Hiiuk V.M. Spin crossover in Fe(II) cyanometallic complexes with bicyclic azine ligands. – Qualifying scientific work on the rights of the manuscript.
Thesis for scientific degree of Doctor of Philosophy in specialty 102 –Chemistry
(10 – Natural sciences). – National University of «Kyiv-Mohyla Academy», Ministry
of Education and Science of Ukraine, Kyiv, 2022.
The thesis is devoted to synthesis and study of physical properties of new spin- crossover (SCO) Fe(II) complexes based on bicyclic azine ligands and linear or square- planar cyanometallate co-ligands, establishment of their crystal structure and study of kinetic effects upon SCO.
Seven new Fe(II) cyanometallic complexes based on bicyclic azine ligands, such as phthalazine (phth), pyridazino[4,5-d]pyridazine (pdzpdz), 2-phenylpyrazine (Phpz), 1,6-naphthyridine (1,6-naphthy), have been synthesized. It is shown that complexes based on phth and pdzpdz with square-planar cyanometallate co-ligands are characterized by a one-step SCO with a wide thermal hysteresis loop at low temperatures. Phpz complexes with linear cyanometallate co-ligands are characterized by a two-step SCO without hysteresis at low temperatures, and cyanometallic complex based on 1,6-naphthy is characterized by a one-step SCO with a wide thermal hysteresis loop centred at room temperature. Thus, it is shown that the variation of azine ligand/co-ligand pair leads to Fe(II) complexes with different SCO parameters (temperature, sharpness, completeness, and hysteresis).
Based on the results of single-crystal X-ray diffraction analysis, it is shown that all the obtained compounds have similar structural features, which are inherent in Hofmann clathrate analogues: Fe(II) ions have a pseudo-octahedral coordination environment [FeN6] with two azine ligands in the axial positions and four dicyanometallic [MI(CN)2]− (where MI = Ag, Au) or tetracyanometallic [MII(CN)4]2− (where MII = Pd, Pt) fragments occupying equatorial positions. Two crystal structures of the same complex (1,6-naphthy complex) in different spin states at room temperature have been obtained for the first time.
It was shown that 3D SCO frameworks based on pdzpdz with square-planar cyanometallate co-ligands are characterized by high porosity having guest available pores.
By comparing the properties of complexes with phthalazine and with pyridazine or complexes with phenylpyrazine and with pyrazine, it was shown that the structural modification of the axial ligand allows to obtain new Fe(II) compounds with different SCO parameters.
It was found that SCO in all the obtained Fe(II) complexes is accompanied by a sharp change in the colour of the complexes (thermochromic effect) upon transition between the high-spin (HS) and the low-spin (LS) states.
It is shown that the sharpness and width of the thermal hysteresis loop of SCO in single crystals of Fe(II) cyanometallic complex based on 1,6-naphthy depends on the temperature sweep rate, which indicates the existence of kinetic effects of the SCO in this compound. It was found that at a temperature change rate of 0.1 K min-1 the complex is characterized by the lowest phase-boundary propagation velocity (3 nm∙s- 1) among those known for SCO Fe(II) complexes. The reason for these kinetic effects is the presence of numerous microstructural defects in single crystals, which is an intrinsic characteristic of this compound.
Key-words: spin crossover, Fe(II) complexes, Hofmann clathrate analogues, bicyclic azine ligands, crystal structure, magnetic properties, optical properties, kinetic studies.
