Activation dihydrogen

In the reactions above we have not explicitly touched upon the reactions of dihydrogen and transition metal complexes. Here the reactions that involve the activation of dihydrogen will be summarised, because they are very common in homogeneous catalysis and because a comparison of the various mechanisms involved may be useful. Three reactions are usually distinguished for hydrogen  

Oxidative addition of dihydrogen commonly involves transformation of a d8 square planar metal complex into a d6 octahedral metal complex, or similar transformations involving d2 — d°, d10 — d8 etc. The oxidative addition of 

In rhodium complexes the reaction has found widespread application in hydrogenation. In model compounds the reaction reads [22]  

The reaction of a metal-dimer with H2 can also be regarded as an oxidative addition reaction. For instance, a dimer of a d7 metal complex reacts with dihydrogen to give two d6 species. In this process dihydrogen also gives formally two hydride anions. A well-known example in the present context is the conversion of dicobaltoctacarbonyl into hydridocobalttetracarbonyl  

In the past this was referred to as a homolytic reaction [23], It is clear that it would be highly unlikely that dihydrogen would be split into a metal hydride and a highly energetic hydrogen radical in this sense the term homolytic splitting is misleading. 

The best studied oxidative addition is that of H2 to the 16e square planar d species, IrCl(CO)(PPh3)2, known as Vaska s complex [97]. This gives anl8e d octahedral Ir(III) dihydride complex (Eq. 2.31). In a concerted addition of this sort, two mutually trans ligands in the starting Ir(I) complex fold back with the result that the cis dihydride isomer is formed, at least initially. Subsequent rearrangement may also occur. 

Activation of Substrates with Non-Polar Single Bonds 

The reactions are usually second order and show negative entropies of activation (ca. —20 eu) consistent with an ordered transition state like a-honded H2 complex. They are little affected hy the polarity of the solvent, hut may be accelerated to some extent by electron releasing phosphines. 

Hydrogenation of unsaturated substrates. Hydrogenation of alkenes and alkynes is one of the most widely useful reactions involving dihydrogen activation. The mechanism of Wilkinson s complex offers a good example of the process (Fig. 2.3) [100]. The cycle shown is just one of several that operate in the real system, depending on the exact conditions. It serves to show a key point, that the activation step is the hrst step in the cycle. As in the Vaska case, the ligands fold back so that the cis-dihydride is formed. An early NMR experiment (Fig. 

4) went far to establish the H2 activation and subsequent steps in the pathway [lOOd]. At low temperature the H2 adduct is stable but at room temperature, the phosphine that is trans to a hydride dissociates, as shown by the loss of coupling to this nucleus. There is no significant formation of free phosphine that would have appeared at the location indicated by the arrow. Sweeping the sample with N2 removes the coordinated H2 showing the reversibility of the oxidative addition in this case. 

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Fryzuk demonstrated the high reactivity of the amide function present in the P-amide-P pincer Ir complex 60 towards dihydrogen [28]. In fact, heterolyhc dihydrogen activation by the Ir(III)-amide bond present in the pincer complex 60 led to the stable Ir(III)-alkyl-amino-hydrido complex 61, as outUned in Equation 6.16. The X-ray crystal structure of the amine product is depicted in Figure 6.14. 

Zhou TJ, Mo YR, Zhou ZH, Tsai K. Density functional study on dihydrogen activation at the H cluster in Fe-only hydrogenases. Inorg Chem. 2005 44(14) 4941-6. 

Lange, S. Brinkmann, A. Trautner, P. Woelk, K. Bargon, J. Leitner, W. Mechanistic aspects of dihydrogen activation and transfer during asymmetric hydrogenation in supercritical carbon dioxide. Chirality 2000, 72 (5/6), 450-457. 

In order to appreciate the mechanism of rhodium asymmetric hydrogenation, it may be useful first to examine the simpler achiral case of alkene hydrogenation by Wilkinson s catalyst. In early studies [13] assumptions were made about the dihydrogen activation step which proved to be incorrect because of the direct... 

This section describes general dihydrogen activation, reactions which form metal hydrides and molecular dihydrogen complexes, and the metal complexes which constitute the main classes of soluble catalysts. 

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