Synthesis and characterization of an amidinate-stabilized cis-1,2-disilylenylethene [cis-LSi{C(Ph)=C(H)}SiL] and a singlet delocalized biradicaloid [LSi(μ2-C2Ph2)2SiL]

Article abstract of DOI:10.1002/chem.201001141

The synthesis and characterization of novel cis-1,2-disilylenylethene [cis-LSi{C(Ph)=C(H)}SiL] (2; L=PhC(NtBu)₂) and a singlet delocalized biradicaloid [LSi(μ₂-C₂Ph₂)₂SiL] (3) are described. Compound

Full text of DOI:10.1002/chem.201001141

DOI: 10.1002/chem.201001141
Synthesis and Characterization of an Amidinate-Stabilized cis-1,2-
Disilylenylethene [cis-LSi{C(Ph)=C(H)}SiL] and a Singlet Delocalized
Biradicaloid [LSi(m2-C2Ph2)2SiL]**
AHCTUNGTRENNUNG
Hui-Xian Yeong,[a] Hong-Wei Xi,[b] Kok Hwa Lim,[b] and Cheuk-Wai So*[a]
ꢁ
Abstract: The synthesis and characteri-
zation of novel cis-1,2-disilylenylethene
[cis-LSi{C(Ph)=C(H)}SiL] (2; L=PhC-
of PhC CPh in toluene. The results
suggest that the reaction proceeds
through an [LSi{C(Ph)=C(Ph)}SiL] in-
termediate, which then reacts with an-
Compounds 2 and 3 have been charac-
terized by X-ray crystallography and
NMR spectroscopy. X-ray crystallogra-
phy and DFT calculations of 3 show
that the singlet biradicals are stabilized
by the amidinate ligand and the deloc-
ACHTUNGTRENNUNG
ꢁ
G
other molecule of PhC CPh to form 3.
A
alization within the “Si
six-membered ring.
ACHTUGNERTN(NUNG m2-C2Ph2)2Si”
Keywords: density functional calcu-
ꢁ
with one equivalent of PhC CH in tol-
uene. Compound 3 was synthesized by
the reaction of 1 with two equivalents
lations
· N ligands · radicals ·
silicon · silylene
Introduction
2,6-(Me2NCH2)2C6H3][1g] were reported and structurally
characterized. That heavier dicarbene analogues have un-
precedented electronic structure prompted our interest in
exploring their chemical reactivity. In our previous commu-
nication, we report the synthesis and characterization of a
Stable heavier Group 14 dicarbene analogues of composi-
tion RED EDR (R=supporting ligand; E=low valent Si, Ge,
ꢀ
Sn, Pb) have attracted much attention in the past few
years.[1] These compounds comprise of an ED ED single bond
stable bromosilylene [{PhC
of [{PhC
(NtBu)2}SiD]2 with bromine.[2] Recently, Roesky and
co-workers have reported the reaction of [{PhC(NtBu)2}SiD]2
ACHTUNGERTN(NUNG NtBu)2}SiBr] from the reaction
ꢀ
and a lone pair of electrons at each E atom. The formal oxi-
dation state of each E atom is +1. They can be synthesized
successfully by incorporating sterically hindered substituents
at the heavier Group 14 elements. For example, the amidi-
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
with Ph2CO to form a four-membered Si2O2 ring with five-
coordinate silicon atoms.[3] Leung et al. have prepared the
germanium analogue of a dithiocarboxylic acid anhydride
ACTHNUTRGNEUNG
nate-stabilized disilylene and digermylene [{R1C(NR2)2}ED]2
(E=Si or Ge, R1 =Ph, R2 =tBu; E=Ge, R1 =tBu, R2 =2,6-
iPr2C6H3),[1a,c,d] N-heterocyclic carbene-stabilized dichlorodi-
[{R3Ge(S)}2S] [R3 =N
ACHTNUGTERN(GUNN SiMe3)C(Ph)CACHTNURGTEG(NUNN SiMe3)ACTHUNGTREN(NUNG C5H4N-2)] by
the reaction of [R3GeD]2 with elemental sulfur.[1b]
!
