The "catalytic nitrosyl effect": NO bending boosting the efficiency of rhenium based alkene hydrogenations.

Diiodo Re(I) complexes [ReI2(NO)(PR3)2(L)] (3, L = H2O; 4 , L = H2; R = iPr a, Cy b) were prepared and found to exhibit in the presence of "hydrosilane/B(C6F5)3" co-catalytic systems excellent activities and longevities in the hydrogenation of terminal and internal alkenes. Comprehensive mechanistic studies showed an inverse kinetic isotope effect, fast H2/D2 scrambling and slow alkene isomerizations pointing to an Osborn type hydrogenation cycle with rate determining reductive elimination of the alkane. In the catalysts' activation stage phosphonium borates [R3PH][HB(C6F5)3] (6, R = iPr a, Cy b) are formed. VT (29)Si- and (15)N NMR experiments, and dispersion corrected DFT calculations verified the following facts: (1) Coordination of the silylium cation to the ONO atom facilitates nitrosyl bending; (2) The bent nitrosyl promotes the heterolytic cleavage of the H-H bond and protonation of a phosphine ligand; (3) H2 adds in a bifunctional manner across the Re-N bond. Nitrosyl bending and phosphine loss help to create two vacant sites, thus triggering the high hydrogenation activities of the formed "superelectrophilic" rhenium centers.