Strong Rail Spin 1/2 Antiferromagnetic Ladder Systems: (Dimethylammonium)(3,5-Dimethylpyridinium)CuX4, X = Cl, Br.

The mixed cation salts, (dimethylammonium)(3,5-dimethylpyridinium)CuX4 (X = Cl, Br), henceforth (DMA)- (35DM P)CuX4, are new examples of spin-ladders based on nonbonded halide…halide interactions between CuX42- anions. In these structures, double rows of the CuX42- anions are sheathed by the 35DMP+ cations, while the edges are capped by the DMA+ cations. For the Br salt, the Br…Br contacts that define the rungs of the ladder are 4.017 Å in length, while those that define the rails are 3.983 Å. For the Cl salt, the corresponding lengths are 3.967 and 4.045 A. The susceptibility data for the Br salt exhibits a maximum at ~5.5 K, and fitting the data to the spin ½ antiferromagnetic ladder model yields 2J(rail)/k = -7.95 K and 2J(rung)/k = -4.07 K. The exchange coupling is much weaker in the Cl salt, no maximum in χ is observed down to 1.8 K, and the corresponding exchange constants are -1.59 and -1.25 K, respectively. An analysis is made of the structural factors involved in the J(rung) pathway. [ABSTRACT FROM AUTHOR]