Thermochemical data on adducts of cyclic ureas and copper chloride: the resemblance to biological systems

Adducts of the general formula CuCl2·4L (L: ethyleneurea (eu), ethylenethiourea (etu) and propyleneurea (pu)) were synthesized and characterized by elemental analysis, infrared spectroscopy, thermogravimetry and calorimetry. The infrared results showed that eu and pu coordinate through carboxylic oxygen atoms, whereas etu uses the nitrogen atom to bond the cation. Thermal degradation of adducts starts at 130, 160 and 140°C, respectively, and is reflected by a one stage mass loss. Decomposition temperatures correlate, to some extent, with metal–ligand bond strength. The standard enthalpies of the reaction: CuCl2(c)+4L(c)=CuCl2·4L(c) in the condensed phase (ΔrHmθ) were determined by reaction–solution calorimetry. The following values were obtained: −42.50±0.92; −48.76±0.66 and −43.64±0.51 kJ mol−1 for eu, etu and pu adducts, respectively. Using ΔrHmθ values and auxiliary enthalpies of sublimation of copper chloride and adducts, the enthalpies of decomposition (ΔDHmθ), lattice enthalpies (ΔMHmθ), enthalpies of reaction in the gaseous phase (ΔgHmθ) and the mean metal–ligand bond enthalpies (D〈M–L〉) were calculated to be: ΔDHmθ=377.3±7.7;518.0±8.4;400.8±10.0; ΔMHmθ=552.0±7.7;692.7±8.5;575.5±10.1; ΔgHmθ=468.3±8.0;575.4±8.8;486.2±10.4 and D〈M–L〉=117.1±2.0;143.8±2.2;121.6±2.6, for eu, etu and pu adducts, respectively.