Incorporation of azo group at axial position of silatranes: Synthesis, characterization and antimicrobial activity

Article abstract of DOI:10.1002/aoc.3330

4-Aminoazobenzene-derived silatranes bearing urea and aminosuccinimide as linker groups at the axial position are reported. The urea functionality is introduced in a silane (2) by the rearrangement reaction between 3-isocyanatopropyltriethoxysilane and 4-aminoazobenzene. N-(3-silatranylpropyl)-N′-[(p-phenyldiazenyl)phenyl]urea and N-[3-(3,7,10-trimethylsilatranyl)propyl]-N′-[(p-phenyldiazene)phenyl]urea were prepared by transesterification reaction of 2 with triethanolamine and trisisopropanolamine, respectively. An efficient method for C-N bond formation is described for the synthesis of 3-(silatranylpropyl)amino-N-[(p-phenyldiazene)phenyl]pyrrolidine-2,5-dione and 3-[(3,7,10-trimethylsilatranyl)propyl]amino-N-[(p-phenyldiazene)phenyl]pyrrolidine-2,5-dione via aza-Michael addition reaction of aminopropylsilatranes with 4-(N-maleimido)azobenzene under mild conditions. All the compounds were well characterized using elemental analysis, spectroscopic techniques, thermogravimetric analysis and X-ray diffraction. UV-visible spectroscopy indicates that the 4-aminoazobenzene-derived silatranes are capable acetate receptors. The synthesized compounds were screened for possible antimicrobial properties with the results showing a modest activity.

Full text of DOI:10.1002/aoc.3330

Full paper
Received: 20 January 2015
Revised: 8 April 2015
Accepted: 12 April 2015
Published online in Wiley Online Library: 25 May 2015
(wileyonlinelibrary.com) DOI 10.1002/aoc.3330
Incorporation of azo group at axial position of
silatranes: synthesis, characterization and
antimicrobial activity
Gurjaspreet Singha*, Amandeep Saroaa, Shally Girdhara, Sunita Rania,
Duane Choquesillo-Lazarteb* and Subash Chandra Sahooa
4-Aminoazobenzene-derived silatranes bearing urea and aminosuccinimide as linker groups at the axial position are reported.
The urea functionality is introduced in a silane (2) by the rearrangement reaction between 3-isocyanatopropyltriethoxysilane
and 4-aminoazobenzene. N-(3-silatranylpropyl)-N′-[(p-phenyldiazenyl)phenyl]urea and N-[3-(3,7,10-trimethylsilatranyl)propyl]-N
′-[(p-phenyldiazene)phenyl]urea were prepared by transesterification reaction of
2
with triethanolamine and
trisisopropanolamine, respectively. An efficient method for C–N bond formation is described for the synthesis of 3-
(silatranylpropyl)amino-N-[(p-phenyldiazene)phenyl]pyrrolidine-2,5-dione and 3-[(3,7,10-trimethylsilatranyl)propyl]amino-N-[(p-
phenyldiazene)phenyl]pyrrolidine-2,5-dione via aza-Michael addition reaction of aminopropylsilatranes with 4-(N-maleimido)
azobenzene under mild conditions. All the compounds were well characterized using elemental analysis, spectroscopic tech-
niques, thermogravimetric analysis and X-ray diffraction. UV–visible spectroscopy indicates that the 4-aminoazobenzene-
derived silatranes are capable acetate receptors. The synthesized compounds were screened for possible antimicrobial properties
with the results showing a modest activity. Copyright © 2015 John Wiley & Sons, Ltd.
Additional supporting information may be found in the online version of this article at the publisher’s web-site.
Keywords: 4-aminoazobenzene; silatrane; urea; aminosuccinimide; acetate receptor; antimicrobial activity
the synthesis of a non-symmetric urea-substituted silane (2). These
substituted urea-trialkoxysilanes have attracted considerable atten-
Introduction
Azo dyes are a well-known family of organic dyes, which are widely
used in diverse areas such as dyes, nonlinear optical chromophores,
photoresponsive molecular switches, pH indicators and photo-
storage units.[1–6] Along with their conventional roles in industrial
applications, azo dyes are known to take part in a number of biolog-
ical reactions such as inhibition of DNA and RNA and biological
activities against bacteria and fungi.[7,8] Recently, metal complexes
of dyes have attracted much attention in both academic and ap-
plied research due to their interesting electronic and geometric
features.[9,10] To the best of our knowledge, only a handful of com-
pounds have demonstrated successful incorporation of azo dyes
into axial position of silatranes. The reason for choosing silatranes
over numerous other metal complexes is the structural aspect of
silatranes, especially the N→Si transannular bond in distorted trigo-
nal bipyramidal geometry at the silicon atom.[11] These modified
compounds are significant because of the stereoelectronic effect
of the silatranyl group in shaping the reactivity of exocyclic func-
tional groups apical to the transannular bond.[12] Silatranes have a
wide range of geometries depending upon these axial groups
which enhance an array of biological and material science
applications.[13] Contemporary chemists have boosted work on
the modification of exocyclic functional groups due to enhanced bi-
ological and material science applications of modified silatranes.[14]
To incorporate azo groups at axial positions of silatranes, two
routes were pursued in the work reported in this paper. In the first
route, derivatization of 4-aminoazobenzene (1) was carried out by
the reaction with 3-isocyanatopropyltriethoxysilane which led to
tion due to applications in nonlinear optical chromophores, hybrid
materials and functionalized mesoporous silicas.[15,16] In the second
route, 4-(N-maleimido)azobenzene (7) acting as a strong electron-
accepting moiety was incorporated at the axial position of
aminopropylsilatranes (5, 6) by an aza-Michael addition reaction.
