Preparation, spectroscopy, and electrochemistry of cyano-bridged bis(oxo-centered-hexa(μ-acetato)-triruthenium) complexes

Preparation, characterization, and properties of a cyano bridged complex containing the Ru₃(μ₃-O) cores, [{Ru₃(μ₃-O) (μ-CH₃COO)₆(py)₂}₂(μ-CN)](PF₆) ([1]PF₆) (py = pyridine), are reported and compared with the properties of [{Ru₃(μ₃-O)(μ-CH₃COO)₆(py)₂}(μ-CN){Ru₃(μ₃-O)(μ-CH₃COO)₆(py)(CO)}] (2). The cyclic voltammogram of [1]⁺ in 0.1 M (n-Bu)₄NPF₆-CH₂Cl₂ solution with an Ag/AgCl reference electrode showed a series of redox waves assigned to the processes Ru₃^II,II,III/Ru₃^II,III,III (-1.70 V, -1.52 V), Ru₃^II,III,III/Ru₃^III,III,III (-0.37 V, -0.19 V), Ru₃^III,III,III/Ru₃^III,III,IV (+1.08 V), and Ru₃^III,III,IV/Ru₃^III,IV,IV (+2.03 V). Upon bridge formation the redox waves of the C-bonded Ru₃ moiety of [1]⁺ stay at the similar potential region to those of Ru₃(μ₃-O)(μ-CH₃COO)₆(py)₂(CN) (3) with a unidentate CN^-, so that the enhancement of the π-back donation does not operate, indicating that the Ru₃O core is not a good π-donor. Complex 2 showed five redox waves for the processes Ru₃^II,II,III/Ru₃^II,III,III (-1.15 V), Ru₃^II,III,III/Ru₃^III,III,III (-0.21 V, +0.55 V), and Ru₃^III,III,III/Ru₃^III,III,IV (+1.05 V, +1.35 V). The redox potential (-1.15 V) of the Ru₃^II,II,III/Ru₃^II,III,III process of the Ru₃(py) (CO) unit in 2 showed a negative shift in comparison to that (-0.84 V) of [Ru₃^II,III,III(μ₃-O) (μ-CH₃COO)₆(py)₂(CO)]. The infrared spectroelectrochemistry of [1]⁺ and 2 in 0.1 M n-Bu₄NPF₆-CH₂Cl₂ solutions revealed that the frequencies of the stretching vibration ν(CN) of the cyanide increased with the increase in the isovalent oxidation states of [1]⁺ but those in the Ru₃O-based mixed-valence states, Ru₃^II,III,III-Ru₃^III,III,III, and Ru₃^III,III,III-Ru₃^III,III,IV, were significantly smaller, which was possibly due to the effective electronic coupling through the CN^- bridge between the two Ru₃ units with different oxidation states. In contrast, such coupling was not observed for 2 that has two Ru₃ units with significantly different ligand environment.