Synthesis and structural characterization of a series of carboxylic acid modified cerium(III) alkoxides

Article abstract of DOI:10.1002/ejic.200600616

A series of cerium alkoxides were synthesized from the reaction of Ce{N[Si(CH₃)₃]₂}₃ and the appropriate alcohol: neopentyl alcohol [H-OCH₂C(CH₃) ₃ = H-ONep], tert-butyl alcohol [H-OC(CH₃)₃ = H-OtBu], o-(tert-butyl)phenol {H-OC₆H₄[C(CH ₃)₃]-2 = H-oBP), 2,6-dimethylphenol [H-OC ₆H₃(CH₃)₂-2,6 = H-DMP], 2,6-diisopropylphenol {H-OC₆H₃[CH(CH₃) ₂]₂-2,6 = H-DIP}, 2,6-di-tert-butylphenol {H-OC ₆H₃[C(CH₃)₃]₂-2,6 = H-DBP}, or 2,6-diphenylphenol [H-OC₆H₃(C₆H ₅)₂-2,6 = H-DPP] using toluene (tol), tetrahydrofuran (THF) or pyridine (py). The precursors were characterized as [Ce(μ-ONep) ₂(ONep)]₄ (1), Ce₄(μ₃-OtBu) ₃(μ-OtBu)₄(OtBu)₅ (2), Ce ₃(μ₃-OtBu)₃(H-OtBu)₂(OtBu) ₃(H-OtBu)₂ (2a), Ce(OBP)₃(THF)₃ (3), [Ce(μ-DMP)(DMP)₂(solv)₂]₂ [solv = THF (4) and py (4a)], Ce(DIP)₃(THF)₃ (5), Ce(DPP) ₃(THF)₂ (6). Once isolated, several of these species were further reacted with a series of sterically varied carboxylic acid modifiers including isobutyric acid [H-O₂CCH(CH₃)₂ = H-OPc] and trimethylacetic acid [H-O₂CC(CH₃)₃ = H-OBc]. The products were isolated as [Ce(OR)(μ-ORc)(μ_c-ORc) (py)]₂ [OR = oBP, OBc: 7; DMP, OPc: 8; DMP, OBc: 9; DIP, OPc: 10]. These compounds were identified by single-crystal X-ray diffraction and powder XRD analyses. Several novel structure types are added to the cerium alkoxide family of compounds. Wiley-VCH Verlag GmbH & Co. KGaA, 2006.

Full text of DOI:10.1002/ejic.200600616

FULL PAPER
DOI: 10.1002/ejic.200600616
Synthesis and Structural Characterization of a Series of Carboxylic Acid
Modified Cerium(III) Alkoxides
Timothy J. Boyle,*[a] Louis J. Tribby,[a] and Scott D. Bunge[a]
Keywords: Metal alkoxides / Cerium alkoxides / Carboxylate ligands / Chelates
A series of cerium alkoxides were synthesized from the reac-
tion of Ce{N[Si(CH3)3]2}3 and the appropriate alcohol: neo-
THF (4) and py (4a)], Ce(DIP)3(THF)3 (5), Ce(DPP)3(THF)2
(6). Once isolated, several of these species were further re-
acted with a series of sterically varied carboxylic acid modifi-
ers including isobutyric acid [H–O2CCH(CH3)2 = H-OPc] and
trimethylacetic acid [H–O2CC(CH3)3 = H-OBc]. The products
were isolated as [Ce(OR)(µ-ORc)(µc-ORc)(py)]2 [OR = oBP,
OBc: 7; DMP, OPc: 8; DMP, OBc: 9; DIP, OPc: 10]. These com-
pounds were identified by single-crystal X-ray diffraction
pentyl alcohol [H–OCH2C(CH3)3
= H-ONep], tert-butyl
alcohol [H–OC(CH3)3 = H-OtBu], o-(tert-butyl)phenol {H–
OC6H4[C(CH3)3]-2
OC6H3(CH3)2-2,6
=
H-oBP}, 2,6-dimethylphenol [H–
H-DMP], 2,6-diisopropylphenol {H–
=
OC6H3[CH(CH3)2]2-2,6 = H-DIP}, 2,6-di-tert-butylphenol {H–
OC6H3[C(CH3)3]2-2,6 = H-DBP}, or 2,6-diphenylphenol [H–
OC6H3(C6H5)2-2,6
= H-DPP] using toluene (tol), tetra- and powder XRD analyses. Several novel structure types are
hydrofuran (THF) or pyridine (py). The precursors were char-
acterized as [Ce(µ-ONep)2(ONep)]4 (1), Ce4(µ3-OtBu)3(µ-
OtBu)4(OtBu)5 (2), Ce3(µ3-OtBu)3(µ-OtBu)3(OtBu)3(H-OtBu)2
(2a), Ce(oBP)3(THF)3 (3), [Ce(µ-DMP)(DMP)2(solv)2]2 [solv =
added to the cerium alkoxide family of compounds.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2006)
tems but the partially hydrolyzed smaller trinuclear species
proved to be the most useful for the generation of highly
dense TiO2 films and simple dinuclear species for ZrO2 thin
films.[1,2]
Introduction
The role of metal alkoxide [M(OR)x] precursors in ce-
ramic materials synthetic routes such as, chemical solution
(or the so-called “sol-gel”), metal organic chemical vapor
deposition (MOCVD), or emerging nanoparticle synthetic
methods have continued to expand. The growth is mainly
due to their commercial availability, low crystallization tem-
peratures, high solubility, high volatility, and other physical
properties. In addition, these compounds are easily modi-
fied through the manipulation of their ligand set which al-
lows for optimization of the final materials properties with-
out increasing the complexity of the processing conditions.
Typically, tailoring hydrolysis and condensation pathways
of the M(OR)x precursors is of interest and accomplished
by introducing a variety of ligands, including carboxylic ac-
ids (H–ORc). We have previously explored the structures
obtained for group 4 alkoxides, modified with a series of H–
ORc reagents, including: formic acid (H–O2CH = H-OFc),
acetic acid (H–O2CCH3 = H-OAc), isobutyric acid [H–
Recently, we have become interested in thin films of ce-
rium oxide (CeOx) as an anticorrosion material. In ad-
dition, CeOx has demonstrated a wide variety of other ap-
plications, such as barrier layers,[3–5] counter electrodes for
electrochromic glasses,[6] catalysts,[7–11] electrolytes for solid
oxide fuel cells,[12–14] polishing agents for soft and hard op-
tical glasses,[15,16] and numerous other applications. Due to
the widespread use of CeOx, it is surprising that only a few
structurally characterized cerium alkoxides [Ce(OR)3] have
been reported in the literature,[17–29] and no systematic stud-
ies were possible from this eclectic mix of potential precur-
sors. Because it is critical to know the structure of the pre-
cursor to be able to optimize the properties of the final
CeOx materials, it was necessary to develop a family of well-
characterized, systematically structurally diverse Ce(OR)3
precursors. Understanding the structural changes available
for these precursors is critical prior to generating tailored-
property CeOx thin films.
