Mechanochemical destruction of crystalline hydrates of cobalt and zinc acetylenedicarboxylates during dehydration

Abstract

Upon the dehydration in vacuo, crystals of cobalt and zinc acetylenedicarboxylates (CoADC•2H2O and ZnADC•2H2O) remain stable only to a certain minimum content of coordination water. Once this content is reached, gaseous decomposition products are abruptly released giving rise to chemically reactive moieties that can initiate the solid-phase polymerization of metal-containing monomers. During the dehydration, stretching and bending bands of carboxyl groups become broader and are shifted, which is indicative of the appearance of a mechanical strain in the crystals. Besides, the removal of one water molecule from the coordination sphere of Zn2+ leads to a strong shift of the Zn—O stretching band to lower frequencies. At the moment of an extensive release of gaseous products (CO2 and H2O), the deformation gradients increase so that narrow reflections of the crystalline phase are absent in the X-ray powder diffraction pattern. The development of strains in the crystals is facilitated also by the accumulation of free CO2 molecules in the solid phase, as evidenced by the appearance of the bands at 2338 and 655 cm–1 in the IR spectra. The energy of dehydration of crystalline hydrates estimated by quantum chemical calculations (DFT) is 150—200 kJ mol–1. This high energy can lead to mechanochemical activation of the destruction of the crystal and coordination structures of CoADC•2H2O and ZnADC•2H2O upon their dehydration.