Self-assembly of supramolecular coordination polymers constructed from AgCN and bipodal spacers

Article abstract of DOI:10.1016/j.poly.2008.12.053

The syntheses and crystal structures of novel supramolecular coordination polymers (SCPs) of AgCN with bipodal ligands; 4,4′-bipyridine (bpy), trans-1,2-bis(4-pyridyl)ethylene (tbpe) and 1,2-bis(4-pyridyl)ethane (bpe) are reported. The syntheses were affe

Full text of DOI:10.1016/j.poly.2008.12.053

Polyhedron 28 (2009) 873–882
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Self-assembly of supramolecular coordination polymers constructed
from AgCN and bipodal spacers
*
Safaa El-din H. Etaiw , Dina M. Abd El-Aziz, Ahmed S. Badr El-din
Chemistry Department, Faculty of Science, Tanta University, El-gash Street, Tanta 31527, Egypt
a r t i c l e i n f o
a b s t r a c t
Article history:
The syntheses and crystal structures of novel supramolecular coordination polymers (SCPs) of AgCN with
bipodal ligands; 4,40-bipyridine (bpy), trans-1,2-bis(4-pyridyl)ethylene (tbpe) and 1,2-bis(4-pyridyl)eth-
ane (bpe) are reported. The syntheses were affected in H2O/acetonitrile/NH3 media at room temperature.
SCP [(AgCN)2 ꢀ bpy] (1) is monoclinic and crystallizes in the space group P21/c, a = 9.0565(4) Å,
b = 16.0063(8) Å, c = 9.1361(4) Å, b = 110.51° and Z = 4. SCP [(AgCN)3 ꢀ (tbpe)2 ꢀ H2O] (2) is triclinic and
Received 28 June 2008
Accepted 23 December 2008
Available online 2 March 2009
Keywords:
Supramolecular
Silver(I) cyanide
Coordination polymers
Organodiimine bridge
crystallizes in the space group P1, a = 9.8427(4) Å, b = 10.3596(4) Å, c = 14.0864(6) Å,
b = 101.739°,
a = 98.873°,
c
= 95.506° and Z = 2. SCP [(AgCN)4 ꢀ bpe] (3) is monoclinic and crystallizes in the space
group P21/c, a = 10.1872(5) Å, b = 9.1610(5) Å, c = 14.4862(6) Å, b = 130.00(18)° and Z = 2. The 3D-topol-
gies of 1–3 resulted from the bridging action of the bipodal ligands and the argentophilic interactions as
well as extensive H-bonding and p–p stacking.
Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction
space group symmetry, and all the inter-chain Ag–Ag separations
exceed 3.88 Å. Also, only a few examples of silver(I) pseudo-halide
coordination polymers with nitrogen bases have been previously
reported [5,7]. In an attempt to encourage the self-assembly of
phases of high dimensionality from silver(I) cyanide, bipyridine
(bpy), trans-bis(4-pyridyl)ethylene (tbpe) and 1,2-bis(4-pyri-
dyl)ethane (bpe) have been used. It was felt that the potential of
multiple coordination sites in ambidentate ligands would allow
the linkage of metal centers through the organodiimine bridges.
In addition to such chaining, it was supposed that the bpy, tbpe
and bpe ligands would not be constrained to a planar conformation
and subsequently allow for the interconnection of chains and prop-
agation of the microstructure in two or three dimensions. Here we
report the synthesis and structural characterization of [(AgCN)2 ꢀ b-
py] (1), [(AgCN)3 ꢀ (tbpe)2 ꢀ H2O] (2) and [(AgCN)4 ꢀ bpe] (3) from
the simple reaction of AgCN and the bpy, tbpe or bpe ligand in
H2O/acetonitrile/NH3 media at room temperature. Further, we re-
port on the crystal packing effects, namely metal–ligand, Ag–Ag
and ligand–ligand interactions, which may have an effect on the
formation and stability of the SCPs 1–3.
Supramolecular chemistry is concerned with preparing assem-
blies of molecules using a combination of secondary chemical
interactions rather than covalent bonding. The comparatively sim-
ple molecules used in these assembles are driven to spontaneously
self-assemble, and then hold together, via such non-covalent inter-
actions as hydrogen bonds, metal–ligand coordination interactions
and
p–p stacking. The underlying self assembly has long been
known: Fischer’s lock-and-key theory of enzyme activity may be
considered as their foundation store because it recognized the cen-
tral role of shape and geometry in molecular interactions [1]. In
fact, self-assembly processes are vital to biosynthesis in nature be-
cause they allow large complicated structures to form quickly.
Silver(I) is good candidate as a soft acid favoring coordination to
soft bases such as ligands containing halogens, sulfur and nitrogen
atoms. Many scientists have exploited the coordination flexibility
of silver(I) in the construction of a large number of coordination
polymers exhibiting interesting structural diversity [2,3]. On the
other hand, although cyanide is a potential bridging ligand, its sil-
ver(I) coordination polymer not yet well documented [4,5]. De-
tailed structural information on AgCN was unavailable until the
appearance of a report on its vibrational spectroscopy and the
determination of the crystal structure using powder neutron dif-
fraction data [6]. The AgCN structure is composed of the packing
2. Experimental
All reagents were commercially available and were used as
received. Microanalyses (C, H and N) were carried out with a Perkin
Elmer 2400 automatic elemental analyzer. The IR spectra were
recorded on a Bruker Vector 22 (Germany) spectrophotometer as
KBr disks. Thermogravimetric analysis was carried out on a
Shimadzu AT 50 thermal analyzer (under N2 atmosphere).
of parallel, linear (AgCN) chains in accordance with the R3 m
1
* Corresponding author. Tel.: +20 2222601379.
