Formation Mechanism of a Nano‐Ring of Bismuth Cations and Mono‐Lacunary Keggin‐Type Phosphomolybdate

Abstract

A new hetero-bimetallic polyoxometalate (POM) nano-ring was synthesized in a one-pot procedure. The structure consists of tetrameric units containing four bismuthsubstituted monolacunary Keggin anions including distorted [BiO 8 ] cubes. The nano-ring is formed via self-assembly from metal precursors in aqueous acidic medium. The compound (NH 4 ) 16 [(BiPMo 11 O 39 ) 4 ] • 22 H 2 O; (P 4 Bi 4 Mo 44 ) was characterized by single-crystal X-ray diffraction, extended X-ray absorption fine structure spectroscopy (EXAFS), Raman spectroscopy, matrix-assisted laser desorption/ionisation-time of flight mass spectrometry (MALDI-TOF), and thermogravimetry/differential scanning calorimetry mass spectrometry (TG-DSC-MS). The formation of the nano-ring in solution was studied by timeresolved in situ small-and wide-angle X-ray scattering (SAXS/ WAXS) and in situ EXAFS measurements at the MoÀ K and the BiÀ L 3 edge indicating a two-step process consisting of condensation of Mo-anions and formation of BiÀ Mo-units followed by a rapid self-assembly to yield the final tetrameric ring structure.Polyoxometalates (POMs) are structurally well-defined nanosized metal-oxo cluster anions consisting of early transition metals like niobium, vanadium, molybdenum, or tungsten cations in high oxidation states. [1] The structural variety of POMs offers the possibility to fine-tune their properties towards applications in biomedicine, [2] catalysis, [3] and material chemistry. [4] Among the POM structures derivatives of the Keggin-type [5] are especially interesting offering access to different substitutions within the POM-based materials. The variability is particularly well-established in polyoxomolybdate and -oxotungstate chemistry, where clusters featuring one or several metal cation binding sites (i. e., lacunary POMs) are formed. [6] Particularly, trivalent cations, such as Ce 3 + , Gd 3 + , and Co 3 + have been employed, since their high coordination number and flexible environment, allows for the formation of more complex architectures and superstructures. [7] Among the main group elements, Bi 3 + is used for connecting POM structures. [2a,b,7d,8] Bi 3 + -containing POMs are investigated for their use in biomedical applications, [2b] as proton conductors, [7d,9] and as catalysts. [10] In Bi-containing POMs, Bi 3 + cations typically adopt a trigonal-pyramidal coordination geometry. [8a,10,11] Hanaya et al. reported the successful preparation of three types of bismuth-tungsten oxide nanoclusters using lacunary silico-tungstates. Structuredirecting properties of Bi 3 + lone electron pair result in the formation of binuclear and tetranuclear cluster architectures. [8a] Villanneau et al. reported two bismuth-substituted structures [Bi{M 5 O 13 (OMe) 4 (NO)} 2 ] 3À (M=Mo, W), in which the Bi 3 + ions connect two monolacunary Lindqvist-type clusters, resulting in an unusual 8-fold coordination geometry around Bi 3 + . [12] First heteropolytungstates exhibiting a similar 8-fold squareantiprismatic coordination geometry were charact...

Oxygen Evolution/Reduction Reaction Catalysts: From In Situ Monitoring and Reaction Mechanisms to Rational Design

“…Our group has recently reported the anion-driven formation of tetrametallic cubane clusters based on the di-2-pyridyl ketone (dpk) ligand, which enables unprecedented control over the synthesis of different cubane types with tailored accessible aqua ligands for the OER . Furthermore, in situ XAS and X-ray scattering techniques have been performed on multinuclear POM-based molecular WOCs to understand their assembly pathways under synthetic reaction conditions and to further analyze their structural integrity during electrocatalysis. , Extension of these in situ approaches to metal complexes would help to determine their synthetic formation pathways and preferred coordination environments.…”

“…354 Furthermore, in situ XAS and X-ray scattering techniques have been performed on multinuclear POMbased molecular WOCs to understand their assembly pathways under synthetic reaction conditions and to further analyze their structural integrity during electrocatalysis. 824,825 Extension of these in situ approaches to metal complexes would help to determine their synthetic formation pathways and preferred coordination environments.…”