Author
Johannes Klein
Bio: Johannes Klein is an academic researcher from Free University of Berlin. The author has contributed to research in topics: Electron paramagnetic resonance & Ligand. The author has an hindex of 8, co-authored 14 publications receiving 193 citations.
Papers
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TL;DR: The results show that donor/acceptor properties in these bidentate ligands are sometimes, but not always, additive with respect to the individual components, and point to the fact that MIC-containing ligands confer remarkable redox stability to their fac-Re(CO)3-containing metal complexes.
Abstract: Bidentate ligands containing at least one triazole or triazolylidene (mesoionic carbene, MIC) unit are extremely popular in contemporary chemistry. One reason for their popularity is the similarities as well as differences in the donor/acceptor properties that these ligands display in comparison to their pyridine or other N-heterocyclic carbene counterparts. We present here seven rhenium(I) carbonyl complexes where the bidentate ligands contain combinations of pyridine/triazole/triazolylidene. These are the first examples of rhenium(I) complexes with bidentate 1,2,3-triazol-5-ylidene-containing ligands. All complexes were structurally characterized through 1H and 13C NMR spectroscopy as well as through single-crystal X-ray diffraction. A combination of structural data, redox potentials from cyclic voltammetry, and IR data related to the CO coligands are used to gauge the donor/acceptor properties of these chelating ligands. Additionally, a combination of UV–vis–near-IR/IR/electron paramagnetic resonance s...
64 citations
TL;DR: Three ReI complexes with mesoionic pyridyl-carbene ligands with potent electrocatalysts for the selective reduction of CO2 to CO and for electrocatalysttic CO2 reduction are presented.
Abstract: Mesoionic carbenes have found wide use as components of homogeneous catalysts. Recent discoveries have, however, shown that metal complexes of such ligands also have huge potential in photochemical research and in the activation of small molecules. We present here three ReI complexes with mesoionic pyridyl-carbene ligands. The complexes display reduction steps which were investigated via UV-vis-NIR-IR spectro-electrochemistry, and these results point toward an EC mechanism. The ReI compounds emit in the visible range in solution at room temperature with excited state lifetimes that are dependent on the substituents of the mesoionic carbenes. These complexes are also potent electrocatalysts for the selective reduction of CO2 to CO. Whereas the substituents on the carbenes have no influence on the reduction potentials, the electrocatalytic efficiency is strongly dependent on the substituents. This fact is likely a result of catalyst instability. The results presented here thus introduce mesoionic carbenes as new potent ligands for the generation of emissive ReI complexes and for electrocatalytic CO2 reduction.
39 citations
TL;DR: It is conclusively proved that the incorporation of MICs into Ru/Os-polypyridyl complexes has a profound influence on the ground and the excited state redox potentials, the position of the emission bands, as well as on the lifetimes of the excited states.
Abstract: In recent years, mesoionic carbenes (MICs) are finding increasing use as building blocks of electro- and photoactive metal complexes. We present here a series of RuII and OsII polypyridine complexes where one or two pyridyl moieties of the well-known tris(bipyridine) analogues are replaced by MICs. We probe the structural, electrochemical, UV-vis-NIR/electron paramagnetic resonance spectroelectrochemical, and photophysical properties of these complexes as a function of the number of MICs in them. Insights from theoretical studies are used to describe the electronic structures of the various redox states. Additionally, electron flux density calculations provide an idea of the flow of electron densities in the excited states of these molecules. This is the first time that such electron flux density calculations are used to probe the excited state properties of transition metal complexes. Our results conclusively prove that the incorporation of MICs into Ru/Os-polypyridyl complexes has a profound influence on the ground and the excited state redox potentials, the position of the emission bands, as well as on the lifetimes of the excited states. These observations might thus be useful for the generation of novel photocatalysts and photosensitizers for dye-sensitized-solar-cells based on MICs.
33 citations
TL;DR: Tuning of the ligand framework was applied to the catalytic oxygenation of alkanes, for which the DMSO complexes were the best catalysts and the yield improved with increasing number of triazole arms, showing how click-derived ligands can be tuned on demand for catalytic processes.
Abstract: Tuning of ligand properties is at the heart of influencing chemical reactivity and generating tailor-made catalysts. Herein, three series of complexes [Ru(L)(Cl)(X)]PF6 (X=DMSO, PPh3, or CD3CN) with tripodal ligands (L1–L5) containing pyridine and triazole arms are presented. Triazole-for-pyridine substitution and the substituent at the triazole systematically influence the redox behavior and photoreactivity of the complexes. The mechanism of the light-driven ligand exchange of the DMSO complexes in CD3CN could be elucidated, and two seven-coordinate intermediates were identified. Finally, tuning of the ligand framework was applied to the catalytic oxygenation of alkanes, for which the DMSO complexes were the best catalysts and the yield improved with increasing number of triazole arms. These results thus show how click-derived ligands can be tuned on demand for catalytic processes.
31 citations
TL;DR: The results conclusively establish that the ease of reduction of an azo group in a particular compound is critically dependent on its substituents and that the noninnocence of the bridging ligands (L2-) in the dinuclear complexes can be decisively tuned by the appropriate choice of ancillary ligands.
