Hydrogen-bonding clusters leading to formation of supramolecular dimers of metalloporphyrin receptors: modulation of Lewis acidity by pi-pi interactions

Two remarkable crystal structures are reported of a cyclic receptor 1, containing two metalloporphyrin units. The overall crystal structure of 1 provides the first direct evidence that pi-stacking between two metalloporphyrins reduces the Lewis acidity of the metal ion and thereby dramatically reduces the affinity of zinc for external ligands; this effect was previously suggested indirectly by solution state binding studies. In addition, crystallising 1 from a different combination of solvents and the ability of 1 to distort its structure leads to the remarkable observation of a supramolecular dimer of inter-penetrating macrocycles, 4, held together by clusters of hydrogen-bonded methanol molecules.

Enantioselective reduction of prochiral ketones by catecholborane catalysed by chiral group 13 complexes

LiGaH4, in combination with the S,O-chelate 2-hydroxy-2'-mercapto-1,1'-binaphthyl (MTBH2), forms an active catalyst for the asymmetric reduction of prochiral ketones, with catecholborane as the hydride source. Enantioface differentiation is on the basis of the steric requirements of the ketone substituents. Aryl/ n-alkyl ketones are reduced in 90-93% ee and RC(O)Me (e.g. R = iPr, cycloC6H11, tBu) in 60-72% ee. Other borane sources and alternative catalyst structures based on indium do not form enantioselective catalysts.

Comparison of the structural motifs of acetoacetanilides and related azo pigments

The structures of three methyl-substituted acetoacetanilides and of an azo pigment derived from one of them are presented and discussed together with a review of related known crystal structures. By considering the position of any aromatic substituents it is possible to predict whether the simple acetoacetanilides adopt planar structures with intramolecular hydrogen bonding or twisted structures featuring intermolecular hydrogen bonding. However, we find that the same crystal engineering rules cannot be applied to the related azo pigments: this is apparently due to the presence of an sp(2) atom which facilitates the adoption of planar conformations. The thermal properties of the acetoacetanilides were measured by DSC and are discussed with reference to their crystal structures.

A synchrotron diffraction study of two polymorphic forms of 2, 5-bis

Two polymorphs, alpha and gamma, of the title compound, C(22)H(20)N(2)O(2), have been characterized by means of single-crystal synchrotron X-ray diffraction. In the alpha form, the molecules pack in a herring-bone fashion, linked via weak C-H.N intermolecular interactions (H.N 2.58 A). In the gamma form, the molecules are arranged in nearly planar sheets, which form a network held together by intermolecular hydrogen bonds of the type C-H.O (H. O 2.49 A) and C-H.N (H.N 2.50 A). The stacking distance between the sheets is 3.40 A.

The First Red Azo Lake Pigment whose Structure is Characterized by Single Crystal Diffraction.

A model for red azo pigment Ca4B was characterized structurally using synchrotron radiation. This highly anisotropic ladder structure represents a new structural class in azo pigment chemistry. The picture shows that the calcium atoms coordinate in a complex manner to three azo ligands (one terdentate, one bidentate, and one monodentate) and two water molecules simultaneously.