Nonadiabatic Charge Transfer within Photoexcited Nickel Porphyrins.

Metalloporphyrins with open d-shell ions can drive biochemical energy cycles. However, their utilization in photoconversion is hampered by rapid deactivation. Mapping the relaxation pathways is essential for elaborating strategies that can favorably alter the charge dynamics through chemical design and photoexcitation conditions. Here, we combine transient optical absorption spectroscopy and transient X-ray emission spectroscopy with femtosecond resolution to probe directly the coupled electronic and spin dynamics within a photoexcited nickel porphyrin in solution. Measurements and calculations reveal that a state with charge-transfer character mediates the formation of the thermalized excited state, thereby advancing the description of the photocycle for this important representative molecule. More generally, establishing that intramolecular charge-transfer steps play a role in the photoinduced dynamics of metalloporphyrins with open d-shell sets a conceptual ground for their development as building blocks capable of boosting nonadiabatic photoconversion in functional architectures through "hot" charge transfer down to the attosecond time scale.

Synthesis of phosphine derivatives of [Fe2(CO)6(µ-sdt)] (sdt = SCH2SCH2S) and investigation of their proton reduction capabilities.

The reactions of [Fe2(CO)6(µ-sdt)] (1) (sdt = SCH2SCH2S) with phosphine ligands have been investigated. Treatment of 1 with dppm (bis(diphenylphosphino)methane) or dcpm (bis(dicyclohexylphosphino)methane) affords the diphosphine-bridged products [Fe2(CO)4(µ-sdt)(µ-dppm)] (2) and [Fe2(CO)4(µ-sdt)(µ-dcpm)] (3), respectively. The complex [Fe2(CO)4(µ-sdt)(κ2-dppv)] (4) with a chelating diphosphine was obtained by reacting 1 with dppv (cis-1,2-bis(diphenylphosphino)ethene). Reaction of 1 with dppe (1,2-bis(diphenylphosphino)ethane) produces [{Fe2(CO)4(µ-sdt)}2(µ-κ1-dppe)] (5) in which the diphosphine forms an intermolecular bridge between two diiron cluster fragments. Three products were obtained when dppf (1,1'-bis(diphenylphosphino)ferrocene) was introduced to complex 1; they were [Fe2(CO)5(µ-sdt)(κ1-dppfO)] (6), the previously known [{Fe2(CO)5(µ-sdt)}2(µ-κ1-κ1-dppf)] (7), and [Fe2(CO)4(µ-sdt)(µ-dppf)] (8), with complex 8 being produced in highest yield. Single crystal X-ray diffraction analysis was performed on compounds 2, 3 and 8. All structures reveal the adoption of an anti-arrangement of the dithiolate bridges, while the diphosphines occupy dibasal positions. Infra-red spectroscopy indicates that the mono-substituted complexes 5, 6, and 7 are inert to protonation by HBF4.Et2O, but complexes 2, 3, 4 and [Fe2(CO)5(µ-sdt)(κ1-PPh3)] (9) show shifts of their ν(C-O) resonances that indicate that protons bind to the metal cores of the clusters. Addition of the one-electron oxidant [Cp2Fe]PF6 does not lead to any discernable shift in the IR resonances. The redox chemistry of the complexes was investigated by cyclic voltammetry, and the abilities of complexes to catalyze electrochemical proton reduction were examined.

Ultrafast Jahn-Teller Photoswitching in Cobalt Single-Ion Magnets.

Single-ion magnets (SIMs) constitute the ultimate size limit in the quest for miniaturizing magnetic materials. Several bottlenecks currently hindering breakthroughs in quantum information and communication technologies could be alleviated by new generations of SIMs displaying multifunctionality. Here, ultrafast optical absorption spectroscopy and X-ray emission spectroscopy are employed to track the photoinduced spin-state switching of the prototypical complex [Co(terpy)2 ]2+ (terpy = 2,2':6',2″-terpyridine) in solution phase. The combined measurements and their analysis supported by density functional theory (DFT), time-dependent-DFT (TD-DFT) and multireference quantum chemistry calculations reveal that the complex undergoes a spin-state transition from a tetragonally elongated doublet state to a tetragonally compressed quartet state on the femtosecond timescale, i.e., it sustains ultrafast Jahn-Teller (JT) photoswitching between two different spin multiplicities. Adding new Co-based complexes as possible contenders in the search for JT photoswitching SIMs will greatly widen the possibilities for implementing magnetic multifunctionality and eventually controlling ultrafast magnetization with optical photons.

