Ring a bell: Disubstituted calix[4]arene as ligand for transition metal chlorides

Article abstract of DOI:10.1016/j.poly.2012.08.004

Lower rim 1,3-disubstituted p-tert-butylcalix[4]arene is studied here in its deprotonated form as complexing ligand for iron and chromium chloride. Crystallization of the two complexes from dry THF gives two different stoichiometries and structures, a trinuclear iron compound and a second, mononuclear structure with chromium. In both cases, the ligand adopts an elliptical shape and the metal ions are bound to the lower rim of the calixarene molecule.

Full text of DOI:10.1016/j.poly.2012.08.004

Polyhedron xxx (2012) xxx–xxx
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Ring a bell: Disubstituted calix[4]arene as ligand for transition metal chlorides
a,b,
Aurelien Crochet a, , Katharina M. Fromm
⇑
⇑
a Fribourg Center for Nanomaterials (FriMat), University of Fribourg, Chemin du Musée 3, CH-1700 Fribourg, Switzerland
b Department of Chemistry, University of Fribourg, Chemin du Musée 9, CH-1700 Fribourg, Switzerland
a r t i c l e i n f o
a b s t r a c t
Article history:
Available online xxxx
Lower rim 1,3-disubstituted p-tert-butylcalix[4]arene is studied here in its deprotonated form as com-
plexing ligand for iron and chromium chloride. Crystallization of the two complexes from dry THF gives
two different stoichiometries and structures, a trinuclear iron compound and a second, mononuclear
structure with chromium. In both cases, the ligand adopts an elliptical shape and the metal ions are
bound to the lower rim of the calixarene molecule.
Dedicated to Alfred Werner.
Keywords:
Iron complex
Ó 2012 Elsevier Ltd. All rights reserved.
Chromium complex
Calix[4]arene
X-ray analysis
Solid state structure
1. Introduction
alkaline earth and mixed-metal alkali and alkaline earth metal
cage compounds obtained from partial abstraction of halide [37–
Calixarenes, in particular calix[4]arenes, are attractive mole-
cules because of their cone-shape and the lower-rim hydroxide
functions. Thus, this class of molecules can be used in many appli-
cations, e.g. as receptors for organic and inorganic analytes [1–4].
Substitution reactions [5] at the hydrophobic upper rim and the
hydroxyl groups of the lower rim lead to the development of mul-
tifunctional sensors [6], capsule systems [7] and even catalytic
properties [8–10]. Recent applications include also channel-type
architectures for ion exchange [11], building blocks in metal organ-
ic frameworks (MOFs) [12], ion extraction [13] and antimicrobial
properties [14]. Substitutions at the upper rim can lead to metal
ion complexation for e.g. cross-coupling reactions [15]. The
hydroxide groups at the lower rim have always been considered
as good binding sites for metal ions in order to mimic oxide surface
conditions and for investigating magnetism, luminescence and/or
redox chemistry [16–28].
41]. Depending of the bulk of the R group on the alkoxide or aryl-
oxide reagent and the nature of the alkali metal and the solvent of
crystallization (binding mode, Lewis basicity, and so forth), differ-
ent structural features can be achieved [29,42,43]. In this context,
we have also studied calixarenes, which can be understood as cyc-
lic polyphenols. Deprotonated with an organo-lithium compound,
the formed compounds can be considered as lithium polyarylox-
ides [44–46]. The sensitivity of such aryloxide complexes, includ-
ing deprotonated calixarenes, to moisture is always described as
a disadvantage for this family of complexes, which are difficult to
handle, store, and analyze.
In previous work, we have synthesized deprotonated alkali
calixarenes [46]. One of them (Fig. S1) is obtained by addition of
lithium tert-butyloxide in THF-solution to a suspension of p-
tert-butylcalix[4]arene in dry THF. In order to avoid dimerization
or oligomerization, we have chosen to limit the quantity of labile
protons on the p-tert-butylcalix[4]arene. We have thus replaced
two hydroxyl groups by two methyl ether groups (Scheme 1) as
these are short and thus reduce the impact of their presence on
the complexation sites (least possible change in the electronegativ-
ity and least bulky as possible in order to have an easy access to the
oxygen atoms for metal ions). For the synthesis of 1,3-dimethyl-
p-tert-butylcalix[4]arene, H2L, we used a method described by
Huang and co-workers (Scheme 1) [47].
