Synthesis, crystal structure and DFT analysis of a phenoxo bridged Cu(II) complex and an azide and μ3-O mixed bridged trinuclear Cu(II) complex

Article abstract of DOI:10.1016/j.poly.2012.09.061

One binuclear Cu(II) complex, [Cu₂(L₁) ₂(N₃)₂] (1), and a trinuclear Cu(II) complex, [Cu₃(L₂)₃(μ_1,1N₃) ₂](ClO₄) (2), of two potentially tridentate NNO-donor Schiff base ligands, [2-(1-(2-(4,6-dimethylpyrimidin-2-yl)hydrazono)ethyl) phenol] (HL₁) and [2-((2-(4,6-dimethyl pyrimidin-2-yl) hydrazono) methyl) phenol] (HL₂), have been synthesized and characterized by elemental analyses, UV-Vis IR spectroscopy, DFT and single crystal X-ray crystallography. The ligands HL₁ and HL₂ are [1 + 1] condensation products of 2-hydrazino-4,6-dimethylpyrimidine with 2-hydroxy acetophenone and salicylaldehyde respectively. In 1, the two Cu(II) centers are bridged by μ-phenoxo groups. In 2, the three Cu(II) centers are held together by two μ_1,1 bridging azide ions and a phenoxo oxygen atom which binds the three metal centers and behaves as a μ₃-O atom. The geometries of the complexes have been optimized using the UB3LYP level of theory. The calculation confirms that all the copper centers are five coordinate with distorted square pyramidal geometries, which is consistent with the experimental data.

Full text of DOI:10.1016/j.poly.2012.09.061

Polyhedron 50 (2013) 51–58
Contents lists available at SciVerse ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Synthesis, crystal structure and DFT analysis of a phenoxo bridged Cu(II)
complex and an azide and l3-O mixed bridged trinuclear Cu(II) complex
Sangita Ray a, Saugata Konar a, Atanu Jana a, Kinsuk Das b, Ray J. Butcher c, Tapan Kumar Mondal d,
Susanta Kumar Kar a,
⇑
a Department of Chemistry, University College of Science, University of Calcutta, 92, A.P.C. Road, Kolkata 700009, India
b Department of Chemistry, Haldia Government College, Debhog, Purba Midnapur 721657, India
c Department of Chemistry, Howard University, 2400 Sixth Street, N.W., Washington, DC 200 59, USA
d Department of Chemistry, Jadavpur University, Kolkata 700032, India
a r t i c l e i n f o
a b s t r a c t
Article history:
One binuclear Cu(II) complex, [Cu2(L1)2(N3)2] (1), and a trinuclear Cu(II) complex, [Cu3(L2)3(l1,1-N3)2]
Received 21 April 2012
Accepted 16 September 2012
Available online 2 November 2012
(ClO4) (2), of two potentially tridentate NNO-donor Schiff base ligands, [2-(1-(2-(4,6-dimethylpyrimi-
din-2-yl)hydrazono)ethyl)phenol] (HL1) and [2-((2-(4,6-dimethyl pyrimidin-2-yl) hydrazono) methyl)
phenol] (HL2), have been synthesized and characterized by elemental analyses, UV–Vis IR spectroscopy,
DFT and single crystal X-ray crystallography. The ligands HL1 and HL2 are [1 + 1] condensation products
of 2-hydrazino-4,6-dimethylpyrimidine with 2-hydroxy acetophenone and salicylaldehyde respectively.
Keywords:
Copper (II) complexes
Pyrimidine derived Schiff base ligands
Spectral properties
X-ray crystal structures
DFT
In 1, the two Cu(II) centers are bridged by
together by two 1,1 bridging azide ions and a phenoxo oxygen atom which binds the three metal centers
and behaves as a 3-O atom. The geometries of the complexes have been optimized using the UB3LYP
l-phenoxo groups. In 2, the three Cu(II) centers are held
l
l
level of theory. The calculation confirms that all the copper centers are five coordinate with distorted
square pyramidal geometries, which is consistent with the experimental data.
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
el systems for the active sites of multinuclear proteins and metallo-
enzymes [17–20]. Most of the reported trinuclear Cu(II) complexes
In recent decades, much attention has been paid to synthesizing
copper(II) trinuclear [1,2] and binuclear [3,4] complexes. Polyden-
tate Schiff base ligands have preparative accessibility, structural
variety and varied denticity, forming complexes of different coordi-
nation numbers and nuclearities with interesting molecular and
crystalline architectures [5–7]. A substantial amount of work on
pyridine and pyrazole derived Schiff base ligands has emerged in
the literature, while works on pyrimidine derived Schiff base li-
gands are scarce. Pyrimidine derived metal ion complexes have
been extensively studied in recent years owing to their great variety
of biological activity, ranging from antimalarial, antibacterial, anti-
tumoral, antiviral activities, etc. [8–13], which have often been re-
lated to their chelating ability with trace metal ions. Nucleic
acids, vitamins and co-enzymes containing pyrimidine ring sys-
have a linear geometry, whereas trinuclear non-linear Cu(II) com-
plexes are comparatively few in number [21].
