Synthesis and characterization of a singlet delocalized 2,4-diimino-1,3-disilacyclobutanediyl and a silylenylsilaimine

Article abstract of DOI:10.1002/chem.201103351

The synthesis and characterization of a singlet delocalized 2,4-diimino-1,3-disilacyclobutanediyl, [LSi(μ-CNAr)₂SiL] (2, L: PhC(NtBu)₂, Ar: 2,6-iPr₂C₆H₃), and a silylenylsilaimine, [LSi(=NAr)-SiL] (3), are described. The reaction of three equivalents of the disilylene [LSi-SiL] (1) with two equivalents of ArN=C=NAr in toluene at room temperature for 12 h afforded [LSi(μ-CNAr)₂SiL] (2) and [LSi(=NAr)-SiL] (3) in a ratio of 1:2. Compounds 2 and 3 have been characterized by NMR spectroscopy and X-ray crystallography. Compound 2 was also investigated by theoretical studies. The results show that compound 2 possesses singlet biradicaloid character with an extensive electronic delocalization throughout the Si₂C₂ four-membered ring and exocyclic C=N bonds. Compound 3 is the first example of a silylenylsilaimine, which contains a low-valent silicon center and a silaimine substituent. A mechanism for the formation of 2 and 3 is also proposed.

Full text of DOI:10.1002/chem.201103351

DOI: 10.1002/chem.201103351
Synthesis and Characterization of a Singlet Delocalized 2,4-Diimino-1,3-
disilacyclobutanediyl and a ACTHUNTGRNEUNGSilylenylsilaimine
Shu-Hua Zhang,[a] Hong-Wei Xi,[b] Kok Hwa Lim,[b] Qingyong Meng,[c]
Ming-Bao Huang,[c] and Cheuk-Wai So*[a]
ꢀ
Abstract: The synthesis and characteri-
zation of a singlet delocalized 2,4-diim-
ino-1,3-disilacyclobutanediyl, [LSi(m-
CNAr)2SiL] (2, L: PhC(NtBu)2, Ar:
(=NAr) SiL] (3) in a ratio of 1:2. Com-
pounds 2 and 3 have been character-
ized by NMR spectroscopy and X-ray
crystallography. Compound 2 was also
investigated by theoretical studies. The
results show that compound 2 possesses
singlet biradicaloid character with an
extensive electronic delocalization
throughout the Si2C2 four-membered
ring and exocyclic C=N bonds. Com-
pound 3 is the first example of a silyl-
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
2,6-iPr2C6H3), and a silylenylsilaimine,
ꢀ
[LSi
G
ACHTUNGTNEReUNGN nylACHTUNRTGENGNsUN ilaCAHTNUGTRNEiNUGN mine, which contains a low-
The reaction of three equivalents of
valent silicon center and a silaimine
substituent. A mechanism for the for-
mation of 2 and 3 is also proposed.
ꢀ
the disilylene [LSi SiL] (1) with two
Keywords: density functional calcu-
lations · N ligands · radicals · sili-
con · silylenes
equivalents of ArN=C=NAr in toluene
at room temperature for 12 h afforded
[LSiACHTUNGTRENNUNG(m-CNAr)2SiL] (2) and [LSi-
Introduction
cause the molecule exists as a short-lived intermediate. Re-
cently, several research groups demonstrated that the intro-
duction of main-group elements into hydrocarbon biradicals
enables their stabilities and spin preferences to be tuned.
One of the well-studied examples is cyclobutane-1,3-diyl,
B,[2] and its heavier main-group analogues C–H
(Scheme 2).[3] Cyclobutane-1,3-diyl, B, is an unstable and lo-
calized triplet biradical. In contrast, the heavier main-group
analogues C–H are stable at room temperature and have
singlet ground states. Moreover, the biradical character can
be regulated by varying the substituents in the heavier
main-group analogues, which is difficult to achieve in the
carbon congener.[4] Therefore, if 2,4-dimethylene-1,3-cyclo-
butanediyl, A, contained heavier main-group element(s) in
the ring skeleton, its geometric and electronic structure
would be of particular interest from a fundamental point of
view. However, the synthesis of a stable main-group ana-
Delocalized biradical, 2,4-dimethylene-1,3-cyclobutanediyl,
A, has attracted much attention, both theoretically and ex-
perimentally, due to its unique electronic structure and po-
tential application in organic magnetic materials.[1] It is a
non-Kekulꢀ molecule and has a triplet ground state. The
radicals delocalize extensively throughout the four-mem-
bered ring and exocyclic C=C double bonds (Scheme 1).
