Catalytic Ionic Hydrogenations With Mo and W Catalysts

In contrast to the formation of the trans-isomer under catalytic conditions, hydride transfer from Cp(CO)2(PPh3)WH to PhsC, followed by addition of Et2CHOH, led to the isolation of cts-[CpW(CO)2(PPh3)(Et2CHOH)r (3.19). 

Conversion of the ktnetically favored trans-isomer to the thermodynamically favored ds-isomer has been observed in several cases of Mo and W complexes with similar structures [23, 48]. 

The next step in the catalytic cycle, ionic hydrogenation of ketones from cationic dihydrides, was separately shown to occur under stoichiometric conditions, as shown in Equation 3.17. Alcohol complexes have been synthesized and isolated, and some were characterized by crystallography [43]. Analogous alcohol complexes were observed spectroscopically during the catalytic reaction, so displacement of the alcohol by H2 regenerates the dihydride and completes the catalytic cycle. 

Evidence for tungsten dihydride complexes was obtained directly, as discussed above, but analogous complexes of molybdenum have not been directly observed. Indirect evidence for molybdenum dihydrides (or dihydrogen complexes) comes from the reaction shown in Equation 3.21, in which the molybdenum ketone complex heterolytically cleaves H2 in the reaction with a hindered base. 

This reaction proceeds in 5 min at room temperature. As a second row metal, Mo may be more likely to form a dihydrogen complex, in contrast to the dihydride found for the third row W analog. 

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