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Synthesis and Structural Characterization of a Series of Mn^IIIOR Complexes, Including a Water-Soluble Mn^IIIOH That Promotes Aerobic Hydrogen-Atom Transfer

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Michael K. Coggins , Lisa M. Brines , and Julie A. Kovacs *

Department of Chemistry, University of Washington, Box 351700, Seattle, Washington 98195-1700, United States

Inorg. Chem., Article ASAP

DOI: 10.1021/ic401234t

Publication Date (Web): October 24, 2013

Copyright © 2013 American Chemical Society

*E-mail: kovacs@chem.washington.edu. Tel.: (206)543-0713. Fax: (206)685-8665.

Synopsis

The synthesis, structure, and reactivity properties of a series of MnIIIOR compounds [R = pNO2Ph (5), Ph (6), Me (7), H (8)], including a rare example of a mononuclear MnIIIOH complex 8, are reported. Complex 8 is water-soluble with a pH-dependent redox potential consistent with 1H+/1e– proton-coupled electron-transfer. Despite their mild redox potentials, 7 and 8 each react with TEMPOH with kH/kD values consistent with concerted H-atom abstraction. The thiolate is proposed to contribute to this reactivity by increasing the basicity of the bound oxygen.

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 pK_a 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^IIIOH (M = Fe, Mn) have been shown to promote these reactions. Herein we describe the synthesis, structure, and reactivity properties of a series of Mn^IIIOR compounds [R = ^pNO₂Ph (5), Ph (6), Me (7), H (8)], some of which abstract H atoms. The Mn^IIIOH complex 8 is water-soluble and represents a rare example of a stable mononuclear Mn^IIIOH. 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 [k_H/k_D = 3.1 (8); k_H/k_D = 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^IIOH₂ 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^Me2N₄(tren))(H₂O)]⁺ (9), was estimated to have a pK_a 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^IIO(H)R pK_a based on their experimentally determined redox potentials. The trend in pK_a [21.2 (R = H) > 16.2 (R = Me) > 13.5 (R = Ph) > 12.2 (R = ^pNO₂Ph)] is shown to oppose that of the oxidation potential E_p,c[−220 (R = ^pNO₂Ph) > −300 (R = Ph) > −410 (R = Me) > −600 (R = H) mV vs Fc^+/0] for this particular series.