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}n (FeNb) 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 FeNb is compared with a simple structural and magnetic pressure response of its analog {[Mn(II)(pyrazole)4]2[Nb(IV)(CN)8]·4H2O}n (MnNb). The FeNb coordination polymer is the first pressure-induced spin-crossover photomagnet.

High pressure can favor the formation of either thiourea hydrates or anhydrates. Above 0.60 GPa thiourea crystallizes as monohydrate (NH 2 ) 2 CS•H 2 O, while only anhydrous thiourea is obtained from aqueous solution at normal conditions. At 0.70 GPa another hydrate, 3(NH 2 ) 2 CS•2H 2 O, 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.

High pressure increases intermolecular interactions in the crystal environment and affects the molecular dimensions of thiourea, CS(NH 2 ) 2 , consistently with changing contributions of the thione and zwitterionic mesomers. The ambient-pressure phase V (space-group Pnma, Z = 4) at 0.34 GPa transforms to phase VI, of the same spacegroup symmetry, but with the larger unit-cell trifold (Z = 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.

Highly hygroscopic pyridine N-oxide, C5H5NO, dissolves in water absorbed from atmospheric air, but it crystallizes in the neat form of the aqueous solution under high pressure. The crystals grown at high-pressure isochoric conditions are of the same phase as that obtained from anhydrous crystallization at ambient pressure. This feature can be employed for retrieving compounds highly soluble in water from their aqueous solutions. The crystal structure is strongly stabilized by CH...O contacts. The crystal compression and thermal expansion as well as three shortest H...O distances comply with the inverse-relationship rule of pressure and temperature changes.

Two new coordination chain compounds based on octacyanoniobate(IV) are reported: {[Mn(II)(bpy)(H2O)2]2[Nb(IV)(CN)8]·5H2O}n (1) and {[Mn(II)(bpy)(H2O)2][Mn(II)(bpy)2][Nb(IV)(CN)8]·5H2O}n (2) (bpy = 2,2'-bipyridine). Both have the topology of vertex-sharing CN-bridged squares. The Mn2Nb2 units are mutually parallel in 1 and perpendicular in 2. The antiferromagnetic intrachain interactions between the Mn(II) and Nb(IV) centers in 1 and 2, typical for Mn(II)-[Nb(IV)(CN)8] family of compounds, lead to ferrimagnetic ground state of each single chain. Both compounds show bulk ferrimagnetic ordering below their critical temperatures (Tc) of 6.4 K in 1 and 4.7 K in 2. Such a behavior is due to the ferromagnetic interchain interactions, assured by the π-π contacts between the 2,2'-bipyridine rings and by the hydrogen bonds between the terminal cyanide ligands of the [Nb(IV)(CN)8]-building blocks and coordination/crystallization water molecules. 1 and 2 are rare examples of ferrimagnets promoted by interchain ferromagnetic exchange, and the Tc of 1 is one of the highest among similar 1-D coordination polymers.