Anion Nature Influence on Spectral and Some Physico-Chemical Properties of 6,7-Dihydroxy-4-methyl-2-phenylchromenylium Salts

Article abstract of DOI:10.1134/S1070363220040064

Abstract: The influence of the nature of anions (Cl^–, Br^–, I^–, ClO₄^–, and PF₆^–) on the physico-chemical properties (solubility, solvatochromism, and spectral characteristics) of

Full text of DOI:10.1134/S1070363220040064

ISSN 1070-3632, Russian Journal of General Chemistry, 2020, Vol. 90, No. 4, pp. 597–601. © Pleiades Publishing, Ltd., 2020.
Russian Text © The Author(s), 2020, published in Zhurnal Obshchei Khimii, 2020, Vol. 90, No. 4, pp. 530–535.
Anion Nature Influence on Spectral and Some Physico-Chemical
Properties of 6,7-Dihydroxy-4-methyl-2-phenylchromenylium
Salts
D. A. Barbalata, A. N. Chebotareva, and D. V. Snigura,*
a I.I. Mechnikov Odessa National University, Odessa, 65082 Ukraine
*e-mail: 270892denis@gmail.com
Received September 17, 2019; revised September 17, 2019; accepted September 20, 2019
Abstract—The influence of the nature of anions (Cl–, Br–, I–, ClO4–, and PF6–) on the physico-chemical properties
(solubility, solvatochromism, and spectral characteristics) of 6,7-dihydroxy-4-methyl-2-phenylchromenylium salts
was studied. The main spectrophotometric characteristics were determined. It was noted that the molar absorption
coefficient increases on passing to polar solvents. The highest molar absorption coefficient was observed for the
hexafluorophosphate anion, which has weak coordinating power that favors more efficient solvation of the organic
cation.
Keywords: 6,7-dihydroxy-4-methyl-2-phenylchromenylium, spectrophotometry, IR spectroscopy, solubility,
solvatochromism
DOI: 10.1134/S1070363220040064
Organic analytical reagents represent a large class of
synthetic compounds that are used to identify, separate,
and concentrate both organic compounds and inorganic
ions. An important aspect of studying organic reagents is
the determination of their physico-chemical and acid-core
properties and spectral characteristics [1]. Among them,
spectrophotometric, fluorescent, and solvatochromic
properties occupy a special position. These latter are
related to the interaction between solvent molecules
differing in dipole moments and chromophoric groups
of dissolved substances [2]. In addition, in solvents of
various natures differing in physico-chemical properties,
spectrophotometric characteristics of equilibrium forms
of a dye can be caused not only by solvatochromism, but
also by tautomerism [3–5].
The aim of this work is to study the effect of the
nature of the anions Cl–, Br–, I–, ClO4–, and PF6– on the
spectral characteristics, solubility, and solvatochromism
of 6,7-dihydroxy-4-methyl-2-phenylchromenylium salts.
One of essential parameters to be taken into account
in rational choosing a reagent for direct or extraction-
(sorption) spectrophotometric determination of metal
ions is the solubility of this reagent and the corresponding
complex compound. The results of HPLC determination
of the solubility of the compounds under study are
presented in Table 1. These data show that among
the studied set of solvents, methanol dissolves best
6,7-dihydroxy-4-methyl-2-phenylchromenylium salts
and water, worst. It should be noted that the solubilities
in water, methanol, and ethanol (96%) decrease almost
linearly (R2 = 0.95, 0.94, 0.85) for the series of anions
Cl–, Br–, I–, ClO4–, and PF6– with increase in the anion radii
(r = 181, 196, 220, 240, 242 pm, respectively) [11, 12].
In turn, derivatives of oxygen-containing heterocyclic
compounds, ortho-dioxychromenoles, are of considerable
interest.Among these latter, the most studied are complex-
forming properties of 6,7-dihydroxychromenylium
derivatives in relation to polyvalent metal ions, and
techniques for their direct or extraction-spectrophotometric
determination are developed [6, 7]. It should be noted that
the publications on the study of these compounds are few
in number and are dedicated to their acid-base properties
[8] or special features of their synthesis [9, 10].
It was interesting to study the influence of the
anion nature on the spectral properties of dihydroxy-
chromenyliums. Thus, in the IR spectra there are a
widened band of OH-groups stretching vibrations at
3270–3460 cm–1 and a band of C-H stretching vibrations
at 2900–3080 cm–1. There are also characteristic bands
597

598
BARBALAT et al.
