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

Abstract

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 Ų for 1 and 2; 6.8 × 6.1 Ų 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.