X-ray Crystal Structure of Triaquacopper(II) Dihydrogen 1,2,4,5- Benzenetetracarboxylate Trihydrate and Raman Spectra of Cu2+, Co2+, and Fe2+ Salts of 1,2,4,5-Benzenetetracarboxylic (Pyromellitic) Acid

Abstract

Some transition metal salts (Fe2+, Co2+, and Cu2+) of 1,2,4,5-benzenetetracarboxylic acid (pyromellitic acid) have been synthesized: hexaaquairon(II) dihydrogen 1,2,4,5-benzenetetracarboxylate (I), hexaaquacobalt(II) dihydrogen 1,2,4,5-benzenetetracarboxylate (II), triaquacopper(II) dihydrogen 1,2,4,5-benzenetetracarboxylate trihydrate (III), and triaquacopper(II) 1,2,4,5-benzenetetracarboxylate tetrahydrate (IV). Single-crystal X-ray diffraction analysis of compound III has been performed. It crystallizes in the non-centrosymmetric space group Pn with two molecules per unit cell. Cu2+ and the ligand form an infinite linear chain. The metal atom is coordinated to five O atoms (three of the aqua ligand and two of the pyromellitate ligand) in a distorted trigonal-bipyramidal coordination. The carboxylic groups are involved in two medium intermolecular hydrogen bonds with water molecules. The O···O distances are 2.629(5) and 2.639(5) A. The absolute structure parameter shows that the crystal and the atomic coordinate sets have the same polarity. Raman spectra of all the compounds and deuterated analogues of Fe2+ and Co2+ salts are reported. Spectral differences have been observed in the spectra of the compounds that present short hydrogen bonds (O···O distance less than 2.5 A) in comparison to compounds that do not. In the crystal of compounds I and II, intramolecular short hydrogen bonds are present. Symmetric and asymmetric stretching vibrations of a short intramolecular hydrogen bond are observed at 307 and 874 cm−1 in salts I and II, respectively. The crystal structures of compounds III and IV do not show short hydrogen bonds, these bands are therefore not observed. These vibrational modes are tentative assignments by deuterium compounds and ab initio calculations of the dihydrogen pyromellitate ion. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)