Two new copper(II) complexes with the shortest (N-N) diazine based rigid ligand: Example of unusual tridentate coordination mode

Article abstract of DOI:10.1016/j.molstruc.2006.04.040

Two new five coordinated Cu(II) complexes, Cu(L)Cl₂,CH₃OH (1) and Cu(L)Br₂ (2) derived from the flexidentate ligand (L), 2-pyridinealdazine, have been synthesised and characterised by spectroscopic and electrochemical studies. Single crystal structures of the complexes were determined. Crystal structures of both the complexes contain monomeric entities of five coordinated copper(II) ions where the Schiff base ligand, 2-pyridinealdazine, acts in a tridentate fashion. The central part of the ligand in complex 2 is disordered over two positions: N8{single bond}N9 make up the major position and N8A{single bond}N9A make up the minor position.

Full text of DOI:10.1016/j.molstruc.2006.04.040

Journal of Molecular Structure 826 (2007) 75–81
www.elsevier.com/locate/molstruc
Two new copper(II) complexes with the shortest (N–N) diazine
based rigid ligand: Example of unusual tridentate coordination mode
a
a
b
a,
*
Ruma Karmakar , Chirantan Roy Choudhury , Stuart R. Batten , Samiran Mitra
a
Department of Chemistry, Jadavpur University, Kolkata – 700 032, India
b
School of Chemistry, PO Box 23, Monash University 3800, Australia
Received 20 February 2006; received in revised form 19 April 2006; accepted 19 April 2006
Available online 14 August 2006
Abstract
Two new five coordinated Cu(II) complexes, Cu(L)Cl2,CH3OH (1) and Cu(L)Br2 (2) derived from the flexidentate ligand (L), 2-pyri-
dinealdazine, have been synthesised and characterised by spectroscopic and electrochemical studies. Single crystal structures of the com-
plexes were determined. Crystal structures of both the complexes contain monomeric entities of five coordinated copper(II) ions where
the Schiff base ligand, 2-pyridinealdazine, acts in a tridentate fashion. The central part of the ligand in complex 2 is disordered over two
positions: N8AN9 make up the major position and N8AAN9A make up the minor position.
ꢀ 2006 Elsevier B.V. All rights reserved.
Keywords: Copper(II) monomers; Flexidentate Schiff base ligand; Crystal structure; Spectral studies; Electrochemical properties
1. Introduction
studies [9–13] showed that such open chain diazine ligands
present several possible mono- and dinucleating coordina-
tion modes due to the flexibility of the ligand around the
N–N single bond. In combination with other donors such
ligand can form several types of, e.g. mono- [13–16], di-
[17–23], tri- [24] and tetranuclear copper(II) complexes
[25,26].
The simplest bis-pyridylimine ligand system is 2-pyri-
dinealdazine (L) in which two pyridylimine binding units
are linked directly (no spacer unit) through the imine nitro-
gen atoms [27].
Many copper(II) complexes are generally biologically
active due to their chelating ability and positive redox
potential. In particular, some copper(II) complexes posses
a wide range of biological activity as antivirals, fungicides,
pesticides and even tracers to medicine [1–4] depending on
the ligand binding sites.
In order to design species presenting specific structural
and functional features, it is of great importance to estab-
lish the rules by which control of the self-assembly process
can be achieved through chemical programming of suitable
components and assembling algorithms [5]. Dinucleating
diazine ligands containing a single N–N bond have
received considerable attention over the years beginning
with the early work of Butch on pyridinealdazine (PAA)
[6,7]. Compared with the diazine moiety in heterocyclic
ring systems the N2 diazine linkages in open chain systems
with N–N single bonds are much more flexible [8]. Previous
The different possible arrangements of L, 2-pyridineald-
azine are shown in Scheme 1.
