Hydrogenase-Inspired Catalysts

Catalytic Metallopolymers from [2Fe-2S] Clusters: Artificial Metalloenzymes for Hydrogen Production 

Metin Karayilan, William P. Brezinski, Kayla E. Clary, Dennis L. Lichtenberger, Richard S. Glass, and Jeffrey Pyun. Angewandte Chemie (International ed.)201958(23),7537-7550. (http://dx.doi.org/10.1002/anie.201813776)

Reviewed herein is the development of novel polymer‐supported [2Fe‐2S] catalyst systems for electrocatalytic and photocatalytic hydrogen evolution reactions. [FeFe] hydrogenases are the best known naturally occurring metalloenzymes for hydrogen generation, and small‐molecule, [2Fe‐2S]‐containing mimetics of the active site (H‐cluster) of these metalloenzymes have been synthesized for years. These small [2Fe‐2S] complexes have not yet reached the same capacity as that of enzymes for hydrogen production. Recently, modern polymer chemistry has been utilized to construct an outer coordination sphere around the [2Fe‐2S] clusters to provide site isolation, water solubility, and improved catalytic activity. In this review, the various macromolecular motifs and the catalytic properties of these polymer‐supported [2Fe‐2S] materials are surveyed. The most recent catalysts that incorporate a single [2Fe‐2S] complex, termed single‐site [2Fe‐2S] metallopolymers, exhibit superior activity for H2 production.


 

[FeFe]‐Hydrogenase Mimetic Metallopolymers with Enhanced Catalytic Activity for Hydrogen Production in Water 

William P. Brezinski, Metin Karayilan, Kayla E. Clary, Nicholas G. Pavlopoulos, Sipei Li, Liye Fu, Krzysztof Matyjaszewski, Dennis H. Evans, Richard S. Glass, Dennis L. Lichtenberger, Jeffrey Pyun. Angewandte Chemie (International ed.)201857(37),11898-11902. (http://dx.doi.org/10.1002/anie.201804661)

Electrocatalytic [FeFe]-hydrogenase mimics for the hydrogen evolution reaction (HER) generally suffer from low activity, high overpotential, aggregation, oxygen sensitivity, and low solubility in water. Using atom transfer radical polymerization (ATRP), we have prepared a new class of [FeFe]-metallopolymers with precise molar mass, composition, and low polydispersity. The synthetic methodology introduced here allows for facile variation of polymer composition to optimize the [FeFe] solubility, activity, and long term chemical and aerobic stability. We find that water soluble functional metallopolymers perform electrocatalytic hydrogen production in neutral water with loadings as low as 2 ppm and operate at rates many orders of magnitude faster than hydrogenases (2.5 x 105 s-1) and with low overpotential requirement. Furthermore, unlike the hydrogenases, these systems are insensitive to oxygen during catalysis with turnover numbers on the order of 40,000 under both anaerobic and aerobic conditions.


 

Effects of alkane linker length and chalcogen character in [FeFe]-hydrogenase inspired compounds 

Mohammad K. Harb, Ahmad Daraosheh, Helmar Goerls, Elliott R. Smith, G. J. Meyer, Matthew T. Swenson, Takahiro Sakamoto, Richard S. Glass, Dennis L. Lichtenberger, Dennis H. Evans, Mohammad El-khateeb and Wolfgang Weigand. Heteroat. Chem.201425, 592-606. (http://dx.doi.org/10.1002/hc.21216)

Models of [FeFe]-hydrogenases containing diselenolato ligands with different bridge linker length have been prepared; Fe2(μ-Se(CH2)4Se-μ)(CO)6 (4DS), and Fe2(μ-Se(CH2)5Se-μ)(CO)6 (5DS) as well as dithiolato Fe2(μ-S(CH2)4S-μ)(CO)6 (4DT) and compared with Fe2(μ-S(CH2)3S-μ)(CO)6 (PDT) and Fe2(μ-Se(CH2)3Se-μ)(CO)6 (PDS). Compounds 4DT, PDS, 4DS, and 5DS were characterized by spectroscopic techniques including NMR, IR, mass spectrometry, ultraviolet photoelectron spectroscopy (UPS), elemental analysis and X-ray crystal structure analysis. Combinations of electrochemical measurements, UPS and DFT calculations indicate that oxidations of these five compounds are not significantly affected by chalcogen character but instead are governed by linker length. Cations for all compounds are calculated to adopt a bridged CO “rotated” structure with a vacant site on one of the Fe-centers. In 4DT, 4DS and 5DS the alkane linker forms an agostic interaction with the vacant site on the rotated Fe. The reduction potentials for these compounds shift positively on average 0.16 V for each carbon added to the alkane linker with shifts being as large as 0.23 V between PDT and 4DT, and as small as 0.09 V between 4DS and 5DS. Catalytic reduction of protons from acetic acid in CH2Cl2 occurs at -1.79 and -1.86 V for PDT and 4DT and -2.02, -2.09, and -2.04 V for PDS, 4DS and 5DS indicating chalcogen character is the primary factor that affects catalytic potential. On average the S-containing compounds catalyze proton reduction at potentials which are 0.23 V less negative than the Se-containing compounds in this study.


