Influence of the nature of the substituent in 3-(2-pyridyl)-1,2,4-triazole for complexation with Pd2+

Article abstract of DOI:10.1007/s11696-017-0194-8

The structure of four new palladium complexes [Pd(HL ² )Cl ₂ and Pd(L ^1-3 ) ₂ ] with 3-(2-pyridyl)-5-R-1,2,4-triazoles (R=H, CH₃, Ph respectively HL ¹, HL ², HL ³ ) wa

Full text of DOI:10.1007/s11696-017-0194-8

Chem. Pap.
DOI 10.1007/s11696-017-0194-8
ORIGINAL PAPER
Influence of the nature of the substituent in 3-(2-pyridyl)-1,2,4-
triazole for complexation with Pd2+
Borys V. Zakharchenko1 Dmytro M. Khomenko1 Roman O. Doroshchuk1
•
•
•
Olga V. Severynovska2 Ilona V. Raspertova1 Victoria S. Starova1
Rostyslav D. Lampeka1
•
•
•
Received: 2 February 2017 / Accepted: 5 May 2017
Ó Institute of Chemistry, Slovak Academy of Sciences 2017
Abstract The structure of four new palladium complexes
[Pd(HL2)Cl2 and Pd(L1–3)2] with 3-(2-pyridyl)-5-R-1,2,4-
triazoles (R=H, CH3, Ph respectively HL1, HL2, HL3) was
proposed based on IR, NMR, UV spectroscopy and
MALDI mass spectrometry data analysis. It is found that
the complexation of HL2 and HL3 with Pd2? ions results in
a decrease of their fluorescence intensity and it is vice versa
in case of HL1. Furthermore, the influence of the sub-
stituent (R) in the 3-(2-pirydyl)-5-R-1,2,4-triazoles on the
fluorescent and protolytic properties of HL1–3 was
investigated.
et al. 2011; Gao and Shreeve 2011; Klingele and Brooker
2003) because of the presence of three donor centers.
Furthermore, the triazole ring deprotonation allows it to act
as a charged bidentate or even tridentate ligand (Liu et al.
2011; Burke et al. 2004). The presence of additional sub-
stituents which contain donor atoms in the triazole moiety
provides a greater number of possibilities for binding to
metal ions. Such kinds of compounds are derivatives of the
3-(2-pyridyl)-1,2,4-triazole. Comparison of the electronic
properties of 3-(2-pyridyl)-1,2,4-triazolyl moiety with the
widely used ligand 2,2‘-bipyridine shows that the former is
a stronger r-donor and weaker p-acceptor (van Diemen
et al. 1991).
Keywords Palladium Á 1,2,4-Triazole Á Fluorescent
properties Á NMR spectroscopy
A significant number of publications in the field of
coordination chemistry are devoted to the study of the
complexes of noble metals, which include derivatives of
1,2,4-triazole (Chi et al. 2014; Muller et al. 2013; Chen
et al. 2013; Castellano et al. 2006). Great attention has been
paid to the study of the catalytic properties of palladium
complexes in a variety of chemical (Diez-Barra et al.2003)
and photochemical reactions (Khlebnikov et al. 2012), as
well as the synthesis of materials with luminescent prop-
erties (Borisov et al. 2010; Drouet et al.2011). Thus,
recently the synthesis, characterization and fluorescent
properties of palladium complexes with ethyl ether of 3-(2-
pyridyl)-1,2,4-triazole-5-acetic acid (HL) have been
described (Khomenko et al. 2015). The ligand mentioned
above forms complexes Pd(HL)Cl2, Pd(L)2 and Pd4(L)4-
Cl4. It was found that HL possesses a high intensity of
fluorescence. At the same time, complexation with Pd2?
ion leads to an essential decrease and significant bath-
ochromic shift of the emission spectra. The complex
Pd(L)2 compared to Pd(HL)Cl2 and Pd4(L)4Cl4 possesses
a rigid planar structure and has the most intense
fluorescence.
Introduction
It is now well known that 1,2,4-triazoles are widely used as
multifunctional ligands in organometallic and coordination
chemistry due to their ability to exhibit extremely diverse
coordination chemical properties (Haasnoot 2000; Aromi
Electronic supplementary material The online version of this
article (doi:10.1007/s11696-017-0194-8) contains supplementary
material, which is available to authorized users.
