Syntheses, crystal structures and (spectro)electrochemical studies of novel clusters [Ru4(µ-H)4(CO)10(L)] [L = 2,2′-bipyrimidine, 2,3-bis(pyridin-2-yl)pyrazine, 2,2′-bipyridine]†

Abstract

Three tetrahedral clusters [Ru₄(µ-H)₄(CO)₁₀(L)] [L = 2,2′-bipyrimidine 1, 2,3-bis(pyridin-2-yl)pyrazine 2 and 2,2′-bipyridine 3] were obtained from a substitution reaction of [Ru₄(µ-H)₄(CO)₁₂] with L, involving the monosubstituted intermediates [Ru₄(µ-H)₄(CO)₁₁(η¹-L)]. The solid-state structures of 1 and 3 have been elucidated by single-crystal X-ray analysis. In solution, dynamic behaviour of the hydride ligands in 1–3 was apparent from ¹H NMR spectra. UV/VIS and resonance-Raman spectroscopy established a significant Ru(d_π)→L(π*) charge transfer (MLCT) character of the lowest electronic transition of 1–3 in the visible region. Consistent with the dominantly L(π*)-localized LUMO, one-electron reduction of 1 and 2 produced corresponding radical anions 1b and 2b which could be characterized by IR, UV/VIS and EPR spectroscopy. Subsequent one-electron reduction of 1b and 2b yielded unstable dianions [Ru₄(µ-H)₄(CO)₁₀(L)]^2– which were found to eliminate H₂. The dihydrido dianionic products 1c, 2c were also formed via slow disproportionation of 1b and 2b. The radical anion 3b, containing the stronger σ-donor 2,2′-bipyridine anion, was detectable only on the subsecond time-scale of cyclic voltammetry. The electrochemically produced dihydrido dianion 3c is similar but different from [Ru₄(µ-H)₂(CO)₁₀(bpy)]^2– 3c′ obtained by deprotonating 3 in reaction with NEt₄OH.