Field-Dependent Magnetic Behaviour in Mn^II(dicarboxylate)-(bipyridyl)-type 3D Metal–Organic Frameworks with Interpenetrated Structures

Three new three-dimensional (3D) Mn^II based metal-organic frameworks (MOFs) [{Mn(tda)(bpy)}₄⋅2CH₃CN]_n (1), [Mn(tda)(bpe)⋅xH₂O]_n (2) and [Mn(tda)(4-bpmh)⋅2CH₃CN]_n (3) have been solvothermally synthesized using MnCl₂⋅4H₂O, thiophene-2, 5-dicarboxylic acid (H₂tda) and different pyridyl N,N′-donor spacer ligands [bpy = 4,4′-bipyridyl, bpe = 1,2-bis-(4-pyridyl)ethylene and 4-bpmh = N,N′-bispyridine-4-yl-methylene-hydrazine]. Structural study reveals twofold interpenetrated 3D pillar-layer structures for all three complexes, constructed from two-dimensional (2D) Mn-carboxylate layers and neutral bipyridyl pillars. The 2D layers represent continuous extension of dinuclear [Mn₂(CO₂)₄N₄] nodes inter-connected via the tda ligands. Topological characterization represents a twofold interpenetrated 6-connected uninodal net with pcu α-Po primitive cubic topology. Variable temperature magnetic measurements in the three complexes reveal the presence of similar antiferromagnetic interactions between neighbouring Mn^II ions, mediated through syn-syn carboxylate bridges. However, temperature dependent susceptibility plots taken at different magnetic fields show that the antiferromagnetic interactions observed in the complexes at lower magnetic fields are overcome by some inter-cluster interactions at higher magnetic fields. This interesting feature has also been perceived from field-dependent magnetization measurements and the hysteresis data.