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 29Si- and 15N 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.