Metal cation- and anion-induced assembly: Structures and luminescent properties

Article abstract of DOI:10.1039/c3ce26685k

Three complexes HgLBr₂ (1), [HgLI₂]·CH ₃CN (2) and HgL(SCN)₂ (3) were synthesized by self-assembly of a multidentate ligand containing pyridyl and pyrazolyl groups, namely, 6-phenyl-4-(4-(1H-pyrazolyl)phenyl)-2,2′-bipyridine (abbreviated as L), with corresponding mercury(ii) salts. While the ligand reacted with Cd(NCS)₂ and Co(NCS)₂, complexes [CdL₂(NCS) ₂]·H₂O (4) and [CoL₂(NCS) ₂]·CH₃OH (5) were obtained. The structures of these complexes were determined by X-ray crystallography. In 1 and 3, the hydrogen bonding or C-H...π interactions based on the anions and the pyrazolyl nitrogen atom contribute to the 2-D and 3-D structures, respectively. Compared with 1 and 3, no hydrogen bonding interaction based on the I^- anion can be found in 2. As a result, only the C-H...N hydrogen bonding interactions based on the pyrazolyl nitrogen atom link the molecules to form a 1-D structure. In complexes 3-5, the difference in their structures is attributed to the different metal center ions with diverse coordination modes. The results reveal that the anions and metal ions have great impact on the final structures of the supramolecular architectures. The relationships between the structures and luminescent properties were investigated in detail.

Full text of DOI:10.1039/c3ce26685k

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Metal cation- and anion-induced assembly: structures
and luminescent properties3
Cite this: CrystEngComm, 2013, 15,
3687
Feng Jin,ab Hui-Zhen Wang,a Ying Zhang,a Yang Wang,a Jun Zhang,a Lin Kong,a Fu-
Ying Hao,ab Jia-Xiang Yang,a Jie-Ying Wu,a Yu-Peng Tianacd and Hong-Ping Zhoua
Three complexes HgLBr2 (1), [HgLI2]?CH3CN (2) and HgL(SCN)2 (3) were synthesized by self-assembly of a
multidentate ligand containing pyridyl and pyrazolyl groups, namely, 6-phenyl-4-(4-(1H-pyrazolyl)phenyl)-
2,29-bipyridine (abbreviated as L), with corresponding mercury(II) salts. While the ligand reacted with
Cd(NCS)2 and Co(NCS)2, complexes [CdL2(NCS)2]?H2O (4) and [CoL2(NCS)2]?CH3OH (5) were obtained. The
structures of these complexes were determined by X-ray crystallography. In 1 and 3, the hydrogen
…
bonding or C–H p interactions based on the anions and the pyrazolyl nitrogen atom contribute to the
2-D and 3-D structures, respectively. Compared with 1 and 3, no hydrogen bonding interaction based on
the I2 anion can be found in 2. As a result, only the C–H N hydrogen bonding interactions based on the
…
pyrazolyl nitrogen atom link the molecules to form a 1-D structure. In complexes 3–5, the difference in
their structures is attributed to the different metal center ions with diverse coordination modes. The results
reveal that the anions and metal ions have great impact on the final structures of the supramolecular
architectures. The relationships between the structures and luminescent properties were investigated in
detail.
Received 15th October 2012,
Accepted 1st March 2013
DOI: 10.1039/c3ce26685k
www.rsc.org/crystengcomm
cular structures of complexes is still a great challenge and
much more work is required to understand the assembly
process.