E-mail address: safaaetaiw@hotmail.com (Safaa El-din H. Etaiw).
0277-5387/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2008.12.053

874
Safaa El-din H. Etaiw et al. / Polyhedron 28 (2009) 873–882
Table 1
Electronic absorption spectra were measured on a Shimadzu
(UV-310PC) spectrometer. Luminescence spectra were recorded
using a Perkin Elmer (LS 50 B) spectrometer.
Crystal data and structure refinement parameters of the SCPs 1–3.
1
2
3
Empirical formula
Formula weight
T (K)
C12H8N4Ag2
423.944
298
C27H22N7OAg3
784.101
298
C16H12N6Ag4
719.794
298
2.1. Synthesis of [(AgCN)2 ꢀ bpy] (1)
Crystal system
Space group
a (Å)
b (Å)
c (Å)
monoclinic
P21/c
9.0565 (4)
16.0063 (8)
9.1361 (4)
90.00
110.516 (3)
90.00
1240.38 (10)
4
3.14
2.270
2.014
808
0.035/0.072
0.121/0.094
0.031
triclinic
P1
monoclinic
P21/c
10.1872 (5)
9.1610 (5)
14.4862 (6)
90.00
130.0 (18)
90.00
974.01 (8)
2
3.97
2.454
1.704
676
0.033/0.076
0.077/0.084
0.033
Seventy eight milligram (0.5 mmol) of bpy in 10 mL MeCN was
added to 33 mg (0.25 mmol) of AgCN in 10 mL H2O. The resulting
mixture was heated at about 70°, while concentrated ammonia
solution was added dropwise with stirring until the solution be-
came clear. Prismatic colorless crystals appeared after standing at
room temperature for two weeks. The crystals were recovered by
filtration, washed with H2O and dried in air (the yield was 65.5%
with respect to AgCN).
9.8431 (4)
10.3592 (4)
14.0880 (6)
98.873 (2)
101.739 (2)
95.506 (3)
1377.54 (10)
2
2.14
1.818
2.127
730
a
(°)
b (°)
c
(°)
V (Å3)
Z
l
(Mo K
a
) (mmꢁ1
)
Anal. Calcd for 1 (C12H8N4Ag2): C, 33.96; H, 1.88; N, 13.20.
Calculated density (Mg mꢁ3
Goodness-of-fit on F2
F(000)
)
Found: C, 33.56; H, 1.79; N, 13.46. IR (KBr, cmꢁ1): 3048 (
m
CH),
2135 (mC„N), 1594 (mC@N), 1529, 1481 (mC@C), 1404, 1317 (dCH),
1218, 1063, 995, 803, 726, 618, 571, 471.
R indices [I>3
r
(I)] R1/wR2
0.039/0.078
0.081–0.087
0.028
R indices (all data)
Rint
Data/restraints/parameters
2.2. Synthesis of [(AgCN)3 ꢀ (tbpe)2 ꢀ H2O] (2)
1370/0/163
3452/0/343
1049/0/113
Ninety one milligram (0.5 mmol) of tbpe in 10 mL MeCN was
added to 33 mg (0.5 mmol) of KCN in 5 mL H2O. The resulting mix-
ture was added to 85 mg (0.5 mmol) of AgNO3 in 10 mL H2O. Con-
centrated ammonia solution was added dropwise with stirring to
the previous mixture until the solution became clear. Yellow nee-
dle crystals appeared after standing at room temperature for two
weeks. The crystals were recovered by filtration, washed with
H2O and dried in air (the yield was 85.6% with respect to AgNO3).
Anal. Calc. for 2 (C27H22N7OAg3): C, 41.32; H, 2.80; N, 12.49.
Table 2
Selected bond distances (Å) and bond angles (°) for SCP 1.
Ag1–C7
2.085 (3)
2.570 (2)
2.080 (3)
2.387 (3)
1.124 (4)
91.26 (9)
164.85 (11)
103.39 (9)
94.86 (9)
110.43 (9)
151.13 (10)
163.3 (3)
154.10 (14)
170.35 (13)
144.60 (13)
Ag1–Ag2d
Ag2–N6
Ag2–N3
N4–C8
3.2550 (4)
2.249 (2)
2.129 (3)
1.127 (4)
Ag1–N5a
Ag1–C8b
Ag2–N4
C7–N3
C7–Ag1–N5a
C7–Ag1–C8b
N5a–Ag1–C8b
N4–Ag2–N6
N4–Ag2–N3
N6–Ag2–N3
Ag1–C7–N3
Ag1c–N5–C9
Ag1c–N5–C10
Ag1c–N5–C11
Ag1c–N5–C13
Ag1c–N5–C12
Ag2–N4–C8
Ag2–N6–C16
Ag2–N6–C14
Ag2–N6–C15
Ag2–N6–C18
Ag2–N6–C17
Ag2–N3–C7
127.0 (2)
117.2 (2)
161.7 (3)
125.9 (2)
173.80 (11)
154.03 (13)
146.26 (13)
117.6 (2)
166.0 (3)
Found: C, 41.21; H, 2.37; N, 11.47%. IR (KBr, cmꢁ1): 3440 (mH O),
2
3046 (mCH), 2117 (mC„N), 1601 (mC@N), 1555, 1498 (mC = C), 1421,
1349 (dCH), 1219, 1067, 971, 826, 669, 550, 468.
