Synthesis and characterization of four dicyanamide bridged copper(II) complexes with N2O donor tridentate Schiff bases as blocking ligands

Article abstract of DOI:10.1016/j.ica.2013.06.025

Four copper(II) complexes [Cu(L¹)(dca)]_n(1), [Cu(L²)(dca)(H₂O)]2 (2), [Cu(L³)(dca)]n(3) and [Cu(L⁴)(dca)]n (4), where HL¹ = 2-[(2-aminoethylimino) methyl]-4-nitrophenol, HL² = 2-[(2-(methylamino)ethylimi-no)methyl]- 4-nitrophenol, HL³ = 2-[(2-(dimethylamino)ethylimino)methyl]-4- nitrophenol and HL⁴ = 2-[(2-(diethylamino)ethylimino)methyl]-4- nitrophenol are tridentate Schiff base ligands, were synthesized and characterized by elemental analysis, IR and UV-Vis spectroscopy and single-crystal X-ray diffraction studies. Complex 1 and 4 are 1D helical, complex 2 is dimeric and complex 3 is 1D linear chain. X-ray single crystal structure analyses reveal that in complexes 1, 2 and 4, dicyanamide (dca) anions act as a purely μ-1,5-bridging ligand whereas in case of complex 3, two consecutive dca units serve as 1,3-bridging ligand while the third one exhibits end-to-end 1,5-bridging mode. Copper(II) centers in case of two complexes (1 and 3) exhibit square pyramidal geometry as depicted by Addison parameter (s) values whereas metal centers of complexes 2 and 4 depict distorted octahedral and square pyramidal geometry respectively. The role of weak forces like lone pair··· π / π··· π interactions in influencing the self-assembly process appears to be of importance. Such types of interactions leading to supramolecular network in the four complexes are described here.

