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 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.

Unprecedented high-temperature CO2 selectivity in N2-phobic nanoporous covalent organic polymers

“…As the aromatic rings present in the azoCOPs cannot be the reason for N 2 phobicity 19,20 , we studied the effects of the -N ¼ N-moieties in the framework on the near rejection of N 2 at higher temperatures, 323 K, as well as the temperature dependence of the increased selectivity. Incidentally, similar observations have been noted in a very recent report 49 , where azo-bridged linkers are compared with ethylenic ones in crystalline metal-organic frameworks (MOFs) for CO 2 uptake. The authors, however, ignored the obvious N 2 -phobic behaviour of azo-MOFs when N 2 uptakes are compared.…”

Photoactive and Physical Properties of an Azobenzene-Containing Coordination Framework

“…In addition to the findings of photoresponsive function, research has suggested that MOFs incorporating azobenzene ligands display an increased affinity for CO 2 , [44] and that azobenzene exhibits a degree of N 2 phobicity that enhances its selectivity for CO 2 over N 2 . [45] Herein, we detail the synthesis and characterisation of a new coordination framework incorporating the novel azo-containing ligand 5-((4-tert-butyl)phenylazo)isophthalic acid.…”

Solvent-Mediated Synthesis of M(II)-Coordination Polymer Part 1: Crystal Structure of Poly(1,2-di(4-pyridyl)ethylene-k2N,N')-bis(1,4-benzenediacetato-k4O,O′,O′′,O′′')zinc(II)], C22H18ZnN2O4

“…Versatile carboxylate ligands derived from 1,4-benzenedicarboxylic acid, 1,3,5-benzenetricarboxylic acid, 1,2,4,5-benzenetetracarboxylic acid, or pyridine-2,4-dicarboxylic acid are commonly used owing to their abundant carboxylate groups possessing high affinity to metal cations [7][8][9]. Other functionalized ligands such as 1,2-bis(4-pyridyl)ethylene, 1,4-benzenediacetic acid, 4,4 -bipyridylethene, or 3-nitro-1,2-benzenedicarboxlic acid have also been extensively used under diverse synthetic conditions to prepare MOFs [10][11][12][13][14][15][16][17], which are used in several applications [18][19][20][21][22]. The interest in the use of phenylenediacetic acid as ligand in the synthesis of coordination polymers stems from the presence of the flexible −CH 2 spacer that enables the −CO 2 − units to freely bend and rotate to meet the requirements of the coordination geometries of metal ions in the assembly process [23].…”