Thymine-functionalised siloxanes: Model compounds and polymers

Article abstract of DOI:10.1016/j.jorganchem.2014.12.007

A novel, efficient synthetic method for the production of organosiloxane polymers functionalised with pendant DNA bases, typified by thymine, is reported. A condensation reaction between an α-amino-ω-alkylsilane or an amino-functionalised siloxane with al

Full text of DOI:10.1016/j.jorganchem.2014.12.007

Journal of Organometallic Chemistry 778 (2015) 29e34
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Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Thymine-functionalised siloxanes: Model compounds and polymers
*
€
Douglas R.G. Smith, Gabriele Kociok-Kohn, Kieran C. Molloy, Gareth J. Price , Rhys D. Short
Department of Chemistry, University of Bath, Bath BA2 7AY, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
A novel, efficient synthetic method for the production of organosiloxane polymers functionalised with
pendant DNA bases, typified by thymine, is reported. A condensation reaction between an a-amino-u-
alkylsilane or an amino-functionalised siloxane with aldehyde-functionalised thymine gave polymers in
higher overall yields than the methods previously reported. Variation in the loading of thymine led to a
range of material properties from a highly viscous fluid at 0.2% functionalization to a hard, brittle solid at
20% loading due to hydrogen bonding interactions.
Received 18 August 2014
Received in revised form
3 December 2014
Accepted 9 December 2014
Available online 18 December 2014
© 2014 Elsevier B.V. All rights reserved.
Keywords:
Siloxane
Thymine
Hydrogen bonding
Silicone
Polymer
Introduction
a backbone carrying pendant vinyl groups onto which functionality
can be added using hydrosilylation chemistry [11,12].
We have previously reported an interest in the synthesis of
organosiloxane polymers functionalised with pendant DNA (pu-
rine, pyrimidine) bases, as progress towards the production of or-
dered polymeric materials in which structure (and properties) are
dictated by hydrogen bonding interactions between the DNA bases
[1]. These well known complementary interactions have been
exploited in forming supramolecular materials from several types
of organic polymer [2,3]. For example, Rowan et al. used [4] the
nucleobase interactions to prepare liquid crystalline polymers that
displayed thermoreversible phase behaviour. Other workers have
prepared stimuli responsive surfaces [5,6] which recognise the
complementary nucleobases and also made use of the ability of
thymine to form sandwich complexes with metals such as mercury
for extraction and sensing [7e9].
Little work has been carried out involving nucleobase contain-
ing silicones although a number of amino acid-functionalised si-
loxanes have been reported [13]. Telechelic siloxanes terminated
with quadruple hydrogen-bonding ureido-pyrimidone units which
assembled into reversible high molecular weight polymers were
reported by Meijer and co-workers [14].
Our initial approach was either hydrosilylation of an alkenyl-
substituted base or, alternatively, base-promoted coupling of a
a-
bromo- -alkylsilane and the appropriate heterocycle (thymine,
u
cytosine, adenine or guanine) [1]. Although these approaches were
successful, the synthetic sequences were complex and the overall
yields low. Thus, alternative, more effective, coupling strategies are
desirable and we report one such herein, namely a condensation
reaction between
a
a-amino-u-alkylsilane or an amino-
Thusfar, equivalent polymers with inorganic backbones have
been relatively unexplored. However, the unique properties offered
by a siloxane backbone [10] led us to explore the possibility of
preparing silicones functionalised by nucleobases. Functional si-
loxanes can be prepared by anionic polymerization of small, three
or four membered ring siloxanes carrying the functionality
although a method with potentially wider applicability is to exploit
functionalised siloxane and an aldehyde-functionalised base,
typified by thymine.
Experimental
Experimental procedures
Starting materials thymine, phenyldimethylsilane, chloroplatinic
acid, benzaldehyde, p-tolualdehyde,
the hydroxydimethylsiloxane oligomers were obtained from
SigmaeAldrich and (3-aminopropyl)diethoxymethylsilane, 3-bis-(3-
a-bromo-p-tolualdehyde and
a,u
* Corresponding author. Tel.: þ44 (0)1225 386504.
E-mail address: g.j.price@bath.ac.uk (G.J. Price).
http://dx.doi.org/10.1016/j.jorganchem.2014.12.007
0022-328X/© 2014 Elsevier B.V. All rights reserved.

30
D.R.G. Smith et al. / Journal of Organometallic Chemistry 778 (2015) 29e34
aminopropyl)tetramethyldisiloxane from Fischer; all were used
without further purification, while the hydroxydimethylsiloxanes
and poly(dimethylsiloxane) functionalised at the 2 mol% level with 3-
aminopropyl groups were gifts from Dow Corning.
