Dihydrogen heterolytic cleavage

The reaction of OsHCl(CO)(P Pr3)2 with HC1 gives the dichloro derivative OsCl2( n2-H2)(CO)(PIPr3)2.35 In solution, this complex is stable under argon for a matter of days. However, the dihydrogen unit is highly activated toward heterolytic cleavage, as demonstrated by deprotonation with NaH and by reactions with carbon monoxide and /ert-butyl isocyanide, which afford OsHCl(CO)L(P Pr3)2 (L = CO, r-BuNC) and HC1. 

One of the important properties of dihydrogen ligands, particularly in charged transition metal complexes, is their ability to nndergo heterolytic cleavage [9]. In addition, protonation of transition metal hydrides with acids is a common method for preparation of transition metal dihydrogen complexes ... 

Scheme 10.2 Heterolytic cleavage of dihydrogen on platinum SPO complex.

The hypothetical reaction mechanism shown in Fig. 10 is a variation of Scheme (3), and is consistent with the redox and pH dependence of the EPR-detectable nickel species (65). Hydrogen is known to undergo heterolytic cleavage (81) it is proposed that this is an intramolecular reaction, leading to the formation of a nickel (II) hydride and a protonated base in the enzyme (Step 1 in Fig. 10). The Ni-C species is postulated to be a protonated Ni(I) species. An alternative formulation for this state would be a dihydrogen complex, Ni(III) H2, as suggested by Crabtree (104). Ultimately the exact mechanism can only be determined by kinetic measurements. 

We have explored both water-free and water-assisted routes to the cleavage of the H-H bond in certain r 2-H2 species of Fig. 3. For the former route, the bridging thiolate sulfur donor and the bridging hydride were considered as possible internal bases for the heterolytic cleavage of H2 from each of the dihydrogen species, Fig. 7. In the water-assisted routes, H20 is used as an external base to deprotonate H2, Fig. 8. 

The hydrogenase enzymes (1) catalyse the heterolytic cleavage of dihydrogen into protons and electrons,... 

Fan HJ, Hall MB. A capable bridging ligand for Fe-only hydrogenase density functional calculations of a low-energy route for heterolytic cleavage and formation of dihydrogen. J Am Chem Soc. 2001 123(16) 3828-9. 

SCHEME 4 SPO platinum catalysts. Top row stoichiometric hydroformylation with 1 middle row, catalyst 4 and hydrogen bond switching in 6 bottom row heterolytic cleavage of dihydrogen (69-71). (For a color version of this figure, the reader is referred to the Web version of this chapter.)... 

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. 

Figure 14 Proposed mechanism of heterolytic cleavage of dihydrogen...

---