Heterocyclic N-oxide ligands: a thermochemical study of adducts with zinc, cadmium, and mercury chlorides
Abstract
The standard enthalpy (ΔHR⊖/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.