Full text of DOI:10.1016/j.ica.2013.06.025

Accepted Manuscript
Synthesis and characterization of four dicyanamide bridged copper(II) com‐
plexes with N O donor tridentate Schiff bases as blocking ligands
2
Prasanta Kumar Bhaumik, Klaus Harms, Shouvik Chattopadhyay
PII:
S0020-1693(13)00348-4
http://dx.doi.org/10.1016/j.ica.2013.06.025
ICA 15520
DOI:
Reference:
To appear in:
Inorganica Chimica Acta
Received Date:
Revised Date:
Accepted Date:
30 March 2013
6 June 2013
14 June 2013
Please cite this article as: P.K. Bhaumik, K. Harms, S. Chattopadhyay, Synthesis and characterization of four
dicyanamide bridged copper(II) complexes with N O donor tridentate Schiff bases as blocking ligands, Inorganica
2
Chimica Acta (2013), doi: http://dx.doi.org/10.1016/j.ica.2013.06.025
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1 Synthesis and characterization of four dicyanamide bridged
2 copper(II) complexes with N2O donor tridentate Schiff bases
3 as blocking ligands
4
5
Prasanta Kumar Bhaumika, Klaus Harmsb, Shouvik Chattopadhyaya,*
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a Department of Chemistry, Inorganic Section, Jadavpur University, Kolkata – 700 032, India. e-
mail: shouvik.chem@gmail.com Tel. +919903756480
b
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Fachbereich Chemie, Philipps-Universität Marburg, Hans-Meerwein-Strasse, D-35032
Marburg, Germany.
10 Abstract
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Four copper(II) complexes [Cu(L1)(dca)]n (1), [Cu(L2)(dca)(H2O)]2 (2), [Cu(L3)(dca)]n
(3) and [Cu(L4)(dca)]n (4), where HL1 = 2-[(2-aminoethylimino)methyl]-4-nitrophenol, HL2 =2-
[(2-(methylamino)ethylimino)methyl]-4-nitrophenol,
(dimethylamino)ethylimino)methyl]-4-nitrophenol,
HL3
HL4
=
2-[(2-
2-[(2-
=
(diethylamino)ethylimino)methyl]-4-nitrophenol are tridentate Schiff-base ligands, were
synthesized and characterized by elemental analysis, IR and Uv–Vis spectroscopy and single-
crystal X-ray diffraction studies. Complex 1 is 1-D helical, complex 2 is dimeric, complex 3 is 1-
D linear chain and complex 4 is helical. X-ray single crystal structure analyses reveal that in
complexes 1, 2 and 4, dicyanamide (dca) anions act as a purely -1,5-bridging ligand whereas in
case of complex 3 , two consecutive dca units serves as 1,3-bridging ligand while the third one
exhibits end-to-end 1,5-bridging mode. Copper(II) centers in case of two complexes (1 and 3)
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exhibit square pyramidal geometry as depicted by Addison parameter (τ) values whereas metal
centers of complex 2 and 4 depict distorted octahedral and inverse square pyramidal geometry
respectively. The role of weak forces like lone pair∙∙∙π/π∙∙∙π interactions in influencing the self-
assembly process appears to be of importance. Such types of interactions leading to
supramolecular network in the four complexes are described here.
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Keywords: Crystal Structures; Copper(II); Polynuclear; Schiff bases; Dicyanamide.
28 1. Introduction
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Coordination polymers with diverse architectures such as the 1-D linear chain, zigzag
chain, 2-D square network, interwoven honeycomb, 3-D square grid, interpenetrated square
grids, railroad-like networks, interwoven diamondoids and even discrete squares have been
extensively developed [1-9]. Many important properties of coordination polymers arise from
these structures and topologies, and thus the rational design and construction of specific
architecture are particularly important. As the network topologies based on molecular building
blocks are usually controlled and modified by the coordination geometry of the central metal and
the structural chemistry of the organic ligands, a careful selection of the proper organic spacer
and metal ion can offer some degree of control over the resultant architecture and properties [10].
The dicyanamide (dca) in conjugation with other co-ligands has already been used extensively
for the synthesis of polymetallic complexes with interesting supra-molecular architectures [11-
12]. Moreover, dca can also act as a linear spacer between metal centers, thus allowing the
investigation of the intra-molecular magnetic interaction between magnetic centers [13-16]. A
large variety of topologies and magnetic properties are seen in dicyanamide based complexes
due to their different bridging modes [17-18]. Dca based coordination polymers have also
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attracted much attention because of their ability to facilitate the synthesis of metal–organic
frameworks containing channels and cavities with various sizes and shapes [19].
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The ease of synthesis, stability under a variety of oxidative and reductive conditions and
their structural versatility associated with diverse applications are some of the various reasons
which made Schiff bases to be used as interesting chelating ligands [20-21]. They are widely
used in designing molecular ferromagnets, in catalysis, in biological modeling applications and
in preparing liquid crystals [22-25]. Tetradentate ligands of salen-type Schiff bases with N2O2
donor sets were a long standing choice of coordination chemists as they provided suitable
coordination environments to form mono- and poly-nuclear complexes with fascinating
structures [26-28]. Now-a-days, N2O donor terdentate Schiff bases, prepared by refluxing N-
substituted diamine with salicylaldehyde [29], are also receiving the attention in preparing high-
nuclearity complexes [30-31]. Incorporation of additional groups in the salicylaldehyde moiety
may increase the denticity of the ligand [32]. Keeping this in mind, we have planned to use the
N2O donor Schiff base, derived from the condensation of N-alkyl ethylenediamine with 5-
nitrosalicyaladehyde, to prepare several copper(II) complexes.
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We will also like to investigate the variation of the supra-molecular architecture of the
complexes with the variation in H-bonding. The supra-molecular chemistry has a great impact on
how efficiently chemists prepare structures of different sizes and shapes [33]. The components of
these structures are held together by a variety of non-covalent forces. Some of these (lone
pair∙∙∙π, π∙∙∙π etc.) are very common and well accepted [34-39]. These are widely used in supra-
molecular chemistry and are very important binding forces that determine the packing of
molecules in crystals. Herein, we report the synthesis, structural features, spectroscopic
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characterization, X-ray crystal structure analysis and supra-molecular architecture involving π
aromatic clouds in solid-state.
69 2. Experimental
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2.1 Materials
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All reagents were of commercially available grade and were used without further
purification.
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2.2 Preparation
2.2.1. Preparation of [Cu(L1)(dca)]n (1)
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A solution of Cu(OCOCH3)2.H2O (199.6 mg, 1 mmol) in methanol (20 ml) was added to
the methanol solution (25 ml) of 5-Nitrosalicylaldehyde (167 mg, 1 mmol) and NaN(CN)2 (130
mg, 2 mmol) and refluxed for 1 hr to give a green solution and then 1,2-diaminoethane (0.1 ml, 1
mmol) was added to it and refluxed for additional 2 hrs to give green precipitate. The precipitate
was separated from the solution by filtration and dissolved in DMSO. Single crystals, suitable for
X-ray diffraction, were obtained on slow evaporation of the DMSO solution in open atmosphere
after several weeks.
Yield: 260 mg (77%). Anal. Calcd. (%) for C44H40Cu4N24O12: C, 39.11; H, 2.98; N,
24.88. Found: C, 39.1; H, 2.8; N, 24.7. IR (KBr, cm-1): 1648 (υC=N), 2303, 2228, 2172 (υdca),
1325 (υNO2), UV/Vis (acetonitrile): λmax (εmax, dm3mol–1cm–1) = 581 (8.6 × 10), 361 (1.6 × 103),
233 (2.0 × 103) nm. Magnetic moment is 1.73 BM.
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2.2.2. Preparation of [Cu(L2)(dca)(H2O)]2 (2)
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Methanol solution of 5-nitrosalicylaldehyde (170 mg, 1 mmol) and N-methyl-1,2-
diaminoethane (0.1 ml, 1 mmol) in 1:1 ratio was refluxed for 1 hr in order to synthesize a yellow
solution of the ligand, HL2. Then methanol solution of Cu(OCOCH3)2.H2O (199.6 mg, 1 mmol)
was added to the ligand solution and refluxed for 1 hr to give a deep green solution. Thereafter
aqueous solution of NaN(CN)2 (130 mg, 2 mmol) was added to it and refluxed again for an
additional hour to obtain deep green solution. It was then kept at room temperature. Compound
obtained from the mother liquor after few days was dissolved in DMF and kept undisturbed.
Single crystals of the complex, suitable for X-ray diffraction, were obtained from the DMF
solution on slow evaporation in open atmosphere after few weeks.
Yield: 450 mg (61%). Anal. Calcd. (%) for C12H14CuN6O4: C, 38.97; H, 3.82; N,
22.72. Found: C, 38.8; H, 3.7; N, 22.5. IR (KBr, cm-1): 1653 (υC=N), 2295, 2239, 2191 (υdca),
3430 (υOH), 1327 (υNO2), UV/Vis (acetonitrile): λmax (εmax, dm3mol–1cm–1) = 586 (3.4 × 102), 360
(6.9 × 103), 250 (8.1 × 103) nm. Magnetic moment is 1.72 BM.
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2.2.3. Preparation of [Cu(L3)(dca)]n (3)
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A methanol solution of of 5-nitrosalicylaldehyde (170 mg, 1 mmol) and N,N-dimethyl-
1,2-diaminoethane (0.1 ml, 1 mmol) in 1:1 ratio was refluxed for 1 hr in order to prepare a clear
yellow solution of ligand, HL3. Then methanol solution of Cu(OCOCH3)2.H2O (199.6 mg, 1
mmol) was added to the ligand solution and refluxed for 1 hr to give a deep green solution.
Thereafter aqueous solution of NaN(CN)2 (130 mg, 2 mmol) was added to it and refluxed again
for an additional hour to obtain shiny green precipitate. The precipitate was separated from the
solution by filtration and dissolved in DMSO and kept undisturbed. Single crystals of the
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complex, suitable for X-ray diffraction, were obtained from DMSO solution on slow evaporation
in open atmosphere after few weeks.
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Yield: 290 mg (79%). Anal. Calcd. (%) for C39H42Cu3N18O9: C, 42.68; H, 3.86; N, 22.97.
Found: C, 42.5; H, 3.7; N, 22.8. IR (KBr, cm-1): 1638 (υC=N), 2297, 2239, 2165 (υdca), 1317
(υNO2), UV/Vis (acetonitrile): λmax (εmax, dm3mol–1cm–1) = 577 (9.5 × 10), 360 (1.9 × 103), 253
(2.4 × 103) nm. Magnetic moment is 1.73 BM.
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2.2.4. Preparation of [Cu(L4)(dca)]n (4)
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Methanol solution of 5-Nitrosalicylaldehyde (170 mg, 1 mmol) with N,N-diethyl-1,2-
diaminoethane (0.1 ml, 1 mmol) in 1:1 ratio was refluxed a for 1 hr in order to prepare a clear
yellow solution of the ligand, HL4. Then methanol solution of Cu(OCOCH3)2.H2O ( 199.6 mg, 1
mmol)) was added to the ligand and refluxed for an additional hr to give a deep green solution.
Thereafter aqueous solution of NaN(CN)2 (130 mg, 2 mmol) was added to it and refluxed again
for an hour to obtain shiny green precipitate. The precipitate was filtered and dissolved in DMF.
Single crystals, suitable for X-ray diffraction, were obtained from the DMF solution on slow
evaporation in open atmosphere after several weeks.
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Yield: 260 mg (66%). Anal. Calcd. (%) for C15H18CuN6O3: C, 45.74; H, 4.61; N, 21.34.
Found: C, 45.6; H, 4.4; N, 21.2. IR (KBr, cm-1): 1640 (υC=N), 2317, 2245, 2200 (dca), 1325
(υNO2), UV/Vis (acetonitrile): λmax (εmax, dm3mol–1cm–1) = 592 (1.1 × 102), 358 (1.6 × 103), 251
(2.0 × 103) nm. Magnetic moment is 1.71 BM.
2.3. Physical measurements
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Elemental analysis (carbon, hydrogen and nitrogen) of the complexes were performed on
a Perkin–Elmer 240C elemental analyzer. Infrared spectra in KBr (4500–500 cm-1) were
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recorded using a Perkin–Elmer RXI FT-IR spectrophotometer. Electronic spectra in acetonitrile
