p-Xylylenediamine derived ligands as flexible connectors in the design of porous coordination polymers

Abstract

The use of flexible ligands in coordination polymers provides both opportunities and unpredictability. A new series of flexible ligands derived from p-xylylenediamine, which vary in the positioning of carboxylic acid functional groups, have been found to form a series of coordination polymers and a discrete coordination complex. Use of the tetra-substituted meta-carboxylic acid ligand N,N,N′,N′-tetra(3-carboxybenzyl)-p-xylylenediamine, isolated as its dihydrochloride salt (H₆M₄pxy)Cl₂, resulted in the simultaneous formation of two products from the same synthetic conditions. One of these products was a discrete cage-type coordination complex of the form [Cu₄(M₄pxy)₂(DMF)₂(OH₂)] (1), while the other product was the two-dimensional coordination polymer poly-[Cu₂(M₄pxy)(OH₂)₂] (2). Synthesis utilising the tetra-substituted para-carboxylic acid ligand, isolated as N,N,N′,N′-tetra(4-carboxybenzyl)-p-xylylenediamine dihydrochloride (H₆P₄pxy)Cl₂, resulted in the formation of multiple two-dimensional coordination polymers, poly-[Cu₂(P₄pxy)(DMF)(OH₂)]·2DMF·3H₂O (3·2DMF·3H₂O), poly-[Mn₂(P₄pxy)(DMA)₂] (4), poly-[Co₄(P₄pxy)₂(DMF)₂(μ-OH₂)₂(OH₂)₂]·6DMF·4H₂O (5·6DMF·4H₂O) and poly-[Cd₂(P₄pxy)(OH₂)₂]·DMA·3H₂O (6·DMA·3H₂O). The last of these materials, a three-dimensional coordination polymer, was found to adsorb approximately 32 cm³ g⁻¹ STP of CO₂ at atmospheric pressure at 273 K.