A tantalum(V) carbene complex: Formation of a carbene-bis(phenoxide) ligand by sequential proton and hydride abstraction

Article abstract of DOI:10.1021/ic0608492

Proton and subsequent hydride abstraction from the bis(phenoxide) ligand of the trimethyltantalum(V) complex affords a cationic tantalum(V) carbene complex, in which two phenoxide groups are linked to the carbene center. The electrophilic nature of the carbene functionality is demonstrated by the reaction with PPh₃.

Full text of DOI:10.1021/ic0608492

Inorg. Chem. 2006, 45, 6580−6582
A Tantalum(V) Carbene Complex: Formation of a
Carbene Bis(phenoxide) Ligand by Sequential Proton and Hydride
−
Abstraction
Takahito Watanabe, Tsukasa Matsuo, and Hiroyuki Kawaguchi*
Coordination Chemistry Laboratories, Institute for Molecular Science, Myodiaji,
Okazaki 444-8787, Japan
Received May 17, 2006
Proton and subsequent hydride abstraction from the bis(phenoxide)
ligand of the trimethyltantalum(V) complex affords a cationic
tantalum(V) carbene complex, in which two phenoxide groups are
linked to the carbene center. The electrophilic nature of the carbene
functionality is demonstrated by the reaction with PPh3.
addition, this ligand system would provide the opportunity
to study the effect of the σ-donor carbene functionality on
the properties of electron-deficient metal complexes. In this
Communication, we report the synthesis and molecular
structure of the tantalum(V) complex having the tridentate
dianionic bis(2-oxyphenyl)carbene ligand [OCO]2- (Scheme
1), in which two phenoxide groups are covalently jointed
by a carbene C. This is a rare example of early-transition-
metal complexes with σ-donor carbene ligands except
N-heterocyclic carbenes.4-7
To avoid the need for preparation of a free carbene, we
examined C-H activation of the [OO] ligand {H2[OO] )
2,2′-methylenebis(6-tert-butyl-4-methylphenol)} that would
provide the desired carbene-bridged bis(phenoxide) [OCO]
complexes upon cyclometalation. There is considerable
precedent for intramolecular C-H activation of 2,6-disub-
stituted phenoxides in metal complexes.8 For instance, the
2,6-dimethylphenoxide ligand of a Ru complex was reported
to undergo double C-H activation, giving an unusual
bidentate carbene-phenoxide ligand.9 We and others have
found that the bridging methylene groups of multidentate
phenoxide ligands are more susceptible to cyclometala-
tion because within the chelate ring the methylene group and
the metal center are brought into relatively close proxim-
ity.10,11
Bis(phenoxides), in which the two phenoxide rings are
linked to a donor atom (X) such as S and P in the ortho
positions, have been useful dianionic ancillary ligands in
coordination chemistry and homogeneous catalysis.1 In these
types of hybrid [OXO] ligands that combine the hard
phenoxide donors with the soft X donor into a chelating
array, electronic and stereochemical parameters can be
manipulated by modifications of X donor groups so as to
accommodate many transition metals in a variety of oxidation
states and induce interesting transformations.2,3 With this in
mind, we have begun to study the chemistry of carbene-
phenoxide hybrid ligands, where a carbene functionality is
introduced at the X donor site.4 The strong two-electron
donor ability of the carbene group was appealing to us.5 In
* To whom correspondence should be addressed. E-mail:
hkawa@ims.ac.jp.
(1) (a) Britovsek, G. J. P.; Gibson, V. C.; Wass, D. F. Angew. Chem.,
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6580 Inorganic Chemistry, Vol. 45, No. 17, 2006
10.1021/ic0608492 CCC: $33.50
© 2006 American Chemical Society
Published on Web 07/26/2006

