Manuela Gross

Versatile Photocatalytic Systems for H2 Generation in Water Based on an Efficient DuBois-Type Nickel Catalyst

The generation of renewable H2 through an efficient photochemical route requires photoinduced electron transfer (ET) from a light harvester to an efficient electrocatalyst in water. Here, we report on a molecular H2 evolution catalyst (NiP) with a DuBois-type [Ni(P2R′N2R″)2]2+ core (P2R′N2R″ = bis(1,5-R′-diphospha-3,7-R″-diazacyclooctane), which contains an outer coordination sphere with phosphonic acid groups. The latter functionality allows for good solubility in water and immobilization on metal oxide semiconductors. Electrochemical studies confirm that NiP is a highly active electrocatalyst in aqueous electrolyte solution (overpotential of approximately 200 mV at pH 4.5 with a Faradaic yield of 85 ± 4%). Photocatalytic experiments and investigations on the ET kinetics were carried out in combination with a phosphonated Ru(II) tris(bipyridine) dye (RuP) in homogeneous and heterogeneous environments. Time-resolved luminescence and transient absorption spectroscopy studies confirmed that directed ET from RuP to NiP occurs efficiently in all systems on the nano- to microsecond time scale, through three distinct routes: reductive quenching of RuP in solution or on the surface of ZrO2 (“on particle” system) or oxidative quenching of RuP when the compounds were immobilized on TiO2 (“through particle” system). Our studies show that NiP can be used in a purely aqueous solution and on a semiconductor surface with a high degree of versatility. A high TOF of 460 ± 60 h–1 with a TON of 723 ± 171 for photocatalytic H2 generation with a molecular Ni catalyst in water and a photon-to-H2 quantum yield of approximately 10% were achieved for the homogeneous system.

Parameters affecting electron transfer dynamics from semiconductors to molecular catalysts for the photochemical reduction of protons

The aim of this work is to use transient absorption spectroscopy to study the parameters affecting the kinetics and efficiency of electron transfer in a photocatalytic system for water reduction based on a cobalt proton reduction catalyst (CoP) adsorbed on a nanocrystalline TiO2 film. In the first approach, water is used as the proton and electron source and H2 is generated after band gap excitation of TiO2 functionalised with CoP. The second system involves the use of a sacrificial electron donor to regenerate the TiO2/CoP system in water at neutral pH. The third system consists of CoP/TiO2 films co-sensitised with a ruthenium-based dye (RuP). In particular, we focus on the study of different parameters that affect the kinetics of electron transfer from the semiconductor to the molecular catalyst by monitoring the lifetime of charge carriers in TiO2. We observe that low catalyst loadings onto the surface of TiO2, high excitation light intensities and small driving forces strongly slow down the kinetics and/or reduce the efficiency of the electron transfer at the interface. We conclude that the first reduction of the catalyst from CoIII to CoII can proceed efficiently even in the absence of an added hole scavenger at sufficiently high catalyst coverages and low excitation densities. In contrast, the second reduction from CoII to CoI, which is required for hydrogen evolution, appears to be at least 105 slower, suggesting it requires efficient hole scavenging and almost complete reduction of all the adsorbed CoP to CoII. Dye sensitisation enables visible light photoactivity, although this is partly offset by slower, and less efficient, hole scavenging.

Photocatalytic Hydrogen Production using Polymeric Carbon Nitride with a Hydrogenase and a Bioinspired Synthetic Ni Catalyst

Solar-light-driven H2 production in water with a [NiFeSe]-hydrogenase (H2ase) and a bioinspired synthetic nickel catalyst (NiP) in combination with a heptazine carbon nitride polymer, melon (CNx), is reported. The semibiological and purely synthetic systems show catalytic activity during solar light irradiation with turnover numbers (TONs) of more than 50 000 mol H2 (mol H2ase)−1 and approximately 155 mol H2 (mol NiP)−1 in redox-mediator-free aqueous solution at pH 6 and 4.5, respectively. Both systems maintained a reduced photoactivity under UV-free solar light irradiation (λ>420 nm).

