Ruthenium complexes carbon dioxide reactions

Krocher, O. Koppel, R.A. Baiker, A. Silica hybrid gel catalysts containing ruthenium complexes influence of reaction parameters on the catalytic behavior in the synthesis of N,N-dimethylformamide from carbon dioxide. J. Mol. Catal. A Chem. 1999, 140 (2), 185-193. 

In the past, this field has been dominated by ruthenium, rhodium and iridium catalysts with extraordinary activities and furthermore superior enantioselectivities however, some investigations were carried out with iron catalysts. Early efforts were reported on the successful use of hydridocarbonyliron complexes HFcm(CO) as reducing reagent for a, P-unsaturated carbonyl compounds, dienes and C=N double bonds, albeit complexes were used in stoichiometric amounts [7]. The first catalytic approach was presented by Marko et al. on the reduction of acetone in the presence of Fe3(CO)12 or Fe(CO)5 [8]. In this reaction, the hydrogen is delivered by water under more drastic reaction conditions (100 bar, 100 °C). Addition of NEt3 as co-catalyst was necessary to obtain reasonable yields. The authors assumed a reaction of Fe(CO)5 with hydroxide ions to yield H Fe(CO)4 with liberation of carbon dioxide since basic conditions are present and exclude the formation of molecular hydrogen via the water gas shift reaction. H Fe(CO)4 is believed to be the active catalyst, which transfers the hydride to the acceptor. The catalyst presented displayed activity in the reduction of several ketones and aldehydes (Scheme 4.1) [9]. 

Ammonium carbamates are readily and reversibly produced on reaction of secondary amines with carbon dioxide. In the presence of a ruthenium catalyst precursors such as Ru3(CO)12 [3], (arene)RuCl2(PR3) [4] or Ru(methallyl)2(dppe) [5] (dppe=bis(diphenylphosphino)ethane) complexes, the three-component combination of a secondary amine, a terminal alkyne, and carbon dioxide selectively provides vinylcarbamates resulting from addition of carbamate to the terminal carbon of the triple bond (Scheme 2). 

Recently, the use of carbon dioxide as a carbon building block [152] has attracted increasing attention. The hydrosilylation of carbon dioxide catalyzed preferably by ruthenium complexes leads to the synthesis of silyl formate esters (Eq. 98) [153]. Results of the reaction of hydrosilylation in supercritical carbon dioxide as a solvent and substrate have recently been reported [154]. 

Carbon dioxide in its supercritical state is a reaction medium of great interest. Noyori et. al. recently detected that Ruthenium(II)-phosphine-complexes of typ [(X)2Ru(PMe3)4] 15 (X = H) and 16 (X = Cl) can act as highly active catalysts for an effective transition metal catalysed hydrogenation of CO2 to formic acid in a supercritical mixture of CO, H, and NEt, without use of any further solvent. 

Since the reverse reaction proceeds by the removal of COj from solution as Li2C03 precipitation, the interconversion of the ruthenium M-CO2 and M-CO could be regarded as very shifted equilibrium. We have already reported that these ruthenium carbon dioxide and carbonyl complexes are in an acid-base equilibria (eq. 7). 

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... 

Terminal alkyne complexes with ruthenium react with carbon dioxide and amines to yield urethanes, as shown in reaction 8. ... 

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). ... 

Also, I wish to mention the catalytic reaction which proceeds via metathesis with heterolytic o-bond activation. Hydrogenation of carbon dioxide to formic acid is one of attractive transition-metal catalyzed CO2 fixation reactions. Rh(I), Rh(III), and Ru(II) complexes were used as a catalyst [54-56]. Of those catalysts, the Ru(II)-catalyzed hydrogenation of CO2 has drawn considerable interest because of its very high efficiency. Its catalytic cycle was theoretically investigated [57]. In this catalytic reaction, the first step is the insertion of CO2 into the Ru-H bond, to afford the ruthenium(II) formate complex, RuHIir -OCOHKPHjIj,... 

Preliminary results of the reaction between vanadium(iii)-tetrasulpho-phthalocyanine complex with oxygen have been reported these data were compared with those obtained for the corresponding reaction of the hexa-aquo complex ion. The oxidation of methyl ethyl ketone by oxygen in the presence of Mn"-phenanthroline complexes has been studied Mn " complexes were detected as intermediates in the reaction and the enolic form of the ketone hydroperoxide decomposed in a free-radical mechanism. In the oxidation of 1,3,5-trimethylcyclohexane, transition-metal [Cu", Co", Ni", and Fe"] laurates act as catalysts and whereas in the absence of these complexes there is pronounced hydroperoxide formation, this falls to a low stationary concentration in the presence of these species, the assumption being made that a metal-hydroperoxide complex is the initiator in the radical reaction. In the case of nickel, the presence of such hydroperoxides is considered to stabilise the Ni"02 complex. Ruthenium(i) chloride complexes in dimethylacetamide are active hydrogenation catalysts for olefinic substrates but in the presence of oxygen, the metal ion is oxidised to ruthenium(m), the reaction proceeding stoicheiometrically. Rhodium(i) carbonyl halides have also been shown to catalyse the oxidation of carbon monoxide to carbon dioxide under acidic conditions ... 

The reaction is insensitive to whether the atmosphere used is dinitrogen or carbon monoxide. The iron complex Fe3(PPh3)2 proved to be much less. active than its ruthenium analogue in this reaction. Analyses o-f the gas evolved during the catalytic reactions conducted under dinitrogen have shown the -formation o-f carbon dioxide, but its quantity was less than calculated on the basis o-f the PhNO converted. On the other hand, in protic media the reaction could proceed via the proton trans-fer -from the solvent to n i trosobenzene, with the intermediate formation o-f hydroxyl ami ne s... 

The reactions of benzyne complexes of zirconium " also occur by electrophilic attack at an M-C bond. The isolated phosphine adduct of a zironocraie-benzyne complex reacts with ketones to imdergo insertion into one of the M-C bonds and with alcohol to make an aryl alkoxo complex, as shown in Equation 12.67. An electron-rich ruthenium-benzyne complex also reacts with electrophiles, such as borzaldehyde or carbon dioxide, to form products from insertion, as shown at the top of Equation 12.68. It also reacts with weak acids, such as aniline, to form products from formal protonation at the Ru-C bond, as shown at the bottom of Equation 12.68. - This reaction with aniline could occur by initial protonation at the metal, followed by C-H bond-forming reductive elimination, or by direct protonation of the M-C bond. Initial protonation of the metal center was proposed. 

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