Reactivity of a disilylene [{PhC(NBut)2}Si] 2 toward bromine: Synthesis and characterization of a stable monomeric bromosilylene

Article abstract of DOI:10.1021/ic9021382

The reaction of the disilylene [{PhC(NBu^t)₂}Si] ₂ (1) with 1 equiv of bromine in toluene afforded novel monomeric bromosilylene [{PhC(NBu^t)₂}SiBr] (2). The result shows that the Si^I-Si

Full text of DOI:10.1021/ic9021382

Inorg. Chem. 2010, 49, 371–373 371
DOI: 10.1021/ic9021382
Reactivity of a Disilylene [{PhC(NBut)2}Si]2 toward Bromine: Synthesis and
Characterization of a Stable Monomeric Bromosilylene
Hui-Xian Yeong,† Kai-Chung Lau,‡ Hong-Wei Xi,§ Kok Hwa Lim,§ and Cheuk-Wai So*,†
†Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang
Technological University, 21 Nanyang Link, Singapore 637371, Singapore, ‡Department of Biology and
Chemistry, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, and §Division of Chemical
and Biomolecular Engineering, School of Chemical and Biomedical Engineering, Nanyang Technological
University, 62 Nanyang Drive, Singapore 637459, Singapore
Received October 28, 2009
The reaction of the disilylene [{PhC(NBut)2}Si]2 (1) with 1 equiv of
bromine in toluene afforded novel monomeric bromosilylene
[{PhC(NBut)2}SiBr] (2). The result shows that the SiI-SiI bond
in 1 was cleaved by bromine. An X-ray structure of compound 2 has
been determined.
close to 1 with a lone pair (LP) of electrons at each Pb atom.2f
Therefore, diplumbyne is considered as diplumbylene. Dis-
tannyne can adopt a multiplybonded structure similar to that
of the germanium analogue or a singly bonded structure
similar to that of the lead analogue, depending on the
bulkiness of the supporting ligand.5 Because of their unique
electronic structures, digermyne and distannyne show differ-
ent reactivities.6 For example, the treatment of [ArMMAr]
[M=Ge, Sn; Ar=C6H3-2,6-(C6H3-2,6-Pri2)2] with N3SiMe3
afforded the cyclic singlet diradicaloid [ArGe{μ2-N-
(SiMe3)}2GeAr] and the imide-bridged complex [ArSn{μ2-
N(SiMe3)}SnAr], respectively.6c It is suggested that distan-
nyne has much less diradical character compared with
digermyne, which leads to different reactivities.
Recently, Roesky and his co-workers reported the first
example of the amidinate-stabilized disilylene [{PhC-
(NBut)2}Si]2 with a SiI-SiI single bond and an LP of
electrons at each Si atom.7 That the disilylene has an
unprecedented electronic structure prompted our interest in
exploring its chemical reactivity. In this Communication, we
report the synthesis and characterization of a stable bromo-
silylene from the reaction of [{PhC(NBut)2}Si]2 (1) with
bromine. The theoretical studies of 1 are also described in
order to understand its bonding nature and, hence, its
reactivity.
Stable heavier group 14 alkyne analogues of composition
RMMR (R=supporting ligand; M=Si, Ge, Sn, Pb) have
attracted much attention in the past several decades.1 These
complexes can be synthesized successfully by incorporating
sterically hindered substituents at the heavier group 14
elements.2 The X-ray structures of heavier group 14 alkyne
analogues show that they have a trans-bent and planar
geometry in which the R-M-M angle decreases from silicon
to lead. Theoretical and reactivity studies show that the Si-Si
bond order in the disilyldisilyne [R0SiSiR0] (R0 =SiPri{CH-
(SiMe3)2}2) is significantly greater than 2.3 The Ge-Ge bond
in the digermyne [ArGeGeAr] (Ar = C6H3-2,6-(C6H3-2,6-
Pri2)2) has a bond order of about 2 and partial singlet
diradical character.4,6c The Pb-Pb bond order in the di-
plumbyne [RPbPbR] (R = C6H3-2,6-(C6H2-2,4,6-Pri3)2) is
*To whom correspondence should be addressed. E-mail: CWSo@ntu.
edu.sg.
