Conformational Flexibility of Hybrid [3]- and [4]-Rotaxanes.

Selena J Lockyer,Neil A Burton,Adam Brookfield,Alice M Bowen,Alistair J Fielding,Richard E P Winpenny,Grigore A Timco,Eric J L Mcinnes,Inigo J Vitorica-Yrezabal,Selina Nawaz

doi:10.1021/jacs.0c06547

Abstract

The synthesis, structures, and properties of [4]- and [3]-rotaxane complexes are reported where [2]-rotaxanes, formed from heterometallic {Cr7Ni} rings, are bound to a fluoride-centered {CrNi2} triangle. The compounds have been characterized by single-crystal X-ray diffraction and have the formulas [CrNi2(F)(O2CtBu)6]{(BH)[Cr7NiF8(O2CtBu)16]}3 (3) and [CrNi2(F)(O2CtBu)6(THF)]{(BH)[Cr7NiF8(O2CtBu)16]}2 (4), where B = py-CH2CH2NHCH2C6H4SCH3. The [4]-rotaxane 3 is an isosceles triangle of three [2]-rotaxanes bound to the central triangle while the [3]-rotaxane 4 contains only two [2]-rotaxanes bound to the central triangle. Studies of the behavior of 3 and 4 in solution by small-angle X-ray scattering and atomistic molecular dynamic simulations show that the structure of 3 is similar to that found in the crystal but that 4 has a different conformation to the crystal. Continuous wave and pulsed electron paramagnetic resonance spectroscopy was used to study the structures present and demonstrate that in frozen solutions (at 5 K) 4 forms more extended molecules than 3 and with a wider range of conformations.

Highlights

The flexibility of supramolecular assemblies has long been of interest
Small-angle X-ray scattering (SAXS) has been previously used to show supramolecular assemblies are present in solution;8,9 we have used SAXS supported by atomistic molecular dynamics simulations (AMDS) to demonstrate that a [13]-rotaxane maintained its structure in solution
A [2]-rotaxane can be prepared, of formula (BH)[Cr7NiF8(O2CtBu)16] (2); the thread is protonated at the amine site during this reaction (Figure S3)

Summary

Introduction

The flexibility of supramolecular assemblies has long been of interest. For example, the rigidity of molecular capsules has been used to allow them to act as reactors to catalyze specific reactions or to stabilize reactive species. Interlocked structures such as the various knots reported− introduce rigidity and have been proposed as means of making less flexible polymers. Studying rigidity is possible using NMR spectroscopy where the species is diamagnetic; for example, the demetalated knots made by Zhang et al show broad NMR spectra that become sharper when diamagnetic Zn2+ ions are added, which increases the rigidity. The rigidity of molecular capsules has been used to allow them to act as reactors to catalyze specific reactions or to stabilize reactive species.. The rigidity of molecular capsules has been used to allow them to act as reactors to catalyze specific reactions or to stabilize reactive species.2,3 Interlocked structures such as the various knots reported− introduce rigidity and have been proposed as means of making less flexible polymers.. Small-angle X-ray scattering (SAXS) has been previously used to show supramolecular assemblies are present in solution; we have used SAXS supported by atomistic molecular dynamics simulations (AMDS) to demonstrate that a [13]-rotaxane maintained its structure in solution.. The studies require the comparison of SAXS and double electron−electron resonance (DEER) spectroscopy while remembering they operate at 300 and 5 K, respectively

Results

Discussion

Conclusion