Confined metal centres in a symmetric cage: mono- and heterodinuclear complexes for photocatalytic hydrogen evolution

We present a supramolecular cage platform that offers modular access to distinct levels of metalation within the same confined, symmetric tris(2-pyridylmethyl)amine (TPMA)-based scaffold. Through a stepwise metalation strategy, well-defined homodinuclear (ZnZn, CoCo), mononuclear (CoH4) and heterodinuclear (CoZn, CoCu) complexes can be obtained in a reproducible manner, overcoming the formation of statistical mixtures typically associated with homoditopic ligands. This molecular selectivity establishes the cage as a versatile platform to systematically compare how confinement and metal nuclearity influence reactivity in a shared supramolecular environment. As a proof of concept, the different cages were evaluated in light-driven hydrogen evolution under visible-light irradiation using [Ru(bpy)3]2+ as photosensitizer and ascorbate as sacrificial electron donor, revealing the mononuclear CoH4 complex as the most active catalyst. DFT calculations suggest that its enhanced activity arises from a confined second-sphere proton relay provided by the protonated, non-coordinated TPMA moiety. Beyond this case study, these findings establish a versatile platform for probing structure–function relationships in confined catalysts.