COMMUNICATION
Scheme 1
A convenient starting material for this chemistry is the
bis(phenoxide) trimethyl complex [OO]TaMe3 (1), which
was obtained as light-yellow crystals in 76% yield by the
reaction of [OO]TaCl3 with 3 equiv of MeMgBr in toluene.12
Complex 1 is thermally unstable, and warming of a toluene
solution of this complex at 80 °C for 3 days induced
abstraction of a methylene proton from the ligand backbone
with extrusion of methane to generate a cyclometalated
species [OCHO]TaMe2 (2) as colorless crystals in 79% yield.
1
In the H NMR spectrum of 2, the Ta-Me protons appear
as two distinct singlets at 0.51 and 0.77 ppm. The cyclo-
metalated methine proton is observed as a singlet at 3.04
ppm. The resonances ascribed to the phenoxide groups are
consistent with 2 having Cs symmetry.
Figure 1. Molecular structures of 2 (above) and the cation part of 3 (below)
showing the atom-labeling scheme and selected bond distances. For 2, only
the methine H atom is shown. Me and tBu groups of the ligand are omitted
for clarity.
To synthesize the alkylidene-bis(phenoxide) complex
“[OCO]TaMe”, we attempted a second R deprotonation of
2.14 However, prolonged thermolysis of 2 caused decomposi-
tion of the complex. Attempts to isolate any products in the
presence of pyridine have met with failure. Owing to the
difficulty in generating the tetraanionic [OCO] ligand via
double deprotonation of the [OO] ligand, we turned our
attention to hydride removal from 2 to prepare the desired
dianionic [OCO] ligand. Treatment of 2 with [Ph3C]-
[B(C6F5)4] at -20 °C in CH2Cl2 afforded thermally unstable
[(OCO)TaMe2][B(C6F5)4] (3) in 72% isolated yield,15 in
which R-hydride abstraction from the [OCHO] ligand by a
trityl cation took place.16,17 A color change from colorless
The solid-state structure of 2, as revealed from an X-ray
diffraction study, is shown in Figure 1. The molecule lies
across a crystallographic mirror plane that passes through
the atoms Ta, C(7), C(13), and C(14). The geometry at
Ta is best described as distorted trigonal bipyramidal. The
tridentate [OCHO] ligand is bound in a meridional fashion,
with the phenoxide donors occupying axial positions [O(1)-
Ta-O(2) ) 148.2(2)°]. The two five-membered chelate
ring planes of the ligand are folded along the Ta-C(7) vector
with an angle of 163.9(1)°. The hydrogen atom [H(3)] of
the methine C(7) carbon was located from Fourier differ-
ence maps and refined isotropically. The short Ta-H(3)
distance (2.36 Å) and the acute Ta-C(7)-H(3) angle (87°)
[Ta-C(7)-C(6), 110.6(3)°; C(6)-C(7)-C(6′), 123.2(5)°]
implicate the presence of R-agostic interaction in the solid
state, whereas this interaction might be weak in solution
as evidenced by the JCH coupling constant (112 Hz) for
the methine C (101.6 ppm) from the 13C NMR spectral
data.13
(13) (a) Brookhart, M.; Green, M. L. H.; Wong, L.-L. Prog. Inorg. Chem.
1988, 36, 1-124. (b) Brookhart, M.; Green, M. L. H. J. Organomet.
Chem. 1983, 250, 395-408.
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5496. (b) Aparna, K.; Babu, R. P. K.; McDonald, R.; Cavell, R. G.
Angew. Chem., Int. Ed. 2001, 40, 4400-4402. (c) Babu, R. P. K.;
McDonald, R.; Cavell, R. G. Chem. Commun. 2000, 481-482.
(15) The half-life of 3 in a CH2Cl2 solution at room temperature was
determined to be ca. 10 min. We have yet to identify any decomposi-
tion products.
(16) (a) Bodner, G. S.; Gladysz, J. A.; Nielsen, M. F.; Parker, V. D. J.
Am. Chem. Soc. 1987, 109, 1757-1764. (b) Hayes, J. C.; Cooper, N.
J. J. Am. Chem. Soc. 1982, 104, 5570-5572. (c) Heinekey, D. M.;
Radzewich, C. E. Organometallics 1998, 17, 51-58. (d) Asaro, M.
F.; Bodner, G. S.; Gladysz, J. A.; Cooper, S. R.; Cooper, N. J.
Organometallics 1985, 4, 1020-1024.
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14255. (b) Akagi, F.; Matsuo, T.; Kawaguchi, H. J. Am. Chem. Soc.
2005, 127, 11936-11937.
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Chem. 1997, 36, 1642-1651.
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D. Polyhedron 1997, 16, 2941-2949.
Inorganic Chemistry, Vol. 45, No. 17, 2006 6581