2.3. Synthesis of [(AgCN)4 ꢀ bpe] (3)
Ninety two milligram (0.5 mmol) of bpe in 10 mL MeCN was
added to 33 mg (0.5 mmol) of KCN in 10 mL H2O. The resulting
mixture was added to 85 mg (0.5 mmol) of AgNO3 in 10 mL H2O
and then concentrated ammonia solution was added dropwise
with stirring to the previous mixture until the solution became
clear. A colorless polycrystalline precipitate appeared after stand-
ing at room temperature for two weeks. The precipitate was recov-
ered by filtration, washed with H2O and dried in air (the yield was
70% with respect to AgNO3).
Hydrogen bonding
Intra-layer H-bonding
C7–H16e
Inter–layer H-bonding
C8–H15
2.989 (3)
2.840 (3)
3.047 (3)
2.895 (3)
2.583 (3)
2.841 (3)
2.923 (3)
3.031 (3)
3.056 (3)
3.033 (3)
3.065 (3)
2.978 (3)
C7–H15e
N3–H18f
C7–H13a
N4–H13a
N4–H17
N3–H16e
C8–H9a
N3–H13a
Anal. Calc. for 3 (C16H12N6Ag4): C, 26.69; H, 1.66; N, 11.67.
Found: C, 27.10; H, 1.59; N, 11.44%. IR (KBr, cmꢁ1): 3060, 2938,
C17–H12e
C8–H17
2861 (mCH), 2130, 2160 (mC„N), 1600 (mC@N), 1558, 1492, (mC@C),
1453, 1416, 1339 (dCH), 1210, 1070, 830, 722, 610, 570 and 469.
Symmetry codes: ax ꢁ 1,1/2 ꢁ y,1/2+z; bꢁ1 ꢁ x,1/2+y,3/2 ꢁ z; c1+x,1/2 ꢁ y,z ꢁ 1/2;
d
f
ꢁ1 ꢁ x, ꢁy,1 ꢁ z;eꢁ x,ꢁy,1 ꢁ z; x, 1/2 ꢁ y,1/2 + z.
2.4. X-ray structural determination
angles are given in Tables 2–4. The C9 atom and N11 atom of the
two bridging cyanide groups in 3 were located at inversion centers
and were modeled as disordered over a pair of sites of equal occu-
pancy. The disorder of these cyano groups in the asymmetric unit
of 3 reflects the observation that Rw remains constant upon adopt-
ing a C atom or N atom at these sites.
Structural measurements for SCPs 1ꢁ3 were performed on a
Kappa CCd Enraf Nonius FR 90 four cycle geniometer with graphite
monochromatic Mo Ka radiation {[k Mo Ka] = 0.71073 Å} at
25 2 °C. All the structures were resolved using direct-methods
and all of the non-hydrogen atoms were located from the initial
solution or from subsequent electron density difference maps dur-
ing the initial stages of the refinement. After locating all of the non-
hydrogen atoms in each structure the models were refined against
F2, first using isotropic and finally using anisotropic thermal dis-
placement parameters. The positions of the hydrogen atoms were
then calculated and refined isotropically, and then the final cycle
of refinements was performed. The cyano groups of all structures
were ordered unless otherwise stated. Crystallographic data for
1–3 are summarized in Table 1. Selected bond distances and bond
3. Results and discussion
The novel SCPs 1–3 exhibit variable stoichiometries. The molar
ratios of AgCN:L are 2:1 for 1, 3:2 for 2 and 4:1 for 3 in MeCN/H2O/
NH3 solution. It seems that self-assembly of either authentic or
in situ prepared AgCN and the bipodal ligand takes place at the
appropriate stoichiometry required by the extended framework

Safaa El-din H. Etaiw et al. / Polyhedron 28 (2009) 873–882
875
Table 3
ing such long bridging ligands. In addition, these ligands behave as
non-rigid spacers which are capable of displaying a variety of con-
formations. SCPs 1–3, which are prepared at room temperature,
can be also considered as unprecedented SCPs that may be com-
pared with those prepared by hydrothermal reactions, described
Selected bond distances (Å) and bond angles (°) for SCP 2.
Ag1–Ag2a
Ag1–N4
Ag1–C31
Ag1–C32
Ag2–N30
Ag2–N18
3.0415 (5)
2.496 (3)
2.076 (4)
2.078 (4)
2.330 (3)
2.294 (3)
117.89 (8)
74.39 (11)
95.26 (11)
96.80 (14)
99.83 (14)
163.2 (2)
89.22 (8)
74.84 (8)
138.25 (9)
130.54 (10)
164.01 (12)
93.56 (12)
90.55 (13)
99.18 (12)
98.78 (13)
91.12 (14)
158.9 (2)
83.78 (13)
86.52 (14)
109.8 (2)
110.7 (2)
82.38 (11)
172.6 (2)
118.4 (3)
124.6 (3)
146.8 (2)
Ag2–N33
Ag2–N34
Ag3–N25
Ag3–C26
Ag3–N33
Ag3–N34
2.409 (3)
2.487 (4)
2.211 (4)
2.062 (5)
2.716 (4)
2.590 (5)
170.1 (2)
130.0 (3)
158.3 (2)
141.8 (2)
114.0 (3)
176.8 (2)
148.9 (2)
120.1 (2)
151.8 (2)
123.7 (3)
170.2 (2)
149.4 (2)
120.4 (3)
148.5 (2)
122.3 (3)
172.3 (4)
92.56 (12)
148.6 (3)
118.0 (3)
176.8 (4)
93.87 (13)
132.8 (4)
132.5 (4)
176.0 (5)
152.6 (2)
for [(CuCN)2.
