Search

Thiele's Hydrocarbons (THs) featuring a 9,10-anthrylene core with switchable geometric and electronic configurations offer exciting possibilities in advanced functional materials. Despite significant advances in main group-based diradicaloids in contemporary chemistry, main group THs containing an anthrylene cores have remained elusive, primarily due to the lack of straightforward synthetic strategies and the inherent high reactivity of these species. In this study, we utilize an anthracene-based phosphine synthon to demonstrate, for the first time, a facile and high-yielding synthetic strategy for robust P-functionalized overcrowded ethylenes (OCEs) within the TH family. These OCEs feature a non-symmetric environment, incorporating (thio) xanthyl and phosphaalkene termini. We systematically probe the electronic structures of these derivatives to illustrate the impact of the isolobal phosphaalkene motif on the quinoidal/diradicaloid character. Notably, the compounds exhibit dynamic redox behavior, leading to orthogonally twisted conformational changes upon oxidation, with a kinetically locked redox-couple., (© 2024 The Author(s). Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

The α-functionalisation of N -containing compounds is an area of broad interest in synthetic chemistry due to their presence in biologically active substances among others. Visible light-induced generation of nucleophilic α-aminoalkyl radicals as reactive intermediates that can be trapped by electron-deficient alkenes presents an attractive and mild approach to achieve said functionalisation. In this work, [Fe(iii)(phtmeimb) 2 ]PF 6 (phtmeimb = phenyl(tris(3-methylimidazol-2-ylidene))borate), an N-heterocyclic carbene (NHC) complex based on Earth-abundant iron, was used as photoredox catalyst to efficiently drive the formation of α-aminoalkyl radicals from a range of different α-trimethylsilylamines and their subsequent addition to a number of electron-deficient alkenes under green light irradiation. Mechanistic investigations elucidated the different reaction steps of the complete photocatalytic cycle. In terms of yields and substrate scope, we show that [Fe(iii)(phtmeimb) 2 ]PF 6 can compete with noble metal photoredox catalysts, for instance outcompeting archetypal [Ru(bpy) 3 ]Cl 2 under comparable reaction conditions, illustrating that iron photocatalysts can efficiently facilitate photoredox reactions of synthetic value., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

Two iron complexes featuring the bidentate, nonconjugated N-heterocyclic carbene (NHC) 1,1'-methylenebis(3-methylimidazol-2-ylidene) (mbmi) ligand, where the two NHC moieties are separated by a methylene bridge, have been synthesized to exploit the combined influence of geometric and electronic effects on the ground- and excited-state properties of homoleptic Fe III -hexa-NHC [Fe(mbmi) 3 ](PF 6 ) 3 and heteroleptic Fe II -tetra-NHC [Fe(mbmi) 2 (bpy)](PF 6 ) 2 (bpy = 2,2'-bipyridine) complexes. They are compared to the reported Fe III -hexa-NHC [Fe(btz) 3 ](PF 6 ) 3 and Fe II -tetra-NHC [Fe(btz) 2 (bpy)](PF 6 ) 2 complexes containing the conjugated, bidentate mesoionic NHC ligand 3,3'-dimethyl-1,1'-bis( p -tolyl)-4,4'-bis(1,2,3-triazol-5-ylidene) (btz). The observed geometries of [Fe(mbmi) 3 ](PF 6 ) 3 and [Fe(mbmi) 2 (bpy)](PF 6 ) 2 are evaluated through L-Fe-L bond angles and ligand planarity and compared to those of [Fe(btz) 3 ](PF 6 ) 3 and [Fe(btz) 2 (bpy)](PF 6 ) 2 . The Fe II /Fe III redox couples of [Fe(mbmi) 3 ](PF 6 ) 3 (-0.38 V) and [Fe(mbmi) 2 (bpy)](PF 6 ) 2 (-0.057 V, both vs Fc +/0 ) are less reducing than [Fe(btz) 3 ](PF 6 ) 3 and [Fe(btz) 2 (bpy)](PF 6 ) 2 . The two complexes show intense absorption bands in the visible region: [Fe(mbmi) 3 ](PF 6 ) 3 at 502 nm (ligand-to-metal charge transfer, 2 LMCT) and [Fe(mbmi) 2 (bpy)](PF 6 ) 2 at 410 and 616 nm (metal-to-ligand charge transfer, 3 MLCT). Lifetimes of 57.3 ps ( 2 LMCT) for [Fe(mbmi) 3 ](PF 6 ) 3 and 7.6 ps ( 3 MLCT) for [Fe(mbmi) 2 (bpy)](PF 6 ) 2 were probed and are somewhat shorter than those for [Fe(btz) 3 ](PF 6 ) 3 and [Fe(btz) 2 (bpy)](PF 6 ) 2 . [Fe(mbmi) 3 ](PF 6 ) 3 exhibits photoluminescence at 686 nm ( 2 LMCT) in acetonitrile at room temperature with a quantum yield of (1.2 ± 0.1) × 10 -4 , compared to (3 ± 0.5) × 10 -4 for [Fe(btz) 3 ](PF 6 ) 3 .

We here report the synthesis of the homoleptic iron(II) N -heterocyclic carbene (NHC) complex [Fe(miHpbmi) 2 ](PF 6 ) 4 (miHpbmi = 4-((3-methyl-1 H -imidazolium-1-yl)pyridine-2,6-diyl)bis(3-methylimidazol-2-ylidene)) and its electrochemical and photophysical properties. The introduction of the π-electron-withdrawing 3-methyl-1 H -imidazol-3-ium-1-yl group into the NHC ligand framework resulted in stabilization of the metal-to-ligand charge transfer (MLCT) state and destabilization of the metal-centered (MC) states. This resulted in an improved excited-state lifetime of 16 ps compared to the 9 ps for the unsubstituted parent compound [Fe(pbmi) 2 ](PF 6 ) 2 (pbmi = (pyridine-2,6-diyl)bis(3-methylimidazol-2-ylidene)) as well as a stronger MLCT absorption band extending more toward the red spectral region. However, compared to the carboxylic acid derivative [Fe(cpbmi) 2 ](PF 6 ) 2 (cpbmi = 1,1'-(4-carboxypyridine-2,6-diyl)bis(3-methylimidazol-2-ylidene)), the excited-state lifetime of [Fe(miHpbmi) 2 ](PF 6 ) 4 is the same, but both the extinction and the red shift are more pronounced for the former. Hence, this makes [Fe(miHpbmi) 2 ](PF 6 ) 4 a promising pH-insensitive analogue of [Fe(cpbmi) 2 ](PF 6 ) 2 . Finally, the excited-state dynamics of the title compound [Fe(miHpbmi) 2 ](PF 6 ) 4 was investigated in solvents with different viscosities, however, showing very little dependency of the depopulation of the excited states on the properties of the solvent used.

