Michael T. Kirchner

C–H⋯F–C hydrogen bonding in 1,2,3,5-tetrafluorobenzene and other fluoroaromatic compounds and the crystal structure of alloxan revisited

The crystal structure of 1,2,3,5-tetrafluorobenzene, 1, has been determined and is compared with those of other polyfluoro-substituted benzenes. Compound 1 has a layered monoclinic structure in which the layers are held by bifurcated C–H⋯F–C interactions. Short F⋯F separations are also observed. The layers are stacked at van der Waals separation to give a 4 A packing. This structure is adopted instead of an alternative tetragonal structure adopted by fluorobenzene and a number of related compounds such as benzonitrile, pyridine N-oxide and alloxan. Compound 1 does not take the tetragonal structure mostly because this structure would demand the formation of a C–F⋯π interaction, which appears improbable for the molecule. The role of weak intermolecular interactions in the crystal packing of predominantly non-polar compounds is highlighted.

Isotopic Polymorphism in Pyridine

The ab initio calculation of the relative stabilities of isomers of gas phase molecules rates as one of the outstanding scientific achievements of the twentieth century. Our understanding of the structure of solid state is, by comparison, much less well advanced. The problem is illustrated by the crystal structure of pyridine. Pyridine (C5H5N or ‘h5’ hereafter) is one of the simplest heteroaromatic compounds but its crystal structure (phase h5-I) is unusually complicated, having four independent molecules in the asymmetric unit (Z’ = 4). [1] Price et al. have surveyed the potential for polymorphism in pyridine using ab initio crystal structure prediction methods, finding over a dozen crystal structures that were energetically competitive with h5-I. [2] In parallel with Price’s theoretical work an intense experimental search was made by one of us (RB group) for new low-temperature polymorphs of pyridine. Though all attempts to crystallize h5 failed to yield anything but the h5-I phase, crystallization of pyridine-d5 (d5) from pentane yielded a new phase, d5-II, at 188 K. Recrystallization from a low-melting solvent such as pentane has been shown in the past to circumvent hightemperature phases because saturation of the solution occurs below the temperature of the phase transition. [3] The new d5-II phase has one molecule in the asymmetric unit (Z’ = 1), but does not correspond to any of the predicted polymorphs of h5. The effect of temperature on the crystal structure of pyridine-d5 was subsequently investigated further using neutron powder diffraction. The sample was ground at 77 K [4] and then rapidly cooled to 2 K. The powder pattern was successfully modelled as d5-I (see Fig. S1a in the Supplementary Information). The sample was then warmed in steps of 2 K, with patterns being acquired at each temperature. When the sample reached 170 K it began to undergo a sluggish phase transition into d5-II. After collecting a clean d5-II neutron powder

Co-crystals with Acetylene: Small Is not Simple!

Acetylene is an amazingly versatile component for the formation of co-crystals. It requires careful handling and special techniques for crystallisation, but the efforts seem to be rewarding when attaining co-crystals with small molecules as partners. Many basic questions such as the dominance of specific heterogeneous intermolecular interactions, their driving force for the formation of multicomponent crystals instead of neat ones are expected to be easily analysed. The underlying packing patterns and resulting stoichiometries based on the known supramolecular synthons seem to be straightforward for such small molecules and crystal engineering, considered as the prototype of supramolecular synthesis, should be a simple task. Nineteen co-crystals with acetylene are presented in this paper, some of which have been previously reported individually. An attempt has been made to find features shared by the groups of co-crystals, including those that could not be co-crystallised. But in spite of clear ideas and experiences from previous experiments, surprisingly almost none of systems reached our expectations. Our intuitive approach was not fulfilled, which demonstrates that multicomponent crystals even of small molecules will remain a great challenge for theoretical methods and the crystal structures shown herein represent good candidates for future testing. On the other hand, we wish to encourage other groups to present their views on the crystal structures with an unbiased approach that may offer a better explanation than we are able to outline in this article.

Solid-State Behaviour of the Dichlorobenzenes: Actual, Semi-Virtual and Virtual Crystallography

The crystal structures of the low-melting 1,2- and 1,3-dichlorobenzene isomers in monoclinic space group P21/n and monoclinic space group P21/c, resp., were detd. by x-ray anal. and in situ crystn. techniques. Attempts to predict these structures in advance by force-field calcns. were not successful, although the known crystal structures of two of the three polymorphs of the 1,4-isomer were successfully a posteriori predicted. Calcd. lattice energies were supplemented with estd. lattice-vibrational entropies obtained in the rigid-body approxn. Energy calcns. for actual and virtual crystal structures indicate that the higher m.p. of the 1,4-isomer can be largely attributed to more efficient crystal packing.

