Published: March 11, 2011 r2011 American Chemical Society 1713 dx.doi.org/10.1021/cg101652e | Cryst. Growth Des. 2011, 11, 1713–1722 ARTICLE pubs.acs.org/crystal Common Structural Features in Calcium Hydroxyphosphonoacetates. A High-Throughput Screening Rosario M. P. Colodrero, ^ Aurelio Cabeza, ^ Pascual Olivera-Pastor, ^ Maria Papadaki, f Jordi Rius, # Duane Choquesillo-Lazarte, ‡ Juan M. García-Ruiz, ‡ Konstantinos D. Demadis, f and Miguel A. G. Aranda* ,^ ^ Departamento de Química Inorg  anica, Universidad de M alaga, Campus Teatinos S/N, 29071-M alaga, Spain f Crystal Engineering, Growth and Design Laboratory, Department of Chemistry, University of Crete, Voutes Campus, Crete GR-71003, Greece # Institut de Ci  encia de Materials de Barcelona, 08193 Bellaterra, Catalunya, Spain ‡ Laboratorio de Estudios Crystalogr  aficos, IACT, CSIC-Universidad de Granada, 18100 Granada, Spain b S Supporting Information ’ INTRODUCTION Coordination polymers, also known as metalÀorganic frame- works (MOFs), are an important class of hybrid frameworks, in which polyfunctional organic molecules bridge metal cations (or clusters) into extended arrays. 1 These materials exhibit a wide structural diversity chiefly as a result of the coordination preferences of the metal and the various ways in which the ligand can coordinate to the metal ion. 1 Various aspects of hybrid materials have been recently reviewed. 2 The possibility of designing materials with predetermined functionalities 3 has prompted investigations of diverse applications for these hybrid systems, including gas separation 4 and storage, 5 heterogeneous catalysis, 6 and photoluminescence. 7 A particular class of multidentate ligands are polyfunctional phosphonic acids, having multiple oxygen-donor groups (and occasionally other groups) capable of binding a number of metal ions into structurally versatile metal phosphonate hybrids. 8 Among these phosphonic acids, 2-hydroxyphosphonoacetic acid (H 3 HPA, where H 3 stands for the number of exchangeable protons and HPA is the acronym of the acid) is a polyvalent ligand, bearing three different coordination groups (ÀOH, ÀCOOH, and ÀPO 3 H 2 ), that recently has attracted consider- able attention as for the synthesis of metal phosphonates. 9 In addition to being a stable and cost-effective compound, H 3 HPA also possesses a chiral carbon in its backbone for potential chiral separations and nonlinear optical applications. 10 Some me- talÀH 3 HPA hybrids also exhibit anticorrosion, 11 catalytic, 12 and photoluminescent capabilities. 13 Received: December 13, 2010 Revised: February 14, 2011 ABSTRACT: 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 Ca 2þ in aqueous solutions (pH values ranging 1.0À7.5) at room temperature and hydrothermally at 180 °C. The tested synthetic conditions yielded five crystalline single- phase CaÀH 3 HPA hybrids: Ca 3 (O 3 PCHOHCOO) 2 3 14H 2 O (1), Ca(HO 3 PCHOHCOO) 3 3H 2 O(2), Ca 5 (O 3 PCHOHCOO) 2 (HO 3 PCHOHCOO) 2 3 6H 2 O(3), CaLi(O 3 PCHOHCOO) (4), and Ca 2 Na(O 3 PCHOHCOO)(HO 3 PCHOHCOO) 3 1.5H 2 O(5). Four new crystal structures, 2À5, are reported (three from powder diffraction data and one from single-crystal data), which allowed us to unravel some key common structural features. The CaÀH 3 HPA hybrids without an extra alkaline cation, 1À3, contain a common structural motif, which has been identified as a linear CaÀH 3 HPAÀCaÀH 3 HPAÀCa trimer. This inorganic motif has a central Ca 2þ in a distorted octahedral environment, whereas the two side Ca 2þ cations are in an eight-coordinated oxygen-rich environment. The H 3 HPA ligands are chelating the central Ca 2þ through two pairs of carboxylate and phosphonate oxygen atoms forming six-membered rings, CaÀOÀCÀCÀPÀOÀCa. This coordination mode allows the peripheral Ca(II) ions to bind the ligand through the ÀOH group and the other carboxylate oxygen, forming a five- membered ring, CaÀOÀCÀCÀOÀCa. The presence of alkaline cations, Li þ and Na þ , disrupt this common structural feature leading to highly dense frameworks. Finally, similarities (and differences) between CaÀH 3 HPA and CdÀH 3 HPA hybrids are also discussed.