Spin transition in the Cu(hfac)₂complex 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)₂L^Et]₂and 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•–N<} exchange clusters in the molecules of the binuclear [Cu(hfac)₂L^Et]₂which led to spin coupling with cooling. The transition occurred in a wide temperature range with a maximum gradient ΔχT at ≈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–ONO distance (2.19 and 1.97 Å at 295 and 50 K, respectively). For the tetranuclear [[Cu(hfac)₂]₄(L^Et)₂], two structural transitions were recorded (at ≈154 K and ≈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 χT(T) curve during the repeated cooling–heating cycles (T½↑ = 122 K, T½↓ = 115 K). This transition is unusual because it causes spin coupling in half of all of the {>N–•O–Cu²⁺} terminal exchange clusters, leading to spin compensation for only two paramagnetic centers of the six centers in the molecule.