Di- and triphenylacetates of lanthanum and neodymium. Synthesis, structural diversity, and application in diene polymerization

Article abstract of DOI:10.1021/om301020r

A new family of lanthanide complexes with di- and triphenylacetate ligands was prepared from LnCl₃. Dimeric diphenylacetates were the only isolated products of the reaction between LnCl₃ (Ln = La, Nd) and Ph₂CHCOONa. The exchange reaction between LnCl₃ and Ph₃CCOONa led to the formation of three structurally different types of neodymium and lanthanum complexes under different reaction conditions. Various coordination modes of the carboxylate ligands were revealed, including κ²- and κ¹-terminal coordination, μ₂-κ¹:κ¹-bridging, and μ₂-κ¹:κ²-semibridging. The treatment of La and Nd tris-triphenylacetates with excess AlEt₃ produces the dimeric tetraethylaluminates [Ln₂((μ₂- κ¹:κ¹-Ph₃CCOO) ₄(AlEt₄)₂]. The structures of the key La and Nd triphenylacetates that were obtained from these reactions were verified by X-ray diffraction, and the complex [(THF)₄Nd₂(Ph ₂CHCOO)₂(μ₂-κ¹: κ²-Ph₂CHCOO)₂(μ₂- κ¹:κ¹-Ph₂CHCOO)₂] was studied by high-resolution X-ray diffraction. Neodymium di- and triphenylacetate complexes can serve as precursors of catalysts for butadiene and isoprene stereospecific 1,4-cis polymerization.

Full text of DOI:10.1021/om301020r

Article
pubs.acs.org/Organometallics
Di- and Triphenylacetates of Lanthanum and Neodymium. Synthesis,
Structural Diversity, and Application in Diene Polymerization
Dmitrii M. Roitershtein,*,†,‡ Alexander A. Vinogradov,‡,§ Alexei A. Vinogradov,†,§
Konstantin A. Lyssenko,∥ Yulia V. Nelyubina,∥ Ivan V. Anan’ev,∥ Ilya E. Nifant’ev,*,†,⊥
Vladimir A. Yakovlev,† and Natalya N. Kostitsyna†
†A. V. Topchiev Institute of Petrochemical Synthesis, RAS, Leninsky pr. 29, Moscow, Russia 119991
‡L. Ya. Karpov Institute of Physical Chemistry, Obukha lane 3-1/12, Moscow, Russia 105064
§N. D. Zelinsky Institute of Organic Chemistry, RAS, Leninsky pr. 47, Moscow, Russia 119991
∥A. N. Nesmeyanov Institute of Organoelement Compounds, RAS, Vavilova str. 28, Moscow, Russia 119991
⊥M. V. Lomonosov Chemistry Department, Moscow State University, Leninskie Gory 1-3, Moscow, Russia 119991
S
* Supporting Information
ABSTRACT: A new family of lanthanide complexes with di-
and triphenylacetate ligands was prepared from LnCl3. Dimeric
diphenylacetates were the only isolated products of the
reaction between LnCl3 (Ln = La, Nd) and Ph2CHCOONa.
The exchange reaction between LnCl3 and Ph3CCOONa led
to the formation of three structurally different types of
neodymium and lanthanum complexes under different reaction
conditions. Various coordination modes of the carboxylate
ligands were revealed, including κ2- and κ1-terminal coordina-
tion, μ2-κ1:κ1-bridging, and μ2-κ1:κ2-semibridging. The treat-
ment of La and Nd tris-triphenylacetates with excess AlEt3
produces the dimeric tetraethylaluminates [Ln2((μ2-κ1:κ1-
Ph3CCOO)4(AlEt4)2]. The structures of the key La and Nd triphenylacetates that were obtained from these reactions were
verified by X-ray diffraction, and the complex [(THF)4Nd2(Ph2CHCOO)2(μ2-κ1:κ2-Ph2CHCOO)2(μ2- κ1:κ1-Ph2CHCOO)2]
was studied by high-resolution X-ray diffraction. Neodymium di- and triphenylacetate complexes can serve as precursors of
catalysts for butadiene and isoprene stereospecific 1,4-cis polymerization.
species, including the starting tricarboxylate and triphosphate
compounds. The use of low-symmetry lipophilic anions causes
neodymium derivatives to be soluble in hydrocarbons,
facilitating efficient polymerization processes.1a However, due
to the high electronic and steric unsaturation of the neodymium
ions in various compounds, such as NdV and NdP, these
neodymium derivatives form oligomers of uncertain composi-
tion because of the coordination of these neodymium ions with
the oxygen atoms of neighboring molecules.6,7
Little data are available from the literature regarding the
structure of carboxylate precatalysts in solution. Kwag
demonstrated that NdV typically exists in oligomers of variable
composition; moreover, he successfully used an additional 1
equiv of carboxylic acid to prepare a monomeric neodymium
tris-neodecanoate adduct. This monomeric complex demon-
strated a productivity that was nearly 10 times higher than the
productivity of the standard neodymium tris-neodecanoate for
INTRODUCTION
■
The most important application of lanthanide compounds in
homogeneous catalysis is the use of organic neodymium
carboxylates and phosphates as precatalysts in the 1,4-cis
polymerization of butadiene.1 In the past decade, several types
of rare-earth complexes with different types of ligands
demonstrating high cis-1,4 selectivity (over 99%) for butadiene
and isoprene polymerization have been described in the
literature.1b−i However, as shown in Scheme 1, the precatalysts
that are most frequently used for this process are saltlike
neodymium carboxylates, such as neodymium versatate (NdV;2
a mixture of isomers of different α,α-disubstituted (α,α-
dialkylated) decanoic acids that is also referred to as
neodymium neodecanoate)1a (A), and neodymium tris[bis(2-
ethylhexyl) phosphate] (NdP) (B).3−5
Surprisingly, although neodymium catalysts have been
extensively used in industry, both the polymerization
mechanism and the nature of the catalytically active
intermediates for these compounds remain uncharacterized.
The main cause of this phenomenon is a dearth of information
about the structures of both of the species that are directly
involved in the catalytic cycle and the precursors to these
Special Issue: Recent Advances in Organo-f-Element Chemistry
Received: October 29, 2012
© XXXX American Chemical Society
A
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Scheme 1
Scheme 2
butadiene polymerization.7 Anwander synthesized certain
individual lanthanide carboxylates by using bulky and
structurally rigid anions that were based on ortho-disubstituted
benzoic acids.8,9 He isolated individual lanthanide carboxylates
and was able to unambiguously characterize several products
that were formed by the alkylation of these carboxylates with
trimethylaluminium. This finding considerably contributed to
our understanding of neodymium catalysis in diene polymer-
ization. However, there is a considerable difference between
substituted lanthanide benzoates and complexes that are based
on α-branched carboxylic acids; thus, it is difficult to compare
these types of benzoates with neodymium versatates. Evans
studied lanthanum and neodymium 2-phenyl- and 2,2-
dimethylbutyrates and their interactions with organoaluminum
compounds.10,11 Unfortunately, these compounds turned out
to be too labile for isolation of heterometallic lanthanide−
aluminum complexes representing the active intermediates in
the catalytic systems for diene polymerization.
The standard workup procedure produced corresponding
carboxylates with the general formula of [Ln-
(Ph2CHCOO)3(THF)2]2 for Ln = La (1), Nd (2). Both of
these compounds were isolated as powders that were soluble in
THF and moderately soluble in chloroform and toluene
(Scheme 2).
In the present study, we suggested that the introduction of
structurally rigid phenyl groups into the α-position of the
carboxylate ligand would be favorable for the isolation of
various lanthanide carboxylates in crystalline form. With this in
mind, we undertook the synthesis of C, lanthanide di- and
triphenylacetates, which are close analogues of NdV. Surpris-
ingly, we found that the synthesis of lanthanum and
neodymium carboxylate complexes is accompanied by the
formation of several types of compounds; we were able to
isolate and characterize most of these compounds. This paper
focuses on the synthesis of these compounds, the investigation
of certain products formed from their reactions with
triethylaluminum, and preliminary data addressing their
applications for stereospecific butadiene and isoprene polymer-
ization.
Figure 1. General view of the complexes 1 (Ln = La, Solv = THF), 2
(Ln = Nd, Solv = THF), and 2b (Ln = Nd, Solv = dioxane) with
atoms shown as thermal ellipsoids (p = 50%). Hydrogen atoms and
carbon atoms of the phenyl groups and solvate molecules of these
complexes are omitted for clarity.
