Oxygenation by ruthenium monosubstituted polyoxotungstates in aqueous solution: Experimental and computational dissection of a Ru(III)-Ru(V) catalytic cycle

Abstract

Molecular polyoxometalates with one embedded ruthenium center, with general formula [RuII/III(DMSO)XW11O39]n- (X=P, Si; n=4-6), are readily synthesized in gram scale under microwave irradiation by a flash hydrothermal protocol. These nanodimensional and polyanionic complexes enable aerobic oxygenation in water. Catalytic oxygen transfer to dimethylsulfoxide (DMSO) yielding the corresponding sulfone (DMSO2) has been investigated with a combined kinetic, spectroscopic and computational approach addressing: (i ) the RuIII catalyst resting state; (ii ) the bimolecular event dictating its transformation in the rate-determining step; (iii ) its aerobic evolution to a high-valent ruthenium oxene species; (iv ) the terminal fate to diamagnetic dimers. This pathway is reminiscent of natural heme systems and of bioinspired artificial porphyrins. The in silico characterization of a key bis-Ru(IV)-μ-peroxo-POM dimeric intermediate has been accessed by density functional theory. This observation indicates a new landmark for tracing POM-based manifolds for multiredox oxygen reduction/activation, where metal-centered oxygenated species play a pivotal role. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.