Origins of dihydrogen binding to metal-inserted porphyrins: Electric polarization and Kubas interaction

Using density functional theory calculations, we have investigated the interactions between hydrogen molecules and metalloporphyrins. A metal atom, such as Ca or Ti, is introduced for incorporation in the central N₄ cavity. Within local density approximation (generalized gradient approximation), we find that the average binding energy of H₂ to the Ca atom is about 0.25 (0.1) eV/H₂ up to four H₂ molecules, whereas that to the Ti atom is about 0.6 (0.3) eV per H₂ up to two H₂ molecules. Our analysis of orbital hybridization between the inserted metal atom and molecular hydrogen shows that H₂ binds weakly to Ca-porphyrin through a weak electric polarization in dihydrogen, but is strongly hybridized with Ti-porphyrin through the Kubas interaction. The presence of d orbitals in Ti may explain the difference in the interaction types.