TEMPO radicals showing magnetic interactions. I. 4-(4-halobenzylideneamino)TEMPO and related compounds

X-ray crystal structure analyses of the 4-(4-halobenzylideneamino)TEMPO (TE MPO = 2,2,6,6-tetramethyl-piperidyl-1-oxyl) radicals 4-(4-fluorobenzylidene amino)-2,2,6,6-tetramethylpiperidyl-1-oxyl, 4-(4-chlorobenzylideneamino)-2, 2,6,6-tetramethylpiperidyl-1-oxyl, 4-(4-bromob enzylidene amino)-2,2,6,6-te tramethylpiperidyl-1-oxyl and 4-(4-iodobenzylideneamino)-2,2,6,6-tetramethy lpiperidyl-1-oxyl, and other 4-Ar-CH=N-TEMPO [Ar = 4-Ph-Ph, 2,2,6,6-tetrame thyl-4-(4-phenylbenzylideneamino)piperidyl-1-oxyl, 4-Py, 2,2,6,6-tetramethy l-4-(4-pyridylmethylideneamino)piperidyl-1-oxyl, and 3,5-diCl-Ph, 4-(3,5-di chlorobenzylideneamino) -2,2,6,6-tetramethylpiperidyl-1-oxyl] radicals have been performed at room temperature. Some of these radicals show intermolec ular ferromagnetic interactions at extremely low temperatures. X-ray analys is revealed that crystals of the 4-I-Ph derivative showed two modifications [(a) and (b)] measurements of the magnetic properties of these crystals sh owed a ferromagnetic transition at an extremely low temperature for modific ation (a) and an antiferromagnetic interaction for modification (b). The st ructural features of these TEMPO radical crystals can be classified into th ree groups: (i) the crystal structures of the 4-Cl-Ph, 4-I-Ph(a) and 4-Ph-P h derivatives, which show a ferromagnetic transition; (ii) the structures o f the 4-Br-Ph and 4-Py derivatives, which show a ferromagnetic interaction (theta > 0); (iii) miscellaneous: the antiferromagnetic 4-F-Ph and 4-I-Ph(b ) derivatives (theta < 0) and the ferromagnetic 3,5-diCl-Ph (theta > 0) der ivative. Sheetlike arrangements of O atoms and intra-sheet interactions thr ough the CH2 or CH3 groups of the TEMPO rings are related to the mechanisms of the ferromagnetic interactions.