Hydrogenation mechanisms in (boratacycle)tantalum analogues of dimethylzirconocene

Article abstract of DOI:10.1021/ja982803g

The hydrogenation of Cp*[C₄H₄B-N(i-Pr)₂]TaMe₂ (1) (Cp* = C₅Me₅) in the presence of PMe₃ affords Cp*[C₄H₄B-N(i-Pr)₂]Ta(H) ₂(PMe₃) (2) in essentially quantitative yield. Similarly, the hydrogenation of Cp*[C₄H₄B-Me]TaMe₂ (3) in the presence of PMe₃ affords Cp*[C₄H₄B-Me]Ta(H)₂(PMe₃) (4). Hydrogenation of 1 and 3 is accompanied by the reversible formation of side products. The most important of these complexes, Cp*[C₄H₄B-N(i-Pr)₂]Ta(PMe ₃)₂ (5) and Cp*[C₄H₄B-Me]Ta(PMe₃)₂ (6), react slowly with dihydrogen forming 2 and 4, respectively. In the early stages of the hydrogenation of 1, the C-H activation product Cp*[C₄H₄B-N(i-Pr)₂]Ta(H)(CH ₂PMe₂) (7) is also present. Mechanistic details of the hydrogenation of 1 and 3 are discussed. Hydrogenation of [C₅H₅B-Ph][C₄H₄B-N(i-Pr) ₂]TaMe₂ (8) in the presence of PMe₃ affords [C₅H₅B-Ph][C₄H₄B-N(i-Pr) ₂]Ta(PMe₃)₂ (9) as the exclusive product. The use of a bulkier phosphine, P(i-Pr)₃, gives [C₅H₅B-Ph]-[C₄H₄B-N(i-Pr) ₂]Ta(H)₂[P(i-Pr)₃] (10). Changing the phosphine to one of intermediate bulk, PEt₃, leads to the formation of trans-[C₅H₅B-Ph][C₄H₄B-N(i-Pr) ₂]Ta(H)₂(PEt₃) (11t). The cis isomer (11c) is observable during early reaction times. 11c is a classical dihydride, perturbed by an unsymmetric three-center/two-electron interaction with the boron of the boratabenzene ligand. Isomerization of 11c to 11t proceeds via phosphine loss followed by kinetically detectable rearrangement of the unsaturated intermediate prior to phosphine recoordination. Treatment of 11c with excess PMe₃ results in the formation of 9 via a mixed-phosphine intermediate, [C₅H₅B-Ph][C₄H₄B-N(i-Pr) ₂]Ta(PEt₃)(PMe₃) (12). The addition of [H(OEt₂)₂][B(C₆H₃(CF ₃)₂] to 11c results in the protonation of the nitrogen atom of the borollide ligand (H-11c⁺). H-11c⁺ is stable at room temperature for over a week. Treatment of 10 with excess PMe₃ affords [C₅H₅B-Ph][C₄H₄B-N(i-Pr) ₂]Ta(H)₂-(PMe₃) (13). Upon thermolysis in the presence of a large excess of PMe₃, 13 is converted to 9. A mechanistic scheme for the hydrogenation of complexes such as 1 is proposed.

Full text of DOI:10.1021/ja982803g

J. Am. Chem. Soc. 1999, 121, 1513-1523
1513
Hydrogenation Mechanisms in (Boratacycle)tantalum Analogues of
Dimethylzirconocene
Caroline K. Sperry,† Guillermo C. Bazan,*,†,§ and W. Donald Cotter*,‡
Contribution from the Department of Chemistry, UniVersity of Rochester,
Rochester, New York 14627-0216, and Department of Chemistry, Mount Holyoke College,
South Hadley, Massachusetts 01075
ReceiVed August 6, 1998
Abstract: The hydrogenation of Cp*[C4H4B-N(i-Pr)2]TaMe2 (1) (Cp* ) C5Me5) in the presence of PMe3
affords Cp*[C4H4B-N(i-Pr)2]Ta(H)2(PMe3) (2) in essentially quantitative yield. Similarly, the hydrogenation
of Cp*[C4H4B-Me]TaMe2 (3) in the presence of PMe3 affords Cp*[C4H4B-Me]Ta(H)2(PMe3) (4). Hydrogenation
of 1 and 3 is accompanied by the reversible formation of side products. The most important of these complexes,
Cp*[C4H4B-N(i-Pr)2]Ta(PMe3)2 (5) and Cp*[C4H4B-Me]Ta(PMe3)2 (6), react slowly with dihydrogen forming
2 and 4, respectively. In the early stages of the hydrogenation of 1, the C-H activation product Cp*[C4H4B-
N(i-Pr)2]Ta(H)(CH2PMe2) (7) is also present. Mechanistic details of the hydrogenation of 1 and 3 are discussed.
Hydrogenation of [C5H5B-Ph][C4H4B-N(i-Pr)2]TaMe2 (8) in the presence of PMe3 affords [C5H5B-Ph][C4H4B-
N(i-Pr)2]Ta(PMe3)2 (9) as the exclusive product. The use of a bulkier phosphine, P(i-Pr)3, gives [C5H5B-Ph]-
[C4H4B-N(i-Pr)2]Ta(H)2[P(i-Pr)3] (10). Changing the phosphine to one of intermediate bulk, PEt3, leads to the
formation of trans-[C5H5B-Ph][C4H4B-N(i-Pr)2]Ta(H)2(PEt3) (11t). The cis isomer (11c) is observable during
early reaction times. 11c is a classical dihydride, perturbed by an unsymmetric three-center/two-electron
interaction with the boron of the boratabenzene ligand. Isomerization of 11c to 11t proceeds via phosphine
loss followed by kinetically detectable rearrangement of the unsaturated intermediate prior to phosphine
recoordination. Treatment of 11c with excess PMe3 results in the formation of 9 via a mixed-phosphine
intermediate, [C5H5B-Ph][C4H4B-N(i-Pr)2]Ta(PEt3)(PMe3) (12). The addition of [H(OEt2)2][B(C6H3(CF3)2] to
11c results in the protonation of the nitrogen atom of the borollide ligand (H-11c+). H-11c+ is stable at room
temperature for over a week. Treatment of 10 with excess PMe3 affords [C5H5B-Ph][C4H4B-N(i-Pr)2]Ta(H)2-
(PMe3) (13). Upon thermolysis in the presence of a large excess of PMe3, 13 is converted to 9. A mechanistic
scheme for the hydrogenation of complexes such as 1 is proposed.
Introduction
LUMO. On the basis of the different hydrogenation rates of
complexes of type Cp2ZrR(X), Schwartz suggested that the
Lewis acidic Zr(IV) center polarizes the coordinated H-H
bond.1e Hu¨ckel-type calculations led Brintzinger to invoke back-
donation from the Zr-C bond into the σ* orbital of the
precoordinated H2 molecule.1f
The hydrogenation of group 4 metallocene dialkyls to their
corresponding dihydride complexes has been studied in some
detail.1 Interest in this type of reaction stems from the
importance of metal-mediated dihydrogen activation in many
stoichiometric and catalytic processes.2 Understanding the
elementary molecular processes involved in this class of
reactions should ultimately lead to more efficient catalysts and
improved industrial processes. The most commonly accepted
mechanism for the reaction of Cp2ZrMe2 (Cp ) C5H5) with H2
involves direct cleavage1d via a four-center/four-electron transi-
tion state (also known as σ-bond metathesis).3 Early proposals
for this mechanism discussed the importance of a weak direct
interaction between the H2 σ-bond and the zirconium-based
Alternative, low-energy pathways for hydrogenolysis appear
to be available. In particular an oxidative addition/reductive
† University of Rochester.
