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Synthesis and Characterization of the Open-Framework Barium Bisphosphonate [Ba3(O3PCH2NH2CH2PO3)2(H2O)4]·3H2O

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journal contribution
posted on 12.12.2005, 00:00 by Sebastian Bauer, Helen Müller, Thomas Bein, Norbert Stock
Following the strategy of using polyfunctional phosphonic acids for the synthesis of open-framework metal phosphonates, the phosphonocarboxylic acid (H2O3PCH2)2NCH2C6H4COOH was used in the hydrothermal synthesis of new Ba phosphonates. Its decomposition led to the first open-framework barium phosphonate [Ba3(O3PCH2NH2CH2PO3)2(H2O)4]·3H2O (1). The synthesis was also successfully performed using iminobis(methylphosphonic acid), (H2O3PCH2)2NH, as a starting material, and the synthesis was optimized to obtain 1 as a pure material. The reaction setup as well as the pH are the dominant parameters, and only a diffusion-controlled reaction led to the desired compound, 1. The crystal structure was solved from single-crystal data:  monoclinic; C2/c; a = 2328.7(2), b = 1359.95(7), and c = 718.62(6) pm; β = 98.732(10)°; V = 2249.5(3) × 106 pm3; Z = 4; R1 = 0.036; and wR2 = 0.072 (all data). The structure of [Ba3(O3PCH2NH2CH2PO3)2(H2O)4]·3H2O is built up from BaO8 and BaO10 polyhedra forming BaO chains and layers, respectively. These are connected to a three-dimensional metal−oxygen−metal framework with the iminobis(methylphosphonic acid) formally coating the inner walls of the pores. The one-dimensional pores (3.6 × 4 Å) are filled with H2O molecules that can be thermally removed. Thermogravimetric investigations and temperature-dependent X-ray powder diffraction demonstrate the stability of the crystal structure up to 240 °C. The uptake of N,N-dimethylformamide and H2O by dehydrated samples is demonstrated. Furthermore, IR, Raman, and 31P magic-angle-spinning NMR data are also presented.