Two new palladium and platinum complexes with a bidentate pyrazole-based ligand: Crystal structure, fluorescence and Hirshfeld surface analysis

Article abstract of DOI:10.1007/s13738-015-0797-3

Two new precious metal coordination complexes [Pd(HL)₂] (1) and [PtL₂] (2) (where H₂L is 1-(carboxymethyl)-1H-pyrazole-3-carboxylic acid) were synthesized by one-pot in situ hydrolysis. The complexes are assembled into a 3D structure by hydrogen bonding interactions and intermolecular contacts. The structures have been established by single-crystal X-ray diffraction, and characterized by FT-IR and liquid state fluorescent spectroscopy. Hirshfeld surface analysis reveals that the OH contacts outnumber the other contacts in both structures (49.4 % of the total interactions for 1 and 41.6 % for 2).

Full text of DOI:10.1007/s13738-015-0797-3

J IRAN CHEM SOC
DOI 10.1007/s13738-015-0797-3
ORIGINAL PAPER
Two new palladium and platinum complexes with a bidentate
pyrazole‑based ligand: crystal structure, fluorescence
and Hirshfeld surface analysis
Chao Feng1 · Xue‑Jing Li1 · Duo Zhang1 · Zhi‑Rong Qu2 · Hong Zhao1
Received: 30 April 2015 / Accepted: 11 December 2015
© Iranian Chemical Society 2016
Abstract Two new precious metal coordination com-
plexes [Pd(HL)2] (1) and [PtL2] (2) (where H2L is
1-(carboxymethyl)-1H-pyrazole-3-carboxylic acid) were
synthesized by one-pot in situ hydrolysis. The complexes are
assembled into a 3D structure by hydrogen bonding interac-
tions and intermolecular contacts. The structures have been
established by single-crystal X-ray diffraction, and charac-
terized by FT-IR and liquid state fluorescent spectroscopy.
Hirshfeld surface analysis reveals that the O···H contacts
outnumber the other contacts in both structures (49.4 % of
the total interactions for 1 and 41.6 % for 2).
bridging ligands [3]. Some of their metal complexes have
been used as precursors for chemical vapor phase depo-
sition and found to show luminescent properties [4]. The
pyrazole–carboxylate segments can be integrated as part of
many polydentate donors [5]. The polypyrazolylborates are
among the most useful and widely employed ligands [6].
Coordination chemistry of palladium(II) and platinum(II)
complexes with heterocyclic ligands with two potential
donor atoms is one of the research areas that has undergone
a fast development during the last years [7]. Among all the
examples reported so far, those containing pyrazole deriva-
tives are especially attractive because they have multiple
applications. For example, the complex of Pt(II) with some
pyridylpyrazole ligand has shown greater antitumor activity
and lower toxicity than the common cis-[PtCl2(NH3)2] com-
plex [8]. Applications of palladium and platinum complexes
including pyrazolic ligands extend also to macromolecular
chemistry [9] and homogeneous catalysis [10].
Keywords Precious metal complex · In situ ·
3D structure · Fluorescent spectroscopy
Introduction
Pyrazolyl ligands are among the important building units
used in ligand designing [1] and crystal engineering [2].
The pyrazole carboxylate are known as multidentate
More recently, our group has strong interests in intro-
ducing a carboxylic group to the pyrazole rings, several
amazing MOFs have been successfully synthesized [11].
Our findings show that the pyrazole carboxylate ligands
bearing bulky conjugate groups can indicate strong coor-
dination ability and various coordination modes. This
prompted us to prepare more novel ligands to investigate
their coordination features. Herein, we adopt ester group
to modify the pyrazole unit to prepare one novel organic
ligand, 1-(2-ethoxy-2-oxoethyl)-1H-pyrazole-3-carboxy-
late (Eopzc). To the best of our knowledge, this ligand has
been never reported so far. However, in order to further
enhance the ability of this ligand, we use one-pot in situ
hydrolysis to obtain two new complexes [Pd(HL)2] (1) and
[PtL2] (2) (where H2L is 1-(carboxymethyl)-1H-pyrazole-
3-carboxylic acid). The photoluminescence property of
complexes 1 and 2 has been investigated.
