Heavy-metal-containing polyhedral metallasiloxane derived from an aminosilanetriol: Synthesis and structural characterization of [(PbO) 6(R2Si2O3)2] (R = (2,6.iPr2C6H3)N(SiMe3))

Article abstract of DOI:10.1021/om049417u

The reaction of RSi(OH)₃ with Pb[N(SiMe₃) ₂]₂ in a 1:1.5 molar ratio in THF/hexane afforded a hexameric lead(II)siloxane [(PbO)₆(R₂Si₂O ₃)₂][R = (2,6-iPr₂C₆H ₃)N(SiMe₃)] 1. The molecular structure of 1 shows a central (PbO)₆ motif enclosed by two outer R₂-Si ₂O₃ siloxane ligands.

Full text of DOI:10.1021/om049417u

5372
Organometallics 2004, 23, 5372-5374
Notes
Hea vy-Meta l-Con ta in in g P olyh ed r a l Meta lla siloxa n e
Der ived fr om a n Am in osila n etr iol: Syn th esis a n d
Str u ctu r a l Ch a r a cter iza tion of [(P bO)6(R2Si2O3)2] (R )
(2,6-iP r 2C6H3)N(SiMe3))†
Umesh N. Nehete,‡ Vadapalli Chandrasekhar,§ Vojtech J ancik,‡
Herbert W. Roesky,*,‡ and Regine Herbst-Irmer‡
Institut fu¨r Anorganische Chemie der Universita¨t Go¨ttingen, Tammannstrasse 4,
D-37077 Go¨ttingen, Germany, and Department of Chemistry, Indian Institute of Technology,
Kanpur-208016, India
Received J uly 28, 2004
Summary: The reaction of RSi(OH)3 with Pb[N(SiMe3)2]2
in a 1:1.5 molar ratio in THF/ hexane afforded a
hexameric lead(II)siloxane [(PbO)6(R2Si2O3)2] [R ) (2,6-
iPr2C6H3)N(SiMe3)] 1. The molecular structure of 1
shows a central (PbO)6 motif enclosed by two outer R2-
Si2O3 siloxane ligands.
zeolites such as TS-1 and TS-2,3 also are themselves
good catalysts, particularly for epoxidation reactions.4
A third and emerging interest in this class of compounds
is the possibility of stabilizing molecular inorganic oxide
fragments.5 Thus, we have recently been able to show
that molecular (SnO)6 could be trapped in a Sn(II)
siloxane.6 Despite the diverse and varied polyhedral
metallasiloxanes that have been synthesized from
silanetriols and monosilanols, the number of structur-
ally characterized metallasiloxanes containing heavy
main-group metals is very rare.7 It must be noted,
however, that uranium-containing silicon-rich metal-
lasiloxanes have been derived from incompletely con-
densed silsesquioxanes. However, the metal/silicon ratio
in these compounds is very low.8 We now report the
synthesis and structural characterization of the novel
hexalead assembly [(PbO)6(R2Si2O3)2] (1; R ) (2,6-
iPr2C6H3)N(SiMe3)). This compound has a Pb/Si ratio
of 6/4. Apart from being a unique example of a metal-
lasiloxane containing two fused cages, compound 1 also
can be envisaged as possessing an embedded internal
(PbO)6 molecular lead(II) oxide enveloped by an external
siloxane sheath.
In tr od u ction
We have a longstanding interest in polyhedral met-
allasiloxanes derived from silanetriols.1 This is due to
many reasons. First, utilizing the silanetriols as syn-
thons allows the preparation of soluble metallasiloxanes
with a high metal content.1 Second, many soluble
metallasiloxanes can function as structural models for
complex metallasilicates and metal-containing zeolites.2
We have, for example, shown that cubic titanasiloxanes
[RSiO3TiR1]4 (R ) (2,6-iPr2C6H3)N(SiMe3), R1 ) Et, iPr),
apart from being good models for titanium-containing
* To whom correspondence should be addressed. Fax: +49-551-
393373. E-mail: hroesky@gwdg.de.
† Dedicated to Professor Nils Wiberg on the occasion of his 70th
birthday.
‡ Universita¨t Go¨ttingen.
§ Indian Institute of Technology.
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10.1021/om049417u CCC: $27.50 © 2004 American Chemical Society
Publication on Web 09/30/2004

