Octakis(tert-butoxo)dicerium(IV) [Ce2(OtBu) 8]: Synthesis, characterization, decomposition, and reactivity

Article abstract of DOI:10.1021/ic4025876

An advanced synthesis for the homometallic derivative [Ce ₂(O^tBu)₈] (1) starting from [Ce(O ^tBu)₂{N(SiMe₃)₂}₂] was developed. Structural characterization of a cerium(IV) complex and its decomposition products confirmed the coexistence of both ether elimination and Ce-O bond cleavage processes, which lead to the formation of [Ce ₃O(O^tBu)₁₀] and [Ce₃(O ^tBu)₁₁] (2) derivatives, respectively. Variable-temperature NMR spectroscopy under strict exclusion of moisture enabled insight into the decomposition processes in noncoordinating solvents and at elevated temperature. In addition, structural analysis of the heterovalent 2 and of two new complexes of the general formula [Ce₂(O ^tBu)₈(L)] [L = HO^tBu (3), OCPh₂ (4)] is described.

Full text of DOI:10.1021/ic4025876

Communication
pubs.acs.org/IC
Octakis(tert-butoxo)dicerium(IV) [Ce2(OtBu)8]: Synthesis,
Characterization, Decomposition, and Reactivity
Johannes Schlafer, Wieland Tyrra, and Sanjay Mathur*
̈
Institute of Inorganic Chemistry, University of Cologne, Greinstraße 6, D-50939 Cologne, Germany
S
* Supporting Information
2 [Ce(OtBu)2{N(SiMe3)2}2]
ABSTRACT: An advanced synthesis for the homometal-
2HOtBu
lic derivative [Ce2(OtBu)8] (1) starting from [Ce-
⎯⎯⎯⎯⎯⎯⎯→ [Ce2(OtBu)8]2 + 4 HN(SiMe3)2
(1)
(OtBu)2{N(SiMe3)2}2] was developed. Structural charac-
terization of a cerium(IV) complex and its decomposition
products confirmed the coexistence of both ether
elimination and Ce−O bond cleavage processes, which
lead to the formation of [Ce3O(OtBu)10] and
[Ce3(OtBu)11] (2) derivatives, respectively. Variable-
temperature NMR spectroscopy under strict exclusion of
moisture enabled insight into the decomposition processes
in noncoordinating solvents and at elevated temperature.
In addition, structural analysis of the heterovalent 2 and of
two new complexes of the general formula
[Ce2(OtBu)8(L)] [L = HOtBu (3), OCPh2 (4)] is
described.
Crystals of 1 were grown by vacuum sublimation, and the
crystal structure was solved in the monoclinic space group P21/c.
The cerium centers exhibit in the dimeric arrangement a
distorted trigonal-bipyramidal coordination sphere (τ = 0.70),14
whereas the polyhedra share one apical edge (Figure 1). This
he tert-butoxides of tetravalent main and transition metals
Tare well-studied volatile compounds that have been widely
used as precursors for the fabrication of functional ceramic
materials and as catalysts.1 In contrast, structurally described
complexes of tetravalent lanthanides and actinides are limited to
solvated products such as [Ce(OtBu)4(py)2]2 and [Th-
(OtBu)4(py)2].3 The homoleptic compounds [MIV(OtBu)4]
(MIV = Ce,4 Th5) have been reported by Bradley et al.; however,
a comprehensive elucidation of the molecular structure was
aggravated by their propensity to form an oxo cluster of the
M3O11 class or solvent adducts triggerd by their intrinsic
coordination unsaturation.2,6 Given the importance of cerium
oxides as photocatalysts and sensing materials, there is a rapidly
growing interest in the chemistry of cerium(IV) tert-butoxide,
not only from a structural point of view but also more so toward a
better chemical understanding of the solution behavior and
stability of the starting materials. This is evident in the large
number of tetravalent cerium complexes that have been
synthesized starting from cerium(IV) alkoxo derivatives.7
Figure 1. Molecular structure of 1. Thermal ellipsoids are shown at the
50% probability level, and hydrogen atoms have been omitted for clarity.
Selected bond lengths (Å): Ce−O1 2.058(3), Ce−O2 2.059(4), Ce−
O3 2.065(3), Ce−O4 2.286(3), Ce−O4′ 2.434(3).
results in an asymmetrical bridging motif with one elongated
[2.434(3) Å] and one shortened [2.286(3) Å] Ce−O distance.
The average Ce−O distance of the terminal alkoxo groups (2.061
Å) is similar to those of literature-known complexes2,6 and shows
no significant difference between the apical and equatorial
positions.