O2CCH(CH3)2
=
H-OPc], trimethylacetic acid [H–
O2CC(CH3)3 = H-OBc], or tert-butylacetic acid [H–
O2CCH2C(CH3)3 = H-ONc], in an effort to understand
how structure affects the final densification of thin films.[1,2]
A number of general constructs were isolated for these sys-
Therefore, we undertook the synthesis and characteriza-
tion of a series of Ce(OR)3 that would fill in several gaps
in the alkyl and aryl oxide derivatives to develop a system-
atic family of structurally characterized Ce(OR)3 complexes
which could be used to explore tailored materials. An
alcohol amide metathesis reaction pathway was selected to
synthesize these compounds [Equation (1)] in toluene (tol),
[a] Sandia National Laboratories, Advanced Materials Laboratory,
1001 University Boulevard SE, Albuquerque, New Mexico,
87105, USA
Fax: +1-505-272-7336
E-mail: tjboyle@Sandia.gov
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T. J. Boyle, L. J. Tribby, S. D. Bunge
FULL PAPER
tetrahydrofuran (THF), or pyridine (py). The products were H-DIP in toluene. Crystallization of the hexanes-soluble
identified as: [Ce(µ-ONep)2(ONep)]4 where ONep fraction of this reaction mixture yielded [Ce(µ-DIP)-
=
OCH2C(CH3)3 (1), Ce4(µ3-OtBu)3(µ-OtBu)4(OtBu)5 (2), (DIP)2]2. Unfortunately, when altering the pendant chain to
Ce3(µ3-OtBu)3(µ-OtBu)3(OtBu)3(H-OtBu)2 (2a), where an alkyl group, Na contamination was observed.[32] This
OtBu = OC(CH3)3, Ce(oBP)3(THF)3 {3, oBP = OC6H4- prevented us from using a standard route to the family of
[C(CH3)3]-2}, [Ce(µ-DMP)(DMP)2(solv)]2 [DMP
=
Ce(OR)3 that we were interested in isolating. It was there-
OC6H3(CH3)2-2,6, solv = THF (4); solv = py (4a)], Ce- fore necessary to adjusted our synthetic strategy, and we
(DIP)3(THF)3 [5, DIP = OC6H3[CH(CH3)2]2-2,6], and switched to an amide alcohol metathesis route [Equation
Ce(DPP)3(THF)3 [6, DPP = OC6H3(C6H5)2-2,6]. Com- (1)] based on our previous success with this route.[32] It is
pounds 1–5 were further reacted with a series of sterically critical for the synthesis of these compounds that a very
varied H–ORc modifiers and their products identified as: pure Ce(NR2)3 be used, which was obtained by repeated
[Ce(OR)(µ-ORc)(µc-ORc) (py)]2 [OR = oBP, OBc (7); sublimations and crystallizations.
DMP, OPc (8); DMP, OBc (9); DIP, OPc (10), py = pyri-
dine; µ = bridging; µc = chelating bridge]. The synthesis,
characterization, and crystal structures of 1–10 are dis-
Synthesis
cussed below.
For each reaction, the Ce(NR2)3 was dissolved in toluene
yielding a pale yellow solution. Upon addition of the de-
Ce(NR2)3 +3 H–ORǞCe(OR)3 +3 H–NR2
(1)
sired alcohol, the reaction mixture was warmed slightly and
turned clear with no precipitates observed for any of these
samples. Crystals were isolated by drastically reducing the
volume of the reaction and cooling the mixture to –25 °C.
FTIR data indicates for each sample that the crystals did
not possess any amide ligands and that the alcohols had
fully reacted. Solution-state NMR could not be obtained
due to the paramagnetic nature of the CeIII species isolated,
therefore, it was necessary to obtain single-crystal X-ray
structures to identify these compounds. The results of these
experiments are shown in Figure 1 to Figure 12 for 1–10,
respectively.
Results and Discussion
As mentioned previously, we are interested in under-
standing the structural types available from simple OR- and
ORc-modified Ce-based compounds prior to generating
tailor-made CeOx thin films. Only a limited number of
X-ray crystallographically characterized Ce(OR)3 species
have been reported: [Ce(µ-OiPr)(OiPr)3(H–OiPr)]2 [OiPr =
OCH(CH3)2],[27,28] Ce4(µ4-O)(µ3-OiPr)2(µ-OiPr)4(OiPr)8,[24,29]
Ce3(µ3-OtBu)2(µ-OtBu)3(OtBu)5(NO3),[20] [Ce(µ-OR)(OR)2]2
where OR = OtBu[23] or OCH(tBu)2,[26] Ce[OC(tBu)3]3-
(CNtBu)2,[22] [Ce{OCH(tBu)2}3(µ-DMAE)]2 [DMAE =
Me2N–C2H4–NMe–C2H4–O–],[21] and [Ce{OC(tBu)3}3]2-
(OC6H4O).[23] In contrast, significantly fewer aryl oxide de-
rivatives have been isolated, including Ce[OC6H3(tBu)2]3,[25]
Ce[OC6H3(tBu)2]3(CNtBu),[25] and Ce[OC6H3(C6H5)2]3.[17]
It is of note that other Ce-containing species are available
with fluorinated ligands, siloxide ligands,[19] or the inclusion
of other metals.[30,31]
This nonsystematic family of compounds does not lend
insight into the possible structure types available for CeOx
precursors and thus necessitated the controlled synthesis of
a series of Ce(OR)3. Initial attempts focused on the aryl
oxide ligands due to the dearth of structures available, the
ease with which steric bulk can be introduced in the ortho
positions, and our previous success using these ligands to
produce high-quality thin films. We were also interested in
several sterically demanding alkyl derivatives ONep and
OtBu due to our previous success using these ligands for
materials synthesis and their lack of availability in the lit-
erature as homoleptic precursors.[20]
Figure 1. Structure plot of 1. Thermal ellipsoids of heavy atoms
drawn at 30% level. Carbon atoms are drawn as ball and stick for
clarity.
