Synthesis and properties of pentacoordinated phenoxysilane and carboxysilanes with intramolecular nitrogen-silicon coordination

Article abstract of DOI:10.1080/10426501003773746

Reactions of a neutral pentacoordinated monohydrosilane bearing a dative N-Si bond with phenol and benzoic acid derivatives gave the corresponding pentacoordinated phenoxysilane and carboxysilanes, respectively. X-ray crystallographic analyses of these silanes revealed that the distance of the N-Si dative bond is shortened and the pentacoordination character of the silicon center becomes greater with the increase in the electron-withdrawing character of the apical substituent on silicon. Copyright Taylor & Francis Group.

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Phosphorus, Sulfur, and Silicon and the
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Synthesis and Properties of
Pentacoordinated Phenoxysilane and
Carboxysilanes with Intramolecular
Nitrogen–Silicon Coordination
Yuya Domoto a , Kowichiro Saruhashi b , Akihiro Fukushima b , Shohei
Sase a , Kei Goto a & Takayuki Kawashima b
a Interactive Research Center of Science, Graduate School of
Science and Engineering , Tokyo Institute of Technology , Tokyo,
Japan
b Department of Chemistry, Graduate School of Science , The
University of Tokyo , Tokyo, Japan
Published online: 27 May 2010.
To cite this article: Yuya Domoto , Kowichiro Saruhashi , Akihiro Fukushima , Shohei Sase , Kei
Goto & Takayuki Kawashima (2010) Synthesis and Properties of Pentacoordinated Phenoxysilane and
Carboxysilanes with Intramolecular Nitrogen–Silicon Coordination, Phosphorus, Sulfur, and Silicon and
the Related Elements, 185:5-6, 1221-1229, DOI: 10.1080/10426501003773746
To link to this article: http://dx.doi.org/10.1080/10426501003773746
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Phosphorus, Sulfur, and Silicon, 185:1221–1229, 2010
Copyright ꢀC Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426501003773746
SYNTHESIS AND PROPERTIES OF PENTACOORDINATED
PHENOXYSILANE AND CARBOXYSILANES WITH
INTRAMOLECULAR NITROGEN–SILICON
COORDINATION
Yuya Domoto,1 Kowichiro Saruhashi,2 Akihiro Fukushima,2
Shohei Sase,1 Kei Goto,1 and Takayuki Kawashima2
1Interactive Research Center of Science, Graduate School of Science and
Engineering, Tokyo Institute of Technology, Tokyo, Japan
2Department of Chemistry, Graduate School of Science, The University of Tokyo,
Tokyo, Japan
Reactions of a neutral pentacoordinated monohydrosilane bearing a dative N Si bond with
phenol and benzoic acid derivatives gave the corresponding pentacoordinated phenoxysilane
and carboxysilanes, respectively. X-ray crystallographic analyses of these silanes revealed
that the distance of the N Si dative bond is shortened and the pentacoordination character
of the silicon center becomes greater with the increase in the electron-withdrawing character
of the apical substituent on silicon.
Keywords Carboxysilanes; intramolecular coordination; pentacoordinated silicon compounds;
phenoxysilanes
INTRODUCTION
Highly coordinated silicon compounds are known to exhibit distinct difference in
reactivities, structures, and properties from their tetracoordinated counterparts.1 Among
them, much attention has been paid to neutral pentacoordinated silicon compounds with an
intramolecular N Si dative bond for examination of their intriguing structures and reactiv-
ities that originate from the characteristics of the dative bond.2 In this class of compounds,
Received 11 February 2009; accepted 24 February 2009.
Dedicated to Professor Naomichi Furukawa on the occasion of his 70th birthday.
This work was partly supported by Grants-in-Aid for The 21st Century COE Program for Frontiers in
Fundamental Chemistry (T.K.), for The Global COE Program for Education and Research Center for Emergence
of New Molecular Chemistry (K.G. and S.S.), and for Scientific Research (K.G. and T.K.) from the Ministry of
Education, Culture, Sports, Science and Technology, Japan. K.G. acknowledges the support by Tokuyama Science
Foundation. We also thank Tosoh Finechem Corporation and Shin-Etsu Chemical Co., Ltd. for the generous gifts
of alkyllithiums and silicon compounds, respectively.
Address correspondence to Kei Goto, Interactive Research Center of Science, Graduate School of Science
and Engineering, Tokyo Institute of Technology, 2-12-1 O-okayama, Meguro-ku, Tokyo 152-8551, Japan. E-mail:
goto@chem.titech.ac.jp and Takayuki Kawashima, Department of Chemistry, Graduate School of Science, The
University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan. E-mail: takayuki@chem.s.u-tokyo.ac.jp
1221

