Neutral coordination polymers based on a metal-mono(dithiolene) complex: Synthesis, crystal structure and supramolecular chemistry of [Zn(dmit)(4,4′-bpy)]n, [Zn(dmit)(4,4′-bpe)]n and [Zn(dmit)(bix)]n (4,4′-bpy = 4,4′-bipyridine, 4,4′-bpe = trans-1,2-bis(4-pyridyl)ethene, bix = 1,4-bis(imidazole-1- ylmethyl)-benzene

Article abstract of DOI:10.1039/c1dt10814j

This article describes a unique synthetic route that enables a neutral mono(dithiolene)metal unit, {Zn(dmit)}, to link with three different organic molecules, resulting in the isolation of a new class of neutral coordination polymers. The species {Zn(dmit)} coordinates with 4,4′-bipyridine (4,4′-bpy), trans-1,2-bis(4-pyridyl)ethene (4,4′-bpe) and 1,4-bis(imidazole-1-ylmethyl)-benzene (bix) as linkers giving rise to the formation of coordination polymers [Zn(dmit)(4,4′-bpy)]_n (1), [Zn(dmit)(4,4′-bpe)]_n (2) and [Zn(dmit)(bix)]_n (3) respectively. Compounds 1-3 were characterized by elemental analyses, IR, diffuse reflectance and single crystal X-ray diffraction studies. Compounds 1 and 3 crystallize in the monoclinic space group P2₁/n, whereby compound 2 crystallizes in triclinic space group P1. In the present study, we chose three linkers 4,4′-bpy, 4,4′-bpe and bix (see Schemes 1-3, respectively, for their structural drawings), that differ in terms of their molecular dimensions. The crystal structures of compounds 1-3 are described here in terms of their supramolecular diversities that include π-π interactions, not only among aromatic stacking (compounds 1 and 3), but also between an aromatic ring and an ethylenic double bond (compound 2). The electronic absorption spectroscopy of compounds 1-3 support these intermolecular π-π interactions.

Full text of DOI:10.1039/c1dt10814j

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PAPER
Neutral coordination polymers based on a metal–mono(dithiolene) complex:
synthesis, crystal structure and supramolecular chemistry of
[Zn(dmit)(4,4¢-bpy)]n, [Zn(dmit)(4,4¢-bpe)]n and [Zn(dmit)(bix)]n
(4,4¢-bpy = 4,4¢-bipyridine, 4,4¢-bpe = trans-1,2-bis(4-pyridyl)ethene,
bix = 1,4-bis(imidazole-1-ylmethyl)-benzene†
Vedichi Madhu and Samar K. Das*
Received 2nd May 2011, Accepted 22nd September 2011
DOI: 10.1039/c1dt10814j
This article describes a unique synthetic route that enables a neutral mono(dithiolene)metal unit,
{Zn(dmit)}, to link with three different organic molecules, resulting in the isolation of a new class of
neutral coordination polymers. The species {Zn(dmit)} coordinates with 4,4¢-bipyridine (4,4¢-bpy),
trans-1,2-bis(4-pyridyl)ethene (4,4¢-bpe) and 1,4-bis(imidazole-1-ylmethyl)-benzene (bix) as linkers
giving rise to the formation of coordination polymers [Zn(dmit)(4,4¢-bpy)]n (1), [Zn(dmit)(4,4¢-bpe)]n (2)
and [Zn(dmit)(bix)]n (3) respectively. Compounds 1–3 were characterized by elemental analyses, IR,
diffuse reflectance and single crystal X-ray diffraction studies. Compounds 1 and 3 crystallize in the
¯
monoclinic space group P21/n, whereby compound 2 crystallizes in triclinic space group P1. In the
present study, we chose three linkers 4,4¢-bpy, 4,4¢-bpe and bix (see Schemes 1–3, respectively, for their
structural drawings), that differ in terms of their molecular dimensions. The crystal structures of
compounds 1–3 are described here in terms of their supramolecular diversities that include p–p
interactions, not only among aromatic stacking (compounds 1 and 3), but also between an aromatic
ring and an ethylenic double bond (compound 2). The electronic absorption spectroscopy of
compounds 1–3 support these intermolecular p–p interactions.