Cerium Alkoxides
Several routes to these Ce(OR)3 were considered prior to
initiating this investigation. The first attempts were made
using a liquid ammonia route wherein CeCl3 was added to
Crystal Structures
For 1, the least sterically hindering ONep ligand led to
Na0 dissolved in NH3(l) at –78 °C followed by addition of the formation of a tetranuclear species, which is identical
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Carboxylic Acid Modified CeIII Alkoxides
FULL PAPER
to what was reported for the all of the lanthanide ONep
derivative, see Figure 1.[32–34] The central core consists of
four Ce metal centers arranged in a square, interconnected
by four bridging µ-ONep ligands and one terminal ONep
ligand per metal. Each of the Ce cations is five-coordinated
adopting a square-base-pyramidal (SBP) geometry. As re-
ported for the other [Ln(ONep)3]4 species,[32–34] there is sig-
nificant H-bonding interaction noted for this compound.
Increasing the steric demands from the ONep to the
OtBu ligand, under stoichiometric conditions, led to the
isolation of another tetranuclear species 2 (Figure 2). For 2,
the four octahedrally bound (Oh) metals are again arranged
in a square linked by µ-OtBu ligands; however, in contrast
to 1, three µ3-OtBu link the metal centers: Ce(1), Ce(2), and
Ce(3) through the use two of the µ3-OtBu while the remain-
ing µ3-OtBu bridges Ce(1), Ce(2), and Ce(4). To complete
the Oh geometry, Ce(1), Ce(2) and Ce(3) bind one terminal
OtBu ligand each and Ce(4) coordinates two ligands. This
structure is similar to what was observed for Ce4(µ4-O)[µ3-
OCH(CH3)2]2[µ-OCH(CH3)2]4[OCH(CH3)2]8,[24,29] which
uses an oxo ligand to achieve this structure; however, this
is the first structural arrangement of this type noted for
homoleptic Ln(OR)3. When excess H-OtBu is used, a cen-
trosymmetric trinuclear species 2a (Figure 3), similar to
other lanthanide alkoxides is isolated in the presence of
Lewis base solvents. For 2a, each Ce is bound by two µ3-
OtBu, two µ-OtBu and two terminal OtBu ligands adopting
a distorted trigonal bipyramidal (TBP) arrangement. By
charge balance, two of these ligands must be protonated, as
is commonly noted for metal alkoxides.[35,36] The increased
steric bulk of the bound Lewis base H-OtBu does not allow
for large clusters to form. The mixed charge NO3 derivative
Ce3[µ3-OC(CH3)3]2[µ-OC(CH3)3]3[OC(CH3)3]5(NO3),[20]
and other solvated Ln(OR)3 are known to adopt this struc-
ture as well.[37] It is interesting to note that the primary
alcohol H-ONep is often considered to be less sterically hin-
dering than the tertiary H-OtBu, but in this system, the
ONep-ligated species yields Ce metal centers with a lower
coordination number. One explanation for this phenome-
non may be the significantly greater cone angle swept out
by the ONep ligand as compared to that of the OtBu li-
gand.
Figure 2. Structure plot of 2. Thermal ellipsoids of heavy atoms
drawn at 30% level. Carbon atoms are drawn as ball and stick for
clarity.
Figure 3. Structure plot of 2a. Thermal ellipsoids of heavy atoms
drawn at 30% level. Carbon atoms are drawn as ball and stick for
clarity.
Because only slight changes in the structure were noted
for the pure alkoxides isolated above, in comparison to the
Systematically increasing the steric bulk by placing a
literature species, further probing of the influence of sterics methyl group in each ortho position, surprisingly led to the
on the structure of these compounds led to an investigation isolation of the dinuclear species 4, shown in Figure 5. Each
of aryl oxide derivatives. For this system, it was necessary of the Ce metal centers adopts a distorted Oh geometry by
to use Lewis base solvents to maintain solubility. Using an binding two µ-DMP, two terminal DMP, and two THF sol-
asymmetrically substituted aryl oxide (oBP) wherein the ste- vent molecules in the axial position. It was thought that
ric bulk of one of the ortho sites is increased to a tert-butyl the delicate balance between steric bulk and coordination
group, the first mono-substituted alkyl phenoxide lantha- strength of the solvent allowed a monomer to form as noted
nide alkoxide complex was isolated as 3 (Figure 4). It is of for 3 vs. the dinuclear for 4. Therefore, a stronger Lewis
note that several lanthanide alkali metal double alkoxides base was used in the synthesis to see if 4 could be converted
with 4-methylphenoxides[38,39] have been isolated pre- to a mononuclear species. Surprisingly, switching to py, lead
viously.[39,40] The tert-butyl groups of the oBP ligands all to the isolation of 4a which adopts an identical dinuclear
point away from the metal center allowing the coordination structure as observed for 4 but the THF ligands are re-
of three THF solvent molecules forming a fac Oh metal placed by py ligands (Figure 6). This is the first py adduct
center.
of Ce(OR)3 reported to date. Compounds 4 and 4a are
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T. J. Boyle, L. J. Tribby, S. D. Bunge
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Figure 4. Structure plot of 3. Thermal ellipsoids of heavy atoms
drawn at 30% level. Carbon atoms are drawn as ball and stick for
clarity.
Figure 6. Structure plot of 4. Thermal ellipsoids of heavy atoms
drawn at 30% level. Carbon atoms are drawn as ball and stick for
clarity.
analogous to [Y(µ-DMP)(DMP)2(THF)]2 structure pre-
viously reported.[41]
rectly related to the size of the cations employed. A similar
mononuclear structural arrangement was observed for the
DPP (6, Figure 8) ligated compound; however, for 6 only
two THF molecules are present. The previously reported
DPP derivative was also a monomer but had π interactions
from the ortho phenyl groups,[17–29] therefore, the addition
of coordinated solvent noted for 6 is not unexpected. For
the Ln(DPP)3(THF)2 Ln = La,[42,44] Nd,[42,44] Yb[45,46] com-
plexes, the large trans effect of the OAr and the THF li-
gands was also observed for 6.
