Photocatalytically active materials with pyridinium-enolate partial structures

Article abstract of DOI:10.1515/znb-2008-0412

2,3-Dichloro-1,4-naphthoquinone and tetrabromo- or tetrachloro-1,4- benzoquinone were converted into pyridinium enolate betaines by reaction with pyridine or 4,4′-bipyridine. The reaction conditions were applied to poly-(4-vinylpyridine) and a modified Me

Full text of DOI:10.1515/znb-2008-0412

Photocatalytically Active Materials with Pyridinium-enolate
Partial Structures
Andreas Schmidt and Marcel Albrecht
Clausthal University of Technology, Institute of Organic Chemistry, Leibnizstraße 6,
D-38678 Clausthal-Zellerfeld, Germany
Reprint requests to Priv.-Doz. Dr. rer. nat. habil. Andreas Schmidt. Fax: +49(0)5323-722858.
E-mail: schmidt@ioc.tu-clausthal.de
Z. Naturforsch. 2008, 63b, 465 – 472; received January 8, 2008
2
,3-Dichloro-1,4-naphthoquinone and tetrabromo- or tetrachloro-1,4-benzoquinone were con-
ꢀ
verted into pyridinium enolate betaines by reaction with pyridine or 4,4 -bipyridine. The reaction
conditions were applied to poly-(4-vinylpyridine) and a modified Merrifield resin to obtain function-
alized polymeric materials. These were examined in thermogravimetric analyses. Reversible pho-
tocatalytic electron transfer reactions in the presence of proflavinium and EDTA as sensitizer and
sacrificial donor, respectively, were examined. All monomeric and polymeric materials, except for
one, proved to be active.
Key words: Mesomeric Betaines, Radicals, Electron Transfer Reactions, Photocatalysis
Introduction
ined from the viewpoint of organic synthesis, nor from
the viewpoint of material sciences, although much in-
terest has been focussed on functional groups and ring
systems including pyridines [9] and bipyridines [10]
as potential radical partial structures of new materi-
als. Some multifunctional spin systems such as pho-
tochromic radicals have also been developed [11].
Recently, we examined the alkaloid 1 from Punica
In heterocyclic mesomeric betaines an even number
of charges is delocalized within a common π-electron
system. In 1985 four distinct types of heterocyclic
mesomeric betaines were classified, i. e. conjugated
mesomeric betaines (CMB), cross-conjugated me-
someric betaines (CCMB), pseudo-cross-conjugated
mesomeric betaines (PCCMB), and ylides which form granatum L. [12] as well as derivatives [13]. The natu-
a subclass of CMB [1]. By recognition of the type ral product forms the mesomeric betaines 2A and 2B
of conjugation of a mesomeric betaine its chemical in equilibrium (“Punicin,” Scheme 1) [12]. Interest-
properties can be predicted. For example, numerous ingly, the betaine 2A belongs to the class of con-
1
,3-dipoles which are used in natural product syn- jugated mesomeric betaines (CMB), whereas its tau-
thesis [2] are conjugated mesomeric betaines. Cross- tomer 2B is a member of the class of cross-conjugated
conjugated mesomeric betaines undergo 1,4-dipolar mesomeric betaines (CCMB). At higher pH values, de-
cycloadditions which were also applied in interesting protonation to the monoanionic species 3 or pericyclic
heterocyclic [3] or natural product syntheses [4]. Very ring cleavage to 4 occurs. Moreover, Punicin pos-
recently it was realized that pseudo-cross-conjugated sesses oxidizing (pyridinium) and reducing (benzene-
mesomeric betaines can be converted into Arduengo 1,4-diolate) partial structures. Internal electron trans-
2
•
carbenes by extrusion of heterocumulenes [5] and vice fers, i. e. disproportionation, resulted in diradicals 2 ,
versa [6]. In view of recent progress in material sci- whereas an intermolecular redox reaction gave a rad-
•
−
•+
ences, CMB such as pyridinium-olates and isoquin- ical anion 2 and a radical cation 2 , or its tau-
olinium-3-olates [7] as well as cross-conjugated sys- tomers. We performed EPR spectroscopy at X as well
tems such as pyrimidinium-olates [8] are photosensi- as W band and DFT computations, and characterized
tive moieties of polymeric materials, as polarities, re- radical species formed from Punicin and some poly-
fractive indices and densities of such materials change meric derivatives [14].