We are interested in cluster compounds, complexes and coordi-
nation polymers of alkali, alkaline earth metal and transition metal
compounds in order to study their behavior in non-aqueous sol-
vents and possible applications as catalysts and/or precursors for
oxide materials [29–36]. We have previously shown that Group 2
metal halides may act as starting materials for both homo-metallic
⇑
Corresponding authors. addresses: Fribourg Center for Nanomaterials (FriMat),
1,3-Dimethyl-p-tert-butylcalix[4]arene can be deprotonated by
several different methods: with sodium [48], sodium hydride
[49,50], mesityllithium [51], butyllithium [49,52], potassium [49],
potassium bis(trimethylsilylamide) [53], rubidium [49], or even ce-
sium [49]. In 1997, Dubberley et al. published the dimeric structure
University of Fribourg, Chemin du Musée 3, CH-1700 Fribourg, Switzerland (A.
Crochet). Department of Chemistry, University of Fribourg, Chemin du Musée 9, CH-
1700 Fribourg, Switzerland (K.M. Fromm).
E-mail addresses: aurelien.crochet@unifr.ch (A. Crochet), katharina.fromm@
unifr.ch (K.M. Fromm).
0277-5387/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2012.08.004
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A. Crochet, K.M. Fromm / Polyhedron xxx (2012) xxx–xxx
Scheme 1. Scheme of the alkylation (top), the deprotonation (middle) and the numbering of atoms of deprotonated calix[4]arene ligand L2À
.
of the sodium dimethylcalix salt [50], and in 2003, he published
two forms of the lithium salt, a dimeric one and one monomeric
form, as established by spectroscopic analyses [49]. We produced
the lithiated calixarene by reaction with two equivalents of lithium
tert-butyloxide, this compound is noted Li2L (Scheme 1). This lith-
iated ligand will be used as reagent for iron and chromium halide
in order to obtain new (mixed) metal complexes. As driving force
for the transition ion complexation by the calix[4]arene ligand,
we will make use of the elimination of LiCl. In first reaction, this
is tested for FeCl2 as this fits well with the two lithium ions of
Li2L and should thus yield an iron complex. All reactions are car-
ried out under inert atmosphere and using dried solvents.
2. Results and discussion
Scheme 2. Schematic representation of (1).
2.1. The iron complex [Li(THF)4][Fe3Cl5(L)(THF)] (1)
one cationic which consists of lithium surrounded by four THF
molecules, and one anionic unit A composed of one functionalized
calixarene, three iron ions, five chloride anions and one THF mole-
cule (Fig. 1). Unit A contains the three Fe(III) ions, which each have
The first compound, [Li(THF)4][Fe3Cl5(L)(THF)] (1) (Scheme 2),
was obtained via addition of dry iron(II) chloride to a THF-solution
of Li2L. The compound crystallizes in the monoclinic space group
P21/n (No. 14) (Fig. 1). The structure is composed of two units,
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A. Crochet, K.M. Fromm / Polyhedron xxx (2012) xxx–xxx
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Fig. 1. Molecular view of 1, 35% of probability, H atoms are omitted for clarity.
different coordination spheres. Fe1 is pentacoordinated and bound
to the four oxygen atoms (Fe1–O1 2.168(3) Å, Fe1–O2 1.955(2) Å,
Fe1–O3 2.160(3) Å, Fe1–O4 1.967(23) Å) of the phenol moieties
of the ligand in an almost square planar fashion (Fe1–plane {O1,
O2, O3, O4} 0.247 Å). It is capped by a chloride ion Cl1 in apical po-
sition (2.391(1) Å), pointing away from the calixarene moiety. The
two other iron ions are tetracoordinated. Fe2 binds to the calixa-
rene via O2 (2.049(2) Å) and to three chloride ions Cl1
(2.433(1) Å), Cl2 (2.256(1) Å) and Cl3 (2.241(2) Å). Cl1 acts thus
as bridging ligand for Fe1 and Fe2. The third ion, Fe3, is connected
to O4 (2.021(2) Å), Cl4 (2.249(1) Å) and Cl5 (2.280(1) Å). Its coordi-
nation is completed by one THF molecule (Fe3–O5 2.083(3) Å).