& Dmytro M. Khomenko
dkhomenko@ukr.net
1
Department of Chemistry, Taras Shevchenko National
University of Kyiv, Volodymyrska Street, 64/13, Kiev 01601,
Ukraine
2
Chuiko Institute of Surface Chemistry National Academy of
Sciences of Ukraine, 17 General Naumov Street, Kiev 03164,
Ukraine
123

Chem. Pap.
This paper presents the results of the study of four
complexes of Pd2? with derivatives of 3-(2-pyridyl)-5-R-
1,2,4-triazoles (HL1–3), where R=H, Me, Ph. The structures
of these ligands and coordination compounds were estab-
pKa values were determined potentiometrically using a pH
meter (pH-340) with ESL-43-07 glass electrode (Belarus).
Ligand solutions were prepared by dissolution of their exactly
weighed portions in DMSO–distilled water mixture. Titra-
tions of ligands were performed in aqueous–DMSO mixture
(xH2O:xDMSO = 20:80 7 80:20) after adding equimolar
amount of HCl (0.01 M). The solutions was titrated with
standardized base (0.06 M KOH) at constant temperature
(23 °C). On the basis of the obtained potentiometric curves,
the values of formal dissociation constants (pKf) were calcu-
latedusingtheHyperquad2000program.AqueouspKa values
were calculated by extrapolation to 100% H2O using a linear
relationship between the pKf and DMSO percentage con-
centration (Britsun et al. 2012).
1
lished by means of H NMR, IR, UV-spectroscopy and
mass spectrometry together with their luminescence prop-
erties. The protolytic properties of HL1–3 are also
discussed.
Experimental
All reagents were obtained commercially unless otherwise
noted and used as received. The respective hydrazides
(Khomenko et al. 2016) were prepared by standard pro-
cedures. All solvents used were laboratory reagent grade.
Elemental analyses were carried out with Perkin-Elmer
2400 CHN Analyzer. Melting points (°C, uncorrected)
were measured with OptiMelt Automated Melting Point
System (MPA 100). 1H NMR spectra in DMSO-d6, CDCl3
or TFA solution were recorded on a Varian 400 spec-
trometer at room temperature. Absorption spectra were
measured using a UV-2800 spectrophotometer (Unico,
USA). The absorbance of the ligand and complex solutions
were registered in cuvettes of l = 1 cm in the range
200–800 nm, using DMSO as the reference. Fluorescence
emission spectra were measured with an LS55 spectroflu-
orimeter (Perkin-Elmer, UK). The excitation and emission
spectra were measured at the optimal conditions at maxi-
mum wavelengths for each investigated system. The
spectra were recorded in the range between 200 and
800 nm. The data collection frequency was 0.5 nm. The
spectra were measured subtracting away background of
solvent. The fluorescence maxima agreed within 0.5–1 nm
in the independent parallel measurements. The IR spectra
(KBr, pellet) were recorded with Spektrum BX Perkin
Elmer spectrometer. The mass spectra were recorded on an
Agilent 1100 Series high-performance liquid chro-
matograph equipped with a diode matrix with an Agilent
LC\MSD SL mass selective detector; the atmospheric
pressure chemical ionization (APCI) was the ionization
method used. For the MALDI-TOF analysis, 1 mg of the
studied complex was dissolved in 1 ml of chloroform.
Aliquots of mixture were applied to the steel tips and dried.
MS analysis was performed by the method of laser des-
orption/ionization on an Autoflex II (Bruker Daltonics,
Germany) mass spectrometer with nitrogen laser
(k = 337 nm). Experiments were carried out in reflectron
mode for positive ions with various times of delayed
extraction (from 1 to 500 ns) in the mass range 50–1000
amu. The resulting mass spectra were obtained by assum-
ing the data of 150 laser shots and processing by the
software FlexAnalysis (BrukerDaltonics, Germany).
Logarithms of ligand distribution coefficients (log D7.4) in
water–n-octanol extraction system were determined at pH 7.4
(phosphate buffer) according to the formula log D7.4 = log
Co/Cw using the approach (Wuitschik et al. 2010). The ligand
concentration in the aqueous phase (Cw) was determined by
standard addition methods with UV-spectrophotometric reg-
istration of maximum optical density values. The ligand
concentration in the octanol phase (Co) was found as the dif-
ference between the content of the substance in the initial
solution and the aqueous phase. The logarithm of the distri-
bution constant (log P) was calculated by the formula: log
P = log D7.4 ? log10 [1 ? 10(pKa-pH)]; the accuracy was
near 5% (Sr B 0.046; n = 3; P = 0.95).