on irradiation. The formation of radical species from
We wish to report here our results on the synthesis
mesomeric betaines, however, has neither been exam- of new materials containing enol pyridinium betaine
0
932–0776 / 08 / 0400–0465 $ 06.00 ꢀc 2008 Verlag der Zeitschrift f u¨ r Naturforschung, T u¨ bingen · http://znaturforsch.com
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466
A. Schmidt – M. Albrecht · Photocatalytically Active Materials with Pyridinium-enolate Partial Structures
Scheme 1.
Scheme 2.
structure elements of dipole 6 and tetrapole 11. We per- solid [15] which crystallized with water of crystalliza-
--- ---- ---- ----
-
formed thermogravimetric analyses and examined the tion (Scheme 2). The O —– C —– C —– C —– O vi-
−
1
capabilities of these new materials in reversible photo- bration can be observed at 1557 cm in the IR spec-
catalytic electron transfer reactions.
trum [16]. Under analogous reaction conditions, poly-
4-vinylpyridine)gave a polymeric material which pre-
(
Results and Discussion
cipitated from the reaction mixture on addition of wa-
ter. An idealized structure of this polymer possess-
ing 66 % of reacted pyridine rings is presented by for-
Syntheses and characterizations
Reaction of 2,3-dichloro-1,4-naphthoquinone (5) mula 7. All polymeric materials described here, in-
with pyridine in acetic acid yielded 2-(1-pyridinio)- cluding 7, were carefully washed or recrystallized un-
1
,4-naphthoquinon-3-olate (6) as an orange colored til the IR spectra no longer displayed the character-
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A. Schmidt – M. Albrecht · Photocatalytically Active Materials with Pyridinium-enolate Partial Structures
467
O
[
9a], pyridine,
Br
N
2 Br
anhyd. EtOH
N
Br
O
1
0
[
9a], pyridine,
MeCN,
O
O
N
N
O
then H O
2
O
O
O
11
X
X
X
X
O
O
O
N
N
[9a],
N
poly-(4-vinylpyridine),
MeCN, then H O
9
9
a: X = Br
b: X = Cl
N
2
O
12
N
O
O
[
4
9b],
,4´-bipyridine,
MeCN
O
N
N
O
13
N
Scheme 3.
istic bands of the unreacted starting materials. Thus, bipyridine as a bisnucleophile with two chloromethyl
-
--- ---- ---- ----
the O —– C —– C —– C —– O vibration of the poly- groups of the Merrifield resin. A comparison of the
meric material could unambiguously be assigned to a CHN analyses before and after the functionalization
−
1
band at 1545 cm in the IR spectrum. As expected, with 2,3-dichloro-1,4-naphthoquinone brought some
the H NMR signals are broad and unresolved so that information on the structure of 8. Thus, the experi-
1
the degree of substitution cannot be determined by this mentally determined nitrogen content is considerably
spectroscopic method. The nitrogen content (6.11%) lower than before (4.74 %) indicative of a reaction be-
reveals that 50 % to 75 % of the pyridine rings of the tween the polymer and 2,3-dichloro-1,4-naphthoquin-
poly-(4-vinylpyridine) are substituted. As examples, one, but higher than calculated for a complete sub-
calculated values of completely and 50 % substituted stitution of all unsubstituted pyridine rings (4.33 %).
