Synthesis and study of Cu^II complex with nitroxide, a jumping crystal analog

Abstract

© 2017, Springer Science+Business Media, LLC. We synthesized 1-ethylimidazolyl-substituted nitronyl nitroxides, i.e., 2-(1-ethylimidazol-4-yl)- (L 4Et ) and 2-(1-ethylimidazol-5-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole 3-oxide-1-oxyl (L 5Et ). The stable radical L 5Et is an ethyl analog of 2-(1-methylimidazol-5-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole 3-oxide-1-oxyl (L 5Me ) described earlier, the reaction of which with Cu(hfac) 2 (hfac is 1,1,1,5,5,5-hexafluoropentane-2,4-dionate) leads to the formation of the [Cu(hfac) 2 (L 5Me ) 2 ] jumping crystals. The reaction of Cu(hfac) 2 with L 5Et with reagent ratios 1: 2 and 1: 1 yields heterospin complexes [Cu(hfac) 2 (L 5Et ) 2 ] and [Cu(hfac) 2 L 5Et ] 2 , respectively. X-ray diffraction study of the mononuclear complex [Cu(hfac) 2 (L 5Et ) 2 ] determined that the compound has a packing similar to that of jumping crystals studied earlier, with the only difference being that the O..O contacts between neigh- boring nitroxide groups were found to be 0.3—0.5 Å longer than in [Cu(hfac) 2 (L 5Me ) 2 ]. As a result of the lengthening of these contacts, [Cu(hfac) 2 (L 5Et ) 2 ] crystals lack chemomechanical activi- ty. We found that when cooling crystals of binuclear complex [Cu(hfac) 2 L 5Et ] 2 below 50 K, the antiferromagnetic exchange between unpaired electrons of the > N—•O groups of neighboring molecules leads to the full spin-pairing of the nitroxides, with only the Cu 2+ ions contributing to the residual paramagnetism of the compound.