Diethyl ether adducts of trivalent lanthanide iodides

Article abstract of DOI:10.1039/c9dt00775j

The synthesis and structural characterization of the first molecular complexes of lanthanide iodides supported by the weak-base, diethyl ether, are reported. Single-crystal diffraction studies reveal a conserved [LnI₃(mer-Et₂O)₃

Full text of DOI:10.1039/c9dt00775j

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DOI: 10.1039/C9DT00775J
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Diethyl Ether Adducts of Trivalent Lanthanide Iodides
a
a
a
a
a
Thaige P. Gompa, Natalie T. Rice, Dominic R. Russo, Luis M. Aguirre Quintana, Brandon J. Yik,
a
a,b
John Bacsa, and Henry S. La Pierre*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
The synthesis and structural characterization of the first molecular
complexes of lanthanide iodides supported by the weak-base,
diethyl ether, are reported. Single-crystal diffraction studies reveal
a conserved [LnI
and Tm). These precursors are for all lanthanides La to Tm
excluding Pm and Eu) from lanthanide metal and iodine in diethyl
ether and provide the THF adducts in good to excellent (60–91%)
yield in a two-step process.
3 2 3
(mer-Et O) ] structure (Ln = Ce, Pr, Nd, Sm, Gd, Tb,
(
Scheme 1. Two-step reaction scheme for synthesis of 2-Ln and 3-Ln
through diethyl ether adduct, 1-Ln. Yields are shown for the two-
step process and metals depicted in red are structurally
characterized as 1-Ln.
Starting material development and characterization remains an
important technical issue facilitating discoveries across the f- of diethyl ether supported lanthanide triiodides, their
1
-8 crystallographic characterization, and their conversion to more
block including the development of single-molecule magnets
and low-valent transuranic complexes.
9
-12
stable precursors.
These studies are
facilitated by anhydrous, hydrocarbon-soluble starting
materials that are well-defined and supported by weakly-
coordinating supporting ligands, often solvent adducts.
Numerous methods have been reported for the preparation of
trivalent lanthanide and actinide halides starting from other
metal halides,1
3-20
metal oxides, or bulk metal
21
22-32
and an
appropriate oxidant.
In order to access divergent properties of low-coordinate
and low-valent lanthanide and actinide complexes, precursor
complexes prepared in low-polarity solvents, including diethyl
2
3-24, 33
ether, have been developed.
isolated as partial solvates [MI
These materials are typically
O) ], where the amount of
3
(Et
2
x
diethyl ether remaining is dependent on isolation conditions
and is variable from batch-to-batch and no structural
information has been reported to-date. In our application of
these methods, it was discovered that the lanthanide triodides
exhibited noticeable, albeit sparing, solubility in diethyl ether.
Herein we report a bulk synthetic method for the preparation
a. Department of Chemistry and Biochemistry, Georgia Institute of Technology,
Atlanta, Georgia 30332-0400, United States.
b.
Nuclear and Radiological Engineering Program, Georgia Institute of Technology,
Atlanta, Georgia 30332-0400, United States.
†Electronic Supplementary Information (ESI) available: Experimental procedures
and crystallographic data (PDF and CIF). CCDC *(1897950-1897957). For ESI and
crystallographic data in CIF or other electronic format See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
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to 3-Ln in tetrahydrofuran is used for these two elements (see
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DOI: 10.1039/C9DT00775J
SI).
In the case of Pr, Nd, and Sm, the final reaction mixtures of
the iodine oxidation in diethyl ether were definitively colored
(pale green, pale blue, and yellow, respectively), indicating
partial solubility of the trivalent iodide complex in diethyl ether.
UV/vis spectra for these compounds were obtained to
accurately determine molar absorptivity of the observed
features (see SI). Figure 2 shows the UV/vis spectra for
saturated solutions of 1-Sm and 1-Nd in diethyl ether. The
saturated solution concentrations obtained by this method for
1
-Sm and 1-Nd were 6.65 mM and 29.1 mM, respectively. Due
to the low solubility and low molar absorptivity of the f-f
transitions for 1-Pr, accurate determination of concentration is
difficult. However, the saturated solution concentration can be
Figure 1. Molecular structure of 1-Pr with thermal ellipsoids shown
at 50% probability and H atoms are omitted for clarity.