Notes
Organometallics, Vol. 23, No. 22, 2004 5373
F igu r e 1. Molecular structure of 1 in the crystal form.
Sch em e 1. Syn th esis of Com p ou n d 1
Resu lts a n d Discu ssion
The synthesis of compound 1 is accomplished in about
51% yield and involves a 1.5:1 reaction of the lead(II)
amide9 Pb[N(SiMe3)2]2 with the aminosilanetriol3a RSi-
(OH)3 (R ) (2,6-iPr2C6H3)N(SiMe3)) (Scheme 1). A
notable feature of the reaction is that the silanetriol
undergoes a self-condensation to generate the disilox-
anetetrol [(RSi(OH)2)2O], which further reacts with the
lead(II) amide. Such a condensation reaction has been
noted by us earlier.10
F igu r e 2. ORTEP core structure of 1. The substituents
on silicon have been omitted for the sake of clarity. Selected
bond lengths (Å) and angles (deg): Pb(1)-O(2) ) 2.155(3),
Pb(1)-O(5) ) 2.267(3), Pb(1)-O(6) ) 2.247(3), Pb(2)-O(5)
)
2.528(3), Pb(2)-O(6A) ) 2.397(3), Pb(2)-O(6) )
2.276(3), Pb(2)-O(1) ) 2.546(3), Pb(2A)-O(6) ) 2.397(3),
Pb(3)-O(1) ) 2.245(3), Pb(3)-O(4A) ) 2.163(3), Pb(3)-O(6)
) 2.242(3); O(2)-Pb(1)-O(6) ) 87.81(11), O(2)-Pb(1)-O(5)
) 91.87(10), O(6)-Pb(1)-O(5) ) 79.09(10), O(6)-Pb(2)-
O(6A) ) 77.28(10), O(6)-Pb(2)-O(5) ) 73.29(9), O(6A)-
Pb(2)-O(5) ) 101.59(9), O(6)-Pb(2)-O(1) ) 72.24(9),
Compound 1 is soluble in a large number of common
organic solvents, including hexane. It is thermally
stable, as evidenced by its high decomposition point of
259 °C. Further, compound 1 is stable under EI-mass
conditions and shows the parent peak at m/z 2541
(100%) [M+]. The 29Si NMR of 1 exhibits two resonances
(4.4 and -74.8 ppm). The latter corresponds to δ(SiO3N),
while the former is due to δ(SiMe3N). In the infrared
spectrum ν˜(Pb-O-Si) is preliminarily assigned to 881.8
O(6A)-Pb(2)-O(1)
)
99.31(9), O(5)-Pb(2)-O(1)
)
134.16(9), O(4A)-Pb(3)-O(6) ) 88.27(11), O(4A)-Pb(3)-
O(1) ) 93.22(10), O(6)-Pb(3)-O(1) ) 78.90(10).
Compound 1 crystallizes in the monoclinic space
group P21/n, along with four molecules of THF per
molecule. The molecular structure of 1 is given in Figure
1. The Pb6O12Si4 core is shown in Figure 2. Selected
metric parameters of 1 are summarized in Figure 2.
The molecular structure of 1 can be visualized in the
following way. The molecule contains two centrosym-
cm-1 7a
.
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2004-2009.
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Roesky, H. W.; Parisini, E.; Teichert, M.; Noltemeyer, M. Chem.
Commun. 1996, 2417-2418.

