The crystal engineering potential and challenge of dl-malic acid, a chemical chameleon for coordination polymers

The conformational flexibility of malate anions, a biomass-derived linker ligand, enables a diverse range of coordination environments that complicates its utility for crystal engineering studies. This work seeks to expand upon our understanding of the malate anion as a linker for the generation of coordination networks. We report six dl-malate coordination compounds that can be classified as 1D coordination polymers, 2D coordination networks or porous hydrogen-bonded frameworks. In addition to analysis of the crystal structures, a CSD mining study was undertaken to examine the coordination behaviour of gauche- and trans-malate conformers upon the structure of 80 reported coordination polymers that comprise malate anions. Our analysis indicates that, whereas the torsional diversity of malate anions represents a crystal engineering challenge, there are possible crystal engineering strategies for biasing malate linkers to adopt a particular conformer in order to enable this green biomass-derived linker to control a specific structural outcome for coordination polymers.