Pyridazine-Based Ligands and Their Coordinating Ability towards First-Row Transition Metals

Tridentate pyridazine ligands with two different substituents at the 3- and 6-positions (pyridyl hydrazone at the 3-position and tolyl 3(tol) or tert-butyl 3(tBu) at the 6-position) were prepared in high yields from 3,6-disubstituted pyridazine hybrazines and pyridine aldehyde by Schiff base condensation. This provides a facile entry into a novel class of heterocyclic molecules exhibiting interesting coordinating abilities. They react with first-row transition metal nitrates to yield complexes of the type [ML2](NO3)(2) (L = 3(tol); M = Co 4, M = Ni 5, M = Zn 7) and [ML(NO3)(2)] (L = 3(tol), M = Cu 6; L = 3(tBu), M = Ni 8, M = Cu 9, M = Zn 10) as crystalline solids. With the exception of copper compound 6 all complexes employing ligand 3(tol) form complexes of the ML2 type, whereas with ligand 3(tBu) exclusively complexes of the ML type are formed, which demonstrates the influence of the substituent at the 6-position despite its being located far from the coordination site. This points to a pronounced electronic effect. Molecular structures of all complexes were determined by single-crystal X-ray diffraction analysis revealing six-coordinate metal centers in a N-6 coordination sphere consisting of two ligands 3 or in a N3O3 sphere with one ligand and two nitrates in the O,- and O,O'-coordination modes, respectively. Diamagnetic zinc complexes 7 and 10 were characterized in solution by H-1 NMR spectroscopy. All other complexes are paramagnetic (Evans' method). The redox behavior of all complexes was investigated by cyclic voltammetry, which revealed that the complexes possess the same stoichiometry in solution and in the solid state.