A Study on Auto‐Catalysis and Product Inhibition: A Nucleophilic Aromatic Substitution Reaction Catalysed within the Cavity of an Octanuclear Coordination Cage

The ability of an octanuclear cubic coordination cage to catalyse a nucleophilic aromatic substitution reaction on a cavity‐bound guest was studied with 2,4‐dinitrofluorobenzene (DNFB) as the guest/substrate. It was found that DNFB undergoes a catalysed reaction with hydroxide ions within the cavity of the cubic cage (in aqueous buffer solution, pH 8.6). The rate enhancement of kcat/kuncat was determined to be 22, with cavity binding of the guest being required for catalysis to occur. The product, 2,4‐dinitrophenolate (DNP), remained bound within the cavity due to electrostatic stabilisation and exerts two apparently contradictory effects: it initially auto‐catalyses the reaction when present at low concentrations, but at higher concentrations inhibits catalysis when a pair of DNP guests block the cavity. When encapsulated, the UV/Vis absorption spectrum of DNP is red‐shifted when compared to the spectrum of free DNP in aqueous solution. Further investigations using other aromatic guests determined that a similar red‐shift on cavity binding also occurred for 4‐nitrophenolate (4NP) at pH 8.6. The red‐shift was used to determine the stoichiometry of guest binding of DNP and 4NP within the cage cavity, which was confirmed by structural analysis with X‐ray crystallography; and was also used to perform catalytic kinetic studies in the solution‐state. An octanuclear cubic coordination cage catalyses the nucleophilic aromatic substitution reaction of a cavity‐bound guest, 2,4‐dinitrofluorobenzene (DNFB). The product 2,4‐dinitrophenolate (DNP) remained bound within the cavity, providing both autocatalysis in the early stages and inhibition in the late stages of the reaction. A red‐shift in the UV/Vis spectrum revealed 1 : 2 host to guest binding, this was confirmed by X‐ray crystallography analysis.