Typical procedure for preparation of ligands
(HL1–3
)
5 ml of sodium methoxide methanolic solution (35 wt%)
was added to a solution of 2-cyanopyridine (0.05 mol) in
20 ml of methanol and stirred at room temperature for 1 h.
Then the respective hydrazide was added to the generated
iminoester solution and refluxed for 10 h. The obtained
mixture was cooled to room temperature and the solvent
removed in vacuo. The resulting residue was diluted with
water and acidified with 2 ml of acetic acid to produce a
white solid. The crystals were separated via filtration, dried
and recrystallized from toluene (HL1–3). The physical
properties and analytical data of compounds HL1-3 are
listed in Table 1, and the spectral data of compounds HL1-3
are listed in Table 2.
Method for preparation of IIa
HL2 (0.08 g, 0.5 mmol) in acetonitrile (10 ml) was added to
a hot solution of cis-PdCl2 9 (MeCN)2 (0.13 g, 0.5 mmol)
in acetonitrile (10 ml). The resulting mixture was stirred,
and the complex was precipitated and collected by filtration.
The physical properties and analytical data of IIa are listed
in Table 1, and the spectral data of IIa are listed in Table 2.
123

Chem. Pap.
Table 1 Characterization data
of the prepared compounds
Compound
Formula
Mr
w1(calc.) (%)
w1(found) (%)
Yield (%)
m.p. (°C)
C
H
N
HL1
HL2
HL3
I
C7H6N4
C8H8N4
C13H10N4
146.1
160.2
222.2
397.0
426.1
338.9
549.1
57.53
57.71
59.99
60.11
70.26
69.95
42.39
42.54
45.24
45.43
28.47
28.31
56.01
56.41
4.14
4.09
5.03
5.10
4.54
4.64
2.57
2.39
3.32
3.11
2.39
2.53
3.74
3.61
38.34
38.28
34.98
34.81
25.21
25.42
28.25
28.11
26.38
25.45
16.61
16.45
21.11
20.82
74
82
77
71
73
68
75
164a
173–174a
220a
C
C
14H10N8Pd
16H14N8Pd
[250 decomp.
II
IIa
III
a
C8H8N4Cl2Pd
C
26H18N8Pd
Muller et al. (2013) reported m.p. of 162 °C (HL1), 172 °C(HL2) and 219 °C(HL3)
Typical procedure for preparation of complexes I–
III
PdCl2 þ 2HL ! 2PdðHLÞCl2 #! PdðLÞ2
# þPdCl2 þ 2HCl:
Different behavior of the HL2 is determined by the fact
that the alkyl substituent acts as electron donor and, as a
consequence, the triazole proton becomes less acidic and
the fourth nitrogen atom of the triazole moiety becomes
more basic. That is why HL2 coordinates in acidic form
and binds to Pd2? via N-4 of the triazole moiety.
cis-PdCl2 9 (MeCN)2 (0.26 g, 1 mmol) in hot acetonitrile
(10 ml) was added to a solution of the respective ligand
(2 mmol) and Et3N (0.29 ml, 2 mmol) in acetonitrile
(10 ml). Precipitated yellow crystals were collected by
filtration, washed with acetonitrile and dried in air.The
physical properties and analytical data of compounds I-III
are listed in Table 1, and the spectral data of compounds
I-III are listed in Table 2.
The IR spectra of the ligands have common features due to
their structural similarity, namely due to the presence of tri-
azol–pyridyl moiety in all molecules and, as a result, we can
observe characteristic bands: m(N–H)Trz = 3160–3015 cm-1
and m(C=N)Trz = 1615–1595 cm-1. The intense sharp band
m(N–H) at 3225 cm-1 in the IR spectrum of complex IIa is
present, whereas in the spectrum of HL2 this band is broad-
ened, which could be explained by participation of the triazole
proton in the formation of intermolecular hydrogen bonds
(Wuitschik et al. 2010). IR spectra of the complexes I–III are
characterized by the absence of the absorption bands of
stretching vibrations m(N–H), which is caused by triazole
deprotonation. At the same time, the high-frequency shift of
bands m(C=N)Trz (Dm = 15–20 cm-1) can be observed. This
shift indicates the coordination of the organic moiety by the
metal ion.
