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).

Figure 1: Synthesis of the Rh(bimca)(CO)-complex (above), as well as crystal structures of Rh(bimca)(CO) and Rh(bimca)(CH3)I(CO).

Rh(I)-carbonyl complexes with this ligand turned out to be very strong nucleophiles that react with methyl iodide already at -78 °C.¹

[1] M. Moser, B. Wucher, D. Kunz, F. Rominger, Organometallics 2007, 26, 1024-1030.

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

Diagram: Correlation between the NCN-angles of the free carbenes and their 13C-NMR-shifts. — Figure 2: Correlation between the NCN-angles of the free carbenes and their 13C-NMR-shifts.

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

Crystal structure of Rh(COD)(dipy)(PPh3)OTf

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

Structure of methylene- and ethylene-bridged Imidazo[1,5-a]pyridine-3-ylidene-Pd-complexes

[7] M. Nonnenmacher, D. Kunz, F. Rominger, T. Oeser, J. Organomet. Chem. 2007, 692, 2554-2563.

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

N-Arylation of N-phenyl-imidazolin-2-one from arylhalogenides, (CuOTf)2*C6H6, trans-1,2-diaminocyclohexane and Cs2CO3. — Figure 4: N-Arylation of N-phenyl-imidazolin-2-one.

The following transformation of the diarylurea to the imidazolium salt or the carbene is also possible.

[8] T. Hafner, D. Kunz, Synthesis 2007, 1403-1411.

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

Figure 4: Synthesis of cp-substituted 2-Imidazolium-ferrocenes (above), as well as crystal structures of an imidazol-fulvene (R=H) and a corresponding ferrocene-complexe (R=t-Bu; beneath).

[9] D. Kunz, E. Ø. Johnsen, B. Breuninger, F. Rominger, Chem. Eur. J. 2008, 14, 10909-10914.