Castner AT, Johnson BA, Cohen SM, Ott S. Mimicking the Electron Transport Chain and Active Site of [FeFe] Hydrogenases in One Metal-Organic Framework: Factors That Influence Charge Transport.
J Am Chem Soc 2021;
143:7991-7999. [PMID:
34029060 PMCID:
PMC8176456 DOI:
10.1021/jacs.1c01361]
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Abstract
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[FeFe] hydrogenase
(H2ase) enzymes are effective proton
reduction catalysts capable of forming molecular dihydrogen with a
high turnover frequency at low overpotential. The active sites of
these enzymes are buried within the protein structures, and substrates
required for hydrogen evolution (both protons and electrons) are shuttled
to the active sites through channels from the protein surface. Metal–organic
frameworks (MOFs) provide a unique platform for mimicking such enzymes
due to their inherent porosity which permits substrate diffusion and
their structural tunability which allows for the incorporation of
multiple functional linkers. Herein, we describe the preparation and
characterization of a redox-active PCN-700-based MOF (PCN = porous
coordination network) that features both a biomimetic model of the
[FeFe] H2ase active site as well as a redox-active linker
that acts as an electron mediator, thereby mimicking the function
of [4Fe4S] clusters in the enzyme. Rigorous studies on the dual-functionalized
MOF by cyclic voltammetry (CV) reveal similarities to the natural
system but also important limitations in the MOF-enzyme analogy. Most
importantly, and in contrast to the enzyme, restrictions apply to
the total concentration of reduced linkers and charge-balancing counter
cations that can be accommodated within the MOF. Successive charging
of the MOF results in nonideal interactions between linkers and restricted
mobility of charge-compensating redox-inactive counterions. Consequently,
apparent diffusion coefficients are no longer constant, and expected
redox features in the CVs of the materials are absent. Such nonlinear
effects may play an important role in MOFs for (electro)catalytic
applications.
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