Phosphine Borane Frustrated Lewis Pairs Derived from a 1,1'-Disubstituted Ferrocene Scaffold: Synthesis and Hydrogenation Catalysis
Abstract
(Dimesitylphosphino)ferrocene (FcPMes2) (1) reacted with HB(C6F5)2 (2 equiv) by disproportionation to give adduct FcPMes2*H2B(C6F5) (4) plus B(C6F5)3, whereas 1-(dimesitylphosphino)-1'-vinylferrocene (2) was cleanly hydroborated with HB(C6F5)2 to afford [Fe(eta(5)-C5H4PMes2)(eta5-C5H4CH2CH2B(C6F5)2)] (7). Compound 7 adopted an open non-interacting P/B frustrated Lewis pair (FLP) structure in the crystal state as well as in a solution.
This frustrated Lewis pair heterolytically cleaved dihydrogen under mild conditions to give the respective zwitterionic [P]H+/[B]H- phosphonium/hydroborate product, [Fe(eta(5)-C5H4PHMes2){eta(5)-C5H4CH2CH2BH(C6F5)2}] (8), which served as a catalyst for the hydrogenation of the electron-rich pi-systems (imine, enamine) as well as well as the electron-deficient carbon carbon double and triple bonds in some enones and an ynone under more forcing conditions (50 bar H-2 pressure, 50 degrees C).