Metal bicarbonate complexes

Inorganic ligands in aqueous solutions, and in particular in natural freshwaters, include, in addition to H2O and OH, the major ions carbonate and bicarbonate, chloride, sulfate and also phosphate [29], The distribution of metal ions between these ligands depends on pH and on the relative concentrations of the ligands. The pH is a master variable with regard to the occurrence of hydrolysed species and to the formation of carbonate and bicarbonate complexes. 

The use of CO2 as a reagent for synthetic purposes would be highly desirable, due not only to the vast availabiUty of this gas but also its environmental concerns. The stoichiometric activation of CO2 has been achieved with the iridium-PCP complex 29 comprising an alkyl rather than an aryl skeleton (Scheme 12.12) [32]. The addition of CO2 to the dihydride complex results in C=0 insertion into the iridium-hydride bond, and affords the formate complex 30. However, this complex is not stable and disproportionates spontaneously into the virtually insoluble bicarbonate complex 31 and the carbonyl dihydride 32. Such disproportionation is suppressed when the iridium metal center is replaced by rhodium [33], which is generally assumed to have a lower hydride affinity than iridium. 

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 Cu(l) bicarbonate complex previously mentioned (77) was synthesized by the reactions summarized in Scheme 7, which includes C02 insertion into copper hydroxide and alkoxide species. The insertion reaction of C02 with metal hydroxides to form bicarbonates is believed to occur... 

Formation of Carbon Dioxide Complexes. As mentioned in the introduction, our initial interest in synthesizing the PCy3 complexes was in their potential for binding C02. However, except for the formation of peroxycarbonate and carbonato complexes from IrCl(02)(PCy3)2 (44), which is well-established chemistry for some platinum metal peroxide complexes (42) (but, to our knowledge, not with PCy3 systems), we have not been able to isolate any C02 complexes or even carbonate or bicarbonate species which are formed sometimes in the presence of adventitious water (16). 

Carbonate complexes are usually synthesized from carbonate or bicarbonate anions in alkaline aqueous solution. The reaction of metal hydroxides with carbon dioxide is another route to carbonate complexes. Other methods include the oxidation of metal carbonyl complexes (equation 17), and the oxidation of metal salts in the presence of carbon dioxide. 

However, for kinetic considerations CO generally requires a transition metal catalyst for its potential as a reducing agent to be realized. Indeed, use of CO as a reductant with consequent formation of CO2 is well documented in transition metal chemistry and represents one of the principal methods for synthesizing metal carbonyl complexes. If the reaction is carried out in aqueous base, then carbonate or bicarbonate results as the CO oxidation product. Several representative reactions are shown ". Further examples are given in ref. 2. 

Beyond the biological process of sullate reduction with subsequent metal precipitation as sulfides, other mechanisms of metal removal can be present during the runs, particularly in the inoculated column (A) precipitation as metals carbonates using the bicarbonate and/or carbonates formed during the reduction of sulfate by the SRB cells or by fermentation from other microorganisms, precipitation as metal hydroxides, complexing with substances excreted by the cells, and accumulation on the surface of cells, through reactions between metal ions and cell wall components [10]. 

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). ... 

Post-synthesis modification comes into its own when preparing molecular sieves with desirable and exploitable properties other than those of ion exchange, be they optical, magnetic, catalytic or adsorptive. Here it is not directly the thermodynamic and kinetic ion exchange properties that are of prime importance but rather which experimental, preparative methods are most commonly used. Thus it is important to assess what are the most appropriate experimental methods of preparation, as well as to review the many pitfalls one can fall into which can subsequently give rise to very inaccurate and inadequate experimental data. These experimental problems can include framework hydrolysis, hydronium exchange, dealvunination, the presence of key trace impurities, ssolution phenomena, carbonate and bicarbonate interference, colloidal phenomena, metal ion complex formation and cation hydrolysis. 

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