Synthesis of a stable four-membered Si2O2 ring and a dimer with two four-membered Si2O2 rings bridged by two oxygen atoms, with five-coordinate silicon atoms in both ring systems

Article abstract of DOI:10.1021/om100164u

The base-stabilized bis-silylene (LSi-SiL, L = PhC(NtBu)₂) was reacted with benzophenone in a 1:2 ratio in THF, which afforded a Si ₂O₂ four-membered ring, stabilized by bulky amidinato ligands. The most striking phenomenon is the abstraction of oxygen from benzophenone and the simultaneous formation of a silicon carbon bond. The four-membered Si₂O₂ ring is planar, and both the silicon atoms are five-coordinate. The two silicon atoms are arranged opposite each other in the four-membered Si₂O₂ ring. Moreover LSi-SiL was treated with N₂O to afford two four-membered Si₂O ₂ rings connected with two oxygen atoms. In this structure also the silicon atoms are five-coordinate.

Full text of DOI:10.1021/om100164u

Organometallics 2010, 29, 2343–2347 2343
DOI: 10.1021/om100164u
Synthesis of a Stable Four-Membered Si2O2 Ring and a Dimer with
Two Four-Membered Si2O2 Rings Bridged by Two Oxygen Atoms,
with Five-Coordinate Silicon Atoms in Both Ring Systems
ꢀ
Sakya S. Sen, Gasper Tavcar, Herbert W. Roesky,* Daniel Kratzert,
ꢀ
Jakob Hey, and Dietmar Stalke
€
Institut fu€r Anorganische Chemie der Universitat Gottingen, Tammannstrasse 4, 37077 Gottingen, Germany
€
€
Received March 1, 2010
The base-stabilized bis-silylene (LSi-SiL, L=PhC(NtBu)2) was reacted with benzophenone in a
1:2 ratio in THF, which afforded a Si2O2 four-membered ring, stabilized by bulky amidinato ligands.
The most striking phenomenon is the abstraction of oxygen from benzophenone and the simulta-
neous formation of a silicon carbon bond. The four-membered Si2O2 ring is planar, and both the
silicon atoms are five-coordinate. The two silicon atoms are arranged opposite each other in the four-
membered Si2O2 ring. Moreover LSi-SiL was treated with N2O to afford two four-membered Si2O2
rings connected with two oxygen atoms. In this structure also the silicon atoms are five-coordinate.
Introduction
acid). After that a series of ab initio calculations have been
done on the four-membered cyclodisiloxane, and to date
most of the theoretical studies conclude that there is no Si-Si
bond.5 The resulting close proximity between the tetravalent
silicon atoms in the Si2O2 ring is due to strong repulsion
between the two highly negatively charged oxygen atoms and
geometric constraint by the four-membered ring.5 Taking
this into account, it is quite interesting to synthesize a com-
pound where two O atoms are connected with two Si atoms
with the support of bulky ligands. It can also be argued that
the fragment of such a cyclic compound (R2SiO) would be a
key monomer in silicon chemistry and one step toward the
isolation of room-temperature stable silanone (R2SidO)
(Kipping’s dream),6 which is still elusive. Moreover the cyclic
siloxanes (R2SiO)n have great commercial importance as pre-
cursors for high molecular weight silicones.7
Interest has increased over the last decades in the chemis-
try of strained ring systems containing silicon atoms. While
many routes to silacyclopropanes and silacyclopropene have
been investigated,1 there are only a few reports on the synthe-
sis of small ring compounds containing silicon and oxygen.2
Although the four-membered ring structure has previously
been suggested for a few silicon-oxygen containing com-
pounds,3 definite evidence for its existence was obtained when
West et al. reported the four-membered cyclodisiloxane.4
It was prepared by the reaction of disilene (R2SidSiR2,
R=Me3C6H2) with triplet oxygen and also by the reaction
of disilaoxirane with m-CPBA (meta-chloroperoxybenzoic
*To whom correspondence should be addressed. E mail: hroesky@
gwdg.de.
(1) (a) Atwell, W. H.; Weyenberg, D. R. J. Am. Chem. Soc. 1968, 90,
3438–3443. (b) Barton, T. J.; Kilgour, J. A. J. Am. Chem. Soc. 1976, 98,
7746–7752.
(2) (a) Ando, W.; Ikeno, M.; Sekiguchi, A. J. Am. Chem. Soc. 1977,
99, 6447–6449. (b) Ando, W.; Ikeno, M.; Sekiguchi, A. J. Am. Chem. Soc.
