Iridacycles as Catalysts for the Autotandem Conversion of Nitriles into Amines by Hydrosilylation: Experimental Investigation and Scope

The set of iridacycles [{C,N}­Cp*IrIII-Cl] ({C,N} = benzo­[h]­quinoline, dibenzo­[f,h]­quinoline) containing the (pentamethylcyclopentadienyl)­iridium­(III) unit were synthesized and derivatized into cations [{C,N}­Cp*Ir-NCMe]+ associated with BArF-type anions. The latter salts were benchmarked for their potential catalytic properties toward HSiEt3 in a H2-releasing test reaction. The best-performing BArF-type salts demonstrated the capability to promote with a low catalytic load of ca. 0.5–1 mol % the autotandem hydrosilylation of acetonitrile, propionitrile, and a series of arylnitrile substrates. Mechanistic investigations confirmed the preliminary formation of a silane–iridacycle adduct by electrophilic and heterolytic activation of the Si–H bond. The molecular structure of a new example of such an adduct was resolved by X-ray diffraction analysis. Theoretical considerations support a donor–acceptor [{C,N}­Cp*IrIII-H]→[SiEt3]+ ({C,N} = benzo­[h]­quinolinyl) formulation where the cationic silyl moiety acting as a Z ligand binds both Ir and H centers. Under the conditions of the catalysis, the latter adduct is assumed to transfer readily the electrophilic [SiEt3]+ moiety to the nucleophilic nitrile substrate to form a N-silylnitrilium cation and the neutral [{C,N}­Cp*Ir-H]. The latter reduces the N-silylnitrilium into the corresponding N-silylimine, which undergoes further N-silylation and reduction to yield the final N,N-disilylamine. Under optimal conditions of low catalyst load (70 °C, 0.5 mol %) the autotandem hydrosilylation of arylnitriles produces the silylated amines in yields >80% in 24 h.