Oxidized Styrylruthenium Ferrocene Conjugates : From Valence Localization to Valence Tautomerism

Abstract

We report on ferrocenyl styryl ruthenium conjugates FcC6H4CH=CHRu(CO)(PiPr3)2(L) where the electron density at the ruthenium alkenyl site is modified by the variation of coligand L (L = Cl, acac, hfac, or dpvm). Crystallographic studies on three derivatives provide snapshots of the conformational degrees of freedom for rotation around the vinyl-phenylene and the phenylene-ferrocenyl linkages. All four complexes undergo two consecutive, reversible one-electron oxidations, whose potentials depend on the ligand L. According to IR spectroelectrochemistry, the first oxidation of the less electron-rich chloro and hfac complexes is strongly biased towards the ferrocenyl site. The radical cation of the acac complex, however, exists as two equilibrating valence tautomers (VTs) Fc+C6H4CH=CH{Ruacac}Fc[C6H4CH=CH{Ruacac}]+ ({Ruacac = Ru(CO)(PiPr3)2(acac)), where the positive charge is either localized at the ferrocenyl site or delocalized over the styryl ruthenium moiety. Variable-temperature EPR and Mössbauer spectroscopy reveal that the ferrocenium valence tautomer dominates at low T. A marked solvent-dependence on the position and relative intensities of the separate Ru(CO) bands in the IR spectra reveals environmental effects on the relative stabilities of the two VTs, while the strong negative solvatochromism of the prominent near IR band of the radical cation of the acac complex argues for a sizable (intervalence) charge-transfer component of the underlying excitation.