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occupancy p* orbital (0.46e) on the boron atom. NBO
analysis shows several strong donor–acceptor interactions
(see Table S3 in the Supporting Information), including the
remarkable electron donation from an oxygen p lone pair to
the boron p* orbital (stabilization energy 87.94 kcalmolÀ1),
indicating the significant B O p bonding.[19] Donor–acceptor
À
interactions are also seen between the oxygen lone-pair
À
orbitals and the imidazolium C H s* orbital to give
stabilization energies of 17.05 and 20.56 kcalmolÀ1, consistent
with the strong hydrogen bond observed in 3b. Calculations
on the anion 3’, in which the imidazolium cation in 3 is
excluded, at the same level indicate a very similar bonding
situation to the anion in 3. The optimized geometry for 3’ is
shown in Figure S3 in the Supporting Information (see Table
S2 in the Supporting Information for the calculated bond
À
parameters). The calculated B O bond length (1.2824 ꢀ) for
the anion 3’ is only 2% shorter than that calculated for 3
(1.3089 ꢀ), thus suggesting that the hydrogen bond in 3 has
À
only a very small effect on the B O bonding.
The
optimized
geometries
for
[HC(CMe)2-
Figure 2. ORTEP diagram of 4a. Thermal ellipsoids are set at the 30%
(NAr’)2]BOBCl3 (4, Figure S4 in the Supporting Information)
at the RB3LYP/6-311 + + G (2d,p) level compare well with
the X-ray results for 4a, except that the calculated B-O-B
angle (156.758) is smaller than that found in 4a (161.208
average) owing to the large size of B(C6F5)3 in 4a. NBO
analysis of 4 showed that the sp2 hybrid boron atom is s-
bonded to two sp2 hybrid nitrogen atoms and the oxygen
probability level. Selected bond lengths [ꢀ] and angles [8] for the
depicted molecule: B1–O1 1.311(3) (second molecule 1.314(3)), B2–
O1 1.484(3), B1–N1 1.498(3), B1–N2 1.483(3), N1–C2 1.349(3), N2–
C4 1.357(3), C2–C3 1.375(3), C3–C4 1.373(3); B1-O1-B2 163.89(19)
(second molecule 158.5(2)), O1-B1-N1 121.4(2), O1-B1-N2 126.1(2),
N1-B1-N2 112.4(2).
À
atom, along with the low-occupancy (1.65e) B1 N3 p bond.
À
The average B O bond length of 1.2915 ꢀ in 3b is 0.021 ꢀ
shorter than that in 4a. The structural features that distinguish
3b from 4a can be seen in their six-membered boracycles: the
A number of strong donor–acceptor interactions (Table S4 in
the Supporting Information) revealed the highly delocalized
structure of 4. Significantly, the oxygen lone pairs (p
character) strongly interact with two s*BÀN, one p*BÀN, and
À
B N bond lengths of 1.492(3) and 1.494(3) in 3b versus
1.483(3) and 1.498(3) ꢀ in 4a; the C2 C3 and C3 C4 bonds
of 1.365(16) and 1.468(7) ꢀ in 3b versus 1.375(3) and
À
À
one s*BÀ orbital. The N3-B-O p conjunction (stabilization
O
energy 47.22 kcalmolÀ1) indicates the existence of some
À
À
1.373(3) ꢀ in 4a; and the relatively short C4 C5 double-
extent of formal B O p bonding. The calculated frontier
bond length of 1.376(6) ꢀ in 3b. These structural differences
are consistent with the dianionic deprotonated form of the b-
diketiminato ligand in 3b and the monoanionic ligand in 4a.
Moreover, it appears that there is electron delocalization over
the N1-C2-C3 and N2-C4-C5 azaallyl segments in 3b, as
MOs for 4 are very similar to those reported for the Lewis
acid stabilized species [HC(CMe)2(NC6F5)2]BOAlCl3.[5,20]
In summary, we have shown that both anionic and Lewis
acid stabilized oxoboranes can be obtained from the borinic
acid 2 in the presence of N-heterocyclic carbenes and Lewis
acids, respectively. Compounds 3a and 3b are the first
examples of metal-free monomeric anionic oxoboranes that
are isoelectronic with urea. The X-ray single-crystal analysis
À
À
reflected by the almost equal N C and C C bond lengths and
the long C3 C4 bond of 1.468(7) ꢀ. These ring features are in
À
sharp contrast to the structure in 4a, which reflects delocal-
ization over N1-C2-C3-C4-N5.
Preliminary DFT calculations on 3, in which the flanking
Ar groups in 3a are replaced by 2,6-Me2C6H3 (Ar’) groups, at
the RB3LYP/6-311 + + G (2d,p) level reveal a very similar
geometry to 3b (Figure S2 and Table S2 in the Supporting
À
of 3b and 4a disclosed short B O bonds in the two
compounds. DFT calculations on model compounds indicated
the existence of a formal B O double bond in these systems.
Studies on the reactivity of these compounds and extension of
the methodology for the synthesis of other species with
multiple bonds involving boron are currently in progress.
À
À
Information). The B O p bonding can be seen from
HOMOÀ7 (HOMO = highest occupied molecular orbital)
with considerable participation by the nitrogen p orbitals. The
HOMO is located primarily on the two nitrogen atoms and
two alkene fragments of the C3N2B ring, while the lowest
unoccupied molecular orbital (LUMO) is on the imidazolium
ring (Figure S2 in the Supporting Information). Natural bond
Experimental Section
3b: Neat 4-(dimethylamino)pyridine (0.05 g, 0.40 mmol) was added
to
a mixture of 2 (0.18 g, 0.40 mmol) and 1,3-diisopropyl-4,5-
dimethylimidazol-2-ylidene (0.07 g, 0.40 mmol) in toluene (20 mL)
at room temperature. The mixture was stirred overnight. All volatiles
were removed, and the remaining solid was washed with n-hexane.
Crystallization from toluene at À308C gave bright yellow crystals of
3b (0.19 g, 75%). Mp: 189–1908C. Elemental anal. calcd for
C40H61BN4O: C 76.90, H 9.84, N 8.97; found: C 76.38, H 9.43, N
À
order (NBO) analysis indicates one polarized B O s bond
formed by the boron sp2 hybrid orbital with the oxygen sp0.86
hybrid orbital, one sp1.42 hybrid, and two p lone pairs (1.93,
1.86, and 1.74e occupancies) on the oxygen atom and a high-
2818
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2011, 50, 2816 –2819