Formation of a 1,4-diamino-2,3-disila-1,3-butadiene derivative

Article abstract of DOI:10.1021/ja4072139

A 1,4-diamino-2,3-disila-1,3-butadiene derivative of composition (Me ₂-cAAC)₂(Si₂Cl₂) (Me ₂-cAAC =:C(CMe₂)₂(CH₂)N-2,6-iPr ₂C₆H₃) was synthesized by reduction of the Me₂-cAAC:SiCl₄ adduct with KC₈. This compound is stable at 0 C for 3 months in an inert atmosphere. Theoretical studies reveal that the silicon atoms exhibit pyramidal coordination, where the Cl-Si-Si-Cl dihedral angle is twisted by 43.3 (calcd 45.9). The two silicon-carbon bonds are intermediates between single and double Si-C bonds due to twisting of the C-Si-Si-C dihedral angle (163.6).

Full text of DOI:10.1021/ja4072139

Communication
pubs.acs.org/JACS
Formation of a 1,4-Diamino-2,3-disila-1,3-butadiene Derivative
Kartik Chandra Mondal,† Herbert W. Roesky,*,† Birger Dittrich,*,‡ Nicole Holzmann,§
Markus Hermann,§ Gernot Frenking,*,§ and Alke Meents∥
†Institut fur Anorganische Chemie, Georg-August-Universitat, Tammannstraße 4, 37077 Gottingen, Germany
̈
̈
̈
‡Martin-Luther-King-Platz 6, Raum AC 15c (Erdgeschoss), 20146 Hamburg, Germany
§Fachbereich Chemie, Philipps-Universitat Marburg, Hans-Meerwein-Straße, 35032 Marburg, Germany
∥Deutsches Elektronen-Synchrotron DESY, Notkestraße 85, 22607 Hamburg, Germany
̈
S
* Supporting Information
to electron-sharing bonds in cAAC−SiCl2−cAAC, with a
singlet biradical spin ground state.7a
ABSTRACT: A 1,4-diamino-2,3-disila-1,3-butadiene de-
rivative of composition (Me2-cAAC)2(Si2Cl2) (Me2-cAAC
= :C(CMe2)2(CH2)N-2,6-iPr2C6H3) was synthesized by
reduction of the Me2-cAAC:SiCl4 adduct with KC8. This
compound is stable at 0 °C for 3 months in an inert
atmosphere. Theoretical studies reveal that the silicon
atoms exhibit pyramidal coordination, where the Cl−Si−
Si−Cl dihedral angle is twisted by 43.3° (calcd 45.9°). The
two silicon−carbon bonds are intermediates between
single and double Si−C bonds due to twisting of the
C−Si−Si−C dihedral angle (163.6°).
This chemical behavior of cAAC has no precedence with
NHC which is due to the small HOMO−LUMO gap of the
former. However, in common both cAAC10 and NHC form a
stable adduct with SiCl4. The NHC:SiCl4 adduct was reduced
to a base-stabilized singlet bis-silylene (NHC:Si(:)Cl)2 (A) and
NHC:SiSi:NHC (B) utilizing KC8 as a reducing reagent.11 In
a recent review,12 Bertrand et al. mentioned that an analogue of
Robinson’s B with cAAC carbene might be expected to form a
biradical. To investigate this further we targeted the synthesis of
the cAAC analogue of A.
The three-electron reduction of Me2-cAAC:SiCl4 (1)10 with
KC8 in a molar ratio of 1:3 in THF resulted in (Me2-
cAAC)2(Si2Cl2) (2). The reaction proceeds already at −78 °C;
it took 30 min to obtain a green solution (Supporting
Information). The temperature was slowly raised, and stirring
was continued at room temperature for 3 h to obtain a red
solution of (Me2-cAAC)2(Si2Cl2) (2) (Scheme 1). Compound
ilicon chemistry, with Si as the sister element of carbon, has
S
seen a number of exciting developments in recent years.
