Reactions of a bis-silylene (LSi-SiL, L = PhC(N t Bu)2) and a heteroleptic chloro silylene (LSiCl) with Benzil: Formation of bis(siladioxolene) and monosiladioxolene analogue with five-coordinate silicon atoms in both ring systems

Article abstract of DOI:10.1021/om100581u

LSi-SiL (L = PhC(NtBu)₂) (4) was reacted with benzil in toluene at room temperature to afford a compound that contains two siladioxolene rings connected by a Si-Si bond. This is the first reactivity study of 4 where the central Si-Si bond was not cleaved during the reaction. Furthermore, LSiCl (L = PhC(NtBu)₂) (6) was treated with benzil at ambient temperature in toluene to yield 2,5-dioxa-1-silacyclopent-3-ene (7). Both 5 and 7 were characterized by single-crystal X-ray diffraction and additionally by NMR spectroscopy, EI-MS spectrometry, and elemental analysis. Investigation of the molecular structure shows that the Si-Si bond length in 5 is 2.36 A, which is more than 2% shorter than the Si-Si bond length in 4. The formal oxidation state of silicon in 5 is +3, which is in agreement with its structure. The existence of 5 and 7 as stable compounds indicates that siladioxolenes can be electronically stabilized by ?-donor ligands.

Full text of DOI:10.1021/om100581u

3930 Organometallics 2010, 29, 3930–3935
DOI: 10.1021/om100581u
Reactions of a Bis-silylene (LSi-SiL, L = PhC(NtBu)2) and a
Heteroleptic Chloro Silylene (LSiCl) with Benzil: Formation of
Bis(siladioxolene) and Monosiladioxolene Analogue with Five-Coordinate
Silicon Atoms in Both Ring Systems
ꢀ
Gasper Tavcar, Sakya S. Sen, Herbert W. Roesky,* Jakob Hey, Daniel Kratzert, and
ꢀ
Dietmar Stalke
Institut fu€r Anorganische Chemie der Universita€t Go€ttingen, Tammannstrasse 4, 37077 Go€ttingen, Germany
Received June 14, 2010
LSi-SiL (L=PhC(NtBu)2) (4) was reacted with benzil in toluene at room temperature to afford a
compound that contains two siladioxolene rings connected by a Si-Si bond. This is the first reactivity
study of 4 where the central Si-Si bond was not cleaved during the reaction. Furthermore, LSiCl
(L =PhC(NtBu)2) (6) was treated with benzil at ambient temperature in toluene to yield 2,5-dioxa-
1-silacyclopent-3-ene (7). Both 5 and 7 were characterized by single-crystal X-ray diffraction and
additionally by NMR spectroscopy, EI-MS spectrometry, and elemental analysis. Investigation of
˚
the molecular structure shows that the Si-Si bond length in 5 is 2.36 A, which is more than 2%
shorter than the Si-Si bond length in 4. The formal oxidation state of silicon in 5 is þ3, which is in
agreement with its structure. The existence of 5 and 7 as stable compounds indicates that
siladioxolenes can be electronically stabilized by σ-donor ligands.
Introduction
Scheme 1 comprehensively illustrates that disilyne com-
pounds can be classified in two types: LSitSiL and LSi-SiL.