Table 1. Solubility of 6,7-dihydroxy-4-methyl-2-phenylchromenylium salts in certain solvents
Solubility, mmol/L
Anion
water
0.55
0.50
0.16
0.10
0.07
methanol
180.31
179.28
53.13
ethanol
13.53
7.69
acetonitrile
1.21
Chloride
Bromide
Iodide
0.92
2.21
12.65
9.33
5.45
Perchlorate
Hexafluorophosphate
33.01
14.75
4.84
3.62
Table 2. Spectral characteristics of solutions of 6,7-dihydroxy-4-methyl-2-phenylchromenylium salts in various solvents
Cl– Br– I– ClO4– PF6–
Solvent
λ,
nm
λ,
nm
λ,
nm
λ,
nm
λ,
nm
ε×10–4 ε×10–4 ε×10–4 ε×10–4
ε×10–4
Polar
protic
Water
Methanol
Ethanol
Isopropanol
n-Propanol
n-Butanol
Isoamyl alcohol
THF
Dioxane
Acetonitrile
Acetone
Ethyl acetate
Butyl acetate
460
470
470
444
473
472
473
460
462
466
460
461
460
2.32
2.31
2.14
2.66
2.13
2.02
2.08
1.74
1.69
1.79
1.81
1.80
1.73
459
462
462
455
471
469
471
446
462
464
455
456
455
2.63
2.55
2.33
2.35
2.48
2.39
2.37
2.34
2.31
2.31
2.34
2.19
2.23
460
454
466
457
470
471
471
456
464
454
442
451
458
2.62
2.42
2.36
2.39
2.39
2.38
2.38
1.99
2.16
2.14
2.13
2.11
1.97
459
469
464
455
469
469
471
438
459
465
448
451
454
2.24
2.09
2.04
1.96
2.07
2.09
2.02
1.79
1.54
1.74
1.86
1.66
1.63
459
461
471
440
471
470
466
447
458
461
452
458
459
3.26
2.90
2.90
3.93
3.06
2.98
2.88
2.63
2.58
2.58
2.56
2.60
2.44
Polar
aprotic
at 1623–630 cm–1 (pronounced in the IR spectrum) and
1596–1602 cm–1 (intense in the IR spectrum), which
correspond to the vibrations of the benzopyrylium C-O
and C–S bonds [13, 14]. There is also a characteristic
band of skeletal vibrations of the benzene ring about
1000 cm–1, of medium intensity in the IR spectrum and of
low in intensity in the IR spectrum. It is interesting to note
that the effect of anions on the intensity of the C–O and
C–C benzopyrylium cycle bands is noticeable in the IR
spectrum: in passing from Cl– to PF6– the intensity of the
band at 1623–1630 cm–1 decreases relative to the band at
1596–1602 cm–1 as the anion radius increases. This effect
may be associated with a change in the charge localization
in the pyrylium cycle, and hence with the change in the
polarity of the benzopyrylium cation, which in turn affects
solubilities of the corresponding salts.
calculations we constructed the dependences of the molar
light absorption logarithm on the Dimrot–Reichardt
empirical parameter of solvents polarity ET(30) [15].
Molar absorption coefficients and positions of absorption
maxima for all studied salts are summarized in Table 2.
Examination of these results allow us to conclude that the
logarithms of molar absorption coefficients in solvents of
different natures almost linearly increase with the increase
in their polarity parameter ET(30) for all the salts under
study (R2 = 0.80–0.90).
According to the data of Table 2, it can be concluded
that in most cases a bathochromic shift of 2–15 nm, as
compared to the aqueous solution, is observed in the
alcohols solutions, but in the isopropanol solutions
a hypsochromic shift of 5–20 nm was recorded. The
anion nature does not significantly affect the position
of the absorption maximum, and the corresponding
Δλ values do not exceed 14 nm in passing from one
6,7-dihydroxy-4-methyl-2-phenylchromenylium salt to
another. The molar absorption coefficient is greater in
polar protic solvents (water and alcohols) than in aprotic
solvents. Among the anions, hexafluorophosphate of
The study of the electronic absorption spectra
of 6,7-dihydroxy-4-methyl-2-phenylchromenylium
salts has shown that the position and intensity of the
absorption bands are affected not only by the solvent
nature (solvatochromism), but also by the counter-anion
nature. On the basis of the experimental results and related
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ANION NATURE INFLUENCE ON SPECTRAL AND SOME PHYSICO-CHEMICAL PROPERTIES
599
6,7-dihydroxy-4-methyl-2-phenylchromenylium has the
highest absorption coefficient, which is probably caused
by the weak coordinating effect of this anion that favors
a more efficient solvation of the 6,7-dihydroxy-4-methyl-
2-phenylchromenyl cation.
