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Two polymorphic forms of caffeine (CAF)–glutaric acid (GLU) cocrystals have been studied via high-resolution X-ray crystallography and Bader’s quantum theory of atoms in molecules (QTAIM). For both the monoclinic, 1, and triclinic, 2, systems the experimental charge density distributions of the 1:1 ratio of CAF and GLU polymorphs have been determined and compared. Previous studies have determined that 1 is less stable than 2, in relative humidity (RH) testing. A topological analysis of the electron density distribution (EDD) revealed little difference between the two polymorph internal systems. The packing densities (0.76 vs 0.74) and lattice energies (−101.1 vs −107.1 kJ mol–1) of 1 and 2, respectively, are nearly equivalent, implying that the differences in hygroscopicity between the two polymorphs are not due to crystal lattice porosity or stability. A topological analysis of the number and strength of hydrogen bonds for 1 and 2 revealed nine hydrogen bonds in both polymorphs. “Classical” (O–H···X) hydrogen bonds were similarly present in both polymorphs, stabilizing the cocrystals. However, the sum of the stability produced from the “nonclassical” (C–H···X) bonds is higher in 2: −27.6 vs −38.2 kJ mol–1 for 1 and 2, respectively. One of the nine hydrogen bonds in 1 and 2 varies from the others, caused by the torsional rotation of the aliphatic carbon chain in GLU. This bond is critical for packing stabilization, creating a parallelogram-like packing arrangement in 2 in comparison to ribboning in 1. A Hirshfeld surface analysis found that the percentages of O–H···X hydrogen bonds were nearly identical in 1 and 2 (23.9% vs 22.1%); however, the H···H contacts were higher in 2 (61.4% vs 65.8% for 1 and 2, respectively), suggesting that more hydrogen-based contacts require competitive displacement by water in the hydration of 2 in comparison to 1. Additionally, a stabilizing aromatic cycle stack between CAF molecules is present in 2 due to the varied parallelogram packing arrangement, which was absent in 1; this provided ∼11.3 kJ mol–1 of stability to the system of 2. The solid-state entropies and molecular dipole moments (MDMs) of 1 and 2 supported the relative stability of the individual polymorphs, with 1 having a higher entropy and dipole moment in comparison to 2 (123.2 vs 112.8 J K–1 mol–1 and 7.45 and 4.93 D for 1 and 2, respectively), implying that it has the potential to hydrate more rapidly. These findings are in good agreement with previous experimental RH stability studies, giving further insight into the information gained from thermally averaged ground-state crystal electron density data.

Experimental charge density distribution studies complemented by quantum mechanical theoretical calculations of 4-hydroxybenzoic acid (4HBA) (1), 4,4'-bipyridine (44BP) (2) and one polymorphic form of the co-crystal containing 4HBA and 44BP molecules in a 2:1 ratio (3), have been carried out via high resolution single-crystal X-ray diffraction. Synthon formation was found to be the main driving force for crystallisation in both (1) and (3) with a carboxylic acid homosynthon present in (1) and a heterosynthon in (3) comprised of a carboxylic acid from 4HBA and a pyridine nitrogen and aromatic hydrogen from 44BP. Topological analysis revealed the bonding in the homosynthon to be stronger than the heterosynthon (305.88 versus. 193.95 kJ mol-1) with a greater number of weak interactions in (3) helping to stabilise the structure. The distance from the hydrogen and hydrogen bond acceptor to the bond critical point (bcp) was also found to be a significant factor in determining bond strength, potentially having a greater effect than lone pair directionality. Two different methods of lattice energy calculations were carried out and both methods found (1) to be more stable than (3) by ~40 and 10 kJmol-1 for the LATEN and PIXEL methods respectively. Energy framework diagrams reveal (1) to be dominated by coulombic forces while both coulombic and dispersion forces are prominent in (3) contributing equally to the lattice energy. This study examined the utility of homosynthons and heterosynthons in future crystal engineering endeavours and concluded that although in this case the single molecule crystal was more thermodynamically stable, the asymmetry of the co-crystal system allowed it to form a wider range of interactions resulting in only a small reduction in stability. This highlights the potential of using heterosynthons to develop co-crystals to improve pharmaceuticals. These findings highlight the utility of high resolution single crystal X-ray crystallography in rationalising observed physical properties.

The reaction of phosphole/arsole starting materials with a series of halide abstraction reagents afforded their respective phosphenium/arsenium complexes. UV–vis absorption and luminescence studies on these cations showed interesting emission profiles, which were found to be dependent upon counterion choice. The addition of a reductant to the phosphole reagent garnered a dimeric species with a central P–P bond, which when heated was found to undergo homolytic bond cleavage to produce an 11π radical complex. Electron paramagnetic resonance (EPR), supported by density functional theory (DFT) calculations, was used to characterize this radical species.

A comprehensive study of the complexes A4[U(NCS)8] (A = Cs, Et4N, nBu4N) and A3[UO2(NCS)5] (A = Cs, Et4N) is described, with the crystal structures of [nBu4N]4[U(NCS)8]·2MeCN and Cs3[UO2(NCS)5]·O0.5 reported. The magnetic properties of square antiprismatic Cs4[U(NCS)8] and cubic [Et4N]4[U(NCS)8] have been probed by SQUID magnetometry. The geometry has an important impact on the low-temperature magnetic moments: at 2 K, μeff = 1.21 μB and 0.53 μB, respectively. Electronic absorption and photoluminescence spectra of the uranium(IV) compounds have been measured. The redox chemistry of [Et4N]4[U(NCS)8] has been explored using IR and UV–vis spectroelectrochemical methods. Reversible 1-electron oxidation of one of the coordinated thiocyanate ligands occurs at +0.22 V vs Fc/Fc+, followed by an irreversible oxidation to form dithiocyanogen (NCS)2 which upon back reduction regenerates thiocyanate anions coordinating to UO22+. NBO calculations agree with the experimental spectra, suggesting that the initial electron loss of [U(NCS)8]4– is delocalized over all NCS– ligands. Reduction of the uranyl(VI) complex [Et4N]3[UO2(NCS)5] to uranyl(V) is accompanied by immediate disproportionation and has only been studied by DFT methods. The bonding in [An(NCS)8]4– (An = Th, U) and [UO2(NCS)5]3– has been explored by a combination of DFT and QTAIM analysis, and the U–N bonds are predominantly ionic, with the uranyl(V) species more ionic that the uranyl(VI) ion. Additionally, the U(IV)–NCS ion is more ionic than what was found for U(IV)–Cl complexes., We thank TCD (R.J.B. and E.H.) for funding this work. J.A.P. is grateful to UK National Service for Computational Chemistry Software. M.E. and G.L. thank MINECO (MAT2012-38318-C03).