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-1H-pyrazol-4-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl (La) resulted in a
paired heterospin complex [[Cu­(hfac)2]3(μ-O,N-La)2]­[Cu­(hfac)2(O-La)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-O,N-La)2]­[Cu­(hfac)2(O-La)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-O,N-La)2]­[Cu­(hfac)2(O-La)2] ⇌ Cu­(hfac)2(μ-O,N-La)]∞. 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; Δd = 1.943 Å) between the terminal
Cu atoms of the trinuclear fragments {[[Cu­(hfac)2]3(μ-O,N-La)2]} and the noncoordinated N atoms of the pyrazole rings
of the mononuclear {[Cu­(hfac)2(O-La)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 χT, 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 χT. This behavior
of χT 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.