Synthesis, structure, and reactivity of cerium(IV) calix[4]arene complexes

Article abstract of DOI:10.1002/ejic.200601004

The equimolar reaction of Ce(hfac)₄ (1) (hfac = 1,1,1,5,5,5-hexafluoropentanedionato) with p-tBu-calix[4](OMe) ₂(OH)₂ in toluene gave the new cerium(IV) calix[4]arene complex {p-tBu-calix[4](OMe)₂(O)₂)Ce(hfac)₂ (2). The single-crystal X-ray structure shows the cone geometry of the calixarene ligand with the methoxy groups coordinated to the cerium; it shows slightly longer cerium-oxygen (acetylacetonate ligand) bond lengths than the corresponding bonds in the analogous nonfluorinated complex {p-tBu-calix[4](OMe) ₂-(O)₂}Ce(acac)₂ (3). The bromination reaction of 3 gave the bisbrominated complex {p-tBu-calix[4](OMe)₂(O) ₂}Ce(Br-acac)₂ (4). ¹H NMR spectroscopic studies and a single-crystal X-ray structure of 4 revealed that the bromination took place in the 3-position of the acac ligand. Furthermore, the first X-ray photoelectron spectroscopy (XPS) evaluation of the Ce oxidation state in cerium calix[4]arene complexes 2 and 3 is presented, and X-ray induced changes of Ce_ox in these complexes are detected. Wiley-VCH Verlag GmbH & Co. KGaA, 2007.

Full text of DOI:10.1002/ejic.200601004

FULL PAPER
DOI: 10.1002/ejic.200601004
Synthesis, Structure, and Reactivity of Cerium(IV) Calix[4]arene Complexes
Jochen Gottfriedsen,*[a] Reimo Hagner,[a] Marlies Spoida,[a] and Yuri Suchorski[a]
Keywords: Cerium / Calixarenes / Photoelectron spectroscopy
The equimolar reaction of Ce(hfac)4 (1) (hfac = 1,1,1,5,5,5-
hexafluoropentanedionato) with p-tBu-calix[4](OMe)2(OH)2
in toluene gave the new cerium(IV) calix[4]arene complex
{p-tBu-calix[4](OMe)2(O)2}Ce(hfac)2 (2). The single-crystal
X-ray structure shows the cone geometry of the calixarene
ligand with the methoxy groups coordinated to the cerium;
it shows slightly longer cerium–oxygen (acetylacetonate li-
gand) bond lengths than the corresponding bonds in the
analogous nonfluorinated complex {p-tBu-calix[4](OMe)2-
(O)2}Ce(acac)2 (3). The bromination reaction of 3 gave the
bisbrominated complex {p-tBu-calix[4](OMe)2(O)2}Ce(Br-
acac)2 (4). 1H NMR spectroscopic studies and a single-crystal
X-ray structure of 4 revealed that the bromination took place
in the 3-position of the acac ligand. Furthermore, the first X-
ray photoelectron spectroscopy (XPS) evaluation of the Ce
oxidation state in cerium calix[4]arene complexes 2 and 3 is
presented, and X-ray induced changes of Ceox in these com-
plexes are detected.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2007)
ports indicated the synthesis of double-decker tetrapyrrole
cerium complexes starting from Ce(acac)3(H2O)n,[8] and our
group reported the synthesis of {p-tBu-calix[4](OMe)2-
(O)2}Ce(acac)2 (3) from Ce(acac)4.[9] A variety of methods
have been used to study the valency of Ce in cerium com-
plexes. Theoretical calculations[10] and XANES (X-ray ab-
sorption near-edge spectroscopy)[11] experiments, as well as
studies of their magnetic properties,[11b] have been used to
study the oxidation states of cerium COT (COT = cyclooc-
tatetraenyl) and substituted COT complexes in detail,[12]
which led to the conclusion that cerium is trivalent in these
complexes. XANES studies of a series of double-decker tet-
rapyrrole cerium complexes[8a] also suggest partial delocal-
ization of the ligand π electrons into a cerium 4f orbital,
which results in a valency of 3.59 to 3.68. The XPS data
of [Ce(Pc)2] (Pc = phthalocyaninate) complexes revealed a
valency that is neither tri- nor tetravalent, but a mixed val-
ent state of them.[13] Calix[4]arene ligand systems are widely
used to form complexes with almost all metal ions.[14] Com-
plexes with cerium are still rare and mainly focused on CeIII
{e.g. cerium(III) calix[4]arene complexes [Ce(LH–2)(Me-
OH)2A]HA (L = 5,11,17,23-tetra-tert-butyl-25,27-bis(di-
ethylcarbamoylmethoxy)-26,28-dihydroxycalix[4]arene, HA
= picric acid)[15] and Ce(H2O)5(p-sulfonatocalix[4]arene +
H+)}.[16] Studies of the coordination behavior of p-tert-bu-
tylcalix[n]arene (n = 4–6) with tetravalent cerium in solution
have also been performed.[17]
Introduction
The coordination chemistry of cerium in the oxidation
state +4 is dominated by its alkoxy and β-diketonate com-
plexes. β-Diketonate complexes of cerium(IV) are well es-
tablished as precursors in MOCVD (metal organic chemical
vapor deposition) or ALE (atomic layer epitaxy) processes.
