Muonium Chemistry at Diiron Subsite Analogues of [FeFe]-Hydrogenase.

The chemistry of metal hydrides is implicated in a range of catalytic processes at metal centers. Gaining insight into the formation of such sites by protonation and/or electronation is therefore of significant value in fully exploiting the potential of such systems. Here, we show that the muonium radical (Mu. ), used as a low isotopic mass analogue of hydrogen, can be exploited to probe the early stages of hydride formation at metal centers. Mu. undergoes the same chemical reactions as H. and can be directly observed due to its short lifetime (in the microseconds) and unique breakdown signature. By implanting Mu. into three models of the [FeFe]-hydrogenase active site we have been able to detect key muoniated intermediates of direct relevance to the hydride chemistry of these systems.

Electronic control of the protonation rates of Fe-Fe bonds.

Protonation at metal-metal bonds is of fundamental interest in the context of the function of the active sites of hydrogenases and nitrogenases. In diiron dithiolate complexes bearing carbonyl and electron-donating ligands, the metal-metal bond is the highest occupied molecular orbital (HOMO) with a "bent" geometry. Here we show that the experimentally measured rates of protonation (kH) of this bond and the energy of the HOMO as measured by the oxidation potential of the complexes (E1/2(ox)) correlate in a linear free energy relationship: ln kH = ((F(c - ßE1/2(ox)))/(RT)), where c is a constant and ß is the dimensionless Brønsted coefficient. The value of ß of 0.68 is indicative of a strong dependence upon energy of the HOMO: measured rates of protonation vary over 6 orders of magnitude for a change in E1/2(ox) of ca. 0.55 V (ca. 11 orders of magnitude/V). This relationship allows prediction of protonation rates of systems that are either too fast to measure experimentally or that possess additional protonation sites. It is further suggested that the nature of the bridgehead in the dithiolate ligand can exert a stereoelectronic influence: bulky substituents destabilize the HOMO, thereby increasing the rate of protonation.

[FeFe] hydrogenase: protonation of {2Fe3S} systems and formation of super-reduced hydride states.

The synthesis and crystallographic characterization of a complex possessing a well-defined {2Fe3S(µ-H)} core gives access to a paramagnetic bridging hydride with retention of the core geometry. Chemistry of this 35-electron species within the confines of a thin-layer FTIR spectro-electrochemistry cell provides evidence for a unprecedented super-reduced Fe(I)(µ-H)Fe(I) intermediate.

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.

Solar fuels: visible-light-driven generation of dihydrogen at p-type silicon electrocatalysed by molybdenum hydrides.

We show that a robust molybdenum hydride system can sustain photoelectrocatalysis of a hydrogen evolution reaction at boron-doped, hydrogen-terminated, p-type silicon. The photovoltage for the system is about 600-650 mV and the current densities, which can be sustained at the photocathode in non-catalytic and catalytic regimes, are similar to those at a photoinert vitreous carbon electrode. The kinetics of electrocatalysed hydrogen evolution at the photocathode are also very similar to those measured at vitreous carbon-evidently visible light does not significantly perturb the catalytic mechanism. Importantly, we show that the doped (1-10â€…Ω cm) p-type Si can function perfectly well in the dark as an ohmic conductor and this has allowed direct comparison of the cyclic voltammetric behaviour of the response of the system under dark and illuminated conditions at the same electrode. The p-type Si we have employed optimally harvests light energy in the 600-700 nm region and with 37 mW cm(-2) illumination in this range; the light to electrochemical energy conversion is estimated to be 2.8 %. The current yield of hydrogen under broad tungsten halide lamp illumination at 90 mW cm(-2) is (91±5) % with a corresponding chemical yield of (98±5) %.

On the use of pH titration to quantitatively characterize colloidal nanoparticles.