l-L] with L = bpy and tbpe [8] and (CuCN)2
[(CuCN)2(
l
-4,40-bpy)] [9]. Hydrothermal reactions, in spite of being
Ag2a–Ag1–N4
Ag2a–Ag1–C31
Ag2a–Ag1–C32
N4–Ag1–C31
N4–Ag1–C32
C31–Ag1–C32
Ag1a–Ag2–N30
Ag1a–Ag2–N18
Ag1a–Ag2–N33
Ag1a–Ag2–N34
N30–Ag2–N18
N30–Ag2–N33
N30–Ag2–N34
N18–Ag2–N33
N18–Ag2–N34
N33–Ag2–N34
N25–Ag3–C26
N25–Ag3–N33
N25–Ag3–N34
C26–Ag3–N33
C26–Ag3–N34
N33–Ag3–N34
Ag1–N4–C9
Ag2–N30–C14
Ag2–N30–C12
Ag2–N30–C13
Ag2–N30–C15
Ag2–N30–C16
Ag2–N18–C22
Ag2–N18–C21
Ag2–N18–C29
Ag2–N18–C23
Ag2–N18–C19
Ag3–N25–C24
Ag3–N25–C40
Ag3–N25–C38
Ag3–N25–C39
Ag3–N25–C37
Ag1–C31–N33
Ag2–N33–Ag3
Ag2–N33–C31
Ag3–N33–C31
Ag1–C32–N34
Ag2–N34–Ag3
Ag2–N34–C32
Ag3–N34–C32
Ag3–C26–N27
Ag1–N4–C8
exploited to prepare numerous SCPs, afford mostly the formation
of mixtures of undesirable materials. The present procedure, in
spite of being surprisingly simple, affords the formation of precip-
itates of 1–3 which consist exclusively of one discrete species and
never of mixtures of different assemblies.
3.1. Crystal Structures
3.1.1. Crystal structure of [(AgCN)2 ꢀ bpy] (1)
The asymmetric unit of 1 contains one fragment which consists
of two silver atoms, two cyanide groups and one bpy molecule,
Fig. 1a. The two silver atoms assume a distorted trigonal planar
geometry where each is coordinated to two ordered cyanide
groups and to the nitrogen atom of the bpy ligand. The two silver
atoms are crystallographically different. Ag1 is strongly bonded to
the cyanide groups, Ag1–C = 2.08 Å, but it is weakly coordinated to
the bpy ligand, Ag1–N5 = 2.570 Å, which in spite of being weak is
only slightly longer than those of previously reported silver com-
plexes [5,7]. In this case the bpy ligand is at right angles to the
C7–Ag1–C8 plane, N5–Ag1–C7 = 91.26°. On the other hand,
whereas the Ag1–C distances are in the normal range, the bonds
from the nitrogen atoms of the cyanide ligands to Ag2 are second-
Ag1–N4–C6
Ag1–N4–C5
Ag1–N4–C7
Intra-layer H-bonds
Water H-bonds
Inter-layer H-bonds
C31–H6
2.878 (4)
C7–H35A
C15–H35B
N27–H35A
N27–H35B
N27–H35Bb
O35–H11
O35–H17b
O35–H7
2.710 (4)
C37–H37c
C26–H13
3.061 (4)
3.053 (5)
C31–H37
N33–H16
N34–H16
N34–H38
N27–H13
C26–H16
N30–H29c
C12–H19b
C17–H23b
C14–H23b
2.982 (4)
3.008 (4)
2.952 (4)
2.862 (5)
2.945 (6)
2.955 (5)
2.812 (3)
3.045 (4)
2.865 (4)
2.729 (4)
3.037 (4)
2.809 (5)
2.839 (5)
2.847 (4)
2.770 (4)
2.735 (4)
2.689 (4)
2.490 (4)
O35–H15
b
Symmetry codes: a1 ꢁ x, ꢁ y,1 ꢁ z; ꢁ x, ꢁ y, ꢁ z; c1 ꢁ x, ꢁ y, ꢁ z.
Table 4
Selected bond distances (Å) and bond angles (°) for SCP 3.
Ag2–C4
Ag2–C9
3.031 (3)
2.060 (4)
2.068 (3)
2.301 (4)
Ag1–Ag2
Ag1–N3
Ag1–C4
3.0565 (4)
2.253 (3)
2.064 (4)
Ag2–N11
Ag1–N8a
Ag2–Ag1–N3
Ag2–Ag1–C4
Ag2–Ag1–N8a
N3–Ag1–C4
N3–Ag1–N8a
C4–Ag1–N8a
C9–Ag2–N11
125.84 (7)
69.52 (9)
71.35 (7)
139.58 (11)
96.94 (11)
123.10 (12)
165.58 (14)
Ag1–C4–N8
Ag1a–N8–C4
Ag2–C9–C9b
Ag2–N11–N11c
Ag1–N3–C5
Ag1–N3–C7
177.9 (3)
152.0 (3)
178.1 (5)
176.8 (5)
122.6 (2)
121.6 (2)
Hydrogen bonding
C6–H5d
2.918 (3)
3.064 (3)
2.813 (3)
C7–H13d
N8–H7a
a
b
Symmetry codes: ꢁx,1/2+y,ꢁ1/2ꢁz; ꢁx,ꢁ y,ꢁ 1ꢁz; c1ꢁx,ꢁ y,ꢁ z; d1 ꢁ x,1/2 + y,1/
2 ꢁ z.
structure to adopt enough space demanded by the bipodal bridging
ligand. The increased ratio of AgCN as the size specificity of the
bridging ligand is increased can be considered as an essential
requirement for the construction of unique (AgCN)n building
blocks which create extended networks suitable for accommodat-
Fig. 1. (a) ORTEP plot of the asymmetric unit of 1. (b) Structure of the 2D-layer of 1
along the diagonal of the unit cell. H-atoms have been omitted for clarity.

