Synthesis of pyridinium - Indan-1,3-dion-2-ide betaine dyes and use as indicators of carboxylic acids' polarity

Article abstract of DOI:10.3184/030823408X324698

2-[4-(2,4,6-Triarylpyridinium-1-yl)phenyl]indan-1,3-dion-2-ides 5a-d, (N⁺C^-) betaine dyes, were synthesised by reaction of 2,4,6-triarylpyrylium perchlorates 6a-d with 2-(4-aminophenyl)indan-1,3-dione 7. The solvatochromic effect of

Full text of DOI:10.3184/030823408X324698

340 RESEARCH PAPER
JUNE, 340–343
JOURNAL OF CHEMICAL RESEARCH 2008
Synthesis of pyridinium – indan-1,3-dion-2-ide betaine dyes and use as
indicators of carboxylic acids' polarity
Piotr Milart*
Faculty of Chemistry, Jagiellonian University, Ingardena 3, 30-060 Kraków, Poland
2-[4-(2,4,6-Triarylpyridinium-1-yl)phenyl]indan-1,3-dion-2-ides 5a–d, (N+C–) betaine dyes, were synthesised by
reaction of 2,4,6-triarylpyrylium perchlorates 6a–d with 2-(4-aminophenyl)indan-1,3-dione 7. The solvatochromic
effect of compounds 5 was investigated. The betaines obtained are suitable for polarity determination of carboxylic
acids and their anhydrides.
Keywords: betaines, solvent effects, dyes, solvatochromism, pyridinium salts
Pyridinium N-phenolate dyes show very large negative
solvatochromic effects. For instance, the most intensively
investigated and commercially available betaine 5'-(2,4,6-
triphenylpyridinium-1-yl)-m-terphenyl-2'-olate (Reichardt's
dye, 1) (Scheme 1) is used as a solvent polarity indicator
(ET(30) scale).1-3 Many structural modifications of 1 have
been proposed to improve its properties including solubility in
aliphatic hydrocarbons 4,5 or in water.6,7 Thus the dye 1 and its
derivatives are almost perfect polarity indicators for different
kind of solvents but not for carboxylic acids. Addition of a
trace of an acid to a solution of 1 changes the colour to pale
yellow due to protonation of the phenolic oxygen atom of
the dye. The protonated form (pyridinium salt) does not
exhibit the solvatochromic absorption band. The same
disadvantage was observed for thiophenolate 28 and aminide
39 betaine dyes.
prepare materials for non-linear optics (NLO) experiments
and host–guest polymer films.12
CH-acids are often relative strong, much stronger than
carboxylic acids.13 For example, the value of pKa for 2-(4-
nitrophenyl)indan-1,3-dione was found as 2.31 in 50% ethanol
and the appropriate value for 2-(4-trimethylammonium-
phenyl)indan-1,3-dione is 2.84.14 A pKa value in the same
range should be observed for the conjugated acid of 5a,
therefore betaine dye 5a should be usable in most carboxylic
acid solutions. The aim of the present paper is to reinvestigate
the preparation of the compound 5a and to examine its
solvatochromic properties.
We found that the best method for synthesis of 5a is a
condensation of 2,4,6-triphenylpyrylium perchlorate 6a with
2-(4-aminophenyl)indan-1,3-dione 7.Amine 7 can be prepared
by several methods. In our laboratory, the best and most
inexpensive method was a two-step procedure: the first step
was the reaction of p-aminophenylacetic acid with phthalic
anhydride leading to 2-(4-acetaminophenyl)indan-1,3-dione15
and the second one was the hydrolysis of this amide.16 Amine
7 reacts readily with pyrylium salt 6a in ethanol solution in
the presence of sodium acetate. The red betaine dye 5a was
isolated by filtration in an almost pure form. The dye 5a is
sparingly soluble in common organic solvents but in most
cases the solubility is too low for solvatochromic experiments.