The diethyl ether complexes of the lanthanide iodides are
prepared by treating lanthanide metal turnings (up to 1 gram)
slurried in diethyl ether with a slightly substoichiometric
amount of iodine (< 1.5 equivalents) dissolved in diethyl ether
(
Scheme 1). A substoichiometic amount of iodine is essential to
1
–
3
avoid the formation of red-brown [I ] . Due to the use of
substoichiometric iodine, it is crucial that high-quality metal
turnings are used to avoid incorporation of Ln in the product.
After 4 days, the complex, [LnI (Et O) ], 1-Ln, is isolated on a frit.
2 3
O
3
2
x
This material can be used directly in further reactions but must
be characterized by elemental analysis to determine amount of
coordinated diethyl ether for reactions that require careful
control of stoichiometry and have any residual metal removed
mechanically. In our laboratory, ether content after exposure to Figure 2. UV-vis spectra of 1-Nd and 1-Sm in diethyl ether.
vacuum depended on the surface area, absolute vacuum, and
estimated at around 3 mM. This observed solubility suggested
that the etherate complexes could be isolated and structurally
characterized. For 1-Ln (Ln = Ce, Pr, Nd, Sm, Gd, Tb, and Tm), X-
ray diffraction quality single crystals are grown by cooling
saturated diethyl ether solutions to -35°C. Each of the
lanthanide iodide ether adduct complexes adopt a pseudo-
octahedral geometry and with a meridional orientation of the
iodides and diethyl ether ligands (Figure 1). The complexes, 1-
Ln (Ln = Ce, Pr, Sm, Gd, and Tb), crystallize in the Pbcn space
group, giving isomorphic structures. Complexes, 1-Nd and 1-Tm
diverge. Complex 1-Nd is isolated in Pna21, while 1-Tm
crystallizes in P-1 and does not yield a satisfactory refinement
metal identity and varied between 1.9 and 0.6 ethers per metal
ion (see SI for details of methodology). In practice, well-defined
solvento- complexes are obtained by tetrahydrofuran Soxhlet
extraction of the [LnI
of the lanthanide iodides, [LnI
LnI (THF) ][LnI (THF) ], 3-Ln. The residual metal and metal
3 2
(Et O)
x
] residue to afford the THF adducts
3
(THF) ], 2-Ln, or
4
[
2
5
4
2
oxide remain on the frit. The yield for this two-step process is
good to excellent (60-91%). The neutral, 2-Ln, or charge-
separated form, 3-Ln, was established by comparison of lattice
parameters with known structures or full structural
1
8, 22, 34
characterization (See Fig. S8).
There is, however,
flexibility in this dichotomy of structures as it has been
demonstrated that recrystallization from toluene will lead to
the isolation of all lanthanides in the charge-separated
(however, connectivity was confirmed: a representation of
connectivity and initial lattice parameters are included in the
SI). These divergent crystal systems demonstrate the relatively
soft potential the complexes have for crystallization and the
sensitivity to diethyl ether loss.
2
4
system. Bulk phase purity was established by complexometric
titration (see SI).35 The isolation of the trivalent lanthanide,
diethyl ether adducts proved to be unsuitable for metals with
accessible divalent oxidation states – namely Eu and Yb.
Oxidation of the lanthanide metal in diethyl ether resulted in
the formation in a mixture of divalent and trivalent products.