The deposition of CeO2 films as buffer layers for YBCO
high temperature super conductor devices[1] were studied,
and doped Ce1–xMxO2–y (M = Sm, Y, Gd) materials, which
exhibit high oxygen ion conductivity, have been presented.
These compounds are considered to be promising materials
for solid oxide fuel cells, oxygen pumps, and methane con-
version reactors.[2] Furthermore, the application of
Ce(tmhd)4 (tmhdH = 2,2,6,6-tetramethyl-3,5-heptanedione)
as a dopant in SrS and CaGa2S4 thin films has been exam-
ined owing to the fact that cerium doping in these materials
can produce blue–green and blue electroluminescent (EL)
phosphors.[3] In addition to the studies on Ce(tmhd)4,[1b,4]
the suitabilities of the following other volatile β-diketonate
complexes were investigated: Ce(fdh)4 (fdhH = 6,6,6-tri-
fluoro-2,2-dimethyl-3,5-hexanedione),[5] Ce(fod)4 (fodH =
1,1,1,2,2,3,3-heptafluoro-7,7-dimethyloctane-4,6-dione),[6]
Ce(tmod)4 (tmodH = 2,2,7-trimethyl-3,5-octanedione),[7]
and Ce(txhd)4 (txhdH
=
2,2,6-trimethyl-3,6-heptane-
dione).[7]
The high synthetic potential of CeIV β-diketonate com-
plexes remains largely unexplored. Recently, literature re-
Here we present the synthesis of novel calix[4]arene com-
plexes of CeIV using two different synthetic approaches. In
addition to the synthesis starting from a homoleptic cerium
β-diketonatocomplex,thereactionof{p-tBu-calix[4](OMe)2-
(O)2Ce(acac)2} (3) with Br2 will be described to demon-
strate its high synthetic potential in this class of cerium
[a] Chemisches Institut der Otto-von-Guericke, Universität Mag-
deburg,
Universitätsplatz 2, 39106 Magdeburg, Germany
Fax: +49-391-67-12933
E-mail: jochen.gottfriedsen@vst.uni-magdeburg.de
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Synthesis, Structure, and Reactivity of CeIV Calix[4]arene Complexes
FULL PAPER
complexes. In addition to the standard characterization 4 exhibits the same solubility as that of 2 and the same blue
methods, NMR spectroscopy, MS, and single-crystal X-ray color. The bromination of Ce(acac)4 was described pre-
determination studies, we performed XPS experiments for viously[18] by using NBS (N-bromosuccinimide) to yield
two of the cerium calix[4]arene complexes to obtain the ef- Ce(Br-acac)4. A ligand transfer to TiCl4 was investigated to
fective oxidation state of Ce in these complexes.
further elucidate the reactivity of complex 3. Red crystals
were obtained after crystallization from toluene, which
where characterized by NMR spectroscopy and shown to
be the known titanium complex {p-tBu-calix[4](OMe)2-
(O)2}TiCl2.[19] 1H and 13C NMR spectroscopic studies re-
Results and Discussion
The equimolar reaction of Ce(hfac)4 (1) (hfac = vealed the typical cone geometry of the calix[4]arene li-
1,1,1,5,5,5-hexafluoropentanedionato) with p-tBu-calix- gands for both complexes 2 and 4 in benzene solution. The
[4](OMe)2(OH)2 in boiling toluene gave an immediate color 1H NMR (Table 1) spectroscopic data show that the chemi-
change upon reaching the boiling point. After workup, the cal shifts of the calix[4]arene ligand of brominated complex
new cerium(IV) calix[4]arene complex {p-tBu-calix[4]- 4 are basically identical with those of starting material 3,
(OMe)2(O)2}Ce(hfac)2 (2) could be isolated as blue crystal- whereas the chemical shifts of 2 are significantly different,
line blocks (Scheme 1). Complex 2 is highly soluble in non- which is due to the electron-withdrawing fluorine substitu-
polar organic solvents such as hexane, pentane, and toluene ents. The resonances of the methoxy group of complexes 2,
as well as in the polar solvents diethyl ether, THF, dichloro- 3, and 4 show the highest change in their chemical shifts
methane, and chloroform.
compared with the corresponding ones of the free calix[4]-
arene ligand. Resonances of the methoxy group in all three
complexes are shifted downfield by nearly 1.5 ppm for 2,
1.25 ppm for 3, and approximately 1 ppm for 4 relative to
the chemical shift of the free ligand (δ =3.47 ppm). This is
due to the electron-withdrawing fluorine atoms that induce
a decrease in the electron density on the cerium atom,
which is compensated by the coordinated methoxy group
thus deshielding the CH3 group. Doublets at 5.11 and
3.27 ppm for 2 and 5.04 and 3.35 ppm for 4 in the 1H NMR
spectra are attributable to the endo and exo protons of the
CH2 bridges of the calix[4]arene ligand. Whereas the chemi-
cal shifts of the endo protons are shifted downfield (nearly
0.7 ppm for 2 and 0.6 ppm for 4) relative to the free calix[4]-
arene ligand, the resonances for the exo protons are not
influenced by coordination to the cerium center. Two sets
of signals for the aromatic hydrogens and the substituted
tertiary butyl groups were found for each new complex, 2
and 4, by assigning the two different ring systems of the
calix[4]arene ligands (Table 1). The 13C NMR spectra also
show the two unequally substituted phenyl groups of the
calixarene ligand in 2 and 4. The different coordinationen-
Scheme 1. Synthesis of 2.
