Multifunctional materials based on spin transition iron-pyrazine complexes

The thesis is devoted to the synthesis and study of the physical (electrical, mechanical, and thermodynamic) properties of functional nano- and composite materials based on Fe(II) spin-crossover (SCO) complexes containing pyrazine.
In the first chapter, a literature review is provided, which briefly describes the phenomenon of spin crossover and its characteristics in a specific class of complexes ‒ the Hofmann-type clathrate analogues. It also outlines the types of functional nano- and composite materials with SCO, methods of their synthesis and characterization of properties. The main application areas of functional materials with SCO and the key scientific achievements in these areas are discussed.
The second chapter describes the experimental techniques for synthesizing SCO coordination compounds, obtaining nanoparticles, and polymer composites based on polyvinylidene fluoride (PVDF) and polymethyl methacrylate (PMMA). The equipment used for instrumental research is described.
In the third chapter, the synthesis of nanoparticles of the SCO coordination polymer [Fe(pz){Au(CN)2}2] (where pz is pyrazine) in an organic medium using Triton X-100 as surfactant and C5/C6 alcohols as cosurfactants is described. It was found that particle size can be controlled by altering the polarity of the reaction medium, which depends on the choice of alcohol (co-surfactant). X-ray powder diffraction analysis revealed that the particles consist of nanoscale crystallites, and the observed changes in the diffractograms suggest a change in symmetry as a size effect. It was demonstrated that the obtained nanoparticles retain the cooperative spin-crossover same as the bulk powder, but size reduction leads to a slight decrease in the transition temperature maintaining the completeness of the transition. IR spectra showed that Triton X-100 is incorporated into the nanoparticles, likely acting as a stabilizing agent and reducing nanoparticle aggregation.
The fourth chapter describes the external pressure effect on the electrical conductivity of [Fe(pz){Au(CN)2}2]. It was shown that the application of external pressure allows to tune the electrical conductivity of the complex. It was found that the direction of conductivity change (decrease or increase) during the transition depends on the thermal activation energy and the shift in spin crossover temperature upon pressure. Additionally, it was shown that the method of synthesis can significantly influence the electrical properties of the obtained materials.
The fifth chapter describes the synthesis and investigation of the mechanical properties of polymer composites based on polyvinylidene fluoride (PVDF) and [Fe(pz){Au(CN)2}2] with a complex content of up to 35% by mass. Changes in optical properties indicate that the spin crossover in the obtained composites is preserved. It was found that increasing the complex concentration to 20% by mass strengthens the films, but further load increase leads to a decrease of the Young's modulus due to particle aggregation. Thermomechanical analysis revealed complex behavior of the composites under different loads, and a mathematical model was developed to explain this behavior. The model considers the elastic properties of the complex, which change due to the spin crossover, and the thermal expansion of the composite components.
The sixth chapter describes the synthesis and investigation of a new spin crossover complex [Fe(pz)2(BH3CN)2]. It was found that the complex has a 2D layered structure, where Fe(II) ion networks and bridging pyrazines are connected together by dihydrogen bonds between cyanoborohydride anions and pyrazine molecules from adjacent layers. It was shown that the obtained complex exhibits a cooperative spin crossover with hysteresis at temperatures above room temperature, accompanied by significant changes in color and volume. Moreover, a polymer composite with polymethyl methacrylate (PMMA) and this complex was synthesized, and it was shown that the spin crossover properties are preserved in the obtained composite. The simplicity of synthesis, the availability of starting ligands, and the stability of the complex within the working temperature range make it a promising switchable material for various applications, such
as thermochromic sensors or markers.
The seventh chapter describes the investigation of the barocaloric effect in the complex [Fe(pz)2(BH3CN)2]. It was found that due to its small molar mass per Fe(II) center, its specific entropy change during spin transition is one of the highest known for solid-state barocaloric refrigerants, while its molar entropy change is comparable to other SCO complexes. It is suggested that the reorientation of disordered pyrazine during the spin crossover makes an additional contribution to the entropy change. Furthermore, the complex demonstrates high sensitivity of the SCO parameters to pressure, making it a highly potential solid-state refrigerant with a barocaloric effect.

Бібік Ю. С. Багатофункціональні матеріали на основі ферум-піразинових комплексів зі спіновим переходом : дис. ... доктора філософії : 102 Хімія / наук. кер. Р.Д. Лампека. Київ, 2024. 167 с.