Z. Anorg. Allg. Chem. 2009, 635, 2391–2395

SHORT COMMUNICATION DOI: 10.1002/zaac.200900351 Structural Transformation of Three›Dimensional Bimetallic Phosphites Containing Corner›Sharing 4›Ring Chains Zhien Lin,[a,b] Hari Pada Nayek,[a] and Stefanie Dehnen*[a] Keywords: Hydrothermal synthesis; X›ray diffraction; Open framework; Bimetallic phosphite; Structural transformation Abstract. Two open›framework bimetallic phosphites, [C6H18N2]› [Co0.3Zn2.7 (HPO3)4] (1) and [C2H8N]2[Co0.3Zn2.7(HPO3)4] (2) were synthesized and characterized. The inorganic frameworks of the two compounds are constructed from strictly alternating MO4 (M = Zn, Co) tetrahedra and HPO3 pseudo pyramids, forming three›dimensional net› Introduction Crystalline microporous and open›framework inorganic sol› ids have been extensively studied because of their widespread applications in catalysis, separation, and ion›exchange proc› esses [1]. Among the most important microporous materials are aluminosilicate and aluminophosphate molecular sieves [2]. The inorganic frameworks of these materials are usually constructed from corner›sharing TO4 tetrahedra (T = Al, Si, P) to form three›dimensional networks with open channels or cages. The utilization of the non›tetrahedral anion, such as the [HPO3]2– pseudo pyramid, as building unit can create novel framework architectures different from those found in 4›con› nected zeolitic networks. So far, a number of open›framework metal phosphites with various pore apertures have been pre› pared and characterized [3]. Notable examples include a trime› tallic phosphite Cr›NKU›24 and four zinc phosphites ZnHPO› CJn (n = 1–4) with 24›ring channels, and an aluminum›zinc phosphite NTHU›5 with the first 26›ring channel structure ever reported in open›framework materials [4]. Despite impressive progress in open›framework metal phos› phites, little is known about their crystallization mechanisms primarily due to the “black box” nature of sealed autoclaves. Some techniques, such as energy dispersive X›ray diffraction, in›situ NMR spectroscopy or small›angle X›ray scattering, were exploited to understand the formation process of three› * Prof. Dr. S. Dehnen Fax: +49›6421›2825653 E›Mail: dehnen@chemie.uni›marburg.de [a] Fachbereich Chemie Philipps›Universität Marburg Hans›Meerwein›Strasse 35043 Marburg, Germany [b] College of Chemistry Sichuan University Chengdu 610064, P. R. China Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/zaac.200900351 or from the author. Z. Anorg. Allg. Chem. 2009, 635, 2391–2395 ' 2009 WILEYVCH Verlag GmbH & Co. KGaA, Weinheim 2391 works with 8›ring and 12›ring channels for 1 and 16›ring channels for 2. The anionic frameworks of the two compounds can be easily transformed to a neutral framework structure with 16›ring channels at room temperature. dimensional open›framework structures [5]. During the course of such investigation, some intermediate phases with low›di› mensional structures were successfully observed and isolated [6]. From the viewpoint of crystal chemistry, the study on the transformation of one crystalline structure to another structure may shed light on how the open›framework structures are formed [7]. We are interested in the study of metal phosphites not only to prepare new open›framework compounds, but also to corre› late their structures and understand the formation process of these complicated structures [8]. In this work, two three›dimen› sional bimetallic phosphites, [C6N2H18][Co0.3Zn2.7(HPO3)4] (1) and [C2NH8]2[Co0.3Zn2.7(HPO3)4] (2), were prepared and struc› turally characterized. Both compounds have (3,4)›connected open›framework structures with corner›sharing 4›ring chains. Interestingly, the two compounds can be easily transformed to a neutral bimetallic phosphite framework with 16›ring channels at room temperature. To the best of our knowledge, this is the first observation of the structural transformations of a three› dimensional metal phosphite to a different