(2) (a) Parshall, G. W.; Ittel, S. D. Homogeneous Catalysis; Wiley: New
York, 1992; pp 25-50. (b) James, B. R. Homogeneous Hydrogenation;
Wiley: New York, 1973. (c) James, B. R. Hydrogenation Reactions
Catalyzed by Tansition Metal Complexes. AdV. Catal. 1979, 17, 319. (d)
James, B. R. In ComprehensiVe Organometallic Chemistry; Abel, E. W.,
Stone, F. G. A., Wilkinson, G., Eds.; Pergamon Press: New York, 1982;
Vol. 8. (e) McQuillen, F. J. Homogeneous Hydrogenation in Organic
Chemisty; D. Reidel: Dordrecht, 1976. (f) Chaloner, P. A. Handbook of
Coordination Catalysis in Organic Chemistry; Butterworth: London, 1986.
(g) Rylander, P. Catalytic Hydrogenation in Organic Sytnheses; Academic
Press: New York, 1979. (h) Rylander, P. Hydrogenation Methods;
Academic Press: New York, 1985. (i) Chaloner, P. A.; Esteruelas, M. A.;
Joo´, F.; Oro, L. A. Homogeneous Hydrogenation; Kluwer Academic
Publishers: Dordrecht, 1994.
‡ Mount Holyoke College.
§ Current address: Department of Chemistry, University of California,
Santa Barbara, CA 93106.
(1) (a) Jordan, R. F.; Bajgur, C. S.; Dasher, W. E.; Rheingold, A. L.
Organometallics 1987, 6, 1041-1051. (b) Couturier, S.; Gautheron, B. J.
Organomet. Chem. 1978, 157, C61-C63. (c) Wolczanski, P. T.; Bercaw,
J. E. Organometallics 1982, 1, 793-799. (d) Halpern, J. J. Phys. Chem.
1959, 63, 398-403. (e) Gell, K. I.; Posin, B.; Schwartz, J.; Williams, G.
M. J. Am. Chem. Soc. 1982, 104, 1846-1855. (f) Brintzinger, H. H. J.
Organomet. Chem. 1979, 171, 337-344. (g) McAlister, D. R.; Erwin, D.
K.; Bercaw, J. E. J. Am. Chem. Soc. 1978, 100, 5966-5968. (h) Lee, H.;
Desrosiers, P. J.; Guzei, I.; Rheingold, A. L.; Parkin, G. J. Am. Chem. Soc.
1998, 120, 3255-3256.
10.1021/ja982803g CCC: $18.00 © 1999 American Chemical Society
Published on Web 02/09/1999

1514 J. Am. Chem. Soc., Vol. 121, No. 7, 1999
Sperry et al.
Table 1. Compound Abreviation Index
elimination sequence is of interest. Such a pathway formally
requires the participation of six electrons, two of which must
originally reside on the metal.4 Given that zirconium in Cp2-
ZrMe2 is in its highest oxidation state, it is difficult to consider
this mechanism directly without prior cyclopentadienyl ligand
rearrangement. Bercaw has suggested that the Cp* (Cp* ) C5-
Me5) ligand can participate directly in an intramolecular
rearrangement, which generates a formally reduced (Zr(II))
metal center.1g Oxidative addition followed by reductive elimi-
nation leads to rapid (t1/2 ≈ 5 min at -15 °C, 1 atm of H2)
formation of products. This mechanism accounts for the
distribution of products derived from isotopomers of Cp*2ZrH-
(CH2CHMe2) and H2.
compd no.
compd formula
Cp*[C4H4B-N(i-Pr)2]TaMe2
1
2
3
Cp*[C4H4B-N(i-Pr)2]Ta(H)2(PMe3)
Cp*[C4H4B-Me]TaMe2
4
5
Cp*[C4H4B-Me]Ta(H)2(PMe3)
Cp*[C4H4B-N(i-Pr)2]Ta(PMe3)2
6
7
8
9
Cp*[C4H4B-Me]Ta(PMe3)2
Cp*[C4H4B-N(i-Pr)2]Ta(H)(CH2PMe2)
[C5H5B-Ph][C4H4B-N(i-Pr)2]TaMe2
[C5H5B-Ph][C4H4B-N(i-Pr)2]Ta(PMe3)2
[C5H5B-Ph][C4H4B-N(i-Pr)2]Ta(H)2[P(i-Pr)3]
[C5H5B-Ph][C4H4B-N(i-Pr)2]Ta(H)2(PEt3)
[C5H5B-Ph][C4H4B-N(i-Pr)2]Ta(PEt3)(PMe3)
[C5H5B-Ph][C4H4B-N(i-Pr)2]Ta(H)2(PMe3)
10
11c/t
12
13
intermediates which may not be apparent in the slower
processes.
In this contribution, we present a more detailed mechanistic
account of the hydrogenation of 1 and related molecules. We
argue that the relative rates of activation of H2 by 1 and related
complexes are understood by a reaction energy profile (σ-bond
metathesis reaction) dominated by the stability of the dihydrogen
adduct as originally suggested by Brintzinger.1f We show
evidence that the final hydride product is a result of H2 oxidative
addition to a low-valent intermediate produced by reductive
elimination of CH4 from Cp*[C4H4B-N(i-Pr)2]Ta(H)Me. Finally,
from an analysis of the hydrogenation profile of related
compounds, we delineate the pathway by which oxidative
addition of H2 produces the dihydride products.
Different mechanistic variants have thus been proposed for the
hydrogenation of ziroconocene complexes, and the choice of
ancillary ligand appears to influence the preferred pathway.
In general, such reactions are slow under moderate (ca. 1
atm) pressures of dihydrogen. Jordan has estimated t1/2 for the
hydrogenolysis (1 atm of H2) of Cp2ZrMe2 in THF at >86 h.1a
Synthetic procedures for preparation of zirconocene dihydrides
via hydrogenolysis of dimethyl precursors typically call for
elevated (60-100 atm) pressures of H2.1b The most facile
reported hydrogenation of a simple dimethylzirconocene deriva-
tive, Cp*2ZrMe2, can be conducted under 1 atm of H2, but
requires an extended reaction time (1 week) and somewhat
elevated temperatures (70 °C).5
Results and Discussion
Hydrogenation of 1. As shown in eq 1, the addition of H2
to 1 in the presence of PMe3 affords Cp*[C4H4B-N(i-Pr)2]Ta-
(H)2(PMe3) (2) in essentially quantitative yield, as determined
by NMR spectroscopy. When Cp*[C4H4B-Me]TaMe2 (3) is
used, the final product is Cp*[C4H4B-Me]Ta(H)2(PMe3) (4).
Phosphine is required in these reactions to stabilize the dihydride
products.
We recently reported a preliminary study on the hydrogenoly-
sis of Cp*[C4H4B-N(i-Pr)2]TaMe2 (1, see Table 1), which
contains the dianionic borollide ligand, [C4H4B-N(i-Pr)2]2- 6
.
Complex 1 is isostructural and may be considered in some
respects (e.g., formal electron count at the metal) isoelectronic
to group 4 metallocene dialkyls. However, the rate of reaction
with dihydrogen of 1 is about 3 orders of magnitude faster than
that of Cp2ZrMe2 and at least 2 orders of magnitude faster than
that of Cp*2ZrMe2.7 This faster rate allows us to probe the
hydrogenation process more readily and to detect short-lived
The rates of disappearance of both 1 and 3 are linearly dependent
on the concentration of starting material and dihydrogen
pressure, at least in the limited range of pressures accessible
for NMR spectroscopy studies (Figure 1). Under identical
reaction conditions 1 is consumed more rapidly than 3 by a
factor of approximately 3.8 (Figure 2).
(3) (a) Thompson, M. E.; Bercaw, J. E. Pure Appl. Chem. 1984, 56,
1-11. (b) Steigerwald, M. L.; Goddard, W. A., III J. Am. Chem. Soc. 1984,
106, 308-311. (c) 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, 203-219. (d) Rappe´, A. K. Organometallics
1990, 9, 466-475. (e) Ziegler, T.; Folga, E.; Berces, A. J. Am. Chem. Soc.
1993, 115, 5, 636-646.
