& Porous Materials Water-Mediated Proton Conduction in a Robust Triazolyl Phosphonate Metal–Organic Framework with Hydrophilic Nanochannels Salma Begum, [a] Zhaoyang Wang, [a] Anna Donnadio, [b] Ferdinando Costantino, [b] Mario Casciola, [b] Rustem Valiullin, [c] Christian Chmelik, [c] Marko Bertmer, [c] Jçrg Kärger, [c] Jürgen Haase, [c] and Harald Krautscheid* [a] Dedicated to Professor Joachim Sieler on the occasion of his 75th birthday Abstract: The development of water-mediated proton- conducting materials operating above 100 8C remains challenging because the extended structures of existing materials usually deteriorate at high temperatures. A new triazolyl phosphonate metal–organic framework (MOF) [La 3 L 4 (H 2 O) 6 ]Cl·x H 2 O(1, L 2À = 4-(4H-1,2,4-triazol-4-yl)phenyl phosphonate) with highly hydrophilic 1D channels was synthesized hydrothermally. Compound 1 is an example of a phosphonate MOF with large regular pores with 1.9 nm in diameter. It forms a water-stable, porous struc- ture that can be reversibly hydrated and dehydrated. The proton-conducting properties of 1 were investigated by impedance spectroscopy. Magic-angle spinning (MAS) and pulse field gradient (PFG) NMR spectroscopies confirm the dynamic nature of the incorporated water molecules. The diffusivities, determined by PFG NMR and IR microscopy, were found to be close to that of liquid water. This porous framework accomplishes the challenges of water stability and proton conduction even at 110 8C. The con- ductivity in 1 is proposed to occur by the vehicle mecha- nism. The fast-growing diverse field of proton-conducting metal–or- ganic frameworks (MOFs) is currently explored for promising applications as proton-exchange membranes, which is one of the key components in fuel-cell technology. [1–3] Nafion and Nafion-like polymer membranes are efficient proton conduc- tors, [4, 5] however the high costs and temperature limitations have driven a number of strategies towards the design of alter- native materials, such as mesoporous silica, [6] coordination polymers (CPs), [1, 7] and porous organic solids. [8] The first study of proton conduction in a 2D copper- and dithioxamide-based CP was reported in 1979 by Kanda et al. [9] Kitagawa et al. [10] ex- tended this work by using derivatives of dithioxamides as or- ganic linkers and reported the first systematic study on proton conduction in CPs. Compared with carboxylate, phosphonate- based MOFs are considerably scarce in the literature, [11] most of the proton-conducting phosphonate MOFs have been report- ed by Shimizu et al., [12, 13] with successful attempts to maintain also uncoordinated phosphonate functionalities protruding to- wards the pores as proton-conduction sites. [14] From the per- spective of synthesizing novel, proton-conducting, porous ma- terials, phosphonate-based MOFs are considered good candi- dates owing to the acidity of the P ÀOH groups, to the high chemical and thermal stability and to the high versatility to crystallize in different structural architectures. [15, 16] The design of phosphonate MOFs with permanent porosity is still consid- ered a challenge as the high number of P ÀO groups potential- ly coordinating to metal atoms and the effect of noncovalent interactions need to be combined for the achievement of the desired porous compounds (Figure 1). Although a number of crystalline porous phosphonate MOFs has been prepared using low-valent cations, [17, 18] trivalent and tetravalent metal phosphonates are rarely crystalline [15] and some of them possess interlayer porosity. [19] By functionaliza- tion of phenyl phosphonate with a triazole ring as additional donor group, it was possible to grow single crystals of 3D porous phosphonate MOFs based on trivalent metal ions— contrary to the established trend of phosphonates that often precipitate as microcrystalline products. Availability of single- crystal structure data of robust proton-conducting materials could potentially fill the gap to establish a basis for the struc- [a] S. Begum, Z. Wang, Prof. Dr. H. Krautscheid Fakultät für Chemie und Mineralogie Universität Leipzig Johannisallee 29, 04103 Leipzig (Germany) E-mail : krautscheid@rz.uni-leipzig.de [b] Dr. A. Donnadio, Prof. Dr. F. Costantino, Prof. Dr. M. Casciola Department of Chemistry University of Perugia Via Elce di Sotto n.8, Perugia (Italy) [c] Dr. R. Valiullin, Dr. C. Chmelik, Dr. M. Bertmer, Prof. Dr. J. Kärger, Prof. Dr. J. Haase Fakultät für Physik und Geowissenschaften Universität Leipzig LinnØstrasse 5, 04103 Leipzig (Germany) Supporting information for this article, including experimental details for ligand and MOF syntheses, characterization by X-ray diffraction and TG/ DTA-MS, 1 H and 31 P solid-state NMR, impedance spectroscopy, IR microsco- py, and PFG NMR data, is available on the WWW under http://dx.doi.org/ 10.1002/chem.201402886. Chem. Eur. J. 2014, 20, 8862 – 8866  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 8862 Communication DOI: 10.1002/chem.201402886