three›dimensional structure under ambient conditions. Results and Discussion Compound 1 was hydrothermally synthesized under autoge› nous pressure. It crystallizes in the orthorhombic space group Pbcn (no. 60) with a = 10.089(2) ¯, b = 14.789(3) ¯, c = 13.988(3) ¯. The asymmetric unit of 1 consists of ten crystal› lographically independent non›hydrogen framework atoms, in› cluding two metal atoms, two phosphorus atoms and six oxy› gen atoms. Both of the M sites are tetrahedrally coordinated by four oxygen atoms. The two unique phosphorus atoms each share three oxygen atoms with adjacent metal atoms, with the fourth vertex occupied by a hydrogen atom The existence of P–H bond is confirmed by the characteristic band [ν˜ = 2391 cm–1] in the IR spectrum. The M–O bond lengths are in Z. Lin, H. P. Nayek, S. DehnenSHORT COMMUNICATION Figure 1. View of the structure of 1 with 8›ring and 12›ring channels running along the (a) [100] and (b) [001] directions, respectively. The structure can be understood as (c) corner›sharing 4›ring chains further linked through metal ions. Color code: MO4 tetrahedra, light grey; HPO3 pseudo pyramids, dark grey; M2+ ions, light grey balls. Figure 2. View of the structure of 2 with 16›ring channels and helical channels with 8 tedrahedra units per turn along the (a) [110] and (b) [010] directions, respectively. The structure can be understood as (c) corner›sharing 4›ring chains further linked through metal ions. Color code: MO4 tetrahedra, light grey; HPO3 pseudo pyramids, dark grey; M2+ ions, light grey balls. the range of 1.893(4)–1.952(4) ¯ and the P–O bond lengths vary from 1.460(4) to 1.536(4) ¯. The stoichiometry of [Zn3(HPO3)4] results in a net charge of –2, which is balanced by one diprotonated diamine (i.e. 2›methylpentamethylenedi› amine) per formula unit. The connectivity between M(1)O4 tetrahedra and HPO3 pseudo pyramids create corner›sharing 4›ring chains, which run along the [001] direction. These chains are further linked by M(2)O4 tetrahedra to form the three›dimensional framework of 1 with 8›ring and 12›ring channels, as shown in Figure 1. The 8›ring channels run along the [100, 010, 110] and [101] directions and the 12›ring channels run along the [001] direc› tion in a zigzag manner. The free pore diameters of the 12› ring window are 5.3 ¯ × 6.8 ¯. The framework density of the structure, defined as the number of polyhedra per 1000 ¯3, is 13.4, which is similar to aluminophosphate molecular sieves such as DAF›1 and VPI›5 [9]. Compound 2 was synthesized under solvothermal conditions. It is isostructural with [(CH3)2NH2]2Zn3(HPO3)4 [10]. The compound crystallizes in the monoclinic space group C2/c (no. 15) with a = 15.783(3) ¯, b = 8.378(3) ¯, c = 15.955(3) ¯, β = 111.50(3)˚. The asymmetric unit of 2 contains thirteen non› hydrogen atoms, of which ten atoms belong to the host net› work and three atoms to the guest templating unit. There are two crystallographically distinct M sites and two crystallo› graphically distinct P sites. The M(1) site locates in a general 2392 www.zaac.wiley›vch.de ' 2009 WILEYVCH Verlag GmbH & Co. KGaA, Weinheim Z. Anorg. Allg. Chem. 2009, 2391–2395 position and the M(2) site locates on the twofold axis. Both of them are tetrahedrally coordinated by oxygen atoms with M–O bond lengths varying from 1.923(2) to 1.965(2) ¯. The phosphorus atoms each share three oxygen atoms with adjacent M sites [P–O: 1.506(2)–1.530(2) ¯], with the fourth vertex oc› cupied by a terminal hydrogen atom [(P–H)av = 1.30 ¯]. The inorganic framework of compound 2 is constructed from strictly alternating MO4 tetrahedra and HPO3 pseudo pyramids, forming a three›dimensional structure with 16›ring channels running along the [110] and [11¯0] directions (Figure 2a). The 16›membered›ring window is somewhat puckered with the free diameters of 2.3 ¯ × 9.4 ¯. When viewed along the [010] direction, the structure seems to possess 8›ring channels (Fig› ure 2b). In fact, the eight›membered rings