Rate of CO2 insertion

Because of the extreme CO lability of the W(C0)50R species, CO loss might be a prerequisite for CO2 insertion. However, the rate of CO2 insertion is not inhibited by the presence of carbon monoxide. Hence, we believe that an open coordination site is unnecessary for the insertion process to occur. The reaction is thought to involve a concerted insertion process, similar to that proposed and well documented for the insertion of CO2 into CH3W(C0)5. Nevertheless, insertion of CO2 into the W-OR bond is more facile than the corresponding process involving W-R. 

As many carbonate complexes are synthesized usually in aqueous solution under fairly alkaline conditions, the possibility of contamination by hydroxy species is often a problem. To circumvent this, the use of bicarbonate ion (via saturation of sodium carbonate solution with COj) rather than the carbonate ion can often avoid the precipitation of these contaminants. Many other synthetic methods use carbon dioxide as their starting point. Transition metal hydroxo complexes are, in general, capable of reacting with CO2 to produce the corresponding carbonate complex. The rate of CO2 uptake, which depends upon the nucleophilicity of the OH entity, proceeds by a mechanism that can be regarded as hydroxide addition across the unsaturated C02. There are few non-aqueous routes to carbonate complexes but one reaction (3), illustrative of a synthetic pathway of great potential, is that used to prepare platinum and copper complexes. Ruthenium and osmium carbonate complexes result from the oxidation of coordinated carbon monoxide by dioxygen insertion (4). ... 

The kinetics of CO2 insertion with cw-[WMe(CO)4L] , L = CO or PR3, yielding ci5-[W(0C(0)Me)(C0)4L] , were determined. When L = PMc3 the rate is 250 times faster than when L = CO (Table 10.10) with the reaction being first order in both [complex] and [CO2]. The structure of [Na(18-crown-6)(THF2)]-[WMe(CO)5] provides support for the proposal that the enhanced rate of insertion is related to neutralization of the negative charge buildup on the acetate ligand by Na coordination. 

The relative rates of the insertion reaction were found to be CS2 > SCO > CO2 with Cr > W. The substitution compounds [M(CO)4(L)(OAr)] were found to undergo insertion of CO2 much more slowly than the parent carbonyl with L = P(OMe)3 > PMc3 > PPh3 and in this case W > Cr. It was argued there was a steric inhibition of the insertion reaction. It was also found that the more bulky aryloxide compound [W(CO)5(OC6H3Ph2-2,6)] did not react with CO2 but would cleanly form the thiocarbonate (S-bound) with SCO. " The mechanism of these reactions is believed to... 

The relative rates of insertion appear to reflect steric as well as electronic factors, and there is a significant correlation with the Cp3Th-R bond disruption enthalpies discussed above. It was also found that the rate of CO insertion could be significantly accelerated by photolysis. However, secondary reactions of the resulting acyl were noted. A comparative study of CO2 migratory insertion to yield bidentate carboxylates ... 

Fig. 12.12 Top Fe and Fe release from syn- tion of goethite in 0.2 M H2C2O4/K2C2O4 mix-thetic goethite, CO2 production and con- tures underthe influence of UV light as measured sumption as a function oftime in the presence of by the level of consumed. Insert oxalate ad-0.025 M oxalate at pH 2.6 and 25 °C. Bottom The sorption by goethite as a function of pH (Cornell effect of pH on the rate of photochemical dissolu- Schindler, 1987, with permission).

