Carbon dioxide reactions with transition metal complexes

Infrared spectral identification of adduct formation involving carbon dioxide and a transition metal complex has often been in error because of subsequent reactions of C02 with concomitant production of carbonato-, hydrogen-carbonato-, or carboxylato-metal complexes. Indeed Mason and Ibers (9) have suggested that the only acceptable structural characterization forjudging the authenticity of a class of transition metal-C02 complexes should be diffraction methods. X-ray structural studies have verified at least six C02 adducts which display all three types of bonding modes of... 

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°). 

The carbon dioxide molecule exhibits several functionalities through which it may interact with transition metal complexes and/or substrates. The dominant characteristic of C02 is the Lewis acidity of the central carbon atom, and the principle mode of reaction of C02 in its main group chemistry is as an electrophile at the carbon center. Consequently, metal complex formation may be anticipated with basic, electron-rich, low-valent metal centers. An analogous interaction is found in the reaction of the Lewis acid BF3 with the low-valent metal complex IrCl(CO)(PPh3)2 (114). These species form a 1 1 adduct in solution evidence for an Ir-BF3 donor-acceptor bond includes a change in the carbonyl stretching frequency from 1968 to 2067 cm-1. 

A new stall in CO cheniistry was found when a few rcscardi groups investigated the reactions of carbon dioxide with transition metal coniplexes. The results, which have been published up to 1981, are summarized in this review. Some of the older reviews which have been published concern 1he reactions of CO with transition metal complexes 1-81 and the syntlietic reactions and chemical utilization of carbon dioxide [9-18]. 

Borowiak MA, Jamroz MH, Dobrowolski JC, Bajdor K, Kazimirski JK, Mascetti J, Quaranta E, Tommasi I, Aresta M (2001) Application of the impulse oscillation model for modeling the formation of peroxocarbonates via carbon dioxide reaction with dioxygen transition metal complexes. A comparison with the experimental results obtained for Rh (q -02)ClP3 [P = phosphane ligand]. J Mol Catal A 165 45-54... 

Reactions of Carbon Dioxide.— The interaction of carbon dioxide with transition-metal complexes has been reviewed. Reaction of cmrbon dioxide with [TiCNMea) and 1,2-epoxycyclohexane followed by hydrolysis gives 2-hydroxy-... 

Reactions of carbon subsulphide and of elementary phosphorus, sulphur and selenium with complexes of the platinum metals Sulphur dioxide insertion reactions of transition metal alkyls and related complexes... 

A recent report 121) on the reactions of tetrakis(trimethylphosphine) iron, Fe(PMe3)4, with carbon dioxide reveals a rich and varied chemistry, illustrating many of the reaction modes of C02 with low-valent transition metal complexes. Two primary reactions of C02 with Fe(PMe3)4 are noted, as a consequence of the two isomers in equilibrium (49). 

Despite the fact that carbon dioxide (C02) is used in a great number of industrial applications, it remains a molecule of low reactivity, and methods have still to be identified for its activation. Both thermodynamic and kinetic problems are connected with the reactivity of C02, and few reactions are thermodynamically feasible. A very promising approach to activation is offered by its coordination to transition metal complexes, as both stoichiometric reactions of C-C bond formation and catalytic reactions of C02 are promoted by transition metal systems. Efforts to enhance the yield of hydrogen in water gas-shift (WGS) reactions have also been centered on C02 interactions with transition metal catalysts. The coordination on metal centers lowers the activation energy required in further reactions with suitable reactants involving C02, making it possible to convert this inert molecule into useful products. 

Less attention has been paid, however, to C02 organometallic chemistry during the past decade. Whilst many reduction or coupling reactions are known to proceed in the presence of stoichiometric or catalytic amounts of transition metal complexes, very few examples remain where the formation of a metal-C02 complex has led to an effective, catalytic reduction reaction of C02. Carbon dioxide complex photoactivation also represents an attractive route to CO bond cleavage, coupled with O-atom transfer. However, progress in the area of C02 utilization requires a better understanding of the reaction mechanisms, of the thermodynamics of reaction intermediates, and of structure-reactivity relationships. 

The study of stoichiometric CO insertions into transition metal complexes is of great importance because this reaction is the first step m the catalytic conversion of carbon dioxide. Hence, these investigations can lead to the possibility of introducing carbon dioxide into transition metal-catalyzed synthetic processes. Analogies with carbon monoxide chemistry may be drawn, for instance. from the CO insertion into metal alkyl bonds leading to such important industrial processes as hydroformylation and carbonylalion. 

