Ketone complexes [CpM(CO)2(PR3)(η1- Et2C=O)]+BAr′4- (R = Ph or Me; M = Mo or W) were prepared from hydride 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 (23°C, <4 atm H2). 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 H2, 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 H2 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 κ 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.
ASJC Scopus subject areas
- Inorganic Chemistry