Results and discussion
Method for the preparation of 3-(2-pyridyl)-5-R-1,2,4-tri-
azoles is based on acylation of carboxylic acid hydrazides
with picolinic acid iminoester followed by cyclization of
the obtained amidrazones in triazole. The above-mentioned
organic ligands and their complexes with palladium(II)
have been synthesized according to the procedures depic-
ted in Fig. 1.
All synthesized organic compounds forms neutral che-
late compounds Pd(L)2 and only HL2 allows to obtain
complex Pd(HL)Cl2. The reaction of equimolar amounts
of HL1 or HL3 with cis-PdCl2 9 (MeCN)2 results in the
formation of Pd(HL)Cl2 significantly contaminated by
Pd(L)2. It may be due to the lability of palladium com-
plexes and the exceptional stability together with low sol-
ubility of Pd(L)2:
One of the features of 1,2,4-triazoles is that they can
exist in several tautomeric forms. In the case of IIa,
coordination leads to stabilization of one of the tautomers
1
which was confirmed by NMR spectroscopy. In the H
NMR spectrum of IIa as compared to the spectrum of non-
coordinated ligand, the doubling of the proton signals is not
123

Chem. Pap.
Table 2 Spectral data of prepared compounds
Compound Spectral data
HL1
IR: m [cm-1] = 3078 (m), 2873 (m), 1792 (w), 1599 (s), 1574 (m), 1484 (s), 1268 (m), 1011 (s), 1094 (m), 720 (s)
1H-NMR (400 MHz, CDCl3): d [ppm] = 14.38 (s, 1H, NH), 8.83 (d, 3JH,H = 5.5 Hz, 1H, Py-H6), 8.30 (d, 3JH,H = 7.5 Hz, 1H, Py-
3
3
H3), 8.24 (s, 1H, H8), 7.90 (t, JH,H = 7.5 Hz, 1H, Py-H4), 7.37 (t, JH,H = 6 Hz, 1H, Py-H5)
1H-NMR (400 MHz, TFA): d [ppm] = 9.50 (d, 1H, Py-H6), 9.27 (d, 1H, Py-H3), 9.17 (s, 2H, Py-H4 andH8), 8.57 (t, 3JH,H = 6 Hz,
1H, Py-H5)
UV spectrum (DMSO): kmax [nm](e, L mol-1 cm-1) = 278 (8.6 9 103). Excitation spectrum (DMSO): kmax [nm] = 296.
Fluorescence spectrum (DMSO): kmax [nm] = 400
LC/MS found: m/z 147 [M ? H]? (calcd 147.0665)
HL2
IR: m [cm-1] = 3016 (m), 2658 (m), 1860 (w), 1596 (s), 1552 (m), 1429 (s), 1170 (m), 1062 (s), 988 (m), 725 (s)
1H-NMR (400 MHz, DMSO-d6): d [ppm] = 14.09 (br. s, 1H, NH), 8.63 (d, 1H, Py-H6), 8.03 (d, 1H, Py-H3), 7.91 (t, 1H, Py-H4),
7.42 (t, 1H, Py-H5), 2.36 (s, 3H, H9).
3
3
1H-NMR (400 MHz, CDCl3): d [ppm] = 8.76 (d, JH,H = 4.6 Hz, 1H, Py-H6), 8.20 (d, JH,H = 7.7 Hz 1H, Py-H3), 7.84 (t,
3JH,H = 7.8 Hz 1H, Py-H4), 7.38 (t, JH,H = 7.5 Hz, 1H, Py-H5), 2.53 (s, 3H,H9).UV spectrum (DMSO): kmax [nm](e,
3
L mol-1 cm-1) = 282 (3.0 9 104). Excitation spectrum (DMSO): kmax [nm] = 305. Fluorescence spectrum (DMSO): kmax
[nm] = 410
LC/MS found: m/z 161 [M ? H]? (calcd 161.0821)
HL3
IR: m [cm-1] = 3460 (w), 3156 (s), 2961 (w), 1860 (w), 1161 (m), 1565 (m), 1472 (s), 1276 (m), 1056 (s), 747 (s)
1H-NMR (400 MHz, CDCl3): d [ppm] = 8.79 (d, JH,H = 4.5 Hz, 1H, Py-H6), 8.37 (d, JH,H = 6.5 Hz, 1H, Py-H3), 8.23 (d,
3
3
3
3JH,H = 7.0 Hz, 2H, H9–9‘), 7.95 (t, JH,H = 5.5 Hz, 1H, Py-H4), 7.47 (m, 4H, H10,10‘,11 and Py-H5)
UV spectrum (DMSO): kmax [nm](e, L mol-1 cm-1) = 277 (1.1 9 105). Excitation spectrum (DMSO): kmax [nm] = 298.
