Influence of solvent and cation on the properties of oxygen–containing organic anions. Part 2. Radical anions derived from 2,5-diarylfurans

Abstract

The e.s.r. spectra of radical anions of 2,5-diarylfurans were studied in several polar aprotic solvents with the cations of Li, Na, K, Rb, and Cs as counter-ions. The electron spin densities obtained from the hydrogen coupling constants for the 3 and 4 ring positions of the 2,5-diphenylfuran radical anions and the electron spin densities of the para-positions of the phenyl groups were analysed by multiple linear regression. A triple linear correlation with Dimroth–Reichardt E_T^N and Gutmann DN parameters of the solvent and the cation radius r_M was found. Hückel–McLachlan electron spin density calculations to fit the experimental values were performed. The values of the Coulomb parameter δ_O for oxygen which had to be used in order to obtain the best fit show that the oxygen–metal distance controls the observed electron spin densities. One can conclude that this distance increases when the solvent has a higher ability for dipolar interaction with an ion pair (measured by E_T^N), a higher capacity for electron-pair donation (measured by DN), and the cation a higher capacity to accept electron pairs from the solvent (smaller radius r_M).