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Accepted Manuscript This article can be cited before page numbers have been issued, to do this please use: T. McCormick, E. Carrera, T. Schon and D. S. Seferos, Chem. Commun., 2013, DOI: 10.1039/C3CC47338D.
Volume 46 | Number 32 | 2010
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This is an Accepted Manuscript, which has been through the RSC Publishing peer review process and has been accepted for publication. Chemical Communications www.rsc.org/chemcomm
Volume 46 | Number 32 | 28 August 2010 | Pages 5813–5976
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COMMUNICATION J. Fraser Stoddart et al. Directed self-assembly of a ring-in-ring complex
FEATURE ARTICLE Wenbin Lin et al. Hybrid nanomaterials for biomedical applications
1359-7345(2010)46:32;1-H
Please note that technical editing may introduce minor changes to the text and/or graphics contained in the manuscript submitted by the author(s) which may alter content, and that the standard Terms & Conditions and the ethical guidelines that apply to the journal are still applicable. In no event shall the RSC be held responsible for any errors or omissions in these Accepted Manuscript manuscripts or any consequences arising from the use of any information contained in them.
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DOI: 10.1039/C3CC47338D
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Reversible Oxidation of a Water-Soluble Tellurophene Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX DOI: 10.1039/b000000x 5
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Oxidation of a novel water-soluble tellurophene [2,5tellurophene-bisphenoxy(octaethylene glycol monomethyl ether)] by peroxide is electrochemically reversible. This tellurophene can also be oxidized by self-photosensitized singlet oxygen in an aqueous solution. The oxidized tellurophenes are studied by optical absorption spectroscopy, 1 H NMR, and electrochemistry. Tellurium-containing compounds can undergo reversible oxidative addition reactions with halogens and peroxide,1 however oxidation of the five-membered ring tellurophene is unknown.2 The utility of these reactions is that two-electron oxidation using peroxides and hydroxides constitutes an important step in water splitting.3, 4 Here we show that a novel water-soluble tellurophene can be oxidized by hydrogen peroxide, and by self-photosensitized singlet oxygen, and is reversibly electrochemically oxidized in aqueous media. One equivalent of hydrogen peroxide produces the oxidized tellurophene compound and addition of excess hydrogen peroxide leads to further oxidation to a secondary product. The original tellurophene compound can be regenerated electrochemically. In the electrochemical and photochemical reactions between tellurophene and water, sub-stoichiometric amounts of peroxide were detected, indicating that water-soluble tellurophene compounds may be useful in activating the two-electron oxidation of water.5 To synthesize a water-soluble tellurophene, we modified 2,5diphenyltellurophene compounds by adding octaethylene glycol monomethyl ether (OEG) to the para-position of the phenyl groups. This is a non-trivial new synthesis that allows for water solubility without disrupting the conjugation of the diphenyl tellurophene compound and produces the first example of a water-soluble tellurophene. To synthesize compound 1, (2,5tellurophene-bisphenoxy(OEG)), we first synthesized iodo-OEG monomethyl ether6 and added it to 4-iodophenol in the presence of base to afford iodo-4-OEG-benzene in quantitative yield.7 The 1,4-substituted butadiyne is generated in a one-pot Sonogashiratype coupling reaction, by deprotecting 1,4-(bis)trimethylsilyl)1,3-butadiyne with tetrabutylammonium fluoride inside a sealed bomb containing iodo-4-OEG-benzene and an appropriate catalyst in toluene. The isolated 1,4-OEG-phenyl-butadiyne is treated with sodium telluride, which is generated in situ through a sodium borohydride reduction of tellurium (0) powder, to give the desired product (Scheme 1). Compound 1 can be purified by This journal is © The Royal Society of Chemistry [year]
O
I + HO
O
I
7
Acetone
R
Cs2CO3
I 100%
R
TBAF
TMS
I + TMS
Pd(PPh3)4 CuI Na2Te R
R
Ethanol
79%
Te R
R R=
O
O
O 7
1 40%
Scheme 1 Synthetic route to 1. 50
55
60
65
70
75
precipitation from a dichloromethane solution into hexanes to give a waxy, light-yellow compound that is indeed soluble in water as well as most polar organic solvents. Oxidative addition of halogens to compound 1 was examined first by optical absorption spectroscopy. Specifically, a solution of compound 1 was prepared in water (λmax = 354 nm). Addition of one equivalent of bromine or chlorine (added as iodobenzene dichloride) causes a bathochromic shift in the absorption spectra (λmax= 444 nm and 447 nm respectively) (see ESI), similar to halogen addition to tellurophenes recently reported.8 No change in the absorption spectrum was observed with the addition of iodine. The reactions with halogens are rapid and are complete by the time the spectra are recorded. Next, we investigated the addition of H2O2. Unlike the oxidative addition of bromine and chlorine, addition of H2O2 is slow. Complete conversion of 1 to the oxidized product occurs over 2 days at room temperature with two isosbestic points at 303 nm and 389 nm. The addition of one equivalent of H2O2 results in a red-shifted absorption peak at 435 nm (Fig. 1), similar to the halogenated species. Excess peroxide causes increased rate of formation of the oxidized product (λmax = 435 nm) and a subsequent reaction to a secondary product with high energy absorption (λmax = 280 nm) that only occurs after complete conversion to the first product. Since the reaction of compound 1 with hydrogen peroxide proceeds slowly, this provides an opportunity to study the rate and reaction order of the first oxidation by monitoring the change [journal], [year], [vol], 00–00 | 1
Chemical Communications Accepted Manuscript
Published on 18 October 2013. Downloaded by Clarkson University on 20/10/2013 20:03:37.