 

Electrochemical, spectroscopic, and computational study of bis(mu-methylthiolato)diironhexacarbonyl: Homoassociative stabilization of the dianion and a chemically reversible reduction/reoxidation cycle 

Orrasa In-noi, Kenneth J. Haller, Gabriel B. Hall, William P. Brezinski, Jacob M. Marx, Taka Sakamoto, Dennis H. Evans, Richard S. Glass and Dennis L. Lichtenberger. Organometallics201433, 5009-5019. (http://dx.doi.org/10.1021/om5004122)

The redox characteristics of (mu-SMe)2Fe2(CO)6 from the 1+ to 2– charge states are reported. This [2Fe-2S] compound is related to the active sites of [FeFe]-hydrogenases but notably without a linker between the sulfur atoms. The 1+ charge state was studied both by ionization in the gas phase by photoelectron spectroscopy and by oxidation in the solution phase by cyclic voltammetry. The adiabatic ionization is to a cation whose structure features a semi-bridging carbonyl, similar to the structure of the active site of [FeFe]-hydrogenases in the same oxidation state. The reduction of the compound by cyclic voltammetry gives an electrochemically irreversible cathodic peak, which often suggests disproportionation or other irreversible chemical processes in this class of molecules. However, the return scan through electrochemically irreversible oxidation peaks that occur at potentials around 1 V more positive than the reduction led to recovery of the initial neutral compound. The dependence of the CVs on scan rate, IR spectroelectrochemistry of reduction and oxidation cycles, chronocoulometry, and DFT computations indicate a mechanism in which stabilization of the dianion plays a key role. Initial one-electron reduction of the compound is accompanied in the same cathodic peak with a second slower electron reduction to the dianion. Geometric reorganization and solvation stabilize the [2Fe-2S]2–dianion such that the potential for addition of the second electron is slightly less negative than that of the first (potential inversion). The return oxidation peaks at more positive potentials follow from rapid pairing of the dianion with another neutral molecule in solution (termed homoassociation) to form a stabilized [4Fe-4S]2– dianion. Two one-electron oxidations of this [4Fe-4S]2– dianion result in regeneration of the initial neutral compound. The implications of this homoassociation for the [FeFe]-hydrogenase enzyme, in which the H-cluster active site features a [2Fe-2S] site associated with a [4Fe-4S] cubane cluster via a thiolate bridge, is discussed.


 

Hydrogen Generation from Weak Acids: Electrochemical and Computational Studies in the [(η5-C5H5)Fe(CO)2]2 System

Greg A. N. Felton, Aaron K. Vannucci, Noriko Okumura, L. Tori Lockett, Dennis H. Evans, Richard S. Glass and Dennis L. Lichtenberger “Hydrogen Generation from Weak Acids: Electrochemical and Computational Studies in the [(η5-C5H5)Fe(CO)2]2 System” Organometallics200827(18), 4671–4679. (http://dx.doi.org/10.1021/om800366h)

5-C5H5)Fe(CO)2H, FpH, is stable to weak acids such as acetic acid. However, reduction of FpH in acetonitrile in the presence of weak acids generates H2 catalytically.  Evidence for the catalytic generation of H2 from just water also was observed.  Since reduction of Fp2 generates Fp-which can be protonated with weak acids, Fp2 serves as a convenient procatalyst for the electrocatalytic production of H2. Electrochemical simulations provided values for the key parameters of a catalytic mechanism for production of H2 in this system.  Protonation of Fp-was found to be the rate-determining step preceding H2 production. The wealth of structural, spectroscopic, and thermodynamic information available on the key Fp2, Fp-, and FpH species provided a variety of checkpoints for computational modeling of the catalytic mechanism.  The computations gave good agreement with the crystal structure of Fp2, the IR spectra of Fp2, Fp-, and FpH, and the photoelectron spectra of Fp2 and FpH.  The computations also accounted well for the reduction potentials and equilibrium constants in the electrochemical simulations. The FpH-anion was found to be susceptible to a direct and rapid attack by a proton to produce H2 and the Fp·radical, which was then reduced and protonated to continue the electrocatalytic cycle. This direct energetically downhill step of metal-hydride protonation to produce molecular hydrogen may be common for sufficiently electron rich metal hydrides and/or sufficiently strong acids among many of the hydrogenase mimics reported thus far.


