High yield synthesis of some phosphonic acid derivatives as surface tethers for energy harvesting technologies

Article abstract of DOI:10.4314/bcse.v28i1.17

Efficient synthesis of novel 6-(2-bromo-2-methyl propanoyloxy)hexyl phosphonic acid, dodecane di-phosphonic acid, 6-(thiophene-3-carbonyloxy)hexyl phosphonic acid, octadecyl phosphonic acid and such other derivatives are reported here. These derivatives have a potential application as tethers to nanoparticle surfaces that can promote efficient electron transfer process in solar energy conversion.

Full text of DOI:10.4314/bcse.v28i1.17

Bull. Chem. Soc. Ethiop. 2014, 28(1), 143-148.
Printed in Ethiopia
DOI: http://dx.doi.org/10.4314/bcse.v28i1.17
ISSN 1011-3924
 2014 Chemical Society of Ethiopia
SHORT COMMUNICATION
HIGH YIELD SYNTHESIS OF SOME PHOSPHONIC ACID DERIVATIVES AS
SURFACE TETHERS FOR ENERGY HARVESTING TECHNOLOGIES
Amshumali Mungalimane*
Department of Industrial Chemistry and Chemistry, Vijayanagara Sri Krishnadevaraya
University, Jnana Sagara campus, VinayakaNagara, Cantonment, Bellary, India-583 104
(Received January 14, 2013; revised November 6, 2013)
ABSTRACT. Efficient synthesis of novel 6-(2-bromo-2-methyl propanoyloxy)hexyl phosphonic acid, dodecane
di-phosphonic acid, 6-(thiophene-3-carbonyloxy)hexyl phosphonic acid, octadecyl phosphonic acid and such
other derivatives are reported here. These derivatives have a potential application as tethers to nanoparticle
surfaces that can promote efficient electron transfer process in solar energy conversion.
KEY WORDS: Surface tethers, Coupling reaction, Phosphonic acid derivatives, Solar energy conversion
INTRODUCTION
The quest for efficient solar energy conversion to produce electricity or chemical fuels has
expanded significantly in the last decade [1-7]. As pointed out in these literatures, it is widely
recognized that many of the key limitations to new energy conversion technology results from a
lack of nanometer scale control of material composition and properties, linked with a clear
understanding of how these properties impact on energy conversion efficiency.
The key steps in solar energy conversion, and solar photochemical formation of fuels, using
small molecule, polymer, and semiconductor nanoparticle (SC-NP) composites are recognized
to be (i) light absorption, (ii) exited state or exciton dissociation, (iii) charge separation, (iv)
charge transport and (v) charge collection (electronic) or electrochemical fuel production.
Limitations in any one of these steps translates to poor energy conversion efficiency for the
entire system [8, 9]. The recent efforts in avoiding the limitations to the above steps consists of
electro deposition of an ultra thin film polymer host on an activated indium tin oxide (ITO)
surface providing a selective (electron blocking) back contact, and CdSe nanoparticles capped
with electroactive ligands are attached electrochemically in the near surface region of the
polymer film [10-12]. This provides for the direct wiring of the SC-NP to the transparent bottom
contact and appears to provide a route for optimizing the use of such nanoparticles as the
photoactive layer in both photo electrochemical and photovoltaic energy conversion
technologies. Notable recent studies using functional ligand capping of SC-NPs have been
directed towards enhancing the compatibility of the nanoparticle with polyphenylenevinylene
(e.g. MEH-PPV) or polythiophene matrices [13-18], where the surface functional groups may be
the trioctylphosphine oxide (TOPO) derivatives or phosphonates. In this work we prepared a
library of such ITO tethering compounds, namely, 6-(2-bromo-2-methyl propanoyloxy)hexyl
phosphonic acid, dodecane di-phosphonic acid, 6-(thiophene-3-carbonyloxy)hexyl phosphonic
acid, octadecyl phosphonic acid have been prepared and characterized along with its
intermediates. We hypothesize that these ligands would passivate the surfaces of metal
chalcogenide nanoparticles and make them ameanable to inclusion in a variety of polymeric
hosts and/or provide for attachment at the termini of the conducting polymeric chains.
