The Activation of Dihydrogen

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-... 

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. 

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. 

As is shown in this book, the activation of dihydrogen by single metal sites encompasses a broad spectrum of research fields, from biochemistry to chemistry and spectroscopy, hence to catalysis, industrial processes, and theoretical analyses. The studies outlined in this chapter are indicative of the range of ongoing applications and challenges of T 2-H2 metal complexes in homogeneous and heterogeneous catalysis. 

One mechanism consistent with many of the experimental observations [3] is shown in Figure 4. The initial activation of the binuclear site by reduction, protonation and consequent dissociation of water primes the site for binding dihydrogen. The activation of dihydrogen involves the formation of a hydride (proposed to bridge between the iron and nickel) and a coordinated thiol. In this manner dihydrogen is formally cleaved heterolytically into H- and H+. 

The activation of dihydrogen is a major topic in organometallic chemistry, and numerous structural as well as functional model systems for the H2ase enzymes were proposed, which have been the topic of many books and reviews. A detailed discussion of all those is found in a recent book chapter. The thoughts particularly intriguing to an organometallic chemist are as follows. 

Acknowledgments We are grateful to the financial support from the Swiss National Science Foundation, Lanxess AG, Leverkusen, Germany, the Funds of the University of Zurich, the DFG and SNF within the project Forschergruppe FORI 175 - Unconventional Approaches to the Activation of Dihydrogen. ... 

In addition to the activation of dihydrogen [29d, 30], the dearomatized pyridine-derived pincer complexes also activate O-H bonds of alcohols [11-18, 31] and water [29e, 32], N-H bonds of amines and ammonia [29f,h], sp C-H [29c] and sp C-H bonds [29a,g], and carbon dioxide [33]. Among these various bond-activation reactions, of particular interest here is the O-H bond activation of alcohols, as complex 8 reacts with alcohols to provide the aromatic coordinatively saturated hydrido-alkoxy complexes 12 (Scheme 1.4), indicating the possibility of catalytic transformations based on dehydrogenation of alcohols. 

Along with the example of Scheme 13, other It2 and compounds show analogous reactivity patterns toward H2 (Scheme 14). Remarkably, the formation of a hydride bridge that replaces the metal-metal bond is observed in all cases, which may be due to the existence of a common mechanism for the activation of dihydrogen by these complexes (Schemes 13 and 14). A concerted pathway that entails coordination of the dihydrogen molecule in cis position to the intermetallic bond as first step has been postulated. Subsequently, single-site oxidative addition assisted by a shift of the electron density at the metal-metal bond would result in the formation of a hydride-bridged bimetallic complex. 

Recall from Section 1.9 that the Lewis acid and base in a frustrated pair are too bulky or otherwise sterically constrained to bond with each other but the cleft between them acts like an enzyme active site and promotes remarkable chemical reactions, perhaps none more so than the activation of dihydrogen at or below room temperature. Two representative reactions mediated by P,B-based FLPs are as follows ... 

The reaction scheme represents the most important steps of the catalytic process the activation of dihydrogen and olefin. Wilkinson assumed that total dissociation of one phosphine ligand in the complex [RhCl(PPh3)3] occurs. However, later investigations by P NMR led to the conclusion that the dissociation of the phosphine ligand is minimal. 

The activation of dihydrogen to form the hydride MH" " is the simplest reaction of an M" " ion, and this was one of the first reactions examined. Numerous studies of bare and ligated rare earth and actinide ions with a variety of small inorganic molecules followed, particularly with O-atom donors, with the purpose of obtaining oxide ions and characterizing their energetics. 

Bertrand also demonstrated that CAAC 110 could activate dihydrogen to give compound 111 (Scheme 1.16). Contrary to the activation of dihydrogen by electrophilic transition metals, the carbene acted primarily as a nucleophile creating a hydride-like hydrogen, which subsequently reacted with the vacant p orbital of the positively polarized carbene carbon atom. Importantly, this activation was not observed with the slightly less nucleophihc imidazol-2-yli-denes and imidazolin-2-ylidenes. 

There is often no reaction whatsoever between a coordinatively unsaturated metal complex and H2. As Morris said The first step in the activation of dihydrogen is the formation of a transition state, intermediate or product stracture in which an essentially intact H2 molecule is bound side-on to the metal center [46]. In order for H2 to bind to a coordinatively unsaturated metal complex, it must displace any intermolecular and/or intramolecular interactions that are stabilizing the vacant site. The formation of an / -H2 complex, even transiently, is often a difficult task since intact H2 is generally the most weakly binding ligand for a given complex [47]. 

Recent Advances in the Activation of Dihydrogen by Synthetic Compiexes... 

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