Structure, reactivity and catalytic properties of manganese-hydride amidate complexes

The high efficiency of widely applied Noyori-type hydrogenation catalysts arises from the N–H moiety coordinated to a metal centre, which stabilizes rate-determining transition states through hydrogen-bonding interactions. It was proposed that a higher efficiency could be achieved by substituting an N–M′ group (M′ = alkali metals) for the N–H moiety using a large excess of metal alkoxides (M′OR); however, such a metal-hydride amidate intermediate has not yet been isolated. Here we present the synthesis, isolation and reactivity of a metal-hydride amidate complex (HMn–NLi). Kinetic studies show that the rate of hydride transfer from HMn–NLi to a ketone is 24-fold higher than that of the corresponding amino metal-hydride complex (HMn–NH). Moreover, the hydrogenation of N-alkyl-substituted aldimines was realized using HMn–NLi as the active catalyst, whereas HMn–NH is much less effective. These results highlight the superiority of M/NM′ bifunctional catalysis over the classic M/NH bifunctional catalysis for hydrogenation reactions.