The Power of Ferrocene, Mesoionic Carbenes, and Gold: Redox-Switchable Catalysis

Catalysis with gold(I) complexes is a useful route for synthesizing a variety of important heterocycles. Often, silver(I) additives are necessary to increase the Lewis acidity at the gold(I) center and to make them catalytically active. We present here a concept in redox-switchable gold(I) catalysis that is based on the use of redox-active mesoionic carbenes, and of electron transfer steps for increasing the Lewis acidity at the gold(I) center. A gold(I) complex with a mesoionic carbene containing a ferrocenyl backbone is presented. Investigations on the corresponding iridium(I) CO complex show that the donor properties of such carbenes can be tuned via electron transfer steps to make these seemingly electron rich mesoionic carbenes relatively electron poor. A combined crystallographic, electrochemical, UV-vis-near-IR/IR spectroelectrochemical investigation together with DFT calculations is used to decipher the geometric and the electronic structures of these complexes in their various redox states. The gold(I) mesoionic carbene complexes can be used as redox-switchable catalysts, and we have used this concept for the synthesis of important heterocycles: oxazoline, furan and phenol. Our approach shows that a simple electron transfer step, without the need of any silver additives, can be used as a trigger in gold catalysis. This report is thus the first instance where redox-switchable (as opposed to only redox-induced) catalysis has been observed with gold(I) complexes.