The convergent assembly of metal-organic frameworks has enabled the design of porous materials using a structural building unit approach, but functional systems incorporating pre-assembled structural building unit 'pore' openings are rare. Here, we show that the face-directed assembly of a ring-shaped macrocyclic polyoxometalate structural building unit, {P(8)W(48)O(184)}(40-) with an integrated 1-nm pore as an 'aperture synthon', with manganese linkers yields a vast three-dimensional extended framework architecture based on a truncated cuboctahedron. The 1-nm-diameter entrance pores of the {P(8)W(48)O(184)}(40-) structural building unit lead to approximately spherical 7.24-nm(3) cavities containing exchangeable alkali-metal cations that can be replaced by transition-metal ions through a cation exchange process. Control over this process can be exerted by either electrochemically switching the overall framework charge by manipulating the oxidation state of the manganese linker ions, or by physically gating the pores with large organic cations, thus demonstrating how metal-organic framework-like structures with integrated pores and new physical properties can be assembled.

Extended modular frameworks that incorporate inorganic building blocks represent a new field of research where "active sites" can be engineered to respond to guest inclusion. [1][2] This process can initiate highly specific chemical reactions [3] that switch the overall nature of the framework, and it may even be developed to facilitate directed chemical reactions similar to those found in enzymatic systems. Achievement of this degree of sophistication requires the ability to control the framework assembly as precisely as in metal-organic frameworks, [1,[4][5][6] combined with the stability and functionality of inorganic zeolites and related systems. [7] Although progress has been made in fine-tuning the reactivity of framework materials, [8] reversible redox single-crystal to single-crystal (SC-SC) transformations that retain long-range order have not yet been observed.[9] Thus, it can be suggested that the best way to engineer redox-and electronically active frameworks would be to incorporate building blocks based on polyoxometalate (POM) clusters, [10][11][12] constructed from {MO x } units where M = Mo, W, V, Nb and x = 4-7. These clusters are attractive units for the construction of such frameworks since they are highly redox active and can incorporate a range of main-group-templating {XO n } units, as exemplified by the Keggin ion [M 12 nÀ . This ion can incorporate anions such as phosphate and silicate, and can bind transition metals within structural vacancies.[13]Herein we show that the directed assembly of a pure metal oxide framework, [(C 4 H 10 NO) 40 [14] based upon substituted Keggin-type POM building blocks, yields a material that can undergo a reversible redox process that involves the simultaneous inclusion of the redox reagent with a concerted and spatially ordered redox change of the framework. Compound 1 ox can also be repeatedly disassembled into its building blocks by dissolution in hot water; subsequent recrystallization results in the reassembly of unmodified 1 ox . These unique properties mean that this compound defines a new class of materials that bridges the gap between coordination compounds, metalorganic frameworks, and solid-state oxides. Furthermore, it has been shown that all the manganese(III) centers in 1 ox can be "switched" to manganese(II) using a suitable reducing agent to give the fully reduced framework 1 red . The redox process occurs with retention of long-range order by cooperative structural changes within the W-O-Mn linkages that connect the Keggin units. The nature of the redox process can be precisely deduced because of the SC-SC transformation between the oxidized and reduced states of the framework. This is important as, until now, covalently connected 3D polyoxometalate-based frameworks with large pockets (greater than 10 ) could be assembled only by the addition of "bridging" electrophiles. However, these solids typically have low stabilities and are not amenable to systematic design strategies, for instance the introduction of redox switchability.The approac...

We describe why the cyclic heteropolyanion [P8W48O184]40– (abbreviated as {P8W48}) is an ideal building block for the construction of intrinsically porous framework materials by classifying and analyzing >30 coordination polymers incorporating this polyoxometalate (POM) ligand. This analysis shows that the exocyclic coordination of first-row transition metals (TMs) to {P8W48} typically yields frameworks which extend through {W–O–TM–O–W} bridges in one, two, or three dimensions. However, despite the rich structural diversity of such compounds, the coordination of TMs to the {P8W48} ring is poorly understood, and therefore largely unpredictable, and had not until now been present with any structural classification that could allow rational design. Herein, not only do we present a new approach to understand and classify this new class of materials, we also present three {P8W48}-based frameworks which complement those frameworks which have previously been described. These new compounds help us postulate a new taxonomy of these materials. This is possible because the TM coordination sites of the {P8W48} ring are found, once fully mapped, to lead to well-defined classes of connectivity. Together, analysis provides insight into the nature of the building block connectivity within each framework, to facilitate comparisons between related structures, and to fundamentally unite this family of compounds. Hence we have tentatively named these compounds as “POMzites” to reflect the POM-based composition and zeolitic nature of each family member, although crucially, POMzites differ from zeolites in the modular manner of their preparation. As the synthesis of further POMzites is anticipated, the classification system and terminology introduced here will allow new compounds to be categorized and understood in the context of the established materials. A better understanding of TM coordination to the {P8W48} ring may allow the targeted synthesis of new frameworks rather than the reliance on serendipity apparent in current methods.

Two polyoxometalate open framework (POMOF) materials have been synthesized using a secondary building unit (SBU) approach that facilitates the convergent assembly of multidimensional framework materials using a preassembled anionic SBU {P(8)W(48)}, with integrated "pore" 1 nm in diameter, and electrophilic manganese {Mn(2+)} linkers. This yields two new POMOFS with augmented hexagonal tiling (2 and 3), related to a known three-dimensional (3D) cubic array K(18)Li(6)[Mn(II)(8)(H(2)O)(48)P(8)W(48)O(184)]·108H(2)O (1), K(12)[Mn(II)(14)(H(2)O)(30)P(8)W(48)O(184)]·111H(2)O (2), and K(8)Li(4)[Mn(II)(14)(H(2)O)(26)P(8)W(48)O(184)]·105H(2)O (3). These frameworks have been crystallized from aqueous Li-buffered solutions of {P(8)W(48)} and Mn(II)(ClO(4))(2)·6H(2)O via careful control of the synthetic strategy akin to a crystal engineering approach using cation and temperature control to isolate different material architectures shown by compounds 1-3.

The design of highly flexible framework materials requires organic linkers, whereas inorganic materials are more robust but inflexible. Here, by using linkable inorganic rings made up of tungsten oxide (P8W48O184) building blocks, we synthesized an inorganic single crystal material that can undergo at least eight different crystal-to-crystal transformations, with gigantic crystal volume contraction and expansion changes ranging from −2,170 to +1,720 Å3 with no reduction in crystallinity. Not only does this material undergo the largest single crystal-to-single crystal volume transformation thus far reported (to the best of our knowledge), the system also shows conformational flexibility while maintaining robustness over several cycles in the reversible uptake and release of guest molecules switching the crystal between different metamorphic states. This material combines the robustness of inorganic materials with the flexibility of organic frameworks, thereby challenging the notion that flexible materials with robustness are mutually exclusive.