We prepared three new betaines 5b–d substituted by methyl
groups in pyridinium moiety which are much more soluble in
various solvents (Scheme 3). Indicators 5 are unfortunately
insoluble in water and in aliphatic hydrocarbons.
Gompper et al.10 synthesised the betaine dye 4 which has
the negative charge at a carbon atom (Scheme 2). Proton
removal from the corresponding CH-acid of 4 is only possible
because of the presence of two electron-withdrawing cyano
groups. Unfortunately, no detailed synthetic procedure and
solvatochromic properties for this compound are available.
Another N+C– betaine dye 5a was described by Utinans and
Neilands.11 This time the negative charge was generated at the
C2 atom of the indan-1,3-dione moiety. The authors proposed
two potential ways for preparation of 5a but no synthetic and
spectroscopic details were given. The only data mentioned
in this paper are lmax in methanol (470 nm) and chloroform
(630 nm). Some modifications of 5a were also undertaken to
N
N
S
N
.
.
.
.
.
.
.
.
.
.
O
N
O
S
1
2
3
O
H3C
Scheme 1 Molecular structure of the betaine dyes 1–3.
* Correspondent. E-mail: milart@chemia.uj.edu.pl

JOURNAL OF CHEMICAL RESEARCH 2008 341
64
62
60
58
56
54
52
50
48
46
44
42
1
5
2
11
6
7
8
10
12
3
9
4
13
14
N
N
15
16
17
40
O
19
O
18
38
CN
NC
4
5a
36
42
44
46
48
50
52
54
56
ET(30) / kcal mol–1
Scheme 2 Molecular structure of the betaine dyes 4 and 5a.
Fig. 1 Linear correlations between the ET(30) values of
standard betaine dye 1 and the ET(5d) values of betaine dye 5d
measured in nine HBD solvents ( , Eqn (2)) and 10 non-HBD
solvents ( , Eqn (3)). Solvents numbering as in Table 1.
The solutions of 5 in acetic, propionic, butyric and valeric
acids were still coloured. After addition of hydrochloric acid
the red tint disappeared. Chloroacetic, trichloroacetic and
formic acids protonated betaine dye 5 just as hydrochloric
acid. A blue solution of 5 in acetone did not bleach after
addition of a variety of benzoic acids, cinnamic acid and
iodoacetic acid. It is concluded that betaine dyes 5 can be used
as solvent polarity indicators for carboxylic acids of moderate
strength and their anhydrides.
smaller than that for the regression line for the non-HBD
solvents. The very weak C–H hydrogen-bond donors CHCl3,
CH2Cl2 and CH3CN are considered as non-HBD solvents.7
The “HBD-line” can be described by Eqn (2) and the “non-
HBD-line” by Eqn (3).
ET(5d) [kcal mol–1] = 0.571 ∙ ET(30) + 29.83
(2)
For the quantitative description of the solvatochromic effect
of betaine 5d, Visible absorption spectra were measured
in 19 HBD (hydrogen-bond donor) and non-HBD solvents
of different polarities. The observed nmax [cm–1] were
recalculated to ET(5d) [kcal mol–1] according to Eqn (1).
(n = 9 HBD solvents, r = 0.929, σ = 0.594 [kcal mol–1])
ET(5d) [kcal mol–1] = 1.680 ∙ ET(30) – 17.57
(3)
(n = 10 non-HBD solvents, r = 0.976, σ = 1.28 [kcal mol–1])
ET(5d) [kcal mol–1] = 2.859 ¥ 10-3 · nmax [cm–1]
(1)
The specific solvation of betaine dye 5d in HBD solvents
leads to a smaller susceptibility (slope 0.571). The same
effect was observed for other betaine dyes possessing
hydrogen-bond-accepting sites.6,7 In the case of compounds 5,
a negative charge is delocalised on five atoms including
The results obtained are given in Table 1 and illustrated in
Fig. 1.
