The metal-mediated (catalytic) activation of strong and typically unreactive bonds under mild conditions requires the development of powerful ligand sets. Two particularly useful strategies have been developed during the last few years. [1] The first relies on electron-deficient systems with high-valent early transition metals, often in a d 0 configuration. [2] Such metal centers have been shown to activate unreactive bonds through agostic interactions and subsequent s-bond metathesis. In a second strategy, electronrich metal centers are used to promote bond activation through oxidative addition, and late transition metals such as the platinum group metals typically accommodate the required electron density for these reactions. [3] The relative basicity can be further increased by using acidic media [4] and by installing strongly donating nontransferable ligands in the metal coordination sphere. [5]
A series of ruthenium(II) complexes have been prepared by using bidentate chelating N-heterocyclic carbene (NHC) ligands that feature different donor groups E (E=olefin, thioether, carboxylate, and NHC). Rigid coordination of all donor sites was concluded from NMR spectroscopy, and the electronic impact of the donor group was evaluated by electrochemical analyses. The chelating donor group had a strong influence on the activity of the metal center in catalyzing direct hydrogenation of styrene. A thioether group or a second NHC donor site essentially deactivates the metal center. Complexes comprising a NHC tethered with an olefin or a carboxylate group showed appreciable activity, though only the carboxylate-functionalized system proved to be a precursor for homogeneous hydrogenation. According to in situ high-pressure NMR analyses, complexes featuring a carboxylate chelating group are remarkably resistant toward reductive elimination even under strongly reducing conditions and may, therefore, be used repeatedly.
Versammlungsverbot für Tau‐Proteine: Die pathologische Aggregation von Tau‐Proteinen hängt eng mit dem Fortschreiten der Alzheimer‐Krankheit zusammen. Von Rhodanin abgeleitete Inhibitoren (z. B. 1) hemmen die Tau‐Aggregation in einem Zellmodell reversibel und in nanomolaren Konzentrationen (siehe SEM‐Bilder).
a Palladation of C2-protected diimidazolium salts with Pd(OAc) 2 afforded complexes comprising C4-bound N-heterocyclic dicarbene ligands. The reactivity of these complexes towards Lewis acids (AgBF 4 , AgOAc) and Brønsted acids (H 2 SO 4 , H 3 PO 4 , HOAc) revealed that abnormal C4 bonding of the carbenes markedly increases the nucleophilicity of the coordinated palladium center as compared to C2 bonding. Despite its formal +2 charge, the palladium center in these complexes is best described as a Lewis base. The abnormal carbene bonding mode induces new reaction patterns such as the formation of a Pd-Ag adduct. Based on metallation studies including the palladation of a dissymmetric diimidazolium salt, a rationale for the selective activation of the C4-H bond in the diimidazolium precursor salts is proposed.
Dedicated to Gerard van Koten with deep appreciation for his outstanding contribution to the organometallic chemistry of late transition metals.
AbstractThe palladation of potentially chelating bisimidazolium ligand precursors with palladium acetate gives bridging bimetallic, chelating monometallic, and homoleptic tetracarbene complexes. The coordination mode of the biscarbene ligand has been identified by spectroscopic analysis and crystallographic characterization of representative complexes, including the first example of a biscarbene A-frame structure. Substantial concentrations of free acetate favor the formation of tetracarbene over biscarbene palladium complexes, while in the absence of a base, the concentration of reactants influences the selectivity for bridging bimetallic versus chelating monometallic species. Preliminary kinetic and mechanistic studies indicate that chelating biscarbene palladium acetate complexes are intermediates in the formation of the homoleptic tetracarbene complexes. Probably due to the high trans effect of the biscarbene ligand, such complexes are more efficient palladating agents for bisimidazolium salts than palladium acetate.
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