The research group is active in the following areas:

 

1. Purposeful synthesis of new organometallic and coordination compounds.

 

The formation, structure and reactivity of new (organo)metallic compounds, especially those containing nitrogen ligands, is studied with the aim to develop new reactivity, particularly bond formation and bond activation induced by the metal(s).

 

2. Homogenous catalysis, ligand design

 

Knowledge of the mechanism of homogeneous catalytic C-C and C-element bond-formation and -breaking reactions is pursued, in order to obtain detailed information which will aid us to steer catalytic reaction sequences for the formation of (fine) chemicals in a rational way.

 

3. NMR of less common (transition metal) nuclei

 

NMR of insensitive spin-1/2 and quadrupolar (transition metal) nuclei is developed and subsequently applied (i) for structure elucidation in organometallic chemistry and (ii) in order to obtain insight into electronic and geometric factors which influence the chemical shift.

 

4. NMR in supercritical media

 

Homogeneous catalysis and NMR in supercritical fluids, aimed at (i) a better understanding of fundamental and practical aspects of catalytic reactions such as hydrogenation reactions in such media and (ii) to render NMR of quadrupolar nuclei with very broad lines accessible.

 

 

Projects 1 and 2. An essential feature in this research is the design of new ligands with specific electronic and steric properties which influence the properties of the metal and the geometry of the compound in such a way, that specific reactivity is introduced. Examples are hydrogenation, C-C bond formation involving e.g. coupling of CO and alkenes, and of other organic substrates. Emphasis is on the study of separate intimate steps by using suitable model compounds. The insights obtained are used to modify the ligand systems employed, in order to fine-tune the reactivity in the desired direction.In these projects we have studied C-C and C-H, C-N and C-X bond formation and bond breaking reactions by using organo-palladium and -ruthenium compounds as the catalysts. We have in this period studied new palladium-catalyzed copolymerization processes, which attract worldwide attention. Particularly our (rigid) a-diimine ligands now enjoy worldwide interest, since their Pd and Ni complexes [(N-N)MMe]X (M=Ni,Pd) can be used not only for efficient catalytic polyketone formation, but also for highly branched polyolefins. In this field, we have been very successful in selective catalytic C-C bond formation between small molecules, e.g. between CO and alkenes or CO and allenes [1]. Other developments include the elaboration of our C-C coupling reactions catalyzed by palladium complexes containing N-ligands instead of phosphines. For instance, catalytic coupling between three components (alkynes, organic halide and organotin reagent) to give conjugated dienes was achieved using Pd(NN) compounds, a reaction which is not catalyzed by Pd(phosphine) complexes [2,3]. In an IOP-catalysis project, we have been successful in the hydrogenation of substrates which are very difficult to hydrogenate, such as esters [4]. These recent seminal results have been obtained on the basis of stoichiometric modelling of C-C and C-H bond formation processes and identification of intermediates. We have observed that nitrogen ligands are in many cases preferred over phosphine ligands in catalytic C-C bond formation reactions. For instance, the three-component catalytic coupling alluded to above has been achieved by using Pd(NN) compounds [2] The reaction is not catalyzed by Pd(phosphine) complexes. Recently, a project involving C-C bond-forming and -breaking reactions by Pd and Pt compounds of new tridentate C,N,N-ligands (bisimino-aryl ligands) has started. The preliminary results are promising.

 

Projects 3 and 4. Apart from the routine NMR interwoven with all projects, we carry out NMR of transition metals, which is very relevant to the studies in projects A and B. The group has consolidated a good internationally recognised position in transition metal NMR, particularly of low-gamma nuclei. A number of projects on Rhodium NMR applied to catalysis have been carried out (J.M. Ernsting) in cooperation with groups of dr. Bianchini (Firenze, Italy) and prof. Milstein (Weizmann Institute, Israel). Systematic studies involving Rh-phosphine complexes have been carried out under the aegis of the "SON Young Chemists" program (dr. H. Donkervoort). In a cooperation with dr. Bühl (Zürich) we have succesfully aimed at further systemization by very accurate calculations (DFT methods) of Rh chemical shifts. We have succesfully implemented methods for detection of insensitive transition metal nuclei and ¹⁵N via gradient-selected inverse spectroscopy. The ¹⁰³Rh NMR of rhodium compounds used for catalytic activation of C-F bonds was elaborated upon with prof. Milstein[5].Currently we have established a working group within the EC COST D10 action "Innovative methods and techniques for chemical transformations", particularly in the section for supercritical fluids, where we wish to apply these techniques and other high pressure NMR techniques together with our synthetic skills for the elucidation of mechanisms of transition metal catalysed hydrogenation reactions. Cooperation with prof. J. Bargon and dr. K. Woelk (Uni Bonn), dr. W. Leitner (MPI Mülheim), prof. B. Heaton (Uni Liverpool), dr. A. Dedieu (Uni Strasbourg) and dr. M. George (Uni Nottingham).S. Gaemers has made the NMR of quadrupolar nuclei with very broad lines, e.g., ¹⁴N, ⁵³Cr, ⁹⁹Ru, better accessible by using supercritical fluids as the solvents [6]. Linewidths have been reduced by a factor of 3-5, thus enabling semi-routine recording of NMR of such nuclei and hence facilitate new applications. A Fulbright fellowship has been awarded to S. Gaemers in 1998, which has resulted in a very successful joint project with prof. A. Bax (NIH Bethesda, USA) aiming at the applications of our methodology to ¹H and ¹⁴N NMR of biomolecules in supercritical media [7].

 

Key Publications:

 

1. R. van Asselt, E.E.C.G. Gielens, R. E. Rülke, K. Vrieze, C.J. Elsevier, J. Am. Chem. Soc., 1994, 116, 977. K. Vrieze, J.H. Groen, J.G.P. Delis, C.J. Elsevier, P.W.N.M. van Leeuwen, New J. Chem., 1997, 21, 807.

 

2. R. van Belzen, H.Hoffmann, C.J. Elsevier, Angew. Chemie, Int. Ed. Engl., 1997, 6, 1743.

 

3. R. van Belzen, R.A. Klein, H. Kooijman, N. Veldman, A.L. Spek, C.J. Elsevier, Organometallics, 1998, 17, 1812.

 

4. H.T. Teunissen, C.J. Elsevier, J.C.S. Chem.Commun., 1997, 667. H.T. Teunissen, C.J. Elsevier, J.C.S. Chem.Commun.., 1998, 1367.

 

5. M. Aizenberg, J. Ott, C.J. Elsevier, D. Milstein, J.Organomet.Chem. 1998, 551, 81.

 

6. S. Gaemers, C.J. Elsevier, Magn. Reson. Chem., 1999, 36, 25.

 

7. S. Gaemers, C.J. Elsevier, A. Bax, Chem. Phys. Lett., in press.