First Example of a Reversible Single-Crystal-to-Single-Crystal Polymerization–Depolymerization Accompanied by a Magnetic Anomaly for a Transition-Metal Complex with an Organic Radical

The reaction of copper­(II) hexafluoroacetylacetonate [Cu­(hfac)₂] with the stable nitronyl nitroxide 2-(1-ethyl-3-methyl-1<i>H</i>-pyrazol-4-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1<i>H</i>-imidazole-3-oxide-1-oxyl (L^a) resulted in a paired heterospin complex [[Cu­(hfac)₂]₃(μ-<i>O</i>,<i>N</i>-L^a)₂]­[Cu­(hfac)₂(<i>O</i>-L^a)₂]. The crystals of the compound were found to be capable of a reversible single-crystal-to-single-crystal (SC–SC) transformation initiated by the variation of temperature. At room temperature, the molecular structure of [[Cu­(hfac)₂]₃(μ-<i>O</i>,<i>N</i>-L^a)₂]­[Cu­(hfac)₂(<i>O</i>-L^a)₂] is formed by the alternating fragments of the pair complex. Cooling the crystals of the complex below 225 K caused considerable mutual displacements of adjacent molecules, which ended in a transformation of the molecular structure into a polymer chain structure. A reversible topotactic polymerization–depolymerization coordination reaction actually takes place in the solid during repeated cooling–heating cycles: [[Cu­(hfac)₂]₃(μ-<i>O</i>,<i>N</i>-L^a)₂]­[Cu­(hfac)₂(<i>O</i>-L^a)₂] ⇌ Cu­(hfac)₂(μ-<i>O</i>,<i>N</i>-L^a)]_∞. Polymerization during cooling is the result of the anomalously great shortening of intermolecular distances (from 4.403 Å at 295 K to 2.460 Å at 150 K; Δ<i>d</i> = 1.943 Å) between the terminal Cu atoms of the trinuclear fragments {[[Cu­(hfac)₂]₃(μ-<i>O</i>,<i>N</i>-L^a)₂]} and the noncoordinated N atoms of the pyrazole rings of the mononuclear {[Cu­(hfac)₂(<i>O</i>-L^a)₂]} fragments. When the low-temperature phase was heated above 270 K, the polymer chain structure was destroyed and the compound was again converted to the pair molecular complex. The specifics of the given SC–SC transformation lies in the fact that the process is accompanied by a magnetic anomaly, because the intracrystalline displacements of molecules lead to a considerable change in the mutual orientation of the paramagnetic centers, which, in turn, causes modulation of the exchange interaction between the odd electrons of the Cu²⁺ ion and nitroxide. On the temperature curve of χ<i>T</i>, this shows itself as a hysteresis loop. The nontrivial character of the recorded spin transition during the cooling of the sample below 225 K lies in the fact that the magnetic moment abruptly increased. In contrast, heating the sample above 270 K led to a drastic decrease in χ<i>T</i>. This behavior of χ<i>T</i> is caused by a stepwise change in the character of the exchange interaction in the {>N–^•O–Cu²⁺–O^•–N<} fragments. The lengthening of distances between the paramagnetic centers on cooling below 225 K led to a transition from antiferromagnetic to ferromagnetic exchange and, vice versa, the shortening of distances between the paramagnetic centers during the heating of the heterospin polymer above 270 K led to a transition from ferromagnetic exchange to antiferromagnetic exchange