Iridium metal carbonyl clusters

Polynuclear Iridium Carbonyl Clusters (See Metal Carbonyl Clusters)... 

Because of the great strength of iridium-iridium bonds, there is a wealth of information on polynuclear metal-carbonyl cluster compounds of iridium (see Dinuclear Organometallic Cluster Complexes mA Polynuclear Organometallic Cluster Complexes). It4(CO)i2 (45) may be synthesized from... 

Carbon monoxide insertions in metal carbonyl clusters have been reported. One example is the reaction of the tetranuclear iridium cluster Ir4Me(CO)g(/r4- 7 -... 

These methods have been seldom used but are potentially quite valuable as they provide quantitative structural data. The diffraction method involves determination of a radial electron distribution (RED) function which gives a distribution of distances and coordination numbers of atoms surrounding a metal atom. This method has been used to characterize the formation of rhodium [79] and iridium [39] carbonyl clusters in NaY zeolite. 

In summary, the interactions of the carbonyl ligands of rhodium and iridium carbonyl clusters with the Lewis add sites in zeolites are indicated by shifts in the stretching frequendes of the CO ligands, and the pattern parallels that observed for metal carbonyl clusters in solutions containing Lewis adds and on surfaces of metal oxides containing Lewis add sites. 

Spectroscopic studies on metal carbonyl complexes were relatively abundant in 1993. They include Iridium carbonyl complexes investigated via NMR O NMR studies on (mesitylene)M CO)3 complexes (M = Cr, Mo, W) an interesting NMR method for optimizing the study of slow chemical exchange has been announced natural abundance 0 NMR spectra of metal carbonyl clusters of the iron triad . 

Rearrangements of clusters, i.e. changes of cluster shape and increase and decrease of the number of cluster metal atoms, have already been mentioned with pyrolysis reactions and heterometallic cluster synthesis in chapter 2.4. Furthermore, cluster rearrangements can occur under conditions which are similar to those used to form simple clusters, e.g. simple redox reactions interconvert four to fifteen atom rhodium clusters (12,14, 280). Hard-base-induced disproportionation reactions lead to many atom clusters of rhenium (17), ruthenium and osmium (233), iron (108), rhodium (22, 88, 277), and iridium (28). And the interaction of metal carbonyl anions and clusters produces bigger clusters of iron (102, 367), ruthenium, and osmium (249). 

Zhao, A., and Gates, B. C., Probing metal oxide surface reactivity with adsorbate organometallic chemistry Formation of iridium carbonyl clusters on P-AI2O3, Langmuir 13,4024 (1997). 

Transition metal ions, within the zeolite framework, may undergo a reductive carbonylation to give mononuclear monovalent carbonyl coumpounds M(I)(CO) and ultimatly to give zerovalent polynuclear carbonyl clusters. The rhodium(I)and iridium(i)carbonyIs were identified using spectroscopic and volumetric methods, the zerovalent rhodium and Iridium clusters M (CO)j were also synthetized in the zeolite matrix and their structure investigated using IR, NMR and spin labelling methods. 

Several other polynuclear metal carbonyls were tested as catalysts for the reduction of nitrobenzene to aniline using carbon monoxide and water as the reducing agent. Rhodium, iridium and osmium clusters were found to be very effective and they were far less susceptible to oxidative degradation than [Fe(CO)5]. 3... 

Chemistry similar to that described above for iridium clusters has also been observed for rhodium clusters. Several authors [16-18] have prepared [Rh6(CO)i6] in NaY zeolite [R1i4(CO)i2] has also been formed [18], and each of these has been decarbonylated with minimal changes in the metal frame, as shown by EXAFS spectroscopy [18]. Thus there appears to be some generality to the method of forming small clusters in zeolite cages by synthesis of stable metal carbonyl precursors followed by decarbonylation. However, the method is limited. Attempts to use it to prepare zeolite-supported platinum clusters that are structurally simple and uniform have apparently not been successfiil. The literature of platinum carbonyl clusters in zeolites is not considered here because it is still contradictory. 

Mixed rhodium-iridium clusters incorporated in zeolites have been formed by thermal decomposition of the appropriate metal carbonyls [286]. 

The cobalt carbonyls are prepared by the disproportionation reaction of [Co2(CO)g] in the presence of Lewis bases or by the reduction of cluster cobalt carbonyls with the alkali metals. The iridium compounds are obtained during reduction of [Ir4(CO)i2] with sodium in ether solution. The rhodium carbonyls are usually synthesized by reduction of [Rh2Cl2(CO)4] or [RhClg] " with carbon monoxide in basic medium or by nucleophilic attack of bases on the carbonyl group of carbonyl clusters (see preparation of [M4(CO)i2] and [M6(CO)i6]). 

Re2(CO)io]/ as well as cluster carbonyls of iron, ruthenium, osmium, rhodium, and iridium. " Catalysts that are obtained in basic aqueous-methanol solution of metal carbonyls of the type M(CO)6 (M = Cr, Mo, W) and M3(CO)i2 (M = Ru, Os) are active even in the presence of sodium sulfide.Most commonly utilized solvents have been mixtures of water with alcohols, methoxyethanol, pyridine, etc. (mainly in the case of neutral solutions and those containing bases, such as KOH, K2CO3). Catalysts that are active in acidic media include Rh2Cl2(CO)4- -HCl- -NaI (solvent H0Ac-hHCl + H20),<"" [Rh(bipy)2]Cl, " > and... 

The encaged clusters considered in this chapter are almost exclusively metal carbonyls and metals (including bimetallics). llte encaged metal carbonyls that have been most investigated include [Rh (CO)i6], [Ir4(CO)i2], and the isomers of [Ir (CO),d the crystal structures of the iridium clusters are shown in Figure 4-6. Some of the most thoroughly characterized encaged metal dusters have been made from these metal carbonyls. Brief mention is made of metal oxide and also nonmetal clusters ionic dusters are scarcely considered. Synthesis, characterization, reactivity, and catalytic and other properties are considered for these materials. 

The other vibrational spectroscopies, although less easily applied, may provide complementary structural information. Raman spectroscopy has been used to detect metal-metal bonds in metal oxide supported osmium [86] and iridium [87] clusters. This method might be expected to find application in the study of zeolite supported metal carbonyl dusters, but it is still far from routine since samples are subject to destruction by laser beams, and fluorescence often prevents measurement of useful spectra. 

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