Polyhedral cobalt(II) and iron(II) siloxanes: Synthesis and X-ray crystal structure of [(RSi(OH)O2)Co(OPMe3)]4 and [(RSiO3)2(RSi(OH)O2)4(μ 3-OH)2Fe8(THF)4] (R = (2,6-iPr 2C6H3)N(SiMe3))

Article abstract of DOI:10.1021/ic050859n

The cobalt(II) and iron(II) siloxane compounds were prepared by the reaction of lipophilic N-bonded silanetriol 1 with metal silylamides M[N(SiMe₃)₂]₂ (M = Co (2), Fe (3)) in a 1:1 and 3:4 molar ratio, respectively. A plot of 1/χ versus temperature in the range of 2-300 K indicates the paramagnetic behavior of 2 and 3. The composition and molecular structures of 2 and 3 were fully determined by IR, elemental analysis, and X-ray crystal structural analyses. Compound 2 possesses a pseudo-4-fold (S₄) symmetry, whereas 3 reveals an inversion center. Compound 2 represents a tetracobalt(II) drum while 3 exhibits an octairon(II) cage containing siloxane ligands.

Full text of DOI:10.1021/ic050859n

Inorg. Chem. 2005, 44, 7243−7248
Polyhedral Cobalt(II) and Iron(II) Siloxanes: Synthesis and X-ray Crystal
Structure of [(RSi(OH)O2)Co(OPMe3)]4 and
[(RSiO3)2(RSi(OH)O2)4( 3-OH)2Fe8(THF)4] (R
µ
) (2,6-iPr2C6H3)N(SiMe3))
Umesh N. Nehete,† Herbert W. Roesky,*,† Hongping Zhu,† Sharanappa Nembenna,†
Hans-Georg Schmidt,† Mathias Noltemeyer,† Dmitrij Bogdanov,‡ and Konrad Samwer‡
Institut fu¨r Anorganische Chemie der Georg-August UniVersita¨t, Go¨ttingen, Tammannstrasse 4,
37077 Go¨ttingen, Germany, and I. Physikalisches Institut, UniVersita¨t Go¨ttingen,
Tammannstrasse 1, D-37077 Go¨ttingen, Germany
Received May 27, 2005
The cobalt(II) and iron(II) siloxane compounds were prepared by the reaction of lipophilic N-bonded silanetriol 1
with metal silylamides M[N(SiMe3)2]2 (M Co (2), Fe (3)) in a 1:1 and 3:4 molar ratio, respectively. A plot of 1/
versus temperature in the range of 2 300 K indicates the paramagnetic behavior of 2 and 3. The composition and
)
ø
−
molecular structures of 2 and 3 were fully determined by IR, elemental analysis, and X-ray crystal structural analyses.
Compound 2 possesses a pseudo-4-fold (S4) symmetry, whereas 3 reveals an inversion center. Compound 2
represents a tetracobalt(II) drum while 3 exhibits an octairon(II) cage containing siloxane ligands.
Introduction
Soluble metallasiloxanes may serve as model compounds for
complex metallasilicate frameworks that either occur natu-
rally or have been synthesized in the laboratory.3 Although,
the molecular species are only models for covalently bonded
transition metal-silica combinations they are an important
source for structural parameters, which are in general not
easily available for complex metallasilicate frameworks.
Many industrially and commercially important processes are
catalyzed by transition metal complexes immobilized on
silica surfaces.4 For example cobalt- and iron-containing
silicate frameworks include zeolites such as Co-ZSM-5 and
The metallasiloxanes containing the Si-O-M functional
group (M ) main group or transition metal) is one of the
most important structural units of inanimate nature. The
various metal silicates are built up from such units, and the
presence of metal in the siloxane framework not only makes
these compounds thermally stable but also improves their
catalytic and conducting properties. We have been utilizing
the multi-functional N-bonded silanetriol RSi(OH)3 (R )
(2,6-iPr2C6H3)N(SiMe3)) (1) for the preparation of various
polyhedral metallasiloxanes with a high metal/silicon ratio.1,2
* Author to whom correspondence should be addressed. E-mail: hroesky@
gwdg.de.
(3) (a) Voigt, A.; Murugavel, R.; Parisini, E.; Roesky, H. W. Angew.