Symmetry-breaking charge separation in molecular materials has attracted increasing attention for optoelectronics based on single-material active layers. To this end, Fe(III) complexes with particularly electron-donating N -heterocyclic carbene ligands offer interesting properties with a 2 LMCT excited state capable of oxidizing or reducing the complex in its ground state. In this Communication, we show that the corresponding symmetry-breaking charge separation occurs in amorphous films of pristine [Fe(III)L 2 ]PF 6 (L = [phenyl(tris(3-methylimidazol-2-ylidene))borate] - ). Excitation of the solid material with visible light leads to ultrafast electron transfer quenching of the 2 LMCT excited state, generating Fe(II) and Fe(IV) products with high efficiency. Sub-picosecond charge separation followed by recombination in about 1 ns could be monitored by transient absorption spectroscopy. Photoconductivity measurements of films deposited on microelectrode arrays demonstrated that photogenerated charge carriers can be collected at external contacts.

Iron N-heterocyclic carbene (FeNHC) complexes with long-lived charge transfer states are emerging as a promising class of photoactive materials. We have synthesized [Fe II (ImP) 2 ] (ImP = bis(2,6-bis(3-methylimidazol-2-ylidene-1-yl)phenylene)) that combines carbene ligands with cyclometalation for additionally improved ligand field strength. The 9 ps lifetime of its 3 MLCT (metal-to-ligand charge transfer) state however reveals no benefit from cyclometalation compared to Fe(ii) complexes with NHC/pyridine or pure NHC ligand sets. In acetonitrile solution, the Fe(ii) complex forms a photoproduct that features emission characteristics (450 nm, 5.1 ns) that were previously attributed to a higher ( 2 MLCT) state of its Fe(iii) analogue [Fe III (ImP) 2 ] + , which led to a claim of dual (MLCT and LMCT) emission. Revisiting the photophysics of [Fe III (ImP) 2 ] + , we confirmed however that higher ( 2 MLCT) states of [Fe III (ImP) 2 ] + are short-lived (<10 ps) and therefore, in contrast to the previous interpretation, cannot give rise to emission on the nanosecond timescale. Accordingly, pristine [Fe III (ImP) 2 ] + prepared by us only shows red emission from its lower 2 LMCT state (740 nm, 240 ps). The long-lived, higher energy emission previously reported for [Fe III (ImP) 2 ] + is instead attributed to an impurity, most probably a photoproduct of the Fe(ii) precursor. The previously reported emission quenching on the nanosecond time scale hence does not support any excited state reactivity of [Fe III (ImP) 2 ] + itself., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

Fe(III) complexes with N -heterocyclic carbene (NHC) ligands belong to the rare examples of Earth-abundant transition metal complexes with long-lived luminescent charge-transfer excited states that enable applications as photosensitizers for charge separation reactions. We report three new hexa -NHC complexes of this class: [Fe(brphtmeimb) 2 ]PF 6 (brphtmeimb = [(4-bromophenyl)tris(3-methylimidazol-2-ylidene)borate] - , [Fe(meophtmeimb) 2 ]PF 6 (meophtmeimb = [(4-methoxyphenyl)tris(3-methylimidazol-2-ylidene)borate] - , and [Fe(coohphtmeimb) 2 ]PF 6 (coohphtmeimb = [(4-carboxyphenyl)tris(3-methylimidazol-2-ylidene)borate] - . These were derived from the parent complex [Fe(phtmeimb) 2 ]PF 6 (phtmeimb = [phenyltris(3-methylimidazol-2-ylidene)borate] - by modification with electron-withdrawing and electron-donating substituents, respectively, at the 4-phenyl position of the ligand framework. All three Fe(III) hexa -NHC complexes were characterized by NMR spectroscopy, high-resolution mass spectroscopy, elemental analysis, single crystal X-ray diffraction analysis, electrochemistry, Mößbauer spectroscopy, electronic spectroscopy, magnetic susceptibility measurements, and quantum chemical calculations. Their ligand-to-metal charge-transfer ( 2 LMCT) excited states feature nanosecond lifetimes (1.6-1.7 ns) and sizable emission quantum yields (1.7-1.9%) through spin-allowed transition to the doublet ground state ( 2 GS), completely in line with the parent complex [Fe(phtmeimb) 2 ]PF 6 (2.0 ns and 2.1%). The integrity of the favorable excited state characteristics upon substitution of the ligand framework demonstrates the robustness of the scorpionate motif that tolerates modifications in the 4-phenyl position for applications such as the attachment in molecular or hybrid assemblies.

Fe-N-heterocyclic carbene (NHC) complexes attract increasing attention as photosensitisers and photoredox catalysts. Such applications generally rely on sufficiently long excited state lifetimes and efficient bimolecular quenching, which leads to there being few examples of successful usage of Fe-NHC complexes to date. Here, we have employed [Fe(iii)(btz) 3 ] 3+ (btz = (3,3'-dimethyl-1,1'-bis( p -tolyl)-4,4'-bis(1,2,3-triazol-5-ylidene))) in the addition of alkyl halides to alkenes and alkynes via visible light-mediated atom transfer radical addition (ATRA). Unlike other Fe-NHC complexes, [Fe(iii/ii)(btz) 3 ] 3+/2+ benefits from sizable charge transfer excited state lifetimes ≥0.1 ns in both oxidation states, and the Fe(iii) 2 LMCT and Fe(ii) 3 MLCT states are strong oxidants and reductants, respectively. The combined reactivity of both excited states enables efficient one-electron reduction of the alkyl halide substrate under green light irradiation. The two-photon mechanism proceeds via reductive quenching of the Fe(iii) 2 LMCT state by a sacrificial electron donor and subsequent excitation of the Fe(ii) product to its highly reducing 3 MLCT state. This route is shown to be more efficient than the alternative, where oxidative quenching of the less reducing Fe(iii) 2 LMCT state by the alkyl halide drives the reaction, in the absence of a sacrificial electron donor., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

Herein we report the first high turnover photocatalytic hydrogen formation reaction based on an earth-abundant Fe III -NHC photosensitiser. The reaction occurs via reductive quenching of the 2 LMCT excited state that can be directly excited with green light and employs either Pt-colloids or [Co(dmgH) 2 pyCl] as proton reduction catalysts and [HNEt 3 ][BF 4 ] and triethanolamine/triethylamine as proton and electron donors. The outstanding photostability of the Fe III -NHC complex enables turnover numbers >1000 without degradation.

