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Müller-Bunz, Helge
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Müller-Bunz, Helge
Official Name
Müller-Bunz, Helge
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Now showing 1 - 10 of 36
- PublicationSynthesis of pincer-type N-heterocyclic carbene palladium complexes with a hemilabile ligand and their application in cross-coupling catalysisBenzimidazolium salts containing both a neutral imine and a masked carboxylate functional group for potential metal chelation were prepared. Palladation of the ester-protected ligand afforded a N,C-bidentate carbene complex 4. Subsequent ester hydrolysis preserved the bidentate coordination mode and yielded complex 5 with a pending COOH group exclusively. However, when ester deprotection was carried out prior to metalation, the N,C,O-tridentate pincer-type coordinated palladium complex 7 was obtained. Proton-abstraction of the dangling COOH group in the bidentate ligand of complex 5 by treatment with a base led to the formation of the N,C,O-tridentate coordinated Pd system 7, and inversely, exposure of the tridentate bound Pd complex 7 with acid afforded the N,C-bidentate ligand coordination mode in complex 5, demonstrating hemilability of the oxygen donor site in the pincer ligand. All three palladium(II) complexes 4, 5, and 7 were evaluated in cross-coupling catalysis and revealed distinct activity differences that are dependent on the type of coupling (Suzuki vs. Heck) and the substrate (Ar-Br vs. Ar-Cl). These differences suggest that judicious choice of donor groups in pincer-type complexes is a viable strategy for catalyst optimization.
508Scopus© Citations 26 - PublicationDinuclear ruthenium complexes containing a new ditopic phthalazin- bis(triazole) ligand that promotes metal-metal interactions(Royal Society of Chemistry, 2014-05-01)
; ; ; ; ; Much attention has been paid to heterocyclic N-containing ligands due to their applicability as bridging ligands in the synthesis of redox active dinuclear metal complexes. With this aim, we report the synthesis and full characterization of a novel phthalazine-triazole ligand (1,4-bis(1-methyl-1H-1, 2,3-triazol-4-yl)phthalazine). Moreover, we show that the phthalazine nitrogen atoms of this N-heterocyclic ligand are more reactive towards alkylating agents than the triazole groups. New ruthenium (ii) complexes containing this ligand have been obtained and characterized both structurally and electrochemically. The geometry imposed by the ligand allows the placement of two ruthenium centers in very close proximity so that efficient through-space interactions take place, a concept of crucial importance for electron transfer processes.437Scopus© Citations 21 - 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 - PublicationDirect evidence of a multicentre halogen bond: unexpected contraction of the P–XXX–P fragment in triphenylphosphine dihalides(Royal Society of Chemistry, 2012-12-21)
; ; Triphenylhalophosphonium halides, Ph3PX2, form crystals comprising bridged linear cations [Ph3P-X-X-X-PPh3]+ where the X3 bridge is shortened from 6.56 Å in Cl-Cl-Cl to 6.37 Å in the Br-Br-Br system. It is proposed that this structure is stabilized by five-center/six-electron (5c-6e) hypervalent interactions.494Scopus© Citations 27 - 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 - PublicationStereospecific synthesis and catalytic activity of L-histidylidene metal complexes(Royal Society of Chemistry, 2012-05)