However, the experimental study of A remains difficult be-
Scheme 1. Delocalized biradical 2,4-dimethylene-1,3-cyclobutanediyl (A).
logue of 2,4-diACHTUNGTRENNmNUG ethACHTNURTGEGyNNNU lCAHTNUGTRENeGUNN ne-1,3-cyclobutanediyl is still a formi-
dable challenge and no examples have been reported.
Recently, an amidinate ligand has been shown to stabilize
electronic delocalization in unsaturated silicon compounds,
such as the germatrisilacyclobutadiene ylide, siladithiocar-
boxylate, zwitterionic 2,4-disila-1,3-diphosphacyclobuta-
diene, tetrasilacyclobutadiene dication, and 1,4-disilaben-
zene derivative.[5] We anticipate that an amidinate ligand is
[a] S.-H. Zhang, Prof. Dr. C.-W. So
Division of Chemistry and Biological Chemistry
Nanyang Technological University
21 Nanyang Link, Singapore 637371
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
capable of stabilizing a main-group analogue of 2,4-di
ylene-1,3-cyclobutanediyl.
In this paper, we describe the synthesis and characteriza-
tion of a singlet delocalized 2,4-diimino-1,3-disilacyclobu-
tanediyl, [LSi(m-CNAr)2SiL] (2, L: PhC(NtBu)2, Ar: 2,6-
ACHTUNGTRENNUNGmeth-
A
A
62 Nanyang Drive, Singapore 637459
[c] Q. Meng, Prof. Dr. M.-B. Huang
ACHTUNGTRENNUNG
College of Chemistry and Chemical Engineering
Graduate University of Chinese Academy of Sciences
P.O. Box 4588, Beijing 100049 (P. R. China)
A
E
N
ACHTUNGTRENNUNG
iPr2C6H3). Its biradicaloid character and electronic delocali-
zation are illustrated by theoretical studies.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201103351.
4258
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Chem. Eur. J. 2012, 18, 4258 – 4263

FULL PAPER
concentration, pure 2 was afforded as dichromic yellow-
brown crystals in 25.7% yield. Subsequently, the dichromic
yellow-red filtrate was concentrated to afford a mixture of 2
(minor product) and yellow crystals of 3 (major product).
An attempt to isolate pure 3 by recrystallization failed.
Compounds 2 and 3 have been characterized by NMR spec-
troscopy and X-ray crystallography. Compound 2 was also
investigated by theoretical studies. The results show that 2 is
best described as 2’, in which it possesses singlet biradicaloid
character with an extensive electronic delocalization
throughout the Si2C2 four-membered ring and exocyclic C=
N bonds (see below). To the best of our knowledge, com-
pound 2 is the first example of a four-membered-ring delo-
calized biradicaloid, which can be considered as a main-
group analogue of 2,4-dimethylene-1,3-cyclobutanediyl.
Other examples of silicon-containing four-membered-ring
biradicaloids such as H (Scheme 2),[3e] a pentasilapropel-
ACTHNUTRGNEUNG
lane[7] and a 1,3-disilabicyclo[1.1.0]butane[8] have been re-
ported. Recently, we and Roesky and co-workers independ-
ently reported the six-membered-ring delocalized biradica-
loid [LSi(m2-CPh=CPh)2SiL], which can be considered as a
1,4-disilabenzene derivative.[5f,g]
Compound 3 is the first example of a silylenylsilaimine,
which contains a low-valent silicon center and a silaimine
substituent. Silaimines, which contain a Si=N bond, are rare.
We reported that the amidinate-stabilized silaimine [LSi-
(=NAr)N(Ar)L’] (L’: Si(Cl)
sized by the reaction of the silylsilylene [LSiSi(Cl)-
{(NtBu)2C(H)Ph}] with excess ArN3.[9] Roesky and co-work-
ers also showed that the chlorosilaimine [LSi(=NAr)Cl] can
ACHTUNGERTN{NUNG (NtBu)2C(H)Ph}) was synthe-
Scheme 2. Cyclobutane-1,3-diyl (B) and its heavier main-group analogues
C–H.
