Cobalt carbonato complexes

These equations are in line with Eq. (30), such that kx denotes the ring-opening rate constant of the protonated carbonato complex and / 2 is the decarboxylation rate constant of the ring-opened bicarbonato complex. Values of these rate constants and the acid dissociation constants of some protonated carbonato complexes of cobalt(TII) (see Table III) reflect the ligand dependence with respect to charge variations, steric constraint, and donor properties of the non-labile ligands. 

The following procedure is based on the reaction of an aqueous solution of cobalt(II) chloride with the equivalent amount of (2-aminoethyl)carbamic acid, followed by oxidation with hydrogen peroxide and the subsequent formation of bis(ethylene-diamine)cobalt(III) ions. The bis(ethylenediamine)cobalt(lII) species are converted to the carbonato complex by reaction with lithium hydroxide and carbon dioxide. During the entire preparation a vigorous stream of carbon dioxide is bubbled through the reaction mixture. This procedure appears to be essential in order to minimize the formation of tris(ethylenediamine)cobalt(III) chloride as a by-product. However, the formation of a negligible amount of the tris salt cannot be avoided. The crude salts have a purity suitable for preparative purposes. The pure salts are obtained by recrystallization from aqueous solution. 

To 27.5 g. (0.1 mole) of crude (carbonato)bis(ethylenediamine)-cobalt(III) chloride is added 200 ml. of 1.00 N hydrochloric acid. The carbonato complex is dissolved with evolution of carbon dioxide gas and formation of a red solution consisting primarily of the corresponding cw-diaqua species. The solution is evaporated in the steam bath until an almost dry paste has been formed. The purple residue is filtered and washed with three 20-ml. portions of ice-cold water. Drying in air yields 19.5 g. of purple crystals of cu-dichlorobis(ethylenediamine)cobalt(III) chloride. The mother liquor and the washings are again evaporated almost to dryness to yield a second crop of crystals, 5.9 g. The total yield is 25.4 g. (84% based on (carbonato)bis(ethylenediamine)cobalt(III) chloride). The analysis and the visible absorption spectrum of the two fractions are identical. Anal. Calcd. for [Co(en)2Cl2 ] C1 H20 Co, 19.42 N, 18.46 C, 15.82 Cl, 35.05 H, 5.98. Found Co, 19.50 N, 18.57 C, 15.77 C1, 35.15 H, 6.01. 

COBALT(III), RHODIUM(III), AND IRIDIUM(III) CARBONATO COMPLEXES OF THE PENTAAMMINE TYPE... 

The procedure described here is based on the observation that amine monohydroxo complexes of cobalt(III), rhodium(IIl), and iridium(III) react directly with carbon dioxide to form the corresponding carbonato complexes,2 3 without effect on the configuration of the amine ligands.4 The amine monoaqua complex is allowed to react with lithium carbonate or carbon dioxide gas at room temperature at pH 8.0 for a few minutes, and the carbonato complex is isolated by adding alcohol. The procedure has been used to prepare salts of the following cations pentaammine(carbonato)-cobalt(III),2 ds-ammine(carbonato)bis(ethylenediamine)cobalt(III),5 trans-... 

Cobalt(III)-carbonato complexes provide a very useful starting point for the preparation of other cobalt(III) complexes so that high yield synthetic methods are of great practical importance. Table 67 lists preparations for the major structural types (241 >-(244) and variations for substituted or related ligand systems are based on these. Recent reviews covering the wider aspects of transition metal carbonates are available.872 874... 

Table 69 Rates of CC>2

Tetraammine(carbonato)cobalt(III)] perchlorate (5.74 g, 0.020 mole) is dissolved in 30 mL of 3 M perchloric acid at room temperature with stirring. The carbonato complex dissolves rapidly evolving carbon dioxide. After 5 minutes the solution is filtered and cooled with ice. To the cold (approximately 5°)... 

Johri, K. N., Kaushik, N. K., Bakshi, K. Thin-layer chromatographic separation of copper (II), nickel(II), and cobalt(II) as thio-carbonato-complexes and determination by ring colorimetry. Chromatographia 5, 326 (1972)... 

The hydrolysis of chelated carbonato complexes of cobalt(III) is much faster in acid than in neutral solution. Explain. 

Some years later, Robert Auten, an undergraduate student at the University of Illinois, achieved an optical inversion in the reaction just mentioned (9). Patterning his work on that of Walden, Auten used silver carbonate instead of potassium carbonate and obtained a levo-rotatory carbonato complex. It was soon found that the choice of carbonate is not the important factor, and Dwyer, Sargeson and Reid, in Australia, showed that the pH of the solution is the deciding factor (10). After his initial success, Auten looked for inversions in the reactions of the dichloro complex with oxalate and nitrite, but did not find them. It is most fortunate that he tried the carbonate reaction first, for otherwise, we would have concluded that optical inversions do not take place in the reactions of cobalt complexes, and probably would not have tried carbonate. 

