STRUCTURES OF STERICALLY OVERCROWDED AND CHARGE-PERTURBED MOLECULES .38. THE DIFFERENT CONFORMATIONS OF N-TRIMETHYLSILYL-SUBSTITUTED P-PHENYLENEDIAMINE DERIVATIVES - INVESTIGATION OF SINGLE CRYSTALS AND IN THE GAS-PHASE

The ESR-proven generation of a Wurster's Blue radical anion by one-electron reduction of N,N,N',N'-tetrakis(trimethylsilyl)-p-phenylenediamine is rationalized by a posterior single-crystal structure determination of the neutral molecule, which demonstrates inter alia that the bulky [(H3C)(3)Si]N-2 substituents are twisted out of the benzene plane by 83 degrees. In contrast, the skeleton of the sterically congested N,N'-bis(trimethylsilyl)-p-phenylenediamine remains almost planar and, accordingly, allows n(N)/pi interaction between the p-type nitrogen electron pairs of the also flattened [(H3C)(3)Si]HN groups and the benzene n system. These conformational differences are reflected in the gas-phase photoelectron spectra, in which n(N)/pi ionisation bands are split by less than 1 eV or more than 3 eV, respectively. The evidence for analogous structures of the two organosilicon p-phenylenediamine derivatives in solution is confirmed by geometry-optimized AM1 calculations, which readily explain the surprising insertion of an extra electron into an already electron-rich molecule.