The binuclear and helical composition are consistent
with the coordination abilities of E–E or Z–E, respectively
[13]. Stratton and Busch described the coordination of L to
octahedral transition metal ions to form complexes with
the formula [M2(L)3] (M = Co, Fe, Ni and L = 2-pyridylm-
ethylketazine) and recognised that the species must consist
of three stands wrapped around two metals in a spiral fash-
ion [6,7,9,28,29]. The authors proposed the term ‘‘flexiden-
tate’’ to describe the coordination behaviour of this type of
ligand.
*
Corresponding author. Tel.: +91 33 2668 4193; fax: +91 33 2414 6266.
E-mail address: smitra_2002@yahoo.com (S. Mitra).
0022-2860/$ - see front matter ꢀ 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2006.04.040

76
R. Karmakar et al. / Journal of Molecular Structure 826 (2007) 75–81
N
H
N
N
N
N
H
N
N
H
N
N
N
H
N
H
N
H
E-E
Z-E
Z-Z
Scheme 1.
No attempt has been made to isolate the various stereo-
acetonitrile as solvent and tetrabutyl ammonium perchlorate
as supporting electrolyte at a scan rate of 50 mV sÀ1. Mag-
netic susceptibility data were collected on a model 155 PAR
vibrating sample magnetometer fitted with a waker scientific
175 FBAL magnet using Co[Hg(SCN)4] as the standard.
isomers of bis-2-pyridylmethylaldazine or ketazine before
reacting the ligand with the copper(II) ions. Previously,
other workers have unsuccessfully attempted to separate
the stereoisomers and come to the conclusion that the sta-
ble form of the ligand is E–E. The Z–E isomer can be gen-
erated by a template rearrangement reaction of solvated
ligand and solvated metal ion. Z–E isomer is only stable
when coordinated to a metal centre [9,10,28,29]. Another
recent study by Dong showed a novel triangular Ni(II)
complex of the 2-pyridinealdazine ligand, where the imine
based ligand behaves as a rigid one [30].
In some cases, the spiral or helical complexes are shown
to undergo exchange reactions to form mononuclear com-
plexes [ML2]2+ (L = 2-pyridinealdazine) in which the
ligand twists to coordinate as a tridentate one but this phe-
nomenon is still not been well explored [27].
2.3. Preparation of the ligand and complexes
2.3.1. Synthesis of the Schiff base ligand (L)
A methanolic solution (20 ml) of pyridine-2-carboxalde-
hyde (1.90 ml, 20 mmol) was mixed with a methanolic solu-
tion (10 ml) of hydrazine hydrate (0.50 ml, 10 mmol). The
resulting mixture was stirred at room temperature for half
an hour when yellow crystalline solid separated out. The
yellow solid Schiff base ligand was filtered and recrystal-
lised from a methanol-acetonitrile (1:1) mixture.
In this paper, we report the synthesis, spectral character-
isation, electrochemical studies and crystal structure of two
new unusual five coordinated Cu(II) complexes, CuL-
Cl2,CH3OH(1) and CuLBr2(2) derived from the flexiden-
tate ligand L, 2-pyridinealdazine which coordinates to the
metal ion in a rare Z–E fashion.
2.3.2. Synthesis of CuLCl2ÆCH3OH (1)
To a methanolic solution of copper(II) chloride (0.170 g,
1 mmol), methanolic solution of the Schiff base ligand
(0.210 g, 1 mmol) was added with constant stirring. The
resulting deep green solution was stirred for 10 min at
room temperature. After 3 days green crystals suitable
for X-ray diffraction were collected and rest were filtered
and air-dried.
2. Experimental
Yield: 65 %,
2.1. Materials
Anal. Found: C, 41.36; H, 3.70; N, 14.79; Cu, 16.85;
calc. for C13H14Cl2Cu N4O, C, 41.41; H, 3.72; N, 14.86;
Cu, 16.89 %.
Reagent grade pyridine 2-carboxaldehyde, hydrazine
hydrate, copper(II) chloride and copper(II) bromide
(Fluka) were obtained commercially. All the solvents used
were of reagent grade.
