Selective PNP Pincer-Ir Promoted Acceptorless Transformation of Glycerol to Lactic Acid and Hydrogen

The catalytic transformation of glycerol (GLY) using [( iPr2 PN HP)Ir(COD)]Cl [ iPr2 PN HP = κ 3-( i Pr 2PCH 2CH 2) 2NH] affords hydrogen and lactic acid (LA), trapped as its sodium salt (Na[LA]) with high yield (96%) and selectivity (99%) in the presence of an equivalent of in situ generated NaOEt at 140 °C within 4 h. A diminution in activity was observed when the PN MeP ligand was used instead of PN HP, or when Cl - was replaced by [BAr F 4] -. An Ir to Rh substitution also resulted in poor activity. Kinetic studies show a first-order dependence of the initial rate of turnovers on the concentrations of [( iPr2 PN HP)Ir(COD)]Cl, NaOEt, and glycerol. An outer-sphere mechanism does not explain the activity of [( iPr2 PN MeP)Ir(COD)]Cl, and DFT studies support an inner-sphere mechanism, with oxidative addition of glycerol to the 14-electron intermediate [( iPr2 PN HP)Ir]Cl determined as the rate-determining step (RDS). A k H/ k D of 2.7 obtained with glycerol- d 8 shows a major contribution from O-H activation in the RDS. The kinetics of the reaction become favorable (Δ G 140 ⧧ = 27.01 kcal/mol) when one of the terminal O-H's of glycerol is hydrogen bonded to the N-H of the pincer backbone, in contrast to cases where no hydrogen bonds are invoked (Δ G 140 ⧧ = 31.96 kcal/mol) or are not possible [( iPr2 PN MeP)Ir]Cl (Δ G 140 ⧧ = 30.36 kcal/mol).