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

The structural and magnetic properties of the planar Cu2Cl6-xBrx2- dinners in the (morpholinium)(2)Cu2Cl6-xBrx salts are investigated. The Cu2Cl62- salt is triclinic, space group P (1) over bar, a = 7.408(7), b = 10.458(10), c = 11.933(12) angstrom, a = 82.26(2), 88.35(2), and gamma = 77.22(2)degrees with Z = 2, while the Cu2Br62- salt is monoclinic, space group P2(1/c), a = 14.148(6), b = 10.814(4), c = 14.298(6) angstrom and beta = 117.108(7)degrees 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 center dot center dot center dot O bonds. This is in contrast to other such dimer systems, where semi-coordinate Cu center dot center dot center dot X bonds link the dinners together into stacks or chains. In the morpholinium salts, hydrogen bonding ties the molecular units into a layered structure. These N-H center dot center dot center dot X hydrogen bonds force short X center dot center dot center dot X contacts between dimers with X center dot center dot center dot 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 center dot center dot center dot X (2H) pathway. Magnetic studies indicate that these systems behave as alternating AFM chains, demonstrating that the halide center dot center dot center dot 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 Cu2Cl62- salt are J(1/k) = -41.6(1) K and J(2/k) = -26(2) K, while those of the Cu2Br62- salt are -149.5(5) and -59(2) K. Comparison with other systems suggests that J(1) value for the chloride salt corresponds to the intradimer coupling while the J(2) value is assigned to the two-halide pathway, while the assignments are reversed for the bromide containing salts. (c) 2005 Elsevier Ltd. All rights reserved.