Angewandte. Communications International Edition:DOI:10.1002/anie.201501659 Rare-Earth Elements German Edition:DOI:10.1002/ange.201501659 An Operationally Simple Method for Separating the Rare-Earth Elements Neodymium and Dysprosium** Justin A. Bogart, Connor A. Lippincott, PatrickJ.Carroll, and Eric J. Schelter* Abstract: Rare-earth metals are critical components of elec- gested that end-of-life product recycling is aviable alternative tronic materials and permanent magnets.Recycling of consu- to the mining of rare-earth metals.[7] Recycling processes have mer materials is apromising new source of rare earths.To been reported for the isolation of mixtures of rare-earth incentivize recycling there is aclear need for simple methods metals from dismantled magnetic materials[4,8] and significant for targeted separations of mixtures of rare-earth metal advances have been made toward processing magnets to salts.Metal complexes of atripodal nitroxide ligand produce rare-earth concentrates.[9] However,the varying t 3 3 [{(2- BuNO)C6H4CH2}3N] À (TriNOx À), feature asize-sensi- blends of Dy,Tb, Nd, and Pr necessitate separations of the tive aperture formed of its three h2-(N,O) ligand arms. rare earths for reblending into new sintered magnets in the Exposure of metal cations in the aperture induces highest-value recycling schemes. aself-associative equilibrium comprising [M(TriNOx)thf]/ Rare-earth metals are separated commercially using [M(TriNOx)]2 (M = rare-earth metal). Differences in the aRhône–Poulenc liquid–liquid extraction process (LLE), an equilibrium constants (Keq)for early and late metals enables energy-, time– and solvent-intensive process that exploits simple Nd/Dy separations through leaching with aseparation differences in binding constants of the metals to organic [10] ratio SNd/Dy = 359. extractants. Recent fundamental studies have worked to enhance the selectivity for one type of rare-earth metal cation The rare-earth elements (M), La–Lu, Y, and Sc, are key over another, based on ionic size,through molecular self- [14] components of vital materials with diverse applications in assembly of helical Ln2L3 dimers, or with the use of electric motors,phosphors,nickel–metal-hydride batteries, lanthanide-based metal–organic frameworks for fractional and catalysts,among others.[1] Their broad, and in many cases crystallization.[15] It is of interest to develop simple and direct irreplaceable,uses result from unique properties arising from chemical methods for efficient separations of early and late their 4f valence electron shell. Neodymium (Nd), typically rare-earth metals,namely dysprosium and neodymium, to mixed with praseodymium (Pr), is akey component of incentivize recycling from their primary consumer use in sintered neodymium magnets (or “neo magnets”; Nd2Fe14B), magnets. which have the highest-energy product (52 MGOe) among Herein, we report rare-earth-metal coordination com- [2] t 3 3 commercialized permanent magnets. Terbium (Tb) and pounds of [{(2- BuNO)C6H4CH2}3N] À (TriNOx À)that dysprosium (Dy) are also key components of this material undergo aself-association (dimerization) equilibrium based which increase its intrinsic coercivity.[3] High-performance on cation size.Asaresult of the relatively larger value of its neo magnets include up to 9% Dy by total magnet weight.[4] self-association equilibrium constant, the Nd–TriNOx system TheU.S.Department of Energy has categorized dysprosium exhibits a50-fold higher solubility than the Dy–TriNOx and neodymium as “critical materials” because of their supply system in benzene.Taken together,the solubility and self- [5] problems and importance to technology. association equilibria achieve aseparation factor SNd/Dy = 359 Apotential new supply stream of neodymium and using simple solvent leaching, providing proof-of-concept for dysprosium is end-of-life magnet recycling. In 2007, global targeted separations of consumer materials. in-use stocks for rare-earth metals in neo magnets were To initiate the separation studies,the tripodal nitroxide t estimated to be 62.6 Gg of Nd, 15.7 Gg of Pr and Dy,and ligand [{(2- BuNOH)C6H4CH2}3N],tris(2-tert-butylhydroxy- [6] 3.1 Gg of Tb. In 2011, however, less than 1% of rare-earth aminato)benzylamine (H3TriNOx), was synthesized in 70% metals were recycled.[4] Arecent life-cycle assessment sug- yield by alithium–halogen exchange reaction between tris-2- bromobenzylamine[16] and nBuLi at 788C, followed by the À [*] J. A. Bogart, C. A. Lippincott, Dr.P.J.Carroll, Prof. E. J. Schelter addition of 2-methyl-2-nitrosopropane dimer (Scheme 1). Department of Chemistry University of Pennsylvania 231 S. 34th St.,Philadelphia, PA 19104 (USA) E-mail:[email protected] [**] E.J.S. acknowledges the U.S. Department of Energy,Office of Science,Early Career Research Program (Grant DE-SC0006518), the Research Corporation for Science Advancement (Cottrell Scholar Award to E.J.S.), and the University of Pennsylvania for financial support of this work. We thank Dr.Andrew J. Lewis for helpful discussions. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/anie.201501659. Scheme 1. Synthesis of the H3TriNOx ligand. 8222 2015 Wiley-VCH Verlag GmbH &Co. KGaA, Weinheim Angew.Chem.Int. Ed. 2015, 54,8222 –8225 Angewandte Chemie H3TriNOx was air stable in both the solid and solution states. Isostructural [M(TriNOx)thf] compounds,where M = La, Nd, Dy,and Y, were formed through asimple protonolysis route III between H3TriNOx and their respective M [N(SiMe3)2]3 reagents (Scheme 2, method A). These compounds exhibited Scheme 2. MethodsAand Bfor the preparationofmetal complexes [M(TriNOx)thf]. OTf = trifluoromethanesulfonate. Figure 1. Crystal structures of a) the monomer [Nd(TriNOx)thf]and only low solubility in THF (see below). Thecompounds could b) the dimer [Nd(TriNOx)]2.Thermal ellipsoidsare set at 30% also be synthesized through an alternative,one-pot synthetic probability.[17] route starting from the M(OTf)3 salts,H3TriNOx (1 equiv), and K[N(SiMe3)2](3equiv) in THF (Scheme 2, method B). Thestructures of the [M(TriNOx)thf] complexes revealed [M(TriNOx)]2 compounds could be converted back into the that each arm of the nitroxide ligand was coordinated h2- respective [M(TriNOx)thf] congeners by addition of THF to (N,O) to the rare-earth cations.This coordination mode their benzene,toluene,ormethylene chloride solutions.Inthe provided a C3-symmetric environment with an open site in the cases of M = Dy or Y, dimers were not detected. apical position, occupied by aTHF molecule. Theexistence of [M(TriNOx)]2 dimers in solutions of Analysis of the solution structures of [M(TriNOx)thf] in CH2Cl2 was also evident from solution-state electrochemistry 1 CD2Cl2 (M = La and Nd) using HNMR spectroscopy experiments.[La(TriNOx)thf] and [Nd(TriNOx)thf] exhibited revealed two species in solution in each case,consistent similar cyclic voltammograms that showed two overlapping, with the presence of [M(TriNOx)thf] complexes and quasi-reversible ligand oxidation waves between 0and C -symmetric dimeric compounds (Scheme 3). Theidentities 0.75 Vversus Fc/Fc+ (ferrocene/ferrocenium) followed by 2 À of the dimeric compounds were confirmed crystallographi- an irreversible oxidation feature at + 0.4 V. In contrast, the cally (Figure 1) and the compounds were isolated in good cyclic voltammograms of [M(TriNOx)thf] (M = Yand Dy) yields by dissolving [M(TriNOx)thf] (M = La, Nd) in toluene showed one quasi-reversible oxidation with Epa =+0.2 Vand and removing the solvent under reduced pressure.The E = 0.5 V(see the Supporting Information). pc À Thevalue of the equilibrium constant for the dimerization of [Nd(TriNOx)thf] in benzene (Scheme 3) was determined using 1HNMR spectroscopy at RT.Asolution of [Nd- (TriNOx)]2 in C6D6 was titrated with THF and the equilibrium concentrations of [Nd(TriNOx)]2 and [Nd(TriNOx)thf] were measured at each point against an internal ferrocene standard (see the Supporting Information). These data yielded avalue of K = 2.4 0.2. Measurement of the equilibrium constant eq Æ was also possible using hypersensitive 4I 2G , 4G 4f–4f 9/2 ! 7/2 5/2 transitions for the Nd–TriNOx system. Aspectrophotometric titration of the Nd dimer in benzene with THF revealed aclear isosbestic point at l = 594.1 nm and the data yielded an The equilibriumbetween monomeric [M(TriNOx)thf]and equilibrium constant of Keq = 2.9 0.4, in good agreement Scheme 3. Æ dimeric [M(TriNOx)]2. with the results from the NMR titration. Angew.Chem. Int.Ed. 2015, 54,8222 –8225 2015 Wiley-VCH Verlag GmbH &Co. KGaA, Weinheim www.angewandte.org 8223 Angewandte. Communications We postulated that the attenuated tendencyfor dimeriza- tion with decreasing ionic radius was due to an effective 2 closing of the (N,O)3 aperture formed by the three h -(N,O) 3 arms of the TriNOx À ligand. To quantify the decrease in aperture size,wedetermined the percent buried volume [18] (%Vbur)for [Nd(TriNOx)thf] and [Dy(TriNOx)thf]. Upon changing the metal from Nd to Dy,wecalculated an increase in %Vbur from 79.9%to81.3%, respectively.Wecontend that this small, but significant, increase in the %Vbur and the change in size of the (N,O)3 molecular aperture is responsible for shifting the thermodynamic preference from dimer to monomer in the TriNOx system. Thepreferential formation of dimeric structures from the 3 larger cations La and Nd with the TriNOx À ligand and monomeric ones from the smaller Dy and Ywas promising for targeted separations of Nd and Dy.Totest for the formation of mixed metal dimers,which would interfere with aclean separation, 1HEXSY NMR spectroscopic experi- ments were performed on mixtures of [Nd(TriNOx)thf] and [Dy(TriNOx)thf] in CD2Cl2.Noexchange was detected between Nd and Dy during these experiments. 1HEXSY NMR spectroscopic experiments were also performed on mixtures of [Nd(TriNOx)thf] and [Y(TriNOx)thf],adiamag- netic analogue of the [Dy(TriNOx)thf] complex, to allow for longer delay times;nocation exchange was detected in the Nd/Y experiments (see the Supporting Information).