Structural and electrochemical characterization of a cerium(IV) hydroxamate complex: Implications for the beneficiation of light rare earth ores

Article abstract of DOI:10.1039/c3cc46486e

Reaction of N-phenyl-pivalohydroxamic acid with Ce^III precursors leads to a homoleptic hydroxamate complex: Ce^IV[^tBuC(O) N(O)Ph]₄. Electrochemical experiments indicate a significant stabilization of the Ce^IV cation at E_p,c = -1.20 V versus SCE in the hydroxamate ligand framework. The spontaneous oxidation of Ce ^III in a hydroxamate ligand field is discussed in the context of beneficiation of the light rare earths from the fluorocarbonate mineral bastn?site. the Partner Organisations 2014.

Full text of DOI:10.1039/c3cc46486e

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Structural and electrochemical characterization
of a cerium(IV) hydroxamate complex: implications
for the beneficiation of light rare earth ores†
Cite this: Chem. Commun., 2014,
50, 5361
Received 25th August 2013,
Accepted 19th October 2013
Heui Beom Lee, Justin A. Bogart, Patrick J. Carroll and Eric J. Schelter*
DOI: 10.1039/c3cc46486e
www.rsc.org/chemcomm
Reaction of N-phenyl-pivalohydroxamic acid with CeIII precursors leads of cerium-hydroxamate chemistry is crucial for rationalizing the models
to a homoleptic hydroxamate complex: CeIV[tBuC(O)N(O)Ph]4. Electro- of collector-surface interactions and improving beneficiation.6
chemical experiments indicate a significant stabilization of the CeIV
Early studies on the coordination chemistry of cerium with
cation at Ep,c = ꢀ1.20 V versus SCE in the hydroxamate ligand frame- hydroxamates, especially that of Agrawal and co-workers,7
work. The spontaneous oxidation of CeIII in a hydroxamate ligand field is included the spectrophotometic determination of cerium and
discussed in the context of beneficiation of the light rare earths from the selective precipitation of rare earths. Notably, Alimarin reported
fluorocarbonate mineral bastnasite.
¨
extraction of CeIII hydroxamate complexes into organic phases
concomitant with oxidation.8 However, none of the products
were structurally characterized and the nature of the cerium
oxidation chemistry remains somewhat ambiguous.
Recent results from our group have established the unprece-
dented stability of the CeIV cation endowed by a pyridyl nitroxide
framework.9 We reasoned that the oxidation of cerium might play an
important role in the froth flotation process that had not been
Due to their unique physical properties, rare earth metals have
become increasingly useful and irreplaceable components of many
materials, especially in hard magnets and optical materials, among
many others.1 To be used in such applications, mixed rare earth
containing minerals are beneficiated and ultimately purified
through a liquid–liquid separations process to attain the required
high purities.2 Depending on the ore type, the beneficiation of
mixed rare earth minerals is accomplished by gravity-, magnetic-, or
electrostatic separation, or by froth flotation.2 In the critical froth
flotation unit operation, hydroxamates are used as collectors to
separate rare earth bearing minerals from barite and calcite gangue
materials.3 The froth flotation process has been applied to bene-
¨
explicitly considered in models of bastnasite beneficiation. In this
respect, the chemistry of related lanthanide catecholates illustrates
the importance of cerium redox chemistry. Notably, the reaction of
insoluble CeIIIPO4 and aqueous catechol results in a stoichiometric
release of CeIII cations under anaerobic conditions while oxidation of
cerium is observed under aerobic conditions.10 The aerobic oxida-
tion of CeIIIPO4 is driven by a strong thermodynamic preference for
the 4+ state of CeIV[O2C6H4]44ꢀ; the reduction potential of
CeIV[O2C6H4]44ꢀ was determined at ꢀ0.69 V versus SCE by Raymond
and co-workers indicating a startling 1036-fold difference in for-
mation constants for the tetravalent versus trivalent forms of the
tetrakis(catecholate) complex.11
Herein, we report the first crystal structure of a simple homoleptic
CeIV hydroxamate complex and its spectroscopic and electrochemical
characterization; the structure is reminiscent of reported lanthanide
and actinide complexes of bidentate hydroxypyridinonates.12 Our
findings suggest that cerium hydroxamate complexes exhibit a strong
thermodynamic preference for the 4+ oxidation state. Extrapolating
these results, we expect that redox chemistry plays an important role
¨
ficiate the light rare earth fluorocarbonate minerals, bastnasite, e.g.
