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Albrecht, Martin
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Albrecht, Martin
Official Name
Albrecht, Martin
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Now showing 1 - 10 of 66
- 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 - PublicationOrganization of spin- and redox-labile metal centers into Langmuir and Langmuir-Blodgett films(RSC Publishing, 2010)
; ; ; ; ; New sal2(trien) ligands that contain alkoxy substituents of various length in meta position of the salicyl entities were coordinated to electronically and magnetically active iron(III) and cobalt(III) centers. The electrochemical and spectroscopic properties of these amphiphilic complexes are virtually unaffected upon alteration of the alkoxy substituents, thus providing a system in which the physical behavior and the metal-centered chemical activity can be tailored independently. The amphiphilic character has been exploited for preparing Langmuir monolayers at the air-water interface and for constructing Langmuir-Blodgett films, hence allowing for hierarchical assembling of electronically and magnetically active systems. While Langmuir films were stable, transfer onto solid supports was limited, which restricted the magnetic analysis of the Langmuir-Blodgett assemblies.505Scopus© Citations 14 - 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 - PublicationThe Potential of N-Heterocyclic Carbene Complexes as Components for Electronically Active MaterialsThe application of N-heterocyclic carbene complexes as active sites in materials other than catalysis has been remarkably scarce. Inspired by the — often misleading — ‘carbene’ label, which implies a substantial degree of M=C π bonding, we have been interested in evaluating the potential of N-heterocylclic carbene complexes as building blocks for constructing electronically active materials. Electron mobility via the metal-carbon bond has been investigated in monometallic imidazol-2-ylidene complexes and subsequently expanded to polymetallic systems. In particular, ditopic benzobisimidazolium-derived ligands have been explored for the fabrication of bimetallic molecular switches and main-chain conjugated organometallic polymers. Electrochemical analyses have allowed for quantifying the degree of electronic coupling between the metal sites and for identifying the key parameters that govern the intermetallic communication.
688Scopus© Citations 35 - 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 - PublicationCircularly polarized lanthanide luminescence from Langmuir-Blodgett films formed from optically active and amphiphilic Eu(III) based self-assembly complexes(Wiley, 2012-01-16)
; ; ; ; ; The development of novel chiral amphiphilic self-assembled complexes 13Eu and 23Eu by using Europium(III) directed synthesis is described. These systems form stable Langmuir films at an air-water interface, which were transferred onto solid supports (quartz slides) resulting in stable monolayers that exhibit time delayed Eu(III) centered emission and circular polarized luminescence (CPL) upon excitation of the chiral naphthyl antennae.713Scopus© Citations 79 - 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.
517Scopus© Citations 48 - PublicationPyridine-derived N-heterocyclic carbenes: An experimental and theoretical evaluation of the bonding in and reactivity of selected normal and abnormal complexes of Ni(II) and Pd(II)(ACS, 2010-10-22)
; ; ; ; ; ; ; We report a thorough investigation of a series of isomeric complexes with the general formula trans-(pyridylidene)M(PPh3)2Cl (M = Pd, Ni). For the first time, a systematic comparison of normal, abnormal and remote bonding modes is presented. X-ray structural and 13C NMR data indicate the importance of carbenoid mesomeric contributions in their compound class. The catalytic performance of the palladium complexes, trans-(pyridylidene)Pd(PPh3)2Cl, as precursors in Suzuki-Miyaura-type cross-coupling suggests a correlation of remote bonding with catalyst robustness and thus effectivity. When metal precursors M(PPh3)4 are reacted with 2,4-dichloro pyridinium salts, preferential formation of remote carbene complexes occurs and indications are that electronic parameters rather than steric influences are responsible for the observed selectivity. Calculations at the BP86/TZ2P level of theory support interpretation of the results.300Scopus© Citations 69 - 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 - PublicationWingtip substituents tailor the catalytic activity of ruthenium triazolylidene complexes in base-free alcohol oxidationA series of RuII (η6-arene) complexes with 1,2,3-triazolylidene ligands comprising different aryl and alkyl wingtip groups have been prepared and characterized by NMR spectroscopy, microanalysis, and in one case by X-ray diffraction. All complexes are active catalyst precursors for the oxidation of alcohols to the corresponding aldehydes/ketones without the need of an oxidant or base as additive. The wingtip groups have a direct impact on the catalytic activity, alkyl wingtips providing the most active species while aryl wingtip groups induce lower activity. An N-bound phenyl group was the most inhibiting wingtip group due to cyclometalation. Arene dissociation was observed as a potential catalyst deactivation pathway.
455Scopus© Citations 54