A Macrocyclic Chelator That Selectively Binds Ln⁴⁺ over Ln³⁺ by a Factor of 10²⁹

A tetravalent cerium macrocyclic complex (CeLK₄) was prepared with an octadentate terephthalamide ligand comprised of hard catecholate donors and characterized in the solution state by spectrophotometric titrations and electrochemistry and in the crystal by X-ray diffraction. The solution-state studies showed that L exhibits a remarkably high affinity toward Ce⁴⁺, with log β₁₁₀ = 61(2) and ΔG = -348 kJ/mol, compared with log β₁₁₀ = 32.02(2) for the analogous Pr³⁺ complex. In addition, L exhibits an unusual preference for forming CeL^4- relative to formation of the analogous actinide complex, ThL^4-, which has β₁₁₀ = 53.7(5). The extreme stabilization of tetravalent cerium relative to its trivalent state is also evidenced by the shift of 1.91 V in the redox potential of the Ce³⁺/Ce⁴⁺ couple of the complex (measured at -0.454 V vs SHE). The unprecedented behavior prompted an electronic structure analysis using L₃- and M_5,4-edge X-ray absorption near-edge structure (XANES) spectroscopies and configuration interaction calculations, which showed that 4f-orbital bonding in CeLK₄ has partial covalent character due to ligand-to-metal charge transfer (LMCT) in the ground state. The experimental results are presented in the context of earlier measurements on tetravalent cerium compounds, indicating that the amount of LMCT for CeLK₄ is similar to that observed for [Et₄N]₂[CeCl₆] and CeO₂ and significantly less than that for the organometallic sandwich compound cerocene, (C₈H₈)₂Ce. A simple model to rationalize changes in 4f orbital bonding for tri- and tetravalent lanthanide and actinide compounds is also provided.