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

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, P 2 1 / 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 13 C 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. [ABSTRACT FROM AUTHOR]