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
Results and Discussion
be synthesized by the reaction of the chlorosilylene [LSiCl]
with ArN=C=NAr.[10] The by-product of the reaction is
[6]
ꢀ
The reaction of three equivalents of [LSi SiL] (1) with
ArNC. Other silaimines, such as [{tBuNCH=CHN
ACHTUNGTRENNUNG(tBu)}-
two equivalents of ArN=C=NAr in toluene at room temper-
A
ACHTUNGTRENNUNG
ature for 12 h afforded the singlet delocalized 2,4-diimino-
1,3-disilacyclobutanediyl [LSiACHTNURGTNEUNG(m-CNAr)2SiL] (2) and the
(R: CH2Ph, Ph, 1-adamantyl, SiMe3), were synthesized by
the reaction of the corresponding silylenes with organo-
silyl
enyl
E
G
azides.[11]
ACHTUNGTRENNUNG
ꢀ
(Scheme 3), a result confirmed by NMR spectroscopy. The
reaction suspension was filtered to give a dichromic yellow-
red filtrate and a dichromic yellow-brown residue. The di-
chromic yellow-brown residue was extracted with toluene to
give a dichromic yellow-brown solution. After filtration and
A mechanism for the formation of 2 and 3 is proposed.
The SiI centers in 1 undergo a [1+2] cycloaddition with
ArN=C=NAr to form a bis(silaaziridinimine) intermediate I,
which then eliminates two nitrene intermediates (DN-Ar) to
release the ring strain and to form
a disilabicyclo-
Scheme 3. Synthesis of 2 and 3.
Chem. Eur. J. 2012, 18, 4258 – 4263
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www.chemeurj.org
4259

C.-W. So et al.
N2-C18-N3 ring is planar and
tilted with respect to the Si2C2
four-membered ring (dihedral
ꢀ
angle=79.48).
The
Si1 C1
ꢀ
(1.834(5) ꢂ)
(1.829(5) ꢂ) bond lengths are
almost identical and are inter-
and
Si1 C1A
ꢀ
mediate between the Si C
single (1.87 ꢂ) and double
(1.70 ꢂ) bond lengths.[14] The
ꢀ
C1 N1
bond
length
(1.323(5) ꢂ) is approximately
intermediate between the C=N
and C N
The results indicate that there
is an electronic delocalization.
(sp2) bond lengths.
ꢀ
Scheme 4. Proposed mechanism for the formation of 2 and 3.
The
Si1···Si1A
distance
ACHTUNGTRENNUNG[1.1.0]butane-2,4-diimine intermediate J (Scheme 4). The ni-
trene intermediate reacts with another molecule of 1 to
ꢀ
form 3. Subsequently, the Si Si bond in J is cleaved homo-
lytically to form 2. The delocalized biradicaloid character of
2 has been confirmed by X-ray crystallography and theoreti-
cal studies (see below).
Compounds 2 and 3 are stable in solution or the solid
state at room temperature under an inert atmosphere. They
are soluble in toluene and THF. Compounds 2 and 3 were
1
characterized by NMR spectroscopy. The H and 13C NMR
spectra of 2 display resonances due to the amidinate ligand
and the Ar substituents. The 29Si{1H} NMR spectrum of 2
exhibits a singlet at d=ꢀ39.9 ppm, which lies between the
three-coordinate silicon atom in [LSiCl] (d=14.6 ppm) and
the five-coordinate silicon atom in [LSiCl3] (d=
ꢀ98.6 ppm).[12] Compound 2 does not show any EPR signals
at room temperature or ꢀ1008C, which indicates that it has
a singlet ground state. With pure 2 at hand, the signals for
compound 3 in the NMR spectra of the mixture of 2 and 3
can be assigned. In the 29Si{1H} NMR spectrum of the mix-
Figure 1. Molecular structure of 2 with thermal ellipsoids at the 50%
probability level. One of two independent molecules is shown. Hydrogen
atoms are omitted for clarity. Selected bond lengths [ꢂ] and angles [8]:
ꢀ
ꢀ
ꢀ
ꢀ
Si1 C1 1.834(5), Si1 C1A 1.829(5), C1 N1 1.323(5), Si1 N2 1.849(4),
ꢀ
ꢀ
ꢀ
Si1 N3 1.841(4), N2 C18 1.357(5), N3 C18 1.339(5); C1-Si1-C1A
91.6(2), Si1-C1-Si1A 88.4(2), Si1-C1-N1 141.1(3), Si1A-C1-N1 130.2(3),
C1A-Si1-N2 117.99(19), C1-Si1-N2 132.85(18), C1A-Si1-N3 127.43(19),
C1-Si1-N3 120.53(19), N2-Si1-N3 71.11(16), Si1-N3-C18 92.1(3), Si1-N2-
C18 91.2(3), N2-C18-N3 105.5(4).
