Oxidized Styrylruthenium–Ferrocene Conjugates: From Valence Localization to Valence Tautomerism

Abstract

We report on ferrocenyl–styrylruthenium conjugates Fc–C6H4–CH=CH–Ru(CO)(PiPr3)2(L) in which the electron density at the alkenylruthenium site is modified by the variation of the coligand L [L = Cl, acetylacetonate (acac), hexafluoroacetylacetonate (hfac), or dipivaloylmethane (dpvm); Fc = ferrocenyl]. Crystallographic studies on three derivatives provide snapshots of the conformational degrees of freedom for rotation around the vinyl–phenylene and phenylene–ferrocenyl linkages. All four complexes undergo two consecutive, reversible one-electron oxidations, the potentials of which depend on the ligand L. On the basis of IR spectroelectrochemistry results, the first oxidations of the less electron-rich chlorido and hfac complexes are biased strongly towards the ferrocenyl site. However, the radical cation of the acac complex exists as two equilibrating valence tautomers (VTs) Fc+–C6H4–CH=CH–{Ruacac} ⇌ Fc–[C6H4–CH=CH–{Ruacac}]+[{Ruacac} = Ru(CO)(PiPr3)2(acac)], in which the positive charge is either localized at the ferrocenyl site or delocalized over the styrylruthenium moiety. Variable-temperature electron paramagnetic resonance (EPR) and Mössbauer spectroscopy reveal that the ferrocenium valence tautomer dominates at low temperature. 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, whereas 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.