Programmed Pore-Shape Fixing in a Soft Porous Molecular Crystal-Navigating the Phase Interconversion Landscape.

Pore-shape fixing effects (PSFEs) in soft porous crystals are a relatively unexplored area of materials chemistry. We report the PSFE in the prototypical dynamic van der Waals solid p-tert-butylcalix[4]arene (TBC4). Starting with the high-density guest-free phase, two porous shape-fixed phases were programmed using the stimuli of CO2 pressure and temperature. A suite of complementary in situ techniques, including variable-pressure (VP) single-crystal X-ray diffraction, VP powder X-ray diffraction, VP differential scanning calorimetry, volumetric sorption analysis, and attenuated total reflectance Fourier-transform infrared spectroscopy was used to track dynamic guest-induced transformations, providing molecular-level insight into the PSFE. The interconversion between the two metastable phases is particle size dependent, making this the second example of the PSFE by crystal downsizing, and the first example involving a porous molecular crystal: larger particles undergo reversible transitions while smaller particles remain fixed in the metastable phase. A complete phase interconversion scheme was constructed for the material, thus allowing navigation of the phase interconversion landscape of TBC4 using the easily applied stimuli of CO2 pressure and thermal treatment.

Dehydration of a crystal hydrate at subglacial temperatures.

Water is one of the most important substances on our planet1. It is ubiquitous in its solid, liquid and vaporous states and all known biological systems depend on its unique chemical and physical properties. Moreover, many materials exist as water adducts, chief among which are crystal hydrates (a specific class of inclusion compound), which usually retain water indefinitely at subambient temperatures2. We describe a porous organic crystal that readily and reversibly adsorbs water into 1-nm-wide channels at more than 55% relative humidity. The water uptake/release is chromogenic, thus providing a convenient visual indication of the hydration state of the crystal over a wide temperature range. The complementary techniques of X-ray diffraction, optical microscopy, differential scanning calorimetry and molecular simulations were used to establish that the nanoconfined water is in a state of flux above -70 °C, thus allowing low-temperature dehydration to occur. We were able to determine the kinetics of dehydration over a wide temperature range, including well below 0 °C which, owing to the presence of atmospheric moisture, is usually challenging to accomplish. This discovery unlocks opportunities for designing materials that capture/release water over a range of temperatures that extend well below the freezing point of bulk water.

Halogen Bonding: From Fundamentals to Applications.

Guest Editors Maté Erdélyi, Catharine Esterhuysen, and Weilang Zhu introduce the joint Special Collection on Halogen Bonding published by ChemPlusChem and The Chemical Record. This collection is organized in association with the 4th International Symposium on Halogen Bonding (ISXB4) and features top multidisciplinary contributions where halogen bonding plays a pivotal role, including computational, synthetic and catalytic, supramolecular and crystal engineering, and biological investigations and applications.

Pressure-Gradient Sorption Calorimetry of Flexible Porous Materials: Implications for Intrinsic Thermal Management.

Thermal management is an important consideration for applications that involve gas sorption by flexible porous materials. A pressure-gradient differential scanning calorimetric method was developed to measure the energetics of adsorption and desorption both directly and continuously. The method was applied to the uptake and release of CO2 by the well-known flexible metal-organic frameworks MIL-53(Al) and MOF-508b. High-resolution differential enthalpy plots and total integral enthalpy values for sorption allow comprehensive assessment of the thermal behavior of the materials throughout the entire sorption process. During adsorption, the investigated materials display the ability to offset exothermic adsorption enthalpy against endothermic structural transition enthalpy, and vice versa during desorption. The results show that flexible materials offer reduced total integral heat over a working range when compared to rigid materials.

Direct Determination of Enthalpies of Sorption Using Pressure-Gradient Differential Scanning Calorimetry: CO2 Sorption by Cu-HKUST.

Enthalpy of sorption (ΔH) is an important parameter for the design of separation processes using adsorptive materials. A pressure-ramped calorimetric method is described and tested for the direct determination of ΔH values. Combining a heatflow thermogram with a single sorption isotherm enables the determination of ΔH as a function of loading. The method is validated by studying CO2 sorption by the well-studied metal-organic framework Cu-HKUST over a temperature range of 288-318 K. The measured ΔH values compare well with previously reported data determined by using isosteric and calorimetric methods. The pressure-gradient differential scanning calorimetry (PGDSC) method produces reliable high-resolution results by direct measurement of the enthalpy changes during the sorption processes. Additionally, PGDSC is less labor-intensive and time-consuming than the isosteric method and offers detailed insight into how ΔH changes over a given loading range.

Steric and Electronic Effects in Gold N-Heterocyclic Carbene Complexes Revealed by Computational Analysis.

