The monograph focuses on Fe(II) spin transition (ST) complexes for sustainable applications. It explores synthesis, properties, and tuning of ST via temperature, pressure, light, and phase changes, using heterocyclic ligands. The work proposes ST regulation via phase transitions for wax-like materials in thin layers, enabling optical bistables, piezo-sensors, chemosensors, solar absorbers, and barocaloric refrigerants, advancing green technology through molecular design.
Table of Contents
- LIST OF ABBREVIATIONS AND SYMBOLS
- INTRODUCTION
- CHAPTER 1. Literature review
- 1.1 Spin transition of the FeII 3d6 ion and the LIESST effect
- 1.2 Effect of external pressure on ST parameters (piezoelectric effect)
- 1.3 Cooperative interactions. Polymorphism
- 1.5 Interrelation between phase and spin transitions
- 1.6 Factors affecting the field strength of a ligand
- 1.7 Polymeric complex compounds. Heterometallic polycyanide complexes of FeII (Hoffmann clathrates)
- 1.8 Liquid crystal complexes with ST (metallic mesogens with ST)
- 1.9 Energy-saving “green” applications of ST compounds
- Differential sunlight absorption for smart optical systems
- Barocaloric effect of ST complexes
- 1.10 Selection of objects and formulation of research tasks
- SECTION 2. Experimental part
- 2.1 Physical methods
- 2.2 Synthesis of compounds
- 2.2.3 Synthetic procedures of precursors
- 2.2.4 Synthetic procedures for FeII complexes
- SECTION 3. Polymeric liquid crystal complexes of FeII
- 3.1 Polycyanate heterometallic two-dimensional FeII complexes based on 4-phenylpyridine
- 3.2 Two-dimensional liquid crystal polycyanometallic complexes of FeII based on 4-phenylpyridine derivatives with aliphatic substituents
- 3.3 Brief conclusions
- SECTION 4. One-, two-, and three-dimensional polymeric FeII complexes
- 4.1 Strongly cooperative ST in coordination polymers based on fluoropyrazine
- 4.2 Coordination FeII polymer based on 2,6-naphthyridine
- 4.3 One- and four-step ST with hysteresis in the polymer complex based on 3,8-phenanthroline
- 4.4 Strongly cooperative one- and two-step ST in polymers with high absorption capacity based on bis(4-pyridyl)butadiene
- 4.5 Two-dimensional coordination polymer with ST based on ionogenic heterocyclic ligand in
- 4.6 Substituted bis(1H-pyrazol-4-yl)selenides for the synthesis of FeII polymer complexes
- 4.7 Brief conclusions
- SECTION 5. Barocaloric effect of a non-hysteretic molecular ST compound at ambient temperature
- 5.1 Magnetic, calorimetric and barocaloric responced of the fluorinated thicyanate complex
- 5.2 Brief conclusions
- REFERENCES
Main Goal & Thematic Focus
This monograph aims to synthesize and establish fundamental regularities between phase transitions and spin transition (ST) parameters in new crystalline, liquid crystal, and meltable Fe(II) complexes, as well as porous Hoffman clathrate-type coordination polymers based on bridging heterocyclic ligands. The ultimate goal is to optimize these materials for applications in barocaloric and light absorption technologies for energy-saving "green" technologies.
- Development of new ligand systems with liquid crystal substituents.
- Characterization of ST properties including thermodynamic and hysteresis parameters.
- Investigation of phase behavior in liquid crystal systems and crystal structures.
- Study of photoinduced STs and associated structural transformations.
- Analysis of host-guest interactions in coordination polymers.
- Evaluation of barocaloric and light absorption potentials for energy-saving applications.
Excerpt from the Book
1.1 Spin transition of the FeII 3d6 ion and the LIESST effect
The phenomenon of thermally induced ST was first reported in the 1930s in the description of a series of FeIII dithiocarbamate complexes, for which an anomalous change in magnetic moment was observed from a value corresponding to five unpaired electrons at room temperature to that corresponding to one unpaired electron at low temperatures [1, 2]. At the same time, Linus Pauling drew attention to the unusual magnetic behavior of hydroxides of heme ferriproteins [3]. About twenty years later, Orgel, relying on ligand field theory, explained the anomalous properties of compounds by the presence of equilibrium between the two spin states of central ions [4]. Later, Griffith described the thermal equilibrium between spin states in heme proteins [5], and Martin and White published the first theoretical interpretation of the magnetic behavior of tris(dithiocarbamates) FeIII [6]. In 1964, unusual cooperative magnetic behavior of the first FeII complexes with NCS– and NCSe– anions and 1,10-phenanthroline in the 175 K region was reported [7], and soon other compounds with ST based on 3d4–7 ions.
From ligand field theory, it is known that the terms of Oh complexes with the 3d4-7 electronic configuration intersect when the critical field value Δcrit [8] is reached (Fig. 1.1). For ions surrounded by weak-field ligands (halides, water, ammonia), the main HS state is realized, and with strong-field ligands (CN−, bipyridine, phenanthroline), only the LS state is realized. For a ligand field strength close in value to the π electron pairing energy (10Dq), the realization of the spin state becomes temperature-dependent as a result of distribution over available energy levels. ST can be considered as an intraionic electron transfer, which in the case of the d6 configuration is described by the transition t2g6eg0 (1A1g) ↔ t2g4eg2 (5T2g), which is accompanied by a change in the total spin from S = 0 (LS, diamagnetic) to S = 2 (HS, paramagnetic), respectively (Fig. 1.1).
Summary of Chapters
INTRODUCTION: This section introduces the growing importance of functional molecular materials, particularly coordination complexes with spin transitions (ST), for modern technologies. It highlights the need to understand and control ST properties, focusing on Fe(II) heterocyclic complexes due to their structural diversity and potential in magnetic, optical, and dielectric applications, and outlines a new approach based on modifying the aggregate state of matter to control ST characteristics.
CHAPTER 1. Literature review: This comprehensive chapter provides a historical and theoretical overview of spin transition (ST) phenomena, covering key aspects like the LIESST effect, the influence of external pressure, cooperative interactions, and polymorphism. It also delves into the interrelation between phase and spin transitions, and the factors affecting ligand field strength, before reviewing polymeric complex compounds (Hoffmann clathrates) and liquid crystal complexes with ST, concluding with an overview of energy-saving applications and the specific research tasks for the monograph.
SECTION 2. Experimental part: This section details the extensive physical methods employed for the characterization of synthesized compounds, including magnetic measurements (SQUID), differential scanning calorimetry (DSC), X-ray diffractometry (XRD), elemental analysis (TGA), infrared (IR) and nuclear magnetic resonance (NMR) spectroscopy, and XANES/EXAFS. It also provides a thorough description of the synthetic procedures for the various precursors and Fe(II) complexes studied in the work.
SECTION 3. Polymeric liquid crystal complexes of FeII: This section focuses on the design, synthesis, and physicochemical characterization of two-dimensional polymeric Fe(II) complexes, specifically those based on 4-phenylpyridine derivatives with aliphatic substituents. It explores their spin transition behavior, magnetic properties, and the interplay between spin state and liquid crystal phases, highlighting their thermochromic properties and the layered structure of these 2D coordination polymers.
SECTION 4. One-, two-, and three-dimensional polymeric FeII complexes: This section investigates the synthesis, structural properties, and spin transition characteristics of various 1D, 2D, and 3D polymeric Fe(II) complexes, including Hoffman clathrates, designed to exhibit strong cooperativity and guest molecule sensitivity. It covers compounds based on diverse bridging ligands like fluoropyrazine, naphthyridine, phenanthroline, bis(4-pyridyl)butadiene, and substituted bis(1H-pyrazol-4-yl)selenides, discussing their magnetic behavior, structural transformations, and the impact of host-guest interactions.