1222
Y. DOMOTO ET AL.
Figure 1 Schematic representation of pentacoordinated silanes with monocyclic (I), bicyclic (II), and tricyclic
(III) motifs.
monocyclic,2a,b bicyclic,2c,d or tricyclic frameworks,2e,f have been utilized to hold the nitro-
gen and silicon atoms in proximity (Figure 1), and it was found that the N Si interaction
is sensitive to the electronic character of the substituents on the silicon atom. To date,
extensive structural investigations concerning pentacoordinated silicon compounds with a
tricyclic motif such as silatranes have been undertaken, which have revealed that electron-
withdrawing substituents on the silicon shorten the N Si distance.3 For pentacoordinated
silanes with the mono- and bicyclic frameworks, however, systematic study from this view-
point is still missing in spite of extensive exploration of their reactivities. Recently, we have
reported the synthesis of neutral pentacoordinated hydrosilanes with a bicyclic motif based
on the dibenzosilazocine framework4a and application to the construction of a tweezer-type
cavitand.4b This type of bicyclic motif has been successfully utilized for the investigation
of the substituent effects on highly coordinated main group elements.5 Several compounds
bearing the dibenzosilazocine framework have been reported by Carre´ et al.,2c although
crystallographic analysis has been performed only for dimethyl- and difluoro-substituted
derivatives. In this article, we report the synthesis of neutral pentacoordinated phenoxy- and
carboxysilanes by the reaction of a pentacoordinated hydrosilane with phenol and benzoic
acid derivatives, respectively, along with their structural characterization.
RESULTS AND DISCUSSION
Treatment of neutral pentacoordinated hydrosilane 14a with phenol in chloroform
under reflux for 13 h gave the corresponding phenoxysilane 2 in 84% yield. The reaction of
1 with benzoic acid proceeded under milder conditions, and silyl benzoate 3 was formed
after 18 h at room temperature, which was isolated in 66% yield. It is of note that salicylic
acid reacts with 1 much faster than benzoic acid. The reaction was completed within 10 min
at room temperature, affording silyl salicylate 4 in 87% yield. Compounds 2–4 (Scheme
1) were unambiguously characterized by NMR spectroscopy (1H, 13C, 29Si) and elemental
analyses. The 29Si NMR (CDCl3) resonances of 2–4 appear at −40.3, −44.1, and −46.1
ppm, respectively, the high-field shifts of which are typical for pentacoordinated silicon
compounds.6
While there have been extensive studies on pentacoordinated aryloxy- and carboxysi-
lanes with intramolecular coordination of an amine unit, their structural properties have
been hardly explored, and crystallographic analyses of the compounds bearing a monoden-
tate aryloxy or carboxy ligand have been performed only for silatrane derivatives.7 The
structures of phenoxysilane 2 and carboxysilanes 3 and 4 were established by X-ray crys-
tallographic analyses. Figure 2 shows the ORTEP drawing of 2. The selected bond lengths
and angles of 2 are given in Table I.

PENTACOORDINATED PHENOXYSILANE AND CARBOXYSILANES
1223
Scheme 1
In the crystal structure of phenoxysilane 2, there are two independent molecules in
the asymmetric unit. The silicon atoms of both fragments adopt slightly distorted trigonal
bipyramidal structures with the nitrogen and oxygen atoms at apical positions and three
carbon atoms at equatorial positions. The N Si distances of 2 [2.439(4) and 2.419(4) Å for
Figure 2 ORTEP drawing of 2 (50% probability). Hydrogen atoms are omitted for clarity.

