Synthesis, crystal structures, and magnetism of the binuclear radical complex [N-methylpyridinium]2[Ni(tdas)2

Abstract

The novel binuclear radical complex [N-methylpyridinium]2[Ni(tdas)2]2 (tdas = 1,2,5-thiadiazole-3,4-dithiolate) has been prepared and its crystal structures determined by X-ray crystallography at three different temperatures in order to investigate the changes in coordination structure and magnetic properties with temperature. In the binuclear radical complex the two nickel ions assume a distorted pyramidal geometry and are bridged by two S atoms of different tdas anionic ligands. At room temperature, the ESR spectrum of the binuclear radical complex in polycrystalline powder and the theoretical calculations reveal a very strong antiferromagnetic interaction, leading to diamagnetic crystals. In contrast, a peak with g = 2.05 appears in the ESR spectrum of the title complex in acetonitrile solution, which indicates that a fraction of the binuclear radical complexes dissociate. The theoretical calculations also reveal that as temperature increases the antiferromagnetic coupling strength gradually decreases, yet at 150 °C the antiferromagnetic coupling strength remains as strong as 2J = −734.26 cm−1. The strong antiferromagnetic coupling strengths should be attributed to the large spin densities on the nickel atoms and the relevant bridging sulfur atoms. This study is the first to report the correlations between the structure, magnetism, and temperature of a binuclear radical nickel complex with tdas as ligand and this study is also the first report the magnetic coupling strength of radical binuclear nickel complex with tdas as bridging ligand and with N-methylpyridinium as counter cation.