title: Synthesis, structure and mechanism of polyoxometalate self-assembly: towards designed nanoscale architectures
Creators: name: Wilson, Elizabeth Frances
Publisher: name: University of Glasgow
Year: 2009
Subjects: subject: polyoxometalates, mass spectrometry

description: Cryospray (CSI-) and electrospray mass spectrometry (ESI-MS) techniques have been utilised to investigate the key features of the ‘in-solution’, self-assembly processes by which complex polyoxometalate systems, such as ((n-C4H9)4N)2n(Ag2Mo8O26)n and ((nC4H9)4N)3[MnMo6O18((OCH2)3CNH2)2], are formed.

CSI-MS monitoring of the rearrangement of molybdenum Lindqvist anions, [Mo6O19] 2-, in the presence of silver(I) ions, into a silver-linked β-octamolybdate structure, has allowed elucidation of the role of small isopolyoxomolybdate fragments and AgI ions in the assembly process. The observation of higher mass fragments, each with increasing organic cation contribution concomitant with their increasing metal nuclearity, has supported the previously proposed hypothesis that the organic cations have a structure-directing role in promoting the mode of POM structure growth in solution. The combined use of UV/vis spectroscopy and real-time CSI-MS monitoring of the reaction solution allowed correlation between the decreasing Lindqvist anion concentration and increasing β-octamolybdate anion concentration. Furthermore, UV/vis spectroscopy was used to show that the rate of decrease in Lindqvist anion concentration, and therefore, the inter-conversion of Lindqvist into β-octamolybdate anions, decreases as the carbon chain length of the alkylammonium cations in the system increases.

This approach was extended to use ESI-MS monitoring in examining the formation of the more complex, organic-inorganic, Mn-Anderson polyoxomolybdate structure ((nC4H9)4N)3[MnMo6O18((OCH2)3CNH2)2]. In this investigation, ESI-MS was used to monitor the real-time, ‘in-solution’ rearrangements of α-octamolybdate anions, [αMo8O26] 4-, and coordination of manganese(III) cations and tris(hydroxymethyl)aminomethane (TRIS) groups in the formation of the Mn-AndersonTRIS structure. These investigations have led to the proposal that the rearrangement of [αMo8O26] 4- anions occurs first through decomposition to [Mo4O13] 2- cluster species, i.e. halffragments of the octamolybdate anion; followed by decomposition to smaller, stable isopolyoxomolybdate fragment ions such as dimolybdate and trimolybdate fragment ions. It has then been proposed these fragments subsequently coordinate with the tripodal TRIS ligands, manganese ions, and further molybdate anionic units to form the final, derivatized Mn-Anderson-TRIS cluster.

Investigations into the encapsulation of the high oxidation state heteroanion templates {IVIIO6} and {TeVIO6} within polyoxomolybdate clusters, have led to the isolation and characterization of two new, molybdenum Anderson-based POM architectures, i.e. Cs4.67Na0.33[IMo6O24]·ca7H2O and Na4((HOCH2CH2)3NH)2[TeMo6O24]·ca10H2O. The use of coordinating caesium and sodium cations allowed the formation of a closely-packed structure composed of the periodate-centred Anderson clusters arranged into two layers, which then form a repeating ABAB pattern through the lattice. In contrast, the main building-blocks of the tellurium-based cluster system features the [TeMo6O24] 6- anions and two coordinated cation arrangements, each composed of a {Na2} dimer and coordinated TEAH+ cation. The presence of this structural motif, and its inter-connection with adjacent clusters, has led to chain-like packing arrangements within the greater lattice structure.

The introduction of three aromatic, phenanthridinium-based cations into polyoxometalate systems has led to the isolation and characterization of three new POM architectures with emergent photoactivity. The polyoxometalate framework in each is composed of tungsten Keggin clusters, i.e. [PW12O40] 3-, which are introduced into the systems as pre-formed building-blocks. Two of the compounds use derivatives of Dihydro-ImidazoPhenanthridinium (DIP) molecules as cations, i.e. (DIP-1)[PW12O40]·5DMSO·ca1H2O and (DIP-2)[PW12O40]·5DMSO·ca4H2O, whereas the final compound uses an ImidazoPhenanthridinium (IP) molecule as the cationic unit, i.e. (IPblue)3[PW12O40]·4DMSO. The use of these cations, which have different steric bulk, geometry and charge states, has led to the formation of interesting packing arrangements within the lattice structures of all three compounds. Additionally, further characterization of these compounds has revealed they all possess emergent photoactivity, in the form of intermolecular charge transfer bands in the solid state. Some degree of intermolecular charge transfer in the solution state has also been detected for the DIP-2-based structure.