Ruthenium containing molecular electrocatalyst on glassy carbon for electrochemical water splitting.

Electrochemical water splitting constitutes one of the most promising strategies for converting water into hydrogen-based fuels, and this technology is predicted to play a key role in the transition towards a carbon-neutral energy economy. To enable the design of cost-effective electrolysis cells based on this technology, new and more efficient anodes with augmented water splitting activity and stability will be required. Herein, we report an active molecular Ru-based catalyst for electrochemically-driven water oxidation (overpotential of ∼395 mV at pH 7 phosphate buffer) and two simple methods for preparing anodes by attaching this catalyst onto glassy carbon through multi-walled carbon nanotubes to improve stability as well as reactivity. The anodes modified with the molecular catalyst were characterized by a broad toolbox of microscopy and spectroscopy techniques, and interestingly no RuO2 formation was detected during electrocatalysis over 4 h. These results demonstrate that the herein presented strategy can be used to prepare anodes that rival the performance of state-of-the-art metal oxide anodes.

The Impact of Ligand Carboxylates on Electrocatalyzed Water Oxidation.

Fossil fuel shortage and severe climate changes due to global warming have prompted extensive research on carbon-neutral and renewable energy resources. Hydrogen gas (H2), a clean and high energy density fuel, has emerged as a potential solution for both fulfilling energy demands and diminishing the emission of greenhouse gases. Currently, water oxidation (WO) constitutes the bottleneck in the overall process of producing H2 from water. As a result, the design of efficient catalysts for WO has become an intensively pursued area of research in recent years. Among all the molecular catalysts reported to date, ruthenium-based catalysts have attracted particular attention due to their robust nature and higher activity compared to catalysts based on other transition metals.Over the past two decades, we and others have studied a wide range of ruthenium complexes displaying impressive catalytic performance for WO in terms of turnover number (TON) and turnover frequency (TOF). However, to produce practically applicable electrochemical, photochemical, or photo-electrochemical WO reactors, further improvement of the catalysts' structure to decrease the overpotential and increase the WO rate is of utmost importance. WO reaction, that is, the production of molecular oxygen and protons from water, requires the formation of an O-O bond through the orchestration of multiple proton and electron transfers. Promotion of these processes using redox noninnocent ligand frameworks that can accept and transfer electrons has therefore attracted substantial attention. The strategic modifications of the ligand structure in ruthenium complexes to enable proton-coupled electron transfer (PCET) and atom proton transfer (APT; in the context of WO, it is the oxygen atom (metal oxo) transfer to the oxygen atom of a water molecule in concert with proton transfer to another water molecule) to facilitate the O-O bond formation have played a central role in these efforts.In particular, promising results have been obtained with ligand frameworks containing carboxylic acid groups that either are directly bonded to the metal center or reside in the close vicinity. The improvement of redox and chemical properties of the catalysts by introduction of carboxylate groups in the ligands has proven to be quite general as demonstrated for a range of mono- and dinuclear ruthenium complexes featuring ligand scaffolds based on pyridine, imidazole, and pyridazine cores. In the first coordination sphere, the carboxylate groups are firmly coordinated to the metal center as negatively charged ligands, improving the stability of the complexes and preventing metal leaching during catalysis. Another important phenomenon is the reduction of the potentials required for the formation of higher valent intermediates, especially metal-oxo species, which take active part in the key O-O bond formation step. Furthermore, the free carboxylic acid/carboxylate units in the proximity to the active center have shown exciting proton donor/acceptor properties (through PCET or APT, chemically noninnocent) that can dramatically improve the rate as well as the overpotential of the WO reaction.

Electrocatalytic proton-reduction behaviour of telluride-capped triiron clusters: tuning of overpotentials and stabilization of redox states relative to lighter chalcogenide analogues.

