New coordination architectures of dithioether ligand with silver salts: Effect of anions on complex structures

Article abstract of DOI:10.1071/CH05290

Reactions of a flexible dithioether ligand, 2,3-bis(5-methyl-1,3,4- thiadiazole-2-thiomethyl)quinoxaline (L), with AgX (X = ClO₄ ^- or PF₆^-) lead to the formation of two new one-dimensional (1D) silver(I) complexes: {[AgL](ClO₄)} _∞ 1 and {[Ag₂L(CH₃OH)](PF ₆)₂(CH₃OH)}_∞ 2, which have been characterized by elemental analysis, IR spectroscopy, and X-ray crystallography. Although 1 and 2 are synthesized under the same conditions, they take different structures due to the difference in anions in the silver salts. In 1, each ligand supplies three N-donors to bridge two silver atoms to result in a chain structure, and all silver atoms in the chain possess the same coordination geometry. In 2, each ligand gives six N-donors to coordinate to two silver atoms of different geometries, forming a ID chain. The changes in counteranions affect the coordination mode of the ligand and the geometry of the Ag(1) centre, and consequently give rise to complexes with different structures. The coordination features of the ligand have also been primarily investigated through density functional theory calculations. CSIRO 2006.

Full text of DOI:10.1071/CH05290

RESEARCH FRONT
Full Paper
CSIRO PUBLISHING
Aust. J. Chem. 2006, 59, 34–39
www.publish.csiro.au/journals/ajc
New Coordination Architectures of Dithioether Ligand with
Silver Salts: Effect of Anions on Complex Structures
Ya-Bo Xie,A,B Jian-Rong Li,A and Xian-He BuA,C
A Department of Chemistry, Nankai University, Tianjin 300071, China.
B College of Environmental and Energy Engineering, Beijing University of Technology, Beijing 100022, China.
C Corresponding author. Email: buxh@nankai.edu.cn
Reactions of a flexible dithioether ligand, 2,3-bis(5-methyl-1,3,4-thiadiazole-2-thiomethyl)quinoxaline (L), with
AgX (X = ClO−4 or PF6−) lead to the formation of two new one-dimensional (1D) silver(i) complexes:
{[AgL](ClO4)}∞ 1 and {[Ag2L(CH3OH)](PF6)2(CH3OH)}∞ 2, which have been characterized by elemental anal-
ysis, IR spectroscopy, and X-ray crystallography. Although 1 and 2 are synthesized under the same conditions, they
take different structures due to the difference in anions in the silver salts. In 1, each ligand supplies three N-donors to
bridge two silver atoms to result in a chain structure, and all silver atoms in the chain possess the same coordination
geometry. In 2, each ligand gives six N-donors to coordinate to two silver atoms of different geometries, forming a
1D chain. The changes in counteranions affect the coordination mode of the ligand and the geometry of the Ag(i)
centre, and consequently give rise to complexes with different structures. The coordination features of the ligand
have also been primarily investigated through density functional theory calculations.
Manuscript received: 21 October 2005.
Final version: 18 December 2005.
Introduction
and rotate when coordinating to metal centres so as to con-
form the coordination geometries of metal ions. Since Ag(i)
is a soft acid inclined to coordinate to soft bases such
as S- and unsaturated N-containing ligands, it is either a
favourable building block or connecting node for coordina-
tion polymers.[7] In our previous work, dithioether ligands
with flexible spacers have been successfully used to con-
struct various structures of complexes including discrete
molecules, and 1D, 2D, and 3D network architectures.[8] As
a continuation of our research on Ag(i) coordination com-
pounds with dithioether ligands, a new dithioether ligand
with an aromatic spacer, 2,3-bis(5-methyl-1,3,4-thiadiazole-
2-thiomethyl)quinoxaline (L, Scheme 1) has been synthe-
sized. Here, two Ag(i) coordination architectures with the
ligand are reported, and the coordination features of the lig-
and as well as the effects of anions on the structures of the
complexes are discussed.
Recently, increasing attention has been paid to the use of
flexible bridging ligands in the construction of supramolec-
ular architectures.[1] This approach is attractive because the
flexibility and conformation freedoms of such ligands offer
the possibility for the construction of new frameworks with
tailored properties and functions.[2] The self-assembly of
metal complexes is a complicated process, and understand-
ing the factors that affect the process between metal ions
and flexible polydentate ligands is a challenging topic in
supramolecular and coordination chemistry. Because of the
conformational freedom, the flexible polydentate bridging
ligands have shown interesting coordination chemistry with
transition metal ions and promise possibilities for generating
novel frameworks.[3] In addition, the flexible bridging lig-
ands may be very sensitive to the changes of conditions, so
they may be suitable candidates for investigating the roles
of various factors such as solvents, counteranions, and weak
supramolecular interactions in the aggregation of molecular
architectures.