400–4000 cm–1 with a baseline correction using an FT-IR-
8400S (Shimadzu) IR spectrometer and a Quest Specac
attachment. The Raman spectra were obtained on a DXR
Raman Microscope Thermo Sientific instrument with a
780 nm laser in the working interval of 100–3500 cm–1
with the baseline correction and fluorescence suppression.
We were not able to detect Raman spectra for the
hexafluorophosphate and chloride, because fluorescence
overlaps with valid signals in their spectra. The 1Н and
13С NMR spectra were obtained on a Bruker Avance III
(400 MHz) spectrometer in DMSO-d6.
Thus, the effect of the anion nature on the physical
and chemical properties of 6,7-dihydroxy-4-methyl-2-
phenylchromenylium salts has been studied. It has been
found that thermochemical radii of anions correlate
with solubilities of the salts in polar protic solvents and
also with characteristic bands in vibrational spectra.
The influence of solvation by protic and aprotic polar
solvents on the position of light absorption maxima of
6,7-dihydroxy-4-methyl-2-phenylchromenylium salts
has been pointed out. Dependences of the molar light
absorption logarithms of the 6,7-dihydroxy-4-methyl-2-
phenylchromenylium salts on the empirical parameters
of solvents polarity ET(30) have been established.
Solubility of 6,7-dihydroxy-4-methyl-2-phenyl-
chromenylium salts was studied by the HPLC method
with a UV detection at λ = 260 nm. Deionized
water, methanol, ethanol (96%), and acetonitrile were
taken as solvents. To construct the calibration graph,
6,7-dihydroxy-4-methyl-2-phenylchromenylium chloride
solutions in acetonitrile with concentrations of 0.2,
0.4, 0.6, 1.0, 2.0, 3.0, 6.0, 9.0, and 12.0 mmol/L were
prepared. The injection volume was 1 μL. Along this
range, the dependence of the 6,7-dihydroxy-4-methyl-
2-phenylchromenylium concentration on the peak area
in the chromatogram is linear with R2 = 0.999. In each
of five 5 mL flasks, 0.1 g of 6,7-dihydroxy-4-methyl-
2-phenylchromenylium chloride, bromide, iodide,
perchlorate, or hexafluorophosfate were injected and 4.0–
4.5 mL of water were added. The mixture was stirred and
after complete dissolution 0.05 g more of a salt was added
in small portions to obtain an undissolved precipitate
and a saturated solution of a 6,7-dihydroxy-4-methyl-2-
phenylchromenylium salt above it. The resulting mixture
was left for 2 h up to reaching equilibrium, and then the
saturated solution was taken off with a syringe, filtered
through a syringe teflon membrane filter with a 2 μm pore
size, and chromatographed. The solubilities in methanol
and ethanol were studied similarly. The injections were
varied so that the signal falls into the middle of the
graduation graph. Further, according to the HPLC-UV
data, the molar concentrations of saturated solutions were
calculated, taking into account the injection volume.
EXPERIMENTAL
The salts (chloride, bromide, and perchlorate)
of 6,7-dihydroxy-4-methyl-2-phenylchromenylium
were obtained by condensation of pyrogallol A with
benzoylacetone in glacial acetic acid with the direct
introduction of the corresponding acid into the reaction
mass or by the exchange reaction of 6,7-dihydroxy-4-
methyl-2-phenylchromhen chloride with the formation of
a soluble salt.All reagents were of no less than analytical
grade.
Identification of the reaction products and their purity
determination were carried out by the HPLC method using
an Infinity 1260 chromatograph with a 6530 Accurate
Mass-TOF (Agilent Technologies, USA) mass detector
and a diode-matrix UV detector. The holding time of the
6,7-dihydroxy-4-methyl-2-phenylchromenylium cation,
regardless of anion, is 3.25±0.05 min. According to the
internal normalization, the purity of the synthesized
products is no less than 99.0%. In the mass-spectra
(ionization with a double electrospray at atmospheric
pressure in a positive field) there is a peak with m/z
253.20±0.05; calculated for the cation 6,7-dihydroxy-
4-methyl-2-phenylchromenylium cation C16H13O3+:
253.27 [M+]. Ionization in the negative field was used
to detect anions (except for Cl–, the m/z value of which
does not reach the working range 50–3000 Da of the
mass detector).
