Chiral Porous Metal–Organic Frameworks of Co(II) and Ni(II): Synthesis, Structure, Magnetic Properties, and CO₂ Uptake

Four isostructural chiral three-dimensional (3D) porous pillared-layer frameworks based on Co­(II) and Ni­(II), {[M­(l-mal)­(azpy)0.5]·2H2O}n (M = Co (1), Ni (2)) and {[M­(l-mal)­(bpee)0.5]·H2O}n(M = Co (3), Ni (4)); (l-mal = l-malate dianion, azpy = 4,4′-bisazobipyridine, and bpee = 1,2-bis­(4-pyridyl)­ethylene), have been synthesized using mixed ligand systems and characterized structurally. All the frameworks are homochiral, based on the chiral l-malate dianion. The bridging of l-malate with Co­(II) or Ni­(II) forms a two-dimensional (2D) layer of {M­(l-mal)}n which is further pillared by azpy or bpee to form a 3D pillared-layer porous framework. The large rectangular channels along the crystallographic b direction (7.0 × 6.2 Å2 for 1 and 2; 6.8 × 6.1 Å2 for 3 and 4) are occupied by the guest water molecules. The binding of −OH and −COO groups of l-malate with the Co­(II) or Ni­(II) render interesting antiferromagnetic and ferrimagnetic type behavior in 1 and 2, respectively. All the frameworks show high thermal stability and guest-induced structural contraction evidenced by the temperature-dependent powder X-ray diffraction patterns. Gas (N2, CO2, H2, O2, and Ar) adsorption studies on the dehydrated frameworks of 1 and 3 show excellent selective CO2 gas uptake at 195 K. The lesser uptake of CO2 in the dehydrated framework of 3 compared to 1 has been rationalized to the different polarity of the pore surface due to the change in the functional group of the pillar module. The more polar azo (−NN−) group in 1 renders strong interaction with CO2 compared to the ethylenic (−CHCH−) group in 3. The difference in polarity in 1 and 3 also is reflected in water sorption studies.本研究通过混合配体体系合成了四种基于Co(II)与Ni(II)的同构手性三维（3D）多孔柱层框架材料，其分子式分别为{[M(l-苹果酸根)(azpy)₀.₅]·2H₂O}ₙ（M=Co(1)、Ni(2)）与{[M(l-苹果酸根)(bpee)₀.₅]·H₂O}ₙ（M=Co(3)、Ni(4)）；其中l-mal指苹果酸二阴离子（l-malate dianion），azpy指4,4'-偶氮联吡啶（4,4′-bisazobipyridine），bpee指1,2-二(4-吡啶基)乙烯（1,2-bis(4-pyridyl)ethylene）。所有产物均经混合配体体系合成，并完成了结构表征。所有框架材料均为同手性，其手性源于手性苹果酸二阴离子配体。苹果酸二阴离子与Co(II)或Ni(II)通过桥联作用形成{M(l-mal)}ₙ型二维（2D）层状结构，随后由azpy或bpee作为柱撑配体进一步构筑为三维柱层多孔框架。沿晶体学b轴方向存在较大的矩形孔道（材料1、2的孔道尺寸为7.0 × 6.2 Ų，材料3、4为6.8 × 6.1 Ų），孔道内填充客体水分子。苹果酸二阴离子上的−OH与−COO基团与Co(II)或Ni(II)配位，使得材料1与2分别表现出有趣的反铁磁与亚铁磁行为。所有框架材料均具备优异的热稳定性，客体分子诱导的结构收缩效应可通过变温粉末X射线衍射图谱得到验证。对脱水后的材料1与3开展气体（N₂、CO₂、H₂、O₂及Ar）吸附研究，结果显示二者在195 K下均表现出优异的CO₂选择性吸附性能。相较于材料1，脱水后的材料3对CO₂的吸附量更低，这一现象可归因于柱撑配体的官能团差异导致孔道表面极性不同：材料1中的偶氮（−N=N−）基团极性更强，与CO₂的相互作用显著强于材料3中的乙烯基（−CH=CH−）基团。材料1与3的极性差异同样体现在水分子吸附测试中。