Synthesis and crystal structures of copper(II) and silver(I) complexes of a semi-rigid bipyrazolyl ligand

Article abstract of DOI:10.1007/s11243-010-9338-1

A semi-rigid bipyrazolyl ligand, namely 5-tert-butyl-1,3-bis[(3′, 5′-diethyl-1H-pyrazol-4′-yl) methylene]ben-zene, and its Ag(I) and Cu(II) complexes have been prepared and structurally characterized. X-ray analysis demonstrates that the Ag(I) complex is

Full text of DOI:10.1007/s11243-010-9338-1

Transition Met Chem (2010) 35:381–386
DOI 10.1007/s11243-010-9338-1
Synthesis and crystal structures of copper(II) and silver(I)
complexes of a semi-rigid bipyrazolyl ligand
•
Guang Yang Shu-Juan Hao Zhao-Yang Wang
•
Received: 19 December 2009 / Accepted: 4 February 2010 / Published online: 19 February 2010
Ó Springer Science+Business Media B.V. 2010
Abstract A semi-rigid bipyrazolyl ligand, namely 5-tert-
butyl-1,3-bis[(30,50-diethyl-1H-pyrazol-40-yl) methylene]ben-
zene, and its Ag(I) and Cu(II) complexes have been prepared
and structurally characterized. X-ray analysis demonstrates
that the Ag(I) complex is based on a dinuclear molecular
rectangle, while the Cu(II) complex displays a mono-strand
helical structure. Two different conformations, namely cis,cis
and cis,trans have been observed for this bipyrazolyl ligand.
form a wide variety of polymeric structures with transition
metal ions. The silver(I) and copper(I) complexes of the
deprotonated bipyrazolyl ligand exhibit rather complicated
3D metal-organic framework (MOF) structures, featuring
a triangular [M(pz)]3 subunit (M = Cu(I) or Ag(I); pz =
pyrazolide) [6–10]. Some bipyrazolide-bridged metallo-
macrocycles with diplatinum or dipalladium corners were
reported recently [11, 12]. We noticed that so far, only
rigid bipyrazolyl ligands have been used in preparation of
metal complexes, although flexible or semi-rigid bipyraz-
oles have been known for some time [13]. As pointed out
by Steel, introducing some flexibility to the ligands can
lead to structural motifs and topologies, which may be
unavailable by using more rigid ligands [14]. To address
this issue, we report here the synthesis and crystal struc-
tures of a semi-rigid C4-linked bipyrazolyl ligand (H2L,
Scheme 1) and its Cu(II) and Ag(I) complexes.
Introduction
Pyrazole is a widely used N-donating ligand in coordina-
tion chemistry. 1H-Pyrazole usually acts as a monodentate
ligand; its deprotonated form, namely pyrazolido ion, can
behave in exo-bidentate or multidentate bridging fashions
˚
with typical MꢀꢀꢀM distances of ca 3.3 A [1–4]. Bipyraz-
olyl ligands, in which two pyrazolyl units are linked via
covalent bonds, are expected to exhibit more versatile
coordination patterns, and therefore be more useful in
construction of complicated supramolecular architectures.
It has been shown that functionalization at the pyrazolyl N1
position results in behavior somewhat like a pyridyl group,
since only one of the two N atoms of pyrazole is available
for coordination in this case [5]. As might be anticipated,
functionalization at the C4 position of a pyrazole imparts
less influence on its coordination behavior. Recent years
have seen several reports involving metal complexes of
the C4-linked bipyrazolyl ligands. 3,30-5,50-Tetramethyl-
4,40-bipyrazole can act as a neutral l2-bridging ligand to
Results and discussion
The bipyraozlyl ligand, 5-tert-butyl-1,3-bis[(30,50-diethyl-
1H-pyrazol-40-yl) methylene]benzene H2L, was prepared
according to the synthetic route shown in Scheme 1. H2L
crystallizes in space group C2/c. In the asymmetric unit,
only half of the molecule is independent and it is related
with the other half by a twofold axis. As depicted in Fig. 1a,
this compound adopts a twisted cis,cis-conformation (the
assignment of cis or trans conformation of each pyrazolyl
ring is made by using the H atoms attached to the phenyl C2
atom as reference, Scheme 2). The two pyrazolyl rings in
each molecule are placed with a dihedral angle of 25.9°.
