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Entropic Control of the Helicity Inversion Rates of Twisted Metallomacrocycles by Reversible and Regioselective
Tomoki Nakajima1, Shohei Tashiro1, Masahiro Ehara2
1Department of Chemistry, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan.
Journal of the American Chemical Society
|February 12, 2026
Summary
Researchers achieved entropic control over molecular motion by altering the helicity inversion rate of a palladium macrocycle. Partial deprotonation significantly slowed this rate, offering new molecular machine design principles.
Area of Science:
- Supramolecular Chemistry
- Chemical Kinetics
- Molecular Machines
Background:
- Molecular motion rates are typically controlled by enthalpy, with activation entropy control being a significant challenge.
- External stimuli like acids, bases, electrons, and light have been used to influence molecular motion rates.
Purpose of the Study:
- To demonstrate entropic control over the helicity inversion rate of a trinuclear palladium(II) macrocycle.
- To investigate the mechanism of helicity inversion influenced by activation entropy.
Main Methods:
- Synthesis of a trinuclear palladium(II) macrocycle with twisted structures.
- Regioselective deprotonation of NH protons using a base.
- Kinetic studies, including kinetic isotope effects, to analyze the inversion rate and mechanism.
Main Results:
- Partial deprotonation of the macrocycle led to a 20-fold decrease in its helicity inversion rate.
- The reduced rate was attributed to the dominant influence of the activation entropy term.
- Kinetic isotope effects indicated an orderly proton relay mechanism involving water molecules.
Conclusions:
- Activation entropy can effectively control the rate of molecular motion, specifically helicity inversion in this Pd(II) macrocycle.
- A proton relay mechanism significantly reduces activation entropy, slowing the inversion process.
- This work provides a framework for designing advanced molecular machines with entropy-controlled dynamics.
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