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 Ru-3(mu(3)-O) cores, [{Ru-3(mu(3)-O) (mu-CH3COO)(6)(py)(2)}(2)(mu-CN)](PF6) ([1] PF6) (py = pyridine), are reported and compared with the properties of [{Ru-3(mu(3)-O)(mu-CH3COO)(6)(py)(2)}(mu-CN){Ru-3(mu(3)-O)(mu-CH3COO)(6)(py)(CO)}] (2). The cyclic voltammogram of [1](+) in 0.1 M (n-Bu)(4)NPF6-CH2Cl2 solution with an Ag/AgCl reference electrode showed a series of redox waves assigned to the processes Ru-3(II,II,III)/ Ru-3(II,III,III) (-1.70 V, -1.52 V), Ru-3(II,III,III)/Ru-3(III,III,III) (-0.37 V, -0.19 V), Ru-3(III,III,III)/Ru-3(III,III,IV) (+1.08 V), and Ru-3(III,III,IV)/Ru-3(III,IV,IV) (+2.03 V). Upon bridge formation the redox waves of the C-bonded Ru-3 moiety of [1](+) stay at the similar potential region to those of Ru-3(mu(3)-O)(mu-CH3COO)(6)(py)(2)(CN) (3) with a unidentate CN-, so that the enhancement of the pi-back donation does not operate, indicating that the Ru3O core is not a good p-donor. Complex 2 showed five redox waves for the processes Ru-3(II,II,III)/Ru-3(II,III,III) (-1.15 V), Ru-3(II,III,III)/Ru-3(III,III,III) (-0.21 V, +0.55 V), and Ru-3(III,III,III)/Ru-3(III,III,IV) (+1.05 V, +1.35 V). The redox potential (-1.15 V) of the Ru-3(II,II,III)/Ru-3(II,III,III) process of the Ru-3(py) (CO) unit in 2 showed a negative shift in comparison to that (-0.84 V) of [Ru-3(II,III,III)(mu(3)-O) (mu-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 n(CN) of the cyanide increased with the increase in the isovalent oxidation states of [1](+) but those in the Ru3O-based mixed-valence states, Ru-3(II,III,III) - Ru-3(III,III,III), and Ru-3(III,III,III) - Ru-3(III,III,IV), were significantly smaller, which was possibly due to the effective electronic coupling through the CN bridge between the two Ru-3 units with different oxidation states. In contrast, such coupling was not observed for 2 that has two Ru-3 units with significantly different ligand environment. (C) 2015 Elsevier B.V. All rights reserved.