Mechanism of Oxidation of Benzaldehyde by Polypyridyl Oxo Complexes of Ru(IV)

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Abstract
The oxidation of benzaldehyde and several of its derivatives to their carboxylic acids by cis-[RuIV(bpy)2(py)(O)]2+ (RuIVO2+; bpy is 2,2‘-bipyridine, py is pyridine), cis-[RuIII(bpy)2(py)(OH)]2+ (RuIII−OH2+), and [RuIV(tpy)(bpy)(O)]2+ (tpy is 2,2‘:6‘,2‘ ‘-terpyridine) in acetonitrile and water has been investigated using a variety of techniques. Several lines of evidence support a one-electron hydrogen-atom transfer (HAT) mechanism for the redox step in the oxidation of benzaldehyde. They include (i) moderate kC-H/kC-D kinetic isotope effects of 8.1 ± 0.3 in CH3CN, 9.4 ± 0.4 in H2O, and 7.2 ± 0.8 in D2O; (ii) a low kH2O/D2O kinetic isotope effect of 1.2 ± 0.1; (iii) a decrease in rate constant by a factor of only ∼5 in CH3CN and ∼8 in H2O for the oxidation of benzaldehyde by cis-[RuIII(bpy)2(py)(OH)]2+ compared to cis-[RuIV(bpy)2(py)(O)]2+; (iv) the appearance of cis-[RuIII(bpy)2(py)(OH)]2+ rather than cis-[RuII(bpy)2(py)(OH2)]2+ as the initial product; and (v) the small ρ value of −0.65 ± 0.03 in a Hammett plot of log k vs σ in the oxidation of a series of aldehydes. A mechanism is proposed for the process occurring in the absence of O2 involving (i) preassociation of the reactants, (ii) H-atom transfer to RuIVO2+ to give RuIII−OH2+ and PhĊO, (iii) capture of PhĊO by RuIII−OH2+ to give RuII−OC(O)Ph+ and H+, and (iv) solvolysis to give cis-[RuII(bpy)2(py)(NCCH3)]2+ or the aqua complex and the carboxylic acid as products.