Molecular and Supramolecular Homochirality: Enantiopure Perfluorocarbon Rotamers and Halogen‐Bonded Fluorous Double Helices

Abstract

The beautiful examples of helical structures provided by nature have inspired huge efforts in the generation of synthetic helical assemblies.[1] Many of the homochiral supramolecular helical structures that have been described were created through the self-assembly of chiral molecules and polymers.[2] A few reports on the preparation of artificial homochiral helices from achiral components also exist.[3] The generation of double-helical architectures retains a unique fascination, as life itself is encoded within a double-helical structure. Both metal-ion coordination [4] and hydrogen bonding [5] are well-established agents for the self-assembly of double helices.Herein, we demonstrate how halogen bonding (XB) [6] directs the self-assembly of naked halide anions and longchain diiodoperfluoroalkanes (diiodo-PFAs), leading to the formation of a new type of artificial molecular double helix. In the homochiral supramolecular complex 4 (Figure 1), the perfluorinated module 3 exclusively adopts a right-handed (P) helical conformation (molecular chirality), which translates into chiral and enantiopure fluorous [7] double helices (supramolecular chirality), owing to strong and directional I À ···IÀC halogen bonding.Naked halide anions function as particularly effective halogen-bond acceptors.[8] Ternary mixtures of calixcrownarenes, KI, and diiodo-PFAs give supramolecular complexes in which the calixarene modules complex the K + ions, and the naked I À ions assemble the diiodo-PFAs into infinite fluorous halogen-bonded chains, which segregate the calixarene domains.[8e]We chose 4-tert-butylcalix[4]arenetetra-N,Ndiethylacetamide (1) for its ability to coordinate alkali and alkaline-earth metal cations, and to form separated ion pairs. Interestingly, calixarene 1 forms a supramolecular complex with KI that is characterized by a columnar stacking of 1&K + supercations intercalated by naked I À ions; this arrangement leaves voids parallel to the stacking direction.[9a] When 1 is combined with a divalent cation in 1&Sr(Pic) 2 (Pic = picrate), Figure 1. Structural formulas of the individual components 1-3, and a section of the crystal structure of the supramolecular complex 4. Hydrogen atoms and water molecules are omitted for clarity.

Understanding the magnitude and origin of bidentate charge-assisted halogen bonds of halo-perfluorocarbons and halo-hydrocarbons with halide anions

“…The C-XÁ Á ÁX angles are all calculated to be very close to 180°, which is in good agreement with the experimental observations [55,56]. Notice that the predicted XÁ Á ÁX À Á Á ÁX angles (100°) are somewhat smaller than those found in experiments (140°) [55,56], which is presumably a result of the effect of steric hindrance in real systems.…”

“…2, it is clear that the Br À or I À ions form symmetrical halogen bonds and the two XÁ Á ÁX interactions in the bifurcated complexes are equivalent; that is to say, halide anions behave as bidentate halogen bond acceptors. At the MP2/lanl2DZ * level, the intermolecular contacts are predicted to be within a range from 3.120 to 3.657 Å , which reproduces the experimentally measured values fairly well [55,56]. These separations are less than sums of van der Waals radii of the atoms involved [66].…”

“…Bidentate charge-assisted halogen bonding interactions between halo-PFCs and naked halide anions (RXÁ Á ÁX À Á Á ÁXR) play crucial roles in determining the formation of double-helix in homochiral supramolecular complexes [55,56]. The use of these interactions as a driving force for the construction of double-helical suprastructures and the use of fluorinated modules are undoubtedly unprecedented.…”

“…DE2 is CCSD(T) interaction energies without CP corrections. Experimental values taken from Refs [55,56]. are given in parentheses.b Total magnitudes of CT from anions to halocarbons at the HF/lanl2DZ * level.…”

Characterization of Halogen Bonded Adducts in Solution by Advanced NMR Techniques

“…Extending the same topic, a double helix held by XBs has also been characterized in the solid state [95], whereas Berryman and co-workers succeeded in synthesizing a triple helix stabilized by XB. For the latter, a complete characterization in solution is also available, including the presence of inter-strand NOE contacts, to verify the helicity and measure the diffusional coefficient in order to further characterize the adduct.…”