For Fe1, a five-coordination is observed. To evaluate between a
trigonal bipyramid or a square pyramidal, an evaluation of the
structural index parameter can be made. This parameter was intro-
duced by Addisson et al. [54]. We have previously applied this
method for a mercury(II) iodide dibenzo-24-crown-8 complex
[32]. In this model, the metal M is coordinated by the five atoms
A, B, C, D and E, as shown in Scheme 3 below. In a square-pyrami-
The distances between the iron ions of 3.1085(7) Å for Fe1–Fe2
and 3.3548(7) Å for Fe1–Fe3, are too long to consider metal–metal
bonds. In order to determine the oxidation states of the iron ions,
the Bond Valence Sum (BVS) method is applied [56,57]. The values
found are 2.12 for Fe1, 1.99 for Fe2 and 2.00 for Fe3, confirming
that we are in the presence of three iron (II) ions.
The packing view shows that one of the THF molecules of the
[Li(THF)4]+ cationic moiety penetrates into the hydrophobic cavity
at the upper rim of the calix[4]arene ligand L. This THF is con-
nected via H-bonding with the aromatic rings of the calix[4]arene
ligand (Fig. 2).
For compound 1, the strategy of substitution of two hydroxide
functions for two methoxy groups successfully leads to the forma-
tion of monomeric complexes with respect to the calixarene ligand.
The three iron atoms are asymmetrically coordinated in the solid
state. However, in solution, a certain dynamic exchange can occur
as exemplified in Scheme 4. THF is then a labile ligand, which can
easily leave the iron ion to which it is coordinated, with the central
chloride atom Cl1 providing the missing electron pair for the metal
ion (intermediate bottom Scheme 4). Upon recoordination, THF can
then bind to both outer iron atoms with the same probability, as
the intermediate is symmetric. This would then give to the com-
plex dynamism in solution.
dal geometry, one would have
a = b = 180° with A as the axial li-
gand (Scheme 3). In most real square pyramidal environments
however, the metal ion M is shifted towards A, such that both an-
gles are <180°, with b being the largest of the basal angles (B–M–C
in Scheme 3 left) and
gonal bipyramidal structures,
axis. In compound 1 the angles are b = O1–Fe1–O3 169.47(9)°
and = O2–Fe1–O4 = 162.4 (1)°.
The geometric parameter a)/60 ‘‘which is applicable to
a
the smallest (D–M–E in Scheme 3). For tri-
According to the literature [58–61], surface covered of FeCl2
[62,63], respectively pentacoordinated Fe(II) complexes [64] can
be used as catalyst for polymerization processes. Compound 1
might thus be a potential catalyst as (i) the THF-ligand on Fe2
might be labile in solution and would free a coordination site on
the metal ion, and (ii) Fe1 has a free coordination site located in-
side the calixarene ligand.
a
is 120° with B–M–C as principle
a
s
= (b À
five coordinated structures as an index of the degree of the trigonality,
within the structural continuum between trigonal bipyramidal and
rectangular pyramidal. For a perfectly tetragonal geometry
s
is equal
In order to verify if compound 1 has some catalytic properties, a
small amount of it was mixed with a solution of styrene in a mix-
ture of THF/DCM. In our hands, no conversion of styrene to polysty-
rene was observed, thus, the complex does not possess catalytic
properties in this context. Smaller monomers did also not yield
to zero, while it becomes unity for perfectly trigonal bipyramidal
geometry’’ [54]. Application of this parameter to our compound 1
gives a value of 0.12 which indicates a geometry clearly nearest a
square pyramidal rather than trigonal bipyramidal geometry [55].
Scheme 3. Determination of the structural index parameter for compound 1.
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Fig. 2. Molecular view of 1; H atoms are omitted for clarity. The two variants show the penetration of a THF molecule of the cation into the calix[4]arene moiety.
