Defense of the dissertation of Amanyazova Botagoz for the degree of Doctor of Philosophy (PhD) in the specialty «8D05306 - Chemistry»

L.N. Gumilyov Eurasian National University, a dissertation defense for the degree of Doctor of Philosophy (PhD) by Amanyazova Botagoz on the topic «Spin-state switching in Iron(II) Complexes with Redox-Active Ligands» to the educational program «8D05306 – Chemistry».

The dissertation was carried out at the «Chemistry education department» of L.N. Gumilyov Eurasian National University.

The language of defense is russian

Official reviewers:

Mataev Mukhametkali Musagalievich – Doctor of Chemical Sciences, Professor, Kazakh National Women's Teacher Training University (Almaty, Republic of Kazakhstan);

Nadirov Rashid Kazimovich – Candidate of Chemical Sciences, Professor, RSE "Institute of Combustion Problems" of the Science Committee of the MSHE RK (Almaty, Republic of Kazakhstan).

Temporary members of the Dissertation Council:

Mironov Maxim - PhD, Associate Professor, University of Warwick (Coventry, UK);

Ospanova Aliya Kapanovnа - Doctor of Chemical Sciences, Professor, Al-Farabi Kazakh National University (Almaty, Republic of Kazakhstan);

Uali Aitolkyn Sailaubekkyzy - Candidate of Chemical Sciences, Associate Professor (docent), L.N. Gumilyov Eurasian National University (Astana, Republic of Kazakhstan).

Scientific advisors:

Erkasov Rakhmetulla Sharapidenovich – Doctor of Chemical Sciences, Professor, Department of Chemistry, L.N. Gumilyov Eurasian National University (Astana, Republic of Kazakhstan);

Shatruk Michael – PhD, Professor, Department of Chemistry and Biochemistry, Florida State University (Tallahassee, USA).

The defense will take place on April 01, 2026, at 03:00 PM in the Dissertation Council for the training direction «8D053 – Physical and chemical sciences» in the specialty «8D05306 – Chemistry» of L.N. Gumilyov Eurasian National University. The dissertation council meetings will be held offline and online.

Link: https://clck.ru/3RjP6j 

Address: Satpayev Street, 2, Room: 302., Astana, Republic of Kazakhstan.

