Towards new molecular photocatalysts for CO2 reduction: photo-induced electron transfer versus CO dissociation within [Os(NN)(CO)2Cl2] Complexes

Optical excitation in the visible region of trans-(Cl)-[Os(bpy)(CO)(2)Cl(2)] (bpy=2,2'-bipyridine; C1) and trans-(Cl)-[Os(dmbpy)(CO)(2)Cl(2)] (dmbpy=4,4'-dimethyl 2,2'-bipyridine; C2) is known to induce the common CO dissociation reaction. However, the quantum yield of the reactions is less than 0.15, although C1 and C2 display pronounced photoluminescence in the visible region at room temperature with a lifetime of few tens of nanoseconds. Taking into account the characteristics of their emitting state, we have investigated the capability of C1 and C2 to act as a photosensitiser in redox reactions in different solvents (MeCN, PrCN and DMF). The efficient oxidation and reduction of both complexes under continuous irradiation in the presence of a sacrificial electron acceptor or donor is reported here. The photo-induced transformations and the nature of the resulting compounds were analysed by UV/Vis and IR spectroscopies and cyclic voltammetry. Photo-induced oxidation of C1 and C2 leads to the corresponding monocarbonyl oxidised species, whereas photo-induced reduction under argon leads mainly to the formation of the corresponding Os-bonded molecular wires P1 and P2 after exchange of two electrons associated with the loss of two chloro ligands. The chemical yield of the latter reaction (around 65%) becomes quantitative by adding [Ru(bpy)(3)](2+) as an external redox photosensitiser. This behaviour has been used to photocatalyse the two electron, two proton conversion of CO(2) to CO. Turnover numbers (TON) of 11.5 and 19.5 have been obtained respectively for C1 and C2 after 4.5 h of irradiation under CO(2) in DMF with triethanolamine as the electron donor. TON can be slightly increased by adding [Ru(bpy)(3)](2+) to the solution.     

Electronic Effects of para‐Substitution on the Melting Points of TAAILs

Owing to numerous new applications, the interest in “task‐specific” ionic liquids increased significantly over the last decade. But, unfortunately, the imidazolium‐based ionic liquids (by far the most frequently used cations) have serious limitations when it comes to modifications of their properties. The new generation of ionic liquids, called tunable aryl–alkyl ionic liquids (TAAILs), replaces one of the two alkyl chains on the imidazolium ring with an aryl ring which allows a large degree of functionalization. Inductive, mesomeric, and steric effects as well as potentially also π π and π π⁺ interactions provide a wide range of possibilities to tune this new class of ILs. We investigated the influence of electron‐withdrawing and ‐donating substituents at the para‐position of the aryl ring (NO₂, Cl, Br, EtO(CO), H, Me, OEt, OMe) by studying the changes in the melting points of the corresponding bromide and bis(trifluoromethanesulfonyl)imide, (N(Tf)₂⁻), salts. In addition, we calculated (B3LYP/6‐311++G(d,p)) the different charge distributions of substituted 1‐aryl‐3‐propyl‐imidazolium cations to understand the experimentally observed effects. The results indicated that the presence of electron‐donating and ‐withdrawing groups leads to strong polarization effects in the cations.     

Theoretical Analysis of the Detailed Mechanism of Native Chemical Ligation Reactions

The method of native chemical ligation between an unprotected peptide α‐thioester and an N‐terminal cysteine–peptide to give a native peptide in aqueous solution is one of the most effective peptide ligation methods. In this work, a systematic theoretical study was carried out to fully understand the detailed mechanism of ligation. It was found that for the conventional native chemical ligation reaction between a peptide thioalkyl ester and a cysteine in combination with an added aryl thiol as catalyst, both the thiol‐thioester exchange step and the transthioesterification step proceed by an anionic concerted S_N2 displacement mechanism, whereas the intramolecular rearrangement proceeds by an addition–elimination mechanism, and the rate‐limiting step is the thiol‐thioester exchange step. The theoretical method was then extended to study the detailed mechanism of the auxiliary‐mediated peptide ligation between a peptide thiophenyl ester and an N‐2‐mercaptobenzyl peptide in which both the thiol‐thioester exchange step and intramolecular acyl‐transfer step proceed by a concerted S_N2‐type displacement mechanism. The energy barrier of the thiol‐thioester exchange step depends on the side‐chain steric hindrance of the C‐terminal amino acid, whereas that of the acyl‐transfer step depends on the side‐chain steric hindrance of the N‐terminal amino acid.     

