Porous coordination polymers based on functionalized Schiff base linkers: enhanced CO₂uptake by pore surface modification

Abstract

We report the synthesis, structural characterization and adsorption properties of three new porous coordination polymers {[Cu(Meazpy)0.5(glut)](H2O)}n (2), {[Zn(azpy)0.5(terep)](H2O)}n (3), and {[Zn(Meazpy)0.5(terep)]}n (4) [glut = glutarate, terep = terephthalate, azpy = N,N'-bis-(pyridin-4-ylmethylene)hydrazine and Meazpy = N,N'-bis-(1-pyridin-4-ylethylidene)hydrazine] composed of mixed linkers systems. Structure determination reveals that all three compounds have three-dimensional (3D) coordination frameworks bridged by dicarboxylates and Schiff base linkers. In all cases 2D dicarboxylate layers are supported by paddle-wheel M2(CO2)4 SBUs extended in three dimensions by designed Schiff base linkers. Compound 1, which has been reported in a paper earlier by our group, is a robust porous three-dimensional (3D) framework whose pore surface was found to be decorated with the -CH=N- groups of a linear Schiff base (azpy) and it showed reversible single-crystal-to-single-crystal transformation and selective CO2 uptake. By using another linear Schiff base linker Meazpy, we have synthesized compound 2 which is isostructural with 1, having an additional methyl group pointing towards the pore. Like 1 it also shows a reversible single-crystal-to-single-crystal transformation upon dehydration and rehydration. The dehydrated framework of 2 exhibits 50% enhanced CO2 uptake compared to 1. This has been achieved by the pore surface modification effected upon changing the pillar backbone from a -CH=N- to -CMe=N- group. It also adsorbs water vapour at 298 K. In the case of the two isostructural 3D MOFs 3and 4, the use of a rigid carboxylate (terephthalate) linker arrested porosity by three-fold interpenetration. We showed that the use of aliphatic dicarboxylate (glutarate) results in a non-interpenetrated framework rather than the common interpenetrated framework with aromatic dicarboxylates in mixed ligand systems.

Interpenetration in coordination polymers: structural diversities toward porous functional materials

“…Recently, we have also reported two analogous threefold Schematic of interpenetrated rigid and flexible PCPs. ylmethylene)hydrazine and Meazpy = N,N 0 -bis-(1-pyridin-4-ylethylidene)hydrazine), which differ by different pillar functionalities [58]. A {Zn 2 (COO) 4 } paddle-wheel and terep form the 2D sheet like structure and these sheets are pillared by azpy or Meazpy to form the 3D structure.…”

Computationally-Guided Synthetic Control over Pore Size in Isostructural Porous Organic Cages

“…The use of methyl groups to reduce pore size has been reported previously for both MOFs and COFs. 25 − 27 Mastalerz et al also reported a series of O-alkylated [4 + 6] cages with different cavity sizes, but the crystal packing of the O-methylated cage was found to be different from that of the unmethylated cage, and the other four alkylated analogues were not sufficiently crystalline to allow structure determination. 28 This highlights the significant difficulty in controlling the pore size of organic cages in an “isoreticular” manner.…”

Five Diverse Multidimensional Polycarboxylate–Based Mixed–Ligand Coordination Polymers with Different N,N′–Donor Ligands: Synthesis, Characterization and Their Sorption Study

“…This hysteresis can be assigned as a result of the intense adsorbate‐adsorbent interaction and also due to the slow kinetics of adsorption ,. The less CO 2 uptake of dehydrated 1 can be better explained by the interaction of quadrupolar CO 2 with the less polar N‐donor ligand moiety present in the framework of 1 and this can be also supported by the lesser pore opening possibility during structural rearrangement in 2D framework due to its bilayer nature. Similarly, the activated framework of 2 also shows nonporous nature indicating CO 2 sorption with maximum value of 15.9 cc/g at 195 K and 1 bar pressure but it exhibits significant uptake for N 2 with type–II adsorption profile (Figure ).…”

“…A moderately large hysteresis was observed in the middle of the adsorption and desorption profile of CO 2 due to the interaction of azine‐functionalized moiety of the framework with the quadrupolar CO 2 molecules. In both the activated frameworks of 2 and 5 , the presence of larger pore size (6.7 × 6.2 Å) for 5 , influence higher uptake capacity of CO 2 for 5 compared to 2 and in 1 though the pore dimension is higher but due to the presence of less polar moiety inside the pore, it is unavailable for high CO 2 adsorption. Complexes 3 and 4 exhibit only surface adsorption for N 2 at 77 K which result type‐II (Figure S12 and Figure S13) sorption profile with moderate hysteresis for both the cases.…”