Activation of dihydrogen

Nagashima reported the hydrogenation of di-, tri- and tetranuclear ruthenium complexes bearing azulenes below 100 °C revealed that only the triruthenium compounds reacted with H2 via triruthenium dihydride intermediates.398 This indicates that there exists a reaction pathway to achieve facile activation of dihydrogen on the face of a triruthenium carbonyl moiety.399... 

Despite the fact that Nature uses nickel for the activation of dihydrogen, and that Raney-Ni is one of the oldest and the most important heterogeneous hydrogenation catalysts, very few nickel complexes are known to catalyze the homoge-... 

Fig. 12.9 Alternative approaches to the sequence of events during the catalytic activation of 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 ... 

The potential of the Niin/Nin couple in hydrogenase is higher than the 2H+/H2 couple by about 0.2 V, so there are mechanistic difficulties in postulating a role for Niin/Nin in the activation of dihydrogen. It is important that further measurements be made on midpoint potentials for nickel in various hydrogenases to check that these values relate to mechanistically important species. 

Fig. 7. Water-free activation of dihydrogen via Path a. The anhydrous activation of dihydrogen using the thiolate sulfur (left) or hydride (right). Relative energies in kcal mol-1.

Fig. 8. Water Assisted Activation of dihydrogen via Path a. The water-assisted cleavage of the H-H bond. Differences in charge does not allow the direct comparison of the energies of 19 and 20. Relative energies given in kcal mol-1.

More recently, Taqui Khan and co-workers (70) introduced the potentially tetradentate ethylenediaminetetraacetate ligand in the ruthenium coordination sphere in order to obtain an efficient water-soluble catalyst precursor. Indeed, starting from the ruthenium(III) aquo EDTA species [Ru(EDTA)(H20)] , carbonylation gives the paramagnetic carbonyl complex [Ru(EDTA)(CO)] which is able to induce the heterolytic activation of dihydrogen (Scheme 3). The hydroformylation of hex-l-ene performed at 50 bar (CO/H2= 1/1) and 130°C in a 80/20 ethanol-water solvent... 

It has been known for some time that polar solvents accelerate the activation of dihydrogen, as was found with HCo(CN)5 [38] and [RhCl(PPh3)3] [39]. A recent study revealed the same phenomenon, a very large rate increase, in aqueous solution [40]. In both dimethyl sulfoxide (DMSO) and water as solvents, the oxidative addition of H2 to trans- lrCI(CO)(TPPMS)2 yielding trans- H2IrCl(CO) (TPPMS)2] could be described by the rate law given by Eq. (15), which is identical with what had been found earlier for the reaction of trans- lrCI(CO)(PPh3)2] with H2 in toluene or DMSO. 

Bonding and Activation of Dihydrogen and Ligands Theory versus Experiment... 

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