A homoleptic η2 hydroxylaminato CeIV complex with S4 symmetry

Article abstract of DOI:10.1039/c4dt00287c

An electron rich, air-stable hydroxylamine ligand and a strongly stabilized cerium(iv) hydroxylaminato complex are described. The synthesis of Ce[η²-ON(^tBu)(2-OMe-5-^tBu-C ₆H₃)]₄ (1) proceeded through a one pot protonolysis and oxidation procedure. Complex 1 crystallized with the molecule in exact S₄ symmetry. The hydroxylaminato ligands were bound in an η² mode that is of interest for potential application in the separations chemistry of rare earth metals. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4dt00287c

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A homoleptic η2 hydroxylaminato CeIV complex
with S4 symmetry†
Cite this: Dalton Trans., 2014, 43,
6300
Walter L. Dorfner, Patrick J. Carroll and Eric J. Schelter*
Received 26th January 2014,
Accepted 24th February 2014
DOI: 10.1039/c4dt00287c
www.rsc.org/dalton
An electron rich, air-stable hydroxylamine ligand and a strongly
stabilized cerium(IV) hydroxylaminato complex are described. The
synthesis of Ce[η2-ON(tBu)(2-OMe-5-tBu-C6H3)]4 (1) proceeded
through a one pot protonolysis and oxidation procedure. Complex
1
crystallized with the molecule in exact S4 symmetry. The
hydroxylaminato ligands were bound in an η2 mode that is of inter-
est for potential application in the separations chemistry of rare
earth metals.
Recent work has shown that ligands bearing amidoxime func-
tional groups covalently bound to solid supports have extracted
up to 1 kg of uranium in 240 days from seawater in the form of
2+ 1–4
uranyl cation, UO2
.
In some cases, the η2 N,O binding
mode of amidoximes has been shown to be highly selective for
binding the Lewis acidic uranyl cation.2 As part of a general Fig. 1 Thermal ellipsoid plot of LH. Thermal ellipsoids set at 50% prob-
ability. Non-hydrogen bonded hydrogens were omitted for clarity.
Selected bond lengths [Å]: N(1)–O(1) 1.4501(9), N(1)–H(1’) 2.126, H(1’)–
O(2) 2.394.
interest in the coordination and separations chemistry of rare
earth metals we recently initiated studies of related hydroxyl-
aminato ligands and their interactions with Lewis acidic lantha-
nide cations.5 Similar to the coordination of amidoximes to
uranyl cations, N,N-diorganylhydroxylamines contribute a hard
the radical form of the ligand was an effective oxidant toward
anionic oxygen donor and a hard neutral nitrogen donor that
cerium(III) cations. The simple synthesis of aromatic hydroxyl-
are well matched to the cationic character of the lanthanides.
amines opens the door to facile exploration of multitopic
In addition to their expected superior coordination
ligands that provide multiple η2 coordination sites.
affinities, hydroxylamines also typically exhibit redox activity.
The synthesis of 2-(tert-butylhydroxylamino)-4-tert-butylani-
Nitroxide radicals find use as catalysts in living radical poly-
sole (LH) was adapted from literature procedures;7 lithiation of
merizations of styrene and other monomers due to the stabi-
2-bromo-4-tert-butylanisole at −78 °C followed by treatment
with 2-methyl-2-nitrosopropane provided LH in 57% isolated
lity of their persistent radical forms.6 In this context, we
initiated a study of the coordination chemistry of the redox-
yield. LH was conveniently isolated as an air-stable white solid.
active hydroxylamine 2-(tert-butylhydroxylamino)-4-tert-butyl-
The air stability of LH stands in contrast to related electron
anisole to observe its binding affinity and tendency to engage
rich hydroxylamines,7,8 which spontaneously oxidize to their
in redox chemistry. In this work, we demonstrate that the
radical form (L•) in the presence of O2. LH can be stored as a
hydroxylamine ligands favored binding in an η2 mode and that
solid on the bench for weeks and up to 24 h in solution with
no sign of oxidation.
The structure of LH was determined by X-ray crystallography.