Scheme 4. Ringing the ‘‘iron bell’’ with the central chloride ion.
polymers in presence of 1. This might be due to the fact that no free
coordination site is accessible at the iron ions Fe2 and Fe3 if our
intermediate, postulated in Scheme 4 forms. Also, the free coordi-
nation site on Fe1 is difficult to access as the substrate would have
to enter the calixarene cavity. There would, if at all, not be enough
space to accommodate one or two monomers in order to connect
them. Unfortunately, we could not verify this hypothesis of reactiv-
ity in solution by NMR as it gave only very broad, shifted signals.
In the literature, only four structures were found, in which two
hydroxyl groups are protected by methanol or benzyl alcohol and
among these four structures only one contains iron(II) [16,65,66].
In all these publications, the calixarene ligand is coordinated to
only one iron ion. Our compound is thus, to the best of our knowl-
edge, the first calixarene complex coordinating with three iron(II)
ions and exposing a FeCl2-face.
Scheme 5. Schematic representation of (2).
Cr1–O3 2.036(2) Å) and O2 and O4 deprotonated (Cr1–O2
1.939(3) Å and Cr1–O4 1.907(2) Å). The coordination sphere of
the chromium ion is completed by two chloride ions Cl1 and Cl2
(Cr1–Cl1 2.308(1) Å and Cr1–Cl2 2.299(1) Å), leading to a slightly
distorted octahedron around the metal ion, with Cl1–Cr1–Cl2
95.22(4)°, O1–Cr1–O3 82.5(4)° and O2–Cr1–O4 171.1(1)° (Fig. 3).
The two hydroxide oxygen atoms O2 and O4 are coordinating to
a lithium ion Li1 (Li1–O2 1.854(8) Å and Li1–O4 1.84(1) Å, which
has its coordination sphere completed by one THF molecule Li1–
O5 1.896(9) Å. This trigonal planar lithium cation (angle sum of
2.2. Chromium complex, [CrCl2(L)Li(THF)], 4THF (2)
The compound [CrCl2(L)Li(THF)], 4THF (2), was obtained by
addition of one equivalent of chromium(II) chloride to a solution
of Li2L in dry THF. The compound crystallizes in the triclinic space
ꢀ
group P1 (No. 2) (Scheme 5 and Fig. 3). The structure consists of
one chromium ion connected to the four oxygen atoms of the cal-
ixarene ligand, with O1 and O3 methylated (Cr1–O1 2.038(2) Å;
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A. Crochet, K.M. Fromm / Polyhedron xxx (2012) xxx–xxx
5
the alcohol than to the CH2-group [67]. The elimination of this rad-
ical on the oxygen atom of a deprotonated calix[4]arene seems
then to be sufficient to oxidize the chromium(II) to chromium(III).
In 2000, Hesschenbrouck et al. published a structure of chro-
mium(III) chloride with dimethylcalix[4]arene [51]. In this com-
plex, chromium(III) is hexacoordinated by four oxygen atoms of
the calixarene ligand, one chloride ion and one THF molecule. This
complex does not have additional lithium in the structure. In 2009,
Redshaw et al. published a similar structure to 2 [23]. The differ-
ence (Table 1) between Redshaw’s compound and ours resides
mainly in the coordinating solvent. In Redshaw’s structure (R),
the lithium ion is tetracoordinated (by two oxygen atoms of the
calixarene ligand and by two acetonitrile molecules).
2.3. General remark on the ligand: opening of the calix[4]arene
Comparing the angle between the planes containing the depro-
tonated phenol moieties with the one between planes containing
the methylated phenol groups, we remark that we are in presence
of elliptical conical frustum scheme and not circular conical frus-
tum. Indeed, in compound 1, planes containing PhOÀ form an angle
of around 69° and planes containing PhOMe are inclined against
each other by ca. 49°. In compound 2, these angles are ca. 151°
and 54° respectively. Thus, the calixarene ligand of 2 is much flat-
ter than in 1. Distances between the central carbon atom of the
tert-butyl group are also good indicators: 9.271(7) Å for the meth-
ylated groups and 10.622(6) Å for the PhOÀ-moieties in 1, and
9.269(5) Å and 13.406(6) Å, respectively, in 2. This opening of the
calixarene is common with dimethylcalix[4]arene and is more pro-
nounced when a metal ion is coordinated endo in the calixarene
cavity (Fig. S4) [16,23,51,52,65,66,68–72].