The complex assembly process can be affected by many
Introduction
Recently, the design and synthesis of organic–inorganic hybrid
complexes based on strong coordinate bonds and multiple
weak non-covalent forces have become a research field of rapid
expansion in coordination chemistry and crystal engineering
not only for their fascinating structural features but also for
their interesting properties as new functional materials with
tremendous potential applications in the areas of lumines-
cence, catalysis, separation, adsorption, biological chemistry,
and so on.1–4 The hybrid materials combine the advantageous
properties of inorganic solids with those of organic com-
pounds. So far, a number of complexes with various topologies
and special properties have been synthesized and many
strategies have been studied and used by researchers.5
However, rational control in the construction of supramole-
factors, among them, the metal ions, anions and ligands are
based-control factors.6 It is well known that different metal
ions possess different properties and coordination modes,
which play key roles in the formation of both molecular
structures and packing structures of complexes. For example,
Hg(II) ion may adopt different coordination modes, such as
four-, five- or six-coordination modes according to the specific
structures of the different ligands.7 Cd(II) and Co(II) ions often
show six-coordination when they react with organic ligands
with N donors.8 The introduction of different small anions can
also have a significant effect on the structural construction of
complexes and their properties. In coordination chemistry,
halogen and thiocyanate ions have been widely used as anions
for the construction of the metal coordination complexes
because they can adjust the topologies of complexes through
different coordinate bonds or non-covalent interactions.9
The organic ligands can control the topology of coordina-
tion complexes. The design and synthesis of new organic
ligands is a key approach for construction of metal–organic
complexes with desired structures and properties. In the past
few decades, a lot of new organic ligands with different
structures, properties and donor groups have been synthesized
and used to construct the coordination complexes. At present,
it is still a challenge to synthesize new organic ligands with
aDepartment of Chemistry, Anhui University and Key Laboratory of Functional
Inorganic Materials Chemistry of Anhui Province, 230039 Hefei, P. R. China.
E-mail: zhpzhp@263.net; Fax: +86-551-5107342; Tel: +86-551-5108151
bDepartment of Chemistry, Fuyang Normal College, Fuyang 236041, P. R. China
cState Key Laboratory of Coordination Chemistry, Nanjing University, Nanjing
210093, P. R. China
dState Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, P. R.
China
Electronic supplementary information (ESI) available: Crystal data and
3
refinement, selected bond lengths and angles, etc. CCDC 854404 (for complex 1),
857231 (for complex 2), 854147 (for complex 3), 850218 (for complex 4) and
872923 (for complex 5). For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c3ce26685k
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The non-hydrogen atoms were refined anisotropically and
hydrogen atoms were introduced geometrically. Calculations
were performed with SHELXTL-97 program package.12 Details
of the crystal parameters, data collections and refinements are
special functionality in supramolecular architectures because
of the synthetic difficulties involved. In designing coordina-
tion complexes, pyridyl and pyrazolyl derivatives have been
widely used as ligands due to their ability to coordinate to
several metal centers in various modes.10
listed in Table S1 (in ESI ), and selected bond distances and
3
angles are given in Table S2 (ESI ).
3
Taking the above into consideration, and in order to
construct new coordination complexes with specific structures
and properties, a multidentate ligand containing pyridyl and
pyrazolyl groups, namely, 6-phenyl-4-(4-(1H-pyrazolyl)phenyl)-
2,29-bipyridine (L) has been designed and synthesized.11 It is a
potential multidentate ligand for the construction of novel
complexes based on the following considerations. On the one
hand, the ligand has different coordination modes with
pyrazolyl and pyridyl groups, which may be completely or
partially coordinated with metal ions to construct different
structural complexes. On the other hand, all the groups of the
ligand are linked through single bonds, and they can rotate
freely around the single bonds to meet the different geometric
requirements of the metal ions and the formation of weak
interactions.
In order to evaluate the role of anions and metal ions with
different geometric requirements in crystal engineering of
organic–inorganic hybridized compounds, reactions of L with
various metal salts were carried out. By self-assembly, a series
of novel Ag(I), Hg(II) and other metal complexes were
synthesized. The Ag(I) complexes with metallomacrocyclic
structures have been previously reported.11 Here, we present
the syntheses, structures and luminescent properties of the
following complexes HgLBr2 (1), [HgLI2]?CH3CN (2), HgL(SCN)2
(3), [CdL2(SCN)2]?H2O (4) and [CoL2(SCN)2]?CH3OH (5).
