Seven rings, eight-coordinated binuclear chelate: [Ca₂(SCZ)₄(NO₃)₂(H₂O)₂]Cl₂ (SCZ = semicarbazide)

A novel eight-coordinated nitrate alkaline-earth metal coordination compound [Ca₂(SCZ)₄(NO₃)₂(H₂O)₂]Cl₂ (1, SCZ = semicarbazide) was synthesized and characterized by elemental analysis and FTIR spectroscopy. Single-crystal X-ray diffraction analysis revealed that 1 crystallizes in the monoclinic space group P2₁/n. In 1, Ca²⁺ was octacoordinated with three SCZ groups, one nitrate anion and a water molecule. Noticeably, SCZ presents typical bidentate coordination and unusual tridentate coordination modes, which provides a new insight into the SCZ based coordination compounds. The nitrate anion is also coordinated with the central calcium in a bidentate fashion. Differential scanning calorimetry (DSC) indicated that 1 is thermally stable with an onset decomposition temperature of more than 600 K. The non-isothermal kinetic parameters were calculated by the Kissinger's method and Ozawa-Doyle's method. The critical temperature of thermal explosion, the entropy of activation (ΔS^≠), the enthalpy of activation (ΔH^≠), and the free energy of activation (ΔG^≠) were calculated as 646 K, 202.7 J K^-1 mol^-1, 314.9 kJ mol^-1, and 185.6 kJ mol^-1, respectively. Sensitivity tests revealed that 1 is insensitive to mechanical stimuli.