These types of compounds are attractive for chemists working in
the field of biomimetic chemistry as new supple receptors for
amines.[17,18] So in the work reported in this article, we integrated
silatranes with azobenzene at the axial position using urea and
aminosuccinimide as linker groups which might lead to the gener-
ation of new materials. Both urea and aminosuccinimide groups are
valuable for making 4-aminoazobenzene-derived silatranes as
anion receptors for acetate groups via hydrogen-bonding
interactions. These interactions are easily monitored using
anion-complexation-induced charge transfer bands in UV–visible
absorption spectra. Due to a wide range of biological properties
*
*
Correspondence to: Gurjaspreet Singh, Department of Chemistry, Panjab
University, Chandigarh 160014, India. E-mail: gjpsingh@pu.ac.in
Duane Choquesillo-Lazarte, Laboratorio de Estudios Cristalográficos, IACT, CSIC-
University of Granada, Avenida de las Palmeras 4, 18100, Armilla (Granada),
Spain. E-mail: duanec@ugr.es
a Department of Chemistry, Panjab University, Chandigarh 160014, India
b Laboratorio de Estudios Cristalográficos, IACT, CSIC-University of Granada,
Avenida de las Palmeras 4, 18100, Armilla (Granada), Spain
Appl. Organometal. Chem. 2015, 29, 549–555
Copyright © 2015 John Wiley & Sons, Ltd.

G. Singh et al.
of silatranes, such as antimicrobial, antifungal and antitumor
activities,[19] we studied the antimicrobial properties of all the syn-
thesized silatranes (3, 4, 8 and 9) against reference microorganisms.
Suitable crystals were mounted on MiTeGenMicromounts™ and
these samples were used for data collection. The data were col-
lected with a Bruker D8 Venture (100 K) diffractometer. The data
were processed with the APEX2 program[23] and corrected for ab-
sorption using SADABS.[24] The structures were solved by direct
methods, which revealed the position of all non-hydrogen atoms.
These atoms were refined on F2 by a full-matrix least-squares proce-
dure using anisotropic displacement parameters.[25] All hydrogen
atoms were located in different Fourier maps and included as fixed
contributions riding on attached atoms with isotropic thermal dis-
placement parameters 1.2 times those of the respective atom.
The geometric calculations were carried out with PLATON[26] and
drawings were produced with Olex2[27] and MERCURY.[28] Addi-
tional crystal data and more information about the X-ray structural
analyses are given in Tables 1 and 2.
Materials and methods
Synthesis and characterization
3-Isocyanatopropyltriethoxysilane, 3-aminopropyltriethoxysilane,
triethanolamine, trisisopropanolamine and maleic anhydride were
purchased from Sigma-Aldrich. Compound 1,[20] aminopropyl-
silatranes 5 and 6[21] and 7[22] were prepared as reported in the lit-
erature. All reactions were performed under nitrogen atmosphere.
1H NMR and 13C NMR spectra were recorded in CDCl3 solution
using 400 MHz (Bruker Avance AL 400) and 300 MHz (JEOL AL
300) FT NMR instruments. Infrared spectra were obtained with a
Thermo Nicolet Nexus 670 spectrometer. C, H and N analyses were
performed with a FLASH-2000 organic element analyser while Si
content was estimated gravimetrically. Mass spectral measure-
ments (ESI source with capillary voltage of 2500 V) were carried
out with a VG Analytical (70-S) spectrometer. Thermal analysis
was conducted using an SDT Q 600V20.9 Build 20TGA instrument.
A weighed amount of sample was loaded in alumina pans and
ramped at 10°C minꢀ1 to 800°C in dry air at 60 ml minꢀ1. UV–visible
spectra were recorded with a Varian Cary 500 spectrophotometer.
Minimal inhibitory concentration (MIC) measurement
The antimicrobial activity of compounds was checked using the
broth microdilution method in a 96-well microtitre plate.[29] Growth
medium (100 μl) was added in each well except the first, in which
200 μl of stock solution (800 μg mlꢀ1) of test compound in
dimethylsulfoxide was added. After appropriate mixing, twofold se-
rial dilutions were made ranging from 0.78 to 400μl mlꢀ1. Actively
growing bacterial culture (100μl) containing 1 × 105 cells was
added to all the dilutions and mixed gently. The plates were incu-
bated at 37°C for 18 h. The MIC of the given compounds was mea-
sured as the maximum dilution of compound showing no visible
growth of bacteria.
