Heterolytic splitting

Heterolytic Splitting of H-H, Si-H, and Other sigma Bonds on Electrophilic Metal Centers Gregory J. Kubas... 

An alternative way in which an electron-poor site could be created would be via the heterolytic splitting of molecular hydrogen i.e.,... 

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.

The efficiency of the orthometallated Pd(II) complexes [Pd(NC)X]2 [39, 40] (3 in Scheme 4.3) in coordinating solvents such as DMF or DMSO was considered to be in favor of the initial dissociation of the dimers 3 into the mononuclear species 6 (Scheme 4.6). The heterolytic splitting of H2 leads to the formation of the hydride 7 plus HC1 or AcOH. The solvato-hydride intermediate 7 was characterized spectroscopically and its elemental analysis furnished [39] regrettably, no data regarding its catalytic activity were reported. 

Scheme 4.9 Heterolytic splitting of H2 and coordination of phenylacetylene in Pd(NN S)CI complexes.

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 2.28. T 2-HSiR3 complex, nucleophilic attack, and heterolytic splitting [20]...

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

Figure 3.5. Continued. The H2-NAD reaction is inhibited neither in air nor in the presence of CO. C,The possible reactions of hydrogen with the Fe-Fe site of active [Fe]-hydrogenases. In the oxidized state, the bimetallic center shows a S = 1/2 EPR signal, presumably due to an Fe -Fe pair (an Fe -Fe pair cannot be excluded). Whether the unpaired spin is localized on iron (Pierik et al. 1998a) or elsewhere (Popescu and Mtlnck 1999) is not known. Hydrogen is presumably reacting at the vacant coordination site on Fe2 (Fig. 3.1C). After the heterolytic splitting, the two reducing equivalents from the hydride are rapidly taken up by the Fe-Fe site (one electron) and the attached proximal cluster (one electron). Subsequently, the electron is transferred from the proximal cluster to the other Fe-S clusters in the enzyme. Under equilibrium conditions, the proximal cluster in the active enzyme appears to be always in the oxidized [4Fe-4S] state (Popescu and Mtlnck 1999). Protons are not shown.

Heterolytic splitting would lead to an initial Co-D/Co-H ratio in the product of 1.0. (R = 0.53). It is apparent that this is not the case. Since D2O does not enter into the homolytic splitting, R should equal 0. At early times this appears to be so. Why might the value slowly increase ... 

In the early nineteen-sixties Halpem, James and co-workers studied the hydrogenation of water-soluble substrates in aqueous solutions catalyzed by rathenium salts [6]. RuCh in 3 M HCl catalyzed the hydrogenation of Fe(III) to Fe(II) at 80 °C and 0.6 bar H2. Similarly, Ru(IV) was autocatalytically reduced to Ru(III) which, however, did not react further. An extensive study of the effect of HCl concentration on the rate of such hydrogenations revealed, that the hydrolysis product, [RuCln(OH)(H20)5. ] " was a catalyst of lower activity. It was also established, that the mechanism involved a heterolytic splitting of H2. In accordance with this suggestion, in the absence of reducible substrates, such as Fe(in) there was an extensive isotope exchange between the solvent H2O and D2 in the gas phase. 

Alternative possibilities, however, involving the formation of HgH+ as the initial product of the rate-determining step, through heterolytic splitting of hydrogen by Hg++ or homolytic splitting by Hgf, cannot be excluded. This species, if it is formed, would probably dissociate in aqueous solution into Hg and H+. 

There is some doubt about the kinetics of the activation of hydrogen by cuprous acetate in the closely related solvent, pyridine. Wright, Weller, and Mills (34) have reported that the rate-law in this solvent (and in dodecyl-amine) is first-order in cuprous acetate, suggesting heterolytic splitting of hydrogen. On the other hand, Wilmarth (33) has observed a second-order dependence similar to that in quinoline. The reasons for this discrepancy and for the difference between pyridine and quinoline, if real, are not clear. 

Aqueous solutions containing cobaltous cyanide complexes (the prevalent complex in a freshly prepared solution is believed to be Co(CN)6 ) absorb hydrogen rapidly ( 5 min.) at temperatures as low as 0°, the total uptake corresponding to that required to reduce all the cobalt to the +1 state. It has been suggested (Mills et al., 50) that the mechanism of the reaction involves the heterolytic splitting of hydrogen, i.e.. 

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