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

The transfer of center dot OH from metal-hydroxo species to carbon radicals (R center dot) 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 center dot) derivatives ((4-X-C6H4)(3)C center dot; 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 center dot OH transfer reaction for both metals. Eyring analyses of reactions for 1 and 3 with (4-X-C6H4)(3)C center dot (X = tBu, CN) also give good linear correlations, and a comparison of the resulting activation parameters highlight the importance of entropy in these center dot 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.