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

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.