Electronic supplementary material The online version of this
article (doi:10.1007/s13738-015-0797-3) contains supplementary
material, which is available to authorized users.
* Zhi-Rong Qu
quzr@hznu.edu.cn
* Hong Zhao
zhaohong@seu.edu.cn
1
School of Chemistry and Chemical Engineering, Southeast
University, Nanjing 211189, People’s Republic of China
2
Key Laboratory of Organosilicon Chemistry and Material
Technology of the Ministry of Education, Hangzhou Normal
University, Hangzhou 311121, People’s Republic of China
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J IRAN CHEM SOC
168.29, 159.50, 139.01, 132.95, 111.87, 61.55, 61.48,
53.77, 14.44, 14.40.
Experimental
General
Synthesis of [Pd(HL)2] (1)
All chemicals were commercially available and used as
received without further purification. The C, H, N micro-
analyses were carried out with an Elemental Vario-EL
CHNS elemental analyzer. The FT-IR spectra were recorded
from KBr pellets in the range of 4000–400 cm−1 on a Bio-
Rad FTS-7 spectrometer. The crystal structure was deter-
mined by single-crystal X-ray diffraction and SHELXL
crystallographic software of molecular structure. The
synthesis procedure of Eopz, complexes 1 and 2 seen in
Scheme 1.
A mixture of PdCl2 (0.0354 g, 0.2 mmol), Eopzc (0.0226 g,
0.1 mmol), KOH (0.0112 g 0.2 mmol), water (5 mL) and
ethanol (10 mL) was stirred for 10 min at room temperature
with the pH value of 8.0. The resulting solution was put into
a Teflon-lined autoclave. The reaction mixture was heated at
110 °C for 4 days, followed by slow cooling to room tem-
perature and phase pure crystals of 1 were obtained by man-
ual separation (yield: 0.0135 g, ca. 30.36 % based on Eopzc
ligand). Anal. Calcd for C12H10N4O8Pd (Mr = 444.64): C,
32.39; H, 2.25; N, 12.60 %, Found: C, 32.28; H, 2.32; N,
12.53 %. IR (cm−1): 3238 (w), 1722 (s), 1640 (s), 1340 (s),
1266 (w), 1240 (s), 1211 (s), 830 (m), 782 (w).
Synthesis of Eopzc
To solution of ethanol (50 mL) and 3-pyrazolecarbox-
ylic acid (1.112 g 10 mmol) at 80 °C was added concen-
trated sulfuric acid (2 mL), and after 2 h of stirring the
ethyl 1H-pyrazole-3-carboxylate was obtained (1.350 g,
9.6 mmol, 96 %). To 20 mL three-necked round bot-
tomed flask equipped with a refluxing condenser was
added 1H-pyrazole-3-carboxylate (0.7 g, 5 mmol), ethyl
bromoacetate (0.835 g, 0.6 mmol) in 30 mL anhydrous
acetone. This suspension was stirred at 60 °C and the
reaction was monitored by TLC. When the 1H-pyrazole-
3-carboxylate was consumed, the yellow brown reaction
mixture was concentrated to dryness. The crude product
was purified by column chromatography on silica gel
with petroleum ether/ethyl acetate (1/1) as eluent to give
the liquid state product Eopzc in yield of 55 % (0.622 g,
Synthesis of [PtL2] (2)
The synthetic method of 2 is similar to 1, only the salt
PdCl2 replaced by K2PtCl6. Finally, phase pure crys-
tals of 2 were obtained by manual separation (yield:
0.0112 g, ca. 21.08 % based on Eopzc ligand). Anal. Calcd
for C12H8N4O8Pt (Mr = 531.31): C, 27.10; H, 1.51; N,
12.05 %, Found: C, 26.05; H, 1.62; N, 11.08 %. IR (cm−1):
3124 (w), 1734 (m), 1654 (s), 1340 (s), 1266 (w), 1238 (s),
1212 (m), 834 (m), 780 (w).
