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

Tris(8-hydroxyquinolinate) metallic complexes, Mq3, are one of the most important classes of organicsemiconductor materials. Herein, the nature of the chemical bond in Mq3 complexes and its implicationson their molecular properties were investigated by a combined experimental and computationalapproach. Various Mq3 complexes, resulting from the alteration of the metal and substitution of the8-hydroxyquinoline ligand in different positions, were prepared. The mer-/fac-isomerism in Mq3 wasexplored by FTIR and NMR spectroscopy, evidencing that, irrespective of the substituent, mer- andfac-are the most stable molecular configurations of Al(III) and In(III) complexes, respectively. The relativeM-ligand bond dissociation energies were evaluated experimentally by electrospray ionization tandemmass spectrometry (ESI-MS-MS), showing a non-monotonous variation along the group (Al 4 In 4 Ga).The results reveal a strong covalent character in M-ligand bonding, which allows for through-ligandelectron delocalization, and explain the preferred molecular structures of Mq3 complexes as resultingfrom 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 strongercovalent M-ligand bonds can be formed due to more extensive through-ligand conjugation mediatedby metal ''d'' orbitals