SYNTHETIC AND STRUCTURAL STUDIES OF A LINEAR BIS-CATECHOL AMIDE, N,N'-BIS(2,3-DIHYDRORRYBENZOYL)-1,7-DIAZAHEPTANE (5-LICAM), AND ITS COMPLEXES WITH NI2+ AND CO2+ - UTILIZATION OF A POLYMER-SUPPORTED, SULFONATED ANALOG, 5-LICAMS, AS A BIOMIMETIC LIGAND FOR DIVALENT METAL-ION REMOVAL FROM AQUEOUS-SOLUTION

The synthesis and structural characterization of the linear bis(catechol) amide ligand, N,N'-bis(2,3-dihydroxybenzoyl)-1 ,7-diazaheptane (5-LICAM, 1), was studied along with several metal complexes, namely, Ni2+ and Co2+. We were able to obtain a single-crystal X-ray analysis of a derivative of 1, the hemimethoxy ligand, 5-LICAM(OCH3)(2), 2, where partial demethylation of the fully methylated derivative of 1 was serendipitiously obtained in the presence of a deficiency of BBr3. The X-ray structure showed why we retained the methoxy group, during the partial demethylation reaction, in each catechol amide ring by the fact that a selective intramolecular hydrogen bond occurs between the one demethylated OH group in each catechol ring and the amide carbonyl in the adjacent position (C=O---HO, 1.67 Angstrom). Reaction of 1 with a Ni2+ salt provided the first structurally characterized square planar, linear bis-catechol amide, dianionic metal complex, with cesium as the counterion, Cs-2[5-LICAM-Ni]. 3DMF, 3, (Ni-O, 1.87 Angstrom, O-Ni-O, 87.9-92.5 degrees). It is also interesting to note that one of the cesium atoms was cis-bonded to the catechol oxygens in each ring (O2---Csl, O6-Csl=3.23 Angstrom, 3.03 Angstrom) and to the carbonyl oxygen of one of the three DMF solvent molecules (Csl-O=C, 3.13 Angstrom), while the other cesium atom was bonded to all three carbonyl oxygens of the DMF molecules in the structure (Cs2-O7, O8=3.13, 3.12 Angstrom). A Co2+ analogue of 3, Cs-2[5-LICAM-Co]. DMF, 4, provided a similar structure as ascertained by FT-IR, FAB/MS, and elemental analysis. In order to test the utilization of 1 in removing Ni2+ from aqueous solution for environmental inorganic applications, we synthesized the polymer pendant ligand version, PS-5-LICAM, bonded to modified 6% macroporous divinylbenzene-polystyrene beads (0.55 mmol/g), with an important modification of a sulfonate group on the catechol ring, PS-5-LICAMS IS, to impart hydrophilicity to the Ligand site. Indeed, we found that the PS-5-LICAMS ligand removed 0.35 mmol of Ni/g of polymer beads from aqueous solution at pH 2.5, but unfortunately, was not selective to Ni2+ in competition with other divalent metal ions. An in depth discussion of the X-ray structure of ligand 2 and that of the Ni complex, 3, will be presented.