__________
*Corresponding author. E-mail: amshumalir@yahoo.com

144
Amshumali Mungalimane
EXPERIMENTAL
General methods. All reactions are carried out under argon atmosphere using standard Schlenk
technique, except where otherwise indicated. All glassware are previously dried in oven at 120
oC and cooled to room temperature. 1-Bromo-6-hexanol, triethylamine, triethylphosphite,
thiophene carboxylic acid, 1-bromooctadecane and 1,12-dibromododecane and 2-bromoiso-
butyrylbromide were used as received from Aldrich or elsewhere. NMR experiments were
carried out in Bruker AM 250 in the solvent indicated in each case at 250 and 62.5 MHz for 1H
and 13 C NMR, respectively. Chemical shifts were noted in parts per million and J values are
given in hertz. Microanalyses were carried out at Indian Institute of Science, Bangalore, on
LECO-CHNS analyzer, and were in a good agreement with the calculated values. High
resolution mass spectrometry (HRMS) was recorded on FAB Shimadzu Instruments.
Synthesis of 6-bromohexyl-2-bromo-2-methylpropanoate. In a dry Schlenk flask equipped with
a stir bar and connected to argon inlet, 2-bromo isobutarylbromide (3 g, 13.1 mmol) was added
via syringe followed by 15 mL of dichloromethane. 1-Bromo-6-hexanol (2.56 g, 14.1 mmol)
was then added to the above mixture and stirred for 15 min. This was kept in an ice bath and
triethylamine (2 mL) was added slowly, via syringe. The mixture was allowed to warm to room
temperature and stirred overnight. Next day the reaction mixture was washed several times with
10% HCl and distilled water, the solvent was removed on rotary evoparator and passed through
the silica plug. Then the solvent was evaporated and concentrated. This gave almost pure
1
3
product. Yield 3.4 g, (10.9 mmol 83%). H NMR (CDCl3, δ ppm) 4.17 (t, 2H, J = 6.5 OCH2),
3
3
3
3.40 (t, 2H, J = 7.0 Br-CH2), 1.93 (s, 6H, -CH3), 1.87 (q, 2H, J = 7.0 CH2), 1.67 (q, 2H, J =
6.75 CH2), 1.45 (broad, 4H, CH2). Mass spectrum: [M+] = 327.96. Elemental analyses:
C10H18Br2O2, calcd. C, 36.39; H, 5.50; found C, 36.37; H, 5.47.
Synthesis of 6-(diethylphosphoryl)hexyl-2-bromo-2-methyl propanoate. 6-Bromohexyl 2-
bromo-2-methylpropanoate (1 g, 5.8 mmol) was mixed with triethylphosphite (1.1 mL, 8.75
o
mmol) and heated to 150 C for 16 h. Excess of phosphate was removed under high vacuum
diluted with dichloromethane and purified by silica gel column using 50:1 ethyl acetate and
methanol mixture. Yield 1.8 g, (4.8 mmol 70%). 1H NMR (CDCl3, δ ppm) 4.11 (q, 6H, 3J = 6.5
CH2), 1.92 (s, 6H, CH3), 1.73 (broad, 6H, CH2), 1.41 (broad, 4H, CH2), 1.31 (t, 6H, J = 9.5
3
CH2). Mass spectrum: [M+] = 386.08. Elemental analyses: C14H28O5PBr, calcd. C, 43.42; H,
7.49; found C, 43.39; H, 7.48.
Synthesis of 6-(2-bromo-2-methylpropanoyloxy)hexyl phosphonic acid. In a Schlenk flask under
argon, the 6-(diethylphosphoryl)hexyl-2-bromo-2-methyl propanoate (1.1 g, 2.84 mmol) was
placed with 10 mL of dichloromethane. Bromotrimethylsilane (1.38 g, 8.5 mmol) was added to
that mixture and stirred for 24 h. The excess of TMSBr in the solvent was removed under
vacuum. Then 10 mL of methanol was added to this mixture and stirred for 2 h. The methanol
was dried and the product was dissolve in dichloromethane and recrystallized in hexane which
1
gave the product. Yield 0.8 g. (2.5 mmol 90%). H NMR (CDCl3, δ ppm) 1.25 (broad, 4H,
CH2), 1.69 (broad, 6H, CH2), 1.93 (s, 6H, CH3), 4.16 (t, 2H, J = 6.5 OCH2), 9.25 (broad, 2H,
3
OH). Elemental analyses: C10 H20O5BrP, calcd. C, 36.27; H 6.09; found C, 36.26; H, 6.04. Mass
spectrum: [M+] = 330.02.
