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 UCl4 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)MCl2 (M = Ni, Cu, Zn; cyclam = 1,4,8,11-tetraazacyclotetradecane) in dichloromethane generate the linear, chloride-bridged clusters (cyclam)M[((sic)Cl-L)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 UIV complexes, with chi T-M decreasing steadily as the temperature drops. Data for the CuU2 cluster show a parallel variance with temperature, indicating the absence of any magnetic exchange coupling. In contrast, subtracting the ZnU2 data from the NiU2 data exposes a rise in chi T-M 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 delta symmetry with respect to the U-Cl bond. Low-temperature magnetization data collected for NiU2 suggest the presence of a large axial zero-field splitting; however, ac magnetic susceptibility experiments gave no indication of single-molecule magnet behavior.