Mechanism-Driven Optimization of Proton Transfer in Mn-Catalyzed Amine-Boranes Dehydrogenation: The Way to Turbo Mode

Improving the efficiency of the individual steps in catalytic cycles is one of the main cornerstones of homogeneous catalysis. In particular, optimization of proton transfer processes by the incorporation of nitrogen donor moieties in the second coordination sphere of transition metal constitutes a privileged strategy to boost the rate and modulate the selectivity of electrocatalysts for small molecules activation. However, proton-relay strategies remain largely unexplored in homogeneous (de)hydrogenation, particularly through the use of exogenous nitrogen bases. We report herein that the rational choice of amine additive as proton transfer mediator can largely improve efficient Mn-based catalytic system for amine-borane dehydrogenation working through the cooperation between Mn + and MnH species. Addition of 1.0-5.0 mol% of simple tertiary amine such as Et 3 N led to 7-45-fold rate acceleration and the increase of TON values by 65-420% for all substrates. This upgrade is due to a combination of lower activation barrier for the rate-determining deprotonation of Mn-coordinated amine-boranes and a higher amount of catalytically active cationic metal species Mn+ present in solution under these conditions. In addition, meticulous mechanistic investigations revealed the crucial role of ammonium salt concentration in this catalytic process thus paving the way for further improvements. Accordingly, the use of a binary Et 3 N/Et 3 NH + mixture instead of pure Et 3 N again speeds up Mn-catalyzed Me 2 NHBH 3 dehydrogenation by a factor of ten to achieve unprecedented TOF of 125 s −1 and TON of 105 at 60°C. Beyond achieving the best performance reported to date for this substrate, our results highlight how the judicious optimization of a priori trivial elementary steps in the catalytic cycle, without altering the intrinsic reactivity of the metal center, can dramatically enhance the activity of an already highly efficient transition metal catalyst.