Theresa M. McCormick,a Elisa I. Carrera,b Tyler B. Schon,b Dwight S. Seferos*b
ChemComm
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DOI: 10.1039/C3CC47338D
Fig. 1 Absorption spectrum, in water of 1 (green), and 1 after the addition of one (red) and two (blue) equivalents of H2O2. 50
5
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20
25
30
35
40
in absorption at 435 nm as a function of time. The addition of excess H2O2 causes an increase in the rate of the formation of the first oxidized product. By taking the initial rate of the reaction at varying peroxide concentrations and by examining the integrated rate law, we determine that the reaction is second order overall, first order in both H2O2 and 1 (see ESI). This, along with total conversion of 1 to the first oxidized product with only one equivalent of H2O2, strongly suggests H2O2 addition occurs through an oxidative addition reaction to form the dihydroxy tellurophene, similar to the structure reported for the oxidative addition of halogens (Scheme 2, 2a). Subsequent loss of water from this compound could give the telluroxide species (2b).9 The identity of this product will be discussed later. The oxidized product with absorption peak at 435 nm will be referred to as compound 2. The further oxidized product with an absorption peak at 280 nm and will be referred to as compound 3. 1 H NMR spectroscopy shows complete conversion of compound 1 to compound 2 with the addition of one equivalent of H2O2. The 1H NMR spectra were taken in CDCl3 solvent after isolation from water solutions. The signal from the protons on the tellurophene shifts from 7.66 ppm in 1 to 7.33 ppm in 2. Signals from protons associated with hydroxides that one would expect for 2a are absent, suggesting that 2b is the oxidized structure (see ESI). After reaction with excess H2O2 the signal from the proton on the tellurophene shifts further upfield to 7.08 ppm. This product is likely the tellurone (3). There are several impurities observed in the aromatic region suggesting possible decomposition after oxidation. Tellurium-containing dyes are known singlet oxygen photosensitizers due to their strong absorption properties and the presence of the heavy tellurium atom.4, 10 Since 1 can be oxidized by H2O2 we were interested in determining whether it could photosensitize singlet oxygen and undergo subsequent oxidation. A solution of 1 in water (under ambient atmosphere) was irradiated with blue light (447 nm LED) overnight. After this, the absorption signature of compound 1 was no longer seen by optical spectroscopy, however the high energy absorption associated with 3 is seen (see ESI). Additionally, 1H NMR spectra match 3 formed previously with excess H2O2 and contains fewer impurities (see ESI).
2 | Journal Name, [year], [vol], 00–00
55
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65
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75
80
85
90
Scheme 2 Reactivity and proposed structures of oxidized tellurophene.
To further prove that 1 generates and subsequently reacts with singlet oxygen, we added a known singlet oxygen sensitizer, eosin, and irradiated with green light (505 nm LED). The oxidized product 3 was observed by absorption spectroscopy. In the absence of eosin, however, no reaction occurs upon irradiation with green light. In addition, no reaction occurs in dichloromethane solutions or in degassed water solutions (see ESI). This indicates that 1 can produce singlet oxygen when irradiated with blue light and singlet oxygen in water will oxidize 1 to 3. Formation of the oxidized product 2 was not observed. It is also interesting to note that these solutions contain peroxide (as determined by test strips), suggesting that the water is being oxidized to peroxide, although in sub-stoichiometric amounts (
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Accepted Manuscript This article can be cited before page numbers have been issued, to do this please use: T. McCormick, E. Carrera, T. Schon and D. S. Seferos, Chem. Commun., 2013, DOI: 10.1039/C3CC47338D.
