Synthesis, structure, and magnetic properties of 2,2′-(buta-1,3-diyne-1,4-diyl)bis(4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole 3-oxide 1-oxyl)

An approach to the synthesis of nitronyl nitroxide 2,2'-(buta-1,3-diyne-1,4-diy1)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 C C fragments, with the distances between the terminal carbon atoms of the neighboring diacetylene groups (T and d) being 6.170 and 4.466 angstrom, respectively, and the angle between the translation vector and the median line passing through the C C-C C fragment of 45.9 degrees. 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 373 K for 1 h 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 400-420 K the product explodes, releasing about 360 kJ/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) = 0.35 mT), indicating the efficiency of the C C-C C fragment as an exchange channel. The character of the experimental mu(eff)(T) dependence for 4 indicates a strong intramolecular antiferromagnetic-type exchange interaction (parallel to k(B) similar to -104 K) and the dominating weak intermolecular ferromagnetic exchange. (C) 2011 Elsevier Ltd. All rights reserved.