Hence it is quite obvious that the bonding situation of 4 is
markedly different from that of 1 and 2, and it is very similar
to that of 3. Compounds 1 and 2 contain two π bonds, while
there are no π bonds in 3 and 4, respectively. However, 3 and
4 have two stereoactive lone pairs on the silicon atoms, which
are absent in 1 and 2. Subsequently it is understandable that
the pattern of the reactivity also will be quite different
between 1 and 4. Sekiguchi et al. already investigated the
reactivity of 1 toward alkenes,3a alkynes,3a tBuLi,3b alkali
metals,3b,c silyl cyanide,3d and nitriles3e to understand the
nature of the triple bond. In the case of 4 NAO and NBO
analysis show that the two Si centers are connected by a σ
bond, characterized by the Wiberg bond order of 0.9768 with
high p-character (s: 18.27%, p: 81.73%).5 Interestingly, the
single nonbonded electron pair on each silicon atom is found
to reside in an orbital with very high s-character (s: ∼65%, p:
∼35%).5 From these theoretical data it can be assumed that 4
is prone to oxidative addition because of the presence of this
stereoactive lone pair on each silicon atom. To explore the
nature of the σ bond between the two Si atoms and the
Investigating the nature of metal-metal bonds is a strong
motivation for the explorative synthetic chemist. The moti-
vation is even more challenging when the investigation is
carried out on the reactivity of disilyne because alkynes
contribute largely to the richness and diversity of organic
chemistry and portray the facility with which carbon forms
multiple bonds. One of the vital research areas in modern
main group chemistry is focused on the synthesis and reac-
tivity of disilyne, although there are very few reports so far
on the synthesis of alkyne analogues of silicon. In 2004,
Sekiguchi and co-workers1a and Wiberg et al.1b were success-
ful in isolating alkyne analogues of silicon, LSitSiL (1, L=
Si(iPr){CH(SiMe3)2}2, and 2, L = SiMe(SitBu3)2), respec-
tively, where the formal oxidation state of silicon is þ1. Very
recently Robinson et al.2a and our group2b independently
reported two compounds, LSi(Cl)Si(Cl)L (3, L=1,3-bis(2,6-
diisopropylphenyl)imidazol-2-ylidene) and LSi-SiL (4, L=
PhC(NtBu)2), with a Si-Si single bond and a lone pair of
electrons on each silicon atom (Scheme 1).2 The latter two
exhibit a novel type of compound with low-valent silicon
atoms, where a diatomic Si2 unit in the formal oxidation state
of þ1 is coordinated by σ-donor ligands.
(3) (a) Kinjo, R.; Ichinohe, M.; Sekiguchi, A.; Takagi, N.; Sumimoto,
M.; Nagase, S. J. Am. Chem. Soc. 2007, 129, 7766–7767. (b) Kinjo, R.;
Ichinohe, M.; Sekiguchi, A. J. Am. Chem. Soc. 2007, 129, 26–27. (c)
Sekiguchi, A.; Kinjo, R.; Ichinohe, M. Synth. Met. 2009, 159, 735–737. (d)
Takeuchi, K.; Ichinohe, M.; Sekiguchi, A. J. Am. Chem. Soc. 2008, 130,
16848–16849. (e) Takeuchi, K.; Ichinohe, M.; Sekiguchi, A.; Guo, J.-D.;
Nagase, S. Organometallics 2009, 28, 2658–2660.
*To whom correspondence should be addressed. E mail: hroesky@
gwdg.de.
(1) (a) Sekiguchi, A.; Kinjo, R.; Ichinohe, M. Science 2004, 305,
1755–1757. (b) Wiberg, N.; Vasisht, S. K.; Fischer, G.; Mayer, P. Z. Anorg.
Allg. Chem. 2004, 630, 1823–1828.
(2) (a) Wang, Y.; Xie, Y.; Wei, P.; King, R. B.; Schaefer, H. F., III;
Schleyer, P. v. R.; Robinson, G. H. Science 2008, 321, 1069–1071. (b)
Sen, S. S.; Jana, A.; Roesky, H. W.; Schulzke, C. Angew. Chem. 2009, 121,
8688–8690; Angew. Chem., Int. Ed. 2009, 48, 8536-8538.
ꢀ
(4) Sen, S. S.; Tavcar, G.; Roesky, H. W.; Kratzert, D.; Hey, J.;
Stalke, D. Organometallics 2010, 29, 2343–2347.
(5) Yeong, H.-X.; Lau, K.-C.; Xi, H.-W.; Lim, K. H.; So, C.-W.
Inorg. Chem. 2010, 49, 371–373.
r
pubs.acs.org/Organometallics
Published on Web 08/12/2010
2010 American Chemical Society

Article
Organometallics, Vol. 29, No. 17, 2010 3931
Scheme 1. Alkyne Analogue of Silicon
stereoactive lone pairs, we reacted 4 with a ketone and N2O.
The reaction of 4 with benzophenone afforded a four-membered
Si2O2 ring by abstracting the oxygen from benzophenone,
whereas it yielded two four-membered Si2O2 rings connected
with two oxygen atoms when it was treated with N2O.4 In
both cases the central Si-Si bond was cleaved, and the
oxygen atom was inserted. In addition So et al. described
that treatment of 4 with Br2 resulted in the heteroleptic
bromo silylene LSiBr (L = PhC(NtBu)2) under cleavage of
the Si-Si bond.5
Scheme 2. Synthesis of 5
Results and Discussion
The 29Si NMR spectrum shows a resonance at δ -79.75 ppm
that differs distinctively from that of 4 (δ 76.15 ppm)2b and is
comparable with that of the bis(silacyclopropene) deriva-
tive prepared by Ando et al. (δ -61.9 ppm).7 Moreover it
matches very well with those of the reported five-coordinate
silicon compounds.8 In the EI-MS spectrum the molecular
ion was observed as the most abundant peak, with highest
relative intensity at m/z 938.4.
