PHOTOELECTRON-SPECTRA AND MOLECULAR-PROPERTIES .138. 1,4-DI[TRIS(TRIMETHYLSILYL)SILYL]BENZENE - SYNTHESIS, STRUCTURAL ANALOGY, PHOTOELECTRON-SPECTRUM AND ESR ENDOR CHARACTERIZATION OF ITS RADICAL-ANION

1,4-Di[tris(trimethylsilyl)silyl)]benzene [(H3C)3Si]3SiC6H4Si[Si(CH3)3]3, 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]3CC6H4C[Si(CH3)3]3, was selected. It shows van der Waals interactions due to close lattice packing. Despite its shorter SiC bonds, however, no severe skeletal distortions due to overcrowding are observed; the contraction of the benzene ipso angles to 114-degrees is due to the donor effect of the tris(trimethylsilyl)methyl substituents. The first vertical ionization energy of the title molecule, IE1v = 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 V. 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 H-1 and Si-29 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 donor in radical cations and as a strong electron acceptor in radical anions.