ꢀ
ture, two singlets at d=ꢀ61.2 (Si=NAr) and 32.4 ppm (DSi
SiNAr) are attributable to the nonequivalent silicon centers
in 3. The 29Si NMR resonance (d=ꢀ61.2 ppm) for the four-
coordinate silicon center in 3 shows a downfield shift com-
pared with that of [LSiACHTUGNTRENNNUG
29Si NMR resonance (d=32.4 ppm) for the three-coordinate
silicon center in 3 is comparable with that of the silylsilylene
ꢀ
(2.553(2) ꢂ) is significantly longer than typical Si Si single
[LSiSi(Cl)
{(NtBu)2C(H)Ph}] (d=26.8 ppm).[13] The UV/Vis
bonds (average 2.34 ꢂ). The N2 C18 (1.357(5) ꢂ) and N3
ꢀ
ꢀ
spectrum of 2 in toluene at room temperature shows absorp-
tion bands at l=436 nm in the visible light region and at
l=285 and 224 nm in the ultraviolet region.
The molecular structure of 2 is shown in Figure 1. In the
asymmetric unit, there are two independent molecules with
slightly different bond lengths and angles. Only one of them
is discussed here for clarity. The Si2C2 four-membered ring
is planar and the sum of the interior angles is 3608. The ami-
dinate ligands are bonded in a N,N’-chelate fashion to the
silicon centers, which adopt a tetrahedral geometry. The Si1-
C18 (1.339(5) ꢂ) bond lengths are approximately intermedi-
ꢀ
ate between the C=N and C N
(sp2) bond lengths. This ge-
ometry shows considerable delocalization throughout the
N2-C18-N3 skeleton.
The molecular structure of 3 is shown in Figure 2. The
amidinate ligands are bonded in a N,N’-chelate fashion to
the Si1 and Si2 atoms, which adopt a distorted tetrahedral
and a trigonal pyramidal geometry, respectively. The sum of
the bond angles at the Si2 atom is 283.88, which is compara-
ble with that of the three-coordinated silylsilylene
4260
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Chem. Eur. J. 2012, 18, 4258 – 4263

Synthesis of a Singlet Delocalized Biradicaloid
FULL PAPER
ꢀ
s orbitals for the Si C bonds (electron occupancy: 1.934 for
ꢀ
ꢀ
ꢀ
ꢀ
Si1 C1, 1.921 for Si1 C1A). The C1 N1 and C1A N1A
bonds are comprised of occupied s and p orbitals. The elec-
ꢀ
ꢀ
tron occupancies for the C1 N1 and C1A N1A s orbitals
ꢀ
ꢀ
are 1.996, whereas those for the C1 N1 and C1A N1A p
orbitals are 1.979. Moreover, the second-order perturbation
ꢀ
ꢀ
theory analysis shows that the C1 N1 and C1A N1A p or-
bitals have very weak donor–acceptor interactions (Table S2
in the Supporting Information). The results indicate that the
ꢀ
ꢀ
p electrons in the C1 N1 and C1A N1A bonds are almost
localized. It is noteworthy that there are 2.028 electrons dis-
ꢀ
tributed among the Si N s* orbitals and the exocyclic C=N
ꢀ
ꢀ
p* orbitals. Moreover, the C1 N1 and C1A N1A p* orbi-
tals form significant p*–s* hyperconjugations with the s*
ꢀ
orbitals of the Si N bonds (sum of the second-order pertur-
bation stabilizing energies: 45.21 kcalmolꢀ1 for the C1 N1
ꢀ
p* orbital, 45.21 kcalmolꢀ1 for the C1A N1A p* orbital).