In a previous publication4 we mentioned that the mecha-
nism for the formation of the four-membered Si2O2 ring by
the reaction of 4 with benzophenone was not clear. However
a [1þ2] cycloaddition was considered 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.4 The result of this reaction motivated
us to investigate the reaction of 4 with diketone, as it is well
known that five-membered heterocycles are more stable than
three-membered species.6 As a result, there might be also a
possibility that in the product the Si-Si bond will not be
cleaved. Accordingly we reacted 4 with 2 equiv of benzil, and
as we anticipated, compound 5 was formed containing two
dioxolene rings. However the most surprising feature is that
the central Si-Si bond in 5 remains intact and the formal
oxidation state of silicon in 5 is þ3. This is the first example of
4 where the Si-Si bond is not cleaved during the reaction.
Compound 5 was formed by treatment of 4 with 2 equiv of
benzil in toluene overnight at room temperature (Scheme 2).
After removal of the solvent under vacuum, n-hexane was
added and the reaction mixture was stirred for 2 h. After that
the solution was concentrated and stored for crystallization,
which afforded colorless crystals of 5 suitable for X-ray
crystallography. The structure of 5 was confirmed by NMR
spectroscopy, EI-mass spectrometry, and elemental analysis.
The mechanism for the reaction seems to be obvious. The
two electrophilic silicon centers in 4 attack the oxygen atoms
of benzil due to their inherent affinity toward oxygen, and 5
is formed unequivocally as a two times [1þ4] cycloaddition
product (Scheme 3). Here it is noteworthy that we recently
communicated the successful isolation of monosila epoxide
stabilized by the same amidinato ligand.9 Therefore the
question arises, why does the reaction of 4 with diketone
afford a compound keeping the Si-Si bond intact, whereas
the similar reaction with ketone resulted in a four-membered
Si2O2 ring under cleavage of the Si-Si bond? Basolo and
Johnson give an answer in their book:6a “In general, it has
been observed that for ligands that do not contain double
bonds, those that form five-membered metal-chelate rings give
the most stable products. Chelate rings that contain either four
atoms or more than six atoms have been observed, but they are
relatively unstable and uncommon.” The reason behind this
observation is presumably the change in internal ring strain
accompanying the change in coordination number of the
ring atom participating in the reaction. For three-membered
rings the internal strain arises primarily from the distortions
of the normal bond angle, whereas for five-membered rings
the strain is attributed primarily to repulsion terms arising
from unfavorable conformations. As a result, five-membered
rings are more stable compared to the three-membered ring
compounds. Therefore, the formation of the bis(silaepoxide)
derivative is energetically not favorable, although the situa-
tion is reverse in case of the bis(siladioxolene) derivative.
(6) (a) Basolo, F.; Johnson, R. C. Coordination Chemistry; The
Benjamin/Cummings Publishing Company: London, 1964; p 130. (b) Omae,
I. Coord. Chem. Rev. 2004, 248, 995–1023.
(7) Ando, W.; Shiba, T.; Hidaka, T.; Morihashi, K.; Kikuchi, O. J.
Am. Chem. Soc. 1997, 119, 3629–3630.
(8) (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) Ghadwal, R. S.; Sen, S. S.; Roesky, H. W.; Tavcar, G.;
Merkel, S.; Stalke, D. Organometallics 2009, 28, 6374–6377. (c) Sen, S. S.;
Roesky, H. W.; Stern, D.; Henn, J.; Stalke, D. J. Am. Chem. Soc. 2010, 132,
1123–1126. (d) Nagendran, S.; Roesky, H. W. Organometallics 2008, 27,
457–492. (e) Metz, S.; Burschka, C.; Tacke, R. Organometallics 2008, 27,
6032–6034. (f) Seiler, O.; B€uttner, M.; Penka, M.; Tacke, R. Organometal-
lics 2005, 24, 6059–6062. (g) Tacke, R.; Burschka, C.; Richter, I.; Wagner,
B.; Willeke, R. J. Am. Chem. Soc. 2000, 122, 8480–8485. (h) Tacke, R.;
Ulmer, B.; Wagner, B.; Arlt, M. Organometallics 2000, 19, 5297–5309.
