Lara Hettmanczyk

Heteromultimetallic Complexes with Redox-Active Mesoionic Carbenes: Control of Donor Properties and Redox-Induced Catalysis

Mesoionic carbenes (MICs) are currently hugely popular as ligands, and triazolylidenes are arguably the most prominent classes of such MICs. Mesoionic carbenes with ferrocenyl substituents are presented that can act as metalloligands for the generation of heteromultimetallic iridium(I) and gold(I) complexes. The ferrocenyl substituents allow for reversible oxidation of these heteromultimetallic complexes, and these oxidation steps have a strong influence on the donor properties of the MICs. Tolman electronic parameters (TEP) determined from analysis of the iridium-carbonyl complexes show that the neutral ferrocenyl-MIC ligands are stronger donors than the imidazolylidene based carbenes, the one-electron oxidized ferrocenyl MICs are in the range of the tricyclohexyl phosphines and the two-electron oxidized forms, which are electron-poor, lie in the range of triphenyl phosphines. Taking advantage of the generation of these electron-poor MICs, we show their gold(I) complexes are potent catalysts for the synthesis of oxazolines, with complexes of the oxidized MIC ligands, without any additional additive, outperforming their neutral counterparts by almost a factor of ten. These results thus present the first examples of MIC ligands that are reversibly electronically tunable, and show the potential of the oxidized MIC ligands in types of catalysis where electron-poor ligands are necessary. The potential of MICs for molecular electroactive materials is also shown.

Phosphinines versus mesoionic carbenes: a comparison of structurally related ligands in Au(I)-catalysis

Gold(i) complexes based on a 2,4,6-triarylphosphinine and a mesoionic carbene derivative have been prepared and characterized crystallographically. Although structurally related, both heterocycles differ significantly in their donor/acceptor properties. These opposed electronic characteristics have been exploited in Au(i)-catalyzed cycloisomerization reactions. For the conversion of the standard substrate dimethyl 2-(3-methylbut-2-enyl)-2-(prop-2-ynyl)malonate the results obtained for both Au-catalysts were found to be very similar and comparable to the ones reported in the literature for other carbene- or phosphorus(iii)-based Au(i)-complexes. In contrast, a clear difference between the catalytic systems was found for the cycloisomerization of the more challenging substrate N-2-propyn-1-ylbenzamide. A combination of the phosphinine-based complex and [AgSbF6] or [Cu(OTf)2] leads to a catalytic species, which is more active than the mesoionic carbene-based coordination compound. We attribute these differences to the stronger π-accepting ability of phosphinines in comparison to mesoionic carbenes. The here presented results show for the first time that phosphinines can be used efficiently as π-accepting ligands in Au(i)-catalyzed cycloisomerization reactions.

Palladium(ii)-Acetylacetonato Complexes with Mesoionic Carbenes: Synthesis, Structures and Their Application in the Suzuki-Miyaura Cross Coupling Reaction

A series of novel palladium(ii) acetylacetonato complexes bearing mesoionic carbenes (MICs) have been synthesized and characterized. The synthesis of the complexes of type (MIC)Pd(acac)I (MIC = 1-mesityl-3-methyl-4-phenyl-1,2,3-triazol-5-ylidene (1), 1,4-(2,4,6-methyl)-phenyl-3-methyl-1,2,3-triazol-5-ylidene (2), 1,4-(2,6-diisopropyl)-phenyl-3-methyl-1,2,3-triazol-5-ylidene (3); acac = acetylacetonato) via direct metalation starting from the corresponding triazolium iodides and palladium(ii) acetylacetonate is described herein. All complexes were characterized by 1H- and 13C-NMR spectroscopy and high resolution mass spectrometry. Additionally, two of the complexes were characterized by single crystal X-ray crystallography confirming a square-planar coordination geometry of the palladium(ii) center. A delocalized bonding situation was observed within the triazolylidene rings as well as for the acac ligand respectively. Complex 2 was found to be an efficient pre-catalyst for the Suzuki-Miyaura cross coupling reaction between aryl-bromides or -chlorides with phenylboronic acid.