A conformational polymorph of Ph3PAu[SC(OEt)=NPh] featuring an intramolecular Au···π interaction

Abstract A conformational polymorph, form β, for Ph3PAu[SC(OEt)=NPh] has been characterised. Like the original structure (form α), the molecule features a linear P–Au–S geometry. The difference between molecular structures rests with the relative disposition of the thiolate ligand which places the N-bound aryl ring in close proximity to the gold centre (form β) rather than the oxygen atom (form α). Density functional theory calculations show the molecule with the Au···π interaction is more stable by 5.2 kcal/mol than the one with the Au···O contact. The molecular packing of both forms are stabilised by C–H···O and C–H···π interactions which make approximately the same contribution to the overall Hirshfeld surfaces. However, key indicators, e.g. crystal packing efficiency and density, and the computational results suggest form β is the thermodynamically favoured form.</jats:p

Snap frozen! Capturing two metastable polymorphs in a tetramorphic one-dimensional coordination polymer constructed from cadmium, dithiophosphate, and 4-pyridinealdazine

Four polymorphs are described for the one-dimensional coordination polymer {Cd[S2P(OMe)2]2(4-pyridine-aldazine)}n (1) which features a trans-N2S4 distorted octahedral coordination geometry. Lattice energy calculations show the room temperature form, 1α, is the thermodynamically stable phase, being 1.52 kcal/mol more stable than the 1β form per unit formula. The latter is formed at 233 K on cooling 1α based on single crystal data. Single crystals of forms 1α and 1β exhibit a reversible single-crystal-single-crystal phase change which also 2 occurs in the bulk form as seen in DSC and PXRD experiments. When 1 was “snap frozen” at 100 K under a cold stream, 1β was observed most of the time but sometimes forms 1γ and 1δ were found. When warmed, 1β converted to 1α by 293 K, and each of 1γ and 1δ first converted to 1β (120-130 and 140-150 K, respectively) and then to 1α (280 K), correlating with the calculated lattice energies; subsequent cooling only yielded 1β, indicating 1γ and 1δ are metastable forms. The polymorphs exhibit different molecular packing patterns based on different non-covalent interactions and these have been evaluated by Hirshfeld surface analyses and NCI plots

Compressibility and phase stability of iron-rich ankerite

ABSTRACT: The structure of the naturally occurring, iron-rich mineral Ca₁․₀₈(₆)Mg₀.₂₄(₂)Fe₀.₆₄(₄)Mn₀.₆₄(₄)(CO₃)₂ ankerite was studied in a joint experimental and computational study. Synchrotron X-ray powder diffraction measurements up to 20 GPa were complemented by density functional theory calculations. The rhombohedral ankerite structure is stable under compression up to 12 GPa. A third-order Birch-Murnaghan equation of state yields V₀ = 328.2(3) ų, bulk modulus B₀ = 89(4) GPa, and its first-pressure derivative B'₀ = 5.3(8)-values which are in good agreement with those obtained in our calculations for an ideal CaFe(CO₃)₂ ankerite composition. At 12 GPa, the iron-rich ankerite structure undergoes a reversible phase transition that could be a consequence of increasingly non-hydrostatic conditions above 10 GPa. The high-pressure phase could not be characterized. DFT calculations were used to explore the relative stability of several potential high-pressure phases (dolomite-II-, dolomite-III- and dolomite-V-type structures), and suggest that the dolomite-V phase is the thermodynamically stable phase above 5 GPa. A novel high-pressure polymorph more stable than the dolomite-III-type phase for ideal CaFe(CO₃)₂ ankerite was also proposed. This high-pressure phase consists of Fe and Ca atoms in sevenfold and ninefold coordination, respectively, while carbonate groups remain in a trigonal planar configuration. This phase could be a candidate structure for dense carbonates in other compositional systems.This research was funded by the Spanish Ministerio de Ciencia, Innovación, y Universidades (MICINN) under the projects MALTA Consolider Ingenio 2010 network MAT2015-71070-REDC and PGC2018-097520-A-I00 (co-financed by EU FEDER funds), and by the Generalitat Valenciana under project PROMETEO/2018/123. D.S.-P. and A.O.R. acknowledge the financial support of the Spanish MINECO for the RyC-2014-15643 and RyC-2016-20301 Ramon y Cajal Grants, respectively. C.P acknowledges the financial support of the Spanish Ministerio de Economia y Competitividad (MINECO project FIS2017-83295-P)