Fluorescence spectrum (DMSO): kmax [nm] = 395
LC/MS found: m/z 223 [M ? H]?(calcd.223.0978)
I
IR: m [cm-1] = 3417 (w), 3034 (w), 1614 (s), 1536 (m), 1478 (s), 1159 (m), 787 (m), 751(s), 439 (w).
1H-NMR (400 MHz, TFA): d [ppm] = 10.39 (d, 1H, Py-H6), 9.03 (d, 1H, Py-H3), 8.75 (s, 2H, Py-H4 and H8), 8.26 (t,
3JH,H = 6 Hz, 1H, Py-H5).
UV spectrum (DMSO): kmax [nm](e, L mol-1 cm-1) = 267 (7.4 9 104), 346 (1.4 9 104). Excitation spectrum (DMSO): kmax
[nm] = 300. Fluorescence spectrum (DMSO): kmax [nm] = 435. MALDI found: m/z 252,398 [PdI(L1) ? H]?; 397,630
Â
Ã
[PdII(L1)2 ? H]?; 504,885 PdI ðL1Þ þ H þ. calcd: m/z 251,963; 397.014; 504,918
2
2
II
IR: m [cm-1] = 3044 (w), 1614 (s), 1536 (m), 1478 (s), 1159 (m), 787 (m), 751(s), 439 (w)
1H-NMR (400 MHz, CDCl3): d [ppm] = 10.03 (d, 1H, Py-H6), 8.01 (m, 2H, Py-H3–4), 7.44 (t, 1H, Py-H5), 2.54 (s, 3H, H9)
UV spectrum (DMSO): kmax [nm](e, L mol-1 cm-1) = 270 (1.5 9 104), 357 (4.5 9 103). Excitation spectrum (DMSO): kmax
[nm] = 285. Fluorescence spectrum (DMSO): kmax [nm] = 402. MALDI found: m/z 266,714 [PdI(L2) ?H]?; 426,018
Â
Ãþ
[PdII(L2)2 ? H]?; 533,845 Pd2I ðL2Þ2 þ H calcd: m/z 265,978; 426,054;532,949
IIa
IR: m [cm-1] = 3480 (w), 3226 (s), 2961 (m), 1615 (w), 1531 (m), 1472 (s), 1282 (m), 1081(w), 743 (s)
3
3
1H-NMR (400 MHz, DMSO-d6): d [ppm] = 15.26 (s, 1H, NH), 9.01 (d, JH,H = 6 Hz, 1H, Py-H6), 8.13 (d, JH,H = 7.8 Hz, 1H,
Py-H3), 8.28 (t, JH,H = 7.7 Hz, 1H, Py-H4), 7.76 (t, JH,H = 6.6 Hz 1H, Py-H5), 2.70 (s, 3H, H9).
3
3
UV spectrum (DMSO): kmax [nm](e, L mol-1 cm-1) = 270 (2.9 9 104), 312 (1.7 9 104). Excitation spectrum (DMSO): kmax
[nm] = 299. Fluorescence spectrum (DMSO): kmax [nm] = 415. MALDI found: m/z 266,676 [PdI(L2) ?H]?; 339,812
[PdII(HL2)Cl2 ? H]?calcd: m/z 265,978; 338,924
III
IR: m [cm-1] = 3441 (w), 3015 (w), 1594 (m), 1554 (w), 1463 (m), 1438 (s), 1149 (w), 1007(w), 718 (s), 684 (m)
1H-NMR (400 MHz, CDCl3): d [ppm] = 9.81 (d, 1H, Py-H6), 7.89 (d, 1H, Py-H3), 8.11 (d, 2H, H9–9‘), 7.13 (t, 1H, Py-H5), 7.48 (m,
4H, H10,10‘,11 and Py-H4)
UV spectrum (DMSO): kmax [nm](e, L mol-1 cm-1) = 277 (5.8 9 104), 396 (5.3 9 104). Excitation spectrum (DMSO): kmax
[nm] = 372. Fluorescence spectrum (DMSO): kmax [nm] = 444. MALDI found: m/z 328,144 [PdI(L3) ?H]?; 549,494
Â
Ãþ
[PdII(L3)2 ? H]?; 654,743 Pd2I ðL3Þ2 þ H calcd: m/z 327,994; 549,077; 654,980
observed. This can be an evidence of stabilization of one
tautomer due to coordination. In IIa, ortho-proton of the
pyridine moiety (DdH6 = ?0.39 ppm) and the methyl
group protons (DdMe = ?0.34 ppm) undergo significant
low field shift compared to the uncoordinated ligand. This
may be due to their proximity to the coordination center
and the deshielding effect from chlorine atoms (see Fig. 1)
(Khomenko et al. 2015).
