Synthesis and physical property studies of cyclometalated Pt(II) and Pd(II) complexes with tridentate ligands containing pyrazole and pyridine groups

Article abstract of DOI:10.1016/j.poly.2020.114799

Novel substituted tridentate ligands, which contains a series of pyrazole and pyridine groups as donors, and their cyclometalated complexes have been successfully synthesized. The ligands and complexes were completely characterized by a variety of techniques, such as ¹H NMR, UV–Vis, FT-IR, mass spectrometry and elemental analysis. Meanwhile, the single crystal structures of ligand 7 and complex 11 were determined by X-ray crystallography. For ligand 7, due to the effect of delocalization, the C(6)-C(7) bond length in the pyrazole ring is 0.05 ? shorter than a normal C-C bond. After coordinating with the metal centre, the configuration was limited by the coordination sphere, so the bond lengths and bond angles of complex 11 are different from those of ligand 7. Through photoluminescence in acetonitrile solution at ambient temperature, two emission bands have been found between 500 and 600 nm for complexes 9 and 10. Moreover, there are several absorbing bands respectively located at 240 to 250 nm and 300 to 310 nm for ligands 7 and 8 (π(L) → π*(L) and n → π*(L)). A red-shift was observed in the UV spectra, on comparing the complexes with electron-donating and electron-withdrawing groups on the ligand, which is due to the electronic effect of the substituent. One absorbing band is also found at 375 to 425 nm for complexes 9 and 10, and this band is attributed to a MLCT (dπ(Pt) → π*(L)).

Full text of DOI:10.1016/j.poly.2020.114799

Polyhedron 191 (2020) 114799
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Polyhedron
journal homepage: www.elsevier.com/locate/poly
Synthesis and physical property studies of cyclometalated Pt(II) and
Pd(II) complexes with tridentate ligands containing pyrazole and
pyridine groups
⇑
Keerthika Kumarasamy, Tamiloli Devendhiran, Mei-Ching Lin , Chi-Wen Chiu
Department of Applied Chemistry, Chaoyang University of Technology, Taichung City 413310, Taiwan, ROC
a r t i c l e i n f o
a b s t r a c t
Article history:
Novel substituted tridentate ligands, which contains a series of pyrazole and pyridine groups as donors,
and their cyclometalated complexes have been successfully synthesized. The ligands and complexes were
completely characterized by a variety of techniques, such as 1H NMR, UV–Vis, FT-IR, mass spectrometry
and elemental analysis. Meanwhile, the single crystal structures of ligand 7 and complex 11 were deter-
mined by X-ray crystallography. For ligand 7, due to the effect of delocalization, the C(6)-C(7) bond length
in the pyrazole ring is 0.05 Å shorter than a normal C-C bond. After coordinating with the metal centre,
the configuration was limited by the coordination sphere, so the bond lengths and bond angles of com-
plex 11 are different from those of ligand 7. Through photoluminescence in acetonitrile solution at ambi-
ent temperature, two emission bands have been found between 500 and 600 nm for complexes 9 and 10.
Moreover, there are several absorbing bands respectively located at 240 to 250 nm and 300 to 310 nm for
Received 9 February 2020
Accepted 6 September 2020
Available online 18 September 2020
Keywords:
Tridentate ligands
Pyridine and Pyrazole
Pd(II) and Pt(II) complexes
Physical property studies
ligands 7 and 8 (p(L) ? p*(L) and n ? p*(L)). A red-shift was observed in the UV spectra, on comparing
the complexes with electron-donating and electron-withdrawing groups on the ligand, which is due to
the electronic effect of the substituent. One absorbing band is also found at 375 to 425 nm for complexes
9 and 10, and this band is attributed to a MLCT (d
p
(Pt) ?
p
*(L)).
Ó 2020 Elsevier Ltd. All rights reserved.
1. Introduction
complexes. The presence of a nitrogen atom improves the catalytic
activity, which results in the formation of higher donating ligands
[5,6]. Cyclometalation in platinum metal complexes often uses
bidentate or tridentate ligands to stabilize the structure, such as
bipyridine (N, N) and terpyridine (N, N, N) ligands [7]. The nature
of these coordination groups and their interaction with the metal
centers also help generate the variety of spectroscopic properties
of the different cyclometalated systems [8]. A group of coordina-
tion functional groups, such as amino [9], iminyl [10], pyridyl
[11], pyrazolyl [12], phosphinyl [13] and thiol [14], have been used
to maintain and stabilize the metal center for cyclometalation [15].
