Synthesis and structural analysis of calix[4]arene-supported iron(III) complexes

Article abstract of DOI:10.1016/j.ica.2003.12.005

The paper describes the reactivity of calix[4]arene dialkyl- or -silylethers H₂R₂calix, R=Me (1), Bz (2), or SiMe ₃ (3) (p-tert.butyl-calix[4]arene=H₄calix), towards the iron(III) complex [FeCl(NSiMe₃)₂(thf)] 4. Bis(silylation) of H₄calix was achieved using a mixture of NEt₃ and Me₃SiCl as silylating agent, which is probably the most convenient and cheapest way for the preparation of H₂(Me₃Si) ₂calix 3. [FeCl(N{SiMe₃}₂)₂(thf)] 4 has been obtained from the reaction of [FeCl₃] and commercially available K[N(SiMe₃)₂] in THF. The reactions of 4 with H₂Me₂calix and H₂Bz₂calix afford mononuclear iron(III) chloro compounds [FeCl(R₂calix)] 5 (R=Me) and 6 (R=Bz). The usage of calix[4]arene silyl ether 3 leads to a dinuclear complex [Fe₂({Me₃Si}calix)₂] 7, presumably under Me₃SiCl cleavage of a mononuclear calixarene iron(III) chloro complex. The calix[4]arene ether stabilized iron(III) chloro complexes are susceptible to nucleophilic substitution reactions, as exemplified by the reaction of 5 with sodium azide yielding an azido complex [Fe(N ₃)(Me₂calix)] 8. The molecular structures of 4, 5, 6, 7, and 8 in the solid state have been determined by X-ray diffraction.

Full text of DOI:10.1016/j.ica.2003.12.005

Inorganica Chimica Acta 357 (2004) 1813–1821
www.elsevier.com/locate/ica
Synthesis and structural analysis of calix[4]arene-supported
iron(III) complexes
*
€
Jurgen Zeller, Sven Koenig, Udo Radius
Institut f€ur Anorganische Chemie, Universit€at Karlsruhe (TH), Engesserstr., Geb. 30.45, D-76128 Karlsruhe, Germany
Received 6 October 2003; accepted 6 December 2003
Dedicated to Professor H. Werner on the occasion of his 70th birthday
Abstract
The paper describes the reactivity of calix[4]arene dialkyl- or -silylethers H2R2calix, R ¼ Me (1), Bz (2), or SiMe3 (3) (p-tert.butyl-
calix[4]arene ¼ H4calix), towards the iron(III) complex [FeCl(NSiMe3)2(thf)] 4. Bis(silylation) of H4calix was achieved using a
mixture of NEt3 and Me3SiCl as silylating agent, which is probably the most convenient and cheapest way for the preparation of
H2(Me3Si)2calix 3. [FeCl(N{SiMe3}2)2(thf)] 4 has been obtained from the reaction of [FeCl3] and commercially available
K[N(SiMe3)2] in THF. The reactions of 4 with H2Me2calix and H2Bz2calix afford mononuclear iron(III) chloro compounds
[FeCl(R2calix)] 5 (R ¼ Me) and 6 (R ¼ Bz). The usage of calix[4]arene silyl ether 3 leads to a dinuclear complex [Fe2({Me3Si}calix)2]
7, presumably under Me3SiCl cleavage of a mononuclear calixarene iron(III) chloro complex. The calix[4]arene ether stabilized
iron(III) chloro complexes are susceptible to nucleophilic substitution reactions, as exemplified by the reaction of 5 with sodium
azide yielding an azido complex [Fe(N3)(Me2calix)] 8. The molecular structures of 4, 5, 6, 7, and 8 in the solid state have been
determined by X-ray diffraction.
Ó 2003 Elsevier B.V. All rights reserved.
Keywords: Crystal structures; Iron complexes; Calix[4]arene complexes
1. Introduction
as models for polyoxo surfaces. In such a role, ca-
lix[4]arenes have some unique geometrical peculiarities.
The fully deprotonated form of the parent calix[4]arenes
act as tetraanionic ligands and almost exclusively as-
sume the cone conformation in metalla-calix[4]arenes,
which keeps the set of the oxygen donor atoms quasi-
planar. The charge of the O4 set can be tuned by
etherification or esterification of the lower rim of the
calix[4]arene. Oxygen alkylation or silylation, for
example, enables the control of the functionalization
degree of the metal and also of the steric protection.
The coordination form of calix[4]arene alkyl ethers
[R2calix]2ꢀ (elliptically distorted cone conformation vs.
paco conformation) as well as the symmetry of the cone
conformation is determined by the coordination number
of the metal and the nature of the co-ligands [5]. The
[R2calix]2ꢀ ligands thus provide robust and well-defined
O4-coordination environments, similar to other widely
successful used supporting dianionic ligands such as O2,
N2-donor ligands of salen type Schiff bases or N4-donor
Calix[n]arenes are macrocyclic molecules made of n-
phenol units connected by ortho methylene groups [1,2].
Calix[4]arenes are the simplest and most common
compounds of this family with four phenolic residues in
the macrocyclic ring. These molecules and their deriva-
tives have been extensively studied for their interesting
properties, e.g. being hosts to cations, anions and neu-
tral molecules, and for the formation of supramolecular
assemblies [3].
Because of the four phenoxyl groups in the ca-
lix[4]arenes (p-tert.butyl-calix[4]arene ¼ H4calix), reac-
tions with transition metal compounds can produce
metal phenolate complexes with substitution of one to
four hydrogen atoms [4]. Thus, these ligands may serve
* Corresponding author. Tel.: +49-721-608-8097; fax: +49-721-608-
7021.
