Synthesis, Spectroelectrochemical, and EPR Spectroscopic Studies of Mixed Bis(alkynyl)biferrocenes of the Type (LnMC≡C)(LnM′C≡C)bfc

Abstract

The synthesis and properties of a series of complexes containing bis(ethynyl)biferrocene as a bridge between different redox-active group 8 metal fragments is described. These metal acetylide compounds of type (LnMC≡C)(LnM′C≡C)bfc (5a, LnM = Fe(Cp*)(η2-dppe), LnM′ = Ru(Cp)(Ph3P)2; 5b, LnM = Fe(Cp*)(η2-dppe), LnM′ = Os(Cp)(Ph3P)2; 5c, LnM = Ru(Cp)(Ph3P)2, LnM′ = Os(Cp)(Ph3P)2; bfc = biferrocene-1′,1‴-diyl, ((η5-C5H4)2Fe)2; dppe = 1,2-bis(diphenylphosphino)ethane, C2H4(PPh2)2; Cp = η5-C5H5, Cp* = η5-C5Me5) were prepared either by treatment of (HC≡C)(LnM′C≡C)bfc (LnM′ = Ru(Cp)(Ph3P)2 (4b), Os(Cp)(Ph3P)2 (4c)) with Fe(Cp*)(η2-dppe)Cl (2a) or by the reaction of 4c with Ru(Cp)(PPh3)2Cl (2b) in the presence of [H4N][PF6] and KOtBu, respectively. Compounds 5a–c show well-separated reversible one-electron redox events in their cyclic voltammograms using [nBu4N][PF6] as supporting electrolyte in dichloromethane solutions. Absorption and vibrational spectroscopic studies were achieved for mixed-valence 5a–c[PF6] and 5a–c[PF6]2 by spectroelectrochemical methods (OTTLE), and in the case of the more robust Fe/Os system the higher oxidation states 5b[PF6]3 and 5b[PF6]4 were also characterized. Taken as a whole, our data indicate that direct electron transfer between the redox termini does not take place. Electron exchange results from dominant interactions between the redox termini and the proximal fc units (fc = Fe(η5-C5H4)2) of the bfc moiety and a weak but sizable interaction between the fc units. Furthermore, EPR spectroscopy of 5a–c[PF6] allowed the simultaneous observation of the EPR signatures of half-sandwich metal-centered radicals and biferrocenium-centered radicals. This feature strongly supports that a multistep electron exchange mechanism takes place between the MLn/M′Ln redox termini of this molecular array, with bridge-centered low-lying mediating states thermally populated even at 66 K. The g tensors of anisotropy (Δg = g∥ – g⊥) of the bis(ethynyl)biferrocenyl moiety ranging between 2.26 and 2.42 for 5a–c[PF6] are consistent with a slow electron exchange rate between the fc units and confirmed that these mixed-valence complexes belong to class II compounds as defined by Robin and Day.