Crystal structure determination
The diffraction data of two complexes were collected on
Agilent G8910A CCD diffractometer with graphite mono-
chromated Mo–Kα radiation (λ = 0.71073 Ǻ) and using
the ω–θ scan mode in the ranges 3.3º ≤ θ ≤ 29.0º (1),
4.1º ≤ θ ≤ 29.2º (2), respectively. Raw frame data were
integrated with the SAINT program. The structures were
solved by direct methods using SHELXS-97 and refined by
full-matrix least-squares on F2 using SHELXS-97 [12]. An
empirical absorption correction was applied with the pro-
gram SADABS [12]. All non-hydrogen atoms were refined
anisotropically. All hydrogen atoms were positioned geo-
metrically and refined as riding. Calculations and graphics
were performed with SHELXTL [12]. The crystallographic
details are provided in Table 1. Selected bond distances and
angles of the complexes 1 and 2 are listed in Table 2. The
hydrogen bonds and angles of complexes 1 and 2 have been
listed in Table 3. Crystallographic data for the structural
analysis have been deposited with the Cambridge Crys-
tallographic Data Centre (CCDC numbers: 1061337 and
1061338).
1
2.7 mmol). H NMR (DMSO-d6, 400 MHz): δ 7.60 (d,
J = 2 Hz, 1H), 6.92 (d, J = 2 Hz, 1H), 5.29 (s, 2H), 4.28–
4.22 (m, 2H), 4.15–4.10 (m, 2H), 1.27–1.24 (m, 3H),
1.20–1.16 (m, 3H); 13C NMR (DMSO-d6, 100 MHz):
Scheme 1 The synthesis procedure of Eopz, complexes 1 and 2
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J IRAN CHEM SOC
Table 1 Crystallographic and experimental data for 1 and 2
Results and discussion
Complexes
1
2
Crystal structure of [Pd(HL)2] (1)
Formula
Mr
C12H10N4O8Pd
444.64
C12H8N4O8Pt
531.31
Complex 1 crystallizes in the monoclinic system with the
P2/c space group, and each asymmetric unit consists of
one-half divalent Pd(II) ion and one deprotonated ligand
(HL), shown in Figure S1. The molecular structure is
illustrated in Fig. 1. The structure consists of discrete cen-
trosymmetric [Pd(HL)2] molecules linked by intermolecu-
lar forces. 1 is a monomeric molecule containing Pd(II)
coordinated in a square planar environment, by two oxygen
atoms from carboxyl groups and two nitrogen atoms in the
pyrazole rings. The angles N1-Pd1-O1 and N1i-Pd1-O1i are
80.60 (7)º, which means that the metallic atoms lie in the
centre of the plane determined by the two nitrogen atoms
and the two oxygen atoms. HL ligand behaves as a biden-
tate ligand and uses only one of its two carboxyl groups,
forming a five membered metallocycle, which has a planar
configuration. The bond distances Pd–N (1.9894 (19) Å)
are similar to those of Pd–Npz (1.979 (3) Å) [13]. And these
distances are consistent with previously described values
(1.960 (10)–2.125 (10) Å) and (1.968 (2)–2.153 (2) Å) [14,
15], respectively. The Pd–O bond lengths 2.0042 (16) and
2.0043 (16) Å, can be regarded as normal compared with
the distances found in the literature (1.9877 (19)–2.014
(2) Å) [16].
Crystal system
Space group
a (Å)
Monoclinic
P2/c
Monoclinic
P2
9.4973 (6)
5.0226 (3)
17.5967 (11)
119.968 (4)
727.16 (8)
440
8.9128 (6)
4.9667 (2)
9.2417 (5)
116.556 (8)
365.94 (3)
250
b (Å)
c (Å)
β (°)
V (Å3)
F(000)
Z
2
1
Dc (g cm−3
)
2.031
2.411
μ (mm−1
)
1.33
9.64
θ (°)
4.1–27.1
2889, 1578
1338
4.1–27.1
1541, 1258
1258
Ref. meas./indep
Obs. ref.[I > 2σ(I)]
Rint
R1[I ≥ 2σ (I)]a
ωR2 (all data)b
Goof
0.018
0.026
0.022
0.041
0.048
0.099
1.01
1.09
Δρ (max, min) (eÅ−3
)
0.37, −0.36
1.82, −1.40
a
R1 = Σ||Fo| − |Fc||/Σ|Fo|
wR2 = [Σw (|F2o| − |F2c|)2/Σw (|Fo2|)2]1/2
b
Table 2 Selected bond lengths (Å) and angles (°) for 1 and 2
There exists intermolecular hydrogen bonds O4–H4···O2ii,
C3–H3A···O2iii, C4–H4A···O3iv and C5–H5A···O3iv con-
structing the complex 1 into 3D structure as shown in Fig. 2
(Symmetry codes: (ii) x − 1, −y + 1, z − 1/2; (iii) –x + 1,
−y + 2, −z + 1; (iv) x, y + 1, z).
