Stable Push-Pull Disilene: Substantial Donor-Acceptor Interactions through the Si-Si Double Bond

Article abstract of DOI:10.1021/jacs.7b09989

The push-pull effect has been widely used to effectively tune -electron systems. Herein, we report the synthesis and properties of 1-amino-2-boryldisilene 1 as the first push-pull disilene. Its spectroscopic and structural features show substantial intera

Full text of DOI:10.1021/jacs.7b09989

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A Stable Push-Pull Disilene: Substantial Donor-
Acceptor Interactions through the Si=Si Double Bond
Tomoyuki Kosai, and Takeaki Iwamoto
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b09989 • Publication Date (Web): 01 Dec 2017
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A Stable PushꢀPull Disilene: Substantial DonorꢀAcceptor Inꢀ
teractions through the Si=Si Double Bond
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Tomoyuki Kosai and Takeaki Iwamoto*
Department of Chemistry, Graduate School of Science, Tohoku University, Aobaꢀku, Sendai 980ꢀ8578, Japan
Supporting Information Placeholder
ABSTRACT: The pushꢀpull effect has been widely used to efꢀ
Mes
N(SiMe3)2
Mes
fectively tune πꢀelectron systems. Herein, we report the synthesis
and properties of 1ꢀaminoꢀ2ꢀboryldisilene 1 as the first pushꢀpull
disilene. Its spectroscopic and structural features show substantial
interactions between the Si=Si double bond and the amino and
boryl substituents. The π→π* absorption band of 1 is remarkably
redꢀshifted compared to that of the corresponding alkylꢀ
substituted disilene 2. Treatment of 1 with H2 resulted in the
cleavage of two molecules of H2 under concomitant formation of
the corresponding trihydridodisilane and hydroborane.
Dsi2(iꢀPr)Si
B
Si Si
Si Si
(Me3Si)2N
H
Si(iꢀPr)Dsi2
A
B
R R
tꢀBu
Tip
Tip
Tip
Si Si
Si Si
ZrClCp2
R R
R R
D
C
Me3Si SiMe3
B
The simultaneous introduction of donors and acceptors to πꢀ
electron systems has been widely used to tune their electronic and
optical properties.1 These soꢀcalled pushꢀpull compounds have
often been applied to functional organic dyes, as their orbital enꢀ
ergy levels can be effectively controlled by judiciously choosing
the donors and acceptors. Over the past decades, πꢀelectron sysꢀ
tems involving Si=Si double bonds, i.e., disilenes, which exhibit
an intrinsically narrow π–π* gap, have been considered as novel
prospective functional πꢀelectron systems.2 Disilenes with organic
πꢀelectron systems,3 transitionꢀmetalꢀbased dꢀelectron systems,4
and siliconꢀbased σꢀelectron systems5 have been reported. Neverꢀ
theless, compared to conventional organic πꢀelectron systems, the
diversity of such functionalized disilenes remains limited, as
bulky protecting groups are inevitable to stabilize the disilenes
sufficiently to be isolated. Although stable disilenes containing
either a πꢀdonor such as an amino group (A),6 or a πꢀacceptor
such as a boryl group (B),7 anthryl group (C),8 or transition metal
(D)4a have been reported (Chart 1), examples of disilenes that
contain both a πꢀdonor and a πꢀacceptor (pushꢀpull disilenes)
remain elusive. Herein, we report 1ꢀaminoꢀ2ꢀboryldisilene 1 as
the first pushꢀpull disilene, which shows, due to the pushꢀpull
effect, a substantial bathochromic shift of its absorption band
compared to that of the corresponding alkylꢀsubstituted disilene 2
(Chart 1). Interestingly, treatment of 1 with H2 at room temperaꢀ
ture resulted in the cleavage of two molecules of H2 under conꢀ
comitant formation of an aminotrihydridodisilane and a hydroboꢀ
rane.