 

.Synthesis and Characterization of Diiron Diselenato Complexes Including Iron Hydrogenase Models

Mohammad K. Harb, Tobias Niksch, Jochen Windhager, Helmar Görls, Rudolf Holze, L. Tori Lockett, Noriko Okumura, Dennis H. Evans, Richard S. Glass, Dennis L. Lichtenberger, Mohammad El-khateeb and Wolfgang Weigand, , Organometallics200928(4), 1039–1048. (http://dx.doi.org/10.1021/om800748p)

Diiron diselenolato complexes were prepared as models of the active site of [FeFe]-hydrogenases. Treatment of Fe3(CO)12 with one equivalent of 1,3-diselenocyanatopropane (1)in THF at reflux afforded the model compound Fe2(μ-Se2C3H6)(CO)6 (2)in 68% yield. The analogous methyl-substituted complex, Fe2(μ-Se2C3H5CH3)(CO)6 (3)was obtained from the reaction of Fe3(CO)12 with the in situ generated compound 3-methyl-1,2-diselenolane (4). In contrast, the reaction of Fe3(CO)12 with 1,3,5-triselenacyclohexane (5)produced a mixture of Fe22,κ-Se,C-SeCH2SeCH2)(CO)6 (6), Fe2[(μ-SeCH2)2Se](CO)6 (7)and Fe2(μ-Se2CH2)(CO)6 (8). Compounds 23and 6-8were characterized by IR, 1H, 13C, 77Se NMR spectroscopy, mass spectrometry, elemental analysis and X-ray single crystal structure analysis. The He I and He II photoelectron spectra for 3were reported and the electronic structure further analyzed with the aid of DFT computations. The calculated reorganization energy of the cation of 3to the “rotated” structure, which has a semi-bridging carbonyl ligand, is less than that of the analogous complexes with sulfur instead of selenium. Complexes 2and 3proved to be catalysts for electrochemical reduction of protons from the weak pivalic and acetic acids to give hydrogen.


 

Review of Electrochemical Studies of Complexes Containing the Fe2S2 Core Characteristic of [FeFe]-hydrogenases Including Catalysis by these Complexes of the Reduction of Acids to Form Dihydrogen

Felton, Greg A. N.; Mebi, Charles A.; Petro, Benjamin J.; Vannucci, Aaron K.; Evans, Dennis H.; Glass, Richard S.; Lichtenberger, Dennis L., J. Organomet. Chem.2009694(17), 2681-2699. (http://dx.doi.org/10.1016/j.jorganchem.2009.03.017)

In this article we reviewed published literature on the electrochemical reduction and oxidation of complexes containing the Fe2S2 core characteristic of the active site of [FeFe]-hydrogenases.  Correlations between reduction and oxidation potentials and molecular structure were developed and presented.  In cases where the complexes had been studied with regard to their ability to catalyze the reduction of acids to give dihydrogen, the overpotentials for such catalyzed reduction were presented and an attempt was made to estimate, at least qualitatively, the efficiency of such catalysis.


 

One-to-two Electron Reduction of an [FeFe]-Hydrogenase Active Site Mimic: The Critical Role of Fluxionality of the [2Fe2S] Core

Greg A. N. Felton, Benjamin J. Petro, Richard S. Glass, Dennis L. Lichtenberger, Dennis H. Evans, J. Am. Chem. Soc.2009131(32), 11290-11291. (http://dx.doi.org/10.1021/ja904520x)

A one- to two-electron reduction of μ-(1,2-ethanedithiolato) diironhexa carbonyl, EDT, was observed under electrochemical conditions depending on scan rate. This variable-electron uptake was attributed to potential inversion which involves significant structural rearrangement of the [2Fe2S] core. DFT computational methods support this assessment. Upon an initial one-electron reduction, the inherent fluxionality of the [2Fe2S] complex anion allows for a second one-electron reduction at less negative potentials to form a dianionic species with a structure characterized by a rotated iron center, bridging carbonyl and, most significantly, a dissociated Fe-S bond. This fluxionality of the [2Fe2S] core as a function of reduction has direct implications for the chemistry of [FeFe]-hydrogenase mimics and for iron-sulfur cluster chemistry in general.


