Hydrogen bonding. Part 71. Hydrogen bonding effects on the spectra, solubility, and F − reactivity of 2,3- and 3,4-dicarboxypyridines: identification of a bifurcated hydrogen bond in 2,3-dicarboxypyridine

Abstract

2,3-Dicarboxypyridine (2,3-H 2DCP) and 3,4-dicarboxypyridine (3,4-H 2DCP) are both zwitterionic in the solid state, with one acid hydrogen on the ring nitrogen. 2,3-H 2DCP has an intramolecular O–H–O bond between carboxylate groups, while 3,4-H 2DCP has intermolecular O–H–O bonds. 2,3-H 2DCP is five times as soluble in H 2O as 3,4-H 2DCP. The calculated heat of solution of 3,4-H 2DCP is higher than that of 2,3-H 2DCP, primarily due to its greater dipole moment; thus the relative insolubility of 3,4-H 2DCP results from stronger intermolecular forces in the crystal; 3,4-H 2DCP forms four intermolecular hydrogen bonds per molecule, compared to two such bonds in 2,3-H 2DCP. The N–H⋯O intermolecular hydrogen bonds in solid 2,3-H 2DCP have an unusual geometry, and we conclude from analysis of the known X-ray data and ab initio molecular orbital calculations that the N–H proton is actually engaged in a bifurcated hydrogen bond with a carboxylate group on a second molecule and with the adjacent carboxylate group on the same molecule. Semi-empirical molecular orbital calculations suggest that both the diacids and their monoanions should exist in zwitterionic form in aqueous solution with the acid hydrogen on nitrogen; however, both 2,3-NaHDCP and 3,4-NaHDCP have their acid hydrogens in O–H–O hydrogen bonds in the solid state. Both diacids are taken into aqueous solution in the presence of an added fluoride ion. Dissolution of 2,3-H 2DCP requires 2.5 F − per diacid molecule, whereas dissolution of 3,4-H 2DCP requires 3.7 F − per diacid molecule.