Spin Transition Resulting From The Generation Of A New Polymorph In The Metastable Phase

The Cu(hfac)(2) complex with a paramagnetic ligand 2-(1-ethyl-imidazol-5-yl)- 4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl (L-5Et) was synthesized in the form of two polymorphs: alpha-[Cu(hfac)(2)L-5Et] (bluish green crystals) and beta-[Cu(hfac)(2)L-5Et] (brown crystals). Despite the structural similarity of the two modifications, whose solid phases are formed by polymer chains with cis coordination of the hfac ligands, their magnetic properties proved quite different. For beta-[Cu(hfac)(2)L-5Et], the repeated cooling-heating cycles at temperatures of 260-180 K lead to a reversible spin transition beta-[Cu(hfac)(2)L-5Et]-HT <->beta-[Cu(hfac)(2)L-5Et]-LT, resulting from a considerable change in the Cu-ONO distances in the Cu-O.-N< heterospin exchange clusters from 2.312(6) angstrom (HT form) to 2.011(2) angstrom (LT form), which leads to a transition of ferromagnetic to antiferromagnetic exchange in them. For alpha-[Cu(hfac)(2)L-5Et], the first cooling also leads to a decrease in mu(eff) from 2.6 mu(B) at 200 K to 0.35 mu(B) at 100 K on the curve of the mu(eff)(T) dependence. However, this is not a spin transition in the starting alpha-[Cu(hfac)(2)L-5Et], but, rather, the consequence of the irreversible phase transformation of alpha-[Cu(hfac)(2)L-5Et] into beta-[Cu(hfac)(2)L-5Et] induced by the cooling of the metastable a modification to T < 200 K. When the cooling-heating cycles are repeated further, the reversible spin transition inherent in beta-[Cu(hfac)(2)L-5Et] is reproduced. For this reason, all the mu eff values recorded in the range of 200-100 K for the cooled alpha-[Cu(hfac)(2)L-5Et] sample are higher than those for beta-[Cu(hfac)(2)L-5Et] up to T similar to 75 K until the whole starting a modification transforms into the beta form. This effect has never been observed earlier for transition metal complexes with nitroxides. The experimental mu(eff)(T) dependence observed during the first cooling of alpha-[Cu(hfac)(2)L-5Et] was called a spin transition in the nascent phase to distinguish it from the mu(eff)(T) curve for beta-[Cu(hfac)(2)L-5Et] corresponding to the typical reversible spin transition for heterospin complexes of transition metals with organic radicals. The results of thermomagnetic measurements correlate with FTIR and EPR data.