Important challenges in the research on coordination and organometallic chemistry are to understand the relation between the ligand and metal-complex structures and the (catalytic, material) properties at the molecular level. New ligands, with various steric and electronic constraints, are designed and synthesized, which is followed by the synthesis and characterisation of coordination compounds and the study of their properties. Methodologies in research vary from fundamental coordination chemistry (spectroscopy, magnetism, X-ray diffraction), including characterisation and structure determination of the solid compounds, to in situ characterisation of the solutions with UV-Vis, NMR and EPR, under (catalytic) reaction conditions.

The focus of the research in our group concerns both fundamental and applied aspects of coordination chemistry in homogeneous catalysis, biomimetics and inorganic materials. The borders between these sub-topics, however, are not very sharp: in all cases the essence of research is the binding of ligands and/or substrates to metal centers and the study of the tunability, the reactivity or their applications.

The main objectives of the catalytic studies are the development of new, atom-efficient reactions that can replace major existing industrial segments. The catalytic reactions studied in my group vary from selective oxidation, aryl-aryl coupling reactions, dehydrative allylation of phenol, reductive carbonylation of nitroarenes to isocyanates, to cascade-type reactions such as hydroamidomethylation. The main objective of this type of research is to gain fundamental understanding of the reactivity of transition metal complexes and to learn how we can direct the reactivity of the catalysts to novel types of intermediates. The approach is to select some desired novel reactions for which new types of catalytic actions are necessary and to determine how the ligand structures, co-catalysts and solvents may be used to direct and fine-tune the properties and reactivity of the metal center. As an example: in the past four years we have developed new catalytic chemistry to convert e.g. pentene and an amide to N-hexylamide, just by using CO/H₂ gas and a catalyst. This reactivity we can now apply to convert biomass-derived levulinic acid to the nylon precursor caprolactam, forming only water as a byproduct!

The objective of the research in biomimetics is to use the knowledge that we have of living systems and their use of transition metals to develop complexes that can be used e.g. as catalysts for the reduction of protons or carbon dioxide, or for the oxidation of water. Related to homogeneous catalysis, the structural and functional modeling of the active sites of a number of enzymes is thus a special topic of consideration. As Nature is using abundantly available first-row transition metals such as nickel or iron for the activation of dihydrogen, we have explored the use of nickel-based catalyst for homogeneous hydrogenation reactions. Our investigations into models for hydrogenases are now resulting in low-molecular weight transition metal complexes with high activity in the electrocatalytic production of dihydrogen from a weakly acidic proton source. Based upon a series of ligands a structure-activity relationship is emerging. As carbenes – a special type of ligand – have proven to be highly useful in homogeneous catalysis, we are now using carbenes as readily available and stable ligands for hydrogenase models.

Metal coordination compounds are also studied for purposes other than catalysis, most notably for binding of small molecules, or for their specific magnetic or optical properties. In the field of inorganic materials our aim is to develop new materials that can be used for e.g. luminescence or sensing applications.

Lanthanoid complexes have long been known to be luminescent. This type of compounds is of particular interest for LEDs (light-emitting diodes); the line-like emission of the lanthanoid ion can be efficiently excited via the ligand-centered absorption bands. The ligands in turn can be tuned to match the wavelength of the excitation source, for instance by introduction of substituents. In the past few years a number of new lanthanide complexes were synthesized in our group, which show superior luminescent properties as compared to classical oxide materials: they are relatively inexpensive, are easily prepared, and their luminescence color is tunable.

Key publications