Synthesis, Electrochemistry, and Reactivity of Half-Sandwich Ruthenium Complexes Bearing Metallocene-Based Bisphosphines

Abstract

The bimetallic complexes CpRu(P−P)X [Cp = η5-C5H5; X = Cl, H; P−P = dppf (1,1′-bis(diphenylphosphino)ferrocene), dppr (1,1′-bis(diphenylphosphino)ruthenocene), dppo (1,1′-bis(diphenylphosphino)-osmocene), dippf (1,1′-bis(diisopropylphosphino)ferrocene), dcpf (1,1′-bis(dicyclohexylphosphino)ferrocene)], Cp*Ru(P−P)X [Cp* = η5-C5Me5; X = Cl, H; P−P = dppf, dippf, dppomf (1,1′-bis(diphenylphosphino)octamethylferrocene), dppc (1,1′-bis(diphenylphosphino)cobaltocene)], [Cp*Ru(P−P)X]+ (X = H, CCPh; P−P = dppc+), and [Cp*Ru(P−P)L]2+ (L = CH3CN, t-BuCN; P−P = dppc+) have been synthesized. Most of the chloride and hydride complexes have been studied by cyclic voltammetry. The X-ray structures of [Cp*Ru(dppc)CH3CN][PF6]2 and [Cp*Ru(dppc)CCPh][PF6] have been determined. Protonation of [Cp*Ru(dppc)CCPh]+ gives the vinylidene complex [Cp*Ru(dppc)CCHPh]2+. The Co(III/II) potential of the dppc+ ligand undergoes a cathodic shift upon coordination in [Cp*Ru-(dppc)H]+ and an anodic shift upon coordination in [Cp*Ru(dppc)CH3CN]2+. The 1H NMR spectrum of Cp*Ru(dppc)H is consistent with its formulation as a Co(II)/Ru(II) complex. As gauged by their reactivity toward iminium cations, the hydride complexes are poor hydride donors; proton and electron transfer are dominant. CpRu(dippf)H and CpRu(dcpf)H deprotonate iminium cations with acidic α-hydrogens. Cp*Ru(dppc)H is oxidized by the N-(benzylidene)pyrrolidinium cation, giving [Cp*Ru-(dppc)H]+ and the vicinal diamine 1,2-bis(N-pyrrolidino)-1,2-diphenylethane. Most of the hydride complexes give trans-dihydride cations upon protonation; an exception is [Cp*Ru(dppc)H]+, which forms a dihydrogen complex [Cp*Ru(dppc)(H2)]2+ with surprising kinetic stability. This dihydrogen complex is more acidic and less thermodynamically stable than its dihydride isomer. The H2 ligand in [Cp*Ru(dppc)-(H2)]2+ is readily replaced by nitriles; the reaction with t-BuCN occurs by a dissociative mechanism.