The methanol adduct [Ln(Ph2CHCOO)3(CH3OH)n] is
most likely formed as the primary product of the reaction of
LnCl3(THF)x with sodium diphenylacetate in methanol.
However, we were unable to isolate this compound and
characterize it because it could not be separated from NaCl.
This problem was circumvented through the use of diethyl-
amine instead of CH3ONa as a base, allowing for the isolation
of methanol adduct 2a, which could easily be separated from
the THF-soluble [Et2NH2]+Cl−. According to the analytical
data, complex 2a corresponded to the formula
RESULTS AND DISCUSSION
■
Synthesis of Lanthanide Carboxylates. Lanthanide
Diphenylacetates. Reactions of methanol solution of
LnCl3(THF)x (Ln = La, Nd) with 3 equiv of sodium
diphenylacetate, which was generated in situ from diphenyl-
acetic acid and CH3ONa, resulted in white (for La) or light
blue (for Nd) precipitates, which were extracted with dry THF.
B
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Scheme 3
Table 1. Selected Bond Distances (Å) in Compounds 1, 2, 2b, 7a, and 8a
1 (La)
2 (Nd)
2b (Nd)
7a (La)
8a (Nd)
Ln−O (THF, dioxane,
2.619(3), 2.647(3)
2.575(1), 2.606(1)
2.6064(9), 2.550(1)
2.620(1), 2.670(1)
2.567(3), 2.609(3)
DME)
Ln−O (κ2-RCOO)
2.605(3)−2.537(3)
2.717(3), 2.586(3),
2.470(3)
2.552(1)−2.476(2)
2.714(1), 2.398(1),
2.500(1)
2.5036(9)−2.504(1)
2.6263(9), 2.4105(9),
2.4967(9)
2.516(1), 2.544(1)
2.432(3)−2.532(3)
Ln−O (μ2-κ1:κ2-RCOO)
Ln−O (μ2-κ1:κ1-RCOO)
Ln- - -Ln
2.444(3), 2.468(3)
4.1434(8)
2.403(1), 2.390(1)
4.0796(3)
2.416(1), 2.4192(9)
4.0546(1)
2.431(1)−2.475(1)
4.428(1)
2.362(3)−2.417(3)
4.379(1)
[Nd2(Ph2CHCOO)6(CH3OH)5]x. The recrystallization of 2a
from THF produces compound 2 (Scheme 2).
The H NMR spectra of compounds 1 and 2 consist of
starting material of LnCl3(THF)x. In contrast to 1 and 2,
triphenylacetates of La and Nd are moderately soluble in
methanol. The solubility of these compounds was sufficient to
isolate and characterize the primary products of this reaction,
which were the methanol adducts of the mononuclear
carboxylate complexes [Ln(κ2-Ph3CCOO)2(κ1-Ph3CCOO)-
(CH3OH)4] for Ln = La (3), Nd (4) (Scheme 3). The
structures of 3 and 4 were resolved by X-ray crystallography
(Figure 2 and Table 2) (see Structural Studies for additional
details).
1
signals from methine protons of the Ph2CH groups and from
the phenyl protons, which are broadened in the case of 2.
Because all of the spectra were nearly uninformative about the
structures of these complexes, X-ray crystallography was used
to determine these structures (Figure 1 and Table 1).
Compounds 1 and 2 appeared to be isostructural binuclear
complexes that possessed a “Chinese lantern” structure with
both bridging and terminal carboxylate ligands (see Structural
Studies for additional details).
The subsequent treatment of 3 and 4 with THF led to the
formation of the THF adducts 5 and 6, respectively (Scheme
3). These compounds represent mononuclear complexes with
three κ2-coordinated carboxylate ligands [Ln(κ2-
Lanthanide Triphenylacetates. A similar synthetic approach
was applied to obtain triphenylacetates of La and Nd from the
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Figure 2. General view of the complexes 3 (Ln = La) and 4 (Ln =N d)
with atoms shown as thermal ellipsoids (p = 50%). Hydrogen atoms
have been omitted for clarity.
Figure 3. General view of one of the independent molecules of the
complex 6 with atoms shown as thermal ellipsoids (p = 50%).
Hydrogen atoms and carbon atoms of the phenyl groups and THF
solvate species have been omitted for clarity.
Ph3CCOO)3(THF)3] for Ln = La (5), Nd (6) (Figure 3 and
Table 2) (see Structural Studies for additional details).
Heating under vacuum caused complexes 5 and 6 to readily
lose THF, producing compounds of composition [Ln-
(Ph3CCOO)3(THF)2]x for Ln = La (7), Nd (8). Presumably,
complexes 7 and 8 possess a dimeric or oligomeric nature. This
perspective is somewhat supported by the fact that the
recrystallization of either 5 and 6 or 7 and 8 from DME
generated the binuclear DME adducts [Ln-
(Ph3CCOO)3(DME)]2 for Ln = La (7a), Nd (8a) (Scheme
3, Figure 4, and Table 1).
1H NMR spectra of 5 and 6 that were obtained in toluene-d8
and chloroform-d were nearly uninformative, due to the
broadening of the signals that were assigned to the phenyl
group protons. This broadening was even observed in the case
of 5, a diamagnetic compound, and apparently reflected the loss
of coordinated THF molecules and the formation of oligomeric
species that were similar to 7 and 8 which had nonequivalent
Ph3C− groups. A spectrum of 5 that was obtained in THF-d8
displayed three well-resolved phenyl group signals, confirming
this hypothesis.
We found that the byproducts that are formed in the reaction
of La and Nd chlorides with sodium triphenylacetate are the
unusual ate complexes 9 and 10 that contain five carboxylate
moieties, including a free acid molecule, and a sodium cation
(Scheme 3). Interestingly, from the starting materials of La
chloride and Nd chloride, the appropriate stoichiometry can be
used to generate compounds 9 and 10, respectively, with yields
of 61 and 81%, respectively. If La and Nd chlorides react with
an excess of sodium triphenylacetate in the presence of DME,
Figure 4. General view of the complexes 7a (Ln = La) and 8a (Ln =
Nd) with atoms shown as thermal ellipsoids (p = 50%). Hydrogen
atoms and carbon atoms of the phenyl groups are omitted for clarity.
Although the complex 8a corresponds to a C1 point group in the
crystal, the distribution of bond lengths is rather similar to those in a
centrosymmetric arrangement, as in 7a.
another ate complex containing four carboxylate moieties (11)
is formed. The molecular structures of compounds 9−11 were
determined by X-ray diffraction studies (Figures 5 and 6 and
Table 3; see Structural Studies for additional details).
Table 2. Selected Bond Distances (Å) in Complexes 3, 4, and 6
a
3 (La)
4 (Nd)
6 (Nd)
Ln−O (CH3OH, THF)
2.514(2) (O(1C))
2.568(2) (O(2C))
2.5650(2) (O(3C))
2.619(2) (O(4C))
2.438(2)
2.4501(14) (O(1C))
2.5207(14) (O(2C))
2.5016(14) (O(3C))
2.5558(15) (O(4C))
2.388(1)
2.541(3)−2.569(3)
Ln−O (κ1-Ph3CCOO−)
Ln−O (κ2-Ph3CCOO−)
2.546(2)−2.613(2)
2.490(1)−2.556(1)
2.441(3)−2.484(3)
a
The bond lengths are given for only one of the three independent molecules in 6.
D
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Table 3. Selected Bond Distances (Å) for Complexes 9−11
and 11a
9 (La)
10 (Nd)
11 (La)
11a (La)
Ln−O (THF,
2.654(2),
2.584(2)
2.604(2),
2.543(2)
2.625(2),
2.656(2)
2.638(2),
2.632(2)
DME)
Ln−O (terminal
2.522(2),
2.557(2)
2.465(2),
2.497(2)
2.564(2),
2.526(2)
2.540(2),
2.574(2)
Ph3CCOO−)
Ln−O (μ2-κ1-κ2-
Ph3CCOO−)
Ln−O
2.516(2),
2.545(2)
2.461(2),
2.490(2)
2.468(2),
2.580(2)
2.542(2),
2.538(2)
Ln−O (Na)
2.580(2),
2.598(2)
2.540(2),
2.564(2)
2.708(2),
2.597(2)
2.628(2),
2.598(2)
Ln−O (μ2-κ1-κ1-
2.519(2)
2.278(3)
2.296(2)
2.445(2)
2.273(2)
2.293(2)
2.397(2)
2.425(2)
Ph3CCOO−)
Na−O (THF,
2.398(3),
2.310(3)
2.316(2),
2.392(2)
DME)
Figure 5. General view of the complexes 9 (Ln = La) and 10 (Ln =
Nd) with atoms shown as thermal ellipsoids (p = 50%). Hydrogen
atoms, carbon atoms of the phenyl groups, and solvate molecules are
omitted for clarity.
