Reversible redox reactions in an extended polyoxometalate framework solid.

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. This process can initiate highly specific chemical reactions 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, combined with the stability and functionality of inorganic zeolites and related systems. Although progress has been made in fine-tuning the reactivity of framework materials, reversible redox single-crystal to single-crystal (SC–SC) transformations that retain long-range order have not yet been observed. Thus, it can be suggested that the best way to engineer redoxand electronically active frameworks would be to incorporate building blocks based on polyoxometalate (POM) clusters, constructed from {MOx} 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 {XOn} units, as exemplified by the Keggin ion [M12O36X O4] n . This ion can incorporate anions such as phosphate and silicate, and can bind transition metals within structural vacancies. Herein we show that the directed assembly of a pure metal oxide framework, [(C4H10NO)40(W72M12O268X7)n] (M=Mn , X= Si, 1ox), [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 1ox can also be repeatedly disassembled into its building blocks by dissolution in hot water; subsequent recrystallization results in the reassembly of unmodified 1ox. These unique properties mean that this compound defines a new class of materials that bridges the gap between coordination compounds, metal– organic frameworks, and solid-state oxides. Furthermore, it has been shown that all the manganese(III) centers in 1ox can be “switched” to manganese(II) using a suitable reducing agent to give the fully reduced framework 1red. 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 A) 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 approach adopted here involves the reaction of the divacant lacunary polyoxometalate [g-SiW10O36] 8 [15] with manganese(II) in the presence of morpholinium cations and potassium permanganate, under strict pH control, to yield 1ox, which has the composition [(C4H10NO)40(W72Mn III 12O268Si7)n]·48H2O. Compound 1ox crystallizes [16] in the cubic space group I4̄3d and has a unit cell of a= 38.5 A with a unit cell volume of 57249 A. The metal oxide framework encloses elliptical pockets of 26.85 E 23.62 E 12.93 A which house solvent molecules and morpholinium cations. Therefore 1ox is the prototype of a 3D polyoxometalate framework that is based on pure metal oxides and constructed without the use of external linkers to connect the polyanionic framework nodes. The assembly of 1ox is achieved by the use of metastable heteropolytungstate ions that are connected directly byW-O-M units whereM is a firstrow transition metal. Here we designate the structure of 1ox as POM-1; the structure type comprises a unit cell in which four 3-connected and three 4-connected Keggin clusters are cross-linked into an infinite 3D framework (see Figure 1). Unlike any other 3D polyoxometalate network material reported to date, the POM-1 framework is composed solely of cluster anions that are directly connected by symmetryequivalent W-O-M linkages over which the tungsten and manganese atoms are statistically disordered with an average metal–oxygen bond length of 1.822 A (for 1ox). The structure of this material can therefore be described as an infinite array of 3and 4-connected Keggin polyanions, where each threeconnected unit is surrounded by three neighboring clusters in a trigonal-planar fashion, and each 4-connected unit features four nearest neighbors located on the vertices of a distorted tetrahedron (see Figure 1). Conceptually, the nodes of the framework can be considered as an equal distribution of tetravacant {SiW8O36} (W8) and trivacant {SiW9O37} (W9) [*] Dr. C. Ritchie, Dr. C. Streb, J. Thiel, S. G. Mitchell, Dr. H. N. Miras, Dr. D.-L. Long, T. Boyd, R. D. Peacock, T. McGlone, Prof. L. Cronin WestCHEM, Department of Chemistry The University of Glasgow University Avenue, Glasgow G12 8QQ, Scotland (UK) Fax: (+44)141-330-4888 E-mail: [email protected] Homepage: http://www.chem.gla.ac.uk/staff/lee/