The reaction of the corresponding nonfluorinated Ce ca-
lix[4]arene complex {p-tBu-calix[4](OMe)2(O)2}Ce(acac)2
(3) with bromine in a 1:2 ratio in diethyl ether resulted in
bromination of the acac ligand in the 3-position with for-
mationofthebisbrominatedcomplex{p-tBu-calix[4](OMe)2-
(O)2}Ce(Br-acac)2 (4) (Br-acac = 3-bromo-pentanedionato)
(Scheme 2).
vironments that are adopted by
1 (–76.51 ppm), 2
(–75.93 ppm), and 1,1,1,5,5,5-hexafluoropentanedione
(–77.40 ppm) are made apparent in their 19F NMR spectra,
Table 1. 1H NMR spectroscopic data of complexes 2, 3, and 4 in
C6D6 at 25 °C (δ in ppm).
2
3
4
(s, 4 H, ArH)
(s, 4 H, ArH)
(d, endo-CH2)
(d, exo-CH2)
(s, 6 H, OCH3)
(s, 18 H, CCH3)
(s, 18 H, CCH3)
(s, 2 H, COCH)
7.11
6.85
5.11
3.27
4.93
1.27
0.90
5.8
7.23
6.97
5.04
3.35
4.72
1.39
0.98
4.96
1.76
7.22
6.96
5.04
3.35
4.58
1.34
0.97
no signal
2.19
Scheme 2. Reactivity of 3 towards Br2 and TiCl4.
The reaction of 3 with bromine in a 1:1 ratio produced
a monobrominated species which was detected by 1H NMR
spectroscopy but not isolated or further studied. Complex
(s, 12 H, COCH3) no signal
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J. Gottfriedsen, R. Hagner, M. Spoida, Y. Suchorski
FULL PAPER
which shows a small influence of the CF3 groups on the
chemical shift for this β-diketone. Mass spectrometric data
of complexes 2 and 4 show the molecular ions as the signals
with the highest mass. The signals with the highest abun-
dance can be interpreted as the molecular ions reduced by
the mass of the corresponding β-diketonate.
X-ray Structure Determinations
A single-crystal X-ray structure determination of the
starting material Ce(hfac)4 (1), which crystallized from tolu-
ene as dark brown crystals, was carried out. The molecular
structure exhibits an eight-coordinate cerium in a distorted
square antiprismatic coordination sphere (Figure 1).
Figure 2. ORTEP plot of the molecular structure of 2. Thermal
ellipsoids are drawn with 50% probability. Hydrogen atoms are
omitted for clarity.
Figure 1. ORTEP plot of the molecular structure of 1. Thermal
ellipsoids are drawn with 50% probability. Hydrogen atoms are
omitted for clarity.
The Ce–O bond lengths of the O,OЈ-bidentate hexafluo-
ropentanedionato ligands range from 2.325(3) to Figure 3. ORTEP plot of the molecular structure of 4. Thermal
ellipsoids are drawn with 50% probability. Hydrogen atoms are
omitted for clarity.
2.346(3) Å, which is in good agreement with similar dis-
tances found for cerium acetylacetonato complexes {e.g. α-
Ce(acac)4,[20a,20b]
β-Ce(acac)4,[20c]
and
[Ce(acac)4]·
10H2O[21]}, which have a mean distance of 2.32 Å. Other
The Ce–O phenoxy bond lengths in
2 [2.069(3)/
cerium β-diketonato complexes display Ce–O distances be- 2.089(3) Å] are slightly shorter than the corresponding dis-
tween 2.24 Å and 2.36 Å [e.g. Ce(fda)4, Ce(tmhd)4, tances in 4 [2.111(4)/2.134(5) Å], which are equal to the
Ce(txhd)4, and Ce(tmod)4].
ones found for {p-tBu-calix[4](OMe)2(O)2}Ce(acac)2 (3)
{p-tBu-calix[4](OMe)2(O)2}Ce(hfac)2 (2) was crystallized (2.130 Å average)[9] (Table 2). They are in the same range as
from toluene at 5 °C and single crystals of the brominated those in other cerium alkoxides {e.g. the hexafluoroisoprop-
complex {p-tBu-calix[4](OMe)2(O)2}Ce(Br-acac)2 (4) were oxide (hfip) adducts Ce(hfip)4(TMEDA) and Ce(hfip)4(di-
obtained from CH2Cl2 at 5 °C. The single-crystal X-ray glyme) (2.13 Å average)M,[22] the terminal Ce–OR alkoxide
structures of both complexes showed the typical cone ge- groups in Ce2(OiPr)8(iPrOH)2 (2.088 Å average),[23] Ce-
ometry of the calixarene ligand with the methoxy groups (OtBu)2(NO3)2(HOtBu)2 (2.088 Å average),[24] and Ce-
coordinated to the cerium, which gave an eight coordinate (OtBu)2(µ-OtBu)2(µ3-OtBu)2Na2(dme)2 [2.136–2.146 Å for
cerium center with square antiprismatic coordination the terminal (OtBu) ligands]},[24] although they are longer
spheres (Figures 2 and 3).
than those observed for Cp3CeIV(OtBu) (2.045 Å).[24] The
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Synthesis, Structure, and Reactivity of CeIV Calix[4]arene Complexes
FULL PAPER
Table 2. Selected bond lengths [Å] and angles [°].
