Increasing the rate of hydrogen oxidation without increasing the overpotential: a bio-inspired iron molecular electrocatalyst with an outer coordination sphere proton relay (original) (raw)
Related papers
Chemical Science, 2023
An azadithiolate bridged CN − bound pentacarbonyl bis-iron complex, mimicking the active site of [Fe-Fe] H 2 ase is synthesized. The geometric and electronic structure of this complex is elucidated using a combination of EXAFS analysis, infrared and Mössbauer spectroscopy and DFT calculations. The electrochemical investigations show that complex 1 effectively reduces H + to H 2 between pH 0-3 at diffusion-controlled rates (10 11 M −1 s −1) i.e. 10 8 s −1 at pH 3 with an overpotential of 140 mV. Electrochemical analysis and DFT calculations suggests that a CN − ligand increases the pK a of the cluster enabling hydrogen production from its Fe(I)-Fe(0) state at pHs much higher and overpotential much lower than its precursor bis-iron hexacarbonyl model which is active in its Fe(0)-Fe(0) state. The formation of a terminal Fe-H species, evidenced by spectroelectrochemistry in organic solvent, via a rate determining proton coupled electron transfer step and protonation of the adjacent azadithiolate, lowers the kinetic barrier leading to diffusion controlled rates of H 2 evolution. The stereo-electronic factors enhance its catalytic rate by 3 order of magnitude relative to a bis-iron hexacarbonyl precursor at the same pH and potential.
ACS Catalysis, 2014
A series of iron hydride complexes featuring P R N R′ P R (P R N R′ P R = R 2 PCH 2 N(R′)CH 2 PR 2 where R = Ph, R′ = Me; R = Et, R′ = Ph, Bn, Me, t Bu) and cyclopentadienide (Cp X = C 5 H 4 X where X = H, C 5 F 4 N) ligands has been synthesized; characterized by NMR spectroscopy, X-ray diffraction, and cyclic voltammetry; and examined by quantum chemistry calculations. Each compound was tested for the electrocatalytic oxidation of H 2 , and the most active complex, (Cp C 5 F 4 N )Fe(P Et N Me P Et )(H), exhibited a turnover frequency of 8.6 s −1 at 1 atm of H 2 with an overpotential of 0.41 V, as measured at the potential at half of the catalytic current and using Nmethylpyrrolidine as the exogenous base to remove protons. Control complexes that do not contain pendant amine groups were also prepared and characterized, but no catalysis was observed. The rate-limiting steps during catalysis are identified through combined experimental and computational studies as the intramolecular deprotonation of the Fe III hydride by the pendant amine and the subsequent deprotonation by an exogenous base.
Journal of the American Chemical Society, 2004
As functional biomimics of the hydrogen-producing capability of the dinuclear active site in [Fe]-H2ase, the Fe I Fe I organometallic complexes, (µ-pdt)[Fe(CO)2PTA]2, 1-PTA2, (pdt) SCH2CH2CH2S; PTA) 1,3,5-triaza-7-phosphaadamantane), and (µ-pdt)[Fe(CO)3][Fe(CO)2PTA], 1-PTA, were synthesized and fully characterized. For comparison to the hydrophobic (µ-pdt)[Fe(CO)2(PMe3)]2 and {(µ-H)(µ-pdt)[Fe(CO)2-(PMe3)]2} + analogues, electrochemical responses of 1-PTA2 and 1-(PTA‚H +)2 were recorded in acetonitrile and in acetonitrile/water mixtures in the absence and presence of acetic acid. The production of H2 and the dependence of current on acid concentration indicated that the complexes were solution electrocatalysts that decreased over-voltage for H + reduction from HOAc in CH3CN by up to 600 mV. The most effective electrocatalyst is the asymmetric 1-PTA species, which promotes H2 formation from HOAc (pKa in CH3CN) 22.6) at-1.4 V in CH3CN/H2O mixtures at the Fe 0 Fe I redox level. Functionalization of the PTA ligand via N-protonation or N-methylation, generating (µ-pdt)[Fe(CO)2(PTA-H +)]2, 1-(PTA‚H +)2, and (µ-pdt)[Fe-(CO)2(PTA-CH3 +)]2, 1-(PTA-Me +)2, provided no obvious advantages for the electrocatalysis because in both cases the parent complex is reclaimed during one cycle under the electrochemical conditions and H2 production catalysis develops from the neutral species. The order of proton/electron addition to the catalyst, i.e., the electrochemical mechanism, is dependent on the extent of P-donor ligand substitution and on the acid strength. Cyclic voltammetric curve-crossing phenomena was observed and analyzed in terms of the possible presence of an η 2-H2-Fe II Fe I species, derived from reduction of the Fe I Fe I parent complex to Fe 0 Fe I followed by uptake of two protons in an ECCE mechanism.
Organometallics, 2014
Active site mimics of [FeFe]-hydrogenase are shown to be bidirectional catalysts, producing H 2 upon treatment with protons and reducing equivalents. This reactivity complements the previously reported oxidation of H 2 by these same catalysts in the presence of oxidants. 2 )] 2+ ([1H] 2+ ). The species [1H] 2+ consists of a ferrocenium ligand, an Nprotonated amine, and an Fe I Fe I core. In the presence of additional reducing equivalents in the form of decamethylferrocene (Fc*), hydrogen evolution is catalytic, albeit slow. The related catalyst Fe 2 (adt Bn )(CO) 3 (dppv)(PMe 3 ) (3) behaves similarly in the presence of Fc*, except that in the absence of excess reducing agent it converts to the catalytically inactive μ-hydride derivative [μ-H3] + . Replacement of the adt in [1] 0 with propanedithiolate (pdt) results in a catalytically inactive complex. In the course of synthesizing [FeFe]-hydrogenase mimics, new routes to ferrocenylphosphine ligands and nonamethylferrocene were developed.
Synthetic Models for Nickel–Iron Hydrogenase Featuring Redox-Active Ligands
Australian Journal of Chemistry, 2017
The nickel-iron hydrogenase enzymes efficiently and reversibly interconvert protons, electrons, and dihydrogen. These redox proteins feature iron-sulfur clusters that relay electrons to and from their active sites. Reported here are synthetic models for nickel-iron hydrogenase featuring redoxactive auxiliaries that mimic the iron-sulfur cofactors. The complexes prepared are Ni II (μ-H)Fe II Fe II species of formula [(diphosphine)Ni(dithiolate)(μ-H)Fe(CO) 2 (ferrocenylphosphine)] + or Ni II Fe I Fe II complexes [(diphosphine)Ni(dithiolate)Fe(CO) 2 (ferrocenylphosphine)] + (diphosphine = Ph 2 P(CH 2) 2 PPh 2 or Cy 2 P(CH 2) 2 PCy 2 ; dithiolate = − S(CH 2) 3 S − ; ferrocenylphosphine = diphenylphosphinoferrocene, diphenylphosphinomethyl(nonamethylferrocene) or 1,1′-bis(diphenylphosphino)ferrocene). The hydride species is a catalyst for hydrogen evolution, while the latter hydride-free complexes can exist in four redox states-a feature made possible by the incorporation of the ferrocenyl groups. Mixed-valent complexes of 1,1′-bis(diphenylphosphino)ferrocene have one of the phosphine groups unbound, with these species representing advanced structural models with both a redoxactive moiety (the ferrocene group) and a potential proton relay (the free phosphine) proximal to a nickel-iron dithiolate. * This paper is dedicated to Emeritus Professor Len Lindoy on the occasion of his 80th birthday.