Synthesis and characterization of 2-quinoxalinol Schiff-base metal complexes

The reaction of uranyl acetate with (2,2′-(1E,1′E)-(2-benzyl-3-hydroxyquinoxaline-6,7-diyl)bis(azan-1-yl-1-ylidene)bis(methan-1-yl-1-ylidene) diphenol) (H2L1) at room temperature in methanol and chloroform yields the UO2L1 complex. Crystals were grown through solvent diffusion of the ligand–metal complex in dimethyl formamide with diethyl ether to prepare: UO2L1 · DMF (1). Complexes with 2,2′-(1E,1′E)-(2-benzyl-3-hydroxyquinoxaline-6,7-diyl)bis(azan-1-yl-1-ylidene)bis(methan-1-yl-1-ylidene)dibenzene-1,4-diol (H2L2) and 2,2′-(1E,1′E)-(2-hydroxy-3-isopropylquinoxaline-6,7-diyl)bis(azan-1-yl-1-ylidene)bis(methan-1-yl-1-ylidene)diphenol (H2L3) were also prepared, and crystals of the uranyl complexes (UO2L2 · DMF (2) and (3)) grown from DMF/ether. A fourth complex UO2L4 · H2O (4) was prepared through layering a solution of the tetra-tert-butyl substituted 2-quinoxalinol salen ligand H2L4 in acetone with an aqueous solution containing uranyl acetate. The complexes exhibit a symmetric core featuring a slightly distorted bicapped pentagonal geometry around the uranium center with two oxo-groups and two imine groups from the ligand chelating the ligand and the fifth site in the coordination plane of the ligand occupied by a solvent molecule. These compounds have been characterized using solution (NMR and UV–Vis) and solid-state (IR, X-ray crystallography) techniques. Complexes of H2L4 with early transition metals; Mn2+, Co2+, Ni2+, and Cu2+ are also prepared and characterized for comparison of solution and spectroscopic characteristics.