Reductive Carbonylation of Mixed Metal Complexes

Ichikawa et al. [207, 208] prepared mixtures of rhodium and iridium carbonyls in zeolite Y by reductive carbonylation of their salts. The carbonyls were suggested to be [Rh cIr6. (CO)i6] on the basis of infrared, EXAFS, and other results. The complexity of the mixture made it difficult to determine which clusters were present and in what proportions. The duster mixture was decarbonylated, giving clusters that were suggested to be Rhir clusters of various compositions. 

Similar experiments have been done to prepare FeRh clusters from such precursors as [Rh4(CO)i2], which were stated to have been formed in the cages of zeolite Y. [209] There is a question about the location and composition of the resultant clusters, in part because [Rh4(CO)i2] is simply too large to enter the cages. 

Ionic clusters in zeolite cages are stabilized by their ionic interactions with the zeolite framework. They have metal-metal distances intermediate between those in metals and those in metal oxides. The subject of ionic clusters in zeolites is dealt with only briefly here, with sodium clusters chosen as an example. 

Rabo and Kasai [210, 211] were the first to show that zeolites can stabilize neutral as well as charged sodium clusters. The ESR spectrum of the material resulting om the interaction of Na vapor with the zeolite NaX was attributed to an Na paramagnetic unit. Harrison et al. [212] showed that the charged clusters are located in the sodalite cages of zeolites A, X, and Y and can only be obtained when an excess of sodium is introduced into the zeolite. 

An alternate method for the preparation of such dusters [213] is based on the impregnation of a dehydrated zeolite with alcoholic solutions of sodium azide, followed by the controlled decomposition of the azide. The ionic sodium clusters were shown by ESR spectroscopy to be Na4 and are formed in the zeolite pores. The clusters exhibit catal)dic properties for both isomerization and hydrogenation reactions of alkenes and alkynes. 

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