1224
Y. DOMOTO ET AL.
Table I Selected bond lengths (Å) and angles (deg) for 2
Bond lengths
Bond angles
Si(1) N(1)
Si(1) O(1)
Si(1) C(1)
Si(1) C(2)
Si(1) C(3)
O(1) C(4)
Si(2) N(2)
Si(2) O(2)
Si(2) C(5)
Si(2) C(6)
Si(2) C(7)
O(2) C(8)
2.439(4)
1.696(3)
1.880(5)
1.890(5)
1.873(4)
1.360(5)
2.419(4)
1.692(3)
1.882(4)
1.884(5)
1.885(4)
1.352(5)
N(1) Si(1) O(1)
N(1) Si(1) C(1)
N(1) Si(1) C(2)
N(1) Si(1) C(3)
O(1) Si(1) C(1)
O(1) Si(1) C(2)
O(1) Si(1) C(3)
C(1) Si(1) C(3)
C(1) Si(1) C(2)
C(2) Si(1) C(3)
Si(1) O(1) C(4)
167.72(14)
77.19(17)
76.64(16)
88.03(15)
100.83(19)
94.71(18)
103.73(16)
113.3(2)
N(2) Si(2) O(2)
N(2) Si(2) C(5)
N(2) Si(2) C(6)
N(2) Si(2) C(7)
O(2) Si(2) C(5)
O(2) Si(2) C(6)
O(2) Si(2) C(7)
C(5) Si(2) C(7)
C(5) Si(2) C(6)
C(6) Si(2) C(7)
Si(2) O(2) C(8)
168.02(15)
77.46(16)
76.82(17)
87.33(16)
94.08(18)
101.10(18)
104.29(17)
118.73(19)
121.7(2)
122.22(19)
116.2(2)
138.0(3)
111.29(19)
140.2(3)
each fragment] are much shorter than the sum of van der Waals radii of silicon and nitrogen
(3.65 Å),8 proving strong interaction between the two atoms in the crystalline state, although
they are longer than the values observed for silatranes bearing an aryloxy ligand (2.07–2.11
Å).7a,b Carre´ et al. reported the crystal structure of a pentacoordinated difluorosilane with
the same type of dibenzosilazocine framework,2c the N Si distance of which [2.430 Å
(average)] is close to those of 2. The Si O bond lengths of 2 [1.696(3) and 1.692(3) Å for
each fragment] are slightly longer than that of (2-bromophenoxy)triphenylsilane (1.645(4)
Å),9 indicating the hypervalent character of the Si O bond of 2.
The ORTEP drawings of carboxysilanes 3 and 4 are shown in Figures 3 and 4,
respectively. The selected bond lengths and angles of 3 and 4 are given in Table II. In
silyl benzoate 3, the N Si distance is 2.315(2) Å, which is much shorter than that of 2.
Silyl salicylate 4 showed even shorter N Si distance (2.2865(17) Å), although it is still
longer than those of silatranes bearing an carboxy ligand (2.05–2.07 Å).7c,d There have
been several examples of pentacoordinated silanes with a salicylate ligand,10 but in all
cases the salicylate serves as an O,O-bidentate ligand to form a cyclic structure. In contrast,
a monodentate coordination of the salicylate ligand is clearly shown in the crystal structure
of 4, where the o-OH group is located in the far side of the silicon atom.
The pentacoordination characters (%TBPa/%TBPe)11 and the N Si bond distances
of 2–4 are summarized in Table III along with their 29Si NMR chemical shifts. The
geometry around the silicon center becomes closer to the ideal TBP as the electron-
withdrawing ability of the substituent at the apical position is higher.12 The N Si bond
distance also depends on the electronic nature of the apical substituent and is shortened as
the pentacoordination character becomes greater, as has been observed for the analogues
with other main group element bearing the same type of framework.5 In the 29Si NMR
spectra, small but apparent high-field shifts were observed as the apical substituent becomes
electron-withdrawing, showing the increase in the degree of pentacoordination. These
results indicate that the correlation between pentacoordination character and electronic
nature of the apical substituent is preserved also in solution.
In summary, we have synthesized neutral pentacoordinated phenoxy- and carboxysi-
lanes with a N Si dative bond bearing a bicyclic framework. We have also performed a
systematic structural analysis of the phenoxy- and carboxysilanes and revealed that, in anal-
ogy with neutral pentacoordinated organosilanes with a tricyclic motif such as silatranes,