reduced pressure to afford the slightly moisture-sensitive imine
product. Some degradation of the imine was noted with time so it
was stored under dry nitrogen. Yield: 0.68 g, 80%. Analysis, calcd for
a,u
C
18H23NSi: C 76.8, H 8.2, N 5.0%, found C 76.2, H 8.3, N 5.0%. 1H NMR
Infra-red spectra were recorded as liquid films on NaCl plates
using a Nexus Nicolet 510P FT-IR spectrometer in the region
4000e400 cmꢀ1. 1H, 13C and 29Si spectra were recorded on Bruker
Avance (300 or 400 MHz) Fourier transform spectrometers, using
TMS as an internal reference. Elemental analyses were performed
using a Carbo Erba Strumentazione E.A. model 1106 analyser. The
results were duplicated and the mean of the duplicated mea-
surements was taken as the final result. Gel permeation chro-
matography, GPC, was carried out on 2 mg mlꢀ1 samples in
chloroform at a flow rate of 1 ml minꢀ1 using refractive index
(300 MHz, CDCl3):
d
¼ 8.20 (s, 1H, HC]N), 7.70 (d, J ¼ 8.0 Hz, 2H,
CH), 7.51 (m, 2H, CH), 7.35 (m, 3H, CH), 7.33 (m, 3H, CH), 3.58 (t,
J ¼ 7.0 Hz, 2H, CH2N]CH), 1.73 (m, 2H, SiCH2CH2), 0.78 (m, 2H,
SiCH2CH2), 0.28 (s, 6H, Si(CH3)2); 13C NMR (75 MHz, CDCl3):
d
¼ 160.9 (HC]N), 139.2 (CH), 136.3 (CH), 133.6 (CH), 130.4 (CH),
128.8 (CH), 128.5 (CH), 128.0 (CH), 127.7 (CH), 65.0 (CH2N]CH),
25.4 (SiCH2CH2), 13.3 (SiCH2), -3.1 (SiCH3); 29Si NMR (60 MHz,
CDCl3):
d
¼ ꢀ2.9.
Also prepared by the same method were
detection and two ‘PLGel’ Mixed bed 30 cm columns with 10 mm
packing at 30 ꢁC. Molecular weights are reported relative to
polystyrene calibration. DSC was carried out using a TA in-
struments calorimeter at a heating rate of 20ꢁ minꢀ1 after cooling
with liquid nitrogen.
[3-(Diethoxy-methyl-silanyl)-propyl]-[1-phenyl-meth-(E)-ylidene]-
amine (4)
Using benzaldehyde (0.32 g, 3.02 mmol) and (3-aminopropyl)
diethoxymethylsilane (1.00 g, 5.23 mmol). Yield: 0.51 g, 60%.
Analysis, calcd for C15H25O2NSi: C 64.5, H 9.0, N 5.0%, found C 65.2,
Synthesis of 1-amino-3-(phenyldimethylsilyl)propane (1)
Allylamine (2.30 g, 40 mmol), phenyldimethyl-silane (2.50 g,
18.4 mmol) and a catalytic amount of chloroplatinic acid in propan-
2-ol was heated at 90 ꢁC for 18 h, after which the IR spectrum no
longer displayed a peak at 2150 cmꢀ1 corresponding to a SieH
stretching vibration. The volatiles were removed under reduced
pressure and the resulting translucent grey liquid distilled under
atmospheric pressure. The only product recovered was found to be
(3-aminopropyl)phenyldimethylsilane (1) (55%). Analysis, calcd for
H 8.9, N 5.0%. 1H NMR (300 MHz, CDCl3):
d
¼ 8.26 (s, 1H, HC]N),
7.72 (m, 2H, CH), 7.36 (m, 3H, CH), 3.76 (q, J ¼ 7.0 Hz, 4H,
SiOCH2CH3), 3.61 (t, J ¼ 7.0 Hz, 2H, CH2N]CH), 1.79 (m, 2H,
SiCH2CH2), 1.21 (t, J ¼ 7.0 Hz, 6H, SiOCH2CH3), 0.66 (m, 2H,
SiCH2CH2), 0.13 (s, 3H, SiCH3); 13C NMR (75 MHz, CDCl3):
d
¼ 160.9
(HC]N), 136.4 (CH), 130.5 (CH), 128.6 (CH), 128.1 (CH), 64.6
(CH2N]CH), 58.2 (SiOCH2CH3), 24.4 (SiCH2CH2), 18.5 (SiOCH2CH3)
C
11H19NSi: C 68.3, H 9.9, N 7.2%, found C 67.8, H 9.9, N 7.2%. 1H NMR
(300 MHz, CDCl3):
11.6 (SiCH2), ꢀ4.9 (SiCH3); 29Si NMR (60 MHz, CDCl3):
d
¼ ꢀ4.8.
d
¼ 7.48 (m, 2H, CH), 7.34 (m, 3H, CH), 2.64 (t,
J ¼ 7.0 Hz, 2H, CH2NH2), 1.43 (m, 2H, SiCH2CH2), 1.17 (s, 2H, NH2),
0.73 (m, 2H, SiCH2), 0.27 (s, 6H, Ph(CH3)2Si); 13C NMR (75 MHz,
CDCl3):
(CH2NH2), 28.2 (CH2CH2NH2), 12.8 (SiCH2), e3.1 (SiCH3); 29Si NMR
(60 MHz, CDCl3):
¼ ꢀ2.9.
