Carbon dioxide-transition metal complexes

The fixation of carbon dioxide into organics may involve its activation by coordination to low-valent transition metal complexes. Carbon-carbon bonds can be thus formed by simultaneous activation at a metal center of both carbon dioxide and an organic substrate. 

The interaction of carbon dioxide with transition metal complexes. I. S. Kolomnikov and M. K. Grigoryan, Russ. Chem. Rev. (Engl. Transl.), 1978,47, 334-353 (306). 

The present volume is a non-thematic issue and includes seven contributions. The first chapter byAndreja Bakac presents a detailed account of the activation of dioxygen by transition metal complexes and the important role of atom transfer and free radical chemistry in aqueous solution. The second contribution comes from Jose Olabe, an expert in the field of pentacyanoferrate complexes, in which he describes the redox reactivity of coordinated ligands in such complexes. The third chapter deals with the activation of carbon dioxide and carbonato complexes as models for carbonic anhydrase, and comes from Anadi Dash and collaborators. This is followed by a contribution from Sasha Ryabov on the transition metal chemistry of glucose oxidase, horseradish peroxidase and related enzymes. In chapter five Alexandra Masarwa and Dan Meyerstein present a detailed report on the properties of transition metal complexes containing metal-carbon bonds in aqueous solution. Ivana Ivanovic and Katarina Andjelkovic describe the importance of hepta-coordination in complexes of 3d transition metals in the subsequent contribution. The final chapter by Sally Brooker and co-workers is devoted to the application of lanthanide complexes as luminescent biolabels, an exciting new area of development. 

A. Carbon Dioxide as a Ligand in Transition Metal Complexes. 121... 

The chemistry of carbon dioxide with transition metal complexes is a field of research that has only recently received wide attention. One goal of research in this area is the development of efficient catalytic processes in which carbon dioxide is reduced by molecular hydrogen and/or incorporated into an organic molecule. Some examples of such desirable transformations... 

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 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). 

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... 

This section will review some recent work in the areas of catalytic and stoichiometric reduction ofC02, as well as incorporation of carbon dioxide into organic compounds, promoted by homogeneous transition metal complexes. 

There have been several recent reports of the reduction of carbon dioxide to carbon monoxide by transition metal complexes. Maher and Cooper (2/) have reported that several metal carbonyl dianions can effect the disproportionation of C02 to metal bound carbon monoxide [Eq. (37)] with Li2C03... 

Transition metal complexes have proved very useful in both the catalytic and stoichiometric production of cyclic lactones. A series of palladium(O)-phosphine complexes have been shown to be effective for the conversion of three-membered ring systems to cyclic lactones [Eq. (45)] (114). When isopropylidenecyclopropane and [Pd(dba)2]-PPh3 (dba = dibenzylidenea-cetone)(4 1) in benzene were treated with 40 atm carbon dioxide at 126°C for 20 hr, 69% of the lactone (34) was formed. In contrast, when [Pd(diphos)2] was used as the substrate under similar conditions 48% of 35 was produced with only trace amounts of 34. None of the complexes appeared to be active for terminal alkenes such as 36 or 37. 

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 first examples of the carbonylation of transition-metal complexes by carbon dioxide under mild conditions have been reported.19 For example, [RhCl(PPh3)3] reacts with C02 at room temperature in the presence of silicon hydrides to give the known [RhCl(CO)(PPh3)2], A polymeric complex has been produced from the interaction of [RhCl(CO)(PPh3)2] with a resin containing grouping (31), as part of a... 

Fig. 15.17. Current in milliamperes cm-2 for CO production from C02 using C02+ tetraphenyl porphyrins modified with various pyridyl derivatives. (Reprinted with permission from T. Atoguchi, A. Aramata, and M. Engo, C02 Reduction by Macrocyclic Transition Metal Complex-Modified Electrodes," in Proc. International Symposium on Chemical Fixation of Carbon Dioxide, p. 338, Fig. 5,1991.)...

Krocher, O., Koppel, R.A., Froba, M. and Baiker, A. (1998) Silica hybrid gel catalysts containing group(VIII) transition metal complexes preparation, structural, and catalytic properties in the synthesis of N, N-dimethylformamide and methyl formate from supercritical carbon dioxide. Journal of Catalysis, 178, 284-298. 

The first step in the activation of carbon dioxide by transition metal compounds is the formation of a M-C02 complex, since it is through coordination that the electronic structure of this molecule, and hence its reactivity, can be substantially modified. Transition metal complexes containing carbon dioxide in its intact form have received considable attention in the last decade (Inone et al, 1982), mainly with the aim of finding model systems for the activation of C02 and subsequent transformation into organic chemicals of comercial interest (Aresta et al, 1987). Despite considerable and intensive work in this area, the number of structurally characterized carbon dioxide complexes is stilt very limited, and they have been found to contain side-on (Alvarez et al, 1986), 72 -coordinated and, l2 C-coordinated (Calabrese et al, 1983) C02. 

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]. 

Insertion of Carbon Dioxide into Transition Metal Complexes... 

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. 

Tlte catalytic fixation of carbon dioxide to formic acid is possible, using a combination of Group Vlil transition metal complexes and bases in the presence of water. Typical catalysts are Pdfdppe) and RuH (PPh3)4, but nickel, rhodium. 

Carbon dioxide, 0=C=6 , mp —57°C (5.2 atm), bp —79 °C (sublimes), is obtained from the combustion of carbon and hydrocarbons in excess air or oxygen or by the pyrolysis ( calcination ) of CaCOs (limestone). The photosynthesis in plants reduces CO2 to organic matter, but the similar reduction of CO2 in a nonliving system ( in vitro ) appears to be very difficult. However, CO2 can be reduced electrochemically to methanol, formate, oxalate, methane, and/or CO depending upon the conditions. Numerous transition metal complexes of CO2 are known,which exhibit the modes of metal-C02 bonding depicted in Figure 2. 

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. 

Independently, in 1981 two groups reported the hydrosilylation of carbon dioxide into formoxysilanes of the type R2R SiOCHO (R,R = alkyl) catalyzed by transition metal complexes, preferably based on ruthenium (eq. (9)) [65],... 

Ryan B. Prince was born in Robbinsdale, MN, in 1972. He graduated from Southeast Missouri State University in 1995 with his B.S. degree in Chemistry, where he worked for Professor Jin K. Gong on the synthesis and characterization of transition-metal complexes that bind and activate carbon dioxide. He then went to the University of Illinois and worked with Professor Jeffrey S. Moore on the synthesis and study of oligo(m-phenylene ethynylene) foldamers. He completed his Ph.D. degree in Organic Chemistry in 2000 and is currently a senior research chemist at 3M Company in St. Paul, MN. 

The activation of C-O multiple bonds in C02 can be seen in the course of hy-drogenolysis of carbon dioxide by promotion of the transition metal complexes [138]. More examples of the C-0 bond cleavage in C02 activated by metal complexes have been reviewed [139]. 

Currently, carbon dioxide is used as a chemical feedstock for the production of carboxylic acids, carbonates, carbon monoxide, and urea (14—16). Despite the fact that numerous chemical reactions utilizing carbon dioxide are thermodynamically advantageous, there is often a substantial kinetic barrier to their occurrence. Transition metal compounds can serve to catalyze reactions of carbon dioxide, i.e., in the utilization of carbon dioxide in synthetic organic chemistry, transition metal complexes can simultaneously activate both carbon dioxide and other substrate molecules such as hydrogen or olefins. 

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