The results show that the biradical is delocalized appreciably
Figure 2. Molecular structure of 3 with thermal ellipsoids at the 50%
probability level. Hydrogen atoms are omitted for clarity. Selected bond
ꢀ
ꢀ
ꢀ
ꢀ
lengths [ꢂ] and angles [8]: Si1 N1 1.8650(16), Si1 N2 1.8517(17), Si1 N3
ꢀ
through the Si N s* orbitals and the exocyclic C=N p* orbi-
ꢀ
ꢀ
ꢀ
ꢀ
1.5921(16), Si1 Si2 2.3636(8), Si2 N4 1.8698(18), Si2 N5 1.8659(18), C1
tals. The electronic delocalization can be illustrated in a rep-
resentation of the highest occupied molecular orbital
(HOMO; Figure 3). The presence of electronic delocaliza-
ꢀ
ꢀ
ꢀ
N1 1.336(3), C1 N2 1.341(2), C28 N4 1.345(3), C28 N5 1.340(3); N3-
Si1-N1 121.32(8), N3-Si1-N2 119.25(9), N3-Si1-Si2 109.06(7), N1-Si1-N2
70.33(7), N1-Si1-Si2 120.93(6), N2-Si1-Si2 110.71(6), N4-Si2-Si1
108.40(6), N5-Si2-Si1 105.77(6), N4-Si2-N5 69.65(8).
[LSiSi(Cl)ACHTUNGTRENNUNG
{(NtBu)2C(H)Ph}] (276.98).[13] This geometry is
consistent with a stereoactive lone pair at the Si2 atom. The
ꢀ
Si1 N3 bond length (1.5921(16) ꢂ) is comparable with that
in [LSi
G
E
(1.589(5) ꢂ; L’: Si(Cl)
bond length (2.3636(8) ꢂ) is comparable with that in
[LSiSi(Cl)
{(NtBu)2C(H)Ph}] (2.381(7) ꢂ).[13]
{(NtBu)2C(H)Ph}).[9] The Si1 Si2
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
The energy difference between the singlet and triplet
states of 2 was calculated at the B3PW91/6-31G(d) level
(Figure S1 in the Supporting Information). The singlet state
of 2 is more stable than the triplet state by 30.1 kcalmolꢀ1.
The results reveal that 2 has a singlet ground state. In con-
trast, the 2,4-dimethylene-1,3-cyclobutanediyl A has a triplet
ground state.
To understand the bonding nature in 2, the simple deriva-
ACHTUNGTRENNUNG
tive [LHSi(m-CNH)2SiLH] (2A, LH: HC(NH)2, Scheme 5)
was investigated by DFT calculations at the B3PW91/6-
311+G
(d,p) level.[15] The optimized structure is in good
agreement with the crystallographic data (Figure S2 in the
ACHTUNGTRENNUNG
Supporting
Information).
The
natural-bond-orbital
(NBO)[16] analysis (Table S1 in the Supporting Information)
shows that the Si2C2 four-membered ring has four occupied
Figure 3. A representation of the HOMO of 2A shows the interaction
ꢀ
between the Si N s* orbitals and the exocyclic C=N p* orbitals: top
view (top), side view (bottom).
tion is also supported by the nucleus-independent chemical
shift (NICS1: ꢀ8.90 ppm; NICS0: ꢀ14.57 ppm).[17] The elec-
ꢀ
tronic delocalization strengthens the Si C bonds and the
substantial electronic population at the exocyclic C=N p*
ꢀ
ꢀ
orbitals (C1 N1: 0.491 electrons, C1A N1A: 0.491 elec-
trons) weakens the C=N double bonds. The results are con-
sistent with the crystallographic data of 2.
Scheme 5. Compound 2A, which was investigated by DFT calculations.
Chem. Eur. J. 2012, 18, 4258 – 4263
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4261

C.-W. So et al.
(15 mL) at room temperature. The resulting yellow/red suspension was
allowed to stir for 12 h. The reaction suspension was filtered to give a di-
chromic yellow-red filtrate and a dichromic yellow-brown residue. The
dichromic yellow-brown residue was extracted with toluene to give a di-
chromic yellow-brown solution. After filtration and concentration, pure 2
was afforded as dichromic yellow-brown crystals. Subsequently, the di-
chromic yellow-red filtrate was concentrated to afford a mixture of 2 and
yellow crystals of 3. An attempt to isolate pure 3 by recrystallization
failed. Therefore, the chemical yield, melting point, and elemental analy-
sis of 3 cannot be obtained. By reference to the NMR spectra of pure 2,
the NMR resonances of 3 were identified in the NMR spectra of the mix-
ture of 2 and 3.