Synthesis of the complexes
HgLBr2 (1). A methanol solution (25 mL) of HgBr2 (0.360 g, 1
mmol) was added to a dichloromethane solution (15 mL) of L
(0.374 g, 1 mmol). The clear mixture solution was allowed to
evaporate slowly at room temperature for a few days, and pale
yellow, needle-like crystals of 1 suitable for single crystal X-ray
diffraction analysis were obtained. Yield: 0.51 g (70%). Anal.
calc. for C25H18HgBr2N4: C, 40.86; H, 2.47; N, 7.62. Found: C,
40.51; H, 2.16; N, 7.90. IR n(cm21): 521(m), 642(w), 703(m),
754(m), 773(m), 786(m), 835(s), 934(s), 1013(m), 1044(m),
1123(w), 1201(w), 1243(m), 1337(m), 1393(s), 1440(m),
1482(m), 1527(s), 1546(m), 1603(s).
[HgLI2]?CH3CN (2). An acetonitrile solution (15 mL) of HgI2
(0.454 g, 1 mmol) was added to a dichloromethane solution
(15 mL) of L (0.374 g, 1 mmol). The clear mixture solution was
allowed to evaporate slowly at room temperature for a few
days, and pale yellow, needle-like crystals of 2 suitable for
single crystal X-ray diffraction analysis were obtained. Yield:
0.566 g (65%). Anal. calc. for C27H21HgI2N5: C, 37.28; H, 2.43;
N, 8.05. Found: C, 37.51; H, 2.16; N, 8.41. IR n(cm21): 520(m),
641(w), 702(m), 737(s), 771(w), 787(m), 834(s), 933(s), 1011(m),
1044(m), 1200(m), 1242(m), 1338(m), 1392(s), 1439(m),
1480(m), 1527(s), 1545(m), 1603(s), 2256(s).
HgL(SCN)2 (3). The reaction was carried out using the same
method used for 2, and the colorless, needle-like crystals of 3
suitable for single crystal X-ray diffraction analysis were
obtained. Yield: 0.51 g (70%). Anal. calc. for C27H18HgN6S2:
C, 46.92; H, 2.62; N, 12.16. Found: C, 46.61; H, 2.33; N, 12.39.
IR n(cm21): 522(m), 641(w), 702(m), 739(m), 770(m), 791(m),
832(s), 894(m), 936(s), 1003(m), 1045(m), 1121(w), 1169(m),
1199(s), 1245(s), 1336(m), 1363(s), 1394(s), 1442(s), 1490(s),
1529(s), 1572(w), 1604(s), 2123(s).
[CdL2(NCS)2]?H2O (4). A clear methanol solution (25 mL) of
Cd(SCN)2 (0.229 g, 1 mmol) was carefully layered onto a
solution of L (0.748 g, 2 mmol) in dichloromethane (25 mL).
Colorless, needle-like crystals suitable for single crystal X-ray
diffraction were obtained by slow interlayer diffusion. Yield:
0.606 g (60%). Anal. calc. for C53H40CdN10OS2: C, 63.06; H,
3.99; N, 13.87. Found: C, 63.33; H, 3.62; N, 13.51. IR n(cm21):
568(s), 678(s), 745(s), 703(m), 759(s), 801(s), 819(m), 885(w),
949(m), 1005(s), 1029(m), 1057(m), 1119(s), 1170(s), 1219(s),
1329(m), 1399(s), 1441(m), 1479(m), 1526(s), 1547(m), 1604(s),
1663(s), 2078(s), 3107(s).
