A DFT/B3LYP study of the mechanisms of the O2 formation reaction catalyzed by the [(terpy)(H2O)MnIII(O)2MnIV(OH2)(terpy)](NO3)3 complex: A paradigm for photosystem II

Abstract

We present a theoretical study of the reaction pathway for dioxygen molecular formation catalyzed by the [(terpy)(HO)Mn(O)Mn(OH) (terpy)](NO) (terpy=2,2':6',2″-terpyridine) complex based on DFT-B3LYP calculations. In the initial state of the reaction, a partial oxido radical (0.44 spins) is formed ligated to Mn. This radical is involved in a nucleophylic attack by bulk water in the OO bond reaction formation step, in which the oxido fractional unpaired electron is delocalized toward the outermost Mn of the μ-oxo bridge, instead of the ligated Mn center. The reaction then follows with a series of proton-coupled electron transfer steps, in which the oxidation state, as well as the bond strength of the OO moiety increase, while the OOMn(1) bond gets weaker until O is released. In this model, basic acetate ions from the buffer solution capture protons in the proton-transfer steps. In each step there is reduction of the OOMn(1) binding strength, with concomitant increase of the OO bond strength, which culminates with the release of O in the last step. This last step is entropy driven, while formation of hydroperoxide and superoxide moieties is enthalpy driven. According with experiments, the rate-limiting step is the double oxidation of Mn(IV,III) or peroxymonosulfate binding, which occur prior to the OO bond formation step. This supports our findings that the barriers of all intermediate steps are below the experimental barrier of 19-21kcal/mol. The implications of these findings for understanding photosynthetic water-splitting catalysis are also discussed.

Quantum Chemical Modeling of Homogeneous Water Oxidation Catalysis

“…Recently,Sproviero considered the use of acetate as ag eneral base to facilitate the attack of water on theM n IV -oxyl moiety. [100] The OÀOb ond formation was suggestedt ot ake place through as tepwise mechanism, with the location of a hydroxyl radicali ntermediate. Quite unexpectedly,t he OÀO bond formationw as almost barrierless.…”

“…Three alternative mechanisms for OÀObond formation suggested on the basis of DFT calculations. [97,98,100] ChemSusChem 2017, 10,4236 -4263…”

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

“…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.…”

Activation by oxidation and ligand exchange in a molecular manganese vanadium oxide water oxidation catalyst

“…As in Siegbahn's studies of the OEC in photosystem II, 28,29 calculations were carried out using the highspin configuration and assuming that antiferromagnetic effects do not significantly affect either the reaction barriers or the geometry of the complex. 30 Geometries were optimized using DFT and the B3LYP functional 31,32 as widely used in other studies of OEC model systems [33][34][35][36][37] and polyoxometalate WOCs - 24,[38][39][40][41][42][43][44][45] combined with the def2-SVP basis set. 46 Local minima were confirmed by the absence of imaginary frequencies.…”