polymers containing nickel bis(dithiolene) complexes that are
Introduction
highly soluble in a number of solvents in the as-made reduced
(dianionic) form.3 A one dimensional coordination polymer,
based on a differentiated bischelate-type ligand bearing both 4,5-
diazafluorene and dithiolene units, has been reported.4 Duval
and co-workers reported a honeycomb network polymer in
which the monomeric units, comprising the trigonal nodes, are
knitted together by interlocking dative Na–N bonds extended
from nitrile groups of bifunctional mnt ligands coordinated
through the sulfur atoms to adjacent cerium centers.5 One
dimensional coordination polymers, formed from sodium–crown
ether supramolecular cationic complexes and [Ni(i-mnt)2]2- com-
plex anions, have been described by Long, Haung and co-
workers.6 Rovira and co-workers reported an interesting 2D
layered coordination polymer based on an unusual mixed valence
Cu(III)/Cu(I) bis-1,2-diselenolene complex.7a They described a
Cu(III) bis-1,2-diselenolene complex with a highly extended 3D
framework through Na+ coordination.7b Kuroda-Sowa and co-
Investigation of self-assembled coordination polymers containing
transition metal complex ions and organic bridging ligands
has been carried out by numerous researchers because of their
potential applications as functional solid state materials and also
their inherent visual appeal.1 Dithiolene complexes are attractive
building blocks for such coordination polymer systems because
of their aromaticity and facile reduction propensity. Pickup and
coworkers reported on a dithiolene-based conjugated metallopoly-
mer prepared by the electrochemical polymerization of bis[cis-1,2-
di(2¢-thienyl)-1,2-ethenedithiolene]nickel.2a Later on, they showed
that a complex with one thiophene substituent on each dithiolene
ligand[cis-1-(2¢-thienyl)-2-phenyl-1,2-ethenedithiolene] results in a
polymer with electrochemistry characteristic of a coordination
polymer.2b Wang and Reynolds described a series of coordination
University of Hyderabad, School of Chemistry, New Chemistry building,
Hyderabad, 500 046, India; Tel: +91-(0)40-2301-1007. E-mail: skdsc@
uohyd.ernet.in, samar439@gmail.com
workers have described a system (NBun )2[M(mnt)2Cu4I4] (M =
4
Ni, Pd, Pt), in which a Cu4I4ring is supported by an M(mnt)2
moiety, two cyano groups out of the four of which coordinate
to two Cu(I) ions in neighboring molecules, resulting in the
formation of a unique doubly-bridged one-dimensional chain-like
† Electronic supplementary information (ESI) available: Tables of com-
plete bond distances and angles for compounds 1–3 and X-ray crys-
tallographic files in CIF format for compounds 1–3. CCDC reference
numbers 824385–824387. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c1dt10814j
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Dalton Trans., 2011, 40, 12901–12908 | 12901
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structure.8 Very recently, Takaishi and co-workers have reported
an electroconductive coordination polymer Cu[Cu(pdt)2] (pdt =
2,3-pyrizinedithiolate) composed of donor and acceptor building
units.9 Fourmigue´ and team have demonstrated the attractive
ability of the CN moieties in the [Ni(tfadt)2]2-, (tfadt = 2-
(trifluoromethyl)acrylonitrile-1,2-dithiolate) complexes to coordi-
nate metallic cationic centers resulting in heterobimetallic chain-
like structure.10 The coordination polymers, based on zinc-triad
1,1-dithiolates {ZnN2S2}, are known; in this context, it worth
mentioning that unlike the compounds of 1,1-dithiolate ligands,
1,2-dithiolene-ligated complexes are more conjugated than 1,1-
dithiolates.11
Synthesis of [Zn(dmit)(4,4¢-bpy)]n (1). 80 mg (0.51 mmol) of
4,4¢-bipyridine was dissolved in 5 mL of acetonitrile. To this
solution was added Cu(BF4)2·xH2O (60 mg, 0.25 mmol) in 5 mL
of water. The color of the reaction mixture became blue. It was
then filtered and the resulting filtrate was taken in a 20 mL round-
bottomed flask. The [Bu4N]2[Zn(dmit)2] (230 mg, 0.24 mmol) was
dissolved in 10 mL of acetonitrile in a 20 mL round-bottomed
flask. Then, these two different reaction (round-bottomed) flasks,
with copper salt and dmit complex, were connected to the two
different terminals of the l-shaped glass tube. Finally, the empty
part of the tube was slowly filled with MeCN (on the top) and
sealed. The tube was then allowed to stand undisturbed at room
temperature for 15–20 days, yielding dark red crystals that were
identified as 1. These, crystals were suitable for a single crystal
X-ray structure determination. Yield 30 mg (24%). Anal. calcd
(%) for C13H8N2S5Zn: C, 37.36; H, 1.93; N, 6.70; S, 38.36; Zn,
15.65. Found: C, 36.67; H, 2.06; N, 6.89; S, 40.18; Zn (ICP), 15.23.
IR (KBr pellet): (n/cm-1) = 1604, 1529, 1485, 1412, 1215, 1055
(–C S), 1032, 885, 806, 636, 461.