Synthesis of tetraethyldodecane-1,12-diyldiphosphonate. 1,12-Dibromododecane (1 g, 3.05
mmol) was place in a Schlenk flask with an argon inlet and triethyl phosphate (1.01 g, 6.1
mmol) was added via syringe. The mixture was heated to 150 oC for 18 h. The excess of triethyl
o
phosphate was removed under vaccumm at 100 C. Dichloromethane was added to the product
1
and the product was extracted with a dilute HCl, and NaHCO3 followed by distilled water. H
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145
NMR CDCl3, ppm 4.00 (q, 8 H, OCH2 J = 6.75), 1.64 (q, 4H, CH2, J = 17.5) 1.01-1.31 (q,
broad, 20H). Mass spectrum: [M+] = 442.26. Elemental analyses: C20H44O6P2, calcd. C, 54.28;
H, 10.02; found C, 54.22; H, 10.06.
Synthesis of dodecane diphosphonic acid. Tetraethyl dodecane-1,12-diyldiphosphonate (1 g, 2.6
mmol) were mixed in 15 mL of dichloromethane and bromotrimethylsilane (1.086 g, 7.1 mmol)
and this mixture was allowed to stir for 24 h. Excess of bromotrimethylsilane was evaporated
along with dichloromethane. Then 10 mL of methanol was added to this mixture and stirred for
3 h. After evaporation of methanol, the off-white solid was obtained. The solid was dispersed in
dichloromethane, and then the dichloromethane was evaporated and dried under vacuum. Yield
1
0.4 g (1.3 mmol 65%). H NMR (DMSO-D6) 1.03-1.24 (broad, CH2, 8H), 1.46 (broad, CH2,
6H), 0.97 (d, CH2, 4H, J = 11.0), 9.5 (broad, OH, 4H). Mass spectrum: MH+ 330.10. Elemental
analysis: C12H28O6P2, calcd. C, 43.64; H, 8.54; found C, 43.60; H, 8.50.
Synthesis of 6-bromohexyl thiophene 3-carboxylate. In a 50 mL Schlenk flask fitted with argon
inlet was added 3-thiophene carboxylic acid (0.5 g, 3.9 mmol), 4 dimethyl amino pyridine (0.48
g, 3.9 mmol) was added and evacuated 3 times and backfilled with argon. 25 mL of
dichloromethane was added to this mixture and stirred for a few min with 0 oC. Then 1-bromo-6
hexanol (0.7 g, 3.9 mmol) was added via syringe to this. A solution of DCC (0.8 g, 3.9 mmol) in
10 mL of dichloromethane was added and this solution was allowed to warm up to room
temperature and stirred overnight. The product was filtered and purified by column
chromatography on silica gel using hexane/EA (8.5/1.5). Yield: 0.7 g, 83%. 1H NMR: (CDCl3, δ
ppm, JHz), 8.09 (d-d, 1H, J = 4.25), 7.53 (m, 1H, J = 4.25), 7.27-7.33 (m, 1H, J = 12), 4.28 (t,
2H, OCH2 J = 8.75), 3.42 (t, 2H, CH2Br, J = 13.5), 1.82 (m, 2H, CH2), 1.57 (m, 2H, CH2), 1.44
(broad, 4H, CH2). 13C NMR: (CDCl3, δ ppm, JHz) 162.70 (C=O), 133.79 (C-quaternary), 132.40
(C-aromatic), 127.81 (C-aromatic), 125.89 (C-aromatic), 64.53 (O-CH2), 34.9-25.3 (C alkyl
chain). Mass spectrum: [M+] = 289.99. Elemental analysis: C11H15O2BrS calcd. C, 45.37; H,
5.19; S, 11.01; found, C, 45.18; H, 5.34; S, 10.94.
Synthesis of 6-(diethoxy phosphoryl)hexyl thiophene-3-carboxylate. 6-Bromo hexyl (3-
thiophne) carboxylic acid (0.7 g, 2.4 mmol) and triethyl phosphate (0.79 g, 4.8 mmol) were
mixed together in a Schlenk flask under argon atmosphere and heated to 145 oC for 18 h with a
closed argon inlet. Then the temperature of the oil bath is reduced to 90 oC and excess of triethyl
phosphite was removed under vacuum. Yield: 0.71 g, 78%. 1H NMR: (CDCl3, δ ppm, JHz), 8.10
(d-d, 1H, J = 4.25), 7.51 (d-d, 1H, J = 6.0), 7.29 (q, 1H, J = 15.75), 4.27 (t, 2H, OCH2), 4.09 (q,
4H, POCH2, J = 21.75), 1.75 (broad, 6H, CH2), 1.45 (broad, 4H, CH2), 1.26 (t, 6H. CH3). 13C
NMR (CDCl3, δ ppm, JHz): 162.73 (C=O), 133.84 (C-quaternary), 132.43 (C-aromatic), 127.94
(C-aromatic), 125.90 (C-aromatic), 64.55 (O-CH2), 34.9-25.3 (C of alkyl chain). Mass
spectrum: [M+] = 348.1. Elemental analyses: C15H25O5PS, calcd. C, 51.71; H, 7.23; S, 9.20;
found, C, 52.00; H, 7.11; S, 8.99.