-
--- ---- ---- ----
polymers are 5.05 % and 7.07 %, respectively. Poly- The O —– C —– C —– C —– O vibration was found
−
1
mer 7, however, includes small amounts of THF and at 1542 cm in the IR spectrum.
water of crystallization as evidenced by thermogravi-
metric analyses (vide infra), so that the elemental anal-
yses gave only approximate values for the degree of
Then, perhalogenated 1,4-benzoquinones were con-
verted into new materials and monomeric model com-
pounds (Scheme 3). Thus, tetrabromo-1,4-benzoquin-
one (9a), prepared by a two-step procedure [16],
yielded air and moisture sensitive 3,6-dibromo-2,5-
di(1-pyridinio)-1,4-benzoquinone (10) by reaction
with pyridine in anhydrous ethanol. Reaction in ace-
tonitrile and subsequent quenching of the reaction
mixture with water resulted in the formation of
ꢀ
substitution. A 4,4 -bipyridinium functionalized Mer-
rifield resin, which we described earlier [14], was re-
acted with 2,3-dichloro-1,4-naphthoquinone in acetic
acid to give a new material which is represented by
the idealized structure 8 (Scheme 2). Cross-linked sub-
structures in the starting material are omitted for the
ꢀ
sake of clarity. These resulted from reaction of 4,4 -
2
,5-di(1-pyridinio)-1,4-benzoquinone-3,6-diolate (11)
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468
A. Schmidt – M. Albrecht · Photocatalytically Active Materials with Pyridinium-enolate Partial Structures
Scheme 4.
creasing chain length of the polymer. Thus, the value
of the nitrogen contents (found: 7.96 %) hints at a high
molecular weight. As example, a structure containing
six quinones and seven bipyridinium segments con-
tains 10.27 % of nitrogen. Water of crystallization, a
typical phenomenon in the chemistry of heterocyclic
mesomeric betaines, prevented us from gaining more
information from the elemental analyses.
The thermogravimetric analyses of the new materi-
als are shown in Fig. 1. The weight loss of 8 – 13 % at
◦
◦
temperatures of 60– 70 C and 100– 120 C is due to
the extrusion of solvents such as THF, acetone, toluene
and water from the polymers, respectively. A consid-
Fig. 1. Thermogravimetric analyses of compounds 7, 8, 12, erable decomposition of the materials begins at tem-
and 13.
◦
peratures above approximately 200 C. In general, the
-
--- ---- thermal stability of the substituted polymer 7 is smaller
which is orange in color. The characteristic O—–C—–
-
--- ----
than that of pure poly(4-vinylpyridine). The weight
C —– C —– O vibration band in the IR spectrum is
◦
loss of 61 % on heating 7 to 301 C is caused by
−
1
found at 1557 cm in this reference material. Un-
der analogous reaction conditions, poly-(4-vinylpyr-
idine) gave a material which displayed an absorption
extrusion of the naphthoquinone-betaine moiety. The
polymer 8 displays three steps of weight loss on heat-
◦
ing: At 254 C the naphthoquinone substituent is ex-
−
1
band at 1556 cm , and which can be represented by
the idealized formula 12. The nitrogen contents de-
termined by elemental analysis revealed that 50 % of
the pyridine rings of the starting polymer were sub-
stituted (calcd. 10.57 %; found: 10.45 %). We cannot
differentiate, however, between inter- and intramolecu-
lar cross-linkings. Tetrachloro-1,4-benzoquinone (9b)
◦
ꢀ
truded (weight loss: 33 %). At 398 C the 4,4 -bipyr-
idine partial structure is cleaved (weight loss: 27 %),
◦
and at 555 C the cleavage of the polymer backbone
can be observed. The polymers 12 and 13 possessing
dibetainic structures decompose to unidentifiable prod-
ucts, because extrusion of one component results in the
destruction of the polymer backbone.