(1000–200 nm) were recorded on a Jasco V-630 UV-VIS spectrophotometer. Fluorescence
spectra were obtained on HORIBA Jobin Yvon FluoroMax-P spectrophotometer at room
temperature. The electrochemical measurements were performed under a dry nitrogen
atmosphere on a CH instrument (model CHI720D). The magnetic susceptibility measurements
were done with an EG & PAR vibrating sample magnetometer, model 155 at room temperature
and diamagnetic corrections were made using Pascal’s constants.
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2.4. X-ray Crystallography
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Single crystals of suitable dimensions were mounted in inert oil and transferred to the
cold gas stream of the cooling device. Data were collected at 100 K on a STOE IPDS 2T
diffractometer using graphite monochromated Mo Kα radiation, and were corrected for
absorption effects using multiscanned reflections. Non hydrogen atoms were refined
anisotropically. H atoms of water molecules were located by difference Fourier maps and were
kept fixed. All other hydrogen atoms were placed in their geometrically idealized positions and
constrained to ride on their parent atoms. Programs used: SIR 2011 [40], SHELXL-97 [41],
DIAMOND [42], ORTEP [43] and STOE IPDS2 [44] software. The crystallographic and
refinement data are summarized in Table 1.
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3. Results and discussion
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3.1. Synthesis of the complexes
The mono-condensed ligand (HL1) has been prepared conveniently as the copper(II)
complex, [Cu(L1)(dca)]n (1) by the reaction of copper(II), 5-nitro salicyaldehyde and NaN(CN)2
in 1:1:1 ratio in methanol, followed by the addition of equimolar amount of 1,2-diaminoethane
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under reflux. It is to be noted that there is a possibility that the diamines may undergo
condensation at both ends to produce a tetradentate di-Schiff base. We, therefore, repeated the
syntheses several times and found that, in presence of the strongly coordinating dicyanamide, the
tridentate ligand and dicyanamide occupy the four equatorial sites of copper(II). The formation
of the complex 1 can be rationalized in light of the templating effect of copper(II) modulated by
the counter anion. A templating agent can be said to contain the required information to organize
a collection of building blocks so that they can be linked together in a specific manner [45]. The
dicyanamide ion, having a more or less similar crystal field stabilization energy as the Schiff
base, occupies a coordination site of copper(II), leaving the other three sites to be coordinated by
a Schiff base and that can be achieved most efficiently by a tridentate Schiff base ligand. Similar
examples are already reported in the literature [46].
The preparation of other tridentate Schiff-base ligands, HL2, HL3 and HL4 were routinely
achieved by the condensation of the N-substituted diamines, N-methyl-1,2-diaminoethane, N,N-
dimethyl-1,2-diaminoethane and N,N-diethyl-1,2-diaminoethane with 5-Nitrosalicylaldehyde
separately in 1:1 molar ratio, following the literature method [32]. They are then reacted with
copper(II) and dca to form the complexes 2, 3 and 4 respectively. The formation of all the
complexes is shown in Scheme 1. The four tridentate Schiff base blocking ligands are very
similar; all the complexes have the same general empirical formula [CuL(dca)]n, yet the bridging
topologies are drastically different. The complexes of the ligands HL1 and HL4 are single -1,5-
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dca bridged polymers, that of HL2 is double -1,5-dca bridged dimer and the complex of HL3 is
a 1-D chain with alternate two -1,3 and one -1,5-dca bridges. The structure of the complex of
HL4 is very interesting for the presence of alternate two -1,3 and one -1,5-dca bridges.
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Scheme 1. Formation of the complexes 1, 2, 3 and 4.
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3.2. Description of the structures
3.2.1. [Cu(L1)(dca)]n (1)
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An ORTEP drawing of the helical complex, [Cu(L1)(dca)]n, is shown in Fig. 1. Selected
bond lengths are summarized in Table 2. Selected bond angles are given in supplementary
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material, Table S1. The structure determination reveals that the copper(II) are bridged singly by
end-to-end dca ligands. The asymmetric unit contains four independent copper(II), Cu(1), Cu(2),
Cu(3) and Cu(4), having different environments. All the copper ions possess distorted square-
pyramidal geometries. Each copper center is linked to two neighboring copper(II) centers by two
dca in end-to-end fashions to form the helix. Cu(1) is coordinated equatorially by one amine
nitrogen atom, N(1), one imine nitrogen atom, N(4) and one oxygen atom O(1), of the tridentate
Schiff base ligand, HL1 and a nitrogen atom, N(12), of the μ1,5 bridged dca; this defines the basal
plane. The apical position is occupied by one nitrogen atom N(316)a {Symmetry code, a=x,-
3+y,z} of the another end-to-end bridged dca ligand from a crystallographically related unit to
complete the square pyramidal geometry of Cu(1). Other three copper ions in the asymmetric
unit also assume square pyramidal geometries (See the supplementary information for details).
Addison parameters [47] for all the four copper(II) are in the range of 0.1 – 0.3. In the equatorial
plane, the Cu-Nimine distances (1.959-1.970 Å) are shorter than the Cu-Namine (1.915-2.028 Å)
distances as were also observed in similar systems [48-49]. The closest conformation of the five-
membered chelate rings [Cu(2)–N(101)–C(102)–C(103)–N(104)] and [Cu(4)–N(301)–C(302)–
C(303)–N(304)], is envelope with puckering parameters [50] q(2)=0.442(8) and 258.3(8) Å;
(2)=258.3(8)°and 254.8(9)° respectively, whereas similar type chelate rings formed by the same
diamine with Cu(1) and Cu(3), [Cu(1)–N(1)–C(2)–C(3)–N(4)] and [Cu(3)–N(2)–C(202)–
C(203)–N(204)] are twisted on C(2)-C(3) and C(202)-C(203) with puckering parameters q(2)
=0.398(8) Å and 0.415(8) Å, (2)=277.3(8)° and 273.7(8)° respectively.
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It has also been observed that the formation of a 2D supramolecular network is ensured
by two strong face-to-face π∙∙∙π stacks (Table 3). Such multilayered π∙∙∙π interactions contribute
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to the formation of the 2D assembly in 1 {Fig. 2}. The details description of π∙∙∙π stacking is
given in supplementary information.
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3.2.2. [Cu(L2)(dca)(H2O)]2 (2)
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The ORTEP diagram of the complex is given in Fig. 3. Selected bond lengths are
summarized in Table 4. Selected bond angles are given in supplementary material, Table S2.
The compound features a double -1,5-dca bridged copper(II) dimer, in which two distorted
octahedral copper(II) centers are bridged by the two centrosymmetrically related dca ions in end-
to-end fashion. The end to end bridges assume asymmetric apical–basal disposition between
neighboring copper(II) ions; i.e., the same dca bridge resides on the apical position of one copper
but in the basal plane of the neighboring copper, with the apical Cu–N distance (2.626 Å) being
significantly longer than the basal one (1.960-2.041 Å). Within the dimeric units, each of the two
copper atoms is six-coordinated, being bonded to one amine nitrogen atom, N(2), one imine
nitrogen atom, N(1) and one oxygen atom O(1), of the tridentate Schiff base ligand and a
nitrogen atom, N(4), of the end-to-end bridged dca in the basal plane. A nitrogen atom, N(6)b,
from a symmetry related (-x,2-y,1-z) end-to-end bridged dca and one oxygen atom O(4) from a
water molecule coordinate axially at rather long distances (Cu–O=2.485(4) Å, Cu(1)–N(6)b =
2.626(4) Å). to form a tetragonally distorted octahedron. In the equatorial plane, the Cu-Nimine
distance (1.960 Å) is shorter than the Cu-Namine (2.041 Å) distance. The closest conformation of
the five-membered chelate ring [Cu(1)–N(1)–C(8)–C(9)–N(2)] is twisted on C(8)–C(9) with
puckering parameters q(2)= 0.425(4) Å; (2)=92.4(4)°. The copper∙∙∙copper separation within the
dinuclear unit is 7.444 Å, and the shortest inter-dimer copper∙∙∙copper distance is 6.542 Å.
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Adjacent dimeric units are connected to one another along a axis by strong self-
complementary O(4)–H(4B)∙∙∙N(6) {2.998(5) Å} hydrogen bonds (Table 5), giving rise to the
2
formation of a R2 (16) hydrogen bonding motif (Fig 4a). This self-assembly propagates along an
axis, generating an infinite 1D chain. Another infinite 1D chain of complex 2 propagates along
the b axis where neighboring [Cu(L2)(dca)(H2O)]2 units are connected to one another by
comparatively weaker self-complementary N(2)–H(2)∙∙∙N(5) {3.269(5) Å} hydrogen bonds,
2
giving rise to the formation of a R2 (12) hydrogen bonding motif (Fig 4b). The nitro group
interacts with water molecule [O(4)–H(4A)∙∙∙O(3) (2.994(4) Å)], giving rise to the formation of a
2
R2 (20) hydrogen bonding motif (Fig 4c). Thus another 1D chain along c axis is formed. The
association of several weak lone pair∙∙∙π interactions (Table 6) contributes to the formation of the
2D assembly in 2 (Fig 5). The further details of H-bonding and lone pair∙∙∙π interactions
interactions are given in the supplementary material.
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3.2.3 [Cu(L3)(dca)]n (3)
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A 1-D linear chain structure is present in complex 3. Two consecutive dca units are
serving as 1,3-bridging ligand, while the third one exhibits end-to-end 1,5-bridging mode. The
structural building block of it is a trinuclear copper(II) moiety. The ORTEP diagram of the
complex is given in Fig. 6a. Three independent copper atoms Cu(1), Cu(2) and Cu(3) having
different environments, possess distorted square-pyramidal geometries (trigonality parameter 
are in the range of 0.04–0.07). The trinuclear units connect each other to form a 1D polymer
where two consecutive dca anions units serving as 1,3-bridging ligand while the third one
exhibits end-to-end 1,5-bridging mode (Fig. 6b). Selected bond lengths are summarized in Table
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7. Selected bond angles are given in supplementary material, Table S3. The dca bridges assume
an asymmetric apical–basal disposition between neighboring copper(II) ions; i.e., the same μ-1,3
or μ-1,5 dca bridge resides on the apical position of one copper but in the basal plane of the
neighboring copper, with the apical Cu–N distance (2.363-2.449 Å) being significantly longer
than the basal one (1.947-2.054 Å), as an obvious result of Jahn Teller effect in d9 copper(II)
system. In the equatorial plane, the Cu-Nimine distances (1.947-1.958 Å) are shorter than the Cu-
Namine (2.023-2.054 Å) distances. . Deviation of the Cu(1), Cu(2) and Cu(3) atoms from the mean
plane towards the apical donor atom is about 0.152, 0.142, and 0.132 Å respectively. The intra-
chain Cu∙∙∙Cu distances are 5.778, 5.680 and 6.467 Å. The five-membered chelate rings [Cu(1)–
N(1)–C(2)–C(3)–N(4)], [Cu(2)–N(101)–C(102)–C(103)–N(104)] and [Cu(3)–N(201)–C(202)–
C(203)–N(204)] connected with Cu(1), Cu(2) and Cu(3) assume envelope conformations with
puckering parameters q(2) = 0.424(5), 0.404(5) and 0.413(6) Å; (2) = 243.7(6)°, 66.2(6)° and
248.2(6)° respectively.
260
261
The π∙∙∙π/lone pair∙∙∙π interactions (Table 3 and Table 6) are responsible for the formation
of the 2D assembly in 3 (Fig. 7).
262
263
3.2.4. [Cu(L4)(dca)]n (4)
264
265
266
267
268
X-ray single crystal structure analyses reveal that complex 4 is a 1D helical system. It
crystallized in the space group P21/c. Fig. 8 shows a perspective ORTEP view of the complex.
Selected bond lengths are summarized in Table 8. Selected bond angles are given in
supplementary material, Table S4. The overall geometry at the metal centre is best described as
a distorted square pyramid,  (Addison parameter) = 0.34 with a CuN4O chromophore. Cu(1) is
13