ꢀ
silylene
R!(Cl)Si Si(Cl)
R
[R=DC{N(2,6-
Herein, we report the synthesis and characterization of a
cis-1,2-disilylenylethene [cis-LSi{C(Ph)=C(H)}SiL] (2; L=
iPr2C6H3)CH}2][1e] and diaryldistannylene [Ar*SnD]2 [Ar*=
PhCACHTNUTRGENN(UG NtBu)2) and a singlet delocalized biradicaloid [LSiACHTUNGTRENNUNG(m2-
ꢀ
C2Ph2)2SiL] (3) by the reaction of [LSi SiL] (1) with phenyl-
acetylene and diphenylacetylene, respectively. The theoreti-
cal studies of 3 are also described to understand its bonding
nature.
[a] H.-X. Yeong, Prof. Dr. C.-W. So
Division of Chemistry and Biological Chemistry
Nanyang Technological University
21 Nanyang Link, 637371 (Singapore)
Fax : (+65)6791-1961
E-mail: CWSo@ntu.edu.sg
[b] Dr. H.-W. Xi, Prof. Dr. K. H. Lim
Division of Chemical and Biomolecular Engineering
Nanyang Technological University
Results and Discussion
ꢁ
The reaction of 1 with one equivalent of PhC CH in tolu-
ene for 2 h afforded the novel cis-1,2-disilylenylethene [cis-
62 Nanyang Drive, 637459 (Singapore)
[**] L=PhCACHTUNGTRENNUNG(NtBu)2
LSi{C(Ph)=C(H)}SiL] (2; L=PhC
stereoisomer, trans-1,2-disilylenylethene, was not observed.
ACHTUNGTERN(NUGN NtBu)2) (Scheme 1). Its
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201001141.
12956
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2010, 16, 12956 – 12961

FULL PAPER
Scheme 1. Synthesis of compounds 2 and 3.
The reaction may proceed through a [1+2] cycloaddition of
a SiI center in 1 with PhCCH to form a silacyclopropene in-
termediate, which then undergoes an insertion with another
SiI center to form 2 (Scheme 2). Recently, we reported that
the reaction of 1 with one equivalent of bromine formed a
show a downfield shift relative to that of [{PhCACTHGUNETRNNU(G NtBu)2}SiR]
[R=NMe2, d ꢀ2.62 ppm; R=OiPr, d ꢀ13.4 ppm).[6] The
29Si NMR signals of 2 are comparable to the theoretical
29Si NMR chemical shift (d=22.7, 49.6 ppm) at the B3LYP/
6-31+G(d) level.
The molecular structure of 2 with an atomic numbering
scheme is shown in Figure 1. The amidinate ligands are
bonded in an N,N’-chelate fashion to the silicon centers and
Scheme 2. Proposed mechanism for the formation of 2.
stable monomeric bromosilylene [LSiBr].[2] In contrast, the
ꢁ
reaction of 1 with two equivalents of PhC CH in toluene af-
forded a mixture of products that cannot be identified. Toki-
ꢁ
toh and co-workers showed that the reaction of [BbtSi
Figure 1. ORTEP drawing of compound 2 (50% thermal ellipsoids). Hy-
drogen atoms are omitted for clarity. Selected bond lengths (ꢁ) and
SiBbt] (Bbt=C6H2-2,6-{CH
G
N
ethylene may proceed through a disilylene intermediate
ꢀ
ꢀ
ꢀ
ꢀ
angles (8): Si1 N1 1.875(1), Si1 N 1.886(1), Si1 C16 1.905(2), Si2 N3
(CH2CH2)SiBbt] to form a bis(silacyclopropane).[4]
ꢀ
ꢀ
ꢀ
ꢀ
1.877(1), Si2 N4 1.874(1), Si2 C17 1.927(2), C16 C17 1.346(2), C1 N1
[BbtSi
ꢀ
ꢀ
ꢀ
1.333(2), C1 N 1.343(2), C24 N3 1.335(2), C24 N4 1.343(2); N1-Si1-N2
69.07(6), N1-Si1-C16 95.54(7), N-Si1-C16 98.48(7), Si1-C16-C17
125.48(13), C16-C17-Si2 118.88(12), N3-Si2-N4 69.34(6), N3-Si2-C17
101.03(6), N4-Si2-C17 99.08(6).
