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

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.