Guest-occupiable space in the crystalline solid state: A simple rule-of-Thumb for predicting occupancy

The generally greater degree of thermal motion of guest molecule(s) relative to the host often impedes their accurate modelling in crystal structures. We propose a 'rule-of-Thumb' for estimating the maximum number of guest molecules that can be accommodated in a given amount of accessible space in an adequately modelled host structure. A survey of the Cambridge Structural Database was carried out to evaluate the fractional occupancy θ of the accessible space for almost 40 000 solvates involving 20 common solvents. Using widely accessible software tools, the volume of a guest is estimated as its van der Waals surface, while the guest-occupiable space of a potentially porous host is determined as that available to a virtual spherical probe. We propose terminology more appropriate to the supramolecular interpretation of surface typology: The probe-Traversable and probe-Accessible boundaries as traced out by the locus and surface of a spherical probe, respectively. High-Throughput analysis using commercial and free software packages yielded a mean θ = 51.1(4)%, ranging from 45.3(6)% for hexane to 60(1)% for acetic acid. © The Royal Society of Chemistry.</p

Elucidating the mechanism responsible for anomalous thermal expansion in a metal-organic framework

The previously reported anisotropic thermal expansion of a three-dimensional metal-organic framework (MOF) is examined by means of theoretical calculations. Inspection of the 100, 190, 280 and 370 K single crystal X-ray diffraction (SCD) structures indicated a concerted change in the coordination sphere of the zinc centre leading to elongation of the coordination helix in the crystallographic c direction (the Zn-O(H)-Zn angle expands), while the largely unaltered ligands (the Zn⋯L⋯Zn distance remains constant) are pulled closer together in the ab plane. This study develops and evaluates a mechanistic model at the DFT level of theory that reproduces the convergent expansion of the coordination helix of the material. The linear increase in energy calculated for extension of a model consisting of six zinc centres and truncated ligands compares favourably with results obtained from a periodic DFT evaluation of the SCD structures. It was also found that the anisotropic thermal expansion trend could be reproduced qualitatively by Molecular Dynamics (MD) simulations in the NPT ensemble. © The Royal Society of Chemistry 2016.</p

Direct in Situ Crystallographic Visualization of a Dual Mechanism for the Uptake of CO2 Gas by a Flexible Metal-Organic Framework

We report a flexible metal-organic framework, [Co2(OBA)2(BPMP)]n (COB), with a new network topology. COB displays structural flexibility under CO2 gas pressure at 298 K, and the resultant porous phases have been characterized by in situ X-ray diffraction analysis. We show that activation yields a framework with discrete voids and substantial reduction in guest-accessible volume. Single-crystal X-ray diffraction analysis under controlled CO2 pressure shows that COB exhibits a breathing mode of flexibility, combined with an overall swelling of the framework. This combination of mechanisms is highly unusual. © 2019 American Chemical Society.</p

A new dynamic framework with direct: In situ visualisation of breathing under CO2 gas pressure

Using in situ single-crystal X-ray diffraction analysis (SCD), direct structural evidence has been obtained for breathing behaviour of a new flexible MOF COB1. Desolvation of the as-synthesised material results in deformation of the coordination geometries due to flexibility of the bridging ligands, and the framework is capable of switching from a wide-pore to a narrow-pore form, with substantial reduction in guest-accessible volume. Upon exposure to CO2 gas the activated framework breathes and switches back to the wide-pore phase. This breathing behaviour is supported by gas sorption analysis, pressure-gradient differential scanning calorimetry, powder X-ray diffraction analysis and molecular modelling. © The Royal Society of Chemistry 2019.</p

Reversible thermosalience of 4-aminobenzonitrile

Crystals of 4-aminobenzonitrile grown by sublimation undergo reversible thermosalient phase changes during cooling and subsequent heating. Single-crystal diffraction studies have been carried out at 20 K intervals during cooling from 300 to 100 K in order to explain the structural change that occurs. © 2018 The Royal Society of Chemistry.</p

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. © 2019 The Royal Society of Chemistry.</p