2.64.3 NONAQUEOUS POLYOXOMETALATE REACTIVITY 763
2.64.3.1 Alkoxide Exchange 763
2.64.3.2 Aryloxide and Other Derivatives 765
2.64.3.3 Hydrolysis 765
2.64.3.4 Oxo Metathesis 765
2.64.3.5 Base Degradation 766
2.64.3.6 Polyoxometalates as Nucleophiles 767
2.64.4 CHARACTERIZATION 769
2.64.4.1 Hexametalates 769
2.64.4.2 Derivatives Based on the Keggin Structure 771
2.64.5 CONCLUSIONS 772
2.64.6 REFERENCES 772
2.64.1 INTRODUCTION
The field of polyoxometalate chemistry continues to expand and diversify, and whilst the majority of chemistry in this area is still carried out in aqueous media, interest in nonaqueous and solid state systems and in the incorporation of polyoxometalates at interfaces (liquid–liquid, liquid–air and solid–air) is growing rapidly.1–3 The complex, pH-dependent equilibria associated with aqueous polyoxometalate chemistry have been studied in detail and several families of structures and related derivatives are now well established. Speciation in nonaqueous solutions is often markedly different, and in organic solvents the choice of reagents for reactivity studies need not be restricted by hydrolytic sensitivity. Tetraalkylammonium cations impart organic solubility to these molecular metal oxides, and nonaqueous studies generally involve Bun4Nþ salts in polar solvents such as MeCN (acetonitrile). The rational syntheses of a series of hexametalates pre- sented in Section 2.64.2 serve to illustrate various nonaqueous aggregation strategies that use moisture-sensitive precursors, whilst reactions that enable the systematic manipulation of struc- ture and surface functionality are discussed in Section 2.64.3. Techniques for studying the structural and electronic properties of polyoxometalates are highlighted in Section 2.64.4 with reference to selected examples from the previous sections.
2.64.2 A HYDROLYTIC STRATEGY FOR NONAQUEOUS HEXAMETALATE SYNTHESIS
In the synthesis of a general polyoxometalate [XxMyOz]n, where X is a heteroelement, starting materials are selected so as to provide the various constituent parts, i.e., the metal oxide, the anionic charge, and (for heteropolyanions) the heteroelement oxide. In recent years,
759
a nonaqueous strategy has evolved that provides access to a range of hexametalates [LM0M5O18]n using metal alkoxides as the metal oxide source and tetraalkylammonium salts of oxometalates, e.g., (Bun4N)2[WO4], (Bun4N)2[MoO4], (Bun4N)2[Mo2O7], and (Bun4N)4- [Mo8O26] as sources of anionic charge.4–6In these reactions, the starting alkoxides are hydro- lyzed to produce polynuclear oxide frameworks in a process which is related to the production of oxide materials by the sol–gel method (see Chapter 1.40). Retrosynthetic analysis (or decon- struction) is a useful aid in the design of synthetic routes to polyoxoanions, as illustrated in the following examples of rational hexametalate assembly. The contribution from the anionic component is first separated and then suitable precursors for the metal oxide and heteroelement fragments can be identified:
5WOðOMeị4ỵ ẵWO42ỵ10H2O! ẵW6O192 ỵ20MeOH ð1ị
The retrosynthetic analysis for the parent hexatungstate (Bun4N)2[W6O19] (Figure 1) is shown in Scheme (1)(boxed items are starting materials). The yield from the associated hydrolysis reaction (seeEquation (1)) is essentially quantitative, emphasizing the stability of this hexanuclear frame- workin organic media. Surface alkoxides have been incorporated to introduce reactive sites into the otherwise inert tungsten oxide shell, andScheme (2) shows the retrosynthetic analysis which led to the synthesis of (Bun4N)3[(MeO)TiW5O18] (Figure 2) by hydrolysis of a mixture of tungsten and titanium alkoxides in the presence of [WO4]2(seeEquation (2)).
[W6O19]2–
[WO4]2–
5WO(OR)4 + 10 H2O + W5O15
Scheme 1
Figure 1 Structure of [W6O19]2.