Cyclometalation of d8 transition metal complexes has recently
experienced an increasing research interest; especially in palla-
dium(II) and platinum(II) based complexes [1]. Lately, scientists
have focused their research on the synthesis and application of
multidentate metal chelates [2]. Therefore, the synthesis and appli-
cation of multidentate metal chelates have gradually received
attention, especially in the field of multidentate ligands [3]. Gener-
ally, pincer complexes illustrate the backbone of catalytic pro-
cesses and this is especially true in the case of the catalytic
formation of CAC bonds. The chelation of metal centers mostly
use strong and efficient catalysts, where pincer compounds are
unique [4]. Recently, the advantages of pincer compounds have
been used in the catalytic production of CAC, CAO, CAN, CAS
and C-B bonds. In coordination chemistry, many pyrazole deriva-
tives are used as ligands. Over the past two-three decades, com-
pounds containing two and more pyrazole fragments have gained
particular popularity as they formed the basis for different metal
In recent years, cyclometalated ligands with
p-conjugated biden-
tate and tridentate systems have considerably increased attention
due to the outstanding photophysical properties of their cyclomet-
alated Pt(II) and Pd(II) complexes. The square-planar d8 cyclomet-
alated complexes also have special properties for substrate binding
and chemosensing [16,17].
Most of these molecules have significant photophysical and
photochemical properties [17], which make them useful in applica-
tions requiring photocatalysis [18,19], photovoltaic devices
[16,20–21], DNA and protein binding [22], cation detection
[23,24], pH sensing [7,25] and cyano-halide detection [26], among
⇑
Corresponding author.
E-mail address: mavis@cyut.edu.tw (M.-C. Lin).
https://doi.org/10.1016/j.poly.2020.114799
0277-5387/Ó 2020 Elsevier Ltd. All rights reserved.

K. Kumarasamy et al.
Polyhedron 191 (2020) 114799
others [24,25]. In particular, palladium(II) and platinum(II) lumi-
nescent complexes with oligopyridine [27] and cyclometallated
ligands have recently been widely studied [29].
These metal complexes affect the strong light-emitting proper-
ties and the color of substances [30], which are photophysical and
photochemical properties, respectively [28,31], because they have
rich and diverse electron transfer processes, such as IL (intra-
involved and hence it does not possess any special properties in
the ligand [11].
The HN-N moiety, through 1-pyrazolyl-NH, will be able to form
more cyclometallic compounds with different characteristics, such
as deprotonation with trimethylamine [11], formation of a binu-
clear metal complex via the pyrazolyl nitrogen atom [37,38]. The
formation of pyrazolyl after deprotonation can be degraded to an
electrophilic functional group, which can generate a bond with
another metal center between molecules. Due to resonance, the
pyrazole group will be more stable and the configuration of the
compound can be controlled by adjusting the pH. Therefore, these
compounds are being widely used as ligands [39,40].
A series of tridentate ligands and metal complexes were pre-
pared with palladium and platinum as the central metals. Func-
tional groups were modified at the ring positions, and the effects
of the functional groups on the optical and physical properties of
the complexes were investigated.
ligand:
p
?
p*) and MLCT (metal–ligand charge transfer)
(d ? p
p
p*). These properties could be applied for cold sensors,
increasing the interest in these areas [8,32,33]. In recent years, sci-
entists have discovered that platinum(II) diamine compounds have
luminescence properties at room temperature and that there is a
long-term excited state in solution [34].
The objective of this study is to synthesize the tridentate
ligands 7 and 8, containing pyrazole, pyridine and benzene rings.
The C, N, N- cyclometalated ligands 7 and 8 (Fig. 1), containing
C-phenyl, N-pyridyl and N-pyrazolyl donors and where the 1-pyra-
zolyl-NH group can provide additional coordination, can form a
variety of bonding modes [8]. Because 1-pyrazolyl-NH is a rela-
tively easy reaction site, it can coordinate in the cyclometallic com-
pound and facilitate other chemical reactions. It also has the
characteristics of ligands that easily form cyclic metal compounds
and can even form more diverse networked compounds [35]. For
most tridentate cyclometalated complexes, substitution of an
ancillary ligand is most common because in comparison to the
direct derivation of the cyclometalating ligand, which involves a
multi-step synthesis, the use of ancillary ligands is an easier
2. Experimental
2.1. Materials and methods
All experiments and manipulations were conducted under an
inert nitrogen atmosphere using standard vacuum techniques.
The starting materials used in this study were obtained from com-
mercial sources and the solvents used were of analytical grade. The
melting points were determined on a MP-ID. The UV–Vis spectra
were recorded with a PERKIN ELMER Lambda 20 Ultraviolet–Visi-
ble spectrometer in acetonitrile solution. Fluorescence spectra
were recorded on a JASCO FP-750 fluorimeter. Fourier Transform
Infrared (FT-IR) spectra were recorded as potassium bromide
(KBr) pellets on a Paragon 500 instrument PERKIN ELMER. 1H
NMR spectra were recorded at 298 K on VARIAN 200 MHz and
BRUKER 500 MHz spectrometers using DMSO d6 and CDCl3 as sol-
approach to functionalization [35,36]. In this system, C, N, Npyrazolyl
,
both the ancillary and cyclometalating ligands are available for
easy derivatization and this approach may be more flexible
towards the functionalization and integrity of cyclometalated com-
plexes with advanced catalytic and spectroscopic properties for
use in supramolecular devices and materials. Here, the pyrazolyl
moiety acts as a donor group, thus the ligands mainly involve
N1-substituted pyrazoles. The 1-pyrazolyl-NH group is thus not
Fig. 1. Chemical structures of the pyrazole containing C,N,N ligands and their cyclometalated complexes.
