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
- 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
- Scheme 1.3. Molecular structure of studied ST complexes.
- Scheme 1. Molecular structure of pr and prF.
- 1.10 Selection of objects and formulation of research tasks
- 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
- 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
- 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
- 4.6 Substituted bis(1H-pyrazol-4-yl)selenides for the synthesis of FeIIpolymer complexes
- 4.7 Brief conclusions
- 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
Research Objectives and Themes
The primary objective of this monograph is to establish fundamental regularities between phase transitions and spin transitions (ST) in new functional molecular materials, specifically Fe(II) coordination compounds. The work addresses the research question of how supramolecular architecture, including liquid crystalline order and porous framework structure, can be designed to control ST characteristics for practical applications, such as barocaloric cooling and smart optical systems.
- Synthesis and characterization of mononuclear and polymeric Fe(II) coordination compounds.
- Investigation of the relationship between phase transitions and ST parameters.
- Development of ligand systems with liquid crystal substituents for structural control.
- Evaluation of the influence of polymer dimensionality and guest molecule incorporation on ST behavior.
- Exploration of "green" technologies, specifically barocaloric effect and differential light absorption.
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
CHAPTER 1. Literature review: Provides a theoretical foundation for spin transition (ST) phenomena, covering ligand field theory, cooperative effects, the LIESST effect, and the relationship between structural phase transitions and spin states.
SECTION 2. Experimental part: Details the methodologies used for synthesis and characterization, including magnetic measurements, differential scanning calorimetry (DSC), X-ray diffraction, and Mössbauer spectroscopy.
SECTION 3. Polymeric liquid crystal complexes of FeII: Explores the synthesis and physical properties of 2D Hoffman-type polycyanometallate complexes designed to exhibit both liquid crystalline behavior and spin transition properties.
SECTION 4. One-, two-, and three-dimensional polymeric FeII complexes: Investigates the impact of framework dimensionality and guest molecule incorporation on the ST behavior and structural properties of various coordination polymers.
SECTION 5. Barocaloric effect of a non-hysteretic molecular ST compound at ambient temperature: Focuses on the development of a specific fluorinated complex designed to achieve a reproducible barocaloric effect with minimal hysteresis for potential cooling applications.
Keywords
Spin Crossover, Iron(II) Complexes, Coordination Polymers, Hoffman Clathrates, Liquid Crystals, Barocaloric Effect, LIESST, Magnetic Bistability, Phase Transitions, Supramolecular Chemistry, Chemosensors, Ligand Design, Molecular Materials, Hysteresis, Thermal Stability.
Frequently Asked Questions
What is the core focus of this research?
This work fundamentally explores the design and synthesis of functional molecular materials based on Fe(II) coordination complexes that exhibit spin transition (ST) properties, aiming to control these transitions for practical technological applications.
What are the primary thematic areas covered?
The monograph covers spin transition mechanisms, the design of polymeric and framework structures, liquid crystalline metallic mesogens, host-guest interactions in porous clathrates, and the application of these materials in barocaloric cooling and smart optical systems.
What is the main goal or research question?
The primary goal is to determine the possibility of regulating the physical properties of polymer compounds through chemical design of ligands, specifically to synchronize phase transitions with spin transitions for creating novel functional materials.
What scientific methods are utilized?
The research employs a variety of physical methods, including SQUID magnetometry, differential scanning calorimetry (DSC), X-ray diffraction (XRD), Mössbauer spectroscopy, and EXAFS/XANES for structural and electronic characterization.
What is covered in the main body of the text?
The main body systematically presents literature reviews on ST, details the experimental synthetic protocols, and discusses the structural and physical properties of 2D and 3D coordination polymers, including their behavior under external stimuli like temperature, light, and pressure.
Which keywords define this work?
Key terms include Spin Crossover, Coordination Polymers, Hoffman Clathrates, Barocaloric Effect, and Liquid Crystals.
How do guest molecules affect the behavior of Hoffman clathrates?
Guest molecules modulate the spin transition temperature and cooperativity by affecting lattice volume, forming specific host-guest interactions like hydrogen bonding or π-stacking, and exerting "internal pressure" on the coordination framework.
Why is the fluorination of ligands important for the barocaloric effect?
Terminal fluorination of propyl groups immobilizes aliphatic chains, blocking symmetry-breaking phase transitions and reducing hysteresis, which is critical for achieving a reproducible and efficient barocaloric effect near room temperature.
- Quote paper
- M. Seredyuk (Author), K. Znovjyak (Author), Y. Moroz (Author), N. Kariaka (Author), 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
