Proton-transfer salts of diphenylphosphinic acid with substituted 2-aminopyridine: crystal structure, spectroscopic and DFT studies.

Three proton-transfer salts of diphenylphosphinic acid (DPPA) with 2-amino-5-(X)-pyridine (AMPY, X = Cl, CN or CH), namely, 2-amino-5-chloropyridinium diphenylphosphinate, CHClN·CHOP (1, X = Cl), 2-amino-5-cyanopyridinium diphenylphosphinate, CHN·CHOP (2, X = CN), and 2-amino-5-methylpyridinium diphenylphosphinate, CHN·CHOP (3, X = CH), have been synthesized and characterized by FT-IR and H NMR spectroscopy, and X-ray crystallography. The crystal structures of compounds 1-3 were determined in the space group P\overline{1} for 1 and 2, and C2/c for 3. All three compounds contain N-H...O hydrogen-bonding interactions due to proton transfer from the O=P-OH group of DPPA as donor to the pyridine N atom of AMPY as acceptor. The proton transfer of compounds 1-3 was also verified by H NMR and FT-IR spectroscopy. The stoichiometry of all three proton-transfer salts was determined to be 1:1 and the Benesi-Hildebrand equation was applied to determine the formation constant (K) and the molar extinction coefficient (ϵ) in each case. Theoretical density functional theory (DFT) calculations were performed to investigate the optimized geometries, the molecular electrostatic potentials (MEP) and the highest occupied molecular orbitals (HOMO) and lowest unoccupied molecular orbitals (LUMO) of all three proton-transfer salts. The results showed good agreement between the experimental data and the DFT computational analysis.