Steric and Electronic Influence on Proton-Coupled Electron-Transfer Reactivity of a Mononuclear Mn(III)-Hydroxo Complex

Abstract

A mononuclear hydroxomanganese(III) complex was synthesized utilizing the N5 amide-containing ligand 2-[bis(pyridin-2-ylmethyl)]amino-N-2-methyl-quinolin-8-yl-acetamidate (dpaq(2Me) ). This complex is similar to previously reported [Mn(III)(OH)(dpaq(H))](+) [Inorg. Chem. 2014, 53, 7622-7634] but contains a methyl group adjacent to the hydroxo moiety. This α-methylquinoline group in [Mn(III)(OH)(dpaq(2Me))](+) gives rise to a 0.1 Å elongation in the Mn-N(quinoline) distance relative to [Mn(III)(OH)(dpaq(H))](+). Similar bond elongation is observed in the corresponding Mn(II) complex. In MeCN, [Mn(III)(OH)(dpaq(2Me))](+) reacts rapidly with 2,2',6,6'-tetramethylpiperidine-1-ol (TEMPOH) at -35 °C by a concerted proton-electron transfer (CPET) mechanism (second-order rate constant k2 of 3.9(3) M(-1) s(-1)). Using enthalpies and entropies of activation from variable-temperature studies of TEMPOH oxidation by [Mn(III)(OH)(dpaq(2Me))](+) (ΔH(‡) = 5.7(3) kcal(-1) M(-1); ΔS(‡) = -41(1) cal M(-1) K(-1)), it was determined that [Mn(III)(OH)(dpaq(2Me))](+) oxidizes TEMPOH ∼240 times faster than [Mn(III)(OH)(dpaq(H))](+). The [Mn(III)(OH)(dpaq(2Me))](+) complex is also capable of oxidizing the stronger O-H and C-H bonds of 2,4,6-tri-tert-butylphenol and xanthene, respectively. However, for these reactions [Mn(III)(OH)(dpaq(2Me))](+) displays, at best, modest rate enhancement relative to [Mn(III)(OH)(dpaq(H))](+). A combination of density function theory (DFT) and cyclic voltammetry studies establish an increase in the Mn(III)/Mn(II) reduction potential of [Mn(III)(OH)(dpaq(2Me))](+) relative to [Mn(III)(OH)(dpaq(H))](+), which gives rise to a larger driving force for CPET for the former complex. Thus, more favorable thermodynamics for [Mn(III)(OH)(dpaq(2Me))](+) can account for the dramatic increase in rate with TEMPOH. For the more sterically encumbered substrates, DFT computations suggest that this effect is mitigated by unfavorable steric interactions between the substrate and the α-methylquinoline group of the dpaq(2Me) ligand. The DFT calculations, which reproduce the experimental activation free energies quite well, provide the first examination of the transition-state structure of mononuclear Mn(III)(OH) species during a CPET reaction.

Oxidation of Naphthalene with a Manganese(IV) Bis(hydroxo) Complex in the Presence of Acid

“…Recently, high‐valent metal hydroxo intermediates have been suggested as reactive species in lipoxygenase and ligninolytic enzymes . In biomimetic studies, a few nonheme high‐valent metal hydroxo complexes have been prepared and exhibited C−H bond activation reactivity towards aliphatic substrates, whereas there are no reports on their reactivity towards aromatic molecules. Herein, we report the very first oxidation reaction of naphthalene with a mononuclear Mn IV complex with terminal hydroxide ligands, [Mn IV (TBDAP)(OH) 2 ] 2+ ( 2 ; TBDAP= N , N ‐di‐ tert ‐butyl‐2,11‐diaza[3.3](2,6)pyridinophane) in the presence of acid, together with detailed kinetic studies.…”

Fast Hydrogen Atom Abstraction by a Hydroxo Iron(III) Porphyrazine

“…10 An Fe(III) hydroxide complex described by Kovacs et al 11 is an unusually weak oxidant and Jackson et al have described phenol oxidations by mononuclear Mn III –OH complexes. 12 Yet, with the exception of the high-valent Cu(III)–OH complex, the reaction rates of hydrogen atom abstraction by these metal(III) hydroxide species are very slow compared with lipoxygenases. 5f,13 …”

Biological functions controlled by manganese redox changes in mononuclear Mn-dependent enzymes

“…Current mechanistic proposals invoke dioxygen binding, with a displacement of the Mn-bound water, leading to the formation of Mn(III) and a superoxide radical, which can then abstract hydrogen from the bis-allylic methylene of the lipid. The involvement of a Mn(III)-hydroperoxide in hydrogen atom abstraction, however, has been proposed on the basis of model studies and DFT calculations [72].…”