Among the unsaturated compounds the disilynes RSiSiR of
Sekiguchi et al. and Wiberg et al. were highlights in this field,1
whereas a compound with a CSi triple bond was
characterized by Baceiredo et al.,2 although the latter is only
stable up to −30 °C. Apeloig, Schwarz, and co-workers3
characterized small silynes HCSiX in the gas phase. The first
compounds with SiC and SiSi double bonds were already
reported 30 years ago,4 followed by a hexaaryltetrasilabuta-1,3-
diene in 1997.5 In recent years, N-heterocyclic carbenes
(NHC) and cyclic alkyl(amino) carbenes (cAAC) have been
used for the stabilization of silylenes.6,7 In the former, the
carbene carbon atom is bound to two σ-withdrawing and π-
donating N-atoms. However, in the case of cAAC one of the σ-
withdrawing and π-donating N-atoms is replaced by one σ-
donating quaternary C-atom. Thus, the cAAC becomes more
nucleophilic but also more electrophilic when compared with
that of NHC.8 Thus, the electronic properties of analogous
compounds of NHC and cAAC may change dramatically. For
example, Robinson et al. have shown9a that NHC:-stabilized P2
allotrope can have two canonical forms; NHC:→P−P←:CHN
and NHCP−PCHN. The former conformer9a was stated
as the predominant product based on the chemical shift values
of 31P NMR, while the latter one9b was shown to be the only
conformer for the cAAC: analogue.9 Furthermore, it was
observed that two radical centers could be easily generated just
next to the SiCl2 unit by replacing one NHC of NHC:SiCl2 by
two cAAC due to their better π-accepting ability and the lower
singlet−triplet gap than that of NHC.7 This explains the
dramatic change from a donor−acceptor bond in NHC→SiCl2
Scheme 1. Synthesis of Compound 2 from 1 under KC8
Reduction
2 was extracted with n-hexane and crystallized as a red solid in
48% yield. The X-ray single-crystal structure analysis of 2
revealed a unprecedented formation of a CSi−SiC chain
(Scheme 1) instead of the monomeric silylene radical of
composition (Me2-cAAC·)−Si(:)Cl. To the best of our
knowledge, a mixed carbon and silicon-centered chain of a
stable and isolable 1,4-diamino-2,3-disila-1,3-butadiene has not
been reported before.
Compounds 1 and 2 are isolated and colorless and rose-red
solid, respectively. Compound 2 is soluble in toluene, benzene,
n-hexane, and THF, whereas 1 is only soluble in THF due to its
Received: July 15, 2013
Published: October 10, 2013
© 2013 American Chemical Society
15990
dx.doi.org/10.1021/ja4072139 | J. Am. Chem. Soc. 2013, 135, 15990−15993

Journal of the American Chemical Society
Communication
zwitterionic nature. Compound 2 is stable in an inert
atmosphere for 3 months at −32 to 0 °C, but it slowly
decomposes to a colorless solid after one week upon storage at
room temperature and loses its color completely in 3−4 weeks.
Compound 2 decomposes above 109 °C and turns to darker
red, above 128 °C adopts a brown red and finally melts to form
a black brown liquid at 200 °C. Compound 2 is stable in
solution in an inert atmosphere but immediately hydrolyzes to
the Me2-cAAC:H+Cl− salt upon exposure to air.