2

K. Kumarasamy et al.
Polyhedron 191 (2020) 114799
vents. Elemental analyses were performed on an Elementary Vario
EL III elemental analyzer. Mass spectra were recorded on a Finni-
gan/Thermo Quest MAT 95XL mass spectrometer.
CDCl3) d, ppm: 8.079 (J = 7.5 Hz, d, 1H), 7.997 (J = 8 Hz, d, 2H),
7.950–7.774 (m, 3H), 7.303 (J = 8 Hz, d, 2H), 3.171 (s, 3H), 3.015
(s, 3H), 2.416 (s, 3H). MS (EI): 266.3 (M+). Elemental analysis
(C17H18N2O), Theoretical: C, 76.66; H, 6.81; N, 10.52; Actual: C,
77.06; H, 6.54; N, 10.45%. The NMR and mass spectra are shown
in the supplementary materials.
2.2. Synthesis of compound 2
5.537 g (0.02 mol) of compound 1 was dissolved in 45 mL dry
ether under nitrogen by stirring 18 mL (0.02 mol) of n-BuLi was
added at À78 °C, followed by stirring for 30 min. Next, 3 mL
dimethylacetamide (0.02 mol) was slowly added at this tempera-
ture. After returning to room temperature, the reaction mixture
was stirred for 17 h. The resulting mixture turned khaki yellow.
The reaction product was extracted with diethyl ether and satu-
rated NH4Cl, then dried over Na2SO4. The product was filtered, con-
centrated by evaporating under vacuum and purified by column
techniques with 8:2 hexane/EA as the eluent, where it gave a col-
orless powder. Yield: 74.62%. Melting point: 44 °C. 1H NMR
(200 MHz, CDCl3) d, ppm: 8.010 (J = 6.8 Hz, dd, 1H), 7.697–7.664
(m, 2H), 2.705 (s, 3H, CH3).
2.6. Synthesis of compound 6
0.42 g (1.61 mmol) of compound 4 was dissolved in N,
N-dimethylformamide dimethyl acetal (100 mL, 8.9 mmol). The
resulting solution was refluxed for 18 hrs at 100 °C and monitored
by TLC. Once the reaction completed, it was cooled, evaporated
under vacuum and purified by column techniques with CH2Cl2/
hexane (1:1) as the eluent. It yielded an off-white powder which
was recrystallized using CH2Cl2 and hexane to obtain white needle
crystals. Yield: 80.8%. Melting point: 155 °C. 1H NMR (500 MHz,
CDCl3) d, ppm: 8.220–8.146 (m, 3H), 7.988–7.816 (m, 3H), 7.772
(J = 8.2 Hz, d, 2H), 6.673 (J = 13 Hz, d, 1H), 3.191 (s, 3H), 3.047
(s, 3H). MS (EI): 320 (M+). Elemental analysis (C17H15F3N2O),
Theoretical: C, 63.75; H, 4.72; N, 8.75; Actual: C, 63.19; H, 4.90;
N, 8.72%. The NMR and mass spectra are shown in the supplemen-
tary materials.
2.3. Synthesis of compound 3
2.07 g (0.01 mol) of compound 2 was dissolved in dry xylene
(80 mL) and 1.80 g (0.015 mL) 4-methylphenylboronic acid was
added under nitrogen, then 0.50 g Pd(PPh3)4 (5% mol) and 2.71 g
(0.02 mol) of potassium carbonate were added. The reaction mix-
ture was refluxed for 5 h at 100 °C and monitored by TLC. Once
the reaction completed, it was extracted with H2O and CH2Cl2
and dried over MgSO4. Next it was filtered, evaporated under vac-
uum and then purified by column techniques with hexane/EA (9:1)
as the eluent. It yielded a white powder. Yield: 91.2%. Melting
point: 70 °C. 1H NMR (500 MHz, CDCl3) d, ppm: 7.996 (J = 8.5 Hz,
d, 2H), 7.936–7.815 (m, 3H), 7.307 (J = 8 Hz, d, 2H), 2.810 (s, 3H),
2.417 (s, 3H). MS(EI): 211.2 (M+). Elemental analysis (C14H13NO);
Theoretical: C, 79.59; H, 6.20; N, 6.63. Actual: C, 79.66; H, 6.52;
N, 6.60%. The NMR and mass spectra are shown in the supplemen-
tary materials.
2.7. Synthesis of ligand 7
1.51 g (5.6 mmol) of compound 5 was dissolved in ethanol
(2 mL). 80% Hydrazine hydrate (2.6 mL) was added and the result-
ing solution was refluxed for 4 hrs at 65 °C. The solution was
poured into 200 mL of water and it produced a white turbid precip-
itate, which was filtered, dried and purified by column techniques
with dichloromethane/hexane as the eluent. It yielded a yellow
powder which was recrystallized using dichloromethane/hexane
to obtain pale yellow needle crystals. Yield: 97.3%. Melting point:
160 °C. 1H NMR (500 MHz, CDCl3) d, ppm: 7.957 (J = 8.5 Hz, d,
2H), 7.756 (J
= 16 Hz, t, 1H), 7.649–7.616 (m, 2H), 7.591
(J = 7 Hz, d, 1H), 7.292 (J = 8 Hz, d, 2H), 6.803 (s, 1H), 2.40 (s,
3H). MS (EI): 235.1 (M+), 236.1 (M + 1). Elemental analysis
(C15H13N3), Theoretical: C, 76.57; H, 5.57; N, 17.86; Actual: C,
76.00; H, 5.46; N, 17.60%. The NMR and mass spectra are shown
in the supplementary materials.