ꢀ
reacted with 4,4 -bipyridine to the dark brown poly-
−
1
mer 13 which displayed a band at 1561 cm in the
IR spectrum. Once again, conclusions about the struc-
ture of this material can be drawn from the elemen-
Classifications
All betaines described here are members of the class
tal analysis, as the nitrogen contents decreases with in- of conjugated mesomeric betaines (CMB). Accord-
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A. Schmidt – M. Albrecht · Photocatalytically Active Materials with Pyridinium-enolate Partial Structures
469
Scheme 5.
ing to the valence bond approach for the classifica-
tion, sites for positive and negative charges exist in
the canonical formulae which is characteristic for this
type of conjugation (Scheme 4). The recognition of
characteristic 1,3-dipole increments from the canoni-
cal formulae is an alternative method to classify the
type of conjugation. In the betaines described here,
1
,3-dipole I (azomethine ylide) can be dissected from
the mesomeric structures which is typical for the class
of CMB. Furthermore, the cationic partial structure is
joined to the negative partial structure, i. e. (E)-3-oxo-
prop-1-en-1-olateII, through a starred position (cf. IV)
of its isoconjugated equivalent hydrocarbon, i. e. (E)-
penta-2,4-dien-1-ide III. This is also characteristic for
the class of conjugated mesomeric betaines.
Scheme 6.
conditions, in the presence of oxygen (air) resulted
in the formation of hydroxide ions. This is indicated
by an increasing pH value with irradiation time. We
therefore first performed a blind probe under analo-
gous reaction conditions without polymers and their
Application
We examined the capabilities of the new materi- model compounds. No change of the pH was observ-
als and their monomeric model compounds in coupled able (Fig. 2). In the presence of 6 – 8, 11, and 12
photocatalytic electron transfers. We intended to pro- the color of the solutions or suspensions changed af-
•
−
•−
duce radical anions such as 6 – 8 on irradiation ter irradiation within a couple of minutes from yel-
of 6 – 8 in systems consisting of proflavinium 14 as low to brown, and the original yellow color recon-
sensitizer and EDTA in aqueous solution under an inert stitutes immediately after exposure to air. This cycle
atmosphere (Scheme 5).
can be started again after rinsing the reaction mixture
Likewise, radical anions of the second series of with nitrogen followed by irradiation and can be re-
compounds can be postulated to be formed as follows peated numerous times. Stable radical species from vi-
from Scheme 6.
ologen [17] or dipunicin derivatives [13] under these
In this cycle the oxidized photocatalyst 14•+ can be conditions are blue or greenish-blue in color and were
regenerated by an electron transfer from the disodium observed by UV spectroscopy. However, the concen-
ethylenediamine-tetraacetate (EDTA) which serves as trations of radical species described here are too small
a sacrificial donor. Reoxidation of the radical an- for a detection by UV spectrometry. We monitored the
•
−
•−
•−
•−
ions 6 – 8 and 11 – 13 , if formed under these reaction by measuring the pH of the solutions or sus-
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470
A. Schmidt – M. Albrecht · Photocatalytically Active Materials with Pyridinium-enolate Partial Structures
ether. The aqueous phase was evaporated, and the result-
ing solid was filtered off and washed with toluene. Recrys-
tallization from DMSO gave 508 mg (40 %) of an orange
◦
◦
solid. M. p. 298 – 299 C (302 – 303 C [15]). – IR (KBr):
−1
ν = 3424, 1687, 1593, 1557, 1470, 1373, 995, 678 cm . –
3
C H NO · 2 H O (789.20): calcd. C 68.44, H 3.96,
15
9
3
2
N 5.32; found C 68.00, H 3.62, N 5.18.