Si Si
Si Si
N
Ad
Tip
Tip
Me3Si SiMe3
1
2
Chart 1. Examples of disilenes that contain either a donor or
an acceptor group, as well as disilenes 1 and 2 [Mes = mesityl;
Dsi = CH(SiMe3)2; Tip = 2,4,6ꢀtriisopropylphenyl; R = SiMe3;
Ad = 1ꢀadamantyl].
Airꢀsensitive dark red crystals of pushꢀpull disilene 1 were obꢀ
tained in 59% yield from the reaction of 2ꢀaminodisilenide 3,
which was prepared by the reduction of an adduct of the cyclic
(alkyl)(amino)silylene (CAASi) E9 and TipSiCl3 (5) with KC8
(for details, see SI), and Bꢀchloroꢀ9ꢀborabicyclo[3.3.1]nonane in
benzene (Scheme 1). Although 1 is stable at room temperature, it
decomposes after 12 h at 70 °C in C6D6. To reveal the effects of
the amino and boryl groups on the Si=Si double bond in 1, we
synthesized disilene 2, which contains alkyl groups instead of
amino and boryl groups in a similar structural motif (Scheme 2).
The molecular structures of 1 and 2 were determined by a combiꢀ
nation of multinuclear NMR spectroscopy and singleꢀcrystal Xꢀ
ray diffraction (XRD) analyses.
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ally optimized at the B3PW91ꢀD3/6ꢀ31G(d) level of theory (1opt).
Scheme 1. Synthesis of 1 (Ad = 1ꢀadamantyl; Tip = 2,4,6ꢀ
triisopropylphenyl).
The 13C NMR spectrum showed only two signals for secondary
and one signal for tertiary carbon atoms in the 9ꢀBBN moiety,
which indicates fast rotation around the Si–B bond in solution on
the NMR time scale. The 29Si NMR spectrum of 1 showed three
signals at 142.4 [(alkyl)(amino)Si=], 2.5 (SiMe3), and –33.0 ppm
[=Si(boryl)Tip]. The difference in chemical shift for the doubleꢀ
bonded silicon nuclei in 1 (ꢁδSi = 175.4 ppm) is much larger than
Me3Si SiMe3
Me3Si SiMe3
Si
THF, rt
1) Tip Cl3,
Cl
B
K(thf)
Tip
1
(59%)
Si
N
Si Si
N
2) 5 KC8,
THF, –30 °C
benzene, rt
Ad
Ad
E
3
Scheme 2. Synthesis of 2.
that in 2 [117.9 ((alkyl)2Si=) and 77.2 ppm (=SiTip(iꢀPr)); ꢁδSi =
40.7 ppm], indicative of a substantially polar Si=Si double bond
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Me3Si SiMe3
Si
Me3Si SiMe3
in 1.15
Si
5 KC8
Tip Cl3,
K
Br
THF, rt, 3 h
Si Si
2 (40%)
The UVꢀvis absorption spectrum of 1 revealed pushꢀpull effects
for the amino and boryl groups on the Si=Si double bond. A hexꢀ
ane solution of 1 exhibited its longestꢀwavelength absorption band
at 482 nm, which is considerably redꢀshifted relative to that of 2
[~407 (shoulder) and 357 nm] (Figure 2).