The UV−Vis spectrum of compound 2 was recorded in n-
hexane, which exhibited absorption bands at 615, 526, 438, 349,
254 nm (Supporting Information). The biradical (Me2-
cAAC)2SiCl2 and the biradicaloid siladicarbene (Me2-cAAC)2Si
show absorptions at higher values (569,7a 569 and 611 nm).7b
The 29Si NMR spectrum of 2 exhibits a resonance at δ = +25.62
ppm which is downfield shifted with respect to that of the
precursor 1 (δ = −103.5 ppm,10 Supporting Information) but
upfield shifted (δ = +38.4 ppm) when compared with
(NHC:Si(:)Cl)2 (A).11
intermediates between single and double Si−C bonds which are
10 pm shorter than the values (193.9(6)−192.9(7) pm)
reported for (NHC:Si(:)Cl)2 (A).11 The Si−Si bond distance
in 2 is 230.58(13) pm and therefore ∼9 pm shorter than the
Si−Si single bond (239.3(3) pm) of (NHC:Si(:)Cl)2 (A) but
∼7 pm longer than the SiSi double bond length of
NHC:SiSi:NHC (B).11 The above-mentioned comparison
suggests that there might be some additional bonding between
the two silicon atoms. The Ccarbene−N bond lengths of 2 are
slightly different (133.6(3)−133.7(2) pm) and are a little
7a
shorter than in the biradical (Me2-cAAC)2SiCl2 (139.94(19)
pm), however, longer (∼ 3 pm) than that (130.6(3) pm)
observed for the zwitterionic precursor 1. Moreover, the N−
Ccarbene−C angles in 2 (107.85(17)−107.95(17)°) are slightly
sharper when compared with that of 1 (111.15(13)°). The C−
Si−Cl bond angles of 2 are in the range of 110.95(8)−
110.85(8)° which are more acute (∼9−10°) than those
(100.7(2)−101.2(2)°) of (NHC:Si(:)Cl)2 (A).11 The Cl−Si−
Si bond angles in 2 are in the range of 106.62(4)−107.30(6)°
which are close to the values (107.96(11)−108.75(11)°)
reported for A. The C−Si−Si bond angles in 2 are
110.40(8)−110.85(8)° which are wider by ∼9−11° than
those of A (98.76(19)−98.7(2)°).11 The sum of the bond
angles around Si1 and Si2 in 2 is 327.96° and 325.34°,
respectively. The distances from each of the silicon atoms to
the C−Cl−Si planes are 68.8 and 71.6 pm, suggesting that the
bond between the two silicon atoms is not a complete double
bond. The X-ray single-crystal structural bond parameters and
the valency of the two carbene carbon atoms and of the two
silicon atoms indicate an interesting bonding situation.
Compound 2 crystallizes in the triclinic space group P1. The
̅
asymmetric unit contains two molecules of 2. Each of the
silicon atoms adopts a distorted trigonal pyramidal geometry
(Figure 1). It is well-known that the Si−Cl bond distances
A comparison of compound A11 with compound 2 clearly
shows that both C−Si bonds as well as the Si−Si bond in 2 are
significantly shorter (Scheme 2). An explanation for the
Scheme 2. Bonding Situation and Comparison of Bond
Lengths of Compound A11 with Compound 2
Figure 1. Molecular structure of compound 2. H-atoms and isopropyl
groups are omitted for clarity. Selected experimental (calculated at
BP86/TZVPP for the singlet state) bond lengths (pm) and angles
(deg) (as averages of two independent molecules): Si1−C1/Si2−C21
182.3(3)/182.6(3) (185.9), Si1−Cl1/Si2−Cl2 210.27(18)/
209.10(13) (214.6/214.7), C1−N1/C21−N2 133.6(3)/133.7(2)
(136.0), Si1−Si2 230.58(13) (236.8), N1−C9/N2−C29 143.8(2)/
143.7(2) (145.1), N1−C1−C2/N2−C21−C22 107.85(17)/
107.95(17) (108.1), C1−Si1−Si2/C21−Si1−Si2 110.40(8)/
110.85(8) (108.0/108.1), C1−Si1−Cl1/C21−Si2−Cl2 110.95(8)/
110.85(8) (111.2/111.4), Cl1−Si1−Si2/Cl2−Si2−Si1 106.62(4)/
107.30(6) (107.9/108.0), Cl1−Si1−Si2−Cl2 43.31 (45.9), C1−Si1−
Si2−C21 163.56 (165.0).
different structures can be given in terms of the bonding
situations of the molecules which are sketched in Scheme 2.