The ligand alone adopts the cone conformation while in princi-
ple, many other are possible for a calix[4]arene [74,75]. Only four
crystal structures of H2L are published, one with an empty cavity
[73], two with solvent in the cavity (toluene [70] or acetonitrile
[74]) and one structure in which one empty calixarene adopts a
1,2-alternate conformation and a second one a cone conformation
with acetonitrile in the cavity [74]. In case of the cone conforma-
tion with an empty cavity, the aromatic rings opposite to each
other form angles of ca. 8° and 63° for the methoxy-substituted,
respectively hydroxy-substituted rings. Presence of solvent in the
cavity slightly increases these angles to 39° and 89° in case of tol-
uene and in case of acetonitrile to around 41° and 71° or 43° and
74° depending of the structure (Table 2). Solvent alone does thus
not have a so big impact on the cavity opening compared to calix-
arene when used as ligand for transition metal ions (see Fig. S5).
Fig. 3. Molecular view of 2; H atoms and free THF molecules omitted for clarity,
25% of probability.
Table 1
Comparison between two Cr(III) calix complexes.
R [23]
2
PhO–Cr (Å)
1.919(1); 1.922(1)
78.90(6)
2.038(2); 2.047(2)
172.01(6)
2.3165(6); 2.3159(6)
94.68(6)
1.939(3); 1.907(2)
78.7(1)
2.036(2); 2.038(2)
171.1(1)
2.299(1); 2.308(1)
95.22(4)
PhO–Cr–OPh (°)
PhO(Me)–Cr (Å)
PhO(Me)–Cr–(Me)OPh (°)
Cr–Cl (Å)
Cl–Cr–Cl (°)
PhO–Li (Å)
PhO–Li–OPh (°)
BVS (Li)
1.954(4); 1.994(4)
76.4(2)
0.90
1.84(1); 1.854(8)
82.5(4)
1.02
Angle between plan OPh (°)
Angle between plan MeOPh (°)
161
41
151
54
357.17°) has a Bond Valence Sum (BVS) of 1.02 and sits in the cav-
ity of the calixarene. It forms also interactions with the -systems
p
of two phenyl rings: The distance between the centroïd of the ring
connected to O1 (Cg1) and Li1 is ca. 2.97 Å and between Cg3 and
Li1 ca. 3.42 Å (Cg1–Li1–Cg3 173.38°).
The BVS value of the chromium ion is 3.12, which assumes an
oxidation state of three, thus an oxidation of Cr(II) to Cr(III) oc-
curred. One possibility for this oxidation can be the formation of
radicals in THF solution. These radicals can be transferred to depro-
tonated calixarene and eliminated by oxidation of the chro-
mium(II) ion. Indeed Giese has studied this phenomenon of
radical formation in a calix[4]arene, in order to identify the posi-
tion of the radical as a function of the protonation of the phenol
group. Calculations and measurements show that if a radical is
formed on the calixarene, it will rather go to the oxygen atom of
Indeed, in
1 the calixarene cavity is empty except for the
[Li(THF)]4+ cation pointing into the cavity and forming C–H. . .
p
(2.5–3.2 Å) interactions. This leads to a more or less spherical cone
conformation with a slight enlargement of this corresponding an-
gle versus the other (see Table 2). However, in compound 2, the
calixarene ligand adopts a flatter conformation than in 1 due to
Table 2
Comparison of angles for the methoxy- and the hydroxy-substituted rings.