∑
-
The above described dithiolene-based polymers are mostly
ionic coordination polymers that are synthesized by chemical
synthesis, except the electrochemical polymerization of bis[cis-
1,2-di(2¢-thienyl)-1,2-ethenedithiolene]nickel.2 We are interested
to synthesize neutral coordination polymers (based on metal–
dithiolene coordination complexes) because neutral coordination
polymers offer possibilities to develop new functional materials.12
In order to synthesize metal–dithiolene-based neutral coordina-
tion polymers, we chose dmit2- dithiolate, 1,3-dithiole-2-thione-
4,5-dithiolate, which has been used extensively in metal coor-
dination chemistry.13 To the best of our knowledge, there is
no report of a coordination polymer that is constructed from
a metal mono(dmit) complex [MII(dmit)], which is basically a
neutral species. There are two reports of ionic coordination
polymers based on [MII(dmit)2]2- complexes.14 Keeping in mind
the fact that [MII(dmit)] is a neutral species, we have chosen
three neutral organic linkers, namely three diimine molecules,
4,4¢-bipyridine (4,4¢-bpy), trans-1,2-bis(4-pyridyl)ethene (4,4¢-bpe)
and 1,4-bis(imidazole-1-ylmethyl)-benzene (bix). Even though this
class of compounds [M(diimine)(dithiolate)] has largely been
investigated based on their electronic structures and photolu-
minescence studies,15–17 they have never been obtained/isolated
as coordination polymers [M(diimine)(dithiolate)]a. We report
here neutral coordination polymers [Zn(dmit)(4,4¢-bpy)]n (1),
[Zn(dmit)(4,4¢-bpe)]n (2) and [Zn(dmit)(bix)]n (3) (4,4¢-bpy = 4,4¢-
bipyridine, 4,4¢-bpe = trans-1,2-bis(4-pyridyl)ethane, bix = 1,4-
bis(imidazole-1-ylmethyl)-benzene)), that are formed by neutral
building block [MII(dmit)] and neutral linker to obtain diverse
architectures ranging from a zig–zag-type chain to curved-type
(concavo–convex) chain depending on the choice and shape of the
linker. We describe the crystal structures of 1–3 in detail, stressing
their supramolecular diversities based on p–p intermolecular
interactions.
Synthesis of [Zn(dmit)(4,4¢-bpe)]n (2). 104 mg (0.57 mmol) of
4,4¢-bpe (trans-1,2-Bis(4-pyridyl)ethane) was dissolved in 5mL of
acetonitrile. To this solution was added Cu(BF4)2·xH2O (60 mg,
0.25 mmol) in 5 mL of water. The reaction mixture color turned to
blue. The filtrate of this was taken in a 20 mL round-bottomed
flask. The [Bu4N]2[Zn(dmit)2] (230 mg, 0.24 mmol) was then
dissolved in 10 mL of acetonitrile and the resulting solution was
taken in another 20 mL round-bottomed flask. Single crystals,
suitable for an X-ray structure determination, were grown based
on the same procedure as that described in the case of compound
of 1. In this case, the l-tube was allowed to stand undisturbed
at room temperature for 20–25 days, yielding dark red crystals
of 2 in the l-shaped glass tube. The resulting crystals were
identified as compound 2. Yield 20 mg (20%). Anal. calcd (%)
for C60H42N8O2S20Zn4: C, 39.82; H, 2.34; N, 6.19; S, 35.43; Zn,
14.45. Found: C, 40.05; H, 2.28; N, 5.87; S, 36.05; Zn (ICP), 14.72.
IR (KBr pellet): (n/cm-1) = 1614, 1530, 1475, 1403, 1215, 1055
(–C S), 1008, 885, 807, 636, 461.
Synthesis of [Zn(dmit)(bix)]n (3)
Preparation of benzoyl protected dmit. 600 mg of
(Bu4N)2[Zn(dmit)2] was dissolved in 10 mL of dry acetone
under N2 atm. To this solution was added 2 mL of benzoyl
chloride in 10 mL of dry acetone drop-wise by using an additional
funnel. While adding the benzoyl chloride solution, the reaction
mixture color turned from red to yellow. The reaction mixture
was continuously stirred overnight. Then, the solvent was
evaporated using a rotary evaporator and the yellow colour
compound was separated. The compound was crystallized from
the CH2Cl2:MeOH (1 : 1) mixture. The spectroscopic data of
(dmit)(COPh)2, obtained in this process, is identical to those for
reported (dmit)(COPh)2.20
Experimental
General
considerations. The
coordination
complex
[Bu4N]2[Zn(dmit)2] was synthesized according to a reported
method.18 The 1,4-bis(imidazol-1-ylmethyl)benzene (bix) ligand
was prepared by a known procedure.19 All other chemicals were
purchased from commercial sources and used without further
purification. Microanalytical (C, H, N, S) data were obtained
with a FLASH EA 1112 Series CHNS Analyzer. The IR spectra
(with KBr pellets) were recorded in the range 400–4000 cm-1 on
a JASCO FT/IR-5300 spectrometer. UV-vis-NIR spectra were
recorded using a 3101 PC/UV/vis-NIR Philips spectrometer
equipped with a diffuse reflectance accessory.
Synthesis of 3. 136 mg (0.334 mmol) of benzoylated dmit
was suspended in a 10 mL of MeOH under dry N2 atm. To
this mixture, 40 mg of sodium (washed with hexane) was added
and continuously stirred for 20 min. This reaction mixture was
treated with imidazole solution, which was prepared in situ by
suspending 150 mg (0.63 mmol) of imidazole based ligand, bix,
12902 | Dalton Trans., 2011, 40, 12901–12908
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in 20 mL of MeOH. The resulting reaction mixture was then
treated with anhydrous ZnCl2 (46 mg, 0.338 mmol), whereby
the white methanolic turbidity was formed. Subsequently, it was
stirred for 10 min at room temperature. The resulting orange
solution was filtered and kept at room temperature. After 10 days,
orange needle-shaped crystals separated out. These crystals are
not soluble in any common organic solvents. Yield 58 mg (34%).
