Brian A. Helfrich

Heteromeric intermolecular interactions as synthetic tools for the formation of binary co-crystals

The crystal structure of 4-nitrobenzamide pyrazinecarboxamide (1 ∶ 1) contains two homomeric amide⋯amide dimers, which is in sharp contrast with the structural motifs displayed by the vast majority of binary co-crystals (illustrated here with five new examples), where the driving force for the supramolecular assembly is one or more strong heteromeric intermolecular interactions.

Making reversible synthesis stick: competition and cooperation between intermolecular interactions

Abstract The ability to predict and control the assembly of molecular and ionic species into ordered networks is becoming an important target area in chemistry, biochemistry and materials science. In this context, the selectivity and directionality of the hydrogen bond have been instrumental in the preparation of distinctive structural aggregates, and the use of hydrogen bonding as a steering force is now emerging as the most important strategy in crystal engineering. A reliable supramolecular synthetic protocol also requires access to a wide range of reactions that are versatile, selective and give rise to high supramolecular yields. Through a systematic structural study we have examined three issues that are relevant to the development of transferable supramolecular synthesis; (a) structural invariance to molecular isomerism; (b) hierarchy of synthons based upon competition between intermolecular forces and (c) changes in supramolecular yield as a function of molecular substitution.

Combining halogen bonds and hydrogen bonds in the modular assembly of heteromeric infinite 1-D chains

Hydrogen bonds and halogen bonds operate in concert in the directed assembly of infinite 1-D chains in binary co-crystals of iodine iso-nicotinamide (2 : 2), 1 and tetrafluorodiiodobenzene iso-nicotinamide (1 : 2) 2.

Tetrachlorocuprate(II) salts of heterocyclic aminoguanidonium (guanylhydrazonium) ions

Abstract Condensation of aminoguanidine with 2-formylpyridine, 2-benzoylpyridine and 2,6-diacetylpyridine results in the aminoguanidones, HFogu, HBzgu and H22,6Acgu2, respectively. Reaction of the heterocyclic aminoguanidone in a mixture of hydrochloric acid–ethanol with copper(II) chloride produces the following mixed salts: [H3Fogu][CuCl4], (1), [H3Bzgu]2 [CuCl4]Cl2·H2O, (2), and [H52,6Acgu2][CuCl4]Cl·H2O, (3). Their crystal structures, as well as their ESR and UV spectra, have been obtained. The influence on the geometry of the [CuCl4]2− of the NH⋯ClCu hydrogen bonds, in the context of charged compensation hypothesis, has been evaluated.

Parallel and perpendicular stacking of ferrocene rings. Syntheses, X-ray structures, and electrochemistry of 1,8-bis-[1-(1'-phenylthio)ferrocenyl]naphthalene and 8,8'-bis-[1-(1'-phenylthio)ferrocenyl]-1,1'-binaphthyl

Abstract The parallel and perpendicular stacking of ferrocene rings in 1,8-bis-[1-(1′-phenylthio)ferrocenyl]naphthalene (1) and 8,8′-bis-[1-(1′-phenylthio)ferrocenyl]-1,1′-binaphthyl (2), respectively, were investigated. They were synthesized from the Suzuki coupling of 2,4,6-tris(1′-phenylthio-1-ferrocenyl)boroxin (3) with 1,8-diiodonaphthalene and 1,8-diiodo-1,1’-binaphthyl, respectively. Their structures were confirmed by single-crystal X-ray analyses. The two cyclopentadienyl (Cp) rings (C9C13 and C19C23) of the two respective ferrocenes Fe(1) and Fe(2) of 1 are not parallel, but are 21.3° away from parallel. The distance between the centroids of these two Cp rings is 3.336 A. The two Cp rings (C9C13 and C9′C13′) of the two respective ferrocenes Fe(1) and Fe(2) of 2 are not perpendicular, but have a dihedral angle of 49.9°, and the distance between two centroids of these two Cp rings is 5.255 A. The electrochemical behaviour of 1 and 2 in dichloromethane solution shows that they undergo reversibly two ferrocene-centred one-electron oxidations to the corresponding dications. The separation between the two anodic processes is greater for 1 with respect to 2, but the absolute value of the separation significantly depends upon the nature of the supporting electrolyte. In the presence of the classical [NBu4][PF6] the ΔE°′ value is 0.15 V for 1 and 0.07 V for 2. In the presence of the new [NBu4][B(C6F5)4] the ΔE°′ value increases to 0.45 V for 1 and 0.13 V for 2. In view of the low ion-pairing ability of the [B(C6F5)4]− counteranion, it is conceivable that the increased intramolecular electronic communication might essentially arise from the increased electrostatic repulsion (or through-space interaction) following the 0/+/2+ redox changes rather than from through-bond electron delocalization.