1978, 100, 3613–3615. (c) Ando, W.; Hamada, Y.; Sekiguchi, A.; Ueno, K.
Tetrahedron Lett. 1982, 23, 5323–5326.
(3) (a) Schwartz, R.; Kuchen, W. Z. Anorg. Allg. Chem. 1955, 279, 84–
93. (b) Sharp, R. R.; Margrave, J. L. J. Inorg. Nucl. Chem. 1971, 33, 2813–
2818. (c) Purakayastha, A.; Baruah, J. B. Appl. Organomet. Chem. 2004, 18,
166–175.
(4) (a) Fink, M. J.; De Young, D. J.; West, R.; Michl, J. J. Am. Chem.
Soc. 1983, 105, 1070–1071. (b) Fink, M. J.; Haller, K. J.; West, R.; Michl, J.
J. Am. Chem. Soc. 1984, 106, 822–823. (c) Michalczyk, M. J.; Fink, M. J.;
Haller, K. J.; West, R.; Michl, J. Organometallics 1986, 5, 531–536.
(d) Sohn, H.; Tan, R. P.; Powell, D. R.; West, R. Organometallics 1994,
13, 1390–1394. (e) Iwamoto, T.; Masuda, H.; Ishida, S.; Kabuto, C.; Kira, M.
J. Am. Chem. Soc. 2003, 125, 9300–9301.
Now it is frequently experienced that the utilization of
compounds with low valent silicon is always an alternative strategy
to prepare silicon containing ring compounds.8 Recently we were
successful in isolating a compound consisting of a Si-Si
(6) (a) Kipping, F. S.; Lloyd, L. L. J. Chem. Soc., Trans. 1901, 449–459.
(b) Greenwood, N. N.; Earnshaw, A. Chemistry of the Elements, 2nd ed.;
Butterworth-Heinemann:Oxford, p 362. (c) Cory, J. Y. Historicaloverviewand
comparison of silicon with carbon. In The Chemistry of Organic Silicon
Compounds; Patai, S., Rappoport, Z., Eds.; Wiley: New York, 1989; pp 1-56.
(7) Noll, W. Chemistry and Technology of Silicones; Academic Press:
New York, 1968.
(8) (a) Ichinohe, M.; Igarashi, M.; Sanuki, K.; Sekiguchi, A. J. Am.
Chem. Soc. 2005, 127, 9978–9979. (b) Lee, V. Y.; Yasuda, H.; Sekiguchi, A.
J. Am. Chem. Soc. 2007, 129, 2436–2437. (c) Lee, V. Y.; Miyazaki, S.;
Yasuda, H.; Sekiguchi, A. J. Am. Chem. Soc. 2008, 130, 2758–2759.
(d) Takeuchi, K.; Ichinohe, M.; Sekiguchi, A.; Guo, J.-D.; Nagase, S.
Organonmetallics 2009, 28, 2658–2660. (e) Xiong, Y.; Yao, S.; Brym, M.;
Driess, M. Angew. Chem. 2007, 119, 4595–4597. Angew. Chem., Int. Ed.
2007, 46, 4511-4513. (f) Yao, S.; van W€ullen, C.; Sun, X.-Y.; Driess, M.
Angew. Chem. 2008, 120, 3294–3297. Angew. Chem., Int. Ed. 2008, 47,
3250-3253. (g) Nagendran, S.; Roesky, H. W. Organometallics 2008, 27,
457–492. (h) Mizuhata, Y.; Sasamori, T.; Tokitoh, N. Chem. Rev. 2009, 109,
3479–3511. (i) Yao, S.; Xiong, Y.; van Wu€llen, C.; Driess, M. Organome-
tallics 2009, 28, 1610–1612. (j) Xiong, Y.; Yao, S.; Driess, M. Chem.;Eur.
J. 2009, 15, 5545–5551. (k) Sen, S. S.; Roesky, H. W.; Stern, D.; Henn, J.;
Stalke, D. J. Am. Chem. Soc. 2010, 132, 1123–1126.
(5) (a) Bachrach, S. M.; Streitwieser, A., Jr. J. Am. Chem. Soc. 1985,
107, 1186–1190. (b) O' Keeffe, M.; Gibbs, G. V. J. Phys. Chem. 1985, 89,
4774–4577. (c) Grev, R. S.; Schaefer, H. F., III. J. Am. Chem. Soc. 1987,
ꢁ
ꢁ
109, 6577–6585. (d) Somogyi, A.; Tamas, J. J. Phys. Chem. 1990, 94, 5554–
5556. (e) Kudo, T.; Nagase, S. J. Am. Chem. Soc. 1985, 107, 2589–2595.
