Synthesis and Magnetic Behavior of Uranium-Transition Metal Clusters

Expanding the study of single molecule magnetic behavior to actinides offers a number of intriguing prospects. The actinides tend to exhibit near-degenerate electronic states which lead to a maximizing of the molecular spin state. Additionally, the free ion anisotropy of block metals is typically more pronounced than that of the d block metals. Despite its potential, this field remains relatively unexplored compared to the field of transition metal magnetism.

Recent work in our group has produced the homoleptic dimer complex [U(Me₂Pz)₄]₂ (Me₂Pz^- = 3,5-dimethylpyrazolate). This dimer is readily cleaved via insertion by trans dichloride complexes of the form (cyclam)MCl₂ (M = Co, Ni, Cu, Zn; cyclam = 1,4,8,11-tetraazacyclododecane) to generate a series of trinuclear chloride-bridged uranium-transition metal assemblies. Comparisons of the crystal structures reveal a close congruence between the coordination geometries of the U^IV centers, suggesting that differences in magnetic behavior from that of the species containing Zn^II (S = 0) should reveal the nature of the magnetic exchange coupling between U^IV and Cu^II (S = ¹/₂), Ni^II (S = 1), or Co^II (S = ³/₂). Indeed, magnetic measurements have demonstrated the presence of a ferromagnetic exchange interaction between U^IV and Ni^II. Work is currently underway to better understand the nature of magnetic exchange coupling in these systems. In addition, we are attempting to synthesize analogous trinuclear species bridged by bromide and iodide, which should serve to increase coupling strength and anisotropy. Furthermore, we are interested in appending U(Me₂Pz)₄ units to tripodal complexes of the form L₃MCl₃ to generate higher-nuclearity clusters.

Due to the extreme air sensitivity of low-valent uranium compounds, their syntheses and manipulation is restricted to the air-free conditions of a Schlenk line and glove box. Typical structural characterization involves single crystal X-ray diffraction, NMR, and mass spectrometry. Magnetic measurements are taken on the Long Group's Superconducting Quantum Interference Device (SQUID). Also, Density Functional Theory (DFT) calculations are employed to study molecular orbital interactions and their ramifications on magnetic exchange pathways.