title: O-H bond oxidation by a monomeric Mn(III)-OMe complex.
abstract: Manganese-containing, mid-valent oxidants (Mn(III)-OR) that mediate proton-coupled electron-transfer (PCET) reactions are central to a variety of crucial enzymatic processes. The Mn-dependent enzyme lipoxygenase is such an example, where a Mn(III)-OH unit activates fatty acid substrates for peroxidation by an initial PCET. This present work describes the quantitative generation of the Mn(III)-OMe complex, [Mn(III)(OMe)(dpaq)](+) (dpaq = 2-[bis(pyridin-2-ylmethyl)]amino-N-quinolin-8-yl-acetamidate) via dioxygen activation by [Mn(II)(dpaq)](+) in methanol at 25 °C. The X-ray diffraction structure of [Mn(III)(OMe)(dpaq)](+) exhibits a Mn-OMe group, with a Mn-O distance of 1.825(4) Å, that is trans to the amide functionality of the dpaq ligand. The [Mn(III)(OMe)(dpaq)](+) complex is quite stable in solution, with a half-life of 26 days in MeCN at 25 °C. [Mn(III)(OMe)(dpaq)](+) can activate phenolic O-H bonds with bond dissociation free energies (BDFEs) of less than 79 kcal mol(-1) and reacts with the weak O-H bond of TEMPOH (TEMPOH = 2,2'-6,6'-tetramethylpiperidine-1-ol) with a hydrogen/deuterium kinetic isotope effect (H/D KIE) of 1.8 in MeCN at 25 °C. This isotope effect, together with other experimental evidence, is suggestive of a concerted proton-electron transfer (CPET) mechanism for O-H bond oxidation by [Mn(III)(OMe)(dpaq)](+). A kinetic and thermodynamic comparison of the O-H bond oxidation reactivity of [Mn(III)(OMe)(dpaq)](+) to other M(III)-OR oxidants is presented as an aid to gain more insight into the PCET reactivity of mid-valent oxidants. In contrast to high-valent counterparts, the limited examples of M(III)-OR oxidants exhibit smaller H/D KIEs and show weaker dependence of their oxidation rates on the driving force of the PCET reaction with O-H bonds.
authors.affiliation: Department of Chemistry and Center for Environmentally Beneficial Catalysis, University of Kansas, 1251 Wescoe Hall Drive, Lawrence, KS 66045, USA. taj@ku.edu.
authors.forename: Gayan B
authors.initials: GB
authors.lastname: Wijeratne
authors.forename: Victor W
authors.initials: VW
authors.lastname: Day
authors.forename: Timothy A
authors.initials: TA
authors.lastname: Jackson
mesh_terms: D006859:Hydrogen / Q000737:chemistry / Q000378:metabolism*; D006860:Hydrogen Bonding; D017895:Manganese Compounds / Q000737:chemistry / Q000378:metabolism*; D000432:Methanol / Q000737:chemistry / Q000378:metabolism*; D015394:Molecular Structure; D010084:Oxidation-Reduction; D010087:Oxides / Q000737:chemistry / Q000378:metabolism*
chemical_list: D017895:Manganese Compounds; D010087:Oxides; C027424:manganese oxide; D006859:Hydrogen; D000432:Methanol