Figure 1 reveals that the strong HBD solvents (alcohols)
follow a regression line (ET(5d) vs ET(30)) with a slope much
R
R
CH3COONa, C2H5OH
- H2O, - NaClO4, - CH3COOH
O
N
ClO4
R'
R'
R'
R'
6a-d
+
NH2
O
O
5a-d
O
O
5a, 6a R = H, R' = H
5b, 6b R = CH3, R' = H
5c, 6c R = H, R' = CH3
5d, 6d R = CH3, R' = CH3
7
Scheme 3 Synthesis of betaine dyes 5a–d.

342 JOURNAL OF CHEMICAL RESEARCH 2008
Table 1 Solvent dependent charge transfer (CT) absorption maxima lmax and the corresponding ET(5d) values of betaine dye 5d
measured in 19 solvents ordered according to decreasing ET(30) values
No
Solvent
ET(30)/kcal mol–1
lmax/nm
n
max/cm–1
ET(5d)/kcal mol–1
HBD solvents
1
2
3
4
5
6
7
8
9
Methanol
55.4
55.1
54.1
52.0
51.9
50.7
50.4
49.7
48.4
460
467
476
482
478
485
496
487
495
21739
21413
21008
20747
20921
20619
20161
20534
20202
62.2
61.2
60.1
59.3
59.8
58.9
57.6
58.7
57.8
2-Chloroethanol
2,2,2-Trichloroethanol
2-Methoxyethanol
Ethanol
Propan-1-ol
Benzyl alcohol
Butan-1-ol
Propan-2-ol
Non-HBD solvents
10
11
12
13
14
15
16
17
18
19
Nitromethane
Acetonitrile
DMSO
46.3
45.5
45.1
43.2
42.2
41.3
40.7
39.1
37.4
37.1
489
484
484
502
528
554
576
615
627
628
20450
20661
20661
19920
18939
18051
17361
16260
15949
15924
58.5
59.1
59.1
57.0
54.1
51.6
49.6
46.5
45.6
45.5
DMF
Acetone
1,2-Dichloroethane
Dichloromethane
Chloroform
THF
Anisole
two oxygen atoms. Partial negatively charged oxygen atoms
are very good sites for hydrogen bonding (Scheme 4).
The results of solvatochromic measurements for carboxylic
acids and anhydrides are presented in Table 2.
Ar
Ar
O
N
Ar
It was decided not to calculate ET(30) values for
the carboxylic acids using Eqn (2). We have no knowledge
as to whether the carboxylic acids are part of the same
family of the HBD solvents as the alcohols or whether they
represent a separate family of compounds. In other words, one
cannot be sure that Eqn (2) should hold for carboxylic
acids. From the values of ET(5d) it appears that acetic acid
is slightly more polar than methanol. Other popular polarity
scales (Grunwald–Winstein's Y scale and Kosower's Z scale)
place methanol as more polar than acetic acid.1 This is further
evidence for a very complicated nature of solute–solvent
interactions. The ET(30) values for anhydrides, which are
always considered as non-HBD solvents, are estimated
according to Eqn (3).
O
H
H
O
R
O
R
Scheme 4 Specific solvation of betaine dye 5 by HBD solvents.
Experimental
Melting points were determined with a Mel-Temp II apparatus
in open capillaries and are uncorrected. Elemental analyses were
carried out with a EuroVector EA 3000 analyser. IR spectra were
recorded using a Bruker Equinox 55 spectrometer as KBr pellets.
1H NMR and 13C NMR spectra were taken at 500.13 MHz and
125.76 MHz respectively with a Bruker AMX 500 spectrometer in
CDCl3 with TMS as internal standard. UV-Vis spectra were recorded
on Helios β (Unicam) spectrophotometer with 0.5, 1.0, and 2.0 cm
quartz cells. Solvents for the solvatochromic measurements were
dried and purified according to literature procedures.17 Pyrylium
salts were prepared by previously described methods: 6a,18 6b,19 6c20
and 6d.21
The last column of Table 2 gives available literature data
for ET(30). These values are secondary values calculated from
Kosower's Z scale by means of the correlation equation given
in a footnote to Table 2.1,4
Table 3 presents the last set of our solvatochromic
experiments results. Visible spectra of all the obtained dyes
5a–d were measured in four solvents of different polarity.
