Synthesis and spectral luminescence properties of 2-aryl-5-methyl-1,3,4-oxadiazoles and zinc(ii) 2-(2-hydroxyphenyl)-5-methyl-1,3,4-oxadiazole complex

Article abstract of DOI:10.1007/s11172-020-2741-7

2-Aryl-5-methyl-1,3,4-oxadiazoles were synthesized by reflux of equimolar amounts of acyl hydrazides with triethyl orthoacetate in o-xylene. The obtained oxadiazoles, except for 2-(2-hydroxyphenyl)-5-methyl-1,3,4-oxadiazole, luminesce with a high quantum

Full text of DOI:10.1007/s11172-020-2741-7

Russian Chemical Bulletin, International Edition, Vol. 69, No. 1, pp. 176—178, January, 2020
176
Synthesis and spectral luminescence properties
of 2-aryl-5-methyl-1,3,4-oxadiazoles and
zinc(II) 2-(2-hydroxyphenyl)-5-methyl-1,3,4-oxadiazole complex
I. E. Mikhailov,a,b Yu. M. Artyushkina,a G. A. Dushenko,a and V. I. Minkina
aInstitute of Physical and Organic Chemistry, Southern Federal University,
194/2 prosp. Stachki, 344090 Rostov-on-Don, Russian Federation.
E-mail: mikhail@ipoc.sfedu.ru
bFederal Research Center "Southern Scientific Center of the Russian Academy of Sciences,"
41 ul. Chekhova, 344006 Rostov-on-Don, Russian Federation
2-Aryl-5-methyl-1,3,4-oxadiazoles were synthesized by reflux of equimolar amounts of acyl
hydrazides with triethyl orthoacetate in o-xylene. The obtained oxadiazoles, except for
2-(2-hydroxyphenyl)-5-methyl-1,3,4-oxadiazole, luminesce with a high quantum yield in
polar and nonpolar solvents (λflmax = 300—339 nm, ϕ = 0.11—0.63). 2-(2-Hydroxyphenyl)-5-
methyl-1,3,4-oxadiazole emits intensely only in highly polar aprotic DMSO (λflmax = 346 nm,
φ = 0.15), whereas its chelate complex with ZnII luminesces in all solvents with a high quantum
yield in a longer-wavelength range of the visible spectrum (λflmax = 413—419 nm, φ = 0.36—0.55).
Key words: 2-aryl-5-methyl-1,3,4-oxadiazoles, metal complexes, luminescence, organic
and metallocomplex luminophores.
1,3,4-Oxadiazoles and related chelate metal complexes
ligand 3b were determined using elemental analysis, IR
1
are widely used for the preparation of the organic1,2 and
metallocomplex luminophores3—5 intensely emitting in
the short-wavelength range of the visible spectrum and are
applied as highly selective fluorescent chemosensors to
"heavy" metal cations.6
spectroscopy, and Н and 13С NMR spectroscopy. In
addition, the spectral luminescence properties of these
compounds were studied.
In the absorption spectra of oxadiazoles 3а—d, the
long-wavelength maximum caused by the electronic tran-
sitions S0 → S1 of the π→π*-type in the mutually conju-
gated 1,3,4-oxadiazole and aryl fragments lies in a range
of 272—316 nm, indicating that the compounds exist in
solutions as benzoid-type structures of the type of 3. The
presence of the electron-donor substituents in the aryl
core results in the shift of these bands to the long-wave-
length range by 8—44 nm, whereas the polarity of the
solvent exerts almost no effect on their position. The
formation of lived phototautomer was detected earlier7 by
fluorescence analysis for 2-(2-hydroxyphenyl)-5-methyl-
1,3,4-oxadiazole (3b). The phototautomer was formed due
In order to extend the scope of compounds of this class
and to study their spectral luminescence properties, we
refluxed equimolar amounts of acyl hydrazides (1а—d)
with triethyl orthoacetate (2) in o-xylene to obtain 2-aryl-
5-methyl-1,3,4-oxadiazoles (3a—d). The chelate complex
of zinc(II), L2Zn (4), was synthesized by reflux of 2-(2-hydr-
oxyphenyl)-5-methyl-1,3,4-oxadiazole (3b) with 0.5 equiv-
alent of zinc acetate dihydrate in methanol using an
equivalent amount of ButOK as a base (Scheme 1).
The compositions and structures of newly synthesized
oxadiazoles 3a,c,d and the chelate complex of ZnII with
Scheme 1
R = H (a), o-OH (b), o-OMe (c), p-OMe (d)
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 0176—0178, January, 2020.
1066-5285/20/6901-0176 © 2020 Springer Science+Business Media, Inc.