Abstract (English): ABSTRACT of the dissertation submitted for the degree of Doctor of Philosophy (PhD) in the educational program “8D05306 – Chemistry” by Amanyazova Botagoz Timurovna “Spin-state switching in Iron(II) Complexes with Redox-Active Ligands” General Characteristics of the Work. Spin crossover (SCO) is a physical phenomenon in which certain transition metal ions exhibit a change in their electronic configuration and magnetic state under the influence of external stimuli such as temperature, pressure, or light. Spin transition is typically observed in Fe(II) cation complexes when the ligand field strength reaches an intermediate level, most often with six coordinating nitrogen atoms. The dissertation sequentially addresses the fundamentals of spin crossover and organic conductors, methods for the synthesis and crystallization of Fe(II) hybrid complexes with radical TCNQ anions, as well as approaches to their magnetic, photomagnetic, and structural characterization. Special attention is given to the light-induced excited spin-state trapping (LIESST) effect as a manifestation of molecular bistability and a promising functional property. The study has expanded understanding of the mechanisms of spin-state switching and established magneto-structural correlations based on X-ray structural, magnetic, and spectroscopic data. Relevance of the Research Topic. Spin-crossover compounds based on Fe(II) represent one of the most intensively developing areas of molecular materials science. Despite the large number of synthesized compounds, only a limited fraction of systems exhibits parameters potentially suitable for practical applications. Functional devices require a spin-transition midpoint temperature T₁/₂ close to 300 K, the presence of hysteresis, and a narrow transition temperature interval (ΔT < 10–20 K). In most known Fe(II) complexes, the spin transition proceeds gradually, without pronounced bistability, and the values of T₁/₂ often lie in the range of 100–250 K. The current global trend involves a transition from the study of isolated SCO complexes toward the development of multifunctional hybrid materials in which magnetic switching is combined with changes in electrophysical characteristics. The integration of Fe(II) centers with π-stacked radical subsystems (e.g., TCNQ) enables the realization of systems with conductivity in the range of 10⁻⁹ to 10⁻³ S•cm⁻¹; however, the number of structures in which the spin transition and charge transport are interrelated and controllable remains limited. Quantitative correlations between the degree of charge delocalization, crystal packing, and SCO parameters have not yet been sufficiently systematized. An additional development direction is associated with the use of non-innocent ligands capable of participating in intra- or intermolecular electron transfer. In such systems, changes in the oxidation state of the ligand can potentially influence the energy gap between the spin states of Fe(II). However, the mechanisms of redox-induced control of the spin transition and the conditions for stabilizing the corresponding ionic structures have been studied fragmentarily, and reproducible synthetic approaches remain limited. Scientific novelty. The scientific novelty of the work consists in an integrated approach to the synthesis and investigation of spin-crossover Fe(II) complexes and includes several fundamentally new aspects: For the first time, a pronounced and quantitatively characterized light-induced spin-state switching (LIESST) effect was experimentally demonstrated in homoleptic Fe(II) complexes with thiazole-based chelating ligands. For the complexes [Fe(2bt)₃]₂•MeOH and [Fe(3tpH)₃]₂•2(3tpH), efficient HS↔LS photoswitching upon irradiation at 5 K was shown, followed by relaxation of the metastable HS state at T ≈ 68–81 K. An unusual two-step relaxation behavior of the photoinduced high-spin state in the complex [Fe(3tpH)₃]₂•2(3tpH) at intermediate temperatures was revealed, which had not been previously described for this class of thiazole-based SCO systems. For the first time, solvent-free hybrid spin-crossover semiconductors of the composition [Fe(L)3](TCNQ)3 (L = 3tpH, 4bt) were obtained and structurally characterized. It was established that these complexes form layered structures with one-dimensional stacks of TCNQ•δ⁻, providing semiconducting behavior with room-temperature conductivities of 2.0×10⁻³ and 2.0×10⁻² S•cm⁻¹, respectively. It was shown that the higher conductivity of the 4bt-ligand complex is due to a more uniform distribution of fractional charge along the TCNQ chains, as confirmed by crystal structure analysis. A quantitative correlation between the degree of charge delocalization in the TCNQ sublattice and the parameters of the spin transition in Fe(II)–TCNQ hybrid systems was established. It was shown that in [Fe(4bt)3](TCNQ)3 a gradual and nearly complete spin transition with a midpoint temperature T₁/₂ ≈ 272 K is realized, whereas in [Fe(3tpH)3](TCNQ)3 the Fe(II) ion is predominantly stabilized in the high-spin state. These differences were correlated for the first time with the features of charge distribution in the TCNQ stacks and the packing density of the cationic layers. A strategy for obtaining desolvation-resistant hybrid spin-crossover materials without the inclusion of solvent molecules in the crystal lattice was developed and experimentally validated. In contrast to most previously described Fe(II)–TCNQ systems, which are prone to the loss of crystallization solvent and degradation of physical properties, the [Fe(L)3](TCNQ)3 complexes synthesized in this work form compact solvent-free crystal structures that retain their magnetic and electrophysical characteristics upon heating above room temperature. Aim of the work. The aim of the work is to establish quantitative relationships between the electronic nature of the ligand subsystem (thiazole, π-acceptor, and non-innocent ligands), crystal packing parameters, and the thermodynamic characteristics of the spin transition (T₁/₂, transition width) in Fe(II) complexes, as well as to determine the influence of these factors on the electrophysical characteristics of hybrid systems. Objectives of the study: 1. To synthesize and investigate Fe(II) complexes based on thiazole ligands; to carry out their structural characterization by single-crystal X-ray diffraction and to determine the parameters of intermolecular interactions (π–π contacts, Fe–N distances). 