The compds. 1,4-bis(dimesitylboryl)benzene (1), 4,4'-bis(dimesitylboryl)biphenyl (2), and dimesitylphenylborane (3) were studied by UV/visible/near-IR spectroelectrochem. In contrast to the colorless precursor compds., all neg. charged species 1.bul.-/2-, 2.bul.-/2-, and 3.bul.- exhibit intense long-wavelength absorption maxima in the visible or near-IR region, e.g., at 1305 nm (e = 31,200 M-1 cm-1; 2.bul.-) or at 736 nm (e = 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 Ph ring was established using the crystal structure results for 3 (C24H27B, monoclinic, space group C2/c, a 13.324(3), b 9.143(2), c 16.303(3) \AA, b 98.93(3)°, Z = 4, R = 0.0493). Ab initio calcns. (6-31G**) on the BH2-substituted analogs of 1-3 and on 3 proper confirmed the concept of an increasingly quinonoid distortion of the organoboron redox systems on redn. and served in assigning the obsd. electronic transitions. A 23-35\% participation of the B p 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 compds. 1.bul.- and 2.bul.-. on SciFinder(R)
Please log in
to take part in the discussion (add own reviews or comments).