Unusual spin transitions

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)₂L^i-Pr], at room temperature, the Cu-O_L distances, where O_L 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-O_L distances (2.143 Å → 2.002 Å) and two longer Cu-O_hfac distances (2.130 Å → 2.293 Å). For the second complex, [Cu(hfac)₂L^BU·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-O_L 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 μ_eff(T) dependence with μ_eff 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.