Iridium(II) Cp* Complexes for the Efficient Hydroamination of Internal Alkynes

A series of iridium(III) and rhodium(III) 1,2,3,4,5-pentamethylcyclopentadienyl (Cp*) complexes of the type [M(Cp*)(LL)Cl]BPh4 were synthesized, where LL is a chelating diimine or mixed pyrazolyl-phosphine or pyrazolyl-N-heterocyclic carbene donor ligand. The majority of these complexes were characterized by X-ray crystallography, allowing for a detailed comparison of their structural properties. These complexes were shown to yield active hydroamination catalysts upon in situ abstraction of the chloride coligand using AgBF4. The activity of these catalysts for the intramolecular hydroamination of alkyne-amines to yield indolyl and pyrolyl heterocycles was investigated. It was found that chelating ligand groups containing a strongly coordinating N-heterocyclic carbene donor led to the most effective catalysis and that an increase in the steric bulk of the ligand was detrimental to the catalyst efficiency.