Polymer 7
2,3-Dichloro-1,4-napthoquinone (1.23 g, 5.4 mmol)
in 50 mL of acetic acid was added to a solution of poly(4-
vinylpyridine) (500 mg) in acetic acid. The mixture was
◦
heated to 120 – 140 C for 2 h. The solvent was then removed,
Fig. 2. Change of the pH of monomeric and polymeric mate- and water was added. Evaporation of the solvent gave a red
rials on irradiation in reversible photocatalytic electron trans-
fers.
polymer (740 mg) which was dissolved in methanol. This
solution was added to THF in which the product precipi-
◦
1
tated. M. p. 200 – 210 C (decomp.). – H NMR (200 MHz,
pensions during irradiation in the presence of air, and
our results are presented in Fig. 2. All compounds de-
scribed here – except for polymer 13 – obviously are
able to undergo these coupled photocatalytic electron
transfers and are therefore interesting candidates for
technical applications. The best result was achieved by
compound 6, followed by polymers 7 and 11 which
cause an increase of the pH value by 0.98 and 0.69,
respectively.
In summary, we present new materials possess-
ing partial structures of conjugated heterocyclic me-
someric betaines which are active in reversible photo-
catalytic electron transfer reactions.
D O): δ = 8.41 (bs), 7.44 (bs), 2.90 – 1.80 (m, 3H, polymer
2
chain) ppm. – IR (KBr): ν = 3419, 1686, 1635, 1588, 1545,
−1
1
366, 1230, 996, 821, 553 cm . – Found C 57.40, H 5.26,
N 6.11.
Polymer 8
2
,3-Dichloro-1,4-naphthoquinone (681 mg, 3 mmol) was
ꢀ
dissolved in 30 mL of hot acetic acid. Then the 4,4 -bipyrid-
inium-substituted Merrifield resin [14] (300 mg) was added.
The suspension was heated at 100 C for 5 h. After addition
◦
of 50 mL of water the solid was filtered off and washed with
hot toluene to give a reddish brown polymer (254 mg). – IR
(
8
KBr): ν = 3424, 1685, 1634, 1589, 1542, 1361, 1265, 1215,
−1
06, 698 cm . – Found C 67.43, H 5.22, N 4.74.
Experimental Section
3
,6-Dibromo-2,5-di(N-pyridinio)-1,4-benzoquinone
The H and 13C NMR spectra were recorded on Bruker
ARX-400 and DPX-200 spectrometers. Multiplicities are
described by using the following abbreviations: s = sin-
1
dibromide (10) [18]
Tetrabromo-1,4-benzoquinone (840 mg, 2 mmol) was
glet, d = doublet, m = multiplet, b = broad. FT-IR spec- suspended in 30 mL of anhyd. ethanol under a nitrogen atmo-
tra were obtained on a Bruker Vektor 22 in the range sphere. After addition of anhyd. pyridine (0.5 mL, 6 mmol)
−1
of 400 to 4000 cm
(2.5 % pellets in KBr). A TQ-150 the mixture was heated at reflux temperature for 6 h. Evapo-
medium-pressure 150 W mercury lamp (UV-Consulting ration of the solvent gave a reddish brown solid which proved
Peschl, Mainz, Germany) in a pyrex photoreactor was to be sensitive towards moisture so that the product could
1
used for the irradiation experiments. Poly(4-vinyl-pyridine) not be characterized completely. – H NMR (200 MHz,
[
[
CAS 25232-41-1] was purchased from Sigma Aldrich, [D ]DMSO): δ = 8.93 (d, J = 5.0 Hz, 2H, α-H), 8.58 (t,
6
◦
Tg =137 C; MW = 60000], and Merrifield resin crosslinked J = 7.8 Hz, 1H, γ-H), 8.06 (dd, J = 7.8 Hz, J = 6.5 Hz, 2H,
with 2 % DVB, 200 – 400 mesh, from Fluka.
β-H) ppm. – IR (KBr): ν = 3061, 1611, 1562, 1474, 1162,
−1
753, 679, 609 cm
.