SECTION 5. Barocaloric effect of a non-hysteretic molecular ST compound at ambient temperature: This section presents the detailed study of a fluorinated thiocyanate complex that exhibits a low-hysteretic spin transition near room temperature, demonstrating a colossal reversible barocaloric effect. It highlights the compound's potential as an efficient and environmentally friendly refrigerant material, attributed to significant isothermal entropy and adiabatic temperature changes at low operating pressures, underscoring the importance of rational molecular design for sustainable cooling technologies.
Keywords
Spin Crossover, Iron(II) Complexes, Ligand Design, Energy-saving Technologies, Green Technologies, Solar Absorption, Barocalorics, Hoffmann Clathrates, Liquid Crystals, Polymer Complexes, Phase Transitions, Molecular Materials, Thermochromism, LIESST Effect, Coordination Polymers
Frequently Asked Questions
What is the main topic of this work?
This work fundamentally deals with the design and study of iron(II) spin crossover compounds, particularly for their application in energy-saving green technologies, focusing on controlled solar absorption and barocaloric effects.
What are the central thematic areas?
The central thematic areas include the molecular design of ligands, synthesis of crystalline and liquid crystal iron(II) complexes, investigation of spin transitions (ST) and phase transitions, and exploration of their potential in smart optical systems and barocaloric refrigeration.
What is the primary objective or research question?
The primary objective is to synthesize and establish fundamental regularities between phase transitions and spin transition parameters in new crystalline, liquid crystal, meltable Fe(II) complexes, and porous coordination polymers, with a view to optimizing them for barocaloric and light absorption applications.
Which scientific method is used?
The scientific methodology involves a combination of synthetic procedures for ligands and complexes, followed by extensive physical characterization using techniques such as magnetic measurements, DSC, X-ray diffractometry, elemental analysis, TGA, IR, NMR, XANES, EXAFS, and optical absorption spectroscopy.
What is covered in the main part?
The main part of the monograph covers detailed studies on polymeric liquid crystal complexes of Fe(II), various one-, two-, and three-dimensional polymeric Fe(II) complexes including Hoffman clathrates, and the barocaloric effect in non-hysteretic molecular ST compounds, elaborating on their structural, magnetic, and thermal properties.
Which keywords characterize the work?
Key terms characterizing this work are: Spin Crossover, Iron(II) Complexes, Ligand Design, Energy-saving Technologies, Green Technologies, Solar Absorption, Barocalorics, Hoffmann Clathrates, Liquid Crystals, Polymer Complexes, Phase Transitions, Molecular Materials, Thermochromism, LIESST Effect, Coordination Polymers.
How do ST compounds contribute to energy-saving 'green' technologies?
ST compounds offer tunable properties like color, magnetism, and volume, allowing for passive control of solar heat gain in smart windows, optimizing light capture for solar energy harvesting, and serving as environmentally friendly refrigerants through the barocaloric effect.
What is the significance of the barocaloric effect in ST compounds for refrigeration?
The barocaloric effect in ST compounds allows for colossal, reproducible temperature changes under low hydrostatic pressure, making them promising alternatives to traditional refrigerants for energy-efficient and sustainable cooling systems, especially at near room temperatures.
What role do Hoffmann clathrates play in the design of ST materials?
Hoffmann clathrates are bimetallic 3D framework compounds recognized for their sharp hysteresis and ability to change properties upon absorption of guest molecules, making them ideal for studying host-guest interactions and designing chemosensors or materials with tailored ST characteristics.
How are liquid crystal properties combined with spin transitions?
Compounds that combine ST and liquid crystal behavior (metallic mesogens) can form thin thermochromic films, exhibit ST in different temperature regimes, and allow for switching of spin states under external fields, opening avenues for applications in optical filters, laser addressing, and polarizers.
- Citation du texte
- M. Seredyuk (Auteur), K. Znovjyak (Auteur), Y. Moroz (Auteur), N. Kariaka (Auteur), 2025, Emerging Trends in Molecular Design of Ligands for Iron(II) Spin Crossover Compounds for Energy-saving “Green” Technologies (Controlled Solar Absorption and Barocalorics), Munich, GRIN Verlag, https://www.grin.com/document/1642652