Solid-state and Solution Studies on a β-Diketiminate Zinc Hydride Complex

[MesnacnacZn(μ-H)]2 (1) was synthesized by reaction of MesnacnacZnI with either an equimolar amount of KNH(iPr)BH3 or an excess of NaH and characterized by multinuclear NMR and IR spectroscopy as well as X-ray diffraction. Two polymorphs of 1 were found and their structures determined on single crystals Graphical Abstract Solid-state and Solution Studies on a β-Diketiminate Zinc Hydride Complex

Additive induced polymorphism. The pentafluorophenol–pentafluoroaniline system

The title system yields polymorph and bi-component crystals readily under cryocrystallisation conditions; the present study clarifies inconsistencies in the literature and defines a new structural landscape.

Supramolecular synthesis of brick wall and honeycomb networks from the T-shaped molecule 5-nitrosalicylic acid

The T-geometry of O–H⋯O and C–H⋯O hydrogen bonding groups in the title molecule (5-NSA) form a brick wall network that is modularly expanded and transformed to chair cyclohexane and honeycomb architectures in molecular complexes with trans-1,4-dithiane-1,4-dioxide and dioxane. The variation in hydrogen bond synthons results in different examples of topologically related (6,3) nets.

Weak C-H···O hydrogen bonds in anisaldehyde, salicylaldehyde and cinnamaldehyde.

In situ cryocrystallization has been employed to grow single crystals of 4-methoxybenzaldehyde (anisaldehyde), C(8)H(8)O(2), 2-hydroxybenzaldehyde (salicylaldehyde), C(7)H(6)O(2), and (2E)-3-phenylprop-2-enal (cinnamaldehyde), C(9)H(8)O, all of which are liquids at room temperature. Several weak C-H···O interactions of the types C(aryl)-H···O, C(formyl)-H···O and Csp(3)-H···O are present in these related crystal structures.

PREPARATION, STRUCTURE AND REACTIVITY OF 2-CHLORO-, 2-FLUORO- AND 2-IODO-2,3-DIHYDRO-1H-1,3,2-DIAZABOROLES

Differently substituted 2-chloro-, 2-fluoro-, and 2-iodo-2,3-dihydro-1H-1,3,2-diazaboroles were prepd. by various methods. 1,3-Di-tert-butyl-2-fluoro-2,3-dihydro-1H-1,3,2-diazaborole (I), 1,3-di-tert-butyl-2-chloro-2,3-dihydro-1H-1,3,2-diazaborole (II), 1,3-bis(2,6-dimethylphenyl)-2-chloro-2,3-dihydro-1H-1,3,2-diazaborole (III), 2-chloro-4,5-dimethyl-1,3-dineopentyl-2,3-dihydro-1H-1,3,2-diazaborole, and 1,3-di-tert-butyl-2-iodo-2,3-dihydro-1H-1,3,2-diazaborole were formed from the corresponding lithiated Z-1,2-diaminoethenes, by treatment with BF3.OEt2, BCl3, or BI3 in n-hexane. I, II, and III are also available by Na amalgam redn. of the adduct (tBu)(BF3)N:CHCH:N(BF3)(tBu), and the borolium salts [RNa:CH-CH:Nb(R)BCl2]X (Na-B) (R = tBu, X = BCl4; R = 2,6-Me2C6H2, X = Cl). The iodo deriv. (2,6-Me2C6H2)-Na-CH:CH-Nb(2,6-Me2C6H2)BI (Na-B) (IV) was synthesized in a redox reaction between the corresponding 1,4-diazabutadiene and BI3. The novel compds. were characterized by 1H, 11B and 13C NMR spectroscopy, as well as by x-ray structure anal. of IV (monoclinic, space group C2/c, a 14.911(2), b 7.9706(8), c 16.9105(12) , b 114.360(6) Deg, V = 1830.9(3) .ANG.3, Z = 4, rc = 1.459 g/cm3, m(MoKa) = 1.747 mm-1, 1884 obsd. reflections with I > 2s(I), 102 refined parameters, R1 = 0.042, wR2 = 0.1043).

Crystal Structures and Packing of 2,4,6-tris(4-Fluorophenoxy)-1,3,5-triazine and 2,4,6-tris(3,4-Dimethylphenoxy)-1,3,5-triazine. New Materials for Octupolar Nonlinear Optics

The crystal structures and packing of 2,4,6-tris(4-fluorophenoxy)-1,3,5-triazine and 2,4,6-tris(3,4-dimethylphenoxy)-1,3,5-triazine are discussed. These structures have been determined as a continuation of a series of octupolar NLO materials we have been investigating. The crystal structures are characterized by C–H...F and C–H...π hydrogen bonds, respectively. A characteristic of these triazine structures is the presence of dimeric Piedfort Units (PU) that are extended into more elaborate two-dimensional (2-D) networks. The structure of the fluoro derivative is compared with that of the corresponding unsubstituted and chloro/bromo-substituted derivatives. The structure of the dimethyl triazine is compared with that of the corresponding 4-methyl derivative. The noncentrosymmetric nature of the dimethyl derivative was confirmed by a powder SHG signal at 1.064 μm of the order of ∼0.5 × KDP. Interestingly, the dimethyl derivative studied here is isostructural with the corresponding 4-methyl triazine. This H/Me isostructurality is shown to be an uncommon phenomenon by an analysis with the CSD.