Functional nanoparticles (NPs) for bioapplications have been achieved, thanks to synthesis providing high quality nanocrystals, efficient procedures for transfer in water, and further conjugation of (bio)active molecules. However, these nanomaterials are still subjected to batch-to-batch variability and investigations of their physicochemical properties and chemical reactivity are still in their infancy. This may be due to lack of a routine, cost-effective, and readily available quantitative method for characterizing functional NPs. In this work, we show that pH titrations can be a powerful tool for investigating the surface properties of charged NPs and quantifying their surface functionalities. We demonstrate how this method can be useful in characterizing the colloidal and chemical stability, composition, and purity of the nanomaterial. The method also shows potential for the optimization of conjugation conditions.

Solution-phase photochemistry of a [FeFe]hydrogenase model compound: evidence of photoinduced isomerisation.

The solution-phase photochemistry of the [FeFe] hydrogenase subsite model (µ-S(CH(2))(3)S)Fe(2)(CO)(4)(PMe(3))(2) has been studied using ultrafast time-resolved infrared spectroscopy supported by density functional theory calculations. In three different solvents, n-heptane, methanol, and acetonitrile, relaxation of the tricarbonyl intermediate formed by UV photolysis of a carbonyl ligand leads to geminate recombination with a bias towards a thermodynamically less stable isomeric form, suggesting that facile interconversion of the ligand groups at the Fe center is possible in the unsaturated species. In a polar or hydrogen bonding solvent, this process competes with solvent substitution leading to the formation of stable solvent adduct species. The data provide further insight into the effect of incorporating non-carbonyl ligands on the dynamics and photochemistry of hydrogenase-derived biomimetic compounds.

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).

The role of CN and CO ligands in the vibrational relaxation dynamics of model compounds of the [FeFe]-hydrogenase enzyme.

The vibrational dynamics of (µ-propanedithiolate)Fe(2)(CO)(4)(CN)(2)(2-), a model compound of the active site of the [FeFe]-hydrogenase enzyme, have been examined via ultrafast 2D-IR spectroscopy. The results indicate that the vibrational coupling between the stretching modes of the CO and CN ligands is small and restricted to certain modes but the slow growth of off-diagonal peaks is assigned to population transfer processes occurring between these modes on timescales of 30-40 ps. Analysis of the dynamics in concert with anharmonic density functional theory simulations shows that the presence of CN ligands alters the vibrational relaxation dynamics of the CO modes in comparison to all-carbonyl model systems and suggests that the presence of these ligands in the enzyme may be an important feature in terms of directing the vibrational relaxation mechanism.

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.).

The mixed diol-dithiol 2,2-bis(sulfanylmethyl)propane-1,3-diol: characterization of key intermediates on a new synthetic pathway.

A new synthetic route to 2,2-bis(sulfanylmethyl)propane-1,3-diol, (II), is described starting from the commercially available 2,2-bis(hydroxymethyl)propane-1,3-diol. The structures of two intermediates on this route are described. 5,5-Dimethenyl-2,2-dimethyl-1,3-dioxane bis(thiocyanate) (systematic name: {[5-(cyanosulfanyl)-2,2-dimethyl-1,3-dioxan-5-yl]sulfanyl}formonitrile), C(10)H(14)N(2)O(2)S(2), (X), crystallizes in the space group P2(1)/c with no symmetry relationship between the two thiocyanate groups. There is a short intramolecular N...S contact for one thiocyanate group, while the second group is positioned such that this type of interaction is not possible. 1,3-(Hydroxymethyl)propane-1,3-diyl bis(thiocyanate), C(7)H(10)N(2)O(2)S(2), (XI), also features a single short N···S contact in the solid state. Hydrogen bonding between two molecules of compound (XI) results in the formation of dimers in the crystal, which are then linked together by a second hydrogen-bond interaction between the dimers. In addition, the structures of two intermediates from an unsuccessful alternative synthesis of (II) are reported. 2,2-Bis(chloromethyl)propane-1,3-diol, C(5)H(10)Cl(2)O(2), (VI), crystallized as an inversion twin with a minor twin fraction of 0.43 (6). It forms a zigzag structure as a result of intermolecular hydrogen bonding. The structure of 9,9-dimethyl-2,4,8,10-tetraoxa-3λ(4)-thiaspiro[5.5]undecan-3-one, C(8)H(14)O(5)S, (VII), shows evidence for a weak S···O contact with a distance of 3.2529 (11) Å.

