Modulation of the magnetic properties of two-dimensional compounds [NiX2(N-N)] by tailoring their crystal structure.

A series of 2-D nickel compounds with the stoichiometry [NiCl2(N-N)] has been prepared, [N-N = pyrazine (1), 4,4'-bipyridine (2), trans-4,4'-azopyridine (3), trans-1,2-bis(4-pyridyl)ethylene (4), and 1,2-bis(4-pyridyl)ethane (5)]. The complex [NiBr2(4,4'-bpy)] (6) was also obtained for comparative reasons. Compound 2 is the β phase of the previously reported complex [NiCl2(4,4'-bpy)]. The syntheses of complexes 1-6 were carried out using solvothermal and microwave techniques. The compounds have been characterized by elemental analysis, infrared spectroscopy, thermogravimetry, and powder X-ray diffraction. The crystal structures of compounds 1, 4, and 5 have been solved using ab initio X-ray powder diffraction methods. Compounds 1-6 show the same arrangement, and their structures are described as layers formed by [NiX2] chains linked perpendicularly by N-N ligands. The magnetic properties of the compounds are explained as a balance between the ferromagnetic interactions along the [NiX2] chains and the antiferromagnetic interactions between chains from different layers. This work demonstrates that this balance can be tuned by the length of the N-N ligand.