NMR AND ELECTRONIC RELAXATION IN PARAMAGNETIC DICOPPER(II) COMPOUNDS

H-1-NMR spectral properties of two paramagnetic binuclear copper(II) c omplexes 1 and 2, which are weakly antiferro- and ferromagnetically co upled, respectively, in the solid state, have been studied in solution ; corresponding parameters are compared to a mononuclear copper(II) an alogue 1a. Compound 1 exhibits unusually sharp and hyperfine shifted l igand 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(I I) systems where much broader signals are observed in addition to redu ced 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 i ts signals. An analysis of the relaxation data and a quantitative comp arison among 1a, 1, and 2 show that the unusual spectral features obse rved for the weakly coupled binuclear copper(II) (S = 1/2) centers is caused by a two orders of magnitude decrease in the electron relaxatio n (tau(s) = 10(-11) s) as compared to tau(s) = 10(-9) s for the mononu clear copper(II) species, 1a. Shortening of tau(s) for homobinuclear c ompounds is not otherwise predicted, and possible mechanisms for the r esults are discussed. The present findings are significant with respec t to the factors determining electronic relaxation in magnetically cou pled systems and to the understanding of proton NMR when applicable to binuclear copper(II) metalloproteins.