Structure and Magnetism of a Spin Ladder System: (C₅H₉NH₃)₂CuBr₄

The crystal structure of bis(cyclopentylammonium)tetrabromocuprate(II) has been determined at room temperature and at −70 °C. The room temperature structure is orthorhombic, space group Pn21a, with a = 12.092(6) Å, b = 8.134(4) Å, and c = 18.698(10) Å. The low temperature structure is also orthorhombic, space group Pna21, with a = 24.111(5) Å, b = 8.089(2) Å, and c = 18.448(4) Å. DSC studies reveal the presence of a weak endotherm at −13 °C. The structures of the two phases are very similar, differing only in the relative orientations of the cyclopentyl rings of the organic cations and slight displacements of the anionic tetrahedra. The CuBr42- anions in the low temperature phase are arranged to define a spin ladder system through Cu−Br···Br−Cu two-halide exchange pathways. Magnetic susceptibility data have been analyzed and yield antiferromagnetic exchange strengths 2Jrail/k = −11.6 K and 2Jrung/k = −5.5 K with a singlet−triplet gap energy Δ/kB = 2.3 K. This is the first report of a spin ladder with a stronger interaction along the axis of the ladder than along the rungs.