Iridium(I) Complexes with Anionic N‑Heterocyclic Carbene Ligands as Catalysts for the Hydrogenation of Alkenes in Nonpolar Media

A series
of lithium complexes of anionic N-heterocyclic carbenes
that contain a weakly coordinating borate moiety (WCA-NHC) was prepared
in one step from free N-heterocyclic carbenes by deprotonation with n-butyl lithium followed by borane addition. The reaction
of the resulting lithium-carbene adducts with [M­(COD)­Cl]2 (M = Rh, Ir; COD = 1,5-cyclooctadiene) afforded zwitterionic rhodium­(I)
and iridium­(I) complexes of the type [(WCA-NHC)­M­(COD)], in which the
metal atoms exhibit an intramolecular interaction with the N-aryl
groups of the carbene ligands. For M = Rh, the neutral complex [(WCA-NHC)­Rh­(CO)2] and the ate complex (NEt4)­[(WCA-NHC)­Rh­(CO)2Cl] were prepared, with the latter allowing an assessment
of the donor ability of the ligand by IR spectroscopy. The zwitterionic
iridium–COD complexes were tested as catalysts for the homogeneous
hydrogenation of alkenes, which can be performed in the presence of
nonpolar solvents or in the neat alkene substrate. Thereby, the most
active complex showed excellent stability and activity in hydrogenation
of alkenes at low catalyst loadings (down to 10 ppm).