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

The catalytic transformation of glycerol (GLY) using [(PN^HP)Ir(COD)]Cl [PN^HP = κ³-(Pr₂PCH₂CH₂)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^F₄]^-. 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 [(PN^HP)Ir(COD)]Cl, NaOEt, and glycerol. An outer-sphere mechanism does not explain the activity of [(PN^MeP)Ir(COD)]Cl, and DFT studies support an inner-sphere mechanism, with oxidative addition of glycerol to the 14-electron intermediate [(PN^HP)Ir]Cl determined as the rate-determining step (RDS). A k_H/k_D of 2.7 obtained with glycerol-d₈ shows a major contribution from O-H activation in the RDS. The kinetics of the reaction become favorable (ΔG₁₄₀^⧧ = 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 (ΔG₁₄₀^⧧ = 31.96 kcal/mol) or are not possible [(PN^MeP)Ir]Cl (ΔG₁₄₀^⧧ = 30.36 kcal/mol).