Synthesis, crystal structure, and π–π stacking magnetism of the mononuclear radical complex [N-hydrogenpyridinium][Pd(mnt) 2 ]

The mononuclear radical anionic complex [N-hydrogenpyridinium][Pd(mnt)2] (mnt = malonitrile-2,3-dithiolate) with a new countercation has been prepared and its crystal structure determined by X-ray crystallography. In the mononuclear radical anionic complex the palladium ion assumes a slightly distorted square-planar geometry and is bridged by four S atoms of two mnt anionic ligands. There are two kinds of slipped π–π stacking interactions between adjacent mononuclear radical anionic complexes in the crystal (i.e., PS-1 and PS-2). The ESR spectra, the variable-temperature susceptibilities, and the theoretical calculations reveal a very strong antiferromagnetic interaction between the adjacent mononuclear radical anionic complexes, leading to diamagnetism of the present complex. Further, the theoretical calculations indicate that the strong antiferromagnetic strength should be attributed to the PS-1 slipped π–π stacking interaction; the large spin densities on the associated short contact atoms should be a major factor leading to the strong magnetic coupling strength. This study is the first to reveal the mechanism of the strong antiferromagnetic interaction of a mononuclear radical anionic palladium complex with mnt as ligand.