H2 Complexes as Precursors for Catalytic and Other Reactions

The loss of H2 (usually trans to H) opens a site for binding of the unsaturated substrate, which in most cases is an alkyne [as in Eq. 9.56)], an alkene, or a ketone. The hydride migrates to substrate, followed by hydrogenolysis to final product, which could involve an q2-H2 intermediate and/or four-center transition state. The H2 complex thus functions as a reservoir of unsaturation and stabilizes the catalyst system against decomposition. Because excess H2 is usually present in hydrogenations, the q2-H2 behaves in a way that is similar to a weakly coordinated solvent molecule and can compete with solvent binding as in Eq. 

The [MH(H2)(pp3)]+ system, which was studied extensively by Bianchini and coworkers,94 105 highlights the importance of differences in M-H2 bond strengths (Os Fe Ru) on the selective hydrogenation of alkynes to alkenes. The Fe congener is one of the most stable H2 complexes and reactions with alkynes under 1 atm ofH2 produce only alkenes (with one exception) via the mechanism in Eq. (9.54), where the H2 remains attached while the alkyne coordinates. In contrast, the complex [RuH(H2)(pp3)]+ catalyzes dimerization of 1-alkynes to nearly 100% Z-l,4-disubstituted enynes.107 

The complex IrH2Cl(PlPr3)2 behaves in a different manner to Eq. (9.56), and heterolytically activates the H2 ligand as discussed above to form HC1 and 1 3( 3)2, where the known H2 complex111 is identified as an intermediate112  

The unsaturated trihydride then catalyzes hydrogenation of benzylideneacetone to 

4-phenylbutan-2-one in isopropanol at 60°C. IrH2Cl(H2)(P Pr3)2 is closely related to the dichloro analogue IrHCl2(H2)(P(Pr3)2, which shows hydrogen bonding betweeen H2 and Cl ligands, which could facilitate HC1 loss. 

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