Dihydrogen heterolytic splitting

Scheme 3.4 The heterolytic splitting of dihydrogen at Ru(ll) to give a hydridic-protonic bond, as proposed by Chu et al. [55] in the mechanism of the homogeneous hydrogenation of carbon dioxide.

We have already reviewed the activation of alkenes, alkynes, and carbon monoxide towards nucleophilic attack. The heterolytic splitting of dihydrogen is also an example of this activation it will be discussed in Section 2.10. The reaction of nucleophiles with silanes co-ordinated to an electrophilic metal can be regarded as an example of activation towards nucleophilic attack (Figure 2.28). Complexes of Ir(III) and Pd(II) give t.o.f. for this reaction as high as 300,000 mol.mol. fh"1. 

Figure 2.32. Heterolytic splitting of dihydrogen by a "base-metal" pair...

The H H interactions (1.75-1.9 A) here and related systems are referred to as proton-hydride bonding by Morris (60-62) and dihydrogen bonding by Crabtree (63-65), who along with others (66-68) have studied or reviewed such unconventional hydrogen bonds that include M-H H-M, M-H H-X, and X-H a interactions in general (X = C, N, P, 0, etc). These complexes represent intermediates in the heterolytic splitting of H2 and illustrate both the basicity of the M-H... 

Dihydrogen is said to be heterolytically cleaved when it is dissociated into a proton and a metal bonded hydride. It has been the topic of much study and discussion, and the evidence for its occurrence has been growing steadily [54,55]. In the ideal case the heterolytic splitting is catalyzed by the metal ion and a base which assists in the abstraction of the proton. In this reaction there is no formal change in the oxidation state of the metal. The mechanism has been proposed for Ru(II) complexes which can react with dihydrogen according to ... 

In addition to catalyzing hydroformylation, the platinum SPO complexes are excellent hydrogenation catalysts for aldehydes (as already demonstrated by the side products of hydroformylation), in particular, in the absence of carbon monoxide. Further, in ibis process, the facile heterolytic splitting of dihydrogen may play a role. The hydrogenation of aldehydes requires the presence of carboxylic acids, and perhaps the release of alkoxides from platinum requires a more reactive proton donor than that available on the nearby SPO. For example, 4 hydrogenates 2-methylpropanal at 95 °C and 40 bar of H2 to give the alcohol, with a TOF of 9000 mol moN h (71). 

Non-classical Hydrogen Bonding along the Pathway to the Heterolytic Splitting of Dihydrogen... 

In the heterolytic splitting of dihydrogen, the acidic dihydrogen complex [M](ti2-H2) transfers a proton to the base (eq 2-4 of the introduction). The proton can then remain associated with the hydride in a 1.7-2.0 A contact in the ion-pair -[MJH—HB+. This may be an intramolecular interaction or an intermolecular interaction. In the following sections, our work on intramolecular hydridic-pro-tonic bonds will be reviewed first, followed by that on such intermolecular non-classical bonds. 

Since that time there have been many reports of intramolecular hydridic-protonic bonds [78-86]. Recently an intermediate with an intramolecular RuH—HN interaction has been implicated in the catalytic asymmetric reduction of ketones [87] and another, in the reduction of CO2 to formic acid [83]. In the last process, the RuH—HN bond is proposed to form via the heterolytic splitting of dihydrogen (see Scheme 4, Section 1.9). 

---