269
270
271
272
273
274
275
276
277
278
279
280
coordinated equatorially by one amine nitrogen atom, N(1), one imine nitrogen atom, N(4) and
one oxygen atom O(1), of the tridentate Schiff base ligand, HL4 and a nitrogen atom, N(20), of
the end-to-end bridged dca; this defines the basal plane. The apical position is occupied by one
nitrogen atom N(20)d {Symmetry code, d=-x,1/2+y,1/2-z} of the another end-to-end bridged dca
ligand from a crystallographically related unit. The dca bridges assume an asymmetric apical–
basal disposition between neighboring copper(II) ions; i.e., the same end-to-end dca bridge
resides on the apical position of one copper but in the basal plane of the neighboring copper, with
the apical Cu–N distance (2.282 Å) being significantly longer than the basal one (1.938-2.096
Å), as an obvious result of Jahn Teller effect in d9 copper(II) system. In the equatorial plane, the
Cu-Nimine distance (1.938 Å) is shorter than the Cu-Namine (2.096 Å) distance. The five-membered
chelate rings [Cu(1)–N(1)–C(2)–C(3)–N(4)] connected with Cu(1) assume envelope
conformation with puckering parameters q(2) = 0.442(4) Å; (2) = 283.7(4)°.
281
282
283
284
285
In complex 4, the nitro oxygen atom, O(2) of the host molecule participates in weak lone
pair∙∙∙π interactions (Table 7) with a chelate ring, Cu(1)–O(1)–C(7)–C(6)–C(5)–N(4) and a
phenyl ring, C(6)–C(7)–C(8)–C(9)–C(10)–C(11) of the neighbouring molecule of symmetry, 1-
x,1-y,1-z. The lone pair∙∙∙π interactions contribute to the formation of the 2D assembly in 4 (Fig
9).
286
287
3.3. IR and electronic spectra, luminescence studies and magnetic moments
288
289
290
The dicyanamide anion has three characteristic IR absorptions located in the 2300–2170
cm-1 region: υas+ υs (C≡N) at 2286 cm-1, υas(C≡N) at 2232 cm-1 and υs(C≡N) at 2179 cm-1 for
NaN(CN)2 [51]. The IR spectrum of complex 1, 2, 3 and 4 also exhibits three strong bands:
14

