Two halide exchange in copper(II) halide dimers: (4,4′-bipyridinium)Cu 2Cl 6 − xBr x

Abstract

The structural and magnetic properties of the series of (4,4′-bipyridinium)Cu 2Cl 6 − x Br x salts are investigated. All of the compounds are isomorphous, triclinic, space group P 1 ¯ . Structurally, the compounds contain pseudo-planar Cu 2 X 6 2 - dimeric species that form stacks due to weak coordination of adjacent dimer anions via semi-coordinate Cu⋯X bonds. Hydrogen bonding from the 4,4′-bipyridinium dications tie the stacks together into a two-dimensional structural framework. Secondary C–H⋯X hydrogen bonds force short X⋯X contacts between the dimers on adjacent layers, with X⋯X distances of 3.645–3.690 Å. In this manner, magnetic chains are expected to exist with alternating exchange pathways: the intradimer exchange coupling and the X⋯X pathway. An additional pathway exists within stacks of dimers via the semi-coordinate Cu⋯X bonds, although this pathway is expected to be relatively weak and is ignored for simplicity. Magnetic studies indicate that these systems behave as alternating AFM chains with very strong coupling, demonstrating that the halide⋯halide contacts are effective exchange pathways. The AFM coupling is observed to increase monotonically as the bromide content increases. The values for the Cu 2 Cl 6 2 - salt are J 1/ k = −57.2(6) K and J 2/ k = −27(4) K while those of the Cu 2 Br 6 2 - salt are −162(2) and −93(5) K, respectively. Analysis of the pathways suggests that J 2 value corresponds to the intradimer coupling while the J 1 value is assigned to the two-halide pathway.