Our research theme is broadly defined by small-molecule activation and its application in (new) catalytic reactions.
We are currently working on metal complexes with non-innocent ligands. These ligands actively participate in (catalytic) reactions of their complexes, either by making/breaking bonds, or by functioning as electron reservior. The judicious design of such ligands will allow us to carry out unusual (redox) transformations with catalysts based on earth-abundant elements (first-row transition metals and/or main group elements).
Redox-active formazanate ligands
We are exploring the use of formazanate ligands in coordination chemistry and catalysis. These ligands are nitrogen-rich analogues of the well-known ß-diketiminates. Despite the fact that the parent formazans have been known for a long time, their use as ligands for the synthesis of metal complexes has not been explored in any detail. Our group has begun to uncover the similarities and differences between ß-diketiminate and formazanate ligands. We have recently shown that bis(formazanate) zinc complexes have very similar structural features as the ß-diketiminate congeners, but provide access to unusual redox-chemistry by using the ligands as electron-reservoir. The formazanate ligands in these L2Zn complexes can be sequentially and reversibly reduced to give the corresponding radical anions L2Zn- and diradical dianions L2Zn2-, which have a singlet diradical ground state. We are currently working on the synthesis of other (transition) metal complexes, with the aim to make use of the reversible redox-chemistry of the ligand framework in catalysis.
Also main group (boron) compounds with formazanate ligands participate in ligand-based redox reactions. These mono(formazanate)boron difluoride complexes are studied for their luminescent properties, which may be switched on/off as a function of ligand oxidation state.
Metal-ligand cooperativity in nitrile activation: an efficient pathway for catalytic oxa-Michael addition to unsaturated nitriles
We study the reactivity of tridentate PNN pincer complexes based on Ru. These compounds, developed by Milstein and co-workers, show a wide variety of catalytic reactions that are based on aromatization/dearomatization of the pyridine fragment of the pincer backbone. We have shown that addition of nitriles to the dearomatized complex leads to formation of an addition product, in which the nitrile CN moiety forms bonds with the metal center (Ru-N) as well as the ligand (C-C). This unusual bond activation occurs with re-aromatization of the pyridine, which provides a driving force for the reaction. The isolated Ru-dieneamido product that is formed upon reaction with alkene-nitriles is shown to tautomerize to a Ru-ketimido intermediate. This Ru-N fragment (derived from the nitrile) is sufficiently basic to activate alcohols towards nucleophic attack at the C=C bond of the α‚β-unsaturated motif. The resulting oxa-Michael addition product is obtained in excellent yield/selectivity under mild reaction conditions, due to this unusal nitrile activation mechanism.
|Laatst gewijzigd:||28 juli 2017 11:16|