Structure of the 2:1 complex of 1-piperidineacetic acid and p-hydroxybenzoic acid studied by X-ray, FTIR, 1H, 13C NMR, and DFT methods

The crystals of the 2:1 complex of piperidineacetic acid with p-hydroxybenzoic acid, (PAA)2·HBA, are monoclinic, space group C2 and Z = 4, a = 29.666(4), b = 6.1208(10), c = 14.200(2) Å, β = 117.755(16)°, R = 0.035. The crystals of the complex contain molecules of three types: 1-piperidiniumacetate (zwitterion, ZPAA), protonated piperidineacetic acid (HPAA), and p-hydroxybenzoate anion (AHBA). Two piperidiniumacetate moieties, ZPAA, and HPAA, form a non-symmetric cyclic dimer through two N+H···OC hydrogen bonds of the lengths of 2.773(3) and 2.820(3) Å. The COOH group of the cation HPAA is engaged in the OH···O hydrogen bond of 2.519(2) Å with the carboxylate group of p-hydroxybenzoate anion (AHBA), while the COO− group of the zwitterion ZPAA interacts with the OH group of p-hydroxybenzoate anion (AHBA) of the neighboring complex molecule through the O···HO hydrogen bond of 2.589(2) Å, which links molecules into infinite chains. The structures of two complexes, denoted as A and B, have been optimized by the B3LYP/6-31G(d,p) method. In both complexes two molecules of ZPAA form a cyclic dimer. In A one molecule of ZPAA interacts with the carboxylic group of HBA via the O···HO hydrogen bond of 2.582 Å, without the proton transfer, while in B with the phenolic group of HBA via the O···HO hydrogen bond of 2.629 Å. Complex B is more stable than A by 1.55 kcal/mol. The FTIR spectrum shows a broad band in the 3120–2070 cm−1 region assigned to the νNH vibrations and a broad absorption in the 1750–800 cm−1 region corresponding to the short OH···O hydrogen bonds. Three bands: one attributed to the νCO and two ones to the νasCOO vibrations at 1675, 1634, and 1606 cm−1, respectively, are observed in the second-derivative spectrum, which is in agreement with the X-ray results. The 1H and 13C NMR spectra have been analyzed to elucidate the complex structure in solution.