AbstractControlling the coordination sphere of heterogeneous single‐metal‐site catalysts is a powerful strategy for fine‐tuning their catalytic properties but is fairly difficult to achieve. To address this problem, we immobilized supramolecular cages where the primary‐ and secondary coordination sphere are controlled by ligand design. The kinetics of these catalysts were studied in a model reaction, the hydrolysis of ammonia borane, over a temperature range using fast and precise online measurements generating high‐precision Arrhenius plots. The results show how catalytic properties can be enhanced by placing a well‐defined reaction pocket around the active site. Our fine‐tuning yielded a catalyst with such performance that the reaction kinetics are diffusion‐controlled rather than chemically controlled.