Carbon dioxide reactions with hydride complexes

The study of the interaction of carbon dioxide (CO2) with covalent-polar M-H bonds reveals quite a different complexity and intriguing reaction mechanisms with respect to the reaction with ionic metal hydrides, as discussed in Sect. 3.1. As a matter of fact, the interaction of M-H bonds with CO2 has been debated for quite a long time [1], and the insertion of the cumulene into M-H bonds has been shown to follow two routes, as depicted in Scheme 4.1, affording either the M-OCHO group or the M-COOH moiety, both having an applicative interest. 

The formato complex 0sH(K2-02CH)(C0)(P Pr3)2 has also been prepared by treatment of the hydride-chloro compound OsHCl(CO)(P Pr3)2 with NaOMe in benzene-methanol under carbon dioxide atmosphere. Under the same conditions, the reaction with carbon disulfide affords OsH(K2-S2COMe)(CO)(P Pr3)2.67... 

Since several cationic carbonyls react with water to give carbon dioxide and a metal hydride (18), it is predicted that Complex XV or XXII should be catalysts for the water gas shift reaction. Preliminary results indicate that this is correct and catalysis occurs slowly at 100°C in aqueous methanol, with the slow step in the catalytic cycle being the reaction of Complex XXII with water to regenerate Complex XV with the evolution of C02. 

In the past, this field has been dominated by ruthenium, rhodium and iridium catalysts with extraordinary activities and furthermore superior enantioselectivities however, some investigations were carried out with iron catalysts. Early efforts were reported on the successful use of hydridocarbonyliron complexes HFcm(CO) as reducing reagent for a, P-unsaturated carbonyl compounds, dienes and C=N double bonds, albeit complexes were used in stoichiometric amounts [7]. The first catalytic approach was presented by Marko et al. on the reduction of acetone in the presence of Fe3(CO)12 or Fe(CO)5 [8]. In this reaction, the hydrogen is delivered by water under more drastic reaction conditions (100 bar, 100 °C). Addition of NEt3 as co-catalyst was necessary to obtain reasonable yields. The authors assumed a reaction of Fe(CO)5 with hydroxide ions to yield H Fe(CO)4 with liberation of carbon dioxide since basic conditions are present and exclude the formation of molecular hydrogen via the water gas shift reaction. H Fe(CO)4 is believed to be the active catalyst, which transfers the hydride to the acceptor. The catalyst presented displayed activity in the reduction of several ketones and aldehydes (Scheme 4.1) [9]. 

Carbon dioxide readily undergoes insertion reactions probably via initial C02 complexing, with metal alkoxides, hydroxides, oxides, dialkylamides, and metal hydrides and alkyls 103... 

Overall then the Pyruvate DH Complex converts pyruvate into acetyl CoA in a physiologically irreversible reaction with the release of carbon dioxide and the capture of an electron pair as a hydride ion on NADH. Note the cofactors involved for this reaction sequence TPP, FAD, Mg2+, lipoamide. Coenzyme A, and NAD+. 

Carbon dioxide is abundant and readily available, but its reaction with transition metal complexes has not been extensively studied. A few examples of carbon dioxide insertion are known. Thus, formic acid can be formed by the insertion of carbon dioxide into the cobalt hydride bond U9>,2°). 

The proposed reaction pathway (Figure 1) is likely to involve i) the reduction of the Pd(II) complex 2 via hydride intermediate 3, by carbon monoxide and water, affording carbon dioxide and a Pd(0) species 4 ii) the oxidation of this Pd(0) species by oxygen with formation of a Pd(II) peroxo-complex 5 in) the reaction of the latter species with an acid, producing hydrogen peroxide and restoring the initial Pd(II) complex [4]. 

The transition metals can be reduced in basic aqueous solutions via other mechanism. For example, the metal carbonyls could be attacked by the hydroxide ions and the metal reduced to metal hydride species by the elimination of carbon dioxide to yield a hydride, which could then be deprotonated with the excess of hydroxide ions (Figure 17). The reduction of the metal complexes by CO in aqueous phase is indeed a very important step in the Reppe-type catalysis and water-gas shift reactions. 

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