Reaction of [Fe3(CO)9(µ3-Te)2] (1) with the corresponding phosphine has been used to prepare the phosphine-substituted tellurium-capped triiron clusters [Fe3(CO)9(µ3-Te)2(PPh3)] (2), [Fe3(CO)8(µ3-Te)2(PPh3)] (3) and [Fe3(CO)7(µ3-Te)2(µ-R2PXPR2)] (X = CH2, R = Ph (4), Cy (5); X = NPri, R = Ph (6)). The directly related cluster [Fe3(CO)7(µ3-CO)(µ3-Te)(µ-dppm)] (7) was isolated from the reaction of [Fe3(CO)10(µ-Ph2PCH2PPh2)] with elemental tellurium. The electrochemistry of these new clusters has been probed by cyclic voltammetry, and selected complexes have been tested as proton reduction catalysts. Each 50-electron dicapped cluster exhibits two reductive processes; the first has good chemical reversibility in all cases but the reversibility of the second is dependent upon the nature of the supporting ligands. For the parent cluster 1 and the diphosphine derivatives 4-5 this second reduction is reversible, but for the PPh3 complex 3 it is irreversible, possibly as a result of CO or phosphine loss. The nature of the reduced products of 1 has been probed by DFT calculations. Upon addition of one electron, an elongation of one of the Fe-Te bonding interactions is found, while the addition of the second electron affords an open-shell triplet which is more stable by 8.8 kcal mol-1 than the closed-shell singlet dianion and has two elongated Fe-Te bonds. The phosphine-substituted clusters also exhibit oxidation chemistry but with poor reversibility in all cases. Since the reduction potentials for the tellurium-capped clusters occur at more positive potentials than for the sulfur and selenium analogues, and the redox processes also show better reversibility than for the S/Se analogues, the tellurium-capped clusters 1 and 3-5 have been examined as proton reduction catalysts. In the presence of p-toluenesulfonic acid (TsOH) or trifluoroacetic acid (TFA), these clusters reduce protons to H2 at both their first and second reduction potentials. Electron uptake at the second reduction potential is far greater than the first, suggesting that the open-shell triplet dianions are efficient catalysts. As expected, the catalytic overpotential increases upon successive phosphine substitution but so does the current response. A mechanistic scheme that takes the roles of the supporting ligands on the preferred route(s) to H2 production and release into account is presented.

Electron Transfer Mediated by Iron Carbonyl Clusters Enhance Light-Driven Hydrogen Evolution in Water by Quantum Dots.

Photocatalytic water splitting has become a promising strategy for converting solar energy into clean and carbon-neutral solar fuels in a low-cost and environmentally benign way. Hydrogen gas is such a potential solar fuel/energy carrier. In a classical artificial photosynthetic system, a photosensitizer is generally associated with a co-catalyst to convert photogenerated charge into (a) chemical bond(s). In the present study, assemblies consisting of CdSe quantum dots that are coupled with one of two molecular complexes/catalysts, that is, [Fe2 S2 (CO)6 ] or [Fe3 Te2 (CO)9 ], using an interface-directed approach, have been tested as catalytic systems for hydrogen production in aqueous solution/organic solution. In the presence of ascorbic acid as a sacrificial electron donor and proton source, these assemblies exhibit enhanced activities for the rate of hydrogen production under visible light irradiation for 8 h in aqueous solution at pH 4.0 with up to 110 µmol of H2 per mg of assembly, almost 8.5 times that of pure CdSe quantum dots under the same conditions. Transient absorption and time-resolved photoluminescence spectroscopies have been used to investigate the charge carrier transfer dynamics in the quantum dot/iron carbonyl cluster assemblies. The spectroscopic results indicate that effective electron transfer from the molecular iron complex to the valence band of the excited CdSe quantum dots significantly inhibits the recombination of photogenerated charge carriers, boosting the photocatalytic activity for hydrogen generation; that is, the iron clusters function as effective intermediaries for electron transfer from the sacrificial electron donor to the valence band of the quantum dots.

Asymmetric hydrogenation of an α-unsaturated carboxylic acid catalyzed by intact chiral transition metal carbonyl clusters - diastereomeric control of enantioselectivity.

Twenty clusters of the general formula [(µ-H)2Ru3(µ3-S)(CO)7(µ-P-P*)] (P-P* = chiral diphosphine of the ferrocene-based Walphos or Josiphos families) have been synthesised and characterised. The clusters have been tested as catalysts for asymmetric hydrogenation of tiglic acid [trans-2-methyl-2-butenoic acid]. The observed enantioselectivities and conversion rates strongly support catalysis by intact Ru3 clusters. A catalytic mechanism involving an active Ru3 catalyst generated by CO loss from [(µ-H)2Ru3(µ3-S)(CO)7(µ-P-P*)] has been investigated by DFT calculations.

Bridgehead isomer effects in bis(phosphido)-bridged diiron hexacarbonyl proton reduction electrocatalysts.

