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.