(f) Bacharach, S. M.; Streitwieser, A. J. Am. Chem. Soc. 1985, 107, 1186–
1190. (g) Lu, W. C.; Wang, C. Z.; Nguyen, V.; Schmidt, M. W.; Gordon,
M. S.; Ho, K. M. J. Phys. Chem. A 2003, 107, 6936–6943. (h) Kimura, M.;
Nagase, S. Chem. Lett. 2001, 1098–1099.
r
2010 American Chemical Society
Published on Web 04/22/2010
pubs.acs.org/Organometallics

2344 Organometallics, Vol. 29, No. 10, 2010
Sen et al.
Scheme 1. Preparation of 2
Scheme 2. Suggested Mechanism for the Formation of 2
single bond and a lone pair of electrons on each Si atom
([PhC(NtBu)2]2Si2) (1).9 The formal oxidation state of each
silicon atom in 1 is þ1, or according to the suggestion of
Frenking et al., compound 1 contains three-valent silicon
atoms of formal oxidation state þ1.10 Anyhow this is a new
entry to the low-valent low-coordinate silicon compound
and may serve as a building block in organosilicon chemistry.
A further impetus came from the theoretical calculation,11
which shows that 1 has two stereoactive lone pairs that prefer
to remain nonbonded in each Si center and the Si-Si bond
does not have multiple-bond character. As a result of this
unprecedented electronic structure, we were interested in
studying the chemistry of 1 with ketone. The question arises
whether it is possible to obtain a [1þ2] cycloaddition product
or a cyclodisiloxane derivative from the reaction of 1 with
benzophenone.
highest relative intensity in the EI-MS spectrum at m/z 884.
Compound 2 is very well soluble in ether, THF, and toluene and
partially soluble in n-hexane. It is stable in the solid state and in
solution at room temperature under an inert atmosphere.
The mechanism for the formation of 2 is not clear at this
moment, and intermediates could not be observed, but we
propose that a [2þ1] cycloaddition occurs as an initial step to
give the respective three-membered SiCO cycles, which sub-
sequently rearrange to the final product under hydrogen
abstraction from THF (Scheme 2).13a To prove the hydrogen
abstraction from the solvent unambiguously, we performed a
NMR-scale reaction in THF-d8, and we did not find any
resonance for the CH proton. Furthermore a hydrogen
abstraction from etheral solvent was also observed by
Robinson et al. during the synthesis of diborane and diborene
compounds.13b The cleavage of the Si-Si bond is not un-
expected because So et al. showed recently that 1 is cleaved
with Br2 under formation of a heteroleptic bromo silylene.11
But the most striking phenomenon is the removal of oxygen
from ketone under formation of a Si-O-Si bond (CdO bond
energy ∼178 kcal/mol).14 Moreover, it is also worth mention-
ing that during the reaction the formal oxidation state of Si
has been changed from þ1 to þ4. There is no comparable
example of oxidative addition at silicon so far known.
Results and Discussion
LSi(I)-Si(I)L was treated with benzophenone in THF at
ambient temperature under stirring overnight. After removal
of the solvent under vacuum, n-hexane was added and the
reaction mixture was stirred for 12 h. After that the solution
was concentrated and kept for crystallization, which affords
colorless crystals of 2 suitable for X-ray crystallography
(Scheme 1). The structure of 2 was confirmed by NMR spectro-
scopy, EI-mass spectrometry, and elemental analysis. The
1H NMR spectrum exhibits a resonance (δ 1.27 ppm) that
corresponds to the tBu protons. A resonance at δ 4.1 ppm
indicates the two CHprotons. The 29Si NMR spectrum shows a
resonance at δ -86.25 ppm, which is comparable with that of
the reported five-coordinate silicon compound12 and differs
distinctively from West’s four-coordinate cyclodisiloxane
(δ -22.02 ppm).4c The molecular ion is observed with the
The molecular structure of 2 2 toluene is shown in Figure 1.15
3
Compound 2 2 toluene crystallizes in the monoclinic space
3
group P21/n (Table 1). The structure consists of a rectangular
cyclodisiloxane ring orthogonal to a slightly distorted planar
skeleton containing the silicons and their pendant nitrogens.
The two independent Si-O bond lengths are very close to each
other (167.91(11) and 171.82(10) pm) and are slightly larger than
the normal bond lengths found for other cyclic siloxanes.4b-d A
2-fold symmetry axis passes through the centroid of the siloxane
ring, which is almost planar (sum of the internal angles 360.02°).
