On Ligand Binding Energies in Porphyrinic Systems

Energy decomposition analysis is a powerful tool to study the bond properties of molecules. Here, we used a recent implementation by P. Su and H. Li (J. Chem. Phys. 131 (2009) 014102) to test the method for investigations of ligand binding to medium sized molecules (30–40 atoms). We studied the properties of the bond between porphyrins or phthalocyanines with central metals without (Mg) or with a closed d shell (Zn and Cd) to ligands binding via various row II and III atoms (nitrogen, oxygen, phosphorus and sulphur) using various methods and basis sets. Several interesting results could be deduced: In the porphyrins, ligands binding via a nitrogen atom were preferred to those binding via oxygen and sixfold coordination was readily available for magnesium as central metal atom as opposed to zinc and cadmium derivatives which also showed a weaker bond strength. In phthalocyanines, a tendency to form fivefold coordination was also seen for magnesium compounds and bonds to nitrogen and oxygen had the same strength. Ligation to row III elements created only weak bonds that were prone to fluctuations.