Bis[2-aryl-5-methyl-1,3,4-oxadiazolyl]zinc(II)
Russ. Chem. Bull., Int. Ed., Vol. 69, No. 1, January, 2020
177
to the intramolecular transfer of the proton in the excited
state from the phenolic oxygen to the nearest nitrogen
atom of the oxadiazole cycle (ESIPT process). This is
indicated by the fluorescence spectra of oxadiazole 3b in
isooctane and acetonitrile in which the long-wavelength
Thus, it is shown that 2-aryl-5-methyl-1,3,4-oxadi-
azoles 3а,c,d intensely luminesce (λflmax = 300—339 nm,
ϕ = 0.11—0.63), which allows their assignment to
widely demanded organic luminophores of UV radiation.
The metallocomplex luminophore L2Zn, which intensely
radiation (λfl
= 475—490 nm, ϕ = 0.001—0.004)
emits in the UV spectral range (λfl
= 413—419 nm,
max
max
from the phototautomer formed via the ESIPT mecha-
ϕ = 0.36—0.55), was synthesized from ligand 3b with the
ortho-phenol substituent in position 2 of the oxadiazole
cycle. This opens prospects for using oxadiazole 3b as
a chemosensor to the zinc cation.
nism is observed along with the short-wavelength band
(λfl
= 332—339 nm, ϕ = 0.005—0.007) from benzoid
max
form 3.7—9 However, the emission spectrum of oxadiazole
3b in DMSO contains only one short-wavelength intense
band (λfl
= 346 nm, ϕ = 0.15)7 assigned to benzoid
structurem3awx ith allowance for the fluorescence excitation
spectra. This is explained by the formation of the strong
intermolecular hydrogen bond between the phenolic hydr-
oxyl of oxadiazole 3b and highly polar aprotic DMSO that
inhibits the ESIPT process. Oxadiazoles 3а,c,d, in which
the ESIPT process is impossible because they have no
mobile proton of ortho-phenolic hydroxyl, emit intensely
in the short-wavelength range of the visible spectrum
(λflmax = 300—339 nm, ϕ = 0.11—0.63), and their emission
spectra exhibit the bathochromic shift of the luminescence
maximum with an increase in the polarity of the solvent
(by 2—15 nm) and upon the introduction of electron-
donor substituents into the aryl ring (by 15—34 nm).
The IR spectrum of metal complex 4 exhibits no ab-
sorption from the phenolic hydroxyl, which is observed in
the spectrum of ligand 3b at 3187 cm–1. In addition, the
band at 1610 cm–1 of vibrations of the C=N bond involved
in complex formation undergoes a downfield shift by
18 cm–1 compared to the corresponding band in the initial
ligand (1628 cm–1). The 1Н and 13С NMR spectra of metal
complex 4 contain the full set of signals from the hydrogen
and carbon atoms appeared in their characteristic ranges.
Experimental
IR spectra were recorded on a Varian Excalibur 3100 FT-IR
spectrometer. 1Н (250.13, 600 MHz) and 13С (62.90, 151 MHz)
NMR spectra were detected on Bruker DPX-250 and Bruker
Avance-600 instruments. Absorption and fluorescence spectra
were measured on a Cary Scan 100 spectrophotometer and on
a Cary Eclipse spectrofluorimeter, respectively. Fluorescence
quantum yields were determined relative to an acetonitrile solu-
tion of anthracene.9
5-Methyl-2-phenyl-1,3,4-oxadiazole (3a). Triethyl ortho-
acetate 2 (1.8 mL, 0.01 mol) in anhydrous o-xylene (10 mL) was
added by small portions with stirring to a solution of benzhydr-
azide 1a (1.36 g, 0.01 mol) in anhydrous o-xylene (50 mL) at
~20 °C. The reaction mixture was refluxed for 6 h monitoring
the formation of the reaction product by TLC. The solvent was
removed in vacuo, and the product was isolated by column chrom-
atography on silica (0.063—0.200 mm, ethyl acetate—petroleum
ether (1 : 10) as eluent) collecting the fraction with Rf = 0.80.
The yield was 1.46 g (83%), colorless crystals, m.p. 66—67 °С.
Found (%): С, 67.53; Н, 5.05; N, 17.44. С9H8N2O. Calculat-
ed (%): С, 67.49; Н, 5.03; N, 17.49. IR (KBr), ν/cm–1: 663, 693,
707, 710; 757 δ(C—H); 798, 812; 851 δ(CAr—H); 929, 960; 1075,
1179, 1250, 1300, 1351, 1422; 1451 ν(N—N); 1500; 1552, 1579
(С=С); 1629, 1641 (C=N); 3337, 3442 ν(C—H). UV, λmax/nm
[ε•10–4/L mol–1 cm–1, λexc = 260 nm]: isooctane, 203 [1.80],
244 sh [1.52], 249 [1.65], 254 [1.60], 260 sh [1.32], 272 sh [0.46],
1
The H NMR spectra of complex 4 contains no signal at
δ 10.22 of the phenolic hydroxyl, which is present in the
λfl
(φ): 300 (0.11); acetonitrile, 244 sh [1.84], 249 [1.95],
spectrum of ligand 3b. Complex formation also results in
25m5asxh [1.84], 259 [1.56], 273 sh [0.56], λfl
(φ): 301 (0.19).