This product mixture suggests that the divalent intermediate is
only slowly oxidized to the trivalent state. Instead, a direct route
3 2 3
The crystal structure of 1-Pr, [PrI (mer-Et O) ], is shown in
Figure 1. This complex is isostructural with the other 1-Ln
complexes and is representative for the series. The
praseodymium atom is coordinated by the three iodides and
three diethyl ether molecules, forming a pseudo-octahedral
coordination sphere. The Iax–Pr–Ieq bond angles are both
8
9.124(6)° and the Iax–Pr–Iax bond angle is 178.248(12)°. The
2
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Pr–Iax bond lengths are 3.0812(3) Å while the Pr–Ieq bond length correlate well with the bond valences of the bonds. The
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DOI: 10.1039/C9DT00775J
is 3.0441(4) Å, giving an average length of 3.069(3) Å. There is volumes of Dirichlet domains (V, Å ) and the corresponding radii
similar variation in the Pr–O distances which span 2.406(2) to of spherical domains (Rsd, Å) are characteristic for the atoms in
2
3
8
.522(4) Å and average 2.445(3) Å. The average Ln–I bond a given oxidation state. There is a systematic reduction in the
length for each structurally characterized complex is consistent polyhedral volumes, average bond lengths, VDP’s and Rsd’s
with lanthanide ion contraction as atomic number increases. across the series. The solid angles for the same atom types stays
Figure 3 demonstrates the linear decrease in bond length of relatively constant across the series. The significant result is
analogous equatorial iodide of the crystallographically that the VDP’s and Rsd’s reflects the atomic sizes of the
characterized 1-Ln complexes in the Pbcn space group (Ln = Ce, lanthanide atom in these compounds. The solid angle is
2
Pr, Sm, Gd, Tb). A linear fit to the data (with an R value of proportional to the portion of the valence electron density from
0
.9908) gives a slope of 1.0776. The y-intercept is 1.974, slightly the lanthanide atom which is taking part in the formation of
3
6
shorter than the iodide ionic radii. This model is consistent bonds. This relationship has been interpreted as an analogue of
with structural indications of ionic bonding.
3
7
valence-electron density in the volume between the interacting
3
9
atoms. Since the sum of the bond valences is equal to the
oxidation state of the metal atom (+3 in all these compounds),
the valences to the same atom types and the solid angles are
expected to be constant for all these compounds.
After Soxhlet extraction, 2-Ln or 3-Ln can be utilized to
produce synthetically useful quantities of standard lanthanide
7 7 3 3
complexes. The tris-benzyl complex, [Ce(C H ) (THF) ], is
produced in comparable yields as reported with similar starting
4
0
reagents, while increased yields are reported for materials
4
1
3 2 3
such as [Ce[N{Si(Me) } ] ], see SI. These reactions are
accomplished by reacting 2-Ln or 3-Ln with an alkali metal salt
of the ligand, i.e. potassium benzyl or potassium
bis(trimethylsilyl)amide in appropriate stoichiometries.
In summary, a standard methodology to produce lanthanide
triiodide etherate complexes is established for early- and late-
lanthanides, La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, and Tm. This
method offers a consistent route for the production of trivalent
lanthanide iodide precursors on reasonable scales for bulk
In these complexes, the Ln ion was six coordinate and the
Figure 3. Plot of the equatorial Ln–I bond distance relative to
trivalent metal ionic radii in structurally characterized 1-Ln
compounds in the Pbcn space group. Error bars are smaller than the synthesis (~1 g). Crystallographic investigation of the 1-Ln
point size.
complexes reveal that these solvated lanthanide triiodides are
isostructural with three weakly bound diethyl ethers to each
lanthanide metal center.
1
correct space group was Pbcn (or its chiral enantiomer Pna2 ,
except for 1-Tm) with the Ln atom located at a special position
with point symmetry 2. Thus, the coordination polyhedron has
2
C symmetry. The coordination geometry is severely distorted
Conflicts of interest
from a regular octahedron because of the very different donor
properties of the diethyl ether and iodide ligands. Not
surprisingly, the ether molecules (and the I atoms) bind to the
Ln atom with substantially different Ln-O distances (and Ln-I
distances) despite having identical donor characteristics
because the trans pair of Ln-O bonds will form substantially
stronger bonds than the ether trans to the Ln-I bond. For
example, the Ce-O distance is 2.3518(11) for the trans ethers
and 2.4480(15) Å for the third ether. There is a correlation
between the asymmetric binding of the ether molecules and the
binding of the iodine atoms. The distortion of the geometry of
the octahedra from regularity was measured by the quadratic
There are no conflicts to declare.
Acknowledgements
Experimental studies were supported by start-up funds
provided by Georgia Institute of Technology. The authors would
like to thank Prof. William J. Evans for sharing data prior to
publication. Single-crystal diffraction experiments were
performed at the Georgia Institute of Technology SC-XRD
facility.
2
elongation (λoct) and the bond angle variance (σ oct). The
distortions are presented in Table S39 in the SI.
A useful method of quantifying the size and distortion of the
coordination geometry is by analyzing the Voronoi–Dirichlet
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DOI: 10.1039/C9DT00775J
The synthesis and structural characterization of molecular
complexes of lanthanide iodides supported by the weak-base,
diethyl ether, are reported.
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