Reversible intramolecular hydride transfer between tantalum and an aromatic ring: An η5-cyclohexadienyl complex as a masked hydride

Article abstract of DOI:10.1021/ja025684k

The methyl triflate complex (2,6-Mes₂C₆H₃N=)(2,6-Mes₂C₆H₃NH)TaMe(OSO₂CF₃) reacts cleanly with H₂ to give the η⁵-cyclohexadienyl complex 3. Comple

Full text of DOI:10.1021/ja025684k

Published on Web 06/27/2002
Reversible Intramolecular Hydride Transfer between Tantalum and an
5
Aromatic Ring: An η -Cyclohexadienyl Complex as a Masked Hydride
John Gavenonis and T. Don Tilley*
Department of Chemistry, UniVersity of California, Berkeley, Berkeley, California 94720-1460
Received January 23, 2002
Tantalum hydrides are known to mediate a number of interesting
1
catalytic transformations including olefin polymerization, alkane
2
3
hydrogenolysis, and arene hydrogenation. In these processes, a
somewhat limited range of ancillary ligands have been employed
4
in tuning the chemical properties of the tantalum complexes. Al-
though considerable recent research has focused on development
5
of the tantalum chemistry of imido ligands, only a few imido-hy-
dride complexes have been reported.6 The few such species that
-8
are known exhibit high reactivities and have been found to mediate
6
interesting C-H and Si-H bond activations. In the exploration
of imido-tantalum complexes in potentially useful transformations
involving hydride transfers, we have employed sterically demanding
aryl-imido ligands in attempts to stabilize monomeric, highly reac-
tive tantalum hydrides. In this communication, we report use of
the [2,6-Mes C H
2 6 3
Nd]2 ([Ar*Nd] , Mes ) 2,4,6-Me
-
2-
3 6 2
C H ) lig-
and in generation of the tantalum hydride [(Ar*Nd)(Ar*NH)Ta-
5
(
H)OTf] (4, OTf ) OSO
cyclohexadienyl tantalum imido complex (Ar*Nd)[2-(η -2,4,6-Me
)-6-MesC NH]Ta(OTf) (3). The latter species exists in equi-
2
CF
3
), which rapidly rearranges to an η -
5
Figure 1. ORTEP diagram of (Ar*Nd)[2-(η -2,4,6-Me
3 6 3 6 3
C H )-6-MesC H NH]-
5
3
-
Ta(OTf) (3).
C
H
6 3
6 3
H
Scheme 1
librium with 4, such that it behaves as a “masked hydride” in its
reaction chemistry. This reaction is therefore relevant to the mech-
anism of Rothwell’s tantalum-catalyzed arene hydrogenations,3b
which appear to involve an analogous, initial migration of hydrogen
from tantalum to an arene ring.9
The imido-amido complex (Ar*Nd)(Ar*NH)TaMe
2
(1) was
prepared in 71% yield by the treatment of TaMe Cl with 2 equiv
3
2
10
of LiNHAr* (prepared via modification of a literature procedure ).
Attempts to convert 1 to a hydride complex by the reaction with
H
2
(e.g., at 1 atm in benzene-d
6
) resulted in complex reaction mix-
. We therefore attempted
tures containing the free amine Ar*NH
2
to obtain a tantalum complex with a more reactive Ta-Me bond,
by conversion of 1 to the triflate derivative (Ar*Nd)(Ar*NH)Ta-
Me)OTf (2) via reaction with 1 equiv of AgOTf.1
1,12
Heating a
(
bromobenzene solution of complex 2 to 95 °C over 2 d in the pre-
sence of H (1 atm) provided a red-orange crystalline solid (3) which
was purified by recrystallization from toluene at -35 °C (Scheme
2
1
1
). The H NMR spectrum of 3 contains two coupled doublets at
the bond lengths from Ta(1) to C(2)-C(6) range from 2.349(6) to
.830(6) Å. The internal angles of the cyclohexadienyl ligand
3
.53 and 4.54 ppm (J ) 16 Hz), consistent with the presence of a
2
methylene group with diastereotopic protons. However, this spec-
involving C(2)-C(6) range from 116.2(6) to 121.0(6)°, suggesting
trum appears to be devoid of a resonance attributable to a Ta hydride
2
ligand.13
sp hybridization, whereas the C(6)-C(1)-C(2) angle (109.6(6)°)
3
is consistent with sp hybridization.
The identity of 3 was established by X-ray crystallography, which
5
Complex 3 presumably forms via the tantalum hydride interme-
diate [(Ar*Nd)(Ar*NH)Ta(H)OTf] species (4, Scheme 1), and
evidence for the existence of this species was obtained by trapping
revealed a structure containing an η -cyclohexadienyl ligand (Figure
). This complex contains a normal imido ligand, as indicated by
1
the Ta(1)-N(2)-C(25) bond angle of 175.6(5)° and the TadN bond
5
5
experiments. When a bromobenzene-d solution of 2 was treated
length of 1.788(5) Å. The amido ligand in 3 is metalated via the
with H
duced along with quantitative amounts of n-hexane and complex
. This observation suggests that 4 reacted with 1-hexene to give
a hexyl triflate complex (5), which then reacted with H to give
2
in the presence of 1-hexene (1 equiv), methane was pro-
transfer of a hydrogen atom to one mesityl group, to give rise to
5
an η -cyclohexadienyl ligand. The reduced mesityl ring is puckered
3
at C(1), such that the Ta(1)‚‚‚C(1) distance is 3.141(7) Å, while
2
*
To whom correspondence should be addressed. E-mail: tdtilley@socrates.
berkeley.edu.
n-hexane and complex 3 (Scheme 1). In fact, the reaction of 2 with
8
536 J. AM. CHEM. SOC. 2002, 124, 8536-8537
9
10.1021/ja025684k CCC: $22.00 © 2002 American Chemical Society

