Zone-Collapse Amorphization Mimicking the Negative Compressibility of a Porous Compound

A strongly anisotropic architecture of stacked metal–organic grids, with pores arranged through their eyes, has been revealed for the new coordination polymer Ni­(bipy)­(hip)­(H₂O)₂­·DMF­·CH₃OH­·H₂O, where bipy is 4,4′-bipyridine and hip is 5-hydroxyisophthalic acid. This porous crystal, when submerged in various nonpenetrating oils used as the hydrostatic media, displays a negative volume compressibility. This counterintuitive effect results in the zone amorphization injecting the contents of collapsing pores into the retained crystalline portions of the sample. This additional contents of the pores supports the crystalline parts and prevents their amorphization

High-Pressure Preference for the Low <i>Z</i>′ Polymorph of a Molecular Crystal

High pressure destabilizes the high <i>Z</i>′ polymorph of 3-hydroxy-4,5-dimethyl-1-phenylpyridazin-6-one (<b>1</b>α); however, recrystallization is needed for obtaining a low <i>Z</i>′, more dense polymorph. Three polymorphs α, β, and γ can be monotonically compressed to 2.0 GPa at least. At ambient pressure <b>1</b> crystallizes in space group <i>C</i>2/<i>c</i>, <i>Z</i>′ = 4 as polymorph <b>1</b>α, or as lower-density polymorph <b>1</b>β, of space group <i>P</i>2₁/<i>c</i> and <i>Z</i>′ = 1. Polymorph β is metastable, and after about one year, it transforms to phase α. The isochoric recrystallization above 0.40 GPa yields a new polymorph γ of space group <i>P</i>2₁/<i>a</i> and <i>Z</i>′ reduced to 1. The γ polymorph retrieved to ambient conditions for months has showed no signs of transformations. The main motif of OH···O bonded chains is retained in all three phases, but high pressure enforced identical conformation of closely packed molecules and their identical crystal environment in phase γ

Singularities in Molecular Conformation

The intramolecular coupling between molecular groups leads to singularities in the molecular conformation, acting like a switch in a molecular-scale rotor. This, in turn, affects the potential-energy (<i>E</i>_p) barriers, acquiring a sharp shape originating from superimposed <i>E</i>_p functions of the molecular conformers with differently coupled methyl groups. The molecular conformation fixed at the switching position results in the disordered methyl orientations in the crystalline state. These general features have been observed for the molecule of pinacolone. The structure of the pinacolone crystals frozen at low-temperature isobaric and high-pressure isochoric conditions has been determined

Conformationally Assisted Negative Area Compression in Methyl Benzoate

Methyl benzoate (MB) freezes isobarically at 0.1 MPa below 261 K and isothermally at 296 K above 0.35 GPa in phase α, with the molecules in energetically unfavorable twisted conformation. At 0.54 GPa, phase α undergoes a most unusual isostructural transition to a new β phase. At the transition point the crystal is compressed along [<i>x</i>] and expands along plane <i>yz</i>, which is analogous to negative Poisson’s ratio of −2.05. Above 0.54 GPa, the MB molecules assume the planar conformation, by 2.70 kJ mol^–1 more stable than the twisted conformation at ambient-pressure, and arene and methyl H-donors are exchanged in the C–H···π bonding pattern. The interplay between the crystal strain, molecular conformation and the intermolecular interactions has been established

High-Pressure Crystallization and Thermodynamic Stability Study on the Resolution of High-Density Enantiomers from Low-Density Racemates

High-pressure recrystallization could be the cheapest clean method of resolving enantiomers from the racemates defying Wallach’s rule. We have investigated the effect of pressure on sodium tartrate monohydrate (NaTa·H2O), a notorious exception from Wallach’s rule: both racemic polymorphs α-dl-NaTa·H2O and β-dl-NaTa·H2O are less dense than the enantiomers. According to the mobile-equilibrium principle, such high-density enantiomorphs should spontaneously separate under high pressures. The pressure dependence of the Gibbs free energy explains the preferential crystallization of mixed enantiomers of NaTa·H2O

