Coordination Polymers of Increasing Complexity Derived from Alkali Metal Cations and (4-Amino-1-hydroxybutylidine)-1,1-bisphosphonic Acid (Alendronic Acid): The Competitive Influences of Coordination and Supramolecular Interactions

• Published in: Crystal growth & design 2015-09, Vol.15 (9), p.4646-4662
• Main Authors: Deacon, Glen B, Forsyth, Craig M, Greenhill, Neil B, Junk, Peter C, Wang, Jun
• Format: Article
• Language: English
• ISSN: 1528-7483; 1528-7505
• DOI: 10.1021/acs.cgd.5b00917

Reactions of (4-amino-1-hydroxybutylidine)-1,1-bisphosphonic acid (alendronic acid = LH5) with 1 equiv of the group 1 metal bases KOH, KHCO3, Rb2CO3, RbOH, or Cs2CO3 in aqueous solution at pH 4–5 gave the corresponding complexes [M­(LH4)­(H2O) n ]·m(H2O), M = K (1), Rb (2), or Cs (3). Crystallization of the products under varying conditions yielded differing hydrates and/or polymorphic phases. Rapid crystallization of the potassium complex 1 from water/EtOH or water/DMSO gave 2­[K­(LH4)]­·3­(H2O) (1a), whereas slow crystallization from water or water/DMSO gave [K2(LH4)2­(H2O)2]­·2H2O (1b phase I). For the rubidium complex 2, rapid crystallization from water/EtOH gave the analogous [Rb2(LH4)2­(H2O)2]­·2H2O (2b phase I), whereas slow crystallization from water/DMSO also gave 2b, but as a mixture of phase I and the structural polymorph [Rb­(LH4)­(H2O)]­·H2O (2b phase II). In contrast, only [Cs­(LH4)­(H2O)]­·H2O (3b phase II) was obtained for the cesium complex 3 under all crystallization conditions. A second complex type [M­(LH4)­(LH5)]­·2H2O (M = Rb 4, Cs 5), incorporating an additional coordinated alendronic acid molecule, was also observed for the larger metals Rb and Cs as a minor product in some syntheses and isolated by fractional crystallization for 4, but as the sole product from Cs­(O2CH) and LH5 for 5. The crystal structures of the complexes [M­(LH4)­(H2O) n ]·m(H2O) 1a, 2b (phase II), and 3b (phase II) comprise edge-shared or corner-shared polyhedral chains that are linked by bridging bisphosphonate ligands into two-dimensional (2-D) sheets. The 4-ammoniobutylidene chains protrude above and below the coordination layers and interact with neighboring layers through hydrogen bonding of the terminal ammonium group forming distinctive supramolecular three-dimensional (3-D) arrays. The phase I structure of 1b and 2b comprises ribbons of parallel M4(LH4)4(H2O)4 subunits arranged in a more close packed 3-D supramolecular network. For [M­(LH4)­(LH5)]­·2H2O (M = Rb 4, Cs 5), isolated M+ cations, bridged by bisphosphonate ligands, are arranged into 2-D sheets, with the pendant 4-ammoniumbutylidene chains resulting in a 3-D lamellar network similar to those of 1a and 2b/3b (phase II). All of the [M­(LH4)­(H2O) n ]·m(H2O) and [M­(LH4)­(LH5)]­·2H2O structures display strong PO–H···O­(phosphonate) and N–H···O­(phosphonate/water) hydrogen bond motifs which significantly impact the observed structures. Reactions of LH5 with 2 equiv of the group 1 metal bases MOH (