Polyhedral ferrous and ferric siloxanes

Article abstract of DOI:10.1002/anie.200453740

The synthesis, isolation, and single-crystal X-ray structures of the first polyhedral ferrous and ferric siloxanes (see scheme) suggest that similar synthetic strategies will allow the preparation of other transition-metal siloxane frameworks.

Full text of DOI:10.1002/anie.200453740

Communications
Cage Compounds
quite valuable in understanding the mechanism of many
catalytic processes particularly those involving silica-sup-
ported transition-metal catalysts.[8] A case in point are the
molecular cubic titanosiloxanes [{Ti(OR1)O3SiR}4] [R1 = tBu,
tBuO, tBuCH2; R = (2,6-iPr2C6H3)N(SiMe3)], which have not
only helped in the elucidation of the Si-O-Ti stretching
frequency observed in TS-1 and TS-2 catalysts (TS is titanium
silicate) but have also themselves been quite effective in
catalytic reactions that involve the epoxidation of 1-hexene
and 1-octene.[9,10] In view of this success, it was of interest to
build analogous molecular iron siloxane frameworks. Previ-
ous work on iron siloxanes centered around the use of
R3SiO[11] or silsesquioxane ligands.[12] However, polyhedral
iron siloxanes have remained elusive and the preparation of
such compounds is a synthetic challenge in view of the
instability of iron alkyls. We have overcome this synthetic
hurdle by choosing bis- and trisamido derivatives of iron[13,14]
as the starting precursor and have been able to assemble two
different iron siloxanes that contain iron atoms in the formal
oxidation states of + ii and + iii, respectively. Accordingly,
herein we report the synthesis and single-crystal molecular
structures of ferrous [{(Me3Si)2NFe}2{LFe}2{O3SiR}2] (L = 1,3-
Polyhedral Ferrous and Ferric Siloxanes**
Umesh N. Nehete, Ganapathi Anantharaman,
Vadapalli Chandrasekhar, Ramaswamy Murugavel,
Mrinalini G. Walawalkar, Herbert W. Roesky,*
Denis Vidovic, Jörg Magull, Konrad Samwer, and
Björn Sass
Dedicated to Professor Michael Veith
on the occasion of his 60th birthday
Iron is the fourth most abundant element in the earthꢀs crust
after oxygen, silicon, and aluminum. Although iron atoms are
present in the ore hematite (Fe2O3) in the formal oxidation
state + iii, in magnetite (Fe3O4) they exhibit both the + ii and
+ iii oxidation states. In the naturally occurring olivine,
MgFeSiO4, iron atoms are present in the + ii oxidation state
while in garnets, [M2+3 M3+2(SiO4)3] (M2+ = Co, Mg, Fe; M3+
=
Al, Cr, Fe) they are found in both oxidation states.[1,2] Other
examples of iron-containing silicate frameworks include
zeolites such as iron-modified ZSM-5, which have been
shown to be important as catalysts, as for example in the
synthesis of phenol from benzene and N2O.[3] Moreover the
Lewis acidity of the iron centers in iron-modified ZSM-5
enables its use as an acid catalyst in the benzylation reaction
of benzene by using benzyl chloride[4] and also in the
esterification of tert-butanol by acetic anhydride.[5]
diisopropyl-4,5-dimethylimidazol-2-ylidene)
siloxanes
[{(L1Fe)(O3SiR)}4] [L1 = PMe3;
iPr2C6H3)N(SiMe3)] 2 (Scheme 1).
1
and ferric
R = (2,6-
The addition of two equivalents of Fe[N(SiMe3)2]2·THF[13]
to a suspension of RSi(OH)3 in hexane/toluene mixture at
room temperature in the presence of the N-heterocyclic
carbene
L (L = 1,3-diisopropyl-4,5-dimethylimidazol-2-yli-
dene) affords 1 in moderate yield (Scheme 1). The use of
the auxiliary coordinating ligand L allowed the isolation of
the ferrous siloxane 1 as a crystalline product. Single crystals
of 1, thus obtained, were found to be soluble in many common
organic solvents. The IR spectrum of 1 shows characteristic
absorptions for the {Fe-O-Si} groups in the range of 900 to
1000 cmÀ1. However no peak attributable to the molecular ion
of 1 can be observed in the EI or other mass spectrometric
methods.
