Polyoxometalate Frameworks DOI: 10.1002/anie.201002672 Modular Inorganic Polyoxometalate Frameworks Showing Emergent Properties: Redox Alloys** Johannes Thiel, Chris Ritchie, Haralampos N. Miras, Carsten Streb, Scott G. Mitchell, Thomas Boyd, M. Nieves Corella Ochoa, Mali H. Rosnes, Jim McIver, De-Liang Long, and Leroy Cronin* The targeted synthesis of new extended modular frameworks exhibiting specific properties is a principal challenge of modern chemistry research. [1–3] Many inorganic frame- works [4, 5] and metal–organic frameworks (MOFs) [6, 7] have been reported, but the fine manipulation of their electronic properties remains challenging. [8] One such approach could be the development of molecular alloys, analogous to metal alloys, yet this idea has rarely been applied, and three dimensional (3D) framework alloys based upon molecular building blocks have not yet been fully realized. Conceptually, the design of 3D framework alloys could be achieved if the components of two isostructural frameworks “A” and “B” could be mixed at the molecular level (in any proportion) forming a crystal of AB units, perfectly arranged, so the AB alloy is also isostructural to frameworks A and B. [9] The potential applications of such an approach are highly appeal- ing, since the combination of coordination-compound-based building blocks, exhibiting different electronic properties, could allow the targeted tuning of frameworks with properties intermediate between A and B; and even the realization of “emergent” or unexpected properties for the alloy. In an attempt to develop an approach to this goal, we opted to use polyoxometalate (POM) building blocks, which are early-transition-metal oxide clusters that are well-known for the versatility of their physical and chemical properties. [10] Their applications cover a wide area of chemical research including, but not limited to, catalysis, [11] molecular magnet- ism, [12] and medicine. [13] However, more importantly in the current context, they exhibit a very diverse and susceptible redox chemistry and therewith offer the prospect to fine-tune their electronic properties. [14] Recently, we reported the first examples of isostructural framework compounds based purely on polyoxometalate clusters, [15] in which the electronic fine-tuning of the building blocks has been taken into account. Furthermore, these materials were the first POMs to undergo genuine reversible single-crystal to single-crystal (SC-SC) redox reactions whereby the oxidation state of the material changed. [16] The networks consist of manganese-substituted a-Keggin-type tungsten oxide clusters, which are fused by multiple Mn-O-W linkages (see Figure 1) and are of the general formula (C 4 H 10 NO) 40 [W 72 Mn III 12 X 7 O 268 ] (X = Si or Ge heteroatom). Embedded in the framework are two types of a-Keggin anions acting as either 3-connected or 4-connected building blocks resulting in the cubic germanium nitride like network structure. [17] Herein we show that it is possible to develop fully modular redox frameworks based upon the structural type, (C 4 H 10 NO) m [W 72 M II/III 12 X 7 O 268 ] by changing both the linking heterometal ions, M, and the heteroatoms, X. As such, the linking metal ion can be either Mn II/III or Co II/III and the heteroatoms can be Si IV or Ge IV so that four pure frameworks Figure 1. Representation of the “Keggin-net” structure (C 4 H 10 NO) m [W 72 M II/III 12 X 7 O 268 ] (where M = Co II or Mn III ;X = Si or Ge) constructed from two types of transition-metal-substituted a-Keggin clusters: 4-connected (red) and 3-connected (blue). The overall struc- ture is represented by linked 3- and 4-connected polyhedra with the oxygen linkers (pink spheres) and the structure of the 3- and 4-connected Keggin-nodes shown in blue and red inserts, respectively. Inserts: The “modular” or interchangeable components are shown as cyan polyhedra (the octahedrally coordinate heterometal center) and pink balls (the tetrahedral coordinate heteroatom center). Heterome- tal (Mn or Co): cyan; W: red, and blue octahedra; O: small red spheres; heteroatom (Si or Ge): pink spheres. [*] J. Thiel, Dr. C. Ritchie, Dr. H. N. Miras, Dr. C. Streb, S. G. Mitchell, T. Boyd, M. N. Corella Ochoa, M. H. Rosnes, J. McIver, Dr. D.-L. Long, Prof. L. Cronin WestCHEM, School of Chemistry, The University of Glasgow Glasgow G12 8QQ (UK) Fax: (+ 44) 141-330-4888 E-mail: l.cronin@chem.gla.ac.uk Homepage: http://www.croninlab.com [**] This work was supported by the ESPRC, the Leverhulme Trust, WestCHEM, and the University of Glasgow. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/anie.201002672. Communications 6984  2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Angew. Chem. Int. Ed. 2010, 49, 6984 –6988