Chem. 1996, 108, 823-825; Angew. Chem., Int. Ed. Engl. 1996, 35,
748-750. (b) Voigt, A.; Walawalkar, M. G.; Murugavel, R.; Roesky,
H. W.; Parisini, E.; Lubini, P. Angew. Chem. 1997, 109, 2313-2315;
Angew. Chem., Int. Ed. Engl. 1997, 36, 2203-2205. (c) Voigt, A.;
Murugavel, R.; Roesky, H. W. Organometallics 1996, 15, 5097-5101.
(d) Nehete, U. N.; Chandrasekhar, V.; Anantharaman, G.; Roesky, H.
W.; Vidovic, D.; Magull, J. Angew. Chem. 2004, 116, 3930-3932;
Angew. Chem., Int. Ed. 2004, 43, 3842-3844. (e) Nehete, U. N.;
Chandrasekhar, V.; Jancik, V.; Roesky, H. W.; Herbst-Irmer, R.
Organometallics 2004, 23, 5372-5374. (f) Nehete, U. N.; Chan-
drasekhar, V.; Roesky, H. W.; Magull, J. Angew. Chem. 2005, 117,
285-288; Angew. Chem., Int. Ed. 2005, 44, 281-284.
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Blackie and Son: Glasgow, 1981. (b) Seiyama, T.; Tanabe, K. New
Horizons in Catalysis; Elsevier: Amsterdam, 1980. (c) Iwasawa, Y.
Tailored Metal Catalysts; Reidel: Dordrecht, The Netherlands, 1986.
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Polymerization; Springer-Verlag: Heidelberg, 1988.
† Institut fu¨r Anorganische Chemie der Georg-August Universita¨t.
‡ I. Physikalisches Institut der Georg-August Universita¨t.
(1) (a) Chandrasekhar, V.; Murugavel, R.; Voigt, A.; Roesky, H. W.
Organometallics 1996, 15, 918-922. (b) Murugavel, R.; Voigt, A.;
Walawalkar, M. G.; Roesky, H. W. Chem. ReV. 1996, 96, 2205-
2236. (c) Murugavel, R.; Chandrasekhar, V.; Roesky, H. W. Acc.
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Budzichowski, T. A. Polyhedron 1995, 14, 3239-3253. (g) Duchateau,
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10.1021/ic050859n CCC: $30.25
Published on Web 08/17/2005
© 2005 American Chemical Society
Inorganic Chemistry, Vol. 44, No. 20, 2005 7243

Nehete et al.
tional procedures and were freshly distilled prior to use. Silanetriol,13
Co[N(SiMe3)2]2,14,15 and Fe[N(SiMe3)2]2‚THF16 were prepared
according to the published procedures. Elemental analyses were
performed by the Analytisches Labor des Instituts fu¨r Anorganische
Chemie der Universita¨t Go¨ttingen. NMR spectra were recorded on
Bruker Advance 500 MHz spectrometers. IR spectra were obtained
on a Bio-Rad FTS-7 spectrometer as Nujol mulls. Mass spectra
were obtained on a Finnigan MAT system 8230 and a Varian MAT
CH5 mass spectrometer by EI-MS methods. Melting points were
recorded on a HWS-SG 3000 apparatus and are uncorrected.
Variable-temperature magnetic measurements were recorded using
a SQUID magnetometer (Quantum Design). Samples were prepared
in a glovebox by crushing single crystals. The crushed single
crystals were restrained between the two halves of a gelatine capsule
by inverting the top half.