Na−O (μ2-κ1-κ1-
2.280(3)
2.272(2)
Ph3CCOO−)
Na−O (μ2-κ1-κ2-
2.298(2),
2.344(2)
2.274(2,)
2.332(2)
2.279(3),
2.336(3)
2.282(2),
2.303(2)
Ph3CCOO−)
Na−O
2.354(2)
2.341(2)
(Ph3CCOOH)
Ln- - -Na
3.810(2)
3.792(2)
3.782(2)
3.803(2)
Nd triphenylacetates with triethylaluminum. We revealed that
the treatment of 6 with excess of AlEt3 at a 1:5 reactant ratio in
toluene generated a pale greenish yellow solution from which
extremely moisture- and air-sensitive pale blue crystals of
[Nd2(Ph3CCOO)4(AlEt4)2] (13) were isolated after the
addition of hexanes. Similar reactions of AlEt3 with the
oligomeric compound 8 and the ate complex 10 also produced
compound 13 (Scheme 4).
It would be particularly interesting to isolate the product of
the reaction of the complex 6 with Al(i-Bu)3, because this
reaction takes place at the first phase of the catalyst system
preparation. Nevertheless, reactions of 5 and 6 with excess Al(i-
Bu)3 gave mixtures of compounds. All of the attempts to
separate these mixtures or to crystallize at least one individual
compound failed, probably due to the low stability of the
heterometallic lanthanide−aluminum complexes with a bridg-
ing isobutyl ligand or the high solubility of these complexes.
The structure of 13 was resolved by X-ray diffraction (Figure
7 and Table 4; see Structural Studies for additional details).
Complex 13 is composed of a dineodymium core and
contains four bridging carboxylate ligands that bear two
[AlEt4]− moieties; i.e., it is the product of the formal
substitution of two [AlEt4]− anions for two terminal
carboxylate ligands in the dimeric complex 8. In this instance,
two tetraethylaluminate anions are η3-coordinated to the
neodymium, causing the ethyl groups of [AlEt4]− to be
nonequivalent.
To assess whether this type of coordination environment
remains unchanged in a toluene solution, the diamagnetic
lanthanum analogue 12 was synthesized. This compound
appeared to be isostructural with 13 (see Structural Studies for
additional details). NMR studies of 12 in toluene solution
demonstrated that all of the ethyl groups of this compound are
equivalent on an NMR time scale; this equivalence likely
reflects a fast exchange process or solvation effects.
Figure 6. General view of the complex 11 with atoms shown as
thermal ellipsoids (p = 50%). Hydrogen atoms and atoms of the
phenyl groups are omitted for clarity.
Thus, we demonstrated that the reaction of LnCl3(THF)x
and sodium triphenylacetate generates a series of products,
including mononuclear, binuclear homobimetallic, and various
ate complexes; the product ratio is controlled by the solvent
and the reactant ratio that are utilized. In this series of
transformations, the mononuclear lanthanide complex is the
primary product, and it is subsequently converted to a binuclear
or ate complex. Because we used the bulky and structurally rigid
triphenylacetate ligand, we were able to isolate and reliably
characterize the broad range of carboxylates that were formed.
Presumably, similar processes may also occur for other
carboxylates, including the well-known neodymium versatates.
However, the impossibility of isolating these compounds in a
pure state precludes their characterization.
Reactions with Organoaluminum Compounds. The
reaction of neodymium carboxylates with trialkylaluminum
followed by reaction with alkylaluminum chlorides is the most
frequently used approach for obtaining a ternary catalyst system
for stereospecific diene polymerization.1a In this work, we
studied the first stage of this process: namely, the reaction of
Two notable points exist with respect to the formation of 8.
First, this compound is formed not only from binuclear
precursors but also from mononuclear compounds 6 and 12,
indicating that this binuclear structure is rather stable.
Therefore, a compound of type 13 can indeed be a prototype
E
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Scheme 4
carboxylate for aluminate appears to be surprisingly high
(greater than 50%), assuming that an excess of AlEt3 is used. It
appears that the structure of type 13 is kinetically stable due to
the presence of bulky triphenylacetate bridging ligands, causing
the further replacement of carboxylate ligands by aluminate
groups to be a complicated process. This characteristic may
produce adverse effects on the formation of catalytically active
species in diene polymerization, given that these species are
believed to result from the double replacement of carboxylate
by aluminate.1a,6
The reaction of 6 with Et3Al followed by treatment with
Et2AlCl produced a mixture of compounds that gave a viscous
oil after the evaporation of the solvent. All of the attempts to
separate any of the Nd-containing products failed; instead, the
only compound that was isolated by crystallization from a
hexane/toluene mixture and subsequently characterized by X-
ray diffraction studies was compound 14, diethylaluminum
triphenylacetate (Figure 8).
Figure 7. General view of the complexes 12 (La) and 13 (Nd) with
atoms shown as thermal ellipsoids (p = 50%). Hydrogen atoms and
atoms of the phenyl groups are omitted for clarity.
Application of Neodymium Di- snd Triphenylacetates
for Butadiene Polymerization in Ternary Catalyst
Systems. We tested several of the compounds that were
obtained in this work as potential precatalysts of butadiene and
of the intermediate formed during the reaction of neodymium
versatate with certain trialkylaluminum derivatives. The yield of
the product that is formed upon the monosubstitution of
Table 4. Selected Bond Distances (Å) in Compounds 12 and 13
12 (La)
13 (Nd)
Ln−O (μ2-κ1-κ1-
2.363(1)−2.419(1)
2.310(2)−2.379(2)
Ph3CCOO−)
Ln−C (C2H5)
Ln···Al
2.823(2)−3.044(2)
2.9960(7)−3.0172(6)
2.768(4)−3.044(3)
2.975(1)−2.944(1)
Al−C (C2H5)
1.971(2)−1.979(2) (noncoordinated), 2.027(2)−2.069(2)
1.972(4)−1.984(3) (noncoordinated), 2.023(3)−2.066(4)
(coordinated)
(coordinated)
F
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be active in the diene polymerization. The complex 6 with
AlEt3 cocatalyst (runs 7 and 11) demonstrated relatively low
activity in the polymerization of butadiene and isoprene.
Polymers that were obtained in these experiments had high
polydispersity indexes (runs 7 and 11) and low cis-1,4
selectivity for butadiene polymerization (run 7).
It was also interesting to compare the catalytic performances
of compounds 6 and 13 as precatalysts. 13 demonstrated
slightly higher activity than 6 (run 8 versus run 6; Table 5), and
the polybutadiene that was obtained with 13 had a significantly
higher Mw and lower polydispersity index than the poly-
butadiene that was obtained with 6. Presumably, complexes
similar to 13 might be responsible for the formation of the
high-molecular-weight fraction in the polymerization of
butadiene that occurs through ternary catalyst systems. The
detailed results from a thorough investigation of this
phenomenon will be described and discussed in a separate
contribution.
Structural Studies. Diphenylacetate complexes 1, 2, and
2b are similar (1 and 2 are isomorphous) in that all of these
complexes contain centrosymmetric Ln2(RCOO)4 dimer units
with four bridging carboxylates (two μ2-κ1-κ2 and two μ2-κ1-κ1
Ph2CHCOO− ligands), two terminal carboxylates, and
terminal O-donor ligands (THF for 1 and 2 but dioxane for
2b); thus, the metal of these complexes has a coordination
number of 9 (Figure 1 and Table 1).
