Hydrogen activation heterolytic splitting

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.

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. 

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. 

As pointed out earlier, the principal requirement for an active catalyst for the heterolytic splitting of hydrogen is the presence of two suitably disposed functional groups—a metal atom to combine with the hydride ion and a base ( B) to act as a proton acceptor. In line with the evidence for the presence of a ferrous complex in hydrogenase, Rittenberg (18) has suggested the following model for the active site of the enzyme. 

This remarkable set of reactions represents another excellent model for hydrogen activation on metal sulfides, and it is to date the only example available of the direct observation of a heterolytic activation of hydrogen assisted by a terminal sulfide ligand. Like Bianchini s Rh complex mentioned above (Eq. 5.7), this is in line with solid state NMR evidence for this latter type of hydrogen splitting on RuSj which has been provided by Lacroix et al., indicating die formation of Ru-H and -SH species on the surface [19] it is possible that other metal sulfides are also capable of activating Hj by similar routes. 

A hydrophobic channel which leads from the protein to this side of the binuclear site is taken as further support that this vacancy or CO-binding site is the site of Fl2 activation (Fl2-binding or Fl2 evolution). As this site is close to basic protein residues (a nearby lysine forms a hydrogen bond with the CN ligand and, at a more oxidized stage, a cysteine residue is Fl-bonded to H2O bound at this site), heterolytic splitting of an with the formation of an Fe-bound hydride, is... 

Additional catalyst development identified the positive effect of 1,2-diamines as additives in the (BlNAP)Ru(OAc)2-catalyzed enantioselective hydrogenations of ketones [24], This discovery ultimately led to the synthesis of a class of (diphosphine) Ru(diamine)X2 (X = H, halide) compounds [25] (Figure 4.1) which have emerged as some of the most active and selective hydrogenation catalysts ever reported [26]. Mechanistic studies by Noyori [14] and Morris [27] have established bifunctional hydrogen transfer to substrate from the cis Ru-H and N-H motifs and identified the importance of ruthenium hydridoamido complexes for the heterolytic splitting of H2. This paradigm allows prediction of the absolute stereochemistry of the chiral alcohols produced from these reachons. 

A likely intermediate in hydrogen activation reactions, including heterolytic splitting, is a hydrogen species where H2 is 7] -bonded to the metal center. The first stable dihydrogen molecule (and recognized as such) has been isolated by Kubas et al. in 1983, and some thermochemical studies on this type of molecules have appeared since then." " The enthalpy of reaction (57), AJP (57) = —30.5, —27.2, and —40.2 kJ moP for M=Cr, Mo, and W, respectively, measures DH° (M-H2) ... 

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