How to quench ferromagnetic ordering in a CN-bridged Ni(II)-Nb(IV) molecular magnet? : a combined high-pressure single-crystal X-ray diffraction and magnetic study

High-pressure (HP) structural and magnetic properties of a magnetic coordination polymer {[NiII(pyrazole)4]2[NbIV(CN)8]·4H2O}n (Ni2Nb) are presented, discussed and compared with its two previously reported analogs {[MnII(pyrazole)4]2[NbIV(CN)8]·4H2O}n (Mn2Nb) and {[FeII(pyrazole)4]2[NbIV(CN)8]·4H2O}n (Fe2Nb). Ni2Nb shows a significant decrease of the long-range ferromagnetic ordering under high pressure when compared to Mn2Nb, where the pressure enhances the Tc (magnetic ordering temperature), or to Fe2Nb exhibiting a pressure-induced spin crossover. The different HP magnetic responses of the three compounds were rationalized and correlated with the structural models as determined by single-crystal X-ray diffraction

Enforcing Multifunctionality: A Pressure-Induced Spin-Crossover Photomagnet

Photomagnetic compounds are usually achieved by assembling preorganized individual molecules into rationally designed molecular architectures via the bottom-up approach. Here we show that a magnetic response to light can also be enforced in a nonphotomagnetic compound by applying mechanical stress. The nonphotomagnetic cyano-bridged Fe^II–Nb^IV coordination polymer {[Fe^II(pyrazole)₄]₂[Nb^IV(CN)₈]·4H₂O}_<i>n</i> (<b>FeNb</b>) has been subjected to high-pressure structural, magnetic and photomagnetic studies at low temperature, which revealed a wide spectrum of pressure-related functionalities including the light-induced magnetization. The multifunctionality of <b>FeNb</b> is compared with a simple structural and magnetic pressure response of its analog {[Mn^II(pyrazole)₄]₂[Nb^IV(CN)₈]·4H₂O}_<i>n</i> (<b>MnNb</b>). The <b>FeNb</b> coordination polymer is the first pressure-induced spin-crossover photomagnet

High-pressure changes of intermolecular interactions in thiocarbamides

Wydział ChemiiCelem rozprawy było wyznaczenie struktur tiomocznika, 1H-benzymidazolo-2-tionu i imidazolidyno-2-tionu oraz określenie wpływu wysokiego ciśnienia na stabilność faz, ściśliwość oraz na zmiany strukturalne wymienionych związków. Badania wysokociśnieniowe przeprowadzone były przy użyciu zmodyfikowanej komory diamentowej Merrilla-Bassetta (DAC). Mimo wielu zastosowań DAC, w rozprawie skupiłam się na przeprowadzeniu eksperymentów opartych na dyfrakcji rentgenowskiej. Wykonane przeze mnie badania dostarczyły informacji o kryształach molekularnych w warunkach ekstremalnych, strukturach rezonansowych badanych substancji oraz właściwościach i przekształceniach wiązań wodorowych typu NH···S, odpowiedzialnych za agregację cząsteczek i wpływających na przekształcenia strukturalne badanych związków chemicznych. Równocześnie zaobserwowałam systematyczne zmiany wymiarów molekularnych pod wpływem wysokiego ciśnienia. Wskazują one, że struktura elektronowa cząsteczki może mieć istotne znaczenie już w ciśnieniu rzędu kilku GPa. W ramach rozprawy doktorskiej wyjaśniłam: istotę przemiany fazowej tiomocznika z fazy V do VI, związaną z reorganizacją wiązań wodorowych oraz stabilność faz atmosferycznych beznzymidazolo-2-tionu i imidazolidyno-2-tionu w warunkach wysokiego ciśnienia.The aim of this study was to determine the structures of thiourea, 2-benzimidazolo-2-thione and imidazolidine-2-thione and to explain the impact of high pressure on phase stability, compressibility and structural changes of the studied compounds. The experiments were performed on the samples in-situ crystallized in a modified high-pressure diamond anvil-cell (DAC). The high-pressure research provided new information about the structure of molecular crystals under extreme conditions. This study allowed us to understand the resonance structures of the studied compounds, as well as properties and transformations of hydrogen bonds NH···S, which are responsible for the aggregation of molecules and the structural transformations. The structural transformations are clearly coupled with the dimensions of hydrogen bonds NH···S. I have also observed systematical changes in the molecular dimensions under high pressure. As part of my doctoral thesis, I explained the nature of the phase transformation from phase V to VI in thiourea, related to the reorganization of hydrogen bonds. Another issue was to investigate the stability of atmospheric structures in 1H-benzimidazole-2-thione and imidazolidine-2-thione under high pressure conditions

