Hydrogen heterolytic activation

Scheme 22.2 Formal heterolytic hydrogen activation via deprotonation of a dihydride intermediate.

The products of reductive cyclization incorporate two non-exchangeable hydrogen atoms. Homolytic and heterolytic hydrogen activation pathways may now be discriminated on the basis of hydrogen-deuterium crossover experiments. Reductive cyclization of the indicated nitrogen-tethered enyne under a mixed atmosphere... 

In the case of HDN, an additional interesting possibility also consistent with the heterolytic mechanism arises, since substrates like the pyridines -or intermediate alkyl or aryl amines- are sufficiently basic to promote the activation of hydrogen so as to form a metal hydride plus a protonated base (e.g. a pyridinium or an alkylammonium cation). Furthermore, some of the most widely accepted amine HDN mechanisms include the initial protonation of the amine nitrogen, followed by elimination of ammonia from the ammonium cation. Therefore, it is very easy to combine the idea of a heterolytic hydrogen activation promoted by, say n-pentylamine, with a subsequent degradation by a Hoffmann mechanism, to conform a reasonable HDN catalytic cycle. A simplified representation of this idea is given in Fig. E4. 

Higher water coverages and the presence of solution both act to lower the barriers to activate water. The intermolecular interactions that result from hydrogen bonding with other water molecules stabilize the activated HO—H complex over the entire dissociation reaction coordinate. For metals with high workfunctions, the aqueous phase can enable heterolytic water activation... 

The role of the solvent is crucial in some cases. For example, in the case of hydrogen activation it was actively participating in the heterolytic cleavage of the hydrogen in both the ruthenium and gold catalysts in water and ethanol, respectively. It must be noted that the so-called heterolytic activation of the hydrogen may be... 

Heterolytic cleavage. This leads to formation of a metal hydride with release of a proton (equation 1). The formal oxidation state of the metal does not change. This mode of hydrogen activation is common in hydrogenation by complexes of ruthenium(II). 

The formation of hydrides by activation of molecular hydrogen implies the transformation of H2 into MH ", MH or MH. The hydrogen activation can be either homolytic or heterolytic, and can occur at single or multiple metal centres. Examples of homolytic activation are given in Equations (1) and (2). 

Both modes of hydrogen activation, heterolytic and homolytic, may be achieved either directly or through a two-step mechanism involving the intermediate formation of ri -H2 complexes. 

This remarkable set of reactions represents another excellent model for hydrogen activation on metal sulfides, and it is to date the only example available of the direct observation of a heterolytic activation of hydrogen assisted by a terminal sulfide ligand. Like Bianchini s Rh complex mentioned above (Eq. 5.7), this is in line with solid state NMR evidence for this latter type of hydrogen splitting on RuSj which has been provided by Lacroix et al., indicating die formation of Ru-H and -SH species on the surface [19] it is possible that other metal sulfides are also capable of activating Hj by similar routes. 

A likely intermediate in hydrogen activation reactions, including heterolytic splitting, is a hydrogen species where H2 is 7] -bonded to the metal center. The first stable dihydrogen molecule (and recognized as such) has been isolated by Kubas et al. in 1983, and some thermochemical studies on this type of molecules have appeared since then." " The enthalpy of reaction (57), AJP (57) = —30.5, —27.2, and —40.2 kJ moP for M=Cr, Mo, and W, respectively, measures DH° (M-H2) ... 

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