Benign synthesis of carboxamide ligands, H2Me2bqb and H2Me2bpb. Preparation, characterization and electrochemistry of Ni(II) complexes: The crystal structure of [Ni II(Me2bqb)]

Article abstract of DOI:10.1016/j.poly.2010.04.017

A novel and highly efficient approach for the synthesis of H ₂Me₂bqb and H₂Me₂bpb using ionic liquid as an environmentally benign reaction medium has been developed, eliminating the need for the pyridine as a toxic solvent. The Ni(II) complex of the dianionic ligand Me₂bqb^2-, [Me₂bqb ^2- = 1,2-bis(quinoline-2-carboxamide)-4,5-dimethyl-benzene dianion], has been synthesized and characterized by elemental analyses and spectroscopic methods, and the crystal and molecular structure of [Ni(Me₂bqb)] (1), has been determined by X-ray crystallography. The complex exhibits distorted square-planar NiN₄ coordination geometry with two short and two long Ni-N bonds (Ni-N ～1.85 and ～1.96 , respectively). The electrochemical behavior of [Ni(Me₂bqb)] (1), has been studied by cyclic voltammetry and compared with the analogous complex, [Ni(Me₂bpb)] (2).

Full text of DOI:10.1016/j.poly.2010.04.017

Polyhedron 29 (2010) 2225–2231
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Benign synthesis of carboxamide ligands, H2Me2bqb and H2Me2bpb. Preparation,
characterization and electrochemistry of Ni(II) complexes: The crystal structure
of [NiII(Me2bqb)]
b
a
a
a
Soraia Meghdadi a, , Kurt Mereiter , Ahmad Amiri , Narges S. Mohammadi , Fahimeh Zamani ,
*
Mehdi Amirnasr a,
**
a Department of Chemistry, Isfahan University of Technology, Isfahan 84156-83111, Iran
b Faculty of Chemistry, Vienna University of Technology, Getreidemarkt 9/164SC, A-1060 Vienna, Austria
a r t i c l e i n f o
a b s t r a c t
Article history:
A novel and highly efficient approach for the synthesis of H2Me2bqb and H2Me2bpb using ionic liquid as
an environmentally benign reaction medium has been developed, eliminating the need for the pyridine as
a toxic solvent. The Ni(II) complex of the dianionic ligand Me2bqb2ꢀ, [Me2bqb2ꢀ = 1,2-bis(quinoline-2-
carboxamide)-4,5-dimethyl-benzene dianion], has been synthesized and characterized by elemental
analyses and spectroscopic methods, and the crystal and molecular structure of [Ni(Me2bqb)] (1), has
been determined by X-ray crystallography. The complex exhibits distorted square-planar NiN4 coordina-
tion geometry with two short and two long Ni–N bonds (Ni–N ꢁ1.85 and ꢁ1.96 Å, respectively). The elec-
trochemical behavior of [Ni(Me2bqb)] (1), has been studied by cyclic voltammetry and compared with the
analogous complex, [Ni(Me2bpb)] (2).
Received 27 February 2010
Accepted 17 April 2010
Available online 27 April 2010
Dedicated to Professor Shadpour
Mallakpour of Isfahan University of
Technology, a pioneer in the synthesis of
Step-Growth Polymers such as polyamides
in Ionic Liquids, on the occasion of his 57th
birthday.
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
Ionic liquid
Benign synthesis
Bispyridylamide
Ni(II) complexes
Crystal structure
Cyclic voltammetry
1. Introduction
Ionic liquids (ILs) are novel solvents, attracting interest as
greener alternatives to conventional hazardous solvents with the
Chemical reactions largely are performed in solutions of volatile
organic compounds which are difficult to contain, and are often
flammable or peroxidizable and toxic by inhalation. Other harmful
compounds, such as pyridine, are also used as solvent in the syn-
thesis of organic compounds such as carboxamides [1–5].