Fe-modified ZSM-5. The Co-ZSM-5 type catalysts have been
found to possess a high activity and selectivity for NO
decomposition in the presence of hydrocarbons, whereas the
Fe-modified ZSM-5 has been important in the synthesis of
phenol from benzene and N2O.5 Nevertheless, due to the
heterogeneous nature of the catalysts, the catalytic species
are normally difficult to characterize, and the exact reactions
occurring at such surfaces are remaining unclear. In these
cases lipophilic metallasiloxanes may serve as useful models
for silica-supported transition metal catalysts.1,6 Metalla-
siloxanes have also been envisaged as single-source precur-
sors for modified zeolites.7 More recently, some of these
metallasiloxanes have also been found to be useful in
homogeneous catalysis reactions such as [RSiO3TiOR1]4 [R
) (2,6-iPr2C6H3)N(SiMe3)] [R1 ) (Et, iPr)] for the epoxi-
dation of olefins.8
Synthesis of [(RSi(OH)O2)Co(OPMe3)]4 [R ) (2,6-iPr2C6H3)N-
(SiMe3)] (2). Co[N(SiMe3)2]2 (1.63 g, 4.28 mmol) in hexane (10
mL) was slowly added to a suspension of silanetriol (1.4 g, 4.28
mmol) in THF/hexane (2 mL, 5 mL). After the addition was
completed, the reaction mixture was stirred for 24 h at room
temperature. The green-colored solution slowly turned to greenish
indigo. A solution of trimethylphosphine oxide (0.4 g, 4.34 mmol)
in THF (20 mL) was added, and the stirring was continued for one
more day. The volatile components were removed to obtain a blue
solid. To this a mixture of toluene (10 mL) and THF (1 mL) was
added. Blue-colored crystals of 2 were obtained at room temperature
over a period of 2 weeks (0.88 g, 43%); mp. 196-198 °C; IR
(Nujol): ν˜ ) 3381 (w), 2363 (w), 2254 (w), 2195 (m), 1924 (w),
1863 (w), 1797 (w), 1701 (w), 1653 (w), 1599 (m), 1579 (m), 1488
(m), 1439 (s), 1407 (s), 1361 (s), 1391 (s), 1249 (s), 1182 (s), 1104
(s), 1027 (s), 933 (s), 910 (s), 880 (s), 838 (s), 803 (s), 755 (s),
685 (m), 641 (w), 618 (w), 599 (m), 546 (m), 489 (m), 440 (m)
In previous papers, we reported the synthesis and single-
9
crystal X-ray structures of zinc [(RSi(OH)O2)Zn(THF)]4
and ferrous siloxanes [{(Me3Si)2NFe}2{LFe}2{O3SiR}2]10
(L ) 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene) using
metal alkyl/amide complexes with kinetically stable amino-
silanetriol. As a part of our continuing study of low-
coordinate transition metal siloxanes, this paper describes
the synthesis and X-ray crystal structures of [(RSi(OH)O2)-
Co(OPMe3)]4 (2) and [(RSiO3)2(RSi(OH)O2)4(µ3-OH)2Fe8-
(THF)4] (3) (R ) (2,6-iPr2C6H3)N(SiMe3)). Compound 2
represents the first example of a tetracobalt(II) drum contain-
ing siloxane ligands. Previously several methods were
reported for the synthesis of cobalt(II)11 and iron(II)12
siloxanes starting from R3SiO or silsesquioxane ligands.
cm-1
. Elemental analysis calcd (%) C72H144Co4N4O16P4Si8
(1906.24): C, 45.37; H, 7.61; N, 2.94. Found: C, 44.76; H, 7.89;
N, 3.12.
Experimental Section
General Comments. All experimental manipulations were
carried out under a dry nitrogen atmosphere, rigorously excluding
air and moisture. The samples for spectral measurements were
prepared in a drybox. Solvents were purified according to conven-
Synthesis of [(RSiO3)2(RSi(OH)O2)4(µ3-OH)2Fe8(THF)4] (3).
The liquid Fe[N(SiMe3)2]2‚THF (2.6 g, 5.79 mmol) was slowly
added to a suspension of RSi(OH)3 (1.4 g, 4.28 mmol) in THF/
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7244 Inorganic Chemistry, Vol. 44, No. 20, 2005

Polyhedral Co(II) and Fe(II) Siloxanes
Scheme 1
ies. The pure crystalline solid of 2 is thermally quite stable
up to its melting point of 196-198 °C, after which its color
slowly turns to brown upon decomposition. However, no
peak attributable to the molecular ion of 2 can be observed
in the EI or other mass spectrometric methods. Only smaller
fragment ion peaks are found. The IR spectrum exhibited a
weak absorption at 3381 cm-1, which is comparable with
that of the OH stretching frequency of the silanol groups
(3344 cm-1).13 The IR spectra of 2 and 3 are dominated by
the typical M-O-Si stretching frequencies observed in the
range of 900-1000 cm-1.
Structural Description of 2. The blue color crystals of 2
were obtained by slow cooling of its saturated solution in
toluene and THF at room temperature over a period of 2
weeks. Compound 2 crystallizes in the monoclinic space
group P1(1)/c, with two molecules in the asymmetric unit.
Selected metric parameters of 2 are summarized in Table 1.