d
¼ 139.2 (CH), 133.5 (CH), 128.9 (CH), 127.7 (CH), 45.5
1-(4-{[(E)-3-(dimethyl-phenyl-silanyl)-propylimino]-methyl}-
benzyl)-5-methyl-1H-pyrimidine-2,4-dione (5)
Using 1 (0.26 g, 1.35 mmol) and 2 (0.21 g, 0.86 mmol):
Yield:0.20 g, 55%. Analysis, calcd for C24H29N3O2Si: C 68.7, H 7.0, N
10.0%, found C 68.2, H 6.9, N 9.9%. 1H NMR (300 MHz, CDCl3):
d
Synthesis of 4-(5-methyl-2,4-dioxo-3,4-dihydro-2H-pyrimidin-1-
ylmethyl)-benzaldehyde (2)
d
¼ 8.21 (s, 1H, HC]N), 7.72 (d, J ¼ 8.0 Hz, 2H, CH), 7.50 (m, 2H, CH),
A solution of thymine (0.63 g, 5 mmol),
a-bromo-p-tolualde-
7.33 (m, 5H, CH, overlapping signals), 6.95 (s, 1H, CH), 4.90 (s, 2H,
CH2N), 3.58 (t, J ¼ 7.0 Hz, 2H, CH2N]CH), 1.88 (s, 3H, CCH3), 1.72 (m,
2H, SiCH2CH2), 0.74 (m, 2H, SiCH2), 0.27 (s, 6H, SiCH3); 13C NMR
hyde (1.04 g. 5.2 mmol) and an excess of potassium carbonate
(2.60 g, 18.8 mmol) in DMSO was stirred at room temperature for
18 h, after which a fine, voluminous precipitate had formed in the
reaction mixture. The mixture was filtered and excess DMSO
removed, affording a slightly oily yellow solid. The resulting solid
was washed with water, dried in a dessicator, then finely divided
with a pestle and mortar before being suspended in chloroform by
vigorous stirring for 1 h. The suspension was filtered and dried to
afford the air-stable product as a fine yellow powder (88%, mp 177-
9 ꢁC). Analysis, calcd for C13H12O3N2: C 63.9, H 5.0, N 11.5%, found C
(75 MHz, CDCl3):
d
¼ 161.9 (C]O), 160.1 (HC]N), 150.0 (C]O),
142.9 (CH), 139.6 (CH), 139.2 (CH),136.6 (CH), 133.6 (CH), 130.0 (CH),
128.8 (CH), 128.2 (CH), 127.8 (CH), 111.4 (CCH3), 64.5 (CH2N]CH),
22.4 (SiCH2CH2), 12.8 (SiCH2), 12.4 (CCH3), ꢀ3.1 (SiCH3); 29Si NMR
(60 MHz, CDCl3):
d
¼ ꢀ2.8.
63.5, H 4.9, N 11.1%. 1H NMR (300 MHz, CDCl3):
d
¼ 10.02 (s, 1H,
1-(4-{[(E)-3-(Diethoxy-methyl-silanyl)-propylimino]-methyl}-
benzyl)-5-methyl-1H-pyrimidine-2,4-dione (6)
CHO), 8.91 (bs, 1H, NH), 7.90 (d, J ¼ 8.0 Hz, 2H, CH), 7.45 (d,
J ¼ 8.0 Hz, 2H, CH), 7.00 (s, 1H, CH), 4.97 (s, 2H, NCH2), 1.91 (s, 3H,
Using
(3-aminopropyl)diethoxymethylsilane
(0.57
g,
CCH3); 13C NMR (75 MHz, CDCl3):
151.0 (C]O), 142.1 (CH), 139.6 (CH), 130.5 (CH), 128.3 (CH), 111.8
(CCH3), 50.9 (NCH2), 12.4 (CCH3).
d
¼ 191.5 (CHO), 163.9 (C]O),
3.02 mmol) and 2 (0.74 g, 3.02 mmol). Yield: 0.90 g, 71%. Analysis,
calcd for C24H29N3O2Si: C 60.3, H 7.7, N 10.0%, found C 60.1, H 7.7, N
9.9%. 1H NMR (300 MHz, CDCl3):
d
¼ 8.26 (s, 1H, HC]N), 7.74 (d,
J ¼ 8.0 Hz, 2H, CH), 7.33 (d, J ¼ 8.0 Hz, 2H, CH), 6.96 (s, 1H, CH), 4.91
(s, 2H, CH2N), 3.76 (q, J ¼ 7.0 Hz, SiOCH2CH3) 3.61 (t, J ¼ 6.8 Hz, 2H,
CH2N]CH), 1.89 (s, 3H, CCH3), 1.78 (m, 2H, SiCH2CH2), 1.21 (t,
J ¼ 7.0 Hz, 6H, SiOCH2CH3), 0.64 (m, 2H, SiCH2), 0.13 (s, 3H, SiCH3);
Synthesis of [3-(dimethyl-phenyl-silanyl)-propyl]-[1-phenyl-meth-
(E)-ylidene]-amine (3)
Benzaldehyde (0.32 g, 3.02 mmol) was added to a stirred sus-
pension of anhydrous magnesium sulphate in dichloromethane.
