TEMPO radicals showing magnetic interactions. II. 4-(benzylideneamino)-TEMPO and related compounds

X-ray crystal structure analyses were performed on 4-(Ar-methyleneamino)-TE MPO radicals at room temperature (TEMPO = 2,2,6,6-tetramethylpiperidyl-1-ox yl): Ar = Ph [4-(benzylideneamino)-2,2,6,6-tetra-methylpiperidyl-1-oxyl], 4 -MeS-Ph [2,2,6,6-tetramethyl-4-(4-methylthiobenzylideneamino)piperidyl-1-ox yl], 4-Me-Ph [4-(4-methylbenzylideneamino)-2,2,6,6-tetramethylpiperidyl-1-o xyl], 4-PhO-Ph [2,2,6,6-tetramethyl-4-(4-phenoxybenzylideneamino)piperidyl- 1-oxyl], 4-MeSO2-Ph [2,2,6,6-tetramethyl-4-(4-methylsulfonylbenzylideneamin o)piperidyl-1-oxyl], 3-Py [2,2,6,6-tetramethyl-4-(3-pyridylmethylideneamino )piperidyl-1-oxyl] and 2-Naph [2,2,6,6-tetramethyl-4-(2-naphthylideneamino) piperidyl-1-oxyl]. Structures of Ph and 4-Me-Ph derivatives were also deter mined at 100 K. Some of these crystals have been revealed to show intermole cular ferromagnetic interactions at an extremely low temperature. Structura l features of crystals of Ph, 4-MeS-Ph and 3-Py derivatives, which show fer romagnetic interactions, are very similar to each other. In these crystals, O atoms are arranged to form a sheet. The ferromagnetic interactions are c onsidered to be transferred through O ... H van der Waals' interactions of the B-H atoms of the neighboring TEMPO rings within the sheet. Between O .. . O sheets, the aryl groups are arranged in a herringbone manner. The cryst al structure of the 4-Me-Ph derivative, which shows an antiferromagnetic in teraction, is also pseudo-isostructural with those of Ph, 4-MeS-Ph and 3-Py derivatives, while the arrangement of the aryl groups is different. The pa cking mode of the crystals of the 4-MeSO2-Ph derivative, of which the Weiss constant theta is nearly zero, is very different from those of the other d erivatives showing magnetic interactions. The fact that the crystal structu re of the paramagnetic 4-MeSO2-Ph derivative does not show the O ... O shee t structure accompanying the O ...beta-H interactions indicates that the in termolecular ferromagnetic interactions through B-H atoms within the O ... O sheet are important for these TEMPO radical crystals.