Tetranuclear uranium clusters by reductive cleavage of 3,5-dimethylpyrazolate.

Actinide metal centers are of fundamental interest as they have electronic structures that are distinct from those of transition metals and lanthanides. Indeed, the combination of increased spin–orbit coupling relative to transition metals and enhanced ligand field effects relative to lanthanides renders modeling the magnetic behavior of actinide-containing molecules particularly challenging. Such factors have a related impact on the chemical reactivity of actinide elements, which has prompted a revival of interest in the coordination chemistry and small-molecule reactivity of uranium. Among the intriguing reactivity patterns uncovered of late are transformations for which transition metal and lanthanide analogues had not previously been identified, as well as some transformations common for transition metal or lanthanide systems, but previously thought to be impossible for actinides. Our recent investigation of the reactivity of tetravalent uranium with 3,5-dimethylpyrazolate (Me2Pz ) led to formation of an unprecedented homoleptic dimer, [{U(Me2Pz)4}2]. [3] Exposure of the dimer to traditional Lewis bases resulted in cleavage to afford mononuclear complexes, as observed for example in its reaction with THF to generate [(Me2Pz)4U(thf)]. Significantly, this reactivity was found to extend to the terminal chloride ligands of certain late transition metal complexes, enabling isolation of a series of linear chloridebridged clusters of the formula [(cyclam)M{(m-Cl)U(Me2Pz)4}2] (cyclam= 1,4,8,11-tetraazacyclotetradecane; M=Co, Ni, Cu, Zn). Moreover, the isostructural nature of this series facilitated a partial deconvolution of the magnetic susceptibility data, thus revealing the presence of ferromagnetic exchange interactions in the Coand Nicentered clusters. In an effort to increase the strength of the exchange coupling within such species, we undertook investigations intended to produce analogues incorporating uranium(III) in place of uranium(IV). Herein, we report the unanticipated ability of uranium(III) to reductively cleave Me2Pz , leading to a remarkable series of tetranuclear clusters incorporating the new 4-ketimidopent-2-ene-2imido (kipi ) ligand [Eq. (1)].