The influence of the substitution, orientation and structure of the phosphido bridges in [Fe2(CO)6(µ-PR2)2] electrocatalysts of proton reduction has been studied. The isomers e,a-[Fe2(CO)6{µ-P(Ar)H}2] (1a(Ar): Ar = Ph, 2'-methoxy-1,1'-binaphthyl (bn')), e,e-[Fe2(CO)6{µ-P(Ar)H}2] (1b(Ar): Ar = Ph, bn') were isolated from reactions of iron pentacarbonyl and the corresponding primary phosphine, syntheses that also afforded the phosphinidene-capped tri-iron clusters, [Fe3(CO)9(µ-CO)(µ3-Pbn')] (2) and [Fe3(CO)9(µ3-PAr)2] (3(Ar), Ar = Ph, bn'). A ferrocenyl (Fc)-substituted dimer [Fe2(CO)6{µ:µ'-1,2-(P(CH2Fc)CH2)2C6H4}] (4), in which the two phosphido bridges are linked by an o-xylyl group, was also prepared. The molecular structures of complexes 1a(Ph), 1b(Ph), 1b(bn'), 2 and 4 were established by X-ray crystallography. All complexes have been examined as electrocatalysts for proton reduction using p-toluene sulfonic acid in tetrahydrofuran. Cyclic voltammograms of the dimers with acid exhibit two catalysis waves for proton reduction. The first wave, which appears at the potential of the primary reduction, reaches maximum current (turnover) at moderate acid concentrations and is rapidly overtaken by the second wave, which appears at more negative potential. Both of these reductive waves show an initial first order dependence on acid. The electrochemistry and electrocatalyses of the [Fe2(CO)6(µ-PR2)2] dimers show subtle variations with the nature of the bridging phosphido group(s), including the orientation of bridgehead hydrogen atoms.

Bioinspired Hydrogenase Models: The Mixed-Valence Triiron Complex [Fe3(CO)7(µ-edt)2] and Phosphine Derivatives [Fe3(CO)7-x (PPh3) x (µ-edt)2] (x = 1, 2) and [Fe3(CO)5(κ2-diphosphine)(µ-edt)2] as Proton Reduction Catalysts.

The mixed-valence triiron complexes [Fe3(CO)7-x (PPh3) x (µ-edt)2] (x = 0-2; edt = SCH2CH2S) and [Fe3(CO)5(κ2-diphosphine)(µ-edt)2] (diphosphine = dppv, dppe, dppb, dppn) have been prepared and structurally characterized. All adopt an anti arrangement of the dithiolate bridges, and PPh3 substitution occurs at the apical positions of the outer iron atoms, while the diphosphine complexes exist only in the dibasal form in both the solid state and solution. The carbonyl on the central iron atom is semibridging, and this leads to a rotated structure between the bridged diiron center. IR studies reveal that all complexes are inert to protonation by HBF4·Et2O, but addition of acid to the pentacarbonyl complexes results in one-electron oxidation to yield the moderately stable cations [Fe3(CO)5(PPh3)2(µ-edt)2]+ and [Fe3(CO)5(κ2-diphosphine)(µ-edt)2]+, species which also result upon oxidation by [Cp2Fe][PF6]. The electrochemistry of the formally Fe(I)-Fe(II)-Fe(I) complexes has been investigated. Each undergoes a quasi-reversible oxidation, the potential of which is sensitive to phosphine substitution, generally occurring between 0.15 and 0.50 V, although [Fe3(CO)5(PPh3)2(µ-edt)2] is oxidized at -0.05 V. Reduction of all complexes is irreversible and is again sensitive to phosphine substitution, varying between -1.47 V for [Fe3(CO)7(µ-edt)2] and around -1.7 V for phosphine-substituted complexes. In their one-electron-reduced states, all complexes are catalysts for the reduction of protons to hydrogen, the catalytic overpotential being increased upon successive phosphine substitution. In comparison to the diiron complex [Fe2(CO)6(µ-edt)], [Fe3(CO)7(µ-edt)2] catalyzes proton reduction at 0.36 V less negative potentials. Electronic structure calculations have been carried out in order to fully elucidate the nature of the oxidation and reduction processes. In all complexes, the HOMO comprises an iron-iron bonding orbital localized between the two iron atoms not ligated by the semibridging carbonyl, while the LUMO is highly delocalized in nature and is antibonding between both pairs of iron atoms but also contains an antibonding dithiolate interaction.

Bio-inspired hydrogenase models: mixed-valence triion complexes as proton reduction catalysts.

Mixed-valence triiron complexes Fe(3)(CO)(7-x)(PPh(3))(x)(µ-edt)(2) (x = 0-2) have been prepared and are shown to act as proton reduction catalysts. Catalysis takes place via an ECEC mechanism with a reduced overpotential of ca. 0.45 V for Fe(3)(CO)(7)(µ-edt)(2) as compared to the corresponding diiron complex.