As a sequel to our long standing interest in pyrimidine derived
ligands [22–24], we have prepared tridentate NNO donor Schiff
base ligands using 2-hydrazino-4,6-dimethylpyrimidine, 2-
hydroxy acetophenone and salicylaldehyde. In the present study
(Scheme 1), using copper perchlorate as the metal precursor, one
new dinuclear copper(II) complex, [Cu2(L1)2(N3)2] (1), and one
new trinuclear copper(II) complex, [Cu3(L2)3(l1,1-N3)2](ClO4) (2),
have been synthesized with HL1 and HL2. We have reported the
synthetic details, spectral characterizations, DFT, a comparison be-
tween the calculations and the experimental data, X-ray crystal
structures and electrochemical properties of 1 and 2 where the
phenoxo groups, and both the l1,1 azide and phenoxo groups act
as bridging units respectively.
tems provide potential binding sites for metal ions. The higher
p
acidity and the presence of more than one hetero atom in pyrimi-
dine play an important role in its coordination chemistry, compared
to that of pyridine bases, and serve as better models in biological
systems [14–16]. Trinuclear and dinuclear copper complexes have
received considerable interest because of their importance as mod-
2. Experimental
2.1. Materials
⇑
All chemicals were of reagent grade, purchased from commer-
cial sources and used without further purification. Salicylaldehyde
Corresponding author. Tel.: +91 033 23508386; fax: +91 033 23519755.
E-mail address: skkar_cu@yahoo.co.in (S.K. Kar).
0277-5387/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2012.09.061

52
S. Ray et al. / Polyhedron 50 (2013) 51–58
The ligand HL2 was synthesized and characterized using the lit-
erature method [26].
HL1: (Yield: 85.30%); M.p. (°C) 159; Anal. Calc. for C14H16N4O: C,
65.62; H, 6.25; N, 21.80 Found: C, 65.52; H, 6.17; N, 21.40%. IR (
cmÀ1): 3247 (
NH), 1620 ( C@C), 1635 ( C@NH), 885 ( Cpym–H), 1255
C–O); 1H NMR (300 MHz, CDCl3, 25 °C, TMS) d/ppm: 6.68 (s,1H,
m,
m
m
m
m
(m
C5–H pyrimidine), 6.8–7.3 (m, 4H, Pym ring), 8.2 (s,1H, –CH@N–),
12.8 (s, 1H, aromatic –OH), 2.28 (s,1H, N–H of hydrazone), 2.46–
2.53 (m, 6H, CH3).
2.4. Preparation of the complexes
2.4.1. Preparation of complex 1
To a methanolic solution (30 ml) of Cu(ClO4)2Á6H2O (0.370 g,
2 mmol), a solution of the Schiff base HL1 (0.512 g, 2 mmol) was
added, followed by a solution of sodium azide (0.260 g, 4 mmol)
in a minimum volume of methanol with constant stirring for 2 h.
This mixture was kept at room temperature, yielding green hexag-
onal crystals suitable for X-ray diffraction after 10 days. The crys-
tals were isolated by filtration and air-dried. (Yield: 65.5%). Anal.
Calc. for C30H30Cu2N14O4: C, 46.20; H, 3.85; N, 25.1. Found: C,
46.10; H, 3.81; N, 24.89%. IR (KBr;
1619 (s) ( c@Npym). UV–Vis (MeOH) kmax/nm: 386 and 607.
m m
/cmÀ1): 2065 (vs) ( as, N3À),
Scheme 1. Schematic representation of the ligands (HL1 and HL2) and their
complexes.
m
2.4.2. Preparation of complex 2
and 2-hydroxy acetophenone were purchased from Aldrich Chem-
ical Company, USA and used without further purification.
Caution! Although we have not encountered any problems, it
should be kept in mind that sodium azide and perchlorate com-
pounds of metal ions are potentially explosive, especially in the
presence of organic ligands. Only a small amount of the material
should be prepared and it should be handled with care.
To a methanolic solution (30 ml) of Cu(ClO4)2Á6H2O (0.370 g,
2 mmol), a solution of the Schiff base HL2 (0.484 g, 2 mmol) was
added, followed by a solution of sodium azide (0.260 g, 4 mmol)
in a minimum volume of methanol with constant stirring for 2 h.
This mixture was kept at room temperature, yielding green hexag-
onal crystals suitable for X-ray diffraction after 10 days. The crys-
tals were isolated by filtration and air-dried. (Yield: 61.7%). Anal.