(9) Ghadwal, R. S.; Sen, S. S.; Roesky, H. W.; Granitzka, M.;
Kratzert, D.; Merkel, S.; Stalke, D. Angew. Chem. 2010, 122, 4044–
4077; Angew. Chem., Int. Ed. 2010, 49, 3952-3955.

ꢀ
Tavcar et al.
3932 Organometallics, Vol. 29, No. 17, 2010
Scheme 3. Suggested Mechanism for the Formation of 5
˚
Moreover, replacement of one R group (R=PhC(NtBu)2Si)
by one chlorine atom decreases the strain of the ring and
favors the formation of monosilaepoxide.9
bit elongated silicon-nitrogen distances (Si(1)-Nax: 1.994 A,
˚
Si(2)-Nax: 2.001 A) compared to the equatorial bond
˚
˚
lengths (Si(1)-Neq: 1.830 A, Si(2)-Neq: 1.816 A). This is
˚
The molecular structure of 5 was unequivocally estab-
lished by single-crystal X-ray structure determination
(Figure 1).10 5 crystallizes in the monoclinic space group
P21/c incorporating two disordered toluene molecules per
unit cell. Important bond lengths and angles are given in
the legend of Figure 1. The structure consists of two five-
membered dioxolene rings connected by a Si-Si bond. To
avoid the steric congestion, two dioxolene rings are situated
in such a way that they are arranged trans to each other. Both
the silicon atoms are five-coordinate and adopt a distorted
trigonal-bipyramidal geometry. For each of them the axial
positions are occupied by one oxygen atom and one nitrogen
atom. The sum of angles between the equatorial atoms and
the silicon atom is 354.45° for Si(1) and 358.24° for Si(2). The
axial substituents include an angle of 164.95° with Si(1) and
167.67° with Si(2). The distorted trigonal-bipyramidal archi-
tecture induces different Si-N and Si-O distances for axial
and equatorial substituents, respectively. This effect is very
apparent for the Si-N bonds in that the axial positions exhi-
also observed for the Si-O distances (1.753 and 1.744 A for
˚
the axial Si-O distances; 1.698 and 1.701 A for the equator-
ial Si-O distances). The bond lengths appear very well com-
parable to previously reported values.11
The most noticeable feature in the structure is that the
Si-Si bond is not cleaved during the reaction, but the bond
length is reduced to 2.36 A when compared with that in the
˚
starting material 4 (2.41 A).2a In 5 the Si-Si bond distance is
˚
˚
only about 0.02 A longer than the sum of the Si covalent radii
12a
˚
(2.34 A)
and matches excellently with the Si-Si single
bond distance in R-silicon (2.36 A).12b The same shortening
was also observed in the related bis(silacyclopropene).7 The
shortening of the Si-Si bond length is probably due to
the nonexistence of the lone pair-lone pair repulsion in 5.
Here it is notable that a similar kind of decrease in the
Ge-Ge bond length was also observed when LGe-GeL
(L = PhC(NtBu)2) was reacted with Fe2(CO)9 to afford the
Lewis acid-base adduct LGe[Fe(CO)4]-Ge[(Fe(CO)4]L.13
Another important facet of the structure is the C-C bond
˚
˚
length in the dioxolene rings (1.34 and 1.35 A), which are in
good agreement with those of C-C double bonds.14
(10) (a) Kottke, T.; Stalke, D. J. Appl. Crystallogr. 1993, 26, 615–619.
(b) Stalke, D. Chem. Soc. Rev. 1998, 27, 171–178.
(11) (a) Kost, D.; Kalikhman, I.; Krivonos, S.; Stalke, D.; Kottke, T.
J. Am. Chem. Soc. 1998, 120, 4209–4214. (b) Kalikhman, I.; Girshberg, O.;
Lameyer, L.; Stalke, D.; Kost, D. J. Am. Chem. Soc. 2001, 123, 4709–4716.
(c) Kost, D.; Gostevskii, B.; Kocher, N.; Stalke, D.; Kalikhman, I. Angew.