A computational analysis of a series of cationic and neutral gold imidazolylidene and benzimidizolylidene complexes is reported. The Bond Dissociation Energies of the various ligands in the complexes calculated at the PBE0-D3/def2-TZVP level of theory increase with increasing ligand volume, except for those of complexes containing t-butyl-substituted ligands, which are anomalously low particularly for the benzimidazolylidene species. Atoms in Molecules studies show the presence of a variety of weak intramolecular interactions, characterised by the presence of bond critical points with a range of different properties. Energy Decomposition Analysis and calculation of Electrostatic Surface Potentials indicate that some interactions are weakly attractive dispersion-type interactions, while others are repulsive. The octanol/water partition coefficients (log P values) were calculated as a measure of the lipophilicities of the complexes and were found to increase with increasing volume.

Computational investigation of Au·H hydrogen bonds involving neutral AuI N-heterocyclic carbene complexes and amphiprotic binary hydrides.

In this computational study, we investigate the ability of various neutral R-AuI-NHC (NHC = N-heterocyclic carbene) complexes [R = H, CH3, Cl, OH] to form hydrogen bonds with the amphiprotic binary hydrides NH3, H2O and HF. Optimized geometries of the adducts calculated at various levels of theory all exhibit Au⋯HX hydrogen bonds. In adducts of complexes containing NHC ligands with α(N)H units, (NH)carbene⋯XH interactions also exist, yielding hydrogen-bonded rings with graph-set notation [Formula: see text] that correspond to pseudo chelates with κ2C,H coordination. AIM analysis at the MP2/aug-cc-pVTZ-pp level of theory indicates that the (NH)carbene⋯XH hydrogen bonds are generally stronger than the Au···HX interactions, except for those involving HF. The Au⋯HX interactions vary with the Lewis basicity of the Au(I) center as a result of the nature of the R ligand, while the (NH)carbene⋯XH hydrogen bonds are unaffected by R. Energy decomposition analysis at the BP86/TZP level of theory identifies the origin of this difference as the greater component of polarization involved in Au⋯HX interactions. Replacing the α(N)Hs with methyl groups prevents formation of a strong (NH)carbene⋯XH interaction, thus reducing the overall stabilization of the adducts. Nevertheless, the Au⋯H interactions remain largely unchanged and are strong enough to sustain the hydrogen-bonded complexes, although weak C-H⋯X interactions are often also present.

CO2-induced single-crystal to single-crystal transformations of an interpenetrated flexible MOF explained by in situ crystallographic analysis and molecular modeling.

A molecular-level investigation is reported on breathing behaviour of a metal-organic framework (1) in response to CO2 gas pressure. High-pressure gas adsorption shows a pronounced step corresponding to a gate-opening phase transformation from a closed (1cp ) to a large-pore (1lp ) form. A plateau is observed upon desorption corresponding to narrow-pore intermediate form 1np which does not occur during adsorption. These events are corroborated by pressure-gradient differential scanning calorimetry and in situ single-crystal X-ray diffraction analysis under controlled CO2 gas pressure. Complete crystallographic characterisation facilitated a rationalisation of each phase transformation in the series 1cp → 1lp → 1np → 1cp during adsorption and subsequent desorption. Metropolis grand-canonical Monte Carlo simulations and DFT-PBE-D3 interaction energy calculations strongly underpin this first detailed structural investigation of an intermediate phase encountered upon desorption.

Prevalent polymorphism in benzophenones.

We report here the crystal structures of dimorphs of 4-hydroxybenzophenone, C13H10O2, and 4-(dimethylamino)benzophenone, C15H15NO, as well as trimorphs of 4,4'-dimethylbenzophenone, C15H14O. The polymorphs were isolated from slow-evaporation conditions or from cocrystallization attempts. The main differences between the polymorphs involve differences in packing rather than differences in conformation, owing to the limited conformational freedom of the three molecules. 4-Hydroxybenzophenone is the exception, exhibiting almost identical packing arrangements in the two polymorphs, with the only major changes being in the interplanar orientations. The lattice energies of the respective polymorphs of the three compounds reported here are all within 1 kcal mol-1 of each other. The existence of nine further polymorphic benzophenone derivatives in the literature suggests that there is a good deal of polymorphic space in this class of compounds.

A thermo-responsive structural switch and colossal anisotropic thermal expansion in a chiral organic solid.

Trianglimine, a chiral triangular-shaped hexaimine, exists in at least three apohost polymorphic forms. Form I had been previously obtained by crystallisation from a mixture of dichloromethane and acetonitrile and we have now crystallised Form II from acetone. Both forms possess similar packing arrangements, but Form II undergoes a reversible phase transition to Form III, as well as colossal anisotropic positive thermal expansion. Form I does not exhibit any remarkable thermal properties.

Solvatochromism as a probe to observe the solvent exchange process in a 1-D porous coordination polymer with 1-D solvent accessible channels.

