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

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 σ²π⁴δ²π⁴ 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 0.1 M ⁿBu₄NPF₆/CH₂Cl₂ solution shows two redox processes, assigned as successive oxidations corresponding to the Ru₂^4+/5+ and Ru₂^5+/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.