COMMUNICATION
Scheme 2
to deep green accompanied the formation of 3. The formula-
tion of 3 is supported by combustion analysis, NMR
spectroscopy, and single-crystal X-ray diffraction. Diagnostic
spectroscopic features for 3 include a 13C NMR singlet at
255.5 ppm, which can be assigned to a carbene C.18 Loss of
the R-hydride results in an increase in symmetry from Cs to
C2V, where two Ta-Me groups of 3 are equivalent. The aryl
ring proton and carbon resonances are shifted downfield
compared with those of 2. An X-ray analysis revealed that
the structure of 3 comprises well-separated cations and anions
(Figure 1).19
[B(C6F5)4] proceeds via R-hydride abstraction to generate 3
and triphenylmethane quantitatively according to NMR
spectroscopy. The preference for hydride abstraction over
methide abstraction is likely a thermodynamic result. Another
plausible explanation is that an initial electron transfer from
the [OCHO] ligand to the trityl cation facilitates subsequent
R-hydrogen abstraction from the ligand-centered radical
species. For electron-rich transition-metal complexes, ab-
straction of R-hydride by the trityl cation has been proposed
to occur by an electron-transfer mechanism.16 Further studies
are required to determine the mechanistic details for the
formation of 3.
Preliminary investigations into the reactivity of 3 reveal
that this complex does not react with simple olefins. The
addition of PPh3 to a CH2Cl2 solution of 3 yielded the
phosphine adduct [(OCO-PPh3)TaMe2][B(C6F5)4] (4). The
formation of 4 is accompanied by a color change to orange.
The 13C{1H} NMR spectrum exhibits a definitive doublet
for the P-bound carbon at 78.5 ppm with 1JPC ) 14 Hz, while
the Ta-Me carbons are observed as two singlets at 67.5 and
76.5 ppm. Attack of PPh3 on 3 indicates the electrophilic
nature of the carbene functionality.17
As with 2, the structure of 3 also exhibits a distorted
trigonal-bipyramidal Ta center. The carbene C is bonded to
Ta with Ta(1)-C(7) ) 2.209(7) Å, which is similar to the
Ta-C(cyclometalated) distance of 2 [2.206(5) Å]. This
distance is significantly longer than those found in high-
oxidation-state Schrock-type alkylidenes and falls in the
range for known TaV-C single bonds.7,20-22 Noteworthy is
planarity at C(7), with the sum of the angles being 359.9-
(6)°. Additionally, the two phenyl rings of the ligand are
almost coplanar with the O(1), O(2), C(7), and Ta(1) atoms,
an alignment that presumably optimizes resonance donation
into the carbene p orbital by the oxyphenyl π electrons. The
short C-O and C(7)-C distances of 3 relative to those of 2
reflect this interaction. Further, short/long alternations for
C-C bonds within the aryl rings indicate significant
contributions from o-quinone methide resonance forms (B
and B′ in Scheme 2).
Although the trityl cation is widely used as a powerful
alkide-abstracting reagent,23 the reaction of 2 with [Ph3C]-
In summary, the present work has demonstrated the
synthesis and structure of the cationic dimethyltantalum(V)
complex with the [OCO] ligand. The dianionic tridentate
[OCO] ligand is formed by the sequential proton and hydride
abstraction from the backbone of the [OO] ligand. The
carbene moiety of 3 is stabilized via π bonding to the
oxyphenyl groups. This is reminiscent of the stabilization
of N-heterocyclic carbenes by the electron-donating effects
of adjacent N atoms.5a The electrophilic nature of carbene 3
has been shown in the reaction with PPh3. Extension of this
work to include other transition-metal complexes and
reactivity studies of 3 are in progress.
(17) (a) Gunnoe, T. B.; White, P. S.; Templeton, J. L.; Casarrubios, L. J.
Am. Chem. Soc. 1997, 119, 3171-3172. (b) Jepsen, A. S.; Vogeley,
N. J.; White, P. S.; Templeton, J. L. J. Organomet. Chem. 2001, 617,
520-529.
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(19) Two crystallographically independent, but chemically equivalent,
molecules are present in the asymmetric unit. The second molecule
has the Ta-C(carbene) distance of 2.216(7) Å, and the sum of the
angles around the carbene C is 359.9(7)°.
(20) (a) Schrock, R. R. Chem. ReV. 2002, 102, 145-179. (b) Schmidt, J.
A. R.; Chmura, S. A.; Arnold, J. Organometallics 2001, 20, 1062-
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Koten, G. Organometallics 1997, 16, 4260-4267. (e) Mashima, K.;
Tanaka, Y.; Kaizu, M.; Nakamura, A. Organometallics 1996, 15,
2431-2433. (f) Abbenhuis, H. C. L.; Feiken, N.; Grove, D. M.;
Jastrzebski, J. T. B. H.; Kooijman, H.; der Sluis, P. V.; Smeets, W. J.
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Acknowledgment. The Institute for Molecular Science
is thanked for financial support. This work was also
supported by a Grant-in-Aid for Scientific Research from
the Ministry of Education, Culture, Sports, Science, and
Technology, Japan.
Supporting Information Available: X-ray crystallographic files
(CIF) and complete synthetic details (PDF). This material is
available free of charge via the Internet at http://pubs.acs.org.
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IC0608492
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6582 Inorganic Chemistry, Vol. 45, No. 17, 2006