Entropic Control of the Helicity Inversion Rates of Twisted Metallomacrocycles by Reversible and Regioselective Deprotonation

TL;DR

This paper shows how the rate of helicity inversion in a twisted metal complex can be controlled through changes in activation entropy.

Contribution

The study introduces a novel method of entropic control over helicity inversion rates via regioselective deprotonation.

Findings

Partial deprotonation reduced the helicity inversion rate to 1/20 of its original rate.

Activation entropy dominates the rate change after deprotonation.

Proton relay via water molecules reduces activation entropy during inversion.

Abstract

The rate of molecular motion has been enthalpically controlled
by external stimuli such as acids and bases, electrons, and light.
However, controlling the rate of molecular motion through activation
entropy remains a challenging task. Here, we report entropic control
of the helicity inversion rate of a trinuclear PdII macrocycle
with right- and left-handed twisted structures. Three of the six NH
protons in this metallo-macrocycle were regioselectively deprotonated
by a moderately strong base, inducing intramolecular proton transfer
from the NH to N– moieties in the helicity inversion.
After the partial deprotonation, the helicity inversion rate of the
twisted macrocycle decreased to 1/20 of that before deprotonation
due to the dominant influence of the activation entropy term. The
kinetic isotope effects on the inversion rate suggest that an orderly
proton relay occurs between the NH and N– moieties
via multiple water molecules during the inversion process, resulting
in a significant reduction in the activation entropy. The mechanism
by which the activation entropy term controls the helicity inversion
rate via a proton relay is expected to provide a guide for the design
of more advanced molecular machines.

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