Work

Multivalent C−H⋅⋅⋅Cl/Br−C Interactions Directing the Resolution of Dynamic and Twisted Capsules

Year: 2019

Type: article

Abstract: In this work, we report a mechanism by which stereoisomeric and twisted capsules P / M ‐ 1 direct their dynamic chirality in the presence of haloalkane guests. The capsule comprises a static, but twisted, cage that is linked to a dynamic tris(2‐pyridylmethyl)amine (TPA) lid at its top. From the results of experimental (NMR spectroscopy and X‐ray crystallography) and computational (DFT) studies, the TPA lid was shown to assume clockwise (+) and counterclockwise (−) folds with diastereomeric (but racemic) capsules M ‐ 1 (+) and M ‐ 1 (−) interconverting at a rapid rate (Δ G ≠ 189K =9.1 kcal mol −1 ). The relative stability of the capsules was found to be a function of guest(s) residing in their interior (243/262 Å 3 ) with small CH 2 Cl 2 (61 Å 3 ) yielding roughly equal population of diastereomeric inclusion complexes. Larger guests, such as CCl 4 (89 Å 3 ) and CBr 4 (108 Å 3 ), however, formed M ‐ 1 (−)⊂CX 4 at the expense of M ‐ 1 (+)⊂CX 4 in circa 3:1 ratio. To account for the observation, theory (DFT:M06‐2X/6–31+G*) and experiments ( 1 H NMR spectroscopy) were used to deduce that CX 4 guests become localized inside the twisted cage of the capsule by forming a C−X⋅⋅⋅π halogen bond [ N c = d/ ( r H + r X )=0.91–0.92] with the benzene “floor” while encountering electrostatic repulsions with closer naphthalimide boundaries. At last, the TPA lid used its central methylene hydrogens to establish, within the M ‐ 1 (−)⊂CX 4 , three stabilizing C−H⋅⋅⋅X−C interactions with the guest. The same C−H⋅⋅⋅X−C interactions, however, became weaker (or possibly vanished) after the conformational reorganization of the lid and the formation of less stable M ‐ 1 (+)⊂CX 4 complex. On individual basis, C−H⋅⋅⋅X−C intermolecular contacts are weak and hardly detectable in the solution phase. In the case of capsule P / M ‐ 1 , however, these contacts were multivalent and altogether strong enough to direct the host's dynamic chirality.

Source: Chemistry - A European Journal

DOI: 10.1002/chem.201903006