Pyrazolin-4-ylidenes: a new class of intriguing ligands.

The immense success of N-heterocyclic carbenes in recent years has initiated the search for even stronger ligands leading to the discovery of abnormal and remote NHCs. This article reflects our particular interest in the coordination chemistry of pyrazolin-4-ylidenes as a contribution to this field. A modular approach to 4-iodopyrazolium salts with different substitution patterns is described, which upon oxidative addition to Pd(0) gave rise to a library of new Pd(ii) pyrazolin-4-ylidene complexes. A preliminary study showed that selected complexes are active precatalysts in Suzuki-Miyaura and Mizoroki-Heck coupling reactions. The 3,5-substituents of pyrazolin-4-ylidenes are found to have significant effects on complexation and catalytic activities. Finally, the nature of this new class of ligands is discussed, and a future direction for further explorations in this exciting field is envisioned.