We have a long-standing interest in molecular assemblies
that contain the {Si-O-M} motif.[6] Firstly, this is in view of the
importance of such compounds as models for naturally
occurring minerals.[7] Secondly, these compounds are also
[*] U. N. Nehete, G. Anantharaman, Prof. Dr. H. W. Roesky, D. Vidovic,
J. Magull
Institut für Anorganische Chemie
Universität Göttingen
Tammannstrasse 4, 37077 Göttingen (Germany)
Fax: (+49)551-39-3373
E-mail: hroesky@gwdg.de
Prof. Dr. K. Samwer, B. Sass
I. Physikalisches Institut
Universität Göttingen
Tammannstrasse 1, 37077 Göttingen (Germany)
Prof. Dr. V. Chandrasekhar
Department of Chemistry
IIT-Kanpur
Kanpur-208 016 (India)
Prof. Dr. R. Murugavel, Dr. M. G. Walawalkar
Department of Chemistry
IIT-Bombay
Powai, Mumbai, 400 076 (India)
[**] This work was supported by the Deutsche Forschungsgemeinschaft
(Graduiertenkolleg: Spektroskopie und Dynamik molekularer
Aggregate, Ketten, Knäuel und Netzwerke) and the Göttinger
Akademie der Wissenschaften. V.C. thanks the Alexander von
Humboldt Foundation, Bonn (Germany), for the Friedrich Wilhelm
Bessel-Research award. M.G.W. and R.M. thank the Alexander von
Humboldt Foundation, Bonn (Germany), for a research fellowship.
Scheme 1. Synthesis of ferrous 1 and ferric siloxanes 2.
DOI: 10.1002/anie.200453740 Angew. Chem. Int. Ed. 2004, 43, 3832 –3835
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The molecular structure of 1 has been determined by
single-crystal X-ray diffraction experiments[15a] and it consists
of an {Fe4O6Si2} core (Figure 1a). The structure of 1 exhibits
two {Fe2O3Si} rings that are fused to each other, which results
in the formation of a drumlike architecture with a pseudoin-
version center. The sides of the drum are defined by two four-
membered {FeO2Si} and two six-membered {Fe2O3Si} rings.
Two types of iron centers are present in the structure of 1.
While Fe1 and Fe4have a coordination environment com-
prising one N and two O atoms with one intact N(SiMe3)2
ligand, Fe2 and Fe3 have a coordination environment of one
C and three O atoms, in which the N-heterocyclic carbene
takes up the fourth coordination side. The tricoordinate iron
centers (Fe1 and Fe4) are in a trigonal planar geometry while
the tetracoordinate iron centers, (Fe2 and Fe3) have an
approximate tetrahedral geometry. Another view of the
structure of 1 (Figure 1b) shows that the four iron centers
are nearly in one plane and are held together by two
essentially bicapping RSiO3 units. This mode of interaction
of the RSiO3 ligand is similar to that of the RPO3 ligand as
found in molecular metal phosphonate and phosphate
structures.[16–18]
One of the three oxygen atoms (O1, O6) of each
silanetriol in 1 is involved in a m-bridging mode (Fe-O-Si),
while the other two oxygen atoms (O2, O3, O4, O5) are
involved in a m3-bridging mode that links two iron centers and
one silicon. In general, the bond lengths that involve the m3-
À
À
oxygen centers are found to be longer (av. Si O, 1.651 Š; Fe
O, 2.055 Š) than the corresponding distances that involve the
À
À
m-oxygen atom (av. Si O, 1.605 Š; Fe O, 1.892 Š).
The reaction of the silanetriol with the ferric amide
[Fe{N(SiMe3)2}3][14] proceeds with complete elimination of
HN(SiMe3)2 to afford the ferric siloxane 2 which was crystal-
lized in the presence of trimethyl phosphane L1 (Scheme 1).
Compound 2, like 1 discussed above is highly lipophilic and is
soluble in common organic solvents including hexane. The
ferric siloxane 2 is thermally stable up to its melting point
(242–2448C). The ESIMS of 2 shows the base peak for
[M+ÀPMe3], which suggest a remarkable stability of the core
structure of 2 under these conditions. Compound 2 exhibits
broad resonances in its NMR spectra, thus indicating that it is
paramagnetic.
The molecular structure of 2 (Figure 2) is made up of a
{Fe4O12Si4} cuboid core at which the corners of the cube are
alternately occupied by iron and silicon atoms.[15b] Each of the
twelve edges of the cube contain the bridging oxygen atoms.
Thus, the overall core structure of the ferric siloxane contains
six {Fe2O4Si2} eight-membered rings as the faces of the cube.