E-mail address: radius@aoc1.uni-karlsruhe.de (U. Radius).
0020-1693/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2003.12.005

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J. Zeller et al. / Inorganica Chimica Acta 357 (2004) 1813–1821
ligands like porphyrines and dibenzotetraaza[14]annu-
lenes [6–8]. The [Me2calix] homologue (where H2-
Me2calix ¼ 1,3-dimethyl ether p-tert-butylcalix[4]arene)
in particular has received much attention in organo-
transition metal chemistry over the last 10 years, and a
rich chemistry has emerged comprising many mononu-
clear complexes, mainly for group 4 and 5 transition
metals. Here we report the preparation and structural
characterization of R2calix iron(III) chloro complexes as
a starting point into the chemistry of calix[4]arene ether
supported Fe(III) compounds, furthermore a convenient
and straight forward synthesis of the silylether
H2(Me3Si)2calix.
calix[4]arene toluene adduct H4calix ꢁ C7H8 in 250 ml
toluene under precipitation of an off-white solid. After
stirring at 80 °C for 24 h and cooling to room temper-
ature a white precipitate was filtered off and the residue
was washed with two 15 ml portions of toluene. The
mother liquor was evaporated to dryness and the re-
sulting residue was redissolved in 60 ml toluene. Addi-
tion of 200 ml methanol gives a white precipitate, which
was filtered off, washed with three portions of methanol
and dried in vacuo. A second fraction was obtained at
)30 °C after concentrating the mother liquor to 100 ml.
Yield: 18.3
g
(89%), colorless crystal powder.
C50H72O4Si2 [793.3]: Calc.: (found) C, 75.70 (75.57); H,
9.15 (9.24). EI/MS m=z (%): 792.8 (100) [M–H]þ. 1H
NMR (CDCl3): d ¼ 0:40 (s, 18H, Si(CH3)3), 0.98 (s,
18H, C(CH3)3), 1.30 (s, 18H, C(CH3)3), 3.26 (d, 4H,
2. Experimental
2
2JHH ¼ 13.0 Hz, CH2), 4.28 (d, 4H, JHH ¼ 13.0 Hz,
2.1. General methods and instrumentation
CH2), 6.78 (s, 4H, Aryl-Hm), 7.05 (s, 4H, Aryl-Hm), 7.31
(s, 2H, OH), 13C{1H} NMR (CDCl3): d ¼ 0:54
(Si(CH3)3), 31.04, 31.70 (C(CH3)3), 32.53 (CH2), 33.72,
33.76 (C(CH3)3), 124.82, 125.19 (Aryl-Cm), 128.13,
130.20 (Aryl-Co), 141.24, 144.34 (Aryl-Cp), 145.62,
150.89 (Aryl-Ci). IR (KBr [cmꢀ1]): 3427 vs m(O–H).
All air/moisture sensitive manipulations were per-
formed using standard Schlenk-line (N2) and dry-box
(Ar) techniques. Solvents were pre-dried and refluxed
over sodium, potassium or sodium–potassium alloy (1:3
w/w) (benzene, toluene, pentane, hexane, diethyl ether,
thf), and P2O5 (acetonitrile, dichloromethane) under N2.
Solvents were distilled at atmospheric pressure prior use.
Deuterated solvents were dried over sodium (C6D6) or
calcium hydride (CDCl3, CD2Cl2) under N2.
1H and 13C-{1H} NMR spectra were recorded on a
Bruker AC 250 spectrometer at room temperature.
Spectra are referenced internally to residual protio-sol-
vent resonances (1H; CDCl3: 7.24 ppm) or solvent res-
onances (13C; CDCl3: 77.0 ppm) and are reported
relative to tetramethylsilane (d ¼ 0:0 ppm). Chemical
shifts are quoted in d (ppm) and coupling constants in
Hertz. Standard DEPT-135 experiments were recorded
to distinguish –CH3 and –CH type carbons from –C or –
CH2 type carbons in the 13C NMR spectrum. IR spectra
were recorded on a Bruker IFS28 spectrometers as KBr
pellets. All data are quoted in wavenumbers (cmꢀ1).
Mass spectra were recorded on a Finnigan MAT 900
XLT. All data are quoted as their mass/charge (m=z)
ratios. Elemental analyses were carried out by the ana-
lytical laboratory of the Institute of Inorganic Chemis-
try of the University Karlsruhe (TH).
2.2.2. [FeCl(N{SiMe3}2)2(THF)] (4)
A suspension of 26.9 g (0.135 mol) K[N(SiMe3)2] in
200 ml THF, cooled to )5 °C, was slowly added to a
cooled ()5 °C) suspension of 16.2 g (0.10 mol) FeCl3 in
200 ml THF to give a red reaction mixture. This mixture
was stirred at 0 °C for 2 h and then allowed to warm up
to room temperature. After stirring for another 8 h it
was filtered through a pad of Celite. The residue was
washed with two portions of 100 ml THF and the filtrate
was evaporated to dryness in vacuo. The resulting dark
red, slightly oily residue was extracted into 750 ml
hexane (three portions of 250 ml) under gentle warming
and filtrated over a pad of Celite. The combined filtrates
were concentrated to approximately 400 ml and stored
at )40 °C over night for crystallization. The resulting
dark red crystals were isolated and dried in vacuo. Re-
ducing the mother liquor to 100 ml and cooling to )40
°C affords a second crop of [FeCl(N{SiMe3}2)(THF)].