PENTACOORDINATED PHENOXYSILANE AND CARBOXYSILANES
1225
Figure 3 ORTEP drawing of 3 (50% probability). Hydrogen atoms are omitted for clarity.
Table II Selected bond lengths (Å) and angles (deg) for 3 and 4
3
4
Bond lengths
Si(1) N(1)
Si(1) N(1)
Si(1) O(1)
Si(1) C(1)
Si(1) C(2)
Si(1) C(3)
O(1) C(4)
C(4) O(2)
2.315(2)
1.768(2)
1.886(3)
1.881(3)
1.878(2)
1.335(3)
1.219(3)
2.2865(17)
1.7772(14)
1.880(2)
1.880(2)
1.868(2)
1.314(2)
1.236(2)
Si(1) O(1)
Si(1) C(1)
Si(1) C(2)
Si(1) C(3)
O(1) C(4)
C(4) O(2)
Bond angles
N(1) Si(1) O(1)
N(1) Si(1) C(1)
N(1) Si(1) C(2)
N(1) Si(1) C(3)
O(1) Si(1) C(1)
O(1) Si(1) C(2)
O(1) Si(1) C(3)
C(1) Si(1) C(3)
C(1) Si(1) C(2)
C(2) Si(1) C(3)
Si(1) O(1) C(4)
O(1) C(4) O(2)
175.86(7)
79.07(10)
78.74(10)
90.73(10)
99.70(11)
98.84(11)
93.37(10)
114.05(11)
123.46(10)
117.53(12)
127.18(16)
123.3(3)
N(1) Si(1) O(1)
N(1) Si(1) C(1)
N(1) Si(1) C(2)
N(1) Si(1) C(3)
O(1) Si(1) C(1)
O(1) Si(1) C(2)
O(1) Si(1) C(3)
C(1) Si(1) C(3)
C(1) Si(1) C(2)
C(2) Si(1) C(3)
Si(1) O(1) C(4)
O(1) C(4) O(2)
171.25(7)
79.49(8)
79.61(8)
89.37(8)
93.45(8)
100.03(8)
98.55(8)
117.64(10)
121.50(9)
116.07(9)
127.82(13)
123.07(18)

1226
Y. DOMOTO ET AL.
Figure 4 ORTEP drawing of 4 (50% probability). Hydrogen atoms, except for that of the hydroxy group, are
omitted for clarity.
the dative N Si bond distance is shortened with the increase in the electron-withdrawing
character of the substituent at the apical position, and the pentacoordination character at
the silicon center concomitantly becomes greater.
EXPERIMENTAL
All reactions were carried out under an argon atmosphere. Melting points were de-
termined on a Yanaco micro melting point apparatus. All melting points were uncorrected.
Chloroform was distilled from calcium hydride under an Ar atmosphere before use. 1H,13C,
and29Si NMR spectra were recorded on a Bruker DRX-500 spectrometer. 1H and 13C chem-
ical shifts were referenced using a residual peak of CDCl3 (1H, δH 7.25, 13C, δC 77.0) as an
Table III Selected structural parameters and 29Si NMR (99 MHz, CDCl3) data of 2, 3, and 4
N
Si/Å
%TBPe/%TBPa
δSi
2
3
4
2.439(4)
2.315(2)
2.2865(17)
74/50
84/63
85/62
−40.3
−44.1
−46.1