Green chemistry searches for alternative, environmentally
friendly reaction media and at the same time strives to increase
the reaction rate and efficiency, and lower the reaction tempera-
ture. One of the key areas of green chemistry is the replacement
of hazardous solvents with environmentally benign ones or the
elimination of solvents altogether [6–11].
aim of facilitating sustainable chemistry [12–17]. However, the
high-cost and the potential risk of toxicity associated with com-
mon room temperature ILs [18–21] has led to the use of more be-
nign salts in the molten state as practical alternatives. Molten TBAB
(tetrabutylammonium bromide), for example, has proved to be an
efficient catalyst in a number of useful synthetic transformations
[22–27]. These reactions catalyzed by molten TBAB are in general,
very fast and clean.
The classical method for the synthesis of carboxamide deriva-
tives, used as ligands, is the reaction of the amines with the appro-
priate carboxylic acids in pyridine in the presence of an activator
such as triphenyl phosphite [1]. Drawbacks of this method include
modest yields, and the health hazards resulting from the use of
pyridine as the reaction solvent. Using this classical method, a
large variety of carboxamido ligands have been synthesized with
the goal of investigating their metal-binding properties [2], and
providing models from the standpoint of bioinorganic chemistry
* Corresponding author. Tel.: +98 311 3913282; fax: +98 311 3912350.
** Corresponding author. Tel.: +98 311 3912351; fax: +98 311 3912350.
E-mail addresses: smeghdad@cc.iut.ac.ir (S. Meghdadi), amirnasr@cc.iut.ac.ir (M.
Amirnasr).
0277-5387/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2010.04.017

2226
S. Meghdadi et al. / Polyhedron 29 (2010) 2225–2231
[28]. These ligands have also been used for asymmetric catalysis
[29–31], dendrimer synthesis [32], design of molecular receptors
[33–35], preparation of metal complexes with antitumor proper-
ties [3,36,37], and controlling the molecular architecture [38,39].
Extensive investigation has been devoted to nickel coordination
chemistry with amide based ligands, and nickel ion binding to pep-
tides has been recently reviewed [40]. Peptides and proteins are
able to form coordination complexes with nickel(II) and copper(II)
through ligation of deprotonated amide nitrogens [41,42]. Such
coordination helps to stabilize the nickel(III) oxidation state, which
may play a key role in the observed DNA strand scission and DNA–
protein cross-links [43,44]. Although many nickel complexes have
been synthesized and their properties studied, it is only recently
that they have found more application in catalysis. Nickel com-
plexes with functionalized bipyridine ligands, potentially tetraden-
tate ligands, have been recently reported to show high catalytic
activity toward norbornene polymerization [30,31].
260°C. Anal. Calc. for C28H22N4O2 (446.50): C, 75.32; H, 4.97; N,
12.55. Found: C, 74.78; H, 4.77; N, 12.33%. ESI-MS: m/z = 469.16
[M+Na]+. FT-IR (KBr, cmꢀ1):
mmax: 3339 (s, N–H), 1688 (s, C@O),
1589 (m, C@C), 1525 (m, C–N). UV–Vis (chloroform): kmax (nm)
(e
, L molꢀ1 cmꢀ1): 330 (15 845), 319 (17 460), 286 (21 440), 241
(90 960). 1H NMR (CDCl3, 500 MHz): d = 2.34 (s, 6H, Me), 7.61 (m,
4H), 7.78 (s, 2H), 7.88 (m, 4H), 8.36 (d, 2H), 8.45 (d, 2H), 10.51
(s, 2H, NH).
