Metallocarboxylic acid complex

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. 

These lead to Fischer-type metal-carbene complexes that are mesomers of formyl, acyl and metallocarboxylic acid complexes (the latter decompose to hydrides). See also Chap. 5. 

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]. 

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... 

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. 

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),... 

As mentioned above, a variety of nucleophiles can form a bond with the carbon of the carbonyl ligand. One feature common to most of the reactions of various nucleophiles is that the bond formation between carbonyl carbon and nucleophile is more or less a reversible process. Alkoxycarbonyl or carbamoyl complexes formed from carbonyl complexes and alcohol or amine readily undergo the reversed course, namely C(0)-0R or C(0)-NR2 bond cleavage, either spontaneously or upon treatment with an acid (see e.g. Scheme 8.5). It is also noted that metallocarboxylic acids, M-COOH (typically M = PtR(PR3)2 R = Cl, Ph) tend to dissociate OH ion, rather than H+ in solution [12,13]. 

If the metal or overall complex is positively charged then H2O reacts with the activated CO and forms metallocarboxylic acid. 

In basic medium, in the first step, nucleophilic attack of OH on the coordinated CO molecule occurs and, as a result, a hydroxycarbonyl complex (metallocarboxylic acid) is formed. Such compounds are formed during the conversion of CO (water-gas shift reaction, wgsr). ... 

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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