Yield: 31.5 g (66%), red crystals. Single crystals suitable
for X-ray diffraction have been obtained from cooling
a saturated hexane solution to )40 °C. C16H44ClFe-
N2OSi4 [484.2]: Calc.: C, 39.69; H, 9.16; N, 5.79. Found:
C, 40.09; H, 9.80; N, 5.52%. EI/MS m=z (%): 485.0 (5)
[M]þ, 411.1 (100) [M–THF]þ, 376.0 (22.6) [M –THF–
Cl]þ. 1H NMR (CDCl3): d ¼ 15:42 (s, br, 18H,
Si(CH3)2), 41.50 (s, br, 8H, CH2).
Literature preparations: H4calix [9] H2Me2calix [10],
and H2Bz2calix [10] were prepared according to
established methods, all other reagents were purchased
from commercial sources and purified by standard
techniques.
2.2. Syntheses
2.2.3. [FeCl(Me2calix)] (5)
A solution of 4.64 g (6.85 mmol) H2Me2calix (1) in 30
ml toluene was added at room temperature to a solution
of 3.32 g (6.85 mmol) [FeCl(N{SiMe3}2)2(THF)] (4) in
30 ml toluene. The reaction mixture was stirred for 2 h
2.2.1. H2(SiMe3)2calix (3)
15.0 ml (108 mmol) NEt3 and 13.6 ml (108 mmol)
Me3SiCl were added to a solution of 20.0 g (27.0 mmol)

J. Zeller et al. / Inorganica Chimica Acta 357 (2004) 1813–1821
1815
at 90 °C. During this time the color changed from dark
red to deep purple. Volatile material was removed in
vacuo and the residue was suspended in approximately
20 ml hexane. The product was filtered off and dried in
vacuo. Crystal suitable for X-ray analysis have been
obtained from saturated hexane solutions at )30 °C.
Yield: 3.83 g (73%), purple powder. C46H58ClFeO4
[766.3]: Calc.: C, 72.10; H, 7.60. Found: C, 72.20; H,
7.53%. EI/MS m=z (%): 765.2 (100) [M]þ, 750.2 (32.3)
[M–CH3]þ, 735.2 (15%) [M–2CH3]þ. IR: (KBr [cmꢀ1]):
m ¼ 2954 vs, 2868 s, 1602 w, 1478 vs br, 1415 m, 1361 s,
1311 vs, 1210 vs, 1165 m, 1120 m, 1092 s, 996 vs, 938 m,
874 vs, 862 s, 852 m, 797 vs, 730 s, 562 s, 358 s.
NaN3. The initially purple suspension was stirred for 3 h
at room temperature to turn red. All volatiles of the
resulting suspension were removed in vacuo and the
residue was extracted into 30 ml hexane. After filtration
through a pad of Celite, the solution was concentrated
to 10 ml and the product was crystallized at )40 °C, the
mother liquor was removed by a cannula yielding 230
mg of red needles of (8). EI-MS data reveal a red
powder obtained from the evaporated mother liquor
mainly consists of (8), although it is not analytically
pure. Crystals suitable for X-ray diffraction have been
obtained from saturated toluene solutions at )30 °C.
Yield: 230 mg (30%), red needles. C46H58FeN3O4
[772.8]: Calc.: C, 71.49; H, 7.56; N, 5.44. Found: C,
71.45; H, 7.85; N, 5.24%. EI/MS (m=z (%)): 772 (100)
[M]þ, 757 (6), [M–CH3], 730 (74) [M–N3]þ. IR (KBr
[cmꢀ1]): m ¼ 3051 m, 2961 vs, 2865 s, 2070 vs mas(N3),
1603 m, 1470 vs br, 1431 s, 1362 s, 1311 vs, 1273 s, 1212
vs, 1168 s, 1122 s, 1059 s, 1001 vs, 875 s, 857 s, 795 vs,
560 s, 345 s, 361 m, 226 vs br.
2.2.4. [FeCl(Bz2calix)] (6)
A mixture of 829 mg (1.00 mmol) H2Bz2calix (2) and
484 mg (1.00 mmol) [FeCl(N{SiMe3}2)2(THF)] (4) was
dissolved in 30 ml toluene and stirred for 4 h at 100 °C.
During this time the color of the reaction mixture
changed from dark red to deep purple. After filtration
through a pad of Celite, all volatile material was re-
moved in vacuo and the resulting solid was suspended in
10 ml pentane. The product was filtered off and dried in
vacuo. Crystals suitable for X-ray analysis have been
obtained from saturated diethyl ether solutions at )40
°C. Yield: 440 mg (48%) deep-purple powder.
C58H66ClFeO4 [918.5]: Calc.: C, 75.85; H, 7.24. Found:
C, 75.58; H, 7.32. EI/MS (m=z (%)): 918 (66) [M]þ. IR
(KBr [cmꢀ1]): m ¼ 2962 vs m, 2867 s, 1754 w br, 1601 m,
1496 vs, 1413 s, 1392 s, 1361 m, 1277 s, 1264 s, 1183 vs,
1168 s, 1118 vs, 1094 s, 1083 s, 1029 vs, 984 vs, 958 s, 927
m, 862 s, 808 vs, 758 s, 670 vs, 640 vs, 610 vs, 577 s, 562
m, 416 m, 355 m.