Calc. for C41H45Cu3ClN18O9: C, 42.4; H, 3.87; N, 21.7 Found: C,
42.3; H, 3.77; N, 21.30%. IR (KBr;
(uncoordinated perchlorate), 2069 (vs)
c@Npym), UV–Vis (MeOH) kmax/nm: 400 and 612.
m
/cmÀ1): 625(s) and 1100 (s, br)
N3À), 1610 (s)
2.2. Physical measurements
(
mas,
(m
Elemental analyses (carbon, hydrogen and nitrogen) of the li-
gands and the metal complexes were determined with a Perkin–
Elmer CHN analyzer 2400 at the Indian Association for the
Cultivation of Science; Kolkata. The electronic spectra of the com-
plexes in methanolic solution were recorded on a Hitachi model U-
2.5. Single crystal X-ray crystallography
Crystal data for the two complexes are given in Table 1. Data
were collected with Mo K radiation, for 1 using a Bruker-AXS
a
3501 spectrophotometer. IR spectra (KBr pellet, 400–4000 cmÀ1
)
SMART APEX II diffractometer and for 2 using an Oxford Diffraction
X-Calibur CCD System. Absorption corrections were carried out
using the SADABS program [27]. Data analysis was carried out with
the CRYSALIS program [28]. Both structures were solved using direct
methods with the SHELXS97 program [29]. An empirical absorption
correction for 2 was carried out using the ABSPACK program [30].
The structures were refined on F2 using SHELXL97 [29].
were recorded on a Perkin–Elmer model 883 infrared spectropho-
tometer. 1H NMR spectra of the ligands were recorded in CDCl3
with a Bruker AM 300L (300 MHz) superconducting FTNMR spec-
trometer. Cyclic voltammetry was carried out with a Sycopel mod-
el AEW2 1820F/L instrument. All electrochemical measurements
were made in DMF on a BAS (Epsilon model) having a three-
electrode setup consisting of glassy carbon (polished with alumina
before measurement) working, platinum wire auxiliary and
Ag/AgCl reference electrodes. Oxygen was rigorously removed
from the DMF solutions of the samples by purging with dry argon
gas of high purity.
2.6. Computational methods
Full geometry optimizations of complexes 1 and 2 have been
carried out in the UB3LYP [31–33] level DFT computational meth-
od in triplet (S = 1) and quartet (S = 3/2) states respectively using
the GAUSSIAN 03 software package [34]. For C, H, N and O atoms,
the 6–31+G(d) basis set was assigned, while for Cu the LANL2DZ
basis set with an effective core potential was employed [35]. Vibra-
tional frequency calculations were performed to ensure that the
optimized geometries represent the local minima and there are
only positive Eigen values. Vertical electronic excitations based
on optimized geometries were computed for the time-dependent
density functional theory (TD-DFT) formalism [36–38] in MeOH
using the conductor-like polarizable continuum model (CPCM)
[39–41]. Gauss Sum [42] was used to calculate the fractional con-
tributions of the various groups to each molecular orbital.
2.3. Syntheses of the ligands HL1 and HL2
The ligand HL1 was synthesized by refluxing a methanol solu-
tion (30 ml) of 2-hydrazino-4,6-dimethylpyrimidine [24,25]
(1.38 g, 10 mmol), with 2-hydroxy acetophenone (1.36 g, 10 mmol)
taken in the same solvent. Refluxing was continued for 1 h at water
bath temperature. The resulting light yellow solution was filtered
and kept at room temperature for slow evaporation. A light yellow
microcrystalline compound separated out after a week. It was fil-
tered off, washed several times with cold methanol and dried in
vacuo over fused CaCl2.

S. Ray et al. / Polyhedron 50 (2013) 51–58
53
Table 1
Experimental data for crystallographic analyses of 1 and 2.
Compound
1
2
Formula
Formula weight
T (K)
C
30H38Cu2N14O4
C41H45ClCu3N18O9
1160.02
295
785.84
150
k (Å)
0.71073
monoclinic
C2/c
1.54178
monoclinic
P21/m
Crystal system
Space group
Unit cell dimensions
a (Å)
b (Å)
c (Å)
20.463(2)
9.8220(8)
18.1022(18)
90
11.1763(1)
18.9296(2)
11.7307(1)
90
a
(°)
b (°)
110.565(5)
90
3406.5(6)
4
98.773(1)
90
2452.75(4)
2
c
(°)
V (Å3)
Z
Densitycal (Mg mÀ3
)
1.532
1.571
Absorption coefficient
(mmÀ1
F(000)
1.307
2.627
)
1624
1186
Crystal size (mm3)
h Range (°) for data
collection
0.11 Â 0.13 Â 0.14
0.12 Â 0.32 Â 0.48
2.1–28.4
4.5–77.5
Index ranges
À27 6 h 6 27
À12 6 k 6 13
À22 6 l 6 24
1.027
À14 6 h 6 13
À23 6 k 6 15
À14 6 l 6 14
1.062
Fig. 1. Structural representation and atom numbering scheme of 1 (H-atoms are
omitted for clarity).
Goodness-of-fit on F2
Completeness to
98.3
99.9
theta = 25.00° (%)
Independent reflections
4222 (Rint = 0.0650)
5288 (Rint = 0.0156)
(Rint
)
Absorption correction
Refinement method
multi-scan
multi-scan
full-matrix least
squares on F2
21549
full-matrix least
squares on F2
11195
Reflections collected
Final R indices [I > 2
r
(I)]
R1 = 0.0444
WR2 = 0.1294
À0.68, 0.38
R1 = 0.0483
WR2 = 0.1348
À0.965, 0.989
Largest difference peak and
hole (e ÅÀ3
)
3. Result and discussion
3.1. Syntheses
The ligands HL1 and HL2 were synthesized by the direct conden-
sation reactions of 2-hydrazino-4,6-dimethylpyrimidine with 2-
hydroxy acetophenone and salicylaldehyde respectively, taken in
methanol in a 1:1 mol proportion. The complexation behavior
of HL1 and HL2 towards Cu(II) perchlorate was investigated.
