Recurrent supramolecular scenarios within complex 3-D hydrogen bond networks derived from organic ammonium salts of (4-amino-1-hydroxybutylidine)-1,1-bisphosphonic acid

Abstract

Reactions of (4-amino-1-hydroxybutylidine)-1,1-bisphosphonic acid (alendronic acid = LH₅) with organic amines or diamines in a 1 : 2 or 1 : 1 ratio in aqueous solution gave a range of nine new compounds of the general formula [(RN)H]₂[LH₃] or [(R′N)H₂][LH₃]. Crystal structures are reported for: (i) [enH₂][LH₃]·2(H₂O) 1; (ii) [MeenH₂][LH₃]·3(H₂O) 2; (iii) [Me₂enH₂][LH₃]·3.5(H₂O) 3; (iv) [Et₂enH₂][LH₃]·2(H₂O) 4; (v) [cdaH₂][LH₃]·3(H₂O) 5; (vi) [xdaH₂][LH₃]·4(H₂O) 6; (vii) [odaH₂][LH₃]·2(H₂O)·0.5(EtOH) 7; (viii) [PheaH]₂[LH₃]·2(H₂O) 8; (ix) [melH]₂[LH₃]·7(H₂O) 9 (en = H₂N(CH₂)₂NH₂; Meen = Me(H)N(CH₂)₂NH₂; Me₂en = Me(H)N(CH₂)₂N(H)Me; Et₂en = Et₂N(CH₂)₂NH₂; cda = trans-rac-1,2-C₆H₁₀(NH₂)₂; xda = m-C₆H₄(NH₂)₂; oda = H₂N(CH₂)₈NH₂; Phea = 2-Ph(CH₂)₂NH₂; mel = 1,3,5-C₃N₃(NH₂)₃). Each structure displays numerous hydrogen bond interactions between bisphosphonate anions, the (alkyl)ammonium cations and water molecules resulting in 3-D network materials. Compounds 1, 7 and 8 display identical 2-D sheets composed of [LH₃]²⁻ anions and water molecules approximately 12 Å thick and pillared by the [enH₂]²⁺ or [onH₂]²⁺ cations or a double layer of [PheaH]⁺ cations. A similar pillared structure is also observed in compound 9 where the 2-D anion layers are separated by regions of hydrogen bonded and π-stacked [melH]⁺ cations. The anion layer, although similar to that of compounds 1, 7 or 8, contains a significantly higher proportion of water molecules. The substituted ethylene diamine derivatives 2, 3, and 4 also display a consistent (but not identical) set of flat 2-D sheets of [LH₃]²⁻ anions and water molecules, with the organic diammonium cations and further water molecules sandwiched in between. In contrast to the above lamella type structures, the cda and xda derivatives, 5 and 6, comprise tessellated columns of anion and cations. A consistent feature of all the above structures is the head-to-tail arrangement of [LH₃]²⁻ anions held by strong, charge assisted, N(H)⋯O hydrogen bonds.