Enhanced Photochemical Hydrogen Production by a Molecular Diiron Catalyst Incorporated into a Metal–Organic Framework

A molecular
proton reduction catalyst [FeFe]­(dcbdt)­(CO)6 (1, dcbdt = 1,4-dicarboxylbenzene-2,3-dithiolate) with
structural similarities to [FeFe]-hydrogenase active sites has been
incorporated into a highly robust Zr­(IV)-based metal–organic
framework (MOF) by postsynthetic exchange (PSE). The PSE protocol
is crucial as direct solvothermal synthesis fails to produce the functionalized
MOF. The molecular integrity of the organometallic site within the
MOF is demonstrated by a variety of techniques, including X-ray absorption
spectroscopy. In conjunction with [Ru­(bpy)3]2+ as a photosensitizer and ascorbate as an electron donor, MOF-[FeFe]­(dcbdt)­(CO)6 catalyzes photochemical hydrogen evolution in water at pH
5. The immobilized catalyst shows substantially improved initial rates
and overall hydrogen production when compared to a reference system
of complex 1 in solution. Improved catalytic performance
is ascribed to structural stabilization of the complex when incorporated
in the MOF as well as the protection of reduced catalysts 1– and 12– from undesirable
charge recombination with oxidized ascorbate.