Iridium(III) 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.