Rare-earth metals are critical components of electronic materials and permanent magnets. Recycling of consumer materials is a promising new source of rare earths. To incentivize recycling there is a clear need for simple methods for targeted separations of mixtures of rare-earth metal salts. Metal complexes of a tripodal nitroxide ligand [{(2-^tBuNO)C₆H₄CH₂}₃N]³⁻ (TriNOx³⁻), feature a size-sensitive aperture formed of its three η²-(N,O) ligand arms. Exposure of metal cations in the aperture induces a self-associative equilibrium comprising [M(TriNOx)thf]/ [M(TriNOx)]₂ (M=rare-earth metal). Differences in the equilibrium constants (K_eq) for early and late metals enables simple Nd/Dy separations through leaching with a separation ratio S_Nd/Dy=359.

Rare-earth metal cations have recently been demonstrated to be essential co-factors for the growth of the methanotrophic bacterium Methylacidiphilum fumariolicum SolV. A crystal structure of the rare-earth-dependent methanol dehydrogenase (MDH) includes a cerium cation in the active site. Herein, the Ce–MDH active site has been analyzed through DFT calculations. The results show the stability of the Ce^III–pyrroloquinoline quinone (PQQ) semiquinone configuration. Calculations on the active oxidized form of this complex indicate a 0.81 eV stabilization of the PQQ⁰ LUMO at cerium versus calcium, supporting the observation that the cerium cation in the active site confers a competitive advantage to Methylacidiphilum fumariolicum SolV. Using reported aqueous electrochemical data, a semi-empirical correlation was established based on cerium(IV/III) redox potentials. The correlation allowed estimation of the cerium oxidation potential of +1.35 V versus saturated calomel electrode (SCE) in the active site. The results are expected to guide the design of functional model complexes and alcohol-oxidation catalysts based on lanthanide complexes of biologically relevant quinones.

Ligand reorganization has been shown to have a profound effect on the outcome of cerium redox chemistry. Through the use of a tethered, tripodal, trianionic nitroxide ligand, [((2-tBuNOH)C₆H₄CH₂)₃N]³⁻ (TriNO_x³⁻), controlled redox chemistry at cerium was accomplished, and typically reactive complexes of tetravalent cerium were isolated. These included rare cationic complexes [Ce(TriNO_x)thf][BAr^F₄], in which Ar^F=3,5-(CF₃)₂-C₆H₃, and [Ce(TriNO_x)py][OTf]. A rare complete Ce–halide series, Ce(TriNO_x)X, in which X=F⁻, Cl⁻, Br⁻, I⁻, was also synthesized. The solution chemistry of these complexes was explored through detailed solution-phase electrochemistry and ¹H NMR experiments and showed a unique shift in the ratio of species with inner- and outer-sphere anions with size of the anionic X⁻ group. DFT calculations on the series of calculations corroborated the experimental findings.

Rare-earth metals are critical components of electronic materials and permanent magnets. Recycling of consumer materials is a promising new source of rare earths. To incentivize recycling there is a clear need for simple methods for targeted separations of mixtures of rare-earth metal salts. Metal complexes of a tripodal nitroxide ligand [{(2-^tBuNO)C₆H₄CH₂}₃N]³⁻ (TriNOx³⁻), feature a size-sensitive aperture formed of its three η²-(N,O) ligand arms. Exposure of metal cations in the aperture induces a self-associative equilibrium comprising [M(TriNOx)thf]/ [M(TriNOx)]₂ (M=rare-earth metal). Differences in the equilibrium constants (K_eq) for early and late metals enables simple Nd/Dy separations through leaching with a separation ratio S_Nd/Dy=359.

Reaction of p-benzoquinone (BQ) with a series of rare-earth metal/alkali metal/1,1′-BINOLate (REMB) complexes (RE: La, Ce, Pr, Nd; M: Li) results in the largest recorded shift in reduction potential observed for BQ upon complexation. In the case of cerium, the formation of a 2:1 Ce/BQ complex shifts the two-electron reduction of BQ by greater than or equal to 1.6 V to a more favorable potential. Reactivity investigations were extended to other RE^III (RE=La, Pr, Nd) complexes where the resulting highly electron-deficient quinone ligands afforded isolation of the first lanthanide quinhydrone-type charge-transfer complexes. The large reduction-potential shift associated with the formation of 2:1 Ce/BQ complexes illustrate the potential of Ce complexes to function both as a Lewis acid and an electron source in redox chemistry and organic-substrate activation.