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Albrecht, Martin
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Albrecht, Martin
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Albrecht, Martin
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Now showing 1 - 10 of 16
- PublicationAbnormal NHC Palladium Complexes: Synthesis, Structure, and ReactivityDevelopments in palladium chemistry have been spurred predominantly by the outstanding application potential of this metal in catalysis. The quest for new ligands in order to modulate the catalytic activity and selectivity of the palladium center has been greatly stimulated by the discovery of N-heterocyclic carbenes as formally neutral, strongly donating, and covalently binding ligands. Abnormal variations of N-heterocyclic carbenes, even though known (yet not recognized) for 30 years, have received very little attention until recently. In parts this may have been due to the fact that the free abnormal carbene ligand is much less stable than the normal carbene analogues. In the last decade, significant progress has been made in abnormal carbene palladium chemistry and reliable synthetic routes as well as promising catalytic applications have been developed. As a consequence, these types of complexes have gradually transformed from laboratory curiosities to unique formally neutral ligands with exceptional donor ability. Here, the advances in abnormal carbene palladium chemistry are summarized. In an attempt to stimulate the entry of newcomers in this fascinating field of research, elementary aspects of synthesis are discussed as well as progress in characterization of the complexes. Most recent (catalytic) applications may highlight the potential of this rapidly growing area of palladium chemistry.
519Scopus© Citations 48 - PublicationCarbene transfer from triazolylidene gold complexes as a potent strategy for inducing high catalytic activity(American Chemical Society, 2013-09-04)
; ; ; ; A series of gold(I) complexes [AuCl(trz)] were synthesized that contain 1,2,3-triazolylidene (trz) ligands with variable wingtip groups. In the presence of AgBF4, these complexes undergo ligand redistribution to yield cationic complexes [Au(trz)2]BF4 in high yields as a result of efficient carbene transfer. Identical reactivity patterns were detected for carbene gold complexes comprised of Arduengo-type IMes ligands (IMes=N,Nâ ²-dimesityl-imidazol-2-ylidene). Reaction of cationic complexes [Au(trz)2]+ with [AuCl(trzâ ²)] afforded the heteroleptic complex [Au(trz)(trzâ ²)]+ and [AuCl(trz)] (trz, trzâ ²=triazolylidene ligands with different wingtip groups). Carbene transfer occurs spontaneously, yet is markeldy rate-enhanced in the presence of Ag+. The facile carbene transfer was exploited as a catalyst activation process to form active gold species for the aldol condensation of isocyanides and aldehydes to form oxazolines. The catalytic activity is strongly dependent on the presence of Ag+ ions to initiate catalyst activation. High turnovers (105) and turnover frequencies (10 4 h-1) were accomplished. Structural analysis at early stages of the reaction support the critical role of triazolylidene dissociation to activate the precatalyst and dynamic light scattering revealed the presence of nanoparticles (±100 nm diameter) as potential catalytically active species. Furthermore, the triazolylidene scaffold had no impact on the diastereoselectivity of the oxazoline formation, and chiral triazolylidenes did not induce any asymmetry in the product. The facile dissociation of carbenes from [AuCl(carbene)] in the presence of Ag+ ions suggests a less stable Au-Ccarbene interaction than often assumed, with potential implications for gold-catalyzed reactions that employ a silver salt as (putative) halide scavenger.487Scopus© Citations 117 - PublicationSolvent-dependent switch of ligand donor ability and catalytic activity of ruthenium(II) complexes containing pyridinylidene amide (PYA) n-heterocyclic carbene hybrid ligands(American Chemical Society, 2014-07-21)
; ; ; ; ; Chelating ligands incorporating both N-[1-alkylpyridin-4(1H)-ylidene]amide (PYA) and N-heterocyclic carbene (NHC) donor sites were prepared and used for the synthesis of ruthenium(II) complexes. Cyclic voltammetry, NMR, and UV–vis spectroscopy of the complexes indicate a solvent-dependent contribution of the limiting resonance structures associated with the ligand in solution. The neutral pyridylidene imine structure is more pronounced in apolar solvents (CH2Cl2), while the mesoionic pyridinium amide form is predominant in polar solvents (MeOH, DMSO). The distinct electronic properties of these hybrid PYA-NHC ligands in different solvents have a direct influence on the catalytic activity of the ruthenium center, e.g., in the dehydrogenation of benzyl alcohol to benzaldehyde. The activity in different solvents qualitatively correlates with the solvent permittivity.643Scopus© Citations 42 - PublicationCarbene Iridium Complexes for Efficient Water Oxidation: Scope and Mechanistic Insights(Royal Society of Chemistry, 2014-07-01)
