Trisphosphine‐Chelate‐Substituted Molybdenum and Tungsten Nitrosyl Hydrides as Highly Active Catalysts for Olefin Hydrogenations

Reaction of M(NO)Cl₃(NCMe)₂] (M=Mo, W) with (iPr₂PCH₂CH₂)₂PPh (etpⁱp) at room temperature afforded the syn/anti‐M(NO)Cl₃(mer‐etpⁱp)] complexes (M=Mo, a ; W, b ; 3 a,b (syn,anti); syn and anti refer to the relative position of Ph(etpⁱp) and NO). Reduction of 3 a,b (syn,anti) produced M(NO)Cl₂(mer‐etpⁱp)] ( 4 a,b (syn)), M(NO)Cl(NCMe)(mer‐etpⁱp)] ( 5 a,b (syn,anti)), and M(NO)Cl(η²‐ethylene)(mer‐etpⁱp)] ( 6 a,b (syn,anti)) complexes. The hydrides M(NO)H(η²‐ethylene)(mer‐etpⁱp)] ( 7 a,b (syn,anti)) were obtained from 6 a,b (syn,anti) using NaHBEt₃ (75 °C, THF) or LiBH₄ (80 °C, Et₃N), respectively. 7 a,b (syn,anti) were probed in olefin hydrogenations in the absence or presence of a hydrosilane/B(C₆F₅)₃ mixture. The 7 a,b (syn,anti)/Et₃SiH/B(C₆F₅)₃ co‐catalytic systems were highly active in various olefin hydrogenations (60 bar H₂, 140 °C), with maximum TOFs of 5250 h^?1 ( 7 a (syn,anti)) and 8200 h^?1 ( 7 b (syn,anti)) for 1‐hexene hydrogenation. The Et₃SiH/(B(C₆F₅)₃ co‐catalyst is anticipated to generate a Et₃Si]⁺ cation attaching to the O_NO atom. This facilitates NO bending and accelerates catalysis by providing a vacant site. Inverse DKIE effects were observed for the 7 a (syn,anti)/Et₃SiH/(B(C₆F₅)₃ (k_H/k_D=0.55) and the 7 b (syn,anti)/Et₃SiH/(B(C₆F₅)₃ (k_H/k_D=0.65) co‐catalytic mixtures (20 bar H₂/D₂, 140 °C).