2.4. Synthesis of compound 4
0.3 g (1.5 mmol) of compound 2 was dissolved in dry xylene
(20 mL) and 0.431 g (2.28 mmol) 4- (trifluoromethyl)phenylboronic
acid was added under nitrogen, followed by the addition of 0.091 g
of Pd(PPh3)4 (5% mol) and 0.427 g (3 mmol) of potassium carbonate.
The reaction mixture was refluxed for 24 hrs at 100 °C and moni-
tored by TLC. Once the reaction completed, it was extracted with
H2O and CH2Cl2 and dried over MgSO4. Next it was filtered, evapo-
rated under vacuum and purified by column techniques with
hexane/EA (9:1) as the eluent. It yielded an off-white powder
(Scheme 1). Yield: 79.74%. Melting point: 76 °C. 1H NMR
(500 MHz, CDCl3) d, ppm: 8.220 (J = 8 Hz, d, 2H), 8.047–7.940
(m, 3H), 7.775 (J = 8.2 Hz, d, 2H), 2.826 (s, 3H). MS (EI): 265.1
(M+). Elemental analysis (C14H10F3NO): Theoretical: C, 63.40; H,
3.80; N, 5.28. Actual: C, 63.84; H, 3.58; N, 5.06%. The NMR and mass
spectra are shown in the supplementary materials.
2.8. Synthesis of ligand 8
0.41 g (1.28 mmol) of compound 6 was dissolved in ethanol
(3 mL). 80% Hydrazine hydrate (1.3 mL) was added and the result-
ing solution was refluxed for 4 hrs at 65 °C. The solution was
poured into 200 mL of water and it produced a white turbid precip-
itate, which was filtered, dried and purified by column techniques
with dichloromethane/hexane as the eluent. It yielded a yellow
powder which was recrystallized using dichloromethane/hexane
to obtain pale yellow needle crystals. Yield 92.3%. Melting point:
173 °C. 1H NMR (500 MHz, CDCl3) d, ppm: 8.168 (J = 8.2 Hz, d,
2H), 7.813 (J = 15.2 Hz, t, 1H), 7.732–7.646 (m, 5H), 6.876 (s, 1H).
MS (EI): 289.1 (M+). Elemental analysis (C15H10F3N3); Theoretical:
C, 62.28; H, 3.48; N, 14.53; Actual: C, 62.25; H, 4.01; N, 14.68%.
The NMR and mass spectra are shown in the supplementary
materials.
2.5. Synthesis of compound 5
0.492 g (2.3 mmol) of compound 3 was dissolved in N,
N-dimethylformamide dimethyl acetal (1.2 mL, 8.9 mmol). The
solution was refluxed for an hour at 100 °C and monitored by
TLC. Once the reaction completed, it was cooled, evaporated under
vacuum and purified by column techniques, with CH2Cl2/hexane
(1:1) as the eluent. This yielded a pale yellow powder which was
recrystallized using CH2Cl2 and hexane to obtain pale yellow crys-
tals. Yield: 87.8%. Melting point: 151.5 °C. 1H NMR (500 MHz,
2.9. Synthesis of complex 9
A mixture of ligand 7 (0.21 g, 0.9 mmol) and K2PtCl4 (0.383 g,
0.9 mmol) was dissolved in glacial acetic acid (25 mL). The result-
ing solution was refluxed for 24 h, whereupon it formed a yellow
precipitate. The precipitate was filtered, washed with H2O,
C2H5OH, CH3OH and ether as washing liquids, and then dried. It
3

K. Kumarasamy et al.
Polyhedron 191 (2020) 114799
Scheme 1. Synthesis of the new substituted C, N, Npyrazolyl ligands: (i) n-BuLi, N,N-dimethylacetamide, ethyl ether, À78 °C; (ii) substituted phenylboronic acid in xylene, Pd
(PPh3)4, potassium carbonate, R = CH3, CF3, 100 °C, 24 h; (iii) N,N-dimethylformamide dimethyl acetal, reflux, 18 h; (iv) N2H4, EtOH, 70 °C, 4 h.
yielded a yellow powder which was recrystallized using acetone
and CH3OH (Scheme 2). Yield: 64.26%. 1H NMR (200 MHz,
DMSO d6) d, ppm: 8.137 (s, 1H), 7.989 (J = 15.8 Hz, t, 1H), 7.784
(J = 16.2 Hz, t, 2H), 7.415 (J = 7.8 Hz, s, 1H), 7.311 (s, 1H), 7.187
(s, 1H), 6.862 (J = 7.8 Hz, d, 1H), 2.260 (s, 3H). MS (FAB): 465
(M+). The NMR and mass spectra are shown in the supplementary
materials.