Unlike in the ferrous siloxane 1, in compound 2 all the oxygen
atoms are involved in a m-bridging mode that interconnects a
silicon and an iron atom. Each iron is tetracoordinate with a
coordination environment of one P and three O atoms. The
coordination geometry around both iron and silicon centers is
À
nearly tetrahedral. The average Fe O distance (1.824Š)
found in the ferric siloxane 2 is much shorter due to the
smaller ionic radius of an FeIII center compared to that of an
FeII in 1. In contrast the Si O distance in 2 is similar (av.
À
1.618 Š) to that found in 1 (m-O).
Magnetic-susceptibility measurements[19] were carried out
in the temperature range from 300 K to 5 K for compounds 1
and 2. A plot of 1/c (OeemuÀ1; c is magnetic susceptibility)
versus T (K) is shown in Figure 3. The plots indicate an
antiferromagnetic behavior for both compounds.
In conclusion we report the first examples of polyhedral
ferrous and ferric siloxanes. The isolation of these compounds
suggests that similar synthetic strategies will allow the
preparation of other transition-metal siloxane frameworks.
These perspectives will lead to interesting new systems.
Figure 1. Molecular structure of 1. a) The substituents on silanetriol
have been removed for the sake of clarity. Selected bond lengths [Š]
and angles [8]: Fe(1)-O(2) 1.940(2), Fe(1)-O(5) 1.945(2), Fe(2)-O(1)
1.891(2), Fe(2)-O(4) 2.143(2), Fe(2)-O(5) 2.098(2), Fe(1)-N(1)
1.938(3), Fe(2)-C(56) 2.123(4), Si(5)-O(1) 1.605(3), Si(5)-O(2)
1.641(3), Si(5)-O(3) 1.653 (2); O(2)-Fe(1)-O(5) 105.15(10), O(2)-Fe(1)-
N(1) 128.58(12), O(5)-Fe(1)-N(1) 126.27(11), O(1)-Fe(2)-O(5)
106.00(10), C(56)-Fe(2)-O(1) 118.92(12), C(56)-Fe(2)-O(5) 124.35(11),
O(1)-Si(5)-O(2) 115.01(14), O(1)-Si(5)-O(3) 105.84 (15), O(2)-Si(5)-
O(3) 100.78(12), Fe(1)-O(2)-Fe(3) 112.67(10), Fe(3)-O(2)-Si(5)
90.79(11), Fe(1)-O(2)-Si(5) 123.09(14), Fe(2)-O(1)-Si(5) 131.19(15).
b) Core structure of 1. Imaginary lines are drawn between iron atoms
to show its planar arrangement.
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Communications
for C64H128Fe4N8O6Si8 (1407.44): C 49.47, H 8.30, N 7.21; found:
C 47.7, H 8.0, N 6.2.
2: [Fe{N(SiMe3)2}3] (2.3 g, 4.28 mmol) in hexane (25 mL) was
slowly added to a suspension of silanetriol (1.4g, 4.28 mmol) in the
same solvent (40 mL). After the addition was complete, the reaction
mixture was stirred for 72 h at room temperature. The green solution
slowly turned to greenish brown. The solvents and hexamethyldisi-
lazane were removed in vacuo to give a brownish-green product. This
was recrystallized from a mixture of toluene and THF (1:0.05) in the
presence of PMe3 as the auxiliary coordinating ligand at room
temperature over a period of two weeks (0.29 g, 14%; it was observed
that the yield of compound 2 was improved to about 25% of single
crystals by the use of a mixture of hexane and THF (1:0.25) along with
PMe3 for crystallization); mp 242–2448C; ESIMS (CH3CN): m/z (%)
1750 (100) [M+ÀPMe3]; IR (nujol): n˜ = 1585, 1576, 1440, 1420, 1321,
1308, 1290, 1245, 1190, 1107, 1096, 1054, 1033, 967, 956, 923, 836, 800,
757, 749, 726, 682, 642, 613, 599, 549, 534, 489, 462, 434 cmÀ1
;
elemental analysis (%) calcd (toluene molecules were removed by
drying under vacuum) C72H140Fe4N4O12P4Si8 (1825.89): C 47.36,
H 7.73, N 3.07; found: C 46.68, H 7.17, N 3.24.
Received: January 13, 2004[Z53740]
Figure 2. Core structure of 2. The substituents on silanetriol and the
phosphane have been omitted for the sake of clarity. Selected bond
lengths [Š] and angles [8]: Fe(1)-O(1) 1.802(3), Fe(1)-O(3B) 1.829(3),
Fe(1)-O(2A) 1.831(3), Fe(1)-P(1) 2.468(1), Si(1)-O(1) 1.628(3), Si(1)-
O(2) 1.616(3), Si(1)-O(3) 1.612(3); Si(1)-O(1)-Fe(1) 147.60(19), O(1)-
Fe(1)-O(3B) 114.61(14), O(3)-Si(1)-O(2) 111.40(16), O(2)-Si(1)-O(1)
108.73(17).
