Synthetic and Structural Studies of a Linear Bis-Catechol Amide, N,N′-Bis(2,3-dihydroxybenzoyl)-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)-l,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 Å). 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, Cs2[5-LICAM-Ni] 3DMF, 3, (Ni-O, 1.87 Å, O-Ni-O, 87.9–92.5°). It is also interesting to note that one of the cesium atoms was cis-bonded to the catechol oxygens in each ring (02---Csl, O6-Csl = 3.23 Å, 3.03 Å) and to the carbonyl oxygen of one of the three DMF solvent molecules (Cs1-O=C, 3.13 Å), while the other cesium atom was bonded to all three carbonyl oxygens of the DMF molecules in the structure (Cs2- 07, 08 = 3.13, 3.12 Å). A Co²⁺ analogue of 3, Cs2[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, 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. © 1995, American Chemical Society. All rights reserved.

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Author Katherine J. Franz Chemistry