Understanding the role of fluorination in the mechanistic nature of the water splitting process catalyzed by cobalt tris-(2-pyridylmethyl)amine complexes†

We report the reaction mechanism of the oxygen evolution reaction catalyzed by penta-coordinated [(Co-V(TPA-alpha F-3)(O))](3+) (TPA = tris-(2-pyridylmethyl)amine) and hexa-coordinated [Co-V(TPA-alpha F-3)(O)OH](2+) cobalt complexes for the formation of the oxygen-oxygen bond and the role of fluorination with the help of density functional theory. The nature of the orbitals involved in the formation of the oxygen-oxygen bond by these complexes is examined. The formation of the oxygen-oxygen bond occurs by the interaction of the d(x)(2)-y(2)* orbital of metal with the sigma*-orbital of the hydroxide in the case of [(Co-V(TPA-alpha F-3)(O)OH](2+), while, in the case of [(Co-V(TPA-alpha F-3)(O))](3+), the d(z)(2)*-orbital accepted the electron from the hydroxide. The activation barrier for the oxygen-oxygen bond formation by the penta coordinated complex is lower than that of the hexa-coordinated complex. The release of oxygen through both the catalytic processes has nearly equal activation free energies. From the spin density analysis, we observe that the oxo-complexes are stabilized by sharing the spin density on the nitrogen atoms with fluorine atoms of the modified TPA ligand. Comparing the four-proton and four-electron process of the catalytic cycle with both catalytic species, it is found that the penta-coordinated complex ([(Co-V(TPA-alpha F-3)(O))](3+)) requires a lower activation free energy barrier than the hexa-coordinated complex [Co-V(TPA-alpha F-3)(O)OH](2).

Automated summary

In this study, the reaction mechanism of the oxygen evolution reaction catalyzed by penta-coordinated and hexa-coordinated cobalt complexes was investigated using density functional theory. It was found that the activation barrier for the oxygen-oxygen bond formation by the penta-coordinated complex was lower than that of the hexa-coordinated complex. Through spin density analysis, it was observed that the oxo-complexes were stabilized by sharing spin density on the nitrogen atoms. Additionally, the penta-coordinated complex required a lower activation free energy barrier in the catalytic cycle compared to the hexa-coordinated complex.