Sven Rau

Carbon dioxide and metal centres: from reactions inspired by nature to reactions in compressed carbon dioxide as solvent

Abstract Aspects of the chemistry of CO2 at metal centres are reviewed with emphasis on the formation of metal carbamato complexes from CO2 (an essential step in the activation of some enzymes containing carbamato groups as ligands), recent results of photochemical activation reactions of CO2 and catalytic reactions at metal complexes in compressed CO2 acting as reaction medium or as both solvent and substrate.

Palladium versus Platinum: The Metal in the Catalytic Center of a Molecular Photocatalyst Determines the Mechanism of the Hydrogen Production with Visible Light

To develop highly efficient molecular photocatalysts for visible light-driven hydrogen production, a thorough understanding of the photophysical and chemical processes in the photocatalyst is of vital importance. In this context, in situ X-ray absorption spectroscopic (XAS) investigations show that the nature of the catalytically active metal center in a (N^N)MCl2 (M=Pd or Pt) coordination sphere has a significant impact on the mechanism of the hydrogen formation. Pd as the catalytic center showed a substantially altered chemical environment and a formation of metal colloids during catalysis, whereas no changes of the coordination sphere were observed for Pt as catalytic center. The high stability of the Pt center was confirmed by chloride addition and mercury poisoning experiments. Thus, for Pt a fundamentally different catalytic mechanism without the involvement of colloids is confirmed.

Recent progress in the development of bimetallic photocatalysts for hydrogen generation

In this contribution recent developments in the design and application of bimetallic photocatalysts for the generation of hydrogen via intramolecular processes are assessed. The basic concepts of such assemblies are discussed together with an overview of the factors and molecular issues that affect their potential as photocatalysts. Issues that so far have limited progress are discussed and suggestions for future directions are made.

Mitochondria Targeted Protein-Ruthenium Photosensitizer for Efficient Photodynamic Applications

Organelle-targeted photosensitization represents a promising approach in photodynamic therapy where the design of the active photosensitizer (PS) is very crucial. In this work, we developed a macromolecular PS with multiple copies of mitochondria-targeting groups and ruthenium complexes that displays highest phototoxicity toward several cancerous cell lines. In particular, enhanced anticancer activity was demonstrated in acute myeloid leukemia cell lines, where significant impairment of proliferation and clonogenicity occurs. Finally, attractive two-photon absorbing properties further underlined the great significance of this PS for mitochondria targeted PDT applications in deep tissue cancer therapy.

Optimization of hydrogen-evolving photochemical molecular devices.

A molecular photocatalyst consisting of a Ru(II) photocenter, a tetrapyridophenazine bridging ligand, and a PtX2 (X=Cl or I) moiety as the catalytic center functions as a stable system for light-driven hydrogen production. The catalytic activity of this photochemical molecular device (PMD) is significantly enhanced by exchanging the terminal chlorides at the Pt center for iodide ligands. Ultrafast transient absorption spectroscopy shows that the intramolecular photophysics are not affected by this change. Additionally, the general catalytic behavior, that is, instant hydrogen formation, a constant turnover frequency, and stability are maintained. Unlike as observed for the Pd analogue, the presence of excess halide does not affect the hydrogen generation capacity of the PMD. The highly improved catalytic efficiency is explained by an increased electron density at the Pt catalytic center, this is confirmed by DFT studies.

Protonation effects on the resonance Raman properties of a novel (terpyridine)Ru(4H-imidazole) complex: an experimental and theoretical case study

The optically active states in a novel (terpyridine)Ru(4H-imidazole) complex displaying an unusually broad and red-shifted absorption in the visible range are investigated experimentally and theoretically. Since this property renders the complex promising for an application as sensitizer in dye-sensitized solar cells, a detailed knowledge on the correlation between features in the absorption spectrum and structural elements is indispensable in order to develop strategies for spectroscopy/theory-guided design of such molecular components. To this aim, time-dependent density functional theory calculations, including solvent effects, are employed to analyze the experimental UV-vis absorption and resonance Raman (RR) spectra of the unprotonated and protonated forms of the complex. This provides a detailed photophysical picture for a complex belonging to a novel class of Ru-polypyridine black absorbers, which can be tuned by external pH stimuli. The complex presents two absorption maxima in the visible region, which are assigned by the calculations to metal-to-ligand charge transfer (MLCT) and intra-ligand states, respectively. RR simulations are performed in resonance with both bands and are found to correctly reproduce the observed effects of protonation. Finally, the examination of the molecular orbitals and of the RR spectra for the MLCT state shows that protonation favors a charge transfer excitation to the 4H-imidazole ligand.

