Catalytic ionic hydrogenations of ketones using molybdenum and tungsten complexes

Ketone complexes [CpM(CO)(2)(PR3)(eta(1)-Et2C=O)]+BAr'(-)(4) (R = Ph or Me; M = Mo or W) were prepared from hydridc transfer from Cp(CO)(2)(PR3)MH to Ph3C+BAr'(-)(4) [Ar'= 3,5-bis(trifluoromethyl)phenyl] in the presence of 3-pentanone. These ketone complexes are catalyst precursors for hydrogenation of Et2C=O under mild conditions (23degreesC, <4 atm H-2). Analogous catalytic hydrogenations are obtained from reaction of the PCy3 complexes CP(CO)(2)(PCy3)MH with Ph3C+BAr'(-)(4). The proposed mechanism involves displacement of the ketone by H-2, producing a cationic metal dihydride [CpM(CO)(2)(PR3)(H)(2)](+). Proton transfer from the dihydride gives a protonated ketone, followed by hydride transfer from the neutral metal hydride CpM(CO)(2)(PR3)H to produce the alcohol complex [CpM(CO)(2)(PR3)(Et2CHOH)](+). The free alcohol product is released from the metal through displacement by H-2 or ketone, completing the catalytic cycle. In most cases, conversion of the ketone or alcohol complexes to the dihydride is the turnover-limiting step of the catalytic cycle, with ketone and alcohol complexes being observed during the reaction. For reactions using the W-PCy3 system, the dihydride [CpW(CO)(2)(PCy3)(H)(2)](+) is observed as the resting state of the catalytic process. Proton transfer is slow and becomes turnover-limiting in this case. The Mo catalysts are more active than W, and the dependence on phosphine is PCy3 > PPh3 > PMe3. The turnover rates are slow, with the fastest initial rate of about 2 turnovers per hour found for the Mo-PCy3 system. This ionic hydrogenation mechanism does not require coordination of the ketone to the metal for the hydrogenation, thus differing from traditional mechanisms where coordination of a ketone to a metal precedes insertion of the ketone into a M-H bond.