(Ce, La, Y)CO3F, from the key global ore bodies at Mountain Pass,
CA, U.S.A. and the Bayan Obo Mining District in the Inner
Mongolia Region of the P.R.C.3a,4
In models of the froth flotation beneficiation process, hydroxamate
collectors are proposed to chemisorb selectively to the rare earth metal
cations at the ore particle surface, and/or extract the ions, creating
multi-layers of lipophilic rare earth complexes that render the ore
particles hydrophobic.5 Since cerium constitutes a major component of
¨
bastnasite with up to 50% content in given ores, a better understanding
P. Roy and Diana T. Vagelos Laboratories, Department of Chemistry,
University of Pennsylvania, 231 South 34th Street, Philadelphia,
¨
in the superior beneficiation of bastnasite using alkyl hydroxamic
Pennsylvania 19104, USA. E-mail: schelter@sas.upenn.edu
acids over other flotation collectors such as tall oil-derived fatty acids.
The synthesis of N-phenyl-pivalohydroxamic acid (HL) was
accomplished conveniently from N-phenyl-hydroxylamine and
pivaloyl chloride (see ESI†). Reaction of 3 or 4 equiv. of HL with
† Electronic supplementary information (ESI) available: Full experimental
details, electrochemistry data, NMR data, UV-vis data, shape parameters and
computational details. CCDC 952851. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c3cc46486e
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Table 1 Average bond distances (Å) in homoleptic, tetravalent metal
hydroxamate complexes
Compound
M–OC (Å) M–ON (Å) N–O (Å) Radiusa (Å) Ref.
Zr[MeC(O)N(O)Me]4 2.198(13) 2.188(11) 1.340(11) 0.84
Hf[PhC(O)N(O)Ph]4 2.258(10) 2.116(10) 1.375(1) 0.83
16
17
—
14
1
2.430(8) 2.265(3) 1.379(3) 0.97
Th[tBuC(O)N(O)iPr]4 2.492(3) 2.357(3) 1.376(4) 1.05
a
Ionic radii based on a coordination number of eight.18
Scheme 1 Synthesis of tetrakis(hydroxamate) cerium(IV) complex 1.
reduced hydroxamate anions (Table 1). The average N–O distance in
1 was determined to be 1.379(3) Å. The average Ce–OC distance is
noticeably longer than the average Ce–ON distance, consistent
with the data for the group 4 hydroxamate complexes (Table 1).
The difference in the Ce–O bond lengths is best explained by a
higher charge density on the nitrogen oxygen. The slightly long
average CQO bond length (1.265(5) Å), the slightly short average
Ce[N(SiMe3)2]3 under anaerobic conditions invariably produced
the neutral, 1 : 4 cerium complex (Scheme 1). Complex 1 was
synthesized reliably in 50% yield by adding four equiv. of HL to
a solution of CeIII[N(SiMe3)2]3 in diethyl ether, though the identity of
the oxidant in this reaction is not currently known. Addition of
TEMPO, 2,2,6,6-tetramethylpiperidine-N-oxyl, or exposure to dioxygen
decreased the reaction time and increased the isolated yield to 75%
and 84% respectively. In each case, B3 equiv. of H[N(SiMe3)2] were
also formed in addition to 1 as observed by 1H NMR spectroscopy. In
the reaction with TEMPO, TEMPO-H/TEMPO-D was also observed as
a byproduct. In addition to the oxidations of CeIII[N(SiMe3)2]3
observed in the presence of HL, reaction of the CeIII precursors
CeCl3 or Ce(NO3)3ꢁ6H2O with HL and LiOtBu in CH2Cl2 under
aerobic conditions resulted in spontaneous formation of 1.
Using Ce(NO3)3ꢁ6H2O, our favoured synthesis, a nearly quanti-
tative yield of 1 was obtained.
The active role of the hydroxylamine moiety in the redox
chemistry of metal complexes is precedented in f-block chemistry.
Reaction of PuIII precursors with the iron siderophore desferriox-
amine E (DFE) resulted in the isolation of only a PuIV(DFE)
complex.13 Tetrakis(hydroxamate) uranium complexes undergo
oxygen transfer reactions to give uranyl bis(hydroxamate) com-
plexes.14 A similar reactivity is observed in the syntheses of the
structurally related tetrakis(b-diketonato) cerium(IV) complexes,
which undergo auto-oxidation under aerobic conditions.15
Cacyl–N bond length (1.324(4) Å), and the planarity of the hydro-
xamate ligand are attributed to the participation of the charge-
separated resonance structure of the hydroxamate ligand in 1.14
Solution phase cyclic voltammetry experiments were performed
on 1 in order to assess the stability of the CeIV cation (see ESI†). The
cyclic voltammogram of 1 exhibited a quasi-reversible redox couple
for the cerium metal centre and a chemically irreversible oxidation
couple for the hydroxamate ligands. Given the experimental rest
potential of ꢀ0.18 V versus SCE, we assigned the cathodic current at
Ep,c = ꢀ1.20 V and the anodic current at Ep,a = ꢀ0.48 V to the
reduction and the oxidation of the cerium metal centre respectively.