Piezochromic Topology Switch in a Coordination Polymer

Abrupt color changes coupled to a giant strain in the crystal of coordination polymer CoCl₂bpp (bpp = 1,3-bis­(4-pyridyl)­propane) mark piezochromic reversible transformations at 1.93 GPa from blue phase α to green phase β and at 2.39 GPa to colorless phase γ. The clearly visible shape and color changes are ideal for calibrating discrete pressure magnitudes associated with these phase transitions. The crystal spectra have been measured and the structures have been determined <i>in situ</i> under pressure in a diamond-anvil cell. In phase α (of monoclinic space group <i>P</i>2₁/<i>m</i>) and phase β (orthorhombic space group <i>Pnmm</i>) the tetrahedral Co-coordination is stepwise modified within the 1D chain topology, but in phase γ (triclinic space group <i>P</i>1̅) the Co²⁺ cations become octahedrally coordinated and the polymer topology transforms to the 2D sheets

Piezochromic Porous Metal–Organic Framework

Pressure changes the color of a new type of metal–organic porous hybrid material CoBbcDabcoH₂O. It is built of Co²⁺ cations linked by 1,4-benzenedicabroxylate (Bdc) anions and 1,4-diazabicyclo[2.2.2]­octane (Dabco) molecules into 2-D grid-like sheets, interconnected through OH···O bonds of water molecules to carboxylate H-acceptors. This first piezochromic MOF, stable in air and in many solvents, is an ideal ultraprecise sensor for pressure calibration. The color changes are due to strains generated by pressure in the highly asymmetric crystal field of Co²⁺ octahedral coordination, involving four different ligand types: a Dabco amine (twice), a monodentate carboxylate, a chelating carboxylate, and a water molecule. At 0.33 GPa/296 K and below 225 K/0.1 MPa a phase transition reduces the crystal symmetry from monoclinic to triclinic system and changes the conformation and orientation of linkers

Hydrogen Bonds NH···N in Compressed Benzimidazole Polymorphs

Two phase transitions in compressed benzimidazole polymorphs reveal a remarkable interplay of the H-site in NH···N hydrogen-bonded aggregates and the crystal structure. The ambient-pressure polar polymorph α, space group <i>Pna</i>2₁, at <i>p</i>₁ = 0.26 GPa transforms into centrosymmetric phase β, space group <i>Pccn</i>, and above <i>p</i>₂ = 2.26 GPa into another centrosymmetric polymorph γ, space group <i>Pbca</i>. Single crystals of forms α, β, and γ have been <i>in situ</i> grown in isothermal and isochoric conditions in a diamond-anvil cell, and their structures were determined by X-ray diffraction. Both at <i>p</i>₁ and <i>p</i>₂ the H-bond distance N···N increases in the higher-pressure phase. However, the H-atom always assumes the site for which the H···N distance in the homoconjugated NH···N bond is shorter

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

Giant Anomalous Strain between High-Pressure Phases and the Mesomers of Urea

At high pressure the urea crystal abruptly collapses in two stages. When transforming from phase I to III at 0.48 GPa, the crystal volume is abruptly compressed by 7.3%. At this transition a huge abrupt linear compression to 65% along <i>a</i> and to 95% along <i>c</i> is partly compensated by an unprecedented abrupt negative linear compression (i.e., expansion) to 148% along <i>b</i>. At another discontinuous transformation, at 2.8 GPa to phase IV, the crystal still displays an abrupt negative linear compression to 106% along <i>c</i>. The intermediate phase III between 0.48 and 2.80 GPa increases the contribution of the zwitterionic mesomeric structure of urea molecules and the formation of weak NH···N bonds has been evidenced in phase III. The formation of phase II, above 373 K and above 0.6 GPa reported by Bridgman, has not been confirmed