are not closed, that is, the channels running along the [010] direction are helical. Two types of helical channels with opposite handedness coex› ist in the open structure. The inorganic framework can be alter› natively described by the use of a chainlike structure with cor› ner›sharing four›membered rings, as shown in Figure 2c. These linear chains run along the [110] and [11¯0] directions, respectively, and are further linked together by metal ions to form the three›dimensional framework. The metal phosphite framework is anionic, which is balanced by protonated dimethylamine molecules per formula unit. It should be noted that no dimethylamine molecule was used as the starting material in the solvothermal reaction. They are Structural Transformation of Three›Dimensional Bimetallic Phosphites formed in situ by the decomposition of DMF molecules under solvothermal conditions. Every 16›ring window hosts two dim› ethylamine cations, which interact with the framework oxygen atoms through hydrogen bonds [N•••O: 2.813–2.889 ¯]. A void space analysis using the program PLATON indicates that these extra›framework species occupy 39.7 % of the unit cell vol› ume [11]. Thermogravimetric analyses of the powder samples of 1 and 2 were carried out under nitrogen atmosphere with a heating rate of 10 °C•min–1. For compound 1, two stags of weight loss are observed over the temperature range 40–800 °C. The initial weight loss between 300–440 °C corresponds to the partial de› composition of the organic molecules (observed: 14.86 %; cal› culated: 18.38 %). The remaining organic species was re› moved from the sample at the second stage between 440– 550 °C with a weight loss of 3.12 %. Compound 2 remains stable up to 210 °C. On further heating, a weight loss of 15.08 % appears, which is continuous up to 410 °C and is as› signed to the decomposition of the organic species (calculated weight loss: 14.87 %). Compounds 1 and 2 are stable in air for several months and insoluble in common non›polar solvents, but slightly soluble in water. By immersing the crystalline samples of 1 and 2 in distilled water at room temperature for two days, the color of compound 1 and 2 obviously changed from deep blue to pink and the shape of the resulting product was totally different from those of 1 and 2. IR spectra and powder XRD patterns of the resulting products are similar with those of the neutral zinc phosphite with 16›ring channels, Zn2(H2O)4(HPO3)2•H2O [3l], indicating that both compounds have been transformed into a new phase representing a structural analogue of Zn2(H2O)4(HPO3)2•H2O. EDS analyses reveal a Co:Zn ratio of 1.00:5.74, suggesting the empirical formula of Co0.3Zn1.7(H2O)4(HPO3)2•H2O (3). In the structure of 3, the metal atoms adopt two different coordination environments: tetrahedral and octahedral. The pink color of compound 3 indi› cates that the cobalt ions in the structure are octahedrally coor› dinated, which is further confirmed by its ESR spectrum. It should be noted here that three›dimensional metal phosphites or metal phosphates templated by organic cations usually re› main stable in water. In many instances, the introduce of inor› ganic cations, such as NH4+, Na+, and K+ ions, in the aqua solution will make their host frameworks collapse and result in the formation of inorganic phases with small pores. One of the driving forces for these structural transformations is the availability of the inorganic cations for solution›mediated ion› exchange processes. The facile structural transformation of compounds 1 and 2 at room temperature without the use of any inorganic cations is noteworthy. Interestingly, the structure of compound 3 can be converted back to the structure of 2 by heating the solid samples of 3 with DMF at 150 °C for 4 days. The resulting solid with deep› blue color was obtained as the only product. The absorption bands associated with dimethylamine cations in the IR spec› trum reappears, and the powder XRD pattern