Polyoxometalate-Based Cobalt–Phosphate Molecular Catalysts for Visible Light-Driven Water Oxidation

A series
of all-inorganic, abundant-metal-based, high-nuclearity
cobalt–phosphate (Co–Pi) molecular catalysts [{Co4­(OH)3­(PO4)}4­(SiW9­O34)4]32– (1), [{Co4­(OH)3­(PO4)}4­(GeW9­O34)4]32– (2), [{Co4­(OH)3­(PO4)}4­(PW9­O34)4]28– (3), and
[{Co4­(OH)3­(PO4)}4­(AsW9­O34)4]28– (4) were synthesized and shown to be
highly effective at photocatalytic water oxidation. The {Co16(PO4)4} cluster contains a Co4O4 cubane which is structurally analogous to the [Mn3CaO4] core of the oxygen-evolving complex (OEC) in photosystem
II (PSII). Compounds 1–4 were shown
to be the first POM-based Co–Pi-cluster molecular catalysts
for visible light-driven water oxidation, thus serving as a functional
model of the OEC in PSII. The systematic synthesis of four isostructural
analogues allowed for investigating the influence of different heteroatoms
in the POM ligands on the photocatalytic activities of these Co–Pi
cluster WOCs. Further, the POM-based photocatalysts readily recrystallized
from the photocatalytic reaction systems with the polyoxoanion structures
unchanged, which together with the laser flash photolysis, dynamic
light-scattering, 31P NMR, UV–vis absorption, POM
extraction, and ICP-MS analysis results collectively confirmed that
compounds 1–4 maintain their structural
integrity under the photocatalytic conditions. This study provides
not only a valuable molecular model of the “Co–Pi”
catalysts with a well-defined structure but also an unprecedented
opportunity to fine-tune high-nuclearity POM clusters for visible
light-driven water splitting.