Experimental section
General procedure
All commercially available chemicals are of analytical grade
and used without further purification. Elemental analyses
were carried out on Perkin-Elmer 240 analyzer. IR spectra were
recorded from KBr discs in the 4000–40 cm21 range on a
nicolet Nexus 870 spectrophotometer. The solid state lumines-
cence spectra were measured on a F-4500 FL spectrophot-
ometer. In the measurements of emission and excitation
spectra, the pass width is 2.5 nm for L and complex 4, and 5
nm for complexes 1, 2, 3 and 5. For time-resolved fluorescence
measurements, the fluorescence signals were collimated and
focused onto the entrance slit of a monochromator with the
output plane equipped with a photomultiplier tube (HORIBA
HuoroMax-4P). The decays were analyzed by least-squares. The
quality of the exponential fits was evaluated by the goodness of
fit (x2).
[CoL2(NCS)2]?CH3OH (5). The reaction was carried out using
the same method used for 4, and the red, block crystals of 5
suitable for single crystal X-ray diffraction analysis were
X-ray crystallography and structure solution
obtained. Yield: 0.525
g
(55%). Anal. calc. for
The X-ray diffraction measurements were performed on Bruker
SMART CCD area detector using graphite monochromated Mo
Ka radiation (l = 0.71069 Å) at 298 (2) K. Intensity data were
collected in the variable v-scan mode. The structures were
solved by direct methods and difference Fourier syntheses.
C53H40CoN10OS2 : C, 66.59; H, 4.22; N, 14.65. Found: C,
66.96; H, 4.58; N, 14.39. IR n(cm21): 521(m), 645(w), 702(m),
733(m), 771(m), 791(m), 835(s), 896(m), 933(s), 1001(m),
1041(m), 1126(w), 1168(m), 1195(s), 1245(s), 1333(m), 1363(s),
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1398(s), 1440(s), 1494(s), 1523(s), 1571(w), 1603(s), 2065(s),
3455(s).
the ligand-to-metal ratio. All complexes are air stable and can
retain their structural integrity at room temperature for a few
months.
In the IR spectra of complexes 3–5, the intense n(CN)
stretching bands of NCS2 at 2123, 2078 and 2065 cm21
,
Results and discussion
respectively, indicate that thiocyanate ions act as terminal
N-bonded ligands in 4 and 5, and intense absorption at 2123
cm21 for 3 corresponds to the terminal S-bonded thiocyanate
groups,9 which agrees with the crystal structures analyses. The
strong and broad absorption bands at 3107 and 3455 cm21 in
4 and 5 are assigned to the characteristic peaks of n(O–H)
stretching vibrations from water and methanol molecules,
respectively.
Syntheses and characterization
In order to investigate the effect of different metal ions on the
crystal structures, the metal salts with the different metal
cations and the same anions, namely, Hg(SCN)2, Cd(NCS)2 and
Co(NCS)2 were used. Hg(SCN)2, HgI2 and HgBr2 were employed
in order to study the influence of different anions on the
construction of structures. Complexes 1–5 were obtained in
methanol or acetonitrile and dichloromethane mixed solvent
system by self-assembly of L with different metal salts. In the
five complexes, the pyrazolyl group is not directly involved in
the coordination, which is different to that in previously
reported Ag(I) complexes.11 However, it can serve as the donor
Description of X-ray crystal structures
Structure of HgLBr2 (1). Complex 1 crystallizes in the
monoclinic system, space group P21/c. In the molecular
structure, there are two crystallographically and conforma-
tionally independent molecules in the asymmetric unit cell, as
shown in Fig. 1a. Each Hg(II) ion is coordinated with two
terminal bromine ions and two N atoms of pyridyl groups to
form distorted tetrahedral geometry. The bond angles around
the Hg(1) and Hg(2) ions are in the range of 69.4(3)–134.78(6)
of hydrogen bonding and still play important roles in
…
formation of higher-dimensional structures through C–H
N
hydrogen bonding interactions. Furthermore, the mononuc-
lear complexes 1–5 were always obtained when reaction
conditions were changed, such as temperature, solvent and
Fig. 1 (a) The structural unit of complex 1. (b) The one-dimensional structure of complex 1 viewed along the a-axis. (c) The one-dimensional structure of complex 1
viewed along the b-axis. (d) The two-dimensional structure of complex 1. Dotted lines represent the weak interactions. Hydrogen atoms are omitted for clarity.