291
292
293
υas(C≡N) + υs(C≡N) at 2303, 2295, 2297 and 2317 cm-1, υas(C≡N) at 2228, 2239, 2239 and 2245
cm-1 and υs(C≡N) at 2172, 2191, 2165 and 2200 cm-1 respectively. These bands can be assigned
to the stretching frequencies υ(C≡N) of the dicyanamide ligand, linked in a bidentate -1,5-1:
1 coordination mode [51], as X-ray diffraction studies demonstrated. In the IR spectra of all the
complexes, stretching vibrations corresponding to the azomethine group appear in the range
1638–1653 cm-1. Two satellite peaks at 2276 cm-1 and 2184 cm-1 may be attributed to -1,3
bridging mode of dca ligand. The infrared spectrum of complex 3 is depicted in Fig. 10. An
intense band within 1317–1327 cm-1 may be attributed to νNO3 vibration [52]. In the IR spectra of
complex 2, a broad band at 3430 cm-1 was observed may be assigned to OH stretching vibration
of the lattice water molecule. The presence of lattice water was also supported by the X-ray
crystal structure analysis.
294
295
296
297
298
299
300
301
302
303
304
305
306
307
The electronic spectra of all the complexes in acetonitrile were recorded in the range
200–1000 nm. The intense absorption bands at short wavelengths, around 360 nm, may be
assigned to ligand to metal ion charge transfer bands (LMCT). The absorptions around 600 nm
may be assigned to d–d transitions. The Uv–Vis spectra in acetonitrile solutions of the
complexes are given in Fig 11. The concentrations of the solutions of 1, 2, 3 and 4 are 2.1×10-4
(M), 5.4×10-5 (M), 1.9×10-4 (M) and 1.0×10-4 (M) respectively.
308
309
310
311
312
313
The complexes 1-4 exhibit luminescence in acetonitrile medium. On
excitation at 350 nm, emissions observed at 423.5, 424, 422.5 and 423 nm respectively for
1
complexes 1, 2, 3 and 4. These are assigned as intra-ligand (π–π*) fluorescence [53]. The
fluorescence spectra of the complexes in acetonitrile solutions are given in Fig 12. The
concentrations of the solutions of 1, 2, 3 and 4 are 8.9×10-6 (M), 4.0×10-6 (M), 8.2×10-6 (M) and
7.6×10-6 (M) respectively.
15