C O MMU N I C A T I O N S
H
2
(1 atm) in the presence of 10 equiv of 1-hexene (in bromoben-
zene, 95 °C, over 2 d) produced the n-hexyl derivative (Ar*Nd)-
Ar*NH)Ta(Hex)OTf (5) as yellow crystals in 66% yield. The
diastereotopic TaCH hydrogens are clearly identified by two triplets
The results described here provide further evidence that imido-
hydride complexes are accessible, potentially reactive species that
can give rise to novel chemistry. Complex 3, with an imido-amido
(
2
2 6 3
ligand set featuring the sterically demanding 2,6-Mes C H aryl
of doublets appearing at 0.58 and 1.00 ppm. The 13C NMR signal
for this methylene group occurs at 66.9 ppm, in approximately the
same region as those for the related carbon atoms in 1 and 2.
Experimental data suggest that in solution, complexes 3 and 4
exist in equilibrium. Thus, 3 reacts with 1-hexene (1 equiv, in the
group, represents an arene hydrogenation intermediate resulting
from the transfer of hydride from tantalum to one of the mesityl
rings. This complex reacts with small molecules via its more
reactive isomer 4, with which it is in equilibrium. Continuing studies
focus on the development of imido-hydrides that are highly reactive
in σ-bond metathesis processes.
absence of H
2 5
) in bromobenzene-d at 95 °C to form the n-hexyl
derivative 5 (Scheme 1). However, attempts to observe 4 by moni-
Acknowledgment is made to the National Science Foundation
for their generous support of this work. We thank Dr. Frederick J.
Hollander and Dr. Allen G. Oliver for assistance with the X-ray
structure determination, Dr. Rudi Nunlist for assistance with the
1
2
toring the reaction of complex 2 with H (1 atm) by H NMR spec-
troscopy in bromobenzene-d at 95 °C were unsuccessful; only
5
resonances due to complexes 2 and 3 were observed. Thus, 3 is
highly favored in its equilibrium with 4.
1
H ROESY NMR experiment, and Professor Ian P. Rothwell
Further insight into the mechanism of formation of 3 was gained
(Purdue University) for insightful discussions.
2
by a deuterium-labeling experiment. Treatment of 2 with D (1 atm)
Supporting Information Available: Procedures for the synthesis
and characterization of 1-5 and H ROESY NMR spectra (PDF). X-ray
crystallographic information for 3 (CIF). This material is available free
in bromobenzene at 95 °C over 3 d provided 3-d as red-orange
1
1
crystals from toluene (Scheme 1). The H NMR spectrum of 3-d
has no signal at ∼3.5 ppm (unlike 3), but contains a broad singlet
of charge via the Internet at http://pubs.acs.org.
2
at 4.52 ppm integrating to 1 H. The H NMR spectrum, as expected,
contains a single, broad resonance at 3.48 ppm, consistent with
deuterium incorporation into only one position of the molecule.
The 13C{ H} NMR spectrum contains a 1:1:1 triplet at 34.5 ppm
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(HMes ) H bonded to C(3), Figure 1) and the doublet at 4.54 ppm
(Hexo bonded to C(1), Figure 1). However, no ROE was observed
between HMes and Hendo (bonded to C(1), Figure 1). In the structure
5
of complex 3, the puckered η -cyclohexadienyl ring places HMes
(
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position), the hydride transfer proceeds in an endo fashion.
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bond, to give a stable η -cyclohexadienyl complex. Complexes of
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5
(
(
9
(
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methylcyclopentane (9 d, quantitative conversion, 50% yield).15 The
slow rates of these hydrogenations probably reflect the sterically
encumbered nature of the catalyst.
689.
5
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at 95 °C (7
(
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JA025684K
J. AM. CHEM. SOC.
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VOL. 124, NO. 29, 2002 8537