Figure 5. Structure plot of 3a. Thermal ellipsoids of heavy atoms
drawn at 30% level. Carbon atoms are drawn as ball and stick for
clarity.
For compound 5 shown in Figure 7, the further increase
in steric bulk of the ortho positions to isopropyl groups cou-
pled with the use of Lewis base solvents, led to the forma-
tion of a monomer. The Ce metal center is Oh bound by
three DIP and three THF ligands in a fac arrangement, as
noted for the oBP ligated species and numerous literature
reports of solvated lanthanide monomers.[40] In compari-
son, the larger La cation only used two solvent molecules to
complete its coordination sphere for the same ligand set;[42]
however, smaller cations have also been found to yield tri-
Figure 7. Structure plot of 5. Thermal ellipsoids of heavy atoms
drawn at 30% level. Carbon atoms are drawn as ball and stick for
clarity.
The bond lengths and angles of this family of CeIII alk-
solvated species such as Sm(DIP)3(THF)3.[43] Therefore, the oxides, 1–6, are consistent with those of previously reported
number of solvent molecules coordinated can not be di- Ce(OR)3.[17–29,32–34,47,48] For 1, as the second largest cation
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Carboxylic Acid Modified CeIII Alkoxides
FULL PAPER
Ce–OR and 2.36 Å for Ce(µ-OR). The monomeric species
that are solvated[17,25,48] also have a similar average Ce–OR
distance of 2.17 Å. The mononuclear solvated 6 was found
to be in agreement with the metrical data reported for the
unsolvated but π-bound DPP;[17–29] the average Ce–OR dis-
tances were 2.21 Å for 5 and 2.24 Å for 6.
Carboxylic Acid Modified Cerium Alkoxides
It was of interest to add more protective groups to the
metal center to reduce the sensitivity to hydrolysis as well as
introduce new constructs for Ce(OR)3. This was undertaken
using H-ORc. Previous studies[1,50,51] have shown that sub-
stantial changes in the rate of hydrolysis and condensation
can be wrought through the introduction of H-ORc modifi-
ers both through esterification oxolation pathways and the
steric hindrance introduced by the bidentate nature of the
ORc ligand. It is of note that the µ- and µc-ORc ligands
(also referred to as bidentate and tridentate, respectively)
have been observed in a variety of lanthanide carboxylate
systems wherein over 1500 structures have been identified
revealing a variety of binding modes available for Ln–ORc
species.[40] A further search of the literature reveals many
Ce–ORc compounds have been structurally characterized
but the majority of these are the homoleptic species.[52–62]
with no structurally characterized carboxylate-modified
alkoxide compounds available.
Figure 8. Structure plot of 6. Thermal ellipsoids of heavy atoms
drawn at 30% level. Carbon atoms are drawn as ball and stick for
clarity.
in the ONep family of [Ln(µ-ONep)2(ONep)]4 it falls on the
long end of the Ln···Ln separations which systematically
decrease from 3.86 to 3.28 Å. Terminal Ln–O distances of
2.14 Å for Ce fall within the range 2.16 Å (La, average
value) to 1.89 Å (Sc) with an overall average distance of
2.06 Å. Ce is the second largest cation so obviously the Ce–
O distance falls at the longer end of the observed bond
lengths.
Synthesis
Therefore, we investigated the modification of the
Ce(OR)3 isolated above with a series of sterically varied car-
boxylic acids. In general, the compounds were dissolved in
py and then added to a stirring solution of the carboxylic
acid dissolved in py. No precipitates were observed. Because
the CeIII metal center is paramagnetic, solution identifica-
tion of these compounds by NMR methods was not pos-
sible. Further, the volatility of the bound solvent prevents
clean identification of the bulk powder by thermal elemen-
tal analyses. Therefore, while we have isolated numerous
powders from these reactions, only those that were structur-
ally characterized will be discussed below.
After numerous attempts, only the OAr–ORc modified
species were structurally characterized. This phenomenon
was mainly attributed to the ability of the steric bulk of the
aryl oxide to prevent larger less soluble precursors to form.
This is clearly illustrated by the larger oligomers observed
for both the ONep and OtBu ligated species 1–2 in com-
parison to the OAr derivatives 3–5.
The OtBu derivative 2 was found to adopt a novel struc-
ture type and no acceptable models were available for struc-
tural comparisons; however, the Ce(III/IV) oxo isopropox-
ide[24,29] was the closest in terms of connectivity and com-
position. The average terminal Ce–OR distances of 2.12,
Ce(µ-OR) of 2.31, and Ce(µ3-OR) of 2.47 Å were not found
to be in agreement with 2 [2.24 (terminal), 2.44 (µ) Å]. This
is most likely a reflection of the varied oxidation state and
oxo species for the Ce(III/IV) oxo isopropoxide[24,29] com-
plex vs. the CeIII alkoxide for 2. There is also a trinuclear
Ce(III/IV) OtBu derivative structurally consistent with 2a;
however, the average Ce–OR, Ce(µ-OR), and Ce(µ3-OR)
distances of 2.08, 2.37, and 2.52 Å, respectively, are not in
agreement with 2a [2.23 (terminal) and 2.45 (µ) Å]. These
differences are not surprising due to the mixed valent
Ce(III/IV) species. Because the lanthanides systematically
vary in cation size due to the lanthanide contraction, it is
valid to use the isostructural La derivatives for metrical
comparisons. In general, the bond lengths and angles of 2a
were found to be in agreement with La3(OtBu)9(H-OtBu)
[49]
.
For example, the average µ3-OR–Ln distances are
2
X-ray Crystal Structure
2.55 Å for 2a and 2.57 Å for La.
The dinuclear species 4 and 4a have average Ce–OR and
Figure 9 shows the structure plot of 7, the OBc-modified
Ce(µ-OR) distances of 2.25 and 2.45 Å, respectively; 5 has species of 3 that was isolated as the substituted dinuclear
an average Ce–OR distance of 2.22 Å. The dinuclear species. The OBc ligands have replaced two of the oBP li-
OtBu/OCtBu3,[23] OiPr,[27,28] OC(CH3)2(iPr),[48] and gands and bridge between the metal centers in two coordi-
[26]
OC(H)(tBu)2
have average distances of 2.14 Å for nation modes: bridging (µ-ORc) and bridging chelating (µc-
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T. J. Boyle, L. J. Tribby, S. D. Bunge
FULL PAPER
ORc) modes. By binding two py and the original oBP li-
gands, this forms eight-coordinate Ce metal centers forming
a dodecahedron[63] arrangement. Independent of an in-
crease in steric bulk of the pendant ORc chain, an identical
core was observed for each of the various modified species
[DMP/OPc 8 (Figure 10); DMP/OBc 9 (Figure 11), and
DIP/OPc 10 (Figure 12)]. Interestingly, no esterification or
redox behavior was noted for any of these reactions which is
in stark contrast to the early transition metal ORc-modified
compounds, wherein, esterification products were isolated
for all but the most sterically hindering ligands.[1,2,64,65]
Figure 11. Structure plot of 9. Thermal ellipsoids of heavy atoms
drawn at 30% level. Carbon atoms are drawn as ball and stick for
clarity.
