Three pyridylphosphonic acids, 3-pyridylphosphonic acid 1, 5-(dihydroxyphosphoryl)nicotinic acid 2, and 3,5-pyridinediyldiphosphonic acid 3, were synthesized and structurally investigated by solid state FT-IR and single crystal X-ray diffraction methods. All compounds appear in zwitterionic forms in the solid state with a proton transferred from the phosphonic group toward the pyridine N-atom. Strong hydrogen-bond interactions O-H/O and N-H/O organize the molecules of the compounds into polar three-dimensional networks. The crystal structures are additionally stabilized via weaker C-H/ O hydrogen bonds and p/p or C]O/p interactions. Powder second harmonic generation and solution NMR spectra were measured as well. The NMR spectra revealed that the double bonds of the functional groups (P]O and C]O), although differently oriented versus the N py atom, are coplanar with the aromatic rings in all stable, low energy conformers of compounds 1-3 in solution. In crystals, however, the P]O bonds are tilted toward the ring by 10.77(7) in 1, 70.42(8) in 2 and by 43.97(7) and 6.56(8) in 3. All three compounds exhibit moderate powder SHG efficiencies compared to that of urea.

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The Co(II) ion in the title complex salt, [Co(H(2)O)(6)](C(14)H(13)O(10)P(2))(2)·2H(2)O or [Co(H(2)O)(6)][H(C(7)H(6)O(5)P)(2)]·2H(2)O, resides on an inversion centre and exhibits an octahedral environment formed by six aqua ligands. Two unique acid residues share an H atom between their phosphonate groups, forming a complex monoanion with a very short (P)O···H···O(P) hydrogen bond of 2.435 (2) Å. The crystal structure is layered and consists of thick organic bilayers with hydrated metal [Co(H(2)O)(6)](2+) ions arranged between them. The interior of the bilayer is occupied by the aromatic portions of the complex monoanions and the carboxyl groups, which form hydrogen-bonded R(2)(2)(8) ring motifs. The phosphonate groups are arranged outwards in order to form the hydrogen-bonded surfaces of the bilayer. Electrostatic and multiple hydrogen-bond interactions, established between the coordination and solvent water molecules and the phosphonate O atoms, hold neighbouring bilayers together.

The rigid organic ligand (pyridine-3,5-diyl)diphosphonic acid has been used to create the title novel three-dimensional coordination polymer, [Ca(C(5)H(6)NO(6)P(2))(2)(H(2)O)](n). The six-coordinate calcium ion is in a distorted octahedral environment, formed by five phosphonate O atoms from five different (pyridin-1-ium-3,5-diyl)diphosphonate ligands, two of which are unique, and one water O atom. Two crystallographically independent acid monoanions, L1 and L2, serve to link metal centres using two different coordination modes, viz. η(2)μ(2) and η(3)μ(3), respectively. The latter ligand, L2, forms a strongly undulated two-dimensional framework parallel to the crystallographic bc plane, whereas the former ligand, L1, is utilized in the formation of one-dimensional helical chains in the [010] direction. The two sublattices of L1 and L2 interweave at the Ca(2+) ions to form a three-dimensional framework. In addition, multiple O-H···O and N-H···O hydrogen bonds stabilize the three-dimensional coordination network. Topologically, the three-dimensional framework can be simplified as a very unusual (2,3,5)-connected three-nodal net represented by the Schläfli symbol (4·8(2))(4·8(8)·10)(8).