2.3.3. Synthesis of CuLBr2 (2)
Complex 2 was synthesised following the same proce-
dure as in complex 1, except using copper(II) bromide
instead of copper(II) chloride.
2.2. Measurements
Yield: 60 %,
Infrared spectra (4000–350 cmÀ1) and far-IR spectra
(400–50 cmÀ1) of complexes 1 and 2 were recorded on a
Perkin-Elmer FT-IR and FTIR Bomen DA 8.3 spectropho-
tometers, respectively, and electronic spectra on a Perkin-El-
mer Lambda-40 spectrophotometer using acetonitrile as
solvent. Elemental analyses were carried out on a Perkin-
Elmer 2400-II instrument. Electrochemical studies were
performed on a CH 600A cyclic voltameter instrument using
Anal. Found: C, 33.20; H, 2.27; N, 12.88; Cu, 14.62;
calc. for C12H10Br2CuN4, C, 33.21; H, 2.31; N, 12.91;
Cu, 14.68 %.
2.4. X-ray data collection and structure refinement
Single crystal data and details of the structure determi-
nations for complexes 1 and 2 are presented in Table 1.

R. Karmakar et al. / Journal of Molecular Structure 826 (2007) 75–81
77
Table 1
Crystallographic data for complex 1 and 2
1
2
Emperical formula
Formula weight
Crystal system
Space group
C13H14Cl2CuN4O
376.72
Monoclinic
P2(1)/c
C12H10Br2CuN4
433.60
Monoclinic
P2(1)/n
˚
a (A)
7.2349(2)
8.1278(6)
14.879(1)
11.2331(7)
92.698(3)
1356.95(16)
4
2.122
836
7.488
123(2)
˚
b (A)
13.8531(3)
14.7814(4)
90.967(1)
1481.27(7)
4
1.689
764
1.838
123(2)
˚
c (A)
b (ꢁ)
Cell volume (A )
3
˚
Z
Dc (Mg mÀ3
F(000)
)
l (mmÀ1
T (K)
)
˚
k (A)
0.71073
0.71073
Theta range (ꢁ)
Index ranges
3.25–27.89
À9 <= h <= 7
À18 <= k <= 18
À19 <= l <= 19
0.8745 and 0.6086
18501
3494 [R(int) = 0.0790]
2614
3494/0/181
R1 = 0.0712, wR2 = 0.1692
R1 = 0.0985, wR2 = 0.1861
3.17–27.86
À10 <= h <= 10
À19 6 k 6 19
À14 6 l 6 14
0.7059 and 0.1913
13700
3198 [R(int) = 0.1125]
2225
3198/2/190
Max. and min. transmission
Total no. of reflections measured
Total no. of unique reflections
No. of observed reflections (I > 2r(I))
Data/restraints/parameters
Final R indices (obs. data)
Final R indices (all data)
R1 = 0.0781, wR2 = 0.2051
R1 = 0.1163, wR2 = 0.2346
Data were collected on a green-blue dichroic block
(0.08 · 0.15 · 0.30 mm) for 1 and on a dark red plate
(0.05 · 0.33 · 0.33 mm) for 2 at 123 K on a Nonius Kap-
paCCD diffractometer with graphite monochromated
the ligand PAA is tridentate or tetradentate, forming
mononuclear complexes with five membered and six mem-
bered chelate rings. The tridentate chelation of the ligand is
accompanied by a distinct spliting of the pyridine ring
breathing vibration (which appear in our case at 1021
and 1010 cmÀ1) and of the C–N stretching mode of the
azine link (at 1628 and 1584 cmÀ1). This also proves that
all the pyridine rings are coordinating. The absorption
bands at 440 and 340 cmÀ1 can be assigned to the m(Cu–
N) modes where N corresponds to the azomethylene and
the pyridine nitrogens, respectively [16].