Chem. 2003, 115, 1053–1056; Angew. Chem., Int. Ed. 2003, 42,
1023-1026. (d) Gostevskii, B.; Adear, K.; Sivaramakrishna, A.; Silbert,
G.; Stalke, D.; Kocher, N.; Kalikhman, I.; Kost, D. Chem. Commun.
2004, 1644–1645. (e) Michalczyk, M. J.; Fink, M. J.; Haller, K. J.; West, R.;
Michl, J. Organometallics 1986, 5, 531–536. (f) Sohn, H.; Tan, R. P.; Powell,
D. R.; West, R. Organometallics 1994, 13, 1390–1394. (g) Iwamoto, T.;
Masuda, H.; Ishida, S.; Kabuto, C.; Kira, M. J. Am. Chem. Soc. 2003, 125,
9300–9301.
(12) (a) Corey, J. Y. In The Chemistry of Organic Silicon Compounds,
Vol. 1; Patai, S.; Rappoport, Z., Eds.; Wiley: Chichester, UK, 1989;
Chapter 1. (b) Nesper, R.; Currao, A.; Wengert, S. In Organosilicon
Chemistry II: From Molecules to Materials; Auner, N.; Weis, J., Eds.;
VCH: Weinheim, Germany, 1996; p 469.
(13) Sen, S. S.; Kratzert, D.; Stern, D.; Roesky, H. W.; Stalke, D.
Inorg. Chem. 2010, 49, 5786–5788.
(14) (a) Wade, L. G. Organic Chemistry, 6th ed.; Pearson Prentice Hall,
2006; p 279. (b) Moss, G. P.; Smith, P. A. S.; Tavernier, D. Pure Appl. Chem.
1995, 67, 1307–1375.

Article
Organometallics, Vol. 29, No. 17, 2010 3933
Figure 2. Crystal structure of 7. Hydrogen atoms are not shown for
clarity. Anisotropic displacement parameters are depicted at the
˚
50% probability level. Selected bond distances (A) and bond angles
(deg): Cl(2)-Si(1) 2.0958(7), N(1)-C(1) 1.314(2), N(1)-C(12)
1.492(2), N(1)-Si(1) 1.9091(16), N(2)-C(1) 1.363(2), N(2)-C(8)
1.493(2), N(2)-Si(1) 1.8133(16), O(1)-C(17) 1.400(2), O(1)-Si(1)
1.6796(13), O(2)-C(16) 1.379(2), O(2)-Si(1) 1.7064(13); C(1)-N-
(1)-C(12) 130.76(16), C(1)-N(1)-Si(1) 90.43(11), C(12)-N-
(1)-Si(1) 138.55(13), C(1)-N(2)-C(8) 132.02(15), C(1)-N(2)-
Si(1) 93.08(11), C(8)-N(2)-Si(1) 134.39(12), C(17)-O(1)-Si(1)
112.30(11), C(16)-O(2)-Si(1) 111.95(11), O(1)-Si(1)-O(2)
90.71(6), O(1)-Si(1)-N(2) 125.79(7), O(2)-Si(1)-N(2) 99.02(7),
O(1)-Si(1)-N(1) 92.29(7), O(2)-Si(1)-N(1) 168.23(7), N(2)-
Si(1)-N(1) 70.09(7), O(1)-Si(1)-Cl(2) 120.80(5), O(2)-Si(1)-
Cl(2) 95.15(5).
Figure 1. Molecular structure of 5. Hydrogen atoms as well as
methyl groups and the toluene lattice molecule are omitted for
˚
clarity. Selected bond distances (A) and bond angles (deg) with stan-
dard deviations: N(1)-Si(1) 1.9938(14), N(2)-Si(1) 1.8304(15),
N(3)-Si(2) 1.8163(14), N(4)-Si(2) 2.0008(14), O(1)-Si(1)
1.7531(12), O(2)-Si(1) 1.7008(12), O(3)-Si(2) 1.7443(12), O-
(4)-Si(2) 1.6984(12), Si(1)-Si(2) 2.3628(7); O(2)-Si(1)-O(1)
89.26(6), O(2)-Si(1)-N(2) 114.81(6), O(1)-Si(1)-N(2) 99.83(6),
O(2)-Si(1)-N(1) 87.50(6),O(1)-Si(1)-N(1) 164.95(6), N(2)-
Si(1)-N(1) 68.43(6), O(2)-Si(1)-Si(2) 128.85(5), O(1)-Si(1)-
Si(2) 97.42(4), N(2)-Si(1)-Si(2) 113.79(5), N(1)-Si(1)-Si(2)
96.05(4), O(4)-Si(2)-O(3) 89.93(6), O(4)-Si(2)-N(3) 118.59(6),
O(3)-Si(2)-N(3) 100.46(6), O(4)-Si(2)-N(4) 90.35(6), O(3)-
Si(2)-N(4) 167.61(6), N(3)-Si(2)-N(4) 68.65(6), O(4)-Si(2)-
Si(1) 124.90(5), O(3)-Si(2)-Si(1) 93.37(4), N(3)-Si(2)-Si(1)
114.75(5), N(4)-Si(2)-Si(1) 96.61(4).
