Unusually large values of the E.P.R. spectra fine structure parameter of Cu(II) dimers with two-bridge exchange mechanisms

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Abstract
E.P.R. study of related dimeric complexes (C5H12N)2[Cu2(OCH3)2 {2,4-NO2)2C6H3O}4]2CH3OH(I) and (C5H12N)2[Cu2(OCH3)2{2,4-(NO2)2 C6H3O}4](II) in X- and Q-bands in the temperature range 292-1·65 K has been carried out. The unusual magnetic properties : (i) the difference of the ground state isotropic exchange for these complexes, J I ≲ 0, J II = 130 cm-1, in spite of the practically unchanged structure of the dimeric fragment; (ii) nearly coincident values of the spin triplet fine structure parameter, |D I| = 1·27 cm-1, |D II| = 1·26 cm-1; (iii) the largest yet observed |D| values in Cu(II) dimers non-correlating with the ground state isotropic exchange, are reported. The value of the exchange interaction in the excited dimer J x 2-y 2,xy (I) = -330 cm-1 is estimated. The large ferromagnetic value of J x 2-y 2,xy is accounted for by the two-bridge exchange mechanisms associated with the virtual electron transfer along the Cu2O2 ring. The change of the isotropic exchange in the ground state when going from I to II is explained by the competition of mechanisms with different signs and the peculiarities of the geometric structure of complexes.

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