Effect of ligands on heme dimethyl ester-based hexa-coordinated complexes as oxygen-carrying carriers

Most of the previous artificial oxygen carriers realized the oxygen-carrying function through a reversible combination of penta-coordinated iron(II) porphyrin with oxygen. The other oxygen transport model, in which oxygen competes with other ligands for binding with central ions, has been rarely studied. In this study, oxygen-carrying properties of a series of hexa-coordinated complexes were investigated, with heme dimethyl ester working as an oxygen-carrying component. In addition, both oxygen uptake and the auto-oxidation rate of the hexa-coordinated complexes were also explored. The results indicate that the hexa-coordinated complexes can effectively carry oxygen, provided that there is at least one strong ligand on the fifth or sixth coordination site of the heme dimethyl ester. Promisingly, when one ligand is strong and the other ligand is weak, the hexa-coordinated complexes can exhibit highly efficient oxygen-carrying capacity. By fixing weak ligands and changing strong ligands, it was found that the coordination ability of strong ligands has a significant impact on the oxygenation curve of the system. Furthermore, the auto-oxidation rate of the hexa-coordinated complexes of heme dimethyl ester is related to the hydrophobicity of the fifth and sixth ligands. The longer the hydrophobic chain of the ligand, the slower the auto-oxidation rate. Consequently, the oxygen-binding mode of this heme derivative has the potential to be an effective oxygen carrier.