The endocyclic Si-O-Si bond angle in the four-membered
ring is 95.08°, which is very close to the Si-O-Si bond angle
(95.8°) in the tBu4Si2O2 ring reported by West et al.4c The
(9) Sen, S. S.; Jana, A.; Roesky, H. W.; Schulzke, C. Angew. Chem.
2009, 121, 8688–8690. Angew. Chem., Int. Ed. 2009, 48, 8536-8538.
(10) Takagi, N.; Shimizu, T.; Frenking, G. Chem.;Eur. J. 2009, 15,
8593–8604.
(11) Yeong, H. -X.; Lau, K. -C.; Xi, H. -W.; Lim, K. H.; So, C.-W.
(13) (a) Malatesta, V.; Ingold, K. U. J. Am. Chem. Soc. 1981, 103,
609–614. (b) Wang, Y.; Quillian, B.; Wei, P.; Wannere, C. S.; Xie, Y.; King,
R. B.; Schaefer, H. F., III; Schleyer, P. v. R.; Robinson, G. H. J. Am. Chem.
Soc. 2007, 129, 12412–12413.
(14) Sanderson, R. T. Chemical Bonds and Bond Energy; Academic
Press: New York, 1976.
Inorg. Chem. 2010, 49, 371–373.
(12) (a) Ghadwal, R. S.; Roesky, H. W.; Merkel, S.; Henn, J.; Stalke,
D. Angew. Chem. 2009, 121, 5793–5796. Angew. Chem., Int. Ed. 2009, 48,
5683-5686. (b) Ando, W.; Shiba, T.; Hidaka, T.; Morihashi, K.; Kikuchi, O.
J. Am. Chem. Soc. 1997, 119, 3629–3630. (c) Ghadwal, R. S.; Sen, S. S.;
Roesky, H. W.; Tavcar, G.; Merkel, S.; Stalke, D. Organometallics 2009, 28,
6374–6377.
(15) (a) Kottke, T.; Stalke, D. J. Appl. Crystallogr. 1993, 26, 615–619.
(b) Stalke, D. Chem. Soc. Rev. 1998, 27, 171–178.

Article
Organometallics, Vol. 29, No. 10, 2010 2345
Scheme 3. Preparation of 3
longer compared to that in 1 (241 pm) and significantly longer
than West’s 1,10,3,30-tetramesitylcyclodisiloxane (231 pm).4c
From these data it can be assumed that there is no bond be-
tween the two Si atoms. This assumption is also supported by
MNDO calculations on the parent cyclodisiloxane, H4Si2O2,4b
which also provided no evidence for bonding between silicon
atoms. In agreement with this, recent ab initio calculations on
H4Si2O2 indicate that the cyclodisiloxane is best described as
containing four equivalent localized Si-O bonds with no app-
reciable σ-bonding between the silicon atoms.5e,f The calculated
Si-O-Si bond angle for H4Si2O2 is 91.5°, similar to that found
Figure 1. Crystal structure of 2 2 toluene. Hydrogen atoms,
3
methyl groups at C(8) and C(12), and two toluene molecules are
not shown for clarity. Anisotropic displacement parameters are
depicted at the 50% probability level. Selected bond distances
(pm) and bond angles (deg): Si(1)-O(1A) 167.91(11), Si(1)-
O(1) 171.82(10), Si(1)-N(2) 182.98(13), Si(1)-C(16) 193.17(15),
Si(1)-N(1) 197.47(12), O(1)-Si(1A) 167.90(11); O(1A)-Si(1)-
O(1) 84.93(5), O(1A)-Si(1)-N(2) 123.96(5), O(1)-Si(1)-N(2)
100.03(5), O(1A)-Si(1)-C(16) 117.07(6), O(1)-Si(1)-C(16)
101.77(6), N(2)-Si(1)-C(16) 116.28(6), O(1A)-Si(1)-N(1)
90.65(5), O(1)-Si(1)-N(1) 162.41(5), N(2)-Si(1)-N(1) 68.51(5),
C(16)-Si(1)-N(1) 95.41(6), O(1A)-Si(1)-Si(1A) 43.07(3),
O(1)-Si(1)-Si(1A) 41.86(3), N(2)-Si(1)-Si(1A) 119.58(4).
