Crystal-structure, magnetic-susceptibility, and EPR studies of bis(piperidinium)tetrabromocuprate(II): A novel monomer system showing spin diffusion

The crystal structure of the compound, bis(piperidinium)tetrabromocuprate(II) [(pipdH${)}_2$ ${\mathrm{CuBr}}_4$] is monoclinic, space group P$2_1$/c, with a=8.487(2) Å, b=17.225(3) Å, c=12.380(2) Å, and β=99.29(2)°. The structure consists of isolated, flattened ${\mathrm{CuBr}}_4^{2\mathrm{-}}$ tetrahedra. Close Br...Br contacts (4.10 Å) occur between tetrahedra related by a center of inversion, defining magnetic dimeric units. Additional Br...Br contacts (4.30 Å) occur between dimeric units related by unit-cell translations in the a direction. This defines a double chain of dimers in which ${\mathrm{CuBr}}_4^{2\mathrm{-}}$ ions are all magnetically equivalent. The crystal symmetry generates a second type of crystallographically equivalent, but magnetically inequivalent, chain in the structure. Magnetic-susceptibility data indicate the presence of dominant antiferromagnetic coupling.