THF, rt
Tip
Me3Si SiMe3
Me3Si SiMe3
4
The molecular structures of 1 and 2 are shown in Figure 1.10 In
1, the boryl group is arranged trans to the amino group (Zꢀisomer
according to the CIP nomenclature). The Si1–Si2 bond in 1
[2.2146(6) Å] is longer than that in 2 [2.1828(6) Å]. The N1–Si1
bond in 1 [1.7215(15) Å] is much longer than typical Si=N double
bonds (1.55ꢀ1.59 Å),11 but relatively short compared to those in
reported aminodisilenes (1.703ꢀ1.775 Å)6 and comparable to that
in E [1.7143(15) Å], which exhibits a moderate but nevertheless
significant N=Si double bond character.9 The Si2–B1 bond in 1
[1.945(2) Å] is longer than conventional Si=B double bonds
[1.8379(17) and 1.859(2) Å],12 but substantially shorter than that
in reported boryldisilenes [1.996ꢀ2.022 Å].7 The geometry around
the N1, Si1, Si2, and B1 atoms is almost planar, i.e., the angle
sum around each atom is ~360° (N1: 358.2°; Si1: 358.1°; Si2:
358.3°; B1: 359.7°). The πꢀorbital axis vectors (POAVs)13 suggest
that the p orbitals on the N1, Si1, Si2, and B1 atoms are almost
parallel to each other, albeit that they include small twist angles
(N1–Si1: 4.5°; Si1–Si2: 15.9°; Si1–B1: 8.7°), which allows effiꢀ
cient interactions between the p orbitals. Although steric effects of
the bulky substituents on the Si=Si double bond cannot be ruled
out at this point,14 these structural features are consistent with a
significant contribution of canonical structures that contain N+=Si
and Si=B− double bonds, which have previously been proposed
for (Dsi2iꢀPrSi)HSi=Si(SiDsi2iꢀPr)(ER2) [E = N or B].7b The Tip
groups adopt an almost perpendicular arrangement relative to the
Si=Si double bond, i.e., the p orbitals on Si2 and Cipso(Tip) are
twisted (1: 77.2°; 2: 84.5°).
Figure 2. UVꢀvis absorption spectra of 1 (red) and 2 (black) in
hexane.
The electronic structure of 1 was also examined by theoretical
calculations (cf. SI). The structures of 1 and 2 were optimized at
the B3PW91ꢀD3/6ꢀ31G(d) level of theory (1opt and 2opt), and are
in good agreement with those obtained from the XRD analyses
(Table S1). The band positions and their oscillator strengths deꢀ
termined by TDDFT calculations on the optimized structures at
the TDꢀB3LYP/6ꢀ31+G(d) level of theory were also consistent
with those obtained from the experimental spectra (Figure S66),
suggesting that the structures of 1 and 2 in solution are close to
those observed in the crystalline state (Table S1). The HOMO of
1opt is the π(Si=Si) orbital, while the LUMO is the π*(Si=Si) orꢀ
bital (Figure 3a). Both the HOMO and the LUMO involve a subꢀ
stantial contribution from the 2p orbital of the N atom and the 2p
orbital of the B atom, which reflects the πꢀdonor effect of the
amino group and the πꢀacceptor effect of the boryl group on the
Si=Si double bond. These orbital features are similar to those
observed in 1ꢀaminoꢀ2ꢀborylacetylene.16 The TDDFT calculations
support the assignment of the absorption band of 1 at 482 nm to
the HOMO→LUMO transition. Although the HOMOs of 1 (–4.45
eV) and 2 (–4.42 eV) are virtually identical in energy, the LUMO
of 1 (–1.53 eV) is substantially stabilized relative to that of 2 (–
0.74 eV). These orbital features suggest that the electronꢀ
withdrawing effect of the boryl group on the Si=Si double bond
dominates the electronꢀdonating effect of the amino group. This
notion is also consistent with the bond lengths determined by the
XRD analysis and the results of the following calculations.17
Figure 1. ORTEP drawings of (a) 1 and (b) 2 (only one of the two
crystallographically independent molecules in the unit cell of 2 is
shown) with thermal ellipsoids set at 50% probability and hydroꢀ
gen atoms omitted for clarity.
