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

• Published in: The Journal of physics and chemistry of solids Vol. 60; no. 11; pp. 1787 - 1796
• Main Authors: Yoshida, T.M, Wada, K, Oguni, M, Chiba, T, Fukuda, Y
• Format: Journal Article
• Language: English
• Published: Oxford Elsevier Ltd 01-11-1999; Elsevier
• Subjects: A. Inorganic compounds; Condensed matter: structure, mechanical and thermal properties; D. Heat capacity; D. Phase transitions; Exact sciences and technology; Heat capacities of solids; Lattice dynamics; Other topics in lattice dynamics; Physics; Thermal properties of condensed matter; Thermal properties of crystalline solids; Thermodynamic properties; D. Phase transitions; A. Inorganic compounds; D. Heat capacity; Temperature dependence; Molecular motion; Enthalpy; Organic ligand; Zinc complexes; Entropy; Heat of fusion; Experimental study; Glass transition; Specific heat; Orientation disorder; NMR; Spin-lattice relaxation; Thermodynamic properties
• ISSN: 0022-3697; 1879-2553
• DOI: 10.1016/S0022-3697(99)00202-4

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.