Figure 9. Structure plot of 7. Thermal ellipsoids of heavy atoms
drawn at 30% level. Carbon atoms are drawn as ball and stick for
clarity.
Figure 12. Structure plot of 10. Thermal ellipsoids of heavy atoms
drawn at 30% level. Carbon atoms are drawn as ball and stick for
clarity.
values ranging from 116° to 128° with an average value of
120.7°. The angles noted for 7–10 (121.3°, 121.3°, 120.9°,
119.6°, respectively) fall within this range. The µ-ORc li-
gands are limited to a few species with a smaller bite angle
range (119° to 126°) with an average value of 123.4°, which
is similar to the average values noted for 7–10: 123.8°,
125.2°, 124.9°, 125.6°, respectively.
Bulk Powder Characterization
Figure 10. Structure plot of 8. Thermal ellipsoids of heavy atoms
drawn at 30% level. Carbon atoms are drawn as ball and stick for
clarity.
In an effort to characterize the bulk powder, several ana-
lytical methods were investigated. Typically, solution NMR
The bond lengths and angles of the alk- can be employed to elucidate purity; however, the paramag-
oxides[17–29,32–34,47,48] and the carboxylates moieties[52–62] netic nature of the CeIII metal centers prevents obtaining
are consistent with literature reports. The terminal distances any meaningful data by these techniques. Solid-state NMR
of the aryl oxides are on the order of 2.24 to 2.27 Å and as spectroscopic data also proved inconclusive due to broad
noted above are within the expected range.[17–29,32,33,47,48] and numerous peaks expected for these structures. These
The bite angle of the µc-ORc in the literature[52–62] have phenomena were also attributed to the paramagnetic nature
4558
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Carboxylic Acid Modified CeIII Alkoxides
FULL PAPER
of the CeIII and packing disorders, because the ligands are appear to agree to a significant degree with the calculated
not free to rotate in the solid state numerous peaks are often patterns.
observed.
The elemental analyses of the bulk powders of 1–10, in
general, agree with the unsolvated compounds. Compound
4a is a solvated species and the slight variation in the ele-
mental analysis was attributed to either “trapped” and/or
bound solvent. These conditions are often reported to cause
variations from ideal conditions.[50,51,66,67] FTIR spectra of
1–10 revealed the presence of the appropriate stretches ex-
pected for the various alkoxide ligands and shifts of the
ORc ligands consistent with them bound to a metal center.
It is of note that there are some reports that indicate these
compounds can undergo a reaction with KBr under high
pressure, yielding spectra that may not reflect the actual
species but a metathesis products instead.[68] However, these
spectra are useful in terms of identifying the resultant prod-
Figure 14. MicroXRD spectrum of 6. Inset is the calculated
pattern.
ucts. Further investigations of these materials by TGA/DTA
revealed that most samples did not show consistent weight
losses or thermal behavior during processing. This is attrib-
uted to high reactivity with the ambient atmosphere as an
artefact with our current experimental setup. This led to
total weight losses that were rarely in agreement with the
crystal structure. Due to the complexity of the above data,
Summary and Conclusion
A family of CeIII alkoxides were isolated through an
alcohol–amide exchange reaction pathway. In general, as
the steric bulk of the pendant alkoxy moiety increased and
the presence of a Lewis base, the nuclearity of the molecule
decreased. Species ranged from tetra- to mono-nuclear spe-
cies with several novel solvents as ligands and arrangements
noted for this family of compounds. The successful modifi-
cation of the aryl oxide complexes led to the first alkoxy
carboxylate structures isolated for Ce. These compounds
were identified as dinuclear species with µ-ORc and µc-ORc
ligands. The change in steric bulk of the OAr or the ORc
ligand had no effect on the final structure. Interestingly,
none of the ORc–Ce(OR)3 possessed an oxo species indicat-
ing that esterification did not occur. These compounds are
being investigated as precursors to CeOx nanoparticles and
thin films wherein the ligand set controls the structure and
reactivity, which imparts control to the final properties of
these CeOx materials.
it is not possible to conclusively deduce the purity of the
bulk powder and alternative methods were investigated.
Therefore, microXRD spectra were obtained on the
Ce(OR)3 bulk powders sealed in glass capillary tubes and
compared to calculated patterns from the obtained single
crystal structures. In general, the resulting spectra matched
what was expected for the powder XRD patterns. Figure 13
and Figure 14 show the obtained microXRD results for 2a
and 6 and their general agreement with the theoretical spec-
trum (inset). The variations noted can be contributed to
two factors (i) the calculated patterns generated from the
crystal structure are based on a truly random powder and
the crystals have preferential orientation and (ii) the pow-
ders are not solvated as are the crystal structures. Therefore,
an exact match was not obtained; however, the powders do
Experimental Section
All reactions were performed under a dry, inert atmosphere using
standard Schlenk line and glovebox techniques. The following
chemicals were used as received (Aldrich): CeBr3, KN[Si(CH3)3]2
(KNR2), H-ONep, H-OtBu, H-DMP, H-DIP, H-DPP, H-oBP, H-
OPc, H-OBc, and H-ONc. H-OFc and H-OAc were dried accord-
ing to literature preparative methods.[69] All solvents were used as
received in anhydrous (Sure/Seal) bottles. Ce(NR2)3 [where R =
Si(CH3)3] was isolated from the reaction of CeBr3 and 3 equiv. of
KNR2.[35,36] FT-IR spectroscopic data were obtained with a Bruker
Vector 22 spectrometer using KBr pellets pressed under argon and
handled under flowing nitrogen. Elemental analyses were per-
formed with a Perkin–Elmer 2400 CHN-S/O elemental analyzer.