876
Safaa El-din H. Etaiw et al. / Polyhedron 28 (2009) 873–882
ary with a much longer range of distances, Ag2–N = 2.129–2.387 Å,
while the Ag2–N6(bpy) bond is relatively strong with a distance
equals to 2.249 Å. The extended structure of 1 consists of infinite
puckered 1D-AgCN chains exhibiting a wave-like structure with a
separation distance between the successive chains of 5.8093 Å,
Fig. 1b. The wave-like appearance of the individual [AgCN]1 chains
results not only from the wide N(C)–Ag–C(N) angles at Ag1 and
Ag2 (C7–Ag1–C8 = 164.85° and N3–Ag2–N4 = 110.43°) but also
from the pronounced deviation from linearity for N3–C7–
Ag1 = 163.3°, C7–N3–Ag2 = 166.0° and C8–N4–Ag2 = 161.7°. This
deviation from linearity at the silver atoms in the [AgCN] chains
1
is in order to allow the coordination of the silver ions with the
bidentate organodiimine ligand. These puckered chains are con-
nected in a unique way by the bpy ligand via the Ag1 and Ag2
atoms creating corrugated 2D-layers with two types of fused rect-
angular rings; the first one consists of Ag8(CN)6(bpy)2 with dimen-
sions of 6.804 ꢂ 23.39 Å, while the second ring consists of
Ag4(CN)2(bpy)2 with dimensions of 6.526 ꢂ 12.142 Å, Fig. 1b. Anal-
ogous connectivity patterns in which infinite [CuCN] chains are
1
bridged by bpy have also been reported for [(CuCN)2(
l
-4,40-
l
-4,40-bpy)] [8,10] and (CuCN)2
bpy)], [{CuCN(
l
-4,40-bpy)}ꢀ2(
[(CuCN)2(
l
-4,40-bpy)] [9]. Interestingly, only one type of hexagonal
Fig. 3. 3D-structure of 1 along the c-axis showing the interwoven layers. H-atoms
have been omitted for clarity.
ring, {Cu4(CN)4(bpy)2}, exists in the sheets forming the lamellar
structure of [(CuCN)2(
l
-4,40-bpy)], which differs however from
those realized in 1. Also, the layer’s structure is stabilized by exten-
sive H-bonding between the cyanide groups and hydrogen atoms
of the bpy ligands, 2.583–3.056 Å, Table 2. The corrugated layers
are arranged in parallel positions along the b-axis with a separation
distance of almost 3.901 Å, creating channels, Fig. 2. These corru-
gated layers are interwoven along the c-axis causing stacking of
the layers via interconnections of Ag–Ag interactions, Ag1–
Ag2 = 3.2548 Å, meanwhile they are further packed by hydrogen
atoms, Fig. 4a. The Ag(I) atoms are coordinated to ordered cyanide
groups in a unique way representing the main building blocks of
the structure of 2. These building blocks form parallel ribbons con-
nected by the tbpe ligands via silver atoms, Fig. 4b. Ag2 and Ag3
atoms are coordinated in tetrahedral fashion, while Ag1 atom
exhibits a distorted planar trigonal configuration. Ag1 is strongly
coordinated to two cyanide groups, Ag1–C31 = 2.076 Å,
Ag1–C32 = 2.078 Å, and one nitrogen atom of the tbpe ligand,
Ag1–N4 = 2.496 Å. The angles of the trigonal configuration at Ag1
assume a large deviation from the ideal value of 120° causing sub-
stantial pyramidal distortion which results essentially from the
strong argentophilic interaction between Ag1 and Ag2, Table 3.
The Ag2 atom exhibits a tetrahedral configuration via coordination
to two cyanide groups and two tbpe ligands. On the other hand, the
Ag3 atom assumes a strong coordination to the nitrogen atom of
the tbpe ligand, Ag3–N25 = 2.211 Å, and to one cyanide group,
Ag3–C26 = 2.062 Å, while it is weakly coordinated to the other cya-
nide groups that complete the distorted tetrahedral surroundings,
Ag3–N33 = 2.716 Å, Ag3–N34 = 2.590 Å. The tetrahedral geometry
around Ag3 is more distorted than that around Ag2 in consequence
of the small chelate angles of 82.38° and 91.120°. In this case, two
cyanide groups are asymmetrically bonded to three silver atoms,
bonding, N3–H18d = 2.975 Å and C8–H15 = 3.056 Å, and
p–p
stacking, 3.120–3.379 Å, creating a novel 3D-network structure,
Figs. 2 and 3. The Ag–Ag bonding interactions look like pillars be-
tween the layers. The argentophilic interaction as well as the close
inter-layers Ag2–N5 and Ag1–N4 contacts of 3.120 and 3.236 Å,
respectively, cause pyramidalisation of the silver atoms in such
way that the angles of the planar trigonal geometry around the sil-
ver sites deviate significantly from 120° in spite of the sum of the
angles being 360°, Table 2. Such behavior is also observed for Cu(I)
in CuCN complexes [8].
