Iridium complexes carbon dioxide reactions

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]. 

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. ... 

The combination of hard oxophilic early transition metals and soft nucleophilic late transition metals with opposite functionalities, provided they do not inhibit one another, is a priori ideal for promoting cooperative effect. A proof of concept can be found in the stoichiometric reactivity of early—late heterobimetallic complexes featuring a metal-metal bond [76]. It has been shown that such complexes are good candidates to realize the heterolytic cleavage of a bond in polar and apolar substrates. An illustrative example by Bergman et al. is the reaction of the Zr-lr complex 20 with carbon dioxide which leads to the rupture of the metal—metal bond (Scheme 18) [77]. The CO2 insertion occurs in the expected fashion with the CO2 bridging the two metals, the carbon atom coordinated to iridium, and the oxygen atom on the zirconium center. 

Rate data are reported for the acid-catalysed aquation of the [M(NH3)5COal+ ions (M = Rh or Ir ), as well as the rates of formation of these carbonato-com-plexes from [M(NH3)50H] + and carbon dioxide. At 298.1 K and p = 0.5 mol 1 , the aquation rate constants are 1.13 and 1.45 s and the formation rate constants 470 and 5901 mol S for the rhodium(m) and iridium(m) complexes respectively. The close similarity of these rate constants for both metal ions indicates carbon-oxygen bond fission for the aquation reactions. 

PHOTOCATALYSIS REDUCTION OF CARBON DIOXIDE AND WATER-GAS-SHIFT REACTION PHOTOCATALYZED BY 2 -BIPYRIDINE OR 1,10-PHENANTHROLINE COBALT(H), RUTHENIUM(H), RHENIUM(I) AND IRIDIUM(ni) COMPLEXES ... 

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... 

The reaction of tra 3 -IrCl(CO)(PPh3)2 with carbon disulfide in the presence of NaBPli4 yields [(49)](BPli4) in which the S-donor ligand is the zwitterion S2C PPh3+. Sulfur dioxide is S-bonded to an iridium(I) atom in the trigonal bipyramidal complex IrH(PPh3)2(CO)(S02) in the related chloro complex (50), the Ir-S distance is 2.45 A. ... 

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