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{CuBr}₄$] is monoclinic, space group P${2}₁/c, with a=8.487(2) A, b=17.225(3) A, c=12.380(2) A, and β=99.29(2)^∘. The structure consists of isolated, flattened{CuBr}₄^{2{{-}}}$ tetrahedra. Close Br...Br contacts (4.10 A{}) occur between tetrahedra related by a center of inversion, defining magnetic dimeric units. Additional Br...Br contacts (4.30 A{}) occur between dimeric units related by unit-cell translations in the a direction. This defines a double chain of dimers in which CuBr₄^2- 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.The system is best characterized magnetically as exchange-coupled CuBr₄^2- dimers (J/k=-6.64 K), with a weaker one-dimensional coupling (J'/k=-0.66 K) between these dimeric units. The room-temperature EPR data in the a-c* and a-b planes indicate the presence of strong spin-diffusive relaxation processes defining the system as a linear chain propagating along the a axis. This is the first copper bromide compound to our knowledge which shows spin-diffusive behavior. In the b-c* plane, the EPR spectra at various orientations are the superposition of the spectra corresponding to two magnetically inequivalent subsystems, with the magnitude of the interaction between the subsystems, Jᵢ, determined to be equal to 0.002 K. Thus, structurally this compound is a monomer, magnetically it can be characterized as a dimer, and EPR studies show that dynamically it behaves as a linear chain propagating along the crystallographic a axis.