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

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 airsensitive. The crystal structure reveals the expected paddlewheel arrangement of ligands around the Ru2 4+ core, with a relatively long Ru-Ru bond (2.4999(8) Å) that is consistent with a σ2 π4 δ2 π*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 0.1 M n Bu4NPF6 / 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