Photosystem II Like Water Oxidation Mechanism in a Bioinspired Tetranuclear Manganese Complex

Abstract

The synthesis of Mn-based catalysts to mimic the structural and catalytic properties of the oxygen-evolving complex in photosystem II is a long-standing goal for researchers. An interesting result in this field came with the synthesis of a Mn complex that enables water oxidation driven by the mild single-electron oxidant [Ru(bpy)3](3+). On the basis of hybrid density functional calculations, we herein propose a water oxidation mechanism for this bioinspired Mn catalyst, where the crucial O-O bond formation proceeds from the formal Mn4(IV,IV,IV,V) state by direct coupling of a Mn(IV)-bound terminal oxyl radical and a di-Mn bridging oxo group, a mechanism quite similar to the presently leading suggestion for the natural system. Of importance here is that the designed ligand is shown to be redox-active and can therefore store redox equivalents during the catalytic transitions, thereby alleviating the redox processes at the Mn centers.

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).…”

“…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...…”

“…[41] It is worth mentioning here that the active species, derived from A,t hat are responsible for the OÀOb ond formation, reach formal the oxidation state Ru1 IV Mn V Ru2 IV .T his is in good agreement with the OEC-PSII,w here af ormal Mn V is also neededi nt he S 4 state. [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.…”

Laccase Redox Potentials: pH Dependence and Mutants, a QM/MM Study

“…Taking 4.28 V as a standard for the SHE 39 , 40 and 0.78 V for the enzyme, 41 the ARP of the investigated system should be at about 5.06 V. As we have employed RRPs in our earlier studies, we will stick to the discussion of RRPs where appropriate.…”

N,N,O Pincer Ligand with a Deprotonatable Site That Promotes Redox‐Leveling, High Mn Oxidation States, and a Mn2O2 Dimer Competent for Catalytic Oxygen Evolution

“…The major benefit of multinuclearity is the sharing of charge and accumulation of redox equivalents between metal centers, a concept that extends to many bioinorganic systems such as the OEC , . Multinuclearity may be part of the design of a catalyst where ligand scaffolds enforce polynuclearity, such as in the work of Åkermark and co‐workers , . However, in many cases, Mn complexes which begin as monomeric units are observed to dimerize to form active catalysts; an example comes from the work of McKenzie and co‐workers, where mononuclear Mn complexes with monoanionic carboxylate‐containing pentadentate ligands evolve oxygen when driven by t BuOOH, and are thought to dimerize during turnover via a bridging carboxylate group from the ligand scaffold.…”