PENTACOORDINATED PHENOXYSILANE AND CARBOXYSILANES
1227
internal standard. 29Si NMR chemical shifts are reported relative to Me4Si (0 ppm) in ppm.
IR spectra were recorded on JASCO V-650. Gel permeation liquid chromatography (GPLC)
was performed by LC-918 and LC-9204 with JAIGEL 1H+2H columns (Japan Analytical
Industry) with chloroform as solvent. Elemental analyses were performed by the Micro-
analytical Laboratory of the Department of Chemistry, Faculty of Science, the University
of Tokyo, or on a Perkin-Elmer PE2400 Series II CHNS/O Analyzer. Pentacoordinated
hydrosilane 1 was prepared according to the reported procedure.4a
Synthesis of Phenoxysilane 2
To the mixture of monohydrosilane 1 (116 mg, 0.325 mmol) and phenol (31 mg, 0.33
mmol), CHCl3 (3 mL) was added. After stirring for 13 h at reflux temperature, the solvent
was removed, and the residue was purified by GPLC to give phenoxysilane 2 as a colorless
1
solid (123 mg, 0.272 mmol, 84%). Mp 78–80◦C; H NMR (500 MHz, CDCl3) δ 0.65 (t,
3JHH = 7.3 Hz, 3H), 0.83–0.87 (m, 2H), 1.17–1.23 (m,2H), 1.44–1.46 (m, 2H), 3.68 (d,
2JHH = 14.7 Hz, 2H), 3.74 (d, 2JHH = 14.7 Hz, 2H), 6.70 (tt, 3JHH = 7.3 Hz, 4JHH = 1.1 Hz,
1H), 6.75 (dd, 3JHH = 8.7 Hz, 4JHH = 1.1 Hz, 2H), 6.98–7.02 (m, 2H), 7.14–7.21 (m, 5H),
7.26–7.28 (m, 2H), 7.34−7.40 (m, 4H), 8.24–8.26 (m, 2H); 13C NMR (126 MHz, CDCl3)
δ 13.6 (q), 20.5 (t), 24.5 (t), 52.3 (t), 55.6 (t), 119.2 (d), 120.1 (d), 125.6 (d), 127.3 (d),
127.4 (d), 128.1 (d), 128.7 (d), 129.9 (d), 132.3 (d), 133.0 (s), 136.7 (s), 138.5 (d), 146.5
1
(s), 157.6 (s); 29Si{ H} NMR (99 MHz, CDCl3) δ –40.3. Anal. Calcd. For C30H31NOSi:
C, 80.13; H, 6.95; N 3.11. Found: C, 80.07; H, 6.97; N, 2.88.
Synthesis of Carboxysilane 3
To the mixture of monohydrosilane 1 (288 mg, 0.806 mmol) and benzoic acid (99
mg, 0.81 mmol), CHCl3 (4 mL) was added. After stirring for 18 h at room temperature,
the solvent was removed and the residue was purified by GPLC to give carboxysilane 3 as
colorless solid (253 mg, 0.529 mmol, 66%). Mp 65–67◦C; 1H NMR (500 MHz, CDCl3) δ
3
0.67 (t, JHH = 7.3 Hz, 3H), 0.84–0.90 (m, 2H), 1.21–1.28 (m, 2H), 1.48–1.51 (m, 2H),
3.72 (d, 2JHH = 14.6 Hz, 2H), 3.76 (d, 2JHH = 14.6 Hz, 2H), 7.18–7.26 (m, 7H), 7.36–7.43
(m, 7H), 7.51 (tt, 3JHH = 7.4 Hz, 4JHH = 1.4 Hz, 1H), 8.10–8.15 (m, 4H); 13C NMR (126
MHz, CDCl3) δ 13.6 (q), 20.5 (t), 24.4 (t), 52.6 (t), 55.5 (t), 125.5 (d), 127.3 (d), 127.5 (d),
128.1 (d), 128.3 (d), 130.1 (d), 130.1 (d), 131.6 (s), 132.0 (d), 132.3 (d), 133.5 (s), 136.5
1
(s), 137.8 (d), 145.8 (s), 166.7 (s); 29Si{ H} NMR (99 MHz, CDCl3) δ –44.1; IR (KBr)
νC 1687 cm−1. Anal. Calcd. For C31H31NO2Si: C, 77.95; H, 6.54; N 2.93, Found: C,
O
77.74; H, 6.83; N, 2.66.
Synthesis of Carboxysilane 4
The carboxysilane 4 (374 mg, 0.758 mmol, 87%) was prepared from monohydrosilane
1 (312 mg, 0.873 mmol) and salicylic acid (122 mg, 0.883 mmol) by the same procedure as
the synthesis of 3, while the reaction time was 0.5 h. Mp 123−125◦C; 1H NMR (500 MHz,
CDCl3) δ 0.66 (t, 3JHH = 7.3 Hz, 3H), 0.85–0.90 (m, 2H), 1.23–1.29 (m, 2H), 1.53–1.56
(m, 2H), 3.76 (d, 2JHH = 14.7 Hz, 2H), 3.80 (d, 2JHH = 14.7 Hz, 2H), 6.88 (dt, 3JHH = 7.9
Hz, 4JHH = 1.0 Hz, 1H), 6.92 (dd, 3JHH = 7.9 Hz, 4JHH = 1.0 Hz, 1H), 7.20–7.29 (m, 5H),
7.37–7.44 (m, 7H), 8.05–8.07 (m, 3H), 11.48 (s, 1H); 13C NMR (126 MHz, CDCl3) δ 13.5
(q), 20.4 (t), 24.5 (t), 52.7 (t), 55.6 (t), 115.4 (s), 117.2 (d), 118.6 (d), 125.6 (d), 127.5 (d),