Calix[4]arene
Dimethyl
Cavity
For MeOPh plans (°)
For (H)OPh plans (°)
C–C for MeOPh
C–C for (H)OPh
Empty [73]
Toluene [70]
Acetonitrile [74]
Acetonitrile [74]
THF (1)
8
39
41
43
49
54
41
63
89
71
75
69
6.041(6)
10.010(7)
8.615(4)
8.462(3)
9.271(7)
9.269(5)
8.223(5)
12.667(6)
11.415(8)
10.213(4)
10.466(3)
10.622(6)
13.406(6)
13.638(8)
Li-THF (2)
R [23]
151
161
Nonsubstituted
Half-lithiated (THF) [46]
Fully lithiated (Li-THF) [46]
62
34
71
79
9.797(5)
8.539(5)
10.543(4)
11.760(6)
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A. Crochet, K.M. Fromm / Polyhedron xxx (2012) xxx–xxx
the endo binding of the lithium cation. This resembles the effect
observed in the fully lithiated calix[4]arene [46], where also one
lithium ion is included in the cone cavity (Fig. S1), and to which
one THF ligand is bound. Thus, the presence of complexed transi-
tion metal ion(s) at the lower rim and lithium complexation inside
the cavity increase the ring opening with an enlargement between
the hydroxy-substituted rings.
Ar–H), 6.68 (b, 4H, Ar–H), 4.28 (b, 8H, ArCH2Ar), 1.27 (s, 36H,
C(CH3)3), 3.93 (s, 6H, CH3). IR: 2953 cmÀ1(m), 2925(sh), 2860(sh),
1601(sh), 1479(m), 1482(s), 1435(sh), 1360(sh), 1326(s),
1303(sh), 1202(m), 1167(m), 1125(m), 1091(m), 1023(sh),
989(sh), 870(m), 796(s), 691(s), 531(m).
[Li(THF)4][Fe3Cl5(Calix(OMe)2)(THF)] 1: Li2L (synthesized from
1.62 g, 2.5 mmol of p-tert-butylcalix[4]arene) in THF solution was
added to FeCl2 (2 g, 16 mmol). The mixture was heated to reflux
under magnetic stirring during 30 min. The solution was filtrated,
concentrated and 20 mL of heptane were added, the mixture was
left at room temperature. Brown single-crystals of 2 suitable for
X-ray analysis grow after some weeks with a yield of ca. 80%. IR:
2952 cmÀ1(m), 2918(s), 2865(sh), 1609(sh), 1458(m), 1392(s),
1360(sh), 1297(s), 1255(s), 1199(m), 1093(sh), 1028(sh), 909(s),
870(m), 849(m), 822(s),797(s), 747(s).
[CrCl2(Calix(OMe)2)Li(THF)], 4THF 2: Li2L (synthesized from
1.62 g, 2.5 mmol of p-tert-butylcalix[4]arene) was dissolved in
THF and was added to CrCl2 (2 g, 16.4 mmol). The mixture was
heated to reflux under magnetic stirring during 30 min. The solu-
tion was filtrated, concentrated and 20 mL of heptane were added.
The mixture was left at room temperature. Green single-crystals of
2 suitable for X-ray analysis grow after some weeks with a yield of
30%. IR 2952 cmÀ1(m), 2925(s), 2856(sh), 1595(sh), 1476(m),
1459(sh), 1361(sh), 1342(sh), 1316(m), 1028(w), 1162(sh),
1093(sh), 1040(m), 1017(m), 917(sh), 860(m), 805(sh), 773(sh),
762(s), 713(sh), 688(m).
3. Conclusions
In conclusion, the difunctionalization of the calix[4]arene at the
lower rim hydroxy groups has provided the formation of mono-
meric species as expected. The formation of complexes with the
lithiated calixarene has been realized with two different metal ha-
lides: FeCl2 and CrCl2. In the case of FeCl2, by adding excess of me-
tal salt, we have obtained a trinuclear complex in which the iron
ions have two different coordination geometries, tetrahedral and
square pyramidal. All three iron ions possess different coordination
spheres. In our hands, this compound did now show any catalytic
effect which can be explained by the good donor properties of the
central chloride ion. In the case of CrCl2, we have obtained a mono-
nuclear chromium(III) complex, obtained after oxidation of the
metal ion. The presence of an endo-bound lithium ion in the cavity
of the calixarene leads to a flat cone conformation of the ligand in
compound 2.