Although flexible thioethers are well-established ligands
in coordination chemistry,[4] less attention has been paid to
symmetric dithioether ligands,[5] in contrast to the exten-
sive studies on the coordination chemistry of other thioethers
such as crown thioether.[6] Heterocyclic dithioether ligands
contain structural information in which the nitrogen and sul-
fur atoms act as donors to coordinate to metal ions, and
the flexible nature of spacers allows the ligands to bend
N
N
S
N
S
S
S
N
N
N
L
Scheme 1.
10.1071/CH05290
© CSIRO 2006
0004-9425/06/010034

RESEARCH FRONT
Coordination Architectures of Dithioether Ligand with Silver
35
(a)
S1
N4
Ag1B
N3
Ag1
Ag1A
S2
S4
S3
N6
N6A
N5
O4
N1
O1
N2
O2
CI1
O3
(b)
Ag
N
N
Ag
Fig. 1. (a) View of the structure of complex 1 with atom labeling (displacement ellipsoids drawn at 30% probability), and
(b) the double-strand chain structure formed by the weak Ag · · · N coordination bonds in 1.
Results and Discussion
along the y-axis. The terminal thiadiazole ring that provides
a bridging N-donor is almost coplanar with the quinoxa-
line ring, and another thiadiazole ring that gives a chelating
N-donorliesabovethequinoxalineplaneinclinedwithadihe-
dral angle of 64.4◦. As a result of the coordination of an
N-atom of the quinoxaline ring, the two aromatic rings of
quinoxaline are twisted relative to each other with a dihe-
dral angle of 4.3◦. The silver atom lies above the quinoxaline
plane with the upright distance of 1.0094 Å. In the chain,
all Ag(i) atoms are in a straight line and the distance of
adjacent ones is approximately 12.372 Å. All quinoxaline
rings in the chain are almost coplanar. The dihedral angle
between two terminal thiadiazole rings of the same ligand is
approximately 77.2◦.
Characterization of the Complexes
The IR spectrum of complex 1 shows absorption bands
resulting from the skeletal vibrations of aromatic rings in
the 1400–1600 cm−1 region, and strong absorption bands of
∼781 and 1080 cm−1 can be assigned to the C–S stretch-
ing vibration and the characteristic absorption band of ClO4−
anion, respectively. Absorption bands resulting from the
skeletal vibrations of aromatic rings for complex 2 appear at
1340, 1491, and 1630 cm−1, and the C–S stretching vibration
absorption band at ∼770 cm−1
.
Crystal Structures
The crystal structure of 1 consists of 1D [(AgL)+]∞ cation
chains and ClO−4 anions. In the [(AgL)+]∞ chain, there
only exists one crystallographically independentAg(i) centre
and each Ag(i) centre is coordinated by three N-atoms from
two distinct L ligands (Fig. 1a). The coordination geome-
try of each Ag(i) centre is best described as trigonal planar
with the Ag(i) centre deviating from the N(3) coordination
plane by 0.2523 Å. ThreeAg–N bond lengths are unequal but
fall within the expected ranges for−such coordination bonds
(Table 1).[8d,8e] In addition, the ClO4 anions form weak coor-
dination bonds to Ag(i) centres with an Ag · · · Cl distance of
2.936 Å.
In 1, each ligand supplies three N-donors and uses two of
them (one from the terminal thiadiazole ring and another
from quinoxaline ring) to chelate a Ag(i) atom forming
a seven-membered ring with chair conformation, and the
remaining N-donor from another terminal thiadiazole ring
to bridge another Ag(i) atom, resulting in an infinite chain
Afeatureof thiscomplexistheformation ofAg · · · Nweak
interactions between adjacent chains (Fig. 1b). The Ag · · · N
distance of 2.643 Å is significantly shorter than the sum of
van der Waals radii of Ag and N (3.27 Å),[9] indicating the
existence of Ag · · · N weak interactions, and a double-strand
chain structure is formed by these weak interactions. In the
double-strand chain, the adjacent quinoxaline and thiadia-
zole ring are aligned at an offset, as they are approximately
parallel to each other with a centroid–centroid separation
approximately 3.747 Å, indicating the presence of face-
to-face π–π stacking interactions.[10] Further investigation
of the packing structure showed that the O · · · S distance of
3.092 Å is shorter than the summed van der Waals radii of
the two atoms (3.32 Å),[9] indicating the existence of weak
O · · · S interactions between ClO−4 anions and thiadiazole
rings. The double-strand chains are further stacked along the
a–c plane by these weak O · · · S interactions, forming a 3D
supramolecular structure.