Once the benzaldehyde had dissolved, 1 was added and the mixture
stirred at room temperature for 18 h, after which the IR spectrum
revealed complete consumption of starting material. The mixture
was filtered and the excess dichloromethane removed under
13C NMR (75 MHz, CDCl3):
(C]O), 143.4 (CH), 139.6 (CH), 136.5 (CH), 128.2 (CH), 111.4 (CCH3),
64.6 (CH2N]CH), 58.1 (SiOCH2CH3), 45.1 (CH2N), 27.2 (SiCH2CH2),
18.4 (SiOCH2CH3), 11.0 (SiCH2), -4.9 (SiCH3); 29Si NMR (60 MHz,
d
¼ 164.0 (C]O),160.2 (HC]N), 151.1
CDCl3):
d
¼ ꢀ4.6.

D.R.G. Smith et al. / Journal of Organometallic Chemistry 778 (2015) 29e34
31
Table 1
Crystal data for 2.
Synthesis of {3-[1,1,3,3-tetramethyl-3-(3-{[1-p-tolyl-meth-(E)-
ylidene]-amino}-propyl)-disiloxanyl]-propyl}-[1-p-tolyl-meth-(E)-
ylidene]-amine (7)
Empirical formula
Formula weight
Crystal system
Space group
a (Å)
C13H12N2O3
244.25
Monoclinic
P 21/n
4.6396(1)
11.7909(3)
21.2558(5)
96.031(1)
1156.36(5)
4
p-Tolualdehyde (0.25 g, 2.01 mmol) and 3-bis-(3-aminopropyl)
tetramethyldisiloxane (0.50 g, 2.01 mmol) were added to a stirred
suspension of anhydrous magnesium sulphate in dichloromethane
and the reaction stirred at room temperature until the IR spectrum
revealed complete loss of starting material. The mixture was
filtered and removal of excess dichloromethane afforded (7) as an
orange-brown oil (Yield: 92%) with no further purification. Anal-
ysis, calcd for C26H40N2Si2O: C 69.0, H 8.9, N 6.2%, found C 68.8, H
b (Å)
c (Å)
b
(o)
V (Å3)
Z
rcalc (Mgmꢀ3
)
1.403
0.102
512
8.8, N 6.2%. 1H NMR (300 MHz, CDCl3):
(d, J ¼ 8.0 Hz, 2H, CH), 7.14 (d, J ¼ 8.0 Hz, 2H, CH), 3.50 (t, J ¼ 7.0 Hz,
2H, CH2N]CH), 2.30 (s, 3H, CCH3), 1.65 (m, 2H, SiCH2CH2), 0.48 (m,
2H, SiCH2CH2), 0.02 (s, 6H, SiCH3); 13C NMR (75 MHz, CDCl3):
d
¼ 8.14 (s, 1H, HC]N), 7.54
m
(Mo-ka) (mmꢀ1
)
F(000)
Theta range for data collection (o) 3.37e24.92
Index ranges
ꢀ5 ꢂ h<¼5; ꢀ13 ꢂ k<¼13; ꢀ25 ꢂ l<¼25
Reflections collected
Independent reflections
Reflections observed (>2
Data completeness
Goodness-of-fit on F@2
16174
d
¼ 160.2 (HC]N), 140.2 (CH), 133.4 (CH), 128.9 (CH), 127.7 (CH),
64.5 (CH2N]CH), 24.5 (CCH3), 21.1 (SiCH2CH2), 15.7 (SiCH2CH2),
-0.4 (SiCH3); 29Si NMR (60 MHz, CDCl3):
¼ 7.7.
2008 [R(int) ¼ 0.0463]
s
)
1774
0.993
1.082
d
Final R1, wR2 indices [I > 2
Final R1, wR2 (all data)
s
(I)]
0.0424, 0.1090
0.0578, 0.1146
0.472, ꢀ0.295
Synthesis of iminopropylthymine functionalised siloxane (8)
Reaction between and poly(dimethyl-co-2% amino-
Largest diff. peak, hole (eÅ3)
2
propylmethyl)siloxane. 2 (0.50 g, 2.05 mmol) was added to a
stirred suspension of anhydrous magnesium sulphate in
dichloromethane and allowed to dissolve. To this was added
PDMS incorporating 2 mol% aminopropyl functionality (7.75 g,
2.05 mmol), and the mixture was stirred at room temperature for
18 h, after which the mixture had visibly thickened and the IR
spectrum indicated no further growth in the C]N imine stretch
(1600e1630 cmꢀ1). The mixture was diluted with dichloro-
methane before being filtered, washed with a small amount of
water and dried over anhydrous MgSO4. Upon removal of the
volatiles a very thick, air stable yellow rubber was recovered and
found to be PDMS containing approximately 2 mol% iminopro-
pylthymine functionality (8) via a propyl imine linkage. 1H NMR
radiation (
l
¼ 0.71073 Å). Structure solution was followed by full-
matrix least squares refinement and was performed using the
WinGX-1.70 suite of programmes [15]. All non-hydrogen atoms
were refined anisotropically, while hydrogen atoms were added in
calculated positions which were located and freely refined, save
H(1) which is involved in hydrogen bonding.
