Xenon Induces Its Own Preferred Heterochiral Host from Exclusive Homochiral Assembly

Abstract

Xenon binding represents a formidable challenge, and efficient hosts remain rare. Here we report our findings that while enantiomeric bis(urea)–bis(thiourea) macrocycles form exclusive homochiral dimeric assemblies, xenon is able to overcome the narcissism and induces an otherwise-nonobservable heterochiral assembly as its preferred host. An experimental approach and fitting model were developed to obtain binding constants associated with the invisible assembly species. The determined xenon binding affinity with the heterochiral capsule reaches 1600 M–1, which is 15 times higher than that with the homochiral capsule and represents the highest record for an assembled host. The origin of the large difference in xenon affinity between the two subtle diastereotopic assemblies was revealed by single-crystal analysis. In the heterochiral capsule with S 4 symmetry, the xenon atom is more tightly enclosed by van der Waals surroundings of the four thiourea groups arranged in a spherical cross-array, superior to the antiparallel array in the homochiral capsule with D 2 symmetry.

Diverse Self-Assembly and Xenon Binding from a Single Mesomeric Macrocycle Component

“…The NH signals moved largely in the downfield direction even in this nonpolar solvent system, suggesting favorable hydrogen-bonding formation. The signal patterns are overall similar to that of the dimeric assembly (M S ) 2 or (M R ) 2 , except for higher complexity. By a combination of 1 H, 13 C, and various two-dimensional (2D) NMR experiments (Figures S4–S10), the two groups of signals can be assigned to the two kinds of dimeric assemblies MM -1 / PP -1 and MM -2 / PP -2 (Figure d).…”

“…This suggested that xenon induces the formation of an otherwise nonobservable heterochiral capsule MP/PM as its own preferred host, parallel to what was observed for the racemic mixture of M S and M R . 52 It seems that the global induced-fit assembly dynamics is universal for this type of bisurea− bisthiourea macrocycle system. The formation of up to six assembly products starting from a single macrocycle component represents an unprecedented example, highlighting the advantage of assembly diversity toward building complex matter.…”

“…Later on, we found that, in a racemic mixture of the two enantiomeric macrocycles, exclusive formation of the homochiral capsules was observed, suggesting narcissistic chiral self-sorting. Surprisingly, the presence of xenon is able to overcome the narcissism tendency and induce the formation of otherwise nonobservable heterochiral capsule M S M R as its preferred host with a recorded affinity of 1600 M –1 among assembly-type hosts (Figure a) …”

“…Surprisingly, the presence of xenon is able to overcome the narcissism tendency and induce the formation of otherwise nonobservable heterochiral capsule M S M R as its preferred host with a recorded affinity of 1600 M −1 among assembly-type hosts (Figure 1a). 52 Inspired by the above exciting findings, we wondered what would occur for achiral, mesomeric macrocycle meso-M (Figure 1b). Here, we report the synthesis, self-assembly, and xenon binding properties with this macrocycle.…”

“…Generally similar structural and interaction motifs are held as those observed in racemic macrocycle assemblies. 51,52 The thiourea groups adopt an unusual twisted syn−anti configuration, leaving the two NH sites, one endo and the other exo arrayed. The two macrocycle components are folded into a bowl-shaped conformation and crossly buckle through forming 4-fold intermolecular C�O••• H−N H-bonds around.…”