Photoelectron spectra and molecular properties. CXXXVIII. 1,4-Dibenzene: synthesis, structural analogy, photoelectron spectrum and ESR/ENDOR cheracterization of its radical anion

Article abstract of DOI:10.1016/0022-328X(93)80042-A

1,4-Dibenzene <(H3C)3Si>3SiC6H4Si3, which contains eight silicon centers, has been synthesized and some of its molecular properties determined.For a single crystal structure determination, its isovalence-electronic and sterically more crowded carbon analogue, <(H3C)3Si>3CC6H4C3, was selected.It shows van der Waals interactions due ro close lattice packing.Despite its shorther SiC bonds, however, no severe skeletal distortions due to overcrowding are observed; the contraction of the benzene ipso angles to 114 deg is due to the donor effect of the tris(trimethylsilyl)methyl substituents.The first vertical ionization energy of the title molecule, IE^v₁ = 7.37 eV, is the lowest of all of these recorded up to now for 1,4-dialkyl or 1,4-disilyl substituted benzene derivatives; for the monosubstituted tris(trimethylsilyl)silyl derivative <(H3C)3Si>3SiC6H5 the value is 8.04 eV.Cyclovoltammetric experiments in aprotic solution reveal a velocity-dependent irreversible oxidation above + 1.28 eV and no electron insertion within the measurement range to -2.9 eV.The reduction to the radical anion, however, can be achieved in THF solution containing <2.2.2>cryptand at a potassium metal mirror.According to the spin populations deduced from 1H and 29Si ENDOR signal patterns, more than half of the spin is located in the two <(H3C)3Si>3Si groups.Results of molecular orbital (MO) calculations and comparison with equivalent molecular states of chemically related compounds allow rationalization of the effects of the <(H3C)3Si>3Si substituent, which acts as a strong electron doenor in radical cations and as a strong electron acceptor in radical anions.