Structural, spectroscopic and theoretical studies of a diruthenium(II,II) tetraformamidinate that reversibly binds dioxygen

Abstract

The reaction of Ru2(O2CMe)4 with N,N′-bis(3,5-dimethoxyphenyl)formamidine (Hdmof) in refluxing toluene solutions yields Ru2(dmof)4 as a diamagnetic red solid that is extremely air-sensitive. The crystal structure reveals the expected paddlewheel arrangement of ligands around the Ru24+ core, with a relatively long Ru–Ru bond (2.4999(8)Å) that is consistent with a σ2π4δ2π∗4 electronic configuration. This is supported DFT calculations that show this electronic structure results from destabilization of the δ∗ orbital due to antibonding interactions with the formamidinate ligands. The cyclic voltammogram of Ru2(dmof)4 in a 0.1M nBu4NPF6/CH2Cl2 solution shows two redox processes, assigned as successive oxidations corresponding to the Ru24+/5+ and Ru25+/6 redox couples. Changes in the electronic absorption spectra associated with these oxidation processes were probed using a UV–Vis spectroelectrochemical study. Ru2(dmof)4 reacts with dioxygen in solution to generate a purple compound that decomposes within an hour at room temperature. Bubbling N2 gas through the purple solution regenerates Ru2(dmof)4, as evidenced by UV–Vis spectrometry and cyclic voltammetry, suggesting that the dioxygen reversibly binds to the diruthenium core.