Cyano‐ and Aqua‐Coordinated Diruthenium(III) Complexes with Oxo‐Bis(acetato) Bridge: Preparation and Steric and Electronic Structures

Abstract

AbstractThree new μ‐oxo‐bis(μ‐acetato)diruthenium(III) complexes,trans(μ‐O,OH2)‐[Ru2(μ‐O)(μ‐CH3COO)2(bpy)2(H2O)2][PF6]2·3H2O ([1][PF6]2·3H2O; bpy = 2,2′‐bipyridine), trans(μ‐O,CN)‐[{Ru2(μ‐O)(μ‐CH3COO)2(bpy)2}2{μ‐Ag(CN)2}{μ‐Ag2(CN)3}][Ag(CN)2][Ag2(CN)3](CH3CN)4(CH2Cl2) (2), and cis(μ‐O,CN)‐[Ru2(μ‐O)(μ‐CH3COO)2(bpy)2(CN)2]·CH3OH·H2O([3]·CH3OH·H2O), have been prepared and their X‐ray crystal structures determined. Whereas [1]2+ and 3 are discrete species, 2 is a polymer in which the Ag–CN groups act as bridges through Ru–CN(or NC)–Ag linkages. The discrete complexes dissolve in common solvents, but 2 is only slightly soluble in acetonitrile and dimethyl formamide (DMF) to give fragmented species (denoted as 2′) in solution. Redox waves of 3 were observed at –0.83 (Epc) and +0.76 V (E1/2) versus Ag/AgCl in a solution of 0.1 M nBu4NPF6 in CH2Cl2, which correspond to the Ru2II,III/Ru2III,III and Ru2III,III/Ru2III,IV processes, respectively. The corresponding redox couples (E1/2) of [1]2+ were observed at –0.31 and +0.76 V in 0.1 M NaClO4 aqueous solution. Significantly more negative potentials of the Ru2II,III/Ru2III,III couple for the two cyano complexes (E1/2 = –0.77 V for 2′) are noted. Complexes [1]2+, 2′, and 3 exhibit strong visible absorption bands at λmax = 566 nm (ϵ = 12600 M–1 cm–1; in H2O), 636 nm (21000 M–1 cm–1; in acetonitrile), and 558 nm (18000 M–1 cm–1; in H2O), respectively. Density functional theory (DFT) molecular orbital calculations have been carried out for [1]2+, trans(μ‐O,CN)‐[Ru2(μ‐O)(μ‐CH3COO)2(bpy)2(CN)2] (2a, a model compound for 2′), 3, [Ru2(μ‐O)(μ‐CH3COO)2(py)6]2+ ([4]2+), and trans(μ‐O,py)‐[Ru2(μ‐O)(μ‐CH3COO)2(bpy)2(py)2]2+ ([5]2+; py = pyridine). It was revealed that the frontier orbitals of these complexes are dominated by the Ru(dπ)–μ‐O(pπ) type π systems. For the cyano complexes, energies of these π‐type MOs are lifted up by the interaction with CN–, so that the energy difference with the upper bpy π* orbitals becomes smaller. Thus, whereas the visible absorption bands of [1]2+, [4]2+, and [5]2+ are ascribed to the transitions within the π system metal‐to‐metal charge transfer (MMCT), those of 2′ and 3 are ascribed to the HOMO‐to‐ligand π* (bpy) transition metal‐to‐ligand charge transfer (MLCT) and to the mixture of MMCT and MLCT (HOMO–1 to π*), respectively.