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and 70.2(3)–130.38(5)u, respectively. The selected bond dis-
As shown in Fig. 2b, similar to complex 1, the pyrazolyl
nitrogen atom of the ligand is not directly involved in
coordination, but it plays an important role in the formation
tances and angles are given in Table S2 (ESI ). C(47)–
3
…
…
H(47) Br(4) hydrogen bonding and C(12)–H(12) p interac-
…
tions occur between the two independent molecules. The
of higher structures through C–H N hydrogen bonding
…
…
H(47) Br(4) and H(12) centroid distances are 2.924 and
2.705 Å, respectively.
interactions. The neighboring molecules are linked by inter-
…
molecular C(16)–H(16) N(4) hydrogen bonding interactions
into zigzag one-dimensional chains along the c-axis. The
distances of H(16) N(4) is 2.615 Å and the C(16)–H(16) N(4)
As shown in Fig. 1b, the pyrazolyl nitrogen atom of the
ligand is not directly involved in coordination, but it plays an
important role in the formation of the higher dimensional
…
…
angle is 149.31u. Every molecule of the complex is connected to
…
…
an acetonitrile molecule through C(18)–H(18) N(5) hydrogen
structures through C–H N hydrogen bonding interactions.
…
…
bonding interactions [d(H(18) N(5) = 2.686 Å]. It is noted that
The independent units are linked through C(48)–H(48) Br(4)
…
…
no H I hydrogen bonding interaction can be found in
and C(28)–H(28) N(8) hydrogen bonding interactions to
…
complex 2.
generate a chain along the b-axis. The H(48) Br(4) and
…
H(28) N(8) distances are 2.836 and 2.736 Å, respectively.
Structure of [HgL(SCN)2] (3). Complex 3 crystallizes in the
monoclinic system, space group P21/c. As shown in Fig. 3a, the
Hg(II) ion is coordinated with two S atoms of thiocyanate
anions and two N atoms of pyridyl groups. The bond angles
around the Hg(II) ion are in the range of 71.10(10)–123.38(9)u.
Hg(II) ion, a soft acid, is preferentially coordinated by the soft
base (S) terminus of the SCN2 ligand. The thiocyanate anion is
coordinated in a bent fashion through the sulfur atom.
The uncoordinated pyrazolyl N atom and SCN2 play key
roles in the formation of the higher-dimensional structures. As
shown in Fig. 3b, the neighboring molecules are linked in a
…
The C(29)–H(29) Br(3) hydrogen bonding interactions link
the one-dimensional structures to form a two-dimensional
…
structure along the bc plane [d(H(29) Br(3) = 2.987 Å], as
…
illustrated in Fig. 1d. In addition, C(41)–H(41) p and C(23)–
…
…
…
H(23) p interactions with H(41) centroid and H(23) centroid
distances of 2.804 and 3.913 Å, respectively, provide further
stability to the layer structure.
Structure of [HgLI2]?CH3CN (2). The single-crystal X-ray
diffraction analysis reveals that complex 2 crystallizes in the
monoclinic system, space group P21/c with one molecule of the
complex in the unit cell together with a solvent acetonitrile
molecule. Details of the crystal parameters, data collections
…
head-to-tail mode by intermolecular C(1)–H(1) N(4) hydrogen
…
bonding interactions (the distance of H O is 2.576 Å, and the
…
and refinements are listed in Table S1 (ESI ), and selected
C–H N angle is 163.78u) to form a one-dimensional structure
3
bond distances and angles are given in Table S2 (ESI ). As
along the b axis. In the bc plane, the molecules are also
3
…
…
depicted in Fig. 2a, the Hg(II) ion is located in a distorted
tetrahedral geometry and coordinated by two terminal iodine
ions and two pyridyl nitrogen atoms from the ligand (Hg–N(1)
= 2.456(3) Å, Hg–N(2) = 2.364(3) Å, Hg–I(1) = 2.634(1) Å, Hg–I(2)
= 2.671(1) Å). The bond angles for Hg are in the range of
68.98(11)–129.59(2)u.