Solvatochromism was studied in the way similar to
the procedure described in the work [16]. In a glass-
stoppered test tube, 0.1 mL of 1×10–3 M methanol
solution of a corresponding 6,7-dihydroxy-4-methyl-2-
phenylchromenylium salt and 5 mLof an organic solvent
were injected, the mixture was stirred, and the absorption
spectrum was recorded on an SF-56 (LOMO-Spectr,
Russia) spectrophotometer in the wavelength range of
The IR spectra were recorded by the frustrated
total internal reflection (FTIR) method in the range of
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BARBALAT et al.
380–780 nm in quartz cells thermostatted at 25°C (l =
1 cm).
washed with several portions of water. Yield 50%. IR
spectrum, ν, cm–1: 3404.7. 3375.5 (О–Н), 3043.4 (C–H,
Ar), 2641.5 (C–H, Alk), 1624.3. 1595.6 (С–O, C–С,
Het), 1580.3, 1556.3, 1522.0, 1489.3, 1407.1, 1382.6,
1367.1, 1295.5, 1252.3, 1238.8, 1216.4, 1195.6, 1158.3,
1100.5, 1081.0, 1034.0, 997.4 (C–С, Ph), 895.9, 849.3,
780.4, 762.6, 727.6, 694.9, 677.8, 661.2, 622.0, 553.4,
525.0, 514.1, 470.7. IR spectrum, ν, cm–1: 1630.2, 1596.9
(С–O, C–С, Het), 1527.8, 1508.9, 1468.0, 1427.2, 1383.8,
1369.1, 1322.9, 1291.5, 1255.2, 1238.3, 1185.1, 1159.0,
1083.4, 1000.5 (C–С, Ph), 898.0, 764.2, 708.6, 679.1,
6.17.9, 529.1. Mass-spectrum (ESI–), m/z: 78.90, 80.90
[calculated for Br–: 78.92 (79Br), 80.92 (81Br)].
6,7-Dihydroxy-4-methyl-2-phenylbenzopyrylium
perchlorate. To a mixture of pyrogallol A (2.52 g,
10 mmol) and benzoylacetone (1.62 g, 10 mmol) in
10 mL of glacial acetic acid, 6 mL of 50% chloric acid
(40 mmol of HClO4) was added. The reaction mixture
was boiled for 30 min.After cooling to room temperature,
a light brown crystalline precipitate was filtered off on
a Schott filter under vacuum and washed with distilled
water (3×5 mL). Yield 70%. IR spectrum, ν, cm–1: 3426.5
(O–H), 3070.2 (C–H, Ar), 1626.0. 1602.2 (С–O, C–С,
Hеt), 1579.1, 1509.0, 1489.7, 1463.7, 1444.9, 1410.3,
1371.3, 1343.5, 1297.0, 1271.0, 1218.4, 1173.5, 1092.4.
1052.1 (Сl–O), 997.9 (C–С, Ph), 896.0, 868.9, 854.0,
780.8, 762.0, 738.5, 681.9, 663.4, 618.5, 517.6, 473.0.
IR spectrum, ν, cm–1: 1625.9. 1596.7(С–O, C–С, Hеt),
1532.0, 1508.5, 1466.2, 1425.7, 1379.5, 1342.9, 1240.9,
6,7-Dihydroxy-4-methyl-2-phenylbenzopyrylium
iodide. Chloride of 6,7-dihydroxy-4-methyl-2-
phenylchromenylium (2.9 g, 10 mmol) was dissolved in
60 mL of a boiling water–ethanol mixture (30 : 70), and
then 10 g (65 mmol) of sodium iodide was added. After
20 min a precipitation was observed. The mixture was
left in air for 24 h, and then the red-brown precipitate was
filtered off on a Schott filter under vacuum and washed
with several portions of water. Yield 60%. IR spectrum,
ν, cm–1: 3278.2 (O–H), 3038.6 (C–H,Ar), 1624.2. 1595.8
(С–O, C–С, Het), 1510.6, 1490.0, 1461.9, 1444.2, 1408.9,
1347.5, 1297.4, 1269.2, 1216.4, 1190.1, 1156.6, 1103.6,
1079.2, 1058.9, 1021.1, 996.2 (C–С, Ph), 895.7, 858.9,
782.5, 761.0, 736.7, 680.9, 662.3, 619.3, 516.1, 472.7.
