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 (∼-120 cm-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 → 2.002 Å) 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.
ASJC Scopus subject areas
- Molecular Biology
- Condensed Matter Physics
- Physical and Theoretical Chemistry