Research article - Peer-reviewed, 2021
Short Strong Hydrogen Bonds can Hinder Complex Formation: A Stability and Structure Study of Copper(II) Alkyl-N-iminodiacetic Acid Complexes in Aqueous Systems and Solid State
Haggman, Leif; Cassel, Anders; Persson, IngmarAbstract
In the present study the solution and coordination chemistry of copper(II)-alkyl-N-iminodiacetate systems are studied in aqueous solution by potentiometry, using ion selective copper and pH electrodes, EXAFS (extended X-ray absorption fine structure) and dye probe molecular absorption spectrophotometry. Alkyl-N-iminodiacetates with varying alkyl chain length, methyl (CH3-), n-hexyl (C6H13-), n-dodecyl (C12H25-) and n-octadecyl (C18H37-) were used to tune the amphiphilic properties of the ligands. The polar head groups have both oxygen (hard Lewis base) and nitrogen donor (border-line Lewis base) atoms. This means that metal ions with different bonding characteristics may bind these ligands differently. Furthermore, the chelating properties of the polar head group may be regulated by pH as the acid-base properties of the imine and carboxylic acid groups are different. Copper(II) forms two stable complexes with alkyl-N-iminodiacetates with short alkyl chains, present as monomers in aqueous solution, log(10)beta(1) = 11.10(2), log(10)beta(2) = 19.5(2) for methyl-N-iminodiacetate, and log(10)beta(1) = 12.22(4), log(10)beta(2) = 21.9(2) for n-hexyl-N-iminodiacetate. n-Octadecyl-N-iminodiacetic acid, present as large aggregates in acidic aqueous solution, has short strong hydrogen bonds between carboxylic acid and carboxylate groups in the surface of the aggregates, which hinder complex formation at pH values below 4, obstructs it in the pH region 4-7, while the complex formation behaves as for short-chained alkyl-N-iminodiacetates at pH > 7. The structure around copper in copper(II)-alkyl-N-iminodiacetate complexes in aqueous solution and solid state formed at different pH values and copper(II):alkyl-N-iminodiacetate ratios has been determined by EXAFS. The coordination chemistry of copper(II) shows four strong bonds in the equatorial plane, and two different Cu-O/N bond distances, ca. 0.2 angstrom apart, in the axial positions of a non-centrosymmetric tetragonally elongated octahedron.Keywords
Alkyl-N-iminodiacetate; Copper(II); Complex formation; StructurePublished in
Journal of Solution Chemistry2021, volume: 50, number: 2, pages: 203-219
Publisher: SPRINGER/PLENUM PUBLISHERS
Authors' information
Häggman, Leif
Swedish University of Agricultural Sciences, Department of Molecular Sciences
Cassel, Anders
Nouryon Surface Chemistry AB
Swedish University of Agricultural Sciences, Department of Molecular Sciences
UKÄ Subject classification
Physical Chemistry
Publication Identifiers
DOI: https://doi.org/10.1007/s10953-020-01050-7
URI (permanent link to this page)
https://res.slu.se/id/publ/111020