Volume 46 | Number 32 | 2010
ccess
This is an Accepted Manuscript, which has been through the RSC Publishing peer review process and has been accepted for publication. Chemical Communications www.rsc.org/chemcomm
Volume 46 | Number 32 | 28 August 2010 | Pages 5813–5976
ChemComm
the chemical
to therapeutic
etic and
Accepted Manuscripts are published online shortly after acceptance, which is prior to technical editing, formatting and proof reading. This free service from RSC Publishing allows authors to make their results available to the community, in citable form, before publication of the edited article. This Accepted Manuscript will be replaced by the edited and formatted Advance Article as soon as this is available. To cite this manuscript please use its permanent Digital Object Identifier (DOI®), which is identical for all formats of publication.
analytical ques for use
es and tools to s.
More information about Accepted Manuscripts can be found in the Information for Authors.
inical systems.
and
les. ion
Registered Charity Number 207890
Pages 5813–5976
org/journals
ISSN 1359-7345
COMMUNICATION J. Fraser Stoddart et al. Directed self-assembly of a ring-in-ring complex
FEATURE ARTICLE Wenbin Lin et al. Hybrid nanomaterials for biomedical applications
1359-7345(2010)46:32;1-H
Please note that technical editing may introduce minor changes to the text and/or graphics contained in the manuscript submitted by the author(s) which may alter content, and that the standard Terms & Conditions and the ethical guidelines that apply to the journal are still applicable. In no event shall the RSC be held responsible for any errors or omissions in these Accepted Manuscript manuscripts or any consequences arising from the use of any information contained in them.
www.rsc.org/chemcomm Registered Charity Number 207890
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Dynamic Article Links ► View Article Online
DOI: 10.1039/C3CC47338D
Cite this: DOI: 10.1039/c0xx00000x
ARTICLE TYPE
www.rsc.org/xxxxxx
Reversible Oxidation of a Water-Soluble Tellurophene Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX DOI: 10.1039/b000000x 5
10
15
20
25
30
35
40
45
Oxidation of a novel water-soluble tellurophene [2,5tellurophene-bisphenoxy(octaethylene glycol monomethyl ether)] by peroxide is electrochemically reversible. This tellurophene can also be oxidized by self-photosensitized singlet oxygen in an aqueous solution. The oxidized tellurophenes are studied by optical absorption spectroscopy, 1 H NMR, and electrochemistry. Tellurium-containing compounds can undergo reversible oxidative addition reactions with halogens and peroxide,1 however oxidation of the five-membered ring tellurophene is unknown.2 The utility of these reactions is that two-electron oxidation using peroxides and hydroxides constitutes an important step in water splitting.3, 4 Here we show that a novel water-soluble tellurophene can be oxidized by hydrogen peroxide, and by self-photosensitized singlet oxygen, and is reversibly electrochemically oxidized in aqueous media. One equivalent of hydrogen peroxide produces the oxidized tellurophene compound and addition of excess hydrogen peroxide leads to further oxidation to a secondary product. The original tellurophene compound can be regenerated electrochemically. In the electrochemical and photochemical reactions between tellurophene and water, sub-stoichiometric amounts of peroxide were detected, indicating that water-soluble tellurophene compounds may be useful in activating the two-electron oxidation of water.5 To synthesize a water-soluble tellurophene, we modified 2,5diphenyltellurophene compounds by adding octaethylene glycol monomethyl ether (OEG) to the para-position of the phenyl groups. This is a non-trivial new synthesis that allows for water solubility without disrupting the conjugation of the diphenyl tellurophene compound and produces the first example of a water-soluble tellurophene. To synthesize compound 1, (2,5tellurophene-bisphenoxy(OEG)), we first synthesized iodo-OEG monomethyl ether6 and added it to 4-iodophenol in the presence of base to afford iodo-4-OEG-benzene in quantitative yield.7 The 1,4-substituted butadiyne is generated in a one-pot Sonogashiratype coupling reaction, by deprotecting 1,4-(bis)trimethylsilyl)1,3-butadiyne with tetrabutylammonium fluoride inside a sealed bomb containing iodo-4-OEG-benzene and an appropriate catalyst in toluene. The isolated 1,4-OEG-phenyl-butadiyne is treated with sodium telluride, which is generated in situ through a sodium borohydride reduction of tellurium (0) powder, to give the desired product (Scheme 1). Compound 1 can be purified by This journal is © The Royal Society of Chemistry [year]
O
I + HO
O
I
7
Acetone
R
Cs2CO3
I 100%
R
TBAF
TMS
I + TMS
Pd(PPh3)4 CuI Na2Te R
R
Ethanol
79%
Te R
R R=
O
O
O 7
1 40%
Scheme 1 Synthetic route to 1. 50
55
60
65
70
75
precipitation from a dichloromethane solution into hexanes to give a waxy, light-yellow compound that is indeed soluble in water as well as most polar organic solvents. Oxidative addition of halogens to compound 1 was examined first by optical absorption spectroscopy. Specifically, a solution of compound 1 was prepared in water (λmax = 354 nm). Addition of one equivalent of bromine or chlorine (added as iodobenzene dichloride) causes a bathochromic shift in the absorption spectra (λmax= 444 nm and 447 nm respectively) (see ESI), similar to halogen addition to tellurophenes recently reported.8 No change in the absorption spectrum was observed with the addition of iodine. The reactions with halogens are rapid and are complete by the time the spectra are recorded. Next, we investigated the addition of H2O2. Unlike the oxidative addition of bromine and chlorine, addition of H2O2 is slow. Complete conversion of 1 to the oxidized product occurs over 2 days at room temperature with two isosbestic points at 303 nm and 389 nm. The addition of one equivalent of H2O2 results in a red-shifted absorption peak at 435 nm (Fig. 1), similar to the halogenated species. Excess peroxide causes increased rate of formation of the oxidized product (λmax = 435 nm) and a subsequent reaction to a secondary product with high energy absorption (λmax = 280 nm) that only occurs after complete conversion to the first product. Since the reaction of compound 1 with hydrogen peroxide proceeds slowly, this provides an opportunity to study the rate and reaction order of the first oxidation by monitoring the change [journal], [year], [vol], 00–00 | 1
Chemical Communications Accepted Manuscript
Published on 18 October 2013. Downloaded by Clarkson University on 20/10/2013 20:03:37.
