Saad K. Ibrahim

Synthesis of the H-cluster framework of iron-only hydrogenase

The metal-sulphur active sites of hydrogenases catalyse hydrogen evolution or uptake at rapid rates. Understanding the structure and function of these active sites—through mechanistic studies of hydrogenases, synthetic assemblies and in silico models—will help guide the design of new materials for hydrogen production or uptake. Here we report the assembly of the iron-sulphur framework of the active site of iron-only hydrogenase (the H-cluster), and show that it functions as an electrocatalyst for proton reduction. Through linking of a di-iron subsite to a {4Fe4S} cluster, we achieve the first synthesis of a metallosulphur cluster core involved in small-molecule catalysis. In addition to advancing our understanding of the natural biological system, the availability of an active, free-standing analogue of the H-cluster may enable us to develop useful electrocatalytic materials for application in, for example, reversible hydrogen fuel cells. (Platinum is currently the preferred electrocatalyst for such applications, but is expensive, limited in availability and, in the long term, unsustainable.)

Water Splitting by Visible Light: A Nanophotocathode for Hydrogen Production

Efficient production of solar fuels is an imperative for meeting future fossil-fuel-free energy demands. Hydrogen that is derived from the splitting of water by solar energy is clearly attractive as a clean energy vector, and there have been many attempts to construct viable molecular and biomolecular devices for photohydrogen production. A common approach in the construction of such devices is the utilization of tris(bipyridine)ruthenium, zinc porphyrin, or related molecular materials as photosensitizers in conjunction with a tethered or free electrocatalyst or enzymic system. Apart from cost, such systems suffer from having limited lifetimes, which may be attributed at least in part to the intrinsic reactivity of the organic N-donor ligands in the radical anion form of the photoexcited state and photodegradation pathways.

Paramagnetic Bridging Hydrides of Relevance to Catalytic Hydrogen Evolution at Metallosulfur Centers

Paramagnetic hydrides are likely intermediates in hydrogen-evolving enzymic and molecular systems. Herein we report the first spectroscopic characterization of well-defined paramagnetic bridging hydrides. Time-resolved FTIR spectroelectrochemical experiments on a subsecond time scale revealed that single-electron transfer to the μ-hydride di-iron dithiolate complex 1 generates a 37-electron valence-delocalized species with no gross structural reorganization of the coordination sphere. DFT calculations support and (1)H and (2)H EPR measurements confirmed the formation an S = ½ paramagnetic complex (g = 2.0066) in which the unpaired spin density is essentially symmetrically distributed over the two iron atoms with strong hyperfine coupling to the bridging hydride (A(iso) = -75.8 MHz).

Ferracyclic carbamoyl complexes related to the active site of [Fe]-hydrogenase.

The active site of the [Fe]-hydrogenase features an iron(II) centre bearing cis carbonyl groups and a chelating pyridine-acyl ligand. Reproducing these unusual features in synthetic models is an intriguing challenge, which will allow both better understanding of the enzymatic system and more fundamental insight into the coordination modes of iron. By using the carbamoyl group as a surrogate for acyl, we have been able to synthesize a range of ferracyclic complexes. Initial reaction of Fe(CO)4Br2 with 2-aminopyridine yields a complex bearing a labile solvent molecule, which can be replaced by stronger donors bearing phosphorus atoms to produce a number of derivatives. Introduction of a hydroxy group using this method is unsuccessful both with a free OH group and when this is silyl-protected. In contrast, the analogous reactions starting from 2,6-diaminopyridine does allow synthesis of complexes bearing a pendant basic group.

Structure and vibrational dynamics of model compounds of the [fefe]-hydrogenase enzyme system via ultrafast two-dimensional infrared spectroscopy

Ultrafast two-dimensional infrared (2D) spectroscopy has been applied to study the structure and vibrational dynamics of (mu-S(CH2)3S)Fe2(CO)6, a model compound of the active site of the [FeFe]-hydrogenase enzyme system. Comparison of 2D-IR spectra of (mu-S(CH2)3S)Fe2(CO)6 with density functional theory calculations has determined that the solution-phase structure of this molecule is similar to that observed in the crystalline phase and in good agreement with gas-phase simulations. In addition, vibrational coupling and rapid (<5 ps) solvent-mediated equilibration of energy between vibrationally excited states of the carbonyl ligands of the di-iron-based active site model are observed prior to slower (approximately 100 ps) relaxation to the ground state. These dynamics are shown to be solvent-dependent and form a basis for the future determination of the vibrational interactions between active site and protein.

Solar Fuels: Photoelectrosynthesis of CO from CO2 at p-Type Si using Fe Porphyrin Electrocatalysts

Photoelectrocatalytic conversion of CO2 to CO can be driven at a boron-doped, hydrogen terminated, p-type silicon electrode using a meso-tetraphenylporphyrin Fe(III) chloride in the presence of CF3CH2OH as a proton source and 0.1 M [NBu4][BF4]/MeCN/5% DMF (v/v) as the electrolyte. Under illumination with polychromatic light, the photoelectrocatalysis operates with a photovoltage of about 650 mV positive of that for the dark reaction. Carbon monoxide is produced with a current efficiency >90% and with a high selectivity over H2 formation. Photoelectrochemical current densities of 3 mA cm(-2) at -1.1 V versus SCE are typical, and 175 turnovers have been attained over a 6 h period. Cyclic voltammetric data are consistent with a turnover frequency of k(Si)(obs)=0.24×10(4) s(-1) for the photoelectrocatalysis at p-type Si at -1.2 V versus SCE this compares with k(Si)(obs)=1.03×10(4) s(-1) for the electrocatalysis in the dark on vitreous carbon at a potential of -1.85 V versus SCE.