The triphenylacetate complexes 7a and 8a are somewhat
similar to the aforementioned diphenylacetate complexes;
however, the chemical bonding pattern for the bridging
carboxylates differs between the examined triphenylacetate
complexes and the diphenylacetate complexes. In particular, all
of the bridging Ph3CCOO− ligands in 7a and 8a are μ2-κ1-κ1
coordinated, whereas in 1, 2, and 2b, μ2-κ1-κ1 and μ2-κ1-κ2
coordinated Ph2CHCOO− ligands are found. Each Ln atom in
7a and 8a is also coordinated by two terminal κ2-
triphenylacetate ligands and by DME; thus, the Ln atoms in
these two compounds have coordination numbers of 8 (Figure
4 and Table 1).
In all five of the aforementioned structures, the Ln−O
distances (for terminal carboxylates, bridging carboxylates, and
O-donor ligands) (Table 1) are in good agreement with the
distances that have been found in the literature for binuclear
carboxylate complexes of rare-earth metals.12 This structural
motif, featuring either four μ2-κ1:κ1 RCOO− ligands or a
Figure 8. General view of 14 with atoms shown as thermal ellipsoids
(p = 50%). Hydrogen atoms are omitted for clarity.
isoprene polymerization (Table 5). Neodymium complexes 2,
6, and 13 demonstrated good performance for both butadiene
and isoprene polymerization. Complex 1 appeared to be
virtually inactive in this role; this result was unsurprising,
because La derivatives are typically less active than Nd
complexes.1 Compound 2 appeared to be active for both 1,3-
butadiene and isoprene polymerization (as evidenced by runs 3,
4, 9, and 10). The low activity of 2a was apparently caused by
its coordinated methanol, which could destroy organo-
aluminum cocatalysts and lead to the formation of oligomeric
Nd alkoxides.
In comparison with compound 6, which bears bulky
triphenylacetate ligands, compound 2, which is less sterically
hindered, consistently demonstrated higher activity (as
evidenced by the comparison of runs 2 and 3 to runs 4 and
5). The polydispersity index of polybutadiene that was obtained
with a diphenylacetate-based catalyst system was lower than the
polydispersity index of polybutadiene that was produced by
triphenylacetate derivatives. It appears that, in the sterically
crowded neodymium triphenylacetate complex, the exchange
rate between carboxylate and alkylaluminate ligands is low,
favoring the simultaneous formation of several structurally
different catalytically active species.
As the heterometallic Ln−Al complexes were isolated only in
the reaction of La and Nd triphenylacetates with AlEt3, not in
the case of Al(i-Bu)3, it was necessary to make sure that the
catalytic system formed using AlEt3 instead of Al(i-Bu)3 would
Table 5. Polymerization of Butadiene and Isoprene in Combination with Ethylaluminum Sesquichloride and Different
a
Quantities of Al(i-Bu)3, AlEt3, and HAl(i-Bu)2
b
c
run
precat.
cocat.
Al :Nd
monomer
yield ,%
cis-1,4, %
trans-1,4, %
1,2 (3,4), %
Mw (× 10−3
)
Mn (× 10−3
)
Mw/Mn
1
1
Al(i-Bu)3
Al(i-Bu)3
Al(i-Bu)3
HAl(i-Bu)2
Al(i-Bu)3
HAl(i-Bu)2
Al(Et)3
30
30
30
15
30
15
30
15
30
15
30
butadiene
butadiene
butadiene
butadiene
butadiene
butadiene
butadiene
butadiene
isoprene
<10
38
2
2a
2
95.0
99.1
98.9
98.4
99.2
82.2
98.6
94.0
96.5
95
0.5
0.8
0.8
1.2
0.7
16.7
1.1
6.0
3.5
3
89
0.1
0.3
0.4
0.1
1.1
0.3
1059
580
421
96
2.53
6.05
4.03
7.59
11.29
3.29
2.10
4.37
9.58
4
2
>95
74
5
6
395
98
6
6
48
514
68
7
6
31
1120
1000
1307
645
105
304
621
147
57.5
8
13
2
HAl(i-Bu)2
Al(i-Bu)3
HAl(i-Bu)2
Al(Et)3
83
9
64
10
11
2
isoprene
95
6
isoprene
22
5
551
a
b
General conditions: [monomer]0/[Nd]0 = 10000 (5000 for runs 7 and 11), [Al2Et3Cl3]/[Nd] = 0.83, reaction time 3 h; T = 50 °C. Al = Al(i-Bu)3,
c
HAl(i-Bu)2, Al(Et)3. Isolated polymer.
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combination of two μ2-κ1:κ2- and two μ2-κ1:κ1 RCOO− ligands,
is quite common for lanthanide carboxylates. However, few
examples of discrete homodinuclear complexes have been
described for lanthanum and neodymium. Most of these
complexes correspond to the general formula of [L(RCOO)-
Ln(μ2-RCOO)4Ln(RCOO)L], where RCOO− is an alkyl or
aryl carboxylate and L is a chelating ligand, such as 1,10-
phenanthroline or 2,2′-bipyidine.13
involves a molecule that is involved in the weakest O−H···π
interaction (O···C = 3.665(2) Å) with one of the Ph rings.
Finally, intermediate values of the Ln−O(H)Me bonds are
observed for the molecules that participate in H bonds of
moderate strength with solvate methanol species. The observed
variation in the Ln−O(H)Me bond lengths is clearly related to
the degree of charge transfer that occurs as a result of H-bond
formation, which should have a maximum value for the
interactions with anionic ligands.14 Clearly, this charge transfer
will influence not only the lengths of the Ln−O bonds
involving solvate molecules but also the lengths of the Ln−O
bonds involving Ph3CCOO− ligands.
It should be noted that the compounds 3−6 represent
relatively rare examples of mononuclear rare-earth-metal
carboxylate complexes. A few examples of these types of
complexes have been structurally characterized; these examples
primarily consist of complexes with substituted benzoate,
pyridinecarboxylato, acetylglycinato, acetate, and trimethyl- and
trichloroacetato ligands.9,15−18
As expected, the differences in the Ln−O bond lengths
between La and Nd complexes is governed by the lanthanide
contraction, with the only exception being the Ln−O bonds for
the μ2-κ1:κ2 type of coordination. The majority of the Ln−O
́
bonds in 2 and 8a are shortened by ca. 0.06−0.08 Å in
comparison with the corresponding bonds in 1 and 7a. The
Ln(1)−O(5A) bond length in the μ2-κ1:κ2-coordinated
RCOO− ligand is, however, the same in 1 and 2 but is
shortened by 0.09 Å in 2b. This finding can be an indication of
the forced character of this type of additional coordination.
Changes in the coordination mode of the RCOO− group
from the combination of μ2-κ1:κ1 and μ2-κ1:κ2 modes that are
observed in compounds 1, 2, and 2b to the μ2-κ1:κ1 mode that
is observed in compounds 7a and 8a causes the elongation of
Ln- - -Ln distances (Table 1).
Another structural type which was obtained in the same
reaction (Scheme 3) as for the complexes 5−8 is ate complexes
9−11 (Figures 5 and 6, Table 3). Two species of this type were
isolated: namely, one containing a coordinated molecule of the
Ph3CCOOH acid (9 and 10) as well as the “regular” ate
complexes 11 and 11a.
The use of triphenylacetates allows for a number of
complexes with different coordination environments to be
obtained. In particular, in the case of triphenylacetates of La
and Nd, the mononuclear complexes 3−6 that were formed by
the reaction of LnCl3(THF)x and Ph3CCOONa (Scheme 3)
were isolated, and the crystal structures of these complexes
were analyzed (Figures 2 and 3).
The coordination environment of La and Nd cations in the
isostructural complexes 9 and 10, respectively, is composed of
the oxygen atoms of two THF molecules and of seven oxygen
atoms of four Ph3CCOO ligands having different types of Ln−
O coordination: terminal, bridging, and two semibridging. The
coordination number of Ln is equal to 9, and the lanthanide
polyhedron is a distorted capped tetragonal antiprism. The
environment of the sodium cation is a distorted trigonal
bipyramid in which two equatorial positions are occupied by
the oxygen atoms of semibridging μ2-κ1:κ2 Ph3CCOO ligands
and the third one by an oxygen atom of the Ph3CCOOH
molecule. In the apical positions, oxygen atoms of the bridging
μ2-κ1:κ1 Ph3CCOO ligand and THF are located. The OH group
of the acid molecule is involved in a hydrogen bond with the
O(7) atom of the bridging Ph3CCOO ligand with an O···O
separation equal to ∼2.62 Å.
The lanthanum ate complex with coordinated DME
molecules was isolated in two crystalline modifications, as
monoclinic (11) and triclinic forms (11a) (see Table 7).
