Disproportional proton tautomers of pipecolic acid and 2,6-dichloro-4-nitrophenol in a 2:3 complex

• R/S-pipecolic acid aggregates with 2,6-dichloro-4-nitrophenol at the 2:3 ratio.
• R/S-pipecolic acids as cation and zwitterion interact separately with phenol and phenolate.
• The crystalline complex was studied by X-ray, IR, Raman and NMR methods.
• In the optimized structure pipecolic acid zwitterion and 2,6-dichloro-4-nitrophenol are present.
• The FTIR and 13C CP MAS spectra confirm the crystal structure.

A racemic mixture of (R/S)-pipecolic acid (piperidine-2-carboxylic acid, P2C) forms a stable crystalline aggregate with 2,6-dichloro-4-nitrophenol (DCNP) at the unusual 2:3 ratio (1). Complex 1 has been characterized by X-ray analysis, Raman, FTIR and NMR spectra, and its structure (2) has been optimized by the B3LYP/6-31G(d,p) approach. The crystals are monoclinic, P21/n space group. The piperidine rings adopt chair conformations with the equatorial positions of the carboxylic groups. A P2C zwitterion is linked to two 2,6-dichloro-4-nitrophenol molecules through two non-equivalent short COO⋯HO–C hydrogen bonds and a cation (P2C+H) is hydrogen bonded with the DCNP− anion. These P2C molecules are NH⋯O hydrogen bonded. The FTIR spectrum is in agreement with the X-ray studies. The 13C CP MAS spectrum confirms the presence of two different P2C and DCNP units in the complex. In the optimized structure of the isolate aggregate, three DCNP molecules are bonded to two pipecolic acid zwitterions. Charge delocalization has been analyzed using the Natural Bond Orbital (NBO) method.

Two molecules of pipecolic acid and three molecules of 2,6-dichloro-4-nitrophenol in co-crystal form the adduct in an unusual 2:3 ratio, in which both pipecolic acid cation, pipecolic acid zwitterion, 2,6-dichloro-4-nitrophenol and its anion are linked through OH⋯O and NH⋯O hydrogen bonds. The 13C CP MAS spectrum of the complex investigated provides the evidence that the complex is built of two subunits.Figure optionsDownload high-quality image (101 K)Download as PowerPoint slide