1228
Y. DOMOTO ET AL.
127.7 (d), 128.5 (d), 130.3 (d), 130.8 (d), 131.2 (s), 132.2 (d), 134.7 (d), 136.2 (s), 137.6
1
(d), 145.5 (s), 162.2 (s), 171.0 (s); 29Si{ H} NMR (99 MHz, CDCl3) δ –46.1; IR (KBr)
νC 1606, 1647 cm−1. Anal. Calcd. For C31H31NO3Si: C, 75.42; H, 6.33; N 2.84, Found
O
C, 75.58; H, 6.57; N, 2.64.
X-Ray Crystallographic Analysis of 2, 3, and 4
Single crystals of 2, 3, and 4 were grown in their chloroform/hexane solutions. The
intensity data were collected at 120 K on a Rigaku/MSC Mercury CCD diffractometer with
graphite-monochromated MoKα radiation (λ = 0.71069 Å). Crystallographic and experi-
mental data are listed in Table IV. The structures were solved by the direct method and
refined by full-matrix least squares on F2 using SHELXL 97.13 The nonhydrogen atoms
were refined anisotropically. The hydrogen atoms were idealized by using the riding models
except for the OH hydrogen of 4, which was located in the difference Fourier map and re-
fined isotropically. Crystallographic data for the structural analysis has been deposited with
the Cambridge Crystallographic Data Centre, CCDC Nos. 721463, 721464, and 721465
for 2, 3, and 4 respectively. Copies of this information may be obtained free of charge from
the director, CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK (fax: +44–1223-336-033;
e-mail: deposit@ccdc.cam.ac.jk or www: http://www.ccdc.cam.ac.uk).
Table IV Crystallographic data for 2, 3, and 4
2
3
4
Formula
Temperature/K
C30H31NOSi
120(2)
C31H31NO2Si
123(2)
C31H31NO3Si
120(2)
Crystal system
Space group
Triclinic
P1
Monoclinic
P21/c
Monoclinic
P21/c
a/Å
b/Å
c/Å
α/deg
β/deg
γ /deg
V/Å3
9.628(5)
10.143(4)
13.930(6)
66.272(14)
82.83(2)
88.67(2)
1235.2(10)
2
9.081(6)
18.032(10)
15.749(10)
8.6593(18)
17.938(4)
17.321(4)
95.935(13)
102.809(3)
2565(3)
4
1.237
2623.4(9)
4
1.250
Z
Calculated density/g cm−3
1.209
Reflections collected
8093
10995
17116
Unique
Rint
6056
0.0326
3720
0.0447
4614
0.0703
F000
480
1016
1048
Limiting indices
−11 ≤ h ≤ 11
−12 ≤ k ≤ 11
−16 ≤ l ≤ 15
3/597
−7 ≤ h ≤ 10
−19 ≤ k ≤ 20
−18 ≤ l ≤ 16
0/317
−10 ≤ h ≤ 7
−21 ≤ k ≤ 21
−20 ≤ l ≤ 20
0/330
Restraints/parameters
Goodness of fit (F2)
R indices (I > 2σ(I))
1.080
1.079
1.001
R1 = 0.0566
wR2 = 0.1298
R1 = 0.0613
wR2 = 0.1336
R1 = 0.0467
wR2 = 0.1084
R1 = 0.0711
wR2 = 0.1243
R1 = 0.0478
wR2 = 0.1132
R1 = 0.0594
wR2 = 0.1270
R indices (all data)

PENTACOORDINATED PHENOXYSILANE AND CARBOXYSILANES
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