˚
Mo-Ka radiation (k = 0.71073 A), using / and x rotations
with 1ꢁ frames. The images were processed with the HKL
suite of programs [31]. Solutions were obtained using
SHELXS-97 [32] followed by successive difference Fourier
methods, and structures were refined against F2 using
SHELXL-97 [32a]. Face-indexed absorption corrections
were applied [32b]. All non-hydrogen atoms were refined
anisotropically. For complex 1 all hydrogen atoms were
assigned to calculated positions except the methanol
hydroxy proton, which was neither found nor assigned.
For complex 2 the central part of the ligand is disor-
dered over two positions: N8-N9 make up the major posi-
tion (60%), with N8A-N9A making up the minor position
(40%). All non-hydrogen atoms were refined anisotropical-
ly. Aromatic hydrogen atoms were assigned to calculated
positions and not refined. The hydrogen atoms bonded to
C7 and C10 were refined with the restraint that they were
For complex 1, the absorption band appearing above
3000 cmÀ1 corresponds to the m(OH) stretching vibration
[37]. Above 1635 cmÀ1, two strong m(C@N) bands are
observed for both the complexes 1 and 2.
Far infrared bands at 276 and 253 cmÀ1 for complexes 1
and 2, respectively, suggest the presence of terminal CuACl
or CuABr bonds, respectively.
3.2. Electronic spectra
˚
0.84(2) A from the corresponding carbons, while hydro-
The electronic spectra of complexes 1 and 2 in acetoni-
trile show bands in the region 210–345, 380–470 and
720 nm corresponding to intraligand charge transfer and
d–d transitions, respectively.
gens were not assigned to the disordered nitrogen atoms.
3. Results and discussion
The bands at 215 and 222 nm (for 1 and 2) are attribut-
3.1. Infrared spectra
able to the ring p–p transitions [38], while absorptions in
*
the 275–295 and 325–340 nm ranges for both 1 and 2 are
Stratton and Busch [33–36] have relied on the C–N
stretching region in the IR spectrum to determine whether
assignable to the n fi p transitions of the pyridine ring
*
[39]. The broad bands at 719 and 722 nm for 1 and 2,

78
R. Karmakar et al. / Journal of Molecular Structure 826 (2007) 75–81
respectively, are consistent with the effective five coordinat-
ed geometries observed at the copper(II) centres [40].
3.4. Magnetic studies
The room temperature magnetic moments for the com-
pounds 1 and 2 are 1.90 and 1.99 BM, respectively. The
results are consistent with the presence of one unpaired
electron in the Cu(II) (d9) system.
3.3. Electrochemical studies
Electrochemical behaviours of both the complexes are
very similar. Fig. 1 represents the cyclic voltammogram
of the complex 1. Both voltammograms show only one
reductive response on the negative side of SCE (at
À1.24 V for complex 1, À1.27 V for complex 2), both of
which are irreversible in nature. These are due to the Cu(II)
to Cu(I) redox couple.
Similarly, irreversible oxidation responses are also
observed on the positive side of SCE for both 1 and 2 (at
1.51 and 1.54 V, respectively) and these are tentatively
assigned to oxidation of the coordinated ligands.
3.5. Description of crystal structures
3.5.1. Crystal structure of complex 1
The crystal structure of complex 1 contains monomeric
entities of five coordinated copper(II) ions and one metha-
nol molecule present in the lattice. A view of the molecular
structure with atom numbering scheme is shown in Fig. 2
and selected bond lengths and angles are given in Table
2. The copper(II) ion is surrounded by three nitrogens
˚
˚
[the N1 (2.039(5) A), N8b (1.997(6) A) and N16
˚
(2.018(5) A) atoms of the C5H4NC@NAN@CNH4C5
ligand], a terminal chloride ion Cl1 (2.5913(15) A) in the
˚
basal plane, and another terminal chloride ion Cl2
˚
(2.2662(13) A) in an apical position.