3.1.2. Crystal structure of [(AgCN)3 ꢀ (tbpe)2 ꢀ H2O] (2)
The reaction of the ternary system AgNO3, tbpe and KCN in H2O/
MeCN/NH3 media affords yellow needle crystals of [Ag3(CN)3(l-
tbpe)2 ꢀ H2O], (2). One molecule of 2 comprises the asymmetric
unit of the structure which consists of two independent fragments
containing three chemically and crystallographically different Ag(I)
resulting in the
cycle [Ag2( 3-CN)2] motif, Fig. 5, similar to that found in the proto-
type CuCN coordination polymers [11].
l3–CN coordination mode forming a quadro mini-
l
Alternatively, the (AgCN) building blocks create unusual paral-
1
lel infinite ribbons which consist of fused hexagonal rings, Ag4(CN)2,
with two double edge sides of the quadro minicyle [Ag2(l3-CN)2]
motifs,Figs. 4b and 5. Surprisingly, the ribbons contain free terminal
cyanide unit at Ag3, Ag3–C26–N27 = 176.0°. Four cyanide angles ex-
hibit significant deviation from linearity, Ag2–N34–C32 = 132.8°,
Ag2–N33–C31 = 148.6°, Ag3–N33–C31 = 118.0°, Ag3–N34–C32 =
132.5°, while the other Ag1–CN bonds are nearly linear, Table 3.
The cyanide ligands turn out to be free of any disordered. 2 is the first
example containing the minicycle [Ag2(
parison to CuCN complexes containing a similar type of the unusual
minicycle [Cu2( 3-CN)2] motif is particularly pertinent. The minicy-
cle [Cu2( 3-CN)2] motif is now realized as being the basic building
block of an increasing number of 2D or 3D supramolecular assem-
blies [11,12]. The unusual character of the [Cu2( 3-CN)2] motif
l3-CN)2] motif, thus a com-
l
l
Fig. 2. 3D-dimensional structure of 1 showing the argentophilic interaction. H-
atoms have been omitted for clarity.
l

Safaa El-din H. Etaiw et al. / Polyhedron 28 (2009) 873–882
877
Fig. 4. (a) ORTEP plot of the asymmetric unit of 2. (b) Structure of the layer of 2 along the b-axis showing the parallel ribbons. H-atoms have been omitted for clarity.
Fig. 5. View of the (AgCN)n ribbon showing the minicyclic [Ag2(l3-CN)2] motif along the c-axis.
comes from the fact that the carbon atoms bridge two copper atoms
while the nitrogen atoms coordinate to one copper atom. In contrast,
be considered as a consequence of the formation of the quadro mini-
cycle [Ag2( 3-CN)2] motifs. Also, the (AgCN)1 building blocks create
l
the nitrogen atoms in the [Ag2(
l
3-CN)2] motif act as
l
2-bridges, as
a cluster-like structure consisting of three silver atoms exhibiting a
strong Ag1–Ag2 interaction with a distance of 3.0145 Å and a weak
Ag2–Ag3 interaction with a distance of 3.715 Å. Examples of weak
Ag–Ag interactions with intermetallic distances from 3.40 to 3.724
Å are known [13]. The parallel ribbons are connected in a unique
way by the tbpe molecules. One set of three nearly parallel tbpe
should be expected. However, while the bifurcation of cyanide
groups is no longer unusual, it greatly exceeds that in CuCN SCPs
[12], while the Ag–N distances are not equal leading to the formation
of a distorted rhombus with the sum of the internal angles being
quite close to 360°, scheme 1. The bifurcated cyanide ligands can

878
Safaa El-din H. Etaiw et al. / Polyhedron 28 (2009) 873–882
manner along the a-axis forming layers, Fig. 6. These layers contain
polygons with dimensions of 9.843 ꢂ 14.602 Å. On the other hand,
one type of the tbpe molecules fill the space of the hexagonal rings
along the c-axis, alternatively one being vertical while the other
one is horizontal, Fig. 7. The second type of tbpe ligands intermingle
around both sides of the ribbon in horizontal positions while the ter-
minal cyanide groups are directed to the space between the layers
which form the 3D-network structure of 2, Figs. 6 and 7. The struc-
ture of the layer is also stabilized by a wide range of intra-layer
hydrogen bonding between the cyanide groups and the H-atoms of
the pyridyl rings, 2.729–3.045 Å, Table 3. These parallel layers exhi-
bit large channels in which water molecules thread, acting as guest
molecules. The water molecules play an essential role in connecting
the layers as they thread along the sides of the layers, H-bonding
with the terminal cyanide groups belonging to one layer, tbpe ligand
in the same layer and with the terminal cyanide groups of the adja-
cent layer, Table 3 and Fig. 6. This was further confirmed by TGA as
SCP 2 starts decomposition by the loss of water molecules between
120 and 160 °C. The layers close packing was also achieved by weak
but not negligible inter-layer hydrogen bonding, C37–H37d =
3.061 Å and C26–H13 = 3.053 Å, as well as p–pstacking interactions
between the pyridyl ring of tbpe from one layer and the ethylenic
group from the adjacent layer, C10–C7 = 3.394 Å.
Scheme 1. Distorted [Ag2(l3-CN)2] rhombus in 2.