The electronic structures of 1 and 3 are affected by the nature
of the exocyclic substituent on boron.8 Complexes containing
(4) (a) Crabtree, R. H. The Organometallic Chemistry of the Transition
Metals, 2nd ed.; Wiley: New York, 1994. (b) Lukehart, C. M. Fundamental
Transition Metal Organometallic Chemistry; Brooks-Cole: Pacific Grove,
1985. (c) Yamamoto, A. Organotransition Metal Chemistry; Wiley-
Interscience: New York, 1986. (d) Collman, J. P.; Hegedus, L. S.; Norton,
J. R.; Finke, R. G. Principles and Applications of Organotransition Metal
Chemistry; University Science Books: Mill Valley, 1987.
(5) Schock, L. E.; Marks, T. J. J. Am. Chem. Soc. 1988, 110, 7701-
7715.
(7) Detailed kinetic studies have not been performed, and qualitative
published reports conflict concerning the rate of hydrogenolysis of Cp*2-
ZrMe2. Sanner (Miller, F. D.; Sanner, R. D. Organometallics 1988, 7, 818-
825) reports that this reaction requires one week at ca. 100 atm of H2. We
base our comparison on Marks’ report (one week to completion at 70 °C,
cited above) and a reaction time of about an hour for complete consumption
of 1; the temperature difference is not accounted for here. Jordan has
cautioned that comparisons of Cp-Zr and Cp*-Zr fragments in this context
are complicated by the fact that Cp* may induce alternative mechanisms.1a
(8) Herberich, G. E.; Hessner, B.; Ohst, H.; Raap, I. A. J. Organomet.
Chem. 1988, 348, 305-316.
(6) Kowal, C. M.; Bazan, G. C. J. Am. Chem. Soc. 1996, 118, 10317-
10318.

(Boratacycle)tantalum Analogues of Dimethylzirconocene
J. Am. Chem. Soc., Vol. 121, No. 7, 1999 1515
Figure 3. Concentration profile of 1, 2, and 5 as a function of [PMe3]:
(A) 5 equiv of PMe3; (B) 9 equiv of PMe3.
Scheme 1
Figure 1. Observed rates for the disappearance of 1 ([Ta]o ) 0.038
M) in C6D6 with 5 equiv of PMe3.
Increasing the concentration of phosphine does not affect the
rate of consumption of 1 or 3 but decreases the rate of
appearance of 2 or 4, producing instead larger amounts of a
new organometallic species, namely Cp*[C4H4B-N(i-Pr)2]Ta-
(PMe3)2 (5) or Cp*[C4H4B-Me]Ta(PMe3)2 (6). Figure 3 shows
the concentration profiles of 1, 2, and 5 for different PMe3
concentrations. As the concentration of PMe3 increases the ratio
of 2 to 5 at early reaction times shifts in favor of 5. Figure 3
also shows that the concentration of 2 increases sharply while
1 is present in solution, after which a second, slower process
takes place. The two distinct slopes for the appearance of product
require two independent paths for its production. In our initial
contribution we proposed that, in the case of 1, the common
intermediate for the two paths is Cp*[C4H4B-N(i-Pr)2]Ta.6
Oxidative addition of H2 followed by trapping with PMe3 forms
2. Alternatively, Cp*[C4H4B-N(i-Pr)2]Ta coordinates 2 equiv
of PMe3 to give 5. Loss of phosphine, followed by H2 addition
represents the slower process for formation of 2. Scheme 1
summarizes our original proposed mechanism.
Figure 2. Representative data for the disappearance of 1 and 3 in C6D6
([Ta]o ) 0.022 M, P(H2) ) 620 Torr, 5 equiv of PMe3).
the aminoborollide ligand are formally ambivalent since three
resonance structures are required to describe the metrical
parameters of their crystallographically determined structures.
Careful determination of mass balance in the 1H NMR spectra
of the reaction mixtures reveals the presence of an additional
species. As shown in Figure 4, this species is observed at
The boron atom in the methylborollide ligand cannot form a π
bond with the exocyclic group and thus 3 is best described by
the high-valent structure corresponding to resonance contribution
A, as shown above.9
(9) Sperry, C. K.; Cotter, W. D.; Lee, R. A.; Lachicotte, R. J.; Bazan,
G. C. J. Am. Chem. Soc. 1998, 120, 7791-7805.

1516 J. Am. Chem. Soc., Vol. 121, No. 7, 1999
Sperry et al.
Figure 4. Full concentration profile for the disappearance of 1, with
Figure 5. Phosphine dependence for conversion of 5 to 2.
mass balance.
Scheme 2
In an attempt to define better the relative importance of the
reactive hydrogenation intermediate (i.e., Int1 or Int2 in Scheme
1), the kinetics of hydrogenation for compound 5, generated in
situ from hydrogenation of 1, were studied (eq 2). The rate of
disappearance of 5 was measured by 1H NMR spectroscopy at
25 °C, after complete consumption of 1. In the presence of
varying concentrations of PMe3 (ca. 0.1-0.6 M, [Ta]0 ) 0.023
M), the rate of loss of 5 was identical to the formation rate of
2 and obeyed pseudo-first-order kinetics in PMe3 over at least
3 half-lives.
significant concentrations during and immediately after con-
sumption of 1. It is characterized by the appearance of an
additional Cp* resonance (15 H) and a pair of high-field doublets
(1 H each). These doublets are coupled to a resonance in the
31P NMR spectrum of the reaction mixture at δ -50.3 (JPH
)
5.0 Hz). The spectral features of this species are consistent with
a phosphinomethanide structure, Cp*[C4H4B-N(i-Pr)2]Ta(H)-
(CH2PMe2) (7), with an uncoordinated phosphorus atom.10 We
have not been able to locate the hydride resonance for 7.
However, its structural assignment is supported by the observa-
tion that when PMe3-d9 is used in place of PMe3, the appearance
of the characteristic doublets is completely suppressed, dem-
onstrating that they originate from the phosphine methyl groups.
The rate of disappearance of 1 is not affected by the isotopic
constitution of PMe3, but the rate of formation of 7 is much
slower in the presence of PMe3-d9 (kH/kD ≈ 3-4), indicating
substantial perturbation of the C-H bond in PMe3 in the rate-
determining step for the formation of 7.
The results of the late-reaction kinetic studies are shown in
Figure 5. Hydrogenation of 5 is suppressed by PMe3, as
-1
anticipated. A plot of (kobs
)
vs [PMe3] is linear in the
concentration range used in these studies11 and passes through
an intercept of approximately zero.12 These observations are
accounted for by a mechanism in which compound 5 participates
in a rapid (relative to subsequent addition of H2) equilibrium
with a small amount of Int2 (Scheme 2). The first-order
suppression of the rate by [PMe3] indicates that loss of both
phosphine ligands to form the “naked” tantalum species Int1
does not occur to a detectable extent. The formation of Int1 in
kinetically significant amounts during the early stages of
hydrogenolysis, i.e., before complete consumption of 1, is not
necessarily contradicted by these observations.
(11) It is not immediately obvious that pseudo-first-order kinetics should
apply in the case of relatively small excesses of phosphine, e.g., 5 equiv,
but they do. In this case, at the point of complete consumption of 1 about
75% of the total Ta is present as the final product, 2. PMe3 is thus present
in approximately a 14-fold excess compared to the reactive species, 5. In
all cases, [PMe3] is corrected for the amount of phosphine retained by 5
and 2 at full consumption of 1.
(10) In isoelectronic zirconocene systems, Cp2Zr(Cl)CH2PR2, the 31P
chemical shift for the η1 isomer resembles closely that of the free phosphine
while the chemical shift of the η2 isomer is similar to that of the coordinated
phosphine. (a) Karsch, H. H.; Denbelly, B.; Hoffmann, J.; Pieper, U.; Mu¨ller,
G. J. Am. Chem. Soc. 1988, 11, 3654-3656. (b) Karsch, H. H.; Mu¨ller, G.;
Kru¨ger, C. J. Organomet. Chem. 1984, 273, 195-212.
(12) The apparent negative intercept (-4(3) × 104) is not significant
compared to the uncertainty in the measurement.