2.3. Crystal structure determination of [FeCl(NSi-
Me3)2(thf)] (4), [FeCl(Me2calix)] (5), [FeCl(Bz2ca-
lix)] ꢁ (C4H10O) (6 ꢁ (C4H10O)), [Fe2({Me3Si}calix)2]
ꢁ 2(C7H8) ꢁ (C6H14) (7ꢁ 2(C7H8) ꢁ (C6H14)), and [FeN3
(Me2calix)] ꢁ 2(C7H8) (8 ꢁ 2(C7H8))
Crystal data collection and processing parameters are
given in Table 1. Crystals were immersed in a film of
perfluoropolyether oil on a glass fibre and transferred to
a Stoe-IPDS image plate diffractometer (Ag Ka radia-
tion) equipped with a FTS AirJet low temperature de-
vice. Data were collected at 203 K. The images were
processed with the STOE IPDS software package and
equivalent reflections were merged. Corrections for
Lorentz-polarisation effects and absorption were per-
formed if necessary and the structures were solved by
direct methods. Subsequent difference Fourier syntheses
revealed the positions of all other non-hydrogen atoms,
and hydrogen atoms were included in calculated posi-
tions. Extinction corrections were applied as required.
Crystallographic calculations were performed using
SHELXS-97 and SHELXL-97 [11].
For compound 4 one of the methylene carbon atoms
of the THF ligand [C(15)] and for compound 5 the
methyl carbon atoms of one of the calix[4]arene tert-
butyl groups [C(17) to C(19)] were disordered over two
sites. These carbon atoms were refined in an isotropic
approximation with occupancy factors of 70% and 30%.
The calix[4]arene tert-butyl groups containing C(37)
to C(39) and C(47) to C(49) were positionally disordered
for compound 6ꢁ(C4H10O). These carbon atoms were
refined in an isotropic approximation with occupancy
factors of 60% and 40%. For compound 7ꢁ2(C7H8)ꢁ
(C6H14), the carbon atoms of one of the calix[4]arene
tert-butyl groups, C(37) and C(38), for 8ꢁ2(C7H8) C(39)
2.2.5. [Fe2({Me3Si}calix)2] (7)
A mixture of 793 mg (1.00 mmol) H2(Me3Si)2calix (3)
and 484 mg (1.00 mmol) [FeCl(N{SiMe3}2)2(THF)] (4)
was dissolved in 30 ml toluene and stirred at 100 °C for 3
h. During this time the color of the reaction mixture
changed from dark red to violet. After filtration through
a pad of Celite, all volatile material was removed in
vacuo and the resulting solid was suspended in 20 ml
hexane. The product was filtered off and dried in vacuo.
Crystals suitable for X-ray analysis have been obtained
from saturated solutions in a 1:1 mixture of toluene and
hexane at )40 °C. Yield: 580 mg (75%), violet powder.
C94H122Fe2O8Si2 [1547.9]: Calc.: C, 72.94; H 7.94.
Found: C, 72.47; H, 8.02%. EI/MS (m=z (%)): 1547.2
(36) [M]þ. IR (KBr [cmꢀ1]): m ¼ 2965 vs, 2924 s, 2865 m,
1604 w, 1587 w, 1468 vs br, 1361 m, 1319 s, 1278 s, 1252
vs, 1185 s, 1171 m, 1121 w, 1094 m, 940 m, 923 s, 875 vs,
851 vs br, 795 s, 779 m, 564 w.
2.2.6. [Fe(N3)(Me2calix)] (8)
20 ml THF were added to a mixture of 600 mg (0.78
mmol) [FeCl(Me2calix)] (5) and 51 mg (0.78 mmol)

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J. Zeller et al. / Inorganica Chimica Acta 357 (2004) 1813–1821
Table 1
X-ray data collection and processing parameters
4
5
6ꢁ(C4H10O)
C62H76ClFeO5
7ꢁ2(C7H8)ꢁ(C6H14
C57H76FeO4Si
)
8ꢁ2(C7H8)
C60H74FeN3O4
Formula
C
484.19
16H44ClFeN2OSi4
C46H58ClFeO4
770.25
Formula weight
Crystal system
Space group
992.53
triclinic
905.09
monoclinic
P21=n
957.07
triclinic
triclinic
ꢀ
monoclinic
P21=c
ꢀ
ꢀ
P1
P1
P1
ꢁ
a (A)
8.4225(9)
11.4664(13)
15.7858(17)
105.596(12)
93.126(13)
109.477(13)
1366.8(3)
2
15.7122(11)
14.0537(6)
19.7315(13)
12.5396(8)
13.0637(8)
19.4183(12)
90.866(8)
94.872(8)
94.450(8)
3159.2(3)
2
13.083(7)
19.7490(16)
20.3516(11)
13.0119(8)
13.0544(7)
18.2136(11)
86.158(7)
77.321(7)
63.210(7)
2692(3)
2
ꢁ
b (A)
ꢁ
c (A)
a (°)
b (°)
c (°)
92.862(8)
100.036(7)
3
ꢁ
V (A )
4351.6(5)
4
5178.8(6)
4
Z
l (mmꢀ1
Independent
reflections
Observed reflections
)
0.432
5031
0.238
7472
0.173
8782
0.193
7473
0.178
8854
4045 [I > 2rðIÞ]
237
0.0343 and 0.0860
5299 [I > 2rðIÞ]
483
0.0504 and 0.1234
5234 [I > 2rðIÞ]
611
0.0926 and 0.2608
4751 [I > 2rðIÞ]
5555 [I > 2rðIÞ]
594
0.0750 and 0.1884
Parameters
Final Ra, and wR2
523
0.0665 and 0.1744
b
P
P
a R ¼ jjFoj ꢀ jFcjj= jFoj.
n
o
1=2
P
P
2
2
b wR2 ¼
½wðFo2 ꢀ Fc2Þ ꢂ= ½wðFo2Þ ꢂ
; for data with I > 2rðIÞ.
and (C49) were positionally disordered and these carbon
atoms were refined in an isotropic approximation using
equal occupancy.