With the price-competitiveness of solar and wind power, hydrogen technologies may be game changers for a cleaner, defossilized, and sustainable energy future. H 2 can indeed be produced in electrolyzers from water, stored for long periods, and converted back into power, on demand, in fuel cells. The feasibility of the latter process critically depends on the discovery of cheap and efficient catalysts able to replace platinum group metals at the anode and cathode of fuel cells. Bioinspiration can be key for designing such alternative catalysts. Here we show that a novel class of iron-based catalysts inspired from the active site of [FeFe]-hydrogenase behave as unprecedented bidirectional electrocatalysts for interconverting H 2 and protons efficiently under near-neutral aqueous conditions. Such bioinspired catalysts have been implemented at the anode of a functional membrane-less H 2 /O 2 fuel cell device.

A new generation of octahedral iron(ii)-N-heterocyclic carbene (NHC) complexes, employing different tridentate C^N^C ligands, has been designed and synthesized as earth-abundant photosensitizers for dye sensitized solar cells (DSSCs) and related solar energy conversion applications. This work introduces a linearly aligned push-pull design principle that reaches from the ligand having nitrogen-based electron donors, over the Fe(ii) centre, to the ligand having an electron withdrawing carboxylic acid anchor group. A combination of spectroscopy, electrochemistry, and quantum chemical calculations demonstrate the improved molecular excited state properties in terms of a broader absorption spectrum compared to the reference complex, as well as directional charge-transfer displacement of the lowest excited state towards the semiconductor substrate in accordance with the push-pull design. Prototype DSSCs based on one of the new Fe NHC photosensitizers demonstrate a power conversion efficiency exceeding 1% already for a basic DSSC set-up using only the I - /I 3 - redox mediator and standard operating conditions, outcompeting the corresponding DSSC based on the homoleptic reference complex. Transient photovoltage measurements confirmed that adding the co-sensitizer chenodeoxycholic acid helped in improving the efficiency by increasing the electron lifetime in TiO 2 . Time-resolved spectroscopy revealed spectral signatures for successful ultrafast (<100 fs) interfacial electron injection from the heteroleptic dyes to TiO 2 . However, an ultrafast recombination process results in undesirable fast charge recombination from TiO 2 back to the oxidized dye, leaving only 5-10% of the initially excited dyes available to contribute to a current in the DSSC. On slower timescales, time-resolved spectroscopy also found that the recombination dynamics (longer than 40 μs) were significantly slower than the regeneration of the oxidized dye by the redox mediator (6-8 μs). Therefore it is the ultrafast recombination down to fs-timescales, between the oxidized dye and the injected electron, that remains as one of the main bottlenecks to be targeted for achieving further improved solar energy conversion efficiencies in future work., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

Photoinduced symmetry-breaking charge separation (SB-CS) processes offer the possibility of harvesting solar energy by electron transfer between identical molecules. Here, we present the first case of direct observation of bimolecular SB-CS in a transition metal complex, [Fe III L 2 ](PF 6 ) (L = [phenyl(tris(3-methylimidazol-1-ylidene))borate] - ). Photoexcitation of the complex in the visible region results in the formation of a doublet ligand-to-metal charge transfer ( 2 LMCT) excited state ( E 0-0 = 2.13 eV), which readily reacts with the doublet ground state to generate charge separated products, [Fe II L 2 ] and [Fe IV L 2 ] 2+ , with a measurable cage escape yield. Known spectral signatures allow for unambiguous identification of the products, whose formation and recombination are monitored with transient absorption spectroscopy. The unusual energetic landscape of [Fe III L 2 ] + , as reflected in its ground and excited state reduction potentials, results in SB-CS being intrinsically exergonic (Δ G CS ° ∼ -0.7 eV). This is in contrast to most systems investigated in the literature, where Δ G CS ° is close to zero, and the charge transfer driven primarily by solvation effects. The study is therefore illustrative for the utilization of the rich redox chemistry accessible in transition metal complexes for the realization of SB-CS.

A homoleptic organometallic Fe IV complex that is stable in both solution and in the solid state at ambient conditions has been synthesized and isolated as [Fe(phtmeimb) 2 ](PF 6 ) 2 (phtmeimb=[phenyl(tris(3-methylimidazolin-2-ylidene))borate] - ). This Fe IV N-heterocyclic carbene (NHC) complex was characterized by 1 H NMR, HR-MS, elemental analysis, scXRD analysis, electrochemistry, Mößbauer spectroscopy, and magnetic susceptibility. The two latter techniques unequivocally demonstrate that [Fe(phtmeimb) 2 ](PF 6 ) 2 is a triplet Fe IV low-spin S=1 complex in the ground state, in agreement with quantum chemical calculations. The electronic absorption spectrum of [Fe(phtmeimb) 2 ](PF 6 ) 2 in acetonitrile shows an intense absorption band in the red and near IR, due to LMCT (ligand-to-metal charge transfer) excitation. For the first time the excited state dynamics of a Fe IV complex was studied and revealed a ≈0.8 ps lifetime of the 3 LMCT excited state of [Fe(phtmeimb) 2 ](PF 6 ) 2 in acetonitrile., (© 2020 The Authors. Published by Wiley-VCH GmbH.)

A facile surface amide-coupling method was examined to attach dye and catalyst molecules to silatrane-decorated NiO electrodes. Using this method, electrodes with a push-pull dye were assembled and characterized by photoelectrochemistry and transient absorption spectroscopy. The dye-sensitized electrodes exhibited hole injection into NiO and good photoelectrochemical stability in water, highlighting the stability of the silatrane anchoring group and the amide linkage. The amide-coupling protocol was further applied to electrodes that contain a molecular proton reduction catalyst for use in photocathode architectures. Evidence for catalyst reduction was observed during photoelectrochemical measurements and via femtosecond-transient absorption spectroscopy demonstrating the possibility for application in photocathodes.

