Synthesis and Structural Characterization of a Series of MnIIIOR Complexes, Including a Water-Soluble MnIIIOH That Promotes Aerobic Hydrogen-Atom Transfer

Abstract

Hydrogen-atom-transfer (HAT) reactions are a class of proton-coupled electron-transfer (PCET) reactions used in biology to promote substrate oxidation. The driving force for such reactions depends on both the oxidation potential of the catalyst and the pKa value of the proton-acceptor site. Both high-valent transition-metal oxo M(IV)═O (M = Fe, Mn) and lower-valent transition-metal hydroxo compounds M(III)OH (M = Fe, Mn) have been shown to promote these reactions. Herein we describe the synthesis, structure, and reactivity properties of a series of Mn(III)OR compounds [R = (p)NO2Ph (5), Ph (6), Me (7), H (8)], some of which abstract H atoms. The Mn(III)OH complex 8 is water-soluble and represents a rare example of a stable mononuclear Mn(III)OH. In water, the redox potential of 8 was found to be pH-dependent and the Pourbaix (E(p,c) vs pH) diagram has a slope (52 mV pH(-1)) that is indicative of the transfer a single proton with each electron (i.e., PCET). The two compounds with the lowest oxidation potential, hydroxide- and methoxide-bound 7 and 8, are found to oxidize 2,2',6,6'-tetramethylpiperidin-1-ol (TEMPOH), whereas the compounds with the highest oxidation potential, phenol-ligated 5 and 6, are shown to be unreactive. Hydroxide-bound 8 reacts with TEMPOH an order of magnitude faster than methoxide-bound 7. Kinetic data [kH/kD = 3.1 (8); kH/kD = 2.1 (7)] are consistent with concerted H-atom abstraction. The reactive species 8 can be aerobically regenerated in H2O, and at least 10 turnovers can be achieved without significant degradation of the "catalyst". The linear correlation between the redox potential and pH, obtained from the Pourbaix diagram, was used to calculate the bond dissociation free energy (BDFE) = 74.0 ± 0.5 kcal mol(-1) for Mn(II)OH2 in water, and in MeCN, its BDFE was estimated to be 70.1 kcal mol(-1). The reduced protonated derivative of 8, [Mn(II)(S(Me2)N4(tren))(H2O)](+) (9), was estimated to have a pKa of 21.2 in MeCN. The ability (7) and inability (5 and 6) of the other members of the series to abstract a H atom from TEMPOH was used to estimate either an upper or lower limit to the Mn(II)O(H)R pKa based on their experimentally determined redox potentials. The trend in pKa [21.2 (R = H) > 16.2 (R = Me) > 13.5 (R = Ph) > 12.2 (R = (p)NO2Ph)] is shown to oppose that of the oxidation potential E(p,c) [-220 (R = (p)NO2Ph) > -300 (R = Ph) > -410 (R = Me) > -600 (R = H) mV vs Fc(+/0)] for this particular series.