3.1.3. Crystal structure of [(AgCN)4 ꢀ bpe] (3)
Half of the formula unit comprises the asymmetric unit of the
structure of 3, which contains two crystallographically different
silver atoms, two cyanide groups and half of a bpe molecule,
Fig. 8. Ag1 is coordinated to two ordered cyanide groups and to
the nitrogen atom of the bpe ligand. Ag1 assumes a distorted trigo-
ligands, with separation distances of 3.617–3.81 Å, and one tbpe li-
gand lying at right angles to the other ligands and to the plane de-
fined by the (AgCN)n ribbon connect these ribbons in an alternate
Fig. 6. View of packing of 2 along the a-axis showing H-bonding through water molecules.
Fig. 7. View of one layer of 2 along the c-axis. H-atoms have been omitted for clarity.

Safaa El-din H. Etaiw et al. / Polyhedron 28 (2009) 873–882
879
nal planar geometry with the sum of bond angles equal to 360°, Ta-
ble 4. The substantial pyramidal distortion of the essentially trigo-
nal planar coordination sphere results mainly from the
argentophilic interaction between Ag1 and Ag2, Table 4. Ag2 is
coordinated to two disordered cyanide groups where it adopts a
quasi-linear geometry, N(C)–Ag2–N(C) = 165.58°. This loss of line-
arity results from close Ag1–Ag2 and Ag2. . .C4 (3.031 Å) contacts.
The structure of 3 consists of two types of (AgCN)n chains,
Fig. 9a and b. The first type, which runs in the ac-plane, is nearly
linear and is composed of {Ag2–CN}n fragments. The second type,
that runs along the b-axis, consists of corrugated chains of
{Ag1CN}n and exhibits a wave-like structure with a separation dis-
tance between the chains of 11.132 Å. These chains are intercon-
nected by the bpe ligands creating a 3D-network structure, while
the two types of (AgCN)n chains are interconnected by the argent-
ophilic interaction, Fig. 10. On the other hand, the 3D-structure is
stabilized by extensive
p–p stacking between the cyanide groups
at both of the silver atoms and the pyridyl rings, 3.03 and
3.756 Å, and H-bonding between hydrogen atoms of the pyridyl
rings and the cyanide groups at the silver atoms, 2.813, 3.159,
2.975 and 3.228 Å. The directions of propagation of the two chains
are perpendicular to each other along the a-axis, Fig. 9b. The planes
of the pyridyl rings are completely parallel in each set of the spacer
ligands along the b-axis, while in the ac-plane each two sets of the
spacer bpe ligands are interwoven in different conformations to
accommodate
11.132 Å, of intersecting (AgCN)n chains, Fig. 11.
a
box-like structure of dimensions 9.161 ꢂ
3.2. Electronic absorption spectra and emission spectra of SCPs 1–3
The electronic absorption spectra of SCPs 1–3 display, generally,
the absorption bands of the bipodal ligands in addition to the CT
*
p transitions of the bipodal li-
bands, Table 5. The bands due to n–
gands disappear in the spectra of SCPs 1–3 due to the participation
of the bipodal ligands in the coordination sphere of the silver(I)
ions. The absorption spectra of SCPs 1 and 2 exhibit an addi-
tional broad band at 301–350 nm, respectively, corresponding to
Fig. 8. ORTEP plot of the asymmetric unit of 3.
Fig. 9. (a) View of the basic building block of 3. H-atoms have been omitted for clarity. (b) View of the connection of (AgCN)n chains in 3 to form a 3D-network, neglecting the
bpe ligand.

880
Safaa El-din H. Etaiw et al. / Polyhedron 28 (2009) 873–882
responding ligands, whereas the intensities of the bands of SCPs 2
and 3 increase significantly from those of the ligands.
3.3. Thermogravimetric analysis of SCPs 1–3
The thermal decomposition bahaviour of SCPs 1–3 show broad
similarity, but they are not identical, Table 6. They show the usual
trend of similar early reported compounds [14]. Firstly dehydra-
tion occurs, followed by loss of the bipodal ligand and finally re-
moval of the inorganic anions. After complete thermolysis the
calculated and observed percentages of the residues are coincident
with metallic silver. The loss of water molecules at 120–160 °C, for
SCP 2, confirms the involvement of the water molecules in hydro-
gen bonding with the cyanide groups, as depicted in the crystal
structure (vide supra). The cyanide groups are lost in the form of
cyanogen molecules at relatively high temperatures, indicating
the bridging capability of the cyanide groups forming stable
(AgCN)n frameworks.
4. General discussion
The SCPs 1–3 illustrate the rich diversity of structures that can
be achieved by variation of the spacer bipodal ligand. The length
and the molecular conformation of the spacer ligands affect signif-
icantly the self-assembly of the (AgCN)n building blocks to produce
enough wide space to accommodate the spacer ligands via the for-
mation of different rings and polygons. The bipodal ligands play
the role of bridging the (AgCN)n building blocks creating 2D-layers
which are interwoven, 1 and 2, or intersecting, 3, to produce 3D-
Fig. 10. View of the 3D-framework of 3 showing the argentophilic interaction. H-
atoms have been omitted for clarity.