1
2
4
Ce(1)–O(1)
Ce(1)–O(2)
Ce(1)–O(3)
Ce(1)–O(4)
2.346(3)
2.325(3)
2.334(3)
2.339(3)
Ce(1)–O(3)
Ce(1)–O(1)
Ce(1)–O(7)
Ce(1)–O(6)
Ce(1)–O(5)
Ce(1)–O(8)
Ce(1)–O(4)
2.069(3)
2.089(3)
2.408(3)
2.409(4)
2.426(3)
2.461(3)
2.560(3)
Ce(1)–O(1)
Ce(1)–O(3)
2.111(4)
2.134(5)
2.365(3)
2.365(3)
2.372(3)
2.372(3)
2.581(3)
Ce(1)–O(4)
Ce(1)–O(4)[a]
Ce(1)–O(5)
Ce(1)–O(5)[a]
Ce(1)–O(2)[a]
Ce(1)–O(2)
2.577(3)
Ce(1)–O(2)
2.581(3)
O(1)–Ce(1)–O(1)[a]
O(2)–Ce(1)–O(1)
O(2)–Ce(1)–O(3)
O(2)–Ce(1)–O(4)
O(2)–Ce(1)–O(3)[a]
O(2)–Ce(1)–O(4)[a]
O(2)–Ce(1)–O(1)[a]
O(2)[a]–Ce(1)–O(2)
O(2)[a]–Ce(1)–O(3)
O(3)–Ce(1)–O(1)
O(3)–Ce(1)–O(1)[a]
O(3)–Ce(1)–O(3)[a]
O(3)[a]–Ce(1)–O(1)
O(3)–Ce(1)–O(4)
O(3)[a]–Ce(1)–O(4)
O(4)–Ce(1)–O(1)
O(4)–Ce(1)–O(1)[a]
O(4)–Ce(1)–O(4)[a]
132.52(14)
70.98(11)
126.42(11)
74.44(10)
126.23(11)
153.33(11)
72.80(11)
78.92(16)
126.23(11)
153.61(11)
73.86(10)
79.77(15)
73.86(10)
71.18(11)
72.61(11)
99.27(11)
99.50(11)
132.22(15)
O(3)–Ce(1)–O(1)
O(3)–Ce(1)–O(7)
O(1)–Ce(1)–O(7)
O(3)–Ce(1)–O(6)
O(1)–Ce(1)–O(6)
O(7)–Ce(1)–O(6)
O(3)–Ce(1)–O(5)
O(1)–Ce(1)–O(5)
O(7)–Ce(1)–O(5)
O(6)–Ce(1)–O(5)
O(3)–Ce(1)–O(8)
O(1)–Ce(1)–O(8)
O(7)–Ce(1)–O(8)
O(6)–Ce(1)–O(8)
O(5)–Ce(1)–O(8)
O(3)–Ce(1)–O(4)
O(1)–Ce(1)–O(4)
O(7)–Ce(1)–O(4)
O(6)–Ce(1)–O(4)
O(5)–Ce(1)–O(4)
O(8)–Ce(1)–O(4)
O(3)–Ce(1)–O(2)
O(1)–Ce(1)–O(2)
O(7)–Ce(1)–O(2)
O(6)–Ce(1)–O(2)
O(5)–Ce(1)–O(2)
O(8)–Ce(1)–O(2)
O(4)–Ce(1)–O(2)
102.07(12)
85.38(13)
146.66(11)
144.93(11)
93.22(13)
98.95(13)
143.00(12)
85.51(11)
70.52(12)
68.77(12)
80.58(12)
144.67(12)
68.32(11)
69.16(12)
113.72(11)
76.17(10)
79.56(11)
70.67(11)
138.20(10)
69.63(11)
133.98(11)
81.08(11)
77.15(10)
136.15(10)
71.74(10)
135.59(11)
68.40(10)
143.03(9)
O(1)–Ce(1)–O(3)
O(1)–Ce(1)–O(4)
O(3)–Ce(1)–O(4)
O(1)–Ce(1)–O(4)[a]
O(3)–Ce(1)–O(4)[a]
O(4)–Ce(1)–O(4)[a]
O(1)–Ce(1)–O(5)
O(3)–Ce(1)–O(5)
O(4)–Ce(1)–O(5)
O(4)[a]–Ce(1)–O(5)
O(1)–Ce(1)–O(5)[a]
O(3)–Ce(1)–O(5)[a]
O(4)–Ce(1)–O(5)[a]
94.11(16)
145.84(7)
87.30(13)
145.84(7)
87.30(13)
68.31(15)
82.72(13)
145.17(8)
77.09(12)
114.41(11)
82.72(13)
145.17(8)
114.41(11)
O(4)[a]–Ce(1)–O(5)[a] 77.09(12)
O(5)–Ce(1)–O(5)[a]
O(1)–Ce(1)–O(2)[a]
O(3)–Ce(1)–O(2)[a]
O(4)–Ce(1)–O(2)[a]
69.09(16)
76.98(7)
75.80(7)
135.76(10)
O(4)[a]–Ce(1)–O(2)[a] 70.29(9)
O(5)–Ce(1)–O(2)[a]
135.80(10)
O(5)[a]–Ce(1)–O(2)[a] 69.68(10)
O(1)–Ce(1)–O(2)
O(3)–Ce(1)–O(2)
O(4)–Ce(1)–O(2)
O(4)[a]–Ce(1)–O(2)
O(5)–Ce(1)–O(2)
O(5)[a]–Ce(1)–O(2)
O(2)[a]–Ce(1)–O(2)
76.98(7)
75.80(7)
70.29(9)
135.76(10)
69.68(10)
135.80(10)
139.56(11)
[a] Atoms that are generated by using the symmetry transformation –x + 1, y, –z + 1 (1), and x, – y, z (4), respectively.
