SPECTROSCOPIC AND BINDING-STUDIES OF AZIDE COPPER(II) MODEL COMPLEXES

The characteristics of azide binding to a series of mononuclear copper(II) complexes with tridentate ligands has been investigated. The ligand donor atoms span NO2 to N3 donor sets, and the resulting copper(II) complexes, overall charges from 0 to +2. The spectroscopic properties of the adducts indicate that azide prefers to bind to Cu(II) in an axial coordination position when the equatorial ligand carries a dinegative charge, while it prefers to bind equatorially when the ligand is neutral or contains a mononegative donor atom in the Cu(II) plane. The equilibrium constants for the formation of the azide adducts have been determined by spectrophotometric titrations. The data show that the affinity of azide increases with the overall charge of the copper complex but depends markedly on the polarity of the medium. Very high affinities (K > 5000 M-1) have been found for dipositive complexes in methanol solution, while all K values are markedly lower in water solution.