314
315
316
Room temperature magnetic susceptibility measurements showed that all have magnetic
moments close to 1.73 BM as expected for discrete magnetically non-coupled spin-only value for
copper(II) ion, as was observed in similar systems [30].
317
318
3.4 Cyclic Voltammetry
319
320
321
322
323
324
The cyclic voltammograms were recorded in acetonitrile solution containing TBAP as
supporting electrolyte at a glassy carbon, Ag/Ag+(nonaquous) and a platinum wire as working,
reference and auxiliary electrodes respectively at ambient temperature (300 K) with no trace of
decomposition as reflected in the smooth curve, in the potential range from −2 to 2 V and were
uncorrected for junction contribution. The value for the Fc–Fc+ couple under our conditions is
0.39 V.
325
326
327
328
329
330
331
332
333
334
335
In the positive range, from 0 to +2 V, the oxidation and reduction processes due to ligand
are observed. Cyclic scanning between 0 and -2 V permitted the study of the copper centered
reduction processes. A representative example of this behavior is shown in Fig. 13 for complex
4. The cyclic voltammogram shows a reduction peak (Ic) at about the potential -343 mV. This
can be assigned as CuII/CuI reduction process. Other peaks at more negative potentials may be
due to the reduction to further lower oxidation states of copper. A wave (Ia) at -606 mV is
observed during the reverse scan. All the redox signals are similar under different scan rates at
ambient temperature (300 K). Electrochemical responses in the total range studied for complexes
1–4 show a similar pattern. However, the differences between cathodic and anodic peaks are
more than 200 mV for the complexes 1 and 4 and this fact suggests the quasi-reversible character
of the electron transfer processes in 1 and 4. On the other hand, the differences between cathodic
16

336
337
338
and anodic peaks are 101 and 191 mV for the complexes 2 and 3 respectively, and this suggests
the reversible nature of the electron transfer processes in 2 and 3. Electrochemical data for the
reduction of the copper(II) centers of the complexes 1, 2, 3 and 4 are given in Table 9.
339
340
4. Conclusion
341
342
343
344
345
346
347
348
349
In the present work, four dca bridged copper(II) complexes with N2O donor Schiff bases
were synthesised and characterized. In all the complexes, we have used N2O donor tridentate
Schiff bases, all of which contain additional nitro groups in them. Although the nitro groups
remain pendant in the complexes, the presence of pendant nitro groups in each of the complexes
is the key advantages of synthesizing the complexes. A set of pendant donors can perturb the
metal ion at additional coordination sites, in competition with external substrates, or can be used
as attachment points for other molecules. That is why these types of structural motif as an
important one with the potential to influence the future development in chemistry in a major
way.
350
351
352
353
354
355
Acknowledgments
This work was supported by the DST, India, under FAST Track Scheme (Order No.
SR/FT/CS-118/2010, dated 15/02/2012). We are thankful to Ms. Durba Ganguly and Dr.
Saurabh Das, Department of Chemistry, Jadavpur University, India for providing facilities for
Uv-Vis spectral studies.
356
17

