Carbon metallocarboxylate

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). 

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. 

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],... 

Metallocarboxylate anions, in particular, are very effective oxide transfer agents.17,19 Reaction of Li2W(C0)5(C02) with additional C02 results in formation of W(CO)6 and Li2C03, a reaction described as reductive disproportionation. This dianion would transfer oxide to CpFe(CO)3+BF4 labeling studies showed that oxygen, but not carbon, of the C02 ligand was incorporated into the iron product [CpFe(CO)2]2. Intermediate metalloanhydrides have been proposed to rationalize intramolecular oxide transfer from coordinated C02 to coordinated CO in several systems. 

These considerations now provide a guideline for the development of other potential catalysts for the use of CO + H2O in the hydroformylation of olefins. If the catalyst is to function in the same manner as just described for Fe(CO)s, then a minimum requirement is that the system form a metal carbonyl which will be readily attacked by a weak base to form an anion analogous to 1. A weak base is essential because CO2 is an inevitable by-product, and only the carbonate salts of weak bases regenerate the base and CO2 upon heating. Thus, if the system is to be catalytic in base as well, then clearly only a weak base can be used. This would appear to be the critical requirement, for the literature indicates that metallocarboxylic acids readily decarboxylate (12), and the final step in Reaction 6, the protonation of a hydridometalcarbonyl anion, would seem to offer no problem provided the catalyst system was not in a highly basic medium. 

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]. 

Anion dissociation during the second reduction step create the vacant site. The key step is the formation of a metallocarboxylic complex, formed by nucleophilic attack of the formal Re(-I) center on the electrophilic carbon of the CO2 substrate. Decomposition of this intermediate by protonation leads to the formation of carbon monoxide, water and the starting rhenium complex. 

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