Complexes 1 and 2 were obtained by mixing the ligands, Cu(II)
perchlorate and sodium azide, taken in a 1:1:2 molar ratio in
methanol. X-ray quality crystals of 1 and 2 were obtained upon
slow evaporation of the reaction mixtures at room temperature.
3.2. Structural description of complex 1
Fig. 2. 3D view of complex 1 with trapped methanol molecules within the chains.
A perspective view of complex 1 with the atom numbering
scheme is shown in Fig. 1. Selected metrical parameters are given
in Table 1. Complex 1 crystallizes in the space group C2/c. The
structure of [Cu2(L1)2(N3)2] (1) is a non-centrosymmetric dimer,
as shown in Fig. 1, with each metal atom in a five-coordinate envi-
ronment. The unit cell of 1 comprises of four molecules. The Addi-
are separated by 3.371 Å and the Cu1–O1–Cu1_a angle is 91.64°.
The copper(II) ion is 0.111 Å above the basal plane. The crystal
structure of these compounds can be described as a 1D layer due
to intermolecular hydrogen bonding. A 3D view of complex 1, with
trapped water molecules within chains is shown in Fig. 2. The
structure is stabilized by a network of intermolecular hydrogen
bonding, H3AB of N3A and O1 of CH3OH and H1 of CH3OH and
N4A of pyrimidine (Fig. 3). Details of the hydrogen bonding are
given in Table 3.
son parameter (s) of the pentacoordinate Cu(II) is 0.314, suggesting
the predominance of a square-based pyramidal (SP) geometry. The
metal atom is bonded to the uninegative tridentate ligand L1 via O1
at 1.870(2), N1 at 1.972(2) and N3 at 2.002(3) Å, together with a
bridging oxygen atom O1_a from a second ligand at 2.751(2) Å
and a terminal azide ion via N5 at 1.964(3) Å. The two Cu atoms

54
S. Ray et al. / Polyhedron 50 (2013) 51–58
Fig. 3. Hydrogen bonding interactions in complex 1.
HOMO
LUMO
HOMO-1
HOMO-2
LUMO+2
LUMO+1
Fig. 4. Contour plots of some selected MOs (b-spin) of 1.
3.3. Structural description of complex 2
and O1B phenoxo oxygen atoms of the ligand molecule. The O1B
atom is a 3-phenoxo oxygen, acting as a three connecting node
l
A perspective view of complex 2 with the atom numbering
scheme is shown in Fig. 5. Selected metrical parameters are given
in Table 2. Complex 2 crystallizes in the space group P21/m and
its unit cell comprises of two molecules. The structure consists of
two distinct entities of [Cu(L2)2] and [Cu(L2)], where two identical
in the structure (Cu2–N1B 1.961, Cu2–N2B 1.999, Cu2–O1B 1.899,
Cu2–N11 2.162 Å). The Cu1–Cu2–Cu1 angle is 113.21°, Cu(1)–
N(11)–Cu(2) is 104.15° and Cu(1)–O(1B)–Cu(2) is 86.15°. The Cu1
center is 0.03 Å above the mean plane. The 3D view of complex 2
with trapped perchlorate molecules within the chains is shown in
Fig. 6. The structure is stabilized by a network of intermolecular
hydrogen bonding, H3AB of pyrimidine and O1 of CH3OH, and H1
of CH3OH and N4A of pyrimidine (Fig. 7). Details of the hydrogen
bonding are given in Table 3.
Cu(L2)2 units and a CuL2 unit are bridged by two
and a 3-phenoxo oxygen atom. The geometry of the two identical
terminal Cu(II) centers are best described as distorted square pyra-
midal with a N3O2 chromophore ( = 0.099). The basal plane of Cu1
l1,1-azido groups
l
s
is occupied by O1A from a phenoxo group, N1A from azomethine,
N2A from pyrimidine, N11 from a bridging azide, and the apical po-
sition by the oxygen atom O1B from another ligand molecule (Cu1–
N1A 1.937, Cu1–N2A 2.034, Cu1–N11 1.983, Cu1–O1B 2.79 Å). The
central Cu(II) center has a trigonal bipyramidal geometry with a
3.4. DFT calculations and electronic structure
The geometry optimizations of complexes 1 and 2 were carried
out using the (U) B3LYP/DFT method, taking the coordinates of their
respective X-ray structures. The calculated bond parameters well
reproduced the experimental structures with minor deviations (Ta-
N4O chromophore,
s = 0.95 [31]. The trigonal plane is formed by
two N11 atoms from two different bridging azides and one azome-
thine, N1B. The apical positions are occupied by N2B pyrimidine
ble 2). The
a-spin occupied and virtual orbitals have HL1 and HL2

S. Ray et al. / Polyhedron 50 (2013) 51–58
55
Fig. 6. 3D view of complex 2 with trapped methanol molecules within the chains.