(Boratacycle)tantalum Analogues of Dimethylzirconocene
J. Am. Chem. Soc., Vol. 121, No. 7, 1999 1517
Scheme 4
Scheme 5
Figure 6. First-order rate plots for the reaction of 1 with H2 and D2
(kH ) 7.2(1) × 10-4 s-1; kD ) 8.3(2) × 10-4 s-1; kH/kD ) 0.87(9)).
Scheme 3
analysis of the methane produced revealed an isotopomer ratio
of 1.3(1):1 (CH4:CH3D).13
The discrepancy between the kinetic isotope effect for
consumption of starting material and the isotopomer distribution
in the products from the 50:50 H2/D2 reaction indicates the
presence of an intermediate capable of H/D exchange. We
propose that the intermediate is Cp*[C4H4B-N(i-Pr)2]TaMeH
(Scheme 4) and point out that [Cp2ZrMe(µ-H)]2 forms in the
reactions of Cp2ZrMe2 with low pressures of H2.1a Consistent
with this proposal, a fleetingly observable species appears, in
very low concentrations, in the early stages of hydrogenation
of 1, identified by a methyl resonance at δ -0.64 ppm. This
resonance is a doublet in reaction mixtures containing H2 (3JHH
) 4.4 Hz), but a singlet under D2, indicating coupling to a single
proton derived from gaseous dihydrogen. We cannot identify
this species more accurately, but these spectroscopic properties
are consistent with a methyl hydride structure. We tentatively
assign it as Cp*[C4H4B-N(i-Pr)2]TaMeH. The higher ratio of
CH4 relative to CH3D in the presence of an excess mixture of
H2 and D2 indicates an equilibrium isotope effect on the
scrambling process, in which Ta-H is preferred relative to
Ta-D.
The hydrogenation of 1 in the absence of phosphine provides
an intermediate that ultimately leads to a yet unidentified
product.6 Spectroscopic characterization for the intermediate is
consistent with the formula Cp*[C4H4B-N(i-Pr)2]TaH2. In one
experiment the intermediate concentration was maximized and
the solution was immediately subjected to 3 freeze-pump-
thaw cycles to remove as much solvated hydrogen as possible.
At that stage an excess of PMe3 was condensed into the reaction
1
mixture and allowed to react. H NMR spectroscopy showed
that only 2 had formed. No 5 could be detected. These
experiments strongly suggest that the intermediate we observe
is Cp*[C4H4B-N(i-Pr)2]TaH2 and that this compound is inert
to reductive elimination of H2. Formation of 5 would have
implicated a facile reductive elimination step.
Hydrogenation of [C5H5B-Ph][C4H4B-N(i-Pr)2]TaMe2 (8).
The relationship between boratabenzene and cyclopentadienyl
ligands has been studied in some detail.14 Since both ligands
are 6 π electron donors it is possible in many instances to
exchange a Cp framework within a complex for a boratabenzene
A set of kinetic deuterium isotope effect measurements were
made to probe the nature of dihydrogen activation by 1 (Scheme
3). When the rate of disappearance of 1 (in benzene) is measured
independently with H2 and D2 (Scheme 3a), the ratio kH /kD is
2
2
approximately 0.87(9) (Figure 6). The magnitude of this ratio
and the error associated with its determination indicate no kinetic
isotope effect within statistical certainty. In a separate experi-
ment, equal pressures of H2 and D2 were thoroughly mixed in
a high-vacuum manifold and added to a benzene solution
containing 1 and 5 equiv of PMe3 (Scheme 3b). 1H NMR
(13) The solubilities of H2 and D2 in benzene are very similar. The
solubility of D2 exceeds that of H2 by 3%. Cook, M. W.; Hanson, D. N.;
Alder, B. J. J. Chem. Phys. 1957, 26, 748-751.
(14) For a recent review of boratabenzene complexes see: Herberich,
G. E. In ComprehensiVe Organometallic Chemistry II; Abel, E. W., Stone,
F. G. A., Wilkinson, G., Eds.; Pergamon Press: Oxford, 1995; Vol. 1, p
197.

1518 J. Am. Chem. Soc., Vol. 121, No. 7, 1999
Sperry et al.
counterpart without gross changes in molecular geometry or
electronic structure. Recent advances in the area of metallocene-
like complexes supported by boratabenzene have shown that
the selectivity of important catalytic reactions can be controlled
by the exocyclic boron substitutent.15 It occurred to us that
replacement of Cp* in 1 with a boratabenzene ligand would
result in complexes which react with H2 in a similar fashion
but which could lead to the detection of intermediates not
observable in the hydrogenation of 1. In this section we describe
the hydrogenation chemistry of [C5H5B-Ph][C4H4B-N(i-Pr)2]-
TaMe2 (8), including the characterization of important inter-
mediates and processes undetectable with Cp* complexes.
depending on the supporting ligands on Ta. In general, electron-
withdrawing ligands favor dihydrogen adduct formation by
reducing the magnitude of back-bonding into σ*HH. Thus, cis-
[Cp2Ta(H2)CO]+ is a dihydrogen adduct in the ground state,
while cis-{Cp2Ta(H)2[P(OMe)3]}+ is a classical dihydride.
Complex 11c adopts a classical structure, as shown by the
absence of detectable HD coupling in the NMR spectrum of
11c-d1.19 Chaudret has shown that the H-H distance (rHH) in
cis-tantalocene dihydrides can be estimated reliably from spin-
lattice relaxation data, using the T1(min) value for the trans
isomer to correct for interactions other than hydride-hydride
relaxation. The T1 value for 11c passes through a minimum of
165 ms at 248 K;20 for 11t, T1(min) ) 358 ms at 208 K. With
use of the approximation that motion about the H-H vector is
slow compared to τc (245 ps at 400 MHz), the equations
described by Morris21 yield rHH ) 1.76 Å for 11c. The H-H
distance in 11c is comparable to that determined for the classical
dihydride cis-{Cp2Ta(H)2[P(OMe)3]}+ (1.67 Å),18 and to the
H-H separation observed in the neutron diffraction structure
of Cp2TaH3 (1.85 Å), and it is thoroughly consistent with a
classical dihydride ground state. The low-valent resonance
structure B presumably renders the tantalum atom in 11c
sufficiently electron-rich to favor the dihydride despite the
inductive effect anticipated from the two boron atoms in the
cyclic ligands.
Further structural information about 11c is found in the 11B
NMR spectra. The boron atom in coordinated borollide and
boratabenzene normally resonates near 30 ppm (relative to BF3‚
OEt2) in electrophilic complexes. For example, the borataben-
zene and borollide borons in 8 resonate at 36.5 and 34.9 ppm,
respectively. Exceptions occur when boron participates in a
three-center interaction with a hydride ligand, as has been
reported previously for 11t.9 The boratabenzene boron is
significantly shifted in 11t, to 19 ppm, and a three-center
interaction with one of the hydride ligands was confirmed by
X-ray analysis. The 11B NMR signal for the boratabenzene boron
in 11c is similarly shifted, resonating at 10.8 ppm, compared
to 30.8 ppm for the borollide boron. Thus, a three-center
interaction is also indicated in 11c, most probably with the exo
hydride. The Ta-H-B interaction for 11c appears to be stronger
than that in 11t, as the boron signal is even further upfield.
11c is thus a classical dihydride complex, perturbed by an
unsymmetric three-center/two-electron interaction with boron.
However, the balance between classical and nonclassical
structures is clearly a delicate one in tantalum sandwich
compounds.19 The possibility of a low-lying dihydrogen tau-
tomer cannot be dismissed. The chemical behavior of 11c is
consistent with this possibility. In particular, the hydride ligands
are very labile. H/D exchange of the hydrides in 11c is complete
within 5 min under 1 atm of D2.
When 8 is treated with H2 in the presence of PMe3 the only
product obtained is [C5H5B-Ph][C4H4B-N(i-Pr)2]Ta(PMe3)2 (9
in Scheme 5). No intermediates are observed by 1H NMR
spectroscopy. Formation of a dihydride complex fails, even at
H2 pressures of 15-20 atm.16 Changing the phosphine to bulkier
P(i-Pr)3 allows for the quantitative formation of [C5H5B-Ph]-
[C4H4B-N(i-Pr)2]Ta(H)2(P(i-Pr)3) (10 in Scheme 5), analogous
to 2.