X-ray crystallography
Crystals for measurement were prepared under inert conditions im-
mersed in perfluoropolyether as protecting oil for manipulation.
Table 1. Crystal data and structure refinement for compounds 2 and 9
2
9
Empirical formula
C
22H32N4O4Si
C28H37N5O5Si
551.72
Formula weight
444.61
100
T (K)
100
λ (Å)
Crystal system
1.54178
Monoclinic
P21/c
1.54178
Monoclinic
C2/c
Space group
Unit cell dimensions
a (Å)
b (Å)
c (Å)
α (°)
15.2270(4)
30.358(3)
9.2457(2)
8.4632(6)
18.4518(5)
25.907(2)
90
90
β (°)
114.2300(11)
119.618(5)
γ (°)
90
90
Volume (Å3)
Z
2368.88(10)
5786.3(9)
4
8
D (calcd; g cmꢀ3
Absorption coefficient (mmꢀ1
)
1.247
1.267
)
1.161
1.092
F (000)
952
2352
Theta range for data collection (°)
Reflections collected
3.18–66.75
3.35–66.56
15 176
20 567
Independent reflections
Data / restraints / parameters
Goodness-of-fit on F2
4122 (R(int) = 0.0246)
4122 / 0 / 283
1.058
4988 (R(int) = 0.0801)
4988 / 0 / 383
1.058
Final R indices (I > 2σ(I))
R indices (all data)
Largest diff. peak and hole (eÅꢀ3
R1 = 0.0559, wR2 = 0.1437
R1 = 0.0609, wR2 = 0.1483
0.908 and ꢀ0.489
R1 = 0.0755, wR2 = 0.1894
R1 = 0.0926, wR2 = 0.2041
0.871 and ꢀ0.665
)
wileyonlinelibrary.com/journal/aoc
Copyright © 2015 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2015, 29, 549–555

Synthesis of silatranes possessing azo group at axial position
Table 2. Selected bond lengths (Å) and angles (°) for 2 and 9
(C7), 155.63 (C13). MS, m/z (relative abundance (%), assignment):
456 (100, (M+ H)+), 477 (49.1, (M + Na)+), 493 (8.2, (M + K)+), 932
(72.4, (2M + Na)+).
Compound 2
1.6238(19)
Compound 9
Si(1)–O(2)
Si(1)–O(1)
1.666(2)
Synthesis of N-[3-(3,7,10-trimethylsilatranyl)propyl]-N′-[(p-phenyldia-
zenyl)phenyl]urea (4)
Si(1)–O(1)
1.6269(18)
1.6092(19)
1.845(2)
Si(1)–O(2)
1.661(2)
Si(1)–O(3)
Si(1)–O(3)
1.658(2)
Si(1)–C(7)
Si(1)–N(1)
2.248(3)
Compound 4 was synthesized using a method similar to that for 3,
with trisisopropanolamine being used as a tripodal ligand instead
of triethanolamine.
O(2)–Si(1)–O(1)
O(2)–Si(1)–C(7)
O(1)–Si(1)–C(7)
O(3)–Si(1)–O(2)
O(3)–Si(1)–O(1)
111.69(11)
104.18(10)
111.87(10)
108.61(11)
106.27(10)
Si(1)–C(10)
1.878(3)
O(1)–Si(1)–C(10)
O(2)–Si(1)–O(1)
O(2)–Si(1)–N(1)
O(2)–Si(1)–C(10)
O(3)–Si(1)–O(1)
O(3)–Si(1)–O(2)
O(3)–Si(1)–N(1)
O(3)–Si(1)–C(10)
C(10)–Si(1)–N(1)
97.92(12)
121.04(13)
80.90(11)
95.50(13)
115.02(13)
117.70(12)
82.25(11)
101.83(13)
175.53(13)
Yield 1.07 g (81%); m.p. 197–199°C. Anal. Calcd for C25H35N5O4Si
(497) (%): C, 60.36; H, 7.04; N, 15.09; Si, 5.63. Found (%): C, 60.28; H,
6.97; N, 14.94; Si, 5.51. IR (cmꢀ1): 544, 695, 1064, 1108, 1538, 1597,
1
1662, 2868, 2937, 3346. H NMR (400 MHz, CDCl3, δ, ppm): 0.42 (t,
2H, J= 8.2 Hz, H16A,16B), 1.13 (m, 9H, CH3), 1.62 (m, 2H, H15A,15B),
2.73 (m, 6H, CH2N), 3.19 (m, 2H, H14A,14B), 3.83 (m, 3H, OCH), 5.63
(s, 1H, NHC14), 7.35–7.81 (m, 9H, Ar–H), 7.68 (s, 1H, NHC10). 13C
NMR (75 MHz, CDCl3, δ, ppm): 15.29 (C16), 20.33, 20.42, 20.77 (CH3),
25.16 (C15), 42.84 (C14), 61.73, 62.04, 63.40 (NCH2), 65.00, 65.16,
65.79 (OCH), 118.41 (C9,11), 122.60 (C8,12), 124.17 (C2,6), 128.87 (C3,5),
130.05 (C4), 143.30 (C10), 151.74 (C1), 151.89 (C7), 155.70 (C13).