Furthermore, the two pyrazolyl rings are skewed in order to
diminish the steric repulsion caused by the 3,5-ethyl groups
of the pyrazolyl ring, which can be described by the torsion
G. Yang (&) ꢀ S.-J. Hao ꢀ Z.-Y. Wang
Department of Chemistry, Zhengzhou University,
Zhengzhou 450001, China
e-mail: yang@zzu.edu.cn
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Transition Met Chem (2010) 35:381–386
Br
The complex [Ag2(H2L)2](NO3)2 adopts a discrete
rectangular structure. As shown in Fig. 2a and b, the Ag(I)
atom is linear two-coordinate with the two coordinated
pyrazolyl rings in a trans-fashion (Scheme 3). For
bis(pyrazole) silver(I) complexes, all the three possible
arrangements in Scheme 3 have been observed [15–18].
Each H2L ligand, in cis,cis-conformation (Scheme 2),
binds to two Ag(I) atoms using its pyrazolyl N2 atoms.
t-Bu
+
H3C
t-BuOK
2
CH3
O
O
Br
CH3
CH3
O
N
NH
O
NH2NH2
CH3
CH3
˚
t-Bu
The intramolecular AgꢀꢀꢀAg distance is 5.825(3) A. On the
t-Bu
CH3
other hand, the intermolecular AgꢀꢀꢀAg distance is
CH3
N
˚
3.192(3) A, less than the sum of the van der Waals radii
O
NH
˚
for the Ag atoms (3.44 A). If this weak interaction is
O
CH3
CH3
taken into account, the disilver complex exhibits a 1D
chain structure running along the a-axis and linking the
dimers through argentophilicity (Fig. 2c). The two pyr-
azolyl rings of each H2L ligand in [Ag2(H2L)2](NO3)2
are nearly parallel, with dihedral angles being 4.9 and
6.2°. The torsion angles are 36.17(57) and 37.60(64)° for
C4–C8–C26–C22 and C30–C34–C52–C48, respectively,
which are much less than that in free H2L (131.40°). A
few similar rectangular dimetallomacrocycles based on
various metals and ligands are also known in the literature
[19–23].
Scheme 1 Synthetic route for H2L
angle defined by the two pyrazolyl C4 atoms and two
methylene C atoms. For H2L, this angle (C4–C8–C8a–C4a)
is 131.40°.
In the crystal of H2L, this compound exhibits a stair-like
chain structure, involving Npz–HꢀꢀꢀNpz and Npz–HꢀꢀꢀOw
i
ii
˚
˚
H-bonds(N2ꢀꢀꢀN2 = 2.922(4)A, N1ꢀꢀꢀO1w = 2.827(3) A;
symmetry codes: (1) –x ? 1, y, -z ? 5/2; (2) x ? 1/2, y - 1/2,
z ? 1), shown in Fig. 1b. Due to the disorder of the H atom
bound to the N atom of the pyrazolyl ring, the occupancy
factor for these H atoms is 0.5. Therefore, the H-bonding
of Npz–HꢀꢀꢀOwꢀꢀꢀH–Npz is only equivalent to one normal
Npz–HꢀꢀꢀOw bond.
The complex [Cu(H2L)Cl2]n exhibits a chiral chain
structure. The Cu(II) atom is tetrahedrally coordinated
by two Cl- ions and two pyrazolyl N2 atoms from two
adjacent ligands (Fig. 3a). The ligand adopts a cis,trans-
conformation and acts as a bridge linking two Cu(II) atoms,
Fig. 1 a Ball-and-stick diagram
of the bipyrazole H2L.