Despite the exchange of THF molecules by one DME molecule
in 11 and 11a, the general features of chemical bonding are
rather close to those observed in 9 except for a less distorted
character of the capped tetragonal prism and significant
shortening of the La−O bond for the μ2-κ1:κ1 Ph3CCOO
ligand in 11 and 11a (Table 3). The same is true for the
sodium atom, for which the positions of THF and Ph3CCOOH
ligands in 9 are replaced by a DME molecule. Furthermore, in
one of the modifications (11) there is a weak coordination
between the sodium atom and one of the phenyl rings of the
Ph3CCOO ligand; the shortest Na···C distance is equal to
3.048(2) Å (Figure 6).
Complexes 3 and 4 are isostructural and crystallize with two
solvate methanol molecules. An interesting feature of their
structures is the coexistence of κ1- and κ2-coordinated
carboxylate ligands, which maintains the coordination number
of the lanthanide cations at 9. In both complexes, the
lanthanide cation is surrounded by two κ2-coordinated
Ph3CCOO− ligands, one κ1-coordinated Ph3CCOO− ligand,
and four CH3OH molecules. The Ln−O bond for the κ1-
coordinated Ph3CCOO is consistently shorter than the Ln−O
bonds for the κ2-coordinated Ph3CCOO by more than 0.1 Å.
Surprisingly, nearly equal average Ln−O bond lengths are
found between compounds 3 and 7a and between compounds
4 and 8a, despite the fact that the two compounds in each of
these pairs involve different lanthanide coordination numbers
(Table 2). It is likely that this effect reflects both the intrinsic
characteristics of the solvate molecules (i.e., steric volume and
donating ability) and crystal-packing effects. In fact, the
recrystallization of complex 4 from THF led to the formation
of the new complex 6, which is characterized by C3 symmetry.
In complex 6, the Nd atom is coordinated with three κ2-
Ph3CCOO− ligands and three THF molecules (Figure 3 and
Table 2). The Nd−O bonds in complex 6 are slightly shortened
(2.441−2.484(3) Å) in comparison to those in complex 4.
The role of crystal packing and H bonds can be illustrated
within the analysis of Ln−O(H)Me bond lengths in 3 and 4.
The Ln−O(H)Me bond lengths vary across a wide range
(2.514(2)−2.619(2) and 2.450(1)−2.556(1) Å in compounds
3 and 4, respectively) and are governed by the peculiarities of
inter-/intramolecular hydrogen bonding. In fact, the shortest
Ln−O(1C) bond involves a methanol molecule that partic-
ipates in the intramolecular O−H···O hydrogen bond (O···O =
2.586(1), 2.595(1) Å) with an κ1-coordinated Ph3CCOO−
ligand, whereas the longest bond of this type, Ln−O(4C),
Analysis of the geometry in the above two types of ate
complexes revealed a significant difference between μ2-κ1:κ2
coordination of Ph3CCOO− ligands in 9−11 and those
observed in 1 and 2 (see above). Indeed, in all the ate
complexes shortening of the Ln−O bond with a μ2-κ1:κ2
Ph3CCOO− ligand is observed for the semibridging oxygen
atom (compare Table 3 and Table 1). This shortening,
H
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Figure 9. Two Newman type projections illustrating the mutual disposition of the [Al(Et)4]− anion with respect to the LaO4 fragment in 12. In each
case, only half of the molecule is shown. The carbon atoms of triphenylacetate ligands are omitted for clarity.
Figure 10. Deformation electron density (DED) maps illustrating the pecularities of chemical bonding in the case of bridging (A) and semibridging
(B) carboxylate groups. The DED contours are drawn with 0.05 e Å−3 steps; negative values of DED are shown as dashed lines.
apparently, results from the lower value of the Na ionic radius
in comparison with that of Ln. The latter point is also reflected
in the decrease of Ln···Na separation in comparison to Ln···Ln
in 1 and 2.
An interesting feature of the above complexes is a clear
interplay between the η3-coordinated Al(C2H5)4 and
Ph3CCOO ligands. Indeed, in the La complex the oxygen
atoms of bridging ligands, which are located in a staggered
conformation with respect to the CH2 groups of the aluminate
anion (Figure 9), are characterized by the longest La−O bonds
(2.419(1), 2.417(1) Å); these CH2 groups (C(45), C(55)
atoms) also form the shortest La−C bonds (2.823(2), 2.908(2)
Å). The observed relation between the bond lengths in the
central fragment and Al(C2H5)4 anions makes possible to
propose the important covalent contribution to the La···Al-
(C2H5)4 interactions, which is in line with the rather high
stability of 12 and 13 in both the solid state and solution.
Diethylaluminum triphenylacetate (14), which was isolated
in the reaction of 6 with organoaluminum compounds,
crystallizes as a centrosymmetric dimer with an almost planar
eight-membered cycle (Figure 8) in which the metal atoms are
coordinated by two ethyl groups and two oxygen atoms of the
carboxylate moiety in a tetrahedral fashion. The corresponding
Al−O and Al−C bonds are 1.816(1), 1.826(1) Å and 1.952(2),
1.953(2) Å, respectively, which is in good agreement with the
In the molecules of isostructural heterometallic complexes 12
and 13, the central fragment is composed of two Ln cations
connected by four bridging μ2-κ1:κ1 Ph3CCOO ligands and two
Al(C2H5)4 anions that are coordinated to the Ln cations in a η3
manner from both sides of the central fragment (Figure 7). The
structure of the central fragments in 12 and 13 is similar to that
observed in 7a and 8a except for some shortening of Ln−O
bond lengths (the average Ln−O lengths in pairs 7a, 12 and 8a,
13 are equal to 2.454, 2.387 Å and 2.388, 2.338 Å, respectively).
This shortening is likely to be the result of the coordination
number decreasing from 9 in 7a and 8a to 7 in 12 and 13. The
average bond lengths between Ln and CH2 groups of the η3-
coordinated Al(C2H5)4 anions are 2.93 Å (La−Cav) and 2.89 Å
(Nd−Cav). It should be noted that this coordination causes
significant elongation of the Al−C bonds by up to 0.1 Å in
comparison with the noncoordinated Et groups, providing an
inverse relation between the Al−C and Ln−C bonds (Table 4).
I
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Table 6. Topological Parameters in the CPs (3,−1) for the Nd−O Bonds in 2
bond
d, Å
ρ(r)/e Å−3
∇2ρ(r)/e Å−5
V(r)/au
he(r)/au
Econt/kcal mol−1
Nd(1)−O(3)
Nd(1)−O(5)
Nd(1)−O(2)
Nd(1)−O(1)
Nd(1)−O(1S)
Nd(1)−O(2S)
Nd(1)−O(4A)
Nd(1)−O(6A)
Nd(1)−O(5A)
2.4025(7)
2.3961(7)
2.4761(9)
2.5542(8)
2.5751(7)
2.6083(8)
3.3908(7)
2.4985(8)
2.7136(8)
0.36
0.36
0.32
0.26
0.24
0.22
0.36
0.29
0.19
5.7
5.8
5.2
4.2
3.8
3.5
6.0
4.8
2.9
−0.06298
−0.06234
−0.05324
−0.03937
−0.03491
−0.03104
−0.06463
−0.04721
−0.02444
−0.00171
−0.00121
0.000186
0.002041
0.002457
0.002548
−0.0011
0.001404
0.002564
19.8
19.6
16.7
12.3
11.0
9.7
20.3
14.8
7.7
metric parameters reported for the similar dialkylaluminum
carboxylate [Me2Al(μ-OOCC6H2(i-Pr)3]2.9
according to Espinosa’s correlation scheme (EML) and are
summarized in Table 6. A summation of Nd−O bond energies
for different types of ligands in 2 (bridging and semibridging)
gave 40.0 and 42.0 kcal/mol, respectively. The similarity
between the latter values explains the variation in the type of
coordination involving tri- and diphenylacetate ligands. Given
that these energies are close, an increase in the steric volume of
a substituent can cause a variation in the coordination mode
observed in tri- and diphenylacetates 1, 2 and 7, 8.
