Cobalt complexes carbon dioxide reactions

Cobalt(II) alkoxides are known and monomeric forms are part of a wider review.413 The interest in these compounds pertains to a potential role in catalysis. For example, a discrete cobalt(II) alkoxide is believed to form in situ from a chloro precursor during reaction and performs the catalytic role in the decomposition of dialkyl pyrocarbonates to dialkyl carbonates and carbon dioxide.414 A number of mononuclear alkoxide complexes of cobalt(II) have been characterized by crystal structures, as exemplified by [CoCl(OC(t-Bu)3)2 Li(THF)].415 The Co ion in this structure and close relatives has a rare distorted trigonal-planar coordination geometry due to the extreme steric crowding around the metal. 

This may be of significance in connetion with the biosynthesis of acetate from carbon dioxide, because the next step, the fixation of carbon monoxide, was demonstrated by B. Krautler. He irradiated methyl cobalamin under Co at 30 atm and obtained the acyl cobalamin as the product. Interestingly, a radical mechanism was iproposed, involving the reaction of methyl radicals with CO to give acyl radicals, which then recombine with the cobalt complex /55/. 

Carbonic anhydrase is a zinc(II) metalloenzyme which catalyzes the hydration and dehydration of carbon dioxide, C02+H20 H+ + HC03. 25 As a result there has been considerable interest in the metal ion-promoted hydration of carbonyl substrates as potential model systems for the enzyme. For example, Pocker and Meany519 studied the reversible hydration of 2- and 4-pyridinecarbaldehyde by carbonic anhydrase, zinc(II), cobalt(II), H20 and OH. The catalytic efficiency of bovine carbonic anhydrase is ca. 108 times greater than that of water for hydration of both 2- and 4-pyridinecarbaldehydes. Zinc(II) and cobalt(II) are ca. 107 times more effective than water for the hydration of 2-pyridinecarbaldehyde, but are much less effective with 4-pyridinecarbaldehyde. Presumably in the case of 2-pyridinecarbaldehyde complexes of type (166) are formed in solution. Polarization of the carbonyl group by the metal ion assists nucleophilic attack by water or hydroxide ion. Further studies of this reaction have been made,520,521 but the mechanistic details of the catalysis are unclear. Metal-bound nucleophiles (M—OH or M—OH2) could, for example, be involved in the catalysis. 

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. 

The carbon dioxide anion radical was used for one-electron reductions of nitrobenzene diazonium cations, nitrobenzene itself, quinones, aliphatic nitro compounds, acetaldehyde, acetone and other carbonyl compounds, maleimide, riboflavin, and certain dyes (Morkovnik Okhlobystin 1979). This anion radical reduces organic complexes of Com and Rum into appropriate complexes of the metals in the valence 2 state (Morkovnik Okhlobystin 1979). In the case of the pentammino-p-nitrobenzoato-cobalt(III) complex, the electron-transfer reaction passes a stage of the formation of the Co(III) complex with the p-nitrophenyl anion radical fragment. This intermediate complex transforms into the final Co(II) complex with the p-nitrobenzoate ligand as a result of an intramolecular electron transfer. Scheme 1-89 illustrates this sequence of transformations ... 

Another important example of COj-hydrogenation is the synthesis of form-amides. In 1970, Haynes c/tf/. of Shell Development Co. discovered the reaction of carbon dioxide, hydrogen and certain amines, when catalyzed under mild conditions by cobalt, rhodium, iridium and palladium complexes [ I70. ... 

In general, the acetylenic triple bond is highly reactive toward hydrogenation, hydroboration, and hydration in the presence of acid catalyst. Protection of a triple bond in disubstituted acetylenic compounds is possible by complex formation with octacarbonyl dicobalt [Co2(CO)g Eq. (64) 163]. The cobalt complex that forms at ordinary temperatures is stable to reduction reactions (diborane, diimides, Grignards) and to high-temperature catalytic reactions with carbon dioxide. Regeneration of the triple bond is accomplished with ferric nitrate [164], ammonium ceric nitrate [165] or trimethylamine oxide [166]. 

The ruthenium-cobalt bimetallic complex system catalyzes the homologation of methanol with carbon dioxide and hydrogen in the presence of iodide salts. A synergistic effect is found between these two metals. The yield of ethanol is also affected by the Lewis acidity of the iodide salt, lithium iodide being most effective. The reaction profile shows that methanol is homologated with CO formed by the hydrogenation of CO2. 

There are two possible pathways to homologate methanol with carbon dioxide the CO2 insertion path and CO insertion path (Scheme 2). As for the former, Fukuoka et al. reported that the cobalt-ruthenium or nickel bimetallic complex catalyzed acetic acid formation from methyl iodide, carbon dioxide and hydrogen, in which carbon dioxide inserted into the carbon-metal bond to form acetate complex [7]. However, the contribution of this path is rather small because no acetic acid or its derivatives are detected in this reaction. Besides, the time course... 

The importance of reactions of cobalt(I) complexes with proton donors in the context of understanding the photo- and electrocatalytic reduction of water to H2 and of carbon dioxide to HCO2 has been highlighted. [146] In a series of reports the thermodynamics and kinetics of proton binding to the cobalt(I) complex of the macrocycle, 5,7,7,12,14,14,-hexamethyl-... 

Carbon dioxide is abundant and readily available, but its reaction with transition metal complexes has not been extensively studied. A few examples of carbon dioxide insertion are known. Thus, formic acid can be formed by the insertion of carbon dioxide into the cobalt hydride bond U9>,2°). 

---