Molecular structures and hydrogen bonding in the 1 : 1 and 1 : 2 complexes of pyridine betaine with 2,6-dichloro-4-nitrophenol; an example of strongly coupled hydrogen bonds, OH⋯OCOH⋯O−

Abstract

Crystalline complexes of pyridine betaine (PBET) with one and two molecules of 2,6-dichloro-4-nitrophenol (DCNP) were prepared and characterized by single-crystal X-ray analysis and Fourier transform IR (FTIR), Raman, 13C CP/MAS NMR and UV spectroscopies. Both complexes are monoclinic, space group P21c, with Z = 4. In PBET·DCNP, the proton from DCNP is transferred to PBET and both residues are linked by an OH⋯O− hydrogen bond of length 2.419(3) Å and angle 165(4)°. In PBET·(DCNP)2, the COOH group is engaged in two hydrogen bonds, resulting in a short OH⋯O− bond of length 2.452(4) Å and angle 178(4)° and a long CO⋯HO bond of length 2.623(4) Å and angle 160(6)°. The 13C NMR spectra in the solid state show signals at 167.7 and 170.2 ppm due to the carboxyl group in PBET·DCNP and PBET·(DCNP)2 respectively. The powdered FTIR spectrum of PBET·DCNP shows a broad absorption with a maximum at 806 cm−1 tentatively assigned to νOH. Although in PBET·(DCNP)2 the analogous hydrogen bond is longer than in the 1 : 1 complex, the absorption is shifted to lower wavenumbers (773 cm−1). The absorption due to the longer hydrogen bond is at 2832 cm−1 and is shifted towards higher wavenumbers relative to the analogous hydrogen bond in the complex of PBET with two molecules of pentachlorophenol (PBET·(PCP)2; O⋯O = 2.582(6) and 2.611(6) Å; νH = 2725 cm−1). The observed absorption shifts suggest that, in PBET·(DCNP)2, the hydrogen bonds perturb each other. Two peaks near 300 and 420 nm in the UV spectra of the title complexes in acetonitrile confirm the presence of phenol and phenolate residues in equilibrium.