The scan rate dependence of this isolated CeIII/IV redox event is
shown in Fig. 2. Following the method used by Raymond in the
study of CeIV[O2C6H4]4
measured for [nBu4N]2[Ce(NO3)6] in CH3CN,19 the relative stability of
the tetravalent form of 1 can be estimated. Approximating the E1/2
,
4ꢀ 11 using a reference potential of E10 = 1.02 V
=
ꢀ0.84 V for 1, a 1031-fold difference in formation constants is
calculated between the neutral and anionic forms of 1. This large,
negative reduction potential and overwhelming thermodynamic
preference of the 4+ state in 1 highlight the ability of the hydrox-
amate framework to stabilize the CeIV cation. In comparison, the
reduction potential of the related compound CeIV(acac)4 appears at
E1/2 = ꢀ0.04 V versus SCE.20 The striking difference in the reduction
potentials of 1 and Ce(acac)4 prompted us to investigate the electro-
nic structure of 1 in more detail using DFT calculations.
The structure of 1 was determined using X-ray crystallography
(Fig. 1). The coordination polyhedron formed by the hydroxamate
framework around the cerium cation is best described as a distorted
square antiprism; shape parameters for 1 are provided in Table S2
(ESI†). A comparison with previously reported structures of homo-
leptic, tetravalent metal hydroxamate complexes show that the
structure is consistent with a CeIV cation coordinated by four fully
Fig. 1 Thermal ellipsoid plot of 1 at 30% probability with hydrogen atoms
omitted for clarity. Selected bond distances (Å) Ce(1)–O(1) 2.259(3), Ce(1)–
O(2) 2.439(3), N(1)–O(1) 1.369(5), C(1)–N(1) 1.324(7), C(1)–O(2) 1.279(6).
Fig. 2 Cyclic voltammogram of complex
[nPr4N][BArF4] in CH2Cl2 versus SCE.
1 measured in 0.1 M
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The authors gratefully acknowledge the Chemical Sciences,
Geosciences, and Biosciences Division, Office of Basic Energy
Sciences, Early Career Research Program of the U.S. Department of
Energy, under Award Nos. DE-SC0006518 and DE-FG02-11ER16239
for support of this work. The University of Pennsylvania and the
Research Corporation for Science Advancement (Cottrell Scholar
Award to E.J.S.) are also acknowledged for financial support. We
thank the U.S. National Science Foundation for support of the
computing cluster (CHE-0131132) used in this work. This work also
Fig. 3 Calculated HOMO and LUMO of complex 1.
DFT calculations on 1 were performed at the B3LYP level of used the Extreme Science and Engineering Discovery Environment
theory, with the ECP28MWB pseudopotential21 for the cerium cation. (XSEDE), which is supported by U.S. National Science Foundation
The geometry optimized bond distances and angles for 1 agree well grant number OCI-1053575.
with the experimental values. The resulting molecular orbital
depiction shows that the HOMO comprises primarily hydroxamate
p* orbital character with an B6% contribution from the cerium
4fz(x2ꢀy2) orbital (Fig. 3). The hydroxamate ligands are oriented at the
Notes and references
1 D. A. Atwood, The rare earth elements: fundamentals and applications,
cerium cation to allow for a symmetry-matched bonding interaction
between the N–O p* orbitals and a 4f orbital of cerium, though the
overlap is small. The LUMO is calculated to be 2.95 eV above the
HOMO and consists of the corresponding antibonding interaction.
Furthermore, the natural population analysis shows a high degree of
charge donation into the central metal cation as indicated by a low
natural charge of the cerium metal centre of 1.41. A similar electronic
structure was observed in the homoleptic Ce[2-(tBuNO)py]4 complex
reported recently by our group,9 providing further evidence that there
is an electronic preference for the pseudo-cubic D2d symmetry
observed in the 8-coordinate k2-nitroxide complexes.
Evidence in support of the calculated electronic structure and
electrochemistry of 1 was determined using UV-visible electronic
absorption spectroscopy (Fig. S9, ESI†). A broad transition at 2.93 eV
was assigned to a LMCT from the hydroxamate ligand field to a
metal 4f orbital. The energy of this transition is in good agreement
with the calculated energy gap between the HOMO and the LUMO
of 2.95 eV. The broadness of this transition can be attributed to
partial mixing of the ligand-based orbitals with the metal-based 4f
orbitals (Fig. 3). A similar spectroscopic interpretation has been
advanced for tetrakis(b-diketonato) cerium complexes.22
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4
´
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Chem. Commun., 2014, 50, 5361--5363 | 5363