High-Pressure Transformations and the Resonance Structure of Thiourea

High pressure increases intermolecular interactions in the crystal environment and affects the molecular dimensions of thiourea, CS­(NH₂)₂, consistently with changing contributions of the thione and zwitterionic mesomers. The ambient-pressure phase V (space-group <i>Pnma</i>, <i>Z</i> = 4) at 0.34 GPa transforms to phase VI, of the same space-group symmetry, but with the larger unit-cell trifold (<i>Z</i> = 12). Another pressure-induced transition to phase VII was previously postulated for thiourea at 0.54 GPa; however, no associated structural transformations were identified. Our high-pressure single-crystal X-ray diffraction study reveals that following the transition at 0.34 GPa the lattice parameters and crystal structure strongly and monotonically change to about 0.65 GPa while the crystal symmetry is retained

High Pressure Effects on Zwitterionic and Thione Mesomeric Contributions in 2‑Benzimidazole-2-Thione

High pressure reduces the zwitterionic mesomeric contribution and increases the thione contribution in 2-benzimidazole-2-thione. These mesomeric changes are manifested in the shortened bond S–C and elongated bond C–N in the S–C–N moiety. These transformations are consistent with the le Chatelier law, as they counteract the increase of electrostatic interactions when the intermolecular distances between electronegative sulfur atoms and arene π-electrons are compressed. The changing interactions affect the crystal strain and its structural transformations. Consequently, the crystal compression and thermal expansion initially, until about 1.0 GPa, are inconsistent with the inverse relationship rule of pressure and temperature effects. Some anomalous features of the thermal expansion can be associated with isostructural transformations of the crystal

Pressure-Dependent Formation and Decomposition of Thiourea Hydrates

High pressure can favor the formation of either thiourea hydrates or anhydrates. Above 0.60 GPa thiourea crystallizes as monohydrate (NH₂)₂CS·H₂O, while only anhydrous thiourea is obtained from aqueous solution at normal conditions. At 0.70 GPa another hydrate, 3­(NH₂)₂CS·2H₂O, is formed, but above 1.20 GPa anhydrous thiourea becomes stable again. The single crystals of both hydrates were grown in situ in a diamond-anvil cell and their structures were determined by X-ray diffraction. The structural factors favoring the formation of hydrates above 0.6 GPa involve new types of hydrogen bonds to water molecules and the more efficient molecular packing. The crystallization of thiourea anhydrate above 1.20 GPa coincides with the stability region of ice VI

Pressure-Dependent Formation and Decomposition of Thiourea Hydrates

High pressure can favor the formation of either thiourea hydrates or anhydrates. Above 0.60 GPa thiourea crystallizes as monohydrate (NH₂)₂CS·H₂O, while only anhydrous thiourea is obtained from aqueous solution at normal conditions. At 0.70 GPa another hydrate, 3­(NH₂)₂CS·2H₂O, is formed, but above 1.20 GPa anhydrous thiourea becomes stable again. The single crystals of both hydrates were grown in situ in a diamond-anvil cell and their structures were determined by X-ray diffraction. The structural factors favoring the formation of hydrates above 0.6 GPa involve new types of hydrogen bonds to water molecules and the more efficient molecular packing. The crystallization of thiourea anhydrate above 1.20 GPa coincides with the stability region of ice VI

Pressure-Dependent Formation and Decomposition of Thiourea Hydrates

High pressure can favor the formation of either thiourea hydrates or anhydrates. Above 0.60 GPa thiourea crystallizes as monohydrate (NH₂)₂CS·H₂O, while only anhydrous thiourea is obtained from aqueous solution at normal conditions. At 0.70 GPa another hydrate, 3­(NH₂)₂CS·2H₂O, is formed, but above 1.20 GPa anhydrous thiourea becomes stable again. The single crystals of both hydrates were grown in situ in a diamond-anvil cell and their structures were determined by X-ray diffraction. The structural factors favoring the formation of hydrates above 0.6 GPa involve new types of hydrogen bonds to water molecules and the more efficient molecular packing. The crystallization of thiourea anhydrate above 1.20 GPa coincides with the stability region of ice VI