Keggin anion structure

CsPWKlO with intact Keggin anion structure gives satisfactory results in the benzoylation of para-xylene wifh BC, affording 2,5-dimefhylbenzo-phenone in 60% yield. 

The most characteristic feature of all heterojwlyacids is the structure of their anions. Even though a variety of structures are known (1] compounds of the so-called Keggin anion structure (dodecaheteropolyacids, H,XMi204o) are most widely studied. This is due to their easy preparation, high stability and to the fcict that they are active catalysts for various chemical reactions. Their catalytic properties have been reviewed several times [2, 3]. 

There are a few features relative to POMs that are necessary for obtaining the best performance. In all cases. Vanadium is present in the structure of the P/Mo Keggin anion, while the cations include different components, that is, protons, divalent transition metal ions (preferably either Fe " " or Cu " "), and alkali metal ions (preferably Cs" "). The role of Cu ions is to catalyze the reduction of molybdenum, thus increasing the activity of the catalyst it also affects the surface acidity. 

Despite the structural equivalence of the twelve metal centers in the o -PWi204o Keggin anion, a stepwise reduction of the tungsten centers is observed [30f]. EPR experiments on this species show that the added electron is delocalized (type II in the scheme proposed hy Rohin and Day) [4]. Over all 12 tungsten centers suggesting that there is communication... 

Several deficit or lacunary derivatives of the Keggin structure exist as individual species or as fragments of other heteropolyanion structures. Removal of one M06 octahedron (stoichiometric loss of MO"+) from a- or /3-Keggin anions leads to isomers of [XM11O39]"-. Removal of three adjacent octahedra leads to A- and B-type XM9034 anions. The structures of these are shown in Figure 11. 

Reduction of Keggin anions beyond 2e reduction leads to modest changes in electronic and molecular structure, although the reductions remain reversible. For example, in the case of molybdates, a 4e -reduced /(-Keggin structure is stabilized, with the bridging oxygen atoms being partially protonated [Eq. (27)] (3, 253, 254) ... 

Isopolyoxometalates and heteropolyoxometalates are formulated as [MmOy]p and [X MmOy] (xc to), respectively, where M is the addenda atom and X is the heteroatom. The most common addenda atoms are the d°-early-transition-metal cations such as W6 +, Mo6+ and Vs+. The heteroatoms can be p-, d- or f-block elements such as P5 +, As5 +, Si4 +, Ge4 + and B3 +. Among a wide variety of heteropolyoxometalates, the Keggin structures are the most stable and more easily available. Keggin anions, typically represented by the formula [X + M O ] 8- , contain one central heteroatom and twelve addenda atoms (four M3013 triads). 

Lacunary Keggin anions, a-XMnO, have a defect Keggin structure, in which one addenda atom and one terminal oxygen are missing. These lacunary hetero-... 

Ce(IV)03)3(H20)2(A-a-PW9034)2] [23] is an example of sandwich type RE-POM clusters with three central RE atoms. The structure is composed of two trivacant Keggin anions [A-a-PW9034] and a central belt of three cerium atoms alternating with three oxygen atoms. Three cerium atoms have two types of coordination one is six-coordinated while... 

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