Power and co-workers have reported that the reaction of
[ArMMAr] [M=Ge or Sn; Ar=C6H3-2,6-(C6H3-2,6-iPr2)2]
with PhN=NPh resulted in the complete cleavage of the
MM bond and a lone pair of electrons was found at the
[5]
ꢀ
metal centers in the product [ArM{(Ph)N N(Ph)}MAr].
On the other hand, there is no reaction between 1 and
ꢁ
ꢁ
Me3SiC CH or Me3SiC CSiMe3 in toluene.
display a trigonal pyramidal geometry. The sum of the bond
angles at the Si1 and Si2 atoms are 263.1 and 269.58, respec-
tively. They are comparable with that of the three-coordinat-
Compound 2 is isolated as a highly air- and moisture-sen-
sitive yellow crystalline solid which is soluble in toluene. It
is stable in the solid state at room temperature under an
inert atmosphere. Compound 2 has been characterized by
elemental analysis, spectroscopic methods and X-ray crystal-
lography. The 1H NMR spectrum of 2 displays resonances
due to the amidinate ligand and -C(H)=C(Ph)- substituent.
The 29Si NMR spectrum of 2 displays two singlets at d=15.1
and 29.5 ppm for the non-equivalent silicon centers, which
ed chlorosilylene [{PhCACHTNUTGRNEUNG
(NtBu)2}SiCl] (260.78).[7] This geome-
try is consistent with a stereoactive lone pair at the silicon
centers. The Si1 atom is displaced from the N1-C1-N2 least-
squares plane by 0.282 ꢁ, whereas the Si2 atom is displaced
from the N3-C24-N4 least-squares plane by 0.337 ꢁ. The
ꢀ
ꢀ
Si1 C16 (1.905(2) ꢁ) and Si2 C17 (1.927(2) ꢁ) bonds are
II
ꢀ
slightly shorter than the Si Caryl bond (1.973(2) ꢁ) in
Chem. Eur. J. 2010, 16, 12956 – 12961
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12957

C.-W. So et al.
[DSi{C6H3-2,6-
G
E
IV
ꢀ
Si1 C16 and Si2 C17 bonds are comparable with the Si
C
G
ACHTUNGTRENNUNG
(SiHMe2)2].[9] The C16 C17 bond (1.346(2) ꢁ) is a typical
C=C double bond (1.34 ꢁ). The C N bond lengths (C1
N1=1.333(2), C1 N=1.343(2), C24 N3=1.335(2), C24
N4=1.343(2) ꢁ) are approximately intermediate between
the C=N double and C N
ometry shows considerable delocalization throughout the
N1-C1-N2 and N3-C24-N4 skeletons.
ꢁ
The reaction of 1 with one or two equivalents of PhC
CPh in toluene for 2 h afforded [LSi
A
results suggest that the reaction proceeds through
a
[LSi{C(Ph)=C(Ph)}SiL] intermediate, which then reacts with
ꢁ
another molecule of PhC CPh to afford a biradicaloid. The
radicals are stabilized by the amidinate ligand and the deloc-
alization within the “SiACHTNUTRGNE(NUG m2-C2Ph2)2Si” six-membered ring, as
confirmed by X-ray crystallography and DFT calculations.
The molecular structure of 3 is shown in Figure 2. The
ꢁ
Similarly, the reaction of [ArGeGeAr] with HC CSiMe3 af-
phenyl rings on the SiACHTGUNTRENNU(G m2-C2Ph2)2Si ring are disordered. The
forded a 1,4-digermabenzene intermediate with considerable
diradical character, which then underwent a cycloaddition
with a flanking aryl ring on the terphenyl ligand.[10] Stable
disordered phenyl rings (C17A–C22A) are omitted for clari-
ty in Figure 2. Moreover, in the asymmetric unit, there are
two disordered toluene molecules, which are treated by
SQUEEZE/PLATON (for details, see the Supporting Infor-
mation). Several attempts to obtain more precise structural
parameters were unsuccessful.
singlet biradicaloid [ArGeACHTNUGRTNEUNG(m-NSiMe3)2GeAr] was synthe-
sized by the reaction of [ArGeGeAr] with N3SiMe3.[11] Re-
cently, Ando and co-workers reported that a stable 1,4-disi-
la(Dewar-benzene) 6 was synthesized successfully by heating
a dimethyldisilacyclopropenyl.[12] The 1,4-disila(Dewar-ben-
zene) 6 underwent a photochemical valence isomerization
to a 1,4-disilabenzene intermediate, which was trapped by
butadiene to form a [4+2] cycloaddition product.
