Christian Herrero

Artificial photosynthetic systems. Using light and water to provide electrons and protons for the synthesis of a fuel

This review covers the progress achieved in the synthesis and characterization of different metal based catalysts designed for the photocatalytic oxidation of water with special focus on molecular designed systems. In those cases where the mechanism of the light-driven catalytic activity of these complexes is not known, we have looked to published mechanisms related to chemically activated catalysts. We also discuss the choice of the chromophores used to sensitize these reactions as well as the recovery of the reaction products and nature of the electron acceptors used.

Click chemistry on a ruthenium polypyridine complex. An efficient and versatile synthetic route for the synthesis of photoactive modular assemblies.

In this Communication, we present the synthesis and use of [Ru(bpy)(2)(bpy-CCH)](2+), a versatile synthon for the construction of more sophisticated dyads by means of click chemistry. The resulting chromophore-acceptor or -donor complexes have been studied by flash photolysis and are shown to undergo efficient electron transfer to/from the chromophore. Additionally, the photophysical and chemical properties of the original chromophore remain intact, making it a very useful component for the preparation of visible-light-active dyads.

Efficient electron transfer through a triazole link in ruthenium(II) polypyridine type complexes

Spectroscopic, electrochemical and theoretical characterisations of photoactive systems readily assembled via click-chemistry show an efficient bi-directional charge shift through the triazole link.

Intramolecular light induced activation of a Salen–MnIII complex by a ruthenium photosensitizer

We have designed a molecular system consisting of a heteroleptic [Ru(bpy)(2)L](2+) chromophore covalently linked to a Mn(III)-Salen unit. We demonstrate the light induced oxidation of the Mn(III) center in this putative photo-catalyst assembly to a Mn(IV) high spin intermediate. Both oxidation states have been characterized by transient absorption and EPR techniques.

Carbon dioxide reduction via light activation of a ruthenium–Ni(cyclam) complex

In this paper we report the synthesis of a chromophore-catalyst assembly designed for the photoreduction of carbon dioxide. The chromophore unit is made up of a ruthenium trisbipyridyl-like unit covalently attached to a nickel cyclam (cyclam = 1,4,8,11-tetraazacyclotetradecane) via a triazole ring. The intramolecular electron transfer activation of the catalyst unit by visible light was studied by nanosecond flash photolysis and EPR spectroscopy. In aqueous solutions (pH = 6.5), activation of the Ru(II)-Ni(II) modular assembly with 450 nm visible light in the presence of a sacrificial electron donor accomplishes the reduction of CO2 into CO and H2 in a ratio of 2.7 to 1.

Light-Driven Activation of the [H2O(terpy)MnIII-μ-(O2)-MnIV(terpy)OH2] Unit in a Chromophore–Catalyst Complex

