Two halide exchange in copper(II) halide dimers: (Morpholinium)2Cu2ClxBr6−x

Abstract

Abstract The structural and magnetic properties of the planar Cu 2 Cl 6 - x Br x 2 - dimers in the (morpholinium)2Cu2Cl6 − xBrx salts are investigated. The Cu 2 Cl 6 2 - salt is triclinic, space group P 1 ¯ , a = 7.408(7), b = 10.458(10), c = 11.933(12) A, α = 82.26(2), β = 88.35(2), and γ = 77.22(2)° with Z = 2, while the Cu 2 Br 6 2 - salt is monoclinic, space group P21/c, a = 14.148(6), b = 10.814(4), c = 14.298(6) A and β = 117.108(7)° with Z = 4. The mixed halide salts are all isomorphous with the Cu 2 Br 6 2 - salt. Structurally, the compounds contain isolated dimeric species due to the weak coordination of the morpholinium cations to the copper ions via semi-coordinate Cu⋯O bonds. This is in contrast to other such dimer systems, where semi-coordinate Cu⋯X bonds link the dimers together into stacks or chains. In the morpholinium salts, hydrogen bonding ties the molecular units into a layered structure. These N–H⋯X hydrogen bonds force short X⋯X contacts between dimers with X⋯X distances of 3.92–4.07 A. In this manner, magnetic chains with alternating exchange pathways exist: the intradimer exchange coupling and the X⋯X (2H) pathway. Magnetic studies indicate that these systems behave as alternating AFM chains, demonstrating that the halide⋯halide contacts are effective exchange pathways. The magnetic studies show that the AFM coupling increases monotonically as the bromide content increases. The values for the Cu 2 Cl 6 2 - salt are J1/k = −41.6(1) K and J2/k = −26(2) K, while those of the Cu 2 Br 6 2 - salt are −149.5(5) and −59(2) K. Comparison with other systems suggests that J1 value for the chloride salt corresponds to the intradimer coupling while the J2 value is assigned to the two-halide pathway, while the assignments are reversed for the bromide containing salts.