Inorganic-Organic Hybrid Molecular Ribbons Based on Chelating/ Bridging, “Pincer” Tetraphosphonates, and Alkaline-Earth Metals Konstantinos D. Demadis,* Eleni Barouda, Nikoleta Stavgianoudaki, and Hong Zhao † Crystal Engineering, Growth and Design Laboratory, Department of Chemistry, UniVersity of Crete, Voutes Campus, Heraklion, Crete, GR-71003, Greece ReceiVed July 11, 2008; ReVised Manuscript ReceiVed January 14, 2009 ABSTRACT: Syntheses and structures of alkaline earth metal ions and EDTMP, ethylenediamine-tetrakis(methylenephosphonate), are reported. The isostructural Ca 2+ and Sr 2+ analogs have 1D topologies, with EDTMP acting as both chelating and bridging ligand. The M-EDTMP compounds act as Fe-oxide removers from corroded surfaces. The renaissance of metal phosphonate chemistry is witnessed by numerous significant publications that originate from several research groups around the world. Apart from “curiosity-driven”, basic research, metal phosphonates have found important applica- tions in medicine 1 (e.g., palliation of metastatic bone cancer, antiresorption agents, osteoarthritis drugs, etc.), water treatment 2 (extraction of metals, mineral scale inhibition, corrosion control), gas storage and capture, 3 dentistry 4 (adhesives in dental composites), and dispersion technology 5 (rheology modification). On the other hand, alkaline-earth metal ions, in particular Ca 2+ and Sr 2+ , play an important role in bone mineral formation and Ca 2+ metabolism. 6 Therefore, the study of interactions between alkaline-earth metals and phosphonates presents a fruitful area of research that has repercussions in several front-line disciplines. In this communication, we report syntheses, structural charac- terization and some functional properties of alkaline-earth metal tetraphosphonate materials of the general formulas {M[(EDTMP)(H 2 O) 2 ] · H 2 O} n (M ) Ca 2+ (1), Sr 2+ (2)) possessing 1D topologies and an unprecedented EDTMP chelating/bridging coordination mode. Iron oxide dissolution mediated by these M-EDTMP materials (M ) Ca 2+ and Sr 2+ ) is also reported. To the best of our knowledge, this is the first use of phosphonates for iron oxide dissolution. EDTMP is a tetraphosphonic acid that can potentially dissociate its 8 protons (two H + from four -PO 3 H 2 ) depending on solution pH (see the Supporting Information for speciation curves). 7 The basic N groups remain protonated up to pH of ∼10. Therefore, EDTMP can be described as a “zwitter-ionic” ligand. At pH 4, each phosphonate group is monodeprotonated, thus EDTMP possesses an overall “-2” charge. It is therefore ideal for the construction of divalent metal phosphonate frameworks based on simple charge neutralization principles. Thus, reactions of water- soluble M 2+ (M 2+ ) Mg 2+ , Ca 2+ , Sr 2+ , Ba 2+ ) inorganic salts with EDTMP in acidic pH’s yield extended coordination polymers (see reaction scheme below and the Supporting Information). 8 All syntheses produce single-phase products based on powder XRD. In the reaction scheme, proton content is highlighted in blue. The structures of (NH 4 ) 2 (EDTMP), 1, and 2 have been deter- mined by single-crystal X-ray crystallography. 9 In the structure of (NH 4 ) 2 (EDTMP), EDTMP exists as a discrete dianion, neutralized by two NH 4 + ions (Figure 1, upper). The whole 2D layered structure is stabilized by hydrogen bonds between the -PdO and -P-O - groups and the NH 4 + cations or the protonated N atom (see the Supporting Information). Other structural features in (NH 4 ) 2 (EDTMP) compare well with those of EDTMP · 2H 2 O. 10 The crystal structures of the Ca(Sr)-EDTMP frameworks reveal extended, 1D coordination polymers with EDTMP 2- acting as both chelating and bridging ligand, Figure 1, middle and lower. There are some interesting features in these structures that warrant some attention. The -PO 3 H - group coordinates to Sr 2+ through only one of its three oxygens. This is in contrast to several literature examples that demonstrate the strong propensity of the -PO 3 H - group to bridge two or more metal centers. Two -PO 3 H - groups, one from each N, form an 11-membered chelate ring with Sr 2+ . The same function occurs on the other side of the -CH 2 CH 2 - linker that renders the whole tetraphosphonate a bridge between two Sr 2+ centers. Thus, one can envision the 1D polymer as a ribbon in a “wavelike” motion composed of Sr-EDTMP-Sr dimers, see Figure 2. To the best of our knowledge, this coordination mode of EDTMP is unprecedented. The Sr 2+ is located in a slightly distorted octahedral environment shaped by four equatorial phosphonate oxygens and two axial H 2 O molecules trans to each other. The Sr-O (P) bond distances are 2.4686(18) and 2.4884(16) Å, whereas the Sr-O water bond distance is 2.521(2) Å. The water of crystallization rests above a plane formed by four phosphonate oxygens. However, the H 2 O molecule forms hydrogen bonds with from the “wire” above, with two protonated phosphonic acid moieties (-P-O 5 -H) and with two phosphoryl oxygens (-PdO 1 ). The specific bridging mode of EDTMP observed in the structure of Ca(Sr)-EDTMP causes the ligand to acquire a strained position. Thus, the H 2 O-Sr-OH 2 axial vectors in the dimeric “unit” are not aligned, but form a dihedral angle of 48.49°. Very few metal- EDTMP compounds have been reported with metal ions such as Eu 3+ , Pb 2+ , and Zn 2+ . 11 The coordination mode of EDTMP is distinctly different in these materials. Chemical cleaning of corroded steel surfaces is an important technological field of great industrial and economical significance. 12 Reported methods include use of polycarboxylate ligands and derivatives. 12 In our experiments in situ corroded carbon steel surfaces were used as specimens for iron oxide removal. 13 * Corresponding author. E-mail: demadis@chemistry.uoc.gr. Web: http:// www.chemistry.uoc.gr/demadis. † Present address: Ordered Matter Science Research Center, Southeast University, Nanjing, P. R. China. CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 3 1250–1253 10.1021/cg800748f CCC: $40.75  2009 American Chemical Society Published on Web 02/09/2009