Scheme 5. Proposed Mechanism for the Formation of 7
Scheme 4. Preparation of 7
resonances of the tBu of 7 exhibit a downfield shift when
compared with those of the precursor 6 (for 6 δ 1.08 ppm for
tBu protons). In the 29Si NMR spectrum a sharp resonance
was observed at δ -92.24 ppm. The chemical shift is con-
sistent with that reported for the reaction product of
RfSiCl2 (R = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-
ylidene) and benzil (δ -99.50 ppm).9 In the EI-MS spectrum
of 7 the molecular ion appeared as the most abundant peak,
with highest relative intensity at m/z 505.
For the reaction of 6 with benzil we propose the following
pathway (Scheme 5). The initial step leads to a formal [1þ2]
cycloaddition product, in which a highly reactive oxasilarane
is formed. This intermediate reacts with the other carbonyl
group of the R-diketone to afford the stable product 7.
Comparable pathways have been described by Ando et al.
and Jutzi et al. for the reaction of dimethyl silylene with
various R-diketones, and decamethyl silicocene with benzil,
respectively.16 Here it is also noteworthy that the reaction of
6 with benzil proceeds smoothly at room temperature unlike
decamethyl silicocene, where drastic conditions were required
to promote the reaction.
After the accomplishment of the reaction between 4 and
benzil we reacted heteroleptic chloro silylene (PhC(NtBu)2SiCl)
(6)15,8c with the same diketone. Treatment of 6 with benzil in
toluene at room temperature led to the formation of the
[1þ4] cycloaddition product (7) in good yield (Scheme 4). 7 is
colorless and soluble in diethyl ether, toluene, and THF. 7
was isolated as an analytically pure, pale yellow solid, which
turned out to be extremely sensitive toward air and moisture
but could be stored under inert atmosphere at room tempe-
rature for months without decomposition. 7 was characte-
rized by NMR spectroscopy, EI-MS spectrometry, elemen-
tal analysis, and single-crystal X-ray diffraction methods. In
the 1H NMR spectrum two sets of resonances were observed:
one singlet for tBu protons (δ 1.20 ppm) and another multi-
plet for the phenyl protons (δ 7.13-7.56 ppm). The proton
(15) So, C. -W.; Roesky, H. W.; Magull, J.; Oswald, R. B. Angew.
Chem. 2006, 118, 4052–4054; Angew. Chem., Int. Ed. 2006, 45, 3948 -
3950.
(16) (a) Ando, W.; Ikeno, M. J. Chem. Soc., Chem. Commun. 1979,
€
655–656. (b) Jutzi, P.; Eikenberg, D.; Bunte, E.-A.; Moheke, A.; Neumann,
B.; Stammler, H.-G. Organometallics 1996, 15, 1930–1934.

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Tavcar et al.
