Mechanistic Insights into Cobalt-Based Water Oxidation Catalysis by DFT-Based Molecular Dynamics Simulations

Abstract

We report the mechanistic details of the water oxidation process by the complex, [CoII(bpbH2)­Cl2], where bpbH2 = N, N′-bis(2′-pyridinecarboxamide)-1,2-benzene. An experimental study reported the complex as the efficient catalyst for the water oxidation process. We performed density functional theory calculations at the M06-L level and first-principles molecular dynamics simulations to study the catalytic nature of the complex. We investigated the energetics of the total catalytic cycle, which combines the oxygen–oxygen bond formation, proton-coupled electron transfer (PCET) steps, and release of oxygen molecule. The formed peroxide and superoxide intermediates in the catalytic cycle were characterized with the help of the Mulliken spin density parameters. Mulliken spin densities of the metal–oxo bond reveal that the triplet state of CoVO has a double-bond nature, but the quintet state of the complex has a radical nature (CoIV–O•–). In an alternative way, the deprotonation of the amide groups of the ligand is also considered. The deprotonation and formation of higher oxidation metal–oxo intermediates are also possible. In addition to this, we have considered the effect of phosphate buffer on water nucleophilic addition. The oxygen–oxygen bond formation is favorable by the catalyst with the deprotonated form of the ligand, with the addition of water as the nucleophile. In the oxidation process, the CO bonds of the ligand transfer the electron density to nitrogen atoms, stabilizing the higher order oxo, peroxide, and superoxide bonds. The oxygen–oxygen bond formation is the rate-determining step in the overall water oxidation process. This bond was further investigated using first-principles molecular dynamics at the PBE-D2 level. The dynamics of proton, hydroxide ion, and the nature of the ligand structure on the oxygen–oxygen bond were examined. We find that the oxygen molecule is released from the superoxide complex with the addition of water molecules.

Insights into the Reduction Mechanisms of Np(VI) to Np(V) by Carbohydrazide

“…Spin density can elucidate the change of oxidation state of Np . Diagrams of spin density and the value of spin density located on the Np atom for the structures of the pathway I are presented in Figure , and corresponding values on the Np, N, and O yl atoms are tabulated in Table S2.…”

“…Spin density can elucidate the change of oxidation state of Np. 54 Diagrams of spin density and the value of spin density located on the Np atom for the structures of the pathway I are presented in Figure 6, and corresponding values on the Np, N, and O yl atoms are tabulated in Table S2. The similar value (2.25−2.28 au) of the spin density on the Np atom for the structures of IC1 I , TS1 I , and INT1 I uncovers that the oxidation state of Np for the three structures is Np(V).…”

Reduction of Np(vi) with hydrazinopropionitrileviawater-mediated proton transfer

“…Spin density is used to illuminate the change of oxidation stage of Np, as reported by Gorantla and Mallik. 79 Diagrams of spin density and the value of spin density located on the Np atom for the structures on the pathway I are shown in Fig. 8 and the corresponding values on the Np, O, and N atoms are listed in Table S8 (ESI †).…”

Unveiling the Reduction Mechanism of Pu(IV) by Acetaldoxime

“…Spin density can elucidate the oxidation state as reported by Gorantla and Mallik . Diagrams of spin density for the structures of ICs, TSs, and INTs are presented in Figure , and the corresponding values on the Pu, O, N, and C atoms are listed in Table S2.…”