A family of heterospin polymer chain complexes Cu(hfac)2 with pyrazole-substituted nitronyl nitroxides (LR, where R = Me, Et, Pr) of the composition Cu(hfac)2LR was found. In the solid state the complexes undergo low-temperature structural rearrangements accompanied by magnetic effects analogous to spin-crossover. Polymer chains with a "head-to-head" or "head-to-tail" motif in Cu(hfac)2LR are formed as a result of the bridging coordination of LR through the imine N atom of pyrazole and one of the O atoms of the nitronyl nitroxide fragment. Despite the low-temperature structural phase transition, the single crystals retain the quality needed for an X-ray investigation, due to which the compounds may be studied at different temperatures and structural dynamics studies are possible. It was found that the major structural changes mainly occur in the coordination polyhedra, leading to phenomena analogous to spin-crossover. This rearrangement is possible due to the John-Teller nature of the Cu(II) ion, which is responsible for the pulled octahedron structure of the coordination unit. The transition of the coordinated nitroxyl O atoms from the axial (dCu-O ∼ 2.2-2.4 Å) to equatorial (dCu-O ∼ 2-0 Å) position is accompanied by a transition of the exchange interaction in the Cu(II)-O•-N< exchange clusters from weak ferromagnetic (or weak antiferromagnetic) to strong anti-ferromagnetic, compensating the spins of the Cu(II) ion and the nitroxyl fragment. The motif of the polymer chain ("head-to-head" or "head-to-tail") proved to be inessential to the occurrence of thermally induced spin transitions. In both cases, the shortening of the Cu-O distance in the Cu(II)-O•-N< exchange cluster decreases the effective magnetic moment of the complex by a factor of √2, because spin compensation occurs in only half of all coordination units (Cu(hfac)2LR, Cu(hfac)2LR, and Cu2(hfac)4LMeLEt). The low-temperature structural rearrangement in Cu(hfac)2LEt is unusual; it leads to longer Cu-O distances in the Cu(II)-O•-N< exchange cluster and to an abrupt transition from antiferromagnetic to ferromagnetic exchange. At reduced temperatures, the unit cell volume decreased by 5-6% in all of the compounds. The cell parameters are large and so is the absolute value of the decrease (up to 400 Å3). The maximal decrease in the cell dimensions was observed in the directions of the chains and in the directions of the maximal shortening of the Cu...Cu distances. Minimal compression or even extension of the cell took place in the direction of lengthening of the Cu-Ohfac distances. The single crystals of Cu(hfac)2LEt and Cu(hfac)2LPr possess high mechanical stability in repeated cooling-heating cycles.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Inorganic Chemistry
- Materials Chemistry