Use of a phosphine of intermediate bulk, PEt3, leads eventu-
ally to formation of trans-[C5H5B-Ph][C4H4B-N(i-Pr)2]Ta(H)2-
(PEt3) (11t in Scheme 5). The crystallographically determined
molecular structure of 11t has been shown to be largely similar
to those of 2 and 4, except that it shows evidence of a Ta-
H-B three-center interaction with the boratabenzene boron
atom.17
Monitoring the progress of the reaction with PEt3 reveals a
different dihydride species during early reaction times. Variable-
temperature 1H and 31P{1H} NMR spectra show that this
intermediate is the cis-dihydride (11c in Scheme 5) that
rearranges to form the thermodynamically preferred 11t. The
room-temperature spectrum of 11c in THF-d8 contains one
doublet for the rapidly exchanging pair of hydrides (δ ) -2.80;
JPH ) 48 Hz). The borollide and boratabenzene protons are
diastereotopic. At low temperature (T ) 198 K), two inequiva-
lent hydride signals are observed. The different values of JPH
for the two inequivalent hydrides allow for the assignment of
the endo and exo hydride signals (δendo ) -2.60, JPH ) 86.5
Hz; δexo ) -3.19, JPH ) 9.2 Hz).18
Displacement of the two hydride ligands by trimethylphos-
phine is also rapid. Thus, treatment of 11c with excess PMe3 in
the presence of dihydrogen gas results in loss of H2 and
formation of 9. 31P{1H} and 1H NMR spectroscopies reveal the
Complex 11c is isoelectronic with a series of tantalum
dihydride complexes studied by Chaudret.18 Both classical and
nonclassical dihydride ground states have been established,
(19) (a) Kubas, G. J.; Ryan, R. R.; Swanson, B. I.; Vergamini, P. J.;
Wasserman, H. J. J. Am. Chem. Soc. 1984, 106, 451-452. (b) Kubas, G.
J. Acc. Chem. Res. 1988, 21, 120-128. (c) Rattan, G.; Kubas, G. J.; Unkefer,
C. J.; Van Der Sluys, L. S.; Kubat-Martin, K. A. J. Am. Chem. Soc. 1990,
112, 3855-3860.
(20) The values for the exo and endo hydrides are, in fact, slightly
different. We use the T1(min) value for the endo hydride in this calculation,
as the slightly shorter T1(min) for the exo hydride (160 ms at 248 K) may
be perturbed by contributions from the quadrupolar boron nucleus.
(21) (a) Bautista, M. T.; Earl, K. A.; Maltby, P. A.; Morris, R. H.;
Schweitzer, C. T.; Sella, A. J. Am. Chem. Soc. 1988, 110, 7031-7036. (b)
Earl, K. A.; Jia, G.; Maltby, P. A.; Morris, R. H. J. Am. Chem. Soc. 1991,
113, 3027-3029.
(15) (a) Rogers, J. S.; Bazan, G. C.; Sperry, C. K. J. Am. Chem. Soc.
1997, 119, 9305-9306. (b) Bazan, G. C.; Rodriguez, G.; Ashe, A. J., III;
Al-Ahmad, S.; Mu¨ller, C. J. Am. Chem. Soc. 1996, 118, 2291-2292. (c)
Bazan, G. C.; Rodriguez, G.; Ashe, A. J., III; Al-Ahmad, S.; Kampf, J. W.
Organometallics 1997, 16, 2492-2494.
(16) We would like to thank Dr. Emilio Bunel for these experiments.
(17) Herberich, G. E.; Carstensen, T.; Ko¨ffer, D. P. J.; Klaff, N.; Boese,
R.; Hyla-Kryspin, I.; Gleiter, R.; Stephan, M.; Meth, H.; Zenneck, U.
Organometallics 1994, 13, 619-630.
(18) Sabo-Etienne, S.; Chaudret, B.; Abou el Makarim, H.; Barthelat,
J.-C.; Daudey, J.-P.; Ulrich, S.; Limbach, H.-H.; Mo¨ıse, C. J. Am. Chem.
Soc. 1995, 117, 11602-11603.

(Boratacycle)tantalum Analogues of Dimethylzirconocene
J. Am. Chem. Soc., Vol. 121, No. 7, 1999 1519
NMR signal from the averaged hydrides appears as a simple
singlet with a temperature-dependent line width. Rate constants
for exchange were extracted from these spectra by conventional
line-shape analysis.23 Eyring analysis over a 50 K range provides
the activation parameters, ∆Hq ) 16(2) kcal/mol, ∆Sq ) 21(8)
eu (Figure 7). The borollide and boratabenzene ring protons
remain diastereotopic at all temperatures, indicating that the
process by which the hydride ligands are interchanged does not
alter the configuration at tantalum. This restriction precludes
exchange via dissociation or coordination of phosphine. Ex-
change most likely occurs by formation of an intermediate
dihydrogen adduct that rotates in place (eq 4).24
∆Hq for hydride exchange in 11c thus incorporates contribu-
tions from (i) the enthalpy required to convert the classical to
the nonclassical structure, (ii) disruption of the exo hydride/
boron interaction, and (iii) in situ rotation of coordinated H2.
Rotation of coordinated H2 in a tantalocene-like wedge is
accompanied by a ∆Hq of about 10 kcal/mol (estimated from
∆Hq for the nonclassical dihydride cis-Cp2Ta(H2)(CO)+, 9.6
kcal/mol), placing an upper bound on contributions i and ii of
about 6 kcal/mol.19 Interestingly, ∆Hq for intramolecular exo/
endo exchange in 11c is essentially identical with the value
measured for cis-{Cp2Ta(H)2[P(OMe)3]}+ (16.6 kcal/mol), a
classical dihydride in which the stabilizing boron-hydride
interaction is not possible.18
Figure 7. Eyring plot for fluxional hydride exchange in 11c.
complete formation of an intermediate species within 3 min of
PMe3 addition. Subsequent conversion to 9 occurs over ap-
proximately 40 min at room temperature ([Ta] ) 0.0228 M).
The intermediate species contains both coordinated PMe3 and
PEt3 ligands. Thus, we identify the intermediate as the mixed-
phosphine complex [C5H5B-Ph][C4H4B-N(i-Pr)2]Ta(PEt3)-
(PMe3) (12 in eq 3).
Isomerization of 11c to 11t. The kinetics of hydrogenation
of 8 show a phosphine dependence unlike that observed for
hydrogenation of 1. Increasing the concentration of PEt3 does
not alter the rate of formation of 11c, nor does it result in the
formation of a bis-phosphine adduct. Rather, a weak but
detectable suppression of the rate of isomerization from 11c to
11t is observed (Figure 8). This is an unexpected observation,
because the most obvious mechanisms for isomerization give
rise to rate laws which are independent of phosphine concentra-
tion. An exclusively intramolecular mechanism, e.g., one in
which rearrangement occurs via hydride migration to the boron
atom of the boratabenzene ring, is immediately excluded by
the observation of phosphine inhibition. A simple dissociative
mechanism in which loss of phosphine is followed by rate-
determining trapping of a symmetrical, intermediate dihydride
to form 11t (Scheme 6a) is also excluded. Phosphine suppression
of the rate can be best accommodated by a kinetically significant
step after phosphine dissociation that does not itself involve
phosphine. As shown in Scheme 6b, an appropriate rate law
results from a mechanism in which the initial dihydride
intermediate (Int4) retains its cis configuration, with the empty
site occupying an exo position. Subsequent rearrangement to a
trans-like geometry, in which the exo positions are occupied
The dissociative nature of hydrogen loss from 11c is shown
by the independence of the rate of formation of 12 on [PMe3].
