Metal chelation aptitudes of bis(o-azaheteroaryl)methanes as tuned by heterocycle charge demands

We describe the synthesis of a number of 1,3-azol-2-yl-, 1,3-benzazol-2-yl-, and azinyl-based bis- (o-azaheteroaryl)methanes (LH, L^- = Het₂CH-) and their coordinating properties toward divalent transition metals (Zn, Cu, Co, Ni, Hg, Pd). This extended investigation includes both symmetrical and unsymmetrical ligands based on several substituted and/or unsubstituted thiazole, benzothiazole, benzoxazole, benzimidazole, pyridine, and quinoline derivatives. Depending on the structure and electron properties of the ligand, a vast set of neutral chelates ML₂ were obtained, where the ligand is present in its carbanionic form L^-. Additionally, we have prepared salt complexes [M(LH)_n]-X_m, where the ligand is present as a neutral system. Neutral chelates were typically obtained by the reaction of the ligand with metal acetates in alcoholic solution; salt complexes were formed by reaction with other metal salts such as chlorides. By exploring the coordinating properties of several bisheteroarylmethane ligands based on heteroaromatics of variable π-electron structure and substitution pattern, we demonstrate that the formation of neutral chelates is strictly dependent on the electron-withdrawing capacity (charge demand) of the heteroaromatic moiety. The latter primarily dictates the efficiency by which the negative charge of the anionic ligand L^- is stabilized by delocalization in ML₂ and, therefore, the stability of the chelate itself. On the basis of the large number and the variable nature of the nitrogen ligands used, we confirm the general validity of the charge-demand-dependent formation of chelates. This key factor can therefore be used for the efficient design of new π-deficient heteroaromatic nitrogen ligands in chelates of great potential in many synthetic, catalytic, and technological fields.