contrast to the excellent reactivity of calix[6]arene and
calix[8]arene, which are easily converted to the corre-
sponding hexakis- and octakis(trimethylsilyl) deriva-
tives, Gutsche et al. [13] reported the recovery of starting
material from the reaction of H4calix with excess of
Me3SiCl and HN(SiMe3)2 in refluxing pyridine. This
reaction behavior of the cyclic tetramer was assigned to
a greater crowding of the hydroxyl groups. However,
when N; O-bis(trimethylsilyl) acetamide was used as si-
lylation reagent, even a tetrakis(trimethylsilyl) derivative
could be detected after prolonged treatment of ca-
lix[4]arene in refluxing acetonitrile. Schmutzler and co-
workers [14] published the synthesis of 3 using
HN(SiMe3)2 in the presence of catalytic amounts of
Me3SiCl in toluene as silylating reagent as well as from
the reaction of H4calix with Et2NSiMe3 in toluene at
50 °C. Contrarily, silylation of H4calix using Me3SiCl
and HN(SiMe3)2 was not successful as reported by
Parkin and co-workers [15,16], but 3 was obtained from
the reaction of H4calix with two equivalents Me3SiI in
the presence of two equivalents pyridine. Floriani and
co-workers [12b] described the synthesis of 3 using N;
O-bis(trimethylsilyl) acetamide as silylation reagent in
detail and most recently, Anwander et al. reported the
synthesis of 3 starting from H4calix and an excess of
HN(SiMe3)2 in refluxing mesitylene. Treatment of the
calix[4]arene in hexane or toluene with an excess of
HN(SiMe3)2 under reflux conditions did not form any
silylation product [17].
3. Results and discussion
Calix[4]aren-ether stabilized iron(II) complexes have
been described recently by Floriani and coworkers [12].
The starting compounds of their investigation,
[Fe(Me2calix)(thf)], [Fe(nBu2calix)(thf)], [Fe(Bz2ca-
lix)(thf)], and [Fe({Me3Si}2calix)], are accessible via the
reaction of the calixarene ether H2R2calix with
[Fe2Mes4]. The use of iron(II) halides as starting mate-
rial led to compounds containing Fe–X bonds that were
very difficult to free from the alkaline metal salts. In the
course of this study, the Fe(III) halide complexes
[FeCl(Me2calix)] and [FeI(Me2calix)] have been synthe-
sized, using [HgCl2], PhCH2Cl, or I2 as oxidizing agents.
In the case of the silyl and benzyl ether complexes, ox-
idation led to calix[4]arene bridged dinuclear iron(III)
complexes [Fe2(Rcalix)2], compounds stabilized with
calix[4]arene monoalkyl ether as ligands. Since ca-
lix[4]arene dialkyl and –silyl ether stabilized iron(III)
halide derivatives are particularly appropriate for fur-
ther functionalization, we sought for a more simple,
direct and more general route to obtain complexes of the
type [Fe(R2calix)Cl] using the methyl (1, R ¼ Me) benzyl
(2, R ¼ Bz) and silyl (3, R ¼ SiMe3) calix[4]arene ether
H2R2calix.
We have found that bis(silylation) of H4calix can
easily be achieved using a mixture of NEt3 and Me3SiCl
as silylating agent. 3 can be prepared in good to excel-
lent yield from the prolonged reaction of a slurry of
The feasibility of calix[4]arene silylation using stan-
dard silylation procedures was controversially discussed
in the literature over the last few years [12b,13–17]. In

J. Zeller et al. / Inorganica Chimica Acta 357 (2004) 1813–1821
1817
calix[4]arene toluene adduct H4calix ꢁ C7H8 in toluene
with an excess of NEt3 and Me3SiCl at 80 °C. This is
probably the most convenient and the cheapest way for
the preparation of H2(Me3Si)2calix 3. Proton and car-
bon NMR spectra of the compound are in accordance
with a pseudo C2v symmetrical species in solution, and
the integration of the resonances of the SiMe3 protons
and the calixarene tBu protons clearly establishes the
twofold substitution of the calixarene. Furthermore, the
sharp doublets for the protons of the methylene bridges
at d ¼ 3:26 ppm and d ¼ 4:28 ppm indicate that 3
adopts a cone conformation in solution. In the infrared
spectrum, the two unreacted phenolic OH groups show
a stretching vibration at 3427 cmꢀ1, which are signifi-
cantly shifted compared to the OH stretch of the parent
Si(3)
N(2)
Si(1)
Si(2)
Si(4)
Fe
N(1)
Cl
O
Fig. 1. Schakal plot of [FeCl(NSiMe3)2(thf)] (4). H atoms and solvent
ꢁ
molecules have been omitted for clarity. Selected bond lengths (A) and
angles (°): Fe–N(1): 1.906(2), Fe–N(2): 1.895(2); Fe–Cl: 2.242(1); Fe–
O: 2.044(2); N(1)–Fe–N(2): 123.76(8); N(1)–Fe–O: 99.35(8); N(2)–Fe–
O: 112.25(8); N(1)–Fe–Cl: 114.33(6); N(2)–Fe–Cl: 106.17(6); O–Fe–Cl:
97.84(5). Si(1)–N(1)–Fe: 120.55(10); Si(2)–N(1)–Fe: 117.18(10); Si(1)–
N(1)–Si(2): 122.28(11); Si(3)–N(2)–Fe: 115.24(10); Si(3)–N(2)–Si(4):
120.54(12); Si(4)–N(2)–Fe: 121.92(11).
calix[4]arene at 3140 cmꢀ1
.