The core structure of 2 is given in Figure 1. The molecular
structure of 2 can be described as a polyhedral drum-like
architecture with a pseudo-4-fold (S4) symmetry. The core
structure consists of four cobalt, four silicon, and eight
oxygen atoms. Importantly, every silicon atom has one
unreactive Si-OH group. The Co4O8Si4 core is surrounded
by eight bridging oxo ligands, four double bridging (µ-O),
and four triply bridged (µ3-O) oxygen atoms. An alternative
way of viewing 2 is that it contains two puckered Co2O4Si2
[Co(1)-O(31)-Si(3)-O(33)-Co(4)-O(12)-Si(1)-
O(13), Co(2)-O(43)-Si(4)-O(42)-Co(3)-O(21)-Si(2)-
O(23)] boat-like eight-membered rings that are fused together
by four Co-O [Co(1)-O(42), Co(2)-O(33), Co(3)-O(13),
Co(4)-O(23)] bonds. Interestingly, this arrangement leads
to the formation of two almost planar four-membered Co2O2
rings [Co(1)-O(42)-Co(3)-O(13), Co(2)-O(23)-Co(4)-
O(33)], and the cobalt atoms of one ring are connected with
the oxygens of the second one.
Each cobalt is tetracoordinate with its coordination envi-
ronment made up of only oxygen atoms, in which OPMe3
takes up the fourth coordination side. The coordination
geometry around both the cobalt and the silicon centers is
nearly tetrahedral. The shortest Co-O and Si-O distances
are seen for Co(1)-O(31) [1.937(8) Å] and Si(2)-O(21)
bonds [1.600(8) Å] while the longest distances are observed
for Co(1)-O(42) [2.016(9) Å] and Si(1)-O(11) bonds
[1.640(9) Å], respectively. The bond lengths involving the
µ3 oxygen centers are found to be longer {Co(1)-O(42)
[2.016(9) Å], Co(2)-O(23) [2.013(8) Å], Co(3)-O(13)
[2.011(9) Å], Co(4)-O(33) [1.988(8) Å]} than the corre-
sponding distances involving the µ oxygens {Co(1)-O(31)
[1.937(8) Å], Co(2)-O(43) [1.944(8) Å], Co(3)-O(21)
[1.938(8) Å], Co(4)-O(12) [1.967(8) Å]}. A similar range
of bond distances was also observed in the case of other
cobalt(II) siloxanes that have been reported earlier. Thus,
in [{Co3(p-tert-butylcalix[4]areneOSiMe3)2(THF)}‚5Ph-
Me],11a [{Co(OSiPh3)2(THF)}2],11b and [{((Me3Si)3SiO)2-
Co}2]11c contain a four-membered Co2O2 ring with the
average Co-O bond distance of 1.985(1) Å, 1.988(3) Å, and
1.952(8) Å, respectively. The O-Co-O bond angles within
the Co2O2 rings are around 90° and reveal the almost perfect
hexane (4 mL, 25 mL) at room temperature. After stirring for 1 h,
the color of the solution turned from green to greenish blue. The
resulting clear solution was stirred for another 48 h at room
temperature and then heated to reflux for 1 h. The solvents and
hexamethyldisilazane were removed in vacuo to give a greenish
blue product. This was recrystallized from a mixture of toluene
and THF (8 mL, 2 mL). The reddish yellow crystals of 3 were
obtained at -26 °C over a period of 2 weeks (0.42 g, 24%); mp.
210-212 °C; IR (Nujol): ν˜ ) 3652 (w), 3265 (w), 1320 (w), 1259
(m), 1249 (s), 1184 (m), 1101 (m), 1044 (s), 1018 (m), 969 (s),
944 (s), 905 (s), 840 (s), 802 (s), 755 (w), 722 (m), 686 (w), 642
(w), 569 (m), 542 (m), 478 (m) cm-1. Elemental analysis calcd
(%) (toluene and THF molecules were removed by drying under
vacuum) for C90H162Fe8N6O20Si12 (2432.06): C, 44.45; H, 6.71;
N, 3.46. Found: C, 44.19; H, 6.96; N, 3.30.
X-ray Structure Determination of 2 and 3. Data for the
structure were collected on a Bruker three-circle diffractometer
equipped with a SMART 6000 CCD detector. Intensity measure-
ments were performed on a rapidly cooled crystal. The structures
were solved by direct methods (SHELXS-97)17 and refined with
all data by full-matrix least squares on F2.18 The hydrogen atoms
of C-H bonds were placed in idealized positions and refined
isotropically with riding model, whereas the non-hydrogen atoms
were refined anisotropically.
