Common Structural Features in Calcium Hydroxyphosphonoacetates. A High-Throughput Screening

R,S-Hydroxyphosphonoacetic acid (H(3)HPA) is an inexpensive multidentate organic ligand widely used for the preparation of organo-inorganic hybrid materials. There are reports of several crystal structures and the variability of the resulting frameworks is strikingly high, in contrast with the simplicity of the ligand. In an attempt to investigate and rationalize some salient structural features of the crystal structures, we have carried out a systematic high-throughput study of the reaction of H(3)HPA with Ca2+ in aqueous solutions (pH values ranging 1.0-7.5) at room temperature and hydrothermally at 180 degrees C. The tested synthetic conditions yielded five crystalline single-phase Ca-H(3)HPA hybrids: Ca-3(O3PCHOHCOO)(2)center dot 14H(2)O (1), Ca(HO3PCHOHCOO)center dot 3H(2)O (2), Ca-5(O3PCHOHCOO)(2)(HO3PCHOHCOO)(2)center dot 6H(2)O (3), CaLi(O3PCHOHCOO) (4), and Ca2Na(O3PCHOHCOO)(HO3PCHOHCOO)center dot 1.5H(2)O (5). Four new crystal structures, 2-5, are reported (three from powder diffraction data and one from single-crystal data), which allowed us to unravel some key common structural features. The Ca-H(3)HPA hybrids without an extra alkaline cation, 1-3, contain a common,structural motif; which has been identified as a linear Ca-H(3)HPA Ca H(3)HPA-Ca trimer. This inorganic motif has a central Ca2+ in a distorted octahedral environment, whereas the two side Ca2+ cations are in an eight coordinated oxygen-rich environment The H(3)HPA ligands are chelating the central Ca2+ through two pairs of carboxylate and phosphonate oxygen atoms forming six-membered-rings, Ca-O-C-C-P-O-Ca. This coordination mode allows the peripheral Ca(B) ions to bind the ligand through the -OH group and the other carboxylate oxygen, forming a five-membered ring, Ca-O-C-C-O-Ca. The presence of alkaline cations, Li+ and Na+, disrupt this common structural feature leading to highly dense frameworks. Finally, similarities (and differences) between Ca-H(3)HPA and Cd-H(3)HPA hybrids are also discussed.