A large majority of diacidobis(l, 2-ethanediamine)cobalt(III) complexes can be synthesized from the (carbonato)bis(l, 2-ethanediamine)cobalt(III) ion. This is now readily available in high yield and purity.1 cis- and trans-Dichlorobis(l, 2-ethanediamine)cobalt(III) chlorides are obtained from the carbonato complex with hydrochloric acid using appropriate conditions. These isomeric dichloro salts are widely used starting materials for further syntheses. In this respect, the analogous dibromo complexes can be more convenient starting materials because bromide is substituted from the cobalt center more readily than is chloride ion. The milder conditions minimize the production of cobalt(II), a common side reaction. 

Into a solution of 3.3 g (0.009 mole) of cis-diamminecarbonatodi-(pyridine)cobalt(III) chloride monohydrate in 10 mL of water is poured 1.2 g (0.0038 mole) of sodium (—, 2-ethanediamine-bis(oxalato)cobaltate(III) monohydrate3 with stirring. The solution is cooled in an ice bath, and the sides of the vessel are scratched with a glass rod, whereupon the diastereoisomeric salt of the (— )589 carbonato complex deposits. (If the diastereoisomer does not deposit, a small amount of ethanol is added.) The whole is kept in an ice bath for a while. The diastereoisomer is collected on a filter, washed with cold water, and recrystallized several times from water at 35° to increase the optical purity. The recrystallized salt is collected on a filter, washed with cold water, ethanol, and diethyl ether, and dried under vacuum. The yield is 0.3 g. Anal. Calcd. for [Co(C03)(NH3)2(py)2][Co(ox)2(en)]-2.5H20 C, 31.35 H, 4.49 N, 12.90. Found C, 31.37 H, 4.67 N, 12.82. The diastereoisomer is separated as before. 

These metal-alkynyl complexes can be protonated to afford the free alkynes and parent cobalt hydroxo complex (comparable reactivity to their alkyl and aryl congeners), but have proven inert toward oxygenation and carbonylation. They are also thermally stable up to 100 °C. Attempts to explore the reactions of these compounds with unsaturated hydrocarbons were typically fruitless. The one exception is the reaction between 53 and its parent alkyne (HC = C02Me, Scheme 6), which under benzene reflux effects catalytic, stereospecific, linear trimerisation of the alkyne to afford ( , )-buta-l,3-dien-5-yne. The reaction was, however, slow (4.5 turnovers in 20 h) and suffered from catalytic deactivation due to hydrolysis of 53, which subsequently reacted with adventitious CO2 to irreversibly form an inert /x-carbonato complex. The catalytic cycle was concluded to involve initially a double coordination-insertion of the C = C bond of methylpropiolate into the Co-Caikyne linkage. Subsequent hydrolysis of the Co-C bond by a third equivalent of HC = CC02Me would then afford the observed product and regenerate 53. However, a definitive explanation for the stereospecificity of the process was not established. 

The /i.-amido-/x-carbonato complex (11) has been prepared and characterized, and the kinetics of acid hydrolysis leading to the iit>amido-/Lt-hydroxo>bis(bis(eth> ylenediamine)cobalt(III)) ion studied in detail/ Values of kobs are independent of [H" ] over the acidity range [H ] = 0.9 M to 0.01 M. The kinetics of base hydrolysis were also studied over the range [OH ] = 0.7 M to 0.025 M The results are consistent with a kinetic scheme involving a rapid preequilibrium followed by a rate-determining step resulting in a /i-amido-dihydroxo complex. 

Table 2. Some copp>er and cobalt carbonato complexes and the corresponding IR absorption bands related to CO32- vibrations, vi, V2 and V4 correspond to vibrations of CO32- in the C2v symmetry notation according to this notation, the doubly degenerate V3 vibration of the free CCh - ion, which splits into two components, is denoted as vi and V4, whereas the activated Vi vibration is denoted as V2 (data compiled from Gatehouse et al., 1958 Fujita et al., 1962 Jolivet et al., 1982 Healy White, 1972).

The reaction of the carbonato complex with C02 has allowed the demonstration of a facile insertion-deinsertion equilibrium. The study of the deinsertion reaction has allowed estimation of the activation parameters as being = 130 4.0 kJ mor and AS = 121.6 11.9 J moP K . From the above values the authors have calculated an approximate value of the equiUbrium cmistant for the carboxylation reaction equal to 3 x 10 M at 195 K (or a AG value for the same reaction of < 50 kJ moP ), showing that the insertion of CO2 into the M-O-alkyl bond is both kineticaUy and thermodynamically very favored. This trend has also been confirmed for the insertion of CO2 into the Nb-OR bond in [Nb(OR)5]2 (R = methyl, ethyl, aUyl) (see Sect. 6.2.2.1), a catalyst for the synthesis of dialkyl carbonates [67]. Very recently, the facile insertion of CO2 into metal-phenoxide bonds has been reported [68] for cobalt and zinc complexes (Fig. 4.2). It should be noted that such metal systems are used as catalysts in the copolymerization of CO2 with epoxides. 

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