N-Heterocyclic Silylene (NHSi) Rhodium and Iridium Complexes: Synthesis, Structure, Reactivity, and Catalytic Ability

Reaction of the zwitterionic N-heterocyclic silylene (NHSi) 1 L'Si: (L' = [HC(CMeNAr)(C(CH2) NAr)], Ar = 2,6-(Pr2C6H3)-Pr-i) with HCl at low temperatures affords the kinetically stable 1,4-addition product of 1, LSiCl (L = [HC(CMeNAr)(2)], Ar = 2,6-(Pr2C6H3)-Pr-i) (9a), which upon reaction with [Rh(Cl)cod](2) and [Ir(Cl)cod](2) (cod = 1,5-cyclooctadiene) selectively affords the NHSi complexes [L(Cl) Si:-> Rh(Cl)cod] (10a) and [L(Cl) Si:-> Ir(Cl)cod] (10b), respectively. The latter were employed as pre-catalysts in the catalytic reduction of amides in the presence of silanes. Remarkably, they show strikingly different activities and selectivities. While complex 10a yields selectively the C-O cleavage product, 10b affords both cleavage products (C-O and C-N). Moreover, the total conversion of the catalytic amide reduction with 10b is significantly higher than the conversion with a benchmark system [Ir(Cl)cod](2) highlighting the enhanced catalytic activity afforded by the coordination of the NHSi ligand. Introducing the hydride source Li[HBEt3] into the catalytic reactions retards the catalyst performance due to a competitive decomposition pathway. This appears to occur via a H-shift onto the cod ligand with concomitant liberation of cyclooctene, which is also presented. The different reactivity of 10a and 10b towards nucleophiles such as MeLi is also discussed. The reaction of 10a with MeLi affords an intractable array of products, while the reaction of 10b with one equivalent of MeLi selectively affords [L(Cl) Si:-> Ir(CH3)cod] (14) with selective methylation at the Ir centre. The analogous reaction with two equivalents of 10b affords the double methylated product [L(CH3) Si:-> Ir(CH3)cod] (15).