[FeFe(Cl 2 -bdt)(CO) 6 ] (1; Cl 2 -bdt=3,6-dichlorobenzene-1,2-dithiolate), inspired by the active site of FeFe-hydrogenase, shows a chemically reversible 2 e - reduction at -1.20 V versus the ferrocene/ferrocenium couple. The rigid and aromatic bdt bridging ligand lowers the reduction potential and stabilizes the reduced forms, compared with analogous complexes with aliphatic dithiolates; thus allowing details of the catalytic process to be characterized. Herein, time-resolved IR spectroscopy is used to provide kinetic and structural information on key catalytic intermediates. This includes the doubly reduced, protonated complex 1H - , which has not been previously identified experimentally. In addition, the first direct spectroscopic observation of the turnover process for a molecular H 2 evolving catalyst is reported, allowing for straightforward determination of the turnover frequency., (© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Electron and proton transfer reactions of diiron complexes [Fe 2 adt(CO) 6 ] ( 1 ) and [Fe 2 adt(CO) 4 (PMe 3 ) 2 ] ( 4 ), with the biomimetic azadithiolate (adt) bridging ligand, have been investigated by real-time IR- and UV-vis-spectroscopic observation to elucidate the role of the adt-N as a potential proton shuttle in catalytic H 2 formation. Protonation of the one-electron reduced complex, 1 - , occurs on the adt-N yielding 1H and the same species is obtained by one-electron reduction of 1H + . The preference for ligand vs. metal protonation in the Fe 2 (i,0) state is presumably kinetic but no evidence for tautomerization of 1H to the hydride 1Hy was observed. This shows that the adt ligand does not work as a proton relay in the formation of hydride intermediates in the reduced catalyst. A hydride intermediate 1HHy + is formed only by protonation of 1H with stronger acid. Adt protonation results in reduction of the catalyst at much less negative potential, but subsequent protonation of the metal centers is not slowed down, as would be expected according to the decrease in basicity. Thus, the adtH + complex retains a high turnover frequency at the lowered overpotential. Instead of proton shuttling, we propose that this gain in catalytic performance compared to the propyldithiolate analogue might be rationalized in terms of lower reorganization energy for hydride formation with bulk acid upon adt protonation.

Iron's abundance and rich coordination chemistry are potentially appealing features for photochemical applications. However, the photoexcitable charge-transfer states of most iron complexes are limited by picosecond or subpicosecond deactivation through low-lying metal-centered states, resulting in inefficient electron-transfer reactivity and complete lack of photoluminescence. In this study, we show that octahedral coordination of iron(III) by two mono-anionic facial tris -carbene ligands can markedly suppress such deactivation. The resulting complex [Fe(phtmeimb) 2 ] + , where phtmeimb is {phenyl[tris(3-methylimidazol-1-ylidene)]borate} - , exhibits strong, visible, room temperature photoluminescence with a 2.0-nanosecond lifetime and 2% quantum yield via spin-allowed transition from a doublet ligand-to-metal charge-transfer ( 2 LMCT) state to the doublet ground state. Reductive and oxidative electron-transfer reactions were observed for the 2 LMCT state of [Fe(phtmeimb) 2 ] + in bimolecular quenching studies with methylviologen and diphenylamine., (Copyright © 2019 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works.)

Metal hydrides are key intermediates in catalytic proton reduction and dihydrogen oxidation. There is currently much interest in appending proton relays near the metal centre to accelerate catalysis by proton-coupled electron transfer (PCET). However, the elementary PCET steps and the role of the proton relays are still poorly understood, and direct kinetic studies of these processes are scarce. Here, we report a series of tungsten hydride complexes as proxy catalysts, with covalently attached pyridyl groups as proton acceptors. The rate of their PCET reaction with external oxidants is increased by several orders of magnitude compared to that of the analogous systems with external pyridine on account of facilitated proton transfer. Moreover, the mechanism of the PCET reaction is altered by the appended bases. A unique feature is that the reaction can be tuned to follow three distinct PCET mechanisms-electron-first, proton-first or a concerted reaction-with very different sensitivities to oxidant and base strength. Such knowledge is crucial for rational improvements of solar fuel catalysts.

The iron carbene complex [Fe II (btz) 3 ](PF 6 ) 2 (where btz = 3,3'-dimethyl-1,1'-bis(p-tolyl)-4,4'-bis(1,2,3-triazol-5-ylidene)) has been synthesized, isolated, and characterized as a low-spin ferrous complex. It exhibits strong metal-to-ligand charge transfer (MLCT) absorption bands throughout the visible spectrum, and excitation of these bands gives rise to a 3 MLCT state with a 528 ps excited-state lifetime in CH 3 CN solution that is more than one order of magnitude longer compared with the MLCT lifetime of any previously reported Fe II complex. The low potential of the [Fe(btz) 3 ] 3+ /[Fe(btz) 3 ] 2+ redox couple makes the 3 MLCT state of [Fe II (btz) 3 ] 2+ a potent photoreductant that can be generated by light absorption throughout the visible spectrum. Taken together with our recent results on the [Fe III (btz) 3 ] 3+ form of this complex, these results show that the Fe II and Fe III oxidation states of the same Fe(btz) 3 complex feature long-lived MLCT and LMCT states, respectively, demonstrating the versatility of iron N-heterocyclic carbene complexes as promising light-harvesters for a broad range of oxidizing and reducing conditions.