Scheme 3. Synthesis of the cyclopalladated complex: (i) K2PdCl4, acetonitrile/ H2O,
2.10. Synthesis of complex 10
reflux, 24hr.
A mixture of ligand 8 (0.26 g, 0.9 mmol) and K2PtCl4 (0.381 g,
0.9 mmol) was dissolved in glacial acetic acid (25 mL). The result-
ing solution was refluxed for 24 h, whereupon it formed a yellow
precipitate. This precipitate was filtered, washed with H2O,
C2H5OH, CH3OH and ether as washing liquids, and then dried. It
yielded a yellow solid which was recrystallized using acetonitrile
7.172 (s, 1H), 6.911 (J = 8 Hz, d, 1H)). The NMR spectrum is shown
in the supplementary materials.
2.12. Crystal structure determination
and ether. Yield: 55.2%. MS (FAB): 520 (M
+
1). 1H NMR
Intensity data were collected at room temperature on a Bruker
Axs SMART 1000 CCD diffractometer, using a molybdenum (Mo)
target (k = 0.71069 Å) as the radiation source and h-2h angle as
the scanning pattern. Data collections were processed with SAINT
software. Absorption data corrections were applied by using the
SADABS [41] software package. The structures of the ligand and
complex were solved by direct methods using SHELXTL [42] with
standard difference Fourier syntheses. All non-hydrogen atoms of
ligand 7 and complex 11 were purified with anisotropic displace-
ment parameters. The crystal data and structural clarification
details for ligand 7 and complex 11 are shown in Table 1.
(200 MHz, DMSO d6) d, ppm: 8.150 (s, 1H), 8.096 (J = 15.8 Hz,
t, 1H), 7.887 (J = 7.8 Hz, d, 2H), 7.766 (J = 5.6 Hz, d, 1H), 7.698
(s, 1H), 7.359 (J = 7.6 Hz, d, 1H), 7.211 (s, 1H). The NMR and mass
spectra are shown in the supplementary materials.
2.11. Synthesis of complex 11
A mixture of ligand 7 (0.1181 g, 0.5 mmol) and K2PdCl4
(0.1624 g, 0.5 mmol) was dissolved in acetonitrile (15 mL) and
water (15 mL). The resulting solution was refluxed for 18 hrs at
75 °C, then cooled, evaporated under vacuum, and then extracted
with CH2Cl2 and dried over MgSO4. After that, it was filtered and
evaporated under vacuum, adding an appropriate amount of ether
to produce a yellow powdered solid. Recrystallized from acetoni-
trile and ether resulted in pale yellow needle crystals (Scheme 3).
1H NMR (200 MHz, DMSO d6) d, ppm: 8.073 (J = 5.4 Hz, d, 2H),
7.799 (J = 7.8 Hz, d, 2H), 7.491 (J = 7.8 Hz, s, 1H), 7.372 (s, 1H),
3. Results and discussion
3.1. Synthesis and characterization
The new cyclometalated ligands 7 and 8 were synthesized fol-
lowing the procedure shown in Scheme 1. Compound 1 was
reacted with n-BuLi and N,N- dimethylacetamide via lithiation at
a low temperature and the bromine group on one side was substi-
tuted by a keto group to obtain compound 2. A palladium-cat-
alyzed (Pd) Suzuki cross-coupling reaction with substituted
phenylboronic acid was performed to synthesize compounds 3
and 4 (ketone) from compound 2. These compounds were con-
verted into the intermediate compounds 5 and 6 by refluxing with
N,N- dimethylformamide dimethyl acetal. These resulting ketones
were obtained after purification by column chromatography and
recrystallization. The C, N, Npyrazolyl ligand derivatives (7 and 8)
were conveniently prepared in excellent yields by the
Scheme 2. Synthesis of the cycloplatinated complex: (i) K2PtCl4, glacial acetic acid,
reflux, 24hr.
4

K. Kumarasamy et al.
Polyhedron 191 (2020) 114799
Table 1
3.19 and 3.01 ppm for -N(CH3)2 disappeared in both ligands, and
the doublet for the C@C peaks around d 6.68 ppm also disappeared
due to cyclization of pyrazole group, followed by dehydration to
form an imine, indicating the formation of a pyrazole ring. After
cyclization, the pyrazole NAH might be exchanged with a deuter-
ated solvent, due to which the –NH hydrogen signal is not observed
in the NMR spectrum (Figs. S1–S6, supplementary materials).
This observation was further proven with a 1H–1H 2D NMR
spectrum. In this spectrum, the middle hydrogen atom on the pyr-
idine ring has a peak at d 7.75 ppm and it splits into triplet due to
the influence of the adjacent carbon atoms. The pyrazole hydrogen
atom peaks appear as doublets in the region d 7.64 ppm, and the
chemical shift is also moved more downfield due to the nitrogen
atom. The position of the multiplets at d 7.64–7.61 ppm indicate
the hydrogen atoms on both sides of the pyridine ring. The benzene
hydrogen atoms appear as doublets in the region around d 7.95 and
7.29 ppm. At d 7.59 and 6.80 ppm, two peaks appear which
indicate the pyrazole hydrogen atoms. Ligand 8 has similar
spectral properties, with slight changes in its functional group.