1
the upfield shift of the signals in the Н and 13С NMR
1Н NMR (250.13 MHz, CDCl3), δ: 2.65 (s, 3mHax, Me); 7.48—7.56
(m, 3 Н, HAr); 8.06 (d, 2 Н, HAr, J = 7.0 Hz). 13C NMR (62.90
MHz, CDCl3), δC: 10.97 (CH3), 123.88 (CAr quat.), 126.51
(2 CAr), 128.91 (2 CAr), 131.41 (CAr), 163.54 (CHt), 164.67 (CHt).
2-(5-Methyl-1,3,4-oxadiazol-2-yl)phenol (3b) was synthe-
sized by the reaction of salicylic acid hydrazide 1b with triethyl
orthoacetate 2 similarly to oxadiazole 3a. The yield was 1.40 g
(81%), colorless crystals, m.p. 74—75 °С (cf. Ref. 7: m.p. 74—75 °С).
The IR, UV, and 1H and 13C NMR spectra of compound 3b
are consistent with the corresponding published spectra of this
substance.7
spectra of metal complex 4 compared to the correspond-
ing signal of its ligand 3b.
The long-wavelength absorption band maximum of
zinc complex 4 is shifted to the long-wavelength range
(λmax = 348—359 nm) compared to the spectra of oxa-
diazoles 3a—d because of the charge-transfer band between
the ligand and metal, and the maximum of its single lumi-
nescence band (λfl
= 413—419 nm) lies between
max
the emission signals of the benzoid structure of com-
pound 3b and its phototautomer. The rigid structure of
metal complex 4 prevents the nonradiative deactiva-
tion of its excited state, and the complex intensely lumi-
nesces (ϕ = 0.36—0.55) in the UV spectral range in
both polar and nonpolar solvents. Therefore, this complex
is an efficient metallocomplex luminophore, and oxa-
diazole ligand 3b can be used as a chemosensor to the
zinc cation.
5-Methyl-2-(2-methoxyphenyl)-1,3,4-oxadiazole (3с) was
synthesized by the reaction of o-anisic acid hydrazide 1с with
triethyl orthoacetate 2 similarly to oxadiazole 3a. The yield was
1.41 g (79%), colorless crystals, m.p. 118—120 °С. Found (%):
С, 63.12; Н, 5.27; N, 14.78. С10H10N2O2. Calculated (%):
С, 63.15; Н, 5.30; N, 14.73. IR (KBr), ν/cm–1: 709; 753 δ(C—H);
774 δ(CAr—H); 980, 1016, 1066, 1140, 1166, 1264, 1285; 1432,
1498 ν(N—N); 1557, 1597 (С=С); 1630, 1646 (C=N); 3335,

Mikhailov et al.
178
Russ. Chem. Bull., Int. Ed., Vol. 69, No. 1, January, 2020
3447 ν(C—H). UV, λmax/nm [ε•10–4/L mol–1 cm–1, λexc
= 290 nm]: isooctane, 211 [2.18], 248 [1.19], 255 sh [1.60],
=
DMSO, 303 [1.69], 313 sh [1.42], 359 [0.29], λfl
(φ): 419
(0.55). 1Н NMR (600 MHz, DMSO-d6, 90 °С), δ:m2a.4x9 (s, 3 H,
Me); 6.40 (dd, 1 Н, HAr, J1 = 7.8 Hz, J2 = 7.9 Hz); 6.64 (d, 1 Н,
HAr, J = 7.9 Hz); 7.12 (dd, 1 Н, HAr, J1 = 7.9 Hz, J2 = 8.0 Hz);
7.51 (dd, 1 Н, HAr, J1 = 1.8 Hz, J2 = 8.0 Hz). 13C NMR
(151 MHz, DMSO-d6), δC: 9.81 (CH3), 108.21 (CAr quat.), 111.55
(CAr), 121.77 (CAr), 127.39 (CAr), 132.32 (CAr), 160.44
(CAr quat.), 165.32 (CHt), 167.60 (CHt).
