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

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.