Intramolecular Activation of a Disila[2]molybdenocenophanedihydride: Synthesis and Structure of a [1],[1]Metalloarenophane†

Metallocenophanes have attracted increasing attention in recent years, as strained ansa complexes have become pivotal precursors for organometallic polymers prepared by ringopening polymerization (ROP), whereas unstrained metallocenophanes, especially those derived from Group 4 metals, serve as catalysts for olefin polymerization. The structural and electronic properties and the particular reactivity of ansa complexes are in the focus of current research. The reactivity of the E Cipso bond (E = bridging element, Cipso = ipso carbon atom of the cyclopentadienyl ring) is of major importance for ROP, which can be induced thermally, by interaction with nucleophiles, or by latetransition-metal catalysts. Furthermore, ligand exchange reactions, which play a role in catalytic processes, as well as haptotropic shifts of cyclopentadienyl ligands under different conditions are being intensely studied. [2]Metallocenophanes, which are not commonly susceptible to ROP owing to their lower molecular strain, nevertheless attracted considerable attention because of their pronounced propensity to oxidatively add to coordinatively unsaturated complexes of late-transition-metal elements through the bridging E E (E = B, Si, Sn) moiety. Owing to the facile activation of the E E bond by, for example, Pd and Pt, subsequent insertions of various unsaturated organic substrates have been achieved. In 1992, Manners and coworkers reported the first Pd-catalyzed insertion of alkynes into the Si Si bridge of tetramethyldisila[2]ferrocenophane, Herberhold et al. described in 1997 the first oxidative addition of a Pt fragment into the Sn Sn bridge of an ansaferrocene and the subsequent insertion of an alkyne, whereas our group accomplished oxidative additions of Pt fragments as well as homogeneously and heterogeneously catalyzed insertions of alkynes and diazobenzene into B B and Si Si bonds of various [2]metalloarenophanes. Motivated by this facile activation of the E E bridge, we wondered whether a corresponding oxidative addition can be achieved intramolecularly and turned our attention to tetramethyldisila[2]molybdenocenophanedihydride (1) as a promising starting material. Herein we report the synthesis and full characterization of this species and its conversion into an unprecedented twofold-bridged [1],[1]metalloarenophane. Compound 1 was synthesized by dilithiation of 1,2bis(cyclopentadienyl)tetramethyldisilane in toluene/diethyl ether (9:1) at 0 8C and subsequent reaction with MoCl5 at 78 8C in the presence of NaBH4 as a reducing agent in THF/ hexane (4:1) (Scheme 1).

Unravelling the pH-dependence of a molecular photocatalytic system for hydrogen production

Photocatalytic systems for the reduction of aqueous protons are strongly pH-dependent, but the origin of this dependency is still not fully understood. We have studied the effect of different degrees of acidity on the electron transfer dynamics and catalysis taking place in a homogeneous photocatalytic system composed of a phosphonated ruthenium tris(bipyridine) dye (RuP) and a nickel bis(diphosphine) electrocatalyst (NiP) in an aqueous ascorbic acid solution. Our approach is based on transient absorption spectroscopy studies of the efficiency of photo-reduction of RuP and NiP correlated with pH-dependent photocatalytic H2 production and the degree of catalyst protonation. The influence of these factors results in an observed optimum photoactivity at pH 4.5 for the RuP–NiP system. The electron transfer from photo-reduced RuP to NiP is efficient and independent of the pH value of the medium. At pH <4.5, the efficiency of the system is limited by the yield of RuP photo-reduction by the sacrificial electron donor, ascorbic acid. At pH >4.5, the efficiency of the system is limited by the poor protonation of NiP, which inhibits its ability to reduce protons to hydrogen. We have therefore developed a rational strategy utilising transient absorption spectroscopy combined with bulk pH titration, electrocatalytic and photocatalytic experiments to disentangle the complex pH-dependent activity of the homogenous RuP–NiP photocatalytic system, which can be widely applied to other photocatalytic systems.

A Decaheme Cytochrome as a Molecular Electron Conduit in Dye-Sensitized Photoanodes

In nature, charge recombination in light-harvesting reaction centers is minimized by efficient charge separation. Here, it is aimed to mimic this by coupling dye-sensitized TiO2 nanocrystals to a decaheme protein, MtrC from Shewanella oneidensis MR-1, where the 10 hemes of MtrC form a ≈7-nm-long molecular wire between the TiO2 and the underlying electrode. The system is assembled by forming a densely packed MtrC film on an ultra-flat gold electrode, followed by the adsorption of approximately 7 nm TiO2 nanocrystals that are modified with a phosphonated bipyridine Ru(II) dye (RuP). The step-by-step construction of the MtrC/TiO2 system is monitored with (photo)electrochemistry, quartz-crystal microbalance with dissipation (QCM-D), and atomic force microscopy (AFM). Photocurrents are dependent on the redox state of the MtrC, confirming that electrons are transferred from the TiO2 nanocrystals to the surface via the MtrC conduit. In other words, in these TiO2/MtrC hybrid photodiodes, MtrC traps the conduction-band electrons from TiO2 before transferring them to the electrode, creating a photobioelectrochemical system in which a redox protein is used to mimic the efficient charge separation found in biological photosystems.

Electronic structure and reactivity of a [1],[1]disilamolybdenocenophane.

The electronic structure of the two-fold bridged [1],[1]disilamolybdenocenophane has been analyzed by means of density functional theory. As predicted, the relatively high charge at the metal center and, in particular, the highly strained geometry determine a noticeable reactivity towards unsaturated organic substrates. Thus, treatment with the nonpolar reagents 2-butyne and azobenzene leads to side-on coordination of the substrate to the metal center, whereas the reaction with polar tert-butylisonitrile gives a highly unusual structural motif in the form of an ansa-carbene.

[2]borametallocenophanes of group 4 metals: synthesis and structure.

We report a series of [2]borametallocenophanes of Ti, Zr, and Hf with various ligand systems. The ligands have been synthesized in high yields starting from 1,2-dibromo-1,2-bis(dimethylamino)diborane(4) upon reaction with Na[C5H5] and Li[C13H9], respectively. All compounds were fully characterized by multinuclear NMR spectroscopy and, for selected examples, by X-ray analysis.

Reactivity of [1],[1] Disilamolybdenocenophane toward Zerovalent Platinum Complexes

The results of studies of the reactivity of strained [1],[1]disilamolybdenocenophane toward zerovalent platinum complexes are reported. Whereas [Pt(PEt(3))(3)] underwent clean insertion into both Si-C(ispo) bonds of the twofold-bridged molybdenocene, the reaction with the tricyclohexylphosphine analogue is considerably more complex. Thus, treatment of the molybdenum compound with 2 equiv of [Pt(PCy(3))(2)] results in a trinuclear cluster. To gain insight into the mechanistic aspects, the reaction was performed in a 1:1 stoichiometry. Multinuclear NMR spectroscopy revealed the presence of different species in solution. Two constitutional isomers were identified by X-ray diffraction analyses, one presumably depicting an intermediate in the formation of the trinuclear cluster. The predominant isomer in solution was identified as the product of C-H oxidative addition to the platinum phosphine fragment. Its solid-state structure displays an unusual coordination mode of platinum, and structural parameters suggest the formulation as the sigma-complex of a Mo-Si bond.

Photoreduction of Shewanella oneidensis Extracellular Cytochromes by Organic Chromophores and Dye-Sensitized TiO2.

The transfer of photoenergized electrons from extracellular photosensitizers across a bacterial cell envelope to drive intracellular chemical transformations represents an attractive way to harness natures catalytic machinery for solar‐assisted chemical synthesis. In Shewanella oneidensis MR‐1 (MR‐1), trans‐outer‐membrane electron transfer is performed by the extracellular cytochromes MtrC and OmcA acting together with the outer‐membrane‐spanning porin⋅cytochrome complex (MtrAB). Here we demonstrate photoreduction of solutions of MtrC, OmcA, and the MtrCAB complex by soluble photosensitizers: namely, eosin Y, fluorescein, proflavine, flavin, and adenine dinucleotide, as well as by riboflavin and flavin mononucleotide, two compounds secreted by MR‐1. We show photoreduction of MtrC and OmcA adsorbed on RuII‐dye‐sensitized TiO2 nanoparticles and that these protein‐coated particles perform photocatalytic reduction of solutions of MtrC, OmcA, and MtrCAB. These findings provide a framework for informed development of strategies for using the outer‐membrane‐associated cytochromes of MR‐1 for solar‐driven microbial synthesis in natural and engineered bacteria.