Michael B. Hursthouse

Two substituted phosphonium tetrahydoxidohexaoxidopentaborate(1-) salts, [ⁱPrPPh₃][B₅O₆(OH)₄]·3.5H₂O (1) and [MePPh₃][B₅O₆(OH)₄]·B(OH)₃·0.5H₂O (2), were prepared by templated self-assembly processes with good yields by crystallization from basic methanolic aqueous solutions primed with B(OH)₃ and the appropriate phosphonium cation. Salts 1 and 2 were characterized by spectroscopic (NMR and IR) and thermal (TGA/DSC) analysis. Salts 1 and 2 were thermally decomposed in air at 800 °C to glassy solids via the anhydrous phosphonium polyborates that are formed at lower temperatures (2/g. Rhe recrystallization of 1 and 2 from aqueous solution afforded crystals suitable for single-crystal XRD analyses. The structure of 1 comprises alternating cationic/anionic layers with the H₂O/pentaborate(1-) planes held together by H-bonds. The cationic planes have offset face-to-face (off) and vertex-to-face (vf) aromatic ring interactions with the ⁱPr groups oriented towards the pentaborate(1-)/H₂O layers. The anionic lattice in 2 is expanded by the inclusion of B(OH)₃ molecules to accommodate the large cations; this results in the formation of a stacked pentaborate(1-)/B(OH)₃ structure with channels occupied by the cations. The cations within the channels have vf, ef (edge-to-face), and off phenyl embraces. Both H-bonding and phenyl embrace interactions are important in stabilizing these two solid-state structures.

The crystal structure of the cis isomer of cis-bis(L-DOPA-κ² N,O)copper(II) monohydrate (L-DOPA is 3,4-dihydroxy-L-phenylalanine) (CuLD), [Cu(C₉H₁₀NO₄)₂]·H₂O, is a singular example of a structurally characterized, homoleptic, crystalline metal L-DOPA complex. CuLD crystallizes in the space group P2₁, with Z′ = 2. The two independent molecules are square planar, and are interconnected by a linear hydrogen-bonded chain containing 12 independent hydrogen bonds. The copper ions in both molecules have weak apical intermolecular Cu...O interactions [2.739 (2) and 2.973 (2) Å] with catechol –OH groups. A survey of the Cambridge Structural Database suggested that cis and trans isomers of Cu(NH₂–C–CO₂)₂ amino acid complexes are equally likely to occur. 12 strong O—H...O and N—H...O hydrogen bonds stabilize an unusual linear arrangement of the Cu complexes. The Cu...Cu′ distances along the chain are nearly equal [5.0739 (3) and 5.1107 (3) Å] and the Cu...Cu′...Cu angles are nearly linear [176.75 (1)°]. The MATCH procedure available in the Oxford University Crystals for Windows package was used to carry out a detailed analysis of the relationship between the two independent molecules. MATCH has some particular advantages in studying the details of pseudosymmetry, which include: (i) no atomic-order requirements; (ii) the pseudosymmetry matrix is readily available, which allows quick insight into the symmetry elements involved and their location; and (iii) the differences between molecular centroids, as well as between all atomic positions and torsion angles, are listed. A tutorial presentation is designed to attract new users to the technique. In the present case, a search for a pseudosymmetric relationship between the two independent molecules showed that they are related by a pseudo-4₂ axis along the crystallographic c direction. A detailed analysis shows that the pseudo-4₂ symmetry is disrupted by torsions about the CH₂—C(ipso) bonds, and that there is no supergroup that can be used to describe the crystal structure.