357
358
359
360
361
362
363
364
Supplementary data
Crystallographic data for the analysis have been deposited with the Cambridge
Crystallographic data Centre, CCDC Nos. 921569 (Complex 1), 921570 (Complex 2), 921568
(Complex 3) and 915573 (Complex 4). Copies of this information may be obtained free of
charge from CCDC, 12 Union Road, Cambridge, CB21EZ, UK (fax: +44-1223-336033; e-mail:
deposit@ccdc.cam.ac.uk or www: http://www.ccdc.cam.ac.uk).
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Table 1: Crystal data and refinement details of complexes 1-4
1
2
C24H28Cu2N12O8
739.66
3
4
Formula
C11H10CuN6O3
337.79
C13H14CuN6O3
365.84
C15H18CuN6O3
393.89
Formula Weight
Temperature (K)
Crystal system
Space group
100(2)
100(2)
100(2)
100(2)
Orthorhombic
Pna21
Triclinic
P-1
Monoclinic
P21/c
Monoclinic
P21/c
22

a(Å)
19.9332(7)
8.3531(2)
30.4128(14)
90
6.5415(7)
6.9196(8)
17.7611(18)
85.719(9)
80.630(9)
63.759(9)
1
14.0892(5)
11.7467(5)
27.6456(12)
90
18.2284(15)
6.6962(4)
14.9753(10)
90
b(Å)
c(Å)
α
β
90
102.090(3)
90
114.218(5)
90
γ
90
Z
16
16
4
dcalc (g cm-3)
μ (mm-1)
1.772
1.726
1.630
1.569
1.747
1.567
1.490
1.339
F(000)
2736
378
2244
812
Total Reflections
Unique Reflections
Observed data [I >2 σ (I)]
No. of parameters
R(int)
36416
9130
5425
28680
11034
2576
8071
3009
5630
1705
4224
2414
757
212
628
229
0.105
0.078
0.095
0.060
R1, wR2 ( all data)
R1, wR2 [I > 2 σ (I)]
0.0893, 0.0596
0.0431, 0.0498
0.0694, 0.0842
0.0385, 0.0789
0.1284, 0.0746
0.0471, 0.0577
0.0402, 0.0686
0.0311, 0.0670
448
449
450
Table 2: Selected bond lengths of complex 1
Cu(1)–O(1)
Cu(1)–N(1)
1.948(4)
2.028(6)
Cu(3)–O(201)
Cu(3)–N(2)
1.922(4)
2.012(6)
23

Cu(1)–N(4)
Cu(1)–N(12)
1.959(6)
1.976(7)
2.270(6)
1.921(5)
1.990(6)
2.025(9)
1.959(6)
2.277(7)
Cu(3)–N(116)
Cu(3)–N(204)
Cu(3)–N(212)
Cu(4)–O(301)
Cu(4)–N(216)
Cu(4)–N(301)
Cu(4)–N(304)
Cu(4)–N(312)
2.268(6)
1.962(6)
2.003(7)
1.915(5)
1.975(6)
1.982(9)
1.970(6)
2.268(7)
Cu(1)–N(316)a
Cu(2)–O(101)
Cu(2)–N(16)
Cu(2)–N(101)
Cu(2)–N(104)
Cu(2)–N(112)
451
Symmetry element, a= x,-3+y,z.
452
453
Table 3: Geometrical Parameters (Å, deg) for the π-Stacking Moieties involved in the π∙∙∙π
interactions for the complexes 1 and 3.
Complex
Rcd
R1ve
R2vf
α
β
γ
symmetry
rings I−J
Cg(9)–Cg(10) 3.650(5) 3.433(3)
Cg(12)–Cg(11) 3.708(5) 3.436(3)
Cg(7)–Cg(8) 3.941(3) 3.223(2)
Cg(9)–Cg(9) 3.963(3) 3.246(2)
3.404(3) 3.8(4) 21.14 19.87
3.588(3) 9.9(4) 14.65 22.09
1/2+x,-1/2-y,z
1/2+x,5/2-y,z
1
3
3.444(2) 6.0(2) 29.12 35.14 2-x,1/2+y,3/2-z
3.247(2)
0
34.99 34.99
1-x,1-y,1-z
e
454
455
456
457
458
459
d Distance between the centroids of ring I and ring J. Perpendicular distance from the centroid
f
of ring I to ring J. Perpendicular distance from the centroid of ring J to ring I. α = Dihedral
Angle between ring I and ring J(°); β=Angle Cg(I)–Cg(J) vector and normal to plane I (°);
γ=Angle Cg(I)–Cg(J) vector and normal to plane J (°).For complex 1, Cg(9) = Centroid of ring
[C(6)–C(7)–C(8)–C(9)–C(10)–C(11)]; Cg(10) = Centroid of ring [C(106)–C(107)–C(108)–
C(109)–C(110)–C(111)]; Cg(11) = Centroid of ring [C(206)–C(207)–C(208)–C(209)–C(210)–
24

460
461
462
463
C(211)] and Cg(12) = Centroid of ring [C(306)–C(307)–C(308)–C(309)–C(310)–C(311)]; for
complex 3, Cg(7) = Centroid of ring [C(6)–C(7)–C(8)–C(9)–C(10)–C(11)]; Cg(8) = Centroid of
ring [C(106)–C(107)–C(108)–C(109)–C(110)–C(111)] and Cg(9) = Centroid of ring [C(206)–
C(207)–C(208)–C(209)–C(210)–C(211].
464
465
Table 4: Selected bond lengths of complex 2
Cu(1)–O(1)
Cu(1)–O(4)
1.940(3)
2.485(4)
1.960(3)
Cu(1)–N(2)
Cu(1)–N(4)
Cu(1)–N(6)b
2.041(3)
1.971(3)
2.626(4)
Cu(1)–N(1)
466
467
468
Symmetry element, b= -x,2-y,1-z
Table 5: Hydrogen bond distances (Å) and angles (°) of complex 2.
D–H∙∙∙A
D–H
D∙∙∙A
H∙∙∙A
D–H∙∙∙A
147.00
N(2)–H(2)∙∙∙N(5)
O(4)–H(4A)∙∙∙O(3)
0.9300
0.8500
0.8500
3.269(5)
2.994(4)
2.998(5)
2.4500
2.2000
2.2100
155.00
O(4)–H(4B)∙∙∙N(6)
155.00
469
470
D: Donor, A: Acceptor, H: Hydrogen
471
472
Table 6: Geometrical Parameters (Å, deg) for the Lone Pair∙∙∙π interactions for the complexes 2-
4.
25