The observed rate constants were found to be identical (0.02-
(2) s-1) in the presence of 0.114 and 0.456 M PMe3. Although
fairly rapid for standard NMR kinetics, these rates could be
measured by generating a C6D12 solution of 11c in an NMR
sample tube, adding PMe3 to the frozen reaction mixture, and
inserting the sample tube into a room temperature probe
immediately upon thawing.
(23) Sandstrøm, J. Dynamic NMR Spectroscopy; Academic Press: New
York, 1982.
Close examination of the endo/exo hydride exchange process
in 11c also suggests the likelihood of an energetically accessible
dihydrogen adduct. Above the coalescence point,22 the 1H{31P}
(24) At the present time we cannot exclude an exchange process whereby
one hydride “hops” to the boratabenzene boron atom, followed by separation
of H and P within the metallocene-like wedge, rotation of the boratabenzene
ligand, and reattachment of the hydride at the alternate site. We currently
disfavor such an exchange on two grounds: (a) such a mechanism would
significantly increase the positive charge at Ta, and (b) the activation
parameters for exchange are very similar to those reported by Chaudret for
systems in which boron is absent.
(22) At temperatures near and below the coalescence point, the spectra
display complex line shapes, particularly when phosphorus coupling is
retained. The unusual appearance of these spectra may be due to interactions
with quadrupolar 11B.

1520 J. Am. Chem. Soc., Vol. 121, No. 7, 1999
Sperry et al.
Figure 8. Rates of isomerization of 11c to 11t in C6D12 ([Ta]o ) 0.023
M; [PEt3] ) 0.096, 0.144, 0.240, 0.480, 0.800 M; P(H2) ) 620 Torr).
-1
Figure 9. (kobs
)
vs [PEt3] for isomerization of 11c to 11t.
Scheme 6
Synthesis and Thermodynamic Instability of [C5H5B-Ph]-
[C4H4B-N(i-Pr)2]Ta(H)2(PMe3) (13). Hydrogenation of the
dimethyl complexes 1 and 3 produces both high-valent (i.e.
hydrides 2 and 4) and low-valent (phosphine complexes 5 and
6) products, but in both cases the high-valent hydrides are the
final, stable products. The sole formation of the bis-phosphine
adduct 9, when 8 is hydrogenated in the presence of PMe3, to
the exclusion of any hydride products, is thus remarkable. To
determine whether this preference for 9 arises from thermody-
namic or kinetic factors, we sought to make the corresponding
hydride complex by an alternative route.
by hydride ligands and the empty site lies in the endo position,
-1
constitutes an intramolecular “trapping” step. A plot of (kobs
)
vs [PEt3] is linear, as shown in Figure 9. The intercept yields
the rate constant for phosphine loss from 11c (k1 ) 6.5 × 10-6
s-1). Rearrangement and phosphine trapping are closely com-
petitive in rate (k-1/k2 ) 1.8); both are presumably quite fast.
Finally, the stablity of the cis dihydride is sensitive to the
nature of exocyclic substitution on the borollide ligand. Thus,
treatment of 11c with [H(OEt2)2][B(C6H3(CF3)2]25 results in
protonation of the nitrogen atom in the borollide ligand (H-
11c+). As has been shown for other borollide complexes, such
protonation eliminates the contribution of the low-valent Ta
resonance structure B.26 H-11c+ is stable in THF-d8 at room
temperature for over a week. We attribute this stability to a
slower rate of phosphine dissociation as a result of the increased
Ta-P bond strength in the more electron deficient H-11c+.
Treatment of [C5H5B-Ph][C4H4B-N(i-Pr)2]Ta(H)2[P(i-Pr)3]
(10) with excess PMe3 rapidly affords [C5H5B-Ph][C4H4B-N(i-
Pr)2]Ta(H)2(PMe3) (13) as the exclusive product, identified by
1
its H and 31P{1H} NMR spectra (eq 5).
(25) Brookhart, M.; Grant, B.; Volpe, A. F., Jr. Organometallics 1992,
11, 3920-3921.
(26) (a) Herberich, G. E.; Englert, U.; Hostalek, M.; Laven, R. Chem.
Ber. 1991, 124, 17-23. (b) Ashe, A. J., III; Kampf, J. W.; Mu¨ller, C.;
Schneider, M. Organometallics 1996, 15, 387-393. (c) Bazan, G. C.;
Donelly, S. J.; Rodriguez, G. J. Am. Chem. Soc. 1995, 117, 2671-2672.
Most diagnostic of 13 is the doublet at -2.64 ppm (2 H, JPH
)
64.3 Hz), corresponding to the two hydride ligands. Upon
thermolysis at 65 °C for one month, in the presence of 1 atm

(Boratacycle)tantalum Analogues of Dimethylzirconocene
J. Am. Chem. Soc., Vol. 121, No. 7, 1999 1521
Scheme 7
of H2 and 10 equiv of PMe3 ([PMe3] ) 0.37 M), 13 is converted
to the bis-phosphine complex 9.27 9 is thus the thermodynami-
cally stable product under these conditions and the activation
barrier separating the two species is rather high. That the
dihydride is favored for every other case in this study suggests
to us that the two structures are thermodynamically quite close
in energy. The Ta centers in 9 and 13 are particularly electron
deficient (vis-a`-vis 2 and 5) due to the substitution of electron-
rich Cp* with the electron-deficient phenylboratabenzene. This
increased electrophilicity, coupled with the smaller size of PMe3
relative to PEt3 and P(i-Pr)3, apparently creates a particularly
strong Ta-P bond in 9.
d), (iii) partitioning of a monophosphine adduct among three
reaction pathways (Steps f, h, and i), and (iv) approach and
activation of dihydrogen by a low-valent tantalum site (Steps d
or i-q).
The evidence reported here does not allow for a definitive
account of the initial activation of dihydrogen by 1, 3, and 8
(Step a). We have argued elsewhere, on the basis of a
comparison of crystal structure data for a large series of
tantalum-borollide complexes, that the ground state of com-
pound 1 is strongly influenced by the low-valent resonance
structure B.9 Bercaw and co-workers also noted that the
electronic spectra of related aminoborollide-zirconium and
-hafnium complexes support the view of an ambivalent, and
correspondingly electron rich, ground state.28 As such, the
formally d2 tantalum raises the possibility that oxidative addition
of H2 is allowed. We currently do not favor such a mechanism
because the consumption of starting material is not significantly
different between 1 and 3. Unlike the aminoborollide complexes,
3 is best defined as strictly Ta(V), and thus is not subject to
further formal oxidation.
At the present time, Brintzinger’s “direct hydrogen transfer”
mode provides the most consistent rationale for the relative rates
of the four-center/four-electron activation of H2 by 1, 3, and
their slower zirnonocene analogues.1f The essential features are
(i) precoordination of H2 to the metallocene-like fragment and
(ii) a transition state in which both H atoms and the carbon
atom maintain substantial overlap with the metal. Although the
Summary and Conclusion
Despite their potential utility, little is systematically known
on how boratacyclic analogues of cyclopentadienyl perturb the
reactivity of classical organometallic fragments. The results of
this contribution show that the hydrogenation mechanisms of
complexes 1, 3, and 8 are considerably different than those
observed previously with the isoelectronic group 4 metallocenes
and reveal new aspects of the elementary reaction chemistry of
dihydrogen. Scheme 7 presents a series of steps that brings
together many of the observations described in this paper. This
reaction sequence can be usefully broken down into four
stages: (i) initial activation of dihydrogen by high-valent
tantalum, release of methane, and generation of a reactive, low-
valent tantalum fragment (Ta in Scheme 7, Steps a and b), (ii)
partitioning of Ta among two reaction pathways (Steps c and
(28) (a) Kiely, A. F.; Nelson, C. M.; Pastor, A.; Henling, L. M.; Day,
M. W.; Bercaw, J. E. Organometallics 1998, 17, 1324-1332. (b) Pastor,
A.; Kiely, A. F.; Henling, L. M.; Day, M. W.; Bercaw, J. E. J. Organomet.
Chem. 1997, 528, 65-75.
(27) One atmosphere of dihydrogen was introduced to the NMR tube
after excess PMe3 was added. The tube was shaken vigorously at regular
intervals while being heated.