Ce–O bond lengths for the two coordinated methoxy (txhd)4, and Ce(tmod)4, which exhibit Ce–O distances be-
groups Ce(1)–O(2) and Ce(1)–O(4) in
2.577(3) Å] and Ce(1)–O(2) and Ce(1)–O(2)# in
2
[2.560(3)/ tween 2.24 Å and 2.36 Å}.
The four donor atoms of the calixarene in 2, 3, and 4
4
[2.581(3) Å] are in the typical range of Ce–O coordination define a mean O4 plane with a rms deviation of 0.2464 Å
bonds[22] and fit well with the corresponding bonds in {p- in 2, 0.2549 Å in 3, and 0.2774 Å in 4 and the cerium atom
tBu-calix[4](OMe)2(O)2}Ce(acac)2 (3) (2.604 Å average).[9] is located at 1.0607(14) Å from this mean plane in 2,
The Ce–O distances of the O,OЈ-bidentate hexafluoropent- 1.1641(10) Å in 3, and 1.1686(20) Å in 4. The macrocycles
anedionato ligands in 2 range from 2.408(3) to 2.461(3) Å in new complexes 2 and 4, as well as in 3,[9] are in a dis-
and are thus slightly longer than the distances found for torted cone conformation, with dihedral angles between the
starting material 1 [2.325(3) to 2.346(3) Å] and the Ce–O four aromatic rings and the O4 plane of 39.50(16),
bonds of the two O,OЈ-bidentate acetylacetonate ligands in 89.50(11), 34.66(8), and 83.31(11)° in 2 [46.16(24), 71.72(9),
the analogous nonfluorinated complex {p-tBu-ca- 37.45(13), and 72.02(11)° in 4]. The dihedral angles of the
lix[4](OMe)2(O)2}Ce(acac)2 (3) (2.338 to 2.371 Å).[9] The cone in 2 are similar to the ones in 3 [38.09(7), 83.86(7),
corresponding Ce–O distances of the O,OЈ-bidentate 3- 36.90(13), and 84.02(7)°], and to those found in [UCl2(Me2-
bromo-2,4-pentanedionato ligands to cerium in 4 [2.365(3) calix)(THF)2], [UCl2(Me2calix)(py)2], and [U(acac)2(Me2ca-
and 2.372(3) Å] match well with the ones of complex 3. The lix)(THF)2].[25] The slightly different values of the homolo-
Ce–O (β-diketonato) lengths of complexes 2 and 4 are only gous angles in 4 might be due to crystal packing effects.
slightly longer than the Ce–O lengths found for the homo-
leptic cerium β-diketonate complexes. However, they are in
XPS Studies
good agreement with these lengths {e.g. α-Ce(acac)4,[20a,20b]
β-Ce(acac)4,[20c] and [Ce(acac)4](H2O)10,[21] with a mean
High-resolution XPS spectra of complexes 2 and 3 were
distance of 2.32 Å, as well as Ce(fda)4, Ce(tmhd)4, Ce- obtained by using monochromatic X-ray excitation. From
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J. Gottfriedsen, R. Hagner, M. Spoida, Y. Suchorski
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this spectra we determined the effective oxidation state of ous low-energy electrons,[33] which could cause desorption
Ce (in the sense of an effective charge state of the Ce atoms due to their large cross-section seems to be less probable, as
in the ground state of the solid), Ceox. The Ceox value does was proven by low-energy electron bombardment from an
not necessary represent the integral “formal valence” of Ce external electron gun. Furthermore, a possible reason for
(in compounds with largely covalent character the formal the decrease in Ceox might be due to decomposition pro-
valence does not approximate the actual ionic charge), but cesses that are induced by the X-ray radiation, which leads
it reflects objectively the electronic structure of the studied to the formation of a mixture of cerium centers in the oxi-
materials. The direct use of established techniques, such as dation state 3 and 4. It should also be noted that the attenu-
the commonly used deconvolution of high-resolution XPS ation length of the Al Kα photons exceeds the XPS infor-
spectra to evaluate the Ceox in coordination complexes, is mation depth of ≈ 2.5 nm, by at least three orders of magni-
still difficult despite a great effort devoted to the electronic tude, in the present case. Therefore, the irradiation-caused
structure of ceria and similar compounds. The main uncer- changes in the Ce oxidation state may extend to a signifi-
tainties are associated with the complex features in the cant depth.