In the field of olefin carboxylation, stoichiometric reactions have been described to occur between non-activated alkenes, CO2 and an electron-rich transition-metal complexes, such as Ni(0) [3], Ti(II) [4] or Fe(0) [5]. A Pd-catalyzed CO2 fixation occurs into methylenecyclopropane derivatives affording lactones [6]. The reaction of carbon dioxide with ethylene is difficult and its carboxylation to propionic acid, catalyzed by Rh derivatives [7], needs drastic experimental conditions. 

Carbon dioxide is obtained by combustion of carbon in the presence of an excess of oxygen or by treating carbonates with dilute acids. Its important properties should already be familiar. It undergoes a number of insertion reactions (Section 24-A-3) similar to those of CS2 noted below, and a few complexes with transition metals are known. 

Though carbon-nitrogen bond cleavage reaction is less explored, allylic C-N bond can be cleaved by low valent late transition metal complexes. Treatment of (/x-dinitrogen or carbon dioxide)nickel(O) complexes with allylammonium... 

Reactions of Carbon Dioxide with Carbon-Carbon Bond Formation Catalyzed by Transition Metal Complexes"... 

Though this review focus on homogeneous catalyzed reactions between unsatured hydrocarbons and carbon dioxide, also some related reactions without transition metal catalysts will be considered. It appears suitable to compare the different possibilities of catalytic and non-catalytic methods in the field of C-C linkage. For instance ionic reactions are well known routes to attach CO2 on a hydrocarbon chain. Living oligomers of ethene obtained with n-BuLi complexed by tertiary amines react with carbon dioxide and yield long-chain car xylic acids [27] (Equation 3). 

Water and carbon dioxide do not absorb light above 200 nm and their monoelectronic reduction requires an energy too high to be performed by classical transition metal complexes. It is therefore necessary to use a photosensitizer and organometallic complexes which are able to transfer more than one electron (e.g. cobalt(I), ruthenium(O), or rhenium(-I) or iridium(I) complexes). In principle, these species could be oxidized to a higher oxidation state, by reaction with water or carbon dioxide. However this poses certain problems (i) the compatibility of redox potentials between the photosensitizer and the catalyst (ii) finding mediators and... 

Braunstein P, Matt D, Nobel D (1988) Reactions of carbon dioxide with carbon-carbon bond formation catalyzed by transition metal complexes. Chem Rev 88 747-764... 

The use of carbon dioxide in the synthesis of functional molecules is of considerable interest. An example is the industrially important reaction of epoxides with carbon dioxide to give cyclic carbonates. Also, functionalization of acetylenes and dienes with carbon dioxide on transition metal catalysts gives rise to the formation of cyclic lactones or dicarboxylic acids. The activation of carbon dioxide by metal complexes was reviewed in 1983 . Reactions of carbon dioxide with carbon-carbon bond formation catalyzed by transition metal complexes was reviewed in 1988 ", heterogenous catalytic reactions of carbon dioxide were reviewed in 1995, and the use of carbon dioxide as comonomers for functional polymers was reviewed in 2005. ... 

Chemical transformation of carbon dioxide has attracted considerable interest because it presents possibilities of converting it into new chemical products and of reducing its concentration in the earths atmosphere [1]. In the chemical community, enormous efforts have been made to develop low-cost and catalyt-ically effective transition metal catalysts and reagents for the transformation of carbon dioxide [2]. Over the years, tremendous progresses have been made in understanding its reactions with various transition metal complexes [3]. At the same time, various interesting catalytic conversions of CO2 have also been discovered. 

Palladium(II) complexes possessing bidentate ligands are known to efficiently catalyze the copolymerization of olefins with carbon monoxide to form polyketones.594-596 Sulfur dioxide is an attractive monomer for catalytic copolymerizations with olefins since S02, like CO, is known to undergo facile insertion reactions into a variety of transition metal-alkyl bonds. Indeed, Drent has patented alternating copolymerization of ethylene with S02 using various palladium(II) complexes.597 In 1998, Sen and coworkers also reported that [(dppp)PdMe(NCMe)]BF4 was an effective catalyst for the copolymerization of S02 with ethylene, propylene, and cyclopentene.598 There is a report of the insertion reactions of S02 into PdII-methyl bonds and the attempted spectroscopic detection of the copolymerization of ethylene and S02.599... 

Equations 1 to 3 show some of fixation reactions of carbon dioxide. Equations la and lb present coupling reactions of CO2 with diene, triene, and alkyne affording lactone and similar molecules [2], in a process catalyzed by low valent transition metal compounds such as nickel(O) and palladium(O) complexes. Another interesting CO2 fixation reaction is copolymerization of CO2 and epoxide yielding polycarbonate (equation 2). This reaction is catalyzed by aluminum porphyrin and zinc diphenoxide [3],... 

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