Dioxide Complexes of Rhodium and Iridium

In our experience, unrecrystallized material gives a very low or no yield of product. Material recrystallized from chloroform also may give erratic results. The checker finds that recrystallization is essential. Addition of a small amount (less than 10% by weight) of P(C6Hs)3 was found to aid the process. 

In recent years, interest has intensified in the chemistry of carbon dioxide, stimulated by the current concern about alternate petrochemical feedstocks. One area under active exploration involves C02 activation via coordination to a transition metal complex.1 Several adducts of C02 have been claimed, and two monometallic complexes, with x-ray structures which have been published, are shown below schematically (1 and 2).2,3 We report here two examples of the preparation of 1 1 C02 adducts of a series of rhodium and iridium complexes and, relatedly, methods for preparing the 2 1 C02 lr complex 2. 

(CARBON DIOXIDE)BIS[ 1,2-ETHANEDIYLBIS(DIMETHYLPHOS PHINE)] IRIDIUM CHLORIDE4 

The following manipulations are all done in an atmosphere of dry nitrogen using dry, deoxygenated, reagent-grade solvents. Coleman Instrument Grade (99.99% minimum) carbon dioxide is used. Dmpe is commercially available. 

To a stirred solution of 3.25 g (3.63 mmole) Ir2Cl2(C8H]4)4s in 100 mL toluene, 2.18 g (14.5 mmole) (CH3)2PC2H4P(CH3)2 (dmpe) in 10 mL toluene is added. The mixture is stirred for 30 minutes then filtered, and the resultant solid is washed with toluene and then with pentane. Drying to 0.1 micron affords 2.9 g of a light-orange solid (75% yield), mp (vac.) 252-253° (dec.). (The checkers (C.K. and H. D. K.) report the formation of the cyclometalation product7 when the procedure is followed without exclusion of light, v(li—H) = 2180 cm 1.) 

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C. M. Bolinger, N. Storey, B. P. Sullivan and T. J. Meyer (1988) Electrocatalytic reduction of carbon dioxide by 2,2 -bipyridine complexes of rhodium and iridium , Inow. Chem. 27, 4582-4587. 

Rasmussen, S.C., Richter, M.M., Yi, E., Place, H. and Brewer, KJ. (1990) Synthesis and characterization of a series of novel rhodium and iridium complexes containing polypyridyl bridging ligands Potential uses in the development of multimetal catalysts for carbon dioxide reduction. Inorg. Chem., 29, 3926—3932. 

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]. 

Scheme 8.20 Difference in interaction of carbon dioxide and dihydrogen with rhodium and iridium pincer complexes.

The procedure described here is based on the observation that amine monohydroxo complexes of cobalt(III), rhodium(IIl), and iridium(III) react directly with carbon dioxide to form the corresponding carbonato complexes,2 3 without effect on the configuration of the amine ligands.4 The amine monoaqua complex is allowed to react with lithium carbonate or carbon dioxide gas at room temperature at pH 8.0 for a few minutes, and the carbonato complex is isolated by adding alcohol. The procedure has been used to prepare salts of the following cations pentaammine(carbonato)-cobalt(III),2 ds-ammine(carbonato)bis(ethylenediamine)cobalt(III),5 trans-... 

A range of metal catalysts have also been studied in aqueous solution for the transformation of carbon dioxide, including rhodium, ruthenium and iridium bipyridine or phenanthroline complexes.One of the most effective systems is the iridium complex shown in Figure 3.14. The ligand design concept used in this study is very clever. The catalytic activity of the complex and its solubility in aqueous solution can be tuned by the pH of the solution.Under acidic... 

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. 

The transition-metal catalyzed decomposition of thiirene dioxides has been also investigated primarily via kinetic studies103. Zerovalent platinum and palladium complexes and monovalent iridium and rhodium complexes were found to affect this process, whereas divalent platinum and palladium had no effect. The kinetic data suggested the mechanism in equation 7. 

All of the carbonato cobalt(III) complexes reported here are reddish in color and extremely soluble in water. The rhodium complex is pale-yellow, whereas the iridium salt is virtually white they are both soluble in water. Treatment with dilute acid immediately gives the corresponding aqua complex with evolution of carbon dioxide. The characterization and the mechanistic details of acid hydrolysis of these complexes have been reported.3,4,11... 

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. ... 

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