Organoboron substituent effects in EPR/ENDOR spectroscopy. Radical ions with B–C and B–N bonds

The consequences of R₃B coordination (R = CH₃, C₂H₅, C₆H₁₁) to nitrogen-containing anion radicals and of Mes₂B substitution (Mes = mesityl, C₉H₁₁) at C or N π centres of radical ions have been studied for a series of closely related carbo- and hetero-cyclic π systems by EPR/ENDOR spectroscopy. Examples include the anion radicals of dinuclear R₃B coordination compounds with pyrazine and 4,4′-bipyridine, para-diboryl substituted benzene and biphenyl radical anions and the correspondingly substituted 1,4-dihydropyrazine and 1,1′-dihydro-4,4′-bipyridine radical cations. Whereas the perturbation of spin distribution is most pronounced for the dimesitylboryl-substituted homoaromatic anions, the isoelectronic cations of the non-aromatic Mes₂B-substituted dihydroheterocycles exhibit a substantial decrease of the heteronuclear (¹¹B, ¹⁴N) EPR/ENDOR coupling constants owing to a considerable degree of B/N π bonding. MO/McLachlan perturbation calculations suggest that both the orbital overlap and the electronegativity differences contribute to the observed effects.