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

Three new μ‐oxo‐bis(μ‐acetato)diruthenium(III) complexes, trans (μ‐O,OH 2 )‐[Ru 2 (μ‐O)(μ‐CH 3 COO) 2 (bpy) 2 (H 2 O) 2 ][PF 6 ] 2 · 3H 2 O ([ 1 ][PF 6 ] 2 · 3H 2 O; bpy = 2,2′‐bipyridine), trans (μ‐O,CN)‐[{Ru 2 (μ‐O)(μ‐CH 3 COO) 2 (bpy) 2 } 2 {μ‐Ag(CN) 2 }{μ‐Ag 2 (CN) 3 }][Ag(CN) 2 ][Ag 2 (CN) 3 ](CH 3 CN) 4 (CH 2 Cl 2 ) ( 2 ), and cis (μ‐O,CN)‐[Ru 2 (μ‐O)(μ‐CH 3 COO) 2 (bpy) 2 (CN) 2 ] · CH 3 OH · H 2 O([ 3 ] · CH 3 OH · H 2 O), 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 ( E pc ) and +0.76 V ( E 1/2 ) versus Ag/AgCl in a solution of 0.1 M n Bu 4 NPF 6 in CH 2 Cl 2 , which correspond to the Ru 2 II,III /Ru 2 III,III and Ru 2 III,III /Ru 2 III,IV processes, respectively. The corresponding redox couples ( E 1/2 ) of [ 1 ] 2+ were observed at –0.31 and +0.76 V in 0.1 M NaClO 4 aqueous solution. Significantly more negative potentials of the Ru 2 II,III /Ru 2 III,III couple for the two cyano complexes ( E 1/2 = –0.77 V for 2′ ) are noted. Complexes [ 1 ] 2+ , 2′ , and 3 exhibit strong visible absorption bands at λ max = 566 nm ( ϵ = 12600 M –1  cm –1 ; in H 2 O), 636 nm (21000 M –1  cm –1 ; in acetonitrile), and 558 nm (18000 M –1  cm –1 ; in H 2 O), respectively. Density functional theory (DFT) molecular orbital calculations have been carried out for [ 1 ] 2+ , trans (μ‐O,CN)‐[Ru 2 (μ‐O)(μ‐CH 3 COO) 2 (bpy) 2 (CN) 2 ] ( 2a , a model compound for 2′ ), 3 , [Ru 2 (μ‐O)(μ‐CH 3 COO) 2 (py) 6 ] 2+ ([ 4 ] 2+ ), and trans (μ‐O,py)‐[Ru 2 (μ‐O)(μ‐CH 3 COO) 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.