Complex
X···Cg(J)
Y···Cg(J)
symmetry
Y−X(I)···Cg(J)
Y−X···Cg(J)
X−Perp
N(3)–O(2)···Cg(3)
N(3)–O(3)···Cg(3)
3.414(4)
3.586(4)
3.187(4)
3.273(5)
3.778(4)
3.792(4)
3.412(3)
3.778(3)
3.342(5)
3.342(5)
3.457(5)
3.370(5)
3.370(5)
3.699(5)
3.643(4)
4.016(4)
76.2(3)
68.7(3)
91.9(3)
83.7(3)
61.6(3)
76.3(3)
90.8(2)
92.1(2)
3.300
3.314
3.159
3.227
3.337
3.404
3.339
3.359
-x,2-y,-z
-x,2-y,-z
1-x,2-y,-z
2
N(3)–O(2)···Cg(3)
N(108)–O(102)···Cg(4)
N(108)–O(103)···Cg(4)
N(208)–O(202)···Cg(9)
N(10)–O(2)···Cg(2)
N(10)–O(2)···Cg(3)
2-x,-1/2+y,3/2-z
1-x,1-y,1-z
3
1-x,1-y,1-z
4
473
474
475
476
477
478
For complex 2, Cg(3) = Centroid of ring [C(2)–C(3)–C(4)–C(5)–C(6)–C(7)]; for complex 3,
Cg(4) = Centroid of ring [Cu(1)–O(1)–C(7)–C(6)–C(5)–N(4)] and Cg(9) = Centroid of ring
[C(206)–C(207)–C(208)–C(209)–C(210)–C(211)] and for complex 4, Cg(2) = Centroid of ring
[Cu(1)–O(1)–C(7)–C(6)–C(5)–N(4)] and Cg(3) = Centroid of ring [C(6)–C(7)–C(8)–C(9)–
C(10)–C(11)]. X−Perp=Perpendicular distance between X and the ring J (Å).
479
480
Table 7: Selected bond lengths of complex 3
Cu(1)–O(1)
Cu(1)–N(1)
Cu(1)–N(4)
Cu(1)–N(14)
Cu(1)–N(114)
1.930(4)
2.034(5)
1.958(4)
2.363(5)
1.982(4)
26

481
482
483
484
485
486
487
488
489
490
491
492
Cu(2)–O(101)
Cu(2)–N(101)
Cu(2)–N(104)
Cu(2)–N(116)
Cu(2)–N(214)
Cu(3)–O(201)
Cu(3)–N(201)
Cu(3)–N(204)
Cu(3)–N(216)
Cu(3)–N(18)c
1.914(4)
2.023(5)
1.956(4)
2.407(4)
1.976(4)
1.919(4)
2.054(5)
1.947(4)
2.449(4)
1.953(6)
Symmetry element, c= -1+x,1/2-y,-1/2+z
Table 8: Selected bond lengths of complex 4
Cu(1)–O(1)
Cu(1)–N(1)
Cu(1)–N(4)
Cu(1)–N(16)
Cu(1)–N(20)d
1.949(3)
2.096(3)
1.938(3)
2.282(3)
1.959(3)
493
494
Symmetry elements d= -x,1/2+y,1/2-z;
27

495
496
497
Table 9: Electrochemical data for the oxidation and reduction of the copper(II) center of the
complexes 1-4.
Complex
Epa(V)
-0.654
-0.604
-0.588
-0.606
Epc(V)
-0.412
-0.503
-0.397
-0.343
E1/2(V)
-0.533
-0.553
-0.492
-0.474
∆Ep(mV)
242
1
2
3
4
101
191
263
498
499
500
501
502
E1/2 is calculated as the average of anodic (Epa) and cathodic (Epc) peak potentials;
∆Ep = (Epa- Epc).
503
504
505
506
507
508
509
510
511
28

512 Legends to the figures
513
Figure 1: ORTEP3 diagram of 1 with 30% ellipsoid probability. Symmetry code, a= x,-3+y,z.
514
515
Figure 2: (a) 2D array in crystal packing of 1, generated through π∙∙∙π interactions. (b) π∙∙∙π
interactions are highlighted in complex 1.
516
Figure 3: ORTEP3 diagram of 2 with 30% ellipsoid probability. Symmetry code, b= -x,2-y,1-z.
517
518
Figure 4: Formation of a 1D hydrogen bonding chain of 2, a) along a axis, b) along b axis and c)
along c axis.
519
Figure 5: Two-dimensional assembly of 2 via lone pair∙∙∙π interactions.
520
521
522
Figure 6: a) ORTEP3 diagram of 3 with 30% ellipsoid probability. Symmetry code, c=-1+x,1/2-
y,-1/2+z. b) -1,5 and -1,5-dca bridged polynuclear chain in 3. Only the relevant atoms are
shown.
523
Figure 7: Two-dimensional assembly in 3, generated through lone pair∙∙∙π/π∙∙∙π interactions.
d
524
525
Figure 8: The ORTEP3 diagram of 4 with 30% ellipsoid probability. Symmetry code, =-
x,1/2+y,1/2-z.
526
527
528
529
530
Figure 9: Supramolecular network in 4, generated through lone pair∙∙∙π interactions.
Figure 10: IR spectrum (500-3500 cm-1) of 3.
Figure 11:The Uv–Vis spectra of the complexes in acetonitrile.
Figure 12:The fluorescence spectra of the complexes in acetonitrile.
Figure 13: Cyclic voltammogram of 4 in acetonitrile.
29