Principally, there are several ways to introduce
R2calix ligands into iron(III) complexes. Since we had
problems to obtain a reaction product free of alkaline
metal chloride from reactions of [FeCl3] with the alka-
line metal salts of the calix[4]arene ether, we decided to
follow a similar way as described by Floriani et al. for
the preparation of the iron(II) complexes, i.e. the me-
tathesis of more basic alkyl, aryl or amide ligands with
the calix[4]arene ether phenolic residues. Using a similar
approach, we already introduced calix[4]arenes in group
6 metal complexes [18]. Suitable Fe(III) starting com-
pounds are complexes of the type [FeR2ClLn], which
contain one chloro ligand and two monofunctional li-
gands R (alkyl, aryl or amide) which are suitable to
undergo metathesis reaction with H2R2calix. However,
a convenient route to a compound of this type was not
available, but one of the most common Fe(III) starting
materials is [Fe(N{SiMe3}2)3] [19], so we reached out
for the synthesis of a compound of the type [FeCl-
(N{SiMe3}2)2]. A complex [FeCl{N(SiMe3)2}2(thf)] 4,
stabilized with one additional molecule of THF ligated
to the Fe(III) center, has been obtained in approxi-
mately 70% yield from the reaction of [FeCl3]
and commercially available K[N(SiMe3)2] in THF. A
similar procedure was published only recently by Lee
and co-workers [20]. The proton NMR spectrum of 4
shows broad, paramagnetically shifted signals at
d ¼ 15:4 ppm and d ¼ 41:5 ppm, which are arbitrarily
assigned to the protons of the SiMe3 group and to the
protons of the THF ligand. The EI mass spectrum of 4
reveals a peak of the molecular ion at m=z ¼ 485 (5%)
and a peak at 411 [M–THF]þ as the basis peak of the
spectrum.
ꢁ
ꢁ
1.895(2) A and the Fe–Cl distance of 2.242(1) A are
within standard range for iron(III) terminal amide or
chloride complexes [19b,19d,21]. The steric demand of
the amide ligands expands the angle N(1)–Fe–N(2) to
123.76(8)° and compresses the angle Cl–Fe–O to
97.84(5)° relative to an ideal tetrahedron. The nitrogen
atoms of 4 are almost planar, which can be attributed
to a development of steric strain in the molecule during
pyramidalization of the amide nitrogen atoms. The sum
of the angles is 360.01° at N(1) and 357.7° at N(2).
The reactivity of 4 with H2R2calix is summarized in
Scheme 1. Gentle heating of the calixarene ether and 4
in toluene leads to deeply colored solutions, from which
the complexes 5–7 can be isolated in reasonable yields.
The infrared spectra of each product lack the stretching
vibration of phenolic OH groups, a clear indicative for
the complexation of the calix[4]aren. Furthermore, the
data of the elemental analyses as well as mass spec-
troscopy reveal, that the reaction of 4 with H2Me2calix
and H2Bz2calix afford mononuclear iron(III) chloro
compounds [FeCl(R2calix)] 5 (R ¼ Me) and 6 (R ¼ Bz).
The analogous reaction of 4 using the calix[4]arene
silyl ether 3 leads, presumably under Me3SiCl cleav-
age of a mononuclear calixarene iron(III) chloro com-
plex, to the dinuclear compound [Fe2({Me3Si}calix)2] 7,
which is stabilized with the calix[4]arene monosilyl
ether.
The molecular structures of the compounds 5, 6, and
7 in the solid state have been determined by X-ray
diffraction. Single crystals of complex 5 and 6 suitable
for X-ray analysis have been obtained from saturated
hexane or ether solutions at )30 °C and )40 °C, re-
spectively, those of complex 7 from saturated solutions
of 7 in a 1:1 mixture of toluene and hexane at )40 °C.
The molecular structures of these complexes are shown
Single crystals of 4 were grown from hexane solutions
of the compound. The X-ray crystal structure of 4
confirms the mononuclear character of the compound in
the solid state (see Fig. 1).
Two amide ligands, a chloride ligand and a THF
molecule bind to the tetrahedrally coordinated Fe(III)
ꢁ
atom of 4. The Fe–N contacts of 1.906(2) A and

1818
J. Zeller et al. / Inorganica Chimica Acta 357 (2004) 1813–1821
Scheme 1.
in Figs. 2–4 (5,6,7). Whereas 5 crystallizes without any
solvent molecule in the unit cell, the benzyl ether
complex 6 crystallizes with an additional ether molecule
that is hosted inside the calixarene cavity and the di-
nuclear compound 7 with two molecules toluene,
located inside the calixarene cavities, and one addi-
tional molecule hexane which occupies a general posi-
tion in the lattice.
Cl
C(6)
Fe
O(1)
O(2)
C(5)
O(3)
O(4)
The molecular structures of 5 and 6 confirm the
mononuclear nature of these compounds. The calixa-
rene macrocycles assume the usual elliptical conforma-
tion observed for calixarene di(alkyl)ether complexes,
the rings of the anisol entities (at O(1) and O(3)) being
slightly pushed inward and the rings of the phenolic
units (at O(2) and O(4)) outward relative to the cavity of
the ligand. The asymmetry of the cavity is monitored by
the separation between the opposite C(25) and C(45)
Fig. 3. Schakal plot of [FeCl(Bz2calix)] (6). H atoms and solvent
ꢁ
molecules have been omitted for clarity. Selected bond lengths (A) and
angles (°): Fe–Cl: 2.222(2); Fe–O(1): 2.210(5); Fe–O(2): 1.814(5); Fe–
O(3): 2.190(4); Fe–O(4): 1.778(5); Cl–Fe–O(1): 94.00(13); Cl–Fe–O(2):
114.79(16); Cl–Fe–O(3): 96.03(13); Cl–Fe–O(4): 117.7(2); O(1)–Fe–
O(2): 88.7(2); O(1)–Fe–O(3): 169.48(16); O(1)–Fe–O(4): 85.4(2); O(2)–
Fe–O(3): 90.0(2); O(2)–Fe–O(4): 127.5(3); O(3)–Fe–O(4): 87.1(2);
C(10)–O(1)–Fe: 116.9(4); C(20)–O(2)–Fe: 144.6(4); C(30)–O(3)–Fe:
115.0(4); C(40)–O(4)–Fe: 176.1(5); C(5)–O(1)–C(10): 113.5(5); C(6)–
O(3)–C(30): 116.0(5).