Compound 3 is isolated as a highly air- and moisture-sen-
sitive dark red crystalline solid, which is soluble in CH2Cl2
and THF. It is stable in the solid state at room temperature
under an inert atmosphere. When crystals of 3 dissolve in
[D8]toluene,
[D6]benzene,
[D2]dichloromethane,
[D5]pyridine or [D8]THF at room temperature, the solution
decomposes after 1 day to give a white precipitate. The iso-
lation of products from these solutions is currently under in-
vestigation. Compound 3 is stable in [D8]1,4-dioxane at
1
Figure 2. ORTEP drawing of compound 3 (30% thermal ellipsoids). Hy-
drogen atoms and the disordered phenyl ring (C17A–C22A) are omitted
room temperature. The H NMR spectrum displays resonan-
ces for the tBu and phenyl protons. The 29Si{1H} NMR
shows a singlet at d=ꢀ51.0 ppm, which shows an upfield
shift as compared with the 29Si NMR spectrum of the 1,2-
disilabenzene 5 in [D6]benzene (d=99.2 ppm)[14] and sila-
ꢀ
for clarity. Selected bond lengths (ꢁ) and angles (8): Si1 C16 1.821(4),
ꢀ
ꢀ
ꢀ
ꢀ
Si1 C23 1.819(3), C16 C23A 1.384(5), Si1 N1 1.807(3), Si1 N2 1.819(3),
ꢀ
ꢀ
C1 N1 1.339(5), C1 N2 1.339(5); C16-Si1-C23 114.8(2), Si1-C16-C23A
123.4(2), Si1-C23-C16A 121.4(2), N1-Si1-N2 72.3(1), Si1-N1-C1 91.2(2),
N1-C1-N2 105.9(3), C1-N2-Si1 90.7(2).
benzene
4
in [D12]cyclohexane (d=92.5 ppm).[13] The
29Si{1H} NMR signal lies between the three-coordinate
silicon in 2 (d=15.1 and 29.5 ppm) and the five-coordinate
silicon in [{PhCACHTUNGTRENNUNG
suggest that compound 3 is a singlet biradicaloid instead
of a 1,4-disilabenzene derivative. The UV/Vis spectrum of 3
in CH2Cl2 at room temperature shows absorption bands
at 431 nm in the visible light region and at 299, 295, 281 and
266 nm in the ultraviolet region. The absorption band
of 3 in the visible light region shows a bathochromic shift
compared with that of the 1,4-disilabenzene [1,4-Si2C4H6]
(408, 396, 385, 340, 275 nm) in low-temperature Ar matri-
ces.[15]
The SiACHTUGNETRN(NUG m2-C2Ph2)2Si ring is almost planar (the dihedral
angle of Si1A-C23A-C16-Si1=7.438) and the sum of the in-
terior angles is 719.28. The amidinate ligands are bonded in
an N,N’-chelate fashion to the silicon centers, which display
a tetrahedral geometry. Unlike 2, the Si1 atom in 3 is dis-
placed from the N1-C1-N2 least-squares plane by 0.021 ꢁ.
Thus, the Si1-N1-C1-N2 ring is planar and almost perpendic-
ular to the “Si
ACHTUGNRTNE(NUNG m2-C2Ph2)2Si” ring (dihedral angle=87.08).
The amidinate rings are tilted (dihedral angle: 5.988). The
12958
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ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2010, 16, 12956 – 12961

Synthesis of an Amidinate-Stabilized cis-1,2-Disilylenylethene and Biradicaloid
FULL PAPER
phenyl rings on the Si
G
tion from the crystallographic data, which is due to the sub-
staggered (dihedral angle of C17-C16-C23A-C24A=35.58;
stantial disorder of the phenyl rings on the SiACHTGNUTERN(NUG m2-C2Ph2)2Si
ꢀ
for the disordered phenyl ring (C17A C22A), dihedral
ring in the X-ray structure of 3.