A one-dimensional porous coordination polymer (PCP) {[Cu2(acetate)2(3,6-di(pyridin-4-yl)-1,2,4,5-tetrazine)]·2CHCl3}n possesses pleochroic properties. Solvent exchange with acetonitrile and nitromethane reveals that single crystals of the PCP are also solvatochromic, with the solvent exchange process occurring in an unexpected double V-shaped pattern. Crystal surface adsorption modeling shows that the origin of this effect is preferential sorption at two faces of the crystal.

Solvent- and Pressure-Induced Phase Changes in Two 3D Copper Glutarate-Based Metal-Organic Frameworks via Glutarate (+gauche ⇄ -gauche) Conformational Isomerism.

Two isoreticular three-dimensional copper(II) glutarate-based pillared-layered metal-organic frameworks (MOFs) with flexible pillars, [Cu2(glu)2(bpa)] and [Cu2(glu)2(bpp)] (bpa = 1,2-bis(4-pyridyl)ethane; bpp = 1,3-bis(4-pyridyl)propane), undergo spontaneous phase changes upon solvent loss at room temperature. Using single-crystal X-ray diffraction analysis (SCXRD), we show that the phase changes result in new narrow-channel forms that experience a large reduction in solvent-accessible volume. Moreover, the [Cu2(glu)2(bpa)] MOF displays a stepped sorption isotherm for the uptake of CO2 at room temperature. This is indicative of reversion of the framework to the wide-channel form under CO2 pressure. Supercritical CO2 was used to isolate the gas-included structures, and by means of SCXRD we were able to determine the positions of the CO2 molecules in the channels of the frameworks. Finally, we report the use of molecular modeling simulations to elucidate the phase-change mechanism, including the energetic changes involved. Structural limitations in both MOFs allow for only direct gauche-gauche enantiomeric interconversion of the glutarate moieties.

Gold setting the "gold standard" among transition metals as a hydrogen bond acceptor - a theoretical investigation.

The Au(i) atom of dimethylaurate (DMA) is shown to behave as a hydrogen-bond acceptor, providing theoretical evidence that it can act as a Lewis base. Calculations at the MP2/aug-cc-pVTZ-pp level of theory confirm that DMA forms hydrogen bonds decreasing in strength from -16.2 kcal mol-1 to -2.4 kcal mol-1 in the order HCN ≈ HF > H2O > HCCH > NH3 > CH4, i.e. following the trend of decreasing proton acidity of the hydrogen-bond donor. The geometrical and Atoms in Molecules (AIM) parameters of the hydrogen-bonded adducts compare well to those obtained with the auride anion, a known hydrogen-bond acceptor. Relativistic effects are shown to play a dominant role in the formation of the hydrogen bonds with DMA: omission of these effects (confirmed using two different approaches) results in the loss of the hydrogen bond. Instead, the hydrogen-bond donor interacts with the carbon atom on one of the methyl ligands, yielding an adduct that is closely comparable to those found with the Cu and Ag analogues of DMA.

Trifluoromethyl: An Amphiphilic Noncovalent Bonding Partner.

The traditional "Fδ- " picture of fluorine suggests that it can only interact with electrophilic centers such as backbone-carbonyl carbon atoms or hydrogen-bond donors in proteins. We show that this view, which neglects polarization, is incomplete and the trifluoromethyl groups can act both as electrophiles and nucleophiles to form noncovalent interactions. The underlying polarization mechanism is based on the anomeric effect and is only fully operative if the geometry is allowed to relax. MP2/aug-cc-pVDZ calculations on model systems demonstrate the effect of the unusual group polarizability of trifluoromethyl. A survey of the Protein Databank reveals more than 600 weak interactions involving a trifluorotoluene moiety. The unique combination of the anomeric effect and the group-polarization process associated with it in CF3 allows its most negative molecular electrostatic potential (MEP) on the surface in contact with a nucleophile to become zero, so that the area of positive MEP on the backside of the carbon atom becomes dominant. However, the reverse polarization is also facile, so that CF3 can also act as an H-bond acceptor for cations such as the guanidinium group of arginine.

Preparing Gold(I) for Interactions with Proton Donors: The Elusive [Au]⋠⋠⋠HO Hydrogen Bond.

MP2 and DFT calculations with correlation consistent basis sets indicate that isolated linear anionic dialkylgold(I) complexes form moderately strong (ca. 10 kcal mol(-1) ) Au⋅⋅⋅H hydrogen bonds with single H2 O molecules as donors in the absence of sterically demanding substituents. Relativistic effects are critically important in the attraction. Such bonds are significantly weaker in neutral, strong σ-donor N-heterocyclic carbene (NHC) complexes (ca. 5 kcal mol(-1) ). The overall association (>11 kcal mol(-1) ), however, is strengthened by co-operative, synergistic classical hydrogen bonding when the NHC ligands bear NH units. Further manipulation of the interaction by ligands positioned trans to the carbene, is possible.