) was dissolved in 0.6 mL of C6D6, the
Synthesis of (η5-NC4Me4)TaMe3(indolyl) (5). Complex 1 (218 mg,
0.57 mmol) was dissolved in 15 mL of diethyl ether and added to a
suspension of indolyllithium (70 mg, 0.57 mmol) in 15 mL of diethyl

3234 Inorganic Chemistry, Vol. 35, No. 11, 1996
Parker et al.
solution was degassed in two freeze-pump-thaw cycles, and the NMR
tube was flame sealed under vacuum. The reaction was monitored by
NMR at room temperature. The decomposition product (η5-NC4Me4)-
TaMe2(indolyl)2 grew in very slowly; and after 44 days, the yield was
25% (by 1H NMR). 1H NMR (C6D6): δ 7.68, 7.57 (2d, each 2H, J )
8 Hz, ind); 7.27 (m, 4H, ind); 6.93, 6.49 (2d, each 2H, J ) 3 Hz, ind);
2.04, 1.78 (2s, each 6H, TMP); 1.41 (s, 6H, TaMe). 13C NMR
(C6D6): δ 145.2, 133.0 (2s, TMP); 137.6, 122.7, 122.5, 121.0, 117.2,
108.2 (6 s, ind); 125.9, 121.3 (2 s, ind quart C); 80.3 (s, TaMe); 12.2,
8.7 (2 s, TMP Me).
Hydrogenation of Toluene-d8. Complex 4 (9 mg, 0.02 mmol) and
hydrogen (2.5 atm) were combined in 0.6 mL of toluene-d8, and the
reaction tube was flame sealed. After 2 weeks the volatiles were
vacuum transferred and GC/MS data qualitatively confirmed the
formation of methylcyclohexane-d8. The product was not successfully
identified by 1H or 13C NMR. GC/MS: m/z 106 (M). In a competition
experiment, 10 mg of 4 was dissolved in 0.3 mL of benzene-d6 (3 mmol)
and 0.3 mL of toluene-d8 (3 mmol) and hydrogen (2.5 atm) were added.
The reaction tube was flame sealed and maintained at room temperature
for 2 weeks. The volatiles were then vacuum transferred and analyzed
by GC/MS. The data established that methylcyclohexane-d8 had formed
but no cyclohexane-d6 was detected.
Hydrogenation of Naphthalene. Complex 4 (18 mg, 0.043 mmol)
and naphthalene (140 mg, 1.09 mmol) were combined in 5 mL of THF.
The Schlenk reaction tube was evacuated, and 2.5 atm of H2 was added.
After 2 weeks stirring at room temperature, the solution was concen-
trated to ca. 2 mL in vacuo, and the GC/MS experiment was performed.
The data gave evidence for the partial hydrogenation of naphtha-
lene to tetrahydronaphthalene. No other reduced species were detected.
GC/MS: m/z 132 (M).
Formation of (η5-NC4Me4)TaMe(NC4H4)(η2-Me2CO). Complex
4 (10 mg, 0.024 mmol) was dissolved in 0.6 mL of C6D6 in an NMR
tube. The yellow solution was frozen under vacuum. Carbon monoxide
(243 Torr, 0.024 mmol) was added, and the tube was flame sealed. On
thawing, the solution became orange at the solvent/gas interface, and
1
upon shaking the solution became dark orange. An initial H NMR
spectrum showed evidence for (η5-NC4Me4)TaMe(pyrr)(η2-Me2CO).
1H NMR (C6D6): δ 7.79 (m, 2H, pyrr); 6.55 (m, 2H, pyrr); 2.14 (s,
3H, η2-Me2CO); 1.70 (s, 3H, η2-Me2CO); 1.94 (s, 6H, TMP); 1.46 (s,
6H, TMP); 0.25 (s, 3H, TaMe). Within 30 min the compound had
decomposed significantly. Free pyrrole and tetramethylpyrrole were
observed in the spectrum at this time.
Acknowledgment. Support for this work by the Division
of Chemical Sciences, Office of Basic Energy Sciences, Office
of Energy Research, U.S. Department of Energy, is gratefully
aknowleged.
Hydrogenation of Benzene-d6. Complex 4 (10 mg, 0.02 mmol)
was dissolved in 0.6 mL of C6D6. The tube was evacuated, and H2
(3.5 atm) was added to the tube. The tube was flame sealed, and the
solution was shaken at room temperature. After 1 week the reaction
appeared to be complete, and the spectrum indicated that cyclohexane-
d6 had formed in 84% yield. The tube was opened, the volatiles were
transferred, and the GC/MS data were obtained. 1H NMR (C6D6): δ
1.36 (s, 6H, C6D6H6). 13C NMR (C6D6): δ 26.5 (t, JC-D ) 19 Hz,
C6D6H6). GC/MS: m/z 90 (M).
Supporting Information Available: Details of data collection and
refinement and complete tables of bond distances and angles and
positional and thermal parameters for 2 and 5 (18 pages). Ordering
information is given on any current masthead page.
IC9509544