Insight into Structural Demand for Cold Crystallization of a Small Molecule. A Case Study for Schiff Base Compounds that Exhibit Prototropic Tautomerization

Katsunori IWASE, Yasuhiro TOYAMA, Isao YOSHIKAWA, Yasuhisa YAMAMURA, Kazuya SAITO and Hirohiko HOUJOU

To elucidate the relationship between prototropic tautomerism and the cold crystallization phenomenon, o-hydroxy aryl Schiff base compounds derived from three different toluidine isomers were examined. Comparing their thermal behavior, we confirmed that cold crystallization occurred to give a different crystalline phase compared to the initial state. To elucidate the mechanisms of the cold crystallization of the materials, their thermal, structural, and energetic properties were investigated. The DFT calculation of the molecules in their OH- and NH-forms suggested that the NH-form has higher molecular flexibility, which may prevent normal crystallization upon cooling, compared to the OH-form. Spectroscopic analyses implied that the molecules are present as a mixture of the OH- and NH-forms, and that they efficiently crystallize when a preferable OH/NH ratio is achieved. For this molecular system, we concluded that the equilibrium between NH- and OH-forms controls the supercooling of the liquid as well as nucleation and crystal growth.
(Bull. Chem. Soc. Jpn., 91, 669-677 (2018))

Structural Isomerization and Cold Crystallization of Bis[1-(2-propyl)iminomethylnaphthalen-2-olato]nickel(II) by Thermal Analysis, X-ray Diffraction and FT-IR

Katsunori IWASE, Isao YOSHIKAWA, Hirohiko HOUJOU, Yasuhisa YAMAMURA and Kazuya SAITO

Two coordination isomers, i.e., square planar and pseudo-tetrahedral forms of the title compound were crystallographically identified and discussed in relation to their thermal behavior revealed by XRD-DSC and FTIR-DSC. Upon the fusion, the coordination geometry partly changes, resulting in the liquid, which is an equilibrium mixture of molecules with the two coordination geometries. In this system, the mixed nature of liquid, in addition to molecular flexibility, is responsible for realization of large supercooling and glass transition, both of which enables the cold crystallization.
(Bull. Chem. Soc. Jpn., 88, 989-995 (2015))

Cold Crystallization in Schiff-Base Nickel(II) Complexes Derived from Three Toluidine Isomers

Katsunori IWASE, Yuta NAGANO, Isao YOSHIKAWA, Hirohiko HOUJOU, Yasuhisa YAMAMURA and Kazuya SAITO

To test metal complexes as potential compounds for heat-storage materials, two Schiff-base Ni(II) complexes derived from two differently N-substituted toluidine isomers were examined. Comparing the behavior with that of a similar complex previously reported, it was found that the cold crystallization was realized in polymorphic complexes. The activation energy for crystallization estimated by using differential scanning calorimetry was well correlated to the ease of supercooling necessary for the cold crystallization. DFT calculations of molecular energy suggested that the cold crystallization phenomenon is closely related to a molecular flexibility that prevents normal crystallization during cooling. The molecular flexibility controls the ease in the supercooling of liquid, and nucleation-growth process of crystallization by altering the activation energy for crystallization.
(J. Phys. Chem. C, 118, 27664-27671(2014))

Cold Crystallization in Bis[1-(3-methylphenyl)iminomethylnaphthalen-2-olato]nickel(II) Studied by Thermal Analysis and X-ray Diffraction

Katsunori IWASE, Hirohiko HOUJOU, Yasuhisa YAMAMURA and Kazuya SAITO

Bis[1-(3-methylphenyl)iminomethylnaphthalen-2-olato] nickel(II) was synthesized to see a potential use of this type of compounds as heat storage materials utilizing cold crystallization. Its thermal, structural and energetic properties were investigated by thermal analysis, X-ray diffraction and DFT calculation. Some principles in designing complexes for this purpose are deduced.
(Chem. Lett., 42, 1040-1042 (2013))

Thermodynamic Relationship between Structural Isomers of the Thermochromic Compound Bis(N-Isopropyl-5,6-benzosalicylideneiminato)nickel(II)

Qi WANG, Akira TAKEUCHI, Yasuhisa YAMAMURA, Kazuya SAITO, Wasuke MORI and Michio SORAI

A pair of structural isomers was isolated at room temperature for the thermochromic nickel complex bis(Nisopropyl-5,6-benzosalicylideneiminato)nickel(II); one is a diamagnetic green form with square-planar coordination geometry (G phase), and the other is a paramagnetic brown form with a tetrahedral geometry (B phase). However, a question as to which form is thermodynamically stable was left open. To solve this problem, thermal and magnetic properties of this complex were investigated by adiabatic heat capacity calorimetry in the 6-508 K temperature range and magnetic measurements in the 2-400 K region. In addition to the two forms previously reported, two metastable crystal forms (G' and B' phases) were found. The stable phase sequence was G phase, B phase, and then liquid upon heating. The supercooled B phase gave rise to a small phase transition with nonmagnetic origin at around 50 K. By rapidly cooling the liquid, a glassy liquid state was realized below ∼290 K. The order of thermodynamic stability at 298.15 K was revealed to be the G, B, G', and then the B' phase. The entropy, enthalpy, and Gibbs energy differences between the B and the G phases at 298.15 K were determined.
(J. Phys. Chem. B, 112, 11039-11048 (2008))

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