interconnected by C(18)–H(18) N(5) and C(22)–H(22) N(6)
…
hydrogen bonding interactions (the distances of H(18) N(5)
…
…
and H(22) N(6) are both 2.502 Å, and the C(18)–H(18) N(5)
…
and C(22)–H(22) N(6) angles are 165.72 and 157.75u, respec-
tively) to aggregate in a two-dimensional net (Fig. 3c), and the
…
…
p
p interactions with centroid centroid distances of 3.634 Å
exist in the sheet. The two-dimensional nets further extend
Fig. 2 (a) The structural unit of complex 2. (b) The one-dimensional structure of complex 2. Dotted lines represent the weak interactions. Hydrogen atoms not
participating in hydrogen bonding are omitted for clarity.
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Fig. 3 (a) The structural unit of complex 3. (b) The one-dimensional structure of complex 3. (c) The two-dimensional structure of complex 3. Dotted lines represent the
weak interactions. Hydrogen atoms not participating in hydrogen bonding are omitted for clarity.
into three-dimensional supramolecular structures through
molecule. As shown in Fig. 5a, the central metal Cd(II) ion is
six coordinated by two N atoms from two thiocyanate anions
and four pyridyl N atoms from two ligands, to form a distorted
octahedral geometry. The bond angles around the Cd(II) ion
are in the range of 69.26(9)–170.34(8)u. The selected bond
…
C(24)–H(24) S(1) hydrogen bonding interactions in another
…
…
direction (the distance of H S is 2.866 Å, and the C–H
S
angle is 163.68u), as shown in Fig. 4.
Structure of [CdL2(NCS)2]?H2O (4). Single-crystal X-ray
diffraction analysis reveals that complex 4 crystallizes in the
orthorhombic system, space group Fdd2 with one molecule of
the complex in the unit cell together with a disordered water
distances and angles are shown in Table S2 (ESI ).
In the molecular packing structure of complex 4, no C–H
3
…
N
hydrogen bonding interaction based on the pyrazolyl N atom
Fig. 4 The three-dimensional structure of complex 3. Dotted lines represent the weak interactions. Hydrogen atoms not participating in hydrogen bonding are
omitted for clarity.
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…
Fig. 5 (a) The structural unit of complex 4. (b) The one-dimensional chain of complex 4 formed by C–H S hydrogen bonding interactions. (c) The three-dimensional
structure of complex 4. Dotted lines represent the weak interactions. Hydrogen atoms not participating in hydrogen bonding and water molecules are omitted for
clarity.
…
…
of the ligand is found, but thiocyanate anions play important
roles in forming the higher dimensional structure. As shown
in Fig. 5b, two adjacent molecules are linked by C(44)–
p
p and C(50)–H(50) p interactions to form a 1-D chain. The
…
centroid-to-centroid and H(50) centroid distances are 3.703
and 2.836 Å, respectively. The neighboring chains are further
linked by p p interactions (the distance of centroid centroid
is 3.809 Å), giving rise to an extended layer structure, as shown
in Fig. 6c. The adjacent layers are stacked through C(19)–
H(19) p and C(44)–H(44) p interactions to form a 3D
supramolecular structure (Fig. 7). The H(19) centroid and
H(44) centroid distances are 3.840 and 3.614 Å, respectively.
…
…
…
H(44) S(1) hydrogen bonding interactions to form a zigzag
…
1-D chain. The H(44) S(1) distance is 2.990 Å. The C(13)–
…
H(13) p interaction based on the thiocyanate anion links the
…
…
neighboring chains to form a 3-D supramolecular structure, as
…
…
shown in Fig. 5c. The distance of H(13) C(52) is 2.776 Å.
…
Further more, the hydrogen bonding interactions based on
disordered water molecules also contribute to the formation of
the 3-D structure.