The multinuclear NMR spectra of 1 in C7D8 from –80 °C to
80 °C indicated the presence of one isomer, which is probably the
Zꢀisomer, i.e., the isomer observed in the solid state. This notion
was also supported by DFT calculations, as the geometric isomer
of 1 (Eꢀ1opt) is 16.2 kJ/mol higher in energy than 1, when structurꢀ
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(a)
isomerize to acyclic silylene 8opt (activation energy: +53.5 kJ
mol–1) via a 1,2ꢀhydrogen migration.23 Subsequently, a second
molecule of H2 should be inserted at the silylene center of 8opt
(activation energy: +62.0 kJ mol–1)24,25 to give 6opt. Although the
direct antiꢀaddition of H2 across the Si=Si double bond of 7opt in a
manner similar to the reaction of 1,2ꢀdiiminodisilene with H2,
which has very recently been reported by Inoue and Rieger,21e,26
was also examined, the activation energy (143.9 kJ mol–1) of this
process was estimated to be substantially higher than that in the
pathway proceeding via 8opt
.
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Scheme 3. Reaction of 1 with H2 (R = SiMe3).
(b)
R
R
R
R
R
R
R
R
R R
B
B
B
B
H
H
H2 (1 atm)
C6D6, rt
H
H
Si Si
Si Si
Si Si
Si Si
1
Si
H
+
1/2
B
B
Si
Tip
(82%)
R3
R3
R3
R3
N
N
N
N
N
R2
R2
R2
R2
Ad
1a
1b
1c
1d
6
(a) Cleavage of the first molecule of H2.
Figure 3. (a) The HOMO and LUMO of 1opt (isosurface value =
0.05); hydrogen atoms are omitted for clarity. (b) Major canonical
resonance structures for 1 (R = SiMe3; R2 = 1ꢀadamantyl; R3 =
Tip).
H
R R
B
C
H
Si Si
C
N
=
B
Si
Si
C
N
115.0
(67.5)
N
N
Si Si
Ad
Tip
Tip
A natural resonance theory (NRT)18 analysis of model comꢀ
pound 1', which bears methyl and 2,6ꢀdimethylphenyl (Dip)
groups instead of the bulky adamantyl and Tip groups (R2 = Me;
R3 = Dip; Figure 3b) and was optimized at the B3PW91ꢀD3/6ꢀ
31G(d) level of theory,19 also showed a dominant contribution of
canonical structures with an Si=B− moiety and a significant conꢀ
tribution of structures with an N+=Si moiety (1'a: 54.8%; 1'b:
7.0%; 1'c: 31.4%; 1'd: 4.1%) (Figure 3b). A secondꢀorder perturꢀ
bation analysis afforded the stabilization energies for the π(Si=Si)
→2p(B) donation (165.0 kJ mol–l) and the n(N)→π*(Si=Si) donaꢀ
tion (93.2 kJ mol–1). The Wiberg bond indices (WBI) of the Si=Si
double bond in 1' (1.44) are lower compared to that in 2opt (1.78),
while the WBI of the N–Si bond in 1' (0.73) is slightly larger than
that in cyclic (alkyl)(amino)dihydridosilane F (a dihydrogenated
derivative of E) (0.67).9 The WBI of the Si–B bond in 1' (1.25)
suggests a slight double bond character for the Si–B bond. Natural
population analysis (NPA) charges in 1' [(alkyl)(amino)Si: +1.34;
boryl(Tip)Si: +0.28] indicated the presence of a polar Si=Si douꢀ
ble bond, which is consistent with the large ꢁδSi observed in the
29Si NMR spectra. Based on these results, the remarkable bathoꢀ
chromically shifted absorption band observed for 1 should be
attributed to the pronounced pushꢀpull effect of the amino and
boryl groups.
1opt
7opt—9ꢀBBN
–3.5
(30.6)
7opt
+ H2
H
+ 1/2 (9ꢀBBN)2
17.3
(–25.1)
B
0.0
(0.0)
H
H
Si Si
C
N
C
N
–43.9
(–49.0)
Si Si
Tip
Tip
7opt
+ 9ꢀBBN
TS1
(b) Cleavage of the second molecule of H2.
