Controlling the Frequency of Macrocyclic Ring Rotation in Benzylic Amide [2]Catenanes

Abstract

A combination of variable-temperature 1H NMR spectroscopy and molecular mechanics calculations have been used to probe the factors that determine the rate of macrocyclic ring rotation in benzylic amide [2]catenanes. The results show that the interlocked macrocycle dynamics are governed by a delicate combination of steric effects, intricate inter-macrocyclic arrays of hydrogen bonds, π−π stacking, and T herringbone-type interactions. A cascade of hydrogen-bond ruptures and formations is the principal event during circumvolution (complete rotation of one macrocyclic ring about the other) but is accompanied by a series of cooperative conformational and co-conformational rearrangements that help to stabilize the energy of the molecule. The experimental picture is consistent both when activation energies are measured from the coalescence of NMR signals and when rate constants are directly measured by spin polarization transfer by selective inversion recovery (SPT-SIR) methods. The nature of the circumrotational process means that the precise structure of the diacylaromatic units has a tremendous effect on the frequency of macrocyclic ring rotation: a 2,5-thiophene-based catenane rotates 3.2 million-fold faster than the analogous 2,6-pyridine-based system at room temperature! The polarity of the environment also plays a crucial role in determining the inter-ring dynamics: reducing the strength of the ground-state hydrogen-bonding network by employing hydrogen bond-disrupting solvents (methanol, DMSO) increases the rate of rotation by lowering the activation energy for circumvolution (normally in the region of 11−20 kcal mol-1) by up to 3.2 kcal mol-1. This allows exquisite control over the kinetics of the translational behavior of the individual components of an interlocked molecular system, a key requirement for their development as nanoscale shuttles, switches, and information storage systems.

Origin of Co-Conformational Selectivity in a [3]rotaxane

“…Deleuze et al 21 provided a theoretical description of the lowest energy pathway for the circumrotation of macrocycles in a catenane system by using molecular mechanics (MM) calculations to model the molecular potential energy surface (PES) and employing unimolecular reaction rate theory. In another MMbased study, Leigh et al 22 investigated the factors that affect the rate of macrocyclic ring rotation in benzylic amide [2]catenanes. Such motions are crucial for switching among different interlocking positions in catenanes.…”

Photoisomerization of a rotaxane hydrogen bonding template: Light-induced acceleration of a large amplitude rotational motion

“…24 and 25. Examples of specifically controlling the frequency of large amplitude internal rotary motions include the redox-mediated acceleration͞deceleration of the spinning of porphyrin ligands in cerium and zirconium sandwich complexes (26), the environmentdependant rate of circumrotation in hydrogen bonded [2]catenanes (27), and the electrochemically induced pirouetting of a macrocycle in a rotaxane (28).…”

Multiscale and Multiphysics Computational Frameworks for Nano- and Bio-Systems

“…In addition, our free energy barrier is quite comparable to the barriers to circumrotation of [2]catenanes. Leigh and co-workers used NMR to determine ∆G ‡ of interlocked catenane molecules as 11 − 20 kcal/mol for various solvents and calculated the free energy barrier as 10 − 20 kcal/mol using force-field based Hessians [69,70,71].…”