The central topic of the thesis is the development of a new approach for tuning the spin transitions (ST) parameters in coordination compounds of 3d6 FeII ions by means of controlled phase transitions. The approach is based on the exploitation of the fundamental dependence of the ST characteristics on the aggregate state of the substance, arising from the influence of the supramolecular structure on the ligand field strength of complex molecules. Controlled solid-solid, solid-liquid-crystalline and solid-liquid phase transitions can act as fundamental factors of the ST, govern its temperature and hysteresis. Ligands with five- and six-membered heterocycles were synthesized using the developed methods and 129 new coordination compounds were derived on their basis, whose structure and physico-chemical properties were investigated. A structure-property correlation of the obtained systems of various types and an analysis of the literature data made it possible to establish the specific features of the molecular structure for the effective interpay of the phase transitions and the ST of the FeII ions.
The first investigation line concerns the study of mononuclear and polymeric FeII complexes without substituents as well as similar systems functionalized with aliphatic substituents, in particular, mononuclear compounds based on symmetric tripodands with heterocyclic donor groups. Magnetic, structural and spectroscopic studies have revealed a diamagnetic low-spin state of the central ion, while comparative structural analysis led to the conclusion that the ligand field is close to the value favoring ST. The change of the quadratic hyperpolarizability during the ST was also demonstrated for the first time for a FeII tripodand complex of the series.
It was revealed that introducing short-chain aliphatic substituents into the ligand molecules does not affect the ability of complex compounds to form a highly organized crystal lattice. Based on the results of X-ray diffraction analysis of single crystal samples of FeII complex cations with the ligand containing n-butylimidazole fragments, the thermally induced structural and spin lability was discovered that can occur either synchronously or independently. Two different STs with significant hysteresis (memory channels A and B) were revealed for the first time.
A series of neutral mononuclear thiocyanate FeII complexes with aliphatic substituents of various lengths was obtained and characterized, as well as a new class of fusible neutral FeII complexes based on the ionogenic tridentate ligands of the aroylhydrazone type with asymmetrically grafted aliphatic substituents. The phase transitions of these compounds cause the appearance of a sharp ST with the possibility of controlling their localization by temperature, hysteresis width and direction of the ST, which can be conventional “forward” or rare “reversed” ones.
It was demonstrated that the grafing one, two or three aliphatic chains to the 4-phenyl-pyridine ligand leads to the formation of two-dimensional polymeric polycyanometallate FeII metallomezogens, exhibiting incomplete and gradual ST and a crystal–smectic type phase transition above room temperature.
The second part of the study is devoted to a systematic investigation of the bridging ligands and infuence of low molecular weight complounds included in the lattice of three-dimensional polycyanomethalate ST FeII complexes. To this end, the structural and physicochemical properties of a series of complexes with bismonodentate ligands of various lengths were studied. It was determined that the fluoropyrazine, 2,6-naphthyridine and 3,8-phenanthroline form framework compounds with a high cooperativity of ST. Influence of three types of guest molecules on the ST of compounds with the longest ligand bis(4-pyridyl)butadiyn has been systematically investigated. Magnetic data indicate a ST process with significant hysteresis, whose localization by temperature depends on the nature of the adsorbed guest molecules, and has a pronounced chemosensory functionality. This behavior is explained assuming the molecular volume and the number of guest molecules, which while increasing stabilize the lattice with high-spin FeII ions.
The synthesis and properties of two FeII complexes based on the ionogenic tetradentate ligand 6,6'-bis(1H-tetrazol-5-yl)-2,2'-bipyridine are described. By changing the synthesis conditions, it is possible to obtain a mononuclear anionic complex with a rare coordination sphere [FeN8] of the central atom as well as a neutral polymeric pseudo-Oh complex with cooperative ST exhibiting LIESST effect at low temperatures.
The reaction of pyrazole and its derivatives with selenium dioxide, leading to the formation of bis(pyrazol-4-yl)selenides in high yields, was studied in details. The possibility of using these compounds as ligands for the synthesis of coordination polymers with FeII ions is demonstrated.
