How Many Ligands Can Be Bound by Magnesium-Porphyrin? A Symmetry-Adapted Perturbation Theory Study.

The stability of complexes of magnesium-porphyrin with one or two identical ligands from the set water, pyridine, imidazole, acetate, acetonitrile, dimethyl sulfoxide (DMSO), ethyl acetate, or acetylacetone was examined using symmetry-adapted perturbation theory (SAPT) for minimum geometries obtained by density-functional theory (DFT). The nonadditive contributions to the interaction energy of the porphyrin ring with two ligands were also included and found to be very small in almost all cases. The stability of the complexes under standard conditions is predicted on the basis of the free Gibbs energy. The analysis of individual components of the SAPT interaction energy allows us to explain why the complexation of the second ligand is not energetically preferred in some cases.