1
Pd1–N1
Pd1–N1i
N1–Pd1–N1i
N1–Pd1–O1i
N1i–Pd1–O1i
1.9894 (19)
1.9894 (19)
180.0
Pd1–O1i
2.0042 (16)
2.0043 (16)
80.60 (7)
Pd1–O1
N1–Pd1–O1
N1i–Pd1–O1
O1i–Pd1–O1
99.40 (7)
80.60 (7)
99.40 (7)
180.000 (1)
Crystal structure of [PtL2] (2)
Symmetry code: (i) −x, y, −z + 1
2
Single-crystal X-ray diffraction analysis of complex 2
reveals that the asymmetric unit consists of one-half Pt(IV)
ion and L2− which deprotonated two H atoms from car-
boxyl groups, shown in Figure S2. The molecular struc-
ture is illustrated in Fig. 3. The coordination geometry
around Pt(IV) is a distorted tetrahedron with bond angles
Pt1–N1i
1.982 (11)
1.982 (11)
93.2 (7)
Pt1–O1i
2.019 (7)
2.019 (7)
101.4 (10)
77.0 (10)
178 (3)
Pt1–N1
Pt1–O1
N1i–Pt1–N1
N1i–Pt1–O1i
N1–Pt1–O1i
N1i–Pt1–O1
N1–Pt1–O1
O1i–Pt1–O1
77.0 (10)
101.4 (10)
Symmetry code: (i) −x, y, −z +1
Table 3 Hydrogen bonds and
angles in 1 and 2
Complex
D–H···A
D–H (Å)
H···A (Å)
D···A (Å)
D–H···A (Å)
1
O4–H4···O2ii
0.67 (3)
0.93
1.99 (3)
2.43
2.653 (2)
3.302 (3)
3.216 (3)
3.287 (3)
173 (4)
156
C3–H3A···O2iii
C4–H4A···O3iv
C5–H5A···O3iv
0.93
2.52
132
0.97
2.40
151
Symmetry codes: (ii) x − 1, −y + 1, z − 1/2; (iii) –x + 1, −y + 2, −z + 1; (iv) x, y + 1, z
2
C4–H4A···O3i
C4–H4A···O3ii
C5–H5B···O3i
0.93
0.93
0.97
2.48
2.50
2.22
3.16 (2)
3.33 (2)
3.12 (3)
130
148
155
Symmetry codes: (i) x, −1 + y, z, (ii) 1 − x, −1 + y, 1 − z)
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J IRAN CHEM SOC
Fig. 1 Molecular structure of 1
N1–Pt1–O1 77.0 (10)º and N1i–Pt1–O1 101.4 (10)º, and the
dihedral angles of [C1, O1, Pt1, N1, C2] and [C1A, O1A,
Pt1, N1A, C2A] is 62.20 (3)º. The metal atom is coordi-
nated to two H2L ligands via two pyrazole nitrogen and two
carboxyl oxygen. H2L behaves as a bidentate ligand and
uses only one of its two carboxyl groups, forming a five-
membered metallocycle, which has a planar configuration.
The bond distances Pt–Npz (1.982 (11) Å) are shorter than
those of Pt–Npz (2.014(3) Å) reported [17–19]. The Pt–O
bond lengths (2.019 (7) Å) can be regarded as normal com-
pared with the distances found in the literature (2.010(6)–
2.035 (5) Å) [20]. The N1–Pt–N1A bite angle, 93.2 (7)º, is
comparable to that found in the structure with the ligands 3,
5- bis(2-pyridyl)pyrazole (pz1) [Cu4(pz1)4(ClO4)4]·2H2O,
80.3(2)º [21].