(1) Power, P. P. Organometallics 2007, 26, 4362–4372.
The treatment of 1 with 1 equiv of bromine in toluene for
2h afforded novel monomeric bromosilylene [{PhC(NBut)2}-
SiBr] (2; Scheme 1). The Si-Si bond in 1 was cleaved by
bromine in the reaction. It is noteworthy that the LP of
(2) (a) Sekiguchi, A.; Kinjo, R.; Ichinohe, M. Science 2004, 305, 1755–
1757. (b) Sasamori, T.; Hironaka, K.; Sugiyama, Y.; Takagi, N.; Nagase, S.;
Hosoi, Y.; Furukawa, Y.; Tokitoh, N. J. Am. Chem. Soc. 2008, 130, 13856–
13857. (c) Stender, M.; Phillips, A. D.; Wright, R. J.; Power, P. P. Angew. Chem.,
Int. Ed. 2002, 41, 1785–1787. (d) Phillips, A. D.; Wright, R. J.; Olmstead, M. M.;
Power, P. P. J. Am. Chem. Soc. 2002, 124, 5930–5931. (e) Pu, L.; Phillips, A. D.;
Richards, A. F.; Stender, M.; Simons, R. S.; Olmstead, M. M.; Power, P. P. J. Am.
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r
2009 American Chemical Society
Published on Web 12/16/2009
pubs.acs.org/IC

372 Inorganic Chemistry, Vol. 49, No. 2, 2010
Scheme 1. Synthesis of 2
Yeong et al.
electrons at the SiI center in 1 did not undergo an oxidative
addition reaction with bromine. In contrast, the reaction of
[:Si{(NCH2But)2C6H4-1,2}] or [R0SiSiR0] (R0 = SiPri{CH-
(SiMe3)2}2) with bromine gave [SiBr2{(NCH2But)2C6H4-
1,2}] and [R0SiBr2-SiBr2R0], respectively.2a,8 Power and
his co-workers have reported that the reaction of [ArM-
MAr] [M = Ge, Sn; Ar = C6H3-2,6-(C6H3-2,6-Pri2)2] with
PhNdNPh resulted in the complete cleavage of the M-M
bond, and an LP of electrons was found at the metal centers
in the product [ArM{(Ph)N-N(Ph)}MAr].6c Similar results
can be found in the reaction of [ArSnSnAr] with N3SiMe3 to
form [ArSn{μ2-N(SiMe3)}SnAr].6c Recently, an amidinate-
stabilized chlorosilylene [{PhC(NBut)2}SiCl] was synthesized
by the reaction of [{PhC(NBut)2}SiCl3] with potassium.9
Density functional theory (DFT) calculations of [{PhC-
(NBut)2}SiCl] have been performed. Carbene-stabilized
dihalosilylenes [L f SiX2] (L = :C{N(C6H3-2,6-Pri2)CH}2;
X=Cl, Br) have also been synthesized by the reaction of [L f
SiCl4] or [{L f SiBr3}Br] with potassium graphite.10
˚
Figure 1. Molecular structure of 2. Selected bond distances (A) and
angles (deg): Si1-Br1 2.326(3), Si1-N1 1.971(5), Si1-N1A 1.882(5),
C1-N1 1.331(5); N1-Si1-N1A 66.9(2), N1A-Si1-Br1 93.3(2),
N1-Si1-Br1 94.8(2), Si1-N1-C1 88.5(3), N1-C1-N1A 105.9(6).
