Syntheses, structures, and magnetic properties of three new chain compounds based on a pentagonal bipyramidal Co(II) building block

Starting from the pentagonal bipyramidal single-ion magnet [Co(tdmmb)(H2O)(2)][BF4](2) and a series of linear bridging ligands of different lengths, three new cobalt(II) one-dimensional (1D) coordination polymers, [Co(tdmmb)(pyz)][BF4](2)center dot CH3CN center dot H2O (1), [Co(tdmmb)(apy)][BF4](2)center dot 2CH(3)CN (2), and [Co(tdmmb)(bpe)][BF4](2)center dot 3CH(3)CN (3) (tdmmb = 1,3,10,12-tetramethyl-1,2,11,12-tetra-aza[3](2,6) pyridino[3](2,9)-1,10-phenanthrolinophan-2,10-diene) (pyz = pyrazine) (apy = 4-aminopyridine) (bpe = 1,2-di(4-pyridyl) ethane)), have been synthesized and characterized structurally and magnetically. Symmetric ligands pyz and bpe result in the nearly linear chains in 1 and 3, while the asymmetric ligand apy leads to the zig-zag chain in 2. As the length of the ligands increases, the intrachain Co center dot center dot center dot Co distance also increases, i.e. 7.56, 7.70, and 13.85 angstrom for 1-3, respectively. Magnetic investigation revealed the gradual decrease in the magnetic interactions between the Co2+ ions from 1 to 3, along with the increase in the intrachain Co-Co distances. Due to the existence of the weak antiferromagnetic (AF) interaction, the SIM behavior of the starting material is surpressed in compounds 1 and 2, and they show a simple paramagnetic behavior. While for 3, field-induced slow magnetic relaxation was observed at a low temperature with an energy barrier of U-eff = 19 K. This observation suggests that the magnetic interaction between the Co2+ centers in 3 is negligible, consistent with the very long Co-Co distance in 3. These 1D compounds are rare chain compounds composed of metal centers with a nearly D-5h symmetry. Notably, compound 3 is the second example of a 1D Co(II) chain compound displaying a SIM behaviour. These studies illustrated that the magnetic interaction between the SIMs can be used to tune slow magnetic relaxation.