Functional POM-OFs DOI: 10.1002/anie.200702698 Modular Assembly of a Functional Polyoxometalate-Based Open Framework Constructed from Unsupported Ag I ···Ag I Interactions** Carsten Streb, Chris Ritchie, De-Liang Long, Paul Kögerler, and Leroy Cronin* Polyoxometalates (POMs), anionic transition-metal oxide clusters, [1] represent a vast class of inorganic materials with a virtually unmatched range of physical and chemical proper- ties relevant for applications as diverse as medicine and biology, [2] magnetism, [3] materials science, [4] and catalysis. [5] It is this unique range of properties which qualifies POM-based materials as prime candidates for the designed construction of molecular-framework materials. Polyoxometalates cover an enormous range in size and structure [1b,c,6] and thereby provide access to a huge library of readily available and controllable secondary building units (SBUs) [7] which can be interconnected by an electrophilic linker. This crucial step in the assembly of functional POM frameworks has so far been achieved by using organic linkers [8] or by ligand-supported transition-metal bridges; [9] both approaches however are limited by the reduced framework stability intrinsic to metal–organic framework materials. In contrast, the assembly of purely inorganic POM-based frameworks offers high potential for the formation of a new type of porous materials which combines the thermodynamic stability of zeolites and mesoporous silicas [10] with the sophistication and versatility of metal–organic frameworks (MOFs). [7,11] The key to the work reported herein is the ability to form structurally supporting {Ag 2 } 2+ building blocks, and we have recently shown that careful solvent and ligand control can be used to encourage the formation of such interactions. [12] This approach has stimulated us to exploit this tendency and to investigate the potential of silver(I) dimers to act as linking units in conjunction with larger isopolyoxometalates to construct 3D frameworks. Herein we present the modular assembly of a 3D porous framework: [Ag(CH 3 CN) 4 ]{[Ag(CH 3 CN) 2 ] 4 [H 3 W 12 O 40 ]} (1). Single-crystal X-ray diffraction studies revealed that the two principal SBUs, protonated a-metatungstate [13] clusters [H 3 W 12 O 40 ] 5 ({W 12 } 5 ) and dimeric {[Ag(CH 3 CN) 2 ] 2 } 2+ ({Ag 2 } 2+ ) bridging units (Figure 1), are linked such as to enclose two sets of colinear channels. T d -symmetric [Ag- (CH 3 CN) 4 ] + units are located in the channels and appear to act as templates in the self-assembly of the framework, so 1 can be represented as {Ag}[{W 12 }{Ag 2 } 8/4 ]. The reaction of ten equivalents AgNO 3 and one equiva- lent (nBu 4 N) 4 [H 4 W 12 O 40 ] in acetonitrile in the presence of aqueous HNO 3 resulted in the isolation of the infinite 3D framework 1 in 60% yield, which was fully characterized. Structurally, the framework can be described as an infinite arrayof{W 12 } 5 a-metatungstate clusters which are connected to eight {Ag 2 } 2+ cations by coordination through the terminal W = O t oxygen ligands. The formation of these W-O-Ag bridges provides flexibility but also stability to allow the assembly of an infinite purely inorganic framework. The key to the framework assembly is the ability of the silver ions to connect the particular clusters by self-organizing into {Ag 2 } 2+ dimers with short silver–silver (Ag I ···Ag I ) contacts which are stabilized by argentophilic metal–metal interactions (see Figure 2). [14] Each dimeric {Ag 2 } 2+ motif cross-links four {W 12 } 5 clusters. To encourage the formation of the Ag···Ag interactions it is vital to exclude coordinating ligands. In other words, we have found that the absence of bridging ligands such as DMSO or 4,4’-bipyridine, which disrupt framework growth by capping the silver linkers and result in the isolation of low-dimensionality structures, can be used to encourage unsupported Ag···Ag interactions. This strategy represents a subtle yet significant departure from our previous synthetic approaches. [12] This new approach allows us to isolate the first example of a framework system constructed by utilizing Figure 1. Illustration of the secondary building units of 1. Left: The {W 12 } 5 Keggin type isopolyoxotungstate [H 3 W 12 O 40 ] 5 ; WO 6 octahe- dra blue, O ligands red. Right: The dimeric {Ag 2 } 2+ linker unit; note that each acetonitrile molecule is disordered over two positions. C gray, N green, Ag purple, O red; this color scheme is used through- out the text. [*] C. Streb, C. Ritchie, Dr. D.-L. Long, Prof. L. Cronin WestCHEM, Department of Chemistry The University of Glasgow University Avenue, Glasgow G128QQ (UK) Fax: (+ 44)141-330-4888 E-mail: l.cronin@chem.gla.ac.uk Homepage: http://www.chem.gla.ac.uk/staff/lee Prof. P. Kögerler Institut für Anorganische Chemie RWTH Aachen 52074 Aachen (Germany) [**] We thank the EPSRC, WestCHEM and the University of Glasgow for supporting this work. Supporting information for this article is available on the WWW under http://www.angewandte.org or from the author. Angewandte Chemie 7579 Angew. Chem. Int. Ed. 2007, 46, 7579 –7582 # 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim