NMR AND ELECTRONIC RELAXATION IN PARAMAGNETIC DICOPPER(II) COMPOUNDS

1H-NMR spectral properties of two paramagnetic binuclear copper(II) complexes 1 and 2, which are weakly antiferro- and ferromagnetically coupled, respectively, in the solid state, have been studied in solution; corresponding parameters are compared to a mononuclear copper(II) analogue 1a. Compound 1 exhibits unusually sharp and hyperfine shifted ligand signals (+230 to −14 ppm) that are about 100 times sharper than corresponding signals that could be detected for 1a (+20.4 to −13 ppm). Complex 2 also displays moderately sharp signals, shifted even to a greater magnitude (+272 to 0 ppm). These observations are in contrast with other moderately antiferromagnetically coupled binuclear copper(II) systems where much broader signals are observed in addition to reduced hyperfine chemical shifts. A complete assignment of signals for 1 and 2 was accomplished by a combination of proton relaxation data and two-dimensional correlated spectroscopy (COSY) measurements, while for 1a only partial analysis could be perfor...