Fuel production using solar power is one of the most important challenges facing the scientific community this century. Nature stores the energy from light in the form of chemical bonds by a process called photosynthesis. In the early stages of this process, Photosystem II uses light to remove electrons and protons from water. In later steps, the electrons taken from water are used to make stable reduced products. This process has inspired the field of “artificial photosynthesis”, the aim of which is to duplicate the processes of light capture, the formation of charge separated states, charge accumulation, and the catalytic formation of high-energy products using molecular devices. One potential strategy involves the design of molecules that contain photoactive and catalytic units that promote photo-chemical oxidation reactions through the accumulation of sufficient light-driven oxidizing equivalents at a catalytic center. Our research group recently developed a ligand designed to hold both a photoactive unit and a putative catalytic center through an imidazole-based electron-relay moiety, which was shown to behave as a redox-active module. In this work, our aim is to expand this chromophore-relay complex in order to create a chromophore-relay-catalyst complex made up of a [Ru ACHTUNGTRENNUNG(bpy)2](L) complex covalently bound to a terpyridine unit through an imidazole bridge. Recently, terpyridine ligands have been increasingly used for manganeseand ruthenium-based catalytic systems with the aim of oxidizing water and organic substrates. Amongst recent publications in this area, one of the more interesting pieces of work concerns the dinuclear manganese complex [(Terpy)2(Mn -di-m-oxo-Mn)], which is one of the few manganese complexes to have been reported to perform the catalytic oxidation of water. Given the presence of the two coordinating sites in our target ligand (1,10-phenanthroline and a 2,2’:6’,2’’-terpyridine), a synthetic pathway had to be designed in order to obtain specific insertion of the manganese and ruthenium metals into each of the two sites. Preparation of the organic ligand prior to metal insertion did not provide this selectivity; therefore, this approach was discarded. Thus, we used a synthetic procedure based on a modified ruthenium–polypyridine complex known to be a useful building block from which our target molecule can be constructed. This approach yields a versatile synthon (1) that could be useful for the preparation of libraries of compounds through simple synthetic transformations; it relies on the inert nature of low-spin ruthenium(II)-polypyridine complexes, which prevent demetalation or ligand-exchange reactions under the conditions used. The ruthenium(II) heteroleptic complex containing two bipyridine ligands and a coordinated 1,10phenanthroline-4,5-dione (phendione) ligand [RuACHTUNGTRENNUNG(bpy)2(Phendione)]2+(1) was prepared in good yields using published methods. This complex was then reacted with 4’-benzaldehyde terpyridine in refluxing acetic acid in pres[a] Dr. C. Herrero, Dr. A. Quaranta, Dr. W. Leibl , Prof. A. W. Rutherford, Prof. A. Aukauloo CEA, iBiTecS, Service de Bio nerg tique Biologie Structurale et M canismes (SB2SM) Gif-sur-Yvette, F-91191 (France) Fax: + (33) 1 69088717 E-mail : christian.herrero@cea.fr [b] Dr. S. Protti Department of Chemistry University of Pavia V.Le Taramelli 12 27100 Pavia (Italy) [c] Prof. A. W. Rutherford Molecular Biosciences Imperial College South Kensington Campus, London SW7 2AZ (UK) [d] D. R. Fallahpour Institute of Organic Chemistry University of Zurich Winterthurerstrasse 190, CH8057, Zurich (Switzerland) [e] Dr. M.-F. Charlot, Prof. A. Aukauloo Laboratoire de Chimie Inorganique Institut de Chimie Mol culaire et des Mat riaux d’Orsay UMR 8182 Universit de Paris-Sud Orsay, F-91405 (France) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/asia.201100030.

Photoinduced multielectron transfer to a multicopper oxidase resulting in dioxygen reduction into water.

By mimicking the naturally evolved enzymatic and photochemical processes of photosynthesis, solar energy can be used to drive catalysis and ultimately convert light to stored chemical energy. The target is to develop robust systems in which light absorption triggers electron-transfer events that subsequently lead to the activation of a catalytic center. Such a process would not only avoid chemical activation by harsh oxidants or reductants, one particular focus of green chemistry, but also diminish the dependence on nonrenewable energy sources. In addition to these now well-established goals of sustainable energy and resource use, the development of photodriven catalysts potentially offers new avenues for the study of catalytic mechanisms. In this field Gray, Winkler, and co-workers pioneered the coupling of photoactive units to enzymes to access the buried active site of different metalloenzymes. The well-documented Ru-polypyridine-type complexes have been used to initiate electron-transfer reactions in the presence of either sacrificial electron acceptors or electron donors for the oxidation or reduction of active sites. One challenging issue when using this strategy to activate metalloenzymes, is the efficient accumulation of multicharges or holes at the catalytic unit. In the majority of cases light-induced transfer processes have been limited to one electron transfer. Only a few examples of multicharge or hole accumulations have been reported recently. Some rare examples using metalloproteins that could lead to photodriven catalytic activities were also reported. Herein, we report the light-driven four-electron reduction of a laccase (our previously studied LAC3 from Trametes sp. C30), which ultimately converts dioxygen into water by using ruthenium(II) polypyridine-type chromophores (complexes 1 and 2, Scheme 1) and ethylenediaminetetraacetic acid (EDTA) as the sacrificial electron donor.

