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.

为系统探究位置异构配体对其配合物结构与磁学性质的影响，我们以三种位置异构联吡啶配体（4,4′-bpt、3,4′-bpt与3,3′-bpt）及苯二羧酸根阴离子为配体，合成了十种二价钴配合物，具体为：[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）与[Co(p-BDC)(3,3′-bpt)₂(H₂O)₂]·2H₂O（10）。其中，o-BDC为1,2-苯二羧酸根阴离子，3-Cl-o-BDC为3-氯-1,2-苯二羧酸根阴离子，m-BDC为1,3-苯二羧酸根阴离子，5-NO₂-m-BDC为5-硝基-1,3-苯二羧酸根阴离子，p-BDC为1,4-苯二羧酸根阴离子。结构分析表明，苯二羧酸根阴离子可通过多样的配位模式调控二价钴离子形成一维链或二维层；此类结构进一步通过不同方向的异构bpt连接体拓展，最终得到多种配位聚合物，例如对应配合物1~5的二维正方形4⁴-sql拓扑层、对应配合物6的三维氯化铯型拓扑网络、对应配合物7的二维蜂窝状6³-hcb拓扑层、对应配合物8的一维双链，以及对应配合物9~10的一维修饰链。本研究表明，在基于相同二价钴中心的这类网络结构构筑中，bpt配体的异构效应具有显著影响。此外，这些配合物展现出各异的磁学行为：在配合物1~3中，尽管二价钴离子具有相似的结构特征，但配合物2相较于1和3更易观测到自旋倾斜现象，这表明bpt异构体中吡啶基的不同取向对配合物的磁学性质具有深远影响；在均由不对称3,4′-bpt配体桥联的配合物4、5与7中，配合物7相较于4和5更易观测到自旋倾斜现象，原因在于4和5中的相邻二价钴离子具有反演对称中心，且配合物4中存在近似各向同性的二价钴离子。另外，本研究还对配合物6和8的磁学行为进行了探究，结果表明其存在反铁磁相互作用。