One-electron reduction and subsequent protonation of a biomimetic proton-reduction catalyst [FeFe(μ-pdt)(CO) 6 ] (pdt = propanedithiolate), 1, were investigated by UV-vis and IR spectroscopy on a nano- to microsecond time scale. The study aimed to provide further insight into the proton-reduction cycle of this [FeFe]-hydrogenase model complex, which with its prototypical alkyldithiolate-bridged diiron core is widely employed as a molecular, precious metal-free catalyst for sustainable H 2 generation. The one-electron-reduced catalyst was obtained transiently by electron transfer from photogenerated [Ru(dmb) 3 ] + in the absence of proton sources or in the presence of acids (dichloro- or trichloroacetic acid or tosylic acid). The reduced catalyst and its protonation product were observed in real time by UV-vis and IR spectroscopy, leading to their structural characterization and providing kinetic data on the electron and proton transfer reactions. 1 features an intact (μ 2 ,κ 2 -pdt)(μ-H)Fe 2 core in the reduced, 1 - , and reduced-protonated states, 1H, in contrast to the Fe-S bond cleavage upon the reduction of [FeFe(bdt)(CO) 6 ], 2, with a benzenedithiolate bridge. The driving-force dependence of the rate constants for the protonation of 1 - (k pt = 7.0 × 10 5 , 1.3 × 10 7 , and 7.0 × 10 7 M -1 s -1 for the three acids used in this study) suggests a reorganization energy >1 eV and indicates that hydride complex 1H is formed by direct protonation of the Fe-Fe bond. The protonation of 1 - is sufficiently fast even with the weaker acids, which excludes a rate-limiting role in light-driven H 2 formation under typical conditions.

The development of a reliable platform for the electrochemical characterization of a redox-active molecular diiron complex, [FeFe], immobilized in a non-conducting metal organic framework (MOF), UiO-66, based on glassy-carbon electrodes is reported. Voltammetric data with appreciable current responses can be obtained by the use of multiwalled carbon nanotubes (MWCNT) or mesoporous carbon (CB) additives that function as conductive scaffolds to interface the MOF crystals in "three-dimensional" electrodes. In the investigated UiO-66-[FeFe] sample, the low abundance of [FeFe] in the MOF and the intrinsic insulating properties of UiO-66 prevent charge transport through the framework, and consequently, only [FeFe] units that are in direct physical contact with the electrode material are electrochemically addressable.

Transition-metal complexes are used as photosensitizers, in light-emitting diodes, for biosensing and in photocatalysis. A key feature in these applications is excitation from the ground state to a charge-transfer state; the long charge-transfer-state lifetimes typical for complexes of ruthenium and other precious metals are often essential to ensure high performance. There is much interest in replacing these scarce elements with Earth-abundant metals, with iron and copper being particularly attractive owing to their low cost and non-toxicity. But despite the exploration of innovative molecular designs, it remains a formidable scientific challenge to access Earth-abundant transition-metal complexes with long-lived charge-transfer excited states. No known iron complexes are considered photoluminescent at room temperature, and their rapid excited-state deactivation precludes their use as photosensitizers. Here we present the iron complex [Fe(btz) 3 ] 3+ (where btz is 3,3'-dimethyl-1,1'-bis(p-tolyl)-4,4'-bis(1,2,3-triazol-5-ylidene)), and show that the superior σ-donor and π-acceptor electron properties of the ligand stabilize the excited state sufficiently to realize a long charge-transfer lifetime of 100 picoseconds (ps) and room-temperature photoluminescence. This species is a low-spin Fe(iii) d 5 complex, and emission occurs from a long-lived doublet ligand-to-metal charge-transfer ( 2 LMCT) state that is rarely seen for transition-metal complexes. The absence of intersystem crossing, which often gives rise to large excited-state energy losses in transition-metal complexes, enables the observation of spin-allowed emission directly to the ground state and could be exploited as an increased driving force in photochemical reactions on surfaces. These findings suggest that appropriate design strategies can deliver new iron-based materials for use as light emitters and photosensitizers.

Photocatalytic water oxidation represents a key process in conversion of solar energy into fuels and can be facilitated by the use of molecular transition metal-based catalysts. A novel straightforward approach for covalent linking of the catalytic units to other moieties is demonstrated by preparation of a dinuclear complex containing two [Ru(pdc)(pic) 3 ]-derived units (pdc = 2,6-pyridinedicarboxylate, pic = 4-picoline). The activity of this complex towards chemical and photochemical oxidation of water was evaluated and a detailed insight is given into the interactions between the catalyst and acetonitrile, a common co-solvent employed to increase solubility of water oxidation catalysts. The solvent-induced transformations were studied by electrochemical and spectroscopic techniques and the relevant quantitative parameters were extracted.

Time-resolved nanosecond mid-infrared spectroscopy is for the first time employed to study the [FeFe] hydrogenase from Chlamydomonas reinhardtii and to investigate relevant intermediates of the enzyme active site. An actinic 355 nm, 10 ns laser flash triggered photodissociation of a carbonyl group from the CO-inhibited state Hox-CO to form the state Hox, which is an intermediate of the catalytic proton reduction cycle. Time-resolved infrared spectroscopy allowed us to directly follow the subsequent rebinding of the carbonyl, re-forming Hox-CO, and determine the reaction half-life to be t1/2 ≈ 13 ± 5 ms at room temperature. This gives direct information on the dynamics of CO inhibition of the enzyme.

Solar energy conversion in photovoltaics or photocatalysis involves light harvesting, or sensitization, of a semiconductor or catalyst as a first step. Rare elements are frequently used for this purpose, but they are obviously not ideal for large-scale implementation. Great efforts have been made to replace the widely used ruthenium with more abundant analogues like iron, but without much success due to the very short-lived excited states of the resulting iron complexes. Here, we describe the development of an iron-nitrogen-heterocyclic-carbene sensitizer with an excited-state lifetime that is nearly a thousand-fold longer than that of traditional iron polypyridyl complexes. By the use of electron paramagnetic resonance, transient absorption spectroscopy, transient terahertz spectroscopy and quantum chemical calculations, we show that the iron complex generates photoelectrons in the conduction band of titanium dioxide with a quantum yield of 92% from the (3)MLCT (metal-to-ligand charge transfer) state. These results open up possibilities to develop solar energy-converting materials based on abundant elements.

Two molecular assemblies with one Ru(II)-polypyridine photosensitizer covalently linked to one Ru(II)(bda)L2 catalyst (1) (bda = 2,2'-bipyridine-6,6'-dicarboxylate) and two photosensitizers covalently linked to one catalyst (2) have been prepared using a simple C-C bond as the linkage. In the presence of sodium persulfate as a sacrificial electron acceptor, both of them show high activity for catalytic water oxidation driven by visible light, with a turnover number up to 200 for 2. The linked photocatalysts show a lower initial yield for light driven oxygen evolution but a much better photostability compared to the three component system with separate sensitizer, catalyst and acceptor, leading to a much greater turnover number. Photocatalytic experiments and time-resolved spectroscopy were carried out to probe the mechanism of this catalysis. The linked catalyst in its Ru(II) state rapidly quenches the sensitizer, predominantly by energy transfer. However, a higher stability under photocatalytic condition is shown for the linked sensitizer compared to the three component system, which is attributed to kinetic stabilization by rapid photosensitizer regeneration. Strategies for employment of the sensitizer-catalyst molecules in more efficient photocatalytic systems are discussed.

