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Abstract: An approach to the synthesis of nitronyl nitroxide 2,2′-(buta-1,3-diyne-1,4-diyl)bis(4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole 3-oxide 1-oxyl) (4) was developed. Compound 4 is the first diradical with nitronyl nitroxide groups directly linked through a diacetylene fragment. In solid phase the diradicals are arranged in stacks with parallel Cts, with the distances between the terminal carbon atoms of the neighboring diacetylene groups (T and d) being 6.170 and 4.466Å, respectively, and the angle between the translation vector and the median line passing through the Ch name="sbnd" />Ct of 45.9°. The values of T and d are outside the range of structural criteria allowing a topochemical reaction. Thus UV irradiation does not initiate solid phase polymerization of 4. After exposure at 373K for 1h the crystals of 4 turn dark-brown, become X-ray amorphous and lose the majority of their paramagnetic centers without significantly changing their mass. Upon further heating up to 400–420K the product explodes, releasing about 360kJ/mol of heat. A diluted solution of 4 in 1,4-dioxane produces an EPR spectrum typical of a strong exchange (a multiplet of nine broadened lines with A 4N =0.35mT), indicating the efficiency of the Ch name="sbnd" />Ct as an exchange channel. The character of the experimental μ eff(T) dependence for 4 indicates a strong intramolecular antiferromagnetic-type exchange interaction (J/k B ∼−104K) and the dominating weak intermolecular ferromagnetic exchange. [Copyright &y& Elsevier]

Abstract: It was established that the reactions of pyrazol-3-yl-substituted nitronyl nitroxide (HL 1) and pyrazol-3-yl-substituted imino nitroxide (HL 3) with Cu(II) acetate lead to self-assembly of the Cu4(OH)2(OAc)4(DMF)2(L1)2 tetranuclear and Cu2(OAc)2(H2O)2(L3)2 dinuclear complexes, respectively. The reaction of Cu(II) acetate with 5-ethoxycarbonyl-pyrazol-3-yl-substituted nitronyl nitroxide (HL 2) gave unexpected solid Cu2(H2O)2(L6)2 ·2DMF, in which L 6 is a deprotonated 5-carboxy-pyrazol-3-yl-substituted nitronyl nitroxide, formed as a result of cleavage of an ester bond in the starting HL 2. A similar transformation of the paramagnetic ligand was observed in the reaction of Cu(II) acetate with 5-ethoxycarbonyl-pyrazol-3-yl-substituted imino nitroxide (HL 4). It led to the formation of Cu2(DMF)2(L7)2, where L 7 is deprotonated 2-(5-carboxy-1H-pyrazol-3-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole 3-oxide. An X-ray diffraction study indicated that in Cu4(OH)2(OAc)4(DMF)2(L1)2 and Cu2(OAc)2(H2O)2(L3)2, the L 1 and L 3 paramagnetic ligands perform the bridging cyclic tridentate function, while in Cu2(H2O)2(L6)2 ·2DMF and Cu2(DMF)2(L7)2, the paramagnetic L 6 and diamagnetic L 7 are bridging bicyclic tetradentate ligands. The magnetic behavior of complexes with coordinated nitronyl nitroxide – Cu4(OH)2(OAc)4(DMF)2(L1)2 and Cu2(H2O)2(L6)2 ·2DMF is dictated by the dominant antiferromagnetic exchange interactions, which is confirmed by quantum-chemical data. The magnetic susceptibility of Cu2(OAc)2(H2O)2(L3)2 reflects the competition between the antiferromagnetic and ferromagnetic components, of which the latter is due to electron coupling in the Cu(II)←N=C–N∸O exchange channels. EPR data confirm the results received from static magnetic measurements for multispin solids. [Copyright &y& Elsevier]