Anal. calcd (%) for C17H14N4S5Zn: C, 40.83; H, 2.82; N, 11.20; S,
32.06; Zn, 13.08. Found: C, 41.53; H, 2.38; N, 10.20; S, 33.21; Zn
(ICP), 12.89. IR (KBr pellet): (n/cm-1) = 1521, 1423, 1350, 1278,
1236, 1111, 1089, 1022, 947, 823, 713, 653, 617, 513, 459.
Scheme 2
Single crystal structure determination
protected dmit and [Bu4N]2[Zn(dmit)2], as shown in Scheme 3.
As shown in Scheme 3, the compound 3 is actually synthesized
from Na2dmit, prepared in situ from benzyol protected dmit and
Na/MeOH, and anhydrous zinc chloride. The reason why the
direct reaction between [Bu4N]2[Zn(dmit)2] and bix does not afford
compound 3, as in the case of the syntheses of compounds 1 and
2, is not clear to us. Probably, the flexible nature of the bix linker
makes this difference to the synthetic routes for compounds 1–3.
Thus, we have shown two different synthetic routes (Schemes 1–3)
that lead to the isolation of 1–3.
Data were measured at room temperature for compounds 1–3 on
a Bruker SMART APEX CCD area detector system [l(Mo-Ka) =
0.7103 A], graphite monochromator; 2400 frames were recorded
˚
with an w scan width of 0.3◦, each for 8 s, crystal-detector distance
60 mm, collimator 0.5 mm. Data reduction was performed with the
SAINTPLUS software,21 absorption correction using an empirical
method SADABS,22 structure solution using the SHELXS-97
program23 and this was refined using the SHELXL-97 program.24
Hydrogen atoms on the aromatic rings were introduced on the
calculated positions and included in the refinement riding on their
respective parent atoms. It has to be mentioned that the value of
R(int) is high (0.26) for compound 2. We could not improve the R(int)
value for compound 2, even after several attempts from different
syntheses. The crystals of this compound do not diffract well.
Results and discussion
Synthesis
The syntheses of compounds 1 and 2 are described in Schemes
1 and 2 respectively. In both syntheses, Cu(BF4)2·xH2O was
used, even though copper is not present in the final products,
1 and 2. Compounds 1 and 2 could not be isolated without
using Cu(BF4)2·xH2O. We believe that copper ion, in these
syntheses, plays important role by dragging a dmit ligand from
[Zn(dmit)2]2-, resulting in the formation of neutral Zn(dmit)
species. The copper species, formed in these syntheses, could not
be isolated/identified; the copper species, once formed, remain
soluble in the present synthesis conditions. Compound 3, which
is stabilized with a more flexible ligand (bix), could not be
synthesized in a straightforward manner, as shown in Schemes
1 and 2. Compound 3 has been synthesized by using a benzoyl
Scheme 3
Description of the crystal structures
Crystal structure description of compound 1. The coordination
polymer [Zn(dmit)(4,4¢-bpy)]n (1) was synthesized as red colored
crystals from [Bu4N]2[Zn(dmit)2] and 4,4¢-bpy in the presence
of [Cu(BF4)2]·xH2O based on the ligand exchange reaction in
acetonitrile solvent (vide supra). The single crystal X-ray structure
analysis shows that the crystals belong to the monoclinic system
with space group P21/n. The asymmetric unit of crystal structure
of 1 consists of two half 4,4¢-bpy units and one dmit unit of zinc
complex, as shown in Fig. 1. The basic crystallographic data for
compound 1 is presented in Table 1. As shown in Fig. 1, the
Zn(II) ion is in a distorted tetrahedral geometry, coordinated to
the two sulfur atoms of the dmit2- ligand and two pyridine nitrogen
atoms from two different 4,4¢-bipyridine molecules, leading to
the formation of a zig–zag chain-like coordination polymer (Fig.
2). The selected bond lengths and bondangles for compound 1
are described in Table 2. In compound 1, the Zn(1)–S(1) and
Scheme 1
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Table 1 Crystal data and structural refinement parameters for compounds 1–3
1
2
3
Empirical formula
Formula weight
T/K
C13H8N2S5Zn
417.88
100(2)
0.71073
Monoclinic
P21/n
9.1288(16)
14.655(3)
11.670(2)
90
101.561(3)
90
1529.6(5)
4
1.815
2.279
C60H42N8O2S20Zn4
1809.70
298(2)
C17H14N4S5Zn
499.99
298(2)
0.71073
Monoclinic
˚
l/A
0.71073
Crystal system
Space group
Triclinic
¯
P1
P21/n
˚
a/A
9.211(2)
19.641(4)
20.970(5)
105.288(3)
97.714(4)
94.959(4)
3596.7(14)
2
10.0567(6)
16.1864(10)
13.0716(8)
90
˚
b/A
˚
c/A
a (◦)
b (◦)
g (◦)
99.3920(10)
90
2099.3(2)
4
1.582
1.677
3
˚
V/A
Z
Dc/Mg m-3
1.671
1.947
1828
m/mm-1
F[000]
840
1016
Crystal size/mm
q range for data collection [deg]
Reflections collected/unique
R(int)
0.23 ¥ 0.08 ¥ 0.03
2.26 to 25.00
14442/2687
0.0472
0.34 ¥ 0.10 ¥ 0.07
1.08 to 25.00
33824/12619
0.2600
0.18 ¥ 0.08 ¥ 0.06
2.02 to 25.00
19854/3689
0.0646
Refinement method
Data/restraints/parameters
Goodness-of-fit on F2
R1/wR2 [I > 2s(I)]
Full-matrix least-squares on F2
3689/0/244
0.953
2687/0/190
1.062
0.0376/0.0884
0.0501/0.0946
0.775/- 0.339
12619/0/847
0.935
0.0873/0.1813
0.2231/0.2424
0.736/-0.694
0.0422/0.0821
0.0639/0.0883
0.588/-0.301
R1, wR2 (all data)
-3
˚
Largest diff. Peak/hole/e A
Fig. 2 (a) A segment of the zig–zag coordination polymer 1 and (b) a
packing diagram of compound 1 viewed down the crystallographic a axis.