Synthesis of 6-(thiophene-3-carbonyloxy)hexyl phosphonic acid. 6-Hexyl (3-thiophene
carboxylic) phosphonite (0.7 g, 2.0 mmol) was placed in a Schlenk flask with 10 mL of
dichloromethane. TMSBr (0.6 g, 4 mmol) was added via syringe and stirred for overnight. Next
day the dichloromethane and the excess of TMS-Br were removed under vacuum. 15 mL of
methanol was added to this mixture and stirred for 3 h. The methanol was removed under
vacuum and the viscous oily product obtained and it solidifies on cooling. This product was
stirred vigorously in boiling hexane and allowed to settle, cooled and filtered. Procedure was
1
repeated 3 times to get rid of oily impurities. And the white product was obtained. H NMR:
(CDCl3, δ ppm, JHz) 10.15 (broad, 2H, OH), 8.09 (d-d, 1H, J = 18.75), 7.52 (d-d, 1H, J = 19),
7.29 (m, 1H, J = 23), 4.27 (q, PCH2, J = 25.5), 1.8 (b, 4H, CH2), 1.75 (b, 2H, CH2), 1.69 (b, 4H,
Bull. Chem. Soc. Ethiop. 2014, 28(1)

146
Amshumali Mungalimane
CH2). Mass spectrum: [M+] = 292.05. Elemental analyses: C11H17O5PS, calcd. C, 45.20; H,
5.86; S, 10.97; found, C 45.00; H, 5.92; S, 10.44.
Synthesis of octadecyl diethyl phosphonate. To 1 g of 1-bromooctadecane, 2 g of diethyl
o
phosphite was added under closed argon atmosphere and heated to 150 C. Then excess of
o
1
triethyl phosphite was removed under high vacuum at 90 C. H NMR: (CDCl3, ppm, J Hz):
4.07 (t, 4H, OCH2 J = 13.25), 1.65 (broad, 4H, CH2-CH2), 1.24 (s, 32H, CH2), 0.866 (t, 3H,
CH3). 13C NMR: 14.58 (CH3), 16.97 (d, CH3), 22.85-32.39 (C-alkyl chain), 33.31 (C-PCH2),
61.81 (d-O-CH2). Mass Spectrum: [M+] = 390.32. Elemental analyses: C22H47O3P, calcd. C,
67.65; H, 12.13; found, C, 67.64; H, 12.10.
Synthesis of octadecyl phosphonic acid. Octadecyl diethyl phosphonite (1.2 g) and 10 mL of
dichloromethane were placed in a Schlenk flask under argon. TMSBr (2.0 g, 4 mmol) was
added via syringe and stirred for overnight. Next day the dichloromethane and the excess of
TMS-Br were removed under vaccum. 15 mL of methanol was added and stirred for 3 h. The
methanol was removed under vacuum, thus the off white crystalline solid was obtained. This
product was stirred vigorously in boiling hexane and allowed to settle, cooled and filtered.
Procedure was repeated 3 times to get rid of oily impurities. And the white product was
1
obtained. H NMR: (CDCl3, δ ppm, JHz) 8.73 (b, 2H, OH), 1.90 (m, 2H, PCH2 = 26.75), 1.616
(broad, 2H, CH2), 1.39 (b, 2H, CH2), 1.19 (s, 30H, CH2-alkyl), 0.877 (t, CH3, 3H, J = 13.0).
Mass spectrum: [M+] = 334.26. Elemental analyses: C18H39O3P, calcd. C, 64.64, H, 11.75;
found, C, 64.62; H, 11.72.
