Calorimetric study of nickel complex compounds with tmen as a ligand: determination of the energetic parameters for the δ–λ conformational change of the ethylenediamine framework

Abstract

Heat capacities of crystalline [Ni(acac)(tmen)(H 2O) 2]I and [Ni(acac) 2(tmen)] were measured with an adiabatic calorimeter in the range 10–300 K, where acac and tmen are acetylacetonate and N, N, N′, N′-tetramethylethylenediamine, respectively. Glass transitions were found at (108±1) and (126±1) K for the respective compounds. They were interpreted as due to the freezing-in of disorder between the δ and λ conformations of tmen moiety. The energy difference (Δ ε) between the two conformations and the activation energy (Δ ε a) for the conformational change were determined by the analyses of the calorimetric data to be (0.87±0.05) and (32±1) kJ mol −1 for the iodide compound, and (1.58±0.05) and (39±1) kJ mol −1 for the molecular compound, respectively. It is discussed that Δ ε is affected both by the coordination structure of complex and by the presence/absence and species of counter anion while that Δ ε a is determined rather by the interaction of tmen with the other ligands within the complex. X-ray diffraction technique was applied to crystalline [Ni(acac)(tmen)(H 2O) 2]ClO 4 to confirm the above interpretation, and the details are given as an appendix.