To illustrate that 2 possesses biradicaloid character, 2A
was optimized at the UB3LYP/6-311+G(d,p) level. The
complete active space self-consistent field CASSCF(4,6) cal-
culations indicate that there are two dominant contributions
to the configuration-interaction (CI) wavefunction for the
1A state (see the Supporting Information), which correspond
to occupation numbers of 1.86 in the HOMO and of 0.13 in
the lowest unoccupied molecular orbital plus two (LUMO+
2). The results indicate that 2 has some biradicaloid charac-
ter.
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
Compound 2: Yield: 0.021 g (25.7%); m.p. 291.28C; 1H NMR
According to the time-dependent DFT (TD-DFT) calcu-
lations for 2 at the B3PW91/6-31G(d) level, the absorption
band (l=436 nm) observed in the UV/Vis spectrum of 2 is
(395.9 MHz, [D6]benzene, 23.68C): d=1.23 (s, 36H; tBu), 1.60 (d, 24H,
3JHꢀH =6.8 Hz; CH(CH3)2), 4.05 (sept, 4H, 3JHꢀH =6.8 Hz; CH
ACHTNUTRGENNUG ACHTUNGTRENNUNG(CH3)2),
6.81–6.89 (m, 6H; Ph), 7.01–7.03 (m, 1H; Ph), 7.12–7.14 (m, 4H; Ph),
7.35 (m, 1H; Ph), 7.39–7.41 ppm (d, 4H; Ph); 13C{1H} NMR (100.5 MHz,
assigned to
a
mixture of HOMO!LUMO+5 and
HOMO!LUMO+6 transitions. The HOMO shows inter-
[D6]benzene, 24.28C): d=23.3 (CHACTHUNGRTE(UNNG CH3)2), 28.3 (CHACTHUNGTRNEN(UNG CH3)2), 30.7
(CMe3), 54.5 (CMe3), 119.6, 122.2, 128.4, 129.8, 131.4, 138.6 (Ph and Ar),
159.1 (C=NAr), 177.1 ppm (NCN); 29Si{1H} NMR (78.7 MHz,
[D6]benzene, 23.58C): d=ꢀ39.9 ppm; UV/Vis (toluene): lmax (e)=224
(4493), 285 (6594), 436 nm (13569 dm3 molꢀ1 cmꢀ1); elemental analysis:
calcd (%) for C56H80N6Si2: C 75.29, H 9.03, N 9.41; found: C 75.11, H
8.75, N 9.23.
ꢀ
action between the Si N s* orbitals and the exocyclic C=N
p* orbitals, whereas LUMO+5 and LUMO+6 involve the
p* orbitals of the Ar rings (see the Supporting Information).
Moreover, the calculations show an allowed transition from
HOMOꢀ1!LUMO (l=417 nm). However, this transition
is calculated to give a very weak absorption band, which
could be overlapped by the absorption band observed (l=
436 nm) in the UV/Vis spectrum of 2.
Crystallographic data for 2: [C56H80N6Si2]; Mw: 893.44; triclinic; space
group Pꢀ1; a=11.0000(8), b=12.8566(10), c=20.603(2) ꢂ; a=
107.790(3), b=99.185(4), g=96.016(3)8; V=2702.1(4) ꢂ3; Z=2; 1calcd
=
1.098 mgcmꢀ3; 27040 measured reflections; 9760 independent reflections;
In view of the experimental and theoretical results, com-
pound 2 is best described as 2’, which is a singlet delocalized
biradicaloid.
652 refined parameters; R1 =0.0729, wR2 =0.1717 (I>2(s)I).
