Structural, spectroscopic and computational studies of the 2:1 complex of nipecotic acid with squaric acid

Abstract The 2:1 centrosymmetric complex of protonated ( R ) and ( S )-nipecotic acids (piperidinium-3-carboxylic acid) with the squarate dianion (3,4-dihydroxy-3-cyclobuten-1,2-dione) ( 1 ), has been synthesized and characterized by single-crystal X-ray analysis, FTIR and NMR spectroscopy. Crystals are monoclinic, space group C 2/ c , Z ′ = 0.5. The squarate dianion lays at the center of symmetry. It is surrounded by six stereoisomers of ( R ) and ( S )-nipecotinium acid cations. The molecules are linked by N–H⋯O and COOH⋯O hydrogen bonds of 2.759(2) and 2.619(2) A, respectively. The piperidine ring adopts a chair conformation with the carboxylic group in equatorial position. The FTIR spectrum shows a broad and intense absorption in the 3400–2000 cm −1 region, typical of medium–strong N–H⋯O and O–H⋯O hydrogen bonds. The 1 H and 13 C NMR spectra of the investigated complex confirm the structure of 1 in aqueous solution. Two models of the isolated 2:1 complexes ( 2 and 3 ) have been optimized at B3LYP/6-311G(d, p) level of theory to predict the more energetically stable structure. In 2 the molecules are linked by the O–H⋯N and O–H⋯O hydrogen bonds, while in 3 by the N–H⋯O and O–H⋯N hydrogen bonds.