Intramolecular Oxyl Radical Coupling Promotes O–O Bond Formation in a Homogeneous Mononuclear Mn-based Water Oxidation Catalyst: A Computational Mechanistic Investigation

Abstract

The mechanism of water oxidation performed by a recently discovered manganese pyridinophane catalyst [Mn(PyNBu)(HO)] is studied using density functional theory methods. A complete catalytic cycle is constructed and the catalytically active species is identified to consist of a Mn-bis(oxo) moiety that is generated from the resting state by a series of proton-coupled electron transfer reactions. Whereas the electronic ground state of this key intermediate is found to be a triplet, the most favorable pathway for O-O bond formation is found on the quintet potential energy surface and involves an intramolecular coupling of two oxyl radicals with opposite spins bound to the Mn-center that adopts an electronic structure most consistent formally with a high-spin Mn ion. Therefore, the thermally accessible high-spin quintet state that constitutes a typical and innate property of a first-row transition metal center plays a critical role for catalysis. It enables facile electron transfer between the oxo moieties and the Mn-center and promotes O-O bond formation via a radical coupling reaction with a calculated reaction barrier of only 14.7 kcal mol. This mechanism of O-O coupling is unprecedented and provides a novel possible pathway to coupling two oxygen atoms bound to a single metal site.

Quantum Chemical Study of the Mechanism of Water Oxidation Catalyzed by a Heterotrinuclear Ru2Mn Complex

“…Then, aP CET of 3 leads to the formationo faRu1 IV Mn V Ru2 IV complex 4 (Figure 2), and the redox potential for this oxidation step was calculated to be 1.54 V. Complex 4 is as eptet and the spin densities on Ru1, Mn, Ru2, O1, O2, and O4 are 0.99, 1.05, 2.77, 0.69, À0.53, and 0.90, respectively.In4,aMn IV is antiferromagnetically coupled with an oxyl radical, which is similar to many other Mn-based water oxidation catalysts, [62,[66][67][68][69][70][71][72] including the oxygen evolvingc omplex in photosystem II. [66][67][68] The effect of spin coupling on the energy of 4 was calculated by using the Noodlemana pproach (see the Supporting Information for details).…”

“…[54] The experimental solvation free energies of water (À6.3 kcal mol À1 ) [54] and acetate (À77.0 kcal mol À1 ) [55,56] in aqueous solution were used. [16,[59][60][61][62][63][64][65] Results and Discussion [57] Importantly,t he use of either the experimental values or the calculated values did not alter any mechanistic conclusions.…”

“…The lower oxidation state transitions from A to B and B to C are not observed experimentally probably owing to the low quality of the electrochemical response for this complex. [66][67][68] OÀObondformation OÀOb ond formation in water oxidationb yR u [17][18][19][20][76][77][78][79][80] and Mn [62,[69][70][71][72][81][82][83][84][85][86] catalysts have been widelys tudied by quantum chemicalc alculations.T he two popularp athways, namely direct coupling (DC)o ft wo adjacent oxygen units andw ater nucleophilic attack (WNA), were studied herein. [66][67][68] OÀObondformation OÀOb ond formation in water oxidationb yR u [17][18][19][20][76][77][78][79][80] and Mn [62,[69][70][71][72][81][82][83][84][85][86] catalysts have been widelys tudied by quantum chemicalc alculations...…”

Quantum Chemical Modeling of Homogeneous Water Oxidation Catalysis

“…The use of aM n IV complex for OÀOb ond formation was considered by Baik and co-workers,a nd the associated barriers were much higher than those for Mn V . [92] In addition, ligand oxidation pathways, involving three possible CÀH activation and two No xidation steps, were explored by calculations. [76] Oxidation of the methyl group turns out to have quite al ow barrier,c lose to that for OÀOb ond formation, which explains the low TON of this catalyst.…”

Mono‐/Multinuclear Water Oxidation Catalysts

“…[71] Duringt he OER, Mn V =O was proposed as the active speciesa sar esult of CV measurements.T he mechanism of OÀOb ond formation was further studied by Gao et al in 2009. [76] Mn V =Ow as demonstrated to be the active intermediate, and the most likely mechanism of OÀOb ond formation was radical coupling, which had an energy barriero f1 4.7 kcal mol À1 according to theoretical calculations. Lassalle-Kaisere tal.…”