Results and Discussion
The addition of Co[N(SiMe3)2]2 in hexane to a suspension
of RSi(OH)3 (1) in THF/hexane mixture at room temperature
in the presence of trimethylphosphine oxide L affords 2 in
moderate yield (Scheme 1).
The reaction proceeds by elimination of eight molecules
of HN(SiMe3)2 with a concomitant Co-O-Si bond forma-
tion containing an unreacted OH group on each silicon atom.
The use of the auxiliary coordinating ligand L allowed the
isolation of the cobalt(II) siloxane 2 as a crystalline product.
Compound 2 is highly soluble in common organic solvents;
2 has been unambiguously characterized by means of
analytical, spectroscopic, and single-crystal diffraction stud-
(17) Sheldrick, G. M. SHELXS-97, Program for Structure Solution. Acta
Crystallogr. Sect. A 1990, 46, 467-473.
(18) Sheldrick, G. M. SHELXL-97, Program for Crystal Structure Refine-
ment; University of Go¨ttingen: Go¨ttingen, Germany, 1997.
Inorganic Chemistry, Vol. 44, No. 20, 2005 7245

Nehete et al.
Table 1. Crystal Data and Structure Refinement for 2 and 3
2
3 6C7H8
C148H242Fe8N6O24Si12
formula
fw
C72H144Co4N4O16P4Si8
1906.23
3273.32
crystal system
space group
temp, K
λ, Å
monoclinic
P1(1)/c
133(2)
triclinic
P1h
133(2)
0.71073
0.71073
a, Å
28.705(6)
27.685(6)
29.579(6)
90
106.41(3)
90
2255(1)
8
1.123
0.769
16.129(13)
16.787(13)
17.464(14)
61.55(4)
81.35(5)
86.55(5)
4109.9(6)
1
1.320
0.837
1734
0.20 × 0.20 × 0.10
1.87 to 24.79
-18 e h e 17
-19 e k e 19
-20 e l e 20
76145
b, Å
c, Å
R, deg
â, deg
γ, deg
V, Å3
Z
F
calcld, g/cm3
µ, mm-1
F(000)
8096
crystal size, mm3
θ range for data collection, deg
index ranges
0.30 × 0.20 × 0.20
1.60 to 24.92
-32 e h e 33
-31 e k e 32
-34 e l e 34
60890
no. of reflns collected
no. of independent reflns (Rint
)
30611 (0.1441)
30611/2771/2032
0.697
0.0676, 0.1319
0.2562, 0.1844
0.777/-0.382
13591 (0.1561)
13591/0/916
0.927
0.0693, 0.1230
0.1466, 0.1433
0.682/-0.491
no. of data/restraints/parameters
GOF on F2
R1,a wR2b (I > 2σ(I))
R1,a wR2b (all data)
largest diff. peak/hole, e‚Å-3
2
2
a R ) ∑||Fo| - |Fc||/∑|Fo|. b wR2 ) [∑w(Fo - Fc )2/∑w(Fo)2]1/2
.
water is capped by three Fe(II) centers by loss of one
molecule of HN(SiMe3)2. Thus, the overall reaction takes
place among six molecules of silanetriol, eight molecules
of Fe[N(SiMe3)2]2, and two molecules of water which leads
to the formation of [(RSiO3)2(RSi(OH)O2)4(µ3-OH)2Fe8-
(THF)4] (R ) (2,6-iPr2C6H3)N(SiMe3)) (3) by loss of 16
molecules of HN(SiMe3)2. Therefore, 3 contains an unreacted
OH group on four silanetriol molecules, and two OH groups
are trapped by three Fe(II) centers. The low yield of 2 may
be due to the water formation in this system. Selected bond
lengths and bond angles for 2 and 3 are given in Table 2.
Albeit the above statement is consistent with the stoichi-
ometry of the reactants, we were not able to resolve the
hydroxyl protons in the molecular structure of 3 due to the
proximity of high electron density metal atoms. The forma-
tion of 3 is insensitive to the slight variations of reactant
stoichiometry. Thus, the equimolar quantity of the reactants
also leads to the same product formation of 3. A remarkable
feature of the Fe(II) siloxane 3 is its high thermal stability.
It is stable up to its melting point of 210-212 °C at which
the color of the compound turns into reddish brown.
Moreover, no peak attributable to the molecular ion of 3 can
be observed in the EI or other mass spectrometric methods.
