Spin Transition in the Cu(hfac)2 Complex with (4-Ethylpyridin-3-yl)-Substituted Nitronyl Nitroxide Caused by the Asymmetric Structural Rearrangement of Exchange Clusters in the Heterospin Molecule

Methods for the synthesis of binuclear [Cu(hfac)(2)L-Et](2) and tetranuclear [[Cu(hfac)(2)](4)(L-Et)(2)] heterospin compounds based on copper hexafluoroacetylacetonate [Cu(hfac)(2)] and 2-(4-ethylpyridin-3-yl)-4,5-bis(spirocyclopentyl)-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl (L-Et), were developed. The crystals of the complexes are elastic and do not crash during repeated cooling-heating cycles. It was found that a singlet-triplet conversion occurred in all of the {Cu(II)-O center dot-N<} exchange clusters in the molecules of the binuclear [Cu(hfac)(2)L-Et](2) which led to spin coupling with cooling. The transition occurred in a wide temperature range with a maximum gradient Delta chi T at approximate to 180 K. The structural transformation of the crystals takes place at T < 200 K and is accompanied by the lowering of symmetry from monoclinic to triclinic, twinning, and a considerable shortening of the Cu-O-NO distance (2.19 and 1.97 angstrom at 295 and 50 K, respectively). For the tetranuclear [[Cu(hfac)(2)](4)(L-Et)(2)], two structural transitions were recorded (at approximate to 154 K and approximate to 118 K), which led to a considerable change in the spatial position of the Et substituent in the nitronyl nitroxyl fragment. The low-temperature process was accompanied by a spin transition recorded as a hysteresis loop on the chi T(T) curve during the repeated cooling-heating cycles (T1/2 up arrow = 122 K, T1/2 down arrow = 115 K). This transition is unusual because it causes spin coupling in half of all of the {>N-center dot O-Cu2+} terminal exchange clusters, leading to spin compensation for only two paramagnetic centers of the six centers in the molecule.