Modulating the oxygen affinity of porphyrins with metals, ligands, and functional groups: A DFT study

The interaction between different metals (M), axial ligands (L), and ring substituents (R) in porphyrins was investigated using density functional theory. Different combinations of iron and cobalt as metal centers; imidazole, chlorine, and an n-heterocyclic carbene (NHC) as axial ligands, and unsubstituted, octaethyl-, and tetraphenyl-porphyrins were explored in their low, intermediate, and high-spin states, alongside oxygen affinity. Remarkably, the n-heterocyclic carbene enhanced the affinity of cobalt porphyrins to oxygen, with binding energies on average 4.4 kcal mol^-1 higher than FeP with the same ligand, and 0.78 kcal mol^-1 higher than FeP with imidazole. The planarity of the iron tetraphenyl porphyrin with imidazole compared to its ruffled cobalt counterpart is noteworthy in both oxy- and deoxy-forms, highlighting imidazole's stabilizing influence on the porphyrin structure, particularly iron porphyrins, alongside imidazole's stabilizing effect on the affinity to O₂. Despite the significant non-planarity induced by NHC as an axial ligand -regardless of the metal or ring substituent used-, it did not hinder the affinity of CoP to O₂ (14.26 kcal mol^-1, on average) as it did with the FeP with NHC (9.88 kcal mol^-1, on average). Cobalt porphyrins with n-heterocyclic carbene ligands show promising potential for O₂ activation or oxygen transport applications. The results show the complex interactions between the different parts of metalloporphyrins and highlight the capability of tailoring their affinity to O₂. It also exemplifies the stabilizing effect of imidazole on the porphyrins, providing a very narrow range of binding energies and smaller differences in their geometries.