Protonation of [FeFe]-hydrogenase sub-site analogues: revealing mechanism using FTIR stopped-flow techniques

The formation of transient metal hydride(s) at the metallo-sulfur active sites of [FeFe]-hydrogenase is implicated in both hydrogen evolution and uptake reactions. Stopped-flow spectroscopic techniques can provide insight into the reactivity patterns of model {2Fe2S} sub-sites towards protons, and this information contributes to understanding the nature of the biological systems. In this study we have focussed on the influence of the nature of the bridging dithiolate ligand in influencing the kinetics and activation energy parameters for protonation in synthetic sub-sites including Fe2{micro-[S(CH2)(n)S]}(CO)4(PMe3)2 [n = 2, ethane-1,2-dithiolate (edt) or n = 3, propane-1,3-dithiolate (pdt)], Fe2[(micro-SCH2)2NH](CO)4(PMe3)2 and (NEt4)2{Fe2[(micro-SCH2)2NH](CO)4(CN)2}. Notably we find that (i) the presence of a nitrogen in the dithiolate bridge does not accelerate metal-metal bond protonation, and that (ii) immobilisation of (NEt4)2[Fe2(micro-pdt)(CO)4(CN)2] in a polymer matrix stabilises otherwise short-lifetime protonation products.

Probing the effect of the solution environment on the vibrational dynamics of an enzyme model system with ultrafast 2D-IR spectroscopy

Ultrafast 2D-IR spectroscopy has been applied to study the structure and vibrational dynamics of (μ-C(CH3)(CH2S)2(CH2S(CH2)2Ph)Fe2(CO)5, an organometallic model of the active site of the FeFe[hydrogenase] enzyme. 2D-IR spectra have been obtained in solvents ranging from non-polar to polar and protic. The influence of the solvent bath on vibrational relaxation, including rapid intramolecular population transfer, has been characterized. In addition, the temporal dependence of the 2D-IR lineshape has been used to extract information relating to hydrogen bond-mediated spectral diffusion via the frequency–frequency correlation function. Comparisons with previous 2D-IR studies of hydrogenase model systems offer insights into the dependence of the rate of population transfer upon vibrational mode separation and solvent environment, with important implications for the composition and reactivity of the active site of the enzyme.

An electrochemical and DFT study on selected β-diketiminato metal complexes

Selected homoleptic metal beta-diketiminates M(I)L and M(II)L2 [M(I) = Li or K, M(II) = Mg, Ca or Yb; L: L(Ph) = [N(SiMe3)C(Ph)]2CH, L(Bu(t)) = N(SiMe3)C(Ph)C(H)C(Bu(t))N(SiMe3), L* = [N(C6H3Pr(i)2-2,6)C(Me)]2CH] have been studied by cyclic voltammetry (CV). The primary reduction (E(p)red, the peak reduction potential measured vs. SCE in thf containing 0.2 M [NBu4][PF6] with a scan rate 100 mV s(-1) at a vitreous carbon electrode at ambient temperature) is essentially ligand-centred: E(p)red being ca. -2.2 V (LiL(Ph) and KL(Ph)) and -2.4 V [Mg(L(Ph))2, LiL(Bu(t)) and Ca(L(Ph))2], while LiL* is significantly more resistant to reduction (E(p)red = -3.1 V). These observations are consistent with the view that the two (L(Ph)) or single (L(Bu(t))) C-phenyl substituent(s), respectively, are available for -electron-delocalisation of the reduced species, whereas the N-aryl substituents of L* are unable to participate in such conjugation for steric reasons. The primary reduction process was reversible on the CV-time scale only for LiL(Bu(t)), Ca(L(Ph))2 and Yb(L(Ph))2. For the latter this occurs at a potential ca. 500 mV positive of Ca(L(Ph))2, consistent with the notion that the LUMO of Yb(L(Ph))2 has substantial metal character. The successive reversible steps, each separated by ca. 500 mV, indicate that there is strong electronic communication between the two ligands of Yb(L(Ph))2. The overall three-electron transfer sequence shows that the final reduction level corresponds to [Yb(II)(L(Ph))2-(L(Ph))3-]. DFT calculations on complexes Li(L(Ph))(OMe2)2 and Li2(L(Ph))(OMe2)3 showed that both HOMO and LUMO orbitals are only based on the ligand with a HOMO-LUMO gap of 4.21 eV. Similar calculations on a doubly reduced complex Yb[(mu-L(Ph))Li(OMe2)]2 demonstrated that there is a considerable Yb atomic orbital contribution to the HOMO and LUMO of the complex.

An intramolecular W–H ⋯ OC hydrogen bond? Electrosynthesis and X-ray crystallographic structure of [WH3(η1-OCOMe)(Ph2PCH2CH2PPh2)2]

Electrochemical reduction of [WH2(η2-OCOMe)(dppe)2]+ in the presence of protons leads to the formation of [WH3(η1-OCOMe)(dppe)2]: NMR and X-ray crystallographic data suggest this complex possesses an unprecedented W–H ⋯ OC intramolecular hydrogen bond.