Electronic Structure of π-Conjugated Redox Systems with Borane/Borataalkene End Groups

The compounds 1,4-bis(dimesitylboryl)benzene
(1), 4,4‘-bis(dimesitylboryl)biphenyl
(2), and dimesitylphenylborane
(3) were studied by UV/vis/near-IR spectroelectrochemistry.
In contrast to the colorless precursor compounds,
all negatively charged species
1•-/2-,
2•-/2-,
and 3•- exhibit intense
long-wavelength absorption maxima in the
visible or near-infrared region, e.g., at 1305 nm (ε = 31 200
M-1 cm-1;
2•-) or at 736 nm (ε =
67 700 M-1
cm-1;
22-). The absorption bands of
the organoborane anion radicals and their appearance, including
their
vibrational structuring, resemble those of corresponding amine radical
cations such as Wurster's Blue. The
conformation of the dimesitylboryl substituent toward a phenyl ring was
established using the crystal structure
results for 3 (C24H27B, monoclinic,
space group C2/c, a = 13.324(3)
Å, b = 9.143(2) Å, c = 16.303(3)
Å, β =
98.93(3)°, V = 1962.0(7) Å3,
Z = 4, R = 0.0493). Ab
initio calculations (6-31G**) on the
BH2-substituted analogues
of 1−3 and on 3 proper confirmed the
concept of an increasingly quinonoid distortion of the organoboron
redox
systems on reduction and served in assigning the observed electronic
transitions. A 23−35% participation of the
boron π centers at the lowest unoccupied MOs is in agreement with
results from EPR/ENDOR measurements of
the radical intermediates, indicating sizable contributions from the
diboratasemiquinone orin the extremefrom
delocalized BIII/BII mixed-valent formulations
in the bifunctional compounds
1•- and
2•-.