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

Abstract

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·CH₃CN·H₂O (1), [Co(tdmmb)(apy)][BF₄]₂·2CH₃CN (2), and [Co(tdmmb)(bpe)][BF₄]₂·3CH₃CN (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⋯Co distance also increases, i.e. 7.56, 7.70, and 13.85 Å 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.