Heterocyclic N-oxide ligands: a thermochemical study of adducts with zinc, cadmium, and mercury chlorides

Abstract

The standard enthalpy (ΔH_R^⊖/kJ mol^–1) of the reaction MCl₂(s)+nL(s)→ MCl₂·nL(s) has been determined by solution calorimetry at 298.15 K, for L = pyridine N-oxide (pyo) and 2,2′-bipyridine N,N′-dioxide (bipyo) and M = Zn, Cd, and Hg: ZnCl₂·2pyo, –72.1 ± 1.4; CdCl₂·pyo, –24.7 ± 0.8; HgCl₂·pyo, –25.7 ± 0.5; ZnCl₂·1.5bipyo, –105.8 ± 1.0; CdCl₂·bipyo, –24.0 ± 1.0; and HgCl₂·bipyo, –4.5 ± 0.4. The standard enthalpies of formation of pyo (7.7 ± 0.8) and bipyo (16.6 ± 5.2 kJ mol^–1) were obtained by solution-reaction calorimetry. The standard enthalpy of formation of the adducts decreases from zinc to mercury for both series of compounds. The enthalpy of sublimation of pyo (81.9 ± 1.5 kJ mol^–1) was determined from vapour pressure measurements and that of bipyo (180 ± 10 kJ mol^–1) was estimated. The mean standard enthalpies of the metal–oxygen bond were also calculated. The results show that pyo is better ligand for these metal chlorides than is bipyo.