Structural, vibrational and DFT studies of di-(pipecolinium acid) squarate

Abstract

A racemic mixture of (R)- and (S)-pipecolic acid (piperidine-2-carboxylic acid, P2C) forms a stable crystalline complex with squaric acid (3,4-dihydroxy-3-cyclobutene-1,2-dione, H2SQ), at the 2:1 ratio (1). The crystals are built of two pipecolic acid cations (P2C+H) and squarate dianion (SQ2−). Complex 1 has been characterized by single-crystal X-ray analysis, Raman, FTIR and 13C CP MAS spectroscopy. The title complex crystallizes in monoclinic C2/c space group with Z=4 and Z’=0.5. In the crystal structure two types of C2-symmetric complexes with different chirality are identified as [(R)-P2C+H]2SQ2− and [(S)-P2C+H]2SQ2−. The molecules are linked through the COOH⋯O–C of 2.587(1) Å and two N+H⋯O–C hydrogen bonds of 2.756(1) and 2.759(1) Å. In the optimized structure, at the B3LYP/6–311++G(d,p) level of theory, pipecolic acid zwitterion and cation interact with the hydrogen squarate anion. Charge delocalization has been analysed using the Natural Bond Orbital (NBO) method. Solid-state Raman, FTIR and 13C CP MAS spectra have been measured. The potential energy distributions (PED) have been used to assign the vibrational spectra. The crystal and optimized structures have been compared.