Unprecedented metal–metal bonded {Ru4(μ3-O)2} butterfly core in oxido-carboxylato bridged mixed valence cluster—structural elucidation and electronic forms in accessible redox states

Abstract

Unprecedented metal–metal bonded oxido-carboxylato bridged mixed valence tetraruthenium cluster [(acac)₆Ru^{III,III,III,IV}₄(μ₃-O)₂(μ-CH₃COO)₃] 1 (S = 1/2) (acac = acetylacetonate) with {Ru₄(μ₃-O)₂} “butterfly” core has been achieved via the reaction of Ru(acac)₂(CH₃CN)₂ with excess CH₃COONa (Ru : CH₃COONa = 1 : 15) in refluxing EtOH–H₂O (5 : 1). Structural analysis of 1 ascertained a unique Ru–Ru bonded (Ru2–Ru3: 2.5187(6) Å (DFT: 2.560 Å)) {Ru₂(Ru–Ru)(μ₃-O)₂} butterfly core, unlike the reported other Fe₄ or Mn₄ derived “butterfly” core. The connectivity of Ru₄(μ₃-O)₂ core in 1 with three Ru₂(μ-CH₃COO)₃ and two each Ru-(acac)₂/Ru-acac units resulted in interconnected four RuO₆ octahedral entities. The doubly bridged μ₃-O²⁻ ions of the nearly planar central metal–metal bonded Ru₂O₂ 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 ¹H NMR over a wide chemical shift range in CDCl₃ (δ, 13 to −30 ppm) and a metal based anisotropic EPR (g₁ = 2.17, g₂ = 2.01, g₃ = 1.86; Δg = g₁ − g₃ = 0.31 and 〈g〉 = [1/3(g₁² + g₂² + g₃²]^1/2 = 2.01) at 100 K in CH₃CN–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 Ru^IIIRu^IIIRu^IVRu^IV (S = 0) and Ru^IIIRu^IIIRu^IIIRu^III (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 CH₃CN. A combined experimental (UV-vis-NIR and EPR spectroelectrochemistry) and theoretical (DFT) analysis indicated a delocalised mixed valence form of 1 ({Ru₂(Ru–Ru)(μ₃-O)₂}).