Compound 3, like 2 discussed above, is highly lipophilic
and is soluble in various organic solvents such as benzene,
toluene, and THF. Compounds 2 and 3 are paramagnetic and
give broad resonances in the NMR spectra. OH stretching
vibrations are observed at 3652 and 3265 cm-1 in the IR
Figure 1. ORTEP diagram of core 2 with 50% probability. Most of the
substituents on silicon and cobalt atoms are omitted for the sake of clarity.
rectangular geometry of the four-membered rings. The
Co(1)-Co(3) and Co(2)-Co(4) distances 2.890(3) Å and
2.889(3) Å are also very close to the reported ones.11a
The reaction of Fe[N(SiMe3)2]2 with the N-bonded silane-
triol RSi(OH)3 (1) in a 6:8 stoichiometric ratio affords the
polyhedral octanuclear iron(II) siloxane 3 in 24% yield
(Scheme 1). The isolation of the crystalline product can be
achieved in the presence of a Lewis base such as THF.
During the course of the reaction a part of silanetriol
undergoes self-condensation to generate the disiloxanetetrol
[(RSi(OH)2)2O] and water. This process of self-condensation
appears to be mediated by the iron(II) amide. It may be noted
that we had earlier observed a similar condensation of two
silanetriol molecules in the presence of hydrazine.19 More-
over, in the presence of Fe[N(SiMe3)2]2 the OH group of
(19) Murugavel, R.; Bo¨ttcher, P.; Voigt, A.; Walawalkar, M. G.; Roesky,
H. W.; Parisini, E.; Teichert, M.; Noltemeyer, M. Chem. Commun.
1996, 2417-2418.
7246 Inorganic Chemistry, Vol. 44, No. 20, 2005

Polyhedral Co(II) and Fe(II) Siloxanes
Table 2. Selected Bond Lengths (Å) and Bond Angles (°) for 2 and 3
Compound 2
Co(1)-O(13)
Co(1)-O(31)
Co(2)-O(33)
Co(2)-O(43)
Co(1)-Co(3)
Si(1)-O(11)
Si(1)-O(13)
Si(2)-O(22)
Si(3)-O(32)
Co(1)-O(42)
Co(1)-O(1)
Co(2)-O(23)
Co(2)-O(2)
Co(2)-Co(4)
Si(1)-O(12)
Si(2)-O(21)
Si(2)-O(23)
Si(4)-O(41)
1.967(8)
1.937(8)
1.961(9)
1.944(8)
2.890(3)
1.640(9)
1.624(9)
1.632(9)
1.636(9)
2.016(9)
1.959(8)
2.013(8)
1.969(8)
2.889(3)
1.605(8)
1.600(8)
1.632(9)
1.622(9)
O(13)-Co(1)-O(42)
O(31)-Co(1)-O(42)
O(33)-Co(2)-O(23)
O(43)-Co(2)-O(23)
O(12)-Si(1)-O(13)
O(13)-Si(1)-O(11)
O(21)-Si(2)-O(22)
O(1)-Co(1)-O(42)
O(1)-Co(1)-O(13)
O(33)-Co(2)-O(2)
O(2)-Co(2)-O(23)
O(12)-Si(1)-O(11)
O(21)-Si(2)-O(23)
O(23)-Si(2)-O(22)
87.2(3)
106.9(3)
86.3(3)
106.9(3)
109.8(4)
106.5(4)
112.2(5)
117.9(4)
111.3(3)
108.3(4)
118.6(3)
113.7(4)
109.6(5)
106.3(5)
Figure 2. ORTEP diagram of core 3 with 50% probability. Most of the
substituents on silicon and iron atoms are omitted for the sake of clarity.
