Heterolytic H2 cleavage

A net heterolytic H2 cleavage was evidenced for [Pd(Salen)] (2 in Scheme 4.2) [29, 30]. Its catalytic activity was pH-dependent namely, it was inhibited by acids and enhanced by bases. A kinetic study of the hydrogenation of 1-hexene in DMF led to the definition of the following mechanism (Scheme 4.10) ... 

Fong H, Moret M-E, Lee Y, PetersJC. Heterolytic H2 cleavage and catalytic hydrogenation by an iron metaHaboratrane. Organometallics. 2013 32 3053-3062. 

The complexes of o bonds have indeed original properties and present a great interest from a theoretical point of view as intermediates of (a) oxidative addition and (b) heterolytic H2 cleavage. 

Radical mechanisms, mechanisms involving a-bond metathesis with d° complexes, and mechanisms involving heterolytic H2 cleavage are also known. 

The heterolytic H2-cleavage reaction of Eq. (1) is reversible. Hence, Cu(II) and Ru(III) can catalyze the isotopic exchange between hydrogen gas and water through forward and backward processes of Eq. (1) ... 

Niu S, Hall MB (2001) Modeling the active sites in metalloenzymes 5. The heterolytic bond cleavage of H2 in the [NiFe] hydrogenase of Desulfovibrio gigas by a nucleophilic addition mechanism. Inorg. Chem. 40 6201—6203... 

The reaction of OsHCl(CO)(P Pr3)2 with HC1 gives the dichloro derivative OsCl2( n2-H2)(CO)(PIPr3)2.35 In solution, this complex is stable under argon for a matter of days. However, the dihydrogen unit is highly activated toward heterolytic cleavage, as demonstrated by deprotonation with NaH and by reactions with carbon monoxide and /ert-butyl isocyanide, which afford OsHCl(CO)L(P Pr3)2 (L = CO, r-BuNC) and HC1. 

Kinetic data were interpreted in terms of a mechanism in which initial heterolytic cleavage of H2 involved the basic 0 ligand coordination of olefin at the generated free equatorial site, followed by insertion to give alkyl and then protonation of the Pd-alkyl bond by the phenolic OH group, completes the catalytic cycle ... 

Eq. (8) requires determination of the two-electron oxidation potential of L M by electrochemical methods. When combined with the two-electron reduction of protons in Eq. (9), the sum provides Eq. (10), the AGh- values of which can be compared for a series of metal hydrides. Another way to determine the AGh-entails the thermochemical cycle is shown in Scheme 7.3. This method requires measurement of the K of Eq. (11) for a metal complex capable of heterolytic cleavage of H2, using a base (B), where the pK., of BH+ must be known in the solvent in which the other measurements are conducted. In several cases, Du-Bois et al. were able to demonstrate that the two methods gave the same results. The thermodynamic hydricity data (AGh- in CH3CN) for a series of metal hydrides are listed in Table 7.4. Transition metal hydrides exhibit a remarkably large range of thermodynamic hydricity, spanning some 30 kcal mol-1. 

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