for 2, while the calculated Si Si bond length (239 pm) is
3 3 3
reasonably shorter than those in 2. A possible qualitative
explanation for the marked differences in bond length observed
in 2 and West’s 1,10,3,30-tetramesitylcyclodisiloxane is as fol-
lows: The 3p orbitals of π-symmetry on silicon have low
electron density compared to the 2p orbitals on oxygen, so that
the diagonal antibonding interactions between oxygens are
stronger than between the silicon atoms, leading to the diamond-
shaped distortion found for West’s disiloxane,4c whereas in 2,
due to the presence of four nitrogen atoms, the electron density
on silicon atoms is shifted toward the amidinato ligand, causing
a longer Si-Si bond length. Here it is also worth point-
ing out that recently Driess and co-workers reported the forma-
tion of cyclodisiloxane, where the two silicon atoms, however,
have different coordination numbers; one silicon atom is five-
and the other one is four-coordinate,17a whereas in 2 both the Si
centers are five-coordinate.18
Table 1. Crystallographic and Structure Refinement
Data at 100.0(2) K
parameter
2 2 toluene
3
3 toluene
3
empirical formula
fw
C
70H84N4O2Si2 C67H100N8O6Si4
1225.91
P21/n
1069.59
P21/n
space group
˚
a (A)
12.4840(17)
11.4379(16)
21.383(3)
100.954(3)
2997.6(7)
2
1.185
1152
0.108
1.94 to 27.10
10.7264(16)
28.415(4)
23.012(3)
101.979(2)
6861.2(18)
4
1.187
2648
0.141
1.15 to 26.37
162 534/14 016
14 016/219/861
1.019
˚
b (A)
˚
c (A)
β (deg)
After being successful in the preparation of the cyclodi-
siloxane derivative by treating 1 with benzophenone, we turned
our focus toward N2O. The latter is well-known to serve as a
mono-oxygen donor. Recently Driess et al. reported the reaction
of N2O with siloxy silylene HSi(L0)-O-(L)Si and NHC-supported
silylene NHCfL0Si [L0 = CH[(CdCH2)CMe(NAr)2], L =
CH(CMeNAr)2, Ar = 2,6-iPr2-C6H3],17 respectively. In the
first case a silanoic silylester resulted, whereas in the second
case a NHC-supported silanone was isolated. Power and co-
workers recently described the complete cleavage of the Cr-Cr
quintiple bond, and instead a Cr2O2 ring was formed when
Ar0CrCrAr0 (Ar0 =C6H3-2,6-(2,6-iPr2-C6H3)2)19 was reacted
with N2O. Following this we also demonstrated the conver-
sion of LGeH to LGeOH (L=CH(CMeNAr)2, Ar=2,6-iPr2-
C6H3) quantitatively by using N2O as a mono-oxygen source.20
3
V (A )
˚
Z
F
calcd (g/cm3)
F(000)
μ (mm-1
)
θ range for data collection (deg)
reflns collected/unique [I>2σ(I)] 60 807/6607
data/restraints/params
Goof on F2
6607/1/362
1.021
R1, wR2 [I>2σ(I)]
R1, wR2 (all data)
largest diff peak/hole (e A
0.0405, 0.0918
0.0569, 0.0999
0.322/-0.301
0.0422, 0.0940
0.0635, 0.1053
0.261/-0.337
-3
˚
)
amidinate ligands and Ph2CH groups are disposed above and
below the Si2O2 ring in such a way that the Si centers exhibit
trigonal-bipyramidal coordination sites.16 To explain the axial
and equatorial arrangement, we selected Si(1), N(2), O(1A),
and O(1), which reside in the equatorial positions, whereas
C(16) and N(1) occupy the axial positions of the trigonal-
bipyramidal geometry. Another striking feature of this struc-
ture is the Si-Si distance of 250.63(8) pm, which is 6.38% longer
than the normal Si-Si σ-bond distance (235 pm) and 3.73%
(17) (a) Yao, S.; Xiong, Y.; Brym, M.; Driess, M. J. Am. Chem. Soc.
2007, 129, 7268–7269. (b) Xiong, Y.; Yao, S.; Driess, M. J. Am. Chem. Soc.
2009, 131, 7562–7563.
(18) The bonding in high-coordinate silicon complexes is elucidated
in: Kocher, N.; Henn, J.; Gostevskii, B.; Kost, D.; Kalikhman, I.;
Engels, B.; Stalke, D. J. Am. Chem. Soc. 2004, 126, 5563–5568.
(19) Ni, C.; Ellis, B. D.; Long, G. J.; Power, P. P. Chem Commun.
2009, 2332–2334.