The N–O bond length 1.4501(9) Å compared well to that of
Me2NOH, 1.452(2) Å.9 The intermolecular NO distances from O
(1) to N(1′) in the NHO hydrogen bonding unit were 2.8432(11) Å,
which were comparable to the NO distance of 2.777(4) Å in the
NHO unit of Me2NOH. The hydrogen bond angle in LH was
P. Roy and Diana T. Vagelos Laboratories, Department of Chemistry, University of
Pennsylvania, Philadelphia, PA 19104, USA. E-mail: schelter@sas.upenn.edu;
Fax: +1 215-573-2112; Tel: +1 215-898-8633
†Electronic supplementary information (ESI) available: Experimental pro-
cedures, synthetic details and characterization, 1H NMR spectra, electrochemical
data and computational details. CCDC 983191–983192. For ESI and crystallo-
graphic data in CIF or other electronic format see DOI: 10.1039/c4dt00287c
6300 | Dalton Trans., 2014, 43, 6300–6303
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the Lewis acidic cerium(IV) centre in accordance with previous
reports of hydroxylamines bound η2 to a single metal cation.14
The TEMPO− anion (TEMPO = (2,2,6,6-tetramethylpiperidin-1-
yl)oxy) bound to a samarium(III) cation exhibited Sm–O bond
lengths of 2.157(3) Å and Sm–N bond lengths of 2.522(4) Å.15 In a
samarium compound bearing an N,N-bis(2-{pyrid-2-yl}ethyl)hydro-
xylaminato ligand the bond lengths were similar, the Sm–O bond
length was 2.2258(19) Å and Sm–N bond length was 2.461(2) Å.16
The hydroxylamine nitrogen atoms in the complex are
highly pyramidalized; the sums of the angles about the nitro-
gen atoms were 329.8°. For other known lanthanide complexes
with η2 bound hydroxylamines, the pyramidalization angles
range from 326.4–339.1°.10,14–17 In the crystal structure of LH,
the hydroxylamine nitrogen was also pyramidalized with an
angle sum of 328.8°, due to hydrogen bonding. These obser-
vations suggested that the electron rich methoxy substituted
ring favored the hydroxylamine being out of plane with the
ring to reduce electron donation into the π-system. DFT calcu-
lations on LH at the B3LYP/6-31G* level of theory also resulted
in a low energy geometry where the hydroxylamine was rotated
out of the plane of the aromatic ring (see ESI†).
Electrochemical characterization of 1 yielded a cyclic vol-
tammogram with a quasi-reversible metal reduction feature at
the strongly reducing potential of −1.73 V versus Fc/Fc+. An
irreversible oxidation, assigned to the ligands, was also
observed at Ep,a = +0.37 V. Assignment of the E1/2 = –1.73 V
wave to the CeIII/IV couple was made in analogy to those pub-
lished recently by our group.5,18 Attempts to chemically reduce
1 using potassium metal in THF resulted in a return of complex
1 and an intractable mixture of other diamagnetic products.
Attempts to synthesize a cerium(III) complex by reaction of
Scheme 1 Synthesis of CeIVL4 (1).
more acute, however, with an ∠N(1)–O(1)–N(1′) of 84.93(5)° com-
pared to 106.9(2)°. Inter- and intra-molecular hydrogen
bonding was implicated to explain the unexpected resistance
of LH to autoxidation. The crystal structure revealed a dimeric
structure comprising four self-complementary hydrogen
bonding interactions (Fig. 1). The oxidative stability was
further illustrated by stirring the ligand over PbO2 to form the
radical L•; the oxidation was only 50% complete after 2 h, and
required 24 h (Fig. S2†) to reach full conversion, characterized
by a broad peak at 1.69 ppm in the 1H NMR. With the air
stable hydroxylamine LH ligand and its radical analogue L•
in hand, we next explored its coordination chemistry with
cerium.
Reaction of 1 equiv. of the red nitroxyl radical L•, three
equiv. of LH, and 1 equiv. Ce[N(SiMe3)2]3 in toluene furnished
a dark red product, Ce[η2-ON(tBu)(2-OMe-5-tBu-C6H3)]4 (1),‡
which was isolated as a pure red powder in 77% yield following
workup (Scheme 1). Complex 1 was characterized by an
1H NMR spectrum that was consistent with a closed-shell
cerium(IV) cation and four anionic ligands. A crystal structure
of 1 revealed the expected 4 : 1 stoichiometry resulting from
oxidation of the cerium(III) cation by the nitroxyl radical L•.
The cerium(IV) complex crystallized with uncommon S4 sym-
metry (Fig. 2), which was consistent with its solution NMR
spectrum (Fig. S3†). In reported ‘-ate’ complexes of the general
formula K[M(η2-ONiPr2)4], M = Y or Sm, symmetrical solution
structures were observed by NMR spectroscopy. However, S4
symmetry was not found in the corresponding crystal struc-
tures of these trivalent rare earth cations.10 In related tetrava-
lent examples, Ti(ONEt2)4 exhibits exact S4 symmetry in the
solid state, while the compounds M(ONMe2)4, M = Si, Ti or Zr,
crystallize in approximate S4 symmetry.11–13
The oxygen atoms of 1 bind the cerium cation in a slightly
distorted square plane, where ∑(∠O–Ce–O) = 361.3° and τ =
0.12. The nitrogen atoms are positioned around the cerium
cation in a pseudo-tetrahedral configuration, where τ = 0.95.
