Biomimetic Oxidations Catalyzed by Transition Metal Complexes
- a B. Meunier, S. P. de Visser, S. Shaik, Chem. Rev
- J. Biol. Inorg. Chem. 2001,
The kinetics of formation of the valence tautomers (tpfc⋅)MnIV(O-LA)] n+ [where LA=ZnII, CaII, ScIII, YbIII, B(C6F5)3, and trifluoroacetic acid (TFA); tpfc=5,10,15-tris(pentafluorophenyl) corrole] from (tpfc)MnV(O) were followed by UV/Vis spectroscopy, giving second-order rate constants ranging over five orders of magnitude from 10-2 for Ca to 103 m -1 s-1 for Sc. Hydrogen atom transfer (HAT) rates from 2,4-di-tert-butyl phenol (2,4-DTBP) to the various Lewis acid valence tautomers of manganese oxo corrole complexes were evaluated and compared. For LA=TFA, ScIII, or YbIII, the rate constants of HAT were comparable to unactivated (tpfc)MnV(O). However, with LA=B(C6F5)3, ZnII, and CaII, 6-, 21-, and 31-fold rate enhancements were observed, respectively. Remarkably, [(tpfc⋅)MnIV(OCa)]2+ gave the most enhancement despite its rate of formation being the slowest. Comparisons of HAT rate constants among the various Lewis acid tautomers revealed that both size and charge are important. This study underscores how valence may affect the reactivity of high-valent manganese-oxo compounds and sheds light on nature's choice of Ca in the activation of Mn-oxo in the oxygen-evolving complex.