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)(35DMP)CuX4, are new examples of spin-ladders based on nonbonded halide...halide interactions between CuX4(2-) anions. In these structures, double rows of the CuX4(2-) 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 A in length, while those that define the rails are 3.983 A. 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 approximately 5.5 K, and fitting the data to the spin 1/2 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 chi 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.