Ferrocenyl-Imidazolylidene Ligand for Redox-Switchable Gold-Based Catalysis. A Detailed Study on the Redox-Switching Abilities of the Ligand

Abstract

An imidazolium salt with a fused benzoferrocenyl was synthesized. This compound was used as an N-heterocyclic carbene (NHC) precursor, and the related ferrocenyl-imidazolylidene complexes Fc-NHC-MLn (MLn = IrCl(COD), IrCl(CO)2, AuCl) were synthesized and fully characterized, including the crystallographic characterization of some of the key structures. The oxidation of the iridium carbonyl compound Fc-NHC-IrCl(CO)2 with acetylferrocenium tetrafluoroborate afforded the oxidized ferrocenium-NHC-IrCl(CO)2 (Fe(III)) species; however the isolated product contained a byproduct resulting from the protonation of the starting Fe(II) complex. The analysis of the electron-donating character of the neutral ligand and the ligand resulting from the oxidation was carried out by studying the variation of the ν(CO) stretching frequencies of Fc-NHC-IrCl(CO)2 and its oxidized analogue, revealing that this shift is 2.9 cm–1, which is consistent with a decrease of the electron-donating character of the ligand produced by the generation of a positively charged metal complex. DFT calculations were carried out in order to rationalize these results. The effects of the oxidation of the ligand in homogeneous catalysis were tested by using a related ferrocenyl-imidazolylidene-gold(I) complex. In the hydroamination of terminal alkynes, the results indicated that the oxidation of the ligand produced a moderate increase of the activity of the gold catalyst. In the cyclization of alkynes with furans, the neutral complex was not active, while the product resulting from its oxidation produced moderate to good yields in the formation of the final products.