ELECTRONIC-STRUCTURE OF PI-CONJUGATED REDOX SYSTEMS WITH BORANE BORATAALKENE END-GROUPS/

The compounds 1,4-bis(dimesitylboryl)benzene (1), 4,4'-bis(dimesitylbo ryl)biphenyl (2), and dimesitylphenylborane (3) were studied by UV/vis /near-IR spectroelectrochemistry. In contrast to the colorless precurs or 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 ab sorption bands of the organoborane anion radicals and their appearance , including their vibrational structuring, resemble those of correspon ding amine radical cations such as Wurster's Blue. The conformation of the dimesitylboryl substituent toward a phenyl ring was established u sing the crystal structure results for 3 (C24H27B, monoclinic, space g roup 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 a nalogues of 1-3 and on 3 proper confirmed the concept of an increasing ly 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 i ntermediates, indicating sizable contributions from the diboratasemiqu inone or-in the extreme-from delocalized B-III/B-II mixed-valent formu lations in the bifunctional compounds 1(.-) and 2(.-).