Combinatorial Libraries of Homogeneous Polyoxometalatebased Catalysts

Early transition metal oxygen anion clusters or polyoxometalates (POMs) have a significant impact in a variety of different research fields. Among catalysis by heteropolyacids, selective hetero- and homogeneous oxidation processes are of utmost importance [144]. Of extraordinary interest are POMs of the so-called Keggin type that possess the general formula {Hp+x)[PVxM(12_x)O40](aq)} (M = Mo, W; x = 0-5).

Hill and Gall published the combinatorial synthesis of phosphorus-centered molybdenum and tungsten polyoxometalates of the Keggin structure [145]. The POM catalysts were evaluated in the selective aerobic oxidation of tetrahydrothio-phene (THT) to its corresponding sulfoxide (THTO) (Scheme 32.33).


Scheme 32.33. Aerobic oxidation of tetrahydrothiophene (THT) to tetrahydrothiophene sulfoxide (THTO) by combinatorially prepared POMs.

Catalyst library synthesis was performed by combinatorially mixing stock solutions of appropriate tungsten, molybdenum, and vanadium precursors under a constant phosphate concentration. Completion of catalyst formation was achieved by equilibration at room temperature, which was followed by 51V-NMR spectroscopy. Conventional gas chromatography was used to quantify product (THTO) yields and selectivities. Promising catalysts were selected and subsequently identified with the aid of 51V-NMR and IR spectroscopy, revealing the complex [a-1,4-

PV2W10O40]5~ as the most effective among them. All the samples evaluated, including the starting materials, exhibited catalytic efficiency to a certain degree in the conversion of THT to THTO, suggesting that Keggin-type polymolybdato- or tung-stophosphates have the ability to oxidize selectively THT to THTO regardless of their vanadium content. It was assumed that the catalytically most active species were primarily Keggin polyoxometalates rather than lower nuclearity polyoxometa-lates. Several other combinatorial libraries containing boron, silicon, and arsenic were evaluated under a variety of conditions, but the results were not quantified.

Selective catalytic oxidations that proceed at satisfactory rates at ambient conditions and use oxygen or air as terminal oxidant are of utmost interest [146]. In a more recent contribution, Hill and his group reported a diversity-based discovery and mechanistic investigation of selective homogeneous thioether oxidation by the Au(III)Cl2NO3(thioether)/oxygen system [147]. A 150-member library of inorganic complexes was constructed by combinatorially combining polyoxometalate anions and redox-active cations. The library design comprised POMs with reversible redox chemistry and redox-active d-block ions or appropriate precursors including HAuCl4, as well as s-block and p-block cations as counterions for the POMs. The catalyst library included also POM-free control formulations such as the chloride, nitrate, or perchlorate salts of the appropriate redox-active d-block cations.

Each library member was evaluated for its ability to catalyze the oxidation of the thioether mustard analog 2-chloroethyl ethyl sulfide (CEES) to the corresponding sulfoxide (CEESO), using only oxygen as the oxidant (Scheme 32.34).


Scheme 32.34. Aerobic oxidation of 2-chloroethyl sulfide (CEES) to 2-chloroethyl sulfide sulfoxide (CEESO) by the POMfree combinatorially discovered catalysts system Au(III)Cl2NO3 (thioether).

Product analysis and distribution was assessed by conventional serial GC. Most catalyst preparations showed little or no activity in the target reaction but three catalyst compositions exhibited considerable catalytic activity. The catalytically active compositions included the POMs [CuPW11O39]5- and [MnPW11O39]5_, and the POM-free system AgNO3, each of them with five equivalents of HAuCl4. The most optimized combination of HAuCl4 and AgNO3 formed a catalyst that exhibited orders of magnitude higher reaction rates and higher turnover numbers (TONs) at ambient temperature and at 1 atm. of air or oxygen than previously reported catalysts. Extensive kinetic and mechanistic studies for the oxygen-based oxidation of thioethers catalyzed by the parent system Au/Ag/NO3- revealed two gold complexes as the catalytically active species (Fig. 32.17).

Fig. 32.17. Proposed catalytically active gold complexes.

Fig. 32.17. Proposed catalytically active gold complexes.

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