It should be noted that the longest Nd(1)−O(5A) bond for
the μ2-κ1:κ2 RCOO− type of coordination has the lowest energy
equal to 7.7 kcal/mol; this explains its unexpected variation
upon the exchange of Ln atom in 1 and 2 or solvate molecule in
2 and 2b (see above). The total interaction energy for the
terminal ligands (29.1 kcal/mol) is significantly lower than for
bridging and semibridging ligands. For comparison, the energy
of interaction for the THF solvate molecules in a crystal is only
9.7−11.0 kcal/mol.
Charges of atoms in 2 were estimated by integration of the
ρ(r) function over atomic basins (Ω) surrounded by zero-flux
surface. The atomic charge for the Nd(1) atom in 2 (+1.2 e) is
very far from its formal value but rather close to one obtained
for a gadolinium complex.14a All the oxygen atoms are
characterized by negative charges varying in the range from
−0.95 to −1.31 e. The minimum negative charge (by the
absolute value) is observed for the oxygen atoms of the THF
molecules and the maximum for the atom O(5) involved in
three coordinated bonds (see Figure 1); thus, the obtained
atomic charges and the estimated energies of the Nd−O bond
are in line with each other. At the same time, the Nd−O bond
energies from high-resolution X-ray diffraction data are in good
agreement with the reactivity of lanthanide di- and
triphenylacetate. Thus, complex 2 easily transforms to 2b,
and the reactions of 6,−8 and 10 with AlEt3 are accompanied
by the exchange of terminal carboxylate ligands, while bridging
carboxylate ligands forming Nd−O bonds of higher energy are
more inert.
Charge Density Analysis. To estimate the relative energy
of the Ln−ligand bonds in the complexes studied, we have
performed a charge density analysis within Bader’s “Atoms in
Molecules” (QTAIM) theory for complex 2, which gave a
crystal of excellent diffraction quality.19
This approach involves the detailed analysis of the electron
density distribution function ρ(r) in a crystal obtained directly
from the X-ray diffraction data. The most important is that the
topological analysis of ρ(r) within QTAIM gives a unique
opportunity to estimate the energy of numerous interatomic
interactions: from weak H···H contacts to H bonds20 and even
to coordinate bonds21 using the correlation scheme suggested
by Espinosa, Mollins, and Lecomte (EML).22 The latter was
successfully applied in many cases, having proved to be suitable
for detailed analysis of chemical bonding patterns in, among
other systems, coordination compounds of lanthanides.14a
The main features of the electron density distribution around
the Nd ion in 2 are similar to those found in
[GdClPhen2(H2O)3]Cl2(H2O).14a In particular, deformation
electron density maps show four maxima formally assigned to
its 4f electrons (Figure 10), and oxygen lone pairs of
carboxylate groups are directed toward the above maxima
around the metal ion. This is somewhat different from the
chemical bond pattern observed in transition-metal complexes
with the so-called “peak−hole” types of interactions.23
Critical points (CP) (3,−1) of the electron density
distribution in the crystal 2 were located for all the expected
Nd−O bonds (Figure 1) as well as for the C−C, C−H, and C−
O bonds within the ligands. According to the topological
parameters of ρ(r) in these CPs (3,−1), the latter bonds
correspond to the shared type of interatomic interactions
(electron energy density he(r) and Laplacian of ρ(r) (∇2ρ(r))
values are negative), while the type of Nd−O bond depends on
the Nd−O bond lengths. Thus, Nd−O bonds with the lengths
less than ∼2.40 Å correspond to the intermediate type of
interactions (he(r) < 0, ∇2ρ(r) > 0) and cannot be considered
as fully ionic (Table 6). The covalent contribution in these
short Nd−O bonds in 2 exceeds at least the corresponding
contribution for closed-shell types of bonds involving alkaline
and alkaline-earth metals;24 thus, they are close to intermediate
bonds formed by transition metals in their complexes.23 In
contrast, the Nd−O bonds with lengths higher than the above
threshold value are characterized by positive values of both
∇2ρ(r) and he(r), which is indicative of a “closed-shell
interaction”.
CONCLUSION
■
We have studied the synthesis of La and Nd di- and
triphenylacetates and isolated a series of mono- and bimetallic
products, including mononuclear complexes, binuclear homo-
bimetallic complexes, and various ate complexes; the product
ratio is controlled by the solvent and reagent stoichiometry that
are selected. We determined that mononuclear carboxylates
that were stabilized with a donor ligand (MeOH or THF) are
the primary reaction products. These products can lose several
of their stabilizing donor ligands and either dimerize or
oligomerize.
The difference in the type of interatomic interaction the
Nd−O bonds belong to is illustrated by their energy. The
interaction energies Eint were estimated as −0.5[V(r)]
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crystals of 2 were grown by the slow diffusion of hexane into a
tetrahydrofuran solution.
Synthesis of [Nd(Ph2CHCOO)3(CH3OH)2.5]x (2a). Et2NH (1.4
Dimerization also occurs within the reaction of La and Nd
carboxylates with Et3Al. For La and Nd tris-triphenylacetates,
we demonstrated that the dimeric aluminate in which the
[AlEt4]− anions are η3-coordinated to the metal and occupy
terminal positions in the dimer is the major product that is
formed from the alkylation of both monometallic and bimetallic
carboxylate complexes.
In our opinion, similar processes can also occur during the
synthesis and interconversion of other saltlike lanthanide
derivatives, including well-known versatates and organic
neodymium phosphates.
mL, 13.5 mmol) was added to a stirred solution of Ph2CHCOOH
(2.860 g, 13.5 mmol) in 30 mL of dry methanol. The resulting
solution was added to a stirred solution of NdCl3(THF)1.8 (1.717 g,
4.57 mmol) in 15 mL of dry methanol. The suspension that was
produced was stirred for 6 h at room temperature, and the liquid was
filtered away. The solid was washed twice with 15 mL of dry methanol
and dried in vacuo to yield 2a as a blue powder (3.2 g, 84%). Anal.
Calcd for Nd2C89H86O17: C, 62.29; H, 5.05; Nd, 16.80. Found: C,
62.35; H, 4.88; Nd 16.82.
Synthesis of [La(Ph3CCOO)3(CH3OH)4]·CH3OH (3). A solution
of Ph3CCOONa (4.30 mmol) in 20 mL of dry methanol, which was
obtained from Ph3CCOOH (1.238 g, 4.30 mmol) and a solution of
CH3ONa in dry methanol (5.05 mL, 0.85 M), was added to a stirred
solution of LaCl3 (0.352 g, 1.43 mmol) in 25 mL of dry methanol. The
resulting white suspension was stirred for 2 h at room temperature,
and the precipitate was removed by decantation. The solution was
cooled to 0 °C, leading to the precipitation of 3 as colorless crystals
(1.3 g, 78%). Anal. Calcd for LaC65H65O11: C, 67.24; H, 5.64; La,
11.96. Found: C, 67.26; H, 5.57; La, 11.99. IR νmax (cm−1): 1550 (s,
COO−terminal). Single-crystal X-ray analysis revealed that the unit cell of
complex 3 contains two free methanol molecules; one of these solvent
molecules is lost after the compound is dried under vacuum.
Synthesis of [Nd(Ph3CCOO)3(CH3OH)4](CH3OH) (4). Crystals of
4 were prepared by the method that was described for 3, without the
removal of NaCl. NdCl3THF1.8 (0.671 g, 1.76 mmol), Ph3CCOOH
(1.525 g, 5.30 mmol), and a solution of CH3ONa in dry methanol
(4.33 mL, 1.22 M) were used in this preparation. A few crystals were
extracted for analysis. Anal. Calcd for Nd2C65H65O11: C, 66.93; H,
5.62. Found: C, 66.91; H, 5.34. IR νmax (cm−1): 1545 (s, COO−terminal).
Single-crystal X-ray analysis revealed that the unit cell of complex 4
contains two free methanol molecules; one of these solvent molecules
is lost after the compound is dried under vacuum.
EXPERIMENTAL SECTION
■
General Considerations. All of the described manipulations were
conducted under argon, using standard Schlenk and vacuum line
techniques. Argon was purified by passage through the columns of a
molecular sieve (4 Å) that was impregnated with a Na/K alloy and
vermiculite that was impregnated with Mn(II) oxide.25 Lanthanide
tetrahydrofuranates were synthesized according to the published
procedure.26 Diphenylacetic and triphenylacetic acids were recrystal-
lized from dry toluene and dried under vacuum prior to their use.
