Co(II) Coordination Polymers: Positional Isomeric Effect, Structural and Magnetic Diversification

To systematically investigate the influence of the positional isomeric ligands on the structures and magnetic properties of their complexes, we synthesized ten Co(II) complexes with three positional isomeric dipyridyl ligands (4,4′-bpt, 3,4′-bpt and 3,3′-bpt), as well as the phenyl dicarboxylate anions, namely, [Co(o-BDC)(4,4′-bpt)(H₂O)] (1), [Co(o-BDC)(3,3′-bpt)(H₂O)] (2), [Co(3-Cl-o-BDC)(4,4′-bpt)(H₂O)] (3), [Co(3-Cl-o-BDC)(3,4′-bpt)] (4), [Co(m-BDC)(3,4′-bpt)] (5), [Co(m-BDC)(3,3′-bpt)]·H₂O (6), [Co₂(5-NO₂-m-BDC)₂(3,4′-bpt)(H₂O)₄] (7), [Co(5-NO₂-m-BDC)(3,3′-bpt)]·H₂O (8), [Co(p-BDC)(3,4′-bpt)₂(H₂O)₂]·2H₂O (9), and [Co(p-BDC)(3,3′-bpt)₂(H₂O)₂]·2H₂O (10) (o-BDC = 1,2-benzenedicarboxylate anion, 3-Cl-o-BDC = 3-Cl-1,2-benzenedicarboxylate anion, m-BDC = 1,3-benzenedicarboxylate anion, 5-NO₂-m-BDC = 5-NO₂-1,3-benzenedicarboxylate anion, p-BDC = 1,4-benzenedicarboxylate anion). Structural analysis reveals that the phenyl dicarboxylate anions display versatile coordination modes to manage the Co(II) ions to form 1-D chains or 2-D layers, which are further extended via the isomeric bpt connectors in different directions, to give rise to a variety of coordination polymers, such as 2-D square 4⁴-sql layer (for 1−5), 3-D CsCl net (for 6), 2-D honeycomb 6³-hcb layer (for 7), 1-D double chain (for 8), and 1-D decorated chain (for 9−10). This work indicates that the isomeric effects of the bpt ligands are significant in the construction of these network structures, based on same Co(II) centers. Furthermore, these compounds exhibit different magnetic behaviors. In 1−3, although Co(II) ions have the similar structural characteristic, the phenomenon of spin-canting in 2 can be observed more easily than 1 and 3, which may reveal that the different orientations of the pyridyl groups in bpt isomer have a profound impact on the magnetic properties of the solids. In 4, 5, and 7, which all are bridged by asymmetric 3,4′-bpt ligand, spin-canting in 7 can be observed more easily than 4 and 5 because the adjacent Co(II) ions are related to a inversion center in 4 and 5. Moveover, there are approximate isotropic Co(II) ions in 4. In addition, the magnetic behavior of compounds 6 and 8 was also studied and indicated the existence of antiferromagnetic interactions.