The hydrogen atoms are omitted for clarity.
Fig. 1 The structure of [Zn(dmit)(4,4¢-bpy)]n (1) in the asymmetric unit
with atom labeling, showing 50% thermal ellipsoids. Hydrogen atoms are
omitted for clarity.
analogue [Zn(S2P(OR)2)2(4,4¢-bpe)]n in terms of conjugation. One
segment of the zig–zag coordination polymer of compound 1 is
described in Fig. 2(a). In the crystal, these chains pack/arrange in
an interesting manner (Fig. 2(b)). The 4,4¢-bypiridine regions of
each chain are used as inter-crossing zones, which exploit their p–p
stacking interactions, leading to the formation of a 3-dimensional
grid-type framework (Fig. 3). The p–p stacking parameters are
characterized by a centroid–centroid (Cg–Cg) distance of 4.13(2)
˚
˚
Zn(1)–S(2) bond distances are 2.2772(11) A and 2.3117(11) A,
respectively, with a S(1)–Zn(1)–S(2) bond angle of 98.87(1)◦.
Along the chain axis, each bipyridine ligand coordinates to
two {Zn(dmit)} complex species (from opposite sides). In each
complex unit, the N(1)–Zn(1)–N(2) bond angle is 105.20(1)◦,
which considerably deviates from the ideal tetrahedral situation
(109◦). Although, the two Zn–S bond distances are not equal
(which is consistent with different bond angles of C(1)–S(1)–
Zn(1) 93.38(13)◦ and C(2)–S(2)–Zn(1) 92.70(13)◦), the two Zn(1)–
N bond distances are virtually identical. The polymer structure
of [Zn(dmit)(4,4¢-bpy)]n (1) has a {ZnN2S2} core that resembles
the [Zn(S2P(OR)2)2(4,4¢-bpe)]n coordination polymer,11 but the
important difference lies in the fact that this reported coordination
polymer is prepared by using 1,1-dithiolate type of ligand.
Obviously, the properties of 1,2-dithiolene system (compound 1
in the present study) would be much different from 1,1-dithiolate
˚
˚
A and a mean plane separation (MPS) of 3.79 A. Interestingly,
when a chain crosses over another chain, they interact roughly
in a perpendicular fashion and it proceeds above and below with
respect to the bipyridine rings (see the inter-crossing region in Fig.
3) to its inter-crossing chains. Thus, the p–p interactions between
the {N1–C4–C5–C6–C7–C8} and {N2–C9–C10–C11–C12–C13}
rings can be considered as an incentive for the formation of such
a three-dimensional arrangement, which is very similar to the
intertwining/weaving situation in making a cloth sheet (Fig. 3(b)).
