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

Abstract

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 LC, in complementary to a bent dicarboxylate linker (LA or LB), allows the integrative self‐sorting to give heteroleptic coordination cages POM8LA/B4LC4 (3 or 4) that are otherwise not accessible through either ligand alone. The aromatic LC can also alter the outcome of the self‐assembly process by acting as a non‐coordinating template, transforming a coordinatively frustrated, homoleptic cage POM8LD6 (5) to POM8LD5 (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.