MS, m/z (relative abundance (%), assignment): 498 (100, (M + H)+),
520 (33.9, (M+ Na)+), 536 (7.4, (M+ K)+), 1017 (9.1, (2M + Na)+).
Syntheses
Synthesis of N-(3-triethoxysilylpropyl)-N′-[(p-phenyldiazenyl)phenyl]urea (2)
3-Isocyanatopropyltriethoxysilane (2.00 g, 8.06 mmol) was added
dropwise to a stirred solution of 1 (1.59 g, 8.06 mmol) in chloroform.
The resulting mixture was refluxed at 70°C for 4 h. Then the solvent
was evaporated under vacuum to afford a dark orange-coloured so-
lution of 2 that eventually solidified at room temperature on resting
for 15min. Silane 2 was highly hygroscopic and its crystals were
grown in tetrahydrofuran.
Yield 2.93 g (82%); m.p. 77–79°C. Anal. Calcd for C22H32N4O4Si
(444) (%): C, 59.45; H, 7.20; N, 12.61; Si, 6.30. Found (%): C, 58.80; H,
7.76; N, 12.49; Si, 6.16. IR (cmꢀ1): 1074, 1511, 1558, 1648, 2888,
2933, 3346. 1H NMR (400 MHz, CDCl3, δ, ppm): 0.59 (t, 2H, J=8.2 Hz,
H16A,16B), 1.16 (t, 9H, J=6.9 Hz, CH3), 1.60 (m, 2H, H15A,15B), 3.64 (m,
2H, H14A,14B), 3.74 (q, 6H, J= 6.9Hz, OCH2), 5.35 (s, 1H, NHC14), 7.09
(s, 1H, NHC12), 7.33–7.81 (m, 9H, Ar–H). 13C NMR (75 MHz, CDCl3, δ,
ppm): 7.82 (C16), 18.45 (CH3), 23.66 (C15), 54.33 (C14), 58.49 (OCH2),
119.23 (C9,11), 122.88 (C8,12), 122.89 (C2,6), 128.93 (C3,5), 130.33
(C4), 142.05 (C10), 152.69 (C1), 152.78 (C7), 155.62 (C13). MS, m/z
(relative abundance (%), assignment): 444 (25.1, M+).
Synthesis of 3-(silatranylpropyl)amino-N-[(p-phenyldiazenyl)phenyl]-
pyrrolidine-2,5-dione (8)
In a 100ml two-neck round-bottom flask, 7 (1.20 g, 4.31 mmol) was
dissolved in 50 ml of chloroform under nitrogen atmosphere. A so-
lution of 3-aminopropylsilatrane 5 (1.00 g, 4.31 mmol) in chloroform
was added dropwise and the mixture was stirred for 10 h at room
temperature. The solvent was evaporated under reduced pressure
and an orange-coloured viscous oil was obtained. Upon addition
of dry hexane, an orange–red solid was extracted.
Yield 1.64 g (74%); m.p. 201–203°C. Anal. Calcd for C25H31N5O5Si
(509) (%): C, 58.93; H, 6.09; N, 13.75; Si, 5.50. Found (%): C, 58.76; H,
6.32; N, 13.67; Si, 5.38. IR (cmꢀ1): 548, 687, 1097, 1118, 1397, 1552,
1
1605, 1710, 2880, 2933, 3288. H NMR (400 MHz, CDCl3, δ, ppm):
0.46 (t, 2H, J = 8.2 Hz, H19A,19B), 1.66 (m, 2H, H18A,18B), 2.16 (s, 1H,
NH), 2.64 (m, 2H, H17A,17B), 2.74 (m, 1H, H15A), 2.80 (t, 6H, J = 5.8Hz,
CH2N), 3.09 (dd, 1H, J= 8.3, 18.0 Hz, H15B), 3.75 (t, 6H, J = 5.8 Hz,
OCH2), 3.97 (dd, 1H, J = 5.0, 8.3 Hz, H16), 7.43–8.02 (m, 9H, Ar–H).
13C NMR (75.5 MHz, CDCl3, δ, ppm): 13.20 (C19), 24.76 (C18), 36.66
(C15), 50.59 (C17), 51.30 (NCH2), 56.23 (C16), 57.92 (OCH2), 123.21
(C9,11), 123.53 (C8,12), 124.06 (C2,6), 129.07 (C3,5), 131.18 (C4), 142.73
(C10), 151.80 (C1), 151.92 (C7), 173.93 (C13), 176.68 (14). MS, m/z
(relative abundance (%), assignment): 510 (100, (M+ H)+).