Compound 3
Fe(1)-O(1)
2.038(4)
2.111(4)
2.116(4)
2.150(4)
1.954(4)
1.960(4)
1.971(4)
2.122(4)
2.201(4)
1.845(4)
1.862(4)
1.883(4)
1.915(4)
1.860(4)
1.865(4)
1.892(4)
1.920(4)
1.624(4)
1.637(5)
1.659(4)
1.611(4)
1.614(5)
1.688(4)
1.612(4)
1.642(5)
1.647(4)
O(1)-Fe(1)-O(31A)
O(1)-Fe(1)-O(13)
O(31A)-Fe(1)-O(13)
O(1)-Fe(1)-O(21)
O(31A)-Fe(1)-O(21)
O(13)-Fe(1)-O(21)
O(22)-Fe(2)-O(21)
O(22)-Fe(2)-O(31A)
O(21)-Fe(2)-O(31A)
O(22)-Fe(2)-O(24)
O(21)-Fe(2)-O(24)
O(31A)-Fe(2)-O(24)
O(33)-Fe(3)-O(3)
O(3)-Fe(3)-O(24A)
O(33)-Fe(3)-O(13)
O(4)-Fe(4)-O(23)
O(23)-Fe(4)-O(24)
O(23)-Fe(4)-O(32)
O(3A)-Si(21)-O(23)
O(3A)-Si (21)-O(22)
O(33)-Si (31)-O(32)
Si(31)-O(33)-Fe(3)
Si(21)-O(23)-Fe(4)
Si(11)-O(4)-Fe(4)
Si(21)-O(22)-Fe(2)
Si(21A)-O(3)-Fe(3)
Si(31)-O(32)-Fe(4)
119.54(18)
96.04(18)
142.88(16)
121.54(17)
80.38(16)
71.48(16)
136.31(19)
134.66(19)
88.80(17)
86.76(17)
90.40(17)
89.00(16)
119.00(2)
99.05(18)
109.80(18)
119.77(18)
99.27(19)
111.6(18)
111.3(2)
107.3(2)
113.6(2)
123.9(2)
118.7(2)
122.7(2)
127.5(3)
119.1(2)
133.9(2)
Fe(1)-O(31A)
Fe(1)-O(13)
Fe(1)-O(21)
Fe(2)-O(22)
Fe(2)-O(21)
Fe(2)-O(31A)
Fe(2)-O(2)
O(31A)-Si(3B)-O(33A)-Fe(3A)-O(3A)-Si(21)-
O(22); Fe(2)-O(22)-Si(21)-O(23)-Fe(4)-O(4)-Si(11)-
O(21)]] eight-membered rings, and one FeSiO2 [Fe(1)-
O(13)-Si(11)-O(21)] four-membered ring. Both of the
Fe4Si3O10 units are fused together to generate two new [Fe-
(4)-O(32)-Si(31)-O(33)-Fe(3)-O(13)-Si(11)-O(4); Fe-
(3A)-O(13A)-Si(1B)-O(4A)-Fe(4A)-O(32A)-Si(3B)-
O(33A)] eight-membered rings, which leads to the assembly
of the cage structure. An interesting feature of 3 is the
coordination environment around the iron centers. Thus, there
are three iron centers [Fe(1), Fe(3A), Fe(4)] with nearly
tetrahedral geometry and one Fe(2) with a trigonal bipyra-
midal geometry. Two of these iron atoms [Fe(3A), Fe(4)]
are surrounded by four oxygen atoms, whereas the remaining
two [Fe(1), Fe(2)] complete their coordination sphere by
accepting an oxygen atom each from the THF molecules.
Another interesting aspect of 3 is the presence of two
different kinds of oxygen environments. Thus, there are six
dicoordinate oxygen atoms that bridge Si and Fe atoms
[O(22), O(23), O(3A), O(33A), O(32A), O(4)]. The angles
at all these oxygen atoms are much less that 180°, the largest
being at O(32) (133.9°) and the smallest at O(23) (118.7°).
The remaining four tricoordinate oxygen atoms that cap each
one silicon and two iron atoms [O(31A), O(21), O(13)] or
cap three iron centers [O(24)].
The Si-O bond distances fall in a narrow range from
1.611(4) Å to 1.688(4) Å for Si(21)-O(3A) and Si(21)-
O(22). The Si-O bond lengths Si(11)-O(13), 1.659(4) Å
and Si(21)-O(22) 1.688(4) Å to which a hydrogen atom is
attached, are longer than the other two Si-O bond distances
around Si(11) [average 1.631(4) Å] and Si (21) [average
1.613(4) Å]. The latter Si-O distances are almost equal to
those observed in [(RSi(OH)O2)6Ti4(µ3-O)2]‚2THF.20 The
Fe-O bond distances range from 1.845(4) Å to 2.201(4) Å
for Fe(3)-O(33) and Fe(2)-O(24), while those involving
the µ oxygens are at the short end of the range 1.845(4) Å
to 1.954(4) Å for Fe(3)-O(33) and Fe(2)-O(22), and those
to the µ3 oxygens are the longest Fe(3)-O(13), 1.915(4) Å
and Fe(1)-O(21), 2.150(4) Å. The similar range of bond
distances is also observed for coordinated THF molecules
Fe(2)-O(24)
Fe(3)-O(33)
Fe(3)-O(3)
Fe(3)-O(24A)
Fe(3)-O(13)
Fe(4)-O(4)
Fe(4)-O(23)
Fe(4)-O(24)
Fe(4)-O(32)
Si(11)-O(4)
Si(11)-O(21)
Si(11)-O(13)
Si(21)-O(3A)
Si(21)-O(23)
Si(21)-O(22)
Si(31)-O(33)
Si(31)-O(32)
Si(31)-O(31)
spectrum of 3. An analogous OH group absorption was
observed in a titanium silicate.20
Structural Description of 3. Single crystals of 3 suitable
for X-ray structural analysis were obtained from a mixture
of toluene and THF at -26 °C over a period of 2 weeks.