IR spectrum, ν, cm–1: 1620.0, 1596.1 (С–O, C–С, Het),
1539.7, 1508.6, 1465.4, 1425.6, 1380.1, 1343.9, 1274.2,
1237.7, 999.6 (C–С, Ph), 897.1, 527.4. Mass-spectrum
(ESI–), m/z: 127.12 (calculated for I–: 126.90).
1
1194.9, 1168.8, 1001.5 (C–С, Ph), 898.2, 526.2. H
NMR spectrum (DMSO-d6), δ, ppm: 2.91 с (3H, CH3),
7.48–7.49 d (2H,Ar, J = 4.64 Hz), 7.70–7.79 m (3Н,Ar),
8.38–8.40 d (2Н, Hеt, J = 7.43 Hz), 8.47 с (1Н, Hеt).
NMR spectrum 13С (DMSO-d6), δС, ppm: 20.72 (CH3),
103.18, 107.11, 114.71, 120.82, 128.53, 129.88, 130.26,
134.84, 150.53, 154.65, 162.43, 164.48, 165.38. Mass-
–
spectrum (ESI–), m/z: 99.66 (calculated for СlO4 : 99.45).
6,7-Dihydroxy-4-methyl-2-phenylbenzopyrylium
chloride. A current of dry hydrogen chloride was passed
through a solution of pyrogallolA(2.52 g, 10 mmol) and
benzoylacetone (1.62 g, 10 mmol) in 10 mL of glacial
acetic acid at room temperature for 30–40 min, then 3 h
more with heating on a boiling water bath. The reaction
mass was poured into 100 mL of a water-ice mixture. A
light brown precipitate was filtered off on a Schott filter
under vacuum and washed with diisopropyl ether. Yield
55%. IR spectrum, ν, cm–1: 3355.9 (O–H), 2903.0 (C–H,
Ar), 2611.7 (C–H,Alk), 1625.0. 1597.2 (С–O, C–С, Het),
1578.5, 1554.6, 1525.1, 1490.8, 1410.4, 1368.3, 1294.8,
1253.8, 1214.9, 1214.9, 1201.4, 1201.4, 997.8 (C–С, Ph),
896.4, 850.4, 780.8, 763.1, 732.2, 705.6, 680.2, 660.4,
620.2, 554.8, 526.1, 502.9, 472.1.
6,7-Dihydroxy-4-methyl-2-phenylbenzopyrylium
hexafluorophosphate. Chloride of 6,7-dihydroxy-4-
methyl-2-phenylchromenylium (2.9 g, 10 mmol) was
dissolved in 60 mL of a boiling water-ethanol mixture
(30:70), and then ammonium hexafluorophosphate (7 g,
43 mmol) was added. After 20 min a precipitation was
observed. The mixture was left in air for 24 h, and then
the red precipitate was filtered off on a Schott filter under
vacuum and washed with several portions of water. Yield
70%. IR spectrum, ν, cm–1: 3457.1 (O–H), 3078.4 (C–H,
Ar), 2980.9 (C–H,Alk), 1629.3, 1596.4 (C–С, С–O, Het),
1574.7, 1531.3, 1505.5, 1490.6, 1403.6, 1371.1, 1342.8,
1256.1, 1227.1, 1204.8, 1103.5, 1079.3, 1027.3, 999.3
(C–С, Ph), 928.3, 877.3, 825.2 (P–F), 768.6, 702.1, 679.4,
658.2, 629.3, 554.0, 516.6, 471.4. Mass-spectrum (ESI–),
m/z: 144.93 (calculated for PF6–: 144.97).
6,7-Dihydroxy-4-methyl-2-phenylbenzopyrylium
bromide. To a solution of pyrogallolA(2.52 g, 10 mmol)
and benzoylacetone (1.62 g, 10 mmol) in 10 mLof glacial
acetic acid, 8 mL of 40% hydrobromic acid (60 mmol
of HBr) was added. The reaction mixture was boiled for
30 min, and then the light brown crystalline precipitate
was filtered off on a Schott filter under vacuum and
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ANION NATURE INFLUENCE ON SPECTRAL AND SOME PHYSICO-CHEMICAL PROPERTIES
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https://doi.org/10.3103/S0027131417040095
CONFLICT OF INTEREST
No conflict of interest was declared by the he authors.
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