Determination of the photolysis products of [FeFe]hydrogenase enzyme model systems using ultrafast multidimensional infrared spectroscopy.

Ultrafast transient 2D-IR (T-2D-IR) spectroscopy has been used to study the photolysis products of the [FeFe]hydrogenase enzyme model compound (µ-propanedithiolate)Fe(2)(CO)(6) in heptane solution following irradiation at ultraviolet wavelengths. Observation of coupling patterns between the vibrational modes of the photoproduct species formed alongside examination of the appearance time scales of these signals has uniquely enabled assignment of the photoproduct spectrum to a single pentacarbonyl species. Comparison of the vibrational relaxation rate of the photoproduct with that of the parent is consistent with the formation of a solvent adduct at the vacant coordination site, while anisotropy data in conjunction with density functional theory simulations indicates substitution in an axial rather than equatorial position. No firm evidence of additional short-lived intermediates is seen, indicating that the subsequent chemistry of these species is likely to be strongly defined by the nature of the first solvation shell.

The iron centre of the cluster-free hydrogenase (Hmd): low-spin Fe(II) or low-spin Fe(0)?

Infrared data for mono-iron complexes possessing two cis-CO together with Mössbauer data for the enzyme and a model complex support the assignment that the iron centre of the cluster-free hydrogenase Hmd is low-spin Fe(II).

Controlling carbon monoxide binding at di-iron units related to the iron-only hydrogenase sub-site.

Carbon monoxide binding by displacement of a pendant hemi-labile ligand at a di-iron site can be substantially 'switched-on' via a ligand protonation pathway which is competitive with metal-metal bond protonation.

Multiple-timescale photoreactivity of a model compound related to the active site of [FeFe]-hydrogenase.

Ultraviolet (UV) photolysis of (mu-S(CH 2) 3S)Fe 2(CO) 6 ( 1), a model compound of the Fe-hydrogenase enzyme system, has been carried out. When ultrafast UV-pump infrared (IR)-probe spectroscopy, steady-state Fourier transform IR spectroscopic methods, and density functional theory simulations are employed, it has been determined that irradiation of 1 in an alkane solution at 350 nm leads to the formation of two isomers of the 16-electron complex (mu-S(CH 2) 3S)Fe 2(CO) 5 within 50 ps with evidence of a weakly associated solvent adduct complex. 1 is subsequently recovered on timescales covering several minutes. These studies constitute the first attempt to study the photochemistry and reactivity of these enzyme active site models in solution following carbonyl ligand photolysis.

Electropolymeric materials incorporating subsite structures related to iron-only hydrogenase: active ester functionalised poly(pyrroles) for covalent binding of {2Fe3S}-carbonyl/cyanide assemblies.

We report the assembly of the first electropolymeric materials incorporating catalytic diiron subsites related to those of the iron-only hydrogenases.

On the structure of a proposed mixed-valent analogue of the diiron subsite of [FeFe]-hydrogenase.

We show that a dinuclear assembly apparently providing the first example of a synthetic molecule exhibiting key features of the diiron subsite of [FeFe] hydrogenase, viz. CO-bridging of a coordinatively unsaturated, dithiolate-bridged mixed-valence diiron centre, is in fact a diamagnetic tetranuclear complex.

On the electronic structure of the hydrogenase H-cluster.

X-ray absorption spectroscopic measurements and density functional calculations suggest that the hydrogenase H-cluster is best described as an electronically inseparable 6Fe-cluster due to extensive delocalization of frontier molecular orbitals of the iron centres, sulfide and the non-innocent dithiolate ligands.