Cl
C(6)
C(5)
Fe O(1)
O(3)
ꢁ
para carbon atoms of the anisole moiety of 9.222 A (5)
ꢁ
and 9.186 A (6), and C(15) and C(35) para carbon atoms
ꢁ
of the phenolate moiety of the ligand of 6.406 A (5) and
O(2)
O(4)
ꢁ
7.414 A (6). In this conformation, for 6, a diethyl ether
molecule is hosted inside the cavity as a guest, the plane
through the ether molecule (C(70), C(71) and O(5)) bi-
secting the equatorial plane through O(2), O(4), and Cl
at an angle of 12.35°.
In both complexes the coordination polyhedron
around the metal is nearly a trigonal bipyramid, with the
O(2), O(4) and Cl atoms defining the equatorial plane
and the O(1) and O(3) atoms at the axial positions. The
Fig. 2. Schakal plot of [FeCl(Me2calix)] (5). H atoms and solvent
ꢁ
molecules have been omitted for clarity. Selected bond lengths (A) and
angles (°): Fe–O(1): 2.228(2); Fe–O(2): 1.811(2); Fe–O(3): 2.184(2); Fe–
O(4): 1.793(2); Fe–Cl: 2.214(1); C(5)–O(1): 1.429(4); C(6)–O(3):
1.432(4); Cl–Fe–O(1): 96.33(6); Cl–Fe–O(2): 114.39(8); Cl–Fe–O(3):
95.95(6); Cl–Fe– O(4): 111.44(8); O(1)–Fe–O(2): 88.23(8); O(1)–Fe–
O(3): 167.46(8); O(1)–Fe–O(4): 86.97(8); O(2)–Fe–O(3): 89.01(8);
O(2)–Fe–O(4): 134.17(10); O(3)–Fe–O(4): 86.20(8); C(10)–O(1)–Fe:
111.94(15); C(20)–O(2)–Fe: 153.6(2); C(30)–O(3)–Fe: 114.83(15);
C(40)–O(4)–Fe: 165.4(2); C(5)–O(1)–C(10): 116.3(2); C(6)–O(3)–C(30):
114.7(2).
ꢁ
ꢁ
metal is displaced by 0.002 A (5) and 0.028 A (6) from
the equatorial plane. The Fe–Cl vector forms a dihedral
angle of 1.86° (5) and 1.31° (6) with the normal to the O4
mean plane of the calix[4]arene ligand. The short Fe–
O(2) and Fe–O(4) bond distances in both 5 and 6 sup-
port significant metal–oxygen p bonding, which is ab-
sent in the cases of O(1) and O(3).

J. Zeller et al. / Inorganica Chimica Acta 357 (2004) 1813–1821
1819
to nucleophilic substitution reactions, as the reaction
with sodium azide exemplifies. This reaction leads to
substitution of the chloride ligand with an azido group,
as the microanalysis as well as mass spectroscopy re-
veals. The infrared spectrum of this compound displays
an intense vibration at 2070 cmꢀ1 which can be attrib-
uted to mas(N3). The azido complex 8 has been struc-
turally characterized, as shown in Fig. 5. Crystals
suitable for X-ray diffraction have been obtained from
saturated toluene solutions at )30 °C.
Si
O(4’) O(2)
O(3’)
Fe
O(1)
O(1’) Fe’
O(3)
O(2’)
O(4)
Compound 8 crystallizes in the triclinic space group
ꢀ
P1 with two formular units and four toluene molecules
Si’
in the unit cell. Two of the additional solvent molecules
are located inside the calix[4]arene cavity the others on
general positions in the lattice. The calixarene macro-
cycles assume an elliptical distorted cone conformation,
the rings of the anisol entities being slightly pushed in-
ward and the rings of the phenolic units outward relative
to the cavity of the ligand. The asymmetry of the cavity
is monitored by the separation between the opposite
C(25) and C(45) para carbon atoms of the phenolate
Fig. 4. Schakal plot of [Fe2({Me3Si}calix)2] (7). H atoms and solvent
ꢁ
molecules have been omitted for clarity. Selected bond lengths (A) and
angles (°): Fe–O(1): 1.794(3), Fe–O(2): 2.307(3); Fe–O(3): 1.780(3); Fe–
O(4): 2.073(3), Fe0 –O(4): 1.909(3); Fe–O(4)–Fe0 : 100.82(13); Fe–O(2)–
Si: 133.01(16); O(1)–Fe–O(2): 86.01(12); O(1)–Fe–O(3): 137.51(16);
O(1)–Fe–O(4): 93.28(13); O(1)–Fe–O(40 ): 110.67(14); O(2)–Fe–O(3):
83.66(13); O(2)–Fe–O(4): 173.22(11); O(2)–Fe–O(40 ): 107.39(12); O(3)–
Fe–O(4): 92.38(13); O(3)–Fe–O(40 ): 111.76(15); O(4)–Fe–O(40 ):
79.18(13); C(10)–O(1)–Fe: 150.0(3); C(20)–O(2)–Fe: 107.6(2); C(30)–
O(3)–Fe: 163.1(3); C(40)–O(4)–Fe: 113.7(2); C(40)–O(4)–Fe0 : 145.2(3).