12904 | Dalton Trans., 2011, 40, 12901–12908
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Table 2 Selected bond lengths [A] and angles [◦] for compound
Table 3 Selected bond lengths [A] and angles [ ] for [Zn(dmit)(4,4¢-bpe)]n
(2)
◦
˚
˚
[Zn(dmit)(4,4¢bpy)]n (1)
Zn(1)–N(1)
Zn(1)–S(1)
S(5)–C(3)
C(11)–C(10)
2.052(3)
2.2772(11) Zn(1)–S(2)
1.660(4)
1.401(5)
1.393(5)
Zn(1)–N(2)
2.063(3)
2.3117(11)
1.335(5)
1.479(7)
1.488(7)
113.01(9)
114.97(9)
92.70(13)
122.6(3)
116.6(3)
122.1(4)
123.0(4)
Zn(1)–N(2)
Zn(1)–S(1)
Zn(2)–S(7)
S(8)–C(22)
2.032(10)
2.271(4)
2.260(4)
1.730(14)
1.288(18)
1.328(16)
97.8(4)
Zn(1)–N(1)
Zn(1)–S(2)
Zn(4)–S(17)
N(7)–C(45)
2.040(10)
2.299(4)
2.268(4)
1.295(17)
1.328(16)
1.287(18)
113.1(3)
107.6(3)
100.0(4)
N(2)–C(9)
C(11)–C(11)a
C(6)–C(7)
C(6)–C(6)b
C(21)–C(28)a
C(60)–C(51)c
N(2)–Zn(1)–N(1)
N(2)–Zn(1)–S(2)
S(1)–Zn(1)–S(2)
C(51)–C(60)b
C(28)–C(21)d
N(2)–Zn(1)–S(1)
N(1)–Zn(1)–S(2)
N(4)–Zn(2)–N(3)
N(1)–Zn(1)–N(2)
N(2)–Zn(1)–S(1)
C(3)–S(4)–C(1)
C(9)–N(2)–C(13)
C(13)–N(2)–Zn(1)
C(12)–C(11)–C(11)a
S(5)–C(3)–S(3)
105.19(12) N(1)–Zn(1)–S(1)
119.15(9)
98.47(18)
117.9(3)
118.7(2)
121.3(4)
124.5(2)
N(1)–Zn(1)–S(2)
C(2)–S(2)–Zn(1)
C(9)–N(2)–Zn(1)
C(12)–C(11)–C(10)
C(10)–C(11)–C(11)a
N(2)–C(9)–C(10)
122.0(3)
99.02(13)
Symmetry transformations used to generate equivalent atoms:a x, y, z - 1;
b x, y - 1, z; c x, y + 1, z; d x, y, z + 1.
Symmetry transformations used to generate equivalent atoms:a -x,-y,-z;
b
-x,-y + 1, z + 1.
Fig. 3 (a) A view of the p–p interaction between the adjacent perpen-
dicular polymer chains (the middle (red) chain is interacting with two
different chains (cyan)). (b) A schematic representation of the 2D-networks
in compound 1, the independent polymer chains are distinguished by
different colors (side view).
Fig. 4 The structural unit of [Zn(dmit)(4,4¢-bpe)]n (2) with atom labeling.
The non-hydrogen atoms are drawn as 50% probability thermal ellipsoids.
Besides these p–p interactions, we found fascinating inter-
chain S ◊ ◊ ◊ S contacts (Fig. S1, Supporting Information†). More
specifically, this contact includes the interaction of S(2) of Zn-
chelate five-member ring with S(5) of C S and vice versa.
˚
The relevant S ◊ ◊ ◊ S separation is 3.5819(16) A (Figure S1(a),
Supporting Information†), which is less than the sum of their
˚
van der Waals radii (3.70 A). Usually, the S ◊ ◊ ◊ S contacts are a
very important requirement for developing conducting and even
superconducting materials; for example, most of the [Ni(dmit)2]-
and tetrathiafulvalene derivatives show high conductivity due to
the presence of S ◊ ◊ ◊ S contacts in relevant solids.25
Crystal structure description of compound 2. Compound 2
crystallizes in triclinic space group P1. The relevant asymmetric
Fig. 5 (a) A segment of the zig–zag coordination polymer 2 and (b) a
packing diagram of compound 2 viewed down the a axis. The hydrogen
atoms are omitted for clarity.
¯
unit consists of two full 4,4¢-bpe units (bridged), four halves of
2-
4,4¢-bpe units and four {dmit} units that are coordinated with
four zinc metals, as depicted in Fig. 4. The basic crystallographic
data for compound 2 is described in Table 1. The geometry around
the Zn(II) ion is distorted tetrahedral, with two sulfur atoms of the
dmit2- ligand and two pyridine nitrogen atoms from two different
trans-1,2-bis(4-pyridyl)ethene (4,4¢-bpe) molecules. In the crystal,
both terminal nitrogen atoms of the (4,4¢-bpe) ligand are involved
in coordination (to two different zinc complexes), thereby leading
to the formation of a one-dimensional chain-like coordination
polymer (Fig. 5(a)). The selected bond lengths and bond angles
for compound 2 are presented in Table 3. The average Zn–S and
respectively. The coordination polymer in compound 2 also exists
in a zig–zag manner, as found in the crystals of compound 1. The
packing diagram of [Zn(dmit)(4,4¢-bpe)]n (2) is shown in Fig. 5(b).
It is to be noted that even though the organic spacer length
is relatively larger in compound 2 than that in compound 1 (the
length is exceeded by a –C C– group in 2), the overall packing of
the chains in the crystal of 2 is very much comparable to that in the
crystals of compound 1. Nevertheless, in the case of compound
2, the inter-crossing zig–zag chains are assembled by the influence
of p–p interactions that involve the C C bond (from one chain)
and the aromatic ring of the 4,4¢-bpe (of the other chain) per inter-
crossing region. This situation of p–p interactions is shown in
Fig. 6.
