Iridium-carbonyl bond

By contrast, reaction of XeFi with the iridium carbonyl complex cation [Ir(CO)3(PEt3)2] in CH2CI2 results in addition across one of the Ir-CO bonds to give the first example of a metal fluoroacyl complex ... 

Wickman and Silverthom [276] have investigated bond properties in molecular adducts of the planar Vaska type compound frans-h s(triphenyl-phosphine)iridium carbonyl chloride, IrCl(CO) ((C6H5)3P)2, with small molecules such as H2, O2, CI2, I2, CH3I, and HCl. They essentially observed a decrease of the isomer shift in the following series of adduct molecules XY H2 > HCl > CH3J > O2 > I2 > CI2,... 

In support of the previous statement that iridium has a lower predisposition to form bridging carbonyl bonds compared to cobalt and rhodium, in this case, only three of the fifteen carbonyl groups assume a doubly bridging disposition whereas all the others have a terminal arrangement. 

Although the precise mechanism has not yet been clarified, a possible mechanism is shown in Scheme 5.2. First, the iridium alkoxide 3 is produced from 1 and an alcohol, this step being stimulated by a base (K2CO3). A ]3-hydride elimination of 3 then yields a carbonyl product and the iridium hydride 4. The insertion of acetone into the iridium-hydride bond in 4, giving metal isopropoxide 5, is followed by exchange of the alkoxy moiety to regenerate 3. 

The use of CO2 as a reagent for synthetic purposes would be highly desirable, due not only to the vast availabiUty of this gas but also its environmental concerns. The stoichiometric activation of CO2 has been achieved with the iridium-PCP complex 29 comprising an alkyl rather than an aryl skeleton (Scheme 12.12) [32]. The addition of CO2 to the dihydride complex results in C=0 insertion into the iridium-hydride bond, and affords the formate complex 30. However, this complex is not stable and disproportionates spontaneously into the virtually insoluble bicarbonate complex 31 and the carbonyl dihydride 32. Such disproportionation is suppressed when the iridium metal center is replaced by rhodium [33], which is generally assumed to have a lower hydride affinity than iridium. 

Chloro- and other halo- containing carbonyl compounds of iridium may also be synthesized under mild conditions. Unlike [Rh(CO)2Cl]2, [Ir(CO)2Cl] is not obtainable by the direct reaction of an iridium chloride solution with CO. Instead, [Ir(CO)2Cl2]n (48) is obtained in low yields by reaction between IrCl3-H20 and carbon monoxide. The predominant mononuclear compound obtained upon carbonylation of iridium chloride salts is the tricarbonyl [Ir(CO)3Cl] (49), which appears in the sohd state to be a polymeric array consisting of stacking square-planar Ir(CO)3Cl units with short fr-Ir bonds. Even though [Ir(CO)3Cl] is polymeric, it is sublimable and is stiU a convenient source of iridium(I) containing carbon monoxide. (49) will react with a number of nucleophiles to form mononuclear iridium carbonyl complexes. 

This chapter will not deal with iridium complexes in which the coordination chemistry of the iridium-carbon bond is implicated inasmuch as Leigh and Richards have very recently (1982) provided an excellent detailed review on such compoimds organoiridium and iridium carbonyl complexes were also previously reviewed. Iridium complex chemistry has been reviewed (1980) along with rhodium," and in annual reviews. Additionally, iridium complexes have been treated in Comprehensive Inorganic Chemistry . ... 

There are a few known examples of Rh(II) and Ir(II) carbonyl complexes. " For rhodium, these are dinuclear with strong Rh —Rh bonds, for example, [Rh2(OOCMe)4 ( 0)2]. Iridium readily forms mononuclear complexes, although dinuclear iridium carbonyls such as [Ir2(tcbi)2 (CO)2 (NCMe)2 P(OEt)3 2]-MeCN, [Ir(H)(/i-SBuO(CO)(PR3)]2, and [Ir2l2(/i-SBu02 are known. 

ALKANE PICOSECOND CARBON-HYDROGEN BOND CLEAVAGE AT THE IRIDIUM CARBONYL CENTER... 

Ca.ta.lysis, Iridium compounds do not have industrial appHcations as catalysts. However, these compounds have been studied to model fundamental catalytic steps (174), such as substrate binding of unsaturated molecules and dioxygen oxidative addition of hydrogen, alkyl haHdes, and the carbon—hydrogen bond reductive elimination and important metal-centered transformations such as carbonylation, -elimination, CO reduction, and... 

L = P(CH3)3 or CO, oxidatively add arene and alkane carbon—hydrogen bonds (181,182). Catalytic dehydrogenation of alkanes (183) and carbonylation of bensene (184) has also been observed. Iridium compounds have also been shown to catalyse hydrogenation (185) and isomerisation of unsaturated alkanes (186), hydrogen-transfer reactions, and enantioselective hydrogenation of ketones (187) and imines (188). 

The formation of C-C bonds is of key importance in organic synthesis. An important catalytic methodology for generating C-C bonds is provided by carbonylation. In the bulk chemicals arena this is used for the production of acetic acid by methanol carbonylation (Eqn. (9)) in the presence of rhodium- or, more recently, iridium-based catalysts (Maitlis et al, 1998). 

Two other publications on Ir (73 keV) Mossbauer spectroscopy of complex compounds of iridium have been reported by Williams et al. [291,292]. In their first article [291], they have shown that the additive model suggested by Bancroft [293] does not account satisfactorily for the partial isomer shift and partial quadrupole splitting in Ir(lll) complexes. Their second article [292] deals with four-coordinate formally lr(l) complexes. They observed, like other authors on similar low-valent iridium compounds [284], only small differences in the isomer shifts, which they attributed to the interaction between the metal-ligand bonds leading to compensation effects. Their interpretation is supported by changes in the NMR data of the phosphine ligands and in the frequency of the carbonyl stretching vibration. 

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