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

TL;DR

This study uses density functional theory to explore how hydrogen molecules bind to metal-inserted porphyrins, revealing different interaction mechanisms for Ca and Ti metals, with implications for hydrogen storage.

Contribution

It provides a detailed analysis of the binding mechanisms, highlighting the role of electric polarization and Kubas interaction in hydrogen binding to metalloporphyrins.

Findings

H2 binds weakly to Ca-porphyrin via electric polarization.

H2 binds strongly to Ti-porphyrin through Kubas interaction.

Ti's d orbitals influence the binding mechanism.

Abstract

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 N$_4$ cavity. Within local density approximation (generalized gradient approximation), we find that the average binding energy of H$_2$ to the Ca atom is about 0.25 (0.1) eV/H$_2$ up to four H$_2$ molecules, whereas that to the Ti atom is about 0.6 (0.3) eV per H$_2$ up to two H$_2$. Our analysis of orbital hybridization between the inserted metal atom and molecular hydrogen shows that H$_2$ 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.