NMR and electronic relaxation in paramagnetic dicopper(II) compounds

H-1-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 performed because of broadness of its signals. An analysis of the relaxation data and a quantitative comparison among 1a, 1, and 2 show that the unusual spectral features observed for the weakly coupled binuclear copper(II) (S = 1/2) centers is caused by a two orders of magnitude decrease in the electron relaxation (tau(s) = 10(-11) s) as compared to tau(s) = 10(-9) s for the mononuclear copper(II) species, 1a. Shortening of tau(s) for homobinuclear compounds is not otherwise predicted, and possible mechanisms for the results are discussed. The present findings are significant with respect to the factors determining electronic relaxation in magnetically coupled systems and to the understanding of proton NMR when applicable to binuclear copper(II) metalloproteins.