RESEARCH FRONT
36
Y.-B. Xie, J.-R. Li, and X.-H. Bu
Table 1. Selected bond lengths [Å] and angles [◦] for complexes 1 and 2
{[AgL](ClO4)} (1)
∞
Ag(1)–N(6)A
Ag(1)–N(3)
2.213(4)
2.365(4)
148.03(14)
99.25(14)
Ag(1)–N(1)
2.310(4)
N(6)A–Ag(1)–N(1)
N(1)–Ag(1)–N(3)
N(6)A–Ag(1)–N(3)
108.33(15)
{[Ag2L(CH3OH)](PF6)2(CH3OH)} (2)
∞
Ag(1)–N(6)B
Ag(1)–N(3)
Ag(2)–N(4)
Ag(2)–N(5)B
2.235(4)
2.436(4)
2.185(4)
Ag(1)–N(1)
Ag(1)–O(2)
Ag(2)–N(2)
2.297(4)
2.492(8)
2.291(4)
2.292(5)
N(6)B–Ag(1)–N(1)
N(1)–Ag(1)–N(3)
N(1)–Ag(1)–O(2)
N(4)–Ag(2)–N(2)
N(2)–Ag(2)–N(5)B
141.89(17)
92.32(15)
109.3(3)
131.17(17)
105.56(16)
N(6)B–Ag(1)–N(3)
N(6)B–Ag(1)–O(2)
N(3)–Ag(1)–O(2)
N(4)–Ag(2)–N(5)B
122.22(16)
87.4(3)
91.0(3)
121.50(16)
A Symmetry transformations: x, y + 1, z.
B Symmetry transformations: −x − 1, y − ½, −z + ½.
S4
S3
C11
C12
C16
C10
N6
S1
C14
C15
N4
S2
C4
O
N5
Ag
Ag2
N
C9
N3
C7
N1
C8
Ag
N
Ag1
N2
C2
N
N
C5
N
N
O2
C3
N
N
S
S
C19
S
Fig. 2. View of the structure of complex 2 with atom labeling (displacement ellipsoids drawn at 30% probability).
The crystal structure of 2 consists of 1D {[Ag2L
(CH3OH)]+}∞ chains, PF−6 anions and uncoordinated
CH3OH molecules. In the {[Ag2L(CH3OH)]+}∞ chain,
there exist two crystallographically independent Ag centres
(Fig. 2). Firstly, the Ag(1) centre is coordinated by three
N-atoms (two thiadiazole nitrogen atoms and a quinoxa-
line nitrogen) from two distinct L ligands and an O-atom
from CH3OH. The coordination geometry of the Ag(1) cen-
tre is best described as a distorted tetrahedron with cis bond
angles around the Ag(1) centre from 87.4(3) to 141.89(17)◦.
Secondly, the Ag(2) site shows trigonal planar coordina-
tion geometry coordinated by three N-atoms (two thiadiazole
nitrogen atoms and a quinoxaline nitrogen) from two distinct
L ligands. TheAg(2) centre deviates from the N(3) coordina-
tion plane by 0.1708 Å. Three Ag–N bond lengths fall within
the expected ranges for such coordination bonds.[8d,8e]
In contrast to 1, each ligand in 2 gives six N-donors and
uses the N-atoms of the terminal thiadiazole rings and the
quinoxaline ring to chelate two Ag(i) centres, to form two
seven-membered rings with chair conformation. The remain-
ing two N-atoms of the ligand from terminal thiadiazole ring
bridge two Ag(i) centres, resulting in an infinite chain along
the b-axis. In the chain, the distances between adjacent Ag(i)
centres are 3.499 and 7.379 Å, respectively. The dihedral
angle between two adjacent quinoxaline rings in the chain
is approximately 20.8◦. Two terminal thiadiazole rings of the
ligand lie in the two sides of the quinoxaline ring and the dihe-
dral angles formed with the central quinoxaline ring are 53.3
and 37.6◦, respectively. The two thiadiazole rings from dif-
ferent ligands bridge twoAg(i) centres with nitrogen atoms to
form a six-membered ring with the four nitrogen atoms, and
the twoAg(1) atoms are coplanar. Further investigation on the
packing structure found that the distance of F · · · S of 3.117 Å
is shorter than the summed van der Waals radii of two atoms
(3.27 Å),[9] indicating the existence of weak F · · · S inter-
actions between PF−6 anions and thiadiazole rings. The 1D
chains are further stacked along the x–z plane by these weak
F · · · S interactions, forming a 3D supramolecular structure.