(Figs. S9–S10, supplementary materials).
Crystal data and structure refinement details for 7 and 11.
Parameter
7
11
Empirical formula
Formula weight
Temperature, K
Wavelength, Å
Crystal system
Space group
C
15H13N3
C15H14ClN3OPd
394.14
297(2)
0.71073
Monoclinic
P 21/n
235.28
297(2)
0.71073
Orthorhombic
P b c a
Unit cell dimensions
a, Å
14.8511(11)
90
8.4187(6)
90
19.9196(14)
90
2490.5(3)
8
5.8803(4)
90
15.0753(10)
98.8100(10)
16.4779(11)
90
1443.49(17)
4
1.814
a,°
b, Å
b,°
c, Å
c
,°
Volume, Å3
Z
Density (calc.), Mg/m3
Absorption coefficient,
mmÀ1
1.255
0.077
1.472
F(000)
992
784
Crystal size, mm3
Theta range for data
collection, °
0.76 Â 0.60 Â 0.50
0.85 Â 0.32 Â 0.30
In complex 9, the 1H NMR spectrum clearly shows a singlet
observed in the region around d 8.13 ppm, assigned to the pyrazole
ring hydrogen. The aromatic protons of the coordinated ligands
appear as multiplets in the region around d 8.13–6.86 ppm. These
are magnetically downfield shifted compared with their ligands,
indicating the coordination of the substituted benzene, pyridine,
and pyrazole rings (C, N, N) to platinum and palladium. The aro-
matic methyl protons were also observed as singlets in the region
d 2.260 and 2.264 ppm. In particular, the benzene ring is uncoordi-
nated to the platinum metal, being 1,4 substituted, and the signals
are for two hydrogen atoms. If the hydrogen doublet of the ben-
zene ring is due to coordination with the metal centre, one of the
hydrogen atoms on the benzene ring will have been eliminated
2.04 to 26.02
1.84 to 26.02
Index ranges
À18ꢀhꢀ17, À10ꢀkꢀ7,
À21ꢀlꢀ24
À7ꢀhꢀ4, À15ꢀkꢀ18,
16ꢀlꢀ20
Reflections collected
Independent reflections 2452 [R(int) = 0.0296]
Completeness to
theta = 26.02°
Absorption correction
Max. and min.
transmission
13,020
7969
2839 [R(int) = 0.0286]
99.8%
99.9%
Empirical
1.000000 and 0.710106
Empirical
0.6665 and 0.3677
Refinement method
Full-matrix least-
squares on F2
2452 / 0 / 179
Full-matrix least-
squares on F2
2839 / 2 / 201
Data / restraints /
parameters
Goodness-of-fit on F2
Final R indices
1.037
1.031
R1 = 0.0383,
wR2 = 0.1022
R1 = 0.0478,
wR2 = 0.1110
0.164 and À0.171
R1 = 0.0296,
wR2 = 0.0827
R1 = 0.0326,
wR2 = 0.0863
0.783 and À0.863
before it coordinates with the metal due to
(Figs. S7–S9, supplementary materials).
p-conjugation.
[I > 2sigma(I)]
R indices (all data)
This observation was further proven with the 1H–1H 2D NMR
spectrum for complex 9 which showed the middle hydrogen atom
on the pyridine ring in the region around d 8.727 ppm. The position
of d 8.466 ppm indicates hydrogen atoms on both sides of the pyr-
idine ring and a doublet for the hydrogen atoms on the benzene
ring were observed in the region around d 8.178 and 7.625 ppm;
also a hydrogen singlet was observed in the region d 8.076 ppm.
The benzene and pyrazole hydrogen peaks were observed in the
regions d 7.956 and 8.903 ppm. So, this proves that the complexes
were successfully synthesized (Fig. S12, supplementary materials).
The EI mass spectrometer values of compounds 3, 4, 5, 6 and
ligands 7 and 8 are shown in Figs. S15–S20, respectively. The
molecular ion peaks for ligands 7 and 8 were observed at
m/z = 236.1 (M + 1) for C15H13N3 and m/z = 289.1 (M + 1) for
C15H10F3N3, respectively, which is in excellent agreement with
the theoretical values. This proves that all the compounds and
ligands were successfully synthesized.
Largest diff. peak and
hole, e ÅÀ3
condensation of the resulting compounds 5 and 6 with hydrazine
under basic conditions.
The neutral cycloplatinated complexes 9 and 10 were obtained
in over 60% yields by refluxing the corresponding ligands with
K2PtCl4 in an equimolar ratio under acidic conditions. Similarly,
the neutral cyclopalladated complex 11 was obtained by refluxing
the corresponding ligand with K2PdCl4 in an equimolar ratio in ace-
tonitrile/water for 24 h. The analytical data of the platinum(II) and
palladium(II) complexes are good agreement with the molecular
structures proposed.