As expected, there are no N–H signals of triazole
1
moiety in H NMR spectra of Pd(L)2. Moreover, in the
spectra of I–III, as well as in the spectrum of complex
IIa, the downfield shift of the proton signals is observed
123

Chem. Pap.
Fig. 1 Synthesis of ligands (HL1–3) and Pd2? complexes (I-III, IIa). Reactions conditions: i) MeONa, MeOH, RT, 1 h; ii) MeOH, reflux, 10 h;
iii) 1 eq cis-PdCl2 9 (MeCN)2, MeCN; iv) 0.5 eq cis-PdCl2 9 (MeCN)2, 1 eq Et3N, MeCN
Table 3 Protolytic and
hydrophobic properties
Compound
Logarithmic acid dissociation constant values
Parameters of hydrophobicity
pKa1
pKa2
log D7.4
log P
HL1
HL2
HL3
3.35 0.02
3.63 0.02
2.31 0.05
8.93 0.03
9.47 0.03
5.94 0.07
0.53 0.06
0.88 0.10
2.06 0.12
0.54 0.06
0.88 0.10
2.08 0.12
compared to their position in the spectra of ‘‘free’’ ligand,
though deprotonation usually leads to opposite effect and
should promote the high field shift. Special attention
should be paid to a significant downfield shift of the
signal of ortho-proton of the pyridine moiety
(DdH6 = ?1.02–1.25 ppm). This indicates unambiguously
the involvement of the pyridine ring nitrogen in the
coordination bond formation and may be due to its
proximity to the lone pair of electrons of the nitrogen
atom of the triazole ring of the second ligand
(Zakharchenko et al. 2015), which results in the formation
of intramolecular hydrogen bond (see Fig. 1).
molecular ions that corresponds to adducts of palladium-
containing fragments with fragments of the ligands and
alkali metals indicates the tendency of the studied com-
pounds with the association in the gaseous phase. Mass
spectra of the complexes also contain peaks with low mass,
mainly corresponding to fragments of HL1–3. The presence
of the fragments [PdI(L) ? H]? and Pd2I ðLÞ2 þ H can be
explained by the reduction of Pd(II) to Pd(I) by trapping of
electrons that generated during fragmentation of com-
pounds. Increasing of ion detection delay time leads to
enhancement of the molecular ion peak fraction corre-
sponding to the peaks of other fragments. This and the other
above-mentioned facts prove the structure proposed for the
investigated coordination compounds.
Â
Ãþ
MALDI studies of complexes revealed that under con-
ditions of the mass spectrometric experiment (identical for
all samples), they all are well ionized. This process takes
place due to protonation or formation of associates with
alkali metal ions. The MALDI mass spectra of complexes I–
III are characterized by the presence of patterns appropriate
for two fragments: [PdI(L) ? H]? and [PdII(L)2 ? H]?.
Complex IIa is also characterized by the formation of the
fragment [PdII(HL2)Cl2 ? H]? with a mass of 339,812 m/z,
There is a tendency to fragment and form a large amount of
associates, with alkali metal ions increasing together with
the molecular weight of the complexes. Moreover, a series
of peaks in the field of mass exceeding the mass of the
It should be noted that the substituents in the 5-position
of the triazole substantially affect the mobility (acidity) of
the triazole proton. Thus, the insertion of electron-with-
drawing substituent in the 5-position, such as phenyl,
causes a decrease in the values of the negative logarithms
of the acid dissociation constants for HL3 compared with
3-(2-pyridyl)-1,2,4-triazole (Table 3).