Micro powder X-ray diffraction was performed with several sam-
ples that were prepared in a glass (Quartz) capillary and mounted
vertically on an XYZ stage of a Bruker D8 Discover diffractometer
Figure 13. MicroXRD spectrum of 2a. Inset is the calculated
pattern.
Eur. J. Inorg. Chem. 2006, 4553–4563
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4559

T. J. Boyle, L. J. Tribby, S. D. Bunge
FULL PAPER
using Cu-Kα radiation (λ = 1.5406 Å). The X-ray beam was colli-
mated to 1 mm size and scans are typically taken from 10 to 80 °
2θ at 0.04 degree steps at 2 s per step. Diffraction scan interpret-
ation was done using MDI, Inc. Jade 7 software.
(without 3THF, 671.92): calcd. C 64.35, H 7.65; found C 63.96, H
8.04.
Ce(DPP)3(THF)2 (6): Used Ce(NR2)3 (0.24 g, 0.39 mmol), H-DPP
(g, 1.2 mmol) in THF. Yield: 99% (0.39 g). FT-IR (KBr, cm–1): ν
˜
= 3054 (m), 3027 (m), 2975 (m), 2874 (m), 1596 (w), 1581 (m), 1495
(w), 1453 (w), 1405 (w), 1307 (w), 1261 (w), 1176 (m), 1156 (s),
1086 (w), 1069 (w), 1025 (w), 914 (m), 856 (w), 803 (s), 702 (w),
626 (s), 600 (w). C62H55CeO5 (1020.24): calcd. C 72.99, H 5.43;
found C 72.64, H 5.79.
General Alkoxide Synthesis: Due to the similarity of the synthesis
of compounds 1–6, a general description will be provided. The ap-
propriate alcohol was dissolved in the solvent of choice and slowly
added to a stirring solution of Ce(NR2)3 dissolved in the same
solvent. The resulting solutions were stirred for 12 h and then the
volume of the volatile material was drastically reduced by rotary
evaporation. After this, the mixture was cooled to –25 °C until crys-
tals formed.
General Modified Alkoxide Synthesis: Due to the similarity of the
synthesis of compounds 7–10, a general description will be pro-
vided. To a stirring solution of the appropriate Ce(OR)3 (1–5),
1 equiv. of the appropriate H-ORc was added. The reaction was
stirred for 12 h and then the volatile portion was allowed to slowly
evaporate until crystals formed.
[Ce(µ-ONep)2(ONep)]4 (1): Used Ce(NR2)3 (0.60 g, 0.97 mmol), H-
ONep (0.25 g, 2.9 mmol) in toluene (tol). Yield: 87% (0.34 g). FT-
IR (KBr): ν = 2951 (w), 2864 (m), 2803 (m), 2687 (s), 1478 (m),
˜
1460 (m), 1390 (s), 1358 (m), 1254 (s), 1215 (s), 1106 (w), 1059 (m),
1019 (m), 932 (s), 896 (s), 749 (s), 702 (s), 592 (m), 561 (s), 426 (s)
cm–1. C60H132Ce4O12 (1606.20): calcd C 44.87, H 8.28; found C
44.67, H 8.17.
[Ce(oBP)(µ-OBc)(µc-OBc)(py)2]2 (7): Used compound 3 (0.26 g,
0.32 mmol), H-OBc (0.066 g, 0.65 mmol) in py. Yield: 92% (0.20 g).
FT-IR (KBr, cm–1): ν = 2962 (m), 1524 (w), 1483 (w), 1459 (w),
˜
1418 (w), 1375 (w), 1224 (m), 1032 (s), 897 (s), 802 (s), 601 (s), 540
(s). C60H82Ce2N4O10 (1299.58): calcd. C 55.45, H 6.36, N 4.31;
found C 55.19, H 6.81, N 4.01.
Ce4(µ3-OtBu)3(µ-OtBu)4(OtBu)5 (2): Used Ce(NR2)3 (0.61 g,
0.98 mmol), H-OtBu (0.22 g, 3.0 mmol) in tol. Yield: 89% (0.32 g).
FT-IR (KBr, cm–1): ν = 2960 (w), 2865 (w), 2150 (s), 2120 (s), 1936
˜
[Ce(DMP)(µ-OPc)(µc-OPc)(py)2]2 (8): Used compound 4 (0.28 g,
(s), 1848 (s) 1466 (w), 1356 (w), 1187 (w), 974 (w), 929 (w), 878
(w), 829 (w), 767 (w), 740 (w), 693 (s), 677 (s), 655 (s), 595 (m).
C48H108Ce4O12 (1437.87): calcd C 40.09, H 7.57; found C 39.78, H
7.68.
0.22 mmol), H-OPc (0.038 g, 0.43 mmol) in py. Yield: 37% (0.10 g).
FT-IR (KBr, cm–1): ν = 2968 (w), 2930 (m), 1581 (w), 1477 (w),
˜
1422 (w), 1373 (m), 1359 (m), 1238 (m), 1217 (w), 1092 (w), 1035
(m), 1004 (m), 850 (w), 753 (w), 702 (w), 663 (s), 619 (m), 547 (m),
531 (m), 518 (m). C52H66Ce2N4O10 (1187.37): calcd. C 52.60, H
5.60, N 4.71; found C 52.39, H 5.52, N 4.19.
Ce3(µ3-OtBu)3(µ-OtBu)3(OtBu)3(H-OtBu)2 (2a): Used Ce(NR2)3
(0.61 g, 0.98 mmol), H-OtBu (0.29 g, 3.9 mmol) in tol. Yield: 90%
(0.36 g). FT-IR (KBr, cm–1): ν = 2965 (w), 2925 (w), 2865 (w), 1467
˜
[Ce(DMP)(µ-OBc)(µc-OBc)(py)2]2 (9): Used compound 4 (0.44 g,
(m), 1370 (m), 1356 (w), 1020 (m), 968 (w), 927 (w), 878 (w), 768
(m), 759 (m), 740 (s), 511 (w), 488 (w). C44H101Ce3O11 (1226.65):
calcd. C 43.08, H 8.30; found C 42.78, H 8.09.
0.34 mmol), H-OBc (0.069 g, 0.68 mmol) in py. Yield: 53% (0.22 g).
