Non-aqueous synthesis of heterometal substituted Lindqvist-type polyoxometalates

Abstract

Mixed metal oxide materials have been successfully applied as active components in environmentally important transformations, e.g. ammonia synthesis, water-oxidation or carbon dioxide reduction. To combat rising demands on energy and resources, improvements to these processes, and in turn the active components that drive them, must be made. This requires a fundamental understanding of the relationship between the structure, composition, and activity of these materials. One way to address this is to develop atomically precise models of these mixed metal oxide materials that can then be systematically modified and the effect of activity noted. This work focuses on the development of a series of such model systems in the form of heterometal substituted polyoxometalates (POMs) A robust synthetic approach for the production of heterometal substituted POMs based on the Lindqvist architecture is developed using the reported (TBA)7[(Co(II)W5O18H)2][X] (X = BF4 or Cl) as a model system. This is then expanded to a range of first row transition metals with evidence of the successful synthesis of magnesium, manganese, iron, nickel, and zinc substituted derivatives presented. The method uses the base degradation of (TBA)2[W6O19] to target a “virtual” lacunary species which efficiently reacts with simple metal salts to give the corresponding heterometal containing derivative. A detailed discussion of non-aqueous solution speciation is presented in part to identify the true nature of this “virtual” lacunary species, with combined 17O NMR, 183W NMR and crystallographic studies indicating it is actually a 2:1 mixture of (TBA)2[WO4] and a novel isopolytungstate, (TBA)5[W7O24H], which is only the third isopolytungstate isolated from non-aqueous solutions. This synthetic approach was also applied to metal(V) - substituted Lindqvist-type POMs, including new rhenium derivatives, and products were fully characterised by multinuclear NMR and X-ray crystallography where appropriate. The effect of metal substitution on the redox properties is discussed, using cyclic voltammetry and density functional theory calculations to support findings.