Exploiting the Pyrazole-Carboxylate Mixed Ligand System in the Crystal Engineering of Coordination Polymers

The utility of the pyrazole-carboxylate
mixed ligand system has
been probed with the synthesis of five new coordination polymers,
derived from bis-pyrazole ligand 4,4′-methylenebis­(3,5-dimethyl-1H-pyrazole) H2L1, large semirigid dicarboxylate coligands, and the deliberately
designed and synthesized ligand para-((3,5-dimethyl-1H-pyrazol-4-yl)­methylene)­benzoic acid H2L4. Complex poly-[Co­(H2L1)­(L2)­(OH2)]·1/2H2O 1 features the coligand (S,S)-1,4,5,8-naphthalenetetracarboxylic
diimide-N,N′-bis­(2-propionate) L2, and comprises chiral two-dimensional sheets, associating via substantial
π–π stacking interactions. Complexes 2 and 3 contain the coligand 1,4-bis­((3-carboxyphenyl)­methyl)­piperazine L3. Complex poly-[Co­(H2L1)2(L3)] 2 is a two-dimensional polymer containing octahedral Co­(II)
ions, whereas modifying the synthesis conditions gave complex poly-[Co2(HL1)2(L3)] 3, a three-dimensional α-Po structure
containing pyrazolate-bridged dimers of tetrahedral Co­(II) ions, displaying
weak antiferromagnetic coupling. Magnetic data for complex 3 fitted well to a S = 3/2 Heisenberg (−2JS1.S2) dimer model with J = −3.2
cm–1 and g = 2.25. We then prepared
the new heteroditopic ligand para-((3,5-dimethyl-1H-pyrazol-4-yl)­methylene)-benzoic acid H2L4, and the complexes poly-[Co­(HL4)2]·H2O 4 and poly-[Cu­(HL4)2]·2MeOH 5, which demonstrate for the first time the tendency of flexible
pyrazole-carboxylate coordination polymers with perfectly commensurate
linker dimensions to lead to low-dimensional assemblies. These results
give fresh insight into the structural properties of flexible bispyrazole-carboxylic
acid systems as a function of coligand dimensions, and provide new
directions for designed polymeric cluster compounds.

本研究以双吡唑配体4,4'-亚甲基双(3,5-二甲基-1H-吡唑)H2L1、大尺寸半刚性二羧酸共配体，以及经定向设计合成的配体对-((3,5-二甲基-1H-吡唑-4-基)亚甲基)苯甲酸H2L4为构筑单元，合成了5种新型配位聚合物，以此探究吡唑-羧酸酯混合配体体系的应用潜力。配合物{[Co(H2L1)(L2)(H2O)]·0.5H2O}1采用共配体(S,S)-1,4,5,8-萘四甲酰亚胺-N,N'-双(2-丙酸根)L2，其结构为手性二维片层，通过显著的π-π堆积相互作用发生缔合。配合物2与3均使用共配体1,4-双((3-羧基苯基)甲基)哌嗪L3：其中{[Co(H2L1)2(L3)]}2为二维聚合物，包含八面体配位的Co(II)离子；通过调整合成条件得到的{[Co2(HL1)2(L3)]}3则为三维α-钋型结构，其结构中存在吡唑酸根桥联的四面体Co(II)离子二聚体，表现出弱反铁磁耦合特性。针对配合物3的磁学数据拟合结果表明，其符合S=3/2的海森堡(-2JS1·S2)二聚体模型，拟合参数为J=-3.2 cm⁻¹、g=2.25。本研究后续合成了新型异双功能配体对-((3,5-二甲基-1H-吡唑-4-基)亚甲基)苯甲酸H2L4，并制备了配合物{[Co(HL4)2]·H2O}4与{[Cu(HL4)2]·2MeOH}5，首次证实了连接基尺寸匹配极佳的柔性吡唑-羧酸酯配位聚合物倾向于形成低维组装体。上述研究结果为深入理解柔性双吡唑羧酸体系的结构性质随共配体尺寸的变化规律提供了全新视角，同时为定向设计合成聚合物簇合物开辟了新的研究方向。