760 Structure of Oxo Metallic Clusters
2[(RO)TiW5O18]3–
3[WO4]2–
7WO(OR)4 + 14H2O
2(RO)TiO1.5
2Ti(OR)4
+ 3H2O + W7O21+
Scheme 2
7WOðOMeị4 ỵ2TiðOMeị4 ỵ 3ẵWO42 ỵ17H2O!2ẵðMeOịTiW5O183 ỵ 34MeOH ð2ị (Bun4N)6[{(MeO)ZrW5O18}2] (Figure 3) has been prepared in a similar fashion from [{Zr(OR)4(ROH)}2] (RẳPrn, Bun), with addition of an excess of methanol subsequent to hydrolysis to ensure exchange of any residual alkoxide groups from the zirconium alkoxide starting material (seeEquation (3), R0ẳmixed Me/R). This hydrolytic approach has been modified to provide access to the chloro derivative [ClTiW5O18]37from the ‘‘virtual’’ nucleophilic oxometalate [W5O18]6 (seeScheme (3)). Volatiles were removed after the initial hydrolysis step (seeEquation (4)), and the residual solid was dissolved in MeCN before addition of a solution of [TiCl4(MeCN)2] (seeEquation (5)).8The group 5 derivatives [VW5O19]3and [(MeO)NbW5O18]2have also been prepared from [VO(OMe)3] and [Nb(OMe)5] respectively (seeEquations (6)and(7)). Following the retrosynthetic analysis shown inScheme (4), the molybdotitanate [(PriO)TiMo5O18]3(Figure 4) has been synthe- sized by hydrolysis of Ti(OPri)4in the presence of oxomolybdates [Mo2O7]2and [Mo8O26]4(see Equation (8)):8
7WOðOMeị4 ỵ ẵfZrðORị4ðROHịg2 ỵ 3ẵWO42 ỵ17H2O! ẵfðR0OịZrW5O18g26 ỵ 36R0OH ð3ị Figure 2 Structure of [(MeO)TiW5O18]3.
TiCl3+ W5O186–
TiCl4 W2O6
[ClTiW5O18]3–
3[WO4]2
2WO(OR)4 + 4H2O +
– +
Scheme 3
2WOðORị4ỵ 3ẵWO42 ỵ 4H2O!‘‘ẵW5O186’’ỵ 8ROH ð4ị
‘‘ẵW5O186’’ỵ ẵTiCl4ðMeCNị2 ! ẵClTiW5O183 ỵ3Cl ỵ2MeCN ð5ị
7WOðOMeị4ỵ 2VOðOMeị3ỵ 3ẵWO42 ỵ 17H2O! ẵVW5O193 ỵ 34MeOH ð6ị
4WOðOMeị4ỵ NbðOMeị5 ỵ ẵWO42 ỵ 10H2O! ẵðMeOịNbW5O182 ỵ20MeOH ð7ị
2TiðOPriị4 ỵ ẵMo2O72 ỵ ẵMo8O264 ỵ 3H2O!2ẵðPriOịTiMo5O183 ỵ 6PriOH ð8ị Figure 3 Structure of [{-MeO)ZrW5O18}2]6.
762 Structure of Oxo Metallic Clusters
2(RO)TiO1.5
2[(RO)TiMo5O18]3–
Mo10O336–
[Mo2O7]2– + [Mo8O26]4–
2Ti(OR)4 + 3H2O +
Scheme 4
These reactions provide considerable scope for the rational manipulation of heteronuclear hexametalates, although little is known about the hydrolytic aggregation processes involved, except that the 1:1 reaction between [WO4]2and [WO(OMe)4] proceeds via rapid ligand redis- tribution to give the structurally dynamic dinuclear species [W2O5(OMe)4]2.9As a result of these exchange processes, stoichiometric hydrolysis with17O-enriched water produces polyoxometalates with17O-enriched oxo sites, and reactions are readily monitored by17O NMR spectroscopy. In addition, these reactions provide an efficient means of producing 17O-enriched samples for subsequent reactivity studies (seeSection 2.64.4).