Unprecedented metal-metal bonded {Ru(μ-O)} butterfly core in oxido-carboxylato bridged mixed valence cluster-structural elucidation and electronic forms in accessible redox states.

Unprecedented metal-metal bonded oxido-carboxylato bridged mixed valence tetraruthenium cluster [(acac)Ru(μ-O)(μ-CHCOO)] 1 (S = 1/2) (acac = acetylacetonate) with {Ru(μ-O)} "butterfly" core has been achieved via the reaction of Ru(acac)(CHCN) with excess CHCOONa (Ru : CHCOONa = 1 : 15) in refluxing EtOH-HO (5 : 1). Structural analysis of 1 ascertained a unique Ru-Ru bonded (Ru2-Ru3: 2.5187(6) Å (DFT: 2.560 Å)) {Ru(Ru-Ru)(μ-O)} butterfly core, unlike the reported other Fe or Mn derived "butterfly" core. The connectivity of Ru(μ-O) core in 1 with three Ru(μ-CHCOO) and two each Ru-(acac)/Ru-acac units resulted in interconnected four RuO octahedral entities. The doubly bridged μ-O ions of the nearly planar central metal-metal bonded RuO core (Ru2-Ru3, "body" or "hinge") linked to the remaining two "wing-tip" Ru atoms (Ru1 and Ru4). Complex 1 with a S = 1/2 spin state displayed paramagnetically shifted H NMR over a wide chemical shift range in CDCl (δ, 13 to -30 ppm) and a metal based anisotropic EPR (g = 2.17, g = 2.01, g = 1.86; Δg = g-g = 0.31 and 〈g〉 = [1/3(g + g + g] = 2.01) at 100 K in CHCN-toluene. The metal based one-electron reversible oxidation at 0.49 V and reduction at 0.485 V versus SCE of 1 led to the EPR inactive (even at 4 K) spin-coupled RuRuRuRu (S = 0) and RuRuRuRu (S = 0) electronic configurations for 1 and 1, respectively. Mixed valence 1 and 1 exhibited low-energy near-infrared (NIR) absorption bands at 1350 nm and 1156 nm, respectively, in CHCN. A combined experimental (UV-vis-NIR and EPR spectroelectrochemistry) and theoretical (DFT) analysis indicated a delocalised mixed valence form of 1 ({Ru(Ru-Ru)(μ-O)}).