Polyoxoanions

A feature of the molybdovanadates in aqueous solution is the domination of polyions whose structures are the same as those of [Vio028]6-, /3-[Mo8026]4 , and [Mo6Oi9]2. The stability of mixed hexametalates is noteworthy because of the nonexistence of this structure in either vanadate or molybdate aqueous solutions [Mo6Oi9]2 is stable only in nonaqueous solution (cf. Section III,C,I). 

Potentiometric data alone is not adequate to establish the specia- 

The ions with six metal atoms have the same structure as the hexa-molybdate ion [Mo6Oi9]2 . The [Mo5VOi9]3 ion, previously identified in acetonitrile medium (166) can be obtained in the solid state by precipitation from an aqueous solution with tetramethyl ammonium as cation (165). The vanadium atoms in both [Mo4V2Oi9]4 and its pro-tonated form [HMo4V2Oi9]3 (pKa = 3.8) are in cis positions. Protonation seems to take place at the oxygen, which bridges the two vanadium atoms. 

Although no polyanions with nine metal atoms have so far been found in either vanadate(V) or molybdate(VI) solutions the [Mo4V5 O27]3- ion as well as its protonated form, [HMo4V5027]4 with pKa 2.5, have been identified in the mixed solutions (162, 165). 

Equilibrium constants for the various mixed polyions in 0.6 M NaCl at 25° have been calculated from combined potentiometic and 61V NMR measurements over wide concentration ranges and 1.4 pH 7 (165). 

Soluble and stable iridium nanoparticles (3.0 0.4nm diameter) have been prepared by reduction of the polyoxoanion-supported lr(l) complex (n-Bu4N)sNa3 [(C0D)lr(P2WisNb3062)] (COD = 1,5-cyclo-octadiene) with molecular hydrogen in 

Iridium Complexes in Organic Synthesis. Edited by Luis A. Oro and Carmen Claver Copyright 2009 WILEY-VCH Verlag GmbH Co. KGaA, Weinheim Tf RN Q7RH- 7.7H199E-1 

---

A variation of the Pd/Cu Wacker-Hoechst process, termed OK Technology, has been proposed by Catalytica Associates (40—46). This process avoids the use of chlorides and uses a Pd/Cu catalyst system which incorporates a polyoxoanion and a nitrile ligand. 

In agreement with the results from the characterization, the SCR activity of VOx/Zr02 also depends only on the V-content, not on the method used for catalyst preparation. The marked increase in SCR activity with the V-content shows that only specific vanadium configurations are active. Although we assess the V=0 modes associated with these active configurations, IR analysis did not specify the structure of active polyoxoanions. 

Ionic compounds such as halides, carboxylates or polyoxoanions, dissolved in (generally aqueous) solution can generate electrostatic stabilization. The adsorption of these compounds and their related counter ions on the metallic surface will generate an electrical double-layer around the particles (Fig. 1). The result is a coulombic repulsion between the particles. If the electric potential associated with the double layer is high enough, then the electrostatic repulsion will prevent particle aggregation [27,30]. 

Scheme 4 Hydrogenation of anisole by polyoxoanion-stabilized Rh(0) nanocluster catalyst...

A similar result is obtained with 30 equivalents of H2O added but a long reaction time is required namely 215 h. Nevertheless, in all cases a black precipitate of bulk Rh(0) is visible at the end of the reaction justifying the destabilization of nanoclusters due to the interaction of H or H2O with the basic P2Wi5Nb3062 polyoxoanion. Finally, the partial hydrogenation of anisole to yield 1-methoxycyclohexane (up to 8%) with a soluble nanocluster catalyst has been reported by Finke and coworkers (see Sect. 3). 

In fact, partial hydrogenations are rarely described with soluble nanoparticle catalysts. Two examples are explained in the Uterature, one reported by Finke and coworkers in the hydrogenation of anisole with polyoxoanion-stabihzed Rh(0) nanoclusters [26] and one reported by Dupont and coworkers in the hydrogenation of benzene with nanoscale ruthenium catalysts in room temperature imidazoUiun ionic Uquids [69]. hi these two cases, the yields are very modest. 

In summary, partial hydrogenation of anisole to 1-methylcyclohexene by polyoxoanion-stabilized Rh(0) nanoclusters is very modest but could in the future be an interesting additive study in the monocyclic arene hydrogenation research area. 

Finke s system discovered in 1994 represents a special subclass of transition metal nanocluster [167,288,289]. A zerovalent transition metal core consisting of, e.g., 300-900 atoms is doubly stabilized by a cationic surfactant (e.g., and by an assembly of polyoxoanions... 

Finke has reported remarkable catalytic lifetimes for the polyoxoanion- and tetrabutylammonium-stabi-lized transition metal nanoclusters [288-292]. For example in the catalytic hydrogenation of cyclohexene, a common test for structure insensitive reactions, the lr(0) nanocluster [296] showed up to 18,000 total turnovers with turnover frequencies of 3200 h [293]. As many as 190,000 turnovers were reported in the case of the Rh(0) analogue reported recently. Obviously, the polyoxoanion component prevents the precious metal nanoparticles from aggregating so that the active metals exhibit a high surface area [297]. 

Electrosteric stabilization can be also obtained from the couple ammonium (Bu4N+)/polyoxoanion (INWnNb C>62)- The significant steric repulsion of the bulky Bu4N+ countercation, when associated with the highly charged polyoxo-anion (coulombic repulsion), provides efficient electrosterical stability towards agglomeration in solution of the resultant nanocatalysts [2, 5, 6]. 

Scheme 9.5 Synthesis of Rh(0) nanoclusters from (1,5-COD)RhP2W15Nb3Oj polyoxoanion-supported nano-cluster-forming precatalyst (space-filling representation). (Adapted from [63].)...

An alternative approach to stabilize nanoparticles is to use polyoxoanions (see Scheme 9.5). Finke and coworkers described polyoxoanion- and ammonium-stabilized rhodium zerovalent nanoclusters for the hydrogenation of classical benzene compounds [95, 108]. This organometallic approach allows reproducible preparation of stable nanoparticles starting from a well-defined complex in terms of composition and structure (see Section 9.3.5). 

The polyoxoanion-stabilized Rh(0) nanoclusters were investigated in anisole hydrogenation [6,95]. The catalytic reactions were carried out in a single phase... 

---