School of Chemistry - Theses

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Start date: 2000 × End date: 2019 × Author: Akhlaghi Bagherjeri, Fateme × Type: PhD thesis × Subject: mixed metal POMs ×

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    Solution behaviour and photochromic properties of a new family of mixed-metal hybrid amino acid-polyoxometalates
    Akhlaghi Bagherjeri, Fateme ( 2017)
    Fifteen members of a new family of hybrid mixed-metal polyoxometalates (POM) that incorporate amino acid ligands of general formula (countercations)x[As4{M4}W44Y4O160(AA)8(H2O)12]n- (M4 = various combination of WVI / MoV/VI / VIV and YIII; AA = Gly or L- and D/L-Nle, countercation = MBAH, GlyH+, BAH, BTEA, n = 9-12) have been synthesized and studied. This work followed on from a preliminary study on the synthesis and characterization of the first compound from this family, which was a mixture of two structurally related hybrid POMs with mixed Mo/W addenda metals and in different redox states. Photo-reduction of the reaction solution by UV light afforded a co-crystallized mixture of a fully oxidized analogue and a 2 electron reduced species. Preliminary studies suggested that a central structural core of these hybrid POMs was the preferred site for both metal substitution and reduction. The first research question addressed in this thesis was whether it is possible to synthesize analogous hybrid POMs that are comprised of a 2e-reduced homometallic core, rather than a mixed-metal core. Careful synthetic investigation, followed by comprehensive characterisation studies using single crystal X-ray diffraction, elemental analysis, 1H NMR spectroscopy, TGA analysis, electrochemistry, infra-red and XPS spectroscopy indicated that a {MoV2MoVIW} mixed-metal, mixed-valence central core was obtained for all reduced analogues of these hybrid POMs. In contrast, the two oxidized members of this family obtained exhibit a homometallic, trinuclear {MoVI3} central core. Density functional theory calculations indicate that the core metal centers are the preferred site of reduction and that the corresponding molecular orbitals are significantly lower in energy for the tetrametallic versus the trimetallic case, consistent with the experimental results. The second research question addressed was whether these compounds are stable in solution and the amino acid ligands remain attached to the POM or exchange. Electronic absorption spectroscopy, SAXS experiments and circular dichroism spectroscopy were performed in aqueous and non-aqueous media, and in the presence or absence of excess amino acid. It was apparent that while the metal-oxo POM framework remains intact in particular solution conditions, exchange of the amino acid ligand can occur. Analogues containing both chiral and achiral amino acid ligands were synthesized, with chiral ligands able to induce chirality in the hybrid POMs. The apparent preference for incorporation of Mo over W in the central core of these hybrid POMs prompted the exploration of the incorporation of other addenda metals and three new vanadium-containing analogues (V/W and (V/Mo/W) were synthesized. All of these compounds possess a tetrametallic {VIV2W2} central core and the selectivity of the core metal sites follows the order: V > Mo > W. The solid and solution UV-Visible absorption experiments along with XPS spectroscopy confirmed the incorporation of V and V/Mo into the POM structures. The solution study revealed that the V-containing analogues are less stable than the Mo-containing species, probably due to higher overall negative charge on the polyanion. The final part of this thesis presents a study of the photochromic behavior of four different salts of a W-only analogue of the hybrid POM family with Gly ligands. Two of the compounds exhibit reversible photochromism associated with photoreduction, cycling from white to blue under UV irradiation and bleaching back to white following re-oxidation in air. The ability of the compounds to undergo photoreduction correlates with the orientation of the zwitterionic ammonium group of the glycine ligands and the intramolecular hydrogen bonding to the POM framework. The kinetics of coloration of these POMs were found to follow a pseudo second-order law, similar to that which governs the photochromism of POMs facilitated by intermolecular hydrogen bonding with organoammonium countercations.