2. To determine the thermodynamic parameters of the spin transition (T₁/₂, transition width) by magnetic measurements (SQUID magnetometry) and to evaluate the character of the transition (abrupt/gradual, presence of hysteresis). 3. To investigate the photoinduced spin transition (LIESST) at low temperatures and to determine the temperature range of stability of the photoinduced state. 4. To obtain co-crystallized homoleptic Fe(II) complexes with fractionally charged TCNQ•⁻ anions and to determine their crystal structures. 5. To measure the temperature dependence of the electrical conductivity of the hybrid systems by the four-probe method and to determine the type of conductivity and the activation energy of charge transport. 6. To correlate the crystal structure parameters, the degree of charge delocalization in the TCNQ subsystem, and the characteristics of the spin transition. 7. To establish the possibility of synthesizing Fe(II) complexes with non-innocent ligands and to carry out their structural characterization by single-crystal X-ray diffraction; to evaluate the synthetic capabilities of this approach and to identify limitations associated with the crystallization of the target compounds. Research Methods. The work employed a combination of solution synthesis and crystallization methods for Fe(II) coordination compounds, single-crystal X-ray diffraction, SQUID magnetometry, techniques for studying photoinduced effects, conductivity measurements, as well as elemental and spectroscopic analysis. Objects of the Study. Fe(II) complexes exhibiting spin crossover, including compounds with thiazole-based ligands, co-crystallized with partially charged radical TCNQ●δ– anions, and complexes with non-innocent ligands. Subject of the Study. The interrelation between the crystal structure and the magnetic, photoinduced, and transport properties of Fe(II) spin-crossover complexes, as well as the influence of cooperative interactions on the character of the spin transition and the possibility of modulating electrical conductivity through changes in the spin state. In addition, the synthetic features and limitations associated with the formation of hybrid materials with controlled properties are considered. Statements to be Defended: 1. In homoleptic Fe(II) complexes with thiazole-containing ligands, intermolecular π–π interactions between the aromatic fragments of the ligands determine the cooperativity of the spin transition and ensure the realization of a pronounced and quantitatively reproducible LIESST effect at low temperatures. 2. In the hybrid complexes [FeII(3tpH)3](TCNQ)3 and [FeII(4bt)3](TCNQ)3 co-crystallized with TCNQ•⁻ radical anions without the participation of solvent molecules, the structural organization of the π-stacked TCNQ sublattice determines the parameters of the spin transition and the nature of thermally activated electrical conductivity, which demonstrates the interrelation of magnetic and transport properties in these systems. 3. In Fe(II) complexes with non-innocent ligands of the dad type, intermolecular electron transfer between the cationic and anionic subsystems takes place, leading to a change in the spin state of the Fe(II) ion; it is shown that the developed synthetic approach provides reproducible access to such systems with a controlled crystal structure. Theoretical Significance. The compounds presented in this dissertation expand the field of hybrid molecular conductors and provide evidence of the effectiveness of the proposed synthetic methods. Application of these methods will enable future researchers to access a greater number of compounds and ultimately realize highly conductive, magnetically sensitive molecular materials. Practical Significance. Designing complexes with new coordination environments is attractive due to the possibility of discovering new families of compounds that exhibit spin crossover. The switching properties make such materials potential candidates for practical applications. Molecular electronics, data storage, display devices, and sensors are currently the most relevant targets in this area. Relation to State Scientific Programs. This dissertation is part of research conducted at Florida State University (Tallahassee, USA) under an agreement with the Department of Chemistry of L.N. Gumilyov ENU. Publications. Two articles on the dissertation topic have been published in a journal included in the Web of Science citation database. 1. The PhD candidate directly participated in the synthesis, acquisition of experimental data, processing, and interpretation of the results for the preparation of the publication: Jo M.; Amanyazova B.; Yergeshbayeva S.; Gakiya-Teruya M.; Ungor O.; Lopez Rivera P.; Jen N.; Lukyanenko E.; Kurkin A.; Erkasov R.; Meisel M.; Hauser A.; Chakraborty P.; Shatruk M.. Light-induced spin-state switching in Fe(II) spin-crossover complexes with thiazole-based chelating ligands // Dalton Transactions. — 2024. — Vol. 53, Iss. 25. — pp. 10511–10520. The article is indexed in the Web of Science Core Collection database. For 2024, Dalton Transactions has an Impact Factor of 3.3 and is ranked in Q1 in the same subject category. 2. As the first author, the PhD candidate directly participated in the synthesis of the compounds, collection of experimental data, their processing and interpretation, as well as in writing the manuscript: Amanyazova B.; Yergeshbayeva S.; Choi Eun S.; Erkasov R.; Shatruk M.. Spin-crossover semiconductors based on homoleptic tris-diimine Fe(II) complexes with fractionally charged TCNQ•⁻ anions // Dalton Transactions. — 2025. The article is indexed in the Web of Science Core Collection database. At the time of its publication in 2025, Dalton Transactions had an Impact Factor of 3.3 (2024) and was ranked in Q1 in the field of Inorganic and Nuclear Chemistry. Structure of the Dissertation. The dissertation is 115 pages long, including an introduction, main part (literature review, experimental section, discussion of experimental data, conclusions, list of 204 references), 60 figures, 10 tables, and 3 appendices.

Dissertation

List of Scientific Works

Consultant's Review

Foreign Consultant's Review

Conclusion of the Research Ethics Committee

Official Reviewer's Review

Official Reviewer's Review