Furthermore, the Cu K α 1,2 splitting is a shortcoming since it aggravates the disappearance of the reflections into the background noise. Absorption is another issue of this rather low X-ray energy, especially in case of iodine-containing sponge crystals. However, Cu K α radiation is limited in resolution to about 0.8 Å. Modern X-ray sources and detectors lead to ever-increasing data quality of the X-ray diffraction pattern collected: higher intensity, higher redundancy and higher resolution all help in dealing with the problems inherent to MOF structures (Hoshino et al., 2016 ). For a more comprehensive overview, the recent reviews by Ramadhar, Cardenal and the Fujita group are recommended (Cardenal & Ramadhar, 2021 Zigon et al., 2021 ).įrom a crystallographic point of view, recent developments in diffractometer technology and software are also of great benefit to the CS method. Many further investigations including, for example, f-block elements as building blocks for crystalline sponges also contributed to the improvement of the CS method (de Poel et al., 2016 ). A new method for the optimization of process parameters for faster guest introduction was recently reported (Rosenberger et al., 2021 ). reported optimized parameters for the preparation of host crystals, soaking procedure and X-ray experimental conditions, as well as on a modification enabling reliable absolute structure determination (Hoshino et al., 2016 Urban et al., 2016 ). Overcoming the many issued faced by the CS method is the subject of the current work: in 2016 Fujita et al. Additionally, twinning and radiation damage both pose serious issues to a successful structural determination of the analyte. In most cases – between these two extremes – more or less severe disorder is encountered. As a result, in the worst case, the compound of interest is no longer detectable in the diffraction experiment. If, on the other hand, the interaction is too weak, the analyte in the sponge may substantially lack orientation. If this interaction between host and guest is too strong, diffusion into the cavities of the sponge is drastically hindered, if not impossible. The obtained crystal structures are often crystallographically challenging because of the nature of the particular host–guest interactions (Cardenal & Ramadhar, 2021 Hoshino et al., 2016 Zigon et al., 2021 ). Anything that can be included in the sponge can be analysed, including solutes in highly dilute solutions, gases and oils, which could not be structurally characterized by X-rays any other way (Inokuma et al., 2013 ).ĭespite the immense potential of the method and recent improvements and successes (Hoshino et al., 2016 Hayes et al., 2016 Ramadhar et al., 2015 a, b Easun et al., 2017 Duplan et al., 2016 Cuenca et al., 2016 Cardenal & Ramadhar, 2021 Bosch et al., 2014 Goh et al., 2018 Inokuma et al., 2016, 2014 Kersten et al., 2017 Lee et al., 2017 Li et al., 2019 Matsuda et al., 2016 Mitsuhashi et al., 2018 Morishita et al., 2020 Morita et al., 2020 Rissanen, 2017 Vinogradova et al., 2014 Wada et al., 2018 Waldhart et al., 2016 Yoshioka et al., 2016 Yuan et al., 2019 Zigon et al., 2015 de Poel et al., 2019 Urban et al., 2016 Sakurai et al., 2017 ) it is still far from being the universal way to determine the structure of troublesome compounds, because the method still faces some serious challenges: the time-consuming process of analysis is highly dependent on the analytes and must be carried out in separate batches for each sample. By this, they direct the guest analytes within the framework into a particular orientation: an analyte molecule can be detected as part of the crystal structure by single-crystal X-ray diffraction (SCXRD) (Hoshino et al., 2016 Hayes et al., 2016 ). Functional groups such as halides bound to the metal centre or nitrogen atoms within the linker molecule can act as hydrogen-bond acceptors and in the presence of aromatic systems allow for π– π interactions. These Fujita-type frameworks feature cavities large enough to be accessible by solvents and other small organic molecules. The linking of late first-row transition metal halides such as Zn X 2 ( X = Cl, I) by organic, nucleophilic compounds results in a three-dimensional metal–organic framework (MOF), the host or sponge. The underlying principle is that the analyte does not itself need to be crystalline, as long as it can be absorbed in a crystalline framework in a sufficiently ordered manner that its structure can be determined as a guest molecule (Inokuma et al., 2013 Stallforth & Clardy, 2013 ). The crystalline sponge (CS) method, as introduced in 2013 by Fujita et al., can facilitate the X-ray diffraction (XRD) structure determination of certain challenging samples, particularly those that are only available in minute quantities, and where single crystals cannot be obtained.
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