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Hydroxyl Transfer to Carbon Radicals by Mn(OH) vs Fe(OH) Corrole Complexes.

ABSTRACT: The transfer of •OH from metal-hydroxo species to carbon radicals (R•) to give hydroxylated products (ROH) is a fundamental process in metal-mediated heme and nonheme C-H bond oxidations. This step, often referred to as the hydroxyl "rebound" step, is typically very fast, making direct study of this process challenging if not impossible. In this report, we describe the reactions of the synthetic models M(OH)(ttppc) (M = Fe (1), Mn (3); ttppc = 5,10,15-tris(2,4,6-triphenyl)phenyl corrolato^3-) with a series of triphenylmethyl carbon radical (R•) derivatives ((4-X-C₆H₄)₃C•; X = OMe, tBu, Ph, Cl, CN) to give the one-electron reduced M^III(ttppc) complexes and ROH products. Rate constants for 3 for the different radicals ranged from 11.4(1) to 58.4(2) M^-1 s^-1, as compared to those for 1, which fall between 0.74(2) and 357(4) M^-1 s^-1. Linear correlations for Hammett and Marcus plots for both Mn and Fe were observed, and the small magnitudes of the slopes for both correlations imply a concerted •OH transfer reaction for both metals. Eyring analyses of reactions for 1 and 3 with (4-X-C₆H₄)₃C• (X = tBu, CN) also give good linear correlations, and a comparison of the resulting activation parameters highlight the importance of entropy in these •OH transfer reactions. Density functional theory calculations of the reaction profiles show a concerted process with one transition state for all radicals investigated and help to explain the electronic features of the OH rebound process.

SUBMITTER: Cummins DC

PROVIDER: S-EPMC8559416 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description: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.

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Hydroxyl Transfer to Carbon Radicals by Mn(OH) vs Fe(OH) Corrole Complexes.

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