Synthesis, structure and mechanism of polyoxometalate self-assembly: towards designed nanoscale architectures

Wilson, Elizabeth Frances (2009) Synthesis, structure and mechanism of polyoxometalate self-assembly: towards designed nanoscale architectures. PhD thesis, University of Glasgow.

Abstract

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 ((n-
C4H9)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 ((n-
C4H9)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, [α-
Mo8O26]4-, and coordination of manganese(III) cations and
tris(hydroxymethyl)aminomethane (TRIS) groups in the formation of the Mn-Anderson-
TRIS structure. These investigations have led to the proposal that the rearrangement of [α-
Mo8O26]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-Imidazo-
Phenanthridinium (DIP) molecules as cations, i.e. (DIP-1)[PW12O40]·5DMSO·ca1H2O and
(DIP-2)[PW12O40]·5DMSO·ca4H2O, whereas the final compound uses an Imidazo-
Phenanthridinium (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.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Keywords: polyoxometalates, mass spectrometry
• Subjects: Q Science > QD Chemistry
• Colleges/Schools: College of Science and Engineering > School of Chemistry
• Supervisor's Name: Cronin, Prof. Leroy
• Date of Award: 2009
• Depositing User: Miss Elizabeth F Wilson
• Unique ID: glathesis:2009-1149
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 17 Sep 2009
• Last Modified: 10 Dec 2012 13:34
• URI: https://theses.gla.ac.uk/id/eprint/1149