Molecule A has NHC→(SiCl)2←NHC donor−acceptor bonds
while 2 possesses cAAC−(SiCl)2−cAAC electron-sharing
bonds. A similar change in the bonding situation was recently
reported for NHC→SiCl2 and cAAC−SiCl2−cAAC.7a
The C−Si−Si−C fragment in 2 could be compared with a
distorted trans-2,3-disila-1,3-butadiene which carries terminal
amino groups. This interpretation agrees with the available
experimental and theoretical data. The Si−Si distance in 2
(230.5 pm) concurs very well with the calculated value for
trans-2,3,-disila-1,3-butadiene (229.9 pm)15 The π-conjugation
in 2 is not fully effective because the C−Si−Si−C fragment is
not planar but has a dihedral angle of 163.6° while the Cl−Si−
Si−Cl moiety has a torsion angle of 43.3°. This explains why
the C−Si distance in 2 (182.3/182.6 pm) is significantly longer
than in trans-2,3,-disila-1,3-butadiene (172.7 pm).15 The
distortion from π-conjugation comes clearly to the fore when
the shapes of the two highest lying occupied orbitals are
depend on the coordination number and formal oxidation state
of the silicon atom. The Si−C1 bond distances are in the range
of 205.81(9) to 219.64(10) pm for the precursor (1)10 and
203.96(4) to 206.48(4) pm for the biradical (Me2-
cAAC·)2SiCl2.7a The experimentally observed Si−Cl and Si−
Ccarbene bond distances in 2 are 209.10(13)−210.27(18) and
182.3(3)−182.6(3) pm. The Si−Ccarbene bond length is
194.4(2) pm in the precursor 1.10
It is noteworthy that the Si−C distances in 2 are longer than
SiC double bonds (170.2−177.5 pm).13 The longer silicon−
carbon bonds in 2 are likely caused by the twisted C−Si−Si−C
unit and particularly by the pyramidal coordination at the
silicon atoms. This becomes obvious from the dihedral angle
Cl1−Si1−Si2−Cl2 of 43.31° which deviates significantly from a
planar arrangement. Two of the silicon−carbon bonds are
15991
dx.doi.org/10.1021/ja4072139 | J. Am. Chem. Soc. 2013, 135, 15990−15993

Journal of the American Chemical Society
Communication
1
considered (Figure 2). The HOMO shows some C−Si and Si−
Si π conjugation, but the HOMO-1 (Figure 2) denotes two
and 254 nm. H NMR (500.133 MHz, C6D6, 298 K, δ ppm):
7.09−7.03 (m, 3H, Har), 3.11 (m, 2H, CHMe2), 1.903 (s, 6H,
NCMe2), 1.69 (s, 2H, CH2), 1.65 (d, 6H, CHMe2), 1.22 (d,
6H, CHMe2), 1.01 (s, 6H, CMe2). 13C NMR (δ ppm): 148.6,
129.2, 128.3, 124.7, 71.9, 55.8, 50.6, 33.1, 31.8, 30.9, 30.7, 29.0,
27.6, 24.6, 22.9. 29Si NMR (δ ppm): +25.62. Compound 5
should not be dried under vacuum at room temperature for
long since it slowly decomposes and slowly loses its color.
ASSOCIATED CONTENT
* Supporting Information
■
S
Figure 2. (a) HOMO and (b) HOMO-1 of compound 2.
Synthesis, UV, crystallographic table, and theoretical details.
This information is available free of charge via the Internet at
http://pubs.acs.org. CCDC 928147 contains the supplemen-
tary crystallographic data of compound 2. These data can be
obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
weakly bonding electrons at the silicon atoms. Thus, while
compound 2 may be considered as substituted trans-2,3-disila-
1,3-butadiene, it becomes obvious that the substituents exert a
strong distortion on the π-conjugation in the parent system.16
Quantum chemical calculations at the BP86/TZVPP level
predict a singlet ground state for 2 which is 27.9 kcal/mol lower
than the triplet state and 54.8 kcal/mol lower than the quintet
state. The caption of Figure 1 provides that the optimized bond
distances and angles are in very good agreement with the
experiment. The silicon atoms exhibit a pyramidal coordination
where the Cl−Si−Si−Cl dihedral angle (calcd 45.9°, expt
43.3°) is even more twisted than the C−Si−Si−C angle (calcd
165.0°, expt 163.6°).
