Dieter Bläser

The Melting Point Alternation in the Short‐Chain n‐Alkanes: Single‐Crystal X‐Ray Analyses of Propane at 30 K and of n‐Butane to n‐Nonane at 90 K

A less dense packing is observed in the odd-numbered n-alkanes compared to the even-numbered members, which consequently lowers melting temperatures. The reason for this is that the even-numbered n-alkanes have optimal intermolecular interactions at both ends (see the picture on the left), while the odd-numbered ones possess these only at one end-at the other end the intermolecular distances are longer (right).

Synthesis, structures and reactions of new thermally stable silylenes

Two representatives 1a and 1b of a new series of stable but reactive bis(amino)silylenes, derived from the N,N′-dineopentyl-1,2-phenylenediamido ligand RC6H3[(CH2But)]2, have been prepared by reductive elimination from RC6H3[[graphic omitted]SiCl2 and characterised by NMR spectroscopy and for 1a X-ray crystallography; the silyienes [graphic omitted](CH2But)]2C6H3-1,2-R (R = H 1a or 4-Me 1b) readily undergo oxidative addition with EtOH or Mel.

1,3,4,5-Tetramethyl-2-methyleneimidazoline—an ylidic olefin

The ylidic properties of 1,3,4,5-tetramethyl-2-methyleneimidazoline 4, obtained by deprotonation of the pentamethylimidazolium ion 3, are revealed both by its physical and chemical properties; the X-ray structure of 4 is reported.

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.

Heterocycles as ligands 211. Stable titanium carbene complexes

The titanium carbene complexes 2 are obtained by the reaction of the imidazol-2-ylidenes 1 with TiCl4 in good yields. Careful hydrolysis of 2c gives the dinuclear complex 3c in which two complex fragments of pentacoordinated titanium are connected by a near linear oxygen bridge.

The crystal structure of D-ribose--at last!

Among the half-million or so chemical compounds whose crystal structures have been determined by X-ray or neutron diffraction, the crystal structure of d-ribose is conspicuously absent. Thus, although most modern chemistry textbooks and handbooks show the molecule of d-ribose in the bfuranose form, as present in countless biochemically important ribose derivatives, it has been known for more than forty years from early NMR observations that d-ribose exists in aqueous solution as a mixture of aand b-pyranoses and aand b-furanoses with the b-pyranose form predominating (Scheme 1).

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.

Synthesis and supramolecular structures of molecular clips

The syntheses of the novel dimethylene-bridged clips I (n = 0, R = H, OAc, OH, OMe, OSO2CF3; n = 1, R = OAc) are reported. They selectively bind electron-deficient neutral and cationic arom. substrates comparable to tetramethylene-bridged tweezers. The geometry of the noncovalently bound complexes with I (n = 0; R = OAc, OH, OMe) as receptors, derived from single-crystal structure analyses, is, however, different from that of the tweezer complexes. In clip complexes the plane of the included arom. substrate mol. is oriented almost parallel to the naphthalene side-walls of the clip, whereas in the tweezer complex the substrate is oriented parallel to the central arene spacer-unit. 1,2,4,5-Benzenetetracarbonitrile (II) as substrate is placed inside the cavity of the hydroquinone clip (I; n = 0, R = OH) in soln. as well as in the cocrystal. In contrast, it was found for the cocrystal with the diacetate clip (I; n = 0, R = OAc) that II is placed between the naphthalene side-wall of two different clip mols. whereas in soln. II is included into the cavity of I (n = 0, R = OAc). Finally, II forms a 1:2 complex with dinaphthonorbornadiene in soln. as well as in the cryst. state. The findings reported here are instructive for the understanding of the weak noncovalent binding forces particularly the arene-arene interaction.

Structural Characterization of a Base-Stabilized [Zn2]2+ Cation

The landmark discovery of decamethyldizincocene [Cp*2Zn2] (1; Cp* = C5Me5) by Carmona et al. in 2004 [1] has led to increasing research on the synthesis of low-valent metal complexes of Group 2 and 12 metals in recent years and several complexes containing Mg Mg, Cd Cd, 4] Hg Hg and Zn Zn bonds, which are typically kinetically stabilized by use of sterically demanding or chelating organic substituents, have been synthesized. However, it should be noted that the first molecular complex with a Zn Zn bond, Zn2H2, was trapped in an argon matrix at 12 K and characterized by vibrational spectroscopy and computational calculations. With respect to the rather large number of organometallic complexes containing a direct Zn Zn bond it is somehow surprising how little is know about the [Zn2] 2+