At the same time, the electron-donating methyl group in
HL2 enhances the basic properties of the ligand, and pKa1
and pKa2 values increase. The experimental data for dis-
sociation constants correlate well with those theoretically
123

Chem. Pap.
Fig. 2 The scheme of the possible acid–base equilibria in ligand solution
Fig. 3 Excitation (1–20) and fluorescence (30–4) spectra of ligands and their palladium(II) complexes in DMSO: CH1 L, = 5 9 10-6 M;
I
C2HL, IIa, II = 1 9 10-5 M; C3HL, III = 4.5 9 10-7
M
calculated using the program ASDLabs. The scheme of the
possible acid–base equilibria in ligand solution is shown in
Fig. 2.
formation metalocycles with two deprotonated ligands. It is
noteworthy that, due to the coordination HL2 with palla-
dium ions, a minor hypsochromic shift (Dk & 8 nm) in the
fluorescence spectrum of II is shown. On the contrary, in
the spectrum of complex III, the bathochromic shift
(Dk & 50 nm) relative to the spectrum of HL3 ligand is
observed. Surprisingly, the intensity of HL1 fluorescence
significantly increases on formation of the palladium
complex. In our opinion, such properties may be due to the
unambiguous planarity of I, which facilitates electron
delocalization and as a result the fluorescence intensity.
Log P value is an important parameter in the design of
new fluorescent probes to determine the amount of metal
ions in biological cells. This parameter describes the ability
of the molecule to penetrate into the lipid bilayer of the cell
membrane (Mottram et al. 2007). Table 3 shows the high
lipophilic properties of the synthesized ligands and correlate
well with the calculated values of log P (for HL1log P the
calculated value is 0.38, for HL2: 0.63, and HL3: 2.82).
Among all investigated compounds, the 3-(2-pyridyl)-5-
phenyl-1,2,4-triazole (HL3) is characterized by the highest
hydrophobicity. This compound also has the most intense
fluorescence, which from our point of view leads to the
prospect of its use as a fluorescent probe for determination
of trace palladium(II) in biological objects.
By comparing the fluorescence spectra of the pyridyl-
triazoles (Fig. 3) it was found that the most intense fluo-
rescence gives the solution of HL3 that implements the most
long p-conjugated system in this compound. At the same
time, the electron-withdrawing nature (-M effect) of the
phenyl substituent increases the rate of electron delocaliza-
tion in a conjugated system, which further enhances the
ligand fluorescence intensity compared with HL1 and HL2.
The methyl group possesses ?I effect and reduces the
probability of electronic transition of the molecule to an
excited state, which is accompanied by a significant
decrease in the fluorescent properties of the ligand. There-
fore, HL2 expectedly has lower fluorescence compared to
HL1. Complexation of HL2 and HL3 with Pd2? is accom-
panied by a decrease of their fluorescence intensity (Fig. 3).
Such fluorescence quenching can occur due to the
excited state energy deactivation through partially unfilled
low-energy d-orbitals of the metal ion. That is why IIa has
the least intensive fluorescence among the investigated
coordination compounds. This is also explained by the fact
that it contains only one chelating ligand, while the p-
systems of compounds I–III are increased twice by the
123

Chem. Pap.
Diez-Barra E, Guerra J, Hornillos V, Merino S, Tejeda J (2003) 1,2,4-
Triazole-based palladium pincer complexes. A new type of
catalyst for the heck reaction. Organometallics 22:4610–4612.
doi:10.1021/om0340600
Drouet S, Paul-Roth CO, Fattori V, Cocchic M, Williams JAG (2011)
Platinum and palladium complexes of fluorenyl porphyrins as
Conclusions
It is concluded from the present study that derivatives of
3-(2-pyridyl)-5-R-1,2,4-triazole (HL1–3, where R=H, Me,
Ph) form mononuclear complexes with palladium(II),
whose structure is proposed on the basis of spectroscopic
and spectrometric studies. It was found that due to the
lower acidity of triazole proton in HL2, the ligand was
capable of forming palladium(II) complexes of M:L = 1:1
and 1:2, whereas HL3 and HL1 preferably form complexes
of M:L = 1:2. The high hydrophobicity of the ligands and
intensive fluorescence of the palladium complexes
appeared to be promising for the use of the triazole ligands
as new analytical reagents for the determination of palla-
dium compounds in biological objects by fluorescence
spectroscopy.
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