Metallocarboxylic

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. 

Pearson and Mauermann/Squires—metallocarboxylate intermediates as C02 precursor over Fe carbonyl catalysts. In 1982, Pearson and Mauermann65 studied two reaction steps of the water-gas shift mechanism involving Fe(CO)5 in basic media using the infrared cell of Ford these included (a) Fe(CO)5 + OH- <-> HFe(CO)4 + C02 and (b) H2Fe(CO)4 <-> H2 + Fe(CO)4. For reaction (a), they proposed the following mechanism, as shown in Scheme 26 ... 

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... 

Carbon dioxide reduction is thought to proceed via metallocarboxylate intermediate (s) formed by coordination of CO2 to the electron-rich Re center, although discrete steps in the process cannot be unambiguously assigned. The timing of Cl displacement from and CO2 adduction to the Re(bpy) (CO)3 unit are important mechanistic parameters. Most interpretations are based on a one-electron pathway, involving the interaction of CO2 with the product of Eq. (5) a two-electron pathway, involving interaction of CO2 with the product of Eq. (6) or a combination of these steps. Additional mechanistic considerations are the role dimeric rhenium intermediates and likely proton sources. 

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. 

All known C02 insertions into a metal-carbon bond result in carbon-carbon bond formation, except in one instance. The insertion of C02 with formation of a metallocarboxylate ester is claimed in the reaction of C02 with a cobalt complex (108, 139). Two species were isolated from the reaction of C02 with a mixture of acrolein and the complex CoH(N2)(PPh3)3, which was assumed to form Co(CO)(C2H5)(PPh3)2 before the C02 was introduced. The two reaction products were characterized by their ir spectra and chemical reactions, and formulated as Co(02CEt)2(PPh3)2 and the metallocarboxylate ester Co(C03) (COOEt)(PPh3), n = 0.5-1.0. Metallocarboxylate esters are well-known products from the attack of alkoxides on metal carbonyls. 

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 known C02 insertion reactions involving metal-carbon bonds have all resulted in carbor. -carbon bond formation with possibly one exception. Infrared spectral and chemical evidence has been presented for the formation of the metallocarboxylate ester Co(C03) (COOEt)(PPh3), n = 0.5-1.0 from the reaction of Co(CO)(C2H5XPPh3)2 with carbon dioxide from Vol-pin s laboratory (68). Although these studies are not conclusive for abnormal C02 insertion, metallocarboxylate esters are well-known compounds which result from the nucleophilic addition of alkoxides on the carbon center in metal carbonyls (69). 

Two different metal-C02 complex intermediates have been proposed for the production of CO-metallocarboxylates and metal formates. The difference between the two species is based on the site of protonation, at the carbon atom in metallocarboxylates and at one of the oxygen atoms in metal formates. Carbon-protonation has not been observed experimentally, while oxygen-protonation is well known [9]. Isomerization can occur between metallocarboxylates and metal formates, and loss of a hydroxide group from the metal formate species yields the M-CO complex. Similarly, the direct reaction of metal complexes with free, dissolved C02 has also been described. In this mechanism, the metal complex reacts with an oxide acceptor, such as C02, generating the metal-CO complex and C032- [9],... 

Formate production stems from similar metal-C02 intermediate species that yield CO as a product. Formate can be formed by the protonation of metal-C02 complexes through intermediates that have not been determined experimentally, namely the metallocarboxylate intermediate described above. A proposed mechanism for formate production by transition metal complexes also involves a metal hydride intermediate, where C02 actually inserts into the metal hydride bond to form the metallocarboxylate intermediate [9]. 

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... 

Re diimine complexes act as photocatalysts and/or electrocatalysts for CO2 reduction to CO. Examples include the tricarbonyl complexes yac-[Re(Q -diimine)(CO)3L]" [n = 0, L = halide n = 1, L = NCMe, P(OR)3 a-diimine = 1,4-disubstituted 1,4-diazabuta-l,3-dienes or bpy and related chelating N-heterocycles], for example, fac-[Re(dmb)(CO)3(NCMe)]+, 5 [Re(dmb)(CO)3]2" and fac-[Re(bpy)(CO)3 P(OPfl)3 ]+. Electron-transfer from an amine electron donor (e g. triethanolamine or triethylamine) to the excited state complex is usually considered as the initiation of the photocatalysis, and metallocarboxylates and metallo-carboxyUc acids have been proposed as intermediates in the formation of CO. The electrocatalytic process is triggered by a 1-electron or a 2-electron cathodically induced chloride (X) or L ligand dissociation to form the catalytic species. ... 

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