Phase relation and molecular motions of mixed-ligand complex [Zn(hfac) 2(tmen)] as studied by calorimetry and 1H NMR measurement

Abstract

Heat capacities of a molecular mixed-ligand complex [Zn(hfac) 2(tmen)] were measured in the temperature range between 13 and 383 K with an adiabatic calorimeter and the proton magnetic spin–lattice relaxation times ( T 1, T 1 ρ and T 1D) were measured in the range between 130 and 330 K, where hfac and tmen denote hexafluoroacetylacetonato and N, N, N′, N′-tetramethylethylenediamine, respectively. The orientationally disordered crystalline phase, designated as phase I, was found to appear only in the supercooled meta-stable state: The temperature, enthalpy and entropy of the fusion were T fus=(370±1) K, Δ fus H=(2.2±0.1) kJ mol −1 and Δ fus S=(5.8±0.3) J K −1 mol −1, respectively. The small entropy was discussed in relation to the low globularity in the shape of the complex molecule. High- and low-temperature stable crystalline phases (II and III) were found: The temperature, enthalpy and entropy of the fusion from the phase II were T fus=(375±1) K, Δ fus H=(24.32±0.02) kJ mol −1 and Δ fus S=(65.4±0.1) J K −1 mol −1, respectively. The temperature, enthalpy and entropy of the III-to-II phase transition were T trs=(293.8±0.1) K, Δ trs H=(4.115±0.005) kJ mol −1 and Δ trs S=(14.02±0.02) J K −1 mol −1, respectively. Glass transitions were found at T g=(114±1) K in phase III and at T g=(116±1) K in the supercooled metastable phase II, respectively, and interpreted as attributed to the freezing-in of the rearrangement motion between δ and λ conformations of tmen. The energy difference between the two conformations was less than 0.25 kJ mol −1, and the activation energy for the conformational change was Δε a=(33±1) kJ mol −1. The activation energy for the C 3 reorientation of methyl groups within the ligand tmen was Δε a =(14±1) kJ mol −1.