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

Abstract

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 (M = Na, K, Rb, Cs) at pH 9–10 gave the dimetalated complexes [M2(LH3)(H2O)n]·m(H2O) (M = Na 6, K 7, Rb 8, Cs 9). Crystallization of 6 from water/MeOH gave [Na2(LH3)(H2O)4]·H2O (6a) as a single phase. For 7, crystallization from water/MeOH gave K2(LH3)·3(H2O) (7a) as the bulk product, whereas crystals of [K2(LH3)(H2O)6] (7b) were grown from water/DMSO solutions. On standing, the crystals of 7b were converted into the bulk product 7a. The structures of 6a and 7b are chain rather than sheet bisphosphonate coordination polymers, with a high number of coordinated water molecules. Moreover, their 3-D supramolecular structures are distinguishable, viz. a pillared array for 6a and a more compact bilayer structure for 7b. The [M2(LH3)(H2O)n]·m(H2O) complexes of the larger group 1 metal cations 8 and 9 each crystallized as a mixture of phases. Crystals of [Rb2(LH3)(H2O)5]·(H2O) (8a) and [Cs4(LH3)2(H2O)9]·2(H2O) (9a), respectively, were identified as the major component after crystallization from DMSO/H2O. The lesser hydrates [Rb4(LH3)2(H2O)8]·(H2O) (8b) and [Cs2(LH3)(H2O)4] (9b) were also identified, but were not isolated as bulk materials. The structures of 8a, 8b, 9a, and 9b each were complex 2-D sheets of bisphosphonate metal coordination polymers, which assembled through supramolecular interactions into compact 3-D arrays.