Two Interpenetrated Cobalt(II) Metal-Organic Frameworks with Guest-Dependent Structures and Field-Induced Single-Ion Magnet Behaviors

Two cobalt(II) metal-organic frameworks with 2-fold vertically interpenetrated (4,4) grids, namely, [Co(bpg)(2)(SCN)(2)]center dot 3MeOH (1) and [Co(bpg)(2)(SCN)(2)]center dot 2DMF (2) (bpg = meso-alpha,beta-bi(4-pyridyl) glycol), were prepared under different conditions. The two complexes crystallize in different space groups (orthorhombic Pccn for 1, and tetragonal (P) over bar 42(1)c for 2) with different guest molecules. Although their structures are very similar, different guest molecules subtly change the coordination environments of the Co-II centers, the intergrid supramolecular interactions (hydrogen bond), and the dihedral angles of those two sets of interpenetrated grids. Magnetically, these modifications of the structures lead to the changes on their magnetism, especially the dynamic magnetic properties at low temperatures. Both compounds exhibit slow magnetic relaxation under an external field. For 1, we could not observe the peak maxima of the ac susceptibilities in our magnetometer at the lowest temperature and highest frequency because the relaxation time is never slow enough. In contrast for compound 2, the estimated energy barrier is significantly higher than that of 1, leading to much slower magnetic relaxation. The difference of magnetic properties between 1 and 2 illustrates the role of the metal-organic frameworks on the development of the tunable single-ion magnets and the prominent guest effect in the MOF based SIMs.