In conclusion, we have synthesized (Me2-cAAC)2(Si2Cl2)
(2) through a controlled reduction of the zwitterionic adduct
Me2-cAAC:SiCl4 (1) with KC8 in THF. Compound 2 can be
regarded as a 1,4-diamino-2,3-disila-1,3-butadiene derivative.
Theoretical calculation revealed that the twisted conformation
about the C−Si−Si−C fragment, which is possibly caused by
steric interactions of the bulky substituents, leads to longer
silicon−carbon bond lengths in 2.17 To the best of our
knowledge the preparation of such species has not been
reported before.
AUTHOR INFORMATION
Corresponding Author
hroesky@gwdg.de
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Dedicated to Professor C. N. R. Rao on the occasion of his 80th
birthday. H.W.R. thanks the Deutsche Forschungsgemeinschaft
(DFG RO 224/60-I) for financial support. We are thankful to
the reviewers for their valuable comments.
REFERENCES
■
(1) (a) Sekiguchi, A.; Kingo, R.; Ichinohe, M. Science 2004, 305,
1755−1757. (b) Wiberg, N.; Niedermeyer, W.; Fischer, G.; Noth, H.;
̈
Suter, M. Eur. J. Inorg. Chem. 2002, 1066−1070. (c) Sasamori, T.;
Hironaka, K.; Sugiyama, Y.; Takaki, N.; Nagase, S.; Hosoi, Y.;
Furukawa, Y.; Tokitoh, N. J. Am. Chem. Soc. 2008, 130, 13856−13857.
(2) Gau, D.; Kato, T.; Saffon-Merceron, N.; De Cozar, A.; Cossio, F.
P.; Baceiredo, A. Angew. Chem. 2010, 122, 6735−6738; Angew. Chem.,
Int. Ed. 2010, 49, 6585−6588.
Previously the NHC:SiCl4 adduct was reduced to bis-silylene
(NHC:Si(:)Cl)2 (A) with KC8. The cAAC analogue of A can
be better described as 2,3-disila-1,3-butadiene derivative (2). It
possesses a singlet ground state and is isolable and stable
between 0° and −32 °C for at least 3 months, but after one
week 2 slowly loses its color if stored at room temperature. The
bond between the silicon and the carbene carbon atom changes
its nature from a coordinate bond in 1 to a covalent electron-
sharing double bond in 2.
(3) Karni, M.; Apeloig, Y.; Schroder, D.; Zummack, W.; Rabezana,
̈
R.; Schwarz, H. Angew. Chem. 1999, 111, 343−347; Angew. Chem., Int.
Ed. 1999, 38, 331−335.
(4) (a) Brook, A. G.; Abdesaken, R.; Gutekunst, B.; Gutekunst, G.;
Kallury, R. K. J. Chem. Soc., Chem. Commun. 1981, 191−192. (b) West,
B.; Fink, M. J.; Michl, J. Science 1981, 14, 1343−1344. (c) Fischer, R.
C.; Power, P. P. Chem. Rev. 2010, 110, 3877−3923.
Synthesis of Compound (Me2-cAAC)2(Si2Cl2) (2). The
1:3 molar solid mixture of Me2-cAAC:SiCl4 (1) (3.3 mmol)
and KC8 (10.0 mmol) and THF (80 mL) were separately
cooled to −78 °C, and then THF was added through a cannula.
The reaction mixture was stirred at −78 °C for 30 min to
produce a light green solution and black residue of graphite.
Then the reaction solution was slowly warmed to room
temperature and stirred for 2−3 h. During this period, the color
of the solution changed from light green to red. The black
residue was separated by filtration, dried under vacuum, and
extracted with n-hexane (80 mL). The volume of n-hexane
solution was decreased to 10−15 mL. Transparent needles of 2
were formed after 1 day either at room temperature or at 0 °C
in a refrigerator. Compound 2 was separated by filtration and
dried under vacuum. C, H, and N analysis found (calcd) for
C40H62Cl2N2Si2; C 68.52 (68.83), H 8.85 (8.95), N 4.08 (4.01).