Keywords: cage compounds · iron · N ligands · silicon ·
siloxanes
.
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Experimental Section
1: [Fe{N(SiMe3)2}2]·THF (3.5 g, 9.17 mmol) was slowly added to a
suspension of RSi(OH)3 (1) (1.5 g, 4.59 mmol) in THF/hexane
(10 mL, 40 mL) at room temperature. The solution turned from
green to dark brown. This solution was stirred for further 16 h at room
temperature.
A solution of N-heterocyclic carbene L (2 g,
11.47 mmol) in toluene (10 mL) was added and the stirring was
continued for one more day. The volatile components were removed
to obtain a brown solid. To this a mixture of toluene (10 mL) and THF
(1 mL) was added. Colorless crystals of 1 were obtained after several
days at 08C: (0.50 g, 13% of single crystals); mp > 2208C; IR (nujol):
n˜ = 1260, 1245, 1221, 1186, 1100, 1043, 1019, 983, 964, 945, 927, 885,
836, 820, 801, 752, 723, 680, 607, 567, 544, 491 cmÀ1; elemental analysis
(%) calcd (toluene molecules were removed by drying under vacuum)
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gelatine capsule by inverting the top half.
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[21] G. M. Sheldrick, SHELX-97, Program for Crystal Structure
Refinement, University of Gꢁttingen, Gꢁttingen (Germany),
1997.
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[15] Crystal data for compounds: a) The single crystals suitable for X-
ray diffraction studies of compound 1·1.5C7H8 were obtained
from toluene at 08C. C74.50H140Fe4N8O6Si8, Mr = 1692.06, triclinic,
¯
space group P1, a = 12.6202(5) Š, b = 14.7837(6) Š, c =
27.2205(12) Š, a = 93.949(4)o, b = 97.850(3)o, g = 109.689(3)o,
V= 4700.8(3) Š3, Z = 2, 1calcd = 1.195 mgmÀ3
, F(000) = 1814,
l = 0.71073 Š, T= 133(2) K, m(MoKa) = 0.755 mmÀ1. The data
were collected using the w scan mode in the range of 1.47 ꢀ q ꢀ
24.84, À14 ꢀ h ꢀ 14, À17 ꢀ k ꢀ 17, À32 ꢀ l ꢀ 27. Of 45117 reflec-
tions collected, 16133 were unique. Final R1 (I > 2s(I)) = 0.0497;
wR2 (all data) = 0.1452. Maximum and minimum heights in the
final Fourier-difference map were 1.031 and À1.016 e·AÀ3
;
b) 2·2C7H8: C86H156Fe4N4O12P4Si8, Mr = 2010.15, tetragonal,
space group P421c, a = b = 19.7056(11) Š, c = 15.1916(11) Š,
a = b = g = 90o, V= 5899.1(6) Š3, Z = 2, 1calcd = 1.132 mgmÀ3
,
F(000) = 2144,
l = 0.71073 Š,
T= 133(2) K,
m(MoKa) =
0.666 mmÀ1. The data were collected by using the w scan mode
in the range of 1.69 ꢀ q ꢀ 24.81, À23 ꢀ h ꢀ 20, À23 ꢀ k ꢀ 19,
À17 ꢀ l ꢀ 11. Of 12247 reflections collected, 4732 were unique.
Final R1 (I > 2s(I)) = 0.0431; wR2 (all data) = 0.1309. Maximum
and minimum heights in the final Fourier difference map were
0.752 and À0.249 e·AÀ3. The single crystals suitable for X-ray
diffraction studies of compound 2·2C7H8 were obtained from a
mixture of toluene and THF in the presence of PMe3 at room
temperature. Diffraction data were collected on a IPDS II Stoe
image-plate diffractometer with graphite-monochromated Mo
Ka radiation (l = 0.71073 Š). The structure was solved by direct
methods (SHELX-97)[20] and refined against F2 on all data by
full-matrix least squares with SHELX-97.[21] The heavy atoms
were refined anisotropically. Hydrogen atoms were included by
using the riding model with Uiso tied to Uiso of the parent atoms.
CCDC-225526 (1) and CCDC-225525 (2) contain the supple-
mentary crystallographic data (excluding structure factors) for
this paper. These data can be obtained free of charge via
www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cam-
bridge Crystallographic Data Centre, 12, Union Road, Cam-
bridge CB21EZ, UK; fax: (+ 44)1223-336-033; or deposit@
ccdc.cam.ac.uk).
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