Distortions for the coordination polyhedron from
extreme square–pyramidal (SPY) and trigonal–bipyramidal
(TBPY) topologies have been analysed by using the method
of Addison et al. [41]. The value of s = 0.122 clearly indi-
cates that the environment of the copper(II) ion is close to
SPY topology. Deviation from ideal SPY geometry is indi-
cated by the difference in cisoid (89.9(3)ꢁ to 94.55(14)ꢁ) and
transoid angles (161.4(2)ꢁ to 168.8(2)ꢁ). Values are compa-
rable to those found in similar systems [13,36]. The N1,
N8b, N16 and Cl2 atoms are displaced 0.117, 0.110, 0.072
˚
and 0.078 A, respectively, from the mean basal plane and
˚
copper(II) ion sits 0.229 A above the plane.
The methanol molecules hydrogen bond in centrosym-
˚
metrically related pairs (O1Á Á ÁO1 = 2.69(3) A). Each meth-
anol molecule also makes a weak interaction to adjoining
˚
Fig. 1. Cyclic voltammogram of complex 1.
Cl1 ligands (O1Á Á ÁCl1 = 3.15(2) A). The complex pack in
Fig. 2. A view of complex 1 with atom numbering scheme.

R. Karmakar et al. / Journal of Molecular Structure 826 (2007) 75–81
79
Table 2
Selected bond lengths (A) and angles (ꢁ) for complex 1
˚
Cu(1)AN(8b)
Cu(1)AN(1)
Cu(1)ACl(1)
1.997(6)
Cu(1)AN(16)
Cu(1)ACl(2)
2.018(5)
2.2662(13)
2.039(5)
2.5913(15)
89.9(3)
N(8b)ACu(1)AN(16)
N(16)ACu(1)AN(1)
N(16)ACu(1)ACl(2)
N(8b)ACu(1)ACl(1)
N(1)ACu(1)ACl(1)
N(1)AC(6)AC(7)AN(8b)
C(7)AN(8b)AN(9b)AC(10)
N(9b)AC(10)AC(11)AN(16)
N(8b)ACu(1)AN(1)
N(8b)ACu(1)ACl(2)
N(1)ACu(1)ACl(2)
N(16)ACu(1)ACl(1)
Cl(2)ACu(1)ACl(1)
C(6)AC(7)AN(8b)AN(9b)
N(8b)AN(9b)AC(10)AC(11)
80.7(4)
168.8(2)
94.55(14)
94.92(16)
91.97(14)
À0.4(7)
161.4(2)
92.50(16)
94.98(13)
102.59(5)
179.5(5)
0(1)
À172.6(5)
À3(1)
Fig. 3. Packing of complex 1 view down a axis (hydrogens omitted for clarity).
centrosymmetrically related pairs ‘back-to-back’ (Fig. 3),
such that the Cl2 atoms of each complex make a long inter-
The ligand is disordered over two positions in the crystal
structure: one position involving C7@N8AN9@C10, and
the other position containing C7@N8aAN9a@C10. The
copper(II) ion is surrounded by two bromide ions, Br1
˚
action to the adjoining copper atom (Cu1Á Á ÁCl2 = 3.304 A)
trans to the axial Cl1 atoms. These pairs then stack in such
a way that the pyridyl rings overlap in a p–p stacking fash-
˚
˚
(2.5103(15) A) and Br2 (2.5074(15) A), and one nitrogen
˚
˚
˚
ion (closest CÁ Á ÁC contact is 3.40 A).
from the ligand, N8a (2.08(2) A) or N9 (2.016(14) A),
which are in the equatorial positions. The apical sites are
As seen in Table 2, the bond distances of the ligand in
complex 1 are similar to those found in the free ligand or
in similar systems [42–44] where the organic portion is sta-
bilised as a neutral species.
˚
occupied by two nitrogen atoms, N1(1.992(8) A), and
N16 (2.005(7) A), of the C5H4NC@NAN@CNH4C5 ligand
˚
forming a trigonal–bipyramidal geometry.
