Highly Electrophilic, Catalytically Active and Redox-Responsive Cobaltoceniumyl and Ferrocenyl Triazolylidene Coinage Metal Complexes.

A convenient access to a triad of triazoles with ferrocenyl and cobaltoceniumyl substituents is reported. N-Alkylation, deprotonation and metalation with Cu ^I /Ag ^I /Au ^I synthons affords the heteroleptic triazolylidene complexes. Due to the combination of neutral, electron-donating ferrocenyl substituents and cationic, strongly electron-withdrawing cobaltocenium substituents, the mesoionic carbene (MIC) ligands of these complexes are electronically interesting "push-pull", "pull-push" and "pull-pull" metalloligands with further switchable redox states based on their fully reversible Fe ^II /Fe ^III , (ferrocene/ferrocenium) and Co ^III /Co ^II , (cobaltocenium/cobaltocene) redox couples. These are the first examples of metal complexes of (di)cationic NHC ligands based on cobaltoceniumyl substituents. DFT calculated Tolman electronic parameter (TEP) of the new MIC ligands, show these metalloligands to be extremely electron-poor NHCs with properties unmatched in other carbene chemistry. Utilization of these multimetallic electronically tunable compounds in catalytic oxazoline synthesis and in antitumor studies are presented. Remarkably, 1 mol % of the Au ^I complex with the dicationic MIC ligand displays full catalytic conversion, without the need for any other additives, in less than 2 hours at ambient temperatures. These results thus firmly establish these new classes of cobaltoceniumyl based (di)cationic MIC ligands as prominent players in several branches of chemistry.
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