High-oxidation-state neutral and cationic tantalum(IV) alkyl complexes that are stable toward β-hydrogen and β-methyl eliminations

Article abstract of DOI:10.1021/ja067011y

The synthesis and solid-state structural characterization of a family of homoleptic and mixed dialkyl d¹Ta(IV) complexes of the formula, (η⁵-C₅Me₅)TaR¹R²[N(i-Pr)C(Me)N(i-Pr)], where R¹ = R² = i-Bu (3), n-Bu (4), and Et (7), and R¹ = Me, R² = i-Bu (10), neopentyl (Np) (11), are reported, along with those for the cationic d1Ta(IV) complex, {(η⁵-C₅Me₅)TaNp[N(i-Pr)C(Me)N(i-Pr)]}[B(C₆F₅)4] (12). All of the new compounds displayed a remarkably high degree of solution stability toward β-hydrogen and β-methyl eliminations/abstractions. Thermolysis of 3 in toluene at 80 °C for 18 h provided the Ta(IV) trimethylenemethane (TMM) complex 13. Copyright

Full text of DOI:10.1021/ja067011y

Published on Web 11/21/2006
High-Oxidation-State Neutral and Cationic Tantalum(IV) Alkyl Complexes That
Are Stable toward â-Hydrogen and â-Methyl Eliminations
Albert Epshteyn, Peter Y. Zavalij, and Lawrence R. Sita*
Department of Chemistry and Biochemistry, UniVersity of Maryland, College Park, Maryland 20742
Received September 29, 2006; E-mail: lsita@umd.edu
Scheme 1
Transition-metal-catalyzed polymerizations and synthetic (asym-
metric) organic transformations will continue to be of dominant
importance to society for the foreseeable future. Accordingly, there
is great need to continuously develop new fundamental organo-
metallic chemistry that can pave the way for the development of
new transition-metal-based catalysts. In this regard, tremendous gaps
still exist in our knowledge of the chemistry of the group 5 metals
in their highest oxidation states, that is, M(IV) and M(V) (M ) V,
Nb, and Ta), as it pertains to the stability and reactivity of
organometallic complexes with saturated alkyl groups possessing
â-hydrogens, and more specifically, those containing M-CH2CHRn
fragments.1 Interest in filling this knowledge gap is more than
academic in that over the past 30 years there have been, for instance,
sporadic reports of Ta-based catalysts for both the Ziegler-Natta
polymerization and selective oligomerization of ethylene and the
selective dimerization of R-olefins.2,3 Indeed, it is through the
seminal discovery and detailed mechanistic investigations of this
last catalytic system by Schrock and co-workers3 that we have the
best evidence to date that such compounds can be stable enough
to isolate for structural characterization.4 However, it must also be
recognized that the increased stability of the Ta(V) metallacyclo-
pentanes that are involved in this process, relative to acyclic alkyl
groups, is most likely the result of conformational restrictions
imposed by the five-membered metallacyclopentane ring that
contribute to a higher barrier for intramolecular â-hydrogen
elimination (abstraction).5 With respect to the organometallic
engage in (1) R-hydrogen deprotonation of an alkyl substituent, to
produce a Ta(V) alkylidene,8 (2) reductive electron transfer to the
metal center to provide Ta(IV) species, and (3) deprotonation of
the methyl group of the acetamidinate ligand to produce an
eneamidate species.9
chemistry of Ta(IV), to the best of our knowledge, the record is
completely silent regarding any report of isolable or spectroscopi-
cally observable alkyl complexes bearing â-hydrogens. Herein, we
now report the successful synthesis and solid-state structural
characterization of a family of neutral and cationic d1 Ta(IV) alkyl
complexes that can be isolated in pure form. Importantly, all these
new compounds display a remarkable stability toward â-hydrogen
and â-methyl eliminations.
Scheme 1 summarizes the synthetic results of the present work
in which all the new compounds, 2-12, have been fully character-
ized, including determination of their solid-state structures by single-
crystal X-ray analysis.6 To begin, alkylation of known 12d at -25
°C with 3 equiv of EtLi in diethyl ether (Et2O) provided the d0
Ta(V) ethyl, ethylidene 2 in a 84% isolated yield. In contrast,
carrying out the same reaction using either 3 equiv of i-BuLi (at
-25 °C) or n-BuLi (at room temperature)7 now provided excellent
isolated yields of the paramagnetic d1 Ta(IV) di(isobutyl) and di-
(n-butyl) compounds, 3 and 4, respectively. Finally, with 3 equiv
of neopentyllithium (NpLi), alkylation of 1 resulted in a 72% yield
of the mononeopentyl Ta(IV) complex 5. While we have not yet
attempted to unequivocally elucidate the mechanistic path(s) that
lead to each of these structurally distinct products, it seems
reasonable to conclude that because of increasing steric demands
that occur about the metal center with each successive Cl f alkyl
exchange, the remaining equivalent(s) of organolithium reagent can
Although the solution stabilities of the new organotantalum(IV)
alkyl compounds, 3-5, were initially surprising (vide infra), these
observations are in keeping with those previously reported by us
for the class of structurally analogous neutral and cationic d0 group
4 (M ) Ti, Zr, Hf) monocyclopentadienylmetal amidinate alkyl
