A theoretical study on the unusual square-planar structure of bis(imino)pyridine-ligated Group 13 complexes

The unusual square-planar (SP) structure of four-coordinate Al^III-complexes with the phenyl-substituted tridentate bis(imino)pyridine (^PhI₂P) ligand has been studied by a combination of density functional theory (DFT) calculations, frontier molecule orbital (FMO), nucleus-independent chemical shift (NICS) and strain/interaction analyses. The calculations disclose that: (i) the aromaticity shifting from the pyridine ring to the alumina-imidazolate metallacycle, and (ii) the small strain energy imposed on the ligand backbone are two favourable factors for driving the overall SP coordination of the Al^III ion. Meanwhile, the calculations reveal that the SP coordination of the Al^III ion is also influenced by the fourth ligand that is bonded to the Al^III ion. The less steric demanding ligand could ensure the stronger aromaticity of the alumina-imidazolate metallacycle and small strain energy on the ligand backbone. Additionally, the calculations predict that Ga^III and In^III ions favour the SP geometry in the analogue ^PhI₂P-ligated complexes, while B^III and Tl^III ions are reluctant to adopt the SP coordination with the ^PhI₂P ligand. Overall, all of these findings would be helpful for the synthesis of more unknown low-valent main-group metal complexes bearing the bis(imino)pyridine ligand.