Photocatalytic Hydrogen Evolution by a Synthetic [FeFe] Hydrogenase Mimic Encapsulated in a Porphyrin Cage

Abstract

The design of a biomimetic and fully base metal photocatalytic system for photocatalytic proton reduction in a homogeneous medium is described. A synthetic pyridylphosphole‐appended [FeFe] hydrogenase mimic was encapsulated inside a supramolecular zinc porphyrin‐based metal–organic cage structure Fe4(Zn‐L)6. The binding is driven by the selective pyridine–zinc porphyrin interaction and results in the catalyst being bound strongly inside the hydrophobic cavity of the cage. Excitation of the capsule‐forming porphyrin ligands with visible light while probing the IR spectrum confirmed that electron transfer takes place from the excited porphyrin cage to the catalyst residing inside the capsule. Light‐driven proton reduction was achieved by irradiation of an acidic solution of the caged catalyst with visible light.

Increasing structural and functional complexity in self-assembled coordination cages

“…136 In a similar fashion, Reek utilized Fe II -based cages built from Zn II -porphyrines for encapsulation of a variety of catalysts. [138][139][140] In these cages, the Zn II -porphyrine-based ligands play a dual function: with their four terminal diimine donors, they coordinate the Fe II vertices, and the central Zn II -cation contributes to catalyst encapsulation by coordinating the pyridine-donors included in the ligands of the catalyst. Especially for reactions in which the redox state and thus, the charge and coordination ability of the catalyst are changing, such additional anchors are quite important.…”

“…In a recent study, Reek encapsulated a [FeFe] hydrogenase active site model compound with the aid of a pyridyl-phosphole ligand as anchor into such a cage. 139 This combination drives the photochemical proton reduction. In an electrochemical follow-up study, 138 the same authors utilized a slightly modified version of the diiron complex and identified a shift to lower overpotential of the [FeFe] catalyst as a result of encapsulation inside the cationic cage.…”

About Control: Kinetics in Molecule- based Photochemical Water Reduction Investigated by Transient IR Spectroscopy

“…In some cases, PS and WRC are combined into a single molecular photocatalyst. [10,35] We will discuss the potential of time-resolved infrared spectroscopy to investigate reaction mechanisms of molecule-based water reduction systems. The main focus will be on a water reduction system, whose reaction mechanism is rather well understood in solution, and we will discuss how the reaction mechanism changes once the system is brought onto a surface.…”

Beyond hydrogen bonding: recent trends of outer sphere interactions in transition metal catalysis

“…The same tetrahedral assembly was used to encapsulate an [FeFe] hydrogenase mimic containing a pyridylphosphole moiety via Zn•••N coordination. 158 The system was applied in the light-harvesting hydrogen evolution reaction, in which the zinc-porphyrins served as photosensitizers allowing electron transfer to the encapsulated iron catalyst upon excitation (Scheme 34). Secondary Zn•••N coordination also drives the encapsulation of a chiral rhodium-coordinated phosphoramidite ligand inside Ribas's tetrahedral cage comprising zinc-porphyrins (Scheme 35).…”