Synthesis of N-(3-silatranylpropyl)-N′-[(p-phenyldiazenyl)phenyl]urea (3)
A two-necked round-bottomed flask fitted with a magnetic stirrer
and Dean–Stark apparatus was sequentially charged in a stream
of nitrogen with silane 2 (1.20 g, 2.70 mmol) dissolved in 30 ml of
dry toluene. The tripodal ligand triethanolamine (0.36 g, 2.70 mmol)
was added slowly to the solution and allowed to stir at 25°C for
10 min in the presence of catalytic amount of sodium ethoxide.
The mixture was refluxed for 4 h and the solvent was removed un-
der reduced pressure to afford an orange-coloured product with
anhydrous hexane.
Synthesis of 3-[(3,7,10-trimethylsilatranyl)propyl]amino-N-[(p-phenyl-
diazene)phenyl]pyrrolidine-2,5-dione (9)
Compound 9 was prepared using a method similar to that for 8,
with 3,7,10-trimethyl-substituted silatrane 6 being used instead of
silatrane 5. Crystals of 9 were grown in chloroform solution by slow
evaporation.
Yield 1.03 g (85%); m.p. 186–188°C. Anal. Calcd for C22H29N5O4Si
(455) (%): C, 58.02; H, 6.37; N, 15.38; Si, 6.15. Found (%): C, 58.17; H,
6.25; N, 15.12; Si, 6.03. IR (cmꢀ1): 583, 679, 1100, 1559, 1597, 1658,
Yield 1.59 g (79%); m.p.: 209–211°C. Anal. Calcd for C28H37N5O5Si
(551) (%): C, 60.98; H, 6.71; N, 12.70; Si, 5.08. Found (%): C, 60.15; H,
6.56; N, 12.59; Si, 4.98. IR (cmꢀ1): 547, 692, 1061, 1149, 1384, 1544,
1
2876, 2929, 3329. H NMR (400 MHz, CDCl3, δ, ppm): 0.42 (t, 2H,
J = 8.2 Hz, H16A,16B), 1.61 (m, 2H, H15A,15B), 2.72 (t, 6H, J = 5.8Hz,
CH2N), 3.20 (m, 2H, H14A,14B), 3.68 (t, 6H, J = 5.8Hz, OCH2), 6.87 (s,
1H, NHC14), 7.37 (s, 1H, NHC10), 7.35–7.81 (m, 9H, Ar–H). 13C NMR
(100.6 MHz, CDCl3, δ, ppm): 14.07 (C16), 24.42 (C15), 47.05 (C14),
50.08 (NCH2), 58.75 (OCH2), 119.03 (C9,11), 122.47 (C8,12), 123.37
(C2,6), 128.30 (C3,5), 128.50 (C4), 142.33 (C10), 151.47 (C1), 151.54
1601, 1710, 2892, 2937, 3305. H NMR (400 MHz, CDCl3, δ, ppm):
1
0.48 (t, 2H, J = 8.2Hz, H19A,19B), 1.19 (m, 9H, CH3), 1.69 (m, 2H,
H18A,18B), 2.21 (s, 1H, NH), 2.72 (m, 2H, H17A,17B), 2.78 (m, 1H, H15A),
2.80 (m, 6H, CH2N), 2.99 (dd, 1H, J = 8.3, 18.0 Hz, H15B), 3.92 (m, 3H,
OCH), 4.10 (dd, 1H, J = 5.0, 8.3 Hz, H16), 7.46–8.03 (m, 9H, Ar–H).
13C NMR (75.5MHz, CDCl3, δ, ppm): 13.27 (C19), 20.41, 20.51, 20.92
Appl. Organometal. Chem. 2015, 29, 549–555
Copyright © 2015 John Wiley & Sons, Ltd.
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G. Singh et al.
(CH3), 24.93 (C18), 36.69 (C15), 50.52 (C17), 56.26 (C16), 61.92, 62.43,
63.50 (NCH2), 65.14, 65.33, 65.57 (OCH), 123.24 (C9,11), 123.55
(C8,12), 124.48 (C2,6), 129.27 (C3,5), 131.22 (C4), 142.80 (C10), 151.76
(C1), 151.81 (C7), 173.84 (C13), 176.58 (14). MS, m/z (relative abun-
dance (%), assignment): 552 (100, (M+ H)+).
characteristic absorption bands of silatranyl group which are
assigned on the basis of the literature.[31] The prominent contribution
from the transannular N→Si bond is evident in the 550–590 cmꢀ1
region. The absorption bands in the region 1550–1600 cmꢀ1 are
assigned to N¼N stretching of azo group. The stretching vibration
bands of C¼O are seen at 1645–1665 cmꢀ1 which are relatively weak
due to resonance involved in NHC(¼O)NH group for compounds 2, 3
and 4. The FT-IR spectra of silatranes 8 and 9 containing C¼O
functionality exhibit a typical absorption band at 1710 cmꢀ1. The
absorption bands at 2800–2950 and 1600–1615 cmꢀ1 correspond
to methylene stretching and aromatic C¼C bending, respectively.
