Theoretical investigation of five-coordinated cobalt(III) oxophlorin complexes with different axial ligands and their interactions with H 2 O, CO and O 2 at various spin states using the B3LYP method

Abstract

Considering the C 2 symmetry axis of the oxophlorin ring, two distinct isomers can be observed for five-coordinated Co(III)-oxophlorin with imidazole or pyridine as the axial ligand, namely perpendicular and parallel isomers. In the parallel isomer, the axial ligand lies in the plane containing the C 2 axis. The results show that these isomers are more stable in the triplet state than those at the other spin states, and that interconversion between them occurs readily. Also, five-coordinated Co(III)-oxophlorin with t-butylcyanide as the axial ligand is stable at singlet state. Non-specific solvent effects on the optimized complexes indicate that dispersion effects and London forces play a major role in the stability of these compounds, making them more stable at singlet spin state than other spin states, compared to gas phase. Data obtained indicate that H 2 O can act as an axial ligand in all spin multiplicities, and CO and O 2 getaways from cobalt of oxophlorin at the quintet state. However, O 2 can react with the parallel isomer of Co(III)-oxophlorin with imidazole or pyridine as the axial ligand at singlet state, similar to heme. Also, the carbon atom of CO molecules is able to attach to the cobalt atom of oxophlorin in the singlet state. Ultimately, a complex at singlet spin state is formed like oxophlorin but that it has two C[Formula: see text]O groups facing each other. Note that this interaction can be supported under conditions of blood and the human body. Besides, it is specified that one and three [Formula: see text]-electrons are on cobalt in each of the complexes at triplet and quintet spin states, respectively. The results obtained are fully consistent with NBO analysis and molecular orbital calculations.