The FT-IR spectra of the pyrazolyl ligands 7 and 8 showed med-
ium to strong bands in the region 3241–3230 cmÀ1, which indi-
cates NAH functional groups. The C@O stretching vibrations
were not observed in the ligands due to cyclization of the pyrazole
group. This is further supported by the appearance of new bands in
The FAB mass spectrometer values of complexes 9 and 10 are
shown in Figs. S14 and S15, respectively. The molecular ion peak
for complex 9 was observed at m/z = 465 (M+) for C15H12ClN3Pt.
This is consistent with the molecular weight of the experimental
formula. The molecular ion peak for complex 10 was observed at
m/z = 520 (M + 1) for C15H9ClF3N3Pt, which is also consistent with
the molecular weight of the experimental formula. Thus, this
proves that the complexes were successfully synthesized.
the region 1640 cmÀ1
, which might be C@N fragments.
(Figs. S21–S26, supplementary materials). Upon complexation,
the bands associated with NAH and C@N stretching frequencies
appeared and shifted to lower frequencies by 27–35 cmÀ1 and this
denotes that the ligands coordinate with the metal through C, N
and N atoms. Hence these shifts confirmed the coordination of
the ligand to the palladium metal.
The bonding arrangement is further supported by the 1H NMR
spectra of ligands 7 and 8; all the aromatic protons appeared as
multiplets in the region around d 8.16–6.87 ppm. In ligand 7, the
aromatic methyl proton also appeared as a singlet in the region
around d 2.40 ppm. In addition, the two singlet peaks around d
3.2. Structural analysis of the compounds
3.2.1. Structure analysis of ligand 7
Single crystals of ligand 7 grew effectively by evaporation of
CH3CN/ether solutions. Fig. 2 shows the perspective view of the
5

K. Kumarasamy et al.
Polyhedron 191 (2020) 114799
Fig. 2. Molecular structure of ligand 7 showing the atom labelling scheme.
crystal structure. A summary of the data collection and refinement
parameters of ligand 7 and complex 11 is listed in Table 1 and
selected bond lengths and bond angles are listed in Table 2. The
C(4)-C(5) bond length is 1.379 Å, which is similar to the average
value of the CAC bond in a benzene ring (1.385 Å). The C(8)-C(9)
bond length is 1.375 Å, which is similar to the average value of
the CAC bond on a pyridine ring (1.369 Å). The common bond
length in a pyrazole ring is 1.356 Å. Generally, the CAC bond length
is 1.54 Å, but due to the influence of electron delocalization, the C
(13)-C(14) bond length (1.384 Å) is shorter than the general CAC
bond length. The C(6)-C(7) and C(11)-C(12) bond lengths are
1.483 and 1.463 Å, respectively, which are shorter than the general
CAC bond length, due to the delocalization of electrons. These
three rings are conjugated systems. The hydrogen atom was also
observed on the N(2) atom, which also proves the part that was dif-
ficult to confirm by 1H NMR spectroscopy.
Fig. 3a. Molecular structure of complex 11 showing the atom labelling scheme.
whole ring is not bonded by the metal it is relatively free. After
coordination, it is bonded by the metal, resulting in the configura-
tion being restricted. Therefore, the phenomena of the bond angles
decreasing and the bond lengths changing occur. As all the plat-
inum(II) and palladium(II) complexes showed similar spectral
properties, the other complexes are considered to have a similar
structure to complex 11.Table 5.
3.4. Spectroscopic and luminescent properties
UV–visible absorption and photoluminescence spectra were
recorded in acetonitrile solutions at room temperature. Figs. 4
and 5 shows the absorption spectra of ligands 7 and 8 and com-
plexes 9 and 10, respectively. The emission spectra of complexes
9 and 10 are shown in Fig. 6, and the complete spectroscopic data
are outlined in Table 4. Ligands 7 and 8 exhibit absorption bands at
3.3. Structure analysis of complex 11
The single crystals of complex 11 grew effectively by evapora-
tion of a CH3CN/ether solution. Fig. 3 shows the perspective view
of the crystal structure. Selected bond lengths and bond angles
are given in Table 3. The crystal lattice of complex 11 is monoclinic
and the space group is p 21 /n. The crystal structure shows three
chelate rings composed of the central palladium metal with the
ligand. The position of the chloride ion bond forms a four-coordi-
nated neutral molecule. The average benzene ring bond length is
1.393 Å, only the C(1)-C(6) bond length is longer (by about
0.03 Å more than before coordination). In the pyridine ring part,
the N(1)-C(7) and N(1)-C(11) bond lengths are also longer than
that before coordination, because the configuration after coordina-
tion is strained by the metal, resulting in a slight distortion. The N
(1)-Pd(1)-Cl(1) bond angle is 177.60° (close to 180°), which is con-
sistent with previous literature values [8]. The difference between
the maximum and minimum values of the angle is 9.34° and after
coordination the difference between the maximum and minimum
bond angles is only 6.1° (Table 4). Before coordination, the pyrazole
ring as a whole is closer to a regular pentagon, because when the
240–250 and 300–310 nm, respectively, corresponding to (
p
(L) ? * (L)) and n ? * ligand–ligand charge transfer transitions.
p
p
From the experimental data, the electronic effect of the sub-
stituents was observed. Compared to a weakly activating com-
pound (H) [7], when a substituent is an electron-donating group,
the absorption wavelength shifts to a longer wavelength by 7 nm
(from 242 nm for the H-substituted compounds), but when a sub-
stituent is an electron-withdrawing group, the absorption wave-
length shifts to a shorter wavelength by 2 nm, which is around
300 nm, the change range of the absorption band is similar. So,
the energy level change is very small due to the red-shift
phenomenon.