Decomposes above 109 °C. NMR data of 2 are given in the
Supporting Information. UV−visible bands: 615, 526, 438, 349,
(5) Weidenbruch, M.; Willms, S.; Saak, W.; Henkel, G. Angew. Chem.
1997, 109, 2612−2613; Angew. Chem., Int. Ed. 1997, 36, 2503−2504.
(6) Ghadwal, R. S.; Azhakar, R.; Roesky, H. W. Acc. Chem. Res. 2013,
44, 444−456.
(7) (a) Mondal, K. C.; Roesky, H. W.; Schwarzer, M. C.; Frenking,
G.; Tkach, I.; Wolf, H.; Kratzert, D.; Herbst-Irmer, R.; Niepotter, B.;
̈
Stalke, D. Angew. Chem., Int. Ed. 2013, 52, 1801−1805; Angew. Chem.
2013, 125, 1845−1850. (b) Mondal, K. C.; Roesky, H. W.; Schwarzer,
M. C.; Frenking, G.; Niepotter, B.; Wolf, H.; Herbst-Irmer, R.; Stalke,
̈
D. Angew. Chem., Int. Ed. 2013, 52, 2963−2967; Angew. Chem. 2013,
125, 3036−3040.
(8) (a) Martin, D.; Soleilhavoup, M.; Bertrand, G. Chem. Sci. 2011, 2,
389−399. (b) Martin, D.; Melaimi, M.; Soleilhavoup, M.; Bertrand, G.
Organometallics 2011, 30, 5304−5313.
(9) (a) Wang, Y.; Xie, Y.; Wie, P.; King, R. B.; Schaefer, H. F., III;
Schleyer, P. v. R.; Robinson, G. J. Am. Chem. Soc. 2008, 130, 14970−
14971. (b) Back, O.; Kuchenbeiser, G.; Donnadieu, B.; Bertrand, G.
Angew. Chem., Int. Ed. 2009, 48, 5530−5533; Angew. Chem. 2009, 121,
5638−5641.
15992
dx.doi.org/10.1021/ja4072139 | J. Am. Chem. Soc. 2013, 135, 15990−15993

Journal of the American Chemical Society
Communication
(10) Mondal, K. C.; Roesky, H. W.; Stuckl, A. C.; Ihret, F.; Kaim, W.;
̈
Dittrich, B.; Maity, B.; Koley, D. Angew. Chem., Int. Ed. 2013,
DOI: 10.1002/anie.201300668.
(11) 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.
(12) Martin, C. D.; Soleilhavoup, M.; Bertrand, G. Chem. Sci. 2013, 4,
3020−3030.
(13) Lee, V. Y.; Sekiguchi, A. Organometallic Compounds of Low-
Coordinate Si, Ge, Sn and Pb: From Phantom Species to Stable
Compounds; Wiley: Chichester, 2010; Chapter 5.
(14) See ref 7a for a detailed description of the bonding interactions.
(15) Fernan
́
dez, I.; Frenking, G. Chem.Eur. J. 2006, 12, 3617−
3629. The value for the C−Si distance is found in the Supporting
Information.
(16) The distortion is mainly caused by electronic effects rather than
steric interactions. Geometry optimization at BP86/TZVPP of (NH2)
HCSi(Cl)−Si(Cl)CH(NH2) show that the equilibrium geometry
has strongly pyramidal coordinated silicon atoms with a dihedral angle
Cl−Si−Si−Cl of 36.6° while the parent systems H2CSi(Cl)−
Si(Cl)CH2 is planar.
(17) Calculations with less bulky amino substituents gave also
strongly twisted conformations which suggest that the twisting of the
structure occurs due to electronic interactions. Holzmann, N. Ph.D.
Thesis, Marburg, 2013.
15993
dx.doi.org/10.1021/ja4072139 | J. Am. Chem. Soc. 2013, 135, 15990−15993