Zeolite-like nitride–chlorides with a predicted topology{ Andrew J. D. Barnes, Timothy J. Prior and M. Grazia Francesconi* Received (in Cambridge, UK) 7th August 2007, Accepted 4th October 2007 First published as an Advance Article on the web 19th October 2007 DOI: 10.1039/b712110e We describe the synthesis and structures of the first tantalum- containing nitride–chlorides, Ba 3 Ta 3 N 6 Cl and Ba 15 Ta 15 N 33 Cl 4 , and the structurally related Ba 3 Si 3 N 5 OCl, and their relationship with a theoretical silaceous framework. Since the discovery of the ion-exchange properties of zeolites, inorganic frameworks have been a burgeoning interdisciplinary research area, due to their unique chemical and physical properties, such as cation exchange and catalysis, and distinctive structural features. 1 Where frameworks are built up from tetrahedral units, they often crystallise in known zeolite types or closely analogous structures, hosting atoms or molecules inside cages or tunnels formed by the framework. 2,3 Most inorganic frameworks are based upon oxygen-containing polyhedra, e.g. MO 4 tetrahedra, but there is a growing list of examples of condensed framework structures based upon non-oxide anions and mixed-anion systems. Examples of nitride frameworks include Ca 3 Ga 2 N 4 , containing two interpenetrating networks formed from corner-sharing GaN 4 92 tetrahedra, 4 Ba 2 Nd 7 Si 11 N 23 , which is the first nitridosi- licate with a zeolitic Si–N network, 5 and HP 4 N 7 6 which contains PN 4 72 tetrahedra. Examples of mixed-anion frameworks include Zn 7 [P 12 N 24 ]Cl 2 7 and Ln 4 [Si 4 O 3+x N 72x ]Cl 12x O x with Ln = Ce, Pr, Nd and x # 0.2, which is the only family of compounds to show a zeolite-like framework containing the oxide, nitride and chloride ions. 8,9 Organic–inorganic hybrid open frameworks show a few examples of zeolite-related compounds containing an electroposi- tive metal (alkali, alkaline-earth metal, or lanthanide) and a transition metal, but, to date, we are not aware of any purely inorganic framework showing this feature. 10 Framework structures based on tetrahedrally coordinated tantalum have not been reported, although several examples of tantalum nitride com- pounds containing TaN 4 72 tetrahedra are known, e.g. Li 4 TaN 3 11 and Sr 2 TaN 3 , 12 but these contain chains of tetrahedra rather than extended 3-D frameworks. Here we report the preparation and structural characterisation of the first two tantalum containing nitride–chlorides, Ba 3 Ta 3 N 6 Cl (1) and Ba 15 Ta 15 N 33 Cl 4 (2), which are also the first multinary nitride–chlorides to show a zeolite-like framework. This is not found in a naturally occurring framework, but has been predicted as a possible low energy structure for SiO 2 from theoretical calculations. 13 We therefore prepared the analogous silicate compound and obtained Ba 3 Si 3 N 5 OCl (3), which shows a 3-D framework based on Si(O/N) 4 tetrahedra and is isostructural with Ba 3 Ta 3 N 6 Cl. The preparation of multinary nitride–chloride compounds is experimentally difficult. In fact, only four such nitride–chlorides are known to date: Ba 25 Nb 5 N 19 Cl 18 , 14 Ba 5 (MoN 4 )O 0.75 Cl 2.5 , 14 Ba 4 [WN 4 ]Cl 2 15 and LiBa 4 [Mo 2 N 7 ]?BaCl 2 . 16 Single crystals of Ba 3 Ta 3 N 6 Cl, Ba 15 Ta 15 N 33 Cl 4 and Ba 3 Si 3 N 5 OCl were prepared using a technique originally employed for nitrides 17 and subse- quently adapted to more complicated systems.{ The synthesis is based on the observation that single crystals of ternary nitride– chlorides containing alkali or alkaline-earth metals and transition metals can be grown on foils of transition metal by reaction of the foil with molten alkali or alkaline-earth metal nitrides and chlorides. This is a productive route to new mixed-anion compounds but tends to yield small single crystals within an amorphous matrix and chemical analysis of products is therefore unreliable. Crystals of Ba 3 Ta 3 N 6 Cl and Ba 15 Ta 15 N 33 Cl 4 were found in the same batch. A scanning electron microscopy (SEM) photograph (Fig. 1) shows that the crystals are hexagonal shaped plates of the order of microns in their longest dimension. Crystal size rendered structural determination using a conventional laboratory X-ray source impossible. Crystals were examined using synchrotron radiation at Station 16.2SMX of the Daresbury SRS, UK. Crystal selection proved very laborious and a large number of possible samples were examined before suitable single crystals were found. Data collection and structure solution were routine, but structural refinement was complicated by the presence of nitrogen bound to tantalum, owing to their vastly different scattering lengths. However, structural studies give definitive information about the frameworks: if Fourier difference maps are calculated with the nitride absent from the final model, peaks are found at the nitride positions. Disordered guest species (Ba, Cl) can be located in a similar manner. Crystal structure analysis 18,19 showed both 1 and 2 Department of Chemistry, University of Hull, Kingston-upon-Hull, UK. E-mail: m.g.francesconi@hull.ac.uk; Fax: +44 1482 466410; Tel: +44 1482 465409 { Electronic supplementary information (ESI) available: Analysis of guest positions in 1 and 2 and comparison of experimental and theoretical frameworks. See DOI: 10.1039/b712110e Fig. 1 SEM photograph of Ba 3 Ta 3 N 6 Cl and Ba 15 Ta 15 N 33 Cl 4 crystals. COMMUNICATION www.rsc.org/chemcomm | ChemComm 4638 | Chem. Commun., 2007, 4638–4640 This journal is ß The Royal Society of Chemistry 2007 Published on 19 October 2007. Downloaded by University of Hull on 28/08/2014 16:16:01. View Article Online / Journal Homepage / Table of Contents for this issue