2.2.1.2. H2Me2bpb. The H2Mebpb was synthesized by a procedure
similar to that used for H2Me2bqb except that picolinic acid was
used instead of quinaldic acid. The viscous solution precipitated
in 10 mL of 1:1 methanol–water mixture. Yield 78%. Anal. Calc.
for C20H18N4O2 (346.38): C, 69.35; H, 5.24; N, 16.17. Found: C,
69.01; H, 5.18; N, 15.84%. ESI-MS: m/z = 369.13 [M+Na]+. FT-IR
(KBr, cmꢀ1
) mmax: 3328, 3221 (m, NH), 1677, 1666(s, C@O), 1594
(m, C@C), 1510 (m, C–N). UV–Vis: kmax (nm) (e )
, L molꢀ1 cmꢀ1
(CHCl3): 290 (12 250), 268 (14 950), 230 (19 550). 1H NMR (CDCl3,
500 MHz): d = 2.29 [s, 6H, Me], 7.44 (m, 2H), 7.62 (s, 2H), 7.88 (m,
2H), 8.30 (d, 2H), 8.55 (d, 2H), 10.16 (s, 2H, NH).
In an attempt towards the development of new methods for the
synthesis of carboxamide ligands and following our earlier studies
on the synthesis of their complexes, we herein report a new syn-
thetic method for the preparation of H2Me2bqb [3] and H2Me2bpb
[45], replacing the pyridine by TBAB (ionic liquid) as the reaction
media. The synthesis, characterization and properties of the new
complex [NiII(Me2bqb)] (1), are also reported and compared with
those of [NiII(Me2bpb)] (2), the synthesis of which has already been
reported [29]. These results obtained in this work give us an oppor-
tunity to elucidate the effect of the fused benzene ring on the spec-
tral and electrochemical properties of these complexes in going
2.2.2. Synthesis of [Ni(Me2bqb)] (1)
To
a solution of nickel(II) acetate tetrahydrate (24.9 mg,
0.1 mmol) in methanol (20 mL) was added slowly a solution of
H2Me2bqb (44.6 mg, 0.1 mmol) in dichloromethane (20 mL). The
resulting dark red solution was stirred for 8 h. Slow evaporation
of this solution afforded dark red crystals suitable for X-ray crystal-
lography. The crystals were filtered-off and washed with diethyl
ether-dichloromethane-methanol (8:1:1 v/v), and dried in vacuum.
Yield 94%. Anal. Calc. for C28H20N4O2Ni (503.18): C, 66.84; H, 4.01;
N, 11.13. Found: C, 66.87; H, 3.85; N, 11.15%. FT-IR (KBr, cmꢀ1):
from Me2bpb2ꢀ to Me2bqb2ꢀ
.
2. Experimental
m
max: 1626 (s, C@O), 1587 (m, C@C), 1559 (m, C–N). UV–Vis (Chlo-
roform): kmax (nm) (
e
, L molꢀ1 cmꢀ1): 535 (2362) 424 (5838), 352
2.1. Materials and general methods
(19 280), 322 (33 923), 244 (101 675).
All solvents and chemicals were from Merck and Aldrich (Grade
Pro Analysi). IR spectra were measured with a FT-IR JASCO 680
spectrometer using KBr pellets. Elemental analyses were per-
formed by using a Perkin–Elmer 2400II CHNS-O elemental ana-
lyzer. The mass spectra were recorded using a Waters micromass
Q-TOF-2 spectrometer in positive ion mode. UV–Vis spectra were
obtained on a JASCO V-570 spectrophotometer. Cyclic voltammo-
grams were recorded by using a SAMA 500 Research Analyzer.
Three electrodes were utilized in this system, a glassy carbon
working electrode, a platinum disk auxiliary electrode and Ag wire
as reference electrode. The glassy carbon working electrode (Metr-
ohm 6.1204.110) with 2.0 0.1 mm diameter was manually
2.2.3. Synthesis of [Ni(Me2bpb)] (2)
The [Ni(Me2bpb)] complex was synthesized according to the lit-
erature procedure [29]. Anal. Calc. for C20H16N4O2Ni (403.06): C,
Table 1
Crystal data and structure refinement for (1).