of the product is similar with that of compound 2. These results demonstrate the Z. Anorg. Allg. Chem. 2009, 2391–2395 ' 2009 WILEYVCH Verlag GmbH & Co. KGaA, Weinheim www.zaac.wiley›vch.de 2393 reversibility of the structural transformation between com› pound 2 and 3. Careful analysis of the structures of 1–3 reveals a close rela› tionship between them. All compounds have three›dimensional structures containing corner›sharing 4›ring chains. In the struc› tures of 1 and 2, the linear metal phosphite chains are linked by tetracoordinate metal ions, whereas in the structure of 3, the linear chains are linked by hexacoordinate metal com› plexes – in agreement with the indication for octahedrally co› ordinated Co2+ ions. The framework structure of 3 can be con› ceptually generated from the three›dimensional structure of 1 and 2 according to the following steps. When immersed in water, the structures of 1 and 2 can undergo dynamic hydroly› sis of the M–O bonds and decompose to form the corner›shar› ing 4›ring chains and M(H2O)62+ complexes. Then, the unsatu› rated oxygen atoms in the linear chains will react with the M(H2O)62+ complexes. Two H2O ligands in trans›position of the M(H2O)62+ complex are replaced by the oxygen atoms of the metal phosphite chains and give rise to the three›dimen› sional framework of 3, as shown in Scheme 1. Scheme 1. Possible pathway for the formation of the neutral metal phosphite framework with 16›ring channels, 3, from 1 or 2 by linear metal phosphite chains with corner›sharing 4›rings, and its re›conver› sion into 2 under solvothermal conditions. Conclusions In summary, two (3,4)›connected open›framework bimetallic phosphites with 12›ring and 16›ring channels, respectively, have been synthesized in the presence of different amines as the structure›directing agents. The two anionic framework structures can be easily transformed to a neutral framework structure at room temperature. The incorporation of cobalt(II) ions in the two open structures make it possible to observe the structural transformation process directly according to the color change caused by the change of coordination environ› ment of cobalt ions. A possible pathway for the structural transformations has been proposed. Moreover, the neutral framework structure is converted back to the (3,4)›connected Z. Lin, H. P. Nayek, S. DehnenSHORT COMMUNICATION structure with 16›ring channels under solvothermal conditions. The unique reversible structural transformation may make it useful for understanding how open›framework structures are formed. Experimental Section Synthesis and Initial Characterization To prepare compound 1, a mixture of Zn(OAc)2•2H2O (0.219 g,), H3PO3 (0.492 g), Co(OAc)2•4H2O (0.249 g), 2›methyl›1,5›pentanedi› amine (0.464 g) and H2O (8.024 g) was stirred under ambient condi› tions for 30 min. The resulting mixture was sealed in a Teflon›lined steel autoclave and heated at 150 °C for 2 days and afterwards cooled to room temperature. The resulting product, which is made up of deep blue single crystals, was recovered by filtration, washed with distilled water and dried in air (32.5 % yield based on zinc). The powder XRD pattern of the crystals is in good agreement with the one simulated on the basis of the single›crystal structure, indicating phase purity. EDS analyses gave a Co:Zn ratio of 1.00:9.14. Elemental analyses con› firmed its stoichiometry (Anal. Found: C, 10.87; H, 3.27; N, 4.34 %. Calcd: C, 11.40; H, 3.51; N, 4.43 %). IR : 3045m, 2957m, 2391m, 2960m, 1633m, 1523m, 1462w, 1391w, 1063s, 999s, 618s, 468m cm–1. To prepare compound 2, a mixture of Zn(OAc)2•2H2O (0.219 g,), H3PO3 (0.533 g), Co(en)2(OAc)2(ClO4) (0.079 g), ethylene glycol (1.023 g) and DMF (4.012 g) was stirred under ambient conditions for 30 min. The resulting solution was sealed in a Teflon›lined steel auto› clave and heated at 110 °C for 3 days and afterwards cooled to room temperature. The resulting product, which is made up of deep blue