˚
˚
Zn–N bond distances are 2.287 A and 2.037 A, respectively, which
are consistent with those of compound 1 and the corresponding
S–Zn–S and N–Zn–N bond angles are 99.13(14)◦ and 99.07◦,
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◦
˚
Table 4 Selected bond lengths [A] and angles [ ] for [Zn(dit)(bix)]n (3)
Zn(1)–N(3)
Zn(1)–S(2)
S(3)–C(3)
N(3)–C(11)
2.014(3)
2.2963(11)
1.714(4)
1.355(5)
1.373(6)
1.373(6)
108.28(12)
119.28(9)
96.50(4)
120.9(4)
121.4(4)
Zn(1)–N(1)
Zn(1)–S(1)
S(3)–C(1)
2.021(3)
2.3329(11)
1.747(3)
1.307(4)
1.378(5)
1.378(5)
116.45(9)
107.51(10)
93.81(12)
121.9(4)
120.4(4)
N(1)–C(4)
C(16)–C(10)a
C(10)–C(16)b
N(3)–Zn(1)–N(1)
N(1)–Zn(1)–S(2)
S(2)–Zn(1)–S(1)
C(15)–C(16)–C(10)a
C(8)–C(10)–C(16)b
C(9)–C(17)b
C(17)–C(9)a
N(3)–Zn(1)–S(2)
N(3)–Zn(1)–S(1)
C(1)–S(1)–Zn(1)
C(8)–C(9)–C(17)b
C(15)–C(17)–C(9)a
Fig. 6 A view of the inter-crossing of the perpendicular chains for
compound 2 and the p–p interaction shown in green shadow (left),
the origin of interaction between C C bond (red) and pyridine ring
(green) shown in middle and a side view of the packing representation of
independent polymer chains are distinguished by different colors (right).
Symmetry transformations used to generate equivalent atoms:a -x + 1/2,
y - 1/2, -z + 1/2; b -x + 1/2, y + 1/2, -z + 1/2.
tetrahedrally coordinated to two sulfur atoms of the dmit2- ligand
and two imidazole nitrogens from two different bix molecules,
leading to the formation of a one-dimensional concavo–convex-
type coordination polymer (Fig. 8). The selected bond lengths
and bond angles for compound 3 are described in Table 4. In
compound 3, the Zn(1)–S(1) and Zn(1)–S(2) bond distances are
The p–p interactions among aromatic rings are common, but
we did not come across such interactions between an aromatic ring
and a C C double bond in the literature. Unlike compound 1,
compound 2 does not exhibit any considerable S ◊ ◊ ◊ S intermolec-
ular contact. This may be due to the relatively longer length of the
organic spacer (4,4¢-bpe). In the case of the compound 1, the N–N
˚
˚
2.332(11) A and 2.296(11) A, respectively, with a S(2)–Zn(1)–
S(1) bond angle of 96.50(4)◦. The two Zn(1)–N bond distances
are virtually same. That two Zn–S bond distances are not equal,
which is consistent with the bond angle of C–S–Zn being 93.8(12)◦.
Along the concavo–convex chain, the Zn–Zn distance is 16.186(1)
˚
distance of 4,4¢-bpy is 7.105(8) A (spacer length), whereas in the
crystal structure of compound 2, the N–N distance of 4,4¢-bpe is
˚
9.368(23) A (spacer length). We believe that free rotation between
˚
A (Fig. 8).
two aromatic rings in the linker 4,4¢-bpe is restricted due to the
presence of C C double bond between them and consequently
we observed similar chain type and packing arrangement in
compounds 1 and 2. Even though, polymers 1 and 2 have similar
kind of packing arrangement, their structures exhibit sufficient
diversities due to the little difference (particularly the spacer
length) between 4,4¢-bpy and 4,4¢-bpe. Thus, the supramolecular
structures of coordination polymers can be altered/tuned by a
slight change of the spacer/linker molecules.
Fig. 8 A segment of the concavo–convex-type coordination polymer 3.
The hydrogen atoms are omitted for clarity.
Crystal structure description of compound 3. The coordination
polymer [Zn(dmit)(bix)]n (3) was synthesized as orange crystals
2-
from sodium salt of {dmit} and bix ligand in the presence
Numerous coordination polymers, known in literature, exhibit
helical chain-like arrangements, double strand chains, and ladder-
like chain structures, etc. Amazingly, the coordination polymer 3
(in the present work) provides the addition of a new type of chain,
i.e., the concavo–convex chain structure. We have succeeded in
constructing this concavo–convex chain in compound 3 because
we chose a much more flexible and larger linker bix (see Scheme 3
for its structural representation) in the case of compound 3.
Furthermore, the adjacent concavo–convex coordination poly-
mers/chains of 3 interact with each other by p–p interactions
that involve imidazole rings of the bix ligand, as well as the
aromatic ring of the bix ligand and the five-member ring of
the dmit ligand, giving rise to a kind of infinite zipper structure
(Fig. 9). The relevant p–p interactions (between the inter-planar
of anhydrous ZnCl2 in EtOH solvent. The crystals belong to
the monoclinic system with space group P21/n. The asymmetric
unit of the crystal structure of 3 consists of two halves of bix
2-
units and one {dmit} unit coordinated to the zinc ion, as
shown in Fig. 7. In the molecular structure, the Zn(II) ion is
˚
distances) are in the range 3.5–4.1 A. The molecular packing
diagram of [Zn(dmit)(bix)]n (3) in the crystallographic ab plane
is shown in Fig. S2 (Supporting Information†). In the crystal
structure of compound 3, the central aromatic ring in the bix
linker is connected to two pyrazine rings by C–CH2–C bonds that
make the bix linker more flexible. As a result, the coordination
polymers in compound 3 acquire the shape of a concavo–convex
curved chain or puckered ribbon. The explanation for this type
Fig. 7 The molecular structure of the asymmetric unit in [Zn(dmit)(bix)]n
(3) with atom labeling, showing 50% thermal ellipsoids. Hydrogen atoms
are omitted for clarity.
