Redox-Active Organometallics: Magnetic and Electronic Couplings through Carbon-Silicon Hybrid Molecular Connectors

Treatment of the triflate complex Cp*(dppe)FeOTf [12; CP* = eta(5)-C-5(CH3)(5), dppe = 1,2-bis(diphenylphosphino) ethane, OTf = CF3SO3] with an excess of HC C-(Si(CH3)(2))(x)-C CH (x = 2-4) in diethyl ether provides the binuclear bis(vinylidene) derivatives [Cp*(dppe)Fe=C=CH(Si(CH3)(2))(x)CH=C=Fe(dppe)Cp*][OTf](2) (x = 2, 13; x = 3, 14; x = 4, 15), which were isolated as ochre solids and rapidly characterized by FT-IR, H-1, P-31, and C-13 NMR spectroscopies. The complexes 13-15 were reacted with potassium tert-butoxide to afford the bis(alkynediyl) complexes [Cp*(dppe)Fe-C -C(Si(CH3)(2))(x)C C-Fe(dppe)Cp*] (x= 2, 1; x= 3, 2; x= 4, 3), which were isolated as orange powders in yields ranging from 76 to 91%. The IR, cyclic voltammetry, and UV-vis data obtained for 1-3 and the X-ray crystal structures determined for 1 and 3 reveal the importance of the sigma-pi conjugation (hyperconjugation) between the Si-Si sigma bond and the adjacent C C pi-symmetric orbitals in the description of the electronic structure of the ground state of these complexes. When reacted at low temperature with 2 equiv of [(C5H5)(2)Fe]X or AgX [X = BPh4, B(3,5-(CF3)(2)C6H3)(4))], compounds 1-3 provide 1[X](2), 2[X](2), and 3[X](2), which can be isolated and stored below -20 degrees C. EPR spectroscopy and magnetization measurements established that the superexchange interaction propagates through the Si-Si bonds (J = -0.97(2) cm(-1) for 3[X](2)). UV-vis-near-IR spectra were obtained with an optically transparent thin-layer electrosynthetic (OTTLE) cell for 1-3[OTf](n) (n = 0-2). A band with a maximum that increases from 6400 cm(-1) (1[OTf]) to 8500 cm(-1) (3[OTf]) observed for the mixed-valence species was ascribed to intervalence charge transfer evidencing photodriven electron transfer through the carbon-silicon hybrid connectors with H-ab parameters ranging from 64 to 285 cm(-1).