angle of C17A-C16-C23A-C24A=2.678). According to the
theoretical calculations, the tilted phenyl rings provide addi-
tional stabilization due to the conjugation between the
The energy difference between the singlet and triplet
state of 3 is calculated (Figure S1 and S2, see the Supporting
Information). The singlet state of 3 is more stable than the
phenyl rings and the C=C double bonds on the Si
(m2-
triplet state by 12.2 kcalmolꢀ1. The calculated Si
ACHTUNGTRENNUNG(m2-
ꢀ
C2Ph2)2Si ring. The C16 C23A bond (1.384(5) ꢁ) is similar
C2Ph2)2Si ring in the triplet state is not planar (the dihedral
angle of SiCCSi=20.38), whereas that in the singlet state is
almost planar (the dihedral angle of SiCCSi=4.848). The re-
sults are consistent with the crystallographic data. It is sug-
gested that compound 3 is a singlet biradicaloid.
to that in benzene (1.39–1.40 ꢁ), 4 (1.381(6)–1.399(6) ꢁ)[13]
and 5 (1.389(6), 1.386(6), 1.452(6) ꢁ).[14] The C16 C23A
bond is longer than the C16 C17 bond (1.346(2) ꢁ) in 2.
The Si1 C16 (1.821(4) ꢁ) and Si1 C23 (1.819(3) ꢁ) bond
lengths are almost equal to each other and intermediate be-
ꢀ
ꢀ
ꢀ
ꢀ
The nucleus-independent chemical shift at 1 ꢁ (NICS(1))
ꢀ
tween the Si C single (1.87 ꢁ) and double (1.70 ꢁ) bond
above the center of the Si
ꢀ3.5 ppm) shows that there is an electron delocalization
within the Si
(m2-C2Ph2)2Si six-membered ring.[21] In contrast,
the NICS(1) of benzene (d=ꢀ11.2 ppm) and the simple 1,4-
disilabenzene derivative (1,4-Si2C4H6) (d=ꢀ7.9 ppm) at the
B3LYP/6-31G(d) level shows aromaticity. The natural-bond-
orbital (NBO) analysis[22] (Table 1) shows that the delocali-
ACHTUNGNERTN(UGN m2-C2Ph2)2Si ring in 3 (d=
lengths.[16] The Si1 C16 and Si1 C23 bonds are significantly
shorter than those in 2 (1.905(2), 1.927(2) ꢁ) and the 1,4-
disila(Dewar-benzene) 6 (average 1.938 ꢁ).[12] They are
longer than those in 5 (1.804(4), 1.799(5) ꢁ)[14] and 4
ꢀ
ꢀ
AHCTUNGTRENNUNG
(1.765(4), 1.770(4) ꢁ).[13] The Si1 Si1A distance (3.343 ꢁ) is
ꢀ
ꢀ
significantly longer than the Si Si bond length (2.248(2) ꢁ)
in 6.[12] This suggests that there
is no Si–Si interaction in 3. The
Table 1. Pertinent bonds and the respective atomic hybridization, contribution (Contr.), occupancy, and
Wiberg bond index of compound 3.
results suggest that the singlet
biradicals are stabilized by the
delocalization within the SiACTHNUTRGNE(NUG m2-
Bond
Atom
1
Contr.
%
Hybrid
Type
Atom
2
Contr.
%
Hybrid
Type
Occupancy
Wiberg
index
C2Ph2)2Si six-membered ring.
ꢀ
Si1 N1
(s)
Si
Si
Si
C
C
C
C
C
15.0
85.0
25.1
50.0
50.0
50.0
39.5
25.1
sp4.28
sp4.28
sp2.15
sp1.83
p
N
N
C
C
C
C
N
N
85.0
15.0
74.9
50.0
50.0
50.0
60.5
74.9
sp2.47
sp2.47
sp1.95
sp1.83
p
1.94
0.24
1.93
1.96
1.81
0.59
1.98
1.95
0.41
ꢀ
The Si N bonds (average
ꢀ
Si1 N1
–
1.813 ꢁ) in 3 are shorter than
those in 2 (average 1.878 ꢁ).
(s*)
ꢀ
Si1 C16
0.94
1.34
–
ꢀ
ꢀ
The C N bond lengths (C1
(s)
ꢀ
N1=1.339(5) ꢁ,
C1 N2=
ꢀ
C16 C23A
(s)
1.339(5) ꢁ) in 3 are approxi-
ꢀ
C16 C23A
(p)
mately intermediate between
ꢀ
the C=N double and C N
(sp2)
ꢀ
C16 C23A
p
p
–
single bond lengths. This geom-
etry shows considerable deloc-
alization throughout the N1-C1-
N2 skeleton.