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Fig. 9 The crystal packing diagram of [Zn(dmit)(bix)]n (3). The solid blue
lines indicate the p–p interactions between the imidazole five-member rings
and the solid red line indicates the p–p interaction between the aromatic
ring of the bix ligand and five-member ring of the dmit ligand.
of shape can be rationalized by the zipper-like arrangement of
these concavo–convex chains that are sustained by p–p inter-chain
supramolecular interactions, as shown in Fig. 9.
Spectroscopy
The solid state (diffuse reflectance) electronic absorption spectra
of [Zn(dmit)(4,4¢-bpy)]n (1) and [Zn(dmit)(4,4¢-bpe)]n (2), which
are identical, exhibit two absorption bands in the 270–700 nm
region, as shown in Fig. 10(a). The corresponding solution
absorption spectra (in DMF) are also comparable (not shown
here). Both bands are assigned to dmit ligand transitions; the
absorption band at 320 nm is assigned to a p–p* transition and
the absorption band at 498 nm to an n–p* transition of the dmit
ligand. However, a careful investigation of the 500 nm region
in the absorption spectrum of [NBu4]2[Zn(dmit)2] by resonance
Raman spectroscopy showed vibrational bands at 1429, 530 and
345 cm-1 with enhanced intensities, out of which the band at
345 cm-1 was assigned to Zn–S stretching. The other two bands,
located at 1429 and 530 cm-1 were assigned to C C stretching
and dmit ligand deformation, respectively.26 Thus, the combined
UV-vis and resonance Raman spectroscopic studies recommend
that the absorption band at 498 nm (for both compounds 1 and 2)
is a MLCT charge transfer band. Both bands (at 320 and 498 nm)
show blue shifts relative to those in [Na2(dmit)] (316, 514 nm
in MeOH),1 [NBu4]2[Zn(dmit)2] (300, 530 nm in MeCN)27 and
[NMe4]2[Zn(dmit)(Sph)2] (327, 512 nm).28 The electronic spectrum
of compound [Zn(dmit)(bix)]n (3) exhibits, in the solid state, two
absorption bands at 305 nm and 487 nm (Fig. 10(b)). We could
not record the absorption spectra of compound 3 in the solution
state because of its insolubility. The assignment feature for these
bands (305 and 487 nm) of compound 3 should be comparable to
that of compounds 1 and 2.
Fig. 10 (a) The diffuse reflactance electronic absorption spectra of com-
pounds [Zn(dmit)(N–N)]n (N–N = 4,4¢-bpy (1) and N–N = 4,4¢-bpe (2)) in
the solid state; (b) the absorption spectrum of compound [Zn(dmit)(bix)]n
(3) in the solid state.
bpy)]n (1), [Zn(dmit)(4,4¢-bpe)]n (2) and [Zn(dmit)(bix)]n (3).
The isolated coordination compounds include one dimensional
polymeric species {Zn(diimine)(dithiolate)}n that are formed by
coordinate covalent bonds (in 1–3), leading to the formation of
an intriguing variety of architectures and topologies. We have
demonstrated a potential approach for the design of various
supramolecular architectures based on a neutral metal–dithiolene
unit, in which metal centers, as well as the 1,2-dithiolene ligand
with various substituents can be varied. The importance of the
present synthetic routes lies in the fact that even though the
coordination polymers of ZnCl2 exist with bridging ligands, the
substitution of two anions by the dmit ligand is not so easy; it
is difficult to take place by a normal chemical synthetic method.
Hence, we have shown a unique synthetic approach to synthesize
this class of Zn coordination polymers.
Conclusion
While most of the supramolecular coordination polymers (a
special class of metallosupramolecular entities) are generally
cationic29 or anionic,3–9 it would be of great interest to assemble
a neutral metal coordination complex unit and neutral organic
linkers to generate neutral coordination polymers of diverse
morphology, which could eventually be used to blend into con-
ventional neutral polymers. We demonstrated that a neutral zinc
coordination complex unit/species, [Zn(dmit)], can be assembled
with three different neutral organic linkers/spacers to generate a
new class of three neutral coordination polymers [Zn(dmit)(4,4¢-
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Met., 2001, 119, 91–92; (b) T. Sheng, X. Wu, P. Lin, W. Zhang, Q. Wang
and L. Chen, Polyhedron, 1999, 18, 1049–1054.
Acknowledgements
We are thankful to the Department of Science and Technol-
ogy (DST), Government of India for funding (project number:
SR/S1/MBD-05/2007). The national single crystal XRD facility
at University of Hyderabad by DST is gratefully acknowledged.
VM thanks CSIR, New Delhi, India for his fellowship.
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