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 (dpaq2Me ). This complex is similar to previously reported [MnIII(OH)­(dpaqH)]+ [Inorg. Chem. 2014, 53, 7622–7634] but contains a methyl group adjacent to the hydroxo moiety. This α-methylquinoline group in [MnIII(OH)­(dpaq2Me)]+ gives rise to a 0.1 Å elongation in the Mn–N­(quinoline) distance relative to [MnIII(OH)­(dpaqH)]+. Similar bond elongation is observed in the corresponding Mn­(II) complex. In MeCN, [MnIII(OH)­(dpaq2Me)]+ 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 k 2 of 3.9(3) M–1 s–1). Using enthalpies and entropies of activation from variable-temperature studies of TEMPOH oxidation by [MnIII(OH)­(dpaq2Me)]+ (ΔH ‡ = 5.7(3) kcal–1 M–1; ΔS ‡ = −41(1) cal M–1 K–1), it was determined that [MnIII(OH)­(dpaq2Me)]+ oxidizes TEMPOH ∼240 times faster than [MnIII(OH)­(dpaqH)]+. The [MnIII(OH)­(dpaq2Me)]+ 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 [MnIII(OH)­(dpaq2Me)]+ displays, at best, modest rate enhancement relative to [MnIII(OH)­(dpaqH)]+. A combination of density function theory (DFT) and cyclic voltammetry studies establish an increase in the MnIII/MnII reduction potential of [MnIII(OH)­(dpaq2Me)]+ relative to [MnIII(OH)­(dpaqH)]+, which gives rise to a larger driving force for CPET for the former complex. Thus, more favorable thermodynamics for [MnIII(OH)­(dpaq2Me)]+ 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 dpaq2Me ligand. The DFT calculations, which reproduce the experimental activation free energies quite well, provide the first examination of the transition-state structure of mononuclear MnIII(OH) species during a CPET reaction.