H
H
H
C
N
C
N
H
Si
Si
Si
Si
H
Si Si
H
C
C
N
Tip
9opt
111.1
(61.0)
H
Tip
Si
H
TS4
53.5
8opt
Si
Tip
6opt
Tip
N
(50.0)
+ H2
7opt
+ H2
114.6
(56.8)
H
109.7
(60.9)
H
H
Si
C
52.6
(47.5)
H
C
–128.0
Si
Tip
0.0
(0.0)
Si Si
N
(–193.1)
N
Tip
TS3
TS2
+ H2
Figure 4. Reaction pathways and energy levels for the reaction of
1opt with two molecules of H2, calculated at the B3PW91ꢀD3/6ꢀ
31G(d) level of theory. Free energy and electron energy values
shown in parentheses are given in kJ/mol (R = SiMe3, Ad = 1ꢀ
adamantyl, Tip = 2,4,6ꢀtriisopropylphenyl).
When a C6D6 solution of 1 was exposed to an atmosphere of H2
(1 atm) at room temperature, the red color of the solution disapꢀ
peared gradually.20,21 Multinuclear NMR spectra indicated the
In summary, we have synthesized pushꢀpull 1ꢀaminoꢀ2ꢀ
boryldisilene 1. The observed large difference in the 29Si NMR
chemical shifts of the Si nuclei in the Si=Si double bond suggests
substantial polarization. The structural features determined by
singleꢀcrystal XRD analysis indicate a significant double bond
character with contributions from N+=Si and Si=B− moieties. The
pushꢀpull effect on the Si=Si double bond is also reflected in a
bathochromic shift of the longestꢀwavelength absorption band of
1. Moreover, 1 cleaves two molecules of H2 at room temperature.
It seems feasible to anticipate that the pushꢀpull effect could beꢀ
come a key interaction in functional compounds based on Si=Si
double bonds, as well as in carbonꢀbased πꢀbonds.
formation of trihydridodisilane
6 and the dimer of 9ꢀ
borabicyclo[3.3.1]nonane [(9ꢀBBN)2] (Scheme 3). When D2 was
used instead of H2, the corresponding deuterated products 6-d3
and (9ꢀBBNꢀd1)2 were obtained. The formation of these products
indicates that the hydrogenation of the Si=Si double bond occurs
under concomitant cleavage of the Si–B bond. Disilene 2 did not
react with H2 at temperatures up to 70 °C. The mechanism for the
reaction of 1 with H2 was investigated theoretically at the
B3PW91ꢀD3/6ꢀ31G(d) level of theory (Figure 4). The results of
these calculations suggest that initially, a molecule of H2 should
coordinate to the empty 2p orbital on the B atom (activation enerꢀ
gy: +115.0 kJ mol–1)22 to cleave the Si–B bond of 1opt and provide
the corresponding hydridodisilene 7opt and 9ꢀBBN, whereupon the
latter should dimerize to afford (9ꢀBBN)2. Then, 7opt should
ASSOCIATED CONTENT
Supporting Information
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(8) (a) Iwamoto, T.; Kobayashi, M.; Uchiyama, K.; Sasaki, S.; Nagenꢀ
The Supporting Information for this article is available free of
charge on the ACS Publications website at DOI:
Experimental and theoretical details (PDF)
dran, S.; Isobe, H.; Kira, M. J. Am. Chem. Soc. 2009, 131, 3156ꢀ3157. (b)
Takeuchi, K.; Ichinohe, M.; Sekiguchi, A. J. Am. Chem. Soc. 2012, 134,
2954ꢀ2957. (c) Kobayashi, M.; Hayakawa, N.; Matsuo, T.; Li, B.; Fukuꢀ
naga, T.; Hashizume, D.; Fueno, H.; Tanaka, K.; Tamao, K. J. Am. Chem.