(p*)
ꢀ
C1 N1
sp2.15
p
sp1.89
p
1.39
–
(s)
ꢀ
C1 N1
(p)
To understand the bonding
nature in compound 3, it was
investigated by means of quan-
tum chemical calculations. The theoretical calculations of 3
were performed by means of DFT[17] B3LYP[18] with the 6-
31G(d) basis set as implemented in the Gaussian 03 pro-
gram[19] due to the large molecular structure of 3. The calcu-
zation system of the SiACHTNGUTRENN(UG m2-C2Ph2)2Si ring in 3 is comprised of
ꢀ
ꢀ
the p and p* orbitals of the C16 C23A and C16A C23
bonds (total electron occupancy for p=3.62; for p*=1.18)
ꢀ
ꢀ
ꢀ
and the s* orbitals of the Si1 N1, Si1 N, Si1A N1A, and
ꢀ
ꢀ
ꢀ
lated structural parameters (Si1 C16 1.813 ꢁ, Si1 N1
Si1A N2A bonds (total electron occupancy=0.97). The p
ꢀ
ꢀ
ꢀ
ꢀ
1.910 ꢁ, C16 C23A 1.432 ꢁ, C1 N1 1.341 ꢁ; C16-Si1-C23
orbitals of the C16 C23A/C16A C23 bonds form p–s* con-
ꢀ
114.68, Si1-C16-C23A 122.68, N1-Si1-N 68.88) are in good
jugation with the s* orbitals of the Si N bonds (second
ꢀ
agreement with the crystallographic data except the Si1 N1
order perturbation stabilizing energy: 5.15 or 4.44 kcal
molꢀ1) (Table S1, see the Supporting Information). The re-
sults suggest that the radicals are stabilized by the amidinate
bond. The 6-31G(d) basis set does not include the diffused
function for electron delocalization, and advanced multiple
polarization functions, although there are pC C!s*SiꢀN
,
ligand and the delocalization within the Si
membered ring.
ACHTUNGNERT(NUNG m2-C2Ph2)2Si six-
=
pC N!s*SiꢀC, and 1N!s*SiꢀC donor–acceptor interactions
=
ꢀ
ꢀ
around the Si1 N1 bond. Therefore, the calculated Si1 N1
bond (1.9103 ꢁ) is slightly longer than the crystallographic
data.[20] Moreover, the calculated dihedral angle of the
In the calculated structure of 3, the amidinate rings are
tilted with respect to the Si(m2-C2Ph2)2Si ring (dihedral angle
of N-Si-Si-N: 12.48). The phenyl rings on the Si(m2-C2Ph2)2Si
ring are also tilted and staggered. The resulting structure is
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
phenyl rings at the SiACHTNUTRGNE(UNG m2-C2Ph2)2Si ring (17.28) shows devia-
Chem. Eur. J. 2010, 16, 12956 – 12961
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12959

C.-W. So et al.
1
The H, 13C, 29Si NMR spectra were recorded on a JEOL ECA 400 spec-
a stable minimum. When the amidinate and phenyl rings are
perpendicular to the Si(m2-C2Ph2)2Si ring (dihedral angle of
the amidinate rings=08, D2h symmetry), the resulting struc-
ture is less stable than compound 3 by 57.0 kcalmolꢀ1. When
the amidinate rings are tilted and the phenyl rings are per-
trometer. The chemical shifts d are relative to SiMe4 for H, 13C and 29Si.
Elemental analyses were performed by the Division of Chemistry and
Biological Chemistry, Nanyang Technological University. Melting points
were measured in sealed glass tubes and were not corrected.
