Hydrogen Atom Abstraction by High-Valent Fe(OH) versus Mn(OH) Porphyrinoid Complexes: Mechanistic Insights from Experimental and Computational Studies.

High-valent metal-hydroxide species have been implicated as key intermediates in hydroxylation chemistry catalyzed by heme monooxygenases such as the cytochrome P450s. However, in some classes of P450s, a bifurcation from the typical oxygen rebound pathway is observed, wherein the Fe(OH)(porphyrin) species carries out a net hydrogen atom transfer reaction to form alkene metabolites. In this work, we examine the hydrogen atom transfer (HAT) reactivity of Fe(OH)(ttppc) (), ttppc = 5,10,15-tris(2,4,6-triphenyl)-phenyl corrole, toward substituted phenol derivatives. The iron hydroxide complex reacts with a series of -substituted 2,6-di--butylphenol derivatives (4-X-2,6-DTBP; X = OMe, Me, Et, H, Ac), with second-order rate constants = 3.6(1)-1.21(3) × 10 M s and yielding linear Hammett and Marcus plot correlations. It is concluded that the rate-determining step for O-H cleavage occurs through a concerted HAT mechanism, based on mechanistic analyses that include a KIE = 2.9(1) and DFT calculations. Comparison of the HAT reactivity of to the analogous Mn complex, Mn(OH)(ttppc), where only the central metal ion is different, indicates a faster HAT reaction and a steeper Hammett slope for . The O-H bond dissociation energy (BDE) of the M(HO-H) complexes were estimated from a kinetic analysis to be 85 and 89 kcal mol for Mn and Fe, respectively. These estimated BDEs are closely reproduced by DFT calculations and are discussed in the context of how they influence the overall H atom transfer reactivity.