Crystal engineering in Africa

This themed collection celebrates the diversity and excellence of research in crystal engineering being carried out across Africa.

2023 has been a momentous year for crystallography in Africa. This year, the African Crystallographic Association (AfCA) was approved as the fifth regional associate of the International Union of Crystallography (IUCr). This is a sound indicator of how far crystallography has developed on the African continent in recent years. There is a strong tradition of crystallography in Africa, but historically this was focused in the far North and the far South. Over the past decade, there has been a significant drive to increase crystallographic education and practice across the continent. Student development, schools and workshops, conferences, and many other activities have been strongly supported by the crystallography community worldwide. The International Year of Crystallography in 2014 provided the impetus to bring people together across the continent, and the inauguration of AfCA in 2021 was a significant milestone for the African crystallography community.

This collection showcases some current crystallographic research in Africa and includes papers from eight countries across the continent.

There is a broad range of science represented in the collection, reflecting how widespread crystal engineering concepts have become in applications in chemistry such as metal–organic frameworks, biomedicine, and solar cell technology. The modular ‘building block’ approach to metal–organic frameworks (MOFs) allows researchers to generate frameworks tuned to applications and function. Hungwe et al. from Zimbabwe (https://doi.org/10.1039/d3ce00039g) used a phenanthroline modulator to tune the topology of a lanthanide MOF from 2D to 3D architectures, yielding a 3D MOF with the rare zbj topology. Network analysis showed that the 3D MOF formed in this process is isoreticular with the previously reported MOF-80, despite being built from different linkers and metal ions. Malaza et al., from the same research group in Zimbabwe, (https://doi.org/10.1039/d1ce00999k) reported two metal–organic frameworks (MOFs) with copper(II) paddlewheel secondary building units (SBUs) as potential sorbents for carbon dioxide. One of the MOFs contains both copper(II) paddlewheels and palladium(II) chelate SBUs, generating a 2D MOF with a large void. While both MOFs showed preferential uptake of carbon dioxide over nitrogen, this was relatively low which was attributed to a crystal-to-crystal transformation of the MOFs on activation.

The study of inclusion compounds, or host–guest chemistry, has generated a wide range of applications including solid-state reactivity, separation, catalysis, and chemical sensors. The selectivity of a given host compound towards particular guests is key to this field of study. The prolific research group of Barton and co-workers from South Africa reported on three different inclusion systems in which an organic host molecule encapsulates a variety of guest species, often demonstrating the ability to separate closely related guest molecules. (https://doi.org/10.1039/d3ce00101f, https://doi.org/10.1039/d3ce00080j and https://doi.org/10.1039/d3ce00174a). Lombard et al. from South Africa (https://doi.org/10.1039/d1ce01286j) investigated the selectivity of 1,10-phenanthrolinium pamoate towards two series of guests both through solution-phase crystallization and mechanochemical methods. They demonstrated that the selectivity profile of a particular host–guest system shows reproducible differences for these two methods of synthesis, possibly owing to the balance of kinetic and thermodynamic parameters. Materials having sigmoidal-shaped sorption isotherms show good potential for separation of guest molecules. Tsobnang and colleagues from Cameroon carried out a study to correlate the structural features related to this isotherm shape. They used in situ powder X-ray diffraction (PXRD) to study the adsorption of water into two nominally non-porous supramolecular complexes, [Co(amp)₃][Cr(ox)₃] and [Cu₂(amp)₄Cl][Cr(ox)₃]·1H₂O (amp = 2-aminomethyl pyridine, ox = oxalate) (https://doi.org/10.1039/d2ce00138a). This study showed that the stepped water sorption isotherm and large hysteresis loop observed for these materials are coordinated with a discontinuous phase transition, with a structural change mainly observed along the b-axis which is the stacking direction of the layers in the crystal structure. By considering the hydrogen bonds in the interlayer regions of the complexes, they could explain the gate effect observed during the hydration/dehydration phase transition.