ꢁ
anisole entities of 9.254 A, and C(15) and C(35) para
carbon atoms of the anisole groups of the ligand of
ꢁ
7.230 A. The coordination polyhedron at metal atom is
nearly trigonal bipyramidal, with the atoms O(2), O(4)
and N(1) defining the equatorial plane and the atoms
O(1) and O(3) at the axial positions. The metal atom is
ꢁ
displaced by 0.003 A from the equatorial plane. The Fe–
N(1) vector forms a dihedral angle of 1.57° with the
normal to the O4 mean plane. The Fe–N(1) distance of
ꢁ
1.916(4) A is slightly shorter compared to that reported
for the tetraphenyl porphyrine stabilized Fe(III) azide
Complex 7 consists of centrosymmetric dimers in the
solid state (Fig. 4). In this complex, the iron atoms ex-
hibit five-coordination through the oxygen atoms of the
distorted O4 core (O(1) to O(4)) and the O(40) bridging
oxo group. The coordination polyhedron resembles a
trigonal bipyramid, with O(1), O(3), and O(40) defining
the equatorial plane, while O(2) and O(4) are in axial
ꢁ
complexes [Fe(N3)(TPP)] (1.953(5) A) [23a] or the salen
ꢁ
positions. The metal is displaced by 0.026 (2) A from the
N(2)
N(1)
C(5) Fe
O(2)
N(3)
C(6)
equatorial plane. The Fe–O(4) vector forms a dihedral
angle of 10.0° with the normal to the equatorial plane,
the Fe–O(2) vector an angle of 16.79°. The Fe–O bond
distances vary according to the degree of p-bonding
formation between the metal and the different oxygen
donor atoms, the distances to the terminal oxygen atoms
O(3)
O(4)
O(1)
ꢁ
O(1) and O(3) are 1.794(3) and 1.780(3) A, respectively,
the bond length to the ether oxygen atom O(2) is sig-
ꢁ
nificantly elongated to 2.307(3) A, and the Fe–O-dis-
0
ꢁ
tances in the bridge are 1.909(3) A to O(4 ) and 2.073(3)
ꢁ
A to O(4).
Fig. 5. Schakal plot of [Fe(N3)(Me2calix)] (8). H atoms and solvent
ꢁ
molecules have been omitted for clarity. Selected bond lengths (A) and
We are currently exploring the usage of the com-
plexes 5 and 6 for the synthesis of further calixarene
stabilized iron(III) to iron(V) complexes, for example
with respect to the synthesis of organometallic iron(III)
compounds. In contrast to iron(II) and iron(0) orga-
nometallic complexes those of iron(III) are relatively
rare. Many of them are stabilized with ligands related to
calixarene ethers such as porphyrins, salenes or tetra-
azaannulenes [22]. The chloride ligand of 5 is susceptible
angles (°): Fe–O(1): 2.167(3); Fe–O(2): 1.820(3); Fe–O(3): 2.198(3); Fe–
O(4): 1.775(3); Fe–N(1): 1.916(4); N(1)–N(2): 1.185(6); N(2)–N(3):
1.137(8); N(2)–N(1)–Fe: 133.6(4); N(3)–N(2)–N(1): 176.0(8); O(1)–Fe–
O(2): 88.92(13); O(1)–Fe–O(3): 167.89(10); O(1)–Fe–O(4): 87.59(14);
O(2)–Fe–O(3): 87.84(13); O(2)–Fe–O(4): 131.67(17); O(3)–Fe–O(4):
85.86(14); N(1)–Fe–O(1): 94.63(17); N(1)–Fe–O(2): 112.88(18); N(1)–
Fe–O(3): 97.38(17); N(1)–Fe–O(4): 115.4(2); C(5)–O(1)–Fe: 129.7(3);
C(6)–O(3)–Fe: 129.4(3); C(10)–O(1)–Fe: 115.5(2); C(20)–O(2)–Fe:
151.4(3); C(30)–O(3)–Fe: 115.3(2); C(40)–O(4)–Fe: 172.3(4).

1820
J. Zeller et al. / Inorganica Chimica Acta 357 (2004) 1813–1821
ꢁ
stabilized compound [Fe(N3)(py)salen] (2.047(8) A)
[23b], but rather unexceptional as compared to other
terminal azide complexes of iron(III) [24]. The linear
azide ligand, the angle N(1)–N(2)–N(3) is 176.0(8)°, is
coordinated with an angle Fe–N(1)–N(2) of 133.6(4)° to
the iron atom, which indicates sp2 hybridization on the
azide N(1) nitrogen atom. The Nꢀ3 ligand is tilted into
the plane of one of the phenolate units of the calixarene
ligand (towards O(4)). The plane defined through the
atoms Fe, N(1), and N(2) almost eclipses with the Fe–
O(4) bond axis as to avoid steric hindrance with the
methyl groups of the calixarene ether. The dihedral
angle N(2)–N(1)–Fe–O(4) is 7.21°. The two N–N dis-
tances N(1)–N(2) of 1.185(6); and N(2)–N(3) of 1.137(8)
are unequal, the longer bond occurs between the central
nitrogen atom and the coordinated nitrogen atom. This
difference is not unusual and reflects one resonance form
of bound azide [25].
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This work was supported by the Deutsche Fors-
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interest in our work. U.R. is grateful to Prof. Dr. H.
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