Chemical formula
Formula weight
C28H20N4NiO2
503.19
T (K)
100(2)
ꢀ
Crystal system, space group
triclinic, P1
a (Å)
b (Å)
c (Å)
10.2852(10)
10.4527(10)
11.7761(12)
64.578(1)
cleaned with 1 lm alumina polish prior to each scan. Tetrabutyl-
ammonium hexafluorophosphate (TBAH) was used as supporting
electrolyte. The solutions were deoxygenated by purging with Ar
for 5 min. All electrochemical potentials were calibrated versus
internal Fc+/0 couple under the same conditions [46].
a
(°)
b (°)
81.846(1)
c
(°)
69.458(1)
1070.67(18)
2, 1.561
V (Å3)
Z, Dcalc (Mg mꢀ3
)
Crystal size (mm)
(mmꢀ1
F(0 0 0)
0.60 ꢂ 0.30 ꢂ 0.12
l
)
0.943
2.2. Synthesis
520
h Range (°)
Index ranges
2.56–30.0
2.2.1. Synthesis of the ligands
ꢀ14 6 h 6 14, ꢀ14 6 k 6 14,
ꢀ16 6 l 6 16
15 752
6161(0.018)
multi-scan
0.75, 0.89
2.2.1.1. H2Me2bqb. A mixture of 0.62 g (2 mmol) triphenyl phos-
phite (TPP), 0.97 g (3 mmol) tetrabutylammonium bromide
(TBAB), 0.35 g (2 mmol) quinaldic acid, and 0.14 g (1 mmol) 4,5-di-
methyl-1,2-phenylenediamine in a 25 mL round bottom flask was
placed in an oil bath. The reaction mixture was heated until a
homogeneous solution was formed. The solution was stirred for
1 h at 120 °C. The viscous solution was precipitated by adding
20 mL methanol and the resulting white solid was filtered-off
and washed with cold ethanol. The crude product was recrystal-
lized from a mixture of MeOH/CHCl3 (1:1, V/V). Yield 75%. m.p.
Reflections collected
Independent reflections (Rint
Absorption correction
Minimum and maximum
transmission
)
Data/restraints/parameters
6161/0/318
1.058
R1 = 0.028, wR2 = 0.075
R1 = 0.029, wR2 = 0.076
0.57 and ꢀ0.53
Goodness-of-fit (GOF) on F2
Final R indices [F2 > 2
R indices (all data)
Maximum/minimum
r
(F)]
D
q )
(e Åꢀ3

S. Meghdadi et al. / Polyhedron 29 (2010) 2225–2231
2227
Table 2
59.60; H, 4.00; N, 13.90. Found: C, 58.96; H, 3.84; N, 14.08%. FT-IR
(KBr, cmꢀ1
max: 1643 (s, C@O), 1604 (s, C@C), 1564 (m, C–N).
Selected bond lengths (Å) and angles (°) for (1).
)
m
Bond lengths
Ni1–N2
Ni1–N4
Ni1–N1
Ni1–N3
O1–C1
O2–C11
N2–C1
N2–C21
N4–C11
N4–C26
1.8468(10)
2.3. X-ray crystallography
1.8488(10)
1.9595(10)
1.9623(10)
1.2372(14)
1.2375(14)
1.3426(14)
1.4055(14)
1.3372(15)
1.4048(14)
The diffraction data for compound (1) were collected on a Bru-
ker Kappa APEX-2 CCD diffractometer employing graphite-mono-
chromated Mo K
a (k = 0.71073 Å) radiation at T = 100 K. Cell
refinement and data reduction were performed with the help of
program SAINT [47]. Correction for absorption was carried out with
the multi-scan method and program SADABS [47]. The structure was
solved with direct methods and refined on F2 using SHELXTL [47]. All
non-hydrogen atoms were refined anisotropically. Hydrogen
atoms were inserted in idealized positions and were refined riding
with the atoms to which they were bonded. The crystallographic
and refinement data are summarized in Table 1.