 

Preparation and Characterization of Diiron-dichalcogenolato Complexes Containing an Oxetane Ring: [FeFe]-Hydrogenase Models

Mohammad K. Harb, Ulf-Peter Apfel, Joachim Kübel, Helmar Görls, Greg A. N. Felton, Taka Sakamoto, Dennis H. Evans, Richard S. Glass, Dennis L. Lichtenberger, Mohammad El-khateeb and Wolfgang Weigand, Organometallics200928(23), 6666-6675. (http://dx.doi.org/10.1021/om900675q)

In order to elucidate the influence of the bridging chalcogen atoms in hydrogenase model complexes, diiron dithiolato, diselenolato, and ditellurolato complexes were prepared and characterized. The new complexes were shown to catalyze electrochemical reduction of protons to give dihydrogen, and the catalytic rate was found to decrease on going from the sulfur to selenium to tellurium compounds. Spectroscopic and computational analysis indicated that the increasing size of the chalcogen atoms from S to Se to Te increases the Fe–Fe distance and decreases the ability of the complex to form the structure with a rotated Fe(CO)3 group that has a bridging carbonyl ligand and a vacant coordination site for protonation.


 

Electronic and Geometric Effects of Phosphatriazaadamantane Ligands on the Catalytic Activity of an [FeFe]-Hydrogenase Inspired Complex

Aaron K. Vannucci, Shihua Wang, Gary S. Nichol, Dennis L. Lichtenberger, Dennis H. Evans, and Richard S. Glass, Dalton Trans.201012, 3050-3056. (http://dx.doi.org/10.1039/b921067a)

The [FeFe] hydrogenase enzyme active site inspired complexes [Fe2(μ‑C6H4S2)-(CO)5PTA] and [Fe2(μ‑C6H4S2)(CO)4-PTA2] (PTA = 1,3,5-triaza-7-phosphaada-mantane) were synthesized and characterized in order to create water-soluble catalysts for the production of molecular hydrogen. The ability of these complexes to catalytically produce molecular hydrogen in solution from the weak acid acetic acid was examined electrochemically and compared to previous studies on the all carbonyl containing analogue [Fe2(μ‑C6H4S2)(CO)6].  Computational methods and cyclic voltammograms indicated that the substitution of CO ligands by PTA in resulted in markedly different reduction chemistry.  Both complexes catalytically produce molecular hydrogen from acetic acid, however, the mechanism by which they catalyze hydrogen differ in the initial reductive processes.


 

Synthesis and Characterization of [FeFe]-Hydrogenases Models with Bridging Moieties Containing (S, Se) and (S, Te)

Mohammad K. Harb, Helmar Görls, Taka Sakamoto, Greg A. N. Felton, Dennis H. Evans, Richard S. Glass, Dennis L. Lichtenberger, Mohammad El-Khateeb, and Wolfgang Weigand, Eur. J. Inorg. Chem.201025, 3975-3985. DOI: 10.1002/ejic.201000278

[FeFe]-Hydrogenase active-site models containing larger chalcogens such as Se or Te have exhibited greater electron richness at the metal centers and smaller gas-phase ionization energies and reorganization energies compared to molecules containing S atoms. Diiron complexes related to the much-studied molecule Fe2(μ-SC3H6S)(CO)6  were prepared with one S atom replaced either by one Se atom giving Fe2(μ-SC3H6Se)(CO)6  or by one Te atom giving Fe2(μ-SC3H6Te)(CO)6.He I photoelectron spectra and DFT computations of the Se and Te moleculesshowa lowering of ionization energies compared to those of the all-sulfur complex, indicating increased electron richness at the metal centers that favors electrocatalytic reduction of protons from weak acids to produce H2. However, chalcogen substitution from S to Se or Te also causes an increase in the Fe–Fe bond distance, which disfavors the formation of a carbonyl-bridged “rotated” structure, as also shown by the photoelectron spectra and computations. This “rotated” structure is believed to be important in the mechanism of H2 production. As a consequence of the competing influences of increased electron richness at the metals with less favorable “rotated” structures, the catalytic efficiency of the Se and Te molecules is found to be comparable to that of the all-S molecule.


Synthesis of Diiron Hydrogenase Mimics Bearing Hydroquinone and Related Ligands. Electrochemical and Computational Studies of the Mechanism of Hydrogen Production and the Role of O–H•••S Hydrogen Bonding

Jinzhu Chen, Aaron K. Vannucci, Charles A. Mebi, Noriko Okumura, Susan C. Borowski, L. Tori Lockett, Dennis L. Lichtenberger, Dennis H. Evans and Richard S. Glass, Organometallics2010XX, XX. DOI: 10.1021/om100396j 

A new advantageous synthesis of hydroquinone-bearing hydrogenase-inspired electrochatalysts was discovered.  These 2Fe2S complexes catalyze the production of H2 from acetic acid.  Spectroscopic studies suggest the presence of intramolecular hydrogen bonding between the phenolic OH groups and the adjacent sulfur atoms. Computations, which are in good agreement with the electrochemical studies and spectroscopic data, indicate that the hydrogen bonding is most important in the reduced forms of the catalysts. This hydrogen bonding lowers the reduction potential for catalysis, but also lowers the basicity and thereby the reactivity of the catalysts.