Organoboron substituent effects in EPR/ENDOR spectroscopy. Radical ions with boron-carbon and boron-nitrogen bonds

, , , und . Journal of the Chemical Society, Perkin Transactions 2: Physical Organic Chemistry (1992)


The consequences of R3B coordination (R = Me, Et, cyclohexyl) to nitrogen-contg. anion radicals and of Mes2B substitution (Mes = mesityl) at C or N p centers of radical ions were studied for a series of closely related carbo- and heterocyclic p systems by ESR/ENDOR spectroscopy. Examples include the anion radicals of dinuclear R3B coordination compds. 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 homoarom. anions, the isoelectronic cations of the non-arom. Mes2B-substituted dihydroheterocyles exhibit a substantial decrease of the heteronuclear (11B, 14N) ESR/ENDOR coupling consts. owing to a considerable degree of B/N p bonding. MO/McLachlan perturbation calcns. suggest that both the orbital overlap and the electronegativity differences contribute to the obsd. effects. on SciFinder(R)

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