Compound 3 crystallizes in the triclinic space group P1h,
along with six molecules of toluene in the asymmetric unit.
Selected metric parameters of 3 are given in Table 1. The
molecular structure of 3 is made up of an Fe8Si6O20 core,
with an inversion center (Figure 2). The core structure of 3
contains two symmetry-related tetranuclear iron(II) units.
Each of these Fe4Si3O10 units contains four iron centers, one
SiO3 subunit, two Si(OH)O2 subunits, and one (µ3-OH)
group. Thus, there are three Fe2SiO3 [Fe(2)-O(31A)-Si-
(3B)-O(33A)-Fe(3A)-O(24);
Fe(2)-O(22)-Si(21)-
O(3A)-Fe(3A)-O(24); Fe(2)-O(22)-Si(21)-O(23)-Fe-
(4)-O(24)] six-membered rings, two Fe2Si2O4 [Fe(2)-
(20) Voigt, A.; Murugavel, R.; Montero, M. L.; Wessel, H.; Liu, F.; Roesky,
H. W.; Uso´n, I.; Albers, T.; Parisini, E. Angew. Chem. 1997, 109,
1020-1022; Angew. Chem., Int. Ed. Engl. 1997, 36, 1001-1003.
Inorganic Chemistry, Vol. 44, No. 20, 2005 7247

Nehete et al.
[{(Me3Si)2NFe}2{LFe}2{O3SiR}2] (L ) 1,3-diisopropyl-4,5-
dimethylimidazol-2-ylidene) (1.891(2)-2.143(2) Å)].10
Magnetic Measurements of 2 and 3. Magnetic suscep-
tibility measurements of crystalline samples of 2 and 3 were
carried out in order to compare their behavior. Figure 3 shows
a plot of 1/ø versus temperature in the range of 2-300 K.
This indicates the paramagnetic behavior for both com-
pounds.
Conclusion
In summary, we report the synthesis and structural
characterization of two novel tetrameric cobalt and octameric
iron siloxanes, in which the cobalt and iron atoms are in
formal oxidation state of +II. Compounds 2 and 3 contain
the M/Si ratio of 4:4 and 8:6, respectively. A plot of 1/ø
versus temperature reveals the paramagnetic behavior for
both compounds. By taking an advantage of reactive Si-
OH units in these molecules, we are currently engaged in
the synthesis of new hetero-trimetallic compounds.
Figure 3. Plot of 1/ø (Oe mol/emu) vs T (K) for 2 and 3.
2.038(4) Å to 2.122(5) Å for Fe(1)-O(1) and Fe(2)-O(2).
As would be expected, the longest Fe-O bond distances are
to the capping (µ3-OH) hydroxide (Fe(3)-O(24A),
1.883(4) Å; Fe(2)-O(24), 2.201(4) Å. To the best of our
knowledge such a structural variation within a single
molecule is quite remarkable and unique. The metric
parameters of 3 are quite normal and are analogous to those
found in earlier examples {[(Me3Si)3SiOFe{µ-OSi(SiMe3)3}2-
Na(DME)] (1.894(2)-1.910(2) Å);12a [{(c-C5H9)7Si7O11-
(OSiMe3)}Fe(dcpe)] (1.866(2)-1.873(3) Å);12b [{(Me3-
Si)3SiO}2Fe(2,2′-dipyridyl)] (1.863(10)-1.899(10) Å);11c
Acknowledgment. We thank the Deutsche Forschungs-
gemeinschaft, the Go¨ttinger Akademie der Wissenschaften,
and the Fonds der Chemischen Industrie for funding.
Supporting Information Available: X-ray data (CIF) for 2 and
3. This material is available free of charge via Internet at
http://pubs.acs.org.
IC050859N
7248 Inorganic Chemistry, Vol. 44, No. 20, 2005