AlEt3 was purchased from Aldrich and was used as received. Methanol
was distilled from Mg/I2.27 Tetrahydrofuran, dioxane, and DME were
predried over NaOH and distilled from potassium/benzophenone
ketyl under argon. Hexane was distilled from an Na/K alloy
(benzophenone ketyl) under argon. Toluene was distilled from
sodium/benzophenone ketyl under argon. Toluene-d8 and THF-d8
(Deutero GmbH) were degassed and dried over an Na/K alloy.
CDCl3 (Deutero GmbH) was distilled over calcium hydride and than
degassed and stored in a Schlenk flask over activated molecular sieves
(4 Å). NMR spectra were recorded on Bruker AM-300, Bruker Avance
400, and Bruker DRX500 instruments. 1H shifts are referenced to
internal solvent resonances and reported in parts per million relative to
TMS. Infrared spectra were recorded on an IFS-66v/s Bruker
instrument. Elemental analyses were performed on a PerkinElmer
2400 Series II elemental analyzer. The lanthanide content was
determined by direct complexometric titration using xylenol orange as
an indicator. Hexane used in the catalyst synthesis and polymerization
was dried by reflux over sodium wire. The microstructures of
polybutadiene and isoprene were measured by IR spectroscopy
according to the literature.28 Gel permeation chromatography was
performed on a high-pressure chromatograph equipped with a
Styrogel HR 5E, HR 4E system of columns, a Rheodyne injector
with a 200-μL loop, and a Waters 2410 refractometer; THF was used
as the eluent.
Synthesis of [La(Ph3CCOO)3(THF)3] (5). Crystals of 5 were
1
prepared from a THF solution of 3 by the addition of hexane. H
NMR (300 MHz, THF-d8): δ 7.31−7.21 (m, 18H, Ph−), 7.05−6.98
(m, 27H, Ph−), 3.67−3.58. Anal. Calcd for La2C72H69O9: C, 71.04; H,
5.71; La, 11.41. Found: C, 71,11; H, 5,66; La, 11.48.
Synthesis of [Nd(Ph3CCOO)3(THF)3] (6). A solution of
Ph3CCOONa (10.33 mmol) in 30 mL of dry methanol was obtained
from Ph3CCOOH (2.973 g, 10.33 mmol) and a solution of CH3ONa
in dry methanol (8.32 mL, 1.24 M) and was then slowly added to a
stirred solution of NdCl3(THF)1.8 (1.308 g, 3.44 mmol) in 60 mL of
dry methanol. The resulting solution was stirred for 5 h at room
temperature, and the solvent was removed in vacuo. After the removal
of this solvent, 50 mL of THF was added, and the resulting precipitate
was removed by decantation. The solution was evaporated to a volume
of 5 mL, and 80 mL of hexane was added. The suspension was cooled
to 0 °C, leading to the precipitation of 6 as a blue powder (3.496 g,
83%). Anal. Calcd for NdC72H69O9: C, 70.73; H, 5.69; Nd, 11.80.
Found: C, 70.68; H, 5.76; Nd, 11.88. IR νmax (cm−1): 1545 (s,
COO−terminal). X-ray-quality crystals of 6 were grown by the slow
diffusion of hexane to a tetrahydrofuran solution.
Synthesis of [La(Ph3CCOO)3(THF)2]x (7). A sample of 5 (0,160 g,
0.131 mmol), was evacuated under dynamic vacuum at 100 °C for 10
h and cooled to room temperature to yield 7 (0.143 g, 95%). Anal.
Calcd for La2C68H61O8: C, 71.32; H, 5.37; La, 12.13. Found: C, 71.20;
H, 5.54; La, 12.09. IR νmax (cm−1): 1548 (m, COO−terminal), 1622 (m,
COO−bridge).
Synthesis of [La(Ph3CCOO)3(DME)]2(THF) (7a). Crystals of 7a
were prepared from a DME-THF solution of 5 or 7 by the slow
addition of hexane. Anal. Calcd for La2C132H118O17: C 70.33; H, 5.28;
La, 12.32. Found: C, 69.92; H, 5.17; La, 12.14.
Synthesis of [La(Ph2CHCOO)3(THF)2]2 (1). A solution of
LaCl3(THF) (892 mg, 2.81 mmol) in 20 mL of dry methanol was
added to 60 mL of a stirred solution of Ph2CHCOONa (8.46 mmol)
in dry methanol, which was obtained from Ph2CHCOOH (1.793 g,
8.46 mmol) and a solution of CH3ONa in dry methanol (7.0 mL, 1.21
M). The resulting white suspension was stirred for 4 h at room
temperature. The precipitate was filtered away and dried in vacuo, and
50 mL of THF was then added. The precipitate was removed by
decantation. The solution was concentrated to 10 mL, and 60 mL of
hexane was added slowly, leading to the precipitation of 1 in the form
1
of colorless crystals (2.143 g, 83%). H NMR (300 MHz, CDCl3): δ
7.10−6.25 (m, 60H, −Ph), 4.85−4.44 (br s, 6H, −Ph), 3.82−3.62 (m,
16H, THF), 1.91−1.78 (m, 16H, THF). Anal. Calcd for
La2C100H98O16: C, 65.50; H, 5.39; La, 15.15. Found: C, 65.55; H,
5.48; La, 15.19.
Synthesis of [Nd(Ph2CHCOO)3(THF)2]2 (2). This compound was
prepared by the method that was described for 1. NdCl3(THF)1.8
(1.357 g, 3.57 mmol), Ph2CHCOOH (2.240 g, 10.56 mmol), and a
solution of CH3ONa in dry methanol (14.1 mL, 0.75 M) were used to
produce 2 (1.940 g, 59%). Anal. Calcd for Nd2C100H98O16: C, 65.12;
H, 5.36; Nd, 15.64. Found: C, 64.79; H, 5.61; Nd, 15.70. IR νmax
(cm−1): 1545 (s, COO−terminal), 1600 (s, COO−bridge). X-ray-quality
Synthesis of [Nd(Ph3CCOO)3(THF)2]x (8). The compound was
prepared by the method that was described for 7. 6 (0.274 g, 0.224
mmol) was used to produce 8 (0.250 g, 97%). Anal. Calcd for
NdC68H61O8: C, 70.99; H, 5.34; Nd, 12.54. Found: C, 70.61; H, 5.15;
K
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Nd, 12.65. IR νmax (cm−1): 1539 (m, COO−terminal), 1575 (m,
COO−bridge).
Synthesis of [Nd(Ph3CCOO)3(DME)]2(DME) (8a). Crystals of 8a
were prepared from a DME-THF solution of 6 or 8 by the slow
addition of hexane. IR νmax (cm−1): 1545 (m, COO−terminal), 1630 (s,
COO−bridge). Anal. Calcd for Nd2C132H120O18: C, 69.45; H, 5.30; Nd,
12.64. Found: C, 69.27; H, 5.23; Nd, 12.59.
added with stirring. After 1 h, a 0.83 M solution of Me3Al2Cl3 (1 mL)
in hexane was added to the reaction mixture. The mixture was stirred
for 20 h at 20 °C and was then considered ready for use.
Butadiene Polymerization. The catalyst (0.3 mL, 0.003 mmol;
see the protocol above) was placed into an ampule under an argon
flow. Hexane (17 mL) was added, and butadiene (2.2 mL, 30 mmol)
was condensed (−78 °C). The ampule was sealed and heated to 50
°C. After 3 h, the ampule was opened and the reaction mixture was
poured into methanol (50 mL). The precipitated polymer was
collected, washed twice with methanol, and dried in vacuo.
Synthesis of [La(Ph3CCOO)3][NaPh3CCOO][Ph3CCOOH](THF)5
(9). A solution of LaCl3(THF) (350 mg, 1.42 mmol) in 20 mL of dry
methanol was slowly added to 20 mL of a stirred solution of
Ph3CCOONa (5.68 mmol) and Ph3CCOOH (1.45 mmol), which was
obtained from Ph3CCOOH (2.053 g, 7.13 mmol) and a solution of
CH3ONa in dry methanol (4.66 mL 1.22 M). After 4 h of stirring, a
white precipitate was formed. The mixture was stirred for 3 h at room
temperature, the solvent was removed in vacuo, and 40 mL of dry
tetrahydrofuran was added. This solution was separated from the
precipitate and evaporated to a volume of 5 mL; subsequently, 80 mL
of hexane was added. The resulting suspension was cooled to 0 °C,
leading to the precipitation of 9 in the form of colorless crystals (1.792
g, 61%). IR νmax (cm−1): 1706 (w, −COOH), 2584 (w, OH), 1550−
1570 (s, COO−bridge Na−Nd), 1550 (s, COO−terminal). Anal. Calcd for
LaNaC120H116O15: C, 73.53; H, 5.97; La, 7.09. Found: C, 73.66; H,
5.94; La, 6.98. X-ray-quality crystals of 9 were grown by the slow
diffusion of hexane into a tetrahydrofuran solution.