À
characters in 1 and 2 respectively, along with a contribution of N3
in the occupied orbitals (Tables 6 and 7 in the supplementary mate-
rials). The b-spin LUMO and LUMO + 1 have 50% Cu(d
1, while LUMO to LUMO + 2 in complex 2 have 53–61% Cu(d
p) in complex
Fig. 5. Structural representation and atom numbering scheme of 2 (H-atoms are
omitted for clarity).
p)
Table 2
Selected bond distances (Å) and angles (°) in 1 and 2.
Selected bonds
(Å)
Calculated value (Å)
Selected angles
(°)
Calculated value (°)
Complex 1
Cu1–O1
Cu1–N5
Cu1–N1
Cu1–N3
1.870(2)
1.964(3)
1.972(2)
2.002(3)
2.751(2)
1.936
1.960
2.037
2.116
2.657
N1–Cu1–N3
N1–Cu1–N5
Cu1–O1–Cu1_a
O1–Cu1–N1
O1–Cu1–N3
O1–Cu1–N5
O1–Cu1–O1_a
O1_a–Cu1–N1
N3–Cu1–N5
O1_a–Cu1–N3
O1_a–Cu1–N5
Cu1–O1–Cu1_a
Cu1–N1–N2
Cu1–N5–N6
82.60(9)
155.22(12)
91.64(9)
91.52(10)
164.98(10)
91.47(11)
83.71(9)
86.31(8)
99.67(11)
82.13(9)
118.47(10)
91.64(9)
110.99(15)
120.3(3)
80.17
154.2
92.72
89.56
164.3
92.88
81.08
87.44
100.6
87.99
118.3
92.72
110.6
120.0
Cu1–O1_a
Complex 2
Cu1–O1A
Cu1–O1B
Cu1–N1A
Cu1–N2A
Cu1–N11
Cu2–O1B
Cu2–N1B
Cu2–N2B
Cu2–N11
Cu2–N11_b
1.895(2)
2.7939(8)
1.937(2)
2.043(2)
1.983(2)
1.899(3)
1.961(3)
1.999(3)
2.162(2)
2.162(2)
1.928
2.790
1.992
2.101
2.003
1.949
2.028
2.064
2.186
2.186
O1A–Cu1–O1B
O1A–Cu1–N1A
O1A–Cu1–N2A
O1A–Cu1–N11
O1B–Cu1–N1A
O1B–Cu1–N2A
O1B–Cu1–N11
N1A–Cu1–N2A
N1A–Cu1–N11
N2A–Cu1–N11
O1B–Cu2–N1B
O1B–Cu2–N2B
O1B–Cu2–N11
N1B–Cu2–N2B
N1B–Cu2–N11
Cu1–O1B–Cu1_b
Cu1–N1A–N3A
Cu2–N1B–N3B
Cu1–N11–N12
Cu2–N11–N12
Cu1–N11–Cu2
89.31(9)
91.26(9)
92.42
90.60
166.6
88.88
99.09
101.6
72.57
79.74
171.5
103.0
91.14
171.2
89.15
80.11
123.5
153.4
112.9
111.4
115.1
117.4
102.8
166.85(10)
87.30(9)
101.82(9)
102.84(8)
71.14(9)
81.44(9)
172.82(10)
101.27(9)
92.72(14)
174.23(14)
89.13(8)
81.51(14)
121.50(6)
155.63(13)
113.51(17)
112.4(2)
116.5(2)
117.14(19)
104.15(10)

56
S. Ray et al. / Polyhedron 50 (2013) 51–58
Table 3
mentary materials). In both the complexes the b-spin occupied orbi-
tals mainly take part in electronic transitions.
Details of hydrogen bond distances (Å) and angles (°) for 1 and 2.
D–H...A
d(D–H)
d(H...A)
d(D...A)
<(DHA)
Complex 1
N2--H2N..O2
O2 --H2O..N5
3.5. Electronic spectra
0.8800
0.67(3)
2.0300
2.23(4)
2.879(4)
2.891(5)
161.00
169(5)
The electronic solution spectra of the complexes were recorded
in MeOH solution. The spectra of the Cu(II) complexes 1 and 2
exhibit a strong band at 386 and 400 nm, along with very weak
low-intensity absorption bands at 607 and 612 nm, respectively.
To assign the electronic transitions corresponding to these bands,
TDDFT calculations on the optimized geometries of 1 and 2 have
been performed in methanol. The TDDFT results show low energy
Complex 2
O1--H1..N4A
N3A--H3AB..O1
N3B--H3BB..O11
0.8200
0.8600
0.8600
2.1400
1.8700
2.0100
2.928(4)
2.718(4)
2.874(6)
162.00
167.00
176.00
Symmetry transformations used to generate equivalent atoms:
For 1 a = [2756.00] = 2 À x, y, 3/2 – z.
