Design, synthesis and noncentrosymmetric solid state organization of three novel pyridylphosphonic acids

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 π⋯π or CO⋯π 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 (PO and CO), 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 PO 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.