1
AHCTUNGTRENNUNG
ꢁ
Compound 2: PhC CH (22.5 mL, 0.21 mmol) was added dropwise to a
pendicular to the Si
is less stable than compound 3 by 35.8 kcalmolꢀ1. When the
phenyl rings on the Si(m2-C2Ph2)2Si six-membered ring are
A
solution of 1 (0.11 g, 0.21 mmol) in toluene (16 mL) at ambient tempera-
ture and stirred for 2 h. After filtration and concentration of the filtrate,
2 was obtained as yellow crystals (0.02 g; 13.6%). M.p. 1158C; 1H NMR
(395.9 MHz, [D6]benzene, 258C): d=1.15 (s, 18H, tBu), 1.22 (s, 18H,
tBu), 6.88–7.16 (m, 10H, C=C(H)+Ph), 7.36–7.40 (m, 3H, Ph), 7.54–
7.58 ppm (m, 3H, Ph); 13C NMR (99.5 MHz, [D6]benzene, 258C): d=31.1
(CMe3), 31.7 (CMe3), 52.7 (CMe3), 52.9 (CMe3), 125.4, 128.3, 128.5, 128.6,
128.8, 130.3, 130.6, 134.9, 135.1, 150.1 (Ph), 157.1 ((Ph)C=C(H)), 158.2
((Ph)C=C(H)), 170.8 (NCN), 173.9 ppm (NCN); 29Si NMR (78.7 MHz,
[D6]benzene, 258C): d=15.1, 29.5 ppm; elemental analysis calcd (%) for
C38H52N4Si2: C 73.50, H 8.45, N 9.03; found: C 73.28, H 8.16, N 8.75.
AHCTUNGTRENNUNG
tilted, this results in the p orbitals of the phenyl rings form-
ꢀ
ing p–p* conjugation with the p* orbitals of the C16 C23A/
ꢀ
C16A C23 bonds (total second order perturbation stabiliz-
ing energy: 19.7 kcalmolꢀ1) (Table S1, see the Supporting in-
formation). This conjugation effect provides an additional
stabilization in compound 3.
Since the Si1-N1-C1-N2 ring is planar, this leads to
Crystal data for
2 (C45H60N4Si2): Mr =713.15; a=10.3544(2), b=
13.7776(3), c=17.4384(5) ꢁ;
a=111.101(2),
b=90.966(2),
¯
g=
pC !s*SiꢀC and 1N!s*SiꢀC hyperconjugation around the
=
ꢀN
111.998(1)8; V=2118.6(9) ꢁ3; Z=2; space group P1 (triclinic); T=
Si N bonds (the sum of second order perturbation stabiliz-
103(2) K; l=0.71073 ꢁ; m=0.118 mmꢀ1
;
1calcd =1.118 gcmꢀ3
;
F
N
ing energy of pC N!s*SiꢀC and 1N!s*SiꢀC =4.78 or 4.65 kcal
=
772; 46961 measured reflections, 13055 independent reflections and 473
molꢀ1). The electronic delocalization (pC N!s*SiꢀC and 1N!
refined parameters; R1=0.0509, wR2=0.1293 (I>2s(I)); largest diff.
=
ꢀ
peak and hole 0.424/ꢀ0.324 eꢁꢀ3
.
s*SiꢀC) shortens the Si N bonds, although 3 has a substantial
ꢁ
ꢀ
Compound 3: A solution of PhC CPh (0.073 g, 0.42 mmol) in toluene
(1.2 mL) was added dropwise to a solution of 1 (0.11 g, 0.21 mmol) in tol-
uene (16.2 mL) at ambient temperature. After stirring for 2 h, a dark red
suspension was formed. Dichloromethane (ꢂ10 mL) was added. The re-
sulting solution was filtered and concentrated to obtain dark-red crystals
of 3. After two hours, the mother liquid was filtered; otherwise the dark
population in the Si N s* orbitals and steric hindrance aris-
ing from the substituents. Similar hyperconjugation is not
found in compound 2, in which the amidinate Si-N-C-N ring
ꢀ
is not planar. The calculated Si N bonds (1.927, 1.924,
1.923, 1.916 ꢁ) in 2 at the B3LYP/6-31G(d) level are longer
than those in 3. The results are consistent with the X-ray
red crystals decomposed slowly to give
a yellow precipitate. Yield:
0.103 g (47.0%). M.p. 1538C (dec); 1H NMR (399.5 MHz, [D8]1,4-diox-
ane, 258C): d=0.983 (s, 36H, tBu), 6.81–6.91 ppm (m, 30H, Ph);
13C{1H} NMR (100.4 MHz, [D8]1,4-dioxane, 258C): d=31.7 (CMe3), 66.08
(CMe3), 125.20, 128.10, 128.15, 128.29, 128.40, 128.89, 129.44, 130.22,
131.50, 138.80 (Ph), 158.46 ((Ph)C=C(Ph)), 181.29 ppm (NCN);
29Si{1H} NMR (79.4 MHz, [D8]1,4-dioxane, 258C): d=ꢀ51.0 ppm; UV/
Vis (CH2Cl2): lmax (e)=266 (19744), 281 (14596), 295 (13047), 299
(13933), 431 nm (2223 molꢀ1 m3 cmꢀ1); elemental analysis calcd (%) for
C58H66N4Si2: C 79.59, H 7.61, N 6.41; found: C 79.32, H 7.51, N 6.24.
