MAGNETOSTRUCTURAL CORRELATION IN A SERIES OF IODIDE SALTS OF P-N-ALKYLPYRIDINIUM NITRONYL NITROXIDES - DEPENDENCE OF THE IODIDE-PYRIDINIUM RING INTERACTION ON THE LENGTH OF THE N-ALKYL CHAIN

Magnetic measurements and X-ray crystal analyses were carried out on iodide salts of p-N-alkylpyridinium a-nitronyl nitroxides [4-(4,4,5,5-tetramethyl-1-oxido-3-oxyl-4,5-dihydro-3H-imidazol-2'yl)-1-R-pyridinium, with R = methyl (1(+)), ethyl (2(+)), n-propyl (3(+)) and n-butyl (4(+))]. The strongly antiferromagnetic crystal of 1(+).l(-) consists of a radical dimer and the iodide ion is out of the plane of the pyridinium ring. 2(+).l(-), which is weakly antiferromagnetic, includes two crystallographically independent molecules, 2A(+) and 2B(+), each of which forms a centrosymmetric dimer. in the pyridinium ring of 2A(+) the iodides are 'out-of-plane' while for 2B(+) they are 'in-plane'. The ferromagnetic 3(+).l(-) and 4(+).l(-) have similar structures: the crystal consists of a two-dimensional (2D) layer formed by a contact between the pyridinium ring and in-plane iodides. In this series, the iodide ion changes position from out-of-plane to in-plane and the magnetism varies from antiferromagnetic to ferromagnetic. It is found that the nitronyl nitroxide with an out-of-plane iodide has a short intermolecular contact between the NO groups (type I), while that with an in-plane iodide forms a contact between the NO group and the pyridinium ring (type II). The observed magnetic behaviour can be interpreted in terms of an antiferromagnetic interaction for the type I contact and a ferromagnetic interaction for type II.