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.