For 2 a = [4565.00] = x, 3/2 À y, z; b = [4565.00] = x, 3/2 À y, z.
weak transitions corresponding to HL1(
plex 1 and HL2( ) ? Cu(d ) transitions in complex 2. The intense
bands at 386 and 400 nm for 1 and 2 respectively correspond to
p p) in com-
)/N3À ? Cu(d
p
p
character (Figs. 4 and 8). Other low-lying virtual orbitals have HL1
p⁄)/L2(p⁄) contributions. The b-spin HOMO to HOMO-5 orbitals
have L1/N3À character for 1 with a reduced, 16–17%, Cu(d
) contri-
HL(p
) ? HL(p⁄) transitions, along with a reduced contribution of
(
N3À ? HL(p⁄) (Tables 4 and 5). The electronic spectrum of the free
ligand HL1 shows a band at 345 nm attributed to the n ? p⁄ tran-
sition of the azomethine group. There is a strong band at 280 nm
p
bution in HOMO-4Àand HOMO-5. The b-spin HOMO to HOMO-10
have L2 and/or N3 character, with only 10–13% contribution of
p ?
p⁄ transition.
Cu(dp) are in HOMO-6 and HOMO-8 (Tables 8 and 9 in the supple-
due to a
Fig. 7. Hydrogen bonding interactions in complex 2.
HOMO
LUMO
HOMO-1
HOMO-2
LUMO+2
LUMO+1
Fig. 8. Contour plots of some selected MOs (b-spin) of 2.

S. Ray et al. / Polyhedron 50 (2013) 51–58
57
Table 4
Selected vertical excitations calculated by the TDDFT method for 1.
Eexcitation kexcitation Osc.
Key transition
Character
kexpt.
(eV)
(nm)
strength (f)
(nm)
1.7904 692.5
1.9223 645.0
2.5917 478.4
3.0453 407.1
3.8335 323.4
4.1727 297.1
0.0257
(37%)HOMO-
2(b) ? LUMO(b)
(35%)HOMO-
L1(
p)/
N3À ? Cu(d
p
p
p
)
)
)
3(b) ? LUMO(b)
0.0134
0.0652
0.1552
0.0956
0.1762
(42%)HOMO-
1(b) ? LUMO(b)
(31%)HOMO-
L1(
p)/
607
N3À ? Cu(d
1(b) ? LUMO+1(b)
(43%)HOMO-
3(b) ? LUMO+1(b)
(27%)HOMO-
L1(
p)/
N3À ? Cu(d
2(b) ? LUMO+1(b)
(47%)HOMO(
LUMO(
(35%)HOMO-
a
) ?
L1(
p)/
386
a
)
N3À ? L1(p⁄
)
1(b) ? LUMO+2(b)
(42%)HOMO-
6(b) ? LUMO+2(b)
(30%)HOMO-
L1(
L1(
p
) ? L1(p⁄
)
)
Fig. 9. Cyclic voltammogram of complex 2.
7(b) ? LUMO+2(b)
(67%)HOMO-
6(a) ? LUMO(a)
p
) ? L1(p⁄
sharp peak at 2069 cmÀ1 for 2 is assigned as the l1,1 azide bridge
[43,44]. The strong mC@N bands occurring at 1619 and 1610 cmÀ1
for 1 and 2 respectively are shifted considerably towards lower fre-
quencies compared to that of the free Schiff base ligand
(1635 cmÀ1), indicating coordination of the imino nitrogen atom
[45]. In 2, the presence of a very broad band at ca. 1100 cmÀ1 (from
1050 to 1150 cmÀ1) and a sharp one at 625 cmÀ1 indicates that the
perchlorate ion still has Td symmetry and is not, therefore, coordi-
nated to the metal ion.
Table 5
Selected vertical excitations calculated by the TDDFT method for 2.
Eexcitation kexcitation Osc.
Key transition
Character
kexpt.
(nm)
(eV)
(nm)
strength
(f)
1.6885
734.3
0.0074
(51%)HOMO-
2(b) ? LUMO+1(b)
(23%)HOMO-
L2(
p
p
) ? Cu(d
) ? Cu(d
) ? Cu(d
p)
p)
p)
4. Conclusion
3(b) ? LUMO+1(b)
Pyrimidine based tridentate ligands HL1 and HL2 have been
used to synthesize a binuclear Cu(II) complex (1) and a trinuclear
Cu(II) complex (2), with Cu(II) perchlorate as the metal salt and
using azide as a bridging ligand. The ligand HL1 forms a binuclear
phenoxo bridged neutral complex with terminal azide ligands,
but the ligand HL2 forms a trinuclear complex in which two types
of bridging have been observed. In 2, the two terminal CuL2 centers
are connected to the central CuL2 center by two l1,1 bridging
2.0707
598.8
0.0123
(57%)HOMO-
1(b) ? LUMO(b)
(31%)HOMO-
L2(
612
400
1(b) ? LUMO+1(b)
2.3324
2.8653
531.6
432.7
0.0153
0.0577
(59%)HOMO-
5(b) ? LUMO(b)
L2(
p
p
(43%)HOMO-
L2(
) ? L(p⁄
)
4(b) ? LUMO+3(b)
(27%)HOMO-
5(b) ? LUMO+3(b)
azides and a
l3-phenoxo oxygen. Each Cu(II) center contains a li-
gand molecule and the phenoxo oxygen atom of the ligand take
3.1215
397.2
0.1105
(39%)HOMO-
L2(p
)/
part in bonding. The phenoxo oxygen atom of the ligand molecule
8(b) ? LUMO+3(b) N3À ? L(p⁄
(33%)HOMO-
)
becomes the
l3-oxygen, O1B, binding the central Cu(II) center by
bridges to the two terminal Cu(II) centers, producing a TBP geom-
etry around the metal ion. This interesting fact is because of the
plasticity of the Cu(II) geometry, which forces the bonding atoms
around the metal center to utilize their variable bonding modes.