Abstract: Ligands containing the azo group are often used in various metal complexes owing to their facile one-electron reduction, which in effect extends the means of degrading environmentally harmful azo dyes. In order to probe the idea of the generally accepted ease of reduction of azo-containing compounds, we present here three different diruthenium complexes [(acac)2RuIII(μ-L2–)RuIII(acac)2] (diastereomeric 1/2), [(bpy)2RuII(μ-L2–)RuII(bpy)2](ClO4)2 ([3](ClO4)2), and [(pap)2RuII(μ-L2–)RuII(pap)2](ClO4)2 ([4](ClO4)2 ) with a bridging ligand (L2– = 1,8-bis((E)-phenyldiazenyl)naphthalene-2,7-dioxido) that contains azo groups in addition to phenoxide-type donors. The RuIII–RuIII complexes (1/2) display interesting one-dimensional-chain effects, as revealed by temperature-dependent magnetic studies. The stability of the RuIII oxidation state in 1/2 under ambient conditions correlates well with the σ-donating acetylacetonato (acac) coligands. However, with π-accepting 2,2/-bipyridine (bpy) or phenylazopyridine (pap)...
29 citations
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TL;DR: Mesoionic carbenes as discussed by the authors are a subclass of the family of N-heterocyclic Carbenes that generally feature less heteroatom stabilization of the carbenic carbon and hence impart specific donor properties and
Abstract: Mesoionic carbenes are a subclass of the family of N-heterocyclic carbenes that generally feature less heteroatom stabilization of the carbenic carbon and hence impart specific donor properties and
316 citations
01 Sep 2009
TL;DR: In this article, the chemistry of these three italicized areas is developed, and from this platform, discovery paths leading to new hydrohalic acid-and water-splitting catalysts are found.
Abstract: Personalized energy (PE) is a transformative idea that provides a new modality for the planet’s energy future. By providing solar energy to the individual, an energy supply becomes secure and available to people of both legacy and nonlegacy worlds and minimally contributes to an increase in the anthropogenic level of carbon dioxide. Because PE will be possible only if solar energy is available 24 h a day, 7 days a week, the key enabler for solar PE is an inexpensive storage mechanism. HY (Y = halide or OH−) splitting is a fuel-forming reaction of sufficient energy density for large-scale solar storage, but the reaction relies on chemical transformations that are not understood at the most basic science level. Critical among these are multielectron transfers that are proton-coupled and involve the activation of bonds in energy-poor substrates. The chemistry of these three italicized areas is developed, and from this platform, discovery paths leading to new hydrohalic acid- and water-splitting catalysts are...
299 citations
TL;DR: In this paper, the diversity and characteristics of C-donor ligands are explored, with particular emphasis on the conceptual design of the electronic properties for applications in coordination chemistry, and the implications of both σ and π effects are discussed.
170 citations
Journal Article•
TL;DR: In situ Raman and ultraviolet-visible spectroscopy alongside spectroelectrochemistry and quantum chemical calculations demonstrate that the redox state of the ligands determines the switching states of the device whereas the counterions control the hysteresis, which may accelerate the technological deployment of organic resistive memories.
Abstract: Non-volatile memories will play a decisive role in the next generation of digital technology. Flash memories are currently the key player in the field, yet they fail to meet the commercial demands of scalability and endurance. Resistive memory devices, and in particular memories based on low-cost, solution-processable and chemically tunable organic materials, are promising alternatives explored by the industry. However, to date, they have been lacking the performance and mechanistic understanding required for commercial translation. Here we report a resistive memory device based on a spin-coated active layer of a transition-metal complex, which shows high reproducibility (∼350 devices), fast switching (≤30 ns), excellent endurance (∼1012 cycles), stability (>106 s) and scalability (down to ∼60 nm2). In situ Raman and ultraviolet-visible spectroscopy alongside spectroelectrochemistry and quantum chemical calculations demonstrate that the redox state of the ligands determines the switching states of the device whereas the counterions control the hysteresis. This insight may accelerate the technological deployment of organic resistive memories.
163 citations
TL;DR: In this article, the authors present recent developments in this field with examples of some selected ligands, developed in their group over the past few years, and focus is on systems developed in our group.
Abstract: Even though the existence of 1,2,3-triazoles has been known for more than a century, the recent discovery of a copper(I) catalyzed version of this reaction has attributed unprecedented importance to these compounds. Coordination and organometallic chemists have benefited from this modular synthetic route, and have accessed ligands based on both the triazoles as well as the triazolylidenes. The wide variation of steric and electronic properties that can be achieved for this ligand class has made them useful for generating metal complexes with various applications. Examples include, among others: magnetically switchable molecules, electro- and photo-active molecules, molecules for light-emitting electrochemical cells, dyes for dye-sensitized solar cells and a host of homogeneous catalytic processes. In this contribution, we present recent developments in this field with examples of some selected ligands. The focus is on systems developed in our group over the past few years.
146 citations