Strongly σ-donating N-heterocyclic carbenes (NHCs) have revived research interest in the catalytic chemistry of iron, and are now also starting to bring the photochemistry and photophysics of this abundant element into a new era. In this work, a heteroleptic Fe(II) complex (1) was synthesized based on sequentially furnishing the Fe(II) center with the benchmark 2,2'-bipyridine (bpy) ligand and the more strongly σ-donating mesoionic ligand, 4,4'-bis(1,2,3-triazol-5-ylidene) (btz). Complex 1 was comprehensively characterized by electrochemistry, static and ultrafast spectroscopy, and quantum chemical calculations and compared to [Fe(bpy)3](PF6)2 and (TBA)2[Fe(bpy)(CN)4]. Heteroleptic complex 1 extends the absorption spectrum towards longer wavelengths compared to a previously synthesized homoleptic Fe(II) NHC complex. The combination of the mesoionic nature of btz and the heteroleptic structure effectively destabilizes the metal-centered (MC) states relative to the triplet metal-to-ligand charge transfer ((3)MLCT) state in 1, rendering it a lifetime of 13 ps, the longest to date of a photochemically stable Fe(II) complex. Deactivation of the (3)MLCT state is proposed to proceed via the (3)MC state that strongly couples with the singlet ground state., (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The structure and reactivity of intermediates in the photocatalytic cycle of a proton reduction catalyst, [Fe2(bdt)(CO)6] (bdt = benzenedithiolate), were investigated by time-resolved spectroscopy. The singly reduced catalyst [Fe2(bdt)(CO)6](-), a key intermediate in photocatalytic H2 formation, was generated by reaction with one-electron reductants in laser flash-quench experiments and could be observed spectroscopically on the nanoseconds to microseconds time scale. From UV/vis and IR spectroscopy, [Fe2(bdt)(CO)6](-) is readily distinguished from the two-electron reduced catalyst [Fe2(bdt)(CO)6](2-) that is obtained inevitably in the electrochemical reduction of [Fe2(bdt)(CO)6]. For the disproportionation rate constant of [Fe2(bdt)(CO)6](-), an upper limit on the order of 10(7) M(-1) s(-1) was estimated, which precludes a major role of [Fe2(bdt)(CO)6](2-) in photoinduced proton reduction cycles. Structurally [Fe2(bdt)(CO)6](-) is characterized by a rather asymmetrically distorted geometry with one broken Fe-S bond and six terminal CO ligands. Acids with pK(a) ≤ 12.7 protonate [Fe2(bdt)(CO)6](-) with bimolecular rate constants of 4 × 10(6), 7 × 10(6), and 2 × 10(8) M(-1) s(-1) (trichloroacetic, trifluoroacetic, and toluenesulfonic acids, respectively). The resulting hydride complex [Fe2(bdt)(CO)6H] is therefore likely to be an intermediate in photocatalytic cycles. This intermediate resembles structurally and electronically the parent complex [Fe2(bdt)(CO)6], with very similar carbonyl stretching frequencies.

A 2e(-) /2H(+) redox platform has been implemented in the ethenyl-bridged bisphosphol-3-ol 1 to afford the first phospholes that feature chemically reversible oxidations. Oxidation of the title compounds to the corresponding bisphosphol-3-one 2 leads to a change in conjugation topology and a concomitant hypsochromic shift of the lowest-energy absorption maximum by 100 nm. Electrochemical oxidation proceeds without any detectable intermediates, whereas the deprotonated form of 1 can be observed in an aprotic medium during the reduction of 2. This dianionic intermediate 3 is characterized by end absorptions that are bathochromically shifted by circa 200 nm compared to those of 2., (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Significance: Control over reversible changes to molecular structure forms the basis for artificial molecular machines that could eventually lead to the development of molecule-based nanotechnology., Recent Advances: Particular applications in information storage and processing could emerge where the structural rearrangements give rise to bistability and molecular hysteresis effects., Critical Issues: Oxidation-state-dependent coordination and bonding preferences in transition metal complexes and organometallic compounds provide a versatile approach to the control of molecular motions by redox input, but so far, few structural motifs have been applied in redox-actuated molecular machines., Future Directions: Further progress toward molecule-based nanoscale devices might be accomplished with molecular components derived from a wider range of structural themes and forms of molecular motion. Examples of redox-stimulated rearrangements in metal complexes and organometallic compounds are described that have been employed in molecular machines or could be considered for the design of new functional molecules.

Early intermediates of catalytic water reduction by a Co(II)-polypyridyl species have been characterized. Electrochemical detection of the Co(III)-hydride and time-resolved spectroscopic detection of the Co(I)-ligand intermediates provide an understanding of their reactivity in electrolytic or light-driven reduction of protons to hydrogen.

A 9 ps (3)MLCT lifetime was achieved by a Fe(II) complex based on C(NHC)^N(py)^C(NHC) pincer ligands. This is the longest known so far for any kind of complexes of this abundant metal, and increased by almost two orders of magnitude compared to the reference Fe(II) bis-terpyridine complex.

Building a detailed understanding of the structure-function relationship is a crucial step in the optimization of molecular photocatalysts employed in water splitting schemes. The optically dark nature of their active sites usually prevents a complete mapping of the photoinduced dynamics. In this work, transient X-ray absorption spectroscopy highlights the electronic and geometric changes that affect such a center in a bimetallic model complex. Upon selective excitation of the ruthenium chromophore, the cobalt moiety is reduced through intramolecular electron transfer and undergoes a spin flip accompanied by an average bond elongation of 0.20 ± 0.03 Å. The analysis is supported by simulations based on density functional theory structures (B3LYP*/TZVP) and FEFF 9.0 multiple scattering calculations. More generally, these results exemplify the large potential of the technique for tracking elusive intermediates that impart unique functionalities in photochemical devices.

