Heterolytic Cleavage and Acidity of Coordinated

Although the heterolytic process here is formally a concerted ionic splitting of H2 as often illustrated by a four-center intermediate with partial charges, the mechanism does not have to involve such charge localization. In other words, the two electrons originally present in the H H bond do not necessarily both go into the newly-formed M H bond while a bare proton transfers onto L or, at the opposite extreme, an external base. The term a-bond metathesis is thus actually a better description and may comprise more transition states than the simple four-center intermediate shown above, e.g., initial transient coordination of H2 to the metal cis to L and dissociation of transiently bound H- L as the final step. Examples of this type of activation will be given in this Section. 

Importantly, H2 gas can be turned into a strong acid on binding to electrophilic cationic complexes. Free H2 is an extremely weak acid with a TpKa near 35 in THF (23), and heterolytic splitting of r 2-H2 in relatively electron-rich neutral complexes is usually achieved only by strong bases. For example, we have shown Eq. (4) that copper alkoxides deprotonate W(CO)3(PR3)2(H2) and FeH2(H2)(PR3)2 to give heterobi-metallic species with bridging hydrides (24). 

However, when H2 is bound to a highly electrophilic cationic metal center, the acidity of H2 gas can be increased spectacularly, up to 40 orders of magnitude. The pK of H2 can become as low as —6 and thus the acidity of rf -H2 becomes as strong as that in sulfuric or triflic acid. As discussed in reviews by Morris (4,5) and Jia (25) and further work by Morris (26,27), such pK values are usually determined by NMR measurement of the concentrations of M H2 complexes in equilibrium with an external base such as a phosphine or amine. Electron deficient cationic and dicationic H2 complexes with strong short H-H bonds ( 0.9 A) and weakly bound H2 such as [Cp Re(H2)(CO)(NO)]+ and [Re(H2)(CO)4(PR3)]+ are among the most acidic complexes (Table I). These acidic complexes typically have relatively high values of Jhd for their r 2-HD isotopomers, although pK values do not correlate 

Reported piTa Values (Pseudo Aqueous scale) and Corresponding Jhd of Selected H2 Complexes, Emphasizing Highly Acidic Species 

Depe = l,2-bis(diethylphosphino)ethane dppe = l,2-bis(diphenylphosphino)ethane  

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It may be reasonable to argue that this further activation is achieved in several ways. The acid-catalysis required for Gal and de Bruin complex [Rh(/c -bpa)(cod)](PF6) to react with dioxygen can be used to protonate the peroxo compoimd (Scheme 10) to a hydroperoxo species. This is a way to achieve further activation of dioxygen, since it decreases the nucleophilic character of the peroxo hgand and makes interaction with the coordinated olefin easier. Recent works by Moro-oka [88,89] and Braun [90] (Scheme 15) have shown that peroxorhodium complexes can be protonated to hydroperoxo compounds. However, the addition of a second mole of acid leads to hydrogen peroxide ehmination rather than to the highly electrophilic oxo species (M = O) that could result from the heterolytic cleavage of the O - O bond with removal of water. 

The protonation of a thiolate donor, formation of a nonclassical r 2-H2 complex, release of H2 and addition of D2, and the heterolytic cleavage of this D2 by the concerted attack of the Lewis acidic metal center and the Brpnsted basic thiolate donor are essential steps. The acidic thiol deuteron can exchange with EtOH protons. The resulting free protons and the deuteride complex yield HD and the coordinatively unsaturated species that is the actual catalyst. The detailed mechanism comprises a considerably larger number of steps (and equilibria) (143). For example, the occurrence of r 2-D2 and [M(D)(SD)] intermediates that exchange with H+ should give rise to [M(D)(SH)]... 

Jia and Morris reported that (dihydrogen)ruthenium(II) complex formulated as [RuCp(H2)L2] produces a proton by heterolytic cleavage of the coordinated H-H bond [77]. Of particular interest is that the pK values of these complexes closely depend on the ancillary ligand employed (pK = 4.9-9.0 in THE), suggesting that a decrease in the electron density of the metal increases the acidity of the dihydrogen complex. 

Alkanes are extremely poor acids, but on analogy with H2 binding to metal complexes, coordination to highly electrophilic M greatly enhances the acidity of the C-H bond and promotes heterolytic cleavage. Soft electrophiles such as Pt" and Hg" are ideal because they bind CH4 and other alkanes transiently even in aqueous... 

Chloro complexes of ruthenium(II) were found to hydrogenate maleic and fumaric adds to succinic acid slowly at 60-80 °C and normal pressure of hydrogen. Non-activated alkenes lead to the production of ruthenium metal. The structures of the species involved are unknown. The mechanism involves coordination of the alkene followed by heterolytic cleavage of hydrogen, giving a ruthenium(II) hydride as the second step. ... 

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