1522 J. Am. Chem. Soc., Vol. 121, No. 7, 1999
Sperry et al.
net result is the formal transfer of H+ to CH3-, the migrating
hydrogen does not build a substantial partial positive charge
along the reaction pathway. In the direct hydrogen transfer
reaction, the reactivity of the weakly coordinated H2 ligand
controls the rate of hydrogenation.29 The decrease in reactivity
toward H2 with increasing electron deficiency (1 > 3 . Cp2-
ZrMe2) is simply accounted for by the electronic demand
anticipated for direct hydrogen transfer. Increasing electron
density at the metal center weakens the H-H interaction by
standard metallocenes, and the sliding of the two hydrides is
expected to require less energy. It is probably effortless.
In summary, substitution of boratacyclic ligands for cyclo-
pentadienyls within an isoelectronic metallocene framework
reveals new metal-mediated elementary reactions. It is not
inherent that the hydrogenation mechanism be the same for
families of compounds represented by 1 and Cp2ZrMe2. Rather,
the study of the [C4H4B-N(i-Pr)2]Ta framework increases our
awareness of alternative mechanistic pathways that may be
operative in compounds containing the Cp2Zr core. From a
broader perspective, the boratacycle for Cp substitution repre-
sents a useful strategy to probe the mechanisms of previously
inaccessible reactions.
promoting back-bonding into the σ*HH
.
The evolution of the second equivalent of methane most likely
takes place by reductive elimination from an intermediate methyl
hydride species, producing the highly reactive, low-valent
tantalum intermediate Ta (Step b). Similar reactivity is observed
with Cp2WHMe, which reductively eliminates CH4 and provides
[Cp2W].30 If one considers the low-valent resonance structure
B for 1 then both 1 and Cp2WHMe can be described as
isoelectronic species with d2 metal centers. With the information
available at this time it is not possible to rule out a phosphine-
induced reductive elimination as shown by Step e. However,
our observations of the hydrogenation of 1 in the absence of
phosphine show that an associative process is not required for
the clean formation of a reactive dihydride (TaH2). Intramo-
lecular elimination of CH4 from the methyl hydride therefore
seems to us the simplest proposal. If a bare intermediate (Ta)
forms, its lifetime in solution is exceedingly short. Once trapped
by phosphine it is not likely to be regenerated (i.e., by ligand
loss) in kinetically significant concentrations.
Experimental Section
General Considerations. All manipulations were carried out with
either high-vacuum or glovebox techniques as previously described.31
1H, 31P, 11B, and 13C NMR spectra were recorded on a Bruker AMX-
400 spectrometer at 400.1, 161.97, 128.3, and 100.6 MHz, respectively.
11B and 31P spectra were taken with use of BF3‚OEt2 and H3PO4
respectively as external references. 1H and 13C were taken with use of
internal references. H2 and D2 gases were purchased from Cambridge
Isotope Labs and passed through oxygen scavengers to remove any
residual water or oxygen. Toluene, benzene, pentane, diethyl ether, and
tetrahydrofuran were distilled from benzophenone ketyl. PMe3-d9, PEt3,
and P(i-Pr)3 were purchased and used as received from Aldrich. The
preparations of 1,26c 2,26c 3,6 4,6 5,6 6,6 8,9 and 99 are available in the
literature.
General Procedure for Hydrogenation Reactions. A stock solution
was prepared by dissolving 1 (60.7 mg, 0.114 mmol) and ferrocene
(10.6 mg, 0.0570 mmol) in C6D12 (2500 µL). Samples were prepared
by adding this stock (250 µL) and the desired quantity of PMe3 to an
NMR tube equipped with a Teflon needle valve, followed by enough
C6D12 to bring the total volume to 500 µL. The sample was then
degassed via 3 freeze-pump-thaw cycles, and H2 was added over the
frozen liquid. Liquid nitrogen coolant was applied only to the level of
solution in the tube. The sample was thawed for at least 1 min prior to
insertion into an NMR probe (298 K). During the kinetic runs, the
sample was removed from the probe and shaken vigorously for 10 s at
regular intervals during the disappearance of starting material to ensure
efficient mixing of gaseous H2 into the solution. After starting material
was completely consumed, the sample was then placed on a rotation
device at room temperature and returned periodically to the NMR probe
for observation.
Preparation of [C5H5B-Ph][C4H4B-N(i-Pr)2]Ta(H)2(P(i-Pr)3) (10).
To a solution of [C5H5B-Ph][C4H4B-N(i-Pr)2]TaMe2 (25 mg, 0.047
mmol) in toluene-d8 was added P(i-Pr)3 (18 µL, 0.095 mmol) via
microliter syringe. The sample was degassed with use of 3 freeze-
pump-thaw cycles and 563 Torr of H2 was placed over the frozen
sample. The solution was thawed and rotated for 3 days. 1H NMR
(toluene-d8): δ 8.00 (d, 2H, o-C6H5), 7.45 (m, 2H, m-C6H5), 7.34 (t,
1H, p-C6H5), 6.24 (m, 2H, CHCHCHB), 5.07 (m, 2H, CHCHB), 5.05
(t, 1H, CHCHCHB), 4.27 (d, 2H, CHCHCHB), 3.34 (sept, 2H, CHMe2),
2.60 (m, 2H, CHCHB), 1.60 (m, 3H, P(CHMe2)3), 1.24 (d, 12H,
CHMe2), 0.8 (m, 18H, P(CHMe2)3), -1.9 (d, 2H, Ta-H, JP-H ) 55.8
Hz). 13C{1H} NMR (toluene-d8): δ 137.2 (C6H5), 134.2 (C6H5), 125.7
(C6H5), 121.7 (C6H5), 120.7 (CHCHCHB), 96.1 (b, CHCHB), 89.7
(CHCHCHB), 77.3 (CHCHB), 58.7 (b, CHCHCHB), 47.0 (CHMe2),
25.6 (P(CHMe2)3), 24.3 (CHMe2), 19.3 P(CHMe2)3). 11B NMR (external
reference BF3‚OEt2): δ 28.5 (η5-C4H4B-N(i-Pr)2), 14.7 (C5H5B-Ph). 31P-
{1H} NMR (toluene-d8): δ 35.15 (Ta(P(CHMe2)3)).
Three choices are available to the monophosphine adduct.
Addition of another equivalent of PMe3 (Step f) forms bispho-
sphine species such as 5, 6, or 9. Intramolecular C-H activation
affords a phosphinomethanide hydride such as 7 (Step h). The
third option is reaction with H2 (Step i).
We have been able to observe the interaction between
dihydrogen and the low-valent fragment derived from 8 in
remarkable detail. Steps i through q in Scheme 7 constitute a
composite of several observations that, when taken together,
delineate the approach and activation of H2 and subsequent
rearrangement of the organometallic species to the final product.
As dihydrogen approaches it chooses a site next to phosphorus
within the metallocene wedge and the formation of a σ-adduct
ensues. Back-bonding from tantalum weakens the H-H interac-
tion giving a classical dihydride (Step k). Supporting these initial
steps are the exchange of hydrides in 11c and the facile
displacement of H2 from 11c, as well as the work by Chaudret
that has shown the small differences in energies between
dihydrogen adducts and dihydrides in the isoelectronic Cp2Ta+
core.18 Isomerization from a cis-dihydride to a trans configu-
ration (Step o) takes place after phosphine decoordination (Step
m). Our studies of the isomerization of 11c to 11t suggest that
rearrangement of the phosphine-free, cis-dihydride species is
competitive with coordination of phosphine (Step n). While most
of these elementary steps have literature precedent it is difficult
to obtain a metal-ligand system for which such an overarching
hydrogenation sequence can be constructed. The ability to detect
evidence for hydrogen motion within the metallocene wedge is
unique and is probably a result of the three-center/two-electron
interaction between Ta, B, and H. No such restraint exists in
Preparation of cis-[C5H5B-Ph][C4H4B-N(i-Pr)2]Ta(H)2(PEt3) (11c).
