Mimicking the Electron Transport Chain and Active Site of [FeFe] Hydrogenases in One Metal-Organic Framework: Factors That Influence Charge Transport.

Ashleigh T Castner,Seth M Cohen,Sascha Ott,Ben A Johnson

doi:10.1021/jacs.1c01361

Abstract

[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.

Highlights

Hydrogenase enzymes are Nature’s catalysts for the interconversion of hydrogen, protons, and electrons
In 2015, Zhou and coworkers reported the preparation of the Zr-based Metal−organic frameworks (MOFs) PCN700 (PCN = porous coordination network) which is composed of dimethyl diphenyl dicarboxylate (Me2dpdc) linkers and Zr6O4(OH)8(H2O)[4] secondary binding units (SBUs).[28]
The components of PCN-700 are similar to those of the canonical Zr-MOF UiO-67, but the presence of methyl groups on the 2- and 2′- positions of the biphenyl linker results in a change to the dihedral angle between the rings, allowing for the formation of MOF crystals with Zr6 clusters that are 8-connected rather than the fully 12connected configuration that is observed for the UiO series of Zr-MOFs

Summary

■ INTRODUCTION

Hydrogenase enzymes are Nature’s catalysts for the interconversion of hydrogen, protons, and electrons. Integration of the current response at slower scan rates (5 mV s−1) reveals that a similar amount of charge is passed in both the first and second waves, observed at −0.94 and −1.34 V, respectively (see Figure S29) This observation is important when investigating the role of the NDI as an electron mediator to the Fe2 site, as not all conceivable reductions may be visible in the CV of PCN700_NDI_FeFe. With every linker reduction being associated with the uptake of a cation from the electrolyte (or expulsion of an anion from the MOF),[17,39] there may be restrictions as to the total concentration of reduced linkers and cations that can be accommodated within the framework. Gas chromatographic analysis of the gas mixture in the headspace of the electrolysis cell identified H2 as the gaseous product, while no H2 could be detected with an identical electrode under identical conditions but in the absence of PCN-700_NDI_FeFe (see the Supporting Information for details, Figures S33−S35)

■ CONCLUSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES