Water Oxidation Catalysis Beginning with Co4(H2O)2(PW9O34)210- When Driven by the Chemical Oxidant Ruthenium(III)tris(2,2′-bipyridine): Stoichiometry, Kinetic, and Mechanistic Studies en Route to Identifying the True Catalyst

Stoichiometry and kinetics are reported for catalytic water oxidation to O-2 beginning with the cobalt polyoxometalate Co-4(H2O)(2)(PW9O34)(2)(10-) (Co4POM) and the chemical oxidant ruthenium(III)tris(2,2'-bipyridine) (Ru(III)(bpy)(3)(3+)). This specific water oxidation system was first reported in a 2010 Science paper (Yin et al. Science 2010, 328, 342). Under standard conditions employed herein of 1.0 mu M Co4POM, 500 mu M Ru(III)(bpy)(3)(3+), 100 mu M Ru(II)(bpy)(3)(2+), pH 7.2, and 0.03 M sodium phosphate buffer, the highest O-2 yields of 22% observed herein are seen when Ru(II)(bpy)(3)(2+) is added prior to the Ru(III)(bpy)(3)(3+) oxidant; hence, those conditions are employed in the present study. Measurement of the initial O-2 evolution and Ru(III)(bpy)(3)(3+) reduction rates while varying the initial pH, [Ru(III)(bpy)(3)(3+)], [Ru(II)(bpy)(3)(2+)], and [Co4POM] indicate that the reaction follows the empirical rate law: -d[Ru(III)(bpy)(3)(3+)]/dt = (k(1) + k(2))[Co4POM](soluble)[Ru(III)(bpy)(3)(3+)]/[H+], where the rate constants k(1) similar to 0.0014 s(-1) and k(2) similar to 0.0044 s(-1) correspond to the water oxidation and ligand oxidation reactions, and for O-2 evolution, d[O-2]/dt = (k(1)/4)[Co4POM](soluble)[Ru(III)(bpy)(3)(3+)]/[H+]. Overall, at least seven important insights result from the present studies: (i) Parallel WOC and Ru(III)(bpy)(3)(3+) self-oxidation reactions well documented in the prior literature limit the desired WOC and selectivity to O-2 in the present system to <= 28%. (ii) The formation of a precipitate from similar to 2 Ru(II)(bpy)(3)(2+)/3 Co-4(H2O)(2)(PW9O34)(2)(10-) with a K-sp = (8 +/- 7) x 10(-25) (M-5) greatly complicates the reaction and interpretation of the observed kinetics, but (iii) the best O-2 yields are still when Ru(II)(bpy)(3)(2+) is preadded. (iv) CoOx is 2-11 times more active than Co4POM under the reaction conditions, but (v) Co4POM is still the dominant WOC under the Co4POM/Ru(III)(bpy)(3)(3+) and other reaction conditions employed. The present studies also (vi) confirm that the specific conditions matter greatly in determining the true WOC and (vii) allow one to begin to construct a plausible WOC mechanism for the Co4POM/Ru(III)(bpy)(3)(3+) system.