; ; ; ; ; Iridium complexes of Cp* and mesoionic carbene ligands were synthesized and evaluated as potential water oxidation catalysts using cerium(IV) ammonium nitrate as a chemical oxidant. Performance was evaluated by turnover frequency at 50% conversion and by absolute turnover number, and the most promising precatalysts were studied further. Molecular turnover frequencies varied from 190 to 451 per hour with a maximum turnover number of 38 000. While the rate of oxygen evolution depends linearly on iridium concentration, concurrent spectroscopic and manometric observations following stoichiometric oxidant additions suggest oxygen evolution is limited by two sequential first-order reactions. Under the applied conditions, the oxygen evolving species appears to be a well-defined and molecular species based on kinetic analyses, effects of careful ligand design, reproducibility, and the absence of persistent dynamic light scattering signals. Outside of these conditions, the complex mechanism is highly dependent on reaction conditions. While confident characterization of the catalytically active species is difficult, especially under high-turnover conditions, this work strongly suggests the primary active species under these conditions is a molecular species.500Scopus© Citations 98 - PublicationSpringloaded porphyrin NHC hybrid rhodium(III) complexes: carbene dissociation and oxidation catalysisPorphyrin rhodium(III) complexes accommodate one or two NHC ligands in the apical position, which leads to severe porphyrin distortion and dearomatization. The strain in the bis(carbene) complex induces facile carbene dissociation and the formation of a catalytically active site for alcohol oxidation.
445Scopus© Citations 26 - PublicationLigand Exchange and Redox Processes in Iridium Triazolylidene Complexes Relevant to Catalytic Water OxidationIridium(III) complexes containing a bidentate spectator ligand have emerged as powerful catalyst precursors for water oxidation. Here we investigate the initial steps of the transformation at the iridium center when using complex [IrCp*(pyr-trz)Cl] 1 (Cp* = pentamethylcyclopentadienyl, pyr-trz = 4-(2-pyridyl)-1,2,3-triazol-5-ylidene), a potent water oxidation catalyst precursor. Ligand exchange with water is facile and is reversed in the presence of chloride ions, while MeCN substitution is effective only from the corresponding aqua complex. A pKa of 8.3 for the aqua complex was determined, which is in agreement with strong electron donation from the triazolylidene ligand that is comparable to aryl anions. Evaluation of the pH-dependent oxidation process in aqueous media reveals two regimes (pH 4–8.5 and above pH 10.5) where proton-coupled electron transfer processes occur. These investigations will help to further optimize water oxidation catalysts and indicate that MeCN as a cosolvent has adverse effects for initiating water coordination in the oxidation process.
355Scopus© Citations 32 - PublicationCarbenes in ActionNew reactions of N-heterocyclic carbenes and the stabilization of abnormal variations of these ligands provide exciting perspectives for organic synthesis and organometallic catalysis.
253Scopus© Citations 34 - PublicationExpanding the family of mesoionic complexes: donor properties and catalytic impact of palladated isoxazolylidenesAbnormal isoxazolylidene complexes, a new subclass of mesoionic complexes containing an isoxazolium-derived carbene type ligand, have been synthesised via oxidative addition and compared to structurally related mesoionic complexes by using 31P NMR spectroscopy as a convenient probe for their donor ability and in catalytic cross-coupling reactions.
521Scopus© Citations 75 - Publication
Scopus© Citations 56 802 - PublicationMesoionic oxides: facile access from triazolium salts or triazolylidene copper precursors and, catalytic relevanceReaction of CsOH with triazolium salts affords mesoionic compounds containing an exocyclic oxygen; the same product is obtained by reaction of the corresponding Cu(I) triazolylidenes with CsOH and represents an unusual reactivity pattern of N-heterocyclic carbene precursors that has implications for carbene copper-catalyzed reactions.
380Scopus© Citations 37