Controlled Redox Chemistry at Cerium within a Tripodal Nitroxide Ligand Framework

Bogart, Justin A.; Lippincott, Connor A.; Carroll, Patrick J.; Booth, C. H.; Schelter, Eric J.

doi:10.1002/chem.201502952

Cited by 54 publications

(57 citation statements)

References 59 publications

(63 reference statements)

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“…The η 2 -(N,O) bonding mode of the three hydroxylaminato arms was retained, and each complex contained a single molecule of THF coordinated to the central metal cation. The η 2 -(N,O) bonding mode had been observed previously by us and others in the RE(TriNOx)THF (RE = La, Ce, Nd, Dy, and Y) (28,40) (44,45). As shown in Table 1, there was a steady decrease in the metalligand bond lengths across the series, consistent with the decrease in ionic radii of the central metal cations.…”

Section: Resultssupporting

confidence: 82%

Accomplishing simple, solubility-based separations of rare earth elements with complexes bearing size-sensitive molecular apertures

Bogart

Cole

Boreen

et al. 2016

Proc. Natl. Acad. Sci. U.S.A.

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Rare earth (RE) metals are critical components of electronic materials and permanent magnets. Recycling of consumer materials is a promising new source of rare REs. To incentivize recycling, there is a clear need for the development of simple methods for targeted separations of mixtures of RE metal salts. Metal complexes of a tripodal hydroxylaminato ligand, TriNOx 3-, featured a size-sensitive aperture formed of its three η 2 -(N,O) ligand arms. Exposure of cations in the aperture induced a self-associative equilibrium comprising RE(TriNOx)THF and [RE(TriNOx)] 2 species. Differences in the equilibrium constants K dimer for early and late metals enabled simple separations through leaching. Separations were performed on RE1/RE2 mixtures, where RE1 = La-Sm and RE2 = Gd-Lu, with emphasis on Eu/Y separations for potential applications in the recycling of phosphor waste from compact fluorescent light bulbs. Using the leaching method, separations factors approaching 2,000 were obtained for early-late RE combinations. Following solvent optimization, >95% pure samples of Eu were obtained with a 67% recovery for the technologically relevant Eu/Y separation. (11)(12)(13)(14). Limitations associated with their beneficiation and separations, especially their solvent-, waste-, and energy intensities, have contributed to the concentration of suppliers in the People's Republic of China. Supply risks for these elements have emerged, particularly in the face of current and growing demand in the next 20 y (15, 16). Because the global marketplace for these elements is dominated by a single source (17), prices for primary rare earth (RE) materials are volatile (18). As a result, the US Department of Energy has classified many of these elements as "critical" (19). There is a clear need to find potential new supplies of these elements.Recent life cycle assessments have indicated that recycling of consumer materials is a promising alternative to conventional production processes (20). Despite this assertion, as recently as 2011, less than 1% of RE-containing materials were being recycled (21). These low recycling rates stem from a combination of sporadic collection procedures and lack of efficient separations and preprocessing steps (22)(23)(24)(25)(26)(27).To contribute to incentivizing the "urban mining" of REcontaining materials, we recently initiated efforts toward new, simplified methods in RE separations (28). Our initial work focused on the separation of neodymium (Nd) and dysprosium (Dy), two key components of neomagnets (Nd 2 Fe 14 B). We disclosed the development of the tripodal nitroxide ligand, [((2-t BuNO)C 6 H 4 CH 2 ) 3 N] 3− (TriNOx 3-), which induced a selfassociation equilibrium between monomeric Nd(TriNOx)THF/ dimeric [Nd(TriNOx)] 2 species. The position of this equilibrium was found to be strongly dependent on the size of the RE cation. We showed proof of concept that differences in the self-association equilibrium constants between Nd and Dy could be exploited to achieve 95% pure materials through a simple leachi...

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Section: Resultssupporting

confidence: 82%

Accomplishing simple, solubility-based separations of rare earth elements with complexes bearing size-sensitive molecular apertures

Bogart

Cole

Boreen

et al. 2016

Proc. Natl. Acad. Sci. U.S.A.

