Thinking inside the box: mechanisms in non-covalent catalysis with Pd₂L₄ capsules

Abstract

Non-covalent catalysis with coordination capsules continues to be an ever-expanding subfield of supramolecular chemistry. With this expansion comes increasingly refined studies of reaction mechanism, revealing how non-covalent catalysts reshape the potential energy landscape around the reactants, offering faster reactions, more favourable selectivities, and milder conditions. However, a key methodology that remains underutilised in contemporary mechanistic studies is modelling the kinetics of reaction networks. This thesis describes the effects of a Pd₂L₄ capsule catalyst on the solution phase chemistry of a variety of guests. This is facilitated by monitoring temporal concentrations of multiple species over the duration of catalytic reactions using NMR and UV-visible spectroscopy, and conducting system-level modelling of reaction kinetics to produce new mechanistic insights.

Chapter one presents a brief introduction to coordination capsules in general, some examples of their use as non-covalent catalysts, and selected examples of mechanistic study in this subfield. This chapter also introduces various classes of techniques used in mechanistic study, and gives a brief introduction to kinetic modelling methods for catalytic reaction systems.

Chapter two concerns the investigation of a catalytic condensation reaction utilising a co-catalyst system of Pd₂L₄ and haloquinone p-fluoranil. Through UV-vis and 1H NMR investigations, the reactivity is proposed to originate from “hidden” Brønsted acid catalysis, initiated by capsule-activated substitution and ionisation chemistry in dichloromethane.

Chapter three concerns the investigation of base-catalysed conjugate addition reactions mediated by catalytic Pd₂L₄. Through direct detection of reactive intermediates by ¹H NMR spectroscopy, the kinetics are studied in detail. An origin for the significant degree of rate enhancement is proposed, and the capsule’s ability to efficiently redirect the reaction mechanism is explored.

Chapter four concerns a theoretical investigation into the kinetics of product inhibition in bimolecular coupling reactions, catalysed by a generalised coordination capsule. Dual encapsulation mechanisms are shown to be kinetically viable in cases where the binding affinity of the two co-substrates is strongly “biased”. The results presented strongly challenge the conventional belief that non-covalent capsule catalysts are inherently unsuited to promoting net coupling reactions.

Chapter five concerns an investigation into the host-guest dynamics of Pd₂L₄-quinone complexes through direct observation of their kinetics using stopped-flow UV-visible spectroscopy. Analysis of the kinetics of binding and exchange processes leads to a more detailed understanding of the chemistry underpinning all catalytic processes using these complexes.