Modulating porosity through conformer-ependent hydrogen bonding in copper(II) coordination polymers

A new divergent ligand, N,N′-bis(4-carboxyphenylmethylene)ethane-1,2-diamine (H4L1), has been prepared in high yield and used to generate two copper(II) coordination polymer materials, poly-[Cu(H2L1)(OH2)]·H2O (1) and poly-[Cu(H2L1)(OH2)]·H2O·DMF (2). Both networks possess (4,4) sheet topologies and have almost identical compositions and coordination modes. The only major difference between the compounds lies with the conformation of the chelating ethylenediamine cores; compound 1 adopts a trans-(R,R/S,S) conformation, while compound 2 exhibits a cis-(R,S) conformation. This seemingly small difference arising from variation in synthetic conditions influences the extended structures of each network through hydrogen bonding interactions, resulting in the formation of a close packed 2-fold 2D → 2D parallel interpenetrated network for 1, while the extended, non-interpenetrated structure of 2 contains aligned one-dimensional solvent channels. After solvent exchange and evacuation, compound 2 was found to adsorb approximately 35 cm3(STP)/g of CO2 at atmospheric pressure at 273 K, with a zero-loading enthalpy of adsorption of -33 kJ/mol, while adsorbing only minimal quantities of N2. These findings are a rare example of conformer-dependent porosity in otherwise geometrically similar frameworks and highlight the importance of understanding weak and fluxional secondary interactions in framework and ligand design. (Chemical Equation Presented)