FT-IR (KBr, cm–1): ν = 3607 (s), 2961 (w), 2929 (w), 2871 (w),
˜
1541 (w), 1483 (w), 1460 (w), 1420 (w), 1375 (w), 1361 (w), 1285
(m), 1263 (w), 1224 (w), 1090 (m), 1033 (m), 896 (w), 839 (m), 803
(m), 791 (m), 756 (m), 700 (m), 601 (w), 552 (m), 517 (s).
C56H74Ce2N4O10 (1243.48): calcd. C 54.09, H 6.00, N 4.50; found
C 54.51, H 6.39, N 4.31.
Ce(oBP)3(THF)3 (3): Used Ce(NR2)3 (1.3 g, 2.9 mmol), H-oBP
(0.91 g, 6.1 mmol) in tol. Yield: 92% (1.5 g). FT-IR (KBr, cm–1): ν
˜
= 3056 (s), 2952 (m), 1587 (m), 1561 (m), 1478 (w), 1439 (w), 1301
(w), 1260 (w), 1217 (m), 1124 (s), 1088 (s), 1068 (s), 1047 (s), 1035
(s), 1003 (s), 868 (w), 823 (m), 748 (w), 703 (w), 597 (s).
C42H63CeO6 (804.09): calcd. C 62.74, H 7.90; found C 62.36, H
7.81.
[Ce(DIP)(µ-OPc)(µc-OPc)(py)2]2 (10): Used compound 5 (0.26 g,
0.29 mmol), H-OPc (0.052 g, 0.58 mmol) in py. Yield: 60% (0.12 g).
FT-IR (KBr, cm–1): ν = 3062 (s), 3036 (s), 2968 (w), 2930 (m), 2870
˜
(m), 1581 (w), 1477 (w), 1442 (w), 1422 (w), 1373 (m), 1359 (s),
1283 (w), 1263 (m), 1238 (s), 1217 (s), 1092 (m), 1035 (m), 1004
(s), 850 (m), 753 (m), 702 (m), 619 (s), 547 (s), 531 (s), 518 (s).
C60H82Ce2N4O10 (1299.58): calcd. C 55.45, H 6.36, N 4.31; found
C 55.90, H 6.81, N 4.33.
General X-ray Crystal Structure Information:[70] Each crystal was
mounted onto a thin glass fiber from a pool of Fluorolube and
immediately placed under a liquid N2 stream, on a Bruker AXS
diffractometer. The radiation used was graphite-monochromatized
Mo-Kα radiation (λ = 0.7107 Å). The lattice parameters were opti-
mized from a least-squares calculation on carefully centered reflec-
tions. Lattice parameter determination and data collection were
carried out using SMART Version 5.054 software. Data reduction
was performed using SAINT Version 6.01 software. The structure
refinement was performed using XSHELL 3.0 software. The data
were corrected for absorption using the SADABS program within
[Ce(µ-DMP)(DMP)2(THF)2]2 (4): Used Ce(NR2)3 (0.22 g,
0.35 mmol), H-DMP (0.13 g, 1.10 mmol) in tetrahydrofuran
(THF). Yield: 95% (0.22 g). FT-IR (KBr, cm–1): ν = 2971 (s), 1591
˜
(m), 1465 (m), 1425 (m), 1367 (s), 1269 (w), 1233 (w), 1211 (w),
1094 (m), 1068 (s), 1027 (m), 850 (w), 761 (m), 697 (m), 678 (s),
565 (s), 524 (s), 492 (s). C64H86Ce2O10 (1295.63): calcd. C 59.33, H
6.69; found C 59.16, H 7.05.
[Ce(µ-DMP)(DMP)2(py)2]2 (4a): Ce(NR2)3 (0.97 g, 1.6 mmol), H-
DMP (0.58 g, 4.7 mmol) in tol and pyridine (py). Yield: 95%
(0.98 g). FT-IR (KBr, cm–1): ν = 3068 (m), 3049 (m), 3012 (m),
˜
2964 (m), 2909 (s), 1596 (w), 1465 (w), 1442 (w), 1424 (w), 1270
(w), 1234 (w), 1210 (w), 1150 (s), 1092 (w), 1067 (m), 1035 (m),
1003 (m), 850 (w), 837 (w), 747 (w), 621 (m), 564 (s), 526 (m).
C68H74Ce2N4O6 (1323.61): calcd. C 61.70, H 5.64, N 4.23; found
C 60.87, H 5.66, N 4.48.
Ce(DIP)3(THF)3 (5): Used Ce(NR2)3 (0.22 g, 0.36 mmol), H-DIP the SAINT software package. General collection parameters for 1–
(0.19 g, 1.1 mmol) in tol and THF. Yield: 96% (0.22 g). FT-IR 3 and 4–10 are shown in Table 1 and Table 2, respectively. Ad-
(KBr, cm–1): ν = 3054 (s), 2958 (w), 2867 (m), 1586 (s), 1380 (s), ditional information concerning the structure of these compounds
˜
1357 (s), 1327 (m), 1264 (w), 1206 (m), 1095 (s), 1042 (m), 1021
(m), 885 (m), 855 (w), 796 (s), 752 (m), 687 (m), 561 (m).
C48H75CeO6 (888.25): calcd. C 64.91, H 8.51; for C36H51CeO3
can be found by accessing the final CIF files through the Cam-
bridge Crystallographic Data Base. In general, each structure was
solved by direct methods. This procedure yielded the heavy atoms,
4560
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© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2006, 4553–4563

Carboxylic Acid Modified CeIII Alkoxides
FULL PAPER
Table 1. Data collection parameters for 1–3.
Compound
1(·8tol)
2
2a(·2tol)
3(·3THF)
Empirical formula
Formula weight
Temp. [K]
C116H196Ce4O12
2343.32
168(2)
tetragonal
P-42(1)c
C48H108Ce4O12
1437.82
168(2)
monoclinic
P21/c
C58H115Ce3O11
1408.92
178(2)
monoclinic
P2(1)
C54H87CeO9
1020.36
168(2)
rhombohedral
R-3
Space group
a [Å]
b [Å]
c [Å]
α [°]
20.5590(18)
20.5590(18)
11.908(2)
18.673(12)
11.382(7)
29.308(18)
14.1032(17)
18.952(2)
14.5646(17)
14.663(2)
14.663(2)
14.663(2)
68.220(3)
68.220(3)
68.220(3)
2617.3(6)
2
1.295
0.922
4.23 (9.60)
5.90 (9.83)
β [°]
91.335(15)
115.902(2)
γ [°]
V [Å3]
5033.1(11)
2
1.060
1.838
7.78 (22.60)
12.30 (25.05)
6227(7)
4
1.534
2.917
8.55 (16.70)
17.00 (19.94)
3501.7(7)
2
1.336
1.965
7.23 (11.87)
11.09 (13.25)
Z
Dcalcd [Mg/m3]
µ (Mo-Kα) [mm–1]
[a]
R1 (%) (all data)
[b]
wR2 (%) (all data)
2 2
[a] R1 = Σ||Fo|–|Fc||/Σ|Fo|×100. [b] wR2 = [Σw(Fo2 –Fc ) /Σ(w|Fo|2)2]1/2 ×100.