Structure of [CoL2(NCS)2]?CH3OH (5). Single-crystal X-ray
diffraction analysis reveals that complex 5 crystallizes in the
In addition, the hydrogen bonding interactions based on
methanol molecules provide further stability to the 3D
structure.
Structural comparison and effect of anion and metal cation
¯
triclinic system, space group P1 with one molecule of the
In complexes 1–3, the pyrazolyl N atom of the ligand does not
directly take part in coordination. However, it can serve as the
donor of hydrogen bonding and still play an important role in
complex in the unit cell together with a solvent methanol
molecule. As shown in Fig. 6a, the central metal Co(II) ion is six
coordinated by two N atoms from two thiocyanate anions and
four pyridyl N atoms from two ligands, to form a distorted
octahedral geometry. The bond angles around the Co(II) ion
are in the range of 74.02(7)–177.64(7)u. The selected bond
the formation of higher-dimensional structures through C–
…
H
N hydrogen bonding interactions. Making a comparison,
in complexes 1–3, which have the same four-coordinated metal
center Hg(II), the different ions are clearly critical in
determining the packing structures of them. It would be
interesting and essential to compare the effect of the anions in
the Hg(II) complexes. The difference in the coordinating
ability, size, geometric shape, symmetry and hydrogen-bond-
ing ability of anions is the underlying reason behind the
distances and angles are shown in Table S2 (ESI ).
3
…
Similar to 4, no C–H N hydrogen bonding interaction
based on the pyrazolyl N atom of the ligand is formed.
…
…
However, the multiple C–H p and p p interactions play a
significant role in the molecular packing of the crystal. As
shown in Fig. 6b, two adjacent molecules are linked by the
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Fig. 6 (a) The structural unit of complex 5. Hydrogen atoms are omitted. (b) The one-dimensional chain of complex 5. (c) The two-dimensional structure of complex 5.
Dotted lines represent the weak interactions. Hydrogen atoms not participating in hydrogen bonding are omitted for clarity.
differences in the structures of complexes.13 In the present
system, anions Br2, I2 and SCN2 all coordinate to Hg(II)
centers in a terminal fashion, due to the restriction of the
coordination geometry of the mercury ion and the chelating
coordination pattern of L as well as the nature of the anions.
The absence or different hydrogen bonding interactions based
on the anions lead to the formation of the supramolecular
structures with different characters in the present series of
found in 2, which may be attributed to the greater size and
smaller electronegativity of the iodide anion.14 As a result, only
…
the C–H N hydrogen bonding interactions based on the
pyrazolyl nitrogen atom link the molecules to form a 1-D
structure.
In complexes 3–5, which possess the same thiocyanate
anion, the different metal center ions determine the structures
of the complexes due to the different coordination nature of
them. For the Hg(II) complex, anion SCN2 coordinates to the
metal ion in terminal g1-S mode. However, anion SCN2 ligates
to Cd(II) and Co(II) ions in terminal g1-N fashion. The Cd(II)
and Co(II) ions both adopt the distorted N6-coordination
mercury complexes. In 1 and 3, the hydrogen bonding or C–
…
H
p interactions based on the anions contribute to the 2-D
and 3-D structures, respectively. Compared with 1 and 3, no
hydrogen bonding interactions based on the anion can be
…
geometry. It is notable that no C–H N hydrogen bonding
interactions based on the pyrazolyl N atom of the ligand are
formed in complexes 4 and 5. However, because two ligands
are involved in coordination, there are more chances to form
…
…
C–H p and p p interactions between the ligands, which play
significant roles in constructing the three-dimensional struc-
tures. All in all, the different structures of complexes 1–3 reveal
that the anions have an effect on the higher-dimensional
architectures of the complexes, and the different structures of
complexes 3–5 indicate that the central metal ions determine
the molecular structure and the crystal packing of complexes.
