Iron Complexes for the Electrocatalytic Oxidation of Hydrogen: Tuning Primary and Secondary Coordination Spheres (original) (raw)
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Chem. Sci., 2015
Oxidation of hydrogen (H 2 ) to protons and electrons for energy production in fuel cells is currently catalyzed by platinum, but its low abundance and high cost present drawbacks to widespread adoption. Precisely controlled proton removal from the active site is critical in hydrogenase enzymes in nature that catalyze H 2 oxidation using earth-abundant metals (iron and nickel). Here we report a synthetic iron complex, (Cp C5F4N )Fe(P Et N (CH2)3NMe2 P Et )(Cl), that serves as a precatalyst for the oxidation of H 2 , with turnover frequencies of 290 s À1 in fluorobenzene, under 1 atm of H 2 using 1,4-diazabicyclo [2.2.2]octane (DABCO) as the exogenous base. The inclusion of a properly tuned outer coordination sphere proton relay results in a cooperative effect between the primary, secondary and outer coordination spheres for moving protons, increasing the rate of H 2 oxidation without increasing the overpotential when compared with the analogous complex featuring a single pendant base. This finding emphasizes the key role of pendant amines in mimicking the functionality of the proton pathway in the hydrogenase enzymes.
Cyclopentadienyl (Cp), a classic ancillary ligand platform, can be chemically non-innocent in electrocatalytic H-H bond formation reactions via protonation of coordinated η5-Cp ligands to form η4-CpH moieties. However, the kinetics of η5-Cp ring protonation, ligand-to-metal (or metal-to-ligand) proton transfer, and the influence of solvent during H2 production electrocatalysis remain underexplored. We report in-depth kinetic details for electrocatalytic H2 production using Fe complexes [CpFe(CO)2(NCMe)]+ with amine-rich cyclopentadienyl ligands (Cp = enCpN; en = ethylenediamine, N = NHiPr, Pyrrolidinyl), which generate H2 with a maximum turnover frequency of 266 s-1 via η4-CpH intermediates. Under reducing conditions, state-of-the-art DFT calculations reveal that coordinated solvent plays a crucial role in mediating stereo- and regioselective proton transfer to generate (endo-CpH)Fe(CO)2(NCMe), followed by rapid solvent release and ligand-to-metal proton transfer to generate CpFeH(C...
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.
Inorganic Chemistry, 2002
The dinuclear iron(II)−hydride complexes [{FeH(dppe) 2 } 2 (µ-LL)][BF 4 ] 2 (LL ) NCCHdCHCN (1a), NCC 6 H 4 CN (1b), NCCH 2 CH 2 CN (1c); dppe ) Ph 2 PCH 2 CH 2 PPh 2 ) and the corresponding mononuclear ones, trans-[FeH(LL)-(dppe) 2 ][BF 4 ] (2a−c) were prepared by treatment of trans-[FeHCl(dppe) 2 ], in tetrahydrofuran (thf) and in the presence of Tl[BF 4 ], with the appropriate dinitrile (in molar deficiency or excess, respectively). Metal−metal interaction was detected by cyclic voltammetry for 1a, which, upon single-electron reversible oxidation, forms the mixed valent Fe II /Fe III 1a + complex. The latter either undergoes heterolytic Fe−H bond cleavage (loss of H + ) or further oxidation, at a higher potential, also followed by hydride-proton evolution, according to ECECE or EECECEC mechanistic processes, respectively, which were established by digital simulation. Anodically induced Fe−H bond rupture was also observed for the other complexes and the detailed electrochemical behavior, as well as the metal−metal interaction (for 1a), were rationalized by ab initio calculations for model compounds and oxidized derivatives. These calculations were used to generate the structural parameters (full geometry optimization), the most stable isomeric forms, the ionization potentials, the effective atomic charges, and the molecular orbital diagrams, as well as to predict the nature of the other electron-transfer induced chemical steps, i.e. geometric isomerization and nucleophilic addition, by BF 4 -, to the unsaturated iron center resulting from hydride-proton loss. From the values of the oxidation potential of the complexes, the electrochemical P L and E L ligand parameters were also estimated for the dinitrile ligands (LL) and for their mononuclear complexes 2 considered as ligands toward a second binding metal center.
Iron catalyzed hydrogenation and electrochemical reduction of CO 2 : The role of functional ligands
Journal of Organometallic Chemistry, 2018
The reduction of CO2 is an attractive route to utilize the greenhouse gas as a C1 building block. In recent years, the scientific progress that could be obtained for CO2 hydrogenation to formate and electrochemical reduction mainly to CO was strongly driven by the development of molecular iron catalysts with high activities and selectivities. However, these advances are also associated with the utilization of functional ligands that facilitate, e.g. H2 heterolysis in thermal hydrogenation or the storage of redox-equivalents in electrochemical transformations. In this review the use of such cooperating and redox non-innocent ligands in iron catalyzed CO2 transformations is discussed with the aim at providing some guidelines for catalyst design and improvement.