N2
a
b
N1
Hydrogen atoms are not shown
for clarity. Symmetry codes: (a)
-x, y, -z ? 3/2. b A stair-like
chain formed by linking the
bipyrazole via hydrogen bonds
(dashed lines). The t-butyl and
ethyl groups have been omitted
for clarity. Only H atoms
C4
C8
C8a
C4a
of water are shown
N1a
N2a
Scheme 2 Three possible
conformations of bipyrazole
H2L
NH
N
H
N
N
N
HN
N
N
H
N
N
H
N
N
H
trans,trans
cis,cis
cis,trans
123

Transition Met Chem (2010) 35:381–386
383
Fig. 2 a Ball-and-stick diagram
of dimeric [Ag2(H2L)2](NO3)2
with atom labels, showing the
coordination environment
N6
N5
a
Ag2
N4
N3
C22
C34
C26
C30
around Ag and the cis,cis-
conformation of the l2-bridging
ligand. Only N-bound H atoms
are shown. Selected distances
˚
(A) and angles (°):
Ag1–N7 = 2.100(5),
Ag1–N1 = 2.105(6),
Ag2–N3 = 2.090(5),
Ag2–N5 = 2.104(5);
C8
C4
C48
N1
N7
C51
N7–Ag1–N1 = 174.9(2),
N3–Ag2–N5 = 176.8(2). b Top
view of [Ag2(H2L)2](NO3)2,
noting that the two pyrazolyl
rings of the ligand in the Ag
complex are much less twisted
than in free H2L and the two
pyrazolyl rings are trans about
each Ag atom. The t-butyl and
ethyl groups of the ligands have
been omitted for clarity. c The
dinuclear silver complexes are
joined into a chain running
along a-axis via argentophilic
interactions.
N2
Ag1
N8
b
i
˚
Ag1ꢀꢀꢀAg2 = 3.1924(11) A,
symmetry code: (i) x-1, y, z
c
H
N
Scheme 3 Various placements
of pyrazoles around linear
two-coordinate Ag-atom
Ag
N
N
Ag
N
N
N
Ag
N
N
N
N
N
N
H
H
H
H
H
trans
cis
gauche
˚
forming 21 helices along the b-axis with a pitch of 9.433 A
(Scheme 2 and Fig. 3b). Within the chain, Npz–HꢀꢀꢀCl
H-bonds are formed, which may further consolidate the
observed chiral chain structure. As shown in Fig. 2b,
each helix is linked, via interstrand H-bonds, with two
neighboring helices of opposite handedness to form two-
dimensional nets. As a result of this kind of arrangement, the
crystal as a whole is centrosymmetric. Other metal–organic
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Transition Met Chem (2010) 35:381–386
a
b
N4
N2
N3
N1
Cu1b
Cu1
Cl2
Cl1
N3a
Cu1a
N4a
Fig. 3 a Ball-and-stick diagram of a fragment of 1D [Cu(H2L)Cl2]n
with atom labels, showing the coordination environment around Cu1,
the cis,trans-conformation of the l2-bridging ligand and the intra-
strand hydrogen bonds (dashed line). The t-butyl and ethyl groups of
the ligands have been omitted for clarity. Only N-bound H atoms are
139.67(7), N1–Cu1–Cl2 = 96.51(5), N3a–Cu1–Cl2 = 100.14(5),
N1–Cu1–Cl1 = 98.88(5), N3a–Cu1–Cl1 = 94.02(6), Cl2–Cu1–
Cl1 = 136.17(3). Symmetry codes: (a) –x ? 3/2, y ? 1/2, -z ? 1/2;
(b) -x ? 3/2, y - 1/2, -z ? 1/2. b Packing diagram of [Cu(H2L)Cl2]n,
viewed along the b-axis. The dashed lines here represent the inter-
strand hydrogen bonds, which join the neighboring helix of opposite
˚
shown. Selected distances (A) and angles (°): Cu1–N1 = 1.9621(17),
˚
Cu1–N3a = 1.9857(16), Cu1–Cl2 = 2.2322(7), Cu1–Cl1 = 2.2581(7),
N4aꢀꢀꢀCl1 = 3.0408(17), N2ꢀꢀꢀCl2 = 3.2116(18); N1–Cu1–N3a =
handedness. N2ꢀꢀꢀCl1c = 3.4494(18) A, symmetry code: (c) -x ? 1,
-y ? 2, -z
ligand. Introducing methylene group to the biprazolyl
ligand provides much flexibility for metal–ligand interac-
tions. We already observed two conformations (cis,cis and
cis,trans) of the ligand upon binding to metals; a third
conformation (trans,trans) awaits to be revealed. In the
cis,cis-conformation, the twist of two pyrazolyl rings varies
from 131.40° in free H2L to 36.17(57) and 37.60(64)° in
[Ag2(H2L)2](NO3)2. The relative orientation of the two
pyrazolyl rings of the ligand is the main factor to determine
the structures of its metal complexes. It is expected that
more complicated and interesting metal–organic structures
can be formed with the deprotonated bipyrazolyl ligand,
which is the subject of our future work.
