Two halide exchange in copper(II) halide dimers: (4,4′-bipyridinium)Cu2Cl6 − xBrx

The structural and magnetic properties of the series of (4,4′-bipyridinium)Cu2Cl6 − xBrx salts are investigated. All of the compounds are isomorphous, triclinic, space group P1¯. Structurally, the compounds contain pseudo-planar Cu2X62- 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 Cu2Cl62- salt are J1/k = −57.2(6) K and J2/k = −27(4) K while those of the Cu2Br62- salt are −162(2) and −93(5) K, respectively. Analysis of the pathways suggests that J2 value corresponds to the intradimer coupling while the J1 value is assigned to the two-halide pathway.

Graphical abstractThe presence of N–H⋯X and C–H⋯X hydrogen bonding interactions between the 4,4′-bipyridinium dications and Cu2X62- dimers force short X⋯X contacts between the dimers with X⋯X distances of 3.645–3.690 Å. In this manner, alternating AFM magnetic chains are formed with alternating exchange pathways: the intradimer exchange coupling, J2, and the X⋯X pathway, J1. Magnetic studies indicate that these systems behave as chains with quite strong coupling, with |J1| > |J2|. The AFM coupling increases monotonically as the bromide content increases.Figure optionsDownload full-size imageDownload as PowerPoint slide