Table 2. Data collection parameters for 4–10.
Compound
4(·2THF)
4a(·2tol)
5(·tol)
6(·2THF)
Empirical formula
Formula weight
Temp. [K]
C72H100Ce2O12
1439.78
168(2)
monoclinic
P21/c
C82H90Ce2N4O6
1507.82
178(2)
monoclinic
P21/n
C55H83CeO6
980.33
178(2)
monoclinic
P21/n
C70H71CeO7
1164.45
203(2)
monoclinic
P2(1)
Space group
a [Å]
b [Å]
c [Å]
12.748(2)
18.883(3)
15.343(3)
14.600(3)
19.504(4)
14.690(3)
11.1537(16)
35.671(5)
13.4253(19)
10.220(2)
21.968(5)
13.461(3)
α [°]
β [°]
112.723(3)
118.254(2)
96.293(3)
101.110(4)
γ [°]
V [Å3]
3406.6(11)
2
1.404
1.379
3.40 (8.51)
4.07 (8.90)
3684.7(11)
2
1.359
1.274
2.74 (6.75)
3.15 (6.97)
5309.2(13)
4
1.226
0.903
7.49 (14.48)
13.89 (17.16)
2965.4(11)
2
1.143
0.810
4.51 (8.54)
5.51 (8.96)
Z
Dcalcd [Mg/m3]
µ (Mo-Kα) [mm–1]
[a]
R1 (%) (all data)
[b]
wR2 (%) (all data)
Compound
7(·py)
8
9
10(·2py)
Empirical formula
Formula weight
Temp. [K]
C65H87Ce2N5O10
1391.36
203(2)
monoclinic
P21/n
C52H66Ce2N4O10
1187.33
168(2)
monoclinic
C2/c
C56H74Ce2N4O10
1243.43
178(2)
C70H92Ce2N6O10
1457.74
168(2)
monoclinic
P21/c
Space group
triclinic
¯
P1
a [Å]
b [Å]
c [Å]
α [°]
11.660(3)
15.714(5)
19.113(6)
20.9692(13)
13.4977(13)
20.1132(14)
11.8503(16)
11.9179(16)
12.9071(17)
86.896(2)
65.609(2)
65.482(2)
1495.3(3)
1
1.381
1.558
4.38 (8.37)
5.63 (8.81)
9.883(3)
20.157(7)
18.208(6)
β [°]
99.253(5)
107.972(2)
103.471(7)
γ [°]
V [Å3]
3456.5(17)
2
1.337
1.358
8.96 (22.76)
9.80 (23.07)
5415.0(7)
4
1.456
1.717
3.82 (10.01)
4.34 (10.30)
3528(2)
2
1.372
1.333
7.76 (12.19)
13.50 (13.95)
Z
Dcalcd [Mg/m3]
µ (Mo-Kα) [mm–1]
[a]
R1 (%) (all data)
[b]
wR2 (%) (all data)
2 2
[a] R1 = Σ | |Fo|–|Fc| |/Σ |Fo|×100. [b] wR2 = [Σ w (Fo2 –Fc ) /Σ (w |Fo|2)2]1/2 ×100.
along with a number of the C, O, and N atoms when present. Sub-
sequent Fourier synthesis yielded the remaining atom positions.
The hydrogen atoms were fixed in positions of ideal geometry and
refined within the XSHELL software. These idealized hydrogen
atoms had their isotropic temperature factors fixed at 1.2 or 1.5
times the equivalent isotropic U of the C atoms to which they were
bonded. The final refinement of each compound included aniso-
tropic thermal parameters on all non-hydrogen atoms. Final crystal
solutions that contain alkoxides are well known to possess ligand
positional disorder issues.
Eur. J. Inorg. Chem. 2006, 4553–4563
© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
4561

T. J. Boyle, L. J. Tribby, S. D. Bunge
FULL PAPER
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CCDC-614454 to -614465 contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif. Any variations from stan-
dard structural solution associated with the representative com-
pounds are discussed below. All CIF files were checked for
errors using the free on-line Checkcif service provided by the
International Union of Crystallography (available on the Web at
http://checkcif.iucr.org/).
[30] L. G. Hubert-Pfalzgraf, V. Abada, J. Vaissermann, Polyhedron
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Acknowledgments
[32] T. J. Boyle, S. D. Bunge, P. G. Clem, J. Richardson, J. T. Daw-
ley, L. A. M. Ottley, M. A. Rodriguez, B. A. Tuttle, G. Avilu-
cea, R. G. Tissot, Inorg. Chem. 2005, 44, 1588.
[33] T. J. Boyle, L. A. M. Ottley, L. J. Tribby, S. D. Bunge, C. J. Gor-
don, Chem. Mater. 2005, (in preparation).
[34] D. M. Barnhart, D. L. Clark, J. C. Gordon, J. C. Huffman,
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[35] D. C. Bradley, R. C. Mehrotra, D. P. Gaur, Metal Alkoxides,
Academic Press, New York, 1978.
For support of this research, the authors would like to thank the
Office of Basic Energy Sciences of the Department of Energy and
the United States Department of Energy. Sandia is a multiprogram
laboratory operated by Sandia Corporation, a Lockheed Martin
Company, for the United States Department of Energy under con-
tract DE-AC04-94AL85000.
[36] D. C. Bradley, R. C. Mehrotra, I. P. Rothwell and A. Singh,
Alkoxo and Aryloxo Derivatives of Metals, Academic Press,
New York, 2001, p. 704.
[37] T. J. Boyle, S. D. Bunge, P. G. Clem, J. Richardson, J. T. Daw-
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Received: June 29, 2006
Published Online: September 20, 2006
Eur. J. Inorg. Chem. 2006, 4553–4563
© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
4563