3934 Organometallics, Vol. 29, No. 17, 2010
To gain more detailed insight into the structural properties
of 7, the molecular structure was unequivocally elucidated by
single-crystal X-ray diffraction studies (Figure 2).10 Com-
pound 7 crystallizes in the monoclinic space group P21/c. The
important bond lengths and bond angles are given in the
legend of Figure 2. The X-ray structure revealed the silicon
atom to be five-coordinate17 and exhibits a distorted square-
pyramidal geometry. The two nitrogen atoms from the amidi-
nato ligand and the two oxygen atoms from the diketone
form the base of the pyramid and occupy the four coordina-
tion sites. The remaining apical coordination site is occupied
by the chlorine atom. The two independent silicon oxygen
Table 1. Crystallographic and Structure Refinement
Data at 100.0(2) K
parameter
5 0.5toluene
7
3
empirical formula
fw
space group
˚
C61.5H70N4O4Si2 C29H33ClN2O2Si
985.39
P21/c
505.11
P21/c
a (A)
13.7468(18)
21.681(3)
19.780(3)
109.503(2)
5556.9(13)
4
12.2971(4)
18.4375(6)
11.5359(3)
95.673(2)
2602.70(14)
4
˚
b (A)
˚
c (A)
β (deg)
3
V (A )
˚
Z
F
calcd (g/cm3)
F(000)
1.178
2108
0.114
1.289
1072
0.222
˚
bond lengths are 1.6796(13) and 1.7064(13) A, respectively,
μ (mm-1
)
which match very well with the reported silicon oxygen single-
bond distance.11 Another notable aspect is the O-Si-O and
N-Si-N bond angles. The former is only 90.71(6)°, which is
smaller compared to the reported O-Si-O angles, whereas
the latter (70.09(7)°) shows a small increase if compared with
that of compound 6 (68.35(8)°).15 The reduction of the
O-Si-O bond angle as well as the increase of the N-Si-N
bond angle is probably the result of the ring strain of the
θ range for data collection (deg) 1.44-26.72
1.66-26.40
59 125/5325
11 781/101/708 5325/0/322
reflns collected/unique [I > 2σ(I)] 84 719/11 781
data/restraints/params
Goof on F2
1.050
0.0450
1.060
0.0392
R1 [I > 2σ(I)]
wR2 (all data)
˚
largest diff peak/hole (e A
0.1142
0.725/-0.628
0.1000
0.285/-0.348
-3
)
Calcd for 5 0.5toluene: C61.5H70N4O4Si2 (985.39) C, 74.96; H,
3
˚
7.16; N, 5.69. Found: C, 74.51; H, 7.22; N, 5.55. 1H NMR (500
MHz, C6D6, 25 °C): δ 1.42 (s, 36 H, tBu), 6.29 (s, 2H, CH),
6.86-6.96 (m, 10 H, Ph); 7.92-8.19 (m, 10 H, Ph) ppm. 13C{1H}
NMR (125.75 MHz, C6D6, 25 °C): δ 30.8(CMe3), 57.2 (CMe3),
125.6, 126.4, 127.0, 127.8, 127.9, 128.5, 128.9, 129.4, 129.9,
130.2, 133.0 (Ph), 171.37 (NCN) ppm. 29Si{1H} NMR (99.36
MHz, C6D6, 25 °C): δ -79.75 ppm. EI-MS: m/z 939 [Mþ]
(100%).
heterocycle in 7. The Si-Cl distance in 7 (2.0958 (7) A) is
shorter when compared with that of 6 (2.156(1) A).
15
˚
In conclusion, we have prepared two siladioxolene deri-
vatives stabilized by a bulky amidinato ligand. Both the
compounds were formed by [1þ4] cycloaddition reaction
between the low-valent silicon compounds and benzil. The
formal oxidation state of silicon in 5 is þ3, whereas that in 7
is þ4. The molecular structure of 5 shows that the Si-Si
bond is not cleaved during the reaction. The formation of 5
and 7 as stable compounds indicates that siladioxolenes can
be electronically stabilized by σ-donor ligands. Here it is also
worth pointing out that reduction of 7 in a molar ratio of 1:1
with KC8 in THF and toluene did not furnish 5 and leads to a
mixture of insoluble products, which could not be identified.
Preparation of 7. Toluene (25 mL) was added to the mixture of
6 (0.29 g, 1.02 mmol) and benzil (0.21 g, 1.00 mmol) at ambient
temperature. The mixture was stirred overnight. The solvent
was removed under vacuum. The residue was treated with 20 mL
of toluene and filtered. Storage of the filtrate at -32 °C in a
freezer for 1 day afforded colorless crystals of 7 (0.39 g, 71.4%).
Mp: 164-170 °C. Anal. Calcd for C29H33ClN2O2Si (505.12): C,
1
68.96; H, 6.58; N, 5.55. Found: C, 68.66; H, 6.43; N, 5.06. H
NMR (500 MHz, THF-d8, 25 °C): δ 1.20 (s, 18H, tBu),
7.13-7.56 (m, 15H, Ph) ppm. 13C{1H}NMR (125.75 MHz,
THF-d8, 25 °C): δ 31.8 (CMe3), 55.6 (CMe3), 127.4, 127.6,
128.5, 128.6, 128.9, 129.1, 129.9, 130.6, 135.6, 135.2, 135.8
(Ph), 175.2 (NCN) ppm. 29Si{1H}NMR (99.36 MHz, C6D6,
25 °C): δ - 92.2 ppm. EI-MS: m/z 505 [Mþ] (100%).