Sugar-Selective Enrichment of a D-Glucose-Substituted Ruthenium Bipyridyl Complex Inside HepG2 Cancer Cells

Ruthenium bipyridyl (bpy) complexes display luminescent properties and a rich photochemistry associated with the visible-light-induced formation of a long-lived MLCT state (t up to 1 ms), which can be efficiently quenched by oxygen under the formation of singlet oxygen (F=0.86). These properties render the complexes interesting candidates for fluorescent sensors and photodynamic-therapy applications. However, the in vivo potential of these complexes depends heavily upon both the molecular (sensor applications) and the cellular (photodynamic therapy applications) recognition of the luminescent probes. In addition, Ru(bpy)3 -type complexes are positively charged ions and therefore possess only limited cellmembrane permeability for intact cells ; this significantly limits their potential applications. It has been shown recently that the latter limitation can be overcome by using more lipophilic derivatives such as Ru(DIP)2dppz 2+ (DIP=4,7-diphenyl-1,10phenanthroline, dppz=dipyridophenazine). Cellular uptake into HeLa cells by nonspecific passive diffusion could be observed by using confocal laser scanning microscopy (CLSM) and flow cytometry. 6] A dinuclear Ru complex was observed to bind to DNA in cellulo. Modification of bipyridyl ruthenium complexes in the 4,4’-position can influence their allocation in cellular matrices as shown for a recently reported 4,4’-di(N,Ndiethyl-amino)-substituted homoleptic compound that selectively binds to the cell membrane. One further example towards the design of biosensors is the modification of bipyridyl ligands with biotin, which acts as a binding site for avidin. Carbohydrate-modified complexes present a highly interesting alternative, as it has been shown that their presence enhances the specificity of the interaction with biological probes. These luminescent sensors for the recognition of lectins were tested in vitro as probes for the influenza virus and show different lectin affinities and amplified luminescence depending on the spacer used. Other advantages of the introduction of these groups include increased biocompatibility, a reduction of toxicity and utilization of the targeting properties of sugar-specific receptors or metabolic pathways. Sugar-containing metal complexes are therefore increasingly examined. Recently, mannosylated bipyridyl based ligands with branched spacers possessing a dendrimeric structure were used to obtain Ru complexes. Under irradiation these complexes were able to produce singlet oxygen and to act as lectin biosensors, in particular if an appropriately attached quencher was displaced by the binding lectin. However, while molecular selectivity can be introduced, cellular selectively has so far not been demonstrated. In this contribution we present the synthesis of new ruthenium polypyridyl complexes with peripherally attached sugar substituents. These complexes show, for the first time, a distinct uptake into cancer cells but still retain their luminescent properties. In all known examples the linkages between saccharides and metal-chelating units was realized by using a biodegradable amide or O-glycosidic bonds with flexible spacers; these compounds focused on lectin binding and did not address cellular uptake. To avoid lectin binding in serum or on cell surfaces, the carbohydrates in the compounds reported in this work were bound close to the bipyridine. For biodistribution studies of the intact glycoconjugates we synthesized S-glycosylated disubstituted bipyridines (L1–L3) because it is known that Sglycosidic bonds resist endogenous hydrolysis catalysed by ubiquitous glycosidases. The symmetric ligands were complexed to Ru by using Ru(DMSO)2Cl2 under reflux in water. This results in the formation of compact “ball-shaped” ruthenium complexes of the general formula Ru(L)3Cl2 (Scheme 1). After purification utilizing a Sephadex LH-20 column, the complexes were obtained as red powders. For comparison, the complex Ru(dmbpy)3Cl2 from 4,4’-dimethyl-2,2’-bipyridine (dmbpy) was synthesized by using the same reaction conditions. [a] Dr. M. Gottschaldt, Prof. U. S. Schubert Laboratory for Organic and Macromolecular Chemistry, Friedrich-Schiller-University Jena Humboldtstrasse 10, 07743 Jena (Germany) Fax: (+49)3641-948202 E-mail : michael.gottschaldt@uni-jena.de [b] Prof. U. S. Schubert Laboratory of Macromolecular Chemistry and Nanoscience, Eindhoven University of Technology Den Dolech 2, 5600MB Eindhoven (The Netherlands) [c] Prof. S. Rau Friedrich Alexander Universit t Erlangen–N rnberg Department for Chemistry and Pharmacy Egerlandstrasse 1, 91058 Erlangen (Germany) [d] Prof. S. Yano Innovative Collaboration Center, Kyoto University Nishikyo-ku, Kyoto-daigaku Katsura, Kyoto 615-8520 (Japan) [e] Prof. J. G. Vos Solar Energy Conversion SRC, School of Chemical Sciences Dublin City University Dublin 9 (Ireland) [f] Dr. T. Kroll, Dr. J. Clement Clinics of Internal Medicine, University Hospital Jena Erlanger Allee 101, 07747 Jena (Germany) [g] Prof. I. Hilger Institute of Diagnostic and Interventional Radiology University Hospital Jena Erlanger Allee 101, 07747 Jena (Germany) E-mail : ingrid.hilger@med.uni-jena.de Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cbic.200900769.

