Syntheses and structures of Group 1 mellitate compounds, K6[C6(COO)6]•8H2O and Cs5[C6(COO)6H]•7H2O

Two Group 1 complexes of mellitic acid, K-6[C-6(COO)(6)]. 8H(2)O (1) and Cs-5[C-6(COO)(6)H]. 7H(2)O (2), have been synthesized and characterized by X-ray crystallography. Potassium mellitate (1) crystallizes in the orthorhombic space group Pbca, with unit cell dimensions of a = 18.091(4), b = 12.634(3), c = 22.151(4) Angstrom, beta = 90 degrees, V = 5062.9(19) Angstrom(3) and Z = 8. In contrast, cesium mellitate (2) crystallizes in the monoclinic space group P2(1)/c, with unit cell dimensions of a = 13.836(3), b = 20.818(4), c = 9.539(2) Angstrom, beta = 103.52(3)degrees V = 2671.5(10) Angstrom(3) and Z = 4. Both structures are comprised of a 3D network of mellitate ion stacks, metal ions and water molecules which are linked by hydrogen bonds. In the case of potassium mellitate (1), two K+ ions (K(1) and K(4)) are located between the mellitate ions in the stacks. The remaining K+ ions occupy positions between the mellitate stacks. In cesium mellitate (2), the Cs+ ions are all located between the mellitate stacks. Two new coordination modes of the carboxylate groups are reported. Potassium mellitate (1) exhibits a tridentate binding mode in which two potassium ions are coordinated to three carboxylates on one mellitate ion. Cesium mellitate displays a similar mode but with an additional cesium ion coordinated to two of the three carboxylate groups. These new modes are attributed to the larger number of cations in these compounds as compared with mellitate compounds containing +2 and +3 cations. (C) 1999 Elsevier Science S.A. All rights reserved.