The thermal stability of 1 is controversially discussed in the
literature. For instance, Bradley et al. reported good volatility and
stability of the complex;4 however, the poor yield of the solvent-
free compound obtained upon sublimation of [Ce-
(OtBu)4(THF)2] contradicts this assumption.8 In order to
clarify these competing observations, a fresh sample of 1 was
sealed under vacuum in a 4 mm NMR tube and heated up to 150
°C at a slow rate. The complex started to melt at 141−143 °C,
Bradley et al. obtained [Ce2(OtBu)8] (1) by refluxing
[Ce2(OiPr)8(HOiPr)2] in a mixture of HOtBu and benzene
followed by repeated fractional distillation of the volatiles.4
Alternatively, sublimation of [Ce(OtBu)4(THF)2] yielded the
tert-butoxide compound in a low amount.8 In order to develop a
more convenient synthesis, the heteroleptic cerium(IV) amide
[Ce(OtBu)2{N(SiMe3)2}2] seems to be a good starting material
that is easily accessible by the salt metathesis reaction of
[Ce(OtBu)2(NO3)2(THF)2] and LiN(SiMe3)2.9 Adding 2 equiv
of tert-butanol to cerium(IV) amide resulted in the formation of
homoleptic tetrakis(tert-butoxo)cerium(IV) exclusively (eq 1).
Received: October 13, 2013
Published: March 3, 2014
© 2014 American Chemical Society
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Communication
and in the course of sublimation, it gradually decomposed with
only a small portion of the complex observed as the sublimate.
The sample was maintained at this temperature for 3 days
followed by NMR spectroscopic analysis. Besides the resonance
corresponding to 1, additional signals for the oxo cluster
[Ce3O(OtBu)10] and the heterovalent complex [Ce3(OtBu)11]
(2) were found to appear. Apparently, the melting process
associated with a higher degree of freedom and molecular
agitation triggers intermolecular reactions under formation of the
trinuclear species, which seem to be the thermodynamically
stable products. Whereas the structure in the melt is not clear,
mass spectrometric measurements indicated the existence of the
monomeric complex [Ce(OtBu)4] in the gas phase, manifested
by the observation of fragments only up to m/z 432 ([M]+).
The structure in solution was first investigated by Bradley et al.
with ebullioscopic measurements of solutions in benzene and
toluene, which revealed a molecular complexity of 2.4.4 However,
the degree of oligomerization is strongly temperature- and
solvent-dependent, and often dynamic equilibrium is present in
metal coordination number, tert-butoxide can, on the one hand,
undergo an ether elimination to form the oxo alkoxide
[Ce3O(OtBu)10]. On the other hand, homolytic cleavage of the
Ce−O bond can be assumed, which results in the formation of 2
•
and the tert-butoxy radical OtBu. The highly reactive alkoxy
radical reacts further to isobutylene and an unidentified
byproduct.
Crystals of 2 could be obtained from a concentrated solution
of the decomposed 1. The heterovalent cerium(III)−cerium(IV)
complex crystallized, contrary to earlier reports,8 in the
monoclinic space group P21/a with two independent molecules
present in the asymmetric unit. The compound adopts the well-
known trinuclear M3O11 structural unit with a doubly capped M3
triangle and six terminal tert-butoxo groups (Figure 2). The
solution.10 In our hands, the H NMR spectrum of 1 at room
1
temperature showed only one resonance for the tert-butoxo
groups at 1.58 ppm. Upon lowering of the temperature to −70
°C, there were no signs of decoalescence of the signal, which
suggested either a fluxional behavior of the ligands or the
presence of monomeric species in solution. In order to estimate
the solution radius of the complex, the self-diffusion constant of 1
1
in CDCl3 was determined by performing a DOSY H NMR
experiment. Using the Stokes−Einstein equation, a radius of 18 Å
could be calculated, which is consistent with a dimeric structure
(see the Supporting Information, SI).
