Experimental demonstration of radicaloid character in a RuV=O intermediate in catalytic water oxidation

Water oxidation is the key half reaction in artificial photosynthesis. An absence of detailed mechanistic insight impedes design of new catalysts that are more reactive and more robust. A proposed paradigm leading to enhanced reactivity is the existence of oxyl radical intermediates capable of rapid water activation, but there is a dearth of experimental validation. Here, we show the radicaloid nature of an intermediate reactive toward formation of the O-O bond by assessing the spin density on the oxyl group by Electron Paramagnetic Resonance (EPR). In the study, an O-17-labeled form of a highly oxidized, short-lived intermediate in the catalytic cycle of the water oxidation catalyst cis,cis-[(2,2-bipyridine)(2)(H2O)Ru-III ORuIII(OH2)(bpy)2](4+) was investigated. It contains Ru centers in oxidation states [4,5], has at least one Ru-V = O unit, and shows vertical bar A(xx)vertical bar = 60G O-17 hyperfine splittings (hfs) consistent with the high spin density of a radicaloid. Destabilization of pi-bonding in the d(3) Ru-V = O fragment is responsible for the high spin density on the oxygen and its high reactivity.