Ionic dihydrides

Stoichiometric Ionic Hydrogenation of Ketones with Metal Dihydrides... 

The cationic tungsten dihydride [Cp(CO)2(PMe3)W(H)2]+ hydrogenates the C=0 bond of propionaldehyde within minutes at 22 °C, leading to the formation of cis and truns isomers of Cp(CO)3W(l IO"Pr) Oif (Eq. (28)) [42]. The cis isomer of the alcohol complex released the free alcohol faster than the trans isomer. A similar stoichiometric ionic hydrogenation of acetone was also observed using [Cp(CO)2(PMe3)W(H)2]+. 

Hydride transfer reactions from [Cp2MoH2] were discussed above in studies by Ito et al. [38], where this molybdenum dihydride was used in conjunction with acids for stoichiometric ionic hydrogenations of ketones. Tyler and coworkers have extensively developed the chemistry of related molybdenocene complexes in aqueous solution [52-54]. The dimeric bis-hydroxide bridged dication dissolves in water to produce the monomeric complex shown in Eq. (32) [53]. In D20 solution at 80 °C, this bimetallic complex catalyzes the H/D exchange of the a-protons of alcohols such as benzyl alcohol and ethanol [52, 54]. 

Molybdenum and tungsten carbonyl hydride complexes were shown (Eqs. (16), (17), (22), (23), (24) see Schemes 7.5 and 7.7) to function as hydride donors in the presence of acids. Tungsten dihydrides are capable of carrying out stoichiometric ionic hydrogenation of aldehydes and ketones (Eq. (28)). These stoichiometric reactions provided evidence that the proton and hydride transfer steps necessary for a catalytic cycle were viable, but closing of the cycle requires that the metal hydride bonds be regenerated from reaction with H2. 

EHT calculations of, 34 173 hydrogen exchange and, 33 104-116 ionic and nonionic types, 33 122 monohydride and dihydride sites, 33 120-122... 

The foregoing remarks do not hold, of course, for the dihydrides of the triva-lent lanthanides. They exhibit metallic conduction (10), as would be expected, since their conduction band is only somewhat depleted. One would expect them to display a tendency to order at low temperatures, but it seems not unreasonable to expect that this tendency would be weaker than the corresponding element, in view of the decreased electron concentration, and the ordering would hence occur at lower temperatures. This was in fact observed for HoH2, which exhibits (6) a Neel point at 8° K., as coippared to 135° K. for Ho. It is also observed in the present work for the terbium dihydrides, whose Neel points are 40° to 50° K., whereas that for the element is 241° K. These properties are compatible with the notion that hydrogen in all the lanthanide hydrides is anionic. On this basis the dihydrides appear as an intermediate form between the truly metallic elements on the one hand and the truly ionic or saline trihydrides on the other. 

The same steps involved in ionic hydrogenation — and H transfer to the substrate ketone — are involved in catalysis of hydrogen transfer from isopropyl alcohol to a prochiral ketone (eq 3). Traditionally, ketone coordination to the catalyst was invoked to rationalise the achievement of enantioselectivity. While transfer hydrogenation is usually done under basic conditions, the base is required more for the generation of the active catalyst than for the reaction itself the dihydride HjRuLj can catalyse the transfer of hydrogen from isopropyl alcohol to various ketones by itself [88, 89]. 

Ionic Hydrogenations from Dihydrides Delivery of the Proton and Hydride from One Metal... 

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. 

In contrast, in the ionic mechanism (Fig. 10), the metal dihydride complex, in the first step, transfers a proton to give a protonated ketone followed by hydride transfer from the neutral metal hydride to produce a coordinated alcohol to be released from the metal through displacement by hydrogen (39). 

A series of molybdenum and tungsten catalysts for the hydrogenation of ketones have been reported by Bullock (101,102). These catalysts operate by an outer-sphere ionic mechanism. The reaction occurs by proton transfer from a cationic metal dihydride (or a metal dihydrogen species), followed by hydride transfer from a neutral metal hydride to a ketone (Fig. 34). 

In table 5 the lattice parameters, a, of the cubic p-RH2+jr phases are presented as a function of x at several temperatures. The static and thermal lattice expansions have been added when available. We note the well-known general contraction (negative Aa/Ax-values) of the dihydride lattice with increasing x, which is an expression of the strong ionic character of its interaction with the excess H atoms on O sites an example is given in fig. 9 for the case of YH2+ (, where the break in the a(x)-curve at x=0.10 H/Y indicates the limit of the pure p-phase. 

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