Hydrogen bonding. Part 69. Inter- and intramolecular hydrogen bonding effects on the structure, solubility, and reactivity of 4,5-dicarboxyimidazoles

1-Methyl-3,5-dicarboxyimidazole (H2MDCI) and 4,5-dicarboxyimidazole (H2DCI) are both zwitterionic, with one acidic hydrogen on a ring nitrogen and one in an O-H-O hydrogen bond between carboxylate groups. H2MDCI is relatively soluble in H2O, and further diacid is taken into saturated aqueous solutio n in the presence of F- with removal of O-H-O hydrogen and formation of HMD CI- and HF2-. In contrast, H2DCI is almost completely insoluble in H2O and, unlike other similar dicarboxylic acids we have studied, is completely una ffected by added F-. We have used titrametric analysis, molecular modeling, infrared spectra, molecular orbital calculations and X-ray crystallography to elucidate the properties of H2DCI and H2MDCI. The crystal structure of H2MDCI was solved in the P2(1)/e space group with Z = 4, a = 4.6402(5), b = 15.070(2), c = 9.6786(11), beta = 94.928(2)degrees and V = 674.32(13) Angs trom(3).. The structure shows two hydrogen bonds, one intramolecular and on e intermolecular per molecule. The reduced solubility of H2DCI arises from facile formation of four N-H ... O intermolecular hydrogen bonds by each mo lecule (2 donor and 2 acceptor). The failure of H2DCI to react with F- is a scribed to lowered acidity of H2DCI, relative to that of structurally quite similar H2MDCI, which in turn results, in part, from the presence of a sym metric O-H-O hydrogen bond in the C-2v H2DCI zwitterion, which is stronger than the unsymmetrical O-H-O bond in the C-s zwitterion of H2MDCI. (C) 1999 Elsevier Science B.V. All rights reserved.