Fig. 4 Emission spectra of H2L (dashed line) and [Ag2(H2L)2]
(NO3)2 (solid line) in the solid state at room temperature. The
excitation wavelength is 410 nm
Experimental section
The CHN microanalyses were carried out with an ELE-
MENTAR vario EL elemental analyzer. IR spectra (KBr
pellets) were recorded on a Niclolet Impact 420 FT-IR
mono-strand chiral structures have been documented in
recent years [24–26].
1
spectrometer. H NMR spectra were recorded on a Bruker
Under UV irradiation (410 nm) at room temperature, the
bipyrazolyl ligand and [Ag2(H2L)2](NO3)2 give off bright
green light. The emission spectra of both exhibit similar
profiles with maximum emission in the range of 460–500 nm
(Fig. 4). We infer that the emission of this Ag complex is
probably ligand-based, with slight modification by Ag(I).
DPX-400 spectrometer.
Preparation of 5-tert-butyl-1,3-bis[(30,50-diethyl-1H-
pyrazol-40-yl)methylene]benzene
Potassium (0.66 g, 17 mmol) was dissolved in tert-butyl
alcohol (80 mL). To the solution, heptane-3,5-dione (2.5 g,
19.5 mmol), 5-tert-butyl-1,3-bis(bromomethyl)benzene (2.71 g,
8.5 mmol) and KI (0.34 g) were added successively. The
mixture was stirred under nitrogen for 3 days and then
filtered. Workup proceeded by distilling off the tert-butyl
alcohol, taking up the residue in CH2Cl2, washing with
Conclusion
As far as we know, this paper is the first one dealing with
metal complexes with a semi-rigid C4-linked bipyrazolyl
123

Transition Met Chem (2010) 35:381–386
385
Table 1 Crystallographic data
H2L
[Ag2(H2L)2](NO3)2
[Cu(H2L)Cl2]n
Formula
C26H38N4ꢀH2O
424.62
C52H76Ag2N10O6
1152.97
C26H38Cl2CuN4
541.04
Mr
Crystal size (mm3)
Crystal system
Space group
0.30 9 0.19 9 0.12
Monoclinic
C2/c
0.49 9 0.36 9 0.23
Triclinic
0.44 9 0.38 9 0.18
Monoclinic
P21/n
ꢀ
P 1
˚
a (A)
13.766(3)
9.0157(18)
13.192(3)
23.786(5)
78.87(3)
89.23(3)
88.64(3)
2775.0(10)
2
11.646(2)
˚
b (A)
23.571(5)
9.4326(19)
25.249(5)
˚
c (A)
9.3333(19)
a (°)
b (°)
c (°)
118.01(3)
93.76(3)
3
˚
V (A )
2673.7(9)
4
2767.8(10)
4
Z
T (K)
Dcalcd (Mg/m3)
298
298
298
1.055
1.380
1.298
l (mm-1
)
0.065
0.761
1.003
F(000) (e)
hkl range
928
1200
1140
-18 B h B18,
-31 B k B 30,
-12 B l B 12
11756
-10 B h B 10,
-15 B k B 15,
-28 B l B 28
18829
-14 B h B 15,
-12 B k B 12,
-33 B l B 27
16619
Refl. collected
Refl. unique
Param. refined
Restraints
3364 (Rint = 0.0548)
182
9491(Rint = 0.0518)
645
6707 (Rint = 0.0428)
329
1
0
6
Final R indices
[I [ 2r(I)]
GoF (F2)
R1 = 0.0829
wR2 = 0.2314
0.954
R1 = 0.0811
wR2 = 0.2201
1.010
R1 = 0.0455
wR2 = 0.1007
1.024
-3
˚
Dqfin (max,min), e A
0.447, -0.165
3.681, -0.887
0.536, -0.385
water, drying over MgSO4, and evaporating to dryness. The
obtained brown syrup was dissolved in MeOH (50 mL) and
N2H4ꢀH2O (0.85 g, 17 mmol) was added. The mixture was
stirred at 50 °C for 12 h and then methanol was distilled
off to give a yellow liquid, which solidified upon cooling to
room temperature. The crude product was recrystallized
from MeOH–H2O to afford colorless prismatic crystals of
H2L. Yield: 50%.
