Iridium compounds

The most common oxidation states, corresponding electronic configurations, and coordination geometries of iridium are +1 (t5 ) usually square plane although some five-coordinate complexes are known, and +3 (t7 ) and +4 (t5 ), both octahedral. Compounds ia every oxidation state between —1 and +6 (<5 ) are known. Iridium compounds are used primarily to model more active rhodium catalysts. 

Binary Compounds. The fluorides of indium are IrF [23370-59-4] IrF [37501-24-9] the tetrameric pentafluoride (IiF ) [14568-19-5], and JIrFg [7789-75-7]. Chlorides of indium include IrCl, which exists in anhydrous [10025-83-9] a- and p-forms, and as a soluble hydrate [14996-61-3], and IrCl [10025-97-5], Other haUdes include IrBr [10049-24-8], which is insoluble, and the soluble tetrahydrate IrBr -4H20 IrBr [7789-64-2]-, and Irl [7790-41-2], Iridium forms indium dioxide [12030-49-8], a poorly characteri2ed sesquioxide, 11203 [1312-46-5]-, and the hydroxides, Ir(OH)3 [54968-01-3] and Ir(OH) [25141-14-4], Other binary iridium compounds include the sulfides, IrS [12136-40-2], F2S3 [12136-42-4], IrS2 [12030-51 -2], and IrS3 [12030-52-3], as well as various selenides and teUurides. 

Synthesis. The principal starting material for synthesis of iridium compounds is iridium trichloride hydrate [14996-61-3], IrCl3-a H2 0. Another useful material for laboratory-scale reactions is [Ir20l2(cod)2] [12112-67-3]. 

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). 

Other soluble iridium compounds, e.g. chloroiridic acid, can also be used, t Phosphorous acid can replace the ester the latter is probably largely or wholly hydrolyzed when it is used as a starting material. 

Na,IrCl6 is a convenient starting material in the synthesis of iridium compounds. 

M(NO)2(PPh3)2]+. The coordination number of the metal in both is four, in a distorted tetrahedral geometry. The position of i/(N—O) in the IR spectrum is essentially the same, and the rhodium and iridium compounds have similar slight bending of the M—N—O linkage. 

M(NO)(OCOCF3)2(PPh3)2. Both these complexes have 5-coordinate geometries with monodentate carboxylates. The rhodium compound has a square pyramidal structure with bent Rh-N-O (122°) but the iridium compound has a tbp structure with straight equatorial Ir-N—O (178°). The position of i/(N—O) reflects this difference (1800 cm-1 (Ir) and 1665 cm-1 (Rh)). 

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. 

Table 7.10 Ir Mossbauer effect studies on various iridium compounds and alloys...

Nearly aU Mossbauer measurements on iridium compounds and alloys have been carried out with the Ir (73 keV) and not with the lr (129 keV) transition, though R.L. Mossbauer originally discovered the nuclear resonance phenomenon with the latter nuchde. An overview of noteworthy pubhcations, which were published after 1978, when the first edition of this book appeared, is collected in Table 7.10. 

The most common use of iridium coordination compounds remains in the catalysis field, although interest is developing in the luminescent properties of iridium compounds. The wide range of accessible oxidation states available to iridium (—1) to (VI) is reflected in the diverse nature of its coordination compounds. 

Szabo-Nagy and Keszethelyi (1999 and 2000) have carried out experiments which show a possible violation of parity in the crystallisation of racemates of tris(l,2-ethylenediamine-Co(III)) and the corresponding iridium compound. 

Today, iridium compounds find so many varied applications in contemporary homogeneous catalysis it is difficult to recall that, until the late 1970s, rhodium was one of only two metals considered likely to serve as useful catalysts, at that time typically for hydrogenation or hydroformylation. Indeed, catalyst/solvent combinations such as [IrCl(PPh3)3]/MeOH, which were modeled directly on what was previously successful for rhodium, failed for iridium. Although iridium was still considered potentially to be useful, this was only for the demonstration of stoichiometric reactions related to proposed catalytic cycles. Iridium tends to form stronger metal-ligand bonds (e.g., Cp(CO)Rh-CO, 46 kcal mol-1 Cp(CO)Ir-CO, 57 kcal mol ), and consequently compounds which act as reactive intermediates for rhodium can sometimes be isolated in the case of iridium. 

Examples of the osmium and iridium complexes are Os(PPh3)2Cl(NO) and Ir(PPh3)3(NO), respectively [216]. The osmium compound gave, on reaction with HC1, the first characterized complex with the feature of an N-coordinated HNO, Os(PPh3)2Cl2(HNO), which was confirmed by X-ray crystallography. On the other hand, the nitrosylated iridium compound gave the hydroxylamine complex [216]. 

Y. Wang, N. Herron, V.V. Grushin, D. LeCloux, and V. Petrov, Highly efficient electroluminescent materials based on fluorinated organometallic iridium compounds, Appl. Phys. Lett., 79 449-451 (2001). 

Iridium (Ir, [Xe + 4/14]5r/9), name from the Latin word iris (rainbow iridium compounds are highly coloured). Discovered (1803) by the English chemist Smithson Tennant. 

Alloys with Cobalt compounds Rhodium compounds Iridium compounds ... 

While the Schrock and Osborn catalysts worked well in the Rh series, the analogous iridium compounds, also studied by them, did not show any special... 

From the outset, iridium compounds have played an important role in the better understanding of the C—H activation process, and consequently in the development of efficient alkane dehydrogenation reactions [8]. Hence, in this chapter we will review the participation of iridium complexes in the optimization of chemical processes for C—H activation which, today, have led to some highly promising... 

Other silicon derivatives containing Si—X—C bonds (where X is O and/or N) can be successfully prepared by using iridium-catalyzed reachons such as the asymmetric hydrosilylation of ketones and amines, the silylcarbonylation of alkenes, and the alcoholysis of Si—H bonds. Indeed, oxygenation of the latter bond to silanol also proceeds smoothly in the presence of iridium compounds. 

More complex iridium compounds are possible as well, including the following ... 

The elemental metal form of iridium is almost completely inert and does not oxidize at room temperatures. But, as with several of the other metals in the platinum group, several of iridiums compounds are toxic. The dust and powder should not be inhaled or ingested. 

For rhodium and iridium compounds alkoxo ligands take over the role of the basic anion. Using /z-alkoxo complexes of ( -cod)rhodium(I) and iridium(I)— formed in situ by adding the /r-chloro bridged analogues to a solution of sodium alkoxide in the corresponding alcohol and azolium salts—leads to the desired NHC complexes even at room temperature [Eq. (10)]. Using imidazolium ethoxyl-ates with [(r " -cod)RhCl]2 provides an alternative way to the same complexes. By this method, it is also possible to prepare benzimidazolin-2-ylidene complexes of rhodium(I). Furthermore, an extension to triazolium and tetrazolium salts was shown to be possible. ... 

The Rh complex 72 shown in Fig. 35 was obtained by simple addition of the N-heterocyclic carbon(O) ligand to [Rh(p-Cl)(CO)2l2 in THF solution [103,104]. The transition metal complexes 73 and 74 of the all carbon four-membered cyclic ligand shown in Fig. 15 were prepared by adding the rhodium or iridium compounds [M(p-Cl)(cod)]2 to the freshly generated allene at 20° C in THF solution [105, 106]. The cod ligand can easily be replaced by admitting CO at room temperature to THF solutions of the complexes to give 75 and 76, respectively, as shown in Fig. 35. 

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