title: Iridicycle-Catalysed Imine Reduction: An Experimental and Computational Study of the Mechanism.
abstract: The mechanism of imine reduction by formic acid with a single-site iridicycle catalyst has been investigated by density functional theory (DFT), NMR spectroscopy, and kinetic measurements. The NMR and kinetic studies suggest that the transfer hydrogenation is turnover-limited by the hydride formation step. The calculations reveal that, amongst a number of possibilities, hydride formation from the iridicycle and formate probably proceeds by an ion-pair mechanism, whereas the hydride transfer to the imino bond occurs in an outer-sphere manner. In the gas phase, in the most favourable pathway, the activation energies in the hydride formation and transfer steps are 26-28 and 7-8 kcal mol(-1) , respectively. Introducing one explicit methanol molecule into the modelling alters the energy barrier significantly, reducing the energies to around 18 and 2 kcal mol(-1) for the two steps, respectively. The DFT investigation further shows that methanol participates in the transition state of the turnover-limiting hydride formation step by hydrogen-bonding to the formate anion and thereby stabilising the ion pair.
authors.affiliation: Kathleen Lonsdale Materials Chemistry, Department of Chemistry, University College London, 20 Gordon Street, London, WC1H 0AJ (UK).|Current address: Quantum Chemistry Laboratory, Department of Materials Science, University of Milan-Bicocca (Italy).
authors.forename: Hsin-Yi Tiffany
authors.initials: HY
authors.lastname: Chen
authors.affiliation: Liverpool Centre for Materials and Catalysis, Department of Chemistry, University of Liverpool, Liverpool, L69 7ZD (UK).|Key Laboratory of Applied Surface and Colloid Chemistry of Ministry of Education, School of Chemistry and Chemical Engineering, Shaanxi Normal University, Xi'an, 710062 ( P. R. China).
authors.forename: Chao
authors.initials: C
authors.lastname: Wang
authors.affiliation: Liverpool Centre for Materials and Catalysis, Department of Chemistry, University of Liverpool, Liverpool, L69 7ZD (UK).
authors.forename: Xiaofeng
authors.initials: X
authors.lastname: Wu
authors.affiliation: Key Laboratory of Applied Surface and Colloid Chemistry of Ministry of Education, School of Chemistry and Chemical Engineering, Shaanxi Normal University, Xi'an, 710062 ( P. R. China).
authors.forename: Xue
authors.initials: X
authors.lastname: Jiang
authors.affiliation: Kathleen Lonsdale Materials Chemistry, Department of Chemistry, University College London, 20 Gordon Street, London, WC1H 0AJ (UK). c.r.a.catlow@ucl.ac.uk.|Current address: Quantum Chemistry Laboratory, Department of Materials Science, University of Milan-Bicocca (Italy). c.r.a.catlow@ucl.ac.uk.
authors.forename: C Richard A
authors.initials: CR
authors.lastname: Catlow
authors.affiliation: Liverpool Centre for Materials and Catalysis, Department of Chemistry, University of Liverpool, Liverpool, L69 7ZD (UK). jxiao@liv.ac.uk.
authors.forename: Jianliang
authors.initials: J
authors.lastname: Xiao
keywords: density functional calculations; homogeneous catalysis; imines; iridium; transfer hydrogenation