Understanding M-ligand bonding and mer-/fac-isomerism in tris(8-hydroxyquinolinate) metallic complexesElectronic supplementary information (ESI) available: Detailed synthesis data and compound characterization; detailed FTIR, UV-Vis, NMR, mass spectrometry and computational results. See DOI: 10.1039/c6cp02608g

• Main Authors: Lima, Carlos F. R. A. C, Taveira, Ricardo J. S, Costa, José C. S, Fernandes, Ana M, Melo, André, Silva, Artur M. S, Santos, Luís M. N. B. F
• Format: Journal Article
• Published: 15.06.2016
• ISSN: 1463-9076; 1463-9084
• DOI: 10.1039/c6cp02608g

Tris(8-hydroxyquinolinate) metallic complexes, Mq 3 , are one of the most important classes of organic semiconductor materials. Herein, the nature of the chemical bond in Mq 3 complexes and its implications on their molecular properties were investigated by a combined experimental and computational approach. Various Mq 3 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 Mq 3 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 Mq 3 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. mer -/ fac -Isomerism in tris(8-hydroxyquinolinate) complexes is ruled by the interplay between steric and bonding factors. The M-ligand bond has strong covalent character and allows for through-ligand electron delocalization.