Layered basic copper anion exchangers: chemical characterisation and X-ray absorption study

Abstract

The local structure around copper in Cu₂(OH)₃OAc·H₂O, and in the anhydrous iodide analogue prepared by ion exchange, has been determined by X-ray absorption spectroscopy at 77 K. For both compounds, the copper environment is compatible with a botallackite-type Cu₂(OH)₃Br arrangement, in which two crystallographically distinct copper atoms lie in 4 + 2 (oxygen + X) and 4 + 1 + 1 (oxygen + oxygen + X) environments (X = exchangeable anion OAc, I, Br etc.). Interatomic distances between copper and the exchangeable anion have been determined to be 2.78 and 3.15 Å in Cu₂(OH)₃OAc·H₂O and Cu₂(OH)₃I, respectively, the latter representing an unusual example of a direct Cu^II—I bond. At least two Cu—Cu distances could be identified in the radial distribution function: r(CuCu)= 3.16 and 6.32 Å, signals from second and more distant neighbours being reinforced by a focussing effect along the linear chain of copper atoms in the layers.