To a solution of [C5H5B-Ph][C4H4B-N(i-Pr)2]TaMe2 (17 mg, 0.0372
mmol) in C6D6 was added 2 equiv of PEt3 (10.99 µL, 0.744 mmol) via
microliter syringe. The sample was degassed with use of 3 freeze-
pump-thaw cycles and 434 Torr of H2 was placed over the frozen
sample. The solution was thawed and allowed to rotate for 1 h. Isolation
of this complex proved unsuccessful as the isomerization to 11t occurs
even at -35 °C.1H NMR (C6D6): δ 8.16 (d, 2H, o-C6H5), 7.47 (m,
(29) Density functional calculations point to the dihydrogen adduct as
the lowest energy species in the hydrogenation of Cp2ScMe. See ref 3e.
(30) (a) Bullock, R. M.; Headford, C. E. L.; Hennessy, K. M.; Kegley,
S. E.; Norton, J. E. J. Am. Chem. Soc. 1989, 111, 3897-3908. (b) Green,
M. L. H. Pure Appl. Chem. 1984, 56, 47.
(31) Burger, B. J.; Bercaw, J. E. In Experimental Organometallic
Chemistry; Wayda, A. L., Darensbourg, M. Y., Eds.; ACS Symp. Ser. 357;
American Chemical Society, Washington, DC, 1987.

(Boratacycle)tantalum Analogues of Dimethylzirconocene
J. Am. Chem. Soc., Vol. 121, No. 7, 1999 1523
2H, m-C6H5), 7.31 (t, 1H, p-C6H5), 6.40 (m, 1H, CHCHCHB), 5.36
(m, 1H, CHCHB), 4.78 (m, 3H, CHCHCHB, CHCHCHB, CHCHB),
4.32 (d, 1H, CHCHCHB), 4.07 (d, 1H, CHCHCHB), 3.26 (b, 2H,
CHMe2), 2.93 (m, 1H, CHCHB), 2.48 (m, 1H, CHCHB), 1.03 (d, 12H,
CHMe2), 1.66 (m, 6H, TaPCH2CH3), 0.57 (m, 9H, TaPCH2CH3), -2.52
(d, 2H, TaH2). 11B NMR (external reference BF3‚OEt2): δ 30.8 (η5-
C4H4B-N(i-Pr)2), 10.8 (C5H5B-Ph). 31P{1H} NMR (THF-d8): δ 18.7
(Ta(PEt3)).
Observation of [C5H5B-Ph][C4H4B-N(i-Pr)2]Ta(PEt3)(PMe3) (12).
Excess PMe3 (15.5 µL, 0.146 mmol) was added to a tube containing
11c (7.03 mg, 0.0114 mmol) and Cp2Fe (1.06 mg, 0.0057 mmol) in
toluene-d8. Within 3 min, 11c had been consumed and the solution
contained 12 and 9 only. 1H NMR (toluene-d8): δ 7.91 (d, 2H, o-C6H5),
7.24 (m, 2H, m-C6H5), 7.19 (t, 1H, p-C6H5), 5.15 (m, 2H, CHCHCHB),
4.56 (t, 1H, CHCHCHB), 3.65 (d, 2H, CHCHCHB), 3.50 (sept, 2H,
CHMe2), 2.30 (b, 2H, CHCHB), 1.85 (b, 2H, CHCHB), 1.45 (m, 6H,
PEt3), 1.35 (d, 9H, PMe3), 1.04 (dd, 12H, CHMe2), 0.86 (m, 9H, PEt3).
31P{1H} NMR (C6D12): δ -2.68 (Ta(PEt3), JPP ) 8.14 Hz), -33.1
(broad, Ta(PMe3)).
Preparation of cis-[C5H5B-Ph][C4H4B-N(i-Pr)2]Ta(H)(D)(PEt3)
(11c-d1). To prepare HD gas, NaH (2.00 g, 0.167 mol) was suspended
in 30 mL of toluene. D2O (1.67 mL, 0.835 mmol) was added via syringe
under an argon purge at -78 °C. HD gas was collected in the vacuum
manifold and the HD gas was placed over a degassed tube of 8 (4.93
mg, 0.00935 mmol) and Cp2Fe (0.9 mg, 0.005 mmol) in 500 µL of
C6D12. The solution was thawed and allowed to rotate for 1 h.
Characterization of [C5H5B-Ph][C4H4B-N(i-Pr)2]Ta(H)2(PMe3)
(13). Excess PMe3 (15.9 µL, 0.150 mmol) was added to a tube
containing 10 (9.88 mg, 0.0150 mmol) in C6D6. Within 5 min, 13 was
1
the exclusive product by NMR spectroscopy. H NMR (THF-d8): δ
7.68 (d, 2H, o-C6H5), 7.22 (m, 2H, m-C6H5), 7.14 (t, 1H, p-C6H5), 6.30
(m, 2H, CHCHCHB), 5.25 (t, 1H, CHCHCHB), 4.91 (m, 2H, CHCHB),
4.25 (d, 2H, CHCHCHB), 3.26 (sept, 2H, CHMe2), 2.29 (m, 2H,
CHCHB), 1.32 (d, 9H, PMe3), 1.09 (d, 12H, CHMe2), -2.64 (d, 2H,
Ta-H, JP-H ) 64.3 Hz). 13C{1H} NMR (C6D6): δ 133.8 (C6H5), 128.2
(C6H5), 127.6 (C6H5), 127.2 (C6H5), 120.6 (CHCHCHB), 96.3 (b,
CHCHB), 87.0 (CHCHCHB), 77.7 (CHCHB), 58.6 (b, CHCHCHB),
46.9 (CHMe2), 21.0 (CHMe2), 16.3 (PMe3). 11B NMR (external
reference BF3‚OEt2): δ 27.6 (η5-C4H4B-N(i-Pr)2), 14.5 (C5H5B-Ph). 31P-
{1H} NMR (C6D6): δ 19.9 (Ta(PMe3).
Preparation of cis-[C5H5B-Ph][C4H4B-N(i-Pr)2]Ta(H)2(PEt3)-
[B(C6H3(CF3)2)4] (H-11c+). One equivalent of [H(OEt2)2][B(C6H3-
(CF3)2)4] (45.0 mg, 0.045 mmol) in THF-d8 was added to freshly
prepared 11c (27.8 mg, 0.045 mmol). An instantaneous color change
1
to a lighter orange/yellow occurred. H NMR (THF-d8): δ 7.80 (b,
10H, o-C6H5, γ-B-C6H3), 7.59 (s, 4H, R-B-C6H3), 7.36 (m, 2H, m-C6H5),
7.32 (m, 1H, p-C6H5), 6.44 (m, 2H, CHCHCHB), 6.25 (d, 1H,
CHCHCHB), 6.03 (b, 1H, N-H), 5.16 (m, 2H, CHCHCHB, CHCHB),
5.09 (m, 1H, CHCHCHB), 4.99 (m, 1H, CHCHB), 3.56 (m, 2H,
CHMe2), 3.38 (m, 8H, Et2O), 2.93 (m, 1H, CHCHB), 2.55 (m, 1H,
CHCHB), 1.80 (m, 6H, PEt3), 1.35 (m, 9H, PEt3), 1.23 (dd, 6H,
CHMe2), 1.11 (m, 12H, Et2O), 0.94 (dd, 6H, CHMe2), -2.12 (d, 1H,
Ta-H, JPH)75.7 Hz), -4.48 (d, 1H, Ta-H, JPH)10.2 Hz). 11B NMR
(external reference BF3‚OEt2): δ 24.8 (η5-C4H4B-N(i-Pr)2), 21.2
(C5H5B-Ph), -2.76 (B-C6H3). 31P{1H} NMR (THF-d8): δ 16.1 (Ta-
(PEt3)).
Acknowledgment. G.C.B. is an Alfred Sloan Fellow and
an Henry and Camille Dreyfus Teacher Scholar. The authors
are grateful to the Department of Energy and the Petroleum
Research Fund for financial support of this work.
JA982803G