metalto-ligand charge transfer (MLCT), where the charge is trans-
*
ferred from the silver(I) center to the unoccupied
p -orbital of
the ligand. The emission spectra of the SCPs 1–3 together with
the emission spectra of the bipodal ligands were measured in the
solid state at room temperature and using the same excitation
wavelength in each case, Fig. 12, Table 5. The emission spectra of
the bipodal ligands display structural bands which correspond
structures via extensive H-bonding and
Agꢀ ꢀ ꢀAg interactions. The structures of 1 and 2 are completely dif-
ferent from those of the analogous [(Cu2(CN)2(
-4,40-bpy)], 10 and
[(CuCN)2(
-tbpe)], 20. 1 exhibits an interwoven but non-interpene-
p–p stacking, as well as
l
l
trating open framework while 10 contains two identical non-inter-
secting but interpenetrating frameworks with a rhombic (Cu2X2)
motif. In spite of both 2 and 20 containing the minicycle (Cu2X2)
and (Ag2N2) motifs, 20 contains large void spaces in the framework
which results in interpenetration by a second independent frame-
work, while 2 contains a cluster-like structure of (AgCN)n ribbons
which are connected in a unique way by the tbpe ligands creating
*
*
to the lowest (p–p ) and close lying (n–p ) states. On the other
hand, the emission spectra of SCPs 1–3 display the structural bands
of the bipodal ligands as well as additional bands at 600–640 nm,
584–624 nm and 600 nm, respectively. These bands may be attrib-
uted to MLCT or metal centered transitions of the type 4d10 ? 4d9
5s1 and 4d10 ? 4d9 5p1 on the silver(I) center. The emission spectra
of SCPs 1–3 exhibit a slight red shift compared to those of the cor-
Fig. 11. View of the box-like structure in 3.

Safaa El-din H. Etaiw et al. / Polyhedron 28 (2009) 873–882
881
Table 5
Absorption and emission bands for bpy, tbpe, bpe and SCPs 1–3.
Ligand
SCP
kabs (nm)
Assignment
kem (nm)
kabs (nm)
Assignment
kem (nm)
bpy
218
278
390
1La
400
452
481
516
660
1
220
255
301
1La
1Lb
CT
405
428
454
484
525
600
640
1Lb
n–p*
tbpe
bpe
*
232
265
325
450
1La
390
478
529
2
3
220
252
350
1La
1Lb
CT
433
454
485
524
584
624b
1Lb
n–p*
n–p*
201
258
315
392
1La
401
432
452
482
522
600
201
268
1La
1Lb
405
425
444
487
527
600b
1Lb
n–p*
b = Broad.
[AgNC], distorted trigonal planar, [AgN3], [AgC2 N], [AgN2C], and
tetrahedral [AgN4], [AgN3C] geometries have been observed, in
addition to the influence of the argentophilic interaction on the
distortion of the geometry around the silver sites.
5. Conclusion
The results reinforce the observation that the cyanide group is
an effective bridging ligand which can link together two or three
silver centers, l2-1,2-CN or l3-1,2,2-CN. Ligand length and flexibil-
ity affect the AgCN–ligand ratio, where the proportion of the ligand
decreases as the length and flexibility of the ligand increase. It is
also clear that the organic ligands may be introduced as structural
components of composite inorganic–organic materials and used, in
addition to conventional and unconventional interactions, to prop-
agate 3D-topologies.
Fig. 12. Emission Spectra of SCPs 1–3; 1 (solid line), 2 (dashed line), 3 (doted line),
(kex is 320 nm for 1, 300 nm for 2 and 270 nm for 3).
Appendix A. Supplementary data
CCDC 682258, 682259 and 682260 contain the supplementary
crystallographic data for 1, 2 and 3. These data can be obtained free
of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or
from the Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: (+44) 1223 336 033; or e-mail: de-
posit@ccdc.cam.ac.uk. Supplementary data associated with this
article can be found, in the online version, at doi:10.1016/
j.poly.2008.12.053.
a non-interpenetrating 3D-network with terminal cyanide ligands
and threaded water molecules. The unprecedented structure of 3
consists of intersecting (AgCN)n chains forming a box-like structure
with wide voids containing the interwoven bridging bpe ligands. In
1 and 2, the Ag1–Ag2 and Ag1–N4 (in 1) and Ag2–C4 (in 2) contacts
produce a distorted geometry at the silver sites. The coordination
versatility of the silver atom is also in evidence as quasi-linear,
Table 6
Thermogravimetric analysis data for SCPs 1–3.
Comp. First step
Trans. temp.
Second step
Third step
Residue
D
(cal.)
m% obs.
Assig.
(%)
Trans. temp.
(°C)
D
(cal.)
m% obs.
Assig.
(%)
Trans. temp.
(°C)
D
(cal.)
m% obs.
Assig.
Form. Temp.
(°C)
D
(cal.)
m% obs.
Assig.
(%)
(°C)
1
2
3
150–300
36.5 (36.8)
1 bpy
1 H2O
1 bpe
380–480
170–360
320–600
6.00 (6.13)
0.5
(CN)2
500–700
7.0 (6.13)
0.5
(CN)2%
1.5
700
750
620
50.5 (50.1)
2 Ag
3 Ag
4 Ag
120–160
150–300
2.19 (2.20)
26.1 (25.6)
45.84 (46.4) 2 tbpe
450–730
9.00 (9.9)
42.9 (41.2)
60.1 (59.9)
(CN)2%
13.8 (14.4) 2 (CN)2

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Safaa El-din H. Etaiw et al. / Polyhedron 28 (2009) 873–882
[6] G.A. Bowmaker, B.J. Kennedy, J.C. Reid, Inorg. Chem. 37 (1998) 3968.
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