(16) (a) Stalke, D.; Wedler, M.; Edelmann, F. T. J. Organomet. Chem.
€
1992, 431, C1–C5. (b) Wedler, M.; Knosel, F.; Pieper, U.; Stalke, D.;
(20) Jana, A.; Roesky, H. W.; Schulzke, C. Dalton Trans. 2010, 39,
132–138.
Edelmann, F. T.; Amberger, H.-D. Chem. Ber. 1992, 125, 2171–2181.

2346 Organometallics, Vol. 29, No. 10, 2010
Sen et al.
Scheme 4. Suggested Mechanism for the formation of 3
All these reactions instigated us to probe the reaction of 1 with
N2O. Exposure of a red solution of 1 in toluene to N2O at room
temperature led to rapid decoloration and formation of a
colorless compound. Recrystallization of the latter in a toluene/
n-hexane solution at room temperature furnished colorless
crystals of 3 suitable for single-crystal X-ray diffraction
(Scheme 3). To our surprise, 3 contains two four-membered
disiloxane rings, which are bridged by two oxygen atoms. This
is a new class of compounds that has not been reported so far.
3 was fully characterized by single-crystal X-ray diffraction,
NMR spectroscopy, EI-MS spectrometry, and elemental analysis.
In the 1H NMR spectrum two types of resonances were observed,
one for tBu protons and another for the phenyl rings. In the 29Si
NMR spectrum a sharp resonance was outlined (δ -111.02 ppm)
that corresponds to the five-coordinate silicon atom12 and differs
distinctly from that of 2 (δ -86.25 ppm) and Driess’ cyclo-
disiloxane (δ -60.7 and -119.2 ppm).17a The molecular ion is
observed with the highest relative intensity in the EI-MS spectrum
at m/z 1132. All the silicon atoms in 3 are five-coordinate.
Figure 2. Crystal structure of 3 toluene. Hydrogen atoms as
3
well as methyl groups and the toluene molecule are omitted for
clarity. Selected bond distances (pm) and bond angles (deg) with
standard deviations: O(1)-Si(3) 164.69(13), O(1)-Si(4) 165.48(13),
O(2)-Si(1) 164.55(13), O(2)-Si(2) 164.94(13), O(3)-Si(2)
166.50(13), O(3)-Si(4) 172.82(13), O(4)-Si(4) 165.77(13), O(4)-
Si(2) 172.41(13), O(5)-Si(1) 166.33(13), O(5)-Si(3) 172.04(13),
O(6)-Si(3) 165.90(13), O(6)-Si(1) 172.52(13), N(1)-Si(1)
197.10(16), N(2)-Si(1) 185.24(16), N(3)-Si(2) 198.55(16), N(4)-
Si(2) 183.97(16), N(5)-Si(3) 201.91(16), N(6)-Si(3) 183.77(16),
N(7)-Si(4) 198.57(16), N(8)-Si(4) 184.80(16); Si(3)-O(1)-Si(4)
129.96(8), Si(1)-O(2)-Si(2) 130.94(8), Si(2)-O(3)-Si(4) 94.28(6),
Si(4)-O(4)-Si(2) 94.70(6), Si(1)-O(5)-Si(3) 94.28(6), Si(3)-
O(6)-Si(1) 94.26(6), O(2)-Si(1)-O(5) 117.48(7), O(2)-Si(1)-
O(6) 102.92(6), O(5)-Si(1)-O(6) 85.25(6), O(2)-Si(1)-N(2)
113.48(7), O(5)-Si(1)-N(2) 127.11(7), O(6)-Si(1)-N(2) 96.67
(7), O(2)-Si(1)-N(1) 96.71(7), O(5)-Si(1)-N(1) 92.40(7), O(6)-
Si(1)-N(1) 158.96(7), N(2)-Si(1)-N(1) 68.21(7), O(2)-Si(2)-
O(3) 117.04(7), O(2)-Si(2)-O(4) 101.85(6), O(3)-Si(2)-O(4)
85.15(6), O(2)-Si(2)-N(4) 116.65(7), O(3)-Si(2)-N(4) 124.52(7),
O(4)-Si(2)-N(4) 96.82(7), O(2)-Si(2)-N(3) 96.58(6).