Results and discussion
1-amino-3-(phenyldimethylsilyl)propane (1) was prepared by
the hydrosilylation of allylamine, catalysed by chloroplatinic acid,
as a colourless, air-stable liquid (55% yield) (Eqn. 1):
(300 MHz, C6D5CD3):
d
¼ 8.05 (s, 1H, HC]N), 7.11 (m, 2H, CH), 7.02
(m, 2H, CH), 6.96 (s, 1H, CH), 3.60 (t, J ¼ 7.0 Hz, 2H, CH2N]CH),
1.90 (s, 3H, CCH3), 1.60 (m, 2H, SiCH2CH2), 0.80 (m, 2H, SiCH2CH2),
0.26 (s, ~ 300H, SiCH3); 13C NMR (75 MHz, C6D5CD3):
d
¼ 1.4
(SiCH3); 29Si NMR (60 MHz, C6D5CD3):
d
¼ ꢀ21.5; Analysis, calcd
for C115H315N3O52Si50 i.e. full conversion of the 2mol% loading: C
34.7, H 8.0, N 1.1%; found C 31.7, H 7.6, N 0.7%; GPC (relative to
polystyrene): Mn 21,000, Mw/Mn 3.6.
Only the u-silyl isomer was recovered, suggesting that the steric
bulk of the silyl group has directed the regiochemistry of the re-
action to produce exclusively the linear isomer of 1 by reacting at
the terminal end of the olefin. Although briefly reported some years
ago by others [16e18], 1 remains poorly characterised. The singlet
Synthesis of poly(dimethyl-co-aminopropylmethyl)siloxane (9)
A equimolar mixture of a,u hydroxydimethylsiloxane oligomer
(with 5, 11 or ~700 siloxane units) and 3-(diethoxymethylsilyl)
propylamine was heated and stirred overnight with one drop of
10% HCl under a nitrogen atmosphere to give approximate load-
ings of aminopropyl sidechains of ~20 mol%, ~10 mol% and
~0.2 mol%, respectively. In each case a clear, very viscous polymer
was obtained. The NMR spectra were qualitatively identical: 1H
at
d
¼ ꢀ2.9 ppm in the 29Si NMR, shows good agreement with the
corresponding
bromobutane [
resonance
of
4-(phenyldimethylsilyl)-1-
d
¼ ꢀ3.0 ppm] [1].
The required purine-substituted tolualdehyde (2) was syn-
thesised in very good yield from 1-bromo-p-tolualdehyde and
thymine (Eqn. 2):
NMR (300 MHz, C6D5CD3):
SiCH2CH2 and CH2NH2), 0.45 (m, SiCH2CH2), 0.00 (s, SiCH3). 13C
NMR (75 MHz, C6D5CD3):
¼ 44.3 (CH2NH2), 26.4 (SiCH2CH2CH2),
13.4 (SiCH2), 0.0-0.5 (SiCH3). 29Si (60 MHz, C6D5CD3):
d
¼ 2.60 (m, CH2NH2), 1.40 (m,
d
d
¼ ꢀ22.0, ꢀ22.6.
The procedure described above for 8 was then used to add 2 to
each of these propylamine-functionalised siloxanes (9) to give
polymers functionalised with varying degrees of thymine (10).
Crystallography
The peak due to the methylene protons
a
to nitrogen in the 1H
Experimental details relating to the single-crystal X-ray crys-
tallographic studies are summarised in Table 1. Data were collected
on a Nonius Kappa CCD diffractometer at 150(2) K using Mo-Ka
NMR of 2 compares well with the analogous peak in benzylthymine
[d
¼ 4.87 ppm] [19], suggesting similar N1-subsitution. The asym-
metric unit of the crystal of 2 (Fig. 1) is formed of a single molecule

32
D.R.G. Smith et al. / Journal of Organometallic Chemistry 778 (2015) 29e34
Fig. 1. The structure of 2 showing the labelling scheme of the asymmetric unit and intermolecular hydrogen bonding interactions; thermal ellipsoids are at the 50% level. Selected
geometric data: N(1)eC(1) 1.386(2), N(1)eC(4) 1.375(2), N(2)eC(3) 1.383(2), N(2)eC(4) 1.367(2), N(2)eC(6) 1.475(2), C(1)eO(1) 1.226(2), C(4)eO(2) 1.226(2), C(2)eC(3) 1.342(3),
C(6)eC(7)ꢁ 1.510(2), C(13)eO(3) 1.202(2) Å; C(1)eN(1)eC(4) 126.85(15), C(3)eN(2)eC(6) 119.26(14), C(3)eN(2)eC(4) 121.62(14), C(4)eN(2)eC(6) 119.10(14), C(2)eC(3)eN(2)
00
123.11(15) . Symmetry operations: 01ꢀx, 1ꢀy, ꢀz; exꢀ1/2, ½þy, ½ z.
