Homoleptic and Heteroleptic Polyoxotungstate-Organic Cages for Efficient Photocatalytic Hydrogen Evolution

Metal-organic cages with polyoxometalate (POM) clusters as nodes are an emerging frontier of coordination-driven self-assembly, but they have been limited to homoleptic cages, which are composed of only one type of organic ligands. We show here that, in the construction of POM-organic cages with Keggin-type {SiW₉Ni₄} cluster nodes, introducing a secondary ligand may change the self-assembly processes in two distinct ways: by coordination or as supramolecular templates. The use of a tetracarboxylate panel L^C, in complementary to a bent dicarboxylate linker (L^A or L^B), allows the integrative self-sorting to give heteroleptic coordination cages POM₈L^A/B ₄L^C ₄ (3 or 4) that are otherwise not accessible through either ligand alone. The aromatic L^C can also alter the outcome of the self-assembly process by acting as a non-coordinating template, transforming a coordinatively frustrated, homoleptic cage POM₈L^D ₆ (5) to POM₈L^D ₅ (6). These octa-Keggin cages were all shown to be efficient molecular catalysts for visible-light-driven hydrogen production; for 4, in particular, an apparent turnover number of 4910 was achieved in 5 h under minimally optimized conditions. Mechanistic studies confirmed the existence of both reductive and oxidative quenching processes, with the former being dominant.