Deprotonation of phosphonic acids with M2+ cations for the design of neutral isostructural organic-inorganic hybrids.

Organic-inorganic hybrids constitute an important class of compounds in the exploratory research area of advanced materials design, because these materials can be reliably designed by integrating highly predictable structural features of pure organic and inorganic solids.1-3 For example, in inorganic materials, variations in the nature of the metal cation or counterion or reaction conditions cause drastic structural changes; likewise in organic compounds, the interplay of strong and weak intermolecular interactions pose problems in structure prediction.2,3 However, by carefully choosing anionic ligands with strong affinity for metal cations and the ability to form robust complementary hydrogen-bonding motifs one can reliably design hybrid materials. When a ligand(s) functions as an anion and balances the charge of a metal cation, the entry of counteranions into the crystal lattice can be avoided and predictable neutral coordination networks will be formed irrespective of the nature of the counteranion. Further, the robust hydrogen bonds between ligands in hybrids exist in consonance with the coordination bonds (in other words, these bonds are insulated from each other) and form preordained network topologies.3a In this context, our experience with the molecular and metal complexes of organophosphonates suggests that phosphonates are among the best candidates for attempting the design of novel organic-inorganic hybrid materials.4-6 Recently, we have demonstrated that mono-deprotonation of nitrilotri(methylphosphonic acid), [N(CH2PO3H2)3]1H6, with organic amines results in predictable three-dimensional (3D) hexagonal structures through very short ionic hydrogen bonds, and double deprotonation of the acid 1H6 leads to the formation of hydrogen-bonded dimers stabilized by as many as four ionic hydrogen bonds.5a,7 Therefore, it occurred to us that if we can doubly deprotonate the acid 1H6 with M2+ metal cations, we could not only form a rigid neutral coordination network but also trigger a self-assembly process among the phosphonate anions of 1H4 through motif X (Figure 1).5 The metallo deprotonated networks are expected to be isomorphic irrespective of M2+ metal salt type. The doubly deprotonated salts of the acid 1H6 were synthesized by reacting the acid with a single or a mixture of metal salts under mild hydrothermal conditions or by using layering techniques. When the acid 1H6 was reacted with more than one metal salt and the ligand-to-metal salts ratio was kept constant at 1:1, the triphosphonic acid became doubly deprotonated, and isostructural metal complexes were formed. For example, when the acid 1H6 (75.0 mg, 0.025 mmol), dissolved in a water-ethanol mixture (10 mL), was layered on top of an ethanolic solution of Co(ClO4)2‚ 6H2O (92.0 mg, 0.025 mmol, 15 mL), a doubly deprotonated complex [Co(1H4)(H2O)3] 2b was formed. The Mn analogue of 2b was synthesized by dissolving MnSO4‚xH2O (151.0 mg, 1 mmol) and acid 1H6 (300.0 mg, 1 mmol) together in deionized water and by placing the reaction mixture in a high-pressure bomb at 120-130 °C for 24 h. The single crystals that resulted from mild (120 °C) hydrothermal synthesis, [Mn(1)(H2O)3], 2a were identical in structure to that of the metal complex synthesized using Mn(O2CCH3)2‚4H2O and layering techniques. By more severe hydrothermal treatment and increasing the ratio of acid to metal, an anhydrous complex and an as yet unknown phase were also obtained. These compounds will be reported upon, subsequently. (1) (a) Hagrman, P. L.; Hagrman, D.; Zubieta, J. Angew. Chem., Int. Ed. 1999, 38, 2638-2684. (b) Ekambaram, S.; Sevov, S. C. Angew. Chem., Int. Ed. 1999, 38, 372-375. (c) Asefa, T.; MacLachlan, M. J.; Coombs, N.; Ozin, G. A. Nature 1999, 402, 867-871. (d) Said, M. A.; Roesky, H. W.; Rennekamp, C.; Andruh, M.; Schmidt, H. Noltemeyer, M. Angew. Chem. Int. Ed. 1999, 38, 661-664. (e) Crystal Engineering: From Molecules and Crystals To Materials; Braga, D., Orpen, A. G., Eds.; NATO Advanced Institute Study Series; Kluwer: Dordecht, The Netherlands, 1999. (2) Zaworotko, M. J. Coordination Polymers. In Crystal Engineering: The Design and Applications of Functional Solids; Seddon, K. R., Zaworotko, M. J., Eds.; NATO Advanced Institute Study Series; Kluwer: Dordecht, The Netherlands, 1998, pp 383-408. (3) (a) Wuest, J. D. In Mesomolecules: From Molecules to Materials; Mendenhall, G. D., Greenberg, A., Liebman, J. F., Eds.; Chapman & Hall: New York, 1995; pp 107-131. (b) Russell, V. A.; Evans, C. C.; Li, W.; Ward, M. D. Science 1997, 276, 575-579. (c) Braga, D.; Grepioni, F.; Desiraju, G. R. Chem. ReV. 1998, 98, 1375-1405. (d) Biradha, K., Dennis, D.; MacKinnon, V. A.; Sharma, C. V. K.; Zaworotko, M. J. J. Am. Chem. Soc. 1998, 120, 11894-11903. (4) (a) Alberti, G. In ComprehensiVe Supramolecular Chemistry; Atwood, J. L., Davies, J. E. D., MacNicol, D. D., Vogtle, F., Lehn, J.-M., Eds.; Pergamon: Oxford, 1996; Vol. 7, pp 151-187. (b) Clearfield, A. Metal Phosphonate Chemistry. In Progress in Inorganic Chemistry; Karlin, K. D., Ed.; John Wiley & Sons: New York, 1998; Vol. 47, pp 371-510. (c) Mallouk, T. E.; Gavin, J. A. Acc. Chem. Res. 1998, 31, 219-227. (5) (a) Sharma, C. V. K.; Clearfield, A. J. Am. Chem. Soc. 2000, 122, 4394-4402. (b) Sharma, C. V. K.; Clearfield, A. J. Am. Chem. Soc. 2000, 122, 1558-1559. (6) (a) Garcia, M. E.; Naffin, J. L.; Deng, N.; Mallouk, T. E. Chem. Mater. 1995, 7, 1968-1972. (b) Wang, X.; Jacobson, A. J. Chem. Commun. 1999, 973-974. (c) Gao, Q.; Guillou, N.; Norgues, M.; Cheetham, A. K.; Ferey, G. Chem. Mater. 1999, 11, 2937-2947. (7) (a) Daly, J. J.; Wheatley, P. J. J. Chem. Soc. A 1967, 212. (b) MartinezTapia, H. S.; Cabeza, A.; Bruque, S.; Pertierra, P.; Garcia-Granda, S.; Aranda, M. A. G. J. Solid State Chem. 2000, 151, 122-129. (c) Cabeza, A.; Aranda, M. A. G.; Bruque, S. J. Mater. Chem. 1999, 9, 571-578. Figure 1. Oversimplified schematic representation of coordinate and hydrogen bonds in isostructural complexes 2-3 The arrows and grooves represent hydrogen bond donors and acceptors, respectively (Motif X). 2885 J. Am. Chem. Soc. 2001, 123, 2885-2886