Porous coordination polymers based on functionalized Schiff base linkers: enhanced CO2 uptake by pore surface modification

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 M-2(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 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.