Magneto–structural 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

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Abstract
Magnetic measurements and X-ray crystal analyses were carried out on iodide salts of p-N-alkylpyridinium α-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+·I– consists of a radical dimer and the iodide ion is out of the plane of the pyridinium ring. 2+·I–, 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+·I– and 4+·I– 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.