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

Abstract

Abstract.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 towards Ce 4+ , with log β110 = 61(2) and ΔG = -348 kJ/mol, compared with log β110 = 32.02(2) for the analogous Pr 3+ 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 redox potential of the Ce 3+ /Ce 4+ couple of the complex (measured at -0.454 V vs. SHE). The unprecedented behavior prompted an electronic structure analysis using L3 and M5,4-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 owing 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 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 orbitals for tri-and tetravalent lanthanide and actinide compounds is also provided.

Synthesis and Crystallographic Characterization of the Tetravalent Actinide-DOTA Complexes [AnIV(κ8-DOTA)(DMSO)] (An = Th, U)

“…1 The average An−O carboxylate distances for 1 and 2 are 2.30 Å (range = 2.27(2) to 2.33(2) Å) and 2.36 Å (range = 2.33(2) to 2. 38(2) Å), respectively. The average An−N distances for 1 and 2 are 2.72 Å (range = 2.67(2) to 2.74(2) Å) and 2.75 Å (range = 2.73(2) to 2.78(2) Å), respectively ( Table 1) Figure S4 in the supporting information).…”

The duality of electron localization and covalency in lanthanide and actinide metallocenes

“…As will be shown below, and also described previously, 62,[82][83][84] the lack of signicant f-orbital overlap can result in a counterintuitive relationship between stability and f-orbital covalency for many lanthanide and actinide compounds. Ground state DFT calculations with the B3LYP hybrid functional were conducted on the closed shell conguration to guide assignments of the experimental spectra.…”

“…As described above, Ce(C 8 H 8 ) 2 and U(C 8 H 8 ) 2 have qualitatively similar amounts of C 2p mixing with the f-orbitals of e 2u symmetry (28% vs. 24% C 2p character, respectively), but different amounts of f-orbital overlap. Consequently, similar amounts of f-orbital mixing does not equate to similar strengths of f-orbital bonding, 83,[127][128][129][130][131][132][133][134][135][136][137] which highlights the counterintuitive relationship between covalency and stability for f-element molecules. 138 Specically, the decrease in overlap for Ce(C 8 H 8 ) 2 results in a smaller covalent contribution to the bond energy and more localized electrons.…”

“…Without a similar degree of overlap, 4f-orbital mixing in Ce(C 8 H 8 ) 2 is accompanied by an overall weakening of the complex due to a comparatively large drop in the strength of ionic bonding. 62,83,84,[127][128][129]…”

Ultra-selective ligand-driven separation of strategic actinides

“…As shown in Fig. 1, a handful of other molecules from the same family (octadentate 3,4,3-LI-CAM, Bis-TREN-Me-3,2-HOPO, and TAM-macrocycle; tetradentate 5LI-Me-3,2-HOPO, 5LIO-Me-3,2-HOPO; and bidentate PR-Me-3,2-HOPO) 17–20 exhibit known solution thermodynamic properties that match those of 343HOPO but none of the chelators typically used for separation methods is nearly as selective. Many additional HOPO and CAM ligands have been designed 15 and previously reported octadentate 1,2-HOPO 21 , mixed 3,4,3-LI(1,2-HOPO/Me-3,2-HOPO) 22 , and DFO/1,2-HOPO 23 structures would certainly make excellent candidates for charge-based selectivity; however, their chelation properties have so far only been investigated with either Eu 3+ , Zr 4+ , or Pu 4+ .…”