Abstract
Supports, ligands and additives can promote heterolytic H2splitting by a cooperative mechanism with metal nanoparticles.
Highlights
1.1 Homogeneous systems mechanistic aspects right from the start
In this perspective we focus on hydrogenation catalysis by metal nanoparticles that are mechanistically based on heterolytic cleavage of dihydrogen
Selective hydrogenation of polar functional groups (C O, C N or C S) has been mostly carried out in the laboratory using homogeneous catalysts, in which H2 is heterolytically activated by a metal ligand cooperation
Summary
After two decades of a dominating position of oxidative addition in mechanistic studies, heterolytic cleavage came to the forefront again in the 1990s with the revival of the “other” Wilkinson catalyst,[17] the one based on Ru instead of Rh, via the work of Noyori and Shvo on hydrogenation and transfer hydrogenation and in relation to the work on hydricity cited above.[18,19,20] The development of the chemistry of dihydrogen complexes contributed to new insights as, for instance, dihydrogen complexes of Ru were found to react with amines giving a ruthenium hydride and an ammonium salt, as was shown in 1990 by Chinn and Heinekey.[21] An early example in which a basic ligand functionality fulfils the role of the base was reported by Fryzuk, who described the heterolytic cleavage of H2 across an Ir–amide bond of a P–N–P ligand.[22] Later, they reported that H2 activation occurred in this particular instance by oxidative addition–reductive transfer rather than as a direct heterolytic cleavage of H2.23 Ever since, a wide variety of ligand involvements in metal-catalysed reactions have been reported and reviewed (pincer catalysis,[24,25,26] outersphere hydrogenation catalysis,[27] functional ligands,[28] models for hydrogenase,[29] cooperating ligands,[30] ligand-assisted proton transfer,[31,32,33,34] outer-sphere hydrogen transfer,[35] etc.). HCD in such systems was reviewed by Ito and Ikariya.[37]
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