Compound 3: 1H NMR (399.5 MHz, [D6]benzene, 23.58C): d=1.25 (s,
18H; tBu), 1.28 (s, 18H; tBu), 1.64 (d, 12H, 3JHꢀH =6.8 Hz; CH
ACHTUNGTRENNUNG
4.58 (sept, 2H, 3JHꢀH =6.8 Hz; CH
ACHTUNGTRENNUNG
7.04 (m, 3H; Ph), 7.06–7.09 (m, 1H; Ph), 7.23–7.25 (m, 1H; Ph), 7.33–
7.35 (m, 1H; Ph), 7.41–7.43 ppm (d, 3H; Ph); 13C{1H} NMR (100.5 MHz,
[D6]benzene, 24.18C): d=24.9 (CHACTHUNRTGENN(GU CH3)2), 27.5 (CHAHCTUNGTREN(NUGN CH3)2), 31.7
Conclusion
(CMe3), 31.8 (CMe3), 53.3 (CMe3), 53.6 (CMe3), 114.5, 122.3, 128.9, 129.3,
129.8, 130.2, 132.2, 134.9, 137.5, 139.2, 148.0, 152.7 (Ph and Ar), 161.9,
177.1 ppm (NCN); 29Si{1H} NMR (79.4 MHz, [D6]benzene, 24.18C): d=
ꢀ61.2 and 32.4 ppm.
The first example of a stable singlet delocalized 2,4-diimino-
1,3-disilacyclobutanediyl, [LSiACHTNURGTNEUNG(m-CNAr)2SiL] (2), was syn-
ꢀ
thesized by the reaction of three equivalents of [LSi SiL]
(1) with two equivalents of ArN=C=NAr in toluene. X-ray
crystallography and theoretical studies show that 2 possesses
singlet biradicaloid character with an extensive electronic
delocalization throughout the Si2C2 four-membered ring and
exocyclic C=N bonds. The stability, spin preference, and
electronic delocalization of compound 2 are different from
the carbon congener 2,4-dimethylene-1,3-cyclobutanediyl
(A). The reactivity of compound 2 is currently under investi-
gation.
Crystallographic data for 3: [C42H63N5Si2]; Mw: 694.15; monoclinic; space
group P21/c; a=22.2880(15), b=12.1140(7), c=15.8683(10) ꢂ; a=90,
b=102.715(2), g=908; V=4179.3(5) ꢂ3; Z=4; 1calcd =1.103 mgcmꢀ3
;
40073 measured reflections; 8525 independent reflections; 458 refined
parameters; R1 =0.0476, wR2 =0.1183 (I>2(s)I).
X-ray data collection and structural refinement: Intensity data for com-
pounds 2 and 3 were collected with a Bruker APEX II diffractometer.
The crystals of 2 and 3 were measured at 103(2) K. The structures were
solved by direct phase determination (SHELXS-97) and refined for all
data by full-matrix least squares methods on F2.[18] All nonhydrogen
atoms were subjected to anisotropic refinement. The hydrogen atoms
were generated geometrically and allowed to ride in their respective
parent atoms; they were assigned appropriate isotopic thermal parame-
ters and included in the structure-factor calculations. CCDC-850083 (2)
and -850082 (3) contain the supplementary crystallographic data for this
paper. These data can be obtained free of charge from the Cambridge
Crystallography Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Experimental Section
General procedure: All manipulations were carried out under an inert at-
mosphere of argon gas by using standard Schlenk techniques. Toluene
was dried and distilled over Na prior to use. 1 was prepared as described
in the literature.[6] 2,6-Diisopropylphenylcarbodiimide was purchased
1
Acknowledgements
from Wako Chemicals and used without further purification. The H, 13C,
and 29Si NMR spectra were recorded on a JEOL ECA 400 spectrometer.
The NMR spectra were recorded in C6D6. The chemical shifts (d) are rel-
ative to SiMe4 for 1H, 13C, and 29Si NMR spectra. 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.
This work was supported by the Academic Research Fund Tier 1 (C.-
W.S.: RG 47/08; K.H.L.: RG 73/10) and the Central Strategic Initiative
for Inter-disciplinary Competitive Fund (C.-W.S. and K.H.L.). C.-W.S.
thanks Dr. Y. Li for X-ray crystallography.
Synthesis of 2 and 3: A solution of ArN=C=NAr (0.078 g, 0.22 mmol) in
toluene (6 mL) was added dropwise to 1 (0.164 g, 0.32 mmol) in toluene
4262
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Chem. Eur. J. 2012, 18, 4258 – 4263

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Received: October 25, 2011
Published online: February 29, 2012
[5] a) H.-X. Yeong, S.-H. Zhang, H.-W. Xi, J.-D. Guo, K. H. Lim, S.
Nagase, C.-W. So, Chem. Eur. J., DOI: 10.1002/chem.201102201;
Please note: Minor changes have been made to this manuscript since its
publication in Chemistry—A European Journal Early View. The Editor.
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