Structural systematics is the comparison of sets of chemically related crystal structures with the aim to establish and describe relevant similarities and relationships. An important topic in this context is the comparison of hydrogen-bonded structures (HBSs) and their representation by suitable descriptors. Three different description methods for HBSs are proposed, a graphical representation, a symbolic representation and connectivity tables. The most comprehensive description is provided by a modified graph of the underlying net topology of an HBS which contains information on the multiplicity of links, the directionality and chemical connectivity of hydrogen bonds and on symmetry relations. By contrast, the alternative symbolic representation is restricted to essential properties of an HBS, i.e. its dimensionality, topology type and selected connectivity characteristics of nodes. A comparison of their connectivity tables readily identifies differences and similarities between crystal structures with respect to the intermolecular interaction modes adopted by their functional groups. The application of these methods to the known polymorphs of sulfathiazole and sulfapyridine is demonstrated and it is shown that they enable the rationalisation of previously reported and intricate relationships. The proposed methods facilitate the comprehensive description of the most important relevant aspects of an HBS, including its chemical connectivity, net topology and symmetry characteristics, and they represent a new way to recognise similarities and relationships in organic crystal structures.

Background: Derivatives of fulgides have been shown to have interesting photochromic properties. We have synthesised a number of such derivatives and have found, in some cases, that crystals can be made to change colour on crushing, a phenomenon we have termed “tribochromism”. We have studied a number of derivatives by X-ray crystallography, to see if the colour is linked to molecular structure or crystal packing, or both, and our structural results have been supported by calculation of molecular and lattice energies. Results: A number of 5-dicyanomethylene-4-diphenylmethylene-3-disubstitutedmethylene-tetrahydrofuran-2-one compounds have been prepared and structurally characterised. The compounds are obtained as yellow or dark red crystals, or, in one case, both. In two cases where yellow crystals were obtained, we found that crushing the crystals gave a deep red powder. Structure determinations, including those of the one compound which gave both coloured forms, depending on crystallisation conditions, showed that the yellow crystals contained molecules in which the structure comprised a folded conformation at the diphenylmethylene site, whilst the red crystals contained molecules in a twisted conformation at this site. Lattice energy and molecular conformation energies were calculated for all molecules, and showed that the conformational energy of the molecule in structure IIIa (yellow) is marginally higher, and the conformation thus less stable, than that of the molecule in structure IIIb (red). However, the van der Waals energy for crystal structure IIIa, is slightly stronger than that of structure IIIb – which may be viewed as a hint of a metastable packing preference for IIIa, overcome by the contribution of a more stabilising Coulomb energy to the overall more favourable lattice energy of structure IIIb. Conclusions: Our studies have shown that the crystal colour is correlated with one of two molecular conformations which are different in energy, but that the less stable conformation can be stabilised by its host crystal lattice.

The reactions of D,L-tartaric acid (D,L-H₂Tart) with alkali metal hydroxides MOH (M = Li, Na, Rb, Cs) in aqueous solution yielded new polymorphs of Na(D,L-HTart)·H₂O (1a) and Cs(D,L-HTart) (2a) as well as crystals of LiCs(D,L-Tart)·2H₂O (3) and the conglomerate (Rb_0.5Cs_0.5)₂(D-Tart) (4)/(Rb_0.5Cs_0.5)₂(L-Tart). The crystal structures of these salts display 1D (1a, 2a), 2D (3) or 3D (4) coordination polymers in combination with a hydrogen bonded layer (1a), framework (2a, 4) or tape (3) structure. The triclinic and monoclinic polymorphs of Na(D,L-HTart)·H₂O (1) exhibit a close one-dimensional packing similarity. Standard O–H⋯OC hydrogen bond motifs for the aggregation of HTart⁻ and Tart²⁻ ions in the solid state were identified from a comparison involving 35 crystal structures of chiral and racemic MHTart and M₂Tart salts (M = alkali metal).