9(b) ? LUMO+3(b)
3.6. Electrochemistry
Acknowledgements
The study of the electrochemical behavior of complex 2 was
carried out in the positive range. Fig. 9 displays a representative
cyclic voltammogram of 2. The complex 2 exhibits a irreversible
reduction peak (E1/2) in DMF solution near +0.55 V versus SCE
Sangita Ray acknowledges the University Grants Commission
(UGC), New Delhi, India for financial assistance. Financial support
[Sanc. No. F.5-1/2007(BSR)/5-16/2007(BSR), dated 12.02.2006]
from the Department of Science & Technology (DST), New Delhi,
India, is also gratefully acknowledged.
due to the Cu(II)/Cu(I) couple and the
DEpa value of 143 mV sug-
gests a irreversible one electron process.
3.7. Infrared spectroscopy
Appendix A. Supplementary data
The infrared spectra of the complexes are consistent with the
structural data given in this paper. The band at 2030 cmÀ1 for 1
is attributed to the terminal azide (N3À) group, while a strong
CCDC 866724 and 866725 contain the supplementary crystallo-
graphic data for 1 and 2. These data can be obtained free of charge
via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the

58
S. Ray et al. / Polyhedron 50 (2013) 51–58
[21] S. Thakurta, C. Rizzoli, R.J. Butcher, C.J. Gomez-Garcia, E. Garribba, S. Mitra,
Inorg. Chim. Acta 363 (2010) 1395.
[22] S. Gupta, S. Pal, A.K. Barik, S. Roy, A. Hazra, T.N. Mandal, R.J. Butcher, S.K. Kar,
Polyhedron 28 (2009) 711–720.
[23] S. Gupta, S. Pal, A. Barik, A. Hazra, S. Roy, T.N. Mandal, S.M. Peng, G.H. Lee,
M.S.El. Fallah, J. Tercero, S.K. Kar, Polyhedron 27 (2008) 2519.
[24] G.M. Kosolapoff, C.H. Roy, J. Org. Chem. 26 (1961) 1895.
[25] M.P.V. Boarland, J.F.W. McOmie, R.N. Timms, J. Chem. Soc. 915 (1952) 4691.
[26] S. Roy, T.N. Mandal, A.K. Barik, S. Gupta, R.J. Butcher, M. Nethaji, S.K. Kar,
Polyhedron 27 (2008) 593.
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail:
deposit@ccdc.cam.ac.uk. Supplementary data associated with this
article can be found, in the online version, at http://dx.doi.org/
10.1016/j.poly.2012.09.061.
References
[27] SAINT, Version 6.02, SADABS, Version 2.03; Bruker AXS Inc., Madison, WI, 2002.
[28] CRYSALIS, Oxford Diffraction Ltd., Abingdon, UK, 2006.
[29] G.M. Sheldrick, Acta Crystallogr., Sect. A 64 (2008) 112.
[30] ABSPACK, Oxford Diffraction Ltd., Oxford, UK, 2005.
[31] A.D. Becke, J. Chem. Phys. 98 (1993) 5648.
[32] P.J. Stevens, J.F. Devlin, C.F. Chabalowski, M.J. Frisch, J. Phys. Chem. 98 (1994)
11623.
[1] S. Sarkar, A. Mondal, D. Chopra, J. Ribas, K.K. Rajak, Eur. J. Inorg. Chem. 45
(2006) 3510.
[2] S. Thakurta, J. Chakraborty, G. Rosair, J. Tercero, Inorg. Chem. 47 (2008) 6227.
[3] E. Ruiz, J. Cano, S. Alvarez, P. Alemany, J. Am. Chem. Soc. 120 (1998) 11122.
[4] C. Adamo, V. Barone, A. Bencini, F. Totti, I. Ciofini, Inorg. Chem. 38 (1999) 1996.
[5] P. Comba, P.W. Hambley, Molecular Modelling of Inorganic and Coordination
Compounds, second ed., VCH, Weinheim, 2001. pp. 161–162.
[33] C.T. Lee, W.T. Yang, R.G. Parr, Phys. Rev. B, 37 (1988) 785–789.
[34] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman,
J.A. Montgomery Jr., T. Vreven, K.N. Kudin, J.C. Burant, J.M. Millam, S.S. Iyengar,
J. Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G.A.
Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa,
M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J.E. Knox,
H.P. Hratchian, J.B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R.E.
Stratmann, O. Yazyev, A.J. Austin, R. Cammi, C. Pomelli, J.W. Ochterski, P.Y.