The absorption bands at 3300–3350 cmꢀ1 are observed due to
symmetric and asymmetric NH stretching for all compounds which
supports the synthesis of desired compounds.
Results and discussion
Synthesis
The non-symmetric urea-substituted silane 2 was synthesized by
the rearrangement reaction of 3-isocyanatopropyltriethoxysilane
and 1 without any catalyst as reported earlier (Scheme 1).[30] This
type of reaction is governed primarily by the basicity or nucleophi-
licity of amines. It involves the attack of electrophilic isocyanate
group at the nucleophilic amino group of 1 having active hydrogen.
The non-symmetric urea-substituted silatranes 3 and 4 were pre-
pared by the transesterification reaction of 2 with triethanolamine
and trisisopropanolamine, respectively.
Traditionally, the Michael addition reaction is catalysed by quan-
titative amount of strong bases and acids in conventional organic
solvents which often leads to undesirable side reactions. To suc-
cessfully catalyse the aza-Michael addition, other methods such as
use of Lewis acids, ionic liquids, silica-supported catalysts and water
as solvent have also been utilized for this transformation. In the
present case, Michael donors and acceptors react via aza-Michael
addition reaction under mild conditions which leads to the synthe-
sis of Michael adduct azobenzene-substituted aminosuccinimide
silatranes 8 and 9 as depicted in Scheme 2. In this reaction,
aminopropylsilatranes 5 and 6 possessing NH2 as nucleophilic
group behave as Michael donors and 7 acts as Michael acceptor.
Since 5 and 6 can act as both nucleophiles and bases, no additional
base is typically required in this reaction.
The NMR (1H and 13C) spectroscopic data were recorded for all
compounds at room temperature and are found to accord with
the structure of the synthesized products. In 1H NMR spectra, the
ethoxy group in silane 2 is replaced by atranyl moiety which is re-
vealed by the appearance of two triplets due to NCH2 (2.72 ppm)
and OCH2 (3.68 ppm) protons for compound 3. As a result of three
stereogenic CH2CHMe carbon atoms, the 1H NMR spectrum of
compound 4 shows complex resonances and hints at the sample
actually measured being composed of different diastereomers.
The methylene protons (CH2) are diastereotopic and they undergo
geminal as well as vicinal coupling with methine proton (CH). Sim-
ilar trends in chemical shifts are observed for atranyl moiety in the
case of compounds 8 and 9. Noteworthy, CH2NH protons are vali-
dated by multiplets in the range 3.19–3.64 ppm for compounds 2,
3 and 4 and in the range 2.64–2.66 ppm for compounds 8 and 9 de-
pending upon exocyclic group attached to the silatrane. The rear-
rangement reaction for azobenzene-substituted urea silatranes is
confirmed by the appearance of a broad singlet signal due to NH
proton adjacent to propyl group in the region of 5.35–6.87 ppm
for the compounds 2, 3 and 4. For these compounds, the signal
of the other NH proton attached to azobenzene appears downfield
at 7.09–7.68 ppm. The olefinic protons of 7 are observed as a singlet
at 6.81 ppm. The aza-Michael addition reaction is indicated by the
disappearance of these olefinic proton signals, which confirms
Michael adduct formation. Both silatranes 8 and 9 exhibit a broad
singlet for NH proton at 2.16 and 2.21 ppm, respectively. In 13C
NMR spectra, methylene carbon of CH2Si appears as the most
shielded carbon signal at 7.82ppm for silane 2. However, in the
case of all silatranes, a downfield shift is observed for the same car-
bon. The aza-Michael reaction is confirmed by the appearance of an
upfield methylene carbon signal for succinimide carbon at 36.66
and 36.64 ppm for silatranes 8 and 9, respectively. The carbonyl
carbons appear as the most downfield signal in the region of
155.38–158.63 ppm for compounds 2, 3 and 4 and in the region
of 170.50–176.67 ppm for compounds 8 and 9.
Spectroscopic studies
The FT-IR spectra of all compounds were recorded as neat spectra
in the range 400–4000 cmꢀ1. All silatranes (3, 4, 8 and 9) exhibit
The mass spectra of all the compounds (2, 3, 4, 8 and 9) show the
respective molecular ion peaks with the addition of H and possess
characteristics silatrane fragmentation pattern. The peaks at
m/z = 274 and 316 are observed due to the cleavage of azobenzene
moiety which leads to the formation of N-propylsilatranylamide cat-
ion for compounds 3 and 4, respectively. The mass spectra of com-
pounds 8 and 9 also involve the formation of aminopropylsilatranyl
cation moiety which appears at m/z = 233 and 248 for 8 and 9, re-
spectively. Besides these peaks, 1-propylsilatranyl ion (m/z: a = 216
(3, 8), b = 233 (4, 9)) peaks are also observed which lose one OCH
(R)CH2 arm to form bicyclic fragment with alkyl chain. The homo-
lytic cleavage of Si–CH2 leads to a silatranyl fragment with direct
Si–N covalent bond (m/z: a = 174 (3, 8), b = 216 (4, 9)) that
Scheme 1. Synthesis of azobenzene-substituted urea silatranes.
