Magnetic properties of polymeric framework materials with cobalt

Single molecule magnets (SMM) are a new class of materials that have not been well studied. Metal-organic frameworks (MOFs) created with cobalt(II) metal centers have unpaired 3d electrons, which gives the frameworks interesting magnetic properties. Cobalt(II) nitrate centers can be linked together by varying numbers of 4,4´-bipyridine (bpy) ligands to create six different framework structures: four-coordinate 1D chain, five-coordinate 1D ladder, 2D bilayer, and 3D brick wall, and six-coordinate 2D square grid and 2D rhombic grid. The effects of framework structure and Co(II)-Co(II) distance on Weiss temperatures and μeff were investigated. By understanding which parameters of a framework has the biggest influence on the magnetic properties, better SMM can be designed. All Weiss temperatures were negative and indicated very weak antiferromagnetic coupling interactions between the magnetic moments of the individual Co2+ with a small relation to framework directionality. The μeff of all frameworks indicated that there was significant spin-orbit coupling, but in all the frameworks all smaller than typically found for octahedral and tetrahedral Co2+ complexes. The Co(II)-Co(II) distances had little effect on the spin-orbit coupling or the coupling of the Co2+ atomic magnetic moments