Ferromagnetic exchange coupling in the linear, chloride-bridged cluster (cyclam)CoII[(μ-Cl)UIV(Me2Pz)4]2

The trinuclear cluster (cyclam)Co[(μ-Cl)U(Me2Pz)4]2 (cyclam = 1,4,8,11-tetraazacyclotetradecane, Me2Pz− = 3,5-dimethylpyrazolate) is synthesized through cleavage of the homoleptic dimer [U(Me2Pz)4]2 by (cyclam)CoCl2. A single crystal X-ray diffraction analysis reveals a linear chloride-bridged structure analogous to that previously reported for (cyclam)M[(μ-Cl)U(Me2Pz)4]2 (M = Ni, Cu, Zn). The magnetic exchange coupling of the CoU2 cluster was probed by analyzing the temperature dependence of its magnetic susceptibility. Comparison of χMT versus T between the CoU2 species and the diamagnetic ZnU2 cluster demonstrates the presence of ferromagnetic coupling between the CoII and UIV centers. We present methods for estimating upper and lower bounds for the exchange interaction energy in such systems and find that for CoU2, the exchange constant, J, lies in the range 15–48 cm−1. Application of these methods to the previously reported NiU2 cluster suggests somewhat weaker ferromagnetic exchange, with J lying in the range 2.8–19 cm−1. AC magnetic susceptibility experiments were not indicative of single-molecule magnet behavior for the CoU2 cluster, although qualitative interpretation of the low-temperature magnetization data suggests the presence of significant zero-field splitting in the ground state.