Scheme 2. Synthesis of azobenzene-substituted aminosuccinimide
silatranes.
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Appl. Organometal. Chem. 2015, 29, 549–555

Synthesis of silatranes possessing azo group at axial position
corresponds to characteristic feature of mass spectrum of C-
substituted silatrane involving the fragmentation of X–Si bond. This
silatranyl fragment (m/z: a= 174 (3, 8), b = 216 (4, 9)) further loses a
cyclic arm to form bicyclic moiety (m/z: a = 132 (3, 8), b = 160). Very
intense peaks due to protonated triethanolamine and
trisisopropanolamine (m/z: a = 150 (3, 8), b = 192 (4, 9)) are also
observed.
X-ray diffraction studies
Suitable crystals of 2 were eventually grown from tetrahydrofuran
solvent at room temperature. The X-ray structure is presented in
Fig. 1, and selected structural parameters are presented in Table 2
for compound 2. The silicon-coordination polyhedra of 2 can be de-
scribed as tetrahedral with all bond lengths and angles within the
range of the expected values. The azobenzene group, which is the
common part of the molecular skeleton of all studied compounds,
is substantially planar in 2. In the crystal, molecules are aligned on
a twofold screw axis passing through the C10–O4 carbonyl group.
The hydrogen-bonding interactions, involving urea moieties
(N2–H2···O4_$1, 2.862(2) Å, 155°; N1–H1···O4_$1, 2.959(2) Å 2.8,
150.3°; symmetry code $1 = ꢀx + 1, ꢀy ꢀ 1, ꢀz) linked to adjacent
molecules, generate a ribbon structure extending along the b-axis
as described in Fig. 2. However along the a-axis, molecules are
stacked via weak π–π interactions of 3.619(15) Å. π–π stacking inter-
actions involve six-membered rings (C11–C12–C13–C14–C15–C16,
dcentroid-centroid = 3.6419(15) Å) which connect ribbons to build a
two-dimeansional layer parallel to the bc plane. Both the interac-
tions are weak however; the only possible interactions and playing
a key role to form an overall three-dimensional lattice arrangement.
Compound 9 crystallizes in space group C2/c with one molecule
in the asymmetric unit (Fig. 3(A)). The molecule has three stereo
carbon centres (C2, C5 and C8) having absolute configurations of
(R, S, R), respectively, and theoretically one would expect eight dia-
stereomers. However, expanding all the eight molecules in the unit
cells and with a close view along the a-axis it is found that half the
molecules (four) are exactly inverted by the other half with
completely opposite absolute configuration (S, R, S). Due to equal
number of racemic mixtures (possible inversion of chiral centres
during course of reaction),[32] the resulting space group (C2/c) is
not a chiral one. Of the three asymmetric carbon centres, two atoms
(C5, C8) and one nitrogen atom (N2) are disordered in two positions
(Fig. 3(B)). Viewing the molecule along the N–Si axis, it looks like a
molecular propeller having three propeller arms around the
nitrogen centre (supporting information, Fig. S1b). The silicon atom
in 9 exhibits a disordered trigonal bipyramidal coordination
polyhedron with three equatorial sites occupied by oxygen atoms
(O1, O2, O3) with two axial positons by carbon atom (C-10) and
Figure 2. Molecular lattice arrangement for compound 2 along b-axis.
Molecules are packed via weak hydrogen-bonding interactions between
adjacent urea moieties of 2.959(2) and 2.862(2) Å.
Figure 3. Molecular structure of 9 with probability of displacement
ellipsoid at 50%. Hydrogen atoms are removed for clarity. (A) Molecule is
depicted without including disordered atoms. (B) Molecule is depicted
with disordered atoms for composition.
N1. The Addison parameter τ = 0.91 and Berry distortion of silicon-
coordination polyhedral is 13.2% (O3 as the pivot atom). Unlike
compound 2, the azobenzene moiety in 9 adopts a non-coplanar
arrangement (dihedral angles between phenyl rings are 37.21°). In
compound 9, two of the three arms of the trisisopropanolamine
moiety are disordered over two positions showing different ratios
of 0.44:0.56 (C5A:C5B) and 0.22:0.79 (C8A:C8B). Similarly, the disor-
dered ratio at the N–H linker group of the aminopropylsilatrane
moiety is 0.82:0.18 for N2A and N2B, respectively.
In the crystal lattice, antiparallel pairs of molecules (along b-axis)
are associated by non-classical hydrogen-bonding interactions via
methyl (C9) and succinimide (O5) groups as shown in Fig. 4(A).