complexes that can function as precatalysts and highly active
species, respectively, for the living (stereospecific) Ziegler-Natta
polymerization of R-olefins.10 Accordingly, on the basis of this
experience, we felt it highly likely that a general route to a much
larger variety of Ta(IV) homoleptic and mixed dialkyl compounds
built around the monocyclopentadienyl, amidinate ligand combina-
tion could be successfully developed. To this end, it was first
determined that 1 could be reduced by 1 equiv of sodium amalgam
in Et2O to provide a near quantitative yield of the Ta(IV) dichloride
6, which was also found to be quite stable in solution and the solid
state.6 Compound 6 then proved to be an excellent starting material
for preparing a range of new, isolable Ta(IV) alkyl derivatives
according to Scheme 1. Thus, alkylation of 6 with 2 equiv of EtLi
cleanly provided an excellent yield of the desired Ta(IV) diethyl
compound 7. Furthermore, using 1 equiv of i-BuMgCl and
NpMgCl, controlled monoalkylation of 6 could be carried out to
9
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J. AM. CHEM. SOC. 2006, 128, 16052-16053
10.1021/ja067011y CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
Figure 1. Molecular structures (30% thermal ellipsoids), from left to right, of compounds 7, 10, and 12. The [B(C6F5)4] anion of 12 and all hydrogen atoms,
except for selected ones in 10 and 12 that are represented as small spheres of arbitrary size, have been removed for the sake of clarity.
Scheme 2
preliminary results only provide the barest of introductions to the
likely beginnings of a new era of discovery for high-oxidation-
state group 5 organometallic chemistry. Further studies regarding
the stability and chemical reactivity of all the new compounds
reported herein are in progress, the results of which will be reported
in due course.
Acknowledgment. Funding for this work was provided by the
provide good yields of the corresponding Ta(IV) chloro, alkyl
NSF (Grant CHE-061794) for which we are grateful. This paper is
complexes, 8 and 9, respectively, which, after isolation and
dedicated to the 2005 Chemistry Nobel Laureates, Profs. Richard
purification through recrystallization, were further methylated using
R. Schrock and Robert H. Grubbs.
1 equiv of MeLi to provide excellent yields of the final desired
mixed Ta(IV) methyl, alkyl complexes, 10 and 11, respectively.6
Supporting Information Available: Experimental details, including
Finally, in related work with the goal of exploring the possible
Ziegler-Natta polymerization activity of well-defined cationic d1
Ta(IV) alkyl species, it was determined that protonation of the
eneamidate 5 could be achieved using 1 equiv of [PhNHMe2]-
[B(C6F5)4] in chlorobenzene to produce a near quantitative yield
of the cationic Ta(IV) neopentyl complex 12 according to Scheme
1. Figure 1 provides a few representative examples of the molecular
structures of 3-12 that were determined by single-crystal X-ray
analyses, and in each case, no structural evidence for any R- or
â-hydrogen agostic11 interactions involving the Ta(IV) metal center
are observed.6
crystallographic analyses of 2-13. This material is available free of
charge via the Internet at http://pubs.acs.org.
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species which are all reported to engage in facile low-temperature
â-methyl group elimination.12 This is not to say, however, that the
new Ta(IV) alkyls are devoid of potentially interesting thermal
behavior. Indeed, as depicted in Scheme 2, heating a toluene
solution of the diisobutyl compound 3 to 80 °C for 18 h re-
sulted in a 50% isolated yield of the trimethylenemethane (TMM)
Ta(IV) complex 13 that was also structurally characterized by X-ray
analysis. Interestingly, the TMM fragment of 13 appears to adopt
a unique η1,η3-bonding mode.6,10b As similar results were obtained
upon thermolysis of a Zr(IV) structural analogue,10b,13 it is now
possible to speculate that a common mode of decomposition for
early transition-metal alkyl compounds bearing â-hydrogens in-
volves intra- and intermolecular γ-hydrogen abstraction rather than
simple â-hydrogen eliminations/abstractions alone.14
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(6) Details are provided in the Supporting Information.
(7) At -25 °C, alkylation of 1 with n-BuLi provided a crystalline material
that X-ray analysis revealed to consist of a 1:1 mixture of 4 and a Ta(III)
n-butyl, (η2-1-butene) species. Efforts to isolate this latter compound in
pure form are currently in progress.
(8) Schrock, R. R. Chem. ReV. 2002, 102, 145-179.
(9) Zhang, Y.; Kissounko, D. A.; Sita, L. R. Organometallics 2003, 22, 21.
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M. B.; Keaton, R. J.; Fettinger, J. C.; Sita, L. R. J. Am. Chem. Soc. 2006,
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4317.
In conclusion, the present work serves to present a family of
neutral and cationic Ta(IV) alkyl complexes that are stable toward
â-hydrogen and â-methyl eliminations. Clearly, however, these
(14) Dioumaev, V. K.; Harrod, J. F. Organometallics 1997, 16, 1452-1464.
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