Magnetic Exchange Coupling in Chloride-Bridged 5f−3d Heterometallic Complexes Generated via Insertion into a Uranium(IV) Dimethylpyrazolate Dimer

The homoleptic dimer complex [U(Me 2 Pz) 4 ] 2 (Me 2 Pz− = 3,5-dimethylpyrazolate) was obtained upon reacting UCl 4 with KMe 2 Pz in THF, followed by extraction into toluene. The structure of the dimer consists of two U IV centers, each coordinated in a pseudo trigonal bipyramidal geometry, connected through two bridging Me 2 Pz - ligands. Bases are capable of cleaving the dimer; for example, reaction with THF affords the mononuclear complex (Me 2 Pz) 4 U(THF). More importantly, the dimer can be cleaved via insertion of terminal chloride ligands, such that reactions with (cyclam)MCl 2 (M = Ni, Cu, Zn; cyclam = 1,4,8,11-tetraazacyclotetradecane) in dichloromethane generate the linear, chloride-bridged clusters (cyclam)M[(μ-Cl)U(Me 2 Pz) 4 ] 2 . Variable-temperature magnetic susceptibility data were collected for all three clusters to probe any possible magnetic exchange coupling. The data for the cluster centered by an S = 0 Zn II ion exhibit behavior typical of U IV complexes, with χ M T decreasing steadily as the temperature drops. Data for the CuU 2 cluster show a parallel variance with temperature, indicating the absence of any magnetic exchange coupling. In contrast, subtracting the ZnU 2 data from the NiU 2 data exposes a rise in χ M T with decreasing temperature, suggesting weak ferromagnetic coupling between the Ni II ( S = 1) and U IV centers. Employing a simple spin-only exchange model, a lower bound for the coupling constant was estimated at J = 2.3 cm -1 . Consistent with a simple superexchange mechanism for the coupling, density functional theory calculations performed on a [(Me 2 Pz) 4 UCl] - fragment of the cluster show the spin to reside in 5f xyz and 5f z ( x 2 - y 2 ) orbitals, exhibiting δ symmetry with respect to the U−Cl bond. Low-temperature magnetization data collected for NiU 2 suggest the presence of a large axial zero-field splitting; however, ac magnetic susceptibility experiments gave no indication of single-molecule magnet behavior.