Polyoxometalate self-assembly : from molecules to framework materials

The work presented in this thesis highlights the influence of water soluble tertiary amines in the synthesis of transition metal substituted heteropolyoxometalates (TMSPs), with particular emphasis on polyanions based on the Keggin structure and lacunary derivatives thereof. In particular, an understanding is gained into the role that N,N’-bis(2-hydroxyethyl)piperazine (bhep) and morpholine (morph) play in the polyanion self-assembly process, with the successful isolation of molecular species, chains, layers and three dimensional frameworks. Each of the ligands were used in two different sets of synthetic investigation to probe their influence on the complete self-assembly of transition metal substituted tungstophosphates using Na2WO4 as the tungstate source, and by using the [XW10O36]8- X = Si, Ge precursors. Indeed, at no point in this work has the same polyanion been isolated using both the bhep and morpholine ligands, with this observation being indicative of each ligand having structure directing properties. Synthesis of TMSPs based on the phosphotungstate clusters in the presence of (bhep) has resulted in the isolation of clusters with nuclearities {W6}, {W7}, {W9}, {W10}, and {W11} which were only possible through fine control of the reaction pH which is aided by the ligands high buffer capacity. Additionally, in these compounds the ligand is observed to adopt a variety of coordination tendencies ranging from monodentate coordination through the ligands hydroxyl groups to chelation between nitrogen and oxygen donors. Specifically, the chelation of the ligand to CoII and NiII ions has allowed the rare inclusion of transition metal complexes into lacunary polyanions through a complimentary self-assembly process. Furthermore, the (bhep) ligand has been used to directly functionalise the [γ-SiW10O36]8- polyanion through the controlled formation of W-OC linkages via a dehydration process. Interactions of the Morpholine ligand with heteropolytungsate clusters and transition metals in aqueous solution results in an array of markedly different structural motifs, which are highly depedant on the synthetic approach. The high basicity of the morpholine ligand results in its function as a buffer and cation within a pH domain that is underdeveloped in TMSP chemistry. Consequently, novel molecular species and functional materials have been isolated through the exploitation of this knowledge. The most significant of all the findings is the isolation of five mesoporous frameworks, which are composed solely of manganese or cobalt substituted Keggin polyanions which act as trigonal and tetrahedral nodes in the purely inorganic frameworks. Furthermore, the manganese containing frameworks have been shown to have cation exchange properties and are also redox active with single-crystal to single-crystal transformations being observed.

Level dc:type.qualificationlevel

PhD

Grantor dc:publisher.institution

University of Glasgow

Year dc:date.issued

2008

Author dc:creator

• Ritchie, Christopher

• QD Chemistry
• Q Science (General)