Ce3d XPS-spectra and result from the variable occupancy
of the Ce4f level, redistribution of the levels as a result of
the core hole creation, and changed hybridization at dif-
ferent oxidation states.[26] Therefore, the interpretation of
the spectra is not always unambiguous and can even be con-
tradictory. In addition, artifacts created merely by the X-
ray exposure have to be considered.[27] In the present work
we evaluated Ceox in 2 and 3 by correlation of the XPS data
for the studied samples with those of reference samples with
a known oxidation state.
Ar-sputtered CeO2 was used as the reference sample
where the preferential release of oxygen atoms in the surface
region, as a result of the impact of Ar+ ions in ultrahigh
vacuum (UHV), allows the creation of CexOy oxides in the
top surface layers with continuously variable artificial Ceox
in the range from 4 to 3.4. The details of the Ceox evalu-
ation are published elsewhere.[28] A similar method was re-
cently applied by us for the determination of the oxidation
state of vanadium in vanadium oxide and vanadium/phos-
phate oxide catalysts.[29] The measurements of samples of
both the studied complexes revealed an X-ray induced
“photoreduction”, an effect in principle known (but not en-
tirely understood) for some rare earth containing materials
such as, for example, Sm3+-doped fluoroaluminate
glasses[30] or for the pure CeO2.[31] Thorough long-time
studies of the X-ray-induced evolution of the Ceox revealed
its exponential decay with increasing irradiation time (from
3.60 to 3.28 for 2 and from 3.62 to 3.27 for 3) (Figure 4),
Figure 4. Dependencies of the Ce4+ content for 2 and 3, respec-
tively, on the X-ray exposure. Extrapolation to “zero irradiation”
where the extrapolation to “zero irradiation” (zero expo-
sure) allows the determination of the initial oxidation state
in the freshly synthesized complexes (3.61 for 2 and 3.65 for
3). It has to be noted that in contrast to vanadium-contain-
ing oxide compounds, no UHV-induced modification of the
cerium oxidation state was observed in the present study,
that is, the observed effect is a pure X-ray-induced “photo-
reduction”. The X-ray modified Ceox of cerium remained
provides the initial values of Ceox
.
Conclusions
Two novel cerium calix[4]arene complexes could be syn-
also stable after exposure to air at room temperature. Con- thesized by using two different synthetic approaches. {p-
cerning the interpretation of the observed “photore- tBu-calix[4](OMe)2(O)2}Ce(hfac)2 (2) was generated start-
duction”, we noted that the local irradiation-induced heat- ing from a homoleptic cerium β-diketonato complex
ing, proposed as a reduction mechanism for ceria,[31] cannot Ce(hfac)4 (1). However, {p-tBu-calix[4](OMe)2(O)2}Ce(Br-
explain our observation as no significant increase in the sur- acac)2 (4) was obtained from the reaction of {p-tBu-calix-
face temperature was observed. Apparently, alternative [4](OMe)2(O)2}Ce(acac)2 (3) with Br2; thus, the high syn-
mechanisms such as the electron-hole pair formation and thetic potential of this class of cerium complexes was dem-
Auger decay have to be taken into account.[32] Another pos- onstrated. Furthermore, we detected X-ray-induced
sible mechanism, associated with an impact of the numer- changes in the effective Ce oxidation state in cerium calix-
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© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2007, 2288–2295

Synthesis, Structure, and Reactivity of CeIV Calix[4]arene Complexes
FULL PAPER
used as received. X-ray photoelectron spectra were obtained from
the dry powders of 2 and 3, which were carefully grinded and fixed
on Mo sample-carrier plates under an argon atmosphere in a glove
box and transferred inertly to the multipurpose UHV surface
analysis apparatus (SPECS, Germany) where the XPS analyses
were performed. To avoid the vacuum-induced reduction of the
samples, a “fast transfer” XPS mode[29] was used: the SPECS load-
lock and transfer systems allows sample-transfer times of less than
5 min between the start of the evacuation of the load lock and the
first XPS spectrum at a pressure better than 5ϫ10–9 mbar. The
monochromatic X-ray source (FOCUS-500, SPECS, excitation en-
ergy 1253.74 eV, Al-Kα, linewidth after monochromatization Ͻ
0.3 eV) was used. High-resolution spectra (pass energy 10 eV, step
size 0.1–0.2 eV) were recorded at room temperature with a hemi-
spherical energy analyzer (PHOIBOS-150, SPECS), which provides
the possibility of simultaneous photo-electron detection on nine
channels, and allows fast data acquisition times (0.5 s per data
point) with a satisfactory signal-to-noise ratio.
[4]arene complexes 2 and 3, and performed the first evalu-
ation of Ceox in these complexes.
Experimental Section
General: All reactions were carried out under an inert atmosphere
of dry nitrogen by using standard dry box and Schlenk techniques.
Melting points were determined in a sealed capillary without cor-
rection. NMR spectra were recorded with a Bruker DPX 400 or
AVANCE 600 NMR spectrometer. Chemical shifts (all reported
in ppm) are referenced to tetramethylsilane as internal standard.