Theresa M. McCormick,a Elisa I. Carrera,b Tyler B. Schon,b Dwight S. Seferos*b
ChemComm
Page 2 of 4 View Article Online
DOI: 10.1039/C3CC47338D
Fig. 1 Absorption spectrum, in water of 1 (green), and 1 after the addition of one (red) and two (blue) equivalents of H2O2. 50
5
10
15
20
25
30
35
40
in absorption at 435 nm as a function of time. The addition of excess H2O2 causes an increase in the rate of the formation of the first oxidized product. By taking the initial rate of the reaction at varying peroxide concentrations and by examining the integrated rate law, we determine that the reaction is second order overall, first order in both H2O2 and 1 (see ESI). This, along with total conversion of 1 to the first oxidized product with only one equivalent of H2O2, strongly suggests H2O2 addition occurs through an oxidative addition reaction to form the dihydroxy tellurophene, similar to the structure reported for the oxidative addition of halogens (Scheme 2, 2a). Subsequent loss of water from this compound could give the telluroxide species (2b).9 The identity of this product will be discussed later. The oxidized product with absorption peak at 435 nm will be referred to as compound 2. The further oxidized product with an absorption peak at 280 nm and will be referred to as compound 3. 1 H NMR spectroscopy shows complete conversion of compound 1 to compound 2 with the addition of one equivalent of H2O2. The 1H NMR spectra were taken in CDCl3 solvent after isolation from water solutions. The signal from the protons on the tellurophene shifts from 7.66 ppm in 1 to 7.33 ppm in 2. Signals from protons associated with hydroxides that one would expect for 2a are absent, suggesting that 2b is the oxidized structure (see ESI). After reaction with excess H2O2 the signal from the proton on the tellurophene shifts further upfield to 7.08 ppm. This product is likely the tellurone (3). There are several impurities observed in the aromatic region suggesting possible decomposition after oxidation. Tellurium-containing dyes are known singlet oxygen photosensitizers due to their strong absorption properties and the presence of the heavy tellurium atom.4, 10 Since 1 can be oxidized by H2O2 we were interested in determining whether it could photosensitize singlet oxygen and undergo subsequent oxidation. A solution of 1 in water (under ambient atmosphere) was irradiated with blue light (447 nm LED) overnight. After this, the absorption signature of compound 1 was no longer seen by optical spectroscopy, however the high energy absorption associated with 3 is seen (see ESI). Additionally, 1H NMR spectra match 3 formed previously with excess H2O2 and contains fewer impurities (see ESI).
2 | Journal Name, [year], [vol], 00–00
55
60
65
70
75
80
85
90
Scheme 2 Reactivity and proposed structures of oxidized tellurophene.
To further prove that 1 generates and subsequently reacts with singlet oxygen, we added a known singlet oxygen sensitizer, eosin, and irradiated with green light (505 nm LED). The oxidized product 3 was observed by absorption spectroscopy. In the absence of eosin, however, no reaction occurs upon irradiation with green light. In addition, no reaction occurs in dichloromethane solutions or in degassed water solutions (see ESI). This indicates that 1 can produce singlet oxygen when irradiated with blue light and singlet oxygen in water will oxidize 1 to 3. Formation of the oxidized product 2 was not observed. It is also interesting to note that these solutions contain peroxide (as determined by test strips), suggesting that the water is being oxidized to peroxide, although in sub-stoichiometric amounts (
