STRUCTURE AND ELECTROCHEMISTRY OF FERROCENYLOLIGOSILANES - ALPHA,OMEGA-BIS(FERROCENYL)OLIGOSILANES AND ALPHA,OMEGA-(1,1'-FERROCENEDIYL)OLIGOSILANES

The structures and electrochemical properties of 1,2-bis(ferrocenyl)-1,1,2,2-tetramethyldisilane (Fc-(SiMe2)nFc, n = 2, Ib) and (1,1'-ferrocenediyl)tetramethyldisilane ((C5H4)2(SiMe2)nFe, n = 2, IIb), a ferrocenophane, are reported together with the electrochemical behavior of their analogs (n = 1 (a), 3 (c)). Complex Ib exhibited a Si-Si bond length of 2.340(2) angstrom and two distinctive reversible redox processes (E(ox) = 0.56 and 0.67 V, E(red) = 0.50 and 0.61 V). Complex IIb exhibited an elongated Si-Si bond length of 2.351(1) angstrom and an elongated C(cyciopentadienyl)-Si bond length compared to Ib (1.881(3) versus 1.862(4) angstrom). These bond length elongations result in minimal cyclopentadienyl dihedral angle strain, 4.3-degrees, in IIb. Electrochemically IIb exhibited a single oxidation process (E(ox) = 0.67 V, E(red) = 0.59 V). The ferrocenophane redox behavior disappeared after a few cycles because of decomposition. Themonosilyl-bridged ferrocenophane IIa underwent decomposition upon electrochemical oxidation, while (1,1'-ferrocenediyl)heptamethyltrisilane (IIc) exhibited a completely reversible oxidation process. This trend reflects the capacity of the three bridges, Si, Si2 and Si3, to incorporate the increase in Fe-cyclopentadienyl ring distance upon oxidation. Electrochemical investigation of the alpha,omega-bis-(ferrocenyl) complexes Ia-c, and complexes with n = 0 (biferrocene) and n = 6 (If), showed that the two redox systems are most separated when the two Fe atoms are adjacent (n = 0) and progressively become closer as the inter-ferrocene distance increases, until at n = 6 only a single process is observed. Complexes IIb,c did not undergo simple linear polymerization at temperatures below 350-degrees-C, and above this temperature cross-linking polymerization occurred to produce insoluble materials. Both Ib and IIb undergo Si-Si bond cleavage upon photolysis in MeOH to produce the methoxysilane derivatives, and in the case of IIb the major product (>95%) was (1,1'-ferrocenediyl)tetramethyldisiloxane.