Structure and Electrochemistry of Ferrocenyloligosilanes: α,ω-Bis(ferrocenyl)- and α,ω-(1,1′-Ferrocenediyl)oligosilanes

Abstract

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) Å and two distinctive reversible redox processes (Eox = 0.56 and 0.67 V, Ered = 0.50 and 0.61 V). Complex IIb exhibited an elongated SiSi bond length of 2.351(1) Å and an elongated Ccyclopentadienyl-Si bond length compared to Ib (1.881(3) versus 1.862(4) Å). These bond length elongations result in minimal cyclopentadienyl dihedral angle strain, 4.3°, in IIb. Electrochemically IIb exhibited a single oxidation process (Eox = 0.67 V, Ered = 0.59 V). The ferrocenophane redox behavior disappeared after a few cycles because of decomposition. The monosilyl-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 α,ω-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 °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. © 1993, American Chemical Society. All rights reserved.