Inherent chirality

Abstract: “Inherent chirality” in molecules like calix[4]arenes, fullerenes, and uranyl-salophen complexes can be related to the presence of curvature. This observation serves as a basis for the introduction of a new chirality descriptor.

Abstract: 2,3-bis(diphenylphosphino)butane enantiomers (chiraphos, L) used as chiral auxiliaries results in the preferential formation of an unprecedented Au24 framework with inherent chirality. The crystal structure of [Au24 L6 Cl4 ]2+ (1) has a square antiprism-like octagold core twinned by two helicene-like hexagold motifs, where the inherent chirality is associated with the helical arrangement. The clusters carrying (R,R)- and (S,S)- diphosphines had right- and left-handed strands, respectively. Circular dichroism spectra showed peaks in the visible to near-IR region, some of which did not coincide with absorption bands, suggesting the enantiomeric Au24 frameworks possess unique chiroptical properties. The Au24 frameworks were thermally robust, which could be attributed to the superatomic concept (18 e- system) and the steric constraint effects of the bridging ligand units.

Abstract: Although chirality is an ever-present characteristic in biology and some artificial molecules, controlling the chirality and demystifying the chirality origin of complex assemblies remain challenging. Herein, we report two homochiral Ag14 nanoclusters with inherent chirality originated from identical rotation of six square faces on a Ag8 cube driven by intra-cluster π···π stacking interaction between pntp− (Hpntp = p-nitrothiophenol) ligands. The spontaneous resolution of the racemic (SD/rac-Ag14a) to homochiral nanoclusters (SD/L-Ag14 and SD/R-Ag14) can be realized by re-crystallizing SD/rac-Ag14a in acetonitrile, which promotes the homochiral crystallization in solid state by forming C–H···O/N hydrogen bonds with nitro oxygen atoms in pntp− or aromatic hydrogen atoms in dpph (dpph = 1,6-bis(diphenylphosphino)hexane) on Ag14 nanocluster. This work not only provides strategic guidance for the syntheses of chiral silver nanoclusters in an all-achiral environment, but also deciphers the origin of chirality at molecular level by identifying the special effects of intra- and inter-cluster supramolecular interactions. The preparation of chiral monolayer-protected metal clusters is interesting for their potential applications in a variety of fields, including catalysis. Here, the authors synthesize chiral Ag14 nanoclusters in an all-achiral environment, and decipher the origin of chirality at the molecular level; the solvent choice is key to achieve homochiral crystallization.

Abstract: A cerium(IV) double decker porphyrin 2 bearing two pairs of 4-pyridyl groups and two pairs of 3,5-dimethoxyphenyl groups was synthesized. In the presence of chiral dicarboxylic acid guests with a two carbon spacer [e.g., (1R,2R)-cyclohexane-1,2-dicarboxylic acid and Boc-L-aspartic acid] 2 gave a CD-active species. The plots of the CD intensity vs. the guest concentration showed a sigmoidal curvature, a sign of homotropic, positive allosterism. Analysis according to the Hill equation indicated that the guests were bound autoacceleratively. Even after the chiral guests were removed by the addition of excess pyridine, 2 remained CD-active because of its inherent chirality. Thereafter, the CD intensity decreased very slowly as a result of internal rotation of the porphyrin subunits. Thermodynamic analysis of this racemization process gave ΔG ‡298 = 23.0 kcal mol–1, ΔH ‡298 = 18.1 kcal mol–1 and ΔS ‡298 = –16.4 cal mol–1 K–1. Observations and calculations indicate that the chiral memory can be preserved for 3 days at 0 °C and for one year at –37 °C. In conclusion, this is a rare artificial system for which a homotropic, positive allosterism is observable and in which the guest chirality is transcribed and stored.