Change in electronic structure in a six-coordinate copper(II) complex accompanied by an anion order/disorder transition

Abstract

A variable-temperature crystallographic study of [Cu(L OH)2][ClO4]2·2(CH3)2CO [L OH = 2,6-bis(hydroxyiminomethyl)pyridine] between 30 and 300 K is presented. The complex exhibits an unusual electronic structure at room temperature with a {d_{z^2}}1 ground state, corresponding to an axially compressed ligand coordination geometry about the copper ion. This reflects a suppression of the pseudo-Jahn–Teller distortion that is normally shown by copper(II) compounds with this ligand geometry [Halcrow et al. (2004). New J. Chem. 28, 228–233]. On cooling the compound undergoes an abrupt structural change at 157 ± 3 K, that does not involve a change in the space group (P\bar 1), but causes significant changes to c and the unit-cell angles. This reflects a conformational rearrangement of the complex dication, towards a more typical pseudo-Jahn–Teller elongated coordination geometry. This occurs concurrently with a crystallographic ordering of one of the two perchlorate anions, and a significant displacement of the two lattice acetone molecules. The transformation involves displacements of up to 0.5 Å in the non-H atoms of the structure at 30 K, compared with their positions at 300 K. The change in coordination geometry of the complex around 157 K is reflected in a small reduction in its magnetic moment near that temperature.