The propensity of homoleptic [Ce(OtBu)4(THF)2] to form
the oxo alkoxide [Ce3O(OtBu)10] in noncoordinating solvents is
a well-known phenomenon commonly observed in other metal
alkoxides of large tri- and tetravalent ions, although compre-
hensive elucidation of the specific reaction pathway is
unequivocally not established so far.2,6,11 Because of a rigid
exclusion of water, the elimination of tBu2O is assumed to be the
most likely formation mechanism for −M−O−M− bridges in
oxo alkoxide derivatives, which has already been proven for some
secondary metal alkoxides.11 The observation that this instability
is caused by an intrinsic coordination unsaturation was further
manifested by isolation of the mixed-valence tert-butoxide 2 from
the attempted synthesis of [Ce(OtBu)4(THF)2] starting from
cerium ammonium nitrate and NaOtBu.7
Figure 2. Molecular structure of 2. Thermal ellipsoids (Ce and O) are
shown at 50% probability. For clarity, carbon atoms are depicted as a
wireframe and hydrogen atoms have been omitted. Selected bond
lengths [Å]: Ce1−O1 2.126(8), Ce1−O2 2.137(7), Ce1−O7 2.438(6),
Ce1−O8 2.428(8), Ce1−O10 2.557(6), Ce1−O11 2.580(6), Ce2−O3
2.090(6), Ce2−O4 2.085(6), Ce2−O7 2.338(6), Ce2−O9 2.367(7),
Ce2−O10 2.499(6), Ce2−O11 2.495(6), Ce3−O5 2.097(8), Ce3−O6
2.104(9), Ce3−O8 2.353(8), Ce3−O9 2.379(6), Ce3−O10 2.521(7),
Ce3−O11 2.509(6).
In this work, the stability of 1 in noncoordinating solvents was
investigated by dissolving 1 in freshly dried benzene-d6 and
sealing the sample under vacuum in a 4 mm NMR tube in order
to stringently prevent any influence of atmospheric moisture or
residual water in glasswares. After 1 week at room temperature,
evolution of the decomposition products could be observed even
if the reaction was found to proceed at a very slow rate. When the
reaction time was prolonged and the temperature was gradually
raised up to 130 °C, the degradation was augmented and
eventually finished by heating the sample further up to 180 °C,
resulting in the formation of 2 and [Ce3O(OtBu)10] as products,
which were verified by NMR to be present in equal amounts.
Further, the resonances corresponding to an equivalent of
metal−oxygen bonds show the expected elongation with
increasing bridging mode of the alkoxo ligands. Further, the
trivalent cerium atom could be identified by significantly longer
mean metal−oxygen distances compared to cerium(IV). Because
of its paramagnetic nature, the complex exhibited three broad
resonances in the H NMR spectra with an integrative ratio of
6:3:2 according to the distinctive bridging motifs.
The pronounced tendency of 1 to oligomerize in order to
expand the coordination number of the cerium centers makes it
an interesting synthon for the synthesis of advanced heteroleptic
or heterometallic compounds. By the addition of a neutral donor
ligand to a solution of 1, the dimeric complex of the general
formula [Ce2(OtBu)8(L)] [L = HOtBu (3), OCPh2 (4)] is
formed according to eq 2.
1
t
isobutene and Bu2O (integrative ratio) could be detected.
Additionally, a broad signal at 1.2 ppm aroused, which showed no
coupling to any carbon nuclei, proven by various C−H
correlation experiments. Taking these findings into account,
we suggest two competing and independent mechanisms
involved in the decomposition of 1. With an increase in the
+L
[Ce (OtBu) ] ⎯→⎯ [Ce (OtBu) L ]
8( )
2
8
2
L = HOtBu, OCPh2
(2)
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Communication
The derivative 3 crystallizes in the orthorhombic space group
Pbcm, with both cerium ions present in a distorted octahedral
coordination environment (see the SI), whereby the neutral
alcohol ligand could not be distinguished from the alkoxo ligands
on the basis of the bonding parameters. The average Ce−O
distances for the terminal alkoxo groups are in the expected range
(2.093 Å),2,6 and the Ce−O bond lengths of the μ2-bridging
groups (2.452 Å) are in good agreement with similar complexes
exhibiting an octahedral face-sharing motif.2 Although analogous
complexes [M2(OtBu)8(HOtBu)] (M = U, Th) are reported, 3
represents the first structurally described example.12 The
heteroleptic derivative 4 crystallizes in the monoclinic space
group P21/n. Both cerium centers display an octahedra
coordination sphere and are linked over a face-sharing motif of
the octahedral (Figure 3). The benzophenone ligand binds in
alkoxides in the synthesis of tetravalent cerium coordination
compounds.7
ASSOCIATED CONTENT
* Supporting Information
■
S
Detailed experimental description and single-crystal structure
refinement data for 1−3 and X-ray crystallographic data files in
CIF format. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: sanjay.mathur@uni-koeln.de.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
The authors thank the University of Cologne for financial
■
1
assistance and Dr. N. Schlorer for performing the DOSY H
̈
NMR experiments. The financial support provided by the
European Commission in the form of SOLAROGENIX Project
310333 in the framework of FP7 is gratefully acknowledged.
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