Transition-metal-substituted POMs

Various catalytic systems for H202- and 02-based oxidations catalyzed by POMs have been developed. Typical examples are listed in Table 13.1. The systems can be classified into four groups according to the stmctures of POMs (1) mixed-addenda POMs, (2) transition-metal-substituted POMs, (3) POMs, and (4) lacunary POMs. In this chapter, liquid-phase homogeneous oxidations by POMs with H202 and 02 are described according to the above classification. 

Transition metal-substituted POMs (TM-SPs) can be considered as the largest subclass of polyanions [6]. Within the class of TMSPs, the sandwich-type compounds represent the largest family. In short, sandwich-type POMs are formed by the fusion of two trivacant a-XW9 034" ... 

Lacunary spedes can act as structural motifs with numerous metal cations, leading to mono-, di- and trinuclear transition-metal-substituted POMs according to the number of vacant sites. The metal incorporation involves the reaction of aquated first-row and occasionally second-row d-block metal ions, M(H20)6z +, with a lacunary POM derived from the Keggin, Wells-Dawson and other POM families. In addition, lacunary spedes assemble into large POM structures, either directly or with incorporation of metal ion linkers. 

Transition-metal-substituted POMs are oxidatively and hydrolytically stable compared with organometallic complexes, and their active sites can be controlled. These advantages have been applied to the development of biomimetic catalysis relating to enzyme analogues. To date, various kinds of transition-metal-substituted POMs have been synthesized and certain kinds of them can efficiently activate 02 and H202, as described below. 

Not only the transition metals incorporated in transition-metal-substituted POMs, but also addenda atoms in POMs can activate H2O2. The reaction of lacunary POM [a-CoWiiOag] leads to the formation of tetraperoxo species [ps-CoWn 035(02)4] ° [47]. Compound [p3-CoWn035(02)4] ° prepared in situ by the reaction of [a-CoWnOsg] with H2O2 showed the catalytic activity for the epoxidation of 2-cyclohexen-l-ol and the corresponding epoxide and ketone were obtained (7). 

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