Like 2, the mechanism for the formation of 3 is unknown,
but it can be suggested that the Si-Si bond is cleaved under
insertion of an oxygen atom from N2O. Then each of the lone
pairs of electrons at silicon react with N2O to give the res-
pective SidO, which in situ dimerizes to afford 3 due to the
highly polar nature of Siδ+dOδ- (Scheme 4).5h
The molecular structure of 3 toluene was unequivocally
3
established by single-crystal X-ray diffraction (Figure 2).15
3 toluene crystallizes in the monoclinic space group P21/n
3
(Table 1). Selected bond lengths and bond angles are given in
the legend of Figure 2. The most important feature of
3 toluene is that two four-membered rings are connected by
3
two oxygen atoms. The ring actually consists of two four-
membered disiloxane rings that are parallel to each other and
connected by a bridging oxygen atom. The amidinato ligands
are each arranged orthogonally to the four-membered disilo-
xane ring. All the silicon atoms are five-coordinate and exhibit
a trigonal-bipyramidal geometry. Three sites of each silicon
atom are occupied by the three oxygen atoms, whereas the
nitrogen atoms from the amidinato ligand occupy the remain-
ing two coordination sites. To explain the axial and equatorial
arrangement of the silicon atoms, we chose one silicon atom in
the ring designated as Si(4). From the bond length and angle
data it is revealed that N(7) and O(3) reside in the axial
positions and O(1), O(4), and N(8) occupy the equatorial
positions. The sum of angles between the equatorial atoms
and Si(4) is 357.9°, and the two axial atoms include an angle of
159.5° with Si(4). The arrangement holds true for all the
silicon atoms in the ring where one oxygen atom of the
disiloxane moiety and one nitrogen atom from the amidinato
ligand occupy axial positions and two oxygen atoms and
another nitrogen atom from the ligand occupy equatorial
positions. It is noticeable that the Si-N as well as the Si-O
bond lengths differ significantly depending on whether equa-
torial or axial positions are occupied. The Si-N distances
range from 197.1 to 201.9 pm for axial nitrogen atoms while
the equatorial Si-N bond lengths are between 183.8 and
185.2 pm and thus significantly shorter. The Si-O bond
lengths in the disiloxane rings exhibit a similar behavior, with
axial Si-O bond lengths ranging from 172.0 to 172.8 pm and
shorter equatorial Si-O bond lengths ranging from 165.8 to
166.5 pm. All these Si-O bond lengths are quite similar to
those found for 2 and HSi(L0)-O-(L)Si.17a
Conclusion
We have prepared a four-membered Si2O2 ring with five-
coordinate silicon atoms. Compound 2 is formed by oxida-
tive addition of benzophenone under cleavage of the ketone
bond and hydrogen abstraction from THF. Furthermore we
reacted 1 with N2O to afford compound 3 with two four-
membered rings bridged by two oxygen atoms. The compo-
sition and constitution of 2 and 3 have been supported by
NMR spectroscopy, EI-MS spectrometry, and single-crystal
X-ray diffraction.
Experimental Section
All manipulations were carried out in an inert atmosphere of
dinitrogen using standard Schlenk techniques and in a dinitro-
gen-filled glovebox. Solvents were purified by MBRAUN sol-
vent purification system MB SPS-800. 1 was prepared by litera-
ture methods.9 All chemicals purchased from Aldrich were used

Article
Organometallics, Vol. 29, No. 10, 2010 2347
1
without further purification. H, 13C, and 29Si NMR spectra
were recorded with Bruker Avance DPX 200 or Bruker Avance
(NCN). 29Si{1H} NMR (99.36 MHz, C6D6, 25 °C): δ -111.02.
EI-MS: m/z 1132 [M+] (100%).
1
DRX 500 spectrometers. The H, 13C, and 29Si NMR spectra
Crystal Structure Determination. Shock-cooled crystals were
selected and mounted under a nitrogen atmosphere using the
were recorded in C6D6. The chemical shifts δ are given relative to
SiMe4. EI-MS spectra were obtained using a Finnigan MAT
8230 instrument. Elemental analyses were performed by the
€
Institut fur Anorganische Chemie, Universitat Gottingen. Melt-
ing point was measured in a sealed glass tube on a Buchi B-540
X-TEMP.15 The data for 3 toluene were collected at 100(2) K
3
on a INCOATEC Mo Microsource21 with Quazar mirror optics
and an APEX II detector on a D8 goniometer. The data of
2 2toluene were measured on a Bruker TXS-Mo rotating anode
€
€
€
3
melting point apparatus.
with Helios mirror optics and an APEX II detector on a D8
goniometer. Both diffractometers were equipped with a low-
temperature device and used Mo KR radiation, λ=71.073 pm.
