Hydrogenation of Imines Catalyzed by Trisphosphine‐Substituted Molybdenum and Tungsten Nitrosyl Hydrides and Co‐Catalytic Acid

Hydride complexes Mo,W(CO)(NO)H(mer‐etp i p) ( i Pr 2 PCH 2 CH 2 ) 2 PPh=etp i p) ( 2 a,b( syn ) , syn and anti of NO and Ph(etp i p) orientions) were prepared and probed in imine hydrogenations together with co‐catalytic [H(Et 2 O) 2 ][B(C 6 F 5 ) 4 ] (140 °C, 60 bar H 2 ). 2 a,b( syn ) were obtained via reduction of syn / anti ‐Mo,W(NO)Cl 3 (mer‐etp i p) and syn , anti ‐Mo,W(NO)(CO)Cl(mer‐etp i p). [H(Et 2 O) 2 ][B(C 6 F 5 ) 4 ] in THF converted the hydrides into THF complexes syn ‐[Mo,W(NO)(CO)(etp i p)(THF)][B(C 6 F 5 ) 4 ]. Combinations of the p ‐substituents of aryl imines p ‐R 1 C 6 H 4 CH=N‐ p ‐C 6 H 4 R 2 (R 1 ,R 2 =H,F,Cl,OMe,α‐Np) were hydrogenated to amines (maximum initial TOFs of 1960 h −1 ( 2 a( syn ) ) and 740 h −1 ( 2 b( syn ) ) for N‐ (4‐ m ethoxybenzylidene)aniline). An ‘ionic hydrogenation’ mechanism based on linear Hammett plots ( ρ= −10.5, p ‐substitution on the C‐side and ρ =0.86, p‐ substitution on the N‐side), iminium intermediates, linear P (H 2 ) dependence, and DKIE=1.38 is proposed. Heterolytic splitting of H 2 followed by ‘proton before hydride’ transfers are the steps in the ionic mechanism where H 2 ligand addition is rate limiting.