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Cobalt(ii) complexes with different dihedral angles between the bidentate ligands show a significant variation in their magnetic relaxation behavior.

A new anionic pillared-layer cobalt(II) metal–organic framework combining 4,4′,4′′-nitrilotribenzoic acid (H3ntb) as a linker and terephthalic acid (H2bdc) as a pillar-ligand with the formula {((CH3)2NH2)2[Co3(ntb)2(bdc)]}n (JUMP-1, JUMP = Jena University Magnetic Polymer) has been solvothermally synthesized and characterized. The crystal structure of JUMP-1 was determined to belong to the orthorhombic space group Cmca. It displays a three-dimensional (3D) framework for which the topological analysis revealed a 3,8-connected net with a tfz-d topology. The 3-connected organic node is linked to an 8-connected secondary building unit composed of a linear trinuclear cobalt(II) cluster with a mixed arrangement of coordination geometries (Td–Oh–Td) leading to two-dimensional layers which are further linked by terephthalic acid to give the pillared-layer arrangement. The magnetic properties are characterized by an antiferromagnetic coupling between a central octahedrally coordinated cobalt(II) ion and two outer tetrahedral cobalt(II) centers, whereas only a very weak inter-cluster coupling is observed. Moreover, a moderate zero-field splitting is detected for the tetrahedral cobalt(II) ions. The organic counterions could successfully be exchanged by lithium ions under retention of the anionic 3D network leading to JUMP-1(Li). For both compounds JUMP-1 and JUMP-1(Li), N2 and CO2 adsorption isotherms were measured. The pore distribution analyses revealed improved microporosity for the lithium exchanged MOF.

The synthesis and magnetic properties of cobalt(II) complexes with sterically demanding Schiff-base ligands are reported. The compounds [Co(L(Br))2] (1) and [Co(L(Ph))2]·CH2Cl2 (2·CH2Cl2) are obtained by the reaction of cobalt(II) acetate with the ligands HL(Br) and HL(Ph) in a dichloromethane/methanol mixture. 1 and 2 crystallize in the space groups P21212 and P1̅, respectively. X-ray diffraction studies revealed mononuclear constitution of both complexes. For 1, relatively short intermolecular Co-Co distances of 569 pm are observed. In compound 2, a hydrogen-bonded dichloromethane molecule is present, leading to a solvent aggregate with remarkable thermal stability for which desolvation is taking place between 150 and 210 °C. Magnetic measurements were performed to determine the zero-field-splitting (ZFS) parameter D for both complexes. Frequency-dependent susceptibility measurements revealed slow magnetic relaxation behavior with spin-reversal barriers of 36 cm(-1) for 1 and 43 cm(-1) for 2 at an applied external field of 400 Oe. This observation is related to an increasing distortion of the pseudotetrahedral coordination geometry for complex 2. These distortions can be decomposed in two major contributions. One is the elongation effect described by the parameter ϵT, which is the ratio of the averaged obtuse and acute bond angles. The other effect is related to a twisting distortion of the chelate coordination planes at the cobalt center. A comparison with literature examples reveals that the elongation effect seems to govern the overall magnetic behavior in pseudotetrahedral complexes with two bidentate chelate ligands. Ab initio calculations for complexes 1 and 2 using the CASPT2 method show strong splitting of the excited (4)T2 term, which explains the observed strong ZFS. Spin-orbit calculations with the RASSI-SO method confirm the single-molecule-magnet behavior because only small transversal elements are found for the lowest Kramers doublet for both complexes.

The synthesis, structure, and magnetic properties of two mononuclear cobalt(ii) complexes [Co(LSal,2-Ph)2] (1) and [Co(LNph,2-Ph)2] (2) are reported. The utilized sterically demanding Schiff-base ligands HLSal,2-Ph (2-(([1,1'-biphenyl]-2-ylimino)methyl)phenol) and HLNph,2-Ph (1-(([1,1'-biphenyl]-2-ylimino)methyl)naphthalen-2-ol) lead to a strong distortion of the [N2O2] coordination environment in the complexes 1 and 2, which can be primarily attributed to the variation in the dihedral angle between the planes of the two chelate ligands. Magnetic susceptibility and FD-FT THz-EPR measurements as well as ab initio calculations reveal that both complexes exhibit an easy-axis type of anisotropy. For both compounds frequency-dependent ac susceptibility measurements show an out-of-phase susceptibility under applied static fields of 400 and 1000 Oe. A detailed analysis of the underlying relaxation process is given, revealing significant differences in the contributions of Orbach, Raman, and direct processes within the observed temperature range. Fitting of the magnetic data leads to a spin-reversal barrier of 49 cm-1 for 1 at an applied field of 1000 Oe. For 2 the barrier is not well defined by the analysis of the relaxation times and is, therefore, approximated by the experimental barrier derived from FD-FT THz-EPR measurements (62.8 cm-1). The results from ab initio calculations and FD-FT THz-EPR measurements show that the distortion of the coordination sphere in complexes 1 and 2 from the pseudotetrahedral towards a square-planar coordination geometry leads to an increase in both the axial (D) and the rhombic zero-field splitting (E).