Understanding M–ligand bonding and mer-/fac-isomerism in tris(8-hydroxyquinolinate) metallic complexes

Abstract

Tris(8-hydroxyquinolinate) metallic complexes, Mq₃, are one of the most important classes of organic semiconductor materials. Herein, the nature of the chemical bond in Mq₃ complexes and its implications on their molecular properties were investigated by a combined experimental and computational approach. Various Mq₃ complexes, resulting from the alteration of the metal and substitution of the 8-hydroxyquinoline ligand in different positions, were prepared. The mer-/fac-isomerism in Mq₃ was explored by FTIR and NMR spectroscopy, evidencing that, irrespective of the substituent, mer- and fac-are the most stable molecular configurations of Al(III) and In(III) complexes, respectively. The relative M–ligand bond dissociation energies were evaluated experimentally by electrospray ionization tandem mass spectrometry (ESI-MS-MS), showing a non-monotonous variation along the group (Al > In > Ga). The results reveal a strong covalent character in M–ligand bonding, which allows for through-ligand electron delocalization, and explain the preferred molecular structures of Mq₃ complexes as resulting from the interplay between bonding and steric factors. The mer-isomer reduces intraligand repulsions, being preferred for smaller metals, while the fac-isomer is favoured for larger metals where stronger covalent M–ligand bonds can be formed due to more extensive through-ligand conjugation mediated by metal “d” orbitals.