Though the hydrogen bonds are of non-classical type, these inter-
actions (3.414 Å) are the only possible interactions and are of ex-
tremely weak type. Along the a-axis the three-dimensional
pattern looks like an alternate layer-type arrangement of silatrane
moiety and the rest of the alkyl chain (Fig. 4(B)). Again weak π–π
Figure 1. Molecular structure of 2 (probability level of displacement
ellipsoids at 50%).
Appl. Organometal. Chem. 2015, 29, 549–555
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G. Singh et al.
UV–visible studies
UV–visible titrations were performed with (1–2) × 10ꢀ5 M solutions
of 3, 4, 8 and 9 in CH3CN. Freshly prepared Bu4NX (X= Clꢀ, Brꢀ, Iꢀ
and CH3COOꢀ) standard solutions were added and the UV–visible
spectra of the samples were recorded. In UV–visible spectra,
absorption peaks at 356 and 352nm are observed for
azobenzene-substituted urea silatranes 3 and 4 and azobenzene-
substituted aminosuccinimide silatranes 8 and 9, respectively,
and these peaks are attributed to PhN¼NPhNHC(¼O)NH and
PhN¼NPhN(CO)2CH2CHNH conjugated frameworks, respectively.
The UV–visible spectra of all the silatranes change dramatically
when a small amount of AcOꢀ ions are added, whereas no changes
are observed upon the addition of Clꢀ, Brꢀ and Iꢀ ions up to 300
equiv. These silatrane-based receptors show strong binding to ace-
tate over other ions such as Clꢀ, Brꢀ, Iꢀ and HSO4ꢀ. In the course of
addition of AcOꢀ ions to silatrane 3, the band around 356 nm is de-
creased and a bathochromic shift is observed passing through the
isosbestic point at 403 nm as depicted in Fig. 5(A). The presence of
the isosbestic point indicates that only two species are present at
equilibrium over the course of the titration experiment.[33] The urea
silatrane 3 moiety can be deprotonated due to the presence of
more free acetate ions which can lead to the formation of highly
stable anion species. The bathochromic shift may be observed
due to an increase in conjugation between azobenzene group
and deprotonated urea silatranes.[34,35] For azobenzene-substituted
urea silatranes 3 and 4, the same UV–visible spectral changes are
observed in both silatranes due to their having the same axial
Figure 4. Molecular structure of 9 with packing and interactions. (A) Pair of
molecules joined by non-classical interactions of methyl C9 and O5 with
bond length of 2.416 Å. (B) Lattice arrangement of three-dimensional
pattern along a-axis showing an alternate layer-type arrangement of
silatrane moiety and the rest of the alkyl chain.
interactions between benzene rings (C17–C18–C19–C20–C21–C22,
dcentriod-centriod = 4.173 Å) provide additional support for the three-
dimensional arrangement.
Thermogravimetric analysis
The thermal stability of compounds 3, 4, 8 and 9 was studied using
thermogravimetric analysis. All the compounds were heated from
25 to 1000°C under nitrogen atmosphere and isothermal condi-
tions at a heating rate of 10°C minꢀ1. Additionally, thermogravimet-
ric analysis is compared with the mass spectra and both analyses
are found to be complementary to each other. For compounds 3
and 4, three steps are observed, the first step involving the loss of
10.82 and 12.34%, respectively, due to ethanol and isopropanol for-
mation. In the second step, cleavage of azobenzene group from 3
and 4 is observed from 200 to 350°C (calc. = 61.78, exp. = 61.48 (3)
and calc. = 69.45, exp. =69.58 (4)) which is further confirmed by
mass spectral peaks at m/z = 274 and 316. Compounds 8 and 9 also
display the same pattern for cleavage of azobenzene group. Com-
pounds 8 and 9 show another step at 350–550°C (calc. = 35.84,
exp. = 35.74 (8) and calc. = 32.84, exp. = 32.89 (9)) that corresponds
to aminopropylsilatranyl moiety as residue which is complimentary
with the mass spectra at m/z = 233 and 258, respectively. After
annealing to 1000°C, SiO2 residue formation is observed for all
silatranes.
Figure 5. UV–visible spectral changes observed for 3 and 8 upon addition
of acetate ions in CH3CN at room temperature. (A) Compound 3 (1 × 10ꢀ5 M)
with CH3COOꢀ (0–10 equiv.) and (B) compound 8 (1.5 × 10ꢀ5 M) with
CH3COOꢀ (0–35 equiv.).
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Synthesis of silatranes possessing azo group at axial position
Table 3. MIC (mg mlꢀ1) of test compounds against reference strains
References
Microorganism
3
4
8
9
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New 4-aminoazobenzene-based silatranes bearing urea and
aminosuccinimide groups as linker moieties at axial position of
silatranes have been successfully synthesized in good yields.
UV–visible spectra of azobenzene-substituted urea and
aminosuccinimide silatranes have been studied. These linker groups
make these silatranes solid candidates for supramolecular chemistry
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Supporting information
Acknowledgments
Additional supporting information may be found in the online ver-
The authors are grateful to UGC, New Delhi for financial support.
sion of this article at the publisher’s web-site.
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