Electronic structure of pi-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 (epsilon = 31 200 M(-1) cm(-1); 2(.-)) or at 736 nm (epsilon = 67 700 M(-1) cm(-1); 2(2-)). 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) Angstrom, b = 9.143(2) Angstrom, c = 16.303(3) Angstrom, beta = 98.93(3)degrees, V = 1962.0(7) Angstrom(3), 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 pi 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 B-III/B-II mixed-valent formulations in the bifunctional compounds 1(.-) and 2(.-).