Phase stability of stress-sensitive Ag2CO3 silver carbonate at high pressures and temperature

Silver carbonate (Ag2CO3) is a material currently used for artificial carbon storage. In this work, we report synchrotron X-ray powder diffraction (XRD) experiments under high pressure and high temperature in combination with density-functional theory (DFT) calculations on silver carbonate up to 13.3 GPa. Two pressure-induced phase transitions were observed at room temperature: at 2.9 GPa to a high-pressure (HP1) phase and at 10.5 GPa to a second high-pressure phase (HP2). The facts that a) the HP2 phase can be indexed with the initial P21/m structure, b) our DFT calculations predict the initial structure is stable in the entire pressure range, and c) the HP2 phase is stable under decompression suggest that the intermediate HP1 phase is a product of the appearance of non-hydrostatic stresses in the sample. The observed structural transformations are associated to a high sensitivity of this compound to non-hydrostatic conditions. The compressibility of Ag2CO3 has also been determined, showing the c axis is the most compressible and that the bulk modulus increases quickly with applied pressure. We attribute both observations to the weak nature of the closed-shell Ag–Ag interactions in this material. The behavior of Ag2CO3 under heating at approximately 3 GPa was also studied. No temperature-induced phase transitions were found at this pressure, and the thermal expansion was determined to be relatively high for a carbonate.Authors thank the financial support from the Spanish Ministerio de Ciencia e Innovación (MICINN) and the Agencia Estatal de Investigación under projects MALTA Consolider Ingenio 2010 network (RED2018-102612-T) and PGC2021-125518NB-I00 (cofinanced by EU FEDER funds), and from the Generalitat Valenciana under projects CIAICO/2021/241 and MFA/2022/007. A.O.R. acknowledges the financial support of the Spanish MINECO RyC-2016-20301 Ramón y Cajal Grant and the project AYUD/2021/51036 of the Principality of Asturias (cofinanced by EU FEDER funds). Authors also thank the MALTA Consolider supercomputing centre and Compute Canada for computational resources and ALBA-CELLS synchrotron for providing beamtime under experiments 2020084419 and 2021024988. These experiments were performed at the MSPD beamline with the collaboration of ALBA staff

Phase stability and dense polymorph of the BaCa(CO3)2 barytocalcite carbonate

The double carbonate BaCa(CO3)2 holds potential as host compound for carbon in the Earth?s crust and mantle. Here, we report the crystal structure determination of a high-pressure BaCa(CO3)2 phase characterized by single-crystal X-ray diffraction. This phase, named post-barytocalcite, was obtained at 5.7 GPa and can be described by a monoclinic Pm space group. The barytocalcite to post-baritocalcite phase transition involves a significant discontinuous 1.4% decrease of the unit-cell volume, and the increase of the coordination number of 1/4 and 1/2 of the Ba and Ca atoms, respectively. High-pressure powder X-ray diffraction measurements at room- and high-temperatures using synchrotron radiation and DFT calculations yield the thermal expansion of barytocalcite and, together with single-crystal data, the compressibility and anisotropy of both the low- and high-pressure phases. The calculated enthalpy differences between different BaCa(CO3)2 polymorphs confirm that barytocalcite is the thermodynamically stable phase at ambient conditions and that it undergoes the phase transition to the experimentally observed post-barytocalcite phase. The double carbonate is significantly less stable than a mixture of the CaCO3 and BaCO3 end-members above 10 GPa. The experimental observation of the high-pressure phase up to 15 GPa and 300 ºC suggests that the decomposition into its single carbonate components is kinetically hindered.Authors thank the fnancial support from the Spanish Ministerio de Ciencia e Innovación (MICINN) and the Agencia Estatal de Investigación under projects MALTA Consolider Ingenio 2010 network (RED2018-102612-T), PID2019-106383GB-C44, FIS2017-83295-P and PGC2018-097520-A-I00 (cofnanced by EU FEDER funds), and from the Generalitat Valenciana under project PROMETEO/2018/123. A.O.R. acknowledges the fnancial support of the Spanish MINECO RyC-2016-20301 Ramon y Cajal Grant. Authors also thank Dr. Nicolescu and the Mineralogy and Meteoritic Department of the Yale Peabody Museum of Natural History for providing the mineral samples, the MALTA Consolider supercomputing centre and Compute Canada for computational resources, the General Services of Research Support (SEGAI) at La Laguna University and ALBA-CELLS synchrotron for providing beamtime under experiments 2020084419 and 2021024988. Tese experiments were performed at the MSPD beamline with the collaboration of ALBA staf

