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

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

doi:10.1021/jacs.0c06547

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

J Am Chem Soc. 2020 Sep 16;142(37):15941-15949. doi: 10.1021/jacs.0c06547. Epub 2020 Sep 3.

Affiliations

¹ Department of Chemistry and Photon Science Institute, The University of Manchester, Oxford Road, Manchester M13 9PL, U.K.
² School of Pharmacy and Biomolecular Sciences, Liverpool John Moores University, Liverpool L3 3AF, U.K.

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)(O₂C^tBu)₆]{(BH)[Cr₇NiF₈(O₂C^tBu)₁₆]}₃ (3) and [CrNi₂(F)(O₂C^tBu)₆(THF)]{(BH)[Cr₇NiF₈(O₂C^tBu)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.