Abstract

Self-assembly of metal ions and amino acids to form coordination polymers is well known. In the past, coordination polymers have been mostly synthesized via solvothermal methods. Herein, the crystallization of coordination polymers in gels is investigated. Crystal growth in gels is dominated by diffusion and often leads to high quality single crystals. We demonstrate the crystallisation of Cu(L-Asp), Cu(D-Asp) and Cu(D/L -Asp) in agar gel which leads to coordination polymers with spherulitic morphology. The crystallization of CuAsp in gel also displayed Liesegang pattern formation. The Liesegang ring phenomena was optimized by investigating various parameters, including amino acid and Cu2+ concentrations and chirality. The coordination polymers were characterized by scanning electron microscopy (SEM), Brunauer, Emmett, and Teller (BET) surface area analysis, circular dichroism (CD), powder X-ray diffraction (PXRD) and infrared spectroscopy (IR). The PXRD of the Cu(L-Asp), Cu(D-Asp) and the Cu(D/L-Asp) were identical which indicated conglomerate formation in the case of Cu(D/L-Asp). This result was further investigated using circular dichroism and with the use of chiral additives in gels. Based on previous studies of copper coordination polymers and the structural similarity between the Cu(L-Asp) and Cu(L-Glu), a square planar geometry is proposed for Cu(D/L-Asp) with a head-to-tail arrangement.