Assembly strategy of polyoxotungstates from nanosized clusters to dimensional frameworks with transition metal linkers

The work presented herein focuses on the assembly of polyoxotungstates (POTs) from nanosized clusters to dimensional frameworks with transition metal (AgI, CuII, CoII and MnII) linkers. It shows two ways of producing such POTs: one is ‘stepwise’ synthesis, where POT building blocks are first produced and then coordinate with other addenda metal ions or links to form multiple POTs. The other method is the frequently used ‘one-pot’ method, where all chemical reagents are added together in a ‘one-pot’ aqueous solution, then acidified. With the ‘stepwise’ method, an all-inorganic polyoxometalate open-framework (POMOF) 1 was obtained based on the {P8W48} building block with intrinsic porosity and Co cations. This compound displays unprecedented structural flexibility. Not only does this material undergo the largest single-crystal to single crystal transformation thus far reported, the framework itself can be ‘reprogrammed’ (1-3) using the addition and removal of small molecule guests (water, ammonia and methanol). All these occur through multiple single-crystal-to-single-crystal transformations, allowing snapshots of the mechanism of solid-state conversion to be obtained. By changing the transition metal to Ag, three totally different three dimensional (3D) frameworks were formed with different reagents ratios and reaction temperatures. 4 is the uninodal 8-connected network, while 5 is a complex network with special building blocks {P10W68}, consisting of one {P8W48} ring with a {P2W15} cap attached. 6 is a cubic nano-sized porous framework assembled from cubic {Ag3-P8W48}6 units, which has two different cavities in the structure: one is the cubic cavity with a diameter of 2.4 nm and the other is the one dimensional (1D) cavity, with a diameter of 1 nm. With the ‘one-pot’ method, a time dependent supramolecular assembly of three giant POTs containing a {Ag12}-in-{W76} cluster-in-cluster was discovered. This work gives the structural hierarchy as Cl- {Ag12Cl} {Ag12Cl}{Te3W76} (monomer) 7 [{Ag12Cl}{Te3W76}]2 (dimer) 8 [{Ag12Cl}{Te3W76}]n (3D) 9. Each of these compounds was isolated in turn by employing a time resolved crystallisation process, allowing an unprecedented degree of kinetic control for this reaction system. The proposed mechanism for the formation of the clusters has been studied with Electrospray Ionisation Mass Spectrometry (ESI-MS) to identify the key {Te3W38} building block. Further, control experiments (forming 10 and 11) demonstrate the crucial role that TeO32-, Cl-, and Ag+ play in the self-assembly of 7-9. In order to further prove the crucial role of hetero-templates and Ag+ cations in forming this kind of high nuclearity POTs, with the hetero-template SeO32- instead of TeO32-, three other similar clusters, 12-14, were produced. Without the heteroatom, four POTs formed with the lacunary {W11} and {W12} building blocks linked by copper cations at higher pH value, especially, a tetrameric POT 15, which contains a hexadecacopper(II) core consisting of four cubane-like [Cu4O4] units was successfully prepared. While, 16 is a 1D polyanion chain constructed by lacunary {W11} clusters with copper dimer linkers. The electrochemical behavior of 15 and 16 is fairly similar, which shows the gradual reduced processes of the CuII/CuI and the CuI/Cu0. Utilising the same strategy, another two novel compounds 17 and 18 have been synthesized by changing the ratio of Cu(OAc)2/Na2WO4, the reaction time and the pH value. 17 is the first polyanionic transition metal POM chain known to date. 18 is a porous two dimensional (2D) network which is built from ‘paddle-wheel’ shaped copper dimers and Keggin type POMs, {W12O40}. Furthermore, a new family of isopolyoxotungstates (isoPOTs) possessing the newly discovered {W21O72} building block was obtained. This unit comprises an open {W19} shell supported internally by two {WO6} octahedral templates. The open shell has growth sites which facilitate the aggregation of the {W21O72} units into the larger architectures of 19-22. 19 represents the first example of a molecular metal oxide cluster containing a chiral ‘double-stranded’ motif, whilst 21 is one of the largest transition metal substituted isoPOTs known and is helical in nature. The four clusters described here each contain pentagonal {W(W5)} motifs and related {WO7} containing species, {W(W3)} and {W(W4)}, observed for the first time in isoPOTs, serving as an important milestone towards the discovery of gigantic POTs compared to those frequently observed in polyoxomolybdates (POMos)