Photoinduced electron transfer in ion pairs where the anion is a tetrasubstituted borate gives a boranyl radical. This species is reactive. When the boranyl radical contains an alkyl group, the carbon-boron bond cleaves in ca. 250 fs. The lifetime of tetraarylboranyl radicals is greater than 40 ps. The short lifetime of boranyl radicals prohibits measurement of the oxidation potentials of borates by electrochemical methods. These quantities were estimated by application of Marcus electron-transfer theory, which relates the energetics to the kinetics of the reaction. We find that the use of the empirical Rehm-Weller equation for this purpose is not appropriate and leads to errors in estimation of both the reorganization energy of the reaction and the oxidation potential of the borates. The oxidation potentials were determined in acetonitrile solution by this approach for two series of borates, (tolyl)n(Ph)4-nB-, where the tolyl group might be ortho or para substituted and n = 0-4. The data reveal that λ, the reorganization energy, is 1.0 eV. The oxidation potentials of the borates vary systematically in the p-tolyl series from 0.81 to 1.03 V vs SCE with decreasing number of tolyl groups on the borate. The o-tolyl series shows higher than expected oxidation potentials for the tetra- and tritolyl cases, which may be due to greater steric hindrance to the acceptor for these two borates.
ASJC Scopus subject areas
- General Engineering
- Physical and Theoretical Chemistry