Computational Investigation on the Mechanism of Electrocatalytic Water Oxidation by Copper(II) Porphyrin

In a recent report ( Chem. Sci. , 2019 , 10 , 2613–2622), it was shown that the copper(II) complex of tetrakis(4-N-methylpyridyl)porphyrin serves as an electrocatalyst towards an efficient oxygen evolution reaction (OER) with an onset overpotential of 310 mV. Motivated by the experimental work, we perform the density functional theory (DFT) calculation in a water medium employing the solvation molecular dynamics (SMD) model to find out the mechanism of the water oxidation process by taking unsubstituted porphyrin (L). Three mechanisms are constructed. Mechanism I starts from the water-coordinated complex [LCu ^II -OH ₂ ] ( 1 ), which undergoes a proton-coupled electron transfer (PCET) reaction to [LCu ^III -OH ₂ ] ⁺ ( 2 ) with a redox potential E  = 0.81 V vs. SHE. Then, it undergoes deprotonation to form [LCu ^III -OH] ( 3 ), followed by one-electron oxidation at E  = 1.33 V vs. SHE to form [LCu ^IV -OH] ⁺ ( 4 ). Then, 4 undergoes deprotonation from [LCu ^IV =O] ( 6 ). Next, water attacks the oxygen atom of 4 , followed by two further PCET processes with potential < 1 V vs. NHE from the oxygen-bound copper species with steps [LCu ^II =O- ⁺ OH ₂ ] ( 7 ) → [LCu ^II -OH-OH] ( 8 ) → [LCu ^II -O ^• -OH] ( 7 ) → [LCu ^II -O = O] ( 9 ). The activation barrier for the step [LCu ^II =O- ⁺ OH ₂ ] ( 7 ) → [LCu ^II -OH-OH] ( 8 ) is calculated to be 5.6 kcal/mol. In mechanism II, [LCu ^III -OH] ( 3 ) → [LCu ^IV =O] ( 5 ) considered with E  = 2.08 V vs. NHE. In mechanism III, the attack of water to 5 with the PCET process to form [LCu ^II - ^– O-OH] ^– ( 10 ) followed by oxidation to [LCu ^II - ^• O-O ^– ] ^– ( 11 ) and [LCu ^II -O = O] ( 9 ). For the PCET oxidation of 1 to [LCu ^III -OH] ( 3 ), the redox potential is 1.70 V vs. SHE. It is concluded that mechanism I is more likely to take place with an onset potential E  = 1.33 V vs. SHE, giving rise to an overpotential of 100 mV. Further modification of the redox potential can be made by employing substituted porphyrin systems.