Unusual spin transitions

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Abstract
New heterospin complexes of Cu(hfac)2 (hfac, hexafluoroacetylacetonate) with pyrazole-substituted nitronyl nitroxides have been found that in the solid state exhibit thermally induced spin transitions analogous to spin crossover. For the first complex, [Cu(hfac)2Li-Pr], at room temperature, the Cu—OL distances, where OL is the oxygen atom of the nitroxyl group, are very short (2.143 Å). This leads to a strong antiferromagnetic exchange (∼-120cm−1) in the > N—·O—Cu2+—O·—N < exchange clusters. The CuO6 coordination units formed by four O atoms of the two hfac anions and by the nitroxyl O atoms of the two bridging nitroxides have a rare form of flattened octahedra, transformed at low temperatures into elongated octahedra with shorter Cu—OL distances (2.143 Å→2.002Å) and two longer Cu—Ohfac distances (2.130 Å→2.293 Å). For the second complex, [Cu(hfac)2LBu·0.5C6H14], unusual low temperature structural dynamics of heterospin systems have been found. It is characterized by the formation of two types of CuO6 unit. The axial Cu—OL distances are lengthened in one unit (2.250 Å→2.347 Å) and shortened in the other (2.250 Å → 2.006 Å). This leads to a sophisticated μeff(T) dependence with μeff drastically decreased at 163 K as a result of full coupling of two spins in half of all >N—·O—Cu2+—O·—N < exchange clusters and to a shift from antiferromagnetic to ferromagnetic exchange in the other half.