4252 Organometallics 2009, 28, 4252–4253
DOI: 10.1021/om900476g
A Borenium Cation Stabilized by an N-Heterocyclic Carbene Ligand
Takeshi Matsumoto and Franc-ois P. Gabbaı*
Chemistry Department, Texas A&M University, 3255 TAMU, College Station, Texas 77843-3255
Received June 4, 2009
Summary: Reaction of Mes2BF with trimethylsilyl triflate
(Me3SiOTf ) and [Ag(IMe)2][Ag2I3] (IMe=1,3-dimethylimi-
dazol-2-ylidene) in refluxing chlorobenzene affords the car-
bene-stabilized borenium salt [(IMe)BMes2]OTf ([1][OTf ]).
As indicated by cyclic voltammetry, [1]þ is reversibly reduced
to produce a radical that has been characterized by EPR
spectroscopy.
The reaction of Mes2BF with trimethylsilyl triflate
(Me3SiOTf ) and [Ag(IMe)2][Ag2I3]9 (IMe=1,3-dimethylimi-
dazol-2-ylidene) in refluxing chlorobenzene affords, after 12 h,
[(IMe)BMes2]OTf ([1][OTf ]) (Scheme 1). The formation of
[1][OTf ] was confirmed by 1H NMR, which showed the
presence of one IMe ligand bound to the boron center. The
detection of two resonances for the o-methyl groups of the
mesityl substituent is indicative of a C2 geometry whose
handedness does not change on the NMR time scale at
room temperature. The presence of a tricoordinate boron
atom was confirmed by the detection of a 11B NMR signal
at 66 ppm. This chemical shift is comparable to the value of 64
ppm reported for [(DMAP)BMes2)]þ (DMAP=p-(dimethy-
lamino)pyridine), which also features a tricoordinate boron
atom.10 To our knowledge, NHCs have been incorporated in
neutral borane4,11 and boraanthracene12 adducts but not in
boreniums.6 Hence, [1][OTf ] represents the first example of a
borenium stabilized by a NHC ligand. The structure of
[1][OTf ] has also been studied using single-crystal X-ray
diffraction (Figure 1).13 The boron center is trigonal planar,
as indicated by the sum of the bond angles, which is equal to
359.9°. The B(1)-C(1) bond connecting the IMe ligand to the
N-heterocyclic carbenes (NHC) have become ubiquitous
ligands in main-group chemistry.1 Because of their strong
σ-donor properties, such ligands have served to stabilize a
number of otherwise highly reactive species, including
unusual cations2 and low-coordinate species,3,4 among
others. Interestingly, however, such NHC ligands have never
been introduced in diarylborenium cations ([(L)BAr2]þ,
with L=neutral ligand and Ar=aryl group).5-8 Taking into
account the unusual ligative properties of NHC ligands,
we postulated that the resulting [(NHC)BAr2]þ cations
may possess a robust core and be amenable to rever-
sible redox chemistry. Stimulated by this possibility, we
have now decided to synthesize such a [(NHC)BAr2]þ
cation.
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boron center (1.579(7) A) is comparable to the B(1)-C(6)
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*To whom correspondence should be addressed. E-mail: francois@
tamu.edu.
(1.562(7) A) and B(1)-C(15) bonds (1.560(7) A), suggesting a
strong coordination of the carbene ligand. The dihedral angle
of 37.3° formed between the boron trigonal plane and the
plane containing the IMe ligand is smaller than the average
dihedral angle of 53.7° formed by the mesityl groups and the
boron trigonal plane. This difference can be assigned to the
lower steric hindrance of the IMe ligand, whose N-methyl
groups adopt a more divergent orientation.
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The cyclic voltammogram of [1]þ in CH2Cl2 displays a
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(13) Crystal data for [1]OTf: crystal dimentions 0.20 ꢀ 0.20 ꢀ 0.15
mm, C25H31BCl3F3N2O3S, Mr = 613.74, orthorhombic, space group
ꢀ
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Pbca, a = 14.1972(14) A, b = 10.9105(11) A, c = 36.276(4) A,
3
V=5619.1(10) A , Z=8, Fcalcd=1.451 g cm-3, Mo KR radiation (λ=
˚
1725.
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0.710 69 A), T =110(2) K, 28 958 measured reflections, 4415 unique
reflections, Rint=0.0609, R1=0.0725, wR2=0.1581 (all data).
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Published on Web 07/14/2009
2009 American Chemical Society