In situ tetrazole ligand synthesis leading to a microporous cadmium–organic framework for selective ion sensingw Yongcai Qiu, ab Hong Deng,* a Jixia Mou, a Shihe Yang,* b Matthias Zeller,* c Stuart R. Batten, d Haohan Wu e and Jing Li e Received (in Cambridge, UK) 17th April 2009, Accepted 7th July 2009 First published as an Advance Article on the web 28th July 2009 DOI: 10.1039/b907783a In situ tetrazole ligand synthesis leads to a luminescent micro- porous cadmium–organic framework {[Cd(l 2 -Cl)(l 4 -5MT)] n (5MT = 5-methyl-1H-tetrazole)} that exhibits a high-sensitivity sensing function with respect to nitrite in both DMF and water. The network approach to the design and synthesis of crystalline solids has produced a wide range of novel and useful coordination polymers, also known sometimes as metal–organic frameworks (MOFs). 1,2 The field of porous MOFs has witnessed remarkable progress, and many novel functionalities and potential applications have been discovered, such as gas storage, 3 separation 4 and sensing. 5 All of these properties have been shown to be heavily reliant on the specific pore size and the nature of the pore surface. The entry of small molecules into a porous MOF, for example, is determined by both the shape of the molecules, and the structural and pore characteristics of the MOF. Thus, similar to active sites in proteins, MOFs can be used for molecular recognition. When combined with a suitable sensing mechanism, such as photo- luminescence (or the quenching of it), one can obtain a molecular sensor for applications in, for example, biological and environmental systems. Recently, intriguing recognition and sensing functions of lanthanide-based porous MOFs with respect to small solvent molecules and ions have been reported. 6 However, work on d 10 metal-based MOFs as luminescent probes for the sensing of small molecules has been very limited. 7 An effective strategy to synthesize robust porous MOFs is the use of multifunctional ligands, and many high-dimensional microporous materials have been isolated based on this strategy. 3,10,11 Demko and Sharpless pioneered a facile approach to the synthesis of 5-substituted 1H-tetrazoles, namely through the [2 + 3]-cycloaddition reaction of an azide with nitriles in water aided by a Lewis acid. 8 A number of research groups have continued to work in this field, and have isolated numerous coordination frameworks with intriguing structural motifs and functional properties. 9 Recently, we reported a high-symmetry cubic coordination framework that exhibits reversible shrinking and expansion in a crystal-to-crystal dehydration/rehydration process. 10 In this Communication, we present another highly- symmetric tetragonal MOF prepared via the in situ synthesis of the ligand 5-methyl-1H-tetrazole (5MT),z namely {[Cd(m 2 -Cl)(m 4 -5MT)] n (1). The compound is thermally stable, luminescence active, and exhibits the capacity to both store hydrogen gas and selectively sense nitrite ions. As-synthesized, compound 1 (Fig. 1a) is insoluble in water and common organic solvents. It was prepared by a simple hydrothermal reaction of cadmium chloride with azide in an acetonitrile/water mixture at 150 1C, and was characterized by elemental microanalysis, IR spectroscopy and single-crystal X-ray diffraction; phase purity was confirmed by powder X-ray diffraction (PXRD) and thermal gravimetric analysis (TGA) (see the ESI, Fig. S3 and S4w). The presence of a peak at 1371 cm 1 in its IR spectrum showed the formation of tetrazole groups. TGA indicated that the framework of 1 was thermally stable up to 390 1C. The PXRD pattern of an as-synthesized power of 1 was almost identical to that calculated from the single-crystal structure. Compound 1 crystallizes in the non-centrosymmetric tetragonal space group I  42m and displays a very complicated 3D network. There are three unique Cd atoms, four unique 5MT ligands and four unique Cl anions (Fig. 2a). Each Cd bonds to four 5MT ligands and two cis-Cl anions, while each 5MT ligand bonds to four metal ions (two of the unique 5MTs lie across mirror planes) and each Cl anion bridges two metal atoms. The ligands bridge the metal atoms in a complicated 3D network. Concentrating on just on the Cd-5MT network, it is best described in terms of 1D columns (ESI, Fig. S5aw) in which Cd2 4 (C1-5MT) 4 rings (C1-5MT is the 5MT ligand containing C1 (green in Fig. 2)) are bridged by pairs of Cd1 2 Cd3 3 (C3-5MT) 2 ‘struts’ to create open-ended cavities ca. 9.65 A ˚ across, which stack in such a way that each cavity is orientated 901 with respect to its neighbours (the columns lie about a  4 axis). These columns are then interconnected (ESI, Fig. S5bw) by the 5MT ligands that lie on mirror planes (C5- and C7-5MT) such that each column is connected to six others to give an overall 3D network (Fig. 2b, where the columns are viewed end-on). This connectivity is simply reinforced by the chloride bridges (ESI, Fig. S6w). a School of Chemistry & Environment and Key Lab of Electrochemical Technology on Energy Storage and Power Generation in Guangdong Universities, South China Normal University, Guangzhou 510006, China b Nano Science and Technology Program, Department of Chemistry, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong c Youngstown State University, Department of Chemistry, One University Plaza, Youngstown, OH 44555-3663, USA d School of Chemistry, Monash University, Victoria 3800, Australia e Department of Chemistry and Chemical Biology, Rutgers University, Piscataway, NJ, USA w Electronic supplementary information (ESI) available: IR spectra, TG, fluorescence spectra, powder X-ray diffraction plots and additional spectra. CCDC 721878. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/b907783a This journal is  c The Royal Society of Chemistry 2009 Chem. Commun., 2009, 5415–5417 | 5415 COMMUNICATION www.rsc.org/chemcomm | ChemComm