Electron transfer activity of a de novo designed copper center in a three-helix bundle fold

In this work, we characterized the intermolecular electron transfer (ET) properties of a de novo designed metallopeptide using laser-flash photolysis. α3D-CH3 is three helix bundle peptide that was designed to contain a copper ET site that is found in the β-barrel fold of native cupredoxins. The ET activity of Cuα3D-CH3 was determined using five different photosensitizers. By exhibiting a complete depletion of the photo-oxidant and the successive formation of a Cu(II) species at 400 nm, the transient and generated spectra demonstrated an ET transfer reaction between the photo-oxidant and Cu(I)α3D-CH3. This observation illustrated our success in integrating an ET center within a de novo designed scaffold. From the kinetic traces at 400 nm, first-order and bimolecular rate constants of 10(5) s(-1) and 10(8) M(-1) s(-1) were derived. Moreover, a Marcus equation analysis on the rate versus driving force study produced a reorganization energy of 1.1 eV, demonstrating that the helical fold of α3D requires further structural optimization to efficiently perform ET. This article is part of a Special Issue entitled Biodesign for Bioenergetics--the design and engineering of electronic transfer cofactors, proteins and protein networks, edited by Ronald L. Koder and J.L. Ross Anderson.

An Artificial Enzyme Made by Covalent Grafting of an FeII Complex into β‐Lactoglobulin: Molecular Chemistry, Oxidation Catalysis, and Reaction‐Intermediate Monitoring in a Protein

An artificial metalloenzyme based on the covalent grafting of a nonheme Fe(II) polyazadentate complex into bovine β-lactoglobulin has been prepared and characterized by using various spectroscopic techniques. Attachment of the Fe(II) catalyst to the protein scaffold is shown to occur specifically at Cys121. In addition, spectrophotometric titration with cyanide ions based on the spin-state conversion of the initial high spin (S=2) Fe(II) complex into a low spin (S=0) one allows qualitative and quantitative characterization of the metal centers first coordination sphere. This biohybrid catalyst activates hydrogen peroxide to oxidize thioanisole into phenylmethylsulfoxide as the sole product with an enantiomeric excess of up to 20 %. Investigation of the reaction between the biohybrid system and H2 O2 reveals the generation of a high spin (S=5/2) Fe(III) (η(2) -O2 ) intermediate, which is proposed to be responsible for the catalytic sulfoxidation of the substrate.

Electronic Structures of Mono-Oxidized Copper and Nickel Phosphasalen Complexes

Non-innocent ligands render the determination of the electronic structure in metal complexes difficult. As such, a combination of experimental techniques and quantum chemistry are required to correctly elucidate them. This paper deals with the one-electron oxidation of copper(II) and nickel(II) complexes featuring a phosphasalen ligand (Psalen), which differs from salen analogues by the presence of iminophosphorane groups (P=N) instead of imines. Various experimental techniques (X-ray diffraction, cyclic voltammetry, NMR, EPR, and UV/Vis spectroscopies, and magnetic measurements) as well as quantum chemical calculations were used to define the electronic structure of the oxidized complexes. These can be modified by a small change in the ligand structure, that is, the replacement of a tert-butyl group by a methoxy on the phenoxide ring. The different techniques have allowed quantifying the amount of spin density located on the metal center and on the Psalen ligands. All complexes were found to possess a multi-configurational ground state, in which the ratio of the +II versus +III oxidation state of the metal center, and therefore the phenolate versus phenoxyl radical ligand character, varies upon the substituents. The tert-butyl group favors a strong localization on the metal center whereas with the methoxy group the metallic configurations decrease and the ligand configurations increase. The importance of the geometrical considerations compared with the electronic substituent effect is highlighted by the differences observed between the solid-state (EPR, magnetic measurements) and solution characterizations (EPR and NMR data).