Lay summary
Homogeneous catalysis has been greatly stimulated by the recent discovery of N-heterocyclic carbenes as ligands for transition metals. These ligands display particularly high sigma donor strength, which provides electron density at the metal center for the activation of less reactive bonds. Upon increasing this electron density further, it should become possible to activate also bonds that are generally considered to be unreactive, such as H-H, C-H and perhaps even C-C bonds.
Our approach towards this goal comprises the modification of N-heterocyclic carbene binding. The large majority of carbene complexes that have been used for catalytic applications thus far have been derived from imidazolium salts that bind via C(2) to the metal center. Recently, it has been shown that the ligand donor strength is enhanced when the heteroatom stabilization is reduced, for example by displacing one of the alpha-positioned heteroatoms to a beta-site. While synthetic methodologies have emerged recently that allow to form transition metal complexes with some of these unusual, non-classical carbenes, not many applications have been developed up to now. The project proposed here aims at exploiting the catalytic potential of platinum group transition metals that are bound to such non-classical carbenes. Based on our previous success in using both pyridylidene and C(4)-bound imidazolylidene ligands, we propose to apply these complexes in catalytic transformations for which an electron-rich metal center is crucial. Such reactions will be strongly promoted by non-classical carbenes as these new ligands appear to be exceptionally basic. In particular, catalytic cycles in which oxidative addition is rate-determining will be considerably accelerated. Consequently, we propose these complexes as catalysts for aerobic oxidation of alcohols (rate-determining addition of O2 to the low-valent metal center), olefin hydrogenation (often limited by slow H2 addition to the catalytically active species), and alkane dehydrogenation (requiring highly basic metals for C-H activation). The latter type of reaction is of particular relevance, as it allows for selective functionalization of otherwise unreactive alkanes via homogeneous catalysis, one of the major unsolved problems in organic chemistry. Ultimately, the development of promising catalytic systems for the activation of alkanes, in particular the oxidation of methane to methanol will allow us to exploit the world’s alkane resources (the major component of natural gas) more efficiently and cleanly.