It is evident that methyl substituents at the phenyl rings of the
pyridinium moiety do not significantly influence the CT band
maximum.
Table 2 CT absorption maxima lmax, the corresponding ET(5d) and ET(30) values for carboxylic acids and anhydrides
Solvent
l
max/nm
n
max/cm–1
ET(5d)/kcal mol–1
ET(30)a/kcal mol–1 ET(30)b/kcal mol–1
Acetic acid
Propionic acid
Butyric acid
Valeric acid
Acetic anhydride
Propionic anhydride
454
481
483
485
466
491
22075
20790
20704
20618
21459
20367
63.1
59.4
59.2
58.9
61.4
58.2
–
–
51.7
50.5
–
–
–
47.0
45.1
–
43.9
–
a Calculated by means of the converted Eqn. (3) for anhydrides (non-HBD solvents):
[ ET(30) = 0.595 · ET(5d) + 10.46 ]
bLiterature data1,4, calculated from Kosower's Z values according to the following equation:
[ET(30) = 0.752 · Z – 7.87; r = 0.998 for 15 solvents]

JOURNAL OF CHEMICAL RESEARCH 2008 343
Table 3 Solvent dependent CT absorption maxima lmax of betaine dyes 5a–d measured in four solvents of different polarity
Compound
Methanol lmax/nm
Acetic acid lmax /nm
Nitromethane lmax/nm
DMF lmax/nm
5a
5b
5c
5d
461
461
462
460
460
457
459
454
494
491
492
489
505
503
503
502
pyridinium and indandionide moieties), 7.05 and 7.12 (2d, J = 8.5 Hz,
8H, AA'BB' system, aromatic H of the 2- and 6-aryl rings), 7.16 and
7.30 (two dd, J1 = 5 Hz, J2 = 3 Hz, 4H, AA'BB' system, aromatic H
of indandionide moiety), 7.40 and 7.74 (2d, J = 9 Hz, 4H, AA'BB'
system, aromatic H of the 4-aryl ring), 7.98 (s, 2H, aromatic H of
the pyridinium ring). 13C NMR (CDCl3) d [ppm] = 21.3 (CH3), 21.6
(CH3), 102.2 (C2), 117.8, 123.5, 124.8, 126.3, 127.7, 129.1, 129.6,
129.7, 129.9, 130.8, 140.7, 141.3, 143.8, 155.4, 157.7 (aromatic C),
191.9 (C=O). C41H31NO2 (569.7): calcd. C 86.4, H 5.5, N 2.5; found
C 86.5, H 5.4, N 2.5%.
CAution: Pyrylium salts are harmful by inhalation,
in contact with skin (can act as photosensitisers) and if
swallowed. Heating of dry salts may cause an explosion.
Synthesis of betaine dyes (5a–d) A suspension of pyrylium salt 6a–d
(1.0 mmol), 2-(4-aminophenyl)indan-1,3-dione 7 (0.3 g, 1.25 mmol)
and sodium acetate (0.25 g, 3.0 mmol) in ethanol (15 ml) was stirred
and refluxed for 2 h. The red precipitate started to separate in a few
minutes (except for 5d). The mixture was stirred for 2 h at room
temperature and then kept overnight in a refrigerator. The solid was
filtered off, washed with cold ethanol and diethyl ether. Compounds
5c and 5d were recrystallised from ethanol. Betaine dyes 5a and 5b
(sparingly soluble) were suspended in ethanol (15 ml) and refluxed
with stirring for 30 min. Separated products were vacuum dried
(ca 1 mmHg) at 50–60°C.
We are grateful to Dr Sebastian Leśniewski from the
Department of Physical Chemistry, Jagiellonian University
for help with the visible absorption spectra measurements.