Simultaneous Cation/Anion Coordination by Bifunctional 1,2‐Diimine/1,2‐Diamine Type Ligands: Synthesis, Structures, and Properties of Tight Ion Pair Complexes of Ruthenium and Iron

The reaction between [(η6-p-cymene)RuCl2]2 and tetra-p-tolyloxalic amidine (oxam1) results in a coupled cation/anion coordination forming the ion-paired complex [(η6-p-cymene)RuCl(oxam1)Cl] (1a). According to an X-ray single crystal diffraction analysis the Ru moiety is coordinated at the 1,2-diimine part yielding a five-membered chelate ring. Furthermore, the 1,2-diamine group on the opposite side binds to a chloride anion via two N−H groups. Exchange of the chloride anion by trifluoractetate yields [(η6-p-cymene)RuCl(oxam1)CF3CO2] (1c) in which the C=O group is bonded to the diamine part. Similarly, bis(mesityl)bis(pyridylmethyl)oxalic amidine (oxam2) reacts with FeX2 to yield the ion-paired complexes 3a (X: Cl), and 3b (X: Br). X-ray diffraction studies of both compounds reveal that the oxalic amidine ligand acts as a four-dentate chelating ligand. One halide is fixed in the same way as in 1a. The complex 1a reacts with Pd(acac)2 (acac: acetylacetonate) to give the tetranuclear heterobimetallic complex [(η6-p-cymene)RuCl(oxam1)PdCl]2 (2). The related complexes [(tbbpy)2Ru(bbimH2)(OOC−CF3)](PF6) (4), [(tbbpy)2Ru(bbimH2){OOC(CF2)7CF3})][OOC(CF2)7CF3] (5), [{(tbbpy)2Ru(bbimH2)}2(OOC−C6F4−COO)](PF6)2 (6), and [{(tbbpy)2Ru(bbimH2)}2(OOC−C6F4−COO)] (7), (bbimH2: bibenzimidazole; tbbpy: 4,4′-di-tert-butyl-2,2′-bipyridine) also form ion-paired compounds. According to the X-ray diffraction analyses of 4−6 both oxygen atoms of the carboxylate ions are coordinated to the two N−H functions via strong N−H···O bonds. Although 5 contains a dication, the compound is soluble in solvents of low polarity, even in supercritical carbon dioxide. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)

Resonance-Raman spectro-electrochemistry of intermediates in molecular artificial photosynthesis of bimetallic complexes

The sequential order of photoinduced charge transfer processes and accompanying structure changes were analyzed by UV-vis and resonance-Raman spectroscopy of intermediates of a Ru(ii) based photocatalytic hydrogen evolving system obtained by electrochemical reduction.

Covalent Photosensitizer–Polyoxometalate-Catalyst Dyads for Visible-Light-Driven Hydrogen Evolution

A general concept for the covalent linkage of coordination compounds to bipyridine-functionalized polyoxometalates is presented. The new route is used to link an iridium photosensitizer to an Anderson-type hydrogen-evolution catalyst. This covalent dyad catalyzes the visible-light-driven hydrogen evolution reaction (HER) and shows superior HER activity compared with the non-covalent reference. Hydrogen evolution is observed over periods >1 week. Spectroscopic, photophysical, and electrochemical analyses give initial insight into the stability, electronic structure, and reactivity of the dyad. The results demonstrate that the proposed linkage concept allows synergistic covalent interactions between functional coordination compounds and reactive molecular metal oxides.