Günther Knör

Synthesis and Characterization of a Stable Bismuth(III) A3–Corrole

An efficient metalation procedure for bismuth complexes with meso-substituted corrole ligands is presented. Reaction of 5,10,15-tris-pentafluorophenylcorrole H(3)(TpFPC) with Bi{N(SiMe(3))(3)} converts the free ligand H(3)(TpFPC) to a neutral low-valent species Bi(TpFPC), which has been characterized by different spectroscopic techniques. (Spectro)electrochemical studies were performed in order to describe the redox potentials of the Bi(TpFPC) complex and to ascribe the sites of electron transfer. The first crystal structure of a bismuth corrole is presented and compared to the geometry-optimized molecular structure obtained with density functional theory (DFT) calculations. We show an example of a 4-coordinate metallocorrole with a very large out-of-plane displacement and significant doming. The electronic structure of the novel bismuth corrole system is discussed in detail. Time-dependent DFT results support the proposed assignment of electronic transitions observed for the Bi(TpFPC) derivative. To account for the reactivity we investigated the photocatalytic properties of the Bi(TpFPC) complex.

Artificial Enzyme Catalysis Controlled and Driven by Light

Bio-inspired chemistry based on photoresponsive molecules is a rapidly developing new strategy to mimic the function of various biological systems. The interaction of electromagnetic radiation with molecular systems is ideally suited for the control and powering of dynamic processes at the speed of light. Besides typical applications in artificial photosynthesis, many other aspects, such as the catalytic turnover of substrates or the controlled release or uptake of small bioactive molecules, are readily verified with light-driven model systems. The potential of this novel approach in biomimetic chemistry is briefly explored in this concept article.

Photocatalytic Reduction of Artificial and Natural Nucleotide Co-factors with a Chlorophyll-Like Tin-Dihydroporphyrin Sensitizer

An efficient photocatalytic two-electron reduction and protonation of nicotine amide adenine dinucleotide (NAD+), as well as the synthetic nucleotide co-factor analogue N-benzyl-3-carbamoyl-pyridinium (BNAD+), powered by photons in the long-wavelength region of visible light (λirr > 610 nm), is demonstrated for the first time. This functional artificial photosynthetic counterpart of the complete energy-trapping and solar-to-fuel conversion primary processes occurring in natural photosystem I (PS I) is achieved with a robust water-soluble tin(IV) complex of meso-tetrakis(N-methylpyridinium)-chlorin acting as the light-harvesting sensitizer (threshold wavelength of λthr = 660 nm). In buffered aqueous solution, this chlorophyll-like compound photocatalytically recycles a rhodium hydride complex of the type [Cp*Rh(bpy)H]+, which is able to mediate regioselective hydride transfer processes. Different one- and two-electron donors are tested for the reductive quenching of the irradiated tin complex to initiate the secondary dark reactions leading to nucleotide co-factor reduction. Very promising conversion efficiencies, quantum yields, and excellent photosensitizer stabilities are observed. As an example of a catalytic dark reaction utilizing the reduction equivalents of accumulated NADH, an enzymatic process for the selective transformation of aldehydes with alcohol dehydrogenase (ADH) coupled to the primary photoreactions of the system is also demonstrated. A tentative reaction mechanism for the transfer of two electrons and one proton from the reductively quenched tin chlorin sensitizer to the rhodium co-catalyst, acting as a reversible hydride carrier, is proposed.

Photocatalytic reactions of porphyrin-based multielectron transfer sensitizers☆

Abstract Individual metal centers assisted by redox-active ligands can serve as active sites for photocatalytic multielectron transfer (MET) substrate transformations. In porphyrin-based mononuclear systems, the primary steps of excitation and charge stabilization are mediated by the tetrapyrrole ligand acting both as photosensitizer and intermediate charge reservoir. Accumulation and transfer of multiple redox equivalents requires a binding site that provides at least two stable oxidation states separated by more than one unit. The reversible photoconversion between low- and high-valent antimony porphyrins is described as an example of MET sensitization.

Electrocatalytic Reduction of Carbon Dioxide to Carbon Monoxide by a Polymerized Film of an Alkynyl‐Substituted Rhenium(I) Complex

The alkynyl‐substituted ReI complex [Re(5,5′‐bisphenylethynyl‐2,2′‐bipyridyl)(CO)3Cl] was immobilized by electropolymerization onto a Pt‐plate electrode. The polymerized film exhibited electrocatalytic activity for the reduction of CO2 to CO. Cyclic voltammetry studies and bulk controlled‐potential electrolysis experiments were performed by using a CO2‐saturated acetonitrile solution. The CO2 reduction, determined by cyclic voltammetry, occurs at approximately −1150 mV versus the normal hydrogen electrode (NHE). Quantitative analysis by GC and IR spectroscopy was used to determine a Faradaic efficiency of approximately 33 % for the formation of CO. Both values of the modified electrode were compared to the performance of the homogenous monomer [Re(5,5′‐bisphenylethynyl‐2,2′‐bipyridyl)(CO)3Cl] in acetonitrile. The polymer formation and its properties were studied by using SEM, AFM, and attenuated total reflectance (ATR) FTIR and UV/Vis spectroscopy.