263 sh [0.68], 295 [0.55], 307 sh [0.38], λfl
(φ): 324 (0.45);
max
acetonitrile, 247 [1.24], 253 sh [1.17], 264 sh [0.62], 296 [0.58],
309 sh [0.37], λflmax (φ): 334 (0.47); DMSO, 298 [0.65], 313 sh
1
[0.38], λfl
(φ): 339 (0.43). Н NMR (250.13 MHz, CDCl3),
max
δ: 2.48 (s, 3 H, Me); 3.85 (s, 3 H, OMe); 6.93—6.98 (m, 2 Н,
HAr); 7.38 (dd, 1 Н, HAr, J1 = 7.5 Hz, J2 = 7.6 Hz); 7.78 (dd, 1 Н,
HAr, J1 = 2.5 Hz, J2 = 7.7 Hz). 13C NMR (62.90 MHz, CDCl3),
δC: 11.03 (CH3), 55.92 (OCH3), 111.89 (CAr), 113.09 (CAr quat.),
120.64 (CAr), 130.27 (CAr), 132.84 (CAr), 157.68 (CAr quat.),
163.26 (CHt), 163.51 (CHt).
This work was carried out in the framework of the
design part of the state tasks of the Ministry of Education
and Science of Russia (Project No. 4.979.2017/4.6) and
the Southern Scientific Center of the Russian Academy
of Sciences (Project No. 01201354239).
5-Methyl-2-(4-methoxyphenyl)-1,3,4-oxadiazole (3d) was
synthesized by the reaction of p-anisic acid hydrazide 1d with
triethyl orthoacetate 2 similarly to oxadiazole 3a. The yield was
1.36 g (76%), colorless crystals, m.p. 92—93 °С. Found (%):
С, 63.21; Н, 5.25; N, 14.76. С10H10N2O2. Calculated (%):
С, 63.15; Н, 5.30; N, 14.73. IR (KBr), ν/cm–1: 708; 761 δ(C—H);
779 δ(CAr—H); 988, 1069, 1140, 1164, 1285, 1307; 1437, 1463
ν(N—N); 1513, 1557, 1597 (С=С); 1630, 1646 (C=N); 3335,
References
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3447 ν(C—H). UV, λmax/nm [ε•10–4/L mol–1 cm–1, λexc
= 270 nm]: isooctane, 259 [0.61], 271 sh [0.52], 285 sh [0.23],
=
λfl
(φ): 312 (0.29); dioxane, 272 sh [0.52], 286 sh [0.26],
λflmmaaxx (φ): 317 (0.40); acetonitrile, 260 [0.64], 272 sh [0.50], 283
sh [0.29], λflmax (φ): 317 (0.63). 1Н NMR (250.13 MHz, CDCl3),
δ: 2.68 (s, 3 H, Me); 3.88 (s, 3 H, OMe); 7.64 (d, 2 H, HAr
,
J = 8.0 Hz); 7.96 (d, 2 H, HAr, J = 8.0 Hz). 13C NMR (62.90 MHz,
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(CAr quat.), 130.05 (2 CAr), 163.96 (СHt), 164.09(СHt).
Bis[2-(2-hydroxyphenyl)-5-methyl-1,3,4-oxadiazolyl]zinc(II)
(4). A solution of ButOK (0.22 g, 0.002 mol) in methanol (5 mL)
was added at ~20 °C with stirring to a solution of 1,3,4-oxadiazole
3b (0.35 g, 0.002 mol) in methanol (15 mL). The mixture was
refluxed for 1 h and cooled down to ~20 °C, and zinc acetate
dihydrate (0.22 g, 0.001 mol) in methanol (5 mL) was added.
The reaction mixture was refluxed for 4 h, and precipitated
complex 4 was filtered off, washed with methanol (2×10 mL),
and recrystallized from acetonitrile (300 mL). The yield was
0.32 g (77%), colorless crystals, m.p. 324—325 °С. Found (%):
С, 52.06; H, 3.37; N, 13.52. С18H14N4O4Zn. Calculated (%):
С, 52.01; Н, 3.39; N, 13.48. IR (Nujol), ν/cm–1: 648, 664; 765,
776 δ(C—H); 851 δ(CAr—H); 1135, 1156, 1224, 1239, 1258,
1276, 1348, 1400, 1436; 1472 ν(N—N); 1531, 1557,1596 (C=C);
1610 (C=N). UV, λmax/nm [ε•10–4/L mol–1 cm–1, λexc = 350 nm]:
dioxane, 305 [1.19], 312 [1.16], 350 [0.40], λflmax (φ): 416 (0.36);
acetonitrile, 236 [1.69], 246 [1.74], 252 [2.01], 257 sh [1.72], 263
[1.58], 304 [0.77], 311 [0.76], 348 [0.36], λflmax (φ): 413 (0.40);
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Received September 10, 2019;
accepted October 24, 2019