The Ce–O bond lengths are 2.204(3) Å whereas the Ce–N bonds
are quite long, at 2.557(4) Å. These observations suggested that
the anionic donor oxygen was the preferred binding atom for
Fig. 2 Thermal ellipsoid plot of the title complex (1). Thermal ellipsoids
set at 50% probability. Hydrogens were omitted and tert-butyl groups
were represented as wireframes for clarity. Selected bond lengths [Å]:
Ce(1)–O(1) 2.204(3), Ce(1)–N(1) 2.557(4), N(1)–O(1) 1.461(9).
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Ce[N(SiMe3)2]3 with three equiv. LH resulted in an NMR spec- calculated structure, at 2.741 Å versus 2.557(4) Å in the crystal
trum that suggested only closed shell products, including structure. These differences in bond lengths suggested that
peaks characteristic of 1. A color change to dark red was crystal-packing effects shorten the Ce–N distances. The average
also observed from this reaction. Both observations suggested Mayer bond order for the Ce–O bonds was determined to be
oxidation of the metal-containing products to the cerium(IV) 0.76 while the Ce–N bonds were 0.25, suggesting that oxygen
state.
has a more significant interaction with the cerium cation than
The electronic structure of the cerium complex was interro- the neutral nitrogen atom, as expected.
gated computationally using hybrid DFT calculations at the
The HOMO of 1′ was determined to be a doubly degenerate
B3LYP/6-31G* level of theory with the core electrons of cerium (e) set of orbitals of primarily N–O π* character with a small
represented by ECP28MWB.19 The tert-butyl groups on the aro- amount of orbital mixing between ligands and the 4f orbitals
matic rings were truncated to hydrogen atoms to reduce the (Fig. 3). The mixing was further investigated using the AOmix
computational cost of the calculations. The truncated complex software, which revealed 11% 4f-orbital parentage in each of
was referred to as 1′. The geometrical parameters matched well the orbitals in the degenerate pair of HOMO orbitals. Another
from 1 to 1′, with Ce–O bond lengths 2.204(3) Å and 2.222 Å, set of orbitals exhibiting appreciable degrees of 4f-orbital and
respectively. In the optimized structure of 1′, the Ce–N bond ligand mixing were the degenerate LUMO+5 and LUMO+6
lengths were significantly lengthened compared to the virtual orbitals, which had 87% f-orbital contribution and 12%
orbital contribution from the ligand fragment. In contrast, the
LUMO exhibits only minor ligand character, (2%). The
HOMO–LUMO gap was computationally determined to be
3.47 eV (357 nm). In total, the DFT calculations were con-
sistent with the electrochemical and spectroscopic data that
indicate a strongly stabilized cerium(IV) cation.
The UV-Vis spectrum (Fig. 3) was recorded in toluene and
the bands fit to Gaussian curves using fityk.20 The broad,
high energy feature, which was fit by a curve with a peak at
359 nm, was assigned to a LMCT transition.5,14 The higher
energy bands were tentatively assigned to electronic transitions
within the ligands. The CT transition was ascribed to the
intense red color of the complex.
The use of η2 N,O binding in uranyl extractions prompted
consideration of ligands that display η2 N,O binding at Lewis
acidic lanthanide cations. In this context, we have synthesized
a homoleptic CeIV complex with hydroxylamine ligands bound
in an η2 mode. The spectroscopic and electrochemical data
indicate that cerium cations have a strong affinity for the
hydroxylamine ligands. Development of the coordination
chemistry of η2 N,O moieties with rare earth cations and
their prospects for separations chemistry are currently in
development.
E.J.S. acknowledges the U.S. Department of Energy, Office
of Science, Early Career Research Program (DE-SC0006518),
the Research Corporation for Science Advancement (Cottrell
Scholar Award to E.J.S.) and the University of Pennsylvania for
financial support. We also thank the U.S. National Science
Foundation for support of the X-ray diffractometer used in this
work (CHE-0840438).
Notes and references
‡Crystal and structure refinement details. For 1: C60H96CeN4O8, λ = 0.71073 Å,
ˉ
M = 1141.56, tetragonal, P421c, a = 16.9469(14), b = 16.9469(14), c = 10.7840(11)
Fig. 3 The UV-Vis spectrum of 1 with Gaussian fits of the absorption
Å, V = 3097.1(5) Å3, Z = 2, T = 100(1) K, 65 612 reflections measured, 3522 unique
bands performed using fityk20 with an ordinate axis in eV (top). And the
(Rint = 0.0531), 2587 observed [I > 2σ(I)], R1 = 0.0362, wR2 = 0.0889, GOF = 1.076.
calculated energy level diagram of 1’ with a split energy scale between
the HOMO (blue) and LUMO (red), (bottom). Relevant orbitals are shown
on the right, with occupied orbitals colored red and blue, and virtual
orbitals green and yellow.
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