AgNO3 (0.03 mmol, 5.1 mg) in a test tube. Colorless
crystals were obtained in 60% yield within several days.
C52H76N10Ag2O6 (1152.97) Found C 54.1, H 6.7, N
12.2; Calcd. C 54.2, H 6.7, N 12.2. IR(KBr pellet,cm-1):
3445 m, 3200 s, 2966 s, 1715w, 1598 m, 1520 m, 1384 s,
1063 m, 961 w, 933 w, 763 w, 708 w. 1HNMR (400 MHz,
CDCl3): d = 1.14 (m, 12H, CH3–), 1.41 (s, 9H, tBu), 2.48
(m, 8H, –CH2–), 3.75 (s, 4H, Ph–CH2–pz), 5.63 (s, 1H,
Ph–H2), 7.20(s, 2H, Ph–H4,H6) ppm.
C26H38N4 (406.61) Found C 76.6, H 9.7, N, 13.7; Calcd. C
76.8, H 9.4, N 13.8. IR (KBr pellet, cm-1): 3430 w, 3226 m,
3129 m, 2966 s, 1712 w, 1597 m, 1474 m, 1390 w, 1045 m,
Preparation of [Cu(H2L)Cl2]n
1
961 w, 840 w, 700 w. HNMR (400 MHz, CDCl3): d =
1.15 (t, 12H, CH3–), 1.32 (s, 9H, tBu), 2.42 (q, 8H, –CH2–),
3.70 (s, 4H, Ph–CH2–pz), 5.81 (s, 1H, Ph–H2), 7.06 (s, 2H,
Ph–H4, H6) ppm.
H2L (8.1 mg, 0.02 mmol) and CuCl2 (2.7 mg, 0.02 mmol)
were mixed in ethanol (5 mL). The resulted solution was
allowed to evaporate for 1 week to afford brown red
crystals in 53% yield.
Preparation of [Ag2(H2L)2](NO3)2
C26H38N4CuCl2 (541.04) Found C 57.7, H 7.1, N 10.4;
Calcd. C 57.7, H 7.1, N 10.4. IR(KBr pellet, cm-1): 3455 m,
3318 s, 2964 s, 1648 w, 1599 m, 1516 m, 1440 m, 1174 m,
1055 m, 687 m, 614 m.
A solution of H2L (0.03 mmol, 12.2 mg) in ethanol (1 mL)
was layered on the surface of an aqueous solution of
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Transition Met Chem (2010) 35:381–386
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Diffraction intensities were collected on a Bruker Apex II
CCD diffractometer with graphite-monochromated Mo-Ka
˚
radiation (0.71073 A). Absorption corrections were applied
using the multiscan program [27]. The structures were
solved by direct methods and refined by least squares
techniques using the SHELXS-97 and SHELXL-97 pro-
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anisotropic displacement parameters; hydrogen atoms were
generated geometrically. Since the disordered tert-butyl
group of H2L is located on a two-fold axis, three half-
occupied C-atoms, located from Fourier map, were used to
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Acknowledgments This work was supported by the Scientific
Research Foundation for the Returned Overseas Chinese Scholars,
State Education Ministry and the Education Department of Henan
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