Copper complexes of a new tridentate imidazole-containing ligand: spectroscopy, structures and nitrite reductase reactivity: The molecular structures of [Cu(biap)(NO2)2] and [Cu(biap)Br2]

The ligand N,N-bis(2-ethyl-5-methyl-imidazol-4-ylmethyl)-aminopropane (abbreviated as biap) forms several copper(II) coordination compounds with general formula [Cu(biap)X2], with X−=Cl−, Br−, NO3 −, NO2 − and NCS−. In all these compounds the ligand biap acts as a tridentate ligand. Single crystals of [Cu(biap)(NO2)2] and [Cu(biap)Br2] were used for X-ray structure determinations. All compounds were characterized by IR, ligand field and EPR spectroscopy, and cyclic voltammetry. The geometry of copper(II) in the (pseudo-) halide complexes is five-coordinate, whereas copper(II) is distorted octahedrally coordinated in the nitrate and nitrite complexes, whereby one of the anions binds didentate chelating and the other one monodentate. The affinity of nitrite for the [Cu(biap)]2+ cation was studied by determining the binding constant of the anion to the perchlorate salt of the complex in methanol solution. The data show that nitrite exhibits high affinity for the complex and its behavior is comparable with that of azide. The reduction of nitrite, to produce nitric oxide, by the reduced complex [Cu(biap)]+ has been studied anaerobically. Kinetic experiments showed that the reaction exhibits saturation behavior with respect to nitrite concentration. The kinetic data indicate that [Cu(biap)]+ has a moderate efficiency in the nitrite reductase reaction.