Chemical shifts of the 13C NMR were assigned to the correspond-
ing carbons by using HSQC, HMBC, and NOSY techniques. In
turn, chemical shifts of the 19F NMR spectra are referenced to tri-
fluorotoluene as external standard. The single-crystal X-ray dif-
fraction studies were performed with a Bruker CCD SMART (2,
4) or Stoe IPDS (1) diffractometer. Crystal data are given in
Table 3. The structures were solved by Patterson methods
(SHELXS-97).[34] Refinements were carried out by using full-ma-
trix least-squares techniques on F2 with the SHELXL-97 pro-
gram.[35] CCDC-624482 (for 1), -616674 (for 2), and -624481 (for
4) contain the supplementary crystallographic 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. Mass spectra (EI, 70 eV) were obtained with a Finnigan
SSQ 7000 or a Finnigan MAT 95 (70 eV). Only characteristic frag-
Ce(hfac)4 (1): (NH4)4[Ce(SO4)4]·2H2O (3.8 g, 6 mmol) in H2O
(200 mL) and 1,1,1,5,5,5-hexafluoro-2,4-pentanedione (5.0 g,
24 mmol) in toluene (100 mL) were mixed and stirred for 15 min,
which resulted in a fast color change in the organic layer to dark
brown. The two phases were separated, and toluene was removed
under vacuum to leave a brown solid that was recrystallized from
dry toluene (15 mL) to yield dark brown crystal blocks in 35%
1
yield (2.03 g). H NMR (400 MHz, C6D6, 25 °C): δ = 6.05 (s, 4 H,
CH) ppm. 19F NMR (376.5 MHz, C6D6, 25 °C): δ = –76.51 ppm.
UV/Vis (250–800 nm, CHCl3): λ = 275.6 nm.
ments containing the isotopes of the highest abundance are listed.
[36]
The starting materials p-tBu-calix[4](OMe)2(OH)2
and {p-tBu-
calix[4](OMe)2(O)2}Ce(acac)2[9] were produced according to a pub-
lished procedure. Ce(hfac)4 was synthesized by using a slightly
modified procedure.[37] Elemental analyses were performed with a
LECO CHNS932 apparatus. Hexafluoroacetylacetone and (NH4)4-
[Ce(SO4)4]·2H2O were purchased from Aldrich Chemical Co. and
{p-tBu-calix[4](OMe)2(O)2}Ce(hfac)2
(2):
Ce(hfac)4
(1.0 g,
1.03 mmol) and p-tBu-calix[4](OMe)2(OH)2 (0.7 g, 1.03 mmol)
were dissolved in toluene (50 mL), stirred at reflux temperature for
2 h, and another 12 h at room temperature. The reaction mixture
was dried under vacuum (10–2 mbar) at 40 °C, and the resulting
Table 3. Crystal data and structural refinement for complexes 1, 2, and 4.
1
2
4
Empirical formula
Formula weight
Temperature [K]
Wavelength [Å]
Crystal system
Space group
a [Å]
b [Å]
c [Å]
α [°]
C20H4CeF24O8
968.35
180(2)
0.71073
monoclinic
I2/a
16.511(3)
11.081(2)
16.519(3)
90
90.42(3)
90
C63H68CeF12O8
1321.29
200(2)
C60H65Br2CeCl4O9
1371.86
180(2)
0.71073
monoclinic
C2/m
28.049(6)
15.198(3)
16.611(3)
90
118.84(3)
90
0.71073
triclinic
¯
P1
13.3831(10)
14.3193(10)
16.9920(12)
102.0560(10)
98.3980(10)
94.188(2)
3132.2(4)
2, 1.401
0.814
1352
β [°]
γ [°]
Volume [Å3]
3022.3(10)
4, 2.128
1.698
6203(2)
4, 1.469
2.246
2772
Z, calcd. density [mgm–3]
Abs coeff [mm–1]
F(000)
1848
Crystal size [mm]
θ range for data coll. [°]
Reflections collected
Unique reflections
Absorption correction
Data/rest./parameters
GOF on F2
0.50ϫ0.50ϫ0.20
2.47 to 27.96
14035
3583 (Rint = 0.0819)
face indexed
3583/0/248
0.945
R1 = 0.0445
wR2 = 0.1064
R1 = 0.0572
wR2 = 0.1094
0.50ϫ0.40ϫ0.20
2.01 to 29.29
21948
12975 (Rint = 0.0268)
SADABS
12975/0/928
1.248
R1 = 0.0595
wR2 = 0.1411
R1 = 0.0659
wR2 = 0.1447
0.60ϫ0.50ϫ0.10
3.54 to 27.96
29279
7645 (Rint = 0.0877)
face indexed
7645/0/395
1.101
R1 = 0.0576
wR2 = 0.1409
R1 = 0.0716
wR2 = 0.1480
Final R indices [IϾ2σ(I)]
R indices (all data)
Eur. J. Inorg. Chem. 2007, 2288–2295
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
2293

J. Gottfriedsen, R. Hagner, M. Spoida, Y. Suchorski
FULL PAPER
blue powder was dissolved in toluene (10 mL). Crystallization at
5 °C gave dark blue crystals in 79% yield (1.08 g, calculated as
toluene solvate). M.p. 295 °C. H NMR (400 MHz, C6D6, 25 °C):
[1]
a) X. D. Wu, R. C. Dye, R. E. Muenchausen, S. R. Foltyn, M.