Soc. 2016, 138, 758ꢀ761. (d) Obeid, N. M.; Klemmer, L.; Maus, D.; Zimꢀ
mer, M.; Jeck, J.; Bejan, L.; White, A. J. P.; Huch, V.; Jung, G.; Scheschꢀ
kewitz, D. Dalton. Trans. 2017, 46, 8839ꢀ8848. (e) Kosai, T.; Ishida, S.;
Iwamoto, T. Dalton Trans. 2017, 46, 11271ꢀ11281.
Crystallographic data for 1-3 and 5 (CIF)
AUTHOR INFORMATION
Corresponding Author
(9) Kosai, T.; Ishida, S.; Iwamoto, T. Angew. Chem. Int. Ed. 2016, 55,
15554ꢀ15558.
*iwamoto@m.tohoku.ac.jp
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ORCID
(10) Disilene 2 contains two crystallographically independent moleꢀ
cules per asymmetric unit. As the doubleꢀbonded silicon atoms of one
molecule were disordered, we used the other molecule for the discussion.
(11) (a) Wiberg, N.; Schurz, K.; Reber, G.; Müller, G. J. Chem. Soc.
Chem. Commun. 1986, 591ꢀ592. (b) Niesmann, J.; Klingebiel, U.; Schäfer,
M.; Boese, R. Organometallics 1998, 17, 947ꢀ953. (c) Iwamoto, T.; Ohniꢀ
shi, N.; Gui, Z.; Ishida, S.; Isobe, H.; Maeda, S.; Ohno, K.; Kira, M. New.
J. Chem. 2010, 34, 1637ꢀ1645.
Tomoyuki Kosai: 0000ꢀ0003ꢀ3435ꢀ1354
Takeaki Iwamoto: 0000ꢀ0002ꢀ8556ꢀ5785
Notes
The authors declare no competing financial interests.
(12) (a) Nakata, N.; Sekiguchi, A. J. Am. Chem. Soc. 2006, 128, 422ꢀ
423. (b) Suzuki, Y.; Ishida, S.; Sato, S.; Isobe, H.; Iwamoto, T. Angew.
Chem. Int. Ed. 2017, 56, 4593ꢀ4597.
ACKNOWLEDGMENT
This work was supported by MEXT KAKENHI grant
JP24109004 (T.I.) (GrantꢀinꢀAid for Scientific Research on Innoꢀ
vative Areas "Stimuliꢀresponsive Chemical Species") and the
JSPS KAKENHI grant JPK1513634 (T.I.). The authors thank
Prof. Satoshi Maeda (Hokkaido University) for helpful discusꢀ
sions on theoretical calculations.
(13) Haddon, R. C. J. Am. Chem. Soc. 1990, 112, 3385ꢀ3389.
(14) The structural parameters were reproduced by the permethylated
model compounds (for details, see SI).
(15) We failed to observe a 11B NMR signal for 1, which is probably
due to the large chemical shift anisotropy of the 11B nucleus.
(16) Onuma, K.; Suzuki, K.; Yamashita, M. Chem. Lett. 2014, 44, 405ꢀ
407.
REFERENCES
(17) Although cyclic voltammetry (CV) measurements should afford
further information on the electronic structures, such measurements were
unfortunately unsuccessful.
(18) For a detailed citation of the NRT analysis, see ref S14 in the SI.
(19) NRT analyses on the real molecule 1opt were unsuccessful as its
molecular size was too large. Therefore, we used a model compound,
whose structural parameters around the Si=Si double bond were conꢀ
(1) For a recent review on the pushꢀpull effect, see: Meier, H. Angew.
Chem., Int. Ed. 2005, 44, 2482ꢀ2506.
(2) For recent comprehensive reviews on species that contain multiply
bonded silicon, see: (a) Lee, V. Ya.; Sekiguchi, A. Organometallic Com-
pounds of Low-Coordinate Si, Ge, Sn, and Pb: From Phantom Species to
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