Compared with the free L, the ligands in complex 1, 2, 3 and
5 have the larger twist angles between the pyrazolyl ring and
the near-by benzene ring plane (Table S3, ESI ), which may be
attributed to the larger number of weak interactions in the
complexes. However, in complex 4, one ligand has better
3
Fig. 7 The three-dimensional structure of complex 5. Dotted lines represent the
weak interactions. Hydrogen atoms not participating in hydrogen bonding are
omitted for clarity.
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Fig. 8 Solid-state emission spectra of L and complexes 1–5 at room tempera-
ture. In the measurements of emission and excitation spectra, the pass width is
2.5 nm for L and complex 4, and 5 nm for complexes 1, 2, 3 and 5.
Fig. 9 Time-resolved fluorescence curves of L and complexes 1, 3 and 4 in the
solid state at room temperature.
solid state at room temperature. The results are shown in
Fig. 9. The fluorescence lifetimes of complexes 1 (0.10 ns) and
3 (0.13 ns) are shorter than that of L (0.16 ns), which may be a
result of the heavy-atom effect of the Hg(II) ion.17 The
fluorescence lifetime of complex 4 is 0.33 ns, which was a
remarkable increase compared with that of L, which may be
derived from the better planarity of L in complex 4 and the
supramolecular structures in the solid state complex.13d,18
planarity than the free ligand, which would have significant
influences on the solid state emission properties of the crystal.
Luminescent properties
Coordination complexes with d10 metal atoms show fluores-
cence properties and are promising candidates for photoactive
materials with potential applications.15 For potential applica-
tions as luminescent materials, the luminescent properties of
ligands and complexes have been investigated in the solid
state at room temperature.
Conclusions
A series of complexes have been synthesized by self-assembly.
In 1–3, the different anions result in different weak inter-
molecular interactions, which are responsible for the crystal
packing of the different complexes. However, in 3–5, the
different weak non-covalent forces are generated because of
the different coordination modes of Hg, Cd and Co ions.
Complexes 1, 3 and 4 all show fluorescence emissions which
are likely to originate from the intraligand p–p* transitions
with red shifts and decreased intensity compared to the free
ligand. The results indicate that both anions and metal center
ions have a marked influence on the final structures and
fluorescence properties of the complexes. Obviously, the
rational choice of the anions and the metal centers may be
an effective way to construct novel coordination complexes
with desired structures and properties.
The compounds are excited at the excitation wavelength of
290 nm. In the measurements of emission and excitation
spectra, the pass width is 2.5 nm for L and complex 4, and 5
nm for complexes 1, 2, 3 and 5. No clear photoluminescence
was observed for complexes 2 and 5. The luminescence
emissions are probably quenched by the iodine and Co(II)
ions, respectively.15 Fig. 8 presents the comparison of the
emission spectra of free ligand and complexes 1, 3 and 4. The
free ligand exhibits an emission maximum at 376 nm, which is
assigned to the p–p* transition. In contrast to the free ligand,
complexes 1, 3 and 4 exhibit broader fluorescent emission
band maxima at 419, 424 and 418 nm, respectively, which may
be assigned to the intraligand p–p* transitions because of the
resemblance of the emission spectra in comparison with the
free ligand. The red shifts may be attributed to the coordina-
tion of ligand to the metal centers.16 Compared to free ligand,
all these complexes exhibited relatively weaker emissions,
which may be a result of the heavy-atom effect of the metal
central ions. The emission intensity of complexes 4 is relatively
stronger than those of 1 and 3, and the possible explanation is
Acknowledgements
This work was supported by the Program for New Century
Excellent Talents in University (China), Doctoral Program
Foundation of Ministry of Education of China
(20113401110004), National Natural Science Foundation of
China (21271003, 21271004, 21101001, 21201005), the 211
Project of Anhui University, the Anhui University Students
the better planarity of L in complex 4 (Table S3, ESI ) and the
3
heavy-atom effect of Hg(II) ion in 1 and 3.
The fluorescence decay profiles of L and complexes 1, 3 and
4 were measured at their optimal excitation wavelengths in the
3694 | CrystEngComm, 2013, 15, 3687–3695
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