Our Research interests
Our research interests are in the area of Organic and Organometallic Chemistry. Organometallic chemistry is the study of compounds having metal-carbon bonds. Carbon-containing (organic) molecular fragments that are bonded to metal atoms often have unique geometries and reactivities on account of the metal's electronic properties. Organometallic compounds are important in catalysis, medicine, and the construction of molecular scale devices (nanoscience).
Research goal: Catalysts!
Catalysts have a huge economical impact on all different kind of syntheses.
Catalysts increase the efficiency and selectivity of chemical reactions,
and they help saving ressources and energy. Organometallic compounds often
have unique reactivities on account of the metal's electronic properties.
and they can serve as catalysts in organic transformations. I
t is our goal to find novel organometallic archtiectures,
which are catalytically active and easy to handle.
We are interested in
a) the development of novel ligand structures
b) the investigation of relationships between the structure of ligands and the properties of their respective metal complexes
c) the application of these metal complexes as catalysts in organic transformations
d) and how ligand structures can influence the transformations
We are especially interested in the development of catalytically active iron complexes.
Iron has a number of advantages over other transition metals regularly employed in catalytic
processes: it is cheap, abundant, nontoxic and environmentally friendly.
Ligand syntheses
In order to investigate relationships between ligand structures and the catalytic activity of their respective metal complexes, we selected phosphoramidites (1) and phosphinooxazolines (PHOX, 2, Figure 1) for steric and electronic tuning experiments. We synthesized small libraries of the two ligand classes consisting of both known and unknown derivatives.
Synthesis of sterically and electronically tuned ruthenium phosphoramidite complexes and their catalytic applications
We incorporated the modified phosphoramidite ligands 1 into ruthenium complexes. A series of half sandwich complexes of the general formula [RuCl2(p-cymene)(1)] (3, Figure 2) were synthesized.
We subsequently determined that all complexes are catalytically active in the formation of
beta-oxo esters (5) from aromatic and aliphatic primary, secondary and tertiary propargylic
alcohols (4) and aromatic and aliphatic carboxylic acids (Scheme 1). Most significantly,
the ruthenium phosphoramidite complex containing an N-benzyl ligand (3c) was catalytically much
more active than the complex bearing N-methyl ligands (3a). Thus, it was established that the
ligand structure can have a profound impact on the catalytic activity of its respective
metal complex.
In these experiments, we found that the p-cymene ligand in the ruthenium complexes 3 readily detaches from the metal complex upon heating. We were looking for more robust architectures and were also interested in investigating complexes chiral at the metal. Thus, we synthesized ruthenium complexes of the general formula [RuCl(Cp)PPh3(1)] (Cp = cyclopentadienyl, 6) (Figure 3). Complexes 6b,c were isolated diastereomerically pure and characterized structurally.
The chloride ligand can be removed employing [Et3O]+[PF6]- and the resulting Lewis acidic fragment of the formula [Ru(Cp)PPh3(1)]PF6 is catalytically active in the Mukaiyama aldol reaction (Scheme 2).
Synthesis of the first iron PHOX complexes
To best of our knowledge, iron PHOX chelate complexes have not been reported in the literature and to date have not been applied as catalysts. To perform tuning experiments as described for the phosphoramidite ruthenium complexes, we were interested to determine if iron can form stable PHOX complexes. Thus, we synthesized the first iron PHOX complexes of the general formula [FeCp(CO)(2)]+I- (10) upon heating of the corresponding PHOX ligands with [Fe(Cp)I(CO)2] (Figure 4). The novel iron complexes 10 were characterized structurally, and catalytic investigations are underway.
Synthesis of a series of amino-dithiaphospholane ligands, ruthenium, rhodium and iridium complexes thereof and catalytic applications
Thus far, we have observed steric but no electronic impacts of different phosphoramidite ligands on their catalytic performance (Scheme 1). We were interested to determine whether electronic tuning closer to the phosphorus atom (which coordinates directly to the metal) can influence the catalytic performance of the corresponding metal complexes. Thus, we replaced the two oxygen atoms directly bonded to the phosphorus center in phosphoramidite ligands with sulfur to obtain the new amino-dithiaphospholanes 11a-c (Figure 5). This class of compounds is known, but has to the best of our knowledge never been employed as ligands in transition metal complexes. Accordingly, we successfully employed the amino-dithiaphospho-lanes 11a-c as ligands in the synthesis of complexes of ruthenium (12), rhodium (13), and iridium (14).
The ruthenium complex 12 was shown to be catalytically active in propargylic substitution reactions (etherification of propargylic alcohols 4 with methanol and ethanol, Scheme 3).
