Tuning the magnetic properties of transition metal MOFs by metal–oxygen condensation control: the relation between synthesis temperature, SBU nuclearity and carboxylate geometry

Abstract

Five novel metal organic frameworks, belonging to three structural types, have been obtained hydrothermally. Three of them, with formula [M₃(hfipbb)₂(OH)₂(H₂O)] (M = Mn, Co, Ni; hfipbb = 4,4′-(hexafluoroisopropylidene)bis(benzoate) dianion) belong to the first structural type. The crystal structure of the Mn-based compound was obtained from single crystal synchrotron X-ray diffraction data, while the isostructural compounds of Co and Ni were studied by means of Rietveld analysis using powder X-ray diffraction data. The magnetic behavior of the compounds with M = Co and Ni is characterized by the onset of long-range ferromagnetic order at very low temperature with Curie temperatures near 8 and 2.5 K, respectively. Such spontaneous magnetization seems to be preceded by low-dimensional magnetic interactions, due to the nature of the secondary building units (SBUs) of the structure, which are triple chains built by M₃O₁₄ units. Structural and topological analyses of [Mn₂(hfipbb)₂(H₂hfipbb)] and [Co₂(hfipbb)₂(H₂hfipbb)] (2 and 3 structural types, respectively; H₂hfipbb = 4,4′-(hexafluoroisopropylidene)bis(benzoic acid)) demonstrate that the packing of the rod-shaped SBUs in 2 prevents the framework interpenetration giving rise to a bnn net whereas 3 is a 2-fold interpenetrated pcu net with isolated paddle-wheel clusters as SBUs pcu. Concerning the magnetic behavior of 2 and 3, antiferromagnetic interactions observed at very low temperature (magnetic susceptibility maximum at 2.8 and 18 K, respectively) are confined to the secondary building units.