Electrochemical reductions of CO2 at a number of metal electrodes have been reported [12, 65, 66]. CO has been identified as the principal product for Ag and Au electrodes in aqueous bicarbonate solutions at current densities of 5.5 mA cm [67]. Different mechanisms for the formation of CO on metal electrodes have been proposed. It has been demonstrated for Au electrodes that the rate of CO production is proportional to the partial pressure of CO2. This is similar to the results observed for the formation of CO2 adducts of homogeneous catalysts discussed earlier. There are also a number of spectroscopic studies of CO2 bound to metal surfaces [68-70], and the formation of strongly bound CO from CO2 on Pt electrodes [71]. These results are consistent with the mechanism proposed for the reduction of CO2 to CO by homogeneous complexes described earlier and shown in Sch. 2. Alternative mechanistic pathways for the formation of CO on metal electrodes have proposed the formation of M—COOH species by (1) insertion of CO2 into M—H bonds on the surface or (2) by outer-sphere electron transfer to CO2 followed by protonation to form a COOH radical and then adsorption of the neutral radical [12]. Certainly, protonation of adsorbed CO2 by a proton on the surface or in solution would be reasonable. However, insertion of CO2 into a surface hydride would seem unlikely based on precedents in homogeneous catalysis. CO2 insertion into transition metal hydrides complexes invariably leads to formation of formate complexes in which C—H bonds rather than O—H bonds have been formed, as discussed in the next section. 

The carboxylation reaction shown in reaction (11) is catalyzed by both nickel and palladium phosphine complexes. For example, Ni(dppe)Cl2 (where dppe is l,2-bis(diphenylphosphino)ethane) and Pd(PPh3)2Cl2 both catalyze reaction (11) [84-86]. Mechanistic studies have been carried out on these two systems, and the results indicate that two different mechanisms are involved. In the case of the Ni complex, the first step is the reduction of Ni(dppe)Cl2 to a transient Ni(dppe) species [85]. This process occurs in two one-electron steps (reaction 12). Bromobenzene then oxidatively adds to Ni(dppe) to form Ni(dppe)(Br)(Ph), reaction (13). The resulting Ni(II) aryl species is reduced in a one-electron process to form Ni(dppe)(Ph), which reacts rapidly with CO2 to form a Ni—CO2 intermediate as shown in reaction (14). The rate-determining step for the overall catalytic reaction is the insertion of CO2 into the Ni-aryl bond, reaction (15) step 1. This reaction is followed by a final one-electron reduction to regenerate Ni(dppe), the true catalyst in the cycle (reaction 15, step 2). 

With tetramethylsilane the only organic reaction product was the trimethylsilyl methyl mercaptan, (CHj)3SiCH2SH, as expected for a C—H bond insertion. Further evidence that the product arose from S( Z)) insertion was provided by the suppressing effect of CO2 on the reaction. A novel feature of this reaction is the large damping effect of (CH3)4Si on the CO rate, as shown by the product rate vs. substrate pressure plot in Figure 12. From simple stoichiometry the following relation should obtain for paraffinic insertion processes ... 

Kinetic studies made on [Pd(PP2)(PEt )[(BF )2> and reported elsewhere (45), indicate that the rate of catalysis is first order in CO2, first order in catalyst, and first order in acid at low acid concentrations. These results are consistent with the mechanism shown in Scheme 2. In comparison with Scheme 1, two important features should be noted. First in Scheme 2, the formation of a coordinatively unsaturated metal hydride complex is necessary for CO2 insertion to occur. A priori there is no way of knowing whether or not the generation of a coordinatively unsaturated metal hydride will be required for catalysis since evidence exists for both associative and dissociative pathways for CO2 insertion into metal hydride and metal carbon bonds (20-25). This is the reason that complexes of the types... 

Th-R bond disruption enthalpies. When this correlation was compared with the CO2 insertion, to generate carbonate complexes, it showed that carboxy-lation is significantly slower than carbonylation, and exhibits different trends on the dependence of rate on the alkyl ligand [223]. 

Figure 5.2. Cyclic voltammogram for the complex of stmcture 1(h) for M = cobalt. [CoL] 5 X 10 " M, in N,N -dimethylformamide. Scan rate 200 mVs (—) in nitrogen (—) in CO2. Insert is for the voltammogram registered in nitrogen atmosphere. Reprinted from Figure 3 M. Isaacs, J.C. Canales, M.J. Aguirre, G. Estiu, F. Caruso, G. Ferraudi and J. Costamagana, Electrocatalytic reduction of CO2 by aza-macrocylic complexes of Ni(II), Co(II) and Cu(II). Theoretical contribution to probable mechanism, Inorganica Chimica Acta, 339 (2002) 224-232. Copyright 2002, with permission of Elsevier.

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