Bond angles
N1–Ni1–N2
N3–Ni1–N4
N2–Ni1–N4
N1–Ni1–N3
N1–Ni1–N4
N2–Ni1–N3
C1–N2–C21
C1–N2–Ni1
C21–N2–Ni1
C11–N4–C26
C11–N4–Ni1
C26–N4–Ni1
N2–C21–C26
N4–C26–C21
O1–C1–N2
83.47(4)
83.04(4)
84.09(4)
110.12(4)
165.18(4)
165.66(4)
125.88(10)
117.31(8)
115.57(8)
126.29(10)
117.30(8)
115.53(8)
112.34(10)
112.34(10)
128.71(11)
121.50(10)
109.73(10)
129.23(11)
121.33(10)
109.39(9)
3. Results and discussion
3.1. Synthesis and characterization
3.1.1. Synthesis of the ligands
Since the recognition of ILs as new reaction media and catalysts,
these compounds have been used for replacing hazardous solvents,
reducing reaction time, and increasing yields of products compared
to conventional methods [6–11]. We have used an efficient and be-
nign procedure for the preparation of the two carboxamide ligands,
H2Me2bqb and H2Me2bpb by eliminating the toxic pyridine and
using TBAB as the reaction media. The significant advantages of
using TBAB are to avoid the use of toxic pyridine, drastic reduction
of the reaction time, and a considerable increase in the yields of
products. The optimization of the reaction condition by varying
the reaction temperature, period of heating, and amount of TBAB
were examined to obtain the products in high yield. Interestingly,
in the preparation of H2Me2bqb carried out by this method, the
reaction time reduced from 5 to 1 h and the yield increased from
57% to 75% as compared to the conventional method reported in
the literature [3]. The reaction time for the synthesis of H2Me2bpb
by this method reduced from 4 to 1 h and the yield increased from
47% [45] to 78%.
O1–C1–C2
N2–C1–C2
N4–C11–O2
C12–C11–O2
N4–C11–C12
2
3.1.2. Synthesis of complexes
The new Ni(II) complex of the dianionic ligand, Me2bqb2ꢀ, was
prepared by the reaction of equimolar quantities of the ligand and
nickel acetate in methanol–chloroform solution. Dark needle like
crystals of Ni(Me2bqb) (1), suitable for X-ray structure analysis
were obtained by slow evaporation of the solvent over a period
of one week at room temperature. Ni(Me2bpb) (2), was prepared
according to the published procedure [29].
Fig. 1. ORTEP diagram of complex (1) with the atom labelling scheme.
strong
r donor. The cis angles in the two complexes span a wide
range, 83.04(4)°–110.12(4)° in (1), and 82.75(7)°–111.72(10)° in
(2), which is usual for complexes of H2bpb and H2bqb ligands
[4,5,29,48].
3.2. Description of structure of [Ni(Me2bqb)] (1)
The dihedral angle between the Ni/Nam/Nam0 and Ni/Npy/Npy0
planes at the nickel atom is 8.88(8)° for (1) and 4.24° for (2) [29].
Two pyridyl rings are tilted a lot with the dihedral angle of
25.80(5)° for (1) and 9.42° for (2) [29]. The steric hindrance due
to the quinoline rings in (1) (side-on contact between C9 and
C19 of only 2.914 Å) leads, as expected, to a larger tetrahedral dis-
tortion of the nickel atom than the modest hindrance from the pyr-
idines in (2) (contact distance between ortho-H atoms 2.2 Å). These
differences show some influence on the electronic spectra and the
redox potentials of the two complexes (vide infra).
Table 1 summarizes the crystal data, together with other rele-
vant information on structure determination, for complex (1). Ta-
ble 2 gives significant bond lengths and angles. A perspective
view of (1), with atom labelling scheme, is shown in Fig. 1.
Fig. 2a and b is the packing illustrations. The nickel complex (1)
has a square-planar structure distorted towards a tetrahedron as
evidenced by the N2–Ni1–N3 and N1–Ni1–N4 bond angles of
165.66(4)° and 165.18(4)°. The average N(amide)–Ni bond length
of 1.848(1) Å in (1) and 1.920(2) Å in (2) [29] agrees with those re-
ported (1.82–2.02 Å) for other deprotonated N(amide)–Ni bonds
[4] and is significantly shorter than the N(quinoline/pyridine)–Ni
bond length of 1.961(1) Å in (1) and 2.02(2) in (2) [29]. This is in
accord with the fact that the deprotonated amide nitrogen is a very
The geometrical properties of the two complexes in the solid
state are comparable to the parent complexes [Ni(bpb)] [48,49],
[Ni(bqb)] [4], [Ni(Mebpb)], [Ni(Mebqb)] [50] and their relatives
[Co(bpb)] [51], [Cu(bpb)] [52], and [Cu(Mebpb)] [38].

