Metallocarboxylic acid

C02-Bridged bimetallic zirconocene complexes have been formed from 1 and metallocarboxylic acids [229]. Reachon of 1 with metal enolates Cp(CO)3WCHR COX (X = OEt, Me, Ph) gives Cp(CO)3WCH(R )CH(R)OZrCp2(Cl). The structure for R = H and R = Me was solved by an X-ray analysis and the chemical reactivity of these organometallic products have been studied [230]. 

These observations illustrate that there are two transformations open to metallocarboxylic acid intermediates reversible loss of OH" accompanied by oxygen exchange, and metal-hydride formation with expulsion of C02. Our entry into this area of chemistry was in 1975 when extensive studies of oxygen lability in metal carbonyl cations were initiated (10). These... 

One involves hydroxide attack, leading to the formation of a metallocarboxylic acid (species 11 in Figure 6), and is evident in the Fe(CO) /base-catalyzed system ( 1). The other involves the formation of a readily hydrolyzable, zwitterionic metallocarboxa-mide, 12, in accord with the work of Edgell (35,36) and is evident in the Ru (CO) 2/NMe3 system. 

Although suggested to operate by a mechanism analogous to Scheme 14, the case of the catalysis by the mononuclear complex Fe(CO)5 was proposed to be more limited.86,88 Relative to the Ru catalyst, the equilibrium constant for formation of the metallocarboxylic acid adduct, Fe(C0)4(C02H)-, was found to be several orders... 

The mechanism is displayed in Scheme 22. The proposed short-lived metalloester intermediate is essentially the analog of the metallocarboxylic acid intermediate that was proposed to be important in the water-gas shift reaction mechanism (e.g., Scheme 20). 

A mechanism similar to Scheme 10 was proposed, involving CO addition, followed by H20 addition (in lieu of hydroxide anion) to form a metallocarboxylic acid complex. Then, decomposition to C02 and a metal hydride was proposed, followed by hydride elimination. Table 15 provides data from reaction testing in the temperature range 140 to 180 °C. In later testing, they compared Rh and Ir complexes for the reduction of benzalacetone under water-gas shift conditions. 

They focused their research on answering the question as to whether catalysis proceeds via formate anion as an intermediate, such that dissociation of a CO ligand is the first step in the mechanism Cr(CO)6 -> Cr(CO)5 + CO followed by nucleophilic attack by formate anion i.e., from CO + OH- -> HCOO-) to produce the formate species Cr(CO)5 + HCOO- > Cr(CO)5(OOCH) , according to King et al.5 in Scheme 18 or whether a metallocarboxylic acid forms upon... 

For the [Pdltriphosphinejlsolvent)] " " complexes, the metallocarboxylic acid formed in step 3 of Sch. 2 is not ready to undergo C—O bond cleavage. In order for this reaction to occur, an additional electron transfer, solvent loss, and a second protonation have to occur. Of particular interest in this sequence is the loss of a weakly coordinated solvent molecule (step 5), to produce a vacant site on the metal for water to occupy as the C—O bond of CO2 is broken to form coordinated CO and coordinated water [34, 35]. This C—O bond cleavage reaction is the slow step in the catalytic cycle for these catalysts at low acid concentrations, and a vacant coordination site is required for this reaction to occur. C—O bond cleavage is also the slow step for Fe(porphyrin) catalysts at low acid concentrations (H+, Mg +, or CO2) [37-39]. In this case, a vacant coordination site is not required. However, the potentials at which catalysis occurs in this case (approximately —2.0 V vs. ferrocene/ferrocen-ium) is much more negative than those... 

The insertion of carbon dioxide into a transition metal-hydrogen bond may be seen as the first and crucial step in the reduction or fixation of C02. This insertion could proceed in either of two ways to produce a formate complex, either mono- or bi-dentate [(31) or (32), respectively], or to form a metallocarboxylic acid, (33). 

The only claim for the production of a metallocarboxylic acid from the insertion of C02 into a metal-hydrogen bond in the opposite sense is based on the reaction of C02 with [HCo(N2)(PPh3)3] (108, 136). The metallocarboxylic acid is said to be implicated since treatment of the product in benzene solution with Mel followed by methanolic BF3 yielded a considerable amount of methyl acetate as well as methyl formate derived from the cobalt formate complex. Metallocarboxylic acid species formed by attack of H20 or OH- on a coordinated carbonyl are considered in the section on CO oxidation. 

In principle the insertion of C02 into a transition metal hydrogen bond can result in either M-0 or M-C bond formation, i.e., production of metalloformate (4) or metallocarboxylic acid (5) derivatives. Thus far,... 

The first step in the formation of HFe(CO)4, from the reaction of Fe(CO)5 and aqueous hydroxide ions, involves an attack of the OH ion on the CO group to give a metallocarboxylic acid.112... 

Conversion of a metallocarboxylic acid to a hydride complex is supposed to occur via a hydrogen transfer to the metal as CO2 is eliminated ... 

Instead, we found a strikingly different route for the macrocychc cobalt systems. As shown in Scheme III, the metallocarboxylic acid (1) is essentially unreactive, but its conjugate base, the metallocarboi late (2), can eliminate CO2 at s to form the Co(I) complex (3). The Co(I) complex can then undergo protonation by available proton sources to form the cobalt(III) hydride (4) (24),... 

The formation of the H2Fe(CO)4 catalyst described above presumably occurs via nucleophilic attack of hydroxyl ion on a CO ligand of Fe(CO)s with generation of the anionic metallocarboxylic acid followed by decarboxylation and protonation (Reaction 6). 

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