Mechanistic Insight into the O₂ Evolution Catalyzed by Copper Complexes with Tetra- and Pentadentate Ligands

Abstract

The mononuclear complexes ([(bztpen)Cu] (BF 4 ) 2 (bztpen = N-benzyl-N,N′,N′-tris (pyridin-2-yl methyl ethylenediamine))) and ([(dbzbpen)Cu(OH 2 )] (BF 4 ) 2 (dbzbpen = N,N′dibenzyl-N,N′-bis(pyridin-2-ylmethyl) ethylenediamine)) have been reported as water oxidation catalysts in basic medium (pH = 11.5). We explore the O 2 evolution process catalyzed by these copper catalysts with various ligands (L) by applying the first-principles molecular dynamics simulations. First, the oxidation of catalysts to the metal-oxo intermediates [LCu(O)] 2+ occurs through the proton-coupled electron transfer (PCET) process. These intermediates are involved in the oxygen−oxygen bond formation through the water-nucleophilic addition process. Here, we have considered two types of oxygen−oxygen bond formation. The first one is the transfer of the hydroxide of the water molecule to the CuO moiety; the proton transfer to the solvent leads to the formation of the peroxide complex ([LCu(OOH)] + ). The other is the formation of the hydrogen peroxide complex ([LCu(HOOH)] 2+ ) by the transfer of proton and hydroxide of the water molecule to the metal-oxo intermediate. The formation of the peroxide complex requires less activation free energy than hydrogen peroxide formation for both catalysts. We found two transition states in the well-tempered metadynamics simulations: one for proton transfer and another for hydroxide transfer. In both cases, the proton transfer requires higher free energy. Following the formation of the oxygen−oxygen bond, we study the release of the dioxygen molecule. The formed peroxide and hydrogen peroxide complexes are converted into the superoxide complex ([LCu(OO)] 2+ ) through the transfer of proton, electron, and PCET processes. The superoxide complex releases an oxygen molecule upon the addition of a water molecule. The free energy of activation for the release of the dioxygen molecule is lesser than that of the oxygen−oxygen bond formation. When we observe the entire water oxidation process, the oxygen−oxygen bond formation is the rate-determining step. We calculated the rates of reaction by using the Eyring equation and found them to be close to the experimental values.

Improvement of electrocatalytic water oxidation activity of novel copper complex by modulating the axial coordination of phosphate on metal center

“…The Cu( iv )-oxo or Cu( iii )-oxyl intermediate was proposed in previous studies about electrochemical water oxidation catalyzed by mononuclear copper complex. 22,40,49,50 Theoretical calculations were carried out to prove the generation of possible intermediates during water catalytic oxidation (Fig. S40†).…”

“…[46][47][48] The Cu(IV)-oxo or Cu(III)-oxyl intermediate was proposed in previous studies about electrochemical water oxidation catalyzed by mononuclear copper complex. 22,40,49,50 Theoretical calculations were carried out to prove the generation of possible intermediates during water catalytic oxidation (Fig. S40 † 2+ to Cu IV -oxo is relatively higher, suggesting that the generation of the Cu IV -oxo species is also feasible though the energy demand of this process is relatively high.…”

Theoretical study on the mechanism of water oxidation catalyzed by a mononuclear copper complex: important roles of a redox non-innocent ligand and HPO42−anion

“…For the step-wise O-O bond formation process, the proton transfer requires a higher energy barrier than the hydroxide coupling with the Cu-oxo, which is similar with the reported mononuclear copper WOC. 52,53 Taking the energy penalty of the formation of 4, the whole O-O bond formation process has a total energy barrier of 19.5 kcal mol −1 . Downhill from the TS1, a hydro-peroxide intermediate (Int1, Fig.…”

Molecular Mechanism of the Mononuclear Copper Complex‐Catalyzed Water Oxidation from Cluster‐Continuum Model Calculations

“…However, our calculations show that the ligand of "Cu III À H 2 O" carries a high radical population of 0.9 (with radical mainly located on the copper coordinated nitrogen atoms, see Figure S1a), indicating the ligand is redox-active [88,89] and the one-electron oxidation leads to * + LÀ Cu II À H 2 O rather than the previously proposed "Cu III À H 2 O" species. [90] The redox potential of 2 0! 3 2 was calculated to be 1.28 V, suggesting that this step is thermodynamically favorable at the experimental potential of 1.42 V. Starting from 3 2, the ligand water either undergoes deprotonation to form the * + LÀ Cu II À OH À species ( 3 2!…”