Magnetic coupling and slow relaxation of magnetization in chain-based Mn^II, Co^II, and Ni^II coordination frameworks

Three isomorphous coordination polymers based on the chain with triple (μ-1,1-N₃)(μ-1,3-COO)₂ bridges have been synthesized from a new zwitterionic dicarboxylate ligand [L^- = 1-(4-carboxylatobenzyl)pyridinium-4-carboxylate]. They are of formula [M(L)(N₃)]_n·3nH₂O [M = Mn^II, Co^II, and Ni^II]. In these compounds, the mixed-bridge chains are linked into 2D coordination networks by the N-benzylpyridinium spacers. The magnetic properties depend strongly on the nature of the metal center. The magnetic coupling through (μ-1,1-N₃)(μ-1,3-COO) ₂ is antiferromagnetic in the Mn^II compound but ferromagnetic in the Co^II and Ni^II analogues. Magnetostructural analyses indicate that the magnitude of the magnetic coupling can be correlated to the M-N-M angle of the azide bridge and the average M-O-C-O torsion angle of the carboxylate bridge. As the values of these parameters increase, the antiferromagnetic coupling for Mn^II decreases while the ferromagnetic coupling for Co^II increases. With strong magnetic anisotropy, the Co^II compound behaves as a single-chain magnet showing hysteresis and Glauber-type slow dynamics probably in the infinite-chain region, with Δ_τ/k = 86 K, Δ_ζ/k = 26 K, and Δ_A/k = 34 K. With weaker anisotropy, the Ni^II species shows slow relaxation of magnetization at much lower temperature.