Steric and Electronic Influence on Proton-Coupled Electron-Transfer Reactivity of a Mononuclear Mn(III)-Hydroxo Complex
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https://figshare.com/articles/dataset/Steric_and_Electronic_Influence_on_Proton-Coupled_Electron-Transfer_Reactivity_of_a_Mononuclear_Mn_III_-Hydroxo_Complex/3517607
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资源简介：
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 k2 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.
创建时间：
2016-08-09