The data of 2 2toluene and 3 toluene were integrated with
Preparation of 2. THF (20 mL) was added to a mixture of 1
(0.5 g, 0.96 mmol) and benzophenone (0.35 g, 1.92 mmol) at
ambient temperature. The mixture was stirred overnight. The
volatiles were removed in vacuo, and n-hexane (20 mL) was
added to the residue. The reaction mixture was once again
stirred overnight. The n-hexane was removed under vacuum,
toluene (5 mL) was added to the reaction mixture, and the
solution was concentrated and stored at room temperature for
3
3
SAINT,22 and an empirical absorption (SADABS) was applied.23
The structures were solved by direct methods (SHELXS-97)
and refined by full-matrix least-squares methods against F2
(SHELXL-97).24 All non-hydrogen atoms were refined with aniso-
tropic displacement parameters. The hydrogen atoms were refined
isotropically on calculated positions using a riding model with their
Uiso values constrained to equal 1.5 times the Ueq of their pivot
atoms for terminal sp3 carbon atoms and 1.2 times Ueq for all other
carbon atoms. Disordered moieties were refined using bond length
restraints and isotropic displacement parameter restraints.
Crystallographic data (excluding structure factors) for the
structures reported in this paper have been deposited with the
Cambridge Crystallographic Data Centre. The crystal data are
listed in Table 1. Copies of the data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
two days to yield colorless crystals of 2 2toluene (0.22 g, 26%).
Mp: 165-170 °C. For the elemental analysis, 2 2toluene was
3
3
treated under vacuum to remove the two toluene molecules.
Anal. Calcd for 2 toluene, C63H76N4O2Si2 (976.55): C, 77.41;
3
1
H, 7.84; N, 5.73. Found: C, 77.56; H, 8.04; N, 5.35. H NMR
(300 MHz, C6D6, 25 °C): δ 1.27 (s, 36 H, tBu), 4.1 (s, 2H, CH),
7.26-7.33 (m, 10 H, Ph); 7.52-7.83 ppm (m, 10 H, Ph). 13C{1H}
NMR (125.75 MHz, C6D6, 25 °C): δ 30.1(CMe3), 37.5 (CH),
57.2 (CMe3), 128.5, 128.9, 129.4, 129.9, 130.2, 133.0 (Ph), 166.45
ppm (NCN). 29Si{1H} NMR (99.36 MHz, C6D6, 25 °C): δ
-86.248. EI-MS: m/z 884 [M+] (100%).
Preparation of 3. Dry N2O was bubbled into a solution of 1
(0.11 g, 0.21 mmol) in toluene (20 mL) at room temperature.
After 5 min the gas flow of N2O was disconnected, and all the
volatiles were removed under vacuum. The residue was treated
with n-hexane (10 mL) and stirred overnight. The n-hexane was
removed under vacuum, toluene (15 mL) was added to the reac-
tion mixture, and the solution was concentrated and stored at
Acknowledgment. We are thankful to the Deutsche
Forschungsgemeinschaft for supporting this work. G.T.
is thankful to the Alexander von Humboldt Stiftung for a
research fellowship.
Supporting Information Available: CIF for 2 2toluene and
3 toluene. This material is available free of charge via the Inter-
net at http://pubs.acs.org.
3
room temperature to yield colorless crystals of 3 toluene (0.12 g,
3
3
50%). Mp: 168-175 °C. For the elemental analysis 3 toluene
3
was kept under vacuum overnight to remove the toluene. Anal.
Calcd for C60H92N8O6Si4 (1132.62): C, 63.56; H, 8.18; N, 9.88.
Found: C, 64.71; H, 8.55; N, 9.09. 1H NMR (500 MHz, C6D6,
25 °C): δ 1.58 (s, 36 H, tBu), 6.94-7.14 (m, 20 H, Ph). 13C{1H}
NMR (125.75 MHz, C6D6, 25 °C): δ 33.1(CMe3), 53.5 (CMe3),
125.6, 127.6, 128.3, 128.5, 128.8, 129.3, 135.9 (Ph), 171.23 ppm
(21) Schulz, T.; Meindl, K.; Leusser, D.; Stern, D.; Ruf, M.; Sheldrick,
G. M.; Stalke, D. J. Appl. Crystallogr. 2009, 42, 885–891.
(22) Bruker, SAINT v 7.68A; Bruker AXS Inc.: Madison, WI, 2009.
€
(23) Sheldrick, G. M. SADABS 2008/2; Gottingen, 2008.
(24) Sheldrick, G. M. Acta Crystallogr., Sect. A 2008, 64, 112–122.