Heterometal-substituted molybdovanadate polyoxometalate (POM) clusters can be linked by bare transition-metal aqua cations into extended all-inorganic frameworks, but the reported structures stop at one dimension (a 2019 chain built from Z-shaped {V9Mo30Cu8} anions) and, in a 2024 follow-up from the same group, a two-dimensional layer from C-shaped {V9Mo23Cu6} anions - neither porous. The open problem is to carry this sequential-synthetic platform to genuinely porous higher-dimensional all-inorganic frameworks: two- or three-dimensional networks of lacunary molybdovanadate clusters that survive guest removal and show permanent, measurable porosity (gas or vapor sorption) while the cluster building block stays intact; making such assembly general rather than serendipitous through linker and condition design is part of the challenge. No framework of this cluster family has been reported to show permanent porosity; only its dimensionality component has been partially resolved.

Background

Polyoxometalates (POMs) are discrete anionic metal-oxide clusters of early transition metals in high oxidation states, above all Mo(VI), W(VI) and V(V); the Mo/V mixed-addenda ones are called molybdovanadates. A lacunary POM is a cluster from which one or more addenda units have been removed, leaving an oxygen-rich vacancy pocket that behaves as a multidentate ligand: it binds first-row transition-metal cations (the '3d' metals, e.g. Cu, Co, Mn) strongly and directionally, which makes lacunary POMs standard inorganic nodes for assembling larger aggregates. In the source study, Gao, Xu, Gong, Hu and Yang (J. Cluster Sci. 30, 1131-1137, 2019) used a sequential aqueous synthesis - first prepare and isolate a Cu-substituted molybdovanadate precursor, then react it with Mn2+ or Co2+ salts - to obtain two isostructural compounds in which Z-shaped {V9Mo30Cu8} lacunary-cluster anions are joined by [M(H2O)4]2+ (M = Co, Mn) linkers into a one-dimensional all-inorganic framework (connected without any organic molecule), the first such chain observed in the molybdovanadate system. The materials show antiferromagnetic coupling between the 3d metals and electrocatalyze ascorbic-acid oxidation at carbon-paste electrodes. The authors frame the next step as the extension of this strategy to truly multidimensional and porous all-inorganic materials - frameworks that would combine the redox activity of the V(V)/Mo(VI) addenda shell and the magnetism of the embedded 3d metals with the mass-transport and shape-selectivity benefits of permanent porosity. Permanent porosity means the pore system survives removal of guest solvent (activation) without framework collapse, demonstrated conventionally by a gas-sorption isotherm on the activated sample (type-I for a microporous material) and a Brunauer-Emmett-Teller (BET) surface area computed from it that reflects uptake by an internal pore system rather than adsorption on external surfaces alone; mere crystallographic voids filled with lattice water and countercations do not qualify. Achieving this in an all-inorganic POM framework is hard because (i) the ionic hydrate lattices of large POM salts usually densify or collapse on dehydration as counterions migrate into the voids, and (ii) the lacunary cluster must tolerate the pH and linker conditions of network assembly without decomposing into simple molybdates/vanadates. Precedents for porosity in POM-based solids exist in adjacent platforms - 'porous ionic crystals' built from compact Keggin-type POMs (e.g. [PMo12O40]3-, a cage-like phosphomolybdate) combined with large metal-oxo countercations, and crystalline porous complex metal oxides assembled from epsilon-Keggin POM units, which show reversible O2 uptake - but none of these routes uses lacunary molybdovanadate clusters, and POM-in-MOF or covalently organo-functionalized POM hybrids introduce organic linkers, giving up the all-inorganic objective. Within the molybdovanadate family itself, dimensional growth has so far produced a dense, morpholine-templated 3D framework based on the small [VMo7O26]5- cluster (2020), organo-functionalized 1D and 2D organic-inorganic hybrids (2020), and the non-porous 2D all-inorganic layer from the C-shaped {V9Mo23Cu6} cluster made by the same sequential method (2024). The porous, higher-dimensional, all-inorganic goal is thus untouched, and a 2025 report of a POM-constructed 2D porous inorganic framework built from Ru-substituted antimonotungstate clusters, with superprotonic conductivity, confirms that such frameworks are chemically reachable in principle in the wider POM field without showing how to reach them from molybdovanadate building blocks.

Problem Statement

Construct, by the sequential aqueous synthesis that is the demonstrated platform of the source work and its 2024 follow-up, from preformed lacunary molybdovanadate cluster precursors (the {VxMo30Cu8}-type platform of the source work, its C-shaped {V9Mo23Cu6} variant, or directly analogous lacunary Mo/V mixed-addenda clusters) reacted in aqueous solution with transition-metal aqua linker cations (including mixed transition-metal/alkali aqua units such as those linking the 2024 two-dimensional layer), a crystalline two- or three-dimensional all-inorganic framework - cluster anions connected to neighbors in a layer or network solely through metal-aqua/hydroxo coordination, with no organic linkers or covalent organic functionalization (the all-inorganic requirement concerns this framework connectivity; charge-balancing or templating countercations within the pores do not alter it) - that exhibits permanent porosity: after guest removal, the framework retains its crystallinity (powder X-ray diffraction evidence) and the integrity of the cluster building block (structural and spectroscopic evidence), and gas or vapor sorption on the activated material demonstrates accessible internal surface area, i.e. uptake by a retained internal pore system rather than adsorption on external surfaces alone, with BET analysis. The design questions that decide whether this can be achieved reproducibly rather than serendipitously are which heterometal linkers (including mixed and multiply substituted linkers beyond the reported Co2+/Mn2+ chains), which linker-to-cluster ratios, pH windows, and templating countercations stabilize the multidentate lacunary clusters during network assembly instead of giving the observed alternatives (discrete clusters, one-dimensional chains, or non-porous dense layers), and how porosity can be engineered and retained while the mixed-addenda cluster stays intact. The mandatory deliverable is at least one such framework meeting all of the requirements above (crystallinity retained after guest removal, cluster integrity, and sorption-supported BET internal surface area); the design questions describe the open challenge of reaching it reproducibly rather than serendipitously, and an answer that proposes design rules or predictions without such a demonstrated, sorption-characterized framework does not by itself resolve the task.

References

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