Porous coordination polymers based on functionalized Schiff base linkers: enhanced CO₂ uptake 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[double bond, length as m-dash]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[double bond, length as m-dash]N– to –CMe[double bond, length as m-dash]N– group. It also adsorbs water vapour at 298 K. In the case of the two isostructural 3D MOFs 3 and 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.