2-[4-(2,4,6-Triphenylpyridinium-1-yl)phenyl]indan-1,3-dion-2-
ide (5a): Yield 0.40 g (76.9%). m.p. 386–387°C. IR (KBr) n = 1621,
1601, 1584 1547, 1510, 1411 cm–1. 1H NMR (CDCl3) d [ppm] = 6.68
and 8.69 (2d, J = 8 Hz, 4H, AA'BB' system, aromatic H of the ring
between pyridinium and indandionide moieties), 7.23–7.39 (m, 14H,
aromatic H of the 2- and 6-aryl rings and aromatic H of indandionide
moiety), 7.55–7.65 (m, 3H, aromatic m- and p-H of the 4-aryl ring),
7.87 (d, J = 8.5 Hz, 2H, aromatic o-H of the 4-aryl ring), 8.11 (s,
2H, aromatic H of the pyridinium ring). C38H25NO2 (527.7): calcd. C
86.5, H 4.8, N 2.7; found C 86.5, H 4.7, N 2.7%.
2-{4-[4-(4-Methylphenyl)-2,6-diphenylpyridinium-1-yl]phenyl}
indan-1,3-dion-2-ide (5b): Yield 0.48 g (75.0%). m.p. >400°C. IR
(KBr) n = 1617, 1601, 1549, 1509, 1413 cm–1. 1H NMR (CDCl3)
d [ppm] = 2.50 (s, 3H, CH3), 6.66 and 8.68 (2d, J = 8 Hz, 4H, AA'BB'
system, aromatic H of the ring between pyridinium and indandionide
moieties), 7.20–7.50 (m, 16H, aromatic H of the 2- and 6-aryl rings,
aromatic H of indandionide moiety and aromatic m-H of the 4-aryl
ring), 7.79 (d, J = 8.5 Hz, 2H, aromatic o-H of the 4-aryl ring), 8.09
(s, 2H, aromatic H of the pyridinium ring). C39H27NO2 (541.7): calcd.
C 86.5, H 5.0, N 2.6; found C 86.45, H 5.0, N 2.7%.
2-{4-[2,6-Bis(4-methylphenyl)-4-phenylpyridinium-1-yl]
phenyl}indan-1,3-dion-2-ide (5c): Yield 0.36 g (66.0%). m.p. 358–
359°C. IR (KBr) n = 1618, 1606, 1585, 1544, 1510, 1415 cm–1.
1H NMR (CDCl3) d [ppm] = 2.29 (s, 6H, CH3), 6.71 and 8.70 (2d,
J = 8 Hz, 4H, AA'BB' system, aromatic H of the ring between
pyridinium and indandionide moieties), 7.08 and 7.13 (2d, J = 8.5 Hz,
8H, AA'BB' system, aromatic H of the 2- and 6-aryl rings), 7.19 and
7.32 (two dd, J1 = 5 Hz, J2 = 3 Hz, 4H, AA'BB' system, aromatic H of
indandionide moiety), 7.55–7.65 (m, 3H, aromatic m- and p-H of the
4-aryl ring), 7.84 (d, J = 8.5 Hz, 2H, aromatic o-H of the 4-aryl ring),
8.03 (s, 2H, aromatic H of the pyridinium ring). C40H29NO2 (555.7):
calcd. C 86.5, H 5.3, N 2.5; found C 86.6, H 5.3, N 2.6%.
2-{4-[2,4,6-Tris(4-methylphenyl)pyridinium-1-yl]phenyl}indan-
1,3-dion-2-ide (5d): Yield 0.45 g (78.9%). m.p. 364–365°C. IR (KBr)
n = 1622, 1606, 1584, 1552, 1509, 1412 cm–1. 1H NMR (CDCl3)
d [ppm] = 2.27 (s, 6H, CH3), 2.48 (s, 3H, CH3), 6.71 and 8.68
(2d, J = 8 Hz, 4H, AA'BB' system, aromatic H of the ring between
Received 7 February 2008; accepted 28 April 2008
Paper 08/5090 doi: 10.3184/030823408X324698
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