4. Experimental
4.2. X-ray analysis
All experiments were carried out under an inert argon atmo-
sphere, using Schlenk technique [76]. THF was dried in drying unit
under argon and stocked on molecular sieve; other solvents were
bought dried and stocked on molecular sieve. The BVS calculations
are performed with VALIST [77]. For single-crystal measurements, a
Stoe IPDS II theta, equipped with monochromated Mo Ka radiation
(0.71073 Å). IR and NMR spectra were recorded on a Bruker Tensor
27 and on a Bruker AM360, respectively.
Single crystal X-ray structure determination: crystals were
mounted on loops and all geometric and intensity data were taken
from this crystal. Data collection took place using Mo Ka radiation
(k = 0.71073 Å). Measurements were performed at 200 K on a STOE
IPDS-II theta diffractometer equipped with an Oxford Cryosystem
open flow cryostat [78]. Absorption correction was partially inte-
grated in the data reduction procedure [79]. The structures were
solved and refined using full-matrix least-squares on F2 with the
SHELX-97 package [80]. All heavy atoms could be refined anisotrop-
ically. Hydrogen atoms were introduced as fixed contributors
when a residual electronic density was observed near their ex-
pected positions. In compound 2, four tetrahydrofuran solvates
4.1. [Li2(calix(OMe)2)] Li2L: First part: synthesis of dimethyl-p-tert-
butylcalix[4]arene [47]
molecule were accounted for by using the program PLATON/SQUEEZE
[55]. Crystallographic table are available in the Supplementary
material (Table S1).
4.1.1. First part: methylation H2L
p-tert-Butylcalix[4]arene (1.62 g, 2.5 mmol) was dissolved in
30 mL chloroform. KOH (20 mmol, 1.12 g) was dissolved in 20 mL
water and added to the calixarene solution. MeI (4.25 g, 30 mmol)
and PEG 400 (5 g) were added and left under magnetic stirring dur-
ing 20 h. Afterwards the solution was neutralized with HCl and ex-
tracted with chloroform. Most of the solvent was removed by
vacuum and methanol was added for crystallization. After 12 h,
white crystals were obtained with a yield of 90%. 1H NMR (CDCl3,
360 MHz): 7.16 (s, 2H, OH), 7.02 (s, 4H), 6.72 (s, 4H, Ar–H), 4.27
(d, 4H, ArCH2Ar), 1.30 (s, 18H, C(CH3)3), 3.90 (s, 6H, CH3). IR:
3444 cmÀ1(w), 2952(m), 2906(sh), 2869(sh), 1601(sh), 1479(m),
1482(s), 1435(sh), 1360(sh), 1326(s), 1303(sh), 1202(m),
1167(m), 1125(m), 1091(m), 1023(sh), 989(sh), 870(m), 796(s),
691(s), 531(m).
Acknowledgments
The authors thank the Swiss National Science Foundation as well
as the Fribourg Center for Nanomaterials (FriMat) for most gener-
ous support.
Appendix A. Supplementary data
CCDC 884409 and 884410 contain the supplementary crystallo-
graphic data for 1 and 2, respectively. These data can be obtained
free of charge via http://www.ccdc.cam.ac.uk/conts/retriev-
ing.html, or from the Cambridge Crystallographic Data Centre, 12
Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033;
or e-mail: deposit@ccdc.cam.ac.uk. Supplementary data associated
with this article can be found, in the online version, at http://
dx.doi.org/10.1016/j.poly.2012.08.004.
4.1.2. Second part: deprotonation
Dimethyl-p-tert-butylcalix[4]arene (H2L) (1.52 g, 2.25 mmol)
was dried under vacuum for 2 h under vacuum at 150 °C, dissolved
in 30 mL THF, and heated to reflux under magnetic stirring during
1 h. 4.5 mL (2 equiv) of a solution of lithium tert-butyloxide 1.0 M
in THF was added slowly to the suspension and the mixture be-
came clear. The solution was filtered and evaporated to dryness
to give of yield of 95%. 1H NMR (THF-d8, 360 MHz): 6.93 (b, 4H,
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