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

Treatment of the triflate complex Cp*(dppe)FeOTf [12; Cp* = η5-C5(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)xCH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, 1H, 31P, and 13C 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)xC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 σ−π conjugation (hyperconjugation) between the Si−Si σ bond and the adjacent CC π-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)2Fe]X or AgX [X = BPh4, B(3,5-(CF3)2C6H3)4)], compounds 1−3 provide 1[X]2, 2[X]2, and 3[X]2, which can be isolated and stored below −20 °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 Hab parameters ranging from 64 to 285 cm−1.