Table 1. Theoretical Topological Features at the BCP of 1 and [H3Si-SiH3]a
2
bond
F
r F
G
H
|V|
Compound 1
Si-Si0
Si-N
Si-N0
N-C
N0-C
0.52
0.59
0.59
2.25
2.27
-2.29
6.87
7.06
-24.56
-24.92
0.06
0.72
0.73
1.72
1.86
-0.22
-0.24
-0.23
-3.49
-3.60
0.27
0.96
0.96
5.21
5.46
[H3Si-SiH3]
Compound 2 was isolated as a highly air- and moisture-
sensitive pale-yellow crystalline solid, which is soluble in
toluene and tetrahydrofuran. It is stable in solution or the
solid state at room temperature in an inert atmosphere. It has
been characterized by elemental analysis, spectroscopic
methods, and X-ray crystallography. The 1H and 13C
NMR spectra of 2 display resonances due to the amidinate
ligand. The 29Si NMR signal of 2 (18.6ppm) shows an upfield
shift compared with that of 1 (76.3 ppm). It is comparable
with that of [{PhC(NBut)2}SiCl]9 (14.6 ppm) and [L f SiBr2]
(10.9 ppm).10b
Si-Si
0.63
-3.91
0.04
-0.31
0.36
a
-3
2
-5
-3
˚
˚
˚
Units: F, e A ; r F, e A ; H, G, |V|, hartree A
.
˚
The C(1)-N(1) and C(1)-N(1A) bond lengths in 2[1.331(5) A]
are approximately intermediate between the C-N double and
C-N(sp2) single bond lengths. This geometry shows consi-
derable delocalization throughout the NCN backbone of the
ligand. The Si-N bonds in 2 [1.882(5) and 1.971(5) A] are
slightly longer than those of 1 [1.866(4) and 1.874(4) A].
In order to understand the bonding nature and, hence,
reactivity, compound 1 was investigated by means of quan-
tum chemical calculations. The calculations were performed
by the DFT13 B3LYP14 with the 6-311þG(d,p) basis set15 as
implemented in the Gaussian 0316 program. The calculated
˚
7
˚
The molecular structure of 2 with an atomic numbering
scheme is shown in Figure 1.11 The amidinate ligand is
bonded in a N,N0-chelate fashion to the silicon center and
displays a trigonal-pyramidal geometry. The sum of the bond
angles at the silicon center is 255.0°, which is comparable
with that of the three-coordinated chlorosilylene [{PhC-
(NBut)2}SiCl] (260.7°).9 This geometry is consistent with a
stereoactive LP at the silicon center. The phenyl group is
orthogonally arranged to the four-membered Si1-N1-
0
˚
˚
structural parameters (Si-Si = 2.472 A, Si-N = 1.903 A,
0
0
˚
Si-N = 1.905 A, and N-Si-N = 68.5°) are in good
agreement with the crystallographic data. The natural bond
orbital (NBO)17 analysis (Table S1; see the Supporting
Information) shows that the Si-Si bond is derived from
the overlapping of p-rich hybrids (sp7.48, 87.8% p character)
on the Si atoms. The Wiberg bond index18 shows that the
Si-Si bond (0.923) in 1 has single-bond character. The LP
˚
C1-N1A ring. The Si1-Br1 bond [2.326(3) A] is comparable
10b
˚
to those of [L f SiBr2] [2.3379(8) and 2.3607(8) A] but
is longer than that of gaseous silicon dibromide (2.243 A).
12
˚
(8) Gehrhus, B.; Hitchcock, P. B.; Jansen, H. J. Organomet. Chem. 2006,
691, 811–816.
(9) So, C.-W.; Roesky, H. W.; Magull, J.; Oswald, R. B. Angew. Chem.,
Int. Ed. 2006, 45, 3948–3950.
(10) (a) Ghadwal, R. S.; Roesky, H. W.; Merkel, S.; Henn, J.; Stalke, D.
Angew. Chem., Int. Ed. 2009, 48, 5683–5686. (b) Filippou, A. C.; Chernov, O.;
Schnakenburg, G. Angew. Chem., Int. Ed. 2009, 48, 5687–5690.
(13) (a) Feller, D. J. Chem. Phys. 1990, 93, 579–589. (b) Hohenberg, P.;
Kohn, W. Phys. Rev. 1964, 136, B864–B871.
(14) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648–5652. (b) Lee, C.; Yang,
W.; Parr, R. G. Phys. Rev. B 1988, 37, 785–789.
(15) (a) Krishnan, R.; Binkley, J. S.; Seeger, R.; Pople, J. A. J. Chem.
Phys. 1980, 72, 650–654. (b) McLean, A. D.; Chandler, G. S. J. Chem. Phys.
1980, 72, 5639–5648.
