Cationic Iridium and Rhodium Complexes with C–N Chelating Primary Benzylic Amine Ligands as Potent Catalysts for Hydrogenation of Unsaturated Carbon–Nitrogen Bonds

Cationic
half-sandwich C–N chelating Ir and Rh complexes [Cp*M­(NCCH3)­{κ2(N,C)-NH2CR2-2-C6H4}]+SbF6– (1a, M = Ir, R = CH3; 1b, M = Ir, R = C6H5; 2a, M = Rh, R = CH3; 2b, M = Rh, R
= C6H5) are synthesized by AgSbF6-mediated halide abstraction from neutral azametallacycles derived
from tritylamine or cumylamine and are fully characterized by NMR
spectroscopy and X-ray crystallography. The treatment of the cationic
complex 1b with H2 gas under ambient conditions
in the presence of triethylamine in THF-d8 quantitatively yielded hydrido­(amine) complex [Cp*Ir­(H)­{κ2(N,C)-NH2C­(C6H5)2-2-C6H4}] (4). The C–N chelating Ir complex shows a higher catalytic activity
than an N–N chelating complex, [Cp*Ir­(NCCH3)­(Tscydn)]+SbF6– (3; Tscydn
= N-(p-toluenesulfonyl)-1,2-cyclohexanediamime),
that has been previously used for the asymmetric hydrogenation of
acyclic imines. For example, the cationic Ir complex 1a promoted the hydrogenation of N-(1-phenylethylidene)­benzylamine
under 30 atm at 30 °C in the presence of excess AgSbF6 to produce the corresponding amine in 97% yield within 2 h. The
cationic Rh complexes 2 serve as efficient catalysts
for hydrogenative condensation of nitriles in the presence of AgSbF6, producing dibenzylamines selectively even at 60 °C.