2-(1-Pyridinio)-1,4-naphthoquinon-3-olate (6)
2,5-Di(N-pyridinio)-1,4-benzoquinone-3,6-diolate (11) [17]
2,3-Dichloro-1,4-napthoquinone (1.14 g, 5 mmol) was
dissolved in 50 mL of hot acetic acid. Then pyridine
Tetrabromo-1,4-benzoquinone (840 mg, 2 mmol) was dis-
(
0.40 mL, 5 mmol) was added. The mixture was heated solved in 45 mL of hot acetonitrile. Then pyridine (0.65 mL,
◦
to 120 – 130 C for 5 h. After the addition of 200 mL 8 mmol) was added. The mixture was heated at reflux tem-
of water the resulting solution was extracted with diethyl perature for 2 h and hydrolyzed with 40 mL of water.
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A. Schmidt – M. Albrecht · Photocatalytically Active Materials with Pyridinium-enolate Partial Structures
Acetonitrile was evaporated, and the resulting solution was Polymer 13
471
cooled in an ice bath until a precipitate was formed. The
ꢀ
A solution of 4,4 -bipyridine (3.90 g, 25 mmol) in 25 mL
of acetonitrile was added to a boiling solution of tetra-
chloro-1,4-benzoquinone (1.53 g, 6.25 mmol) in acetoni-
trile. The mixture was refluxed for 2 h and then hydrolyzed
with 100 mL of water. After cooling the reddish brown pre-
cipitate was filtered off and washed with water. Recrystal-
lization from water / formic acid and washing with water
and hot toluene gave 920 mg of a dark brown solid. M. p.
solid was filtered off and washed with water. Recrystalliza-
tion from acetic acid gave 241 mg (41 %) of an orange
◦
1
solid. M. p. > 355 C. – H NMR (200 MHz, [D ]DMSO):
6
δ = 8.77 (dd, J = 6.7 Hz, J = 1.3 Hz, 2H, α-H), 8.54 –
8
.46 (m, 1H, γ-H), 8.10 (t, J = 6.7 Hz, 2H, β-H) ppm. –
IR (KBr): ν = 3511, 3459, 3114, 1557, 1467, 1346, 1140,
−1
8
53, 779, 685, 622 cm . – C H N O · H O (312.07):
16
10
2
4
2
calcd. C 61.54, H 3.87, N 8.97; found C 61.01, H 3.46,
N 8.88.
◦
340 C. – H NMR (200 MHz, [D ]DMSO): δ = 8.97 (m,
6
1
>
4H, overlapping signals), 8.62 (d, J = 6.4 Hz, 2H), 8.17 (d,
J = 4.9 Hz, 2H) ppm. – IR (KBr): ν = 3427, 1698, 1561,
Polymer 12
−1
1
337, 814, 735 cm . – Found C 55.19, H 2.58, N 7.96.
To a suspension of poly(4-vinylpyridine) (310 mg)
in 30 mL of acetonitrile was added a solution of tetra-
bromo-1,4-benzoquinone (500 mg, 1.2 mmol) in 15 mL of
Coupled photocatalytic electron transfer
100 mg of the polymers and their model compounds,
acetonitrile. After stirring at reflux temperature for 3 h the respectively, were dissolved or suspended in a solution
solvent was removed, and 30 mL of water were added. of 200 mg of EDTA and 3 mg of proflavine hemisulphate
The solvent was removed, and the resulting precipitate was dihydrate in 10 mL of water. The mixture was then irradiated
refluxed with acetone for 30 min. Filtration and washing with a UV lamp. During the irradiation aliquots were taken
with acetone gave 273 mg of a dark brown polymer. M. p. from the solution, and the pH value of these samples was de-
◦
1
1
85 C (decomp.). – IR (KBr): ν = 3424, 1635, 1601, termined with a pH meter.
−1
556, 1454, 1417, 1220, 1068, 1000, 824 cm . – Found
Acknowledgement
Petra Dr o¨ ttboom is gratefully acknowledged for perform-
ing the TGA experiments.
C 63.23, H 5.27, N 10.45. NMR spectra could not be mea-
sured because of the insolubility of the product in organic
solvents.
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