COMPLEXATION OF COPPER(II) WITH CARBONATE LIGANDS IN AQUEOUS-SOLUTION- A CW AND PULSE EPR STUDY

The pH dependent complexation between copper and C-13-labeled carbonat e ligands in aqueous solution is investigated by optical spectroscopy and continuous wave and pulse EPR. The small C-13 hyperfine coupling o bserved in the pulse EPR spectra at low temperature and pH 5.5 is assi gned to weak, monodentate coordination of carbonates to the Cu2+ ion. The C-13 hyperfine couplings found at pH 6.5 and 8 are of the same mag nitude as twice the nuclear Larmor frequency (matching range) and are assigned to bidentate coordination of carbonate. The larger coupling f or the bidentate complexes is explained by a higher covalency of the i t bonding in the four-membered ring. The elongation of the axial bonds to the water Ligands caused by coordination of carbonate ligands in t he equatorial plane of the copper complex leads to smaller proton coup lings. The observation of both mono-and bidentate coordination at pH 6 .5 and 8 by pulse EPR at low temperature is in contradiction to the ca lculated concentrations of copper-carbonate complexes at room temperat ure. Although equilibrium shifts during the freezing of the samples ca nnot be excluded, the basic mechanisms of mono-and bidentate complexat ion observed in frozen solution should also apply in aqueous solution at ambient temperature.