ARTICLE DOI: 10.1002/zaac.201000104 Titanium 3+ Hexacyanometallates(II): Preparation and Porous Framework Manuel Ávila, [a] Claudia Vargas, [a] Hernani Yee-Madeira, [b] and Edilso Reguera* [a,c] Keywords: Prussian blue analogues; Porous solids; Hydrogen storage; Crystal structure; Titanium Abstract. The studied compounds were prepared by the precipitation method mixing Ti 3+ in concentrated HCl with aqueous solution of [M(CN) 6 ] 4– where M = Fe, Ru, Os. The formed solids, Ti 3 Cl[M(CN) 6 ] 2 ·10H 2 O, were characterized by IR spectroscopy, X-ray diffraction, thermogravimetry, Mössbauer spectroscopy, energy-dis- perse X-ray spectroscopy, UV/Vis spectroscopy, adsorption data, and chemical analyses. Their crystal structures were solved and refined from the recorded X-ray powder patterns in the Fm3 ¯ m space group. This series of compounds has a porous framework with a relatively Introduction Within titanium hexacyanometallates the most studied mem- ber is Ti 4+ hexacyanoferrate(II) because of its ability for 137 Cs sorption [1, 2]. This compound is usually prepared from solu- tions of Ti 4+ chloride and K + or Na + hexacyanoferrate(II), where the mixed complex salt TiA 2 [Fe(CN) 6 ]·xH 2 O(A = Na, K) precipitates. Its crystal structure is formed by a 3D frame- work of –Ti–N≡C–Fe–C≡N–Ti– chains with the alkali metal atoms occupying all the available interstitial spaces. Its ability for the cesium sorption is supported in the ionic exchange of these interstitial alkali metal atoms by cesium. Cesium is a big atom, which practically occupies all the available volume of the interstitial void and from this fact the ionic exchange is highly favorable [3]. For the 137 Cs sorption other divalent tran- sition metal (T) mixed salts, TA 2 [Fe(CN) 6 ]·xH 2 O can also be used [4, 5]. In addition, metal hexacyanoferrates are relatively stable materials in acid media, which favors their application for the 137 Cs recovery from nuclear waste plants [1–3]. The formed precipitate from solutions of Ti 3+ chloride and K + hex- acyanoferrate(III) has also been studied, in which the solid pre- cipitation is accompanied of an inner charge transfer to form * Prof. Dr. E. Reguera E-Mail: ereguera@yahoo.com [a] Centro de Investigación en Ciencia Aplicada y Tecnología Avanzada del IPN Unidad Legaria Legaria 694, México, D.F [b] Escuela Superior de Física y Matemáticas del IPN UP ALM Lindavista, México, D.F [c] Instituto de Ciencia y Tecnología de Materiales Universidad de La Habana La Habana, Cuba Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/zaac.201000104 or from the author. 1968 View this journal online at wileyonlinelibrary.com © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Z. Anorg. Allg. Chem. 2010, 636, 1968–1973 high free volume, which is occupied by coordinated and hydrogen bonded water molecules. The charge balancing Cl – ion was found coor- dinated to titanium atoms. This series of porous solids was studied in order to explore the hydrogen interaction with the titanium atoms found at the surface of the cavities. On the water removal by moderate heating their porous framework collapses as reveal the nitrogen, CO 2 , and hydrogen adsorption but without complex salt decomposition. On the solids rehydration the porous framework is partially restored. Ti 4+ hexacyanoferrate(II) [6]. From Ti 3+ and ferrocyanic acid, H 4 [Fe(CN) 6 ], the formation of a solid with formula unit T 4 [Fe(CN) 6 ] 3 ·xH 2 O was reported [6]. The structure of these solids can be interpreted according to a structural model for Prussian blue (PB) analogues nowadays surpassed. To the best of our knowledge, the preparations and character- izations of Ti 3+ hexacyanometallates(II) with Ru II and Os II as inner metal atoms, have not been reported. In this contribution, the preparation of the Ti 3+ hexacyanometallates(II) series and their characterization from energy-disperse X-ray spectroscopy (EDS), X-ray diffraction (XRD), infrared (IR) spectroscopy, Mössbauer spectroscopy, thermogravimetry (TG), UV/Vis spectroscopy, adsorption data and chemical analyses are dis- cussed. The interest for the Ti 3+ hexacyanometallates(II) series was motivated by the possibility of obtaining porous solids of PB type, Ti 4 [M(CN) 6 ] 3 , with Ti 3+ atoms at the surface of the cavi- ties with available coordination sites in the anhydrous material. Such a solid could be a prototype of porous material for molec- ular hydrogen storage through formation of a coordination bond between the metal and the hydrogen molecule [7]. The availability of open metal sites at the surface of cavities in PB analogues has stimulated their study for hydrogen storage [8– 16]. For Ti 3+ located at a silica surface hydrogen adsorption heats close to –22 kJ·mol –1 were reported [17]. That value is in the required ideal range of adsorption energy for technological applications of hydrogen storage in porous solids [18]. Tita- nium has extended 3d orbitals and this facilitates its coordina- tion interaction with the hydrogen molecule. The obtained sol- ids, Ti 3 Cl[M(CN) 6 ] 2 ·10H 2 O(M = Fe, Ru, Os) in the following Ti 3 ClM 2 , were found to be not appropriate for studies related to the hydrogen adsorption in porous materials because on the crystal water removal a partial collapse for the porous frame- work was observed; nevertheless, valuable information on the