Crystal structures of fourteen 4,5′-substituted benzenesulfonamido-2-pyridines, with tautomeric forms R¹–C₆H₄–SO₂–NC₅NH₄–R² or R¹–C₆H₄–SO₂–NH–C₅NH₃–R², and with R¹ = CF₃, I, Br, Cl, Me, F, H and R² = CF₃ or I are reported. Comparisons carried out with the program XPac show that all investigated structures display a common 3D arrangement of N–H⋯N bonded centrosymmetric dimers. This isostructural series is exceptional in its completeness and in the diversity of the substituents involved. Accordingly, the XPac dissimilarity index x, a measure of how far two structures deviate from perfect geometrical similarity, varies from 0.9 for the interchange R¹ = Br → Cl to 9.5 for the simultaneous interchange of R¹ = I → H and R² = CF₃ → I. XPac plots of individual dissimilarity parameters have been used to elucidate details of geometrical similarities and differences between structures. This indicated that the geometry is preserved most closely within bc-layers that are composed of 1D slipped stacks of N–H⋯N bonded dimers, with interlayer van der Waals interactions being dominated by R¹⋯R² contacts. The main mode of geometrical adjustment for the size of R¹ is a parallel shift of neighbouring layers against one another in the direction perpendicular to the layer plane, whilst the relative orientations of molecules within each layer are altered to a much smaller extent, even for those structure pairs representing R¹ substituents that are very different in size. This study shows that the ability of two compounds to crystallise in fundamentally the same crystal structure depends not only on how much their molecules differ in shape but also, critically, on the specific tolerance characteristics of the crystal packing arrangement concerned. As a comparison with the present study, dissimilarity indices are reported for a set of 24 isostructural 4,4′-substituted benzenesulfonamidobenzenes previously subjected to XPac analysis.

[C₆H₅CH₂NH₃][B₅O₆(OH)₄] was obtained as colourless crystals in high yield from a MeOH/ H₂O (1:1) solution of benzylamine and boric acid (1:5). A single-crystal X-ray study confirmed that the solid-state structure was comprised of a supramolecular H-bonded pentaborate anion lattice, templated by the benzylammonium cations which occupy positions within the lattice cavities. Each pentaborate anion formed 4 H-bonds to 4 neighbouring pentaborate anions at α,α,α,β acceptor sites. Additionally, each cation H-bonds to 3 pentaborate anions. Crystals were monoclinic, P2₁/c, with a = 9.3511(2) Å, b = 14.5157(4) Å, c = 10.4670(2) Å, β = 90.778(2)°, T = 120 K, V = 1420.64(6) ų, and Z = 4. TGA/DSC analysis showed that [C6H₅CH₂NH₃][B₅O₆(OH)₄] thermally decomposed in air at 800 °C to 2.5B₂O₃, via a low-temperature (200–250 °C) dehydration step to a condensed pentaborate.

The full study of polymorphism in the cobaltacarborane closo-[3-Co(η⁵-C₅H₅)-1,2-C₂B₉H₁₁] (1) compound by a combination of X-ray diffraction, thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), IR and hot-stage microscopy (HSM) is reported for the first time. The previously reported Form I (orthorhombic Form, noncentrosymmetric space group P2₁2₁2) crystallizes as dark yellow to orange flat prisms from polar dichloromethane. A new centrosymmetric modification (Form II) that crystallizes from a mixture of polar and non-polar solvents has been found. Form II crystallizes as pleochroic orange blocks in the monoclinic crystal system and space group P2₁/c. Phase transformation between both polymorphs has been observed by DSC, IR and HSM, showing that the centrosymmetric Form II (kinetic) converts irreversibly to the noncentrosymmetric Form I (thermodynamic).

Reaction of [RuCl₂(η⁶-C₆H₆)]₂ with [10-(CH₃)₂S-7,8-nido-C₂B₉H₁₀]⁻ or [9-(CH₃)₂S-7,8-nido-C₂B₉H₁₀]⁻ afforded the expected cationic complexes [Ru(η⁵-n-(CH₃)₂S-7,8-C₂B₉H₁₀)(η⁶-C₆H₆)]⁺ (n = 10, (1); 9, (3)), but also the unexpected neutral Ru(η⁵-10-HS-7,8-C₂B₉H₁₀)(η⁶-C₆H₆) (2) or Ru(η⁵-9-(CH₃)S-7,8-C₂B₉H₁₀)(η⁶-C₆H₆) (4) by double and mono demethylation of the (CH₃)₂S moiety, respectively.