Crystal engineering of organic materials is well-represented in this collection. Lau, Khorasani and Fernandes from South Africa (https://doi.org/10.1039/d3ce00388d) found that, by encapsulating the crystals in epoxy glue, the Diels–Alder reaction in a charge-transfer crystal could proceed with predictable topochemistry to a greater conversion extent than for crystals exposed to air. They suggest that this may prove to be extendable to other single-crystal to single-crystal reactions. Another intriguing method of producing crystalline materials is the use of sublimation. In their article, Volkwyn and Haynes from South Africa (https://doi.org/10.1039/d3ce00732d) show that hydrates of a series of organic molecules can be crystallized through co-sublimation in the presence of water. While a mixture of hydrated and anhydrous forms is generally obtained, in several cases an increase in the quantity of water used in the sublimation system results in an increase in the water content of the sublimed crystals. Multicomponent crystals, such as co-crystals, solvates or salts, often display improved physicochemical properties over their parent molecule(s). Polymorphism, and its relevance to active pharmaceutical ingredients (APIs), is a well-recognized and productive field of study in many countries in Africa. One-third of APIs are believed to be prone to forming hydrates, so the study of the polymorphism of hydrates is attracting interest. Ghouari et al. from Algeria (http://doi.org/10.1039/d3ce00766a) report on the characterization of the seventh polymorph of gallic acid monohydrate (GAM-VII). This new polymorph forms a centrosymmetric (COOH)₂ dimer such as has been observed in GAM-I, GAM-III and GAM-V. However the intermolecular interactions in this polymorph include hydrogen-bonded water bridges stabilizing the stacking of GA molecules, and a further disordered water molecule is a triple donor and double acceptor of hydrogen bonds. Theoretical calculations offer further insight into the properties of the polymorphs and are consistent with the experimental structural results. The work highlights the importance of water in crystal engineering.

The crystal engineering of coordination compounds and coordination polymers (CPs) is rapidly expanding on the continent. Zazouli et al. from Morocco (https://doi.org/10.1039/d2ce01001a) synthesized five coordination polymers with M(II) ions and a flexible organic linker with an ester junction. The free rotation around the central ethylene bond allows the CPs to be formed with a range of dimensionalities and conformations. Both syn–syn–syn and anti–anti–anti conformations were observed. A research group from Algeria, led by Setifi (https://doi.org/10.1039/d2ce00485b), performed a detailed topological study of dicyanamides of d-metals. Interestingly, although 74 different topologies are possible, 80% of the 1295 dicyanamides reported have one of only 10 topologies. Using this data, they showed that a knowledge-based system could be used to predict the possible architectures of novel dicyanamides, testing this system on five new complexes. From Cameroon, Dazem et al. (https://doi.org/10.1039/d2ce01513g) have published the latest in a series of papers studying the structural diversity shown by copper(II)oxalate when incorporated into salts with pyridine-derived cations. Several different coordination modes have been observed and the authors note that the cationic species are key to tuning the architectures and supramolecular networks formed by these compounds. Magnetic studies showed that the spin centres in these CPs are antiferromagnetically coupled. Zacharias, Ramon and Bourne from South Africa (https://doi.org/10.1039/d2ce00039c) investigated the effect of guest inclusion on the flexibility of a 2D coordination polymer {[Co(btc)(DMF)₂]·solvent}_n (where btc is benzene-1,3,5-tricarboxylate and DMF is N,N-dimethylformamide). The CP showed reversible solvatochromism on exchange of the guest species. A survey of related structures in the Cambridge Structural Database found four motifs which cannot be predicted from either the composition or space group symmetry of the compound.

