Selective, Base-Free Hydrogenation of Aldehydes Catalyzed by Ir Complexes Based on Proton-Responsive Lutidine-Derived CNP Ligands

Metal catalysts based on ligands
containing proton-responsive sites
have found widespread applications in the hydrogenation of polar unsaturated
substrates. In this contribution, Ir complexes incorporating lutidine-derived
CNP (C = N-heterocyclic carbene, NHC; P = phosphine) pincer ligands
with two nonequivalent Brønsted acid/base sites have been examined
in the hydrogenation of aldehydes. To this end, Ir­(CNP)­H2Cl complexes were synthesized in two steps from the CNP ligand precursors
and Ir­(acac)­(COD). These derivatives react with an excess of NaH to
yield the trihydride derivatives Ir­(CNP)­H3, which were
assessed as catalyst precursors in the hydrogenation of a series of
aldehydes. The catalytic reactions were performed using commercial-grade
substrates under neutral, mild conditions (0.1 mol % Ir-CNP; 4 bar
H2, room temperature) with high conversions and selectivities
for the reduction of the carbonyl function in the presence of other
readily reducible groups such as CC, nitro, and halogens.
Reaction of an Ir­(CNP)­H2Cl complex with base in the presence
of an aromatic aldehyde produces the reversible formation of alkoxide
Ir complexes in which the aldehyde is bound to the deprotonated pincer
framework (CNP*) through the CH-NHC arm of the ligand. These species,
along with a carboxylate complex resulting from the Ir mediated oxidation
of the aldehyde by water, is observed in the reaction of Ir­(CNP)­H3 with benzaldehyde. Finally, investigation of the mechanism
of the hydrogenation of aldehydes has been carried out by means of
DFT calculations considering the involvement of each arm of the Ir-CNP/CNP*
derivatives. Calculations support a mechanism in which the catalyst
switches its metal–ligand cooperation sites to follow the lowest
energy pathway for each step of the catalytic cycle.