Ayala, K. Morokuma, G.A. Voth, P. Salvador, J.J. Dannenberg, V.G. Zakrzewski, S.
Dapprich, A.D. Daniels, M.C. Strain, O. Farkas, D.K. Malick, A.D. Rabuck, K.
Raghavachari, J.B. Foresman, J.V. Ortiz, Q. Cui, A.G. Baboul, S. Clifford, J.
Cioslowski, B.B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R.L.
Martin, D.J. Fox, T. Keith, M.A. Al-Laham, C.Y. Peng, A. Nanayakkara, M.
Challacombe, P.M.W. Gill, B. Johnson, W. Chen, M.W. Wong, C. Gonzalez, J.A.
Pople, GAUSSIAN 03, Revision D.01, GAUSSIAN Inc., Wallingford, CT, 2004.
[35] P.J. Hay, W.R. Wadt, J. Chem. Phys. 82 (1985) 270.
[36] R. Bauernschmitt, R. Ahlrichs, Chem. Phys. Lett. 256 (1996) 454.
[37] R.E. Stratmann, G.E. Scuseria, M.J. Frisch, J. Chem. Phys. 109 (1998) 8218.
[38] M.E. Casida, C. Jamorski, K.C. Casida, D.R. Salahub, J. Chem. Phys. 108 (1998)
4439.
[39] V. Barone, M. Cossi, J. Phys. Chem. A 102 (1998) 1995.
[40] M. Cossi, V. Barone, J. Chem. Phys. 115 (2001) 4708.
[41] M. Cossi, N. Rega, G. Scalmani, V. Barone, J. Comput. Chem. 24 (2003) 669.
[42] N.M. O’Boyle, A.L. Tenderholt, K.M. Langner, J. Comput. Chem. 29 (2008) 839.
[43] K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination
Compounds, fifth ed., Wiley, New York, 1997. pp. 59–60.
[44] P. Govindaswamy, Hemant P. Yennawar, Mohan Rao Kollipara, J. Organomet.
Chem. 689 (2004) 3108.
ˇ ˇ
[6] M.K. Milcic, V.B. Medakovic, D.N. Sredojevic, N.O. Juranic, S.D. Zaric, Inorg.
Chem. 45 (2006) 4755.
[7] Q. Ye, D.-W. Fu, H. Tian, R.-G. Xiong, P.W.H. Chen, S.D. Huang, Inorg. Chem. 47
(2008) 772.
[8] J.S. Casas, E.E. Castellans, M.D. Louce, J. Ellena, A. Sanchez, J. Sordo, C. Taboada,
J. Inorg. Biochem. 11 (2006) 1858.
[9] D.X. West, A.E. Liberta, S.B. Padhye, R.C. Chilate, P.B. Sonawane, A.S. Kumbhar,
R.G. Yerande, Coord. Chem. Rev. 123 (1993) 49.
[10] M.C. Rodriguez-Arguelles, S.J. Pinelli, J. Inorg. Biochem. 58 (1995) 157.
[11] J.S. Casas, M.S. Garcia-Tasende, C. Maichel-Mossmer, M.C. Rodriguez-Arguelles,
A. Sanchez, J. Sordo, A. Vazquez-Lopez, S. Pinelli, P. Lunghi, R. Albertini, J. Inorg.
Biochem. 62 (1996) 41.
[12] M.B. Ferrari, G.G. Fava, G. Tarasconi, R. Albertini, S. Pinelli, R. Starcich, J. Inorg.
Biochem. 53 (1994) 13.
[13] U. Koch, B. Attenni, S. Malancona, S. Colarusso, I. Conte, M.D. Filippo, S. Harper,
B. Pacini, C. Giomini, S. Thomas, I. Incitti, L. Tomei, R.D. Francesco, S. Altamura,
V.G. Matassa, F. Narjes, J. Med. Chem. 49 (2006) 1693.
[14] F. Zamora, M. Kunsman, M. Sabat, B. Lippart, Inorg. Chem. 36 (1997) 1583.
[15] F. Jolibois, J. Cadet, A. Grand, R. Subra, N. Raga, V. Barone, J. Am. Chem. Soc. 120
(1998) 1864.
[16] A.R. Katritzky, C.W. Pees, A.J. Boulton, C. Mckillop, J. Heterocycl. Chem. 3
(1984) 57.
[17] D.J. Spira-Solomon, M. Allendorf, E.L. Solomon, J. Am. Chem. Soc. 108 (1986)
5318.
[18] P. Kyritsis, A. Messerschmidt, R. Huber, G.A. Salmon, A.G. Sykes, J. Chem. Soc.,
Dalton Trans. 5 (1993) 731.
[19] M. Tremdiere, T.B. Bieth, Phytochemistry 23 (1983) 501.
[20] A. Rompel, H. Fischer, K. Büldt-Karentzopoulos, D. Meiwes, F. Zippel, H.F.
Nolting, C. Hermes, B. Krebs, H. Witzel, J. Inorg. Biochem. 59 (1995) 715.
[45] L. Xue, Q. Liu, H. Jiang, Org. Lett. 11 (2009) 3454.