Iridium trivalent

Fig. 7.63 Mossbauer spectra of some hexacoordinated ammine and pyridine complexes of trivalent iridium taken at 4.2 K with a source of Os in Os metal. The stick spectra indicate the positions and relative intensities of the individual resonance lines (from [285])...

Et4N]2[Fe2lr2(CO)i2] cluster precursor, which exhibit a high activity in the synthesis of methanol from CO and H2, were studied by Ir and Fe Mossbauer spectroscopy. The study extends from the precursors via the fresh to the aged catalysts. The presence of iridium in the metallic state as well as the presence of trivalent, divalent and alloyed iron is detected. Representative Ir and Fe Mossbauer spectra are shown in Fig. 7.69. Information about the adsorption on the surface of MgO... 

The rate of the methanol carbonylation reaction in the presence of iridium catalysts is very similar to that observed in the presence of rhodium catalysts under comparable conditions (29). This is perhaps initially surprising in view of the well-recognized greater nucleophilicity of iridium(I) complexes as compared to their rhodium(I) analogues. It can be seen from the above studies that the difference in the chemistry of the metals at the trivalent stage of the catalytic cycle serves to produce faster rates of alkyl migration with the rhodium system thus, overall the two metal catalysts give comparable rates. 

Complexes 6 undergo the second migratory insertion in this scheme to form the acyl complexes 7. Complexes 7 can react either with CO to give the saturated acyl intermediates 8, which have been observed spectroscopically, or with H2 to give the aldehyde product and the unsaturated intermediates 3. The reaction with H2 involves presumably oxidative addition and reductive elimination, but for rhodium no trivalent intermediates have been observed. For iridium the trivalent intermediate acyl dihydrides have been observed [29], The Rh-acyl intermediates 8 have also been observed [26] and due to the influence of the more bulky acyl group, as compared to the hydride atom in 2e and 2a, isomer 8ae is the most abundant species. 

All three elements form complex ammino-derivatives. Those of osmium have been very little investigated those of iridium are analogous to the anunino-derivatives of platinum on the one hand and to the ammincs of cobalt and chromium on the other whilst the platinum derivatives resemble those of cobalt, save that the metal in the platinic derivatives is tetravalent and not trivalent as in the cobalt-ammines. 

Two series of complex iridium salts corresponding to the hexammino-and diacido-iridium salts have been prepared containing ethylene-diamine. These have the same general characteristics as the ammino-salts. Both series have been resolved into optically active isomers, the dinitrito-series by means of d- and 1-bromo-camphor sulphonic acid, and the triethylenediamino-series by d- and 1-nitro-camphor. Attempts have been made to prepare etliylenediamino-derivatives containing tetravalent iridium. Thus, sodium hexaehloro-iridate, [IrCl6]Na2, reacts with ethylenediamine, but the products are syrupy and cannot be purified. Derivatives of trivalent iridium, however, have been prepared. These are crystalline and fairly easily purified. The... 

An interesting scries of salts is afforded by the alums of trivalent iron, cobalt, rhodium, and iridium. These have the general formula... 

The more important simple derivatives of cobalt are divalent, the metal only yielding stable trivalent salts in conjunction with other metallic derivatives, as, for example, the cobalti-nitrites and eobalii-cyanides, or in the complex ammino derivatives. Rhodium and iridium function almost exclusively as trivalent metals in their salts. 

Reactions of Salts of Iridium,.—Hydrogen sulphide in excess causes the precipitation of dark brown iridium sesquisulphide, Ir2S3. If the iridium is present in solution in the tetravalent condition, as for example in the form of alkali chlor-iridates, M2IrCl8 or 2MQ.IrCld, the dark solution is first decolorised and a yellowish white precipitate of sulphur forms in consequence of the reduction of the iridium salt to the trivalent condition, namely, as IrCl3. When this has been accomplished the brown sesquisulphide is obtained. 

In most of its compounds iridium is either trivalent or quadrivalent, the latter being tailed iridic. In both states of valence, the halogen compounds form double salts with the corresponding alkali halides, giving chloriridites and chloriridatcs respectively. These are to be considered as alkali salts of complex acids. 

In its trivalent form, iridium forms the sesquisulfate, Ir2(S04)j, which like the corresponding salts of cobalt and rhodium forms a series of alums. 

Iridium-catalyzed intramolecular l,n-enyne metathesis has been studied as a unique tool for the synthesis of various types of cyclic compounds. Reactions of this type depend on both the structure of substrates and the nature of catalyst systems used (411). Recently, the cycloisomerization of various 1,6-enynes have been shown to be catalyzed by [Ir(cod)Cl]2/dppf (494). These reactions are highly stereoselective, and generate the (Z)-isomer preferentially over the ( )-isomer (Scheme 63). The proposed mechanism (Scheme 64) involves oxidative cyclization of the enyne at Ir(I) to give the trivalent iridacyclopentene. The intermediate undergoes (3-hydride elimination to give the irida-1,3-diene, which experiences steric repulsion between the metal fragment and the cis substituent on the... 

Indeed, these calculations correspond to the experimental results of Beller s group with monodentate phosphine ligands and may explain to some extent the lower reactivity for iridium in comparison to rhodium catalysts found in the past [10]. By comparing the efficiency of trivalent phosphorus ligands on the aldehyde yield, the following order was established (Figure 1.14). 

Oxidative addition to iridium [125] is much faster than that to the corresponding rhodium complexes. Also the equilibrium is on the side of the trivalent state. 

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