Dihydrogen homogeneous activation

Homogeneous Activation of Dihydrogen by Metal Complexes in Solution " ... 

In addition to the polymeric rhodium catalysts previously discussed, monomeric rhodium systems prepared from [Rh(CO)2Cl]2 by addition of strong acid (HC1 or HBF4) and Nal in glacial acetic acid have also been shown to be active homogeneous shift catalysts (80). The active species is thought to be an anionic iodorhodium carbonyl species, dihydrogen being produced by the reduction of protons with concomitant oxidation of Rh(I) to Rh(III) [Eq. (18)], and carbon dioxide by nucleophilic attack of water on a Rh(III)-coordinated carbonyl [Eq. (19)]. 

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 ... 

Calcium chloride solutions (pH =6.2) labeled with Ca or 36ci were displaced vertically downward through columns of homogeneous, repacked, water-saturated sandy soil by a chemically identical solution labeled with Cl or Ca, respectively. Constant water fluxes, and solution activities of 1 to 15 pCi/dm, were used. Calcium solutions were analyzed by titration with disodium dihydrogen ethylenediamine tetraacetate to a murexide end point (11). The activity of radioactively labeled solutions was obtained by liquid scintillation techniques. Concentrations of adsorbed calcium were calculated from isotope dilution. The concentration of calcium chloride in the influent solution was 0.08 N. Because exchange of calcium for itself was the only chemical process affecting transport, the calcium chloride concentration remained constant at 0.08 N throughout each experiment, both within the column and in the effluent. 

C-H and C-C bond activations by ruthenium complexes have formed the focus of this chapter, and consequently other important reactions to cleave chemical bonds such as dihydrogen, C-S and M-R have not been described. Today, ruthenium is regarded as a powerful tool for cleaving a variety of both activated and unactivated chemical bonds under homogeneous conditions. Important factors that provide these activities include 1) coordinative unsaturation of the ruthenium center 2) a close proximity of the bond to the ruthenium metal and 3) kinetic preference and thermodynamic stability of the products. It is likely that the combined use of ruthenium complexes and modern strategies in organic synthesis and catalysis will provide many opportunities for the creation of new reaction processes in the futtue. 

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