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“…This suggests an enhanced stabilization of the Ce IV oxidation state in 2 vs. homoleptic [Ce{OSi(O t Bu) 3 } 4 ] { E pc = –1.61 V; CH 2 Cl 2 ; 0.1 m [ n Pr 4 N][B{C 6 H 3 ‐3,5‐(CF 3 ) 2 } 4 ]},[32d] apparently originating from the presence of a nitrogen donor in 2 . [9f], Complex 1 revealed also relative similar cathodic potentials ( E pc ) in CH 2 Cl 2 and THF solution. Presumably, due to a preferred ion pairing in THF donor solvent the respective oxidation of complex 1 ( E pa ) occurred at more positive values than in CH 2 Cl 2 .…”

Section: Resultsmentioning

confidence: 84%

“…[9f], Complex 1 revealed also relative similar cathodic potentials ( E pc ) in CH 2 Cl 2 and THF solution. Presumably, due to a preferred ion pairing in THF donor solvent the respective oxidation of complex 1 ( E pa ) occurred at more positive values than in CH 2 Cl 2 . The electrochemical reduction ( E pc ) of nitrato‐containing complex 1 proceeded at less negative values vs. the ferrocene couple in both solvents (Table ).…”

Section: Resultsmentioning

confidence: 99%

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Ceric Ammonium Nitrate and Ceric Ammonium Chloride as Precursors for Ceric Siloxides: Ammonia and Ammonium Inclusion

et al. 2018

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Treatment of ceric ammonium nitrate (CAN) with sodium tris(tert‐butoxy)siloxide in THF afforded crystalline complexes [Ce{OSi(OtBu)3}3(NO3)(NH3)2(THF)] and [Ce{OSi(OtBu)3}4(NH3)2] featuring strong ammonia coordination. Conducting the CAN/NaOSi(OtBu)3 (1:4) reaction in acetonitrile gave ceric ate complex [Ce{OSi(OtBu)3}3(NO3)2(NH4)(NCCH3)] where the ammonium cation is embedded in the siloxy ligand sphere via hydrogen bonding. Salt‐metathetic conversion of ceric organo(R)ammonium hexachlorides (CACR, R = Me, Et) with NaOSi(OtBu)3 and sodium siloxides NaOSiR3 (R = Et, iPr) in THF allowed the isolation of ceric siloxide complexes [Ce{OSi(OtBu)3}4], [Ce(OSiEt3)4(THF)0.5], and [Ce(OSiiPr3)4]. Although the reaction products are prone to ate complex formation with the organoammonium chloride side products, the salt‐metathesis route gave good yields for homoleptic [Ce{OSi(OtBu)3}4]. The CACR/alkylsiloxide reaction mixtures also suffer from (photo)redox processes as revealed by the formation of trivalent [Ce(OSiiPr3)4(NMe4)], but can be further stabilized by pyridine coordination as shown for the solid‐state structure of [Ce(OSiiPr3)4(py)]. The siloxide complexes were characterized by NMR, DRIFT, UV/Vis spectroscopy and cyclic voltammetry, as well as paramagnetic susceptibility measurements, X‐ray structure, and elemental analysis. The electrochemical studies revealed an effective stabilization of the CeIV ion within the siloxy ligand environments.

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“…96,98 More recently, an in-depth chlorine K-edge X-ray absorption spectroscopy (XAS) and DFT study showed that Cl 3p and Ln 4f covalency was significant in tetravalent CeCl6 2-but virtually nonexistent in the trivalent analog, CeCl6 3-. 57 In spite of this and other notable synthetic progress, 57,72,87,98,[104][105][106] molecular Ce compounds are seldom the subject of emerging methods for physical and theoretical characterization. Molecular solids are promising candidates for further study because they can often be prepared as pure, crystalline compounds, can be designed with high symmetry, and can be modeled as a complete system without requiring periodic boundary conditions or other approximations.…”

Section: Introductionmentioning

confidence: 99%

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

et al. 2016

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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.

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Controlled Redox Chemistry at Cerium within a Tripodal Nitroxide Ligand Framework

Cited by 54 publications

References 59 publications

Accomplishing simple, solubility-based separations of rare earth elements with complexes bearing size-sensitive molecular apertures

Accomplishing simple, solubility-based separations of rare earth elements with complexes bearing size-sensitive molecular apertures

Ceric Ammonium Nitrate and Ceric Ammonium Chloride as Precursors for Ceric Siloxides: Ammonia and Ammonium Inclusion

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

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Controlled Redox Chemistry at Cerium within a Tripodal Nitroxide Ligand Framework

Cited by 54 publications

References 59 publications

Accomplishing simple, solubility-based separations of rare earth elements with complexes bearing size-sensitive molecular apertures

Accomplishing simple, solubility-based separations of rare earth elements with complexes bearing size-sensitive molecular apertures

Ceric Ammonium Nitrate and Ceric Ammonium Chloride as Precursors for Ceric Siloxides: Ammonia and Ammonium Inclusion

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

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