Characterization of intermediary redox states of the water oxidation catalyst, [Ru(bpy)(2)(OH(2))](2)O(4+).

Higher oxidation states of the mu-oxo bridged ruthenium "blue dimer" ([Ru(bpy)(2)(OH(2))](2)O(4+)) have been characterized by redox titration measurements, resonance Raman (RR) spectroscopy, EPR spectrometry, and pulse radiolysis. The cumulative results indicate that the progression of accessible oxidation states in acidic media is {3,3} --> {3,4} --> {4,4} --> {5,5}, but changes to {3,3} --> {3,4} --> {4,5} --> {5,5} above pH 2. Although the reaction 2{4,5} + 2H(2)O --> 2{3,4} + O(2) is thermodynamically favorable, no O(2) was detected during the decay of {4,5} to {3,4}.