ꢀ
in which 1-bromo-p-tolualdehyde has added to thymine at the N-1
position [N(2)-C(6) 1.475(2) Å], at a distance which compares well
with N1-Ca distances in similar molecules such as (trimethylsilyl)-
1-allylthymine [1.474(2) Å] and (4-phenyldimethylsilyl)butylth-
ymine [1.4778(19) Å] [1]. The C]C double bond [C(2)eC(3) 1.342(3)
Å], the endocyclic N3eC bonds [N(1)eC(1) 1.386(2) Å, N(1)eC(4)
1.375(2) Å] and the endocyclic N1eC bonds [N(2)eC(3) 1.383(2) Å,
N(2)eC(4) 1.367(2) Å] in the thymine ring each compare well in
length to the analogous bonds in thymine [C(2)eC(3) 1.343(4),
N(1)eC(1) 1.401(5), N(1)eC(4) 1.361(4), N(2)eC(3) 1.384(5), N(2)e
C(4) 1.358(4) Å] [20], showing that little or no ring distortion has
taken place in 2 to accommodate the p-tolualdehyde group. The
angle at bridging C(6) [N(2)eC(6)eC(7) 112.23(14)ꢁ] is relatively
wide to accommodate the bulk of the two rings, though the het-
erocyclic bond angles around the two nitrogen atoms [C(3)eN(2)e
C(4) 121.62(14), N(1)eC(4)eN(2) 115.01(14), C(1)eN(1)eC(4)
126.85(15)ꢁ] are again similar to the analogous angles in thymine
[C(3)eN(2)eC(4) 122.5(3), N(1)eC(4)eN(2) 115.5(2), C(1)eN(1)e
C(4) 126.3(3)ꢁ] [20]. The angles around the N1-Ca bond [C(3)e
N(2)eC(6) 119.26(14)ꢁ, C(4)eN(2)eC(6) 119.10(14)ꢁ, C(3)eN(2)e
C(4) 121.62(14)ꢁ] compare well with those in (4-
phenyldimethylsilyl)-butylthymine [C(3)eN(2)eC(6) 119.59(13)ꢁ,
C(4)eN(2)eC(6) 118.77(13)ꢁ, C(3)eN(2)eC(4) 121.48(13)ꢁ].
O/HeN bonding [ca. 1.245 Å] is seen in comparison to the passive
C]O [ca. 1.215e1.225 Å]. In both thymine and uracil, one C]O is
long (ca 1.244 Å) and is associated with a strong C]O/HN
hydrogen bond, while a shorter C]O (ca 1.22 Å) is associated with a
much weaker C]O/HC hydrogen bond. In 2, the non-H bonded
C]O, at 1.226 Å, is similar to the latter while some lengthening of
the other, relatively short, C]O (also 1.226 Å) associated with the
C]O/HN hydrogen bond might be anticipated. There is no
obvious electronic reason for this not occurring, so it is presumably
a consequence of packing factors.
Having established that (2) could be synthesised in high yield
and purity, the next stage involved coupling of the aldehyde (or, as a
model reaction, of benzaldehyde) (Eqn. 3) with an aminosilane to
form precursors for siloxanes via imine formation. The reactions
were monitored until there was no further observable increase in
the intensity of the IR band due to
y
(C]N) at 1600e1630 cmꢀ1 or
no further decrease in the
y
(C]O) stretch at 1710 cmꢀ1
.
The imines (Eqn. 4) were isolated as slightly moisture-sensitive
solids in 55e80% yield. In addition to 1H and 13C NMR spectra which
confirm imine formation [
160.1e160.9 ppm], 3e6 display singlet (
the range ꢀ2.8 to ꢀ4.6 ppm.
d
(HC]N)j: 8.20e8.26 ppm;
d
(HC]N):
d
29Si) peaks with shifts in
The molecule forms a dimer via two symmetry related NeH/O
hydrogen bonds [H(1)/O(2) 1.89(3) Å; N(1)-H(1)/O(2) 176(2)ꢁ],
the same length within experimental error as hydrogen bonds in
(4-phenyldimethylsilyl)butylthymine [1.97(2) Å; 172(2)ꢁ] [1]. 2
further interacts via a CeH/O hydrogen bond [H(3)/O(3) 2.30 Å]
to form a supramolecular chain of dimers, longer than analogous
bonds in compounds such as 1-methylcytosine [H(1)/O(2) 2.04(2)
Å] [21], but similar in length to that found in (trimethylsilyl)-1-
allylthymine [2.29, 2.34 Å] [1]. Notably, this hydrogen bond in-
volves the carbonyl group of the aldehyde, not the second exocyclic
C]O of the heterocycle. The bond in 2 is, however, considerably
closer to linearity [C(3)-H(3)/O(3) 172.8ꢁ] than the analogous
bond in (trimethylsilyl)-1-allylthymine (158.8, 164.0ꢁ) [1]. Analo-
gous CeH/O hydrogen bonds are also seen in the lattice structures
of uracil [22] and 1-methylcytosine [21]. Despite the differing
hydrogen bond interactions in 2, the two C]O bond lengths
associated with the thymine residue are identical [1.226(2) Å]
despite the fact that O(1) is not involved in any intermolecular
interaction, while weakly hydrogen bonded C]O(3) retains sig-
nificant double-bond character [C13)eO(3) 1.202(2) Å]. This is in
marked contrast to, for example, thymine [20] and uracil [22],
where a marked lengthening of the carbonyl bond involved in C]
Similarly, coupling of an aldehyde to an amino functionalised
disiloxane via imine formation as a prototype for polymer func-
tionalization was performed (Eqn. 5):

D.R.G. Smith et al. / Journal of Organometallic Chemistry 778 (2015) 29e34
33
In order to vary the amount of thymine available for hydrogen
bonding within the material, polymers with variable amine content
were needed. These (9) were produced (Eqn. 7) by acid-catalysed
7 was isolated as an orange-brown oil in excellent yield (92%)
and was analytically pure without further treatment. The 29Si
spectrum of 7 displays a peak at
d
¼ 7.7 ppm, corresponding well to
reaction of 3-(diethoxymethylsilyl)propylamine with
a poly(-
the analogous peak in the spectrum of 1,3-bis(4-bromobutyl)tet-
dimethyl siloxane) diol of various chain lengths (x ¼ 5, 11, ~700),
where loss of ethanol afforded the amine-functionalised co-poly-
mers with varying amine loadings (~0.2, ~10, ~20%). The co-
polymers were soluble in toluene and the NMR spectra were un-
remarkable with peak assignments similar to those of 1 and 8.