Interplay between local structure and electronic properties on CuO under pressure

The electronic and local structural properties of CuO under pressure have been investigated by means of X-ray absorption spectroscopy (XAS) at Cu K edge and ab-initio calculations, up to 17 GPa. The crystal structure of CuO consists of Cu motifs within CuO$_4$ square planar units and two elongated apical Cu-O bonds. The CuO$_4$ square planar units are stable in the studied pressure range, with Cu-O distances that are approximately constant up to 5 GPa, and then decrease slightly up to 17 GPa. In contrast, the elongated Cu-O apical distances decrease continuously with pressure in the studied range. An anomalous increase of the mean square relative displacement (EXAFS Debye Waller, \sigma$^2$) of the elongated Cu-O path is observed from 5 GPa up to 13 GPa, when a drastic reduction takes place in \sigma$^2$. This is interpreted in terms of local dynamic disorder along the apical Cu-O path. At higher pressures (P>13 GPa), the local structure of Cu$^{2+}$ changes from a 4-fold square planar to a 4+2 Jahn-Teller distorted octahedral ion. We interpret these results in terms of the tendency of the Cu$^{2+}$ ion to form favorable interactions with the apical O atoms. Also, the decrease in Cu-O apical distance caused by compression softens the normal mode associated with the out-of-plane Cu movement. CuO is predicted to have an anomalous rise in permittivity with pressure as well as modest piezoelectricity in the 5-13 GPa pressure range. In addition, the near edge features in our XAS experiment show a discontinuity and a change of tendency at 5 GPa. For P < 5 GPa the evolution of the edge shoulder is ascribed to purely electronic effects which also affect the charge transfer integral. This is linked to a charge migration from the Cu to O, but also to an increase of the energy band gap, which show a change of tendency occurring also at 5 GPa

Hydrogen bonding in 2,6-bis(4-fluorophenyl)-3,5-dimethylpiperidin-4-one methanol solvate

The crystal structure analysis of a 2,6-diaryl 4-piperidone derivative, isolated as a mono-methanol solvate, reveals that both the piperidone and the methanol molecule lie on a crystallographic mirror plane. A chair conformation is found for the piperidone ring with the aryl and methyl groups in equatorial positions. The most prominent feature of the molecular packing is the formation of supramolecular zigzag chains mediated by amine-N–H···O(methanol) and hydroxyl-O–H···N(amine) hydrogen bonds, i.e. the methanol molecule serves as a bridge between piperidone molecules. The molecular structure is compared with that determined in an unsolvated form and the gas-phase equilibrium structure, obtained using density-functional theory (DFT); differences relate, in the main, to the relative dispositions of the aryl rings. An analysis of the Hirshfeld surfaces of the experimental structures indicates very similar relative contributions with the notable exception being the contribution by O···H/H···O which at 13.7% in the methanol solvate is >8.5% in the unsolvated form

Exploring C-H∙∙∙metalloaromatic interactions

The C-H···metalloaromatic interaction is an of intermolecular contact which is less explored compared to the common secondary interactions such as hydrogen bonding and π··· π stacking. A series of 12 palladium(II) xanthate complexes was prepared by varying the xanthate side group from one carbon to six carbons, i.e. primary, secondary and tertiary hydrocarbons. All 12 complexes were crystallised and the intermolecular interactions present in each system were delineated. Among the series, 11 out of 12 palladium(II) xanthates showed at least one pair of the anticipated C-H···metalloaromatic π interaction, and up to a maximum of six pairs as identified in Pd[S2CO(neo-hexyl)]2. Further, the CH···metalloaromatic π interactions led to the formation of zero, one and even two dimensional motifs in the respective crystals. From a computational chemistry study performed on Pd[S2CO(n-propyl)]2, it is revealed that a C-H···metalloaromatic π interaction is of similar strength as intermolecular Pd···S secondary bonding which led to the dimerisation of palladium(II) xanthate in the condensed phase