2003 – Robert H. Crabtree
An Organometallic Carousel: Abnormal Binding of Heterocyclic Ligands
Organometallic catalysis, widely employed in organic and industrial chemistry, relies on metal complexes that contain permanent ligands that endow the catalyst with the desired selectivity properties, such as enantioselectivity in asymmetric catalysis. Phosphines (PR3), the ligands used most widely to date, are extremely effective but give catalysts that are generally air- and heat-sensitive. In recent years, 'carbene' ligands derived from imidazolium salts have given equally impressive catalysts in terms of selectivity and activity, but these now tend to be much more air and thermally stable.
We now show that such carbenes can be formed for Rh, Ir, Pd and Pt by simpler and milder synthetic pathways than currently used. These allow access to chelating carbenes with more complex structures than previously possible. Not only imidazolium, but also triazolium-derived ligands are possible. Abnormal binding at the 'wrong' carbon — never previously seen — can occur for both heterocycles. The ratio of normal to abnormal binding is very sensitive to the counterion, posing interesting mechanistic issues. NMR work allows the ion pair solution structure to be deduced and compared with that predicted from DFT calculations. High catalytic activity is seen in several cases, strongly depending on the structure of the catalyst. Abnormal imidazolium binding may have bioinorganic significance because histidine could in principle bind via carbon as well as the classical binding via N. Abnormal ligand is discussed.