Trisphosphine-chelate-substituted molybdenum and tungsten nitrosyl hydrides as highly active catalysts for olefin hydrogenations

Abstract

Reaction of [M(NO)Cl3 (NCMe)2 ] (M=Mo, W) with (iPr2 PCH2 CH2 )2 PPh (etp(i) p) at room temperature afforded the syn/anti-[M(NO)Cl3 (mer-etp(i) p)] complexes (M=Mo, a; W, b; 3 a,b(syn,anti); syn and anti refer to the relative position of Ph(etp(i) p) and NO). Reduction of 3 a,b(syn,anti) produced [M(NO)Cl2 (mer-etp(i) p)] (4 a,b(syn)), [M(NO)Cl(NCMe)(mer-etp(i) p)] (5 a,b(syn,anti)), and [M(NO)Cl(η(2) -ethylene)(mer-etp(i) p)] (6 a,b(syn,anti)) complexes. The hydrides [M(NO)H(η(2) -ethylene)(mer-etp(i) p)] (7 a,b(syn,anti)) were obtained from 6 a,b(syn,anti) using NaHBEt3 (75 °C, THF) or LiBH4 (80 °C, Et3 N), respectively. 7 a,b(syn,anti) were probed in olefin hydrogenations in the absence or presence of a hydrosilane/B(C6 F5 )3 mixture. The 7 a,b(syn,anti)/Et3 SiH/B(C6 F5 )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 Et3 SiH/(B(C6 F5 )3 co-catalyst is anticipated to generate a [Et3 Si](+) cation attaching to the ONO atom. This facilitates NO bending and accelerates catalysis by providing a vacant site. Inverse DKIE effects were observed for the 7 a(syn,anti)/Et3 SiH/(B(C6 F5 )3 (kH /kD =0.55) and the 7 b(syn,anti)/Et3 SiH/(B(C6 F5 )3 (kH /kD =0.65) co-catalytic mixtures (20 bar H2 /D2 , 140 °C).

Keywords: hydrogenation; isomers; molybdenum; olefins; tungsten.