EPR spectra of binuclear triazolato- and imidazolato-bridged copper(II) complexes including the four-copper form of bovine erythrocyte superoxide dismutase

Single-crystal EPR spectra of bis[p-3,5-bis(pyridin-2-y1)-l ,2,4-triazolato-N',N1,~b,iNs '[a~q ua(trifluoromethanesulfonato-0)- copper(II)], [CU(~~~)(CF,SO,)(H,wOer)e ]re~c,o rded at room temperature. They are typical of a triplet with gxx = 2.055 ( l ) , gyy = 2.051 (l), g,, = 2.232 (l), Ddd = 0.0026 ( 5 ) cm-I, Dff = 0.0338 (3) cm-', and Dii = -0.0364 (4) cm-I. The z and z' axes are practically parallel to each other, making an angle of 4 (1)O, while they and y'axes are rotated by 51 (9)O. The fact that the largest zero-field splitting component is observed parallel to g,,, and orthogonal to the copper-copper direction, is taken as evidence of a dominant exchange contribution, which is propagated by the triazolate bridges between the two copper(I1) ions separated by 408.5 pm. The comparison of the EPR spectra of [Cu(bpt)(CF3SO3)(H,O)1, with those previously reported for [(TMDT),CU,(~~)(CIO~)~a]n(dC f~orO t~h)e four-copper form of bovine erythrocyte superoxide dismutase, Cu2Cu2SOD,a llows us to suggest an assignment for the latter compounds and shows that even in these cases the exchange contribution to the zero-field splitting is dominant