The distortions for the coordination polyhedron from
the extreme square–pyramidal (SPY) and trigonal–bipyra-
midal (TBPY) topologies have been analysed by using the
method of Addison et al. [41]. The value of s = 1.02 clearly
indicates that the environment of the copper(II) ion is close
to TBP topology. The deviation from ideal TBP geometry
is indicated by the difference in cisoid (106.59(5)ꢁ to
3.5.2. Crystal structure of complex 2
The
NAN@CNH4C5)], complex 2 contains monomeric entity
of five coordinated copper(II) ion. A view of the molecular
structure is shown in Fig. 4. The selected bond lengths and
angles are given in Table 3.
crystal
structure
of
[CuBr2(C5H4NC@

80
R. Karmakar et al. / Journal of Molecular Structure 826 (2007) 75–81
Fig. 4. A view of complex 2 with atom numbering scheme.
Table 3
˚
Selected bond lengths (A) and angles (ꢁ) for complex 2
Cu(1)AN(1)
Cu(1)AN(9)
Cu(1)ABr(2)
1.992(8)
Cu(1)AN(16)
Cu(1)AN(8A)
Cu(1)ABr(1)
N(1)ACu(1)AN(9)
N(1)ACu(1)AN(8A)
N(9)ACu(1)AN(8A)
N(16)ACu(1)ABr(2)
N(8A)ACu(1)ABr(2)
N(16)ACu(1)ABr(1)
N(8A)ACu(1)ABr(1)
2.005(7)
2.08(2)
2.5103(15)
93.4(5)
72.8(7)
21.0(6)
92.0(3)
136.0(6)
93.3(3)
2.016(14)
2.5074(15)
168.1(4)
75.2(5)
96.1(7)
93.3(2)
133.1(4)
95.4(2)
118.9(4)
106.59(5)
6(3)
N(1)ACu(1)AN(16)
N(16)ACu(1)AN(9)
N(16)ACu(1)AN(8A)
N(1)ACu(1)ABr(2)
N(9)ACu(1)ABr(2)
N(1)ACu(1)ABr(1)
N(9)ACu(1)ABr(1)
Br(2)ACu(1)ABr(1)
N(1)AC(6)AC(7)AN(8)
C(6)AC(7)AN(8)AN(9)
C(7)AN(8)AN(9)AC(10)
N(8)AN(9)AC(10)AC(11)
N(9)AC(10)AC(11)AN(16)
115.9(6)
N(1)AC(6)AC(7)AN(8a)
C(6)AC(7)AN(8)AN(9a)
C(7)AN(8a)AN(9a)AC(10)
N(8a)AN(9a)AC(10)AC(11)
N(9a)AC(10)AC(11)AN(16)
0(1)
À3(3)
À178(1)
172(2)
À11(4)
6(4)
180(1)
179(1)
À4(1)
Fig. 5. Packing diagram of complex 2 view down a axis (b axis horizontal; c axis vertical, hydrogen atoms omitted for clarity).

R. Karmakar et al. / Journal of Molecular Structure 826 (2007) 75–81
81
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˚
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4. Supplementary material
Crystallographic data for structural analysis have been
deposited with the Cambridge Crystallographic Data Cen-
tre, CCDC Nos. 295773 for 1 and 295774 for 2. Copies of
this information may be obtained free of charge from The
Director, CCDC, 12 Union Road, Cambridge, CB2 1EZ,
UK (fax: +44-1223-336033; deposit@ccdc.cam.ac.uk or
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This work was supported by the grants from CSIR, New
Delhi, India, and the Australian Research Council. Our
thanks are also extended to Dr. S. Bhattacharya, Depart-
ment of Chemistry, Jadavpur University, for his valuable
discussion in the cyclic voltametric studies.
Appendix A. Supplementary data
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Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/
j.molstruc.2006.04.040.
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