5374 Organometallics, Vol. 23, No. 22, 2004
Notes
spectrometers. Chemical shifts are reported in ppm with
reference to TMS. The upfield shifts are negative. IR spectra
were recorded 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.
metrically related Pb3O6Si2 units that are fused with
each other to generate a central diplumboxane motif
(Pb2O2). Each half of the molecule can be described as
possessing a drumlike cage structure. The top and
bottom of the drum are comprised of two puckered
Pb2O3Si six-membered rings, while the sides are made
up of three contiguous Pb2O2 four-membered rings. In
contrast, the recently reported6 tin(II) siloxane contains
two centrosymmetrically related bicyclic Si2Sn2O5 rings
that are connected to each other by the central four-
membered Sn2O2 ring, where each tin is three-coordi-
nate. Interestingly, this arrangement leads to an in-
complete cage on either side of the distannoxane motif.
An alternative way of viewing compound 1 is that it
contains a (PbO)6 motif, in the form of fused Pb2O2 rings,
which are enclosed within two R2Si2O3 ligands. While
the central lead centers Pb(2) and Pb(2A) are four-
coordinate (4O), the others are three-coordinate (3O).
Interestingly, in the solid-state structure of PbO the
coordination number of lead is 4 (4O), where lead
occupies the apex of a square pyramid.11
Three types of Pb-O distances are found in 1. The
shortest distances observed are for Pb(1)-O(2)
(2.155(3) Å) and Pb(3)-O(4A) bonds (2.163(3) Å). The
longest distances found are for Pb(2)-O(5) (2.528(3) Å)
and Pb(2)-O(1) bonds (2.546(3) Å). These metric pa-
rameters may be compared with those of other lead(II)
siloxanes: [Pb4(OSiPh3)6O], 2.25-2.49 Å;7a NaPb[OSi-
(OtBu)3]3, 2.12-2.15 Å;7b [Pb7(OSiMe3)10O2], 2.263(8)-
2.660(9) Å.7d
There are three different kinds of oxygens in the
structure of 1. Three of these are two-coordinate and
bridge either silicon and lead centers (O(2) and O(4))
or two silicon centers (O(3)). There are two three-
coordinate oxygens (O(1) and O(5)) that cap two lead
centers and one silicon center. The other oxygen (O(6))
is tetra-coordinate and bridges four lead centers. The
angles at all of these oxygens are much less than 180°.
Thus, the bond angles at oxygens that bridge silicon and
lead vary from 128.51(16)° for Si(3)-O(5)-Pb(1) to
109.41(15)° for Si(3)-O(5)-Pb(2). In contrast, the bond
angles that bridge two lead centers vary from
114.91(12)° for Pb(1)-O(6)-Pb(2A) to 97.79(10)° for
Pb(1)-O(5)-Pb(2).
Syn th esis of [(P bO)6(R2Si2O3)2] (1; R ) (2,6-iP r 2C6H3)N-
(SiMe3)). Pb[N(SiMe3)2]2 (2.42 g, 4.59 mmol) was slowly added
to a stirred suspension of the silanetriol (1.0 g, 3.06 mmol) in
hexane (25 mL) and THF (7 mL). After the addition was
complete, the reaction mixture was stirred for 1 h at room
temperature. Subsequently, the reaction mixture was refluxed
for 1 h. The volatile components were removed to obtain a
white solid. To this was added a mixture of hexane (10 mL)
and THF (1 mL). The pale yellow crystals of 1 were obtained
by slow cooling of its saturated solution after 4 days at room
temperature (yield 0.99 g, 51%). Mp: 259 °C dec. 1H NMR (500
MHz, C6D6, TMS): δ 0.28 (s, 36H, Si(CH3)3), 1.41, 1.43 (d, 48H,
CH(CH3)2), 3.56 (sept, 8H, CH(CH3)2), 7.6 (m, 12H, aromatic).
29Si NMR (99 MHz, C6D6, TMS): δ 4.41 (SiMe3), -74.8 (SiO3).
IR (Nujol): ν˜ 1439 (m), 1357 (m), 1322 (m), 1257 (s), 1244 (s),
1185 (m), 1100 (m), 1044 (s), 1019 (m), 936 (s), 909 (s), 881
(m), 835 (s), 800 (s), 757 (w), 721 (w), 684 (w), 600 (w), 556
(w), 546 (w), 512 (m), 465 (m), 439 (m) cm-1. MS (70 eV; m/z
(%)): 2541 (100) [M+]. Anal. Calcd for C60H104N4O12Pb6Si8
(2541.4): C, 28.36; H, 4.12; N, 2.20. Found: C, 28.85; H, 4.31;
N, 2.14.
Cr ysta l Da ta for 1‚4C4H8O: C76H136N4O16Pb6Si8, Mr
)
2829.75, monoclinic, space group P21/n, a ) 18.664(1) Å, b )
12.693(1) Å, c ) 20.651(1) Å, R ) γ ) 90°, â ) 97.19(1)°, V )
4854 (1) Å3, Z ) 2, Fcalcd ) 1.936 Mg/m3, F(000) ) 2704, T )
100(2) K, absorption coefficient 21.222 mm-1. The data were
collected using the ω-scan mode in the ranges of -20 e h e
20, -14 e k e 13, and -22 e l e 22. Of 21 054 reflections
collected, there were 6771 independent reflections (R(int) )
0.0300). Final R1 (I > 2σ(I)) ) 0.0205; wR2 (all data) ) 0.0485.
Maximum and minimum heights in the final Fourier difference
map were 0.723 and -0.774 e Å-3. The pale yellow single
crystal suitable for X-ray diffraction studies of the compound
1‚4C4H8O were obtained from hexane/THF at room tempera-
ture. Diffraction data were collected on a IPDS II Stoe image-
plate diffractometer with graphite-monochromated Mo KR
radiation (λ ) 0.710 73 Å). The structure was solved by direct
methods (SHELX-97)12 and refined against F2 on all data by
full-matrix least squares with SHELX-97.13 The heavy atoms
were refined anisotropically. Hydrogen atoms were included
using the riding model with Uiso tied to the Uiso values of the
parent atoms.
Ack n ow led gm en t. This work was supported by the
Deutsche Forschungsgemeinschaft and the Go¨ttinger
Akademie der Wissenschaften. V.C. thanks the Alex-
ander von Humboldt Foundation, Bonn, Germany, for
a Friedrich Wilhelm Bessel-Research award.
Exp er im en ta l Section
Gen er a l In for m a tion a n d Ma ter ia ls. All experimental
manipulations were carried out under a dry nitrogen atmo-
sphere, rigorously excluding air and moisture. The samples
for spectral measurements were prepared in a drybox. Solvents
were purified according to conventional procedures and were
freshly distilled prior to use. Silanetriol and Pb[N(SiMe3)2]2
were prepared according to the published procedures. Elemen-
tal 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
Su ppor tin g In for m ation Available: Tables giving single-
crystal X-ray structure data of compound 1; data are also
available as a CIF file. This material is available free of charge
via the Internet at http://pubs.acs.org.
OM049417U
(12) Sheldrick, G. M. Acta Crystallogr., Sect. A 1990, 46, 467-473.
(13) Sheldrick, G. M. SHELX-97: Program for Crystal Structure
Refinement; University of Go¨ttingen, Go¨ttingen, Germany, 1997.
(11) Greenwood, N. N.; Earnshaw, A. Chemistry of the Elements,
1st ed., Pergamon Press: Oxford, U.K., 1984.