Organofunctionalization of polyoxometalates and investigation of their reactivity towards heterometals

  • Organofunktionalisierung von Polyoxometallaten und Untersuchung ihrer Reaktivität gegenüber Heterometallen

Iftikhar, Tuba; Kögerler, Paul (Thesis advisor); Englert, Ulli (Thesis advisor)

Aachen : RWTH Aachen University (2021)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2021

Abstract

Polyoxometalates (POMs) are inorganic metal-oxygen cluster anions displaying exclusive nanoscale topologies of a large structural and compositional variety, and exhibiting a multitude of appealing properties like electronic versatility, structural rigidity, adjustable by countercations solubility as well as thermodynamic and redox stability. Among POMs, a sub-class of lacunary polyoxotungstates (POTs) attracts special attention due to their ability to incorporate a vast number of transition metals (TMs) and lanthanides (LnIII) as well as stabilize their multinuclear assemblies. Reports on potential applications of the heterometallic POT derivatives in different areas, such as catalysis, medicine, and materials science, frame a diverse range of properties of these species. Organofunctionalization has been widely recognized as a possibility to further extend the structural variety of polyanions, tune their electronic properties, and can also be used for stabilization of otherwise unstable discrete metal-oxo moieties. Organophosphonates and arsonates RXO32− (X = PV, AsV) providing a rigid tetrahedral XO3 site able to anchor on metal oxide surface as along with an organic group R which could be used for post-functionalization of hybrid organo-POT derivatives as well as for their controlled grafting on various surfaces represent a special interest from this prospect. Motivation behind this research work was to explore the potential of organophosphonate-functionalized POTs as innovative precursors for the design of new hybrid heterometal-substituted POTs and POT-based supramolecular assemblies of interest for applications in magnetism and catalysis as well as preparation of novel organophosphonate- and arsonate-decorated polyanions. Chapter 1 of this dissertation summarizes the history of POMs and discusses their intrinsic properties and developments concerning their applications in magnetism, medicine, catalysis, and preparation of hybrid materials. Chapter 2 gives an overview of materials and methods employed in this research work. The results obtained in the course of the Ph.D. work are presented in Chapters 3 – 5. In Chapter 3 results on the investigation of reactivity of phenylphosphonate-functionalized POTs [P4W24O92(C6H5PO)2]16− ({P4W24(PhP)2}) towards various heterometals in aqueous media are discussed. Innovative synthetic strategies utilized in this work led to the formation of novel transition metal and lanthanide-containing POTs based on unique polyanionic moieties that have not been ever reported before and cannot be synthesized using conventional POT precursors. Five sub-chapters summarize synthesis and investigation of twenty-three discrete polyanions, most of which are complexes of POT with magnetic TMs or LnIII ions. In the course of this research line, I observed a great tendency to the dissociation of organophosphonate groups from the POT precursor following by a rearrangement of the POT skeleton. Only in CoII and VV derivatives the original {P4W2(PhP)2,} structural moiety has retained its integrity. Most novel polyanions showed stability in solution and interesting electrochemical behavior which can serve as an important base for their potential applications. Chapter 4 summarizes the homo- and heterometallic derivatives obtained by self-assembly reactions of [-H2P2W12O48]12− ({-P2W12}) with 2-aminophenylarsonic acid. It appeared that the addition of divalent heterometals (MnII, CoII, and NiII) in the reaction mixture strongly influences the dimerization path of {-P2W12} moieties and leads to coordination polymers based on POTs with a completely different structure than that obtained in the absence of a heterometal. In total, one novel organoarsonate-functionalized homometallic POT, as well as three heterometallic derivatives with chain-like structures, have been prepared and characterized by various analytical techniques. Chapter 5 describes a 3D supramolecular assembly based on unique organophosphonate-functionalized POTs, [-P2W12O48(C6H5PO)2]10−, barrel-shaped -cyclodextrin molecules as well as well-known polyanions of Wells-Dawson type, [-P2W18O62]6−, obtained in one-pot self-assembly reaction of {-P2W12}, phenylphosphonic acid and -cyclodextrin. Solution studies unambiguously confirmed that the interactions between the building units of the supramolecular assembly retain also in aqueous media. Chapter 6 recapitulates the specific conclusions and outlook of individual sub-chapters. It also involves the discussion about the scope of synthesized advanced polyanionic compounds in a broader context, e.g. molecular electronics, spintronics, or in competitive heterogeneous catalytic systems.

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