Rhodium-Coordinated Poly(arylene-ethynylene)-alt-Poly(arylene- vinylene) Copolymer Acting as Photocatalyst for Visible-Light- Powered NAD + /NADH Reduction

A 2,2′-bipyridyl-containing poly(arylene-ethynylene)-alt-poly(arylene-vinylene) polymer, acting as a light-harvesting ligand system, was synthesized and coupled to an organometallic rhodium complex designed for photocatalytic NAD+/NADH reduction. The material, which absorbs over a wide spectral range, was characterized by using various analytical techniques, confirming its chemical structure and properties. The dielectric function of the material was determined from spectroscopic ellipsometry measurements. Photocatalytic reduction of nucleotide redox cofactors under visible light irradiation (390–650 nm) was performed and is discussed in detail. The new metal-containing polymer can be used to cover large surface areas (e.g. glass beads) and, due to this immobilization step, can be easily separated from the reaction solution after photolysis. Because of its high stability, the polymer-based catalyst system can be repeatedly used under different reaction conditions for (photo)chemical reduction of NAD+. With this concept, enzymatic, photo-biocatalytic systems for solar energy conversion can be facilitated, and the precious metal catalyst can be recycled.

Non-Luminescent 1,2-Diiminetricarbonylrhenium(I) Chloride Complexes – Synthesis, Electrochemical and Spectroscopic Properties of Re(DIAN)(CO)3Cl with DIAN = p-Substituted Bis(arylimino)acenaphthene

Re(1,2-diimine)(CO)3Cl compounds containing the p-substituted bis(arylimino)acenaphthene derivatives DIAN-R with R = H, Me, OMe, Br, and Cl were prepared and characterized. The diimine ligands of this series provide unoccupied orbitals at rather low energies and display a pronounced π-accepting strength. This feature determines the electrochemical and spectroscopic properties of the Re(DIAN-R)(CO)3Cl complexes. All compounds show rhenium(I) to π*(diimine) metal-to-ligand charge transfer (MLCT) absorptions in the visible spectral region. The maxima of these CT bands systematically shift upon modification of the substituents R and are sensitive to variations of the solvent polarity (negative solvatochromism). Re(DIAN-R)(CO)3Cl complexes are non-luminescent both in fluid solution at 298K and in solvent glass at 77K.

Photosensitization and photocatalysis in bioinorganic, bio-organometallic and biomimetic systems

Abstract Inorganic photochemistry has experienced an enormous progress within the past decades. Many branches of current frontier science including sustainable chemistry, solar fuel production, or modern therapeutic strategies critically depend on light-responsive metal complexes. More recently, the various aspects of bioinorganic and bioorganometallic photochemistry have been systematically explored. In the present review, an attempt is made to provide some unifying concepts and rational design guidelines for the development of novel biomimetic and bioinspired systems controlled and driven by photons. Spectral sensitization of such photoprocesses remains a central challenge for utilizing sunlight as the energy source for enzyme mimetics, artificial photosynthesis, and chemical photocatalysis. Several applications of metal-based drugs in molecular photomedicine also require light sensitivity in clearly defined spectral regions. Therefore, a brief overview on bioinorganic chromophores and their synthetic counterparts is provided. We also focus on the integration of abundant natural resources and the search for novel photocatalysts based on nonprecious metals and environmentally benign materials.

Electronic spectra of 1,2-diiminetricarbonylrhenium(I)chloride complexes with imidazole derivatives as ligands

A series of Re(CO)3(1,2-diimine)Cl complexes with bisimidazoles (BIIM) as 1,2-diimines have been synthesized and their electronic spectra studied. Compared with the absorption spectra of the free BIIM ligands, the spectra of the metal complexes display a shoulder at the low energy side of the intraligand (IL) bands. This additional absorption is assigned to the Re(I) to π* (BIIM) metal-to-ligand charge transfer (MLCT) transition. Since imidazoles are weak π-acceptor ligands, the MLCT band occurs at rather short wavelengths and is partially hidden by the intense IL bands of the BIIM moiety. The complexes are photoluminescent both at room temperature and at 77 K.

Switchable photoreduction pathways of antimony(V) tetraphenylporphyrin. A potential multielectron transfer photosensitizer

Photoreduction of [SbV(tpp)(OH)2]+(tpp = dianion of 5,10,15,20-tetraphenylporphyrin) yields either the metastable π-radical anion [SbV(tpp˙–)(OH)2] or the two-electron reduced[SbIII(tpp)]+ complex; product formation can be channelled in the desired direction by an appropriate choice of the reaction medium and dioxygen regenerates the starting compound.