Intermediate dihydrogen complexes

In the vast majority of studies on the protonation of hydride complexes, the kinetic site was the hydride ligand. This has been proven for many cases in which the thermodynamics of protonation favour a dihydride complex or a mixture of dihydride and dihydrogen complexes. In the case of CpW(CO)2(PMe3)H we have shown that hydride protonation (as monitored by exchange) is faster than protonation at the metal the intermediate dihydrogen complex rearranges too quickly to be observed [46]. 

Some hydride complexes (Equation 3.100) are produced by the heterolytic cleavage of Hj, a reaction that probably proceeds through the deprotonation of an intermediate dihydrogen complex. Other hydride complexes (Equation 3.101) are produced by the hydrogenolysis of metal-alkyl bonds. ... 

The dinitrogen complex [Os(NH3)5N2]2+ is a useful synthetic intermediate, while the presence of the weakly nucleophilic triflate group enables it to be easily removed in the synthesis of the dihydrogen complex. 

In favor of a dihydrogen intermediate, the following should be noted (i) the addition of HX molecules to OsHCl(CO)(P Pr3)2 is a useful synthetic route for dihydrogen compounds (vide infra), (ii) the electrophilic character of the dihydrogen complexes has been demonstrated,38 and (iii) the addition of electrophiles to metal alkynyl complexes is a general method of preparing vinylidene compounds.39... 

A ruthenium dihydrogen complex G or a ruthenacycle D, which was proposed as a potential intermediate, catalyzed the insertion of ethylene into sp2-C-H bonds, with TONs reaching 19 after 48 h of reaction and under very mild conditions (room temperature as opposed to the usual 135 °C) (Equation (96)).91,91a91c... 

Schemes 3.2 and 3.3 show intermediates containing dihydrogen ligands with the H-H bond intact. It has only been appreciated since the discovery of the first dihydrogen complexes by Kubas and coworkers in 1984 [14] that such complexes are key intermediates in catalytic cycles [11, 13, 14].

More recently, dihydrogen complexes have been patented for nitrile hydrogenation. For example, the complex Ru(7/2-H2)2(H)2(PCy3)2 (Fig. 3.6) catalyzes the hydrogenation of adiponitrile to hexamethylenediamine (HMD) in toluene at 90 °C, 70 bar H2 with TON 52, TOF 5 IT1 [68]. At intermediate conversions, the... 

Noyori et al. recently used ESI-MS to characterize species present in catalytically active solutions during the hydrogenation of aryl-alkyl ketones using their base-free catalyst precursors trans-[Ru((R)-tol-BINAP)((R, RJ-dpenJfHXf/ -BH ] (tol-BI-NAP = 2,2 -bis(ditolylphosphino) -1, T-binaphthyl dpen = 1,2-diphenylethylenedia-mine) in 2-propanol [9b]. Based upon ESI-MS observations, deuterium-labeling studies, kinetics, NMR observations, and other results, the authors proposed that the cationic dihydrogen complex trans-[Ru((R)-tol-BINAP)((R, R)-dpen)(H)( 2-H2)]+ is an intermediate in hydrogenations carried out in the absence of base. 

Inhibition of a catalytic reaction by impurities present may take place and sometimes this may have a temporary character. If it is permanent one cannot be mistaken in the kinetic measurements. Impurities that are more reactive than the substrates to be studied may block the catalyst if they react according to a scheme like that of Figure 3.7. Only after all inhibitor has been converted the conversion of the desired substrate can start. Another type of deactivation that may occur is the formation of dormant states, which is very similar to inhibition. Either the regular substrate or an impurity may lead to the formation of a stable intermediate metal complex that does not react further. There are examples where such intermediates can be rescued from this dormant state for instance by the addition of another reagent such as dihydrogen (Chapter 10, dormant states in propene polymerisation). 

This means that the less stable intermediate alkene complex reacts faster in the subsequent reactions. The next step in the hydrogenation sequence involves the oxidative addition of dihydrogen to the alkene complex (Figure 4.10). 

When the initial hydride CpRuH(CO)(PH3), It, is placed near H3O+, a proton transfer occurs without the energy barrier to form the dihydrogen complex [CpRu(H2)(CO)(PH3)]+, in good agreement with the data of Orlova and Scheiner [42]. Dihydrogen-bonded intermediates appear on the protonation pathway if the hydride It interacts with the weaker acids TFA and PFTB. However, the... 

The proposed intermediate m this scheme represents an alternate mode of coordination for two H ligands—as a coordinated H2 molecule. Although the intermediate in Eq. 15.28 has not been isolated, a number of other dihydrogen complexes have been The first was prepared in I983 32... 

Dihydrogen complexes may be intermediates in the oxidative addition of H2 to a metal14b (equation 1) and in the hydrogenolysis of d° alkyls. They will also need to be considered as possible species present in hydrogenases and nitrogenase.14f... 

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