; ; ; ; ; We report on the synthesis, metal coordination, and catalytic impact of histidylidene, a histidine-derived N-heterocyclic carbene (NHC) ligand. The histidinium salt 3, comprising methyl substituents at both heterocyclic nitrogens and protected at the C- and N-terminus of the amino acid, was rhodated and iridated by a transmetallation protocol using Ag2O. Ambient temperature and short reaction times were pivotal for full retention of configuration at the a-carbon. The stereospecificity of the reaction was conveniently probed by P-31 NMR spectroscopy after transmetallation with rhodium(I) and coordination of enantiopure (S)-Ph-binepine. The histidylidene rhodium complexes are highly efficient catalysts for the mild hydrosilylation of ketones. For the cationic complexes [Rh(cod)(histidylidene)(phosphine)](+), lowering the temperature shifted the rate-limiting step of the catalytic reaction to an earlier stage that is not enantioselective. Hence the asymmetric induction-which is governed by the chiral phosphine-did not improve at low temperature.514Scopus© Citations 17 - PublicationIridium, ruthenium, and palladium complexes containing a mesoionic fused imidazolylidene ligandImidazo[1,2-a]pyridine consisting of a pyridine fused to an imidazolium salt at the imidazolium N1–C2 bond and hence protected from forming normal imidazole-2-ylidene complexes undergoes selective activation of the C5–H bond with Ag2O, i.e. at the imidazolium carbon that is proximal to the pyridine nitrogen. While the silver carbene complex is unstable, transmetallation with [IrCp*Cl2]2, [RuCl2(cym)]2, and [PdCl(allyl)]2 afforded stable mesoionic carbene complexes. Two iridium(III) complexes containing one fused carbene ligand and one palladium(II) complex containing two carbene ligands at the metal centre were structurally characterized. The absence of substituents adjacent to the carbene carbon prevents wingtip group activation, and it imparts a reduced stability of the complexes in particular under (mildly) acidic conditions.
430Scopus© Citations 14 - PublicationSynthesis and Tunability of Abnormal 1,2,3-Triazolylidene Palladium and Rhodium Complexes(American Chemical Society, 2011-01-18)
; ; ; ; ; Palladation of N3-alkylated 1,2,3-triazolium salts with Pd(OAc)2 afforded a μ2−I2 bridged bimetallic complex [Pd(trz)I2]2 and monometallic bis(carbene) complexes Pd(trz)2I2 as a mixture of trans and cis isomers (trz = 1,2,3-triazol-5-ylidene). Addition of excess halide or modification of the palladation procedure from direct functionalization to a transmetalation sequence involving a silver intermediate allowed for chemoselective formation of the bis(carbene) complex, while subsequent anion metathesis with NaI produced the monometallic bis(carbene) complexes exclusively. Modification of the wingtip group had little influence on the metalation to palladium or rhodium(I) via transmetalation. According to NMR analysis using δC and 1JRh−C, subtle but noticeable tunability of the metal electronic properties was identified. In addition, phenyl wingtip groups as N-substituents in the triazolylidene ligands were susceptible to cyclopalladation in the presence of NaOAc and are thus not chemically inert.349Scopus© Citations 128 - PublicationMesoionic triazolylidene nickel complexes: synthesis, ligand lability, and catalytic C–C bond formation activity(American Chemical Society, 2014-07-28)
; ; ; A set of triazolylidene (trz) nickel(II) complexes [NiCpX(trz)] was synthesized by a direct metalation of the corresponding triazolium salt with nickelocene, NiCp2. While at short reaction times and in the presence of a coordinating anion X the mono-carbene complex is preferably formed, long reaction times induce the gradual transformation of [NiCpX(trz)] to the bis-carbene complexes [Ni(Cp)(trz)2]+. Kinetic analyses lend strong support to a consecutive pathway involving triazolylidene dissociation from [NiCpX(trz)] en route to the bis-carbene complex. Similar carbene transfer is observed in a solid-state reaction upon heating the complex [NiCpI(trz)] in vacuo, which induces disproportionation to [NiI2(trz)2] and NiCp2, confirming that the Ni–C(trz) bond is kinetically labile. The complexes [Ni(Cp)(trz)2]+ and [NiCpX(trz)] were both efficient catalyst precursors for Suzuki–Miyaura cross-coupling of aryl bromides and phenylboronic acid, with turnover frequencies exceeding 228 h–1. Complex degradation after short reaction times, identified in separate experiments, prohibits high turnover numbers, and for high conversions, repetitive additions of triazolylidene nickel complex and phenylboronic acid are necessary.535Scopus© Citations 57 - 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