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 ]·2H 2 O} n (M = Co ( 1 ), Ni ( 2 )) and {[M( l -mal)(bpee) 0.5 ]·H 2 O} 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 Å 2 for 1 and 2; 6.8 × 6.1 Å 2 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 (N 2, CO 2, H 2, O 2, and Ar) adsorption studies on the dehydrated frameworks of 1 and 3 show excellent selective CO 2 gas uptake at 195 K. The lesser uptake of CO 2 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 CO 2 compared to the ethylenic (−CH═CH−) group in 3 . The difference in polarity in 1 and 3 also is reflected in water sorption studies.