Two New Crystalline Organogermanate‐Based Inorganic‐Organic Hybrid Compounds

The bifunctional metalloligand bis(carboxyethylgermanium)sesquioxide, (HOOCCH₂CH₂Ge)₂O₃, was employed in the systematic high‐throughput (HT) investigation of the system Zn²⁺/(HOOCCH₂CH₂Ge)₂O₃/H₂O/C₄H₉OH. Two new metal‐organogermanates Zn[(OOCCH₂CH₂Ge)₂O₃] ( 1 ) and Zn₂(O₃GeCH₂CH₂COO) ( 2 ) were discovered that show two new structural motifs for this class of compounds. Whereas in compound 1 a formal intercalation in the structure of (HOOCCH₂CH₂Ge)₂O₃ is observed, 2 exhibits a new layered structure composed of CGeO₃ and ZnO₄ unit linked by μ₃‐oxygen atoms. Both connectivity modes lead to dense three‐dimensional framework structures.     

Spectral Diffusion of Single Molecules in a Hierarchical Energy Landscape

Spectral diffusion as a result of both the transitions between different molecular conformers and the ′′molecular softness′′ of quasi‐free perylene diimides on a SiO₂ surface is investigated by means of single‐molecule spectroscopy, which reveals the time dependence of both the fluorescence spectra and the three‐dimensional orientation. Spectral wavelengths of all single emitters cover a wide energy range of about 0.27 eV, which is due to different types of conformers with large differences in optical transition energy. Time‐dependent spectral trajectories of single emitters within this wavelength manifold are evaluated with a model transcribed from the analysis of spatial diffusion. Spectral diffusion processes are closely correlated with fluorescence emission and excitation power. The overall analysis of spectral diffusion reveals, similar to proteins, a hierarchy of energy barriers in a broad energy landscape.     

Direct Patterning of Copper on Polyimide by Site‐Selective Surface Modification via a Screen‐Printing Process

We demonstrate a simple route to fabricating copper circuit patterns on the surface of polyimide film. The copper pattern can be obtained in three steps: 1) Formation of partially potassium hydroxide modified pattern via a screen‐printing process, 2) formation of macromolecular metal complex with copper, and 3) copper metallization by DMAB reduction. The morphologies of these copper patterns are determined by cross‐sectional transmission electronic microscopy (TEM), scanning electronic microscopy (SEM), and atomic force microscopy (AFM). Furthermore, the growing process of the metallic copper film is investigated. The direct patterning of copper patterns onto polyimide substrates is promising for use in electronics industry as a large‐area and low‐cost processing technique.     

Mechanistic Insights into the Ni‐Catalyzed Reductive Carboxylation of C−O Bonds in Aromatic Esters with CO2: Understanding Remarkable Ligand and Traceless‐Directing‐Group Effects

The mechanism of the Ni⁰‐catalyzed reductive carboxylation reaction of C(sp²)?O and C(sp³)?O bonds in aromatic esters with CO₂ to access valuable carboxylic acids was comprehensively studied by using DFT calculations. Computational results revealed that this transformation was composed of several key steps: C?O bond cleavage, reductive elimination, and/or CO₂ insertion. Of these steps, C?O bond cleavage was found to be rate‐determining, and it occurred through either oxidative addition to form a Ni^II intermediate, or a radical pathway that involved a bimetallic species to generate two Ni^I species through homolytic dissociation of the C?O bond. DFT calculations revealed that the oxidative addition step was preferred in the reductive carboxylation reactions of C(sp²)?O and C(sp³)?O bonds in substrates with extended π systems. In contrast, oxidative addition was highly disfavored when traceless directing groups were involved in the reductive coupling of substrates without extended π systems. In such cases, the presence of traceless directing groups allowed for docking of a second Ni⁰ catalyst, and the reactions proceed through a bimetallic radical pathway, rather than through concerted oxidative addition, to afford two Ni^I species both kinetically and thermodynamically. These theoretical mechanistic insights into the reductive carboxylation reactions of C?O bonds were also employed to investigate several experimentally observed phenomena, including ligand‐dependent reactivity and site‐selectivity.     

Visible‐Light‐Driven Chemoselective Hydrogenation of Nitroarenes to Anilines in Water through Graphitic Carbon Nitride Metal‐Free Photocatalysis

Green and efficient procedures are essential for the chemoselective hydrogenation of functionalized nitroarenes to form industrially important anilines. Herein, it is shown that visible‐light‐driven, chemoselective hydrogenation of functionalized nitroarenes with groups sensitive to forming anilines can be achieved in good to excellent yields (82–100 %) in water under relatively mild conditions and catalyzed by low‐cost and recyclable graphitic carbon nitride. The process is also applicable to gram‐scale reaction, with a yield of aniline of 86 %. A study of the mechanism reveals that visible‐light‐induced electrons are responsible for the hydrogenation reactions, and thermal energy can also promote the photocatalytic activity. A study of the kinetics shows that this reaction possibly occurs through one‐step hydrogenation or stepwise condensation routes. A wide range of applications can be expected for this green, efficient, and highly selective photocatalysis system in reduction reactions for the synthesis of fine chemicals.     

Beitrag zur Kenntniss des Kobalts

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