Research
1. Multidentate N-heterocyclic carbene ligands
N-Heterocyclic carbenes feature the beneficial property of a combined strong σ-donor- and weak π-acceptor character. We want to make use of this in the design of transition metal catalysts.
1.1 Synthesis and reactivity of pincer-complexes
An enhancement of this property could be realized by combining more NHC-moieties within one ligand. Therefore we have substituted two donor atoms of well-established pincer-ligands by two NHC-moieties. The carbazole backbone turned out to be the backbone of choice as the complete sp2-hybridization of the connecting carbon atoms leads to a stable biscarbene ligand (bimca).
1.2 Catalytic properties of the Rh-pincer complexes
Reaction of the Rh(I)-pincer-complex with allyl halides proceeds fast and leads to formation of complexes with exclusively η1-coordinated allyl ligands. These complexes were tested as catalysts in the allylic alkylation.2
[2] B. Wucher, M. Moser, S. A. Schumacher, F. Rominger, D. Kunz*, Angew. Chem. 2009, 121, 4481-4485; Angew. Chem. Int. Ed. 2009, 48, 4417-4421.
1.3 Coordination chemistry of tetradentate electronrich ligands
[3] C. Deißler, D. Kunz, F. Rominger, Dalton Trans. 2009, 35, 7152-7167.
2. Mono- and dipyrido carbene
2.1 Dipyrido carbenes and their Rh-complexes
We managed to obtain the first X-ray crystal structure analyses of a mono- and a dipyrido carbene. By comparing the NCN-angle of all structurally characterised five-membered ring carbenes and their 13C NMR chemical shift, we found a strong correlation between these data.4
Due to their unique planar geometry, the dipyrido carbenes offer interesting properties. As we could show by means of their (CO)5Cr- and (CO)5W complexes they show a similiar net-donor character as saturated or unsaturated NHCs.5
The complex Rh(COD)(dipy)(PPh3)OTf turned out to be a good catalyst in the hydrosilylation of dialkyl- and diaryl ketones as well as α,β-unsaturated ketones.6
[4] M. Nonnenmacher, D. Kunz, F. Rominger, T. Oeser, Chem. Commun. 2006, 1378-1380.
[5] M. Nonnenmacher, D. Kunz, F. Rominger, T. Oeser, J. Organomet. Chem. 2005, 690, 5647-5653.
[6] M. Nonnenmacher, D. Kunz, F. Rominger, Organometallics 2008, 27, 1561-1568.
2.2 Bis(monopyrido)Pd(II)-complexes
The geometric advantage of planar mono- and dipyridocarbenes is the option to add subsituents at the ortho-positions right next to the metal centre, which could influence the active centre of the catalyst in the desired way. So far, we showed that methylene and ethylene bridged bis(monpyridocarbene)PdX2 complexes without additional ortho-substituents are as active in C-C-coupling reactions as their (NHC)2PdX2 analogues.7
3. New synthetic pathways to N-heterocyclic carbenes
3.1 N-Arylation of imidazolin-2-one and synthesis of imidazolin-2-ylidenes
We investigated the copper catalysed N-arylation of imidazolinone to obtain a simple access to unsymmetric diaryl-substituted N-heterocyclic carbenes and found a general synthetic procedure.8
4. Ionic ligands
A project that was recently started deals with the synthesis of ionic ligands for applications in Ionic Liquids. Many of the conventional catalysts are better soluble in the organic than in the IL phase, so that the reactions proceed rather on the phase surface or the catalyst gets extracted into the organic phase. To avoid this, we want to prepare catalysts that offer similar solubility properties as the IL. We have prepared an imidazol-fulvene (R = H, t-Bu) that features an extremely strong zwitterionic character which is prooved by the very long exocyclic double bond of 1.430 Å. This compound is an ideal substrate for the synthesis of cyclopentadienyl complexes. So far, the respective ferrocene and ruthenium complexes were synthesised and could be structurally characterized by X-ray analysis in the case of the ferrocene complex.9
