Development of a building block strategy to access gigantic nanoscale heteropolyoxotungstates by using SeO3(2-) as a template linker.

Polyoxometalates (POMs) have potential in nanoscience due to the range of available building blocks that can allow the assembly of new nano-objects with configurable architectures. This, in turn, promises to lead to nanosystems with predefined functions, applications, and the discovery of new phenomena. Heteropolyoxometalates are an important subset of POMs. Within this class, the choice of the heteroelement not only determines certain physical properties of the cluster, but increasingly has been found to control the range and connectivity of the building blocks. High nuclearity POM clusters based on Mo centers are well known; for example, {Mo132} and {Mo154} [13–16] clusters are synthetically accessible due to the occurrence of pentagonal and mixed-valence building blocks. In contrast, the exploration of high nuclearity polyoxotungstate clusters has yielded different results with somewhat lower nuclearity isopolyoxotungstates: {W34} and {W36}, [20,21] and has been more limited since tungsten-based building blocks with pentagonal geometries have only very recently been accessed. Instead, the use of extra linking heteroatoms are required to generate large architectures, as shown by the heteropolyoxotungstate anion [As6W65O217(H2O)7] 26 , which is the largest cluster in this class with six {W9As} units linked by {WO6} units. [24] Further, the introduction of electrophilic linkers can result in the assembly of even bigger clusters such as the unsurpassed [As12Ce16(H2O)36W148O524] 76 , and [{Sn(CH3)2(H2O)}24{Sn(CH3)2}12(As W9O34)12] 36 , and most recently the synthesis of the spectacular {W72Fe30} “Keplerate”. Herein, we demonstrate that it is possible to generate gigantic heteropolyoxotungstates by utilizing the SeO3 2 heteroanion. Furthermore, it is possible to control the size, shape, and nuclearity as a function of the ratio of W:Se employed during the synthesis. Structurally, the use of the SeO3 2 heteroanion effectively prevents the closure of the cluster assemblies to the Keggin-type cluster, and instead gives rise to “open” lacunary {W9Se} units, which can be viewed as “inorganic ligands”. As a result, by using {W9Se}-based units, a series of polyoxotungstate anions have been isolated and characterized, such as K22Na2[H2W43Se3O148]·65H2O (1), K20Na24[H4W77Se5O265]· 220H2O (2), and K16Na18[H6W63Se6O221]·105H2O (3). Not only are all these species synthesized under simple roomtemperature “one-pot” conditions, these clusters also contain W-based “defect” {W(W4)} pentagonal units. Furthermore we present the first system that employs a lacunary building block to capture a high nuclearity tungstate fragment, leading to the assembly and isolation of the gigantic nanosized cluster K52[(H8W100Se16O364)WO(H2O)2]·174H2O (4). This represents the largest pure heteropolyxoxotungstate so far characterized which does not employ heterometallic electrophilic linkers connecting the POM subunits. Compound 1 is synthesized at pH 4.0 from the reaction of K2WO4 and Na2SeO3 (W:Se molar ratio 5:1) and structural analysis of [H2W43O139(SeO3)3] 24 (1a) shows that the cluster contains three {W9Se} subunits with an average Se O bond length of 1.72 and a metal core assembled from 16 W centers. The {W16} “core” contains a unique {W7} building block comprising a pentagonal W{W4} unit plus two cornersharing {WO6}, as well as a {W3O13} unit and six {WO6} linkers to give a core that has the three {W9Se} units bolted on to give the overall cluster. The average W O bond length in the pentagonal {WO7} unit is 2.01 and it is slightly longer than that found in the {WO6} units. Finally, the central {W3O13} unit (Figure 1) appears to be doubly protonated with the protons located near the central m3-O ligand according to the BVS calculations, and the clusters are themselves paired into a