Peculiarities of synthesis of chain polymeric, copper(II) hexafluoroacetylacetonate based complexes with stable nitroxyl radicals and the results of studies on correlations between the magnetic properties and structure of these compounds are summarized. Temperature variation causes structural rearrangements in the solid phases of the compounds, accompanied by the magnetic effects similar to spin crossover phenomena. Magnetic anomalies induced by phase transitions originate from specific motions in the Jahn-Teller coordination units containing two types of exchange clusters, Cu 2+ -O • -N< or >N-• O-Cu 2+ -O • -N<, and are ac companied by significant changes in the crystal volume after multiple cooling/heating cycles. Chemical methods of controlling the character and temperature of spin transitions by both the formation of solid solutions of mixed metal hexafluoroacetylacetonates with the same nitroxyl radical, {Cu 1-x M x (hfac) 2 L} (M = Mn, Ni, Co), and by the formation of solid solutions based on copper(II) hexafluoroacetylacetonate with different nitroxyl radicals, {Cu(hfac) 2 L x L´1 -x }, are discussed. Specific influence of isotope substitution CH 3 → CD 3 in the paramagnetic ligand on both the structure of the heterospin polymer chain and the temperature of the magnetic anomaly is discussed.

Thermally induced spin transitions in a family of heterospin polymer chain complexes of Cu2+ hexafluoroacetylacetonate with two pyrazole-substituted nitronyl nitroxides are studied using electron paramagnetic resonance (EPR) spectroscopy. The structural rearrangements at low temperatures induce spin transitions in exchange-coupled spin triads of nitroxide-copper(II)-nitroxide. The values of exchange interactions in spin triads of studied systems are typically on the order of tens to hundreds of inverse centimeters. The large magnitude of exchange interaction determines the specific and very informative peculiarities in EPR spectra due to the predominant population of the ground state of a spin triad and spin exchange processes. The variety of these manifestations depending on structure and magnetic properties of spin triads are described. EPR is demonstrated as an efficient tool for the characterization of spin transitions and for obtaining information on the temperature-dependent sign and value of the exchange interaction in strongly coupled spin triads.

The stereochemically flexible Cu(hfac)(2) metal-ligand system when combined with polyfunctional nitroxides leads to a variety of solids with varying structure and composition. While investigating the products of Cu(hfac)(2) interaction with spin-labeled pyrazole 4,4,5,5-tetramethyl-2-(1-methyl-1H-pyrazol-4-yl)-imidazoline-3-oxide-1-oxyl, we have isolated a family of (12) heterospin compounds differing in structure and composition in the solid state. In synthetic systems, these compounds often cocrystallize and must be separated mechanically. It is also shown that minor variation of the structure of the solid heterospin complex can substantially change the magnetic properties of compounds.