Compound 2 is stable in solution and the solid state
under air with an unusual structure. It has a rare C,O-
chelate around the four-coordinate boron atom. The B-C
˚
bonds are much longer (1.617(7) to 1.632(7) A) than those
of 1 and 3, but similar to those of previously reported four-
coordinate BMes2 compounds.8 The B-O bond is1.609(6)
˚
A, somewhat longer than those observed in noncongested
four-coordinate boron compounds (e.g., BPh2(q), q =
8-hydroxyquinolato).9 The alkyne bond has been trans-
formed to an olefin bond with a typical double bond
Figure 1. Structure of 1, 2, and 3.
˚
distance, 1.352(6) A, and an E-geometry with respect to
the pyridyl and phenyl groups. A proton bound to the py
nitrogen atom was located and refined successfully in
X-ray analysis. This proton forms a hydrogen bond with
Sonogashira coupling between o-(dimesitylboryl)ethynyl-
benzene and 2-bromopyridine in refluxing wet THF/Et3N
using Pd(PPh3)2Cl2 and CuI as catalysts (see the Support-
ing Information for details). From the same reaction, a
yellow precipitate, compound 2, with bright yellow-green
fluorescence was isolated in 22% yield. When freshly dried
THF was used in the synthesis, the yield of compound 1
was improved to 35% and no compound 2 was obtained.
The control compound, 2-(20-BMes2-phenylethynyl)benzene
(3), was obtained by a similar procedure in 45% yield. All
three compounds are air stable in solution and in the solid
state. They were fully characterized by NMR, HRMS, and
single-crystal X-ray diffraction analyses (see the Support-
ing Information).
˚
˚
the oxygen atom (N-H = 0.98(3) A, O H = 1.98(3) A),
3 3 3
holding the py ring in plane with the phenyl ring. The C-O
bond length is 1.322(7) A, typical of conjugated C-O
˚
bonds.10 Thus, based on the crystal structure, compound 2
can be described as a zwitterionic BMes2 enolate (Figure 3).
Between the enol and keto tautomers, the keto form is
usually much more stable, thus more commonly observed
than the enol tautomer.11 Clearly, binding to the BMes2
unit by the oxygen atom stabilizes the otherwise unstable
enol tautomer.
Both 1 and 2 crystallize in the monoclinic space
group P21/c while compound 3 crystallizes in the
triclinic P1 space group. The structures of 1 and 3 are
shown in Figure 2 for comparison. The B-C bond
lengths in both molecules span the range of 1.55(1)
˚
to 1.58(1) A, which are typical of triarylboron com-
pounds. In 1, one mesityl is parallel and directly above
the pyridylalkyne group with a dihedral angle of 16.4°
and short separation distances (e.g., 3.00, 3.48 A). Such
˚
π-stacking interaction may be faciliated by the attrac-
tion between the electron-rich mesityl ring and the
electron-deficient pyridylalkynyl group. In contrast,
the mesityl ring in 3 is nearly perpendicular to the
phenylakynyl group with a dihedral angle of 106.4°.
This conformation of 3 may be attributed to the dimer
Figure 3. Crystal structure of 2 with 35% thermal ellipsoids.
Compound 2 has a distinct yellow color in solution due
to an intense and broad absorption band between 360
and 470 nm (λmax = 435 nm, ε = 27 000 M-1 cm-1) in
the absorption spectrum (Figure 4) that can be attrib-
uted to a π to π* transition on the backbone with a small
charge transfer contribution from the mesityl to the π*
orbital of the chelate backbone, based on TD-DFT
calculation results (see the Supporting Information). In
contrast, 1 and 3 are colorless with little absorption
in the visible region.
formation via intermolecular C-H (methyl)
(alkynylphenyl) interactions in the solid state (see the
π
3 3 3
Supporting Information).
(8) (a) Baik, C.; Hudson, Z. M.; Amarne, H.; Wang, S. J. Am. Chem.
Soc. 2009, 131, 14549. (b) Amarne, H.; Baik, C.; Murphy, S. K.; Wang,
S. Chem.;Eur. J. 2010, 16, 4750.
(9) Cui, Y.; Wang, S. J. Org. Chem. 2006, 71, 6485.
(10) Bond lengths in crystalline organic compounds. In CRC Hand-
book of Chemistry and Physics, 91st ed.; CRC Press: Boca Raton, FL, 2010;
pp 9-10 .
(11) March, J. Advanced Organic Chemistry: Reactions, Mechanisms,
and Structures, 4th ed.; John Wiley & Sons: New York, 1992; p 762.
Figure 2. Crystal structures of 1 (left) and 3 (right) with 35%
thermal ellipsoids.
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