Mechanistic investigations of imine hydrogenation catalyzed by dinuclear iridium complexes

Abstract

Treatment of [Ir2(-H)(-Pz)2H3(NCMe)(PiPr3)2] (1) with one equivalent of HBF4 or [PhNHCHPh]BF4 affords efficient catalysts for the homogeneous hydrogenation of N-benzylideneaniline. The reaction of 1 with HBF4 leads to the trihydride-dihydrogen complex [Ir2(-H)(-Pz)2H2(2-H2)(NCMe)(PiPr3)2]BF4 (2), which has been characterized by NMR spectroscopy and DFT calculations on a model complex. Complex 2 reacts with imines such as tBuNCHPh or PhNCHPh to afford amine complexes [Ir2(-H)(-Pz)2H2(NCMe){L}(PiPr3)2]BF4 (L = NH(tBu)CH2Ph, 3; NH(Ph)CH2Ph, 4) through a sequence of proton- and hydride-transfer steps. Dihydrogen partially displaces the amine ligand of 4 to form 2; this complements a possible catalytic cycle for the N-benzylideneaniline hydrogenation in which the amine-by-dihydrogen substitution is the turnover-determining step. The rates of ligand substitution in 4 and its analogues with labile ligands other than amine are dependent upon the nature of the leaving ligand and independent on the incoming ligand concentration, in agreement with dissociative substitutions. Water complex [Ir2(-H)(-Pz)2H2(NCMe)(OH2)(PiPr3)2]BF4 (7) hydrolyzes N-benzylideneaniline, which eventually affords the poor hydrogenation catalyst [Ir2(-H)(-Pz)2H2(NCMe)(NH2Ph)(PiPr3)2]BF4 (11). The rate law for the catalytic hydrogenation in 1,2-dichloroethane with complex [Ir2(-H)(-Pz)2H2(OSO2CF3)(NCMe)(PiPr3)2] (8) as catalyst precursor is rate = k[8]{p(H2)}; this is in agreement with the catalytic cycle deduced from the stochiometric experiments. The hydrogenation reaction takes place at a single iridium center of the dinuclear catalyst, although ligand modifications at the neighboring iridium center provoke changes in the hydrogenation rate. Even though this catalyst system is also capable of effectively hydrogenating alkenes, N-benzylideneaniline can be selectively hydrogenated in the presence of simple alkenes.