A series of homoleptic bis(tridentate) [Ru(L)2](2+) (1, 3) and heteroleptic [Ru(L)(dqp)](2+) complexes (2, 4) [L = dqxp (1, 2) or dNinp (3, 4); dqxp = 2,6-di(quinoxalin-5-yl)pyridine, dNinp = 2,6-di(N-7-azaindol-1-yl)pyridine, dqp = 2,6-di(quinolin-8-yl)pyridine] was prepared and in the case of 2 and 4 structurally characterized. The presence of dqxp and dNinp in 1-4 result in anodically shifted oxidation potentials of the Ru(3+/2+) couple compared to that of the archetypical [Ru(dqp)2](2+) (5), most pronounced for [Ru(dqxp)2](2+) (1) with a shift of +470 mV. These experimental findings are corroborated by DFT calculations, which show contributions to the complexes' HOMOs by the polypyridine ligands, thereby stabilizing the HOMOs and impeding electron extraction. Complex 3 exhibits an unusual electronic absorption spectrum with its lowest energy maximum at 382 nm. TD-DFT calculations suggest that this high-energy transition is caused by a localization of the LUMO on the central pyridine fragments of the dNinp ligands in 3, leaving the lateral azaindole units merely spectator fragments. The opposite is the case in 1, where the LUMO experiences large stabilization by the lateral quinoxalines. Owing to the differences in LUMO energies, the complexes' reduction potentials differ by about 900 mV [E(1/2)(1(2+/1+)) = -1.17 V, E(c,p)(3(2+/1+)) = -2.06 V vs Fc(+/0)]. As complexes 1-4 exhibit similar excited state energies of around 1.80 V, the variations of the lateral heterocycles allow the tuning of the complexes' excited state oxidation strengths over a range of 900 mV. Complex 1 is the strongest excited state oxidant of the series, exceeding even [Ru(bpy)3](2+) by more than 200 mV. At room temperature, complex 3 is nonemissive, whereas complexes 1, 2, and 4 exhibit excited state lifetimes of 255, 120, and 1570 ns, respectively. The excited state lifetimes are thus somewhat shortened compared to that of 5 (3000 ns) but still acceptable to qualify the complexes as photosensitizers in light-induced charge-transfer schemes, especially for those that require high oxidative power.

Mononuclear pentacoordinate iron complexes with a free coordination site were prepared as mimics of the distal Fe (Fe(d)) in the active site of [FeFe] hydrogenases. The complexes catalyze the electrochemical reduction of protons at mild overpotential., (This journal is © The Royal Society of Chemistry 2011)

Photosynthesis is performed by a multitude of organisms, but in nearly all cases, it is variations on a common theme: absorption of light followed by energy transfer to a reaction center where charge separation takes place. This initial form of chemical energy is stabilized by the biosynthesis of carbohydrates. To produce these energy-rich products, a substrate is needed that feeds in reductive equivalents. When photosynthetic microorganisms learned to use water as a substrate some 2 billion years ago, a fundamental barrier against unlimited use of solar energy was overcome. The possibility of solar energy use has inspired researchers to construct artificial photosynthetic systems that show analogy to parts of the intricate molecular machinery of photosynthesis. Recent years have seen a reorientation of efforts toward creating integrated light-to-fuel systems that can use solar energy for direct synthesis of energy-rich compounds, so-called solar fuels. Sustainable production of solar fuels is a long awaited development that promises extensive solar energy use combined with long-term storage. The stoichiometry of water splitting into molecular oxygen, protons, and electrons is deceptively simple; achieving it by chemical catalysis has proven remarkably difficult. The reaction center Photosystem II couples light-induced charge separation to an efficient molecular water-splitting catalyst, a Mn(4)Ca complex, and is thus an important template for biomimetic chemistry. In our aims to design biomimetic manganese complexes for light-driven water oxidation, we link photosensitizers and charge-separation motifs to potential catalysts in supramolecular assemblies. In photosynthesis, production of carbohydrates demands the delivery of multiple reducing equivalents to CO(2). In contrast, the two-electron reduction of protons to molecular hydrogen is much less demanding. Virtually all microorganisms have enzymes called hydrogenases that convert protons to hydrogen, many of them with good catalytic efficiency. The catalytic sites of hydrogenases are now the center of attention of biomimetic efforts, providing prospects for catalytic hydrogen production with inexpensive metals. Thus, we might complete the water-to-fuel conversion: light + 2H(2)O --> 2H(2) + O(2). This reaction formula is to some extent already elegantly fulfilled by cyanobacteria and green algae, water-splitting photosynthetic microorganisms that under certain conditions also can produce hydrogen. An alternative route to hydrogen from solar energy is therefore to engineer these organisms to produce hydrogen more efficiently. This Account describes our original approach to combine research in these two fields: mimicking structural and functional principles of both Photosystem II and hydrogenases by synthetic chemistry and engineering cyanobacteria to become better hydrogen producers and ultimately developing new routes toward synthetic biology.

Biphenyl-2,2'-dithiolate (bpdt) bridged Fe(2)(bpdt)(CO)(6) (1) undergoes two sequential electrochemically quasi-reversible reductions. The one-electron reduction product 1(-) is unusually stable against irreversible structural changes and could be characterized by IR and EPR spectroscopy supported by computational methods. Reduction to the (I,0) state does not trigger bridging coordination of CO but partial deligation of the dithiolate in 1(-) that ultimately forms a diamagnetic dimerization product.

The light-driven splitting of water into its constituting elements gives access to a valuable fuel from an abundant substrate, using sunlight as the only energy source. Synthetic diiron complexes as functional models of the [FeFe] hydrogenase H2ase enzyme active site have moved into the centre of focus as potentially viable catalysts for the reductive side of this process, i.e. the reduction of protons to molecular hydrogen. The active site of the enzyme, as well as its mimics in an artificial system, are required to accumulate two electrons from single electron transfer events and to combine them with two protons to form hydrogen. Whereas in biology this reaction is not coupled to photosynthesis and thus proceeds in the dark, additional aspects need to be considered when designing a functional artificial system for the light-driven reduction of protons. Suitable photosensitizers have to be chosen that not only provide sufficient driving force for the reduction of the synthetic diiron catalyst, but also allow for selective excitation to minimize photodegradation. Electron transfer efficiencies have to be optimized for all steps and the sequential nature of the catalyst reduction requires a sufficient stability of potentially labile intermediates of the catalytic cycle. In this perspective, systems for the light-driven conversion of protons to molecular hydrogen are discussed where the catalyst is based on model complexes of the [FeFe] H2ase active site. Covalently linked dyads, supramolecular assemblies and multi-component systems will be examined with an emphasis on mechanistic electron transfer schemes, the properties of the individual components, their scope and their potential limitations.