ramethyldisiloxane [
ramethyldisiloxane [
d
¼ 7.0 ppm] and 1,3-bis(1-butylthymine)tet-
¼ 6.4, 7.1 ppm] [1].
d
Having established routes to both monosilane and disiloxane
model compounds, the synthetic protocols were extended to the
formation of siloxane polymers with pendant imine-bound thymine
groups. Commercially available poly(dimethylsiloxane) functional-
ised with ~ 2 mol% of 3-aminopropyl sidechains was reacted with 2
to afford, after solvent removal, polymer 8 as an air-stable rubbery
material (Eqn. 6).
Functionalisation of 9 with 2 proceeded as described earlier
(Eqn. (6)). The resulting polymers (10) were very sparingly soluble
although NMR characterisation confirmed the nature of the mate-
rials and inclusion of 2. In each case, marked physical changes were
noted. Prior to reaction with 2, the amino-functionalised polymers
9 were clear fluids which, while viscous, retained flow character-
istics. Inclusion of ~0.2 mol% 2 gave a material which was more
viscous but retained the ability to flow. Higher levels of 2 gave
rubbery materials, each pale yellow in colour (Fig. 2) while the
20 mol% polymer formed a powdery, brittle solid. These changes
are suggestive of increasing interaction between the chains, most
likely between the pendant thymine molecules forming hydrogen-
bonded links analogous to those reported in the model compound
(2) above (Fig. 1). While it is not possible to state definitively
whether hydrogen bonding interactions are intra-polymer or
crosslinks between different polymer strands, such crosslinks
would most readily explain the very limited solubility of these
materials which has hampered fuller characterisation, particularly
by GPC. A preliminary small-angle X-ray scattering experiment on
samples of 8 suggested that there may be some degree of order in
the material with a repeat distance of 7.8 Å. However, further work
is required to confirm this and to identify the precise nature of the
interactions.
Fig. 3 shows DSC thermograms recorded on the materials. Prior
to functionalization with 2, the polymers with low amine loadings
(<10 mol%) show a PDMS melting transition around ~ ꢀ40 ꢁC,
consistent with literature values [23]. The same transition was
noted in the thymine modified materials although it was less
prominent. These materials also show a second, broad melting
transition around 70e100 ꢁC. This is significantly below the melting
point of 2 (177-9 ꢁC) but does provide further evidence for some
interchain interactions. The transition becomes more prominent as
the amount of thymine in the polymer increases. At the highest
loading, some interaction between the aminosiloxanes is apparent
even in the absence of thymine.
The low polymer loading of functionalised side-chains means
that the NMR spectra of 8 are essentially those of the siloxane
backbone i.e.1H
d
¼ 0.26; 13C
d
¼ 1.4; 29Si
d
¼ ꢀ21.5 ppm, cor-
responding to the dominant Me2Si group; the loading of func-
tionalised silyl groups is too low for a distinct signal to be
observable. However, 8 also displays weak signals at
1.60 and 3.60 ppm, corresponding to the methylene protons
and to silicon respectively, while resonances at
6.96 ppm were assigned to the methyl group attached to thymine
and the heterocyclic ring proton, respectively, and those at
d
¼ 0.80,
a, b
g
d
¼ 1.90 and
d
¼ 7.02 and 7.11 ppm were assigned to the protons in the
pendant phenyl group; a signal at
imine proton.
d
¼ 8.05 was observed for the
The number average molecular weight of 8, measured by gel
permeation chromatography relative to polystyrene, was 21,000,
with a polydispersity of 3.6, indicating that the grafting of 2 onto
the aminopropyl functionalised PDMS resulted in a stable siloxane
polymer. The molecular weight of the repeat unit of PDMS is 74,
suggesting a chain backbone containing the order of a few hundred
repeat units although it should be noted that the GPC calibration
with polystyrene rules out a fuller quantitative analysis. The end-
group signals in the NMR were too weak to allow accurate quan-
tification of chain lengths.