Here, complexes 9 and 10 exhibit intense absorption bands in
the region 410–240 nm. The absorption spectra of the cyclometa-
lated platinum(II) complexes exhibit three strong intense absorp-
tion bands around 275–375 nm which are assigned to (
(L)) and n ? * ligand–ligand charge transfer (LLCT) transitions.
The lowest energy absorption bands appear in the region around
390 nm and are described as (d (Pt) ? *(L)) metal–ligand charge
p(L) ? p*
p
p
p
transfer (MLCT) transitions. Both complexes show an absorption
tail at 410–475 nm, which indicates a spin forbidden metal–ligand
charge-transfer transition. From the experimental data, an elec-
tronic effect for the substituent is also observed. So, these com-
plexes exhibit the longest absorption wavelength.
The variation range is from 340 to 353 nm. When comparing
complex 9 to complex 10, complex 9 displays the longest absorp-
tion wavelength in the region 353 nm and the absorption wave-
length shifted to a longer wavelength by 13 nm (340 nm) for
Table 2
Selected bond lengths (Å) and angles (deg) for ligand 7.
Bond lengths
C(6)-C(7)
C(13)-C(14)
1.4839(17)
1.384(2)
Bond angles
116.53(11)
115.31(11)
N(2)-N(3)
C(12)-N(2)
1.3505(16)
1.3392(17)
C(6)-C(7)-N(1)
N(1)-C(11)-C(12)
C(11)-C(12)-N(2)
C(1)-C(6)-C(7)
120.55(11)
120.45(11)
6

K. Kumarasamy et al.
Polyhedron 191 (2020) 114799
Fig. 3b. Crystal packing pattern of complex 11.
Table 3
Table 5
Selected bond lengths (Å) and angles (deg) for complex 11.
UV–Visible absorption data of ligands 7 and 8, and complexes 9 and 10 in acetonitrile
solution at ambient temperature.
Bond lengths
Ligands and Complexes
k/nm(emax /dm3, molÀ1 cmÀ1
)
Pd(1)-C(1)
Pd(1)-N(1)
Pd(1)-N(2)
1.979(2)
1.982(2)
2.177(2)
Bond angles
81.57(9)
77.49(8)
104.43(6)
N(2)-N(3)
Pd(1)-Cl(1)
1.342(3)
2.3292(7)
7
8
9
10
249(26373), 305(12359)
240(25315), 303(10830)
249(27627), 304(16720), 353(7260), 390(2060)
249(19403), 301(13841), 340(5816), 391(1867)
C(1)-Pd-N(1)
N(1)-Pd-N(2)
N(2)-Pd-Cl(1)
C(1)-Pd-Cl(1)
C(1)-Pd-N(2)
N(1)-Pd-Cl(1)
96.50(7)
159.06(9)
177.60(6)
Table 4
Selective bond lengths (Å) and bond angles (deg) before and after coordination for
ligand 7 and complex 11.
Bond lengths
Ligand 7
Complex 11
(Before coordination)
1.3904
1.3392
(After coordination)
C(1)-C(6)
N(1)-C(7)
N(1)-C(11)
1.419
1.345
1.352
1.3457
Bond angles
C(12)-N(2)-N(3)
N(2)-C(12)-C(13)
C(12)-C(13)-C(14)
C(13)-C(14)-N(3)
C(14)-N(3)-N(2)
Max-Min
113.11
105.95
105.12
112.03
103.77
9.34
106.0
110.5
104.5
108.4
110.6
6.1
Fig. 4. UV–visible spectra of ligands 7 and 8 in acetonitrile solution at room
temperature.
7

K. Kumarasamy et al.
Polyhedron 191 (2020) 114799
observed that the bond length of the pyrazole ring, C(13)-C(14),
was shorter than ordinary C-C bonds, mainly due to the effect of
delocalization. In future work, we will focus on double coordina-
tion and protonation with different transition metals to explore
the optical and physical properties.
CRediT authorship contribution statement
Keerthika Kumarasamy: Validation, Investigation, Resources,
Data curation, Writing - original draft, Visualization. Tamiloli
Devendhiran: Investigation, Writing - original draft. Mei-Ching
Lin: Conceptualization, Methodology, Software, Formal analysis,
Resources, Writing - original draft, Visualization, Supervision, Pro-
ject administration. Chi-Wen Chiu: Software, Investigation, Data
curation.
Declaration of Competing Interest
Fig. 5. UV–visible spectra of complexes 9 and 10 in acetonitrile solution at room
temperature.
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared
to influence the work reported in this paper.
Acknowledgments
We thank the Ministry of Science and Technology (MOST
98-2113-M-324-003-MY2), Taiwan for support to this research.
Appendix A. Supplementary data
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.poly.2020.114799.
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9