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1
δ = 7.13 (m, 2 H, toluene), 7.11 (s, 4 H, ArH), 7.05–7.0 (m, 3 H,
toluene), 6.85 (s, 4 H, ArH), 5.8 (s, 2 H, COCH), 5.11 [d, 1J(1H,1H)
= 12.6 Hz, endo-CH2], 4.93 (s, 6 H, OCH3), 3.27 [d, 1J(1H,1H) =
12.6 Hz, exo-CH2], 2.11 (s, 3 H, toluene), 1.27 (s, 18 H, CCH3),
0.90 (s, 18 H, CCH3) ppm. 13C NMR (100.6 MHz, C6D6, 25 °C):
δ = 177.57 (CF3C-O), 173.57 [C(Ar)OCe], 152.54 [C(Ar)OCH3],
148.53 (tBuC), 145.92 (tBuC), 133.45 (CCH2), 133.4 (CCH2),
129.30 (toluene), 128.31 (toluene), 126.29 [CH(Ar)], 125.66 (tolu-
ene), 122.19 [CH(Ar)], 118.5 (CF3CO), 93.09 (COCHCO), 67.96
(OCH3), 34.02 (CH3C), 33.48 (CH3C), 32.46 (CCH3), 31.31 (CH2),
30.94 (CCH3), 21.40 (toluene) ppm. 19F NMR (376.5 MHz, C6D6,
25 °C): δ = –75.93 ppm. MS (EI, 140Ce): m/z (%) = 1228 (5) [M]+·,
1021 (100) [M – hfac]+, 1021 (40) [M – hfac – CH3]+. UV/Vis (250–
800 nm, CHCl3): λ = 276.8 nm. C56H60CeF12O8C7H8 (1321.31):
calcd. C 57.27, H 5.19; found C 57.24, H 5.22.
[2] a) F. Eguchi, T. Setoguchi, T. Inoue, H. Arai, Solid State Ionics
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{p-tBu-calix[4](OMe)2(O)2}Ce(Br-acac)2(4):{p-tBu-calix[4](OMe)2-
(O)2}Ce(acac)2 (0.31 g, 0.31 mmol) was dissolved in diethyl ether
(30 mL), and Br2 (0.098 g, 0.62 mmol) was added by syringe at
room temperature. The reaction mixture was heated under reflux
for 3 h. The color of the solution changed from deep purple to
deep blue. The solvent was removed under vacuum at 40 °C to give
a blue powder, which was solved in dichloromethane (5 mL) and
recrystallized at 5 °C to yield small cube-like crystals in 45% yield
1
(0.16 g). M.p. 310 °C. H NMR (400 MHz, C6D6, 25 °C): δ = 7.22
(s, 4 H, ArH), 6.96 (s, 4 H, ArH), 5.04 [d, 1J(1H,1H) = 12.1 Hz,
endo-CH2], 4.58 (s, 6 H, OCH3), 3.35 [d, 1J(1H,1H) = 12.1 Hz, exo-
CH2], 2.19 (s, 12 H, COCH3), 1.34 (s, 18 H, CCH3), 0.97 (s, 18 H,
CCH3) ppm. 13C NMR (100.6 MHz, C6D6, 25 °C): δ = 187.72
(CH3C-O), 170.57 [C(Ar)OCe], 153.71 [C(Ar)OCH3], 147.56
(tBuC), 141.39 (tBuC), 133.17 (CCH2), 132.39 (CCH2), 125.94
(CH), 123.49 (CH), 100.66 (COCBrCO), 66.79 (OCH3), 34.01
(CH3C), 33.82 (CH3C), 32.43 (CH2), 32.32 (CCH3), 31.13 (CCH3),
28.82 (CH3CO) ppm. MS (EI, 140Ce): m/z (%) = 1170 (5) [M]+·,
993 (100) [M – (Br-acac)]+, 912 (60) [M – (Br-acac) – Br]+. UV/Vis
(250–800 nm, CHCl3): λ = 289.8 nm. C56H70Br2CeO8 (1171.07):
calcd. C 57.43, H 6.02; found C 57.23, H 6.04.
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[12]
Synthesis of {p-tBu-calix[4](OMe)2(O)2}TiCl2 from {p-tBu-ca-
lix[4](OMe)2(O)2}Ce(acac)2 and TiCl4: {p-tBu-calix[4](OMe)2(O)2-
Ce(acac)2} (1.7 g, 1.7 mmol) was dissolved in toluene (50 mL) and
TiCl4 (0.096 g, 1.7 mmol) was added by syringe at room tempera-
ture, which caused the color of the solution to change to red. Sub-
sequent stirring for another 12 h and removal of the solvent under
vacuum gave a red–brown powder that was recrystallized from tol-
uene (10 mL) to afford deep red crystals. 1H NMR (400 MHz,
C6D6, 25 °C): δ = 7.17 (s, 4 H, ArH), 6.82 (s, 4 H, ArH), 4.61
[13]
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[d, J(1H,1H) = 13.4 Hz, endo-CH2], 4.18 (s, 6 H, OCH3), 3.18 [d,
[15]
[16]
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[18]
[19]
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helpful discussions, as well, R. Wrobel and B. Strzelczyk are
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Synthesis, Structure, and Reactivity of CeIV Calix[4]arene Complexes
FULL PAPER
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Received: October 24, 2006
Published Online: April 17, 2007
Eur. J. Inorg. Chem. 2007, 2288–2295
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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