Experimental Section
All manipulations were carried out in an inert gas atmosphere
of dinitrogen using standard Schlenk techniques and in a
dinitrogen-filled glovebox. The solvents used were purified by
a MBraun solvent purification system, MB SPS-800. Com-
pounds 4 and 6 were prepared by literature methods.2a,8c All
chemicals purchased from Aldrich were used without further
purification. 1H, 13C, and 29Si NMR spectra were recorded
using a Bruker Avance DPX 200 or a Bruker Avance DRX 500
spectrometer. The 1H, 13C, and 29Si NMR spectra were recorded
in C6D6 and THF-d8. The chemical shifts δ are reported in ppm
relative to SiMe4 as external standard. EI-mass spectra were
obtained using a Finnigan MAT 8230 instrument. Elemental
Crystal Structure Determination. Shock-cooled crystals were
selected and mounted under nitrogen atmosphere using the
X-TEMP.10 The data for 5 0.5toluene were collected at 100(2)
3
K on a Incoatec Mo microsource18 with Quazar mirror optics
and an APEX II detector on a D8 goniometer. The data of 7
were measured on a Bruker TXS-Mo rotating anode with Helios
mirror optics and an APEX II detector on a D8 goniometer.
Both diffractometers were equipped with a low-temperature
˚
device and used MoKR radiation, λ = 0.71073 A. The data of
€
analyses were performed by the Institut fur Anorganische
Chemie, Universitat Gottingen. Melting points were measured
5 0.5toluene and 7 were integrated with SAINT,19 and an
€
€
3
empirical absorption (SADABS) was applied.20 The structures
were solved by direct methods (SHELXS-97) and refined by full-
matrix least-squares methods against F2 (SHELXL-97).21 All
non-hydrogen atoms were refined with anisotropic displace-
ment parameters. The hydrogen atoms were refined isotropi-
cally on calculated positions using a riding model with their Uiso
values constrained to equal to 1.5 times the Ueq of their pivot
atoms for terminal sp3 carbon atoms and 1.2 times for all
other carbon atoms. Disordered moieties were refined using
€
in a sealed glass tube on a Buchi B-540 melting point apparatus.
Preparation of 5. Toluene (20 mL) was added to a mixture of 4
(0.1 g, 0.19 mmol) and benzil (0.08 g, 0.37 mmol) at ambient
temperature. The mixture was stirred overnight. The volatiles
were removed in vacuo, and n-hexane (10 mL) was added to the
residue. The reaction mixture once again was stirred overnight.
The n-hexane was removed in vacuo, and toluene (10 mL) was
added to the reaction mixture. The solution was concentrated
and stored at room temperature for two days to yield colorless
crystals of 5 0.5toluene (0.07 g, 38.9%). Mp: 154-158 °C. For
the elemental analysis 5 was treated in vacuo overnight. Anal.
3
(18) Schulz, T.; Meindl, K.; Leusser, D.; Stern, D.; Ruf, M.; Shel-
drick, G. M.; Stalke, D. J. Appl. Crystallogr. 2009, 42, 885–891.
(19) Bruker, SAINT v 7.68A; Bruker AXS Inc.: Madison, WI, 2009.
(17) 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.
€
(20) Sheldrick, G. M. SADABS 2008/2; University of Gottingen, 2008.
(21) Sheldrick, G. M. Acta Crystallogr., Sect. A 2008, 64, 112–122.

Article
Organometallics, Vol. 29, No. 17, 2010 3935
bond length restraints and isotropic displacement parameter
restraints.
Crystallography for supporting this work. G.T. is thank-
ful to the Alexander von Humboldt Stiftung for a re-
search fellowship. We are also indebted to Mr. Sudipta
Dutta and Prof. Swapan K. Pati from Theoretical Science
Unit, JNCASR, India, for doing the theoretical calcula-
tions of 4. Comparable theoretical results were however
published in ref 5.
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.
Supporting Information Available: CIF for 5 and 7. This
material is available free of charge via the Internet at http://
pubs.acs.org.
Acknowledgment. We are thankful to the Deutsche
Forschungsgemeinschaft and the Center of Materials