An electrochemically bistable ruthenium polypyridyl complex was immobilised on platinum electrodes via amide condensation with an amine-terminated self-assembled thiol monolayer and underwent rapid electron transfer-induced linkage isomerism.

Electron-transfer-induced linkage isomerization was investigated in a series of bis-tridentate Ru polypyridyl complexes [Ru(L-X-OH)(Y-tpy)](2+) with ambidentate ligand L-X-OH=bpy-C(R)(OH)-py (bpy=2,2'-bipyridine; py=pyridine; R=H, Me, Ph, or tBu) and spectator ligand Y-tpy (tpy=2,2':6',2''-terpyridine; Y=p-tolyl, p-PhCO(2)Me, Cl, OEt, N-pyrrolidine). The ligand-bound ambidentate motif switches reversibly between N and O coordination in the Ru(II) and Ru(III) state, respectively. The potentials of the Ru(III/II) couple differ by about 0.5 V between the isomers, and this results in a bistable electrochemical response of the molecular switches. The effects of structural modifications in form of substituents on the linking carbon atom of the ambidentate ligand and on the central pyridine moiety of the spectator ligand were investigated by electrochemical and computational methods. Differences in isomerization behavior span six orders of magnitude in rate constants and two orders of magnitude in equilibrium constants. The results can be interpreted in terms of steric and electronic substituent effects and their influence on rotational barriers, ligation geometry, and electron deficiency of the metal center.

An amine-linked bis(terpyridyl) ligand, prepared via Pd-catalyzed diarylation of aniline, mediates unusually strong metal-metal interaction in its Ru(2) polypyridyl complex.

Two alternative Ru 2 (2+/3+) mixed-valence states are formed in the first dinuclear Ru complex with a ligand-bound ambidentate motif. The hysteretic electrochemical response follows a double-square scheme where the structure of the mixed-valence state depends on the previous isovalent state. The Ru (3+) state of the pyrrolidine-substituted bisterpyridine unit is characterized by intense ligand-to-metal charge-transfer absorptions that provide a distinctive signature of the corresponding mixed-valence state.

The IR carbonyl stretching frequencies of [Fe2(SRS)(CO)6] complexes correlate well with their first reduction potential; an [FeFe] hydrogenase model with a very mild reduction potential has been realized by using a strongly electron deficient carborane-dithiolate bridge.

The dinuclear RuII-PdII complex shows efficient H2 production in the presence of triethylamine as a sacrificial electron and proton donor under visible light irradiation. XPS and TEM analyses reveal that photoreduction of PdII to Pd0 causes dissociation of Pd from the complex to form colloids that are suggested to be the actual catalyst for H2 production.

A series of bis(terpyridine)RuII complexes have been prepared, where one of the terpyridines is functionalized in the 4'-position by a phosphonic or carboxylic acid group for attachment to TiO2. The other is functionalized, also in the 4'-position, by a potential electron donor. In complexes 1a, 3a, and 4a,b, this donor is tyrosine or hydrogen-bonded tyrosine, while in 2a it is carotenoic amide. The synthesis and photophysical properties of the complexes are discussed. On irradiation with visible light, the formation of a long-lived charge-separated state was anticipated, via primary electron ejection into the TiO2, followed by secondary electron transfer from the donor to the photogenerated RuIII. However, such a charge-separated state could be observed with certainty only with complex 2a. To explain the result, quantum chemical calculations were performed on the different types of complexes.

The protonation behavior of the iron hydrogenase active-site mimic [Fe2(mu-adt)(CO)4(PMe3)2] (1; adt=N-benzyl-azadithiolate) has been investigated by spectroscopic, electrochemical, and computational methods. The combination of an adt bridge and electron-donating phosphine ligands allows protonation of either the adt nitrogen to give [Fe2(mu-Hadt)(CO)4(PMe3)2]+ ([1 H]+), the Fe-Fe bond to give [Fe2(mu-adt)(mu-H)(CO)4(PMe3)2]+ ([1 Hy]+), or both sites simultaneously to give [Fe2(mu-Hadt)(mu-H)(CO)4(PMe3)2]2+ ([1 HHy]2 +). Complex 1 and its protonation products have been characterized in acetonitrile solution by IR, (1)H, and (31)P NMR spectroscopy. The solution structures of all protonation states feature a basal/basal orientation of the phosphine ligands, which contrasts with the basal/apical structure of 1 in the solid state. Density functional calculations have been performed on all protonation states and a comparison between calculated and experimental spectra confirms the structural assignments. The ligand protonated complex [1 H]+ (pKa=12) is the initial, metastable protonation product while the hydride [1 Hy]+ (pKa=15) is the thermodynamically stable singly protonated form. Tautomerization of cation [1 H]+ to [1 Hy]+ does not occur spontaneously. However, it can be catalyzed by HCl (k=2.2 m(-1) s(-1)), which results in the selective formation of cation [1 Hy]+. The protonations of the two basic sites have strong mutual effects on their basicities such that the hydride (pK(a)=8) and the ammonium proton (pK(a)=5) of the doubly protonated cationic complex [1 HHy]2+ are considerably more acidic than in the singly protonated analogues. The formation of dication [1 HHy]2+ from cation [1 H]+ is exceptionally slow with perchloric or trifluoromethanesulfonic acid (k=0.15 m(-1) s(-1)), while the dication is formed substantially faster (k>10(2) m(-1) s(-1)) with hydrobromic acid. Electrochemically, 1 undergoes irreversible reduction at -2.2 V versus ferrocene, and this potential shifts to -1.6, -1.1, and -1.0 V for the cationic complexes [1 H]+, [1 Hy]+, and [1 HHy]2+, respectively, upon protonation. The doubly protonated form [1 HHy]2+ is reduced at less negative potential than all previously reported hydrogenase models, although catalytic proton reduction at this potential is characterized by slow turnover.

The first model of the iron hydrogenase active site has been prepared in which an amine ligand is loosely coordinated to an Fe(i) centre, and can be replaced by a solvent molecule; irrespective of the ligand set, the one electron reduction of both complexes is chemically reversible and is shown to proceed through the same species which features a bridging CO ligand.