Two new three-dimensional (3D) Co(II)- and Cu(II)-azido frameworks, [Co₂(N₃)₄(bpym)₂]n (1) and [Cu₂(N₃)₄(bpym)]n (2), were successfully synthesized by introducing a semi-flexible N-donor ligand, 5,5'-bipyrimidin (bpym), with different bridging modes and orientations. Compounds 1 and 2 were structurally characterized by X-ray crystallography, IR spectroscopy, thermogravimetry and elemental analysis. Compounds 1 and 2 are 3D pillared-layer frameworks with double end-on (EO) azido bridged dinuclear motifs, [M₂(EO-N₃)₂]. In Compound 1, the bpym ligands show trans μ₂-bridging mode and the role as pillars to connect the Co(II)-azido layers, composed of [Co₂(EO-N₃)₂] motifs and single end-to-end (EE) azido bridges, to a 3D network with BN topology. In contrast, in 2, the bpym ligand adopts a twisted μ₄-bridging mode, which not only connects the adjacent [Cu₂(EO-N₃)₂] units to a layer, but also functions as a pillar for the layers of the 3D structure. The structural diversities between the two types of architectures can be attributed to the coordination geometry preference of the metal ions (octahedral for Co2+ and square pyramidal for Cu2+). Magnetic investigations revealed that Compound 1 exhibits ferromagnetic-like magnetic ordering due to spin canting with a critical temperature, TC = 33.0 K, and furthers the field-induced magnetic transitions of metamagnetism at temperatures below TC. Compound 2 shows an antiferromagnetic ordering with TN = 3.05 K and a field-induced magnetic transition of spin-flop at temperatures below the TN.
Keywords: coordination polymer; magnetic properties; metamagnetism; semi-flexible N-donor ligand; spin canting; spin-flop.