Current Inorganic Chemistry (Discontinued) - Volume 4, Issue 1, 2014

Various experimental tools have been applied for characterization of materials and evaluation of their functionalities. Among them, thermodynamic methods are unique in the sense that the energetic and entropic aspects inherent in materials can be directly observed. Physical quantities obtained from thermodynamic measurements reflect macroscopic aspects of materials. However, because those quantities are closely related to the microscopic energy schemes of all kinds of molecular degrees of freedom in a statistical manner, one can gain detailed knowledge on the microscopic level on the basis of precise thermodynamic investigations. Among them, heat capacity calorimetry is an extremely useful tool to investigate thermal properties of materials, in particular at low temperatures. For correct understanding of functionality of materials, it is crucially important to complementarily adopt both spectroscopic and/or structural methods leading to microscopic aspects of materials and thermodynamic methods revealing energetic aspects. The aim of this thematic issue is to review calorimetric studies on functional inorganic materials to show the important roles played by thermodynamic studies. The topics picked up in Part I of this thematic issue are the following four functional materials: (1) Host-guest systems under high pressure: Host-guest systems have potential possibility for selective storage of the guest molecules. (2) Mixedvalence metal complexes: Electron transfer between the mixed-valence metal ions, such as M(II)M(III)X, M(II)M(III)(C2O4)3 or Fe(II)Fe(III)(C2O2S2)3, provide a variety of phase transitions in which the magnetic and electronic properties are dramatically altered. (3) Materials showing negative thermal expansion: Materials that exhibit negative thermal expansion behavior are of interest because of the potential to couple them with materials that exhibit positive thermal expansion to create composites whose thermal expansion properties can be finely tuned. (4) Metal oxide nanocrystals: Metal oxide nanocrystals have potential for applications in many technologies including solar energy conversion, batteries, and ductile ceramics. These materials have properties that are highly dependent on the physical dimensions or grain size. Thermodynamic method can detect some of these peculiar aspects. Papers concerning calorimetry of other functional materials will be published in the forthcoming Part II and Part III. We would like to thank Prof. Yann Garcia, the Editor-in-Chief of Current Inorganic Chemistry, who invited us to edit a thematic issue concerning calorimety of inorganic materials. We would like to acknowledge all the authors who accepted our invitation to contribute to this thematic issue, as well as the reviewers who invested their valuable time to ensure the high scientific quality of all contributions.

The thermal properties and transition behavior of the host-guest inclusion compounds: urea, thiourea, Dianin’s compound, clathrate hydrates and hydroquinone have been reviewed. In particular, we summarize their thermal conductivities, heat capacities and transitions at high pressures. Two of the systems: urea inclusion compounds and clathrate hydrates, show unusual glass-like thermal conductivity κ, i.e. their κ is low and only weakly dependent on temperature despite their crystalline structure. Moreover, results for κ of Dianin’s compound with guests such as ethanol and CCl4 indicate a change from glass-like κ at atmospheric pressure to crystalline-like κ at elevated pressure, whereas κ of hydroquinone and thiourea inclusion compounds appears not to have been studied. Despite the technological and fundamental importance of the unusual glass-like κ, e.g. the use of inclusion compounds as structural model systems for finding improved thermoelectrical materials, the origin of the glass-like κ is not established. More specifically, the commonly employed rattling model, in which rattling guest motions cause resonance scattering of the acoustic host phonons, has recently been challenged, and we discuss alternative models. Heat capacity studies of these compounds reveal numerous transitions, which are associated with guest and host disorder-order transitions upon cooling and pressurization. A transition in hydroquinone may be of second order, or have only a small first-order component, which can explain discrepancies in the observed transition behavior. On pressurization at low temperatures, clathrate hydrates collapse to an amorphous state, which appears to be a glassy state of a water solution with perfectly spaced solute molecules.

Oxalate-ligand based two-dimensional bimetallic molecular materials of general formula A[M(II)M(III) (C2O4)3] have drawn much research interest in the field of molecular magnetism. In this article, these materials and their magnetic properties have been introduced. The heat capacity calorimetric studies on these magnetic materials reported so far have been revisited to highlight the scope of application as well as the importance of the thermodynamic technique - heat capacity calorimetry in the field of molecular magnetism.

In this paper, various thermodynamic approaches to the structural phase transitions and the phonon properties of the materials showing negative thermal expansion (NTE) in the wide temperature range are briefly reviewed through three families of the NTE compounds; ZrW2O8, ZrV2O7, A2B3O12 (A = trivalent cation, B = W or Mo) families. As for the structural phase transitions, the thermodynamic approaches provide us with elucidation of their mechanism from an entropic point of view. The NTE is closely related to phonon properties. Heat capacity measured by calorimetry enables us to estimate not only a Gruneisen function but also a distribution of phonon density of states, leading to specifying the phonon modes with negative mode-Gruneisen parameters. It is demonstrated that the thermodynamic approach is powerful to research the properties of the NTE materials.

Porous metal oxide nanoparticles is a new class of material of great scientific and technological importance with a wide range of applications. In this article, we briefly review the synthetic methods and thermodynamic properties of such materials. We compare and summarize common synthetic routes of such materials including solid-state, solutionphase (co-precipitation, sol-gel, microemulsion, solvothermal/hydrothermal, non-aqueous), and vapor-phase methods. As for the thermodynamics of porous metal oxide nanoparticles, we review experimental determinations, mainly by calorimetry, on surface and interfaces energetics. The interplay among particle size, surface area, morphology, surface stabilizer, phase stability, and redox potentials is discussed.

Over the last two decades, we have proposed several types of orally active insulin–mimetic mononuclear zinc(II) complexes based on several coordination environments. Herein, we demonstrate the hypoglycemic activity and potentially low testicular toxicity of a binuclear Zn2+/VO2+ complex of N,N'-bis(salicylidene)ethylenediamine ([(VOZn)fsaen]). The chemical structure of this complex was evaluated using an extended X-ray absorption fine structure (EXAFS) technique. The in vivo hypoglycemic effect of [(VOZn)fsaen] was evaluated using type-2-like diabetic model mice treated by daily oral administrations. The hetero-binuclear complex [(VOZn)fsaen] exhibited glucose-lowering effect without triggering morphological changes in the testes, despite the recognized potentially significant toxicity of oxovanadium(IV) (VO2+) complexes in the testes.

A series of meso-substituted phenyl porphyrin derivatives were selected for investigating their photochromic behavior in several aryl chloride solutions. After ultraviolet irradiation (UVI, λ = 360 nm), the solutions’ color changed rapidly and sensitively from pink to bright green; and their pink color can be recovered gradually when the green samples are radiated under visible light (VR, λ > 500 nm) or placed around diffused light at ambient conditions. Notably this photochromic behavior can be completely stimulated by a halochromic method. A porphyrin H2TMPP 3 was selected as a typical sample for revealing this chromic mechanism, and molecular configuration for the green porphyrin chromophores was proposed based on crystal assembly of H2TPP 1. Finally the sensitive molecular recognition of H2TMPP 3 towards hydrochlorides was examined.

Three coordination polymers with triangularly secondary building units based on trinuclear metal centers and three tetrazolyl ligands were successfully assembled, and their crystal structures were finely determined by X-ray diffraction method. Crystallographic analysis indicated that in every triangular sub-construction unit, three metal ions (Co2+ or Cd2+) adopted an octahedral coordination model to be linked together by three tetrazolyl ligands and one µ3-OH group. The triangular sub-unit configuration supplied more stability during thermal decompositions.