Beyond Charge Balance: Counter-Cations in Polyoxometalate Chemistry

Abstract

Polyoxometalates (POMs) are molecular metal-oxide anions applied in energy conversion and storage, manipulation of biomolecules, catalysis, as well as materials design and assembly. Although often overlooked, the interplay of intrinsically anionic POMs with organic and inorganic cations is crucial to control POM self-assembly, stabilization, solubility, and function. Beyond simple alkali metals and ammonium, chemically diverse cations including dendrimers, polyvalent metals, metal complexes, amphiphiles, and alkaloids allow tailoring properties for known applications, and those yet to be discovered. This review provides an overview of fundamental POM-cation interactions in solution, the resulting solid-state compounds, and behavior and properties that emerge from these POM-cation interactions. We will explore how application-inspired research has exploited cation-controlled design to discover new POM materials, which in turn has led to the quest for fundamental understanding of POM-cation interactions.

Keywords: Cations; Composites; Interactions; Polyoxometalates; Supramolecular Chemistry.

Figure 1

General solubility trends observed for POM anions (the Keggin ion [XM₁₂O₄₀]ⁿ⁻ (X often P, Si, M=Mo, W) is used for illustrative purposes) salts with alkali metal cations. Normal solubility trend is observed for most POM salts including Mo, V, and W, while the anomalous trend is experienced mostly for Nb and Ta POMs. (Blue represents the least soluble, green the most soluble agents.).

Figure 2

Structural representation of the cation‐stabilized POMs {Bi₆Fe₁₃},67 {Bi₄V₁₃}, 68 and {Ce₂V₁₂} described in Section 2.1; {Ce₂V₁₂}=[(Ce(dmso)₃)₂V^IVV^V ₁₁O₃₃Cl]²⁻.69, 70

Figure 3

Structural representation of cation–POM frameworks described in Sections 2.2 and 2.3. Left: ionic zeotype frameworks reported by Mizuno, Uchida, and colleagues;77 center: Keggin‐net reported by Cronin and colleagues;79 right: guanidinium‐blocked {Mo₁₃₂} Keplerate capsule reported by Müller and colleagues.80, 81

Figure 4

Schematic of “blackberry” macromolecular self‐assembly. From left to right: dilute solutions of POMs (green polyhedral representation of the Keggin ion) with added alkali metal cations (black spheres) enable assembly of the blackberry structures (hollow capsule; green dots: POMs, see inset for close‐up view of the POM environment) Right: the size of the blackberries and the assembly rate increase with increasing alkali metal cation radius. (Blackberry cartoons courtesy of Tianbo Liu).

Figure 5

Illustration of a model POM‐IL with a) symmetric QAAs (black), using the Keggin ion (green polyhedra) as a generic POM model. Little is known about the structure and interactions between the POMs and the QAAs in the POM‐IL liquids, this represents opportunity for fundamental discovery and optimization of these compounds. This cartoon depicts a presumed lack of order in the liquid, with POM–POM, QAA—QAA, and POM–QAA interactions.

Figure 6

Illustration the different assemblies of POMs plus surfactant QAAs. Surfactants can be either single‐tail (left) or double‐tail (right). Left: The POMs that are small with low charge (i.e. −2 to −4; green spheres) can readily form lipid bilayers where the main forces of the assembly are strong interdigitation of the parallel surfactant tails, and also electrostatic attraction between the POM anions and the small ammonium cation heads. These lipid bilayer assemblies are conducive to crystal formation (middle). Right: Larger POMs with higher charge (red spheres) form surfactant‐encapsulated clusters (SECs; shown with double‐tail surfactants). These assemblies do not have strong interactions between the lipid tails, due to curvature, rather than parallel orientation; and the lack of order and strong interactions prohibits crystallization.

Figure 7

Structural representation of the cation‐templated POMs described in Section 4.1.

Figure 8

a) Nanofiber formation by electrostatic aggregation between [SiW₁₂O₄₀]⁴⁻ and cationic oligopeptides capable of π stacking.156 b) Adhesive formation by electrostatic and hydrogen‐bonding aggregation between [SiW₁₂O₄₀]⁴⁻ and hydrogen‐bonded cationic histidine dimers.157