The self-assembly of the supramolecular coordination
polymer is the result of combinations of forces including
strong (for example metal–ligand coordination) and weak
(for example π–π stacking, halogen–halogen, and C–H · · · X
(X = O, N, Cl, or I) interactions.[11] Although the latter
supramolecular forces are weaker than the former, they
have been recognized as critical in stabilizing and regulat-
ing the supramolecular constructs, due to their significant
contribution to the self-assembly and molecular recognition
processes. With the development of supramolecular chem-
istry, there has been an increasing demand to understand
various weak interactions by which the molecular building

RESEARCH FRONT
Coordination Architectures of Dithioether Ligand with Silver
37
(a)
blocks self-assemble into different aggregation states. The
study of weak interactions is of great significance for the
development of supramolecular chemistry, and their use in
the construction of supramolecular coordination polymers
has increased. Complexes 1 and 2 are good examples of
supramolecular compounds self-assembled by a combination
of strong (coordination bonds) and weak (includingAg · · · N,
O · · · S, and F · · · S contacts) interactions.
Ϫ0.6177
N2
Ϫ0.5900
N1
0.6446
Ϫ0.2899
N3
N4
Ϫ0.2794
The structural differences between 1 and 2 imply that
counteranions play important roles in the construction of
coordination architectures. In the related Ag(i) coordination
polymers, different structural complexes can be achieved by
subtle changes of the nature of the counteranions.[12] In gen-
eral, the effect of anions on the frameworks of complexes can
be explained from their differences in sizes and coordination
ability.[13] In the two complexes, due to the weak coordination
ability of BF−4 and PF6− ions, both of them do not coordinate
to Ag(i). However, because BF−4 and PF6− ions are differ-
ent sizes, the different template roles of BF−4 and PF6− affect
the self-assembly process of the ligand and Ag(i) centres.
The counteranion influences the coordination mode of the
ligand and the geometry of the Ag(i) centre, consequently
producing different structural complexes. Investigating the
effects of solvents on the structures of complexes has been
attempted, without success because of the failure to obtain
single crystal of complexes. Complexes 1 and 2 can also be
isolated using different metal/ligand reaction stoichiometries
by the synthetic methods of complexes described here. This
result was confirmed by X-ray diffraction, IR spectra, and ele-
mental analyses, indicating that the self-assembly process of
these complexes are almost independent of the metal/ligand
ratios.
S3
Ϫ0.2855
N5
0.5616
S2
N6
Ϫ0.2854
S1
0.6446
0.6634
S4
(b)
Ϫ0.5983
N1
Ϫ0.5977
N2
Ϫ0.2813
Ϫ0.2837
Ϫ0.2770
N3
Ϫ0.2872
N6
N5
N4
S1
0.5758
S2
0.5573
S3
0.6918
S4
0.6802
Fig. 3. Charge distributions of the coordination atoms of (L) in (a) 1,
and (b) 2.
In 1 and 2, the ligand adopts different coordination modes.
In 1, only three N-atoms of each ligand among nine potential
donors take part in coordination to Ag(i); while in 2, there
are six N-atoms coordinating toAg(i) atoms. In the two com-
plexes, no S-atoms of L coordinates to Ag(1). In order to
investigate the coordination feature of this dithioether ligand,
single-point energy calculations based on the geometry of the
coordinated ligands were carried out by the density functional
theory (DFT) method using the B3LYP/3–21G basis set in the
Gaussian 94 program.[14,15] The charge distributions of the
ligands are shown in Fig. 3. The calculated results show that
all N-atoms of the ligand bear a negative charge, while all
S-atoms bear a positive charge, indicating that the coordina-
tion ability of the N-atoms of the ligand is stronger than that of
the S-atoms.
complex structures in the solid state. The DFT calculated
results show that the coordination ability of the N atom is
stronger than that of S-atom in the dithioether ligand.
Experimental
Materials and General Methods
All reagents for syntheses were commercially available and were either
used without further purification, or purified by standard methods
before use. Elemental analyses were performed on a Perkin–Elmer
240C analyzer and IR spectra were measured on a Nicolet 170SX FT-
IR spectrometer with KBr pellets. 1H NMR spectra were recorded on
a Brüker AC–P500 spectrometer (300 MHz) at 25◦C in CDCl3 with
tetramethylsilane as the internal reference.
