Assembly, disassembly and reassembly: New synthetic approaches towards hybrid, mixed-metal and framework materials

Abstract

The research presented in this thesis focuses on the synthesis of novel examples of advanced materials. The project aimed to synthesise new examples of three particular classes of materials; namely organic-inorganic hybrid polyoxometalates (hybrid POMs), transition metal substituted polyoxometalates (TMS-POMs) and extended materials (polymer-type and framework-type materials). The thesis explores a synthetic method based on distinct assembly, disassembly and reassembly steps to access novel examples of the target materials (Figure A.1). This methodology represents a facile and potentially fertile alternative to the more traditional "bottom-up" synthetic pathways employed elsewhere. The resulting materials were characterised using single crystal X-ray diffraction (XRD) and a variety of complementary techniques, and their physicochemical properties were examined. The Lindqvist hexamolybdate [Mo6O19]2- was identified as a suitable material for this methodology, and its disassembly behaviour is examined using electrospray ionisation mass spectrometry (ESI-MS). It was demonstrated that the Lindqvist species undergoes disassembly into smaller molybdate oligomers when dissolved in methanol under ambient conditions, without applying any chemical or physical stress to the system. Cone voltage (CV) variation experiments are used to demonstrate that these oligomers are truly present in solution, and are not a result of collision-induced dissociation (CID) events within the spraying chamber of the ESI-MS instrument. Subsequent reassembly was demonstrated by addition of phosphonate ligands to the system, which results in the reassembly of the molybdate oligomers into Strandberg-type hybrid molybdate ring systems; [TBA]4[Mo5O15(t-BuPO3)2] (1) and [TDP-H2]2[Mo5O15(tBuPO3)2] (2). Addition of copper salts to the reaction along with phosphonate ligands generates two mixed-metal compounds, [TBA]2[MoVI6CuII4O16(OH)2(t-BuPO3)4(py)2(CH3O)4(H2O)] (3) and [TBA]2[MoVI7CuVI7O19(OH)(CH3O)7(t-BuPO3)6(py)2] (4). The {Mo2} and {Mo3} moieties in these compounds are identified as arising from the disassembly of the Lindqvist hexamolybdate into dinuclear and trinuclear molybdate oligomers. Additionally, the magnetic behaviour of the compounds was examined. Extending the methodology to cobalt-molybdate systems generates [TBA]2[MoVI10CoII6O12(μ-O)14(μ3-O)4(tBuPO3)6(MeCOO)2(py)2(H2O)6] (5). The compound is stabilised by phosphonate, carboxylate and pyridyl ligands; orthogonal substitution of these ligands results in a series of 20 further compounds being isolated with a range of steric and electronic functionalities grafted on to the cluster core via the supporting organic ligands. These analogues are briefly discussed and the effects that ligand substitution has on the physicochemical properties of the compounds is discussed. The manganese analogue of compound 5, [TBA]2[MoVI10MnII6O12(μ-O)14(μ3-O)4(tBuPO3)6(MeCOO)2(py)4(H2O)6] (26) is generated using the same methodology. The phosphonate and pyridyl ligands are shown to be amenable to the same simple orthogonal ligand substitution as observed for the cobalt systems (27-30). Moreover, the carboxylate ligands are demonstrated to arise via in-situ carbon-carbon bond cleavage of acetylacetonate-type ligands. Modification of the conditions generates [MoVI2MnIII11O2(μ-O)4(μ3-O)4(μ4-O)2(μ-OH)2(tBuPO3)10(py)4]--based compounds (31-33). Compound 33 is demonstrated to be a competent catalyst for the water oxidation reaction. Finally, the effects of disassembly and reassembly on metal-organic framework (MOF) materials is considered. Three MOF-type compounds based on {Mn6} nodes are prepared (34-36). It is demonstrated that the addition of a competing ligand (pyridine) can displace the linker units, releasing the building units and any encapsulated anionic guest molecules into solution. Subsequent removal of the competing ligand causes the building units to reassemble into their parent framework. Some potential applications of this effect are proposed.