ABSTRACT
The kinetics of formation of the valence tautomers (tpfcâ )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, [(tpfcâ )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.
ABSTRACT
The addition of an organic acid to the manganese(V)-oxo corrole complex (tpfc)Mn(V)(O) (tpfc = 5,10,15-tris(pentafluorophenyl)corrole) induces valence tautomerization resulting in the formation of (tpfc(+â¢))Mn(IV)(OH) in acetonitrile at 298 K. The corrole radical cation manganese(IV) hydroxo complex has been fully characterized by EPR, (1)H NMR, and UV-vis spectroscopy. The reactivity of the valence tautomer (tpfc(+â¢))Mn(IV)(OH) is compared to that of (tpfc)Mn(V)(O) in three reaction types: hydrogen atom transfer (HAT), electron transfer (ET), and oxygen atom transfer (OAT). (tpfc(+â¢))Mn(IV)(OH) shows a dramatic 5 orders of magnitude enhancement in the rate of ET but surprisingly does not undergo OAT with PhSMe. The high-valent (tpfc)Mn(V)(O) complex is moderately more reactive toward HAT with substituted phenol and shows superior activity in OAT.
ABSTRACT
Rate constants of electron self-exchange of high-valent oxo and imido complexes of chromium(V/IV) corrole have been determined in acetonitrile and toluene at various temperatures by electron paramagnetic resonance (EPR) line width variation of the EPR spectra. The observed activation enthalpies (ΔHobs(⧧)) of electron self-exchange of chromium(V)-oxo and -imido corrole with the corresponding chromium(IV) complexes are zero in toluene, whereas the ΔHobs(⧧) values are slightly positive in acetonitrile. Such activationless electron self-exchange transfer resulted in extremely fast electron-transfer reactions of chromium(V)-oxo and -imido corrole in sharp contrast with slow electron-transfer reactions of other high-valent metal-oxo and -imido complexes.
ABSTRACT
Efficient catalytic two-electron reduction of dioxygen (O2) by octamethylferrocene (Me8Fc) produced hydrogen peroxide (H2O2) using a high-valent chromium(V)-oxo corrole complex, [(tpfc)Cr(V)(O)] (tpfc = tris(pentafluorophenyl)corrole) as a catalyst precursor in the presence of trifluoroacetic acid (TFA) in acetonitrile (MeCN). The facile two-electron reduction of [(tpfc)Cr(V)(O)] by 2 equiv of Me8Fc in the presence of excess TFA produced the corresponding chromium(III) corrole [(tpfc)Cr(III)(OH2)] via fast electron transfer from Me8Fc to [(tpfc)Cr(V)(O)] followed by double protonation of [(tpfc)Cr(IV)(O)](-) and facile second-electron transfer from Me8Fc. The rate-determining step in the catalytic two-electron reduction of O2 by Me8Fc in the presence of excess TFA is inner-sphere electron transfer from [(tpfc)Cr(III)(OH2)] to O2 to produce the chromium(IV) superoxo species [(tpfc)Cr(IV)(O2(â¢-))], followed by fast proton-coupled electron transfer reduction of [(tpfc)Cr(IV)(O2(â¢-))] by Me8Fc to yield H2O2, accompanied by regeneration of [(tpfc)Cr(III)(OH2)]. Thus, although the catalytic two-electron reduction of O2 by Me8Fc was started by [(tpfc)Cr(V)(O)], no regeneration of [(tpfc)Cr(V)(O)] was observed in the presence of excess TFA, regardless of the tetragonal chromium complex being to the left of the oxo wall. In the presence of a stoichiometric amount of TFA, however, disproportionation of [(tfpc)Cr(IV)(O)](-) occurred via the protonated species [(tpfc)Cr(IV)(OH)] to produce [(tpfc)Cr(III)(OH2)] and [(tpfc)Cr(V)(O)].