RESULTS AND DISCUSSION
The compounds selected for the synthesis is based on their suitability as ITO, SC-NP tethers and
the simplicity in the synthesis. Thus for synthesizing the 6-(2-bromo-2-methyl propanoyloxy)
hexyl phosphonic acid, (3), 2-bromoisobutyryl bromide was made to react with 1-bromo-6-
hexanol in dichloromethane, gives the product (1) with the yield of 83%. Product (1) was then
o
mixed with triethylphosphite and heated to 150 C for 16 h to give the product (2) which was
further purified by using silica gel column using 50:1 ethyl acetate and methanol mixture. The
product (2) was treated with TMSBr for hydrolysis. The excess TMSBr was removed and
washed with methanol several times to remove phosphonite impurities. After several washings
in the methanol and dissolving the product in the minimum amount of dichloromethane, and
processed for re crystallization in hexane to give pure product (3). The overall reaction is as
shown in the Scheme 1.
O
O
H C
3
Br
TEA/CH2Cl2
H C
3
+
Br
HO
Br
O
Br
3h
CH
Br
3
CH
3
1, 83 %
O
O
O
P
O
H C
3
Br
H C
3
0
P(OEt) / 150
C
O
3
O
O
Br
CH
Br
3
CH
3
2 , 70%
O
O
HO
O
O
O
H C
3
H C
3
P
P
TMSBr/MeOH
O
O
OH
O
Br
Br
CH
CH
3
3
3, 90%
Scheme 1. Synthesis of 6-(2-bromo-2-methylpropanoyloxy)hexyl phosphonic acid.
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Short Communication
147
In the same manner, the products 4 and 5, prepared from 1,12-dibromododecane and the
products 9, and 10 are prepared from 1-bromooctadecane. The reaction of triethylphosphite with
1,12-dibromododecane gives the product 4. Further treatment of product 4 with TMSBr in
methanol followed by the same purification method as in the synthesis of product 3, gave the
product 5. The reaction is shown in the Scheme 2. In the similar method, product 9 and 10 were
synthesized using 1-bromooctadecane with the overall yield of about 70%. The reaction is
shown in the Scheme 3.
H3C
H3C
CH3
O
O
O
CH3
150 0C/18h
O
Br
O
P
P
2
Br
P
+
O
O
H3C
O
O
CH3
CH3
4, 87%
H C
3
CH
3
O
HO
O
O
O
O
O
P
TMSBr/MeOH
P
P
P
H C
3
O
OH
HO
O
OH
5, 65%
CH
3
Scheme 2. Synthesis of dodecane di-phosphonic acid.
0
Br
P(OEt) / 150
3
C
O
TMSBr/MeOH
P
O
O
9, 85%
O
P
OH
HO
10, 70%
Scheme 3. Synthesis of octadecyl phosphonic acid.
The product 6 was synthesized by coupling reaction of 6-thiophene carboxylic acid and 1-
bromo-6-hexanol with N,N-dimethylaminopyridine and dicyclohexylcarbodiimide
(DMAP/DCC). After this reaction, the product 6 was purified by column chromatography using
hexane/ethyl acetate (85/15) gives the yield of 83%. The product 7 was prepared in excellent
yield by following the reaction of product 6 with triethyl phosphite and work up as described
before. The 6-hexyl(3-thiophene carboxylic) phosphonite, 7, was treated with TMSBr in
dichloromethane for 18 h, after removing the unreacted TMSBr under vacuum, and stirred the
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148
Amshumali Mungalimane
product with hot hexane several times, followed by filtration which gave pure product 8. The
overall reaction is shown in the Scheme 4.
O
O
OH
+
Br
Br
DCC/DMAP
O
HO
S
S
6, 83 %
O
P
O
O
O
o
P(OEt) /150
C
O
O
Br
3
O
O
S
7, 78%
S
O
O
O
O
P
TMSBr/MeOH
OH
O
O
P
O
S
S
8, 80%
OH
Scheme 4. Synthesis of 6-(thiophene-3-carbonyloxy)hexyl phosphonic acid.
CONCLUSIONS
We have reported a straightforward and versatile route for the high yield synthesis of many
phosphonic acid linkers to control both the ligating functionality, as well as the chain length of
the alkyl spacers (5-10 carbons) between ligating and polymerizable groups. These derivatives
have a potential application as tethers to the SC-NP surfaces that promote the efficient electron
transfer. Further investigations on the applicability of these compounds along with developing
other possible compounds of same application are underway.
ACKNOWLEDGEMENTS
Authors gratefully acknowledge the financial support and facilities of VSK University, Bellary.
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