Calorimetric study of nickel complex compounds with tmen as a ligand: determination of the energetic parameters for the delta-lambda conformational change of the ethylenediamine framework

Heat capacities of crystalline [Ni(acac)(tmen)(H2O2)(2)] and [Ni(acac)(2)(t men)] were measured with an adiabatic calorimeter in the range 10-300 K, wh ere acac and tmen are acetylacetonate and N,N,N',N'-tetramethylethylenediam ine, respectively. Glass transitions were found at (108 +/- 1) and (126 +/- 1)K for the respective compounds. They were interpreted as due to the free zing-in of disorder between the delta and lambda conformations of tmen moie ty. The energy difference (Delta epsilon) between the two conformations and the activation energy (Delta epsilon (a)) for the conformational change we re 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) an d (39 +/- 1) kJ mol(-1) for the molecular compound, respectively. It is dis cussed that he is affected both by the coordination structure of complex an d by the presence/absence and species of counter anion while that he, is de termined rather by the interaction of tmen with the other ligands within th e complex. X-ray diffraction technique was applied to crystalline [Ni(acac) (tmen)(H2O)(2)]ClO4 to confirm the above interpretation, and the details ar e given as an appendix. (C) 2001 Elsevier Science Ltd. All rights reserved.