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

The catalytic transformation of glycerol (GLY) using [(^<i>i</i>Pr2PN^HP)Ir(COD)]Cl [^<i>i</i>Pr2PN^HP = κ³-(^<i>i</i>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 [(^<i>i</i>Pr2PN^HP)Ir(COD)]Cl, NaOEt, and glycerol. An outer-sphere mechanism does not explain the activity of [(^<i>i</i>Pr2PN^MeP)Ir(COD)]Cl, and DFT studies support an inner-sphere mechanism, with oxidative addition of glycerol to the 14-electron intermediate [(^<i>i</i>Pr2PN^HP)Ir]Cl determined as the rate-determining step (RDS). A <i>k</i>_H/<i>k</i>_D of 2.7 obtained with glycerol-<i>d</i>₈ shows a major contribution from O-H activation in the RDS. The kinetics of the reaction become favorable (Δ<i>G</i>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<i>G</i>₁₄₀^⧧ = 31.96 kcal/mol) or are not possible [(^<i>i</i>Pr2PN^MeP)Ir]Cl (Δ<i>G</i>₁₄₀^⧧ = 30.36 kcal/mol).