Fig. 2. Siloxane polymers functionalised with varying levels of 2.

34
D.R.G. Smith et al. / Journal of Organometallic Chemistry 778 (2015) 29e34
EPSRC funded service at RAPRA Technology for the GPC
measurements.
References
€
[1] G. Kociok-Kohn, M.F. Mahon, K.C. Molloy, G.J. Price, T. Prior, D.G. Smith, Dalton
Trans. 43 (2014) 7734.
[2] M. Fathalla, C.M. Lawrence, N. Zhang, J.L. Sessler, J. Jayawickramarajah, Chem.
Soc. Rev. 38 (2009) 1608.
[3] E. Elacqua, D.S. Lye, M. Weck, Acc. Chem. Res. 47 (8) (2014) 2405.
[4] S. Sivakova, D.A. Bohnsack, M.E. Mackay, P. Suwanmala, S.J. Rowan, J. Am.
Chem. Soc. 127 (2005) 18202.
[5] K. Viswanathan, H. Ozhalici, C.L. Elkins, C. Heisey, T.C. Ward, T.E. Long, Lang-
muir 22 (3) (2006) 1099.
[6] H. Xu, T.B. Norsten, O. Uzun, E. Jeoung, V.M. Rotello, Chem. Commun. (2005)
5157.
[7] A.I. Mikulska, K. Masamichi, K. Kuroda, A. Iwanowska, S. Yusa,
M. Nowakowska, K. Szczubialka, Eur. Polym. J. 59 (2014) 230.
[8] D. He, X. Wang, K. Zhao, Y. Zou, Langmuir 29 (19) (2013) 5896.
[9] Y. Huang, D. Hu, S. Wen, S. Shen, Z. Zhu, M. Shi, S. Xiangyang, New. J. Chem. 38
(4) (2014) 1533.
[10] E. Yilgor, I. Yilgor, Prog. Polym. Sci. 39 (6) (2014) 1165.
[11] P. Boehm, M. Mondeshki, H. Frey, Macromol. Rapid Commun. 33 (21) (2012)
1861.
[12] D.J. Lunn, C.E. Boott, K.E. Bass, T.A. Shuttleworth, N.G. McCreanor, S. Papadouli,
I. Manners, Macromol. Chem. Phys. 214 (24) (2013) 2813.
[13] J.G. Matisons, A. Provatas, ACS Symp. Ser. 729 (1999) 128.
[14] R.F.M. Lange, M. Van Gurp, E.W. Meijer, J. Polym. Sci. Polym. Chem. 37 (1999)
3657; R.P. Sijbesma, E.W. Meijer, E.W. Chem. Commun. (2003) 1.
[15] L.J. Farrugia, J. Appl. Crystallogr. 32 (1999) 837.
[16] N.S. Nametkin, A.V. Topchiev, T.I. Chernysheva, I.N. Lyashenko, Doklady Aka-
demii Nauk SSSR, 1961, p. 384.
[17] N.S. Nametkin, A.V. Topchiev, T.I. Chernysheva, Issled. v Obl. Kremniiorgan.
Soedin., Sintez i Fiz.-Khim. Svoistva, Akad. Nauk SSSR, Inst. Neftekhim. Sin-
teza, Sb. Statei, 1962, p. 56.
[18] E. Lukevics, E.F. Granin, L.P. Charuiskaya, N.K. Sokolova, N.P. Erchak,
R.Y. Sturkovich, S. Spirina, Y.I. Khudobin, N.A. Andreeva, N.P. Kharitonov, Latv.
PSR Zinat. Akad. Vestis, Kim. Ser. 3 (1978) 343.
[19] R. Shenhar, H. Xu, B.L. Frankamp, T. Mates, A. Sanyal, U. Otkay, V.M. Rotello,
J. Am. Chem. Soc. 127 (2006) 16318.
Fig. 3. DSC thermograms of amino- and thymine functionalised polysiloxanes.
Conclusions
While the intractable nature of the materials containing high
concentrations of thymine make complete characterisation diffi-
cult, we have established a reliable methodology for functionalising
polysiloxanes with thymine. Related chemistry with other purine
and pyrimidine bases should follow the same procedures allowing
a fuller exploration of the potential for these polymers to form
materials with interesting and useful properties.
[20] G. Portalone, L. Bencivenni, M. Coleapietro, A. Pieretti, F. Ramondo, Acta Chem.
Scand. 53 (1999) 57.
[21] M. Rossi, T. Kistenmacher, J. Acta. Crystallogr. Sect. B Struct. Crystallogr. Cryst.
Chem. 33 (1977) 3962.
Acknowledgements
[22] R.F. Stewart, L.H. Jensen, Acta Crystallogr. 23 (1967) 1102.
[23] J. Brandrup, E.H. Immergut, E.A. Grulke (Eds.), Polymer Handbook, fourth ed.,
Wiley, Chichester, 2003, ISBN 978-0-471-47936-9.
We thank EPSRC and Dow Corning for financial support in the
form of a CASE studentship (to D. R. G. S.). We also acknowledge the