Coordination compounds are also studied in many groups for their potential applications. Tella et al. (https://doi.org/10.1039/d3ce00236e) from Nigeria have used mixed-ligand coordination polymers to study the adsorption of refractory sulfur compounds such as those found upon combustion of fuel oils. The physisorption of dibenzothiophene by these CPs was shown to be enhanced by the formation of dispersion interactions between the linkers and dibenzothiophene. As part of a multinational collaboration involving scientists from Egypt, Saudi Arabia, Finland and Sweden, Yousri et al. (https://doi.org/10.1039/d3ce00417a) screened several silver(I) quinoline salts for their antimicrobial activity. Both [Ag(5NO₂Qu)₂]BF₄ and [Ag(Qu3CN)(H₂O)]BF₄ (where 5NO₂Qu is 5-nitroquinoline and Qu3CN is quinoline-3-carbonitrile) show high antifungal potency against C. albicans. Both salts have better activity against P. mirabilis than the commonly used antibiotic amoxicillin. Jabeur et al. from Tunisia (https://doi.org/10.1039/d2ce01138g) have shown that the new hybrid complex (AmpH)₂[MnCl₄(Amp)₂], where Amp is 2-amino-4-methylpyrimidine, has an emission which can be varied from white light to red-NIR by variation of the excitation wavelengths. Potential applications of this material include organic light-emitting diodes (OLEDs) and bio-sensing and bio-imaging applications.

The synthesis of new materials for specific applications remains a key study in crystal engineering. Bendahhou et al. from Morocco (https://doi.org/10.1039/d0ce01561j) reported the synthesis of a ceramic material in which the effects of the Ti/Zn ratio on structural and dielectric properties were studied. Refinement of the XRD data by the Rietveld method found a tetragonal crystal structure at room temperature across a range of Ti/Zn ratios and confirmed the formation of oxygen vacancies. Scanning electron microscopy (SEM) showed that the ceramics have a homogeneous microstructure and low porosity. Metal oxides have many applications in optoelectronic devices and as charge-transporting layers in perovskite solar cells. Mohamed from Egypt, with co-authors from China, investigated Ta-doped β-Ga₂O₃ crystals before and after annealing in air (https://doi.org/10.1039/d0ce01639j). They established that the crystal quality of the doped crystal improves and the work function increases after annealing, which could be attributed to alterations in the oxygen sites during the annealing process. Sa'aedi, Akl and Hassanien from Egypt (https://doi.org/10.1039/d2ce00483f) considered the role of rubidium as a dopant to control the microstructure and morphology of zinc(II) oxide nanoparticles. Using a sol–gel technique, a series of doped samples were produced. Increasing the Rb concentration causes shifts of the PXRD peaks to lower diffraction angles, correlating with the decrease in size of the nanoparticles. The complex perovskite oxides or BCZT (Ba_0.85Ca_0.15Zr_0.1Ti_0.9O₃) materials have remarkable dielectric, ferroelectric and piezoelectric properties which suggest their relevance in energy storage and energy harvesting applications. Hanani et al. from Morocco (https://doi.org/10.1039/d1ce00591j) have used templated-growth methods with hydrogen zirconate titanate nanowires (HZTO-NWs) to obtain nanoscale BCZT under hydrothermal conditions. The synthesis proceeds by a morphological transition and the parameters have been determined to ensure that the desirable BCZT phase is obtained reproducibly. Hamdi and Tlili from Tunisia (https://doi.org/10.1039/d2ce00099g) studied the influence of sodium and magnesium salts on the nucleation of calcium carbonate crystals. Using electrical conductivity modelling applied to the fast-controlled precipitation data they showed the existence of mixed ion pairs during the prenucleation stage. While the foreign salt does not completely inhibit the formation of ion pairs, it influences the nucleation kinetics and thermodynamics. This study thus provides improved insight into the crystallization of calcium carbonate. Lithium molybdate (LMO) is useful as a corrosion inhibitor in industrial air conditioning, but LMO crystals have also found application as cryogenic phonon-scintillation detectors for the detection of neutrinoless double beta decay from isotopes. Since cracks in LMO crystals can affect their material performance in these high-tech applications, Bouzouaoui et al. from Algeria (https://doi.org/10.1039/d1ce00766a) calculated the three-dimensional stress field over different stages in the crystal growth of an LMO crystal grown in a Czochralski furnace. This numerical model showed that thermal stress is related to the growth direction, thus it is important to take into account the crystal anisotropy.

This collection highlights the breadth and relevance of research in crystal engineering that is being performed across the African continent. We thank all contributors to this themed collection, as well as reviewers for their feedback, and also express our appreciation to the CrystEngComm editorial team.

It is clear that the future of crystallographic research in Africa is bright.

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