˚
(11) Crystal data for 2 (C15H23BrN2Si): M=339.35; a=14.239(1) A, b=
3
˚
˚
˚
11.164(8) A, c=11.982(9) A; R=90°, β=116.28(4)°, γ=90°; V=1707.9(2) A ;
˚
Z=4; space group C2/c (monoclinic); T=103(2) K; λ=0.710 73 A; μ=2.468
(16) Frisch M. J.; et al. Gaussian 03, revision B.05; Gaussian, Inc.:
Pittsburgh, PA, 2003. See the Supporting Information for the complete citation.
(17) Weinhold, F.; Landis, C. R. Valency and Bonding: A Natural Bond
Orbital Donor-Acceptor Perspective; Cambridge University Press: Cambridge,
U.K., 2005; Vol. 3, pp 215-275.
mm-1; Fcalc=1.320 g cm-3; F(000)=704; 7803 measured reflections, 1507
independent and 100 observed reflections [I > 2σ(I )]; R1=0.0636, wR2=
-3
˚
0.1665; largest diffraction peak and hole 1.190/-0.380 e A
.
(12) Hargittai, I.; Shultz, G.; Tremmel, J.; Kagramanov, N. D.; Maltsev,
A. K.; Nefedov, O. M. J. Am. Chem. Soc. 1983, 105, 2895–2896.
(18) Wiberg, K. B. Tetrahedron 1968, 24, 1083–1096.

Communication
Inorganic Chemistry, Vol. 49, No. 2, 2010 373
The topological analysis of the electron densities of com-
pound 1 and [H3Si-SiH3] according to Bader’s quantum
theory of atoms in molecules was performed using the
optimized structure at the B3LYP/6-31þG(d) level
2
(Table 1).19 The Laplacian of the electron density r F and
the total energy density H at the (3, -1) bond critical point
(BCP) show that the Si-Si bond in compound 1 and
[H3Si-SiH3] are nonpolar and covalent (Figure 2). The
electron density is locally concentrated between two Si atoms
in 1 and [H3Si-SiH3]. The bond strength of the Si-Si bond in
1 is weaker than that in [H3Si-SiH3]. There are four valence-
shell charge concentrations at the Si atoms. One of them in
the nonbonding region is regarded as the LP density. The
result shows that the LP of electrons is stereochemically
active, consistent with the NBO analysis.
In summary, DFT calculations and the topological ana-
lysis of the electron densities show that 1 comprises a Si-Si
single bond and an LP of electrons at each Si atom. A novel
monomeric bromosilylene complex 2 has been synthesized by
the reaction of 1 with bromine in toluene. The results show
that the SiI-SiI bond in 1 was cleaved by bromine. The LP of
electrons at the SiI center did not undergo oxidative addition
with bromine. Compound 2 can serve as a functionalized
silylene in a metathesis reaction.
2
Figure 2. r F in the (a) SiSiN plane and (b) SiSiC plane in compound 1.
The N atom on the left of part a is slightly off the plane. LP stands for the
Acknowledgment. This work was supported by the Aca-
demic Research Fund Tier 1 (Grant RG 47/08 to C.-W.S.
Grant RG 28/07 to K.H.L.) and by a grant from the
Research Grants Council of Hong Kong Special Adminis-
trative Region, China (Project CityU 101308 to K.-C.L.).
2
LP electron on silicon. Green lines indicate negative values in r F, and
black lines stand for positive values.
electrons at the silicon centers are high in s character (sp0.43
69.9% s character) with some directionality.
,
The presence of a π bond in a molecule can be measured by
the chemical shift anisotropy (CSA).3a Large CSA values
indicate the presence of π bonds. When the calculated 29Si
CSA of 1 (80.5 ppm) is compared with that of [H2SidSiH2]
(362.6 ppm), it is suggested that the Si-Si bond in 1 does not
have multiple-bond character (Table S2; see the Supporting
Information).
Supporting Information Available: Complete citation for
ref 16, selected calculation results of 1, and a CIF file giving
X-ray data for 2. This material is available free of charge via the
Internet at http://pubs.acs.org.
(19) Bader, R. F. W. Atoms in Molecules;A Quantum Theory; Oxford
University Press: New York, 1990.