Uranium and Lanthanide Complexes with the 2-Mercapto Benzothiazolate Ligand: Evidence for a Specific Covalent Binding Site in the Differentiation of Isostructural Lanthanide(III) and Actinide(III) Compounds

Treatment of [U(Cp*)2Cl2] with KSBT in THF gave [U(Cp*)2(SBT)2], which exhibits the usual bent sandwich configuration in the solid state with the two SBT ligands adopting the bidentate ligation mode. The monocyclopentadienyl compound [U(Cp*)(SBT)3] was synthesized by reaction of [U(Cp*)(BH4)3] with KSBT in THF, and its reduction with potassium amalgam in the presence of 18-crown-6 afforded the corresponding anionic complex [K(18-crown-6)(THF)2][U(Cp*)(SBT)3]. The lanthanide analogues [K(THF)2Ln(Cp*)(SBT)3] were obtained by treating [Ln(BH4)3(THF)3] with KSBT and KCp*; isomorphous crystals of [K(15-crown-5)2][Ln(Cp*)(SBT)3]·THF [Ln = La, Ce, Nd] were formed upon addition of 15-crown-5. Comparison of the crystal structures of the pentagonal bipyramidal complexes [M(Cp*)(SBT)3]− reveals that the M−Nax distances are shorter than the M−Neq distances, whatever the metal, the phenomenon being enhanced in the U(III) compound versus the Ln(III) analogues. The structural data obtained by relativistic density functional theory (DFT) calculations reproduce experimental trends. Electronic population and molecular orbital analyses show that the structural differences in the series of [M(Cp*)(SBT)3]− anions are related to the uranium 5f orbital−ligand mixing, which is greater than the lanthanide 4f orbital−ligand mixing. Moreover, the consideration of the corresponding bond orders and the analysis of the bonding energy bring to light a strong and specific interaction between the uranium and apical nitrogen atoms.