Molecular Turnstiles Featuring Bicyclic Rotators: Solution and Solid‐State Investigation of Steric and Electronic Concerns

Bicyclic rotators connected to rigid stators provide opportunities to study rotational movement via ¹H NMR, X-ray crystallography, and UV–vis spectroscopy. Solution studies suggest rotation or wobbling of the central arene with respect to the rigid stator. Similar motion in the solid-state may be limited by intermolecular attractions between neighboring compounds.

Abstract

A molecular rotor is created when a 2,1,3-benzothiadiazole rotator is incorporated into a rigid arylene ethynylene framework supported by pyridine coordination to a metal (Ag⁺ or PdCl₂) guest. Comparisons to a similarly sized naphthyl rotator via ¹H NMR spectroscopy provide insights into the movement of these bicyclic rotators relative to the rigid stator framework. Chemical shift increases of 0.3 ppm, or more, upon metal complexation are consistent with through-space interaction of the central arene with a bound PdCl₂ guest. Further study via X-ray crystallography illustrates that rotation of the 2,1,3-benzothiadiazole unit in the solid state is likely hampered by relatively strong chalcogen bonding (N⋅⋅⋅S distance of 2.93 Å), forming 2S-2N squares between benzothiadiazoles of neighboring complexes. Strong π–π interactions (3.29–3.36 Å) between neighboring complexes likewise restrict solid-state rotation of the potential benzothiadiazole rotator. Modest changes to UV–vis spectra upon metal coordination suggest that electronic properties are mostly independent of stator configuration.

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