Fig. 3 Molecular structure of 2
In this complex, the centroid–centroid separations of the
pyrazole rings from adjacent H2L are 4.596 (9) Å, show-
ing the existence of intramolecular weak π···π stacking
interactions. The adjacent [PtL2] units are linked together
through intermolecular C–H···O H-bonding interactions
between the uncoordinated O atoms of carboxyl groups
and H-atoms of pyrazole rings (C4–H4A···O3i, ii, C5–
H5B···O3i, symmetry codes: (i) x, −1 + y, z, (ii) 1 − x,
−1 + y, 1 − z), thus 2D layers are formed in Fig. 4. The
uncoordinated O2 and O4 further linked the layers into 3D
structure by intermolecular contacts with distance O···O is
1.924 (12) Ǻ.
Fig. 2 3D structure formed by intermolecular hydrogen bonds
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J IRAN CHEM SOC
Fig. 4 3D structure formed by C–H···O hydrogen bonds and O···O contacts
complexes are much weaker than that of the ligand. This
could be influenced by the change of the electron confor-
mation when the ligands coordinate to the metals, and the
hydrogen bonding interactions among the complexes.
Fluorescent spectra
Both the coordinated metals in complexes 1 and 2 have d10
electron configuration. The luminescence spectra of 1 and
2 and the free ligand Eopzc were investigated in the liq-
uid state at room temperature (Fig. 5). The luminescence
spectrum of Eopzc shows emission at λmax = 327 nm
(λex = 298 nm). The emissions of 1 and 2 are also at
327 nm (λex = 298 nm) assigned to intraligand (π–π*) flu-
orescence, which is modified by the Pd(II) and Pt(IV) ions.
On comparison with the free ligand, the emissions of the
Hirshfeld surface
Hirshfeld surface and fingerprint plot analysis could offer
rapid reckonable understanding into the intermolecular
interactions in complex molecular solids as well as crystal
structures by color-coding short or long contacts, which acts
as an easier and considerably faster graphical tool which
is based on 3D Hirshfeld surface and 2D fingerprint plots,
and gives a quantitative summary of the nature and type
of intermolecular contacts experienced by the molecule in
complexes 1 and 2. The Hirshfeld surface is mapped with
dnorm, and 2D fingerprint plots presentedin this paper were
generated using CrystalExplorer 3.1 [22]. In this section, we
study the molecular Hirshfeld surface of H2L in the com-
plexes, to elucidate the features of different supramolecular
synthons. The 3D Hirshfeld surface and 2D fingerprint plots
of 1 and 2 are shown in Fig. 6. They clearly show the influ-
ences of different relationship on the intermolecular inter-
actions of the complexes. The large and deep red spots on
the 3D Hirshfeld surfaces indicate the close-contact inter-
actions, which are mainly responsible for the significant
hydrogen bonding contacts, while the blue color points in
Fig. 5 Liquid-state fluorescent emission spectra of 1, 2 and Eopza at
room temperature
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J IRAN CHEM SOC
Fig. 6 Hirshfeld dnorm surface and fingerprint plots for complexes 1 and 2
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J IRAN CHEM SOC
Fig. 7 Hirshfeld surface calcu-
lations for complexes 1 and 2
the 2D fingerprint plots are indicative of short contacts of
H···H, O···O, and H···O interactions [23–27].
when the ligands coordinate to the metals. Hirshfeld surface
analysis was studied on complexes 1 and 2, and reveals that
the most interactions in both complexes are O···H contacts.
On the basis of the present work, our group will study anti-
bacterial based on the palladium and platinum complexes.
For complex 1, H···H interactions, which are reflected in
the middle of scattered points in the 2D fingerprint plots,
have the most significant contribution (11.6 %) to the total
Hirshfeld surface. The O···H hydrogen bonding intermolec-
ular interaction appear as a fork in the 2D fingerprint plots,
which has 49.4 % contribution to the total Hirshfeld surface
and match with O–H···O hydrogen bonds in 1. For complex
2, O···O interaction appear as a spike in the 2D fingerprint
plots, which has 15.2 % contribution to the total Hirshfeld
surface. The main interaction of 2 is also O···H contact and
match the C–H···O hydrogen bonds, as shown in Fig. 7.
Acknowledgments We gratefully acknowledge the financial sup-
port of the National Natural Science Foundation of China (20801012)
and the financial support from Jiangsu Ainaji Neoenergy Science and
Technology Co., Ltd (8507040091).
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