Synthesis of [Nd(Ph3CCOO)3][NaPh3CCOO][Ph3CCOOH]-
(THF)5 (10). The compound was prepared by the method that was
described for 9. NdCl3(THF)2.14 (435 mg, 1.07 mmol), Ph3CCOOH
(1.548 g, 5.37 mmol), and a solution of CH3ONa in dry methanol (3.5
mL, 1.22 M) were used to produce 10 (1.71 g, 81%). IR νmax (cm−1):
1706 (w, −C(O)OH), 2584 (w, OH), 1550−1570 (s,
COO−bridge Na−Nd), 1550 (s, COO−terminal). Anal. Calcd for NdNa-
C120H116O15: C, 73.33; H, 5.95; Nd, 7.34. Found: C, 73.28; H, 5.88;
Nd, 7.57. X-ray-quality crystals of 10 were grown by the slow diffusion
of hexane into a tetrahydrofuran solution.
Synthesis of [La(Ph3CCOO)2]2[Al(C2H5)4]2 (12). A solution of
Et3Al in toluene (1 mL, 1.03 M) was added to 10 mL of a stirred
solution of [La(Ph3CCOO)3(THF)2]x (7; 225 mg, 0.101 mmol) in
dry toluene. The mixture was stirred overnight at room temperature.
The slow addition of hexane to the solution resulted in the
precipitation of 12 in the form of colorless crystals (98 mg, 57%).
1H NMR (500 MHz, toluene-d8, δ): 7.04−6.87 (m, 60H, −Ph), 0.96
(t, 24H, −CH2CH3), 0.14−0.05 (m, 16H, −CH2CH3). Anal. Calcd for
La2Al2C96H100O8: C, 67.29; H, 5.88. Found: C, 67.98; H, 5.69.
Synthesis of [Nd(Ph3CCOO)2]2[Al(C2H5)4]2 (13). Method a. The
compound was prepared by the method that was described for 12.
[Nd(Ph3CCOO)3(THF)2]x (8; 965 mg, 0.839 mmol) and a solution
of Et3Al in toluene (4.18 mL, 1.07 M) were used to produce 13 in the
form of blue crystals (440 mg, 61%). Anal. Calcd for for
Nd2Al2C96H100O8: C, 66.87; H, 5.68. Found: C, 66.80; H, 5.99. X-
ray-quality crystals of 13 were grown by the slow diffusion of hexane
into a toluene solution.
X-ray Crystallography. Crystalline samples of 1, 2, 5, 6, 9, and 10
were grown from THF/hexane solution. X-ray-quality crystals of 7a
and 8a were obtained by the recrystallization of 7 and 8, respectively,
from a 1/1 mixture of THF and DME. Appropriate crystals for 2b
were obtained by the recrystallization of 2 from dioxane. Single-crystal
samples of 4 and 5 were obtained from methanol at 0 °C, whereas
crystalline samples of 12−14 were obtained by the slow diffusion of
hexane into a toluene solution. Single-crystal samples of 11 and 11a
were isolated as DME adducts from the THF/DME mixture during
the course of attempts to isolate 7a.
All diffraction data were collected on a Bruker SMART APEX II
CCD diffractometer (λ(Mo Kα) = 0.71072 Å, ω scans) at 100 K. The
substantial redundancy in data allows an empirical absorption
correction to be performed with SADABS, using multiple measure-
ments of equivalent reflections. The structures were solved by direct
methods and refined by full-matrix least-squares techniques against F2
in the anisotropic-isotropic approximation. For disordered Ph groups
and THF molecules in some of the crystals the refinement was
performed with additional constraints of C−C bond lengths and
anisotropic displacement parameters of corresponding atoms. The
positions of hydrogen atoms were calculated from a geometrical point
of view and refined with the riding model. In crystals of 7a, 8a, and
11a the analysis of the Fourier electron density synthesis has revealed
that some of the solvate THF (in 7a, 8a) and DME molecules (in 11a)
are strongly disordered. We could not establishe an appropriate model
for refinement of the above residual electron density corresponding to
these molecules. Thus, we had to exclude unresolved solvent by means
of the SQUEEZE procedure.29 All calculations were performed with
the SHELXTL software package.30 Crystal data and structure
refinement parameters are given in Table 7. CCDC 907196−907206
and 907208−907211 contain the supplementary crystallographic data
for this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data via www.ccdc.cam.ac.uk/data_
request/cif.
The multipole refinement of 2 was carried out within the Hansen−
Coppens formalism31 using the XD program package.32 Scattering
factors obtained from STO atomic relativistic wave functions were
obtained at PBE/QZ4P level of theory. The refinement was carried
out against F and converged to R = 0.0204, Rw = 0.0207 and GOF =
0.9269 for 28725 merged reflections with I > 3σ(I) and (sin θ)/λ > 1.0
Å−1. All C−C, O−C, and Nd−O bonded pairs of atoms satisfy the
Hirshfeld rigid-bond criteria (the maximum difference of the mean
square displacement amplitudes was 11 × 10−4 Å2). The only
exceptions are the C−C bonds of one of the disordered THF cycles.
The residual electron density for reflections with 2θmax < 75° was not
more than 0.31 e Å−3. Topological analysis of the experimental ρ(r)
function was carried out using the WinXPRO program package.33 The
estimation of the kinetic energy (g(r)) was based on Kirzhnits’s
approximation,34 relating it with values of the ρ(r) and its derivatives:
g(r) = (3/10)(3π2)2/3[ρ(r)]5/3 + (1/72)|∇ρ(r)|2/ρ(r) + (1/6)∇2ρ(r).
The usage of this relationship in conjunction with the virial theorem
(2g(r) + v(r) = 1/4∇2ρ(r)) provided the value of potential energy
density (v(r)) in the critical points from experimental diffraction data.
The accuracy of this approach for closed-shell and intermediate types
of interatomic interactions was shown for various complexes.35
Method b. A solution of Et3Al in toluene (2.7 mL 1.07 M) was
added to 10 mL of a stirred solution of 10 (568 mg, 0,289 mmol) in
dry toluene. The mixture was stirred overnight at room temperature.
The slow addition of hexane to the solution resulted in the
precipitation of 11 as blue crystals (102 mg, 41%). The results from
the elemental analysis of this compound were consistent with its
identification as 13.
Method c. A solution of Et3Al in toluene (2.47 mL 1.47 M) was
added to 10 mL of a stirred solution of 6 (0.633 mg, 0.518 mmol) in
dry toluene. The mixture was stirred overnight at ambient temper-
ature. The slow addition of hexane to the solution resulted in the
precipitation of 13 in the form of blue crystals (200 mg, 45%). The
results from the elemental analysis of this compound were consistent
with its identification as 13.
Preparation of the Polymerization Catalyst from Carbox-
ylates 1, 2, 2a, 6, and 13. A glass reactor was charged under argon
with 0.1 mmol of a carboxylate and hexane (7.5 mL); butadiene (0.15
mL, 2 mmol) was recondensed (−78 °C). The mixture was cooled to
−78 °C, and a 1 M solution of DIBAL-H (1.5 mL) in hexane was
M
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ASSOCIATED CONTENT
* Supporting Information
CIF files giving X-ray crystallographic ata for compounds 1, 2,
2b, 3, 4, 6, 7a, 8a, 9−11, 11a, and 12−14. This material is
available free of charge via the Internet at http://pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
*E-mail: roiter@server.ioc.ac.ru (D.M.R.); inif@org.chem.msu.
ru (I.E.N.).
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by Russian Foundation for Basic
Research (grant No. 10-03-00953, 11-03-00043, 12-03-33107),
the Program of the Russian Academy of Science, Chemistry and
Material Science Division and by the Council on Grants at the
President of Russian Federation (Program for State Support of
Young Doctors of Science, Project MD-1020.2012.3).
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Commun. 2005, 5787.
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