single crystals, was recovered by filtration, washed with distilled water and dried in air (68.2 % yield based on zinc). The powder XRD pattern of the crystals is in good agreement with the one simulated on the basis of the single›crystal structure, indicating phase purity. EDS anal› yses gave a Co:Zn ratio of 1.00:8.94. Elemental analyses confirmed its stoichiometry (Anal. Found: C, 6.89; H, 3.23; N, 4.58 %. Calcd: C, 7.92; H, 3.32; N, 4.62 %). IR: 3068w, 2802w, 2362m, 1597w, 1467m, 1070s, 1022s, 857m, 561s, 455s cm–1. The X›band ESR spectrum of 2 was recorded on a power sample at room temperature. The spectrum shows two broad signals with effective g values (i.e. the values directly read on the spectrum) of g’^ = 4.46 and g’s = 2.27, which are in agree- ment with the existence of tetrahedrally coordinated CoII ions in the framework structure. X-ray Crystallography Data collection was performed with a STOE IPDS-II diffractometer with graphite-monochromated Mo-KÆ (º = 0.71073 Å) radiation at 100 K. The structure was solved by direct methods and refined on F2 by full-matrix least-squares methods using the SHELXTL program package [12]. Crystal data for 1: C6H22N2O12P4Co0.3Zn2.7 (M = 632.31 g·mol–1), orthorhombic, space group Pbcn (no. 60), a = 10.089(2), b = 14.789(3), c = 13.988(3) Å, V = 2087.2(7) Å3, Z = 4, Dc = 2.012 g·cm–3, ì = 3.675 mm–1, 11229 reflections measured, 2206 unique (Rint = 0.0727). Final wR2 (all data) = 0.1288, R1 = 0.0479. Crystal data for 2: C4H20N2O12P4Co0.3Zn2.7 (M = 606.28 g·mol–1), monoclinic, space group C2.c (no. 15), a = 15.783(3), b = 8.378(3), c = 15.955(3) Å, â = 111.50(3), V = 1962.8(8) Å3, Z = 4, Dc = 2.011 g·cm–3, ì = 3.901 mm–1, 7165 reflections measured, 2072 unique (Rint = 0.0265). The final wR2 (all data) was 0.0832, and R1 was 0.0302. The supplementary crystallographic data CCDC- 731848 for 1 and CCDC-731849 for 2 can be obtained free of 2394 www.zaac.wiley›vch.de ' 2009 WILEYVCH Verlag GmbH & Co. KGaA, Weinheim Z. Anorg. Allg. Chem. 2009, 2391–2395 charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk.data_request.cif. Supporting Information (see footnote on the first page of this article): X-ray data in CIF format, additional crystallographic figures, ESR spectrum, IR spectra, TGA curves, experimental and simulated powder XRD patterns. Acknowledgement This work was supported by the NSF of China (Grant 20801037) and the Alexander von Humboldt Foundation. The authors are grateful to Dr. O. Burghaus for performing the ESR measurement, and M. Ger- lach for performing TGA measurements. References [1] a) S. Natarajan, S. Mandal, Angew. Chem. Int. Ed. 2008, 47, 4798–4828; b) S. Dehnen, M. Melullis, Coord. Chem. Rev. 2007, 251, 1259–1280; c) A. Corma, M. J. Díaz-Cabañas, J. L. Jordá, C. Martínez, M. Moliner, Nature 2006, 443, 842–845; d) Manos, M. J. K. Chrissafis, M. G. Kanatzidis, J. Am. Chem. Soc. 2006, 128, 8875–8883; e) P. Feng, X. Bu, N. Zheng, Acc. Chem. Res. 2005, 38, 293–303. [2] a) J. Yu, X. Xu, Chem. Soc. Rev. 2006, 35, 593–604; b) M. E. Davis, Nature 2002, 417, 813–821; c) A. K. Cheetham, G. Férey, T. Loiseau, Angew. Chem. Int. Ed. 1999, 38, 3268–3292; d) X. Bu, P. Feng, G. D. Stucky, Science 1997, 278, 2080–2085. [3] a) T. Rojo, J. L. Mesa, J. Lago, B. Bazan, J. L. Pizarro, M. I. Arriortua, J. Mater. Chem. 2009, 19, 3793–3818; b) S. Natarajan, S. Mandal, Inorg. Chem. 2008, 47, 5304–5313; c) Y. Yang, Y. Zhao, J. Yu, S. Wu, R. Wang, Inorg. Chem. 2008, 47, 769–771; d) L. Zhao, J. Li, P. Chen, G. Li, J. Yu, R. Xu, Chem. Mater. 2008, 20, 17–19; e) L. Chen, X. Bu, Chem. Mater. 2006, 18, 1857–1860; f) W. Liu, H.-H. Chen, X.-X. Yang, J.-T. Zhao, Eur. J. Inorg. Chem. 2005, 946–951; g) J.-X. Pan, S.-T. Zheng, G.-Y. Yang, Cryst. Growth Des. 2005, 5, 237–242; h) Z.-E. Lin, J. Zhang, S.-T. Zheng, G.-Y. Yang, Eur. J. Inorg. Chem. 2004, 953– 955; i) W. Fu, L. Wang, Z. Shi, G. Li, X. Chen, Z. Dai, L. Yang, S. Feng, Cryst. Growth Des. 2004, 4, 297–300; j) S. Fernández, J. L. Mesa