title: Lewis-Acid-assisted Hydrogen Atom Transfer to Manganese(V)-Oxo Corrole through Valence Tautomerization.
abstract: 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 10-2for Ca to 103m-1s-1for 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.
authors.affiliation: Department of Chemistry Purdue University 560 Oval Drive West Lafayette IN 47906 USA.
authors.forename: Curt J
authors.initials: CJ
authors.lastname: Bougher
authors.affiliation: Department of Chemistry Purdue University 560 Oval Drive West Lafayette IN 47906 USA; Current address: Department of Chemistry and Biochemistry Department of Chemical Engineering University of California Santa Barbara CA 93106 USA.
authors.forename: Mahdi M
authors.initials: MM
authors.lastname: Abu-Omar
keywords: hydrogen atom transfer; manganese; metal-oxo compounds; porphyrinoids; valence tautomers