ꢀ
data of 2 and 3, in which the Si N bonds (average 1.813 ꢁ)
in 3 are shorter than those in 2 (average 1.878 ꢁ).
The calculated enthalpies of reaction for compounds 2
and cis-[LSi{C(Ph)=C(Ph)}SiL] show that compound 2 is
more stable than [LSi{C(Ph)=C(Ph)}SiL] by 14.5 kcalmolꢀ1
(2: DH0reaction =ꢀ35.5 kcalmolꢀ1
;
[LSi{C(Ph)=C(Ph)}SiL]:
DH0reaction =ꢀ21.0 kcalmolꢀ1) in view of thermodynamics.
The higher stability of compound 2 means that it may not
Crystal data for
3 (C72H82N4Si2): Mr =1059.60; a=13.8659(2), b=
13.8659(2), c=28.2606(6) ꢁ; a=90, b=90, g=1208; V=4705.5(1) ꢁ3;
Z=3; space group P3(1)21 (trigonal); T=173(2) K; l=0.71073 ꢁ; m=
ꢁ
undergo further reaction with another molecule of PhC CH
to form a singlet delocalized biradicaloid in the reaction of 1
0.101 mmꢀ1; 1calcd =1.122 gcmꢀ3; F
ACTHNUTRGNEU(GN 000)=1710; 29415 measured reflec-
ꢁ
with PhC CH.
tions, 5553 independent reflections and 350 refined parameters; R1=
0.0743, wR2=0.2156 (I>2s(I)); largest diff. peak and hole 0.437/
ꢀ0.323 eꢁꢀ3
.
X-ray data collection and structural refinement: The crystal data were
collected on a Bruker APEX II diffractometer. The structures were
solved by direct phase determination (SHELXS-97) and refined for all
data by full matrix least squares methods on F2.[23] All non-hydrogen
atoms were subjected to anisotropic refinement. The hydrogen atoms
were generated geometrically and allowed to reside in their respective
parents atoms; they were assigned appropriate isotopic thermal parame-
ters and included in the structure-factor calculations. SQUEEZE/
PLATON was employed to treat two highly disordered toluene mole-
cules in the asymmetric unit of 3. The toluene molecules were removed
from the model, but included in the empirical formula. Additional
PLATON/SQUEEZE details can be found in the Supporting Information
and the CIF file of 3. CCDC-757725 (3) and 757726 (2) contain the sup-
plementary crystallographic data for this paper. These data can be ob-
tained free of charge from The Cambridge Crystallographic Data Center
via www.ccdc.cam.ac.uk/data_request/cif.
Conclusion
A novel cis-1,2-disilylenylethene 2 has been synthesized suc-
cessfully by the reaction of 1 with one equivalent of phenyl-
acetylene. Moreover, a singlet delocalized biradicaloid 3 has
been synthesized successfully by the reaction of 1 with two
equivalents of diphenylacetylene. X-ray crystallography and
DFT calculations show that the singlet biradicals are stabi-
lized by the amidinate ligand and the delocalization within
the SiACHTUNGTRENNUNG
(m2-C2Ph2)2Si six-membered ring.[24]
Experimental Section
General procedure: All manipulations were carried out under an inert at-
mosphere of argon by using standard Schlenk techniques in an argon
filled glove box. Solvents were dried over and distilled over Na/K alloy
prior to use. Compound 1 was prepared as described in the literature.[1d]
12960
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ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2010, 16, 12956 – 12961

Synthesis of an Amidinate-Stabilized cis-1,2-Disilylenylethene and Biradicaloid
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Received: April 29, 2010
Published online: September 28, 2010
Chem. Eur. J. 2010, 16, 12956 – 12961
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
12961