Modulating Porosity through Conformer-Dependent 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.