Preparation of Ligands
Theligand, 2,3-bis(5-methyl-1,3,4-thiadiazole-2-thiomethyl)quinoxaline
(L), was prepared according to Scheme 2. 1,4-Dibromobutane-2,3-
dione was synthesized by the adoption of a literature procedure.[16]
2,3-Bis(bromomethyl)quinoxaline was prepared by the condensation
reaction of an equimolar amount of 1,4-dibromobutane-2,3-dione and
1,2-diaminobenzene in ethanol at reflux (∼2 h) with a yield of 80%.
The reaction of 2,3-bis(bromomethyl)quinoxaline with the appropriate
5-methyl-2-sulfanyl-1,3,4-thiadiazole in ethanol, in the presence of
KOH, gave the ligand L with a yield of 80%, and the product was char-
acterized by 1H NMR and elemental analyses. δH 2.72 (s, 6H, CH3),
5.10 (s, 4H, SCH2), 7.73∼8.07 (m, 4H, ArH). The elemental analysis is
satisfactory.
Conclusions
The coordination chemistry of the dithioether ligand with
Ag(i) salts has been investigated by experimental and cal-
culation methods. In such complexes, in addition to the
strongcoordinationbonds, therealsoexistotherweakinterac-
tions including π–π stacking, theAg · · · N weak coordination
bond, andF · · · S, O · · · Sweakinteractions.Theseweakinter-
actions play an important role in the self-organization of
supramolecular polymers, as well as the stabilization of the

RESEARCH FRONT
38
Y.-B. Xie, J.-R. Li, and X.-H. Bu
S
NH2
NH2
SH
O
O
O
O
N
N
Br2
L
N
N
BrH2C
CH2Br
H3C
CH3
BrH2C
CH2Br
Scheme 2.
Table 2. Crystal data and details of measurements for 1 and 2
Complex
Formula
Formula weight
T (K)
System
Space group
a [Å]
1
2
C16H14AgClN6O4S4
625.89
293(2)
Monoclinic
C2/c
20.423(7)
12.372(5)
18.104(6)
108.185(5)
4346(3)
1.913
C18H22Ag2F12N6O2P2S4
988.83
293(2)
Tetragonal
Pccn
19.085(5)
21.093(6)
14.986(4)
90
b [Å]
c [Å]
β [◦]
V [Å3]
6033(3)
2.160
Dc [g cm−3
]
Z
8
8
µ(MoKα) [mm−1
]
1.474
1.788
Measured reflections
Unique reflections
RA/wRB
10063
4323
0.0453/0.1209
33037
6201
0.0500/0.1304
A R = ꢀ(||Fo| − |Fc||)/ꢀ|Fo|; B wR = [ꢀw(|Fo|2 − |Fc|2)2/ꢀw(F2o)]1/2, where Fo = observed and Fc = calculated
structure factors, respectively.
Preparation of Complexes
Road, Cambridge CB2 1EZ, UK; Fax: +44–1223–336033; E-mail:
deposit@ccdc.cam.ac.uk.
{[AgL](ClO4)} (1). The 5 mL CH3OH solution of AgClO4·H2O
∞
(0.1 mmol) was carefully layered on top of a 10 mL CHCl3 solution of L
(0.1 mmol) in a test-tube.After one week at room temperature, colorless
block single crystals suitable for X-ray investigation appeared at the wall
of the test-tube at the boundary between the CH3OH and CHCl3 solu-
tions (36% yield). (Found: C 30.6, H 2.4, N 13.6. C16H14AgClN6O4S4
requires C 30.7, H 2.3, N 13.4%). νmax (KBr)/cm−1 3441w, 2926w,
1644w, 1562w, 1484m, 1463w, 1429m, 1411m, 1397m, 1383m, 1210w,
1189w, 1106s, 1080vs, 781s, 621s.
Accessory Materials
Views of the supramolecular structures in 1 and 2 are avail-
able from the author or, until January 2011, the Australian
Journal of Chemistry.
Acknowledgments
{[Ag2L(CH3OH)](PF6)2(CH3OH)} (2). Colorless crystals of 2
∞
This work was financially supported by the National Natural
Science Foundation of China (no. 20373028).
suitableforX-rayanalysiswasobtainedbyasimilarmethodasdescribed
for 1 (using AgPF6 instead of AgClO4·H2O) (22% yield). (Found: C
21.4, H 2.4, N 8.8. C18H22Ag2F12N6O2P2S4 requires C 21.9, H 2.2,
N 8.5%). νmax (KBr)/cm−1 1630w, 1571w, 1491m, 1340m, 1298m,
1208w, 1134m, 1020m, 840vs, 771s, 558s.
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