2228
S. Meghdadi et al. / Polyhedron 29 (2010) 2225–2231
Fig. 2. Packing diagrams of (a) [Ni(Me2bqb)] (1) viewed down the b-axis of the triclinic primitive unit cell (black parallelogram), and (b) [Ni(bqb)] [4] viewed along the c-axis
of the monoclinic C-centered unit cell. In diagram (a) the orange parallelogram outlines the triclinic C-centered unit cell (see text) and the pink lines on the left symbolize the
two different kinds of p–p-stacking interactions between the quinoline moieties. (For interpretation of the references to colour in this figure legend, the reader is referred to
the web version of this article.)
The architecture of the crystal structure of [Ni(Me2bqb)] (1)
shows interesting relationships with its homologues [Ni(bqb)] [4]
and [Ni(Mebqb)] [50]. The parent compound [Ni(bqb)] crystallizes
in a monoclinic C-centered lattice of space group C2/c, a = 17.425 Å,
b = 12.968 Å, c = 9.726 Å, b = 115.28°, V = 1987.4 Å3, Z = 4 [4]. The
[Ni(bqb)] complexes adopt a crystallographic C2 symmetry and
are aligned with their longest extension parallel to the b-axis.
ship to the parent lattice of [Ni(bqb)], which becomes evident by
using a unit cell with following new base vectors: anew = a + c,
bnew = c ꢀ a, cnew = ꢀb (cf. Fig. 2). This new unit cell has space
group symmetry C1 and cell dimensions a0 = 16.698 Å,
ꢀ
b0 = 14.495 Å, c0 = 10.453 Å, a0 = 95.73°, b0 = 121.26°, c0 = 82.19°,
V0 = 2135.3 Å3, Z = 4. While the basic alignment and spatial
arrangement of the Ni complexes in [Ni(bqb)] is retained in
[Ni(Me2bqb)], their interactions are different. For example, the
The structure is dominated by one-sided p–p-stacking interactions
between the quinoline moieties of neighbouring [Ni(bqb)] com-
plexes forming thereby columns parallel to [1 0 1]. On transition
to [Ni(Mebqb)], i.e. by addition of a methyl group, the previous
packing is largely retained and apart from modest changes in unit
cell dimensions, essentially only the symmetry of the complex as
well as that of the lattice is lowered by the loss of C2 symmetry
whereas the c-glide planes are retained. Thus, the unit cell data
of [Ni(Mebqb)] are perceptibly similar to the parent compound:
space group Cc, a = 17.092 Å, b = 13.791 Å, c = 9.692 Å,
b = 115.18°, V = 2067.5 Å3, Z = 4 [50]. The addition of two methyl
groups to [Ni(bqb)] under formation of [Ni(Me2bqb)] (1) leads to
a more significant change in the crystal lattice. As shown in Table 1,
quinoline moiety 1 (N1, C2 to C10) shows a p–p-stacking interac-
tion over its full length of two rings with a centrosymmetric equiv-
alent, but the quinoline moiety 2 (N3, C12 to C20) shows such an
interaction only for its terminal benzene ring (C15 to C20) but
not for its pyridine part (Fig. 2a). As a result the distortion of the
complex [Ni(Me2bqb)] in solid state is notably asymmetric and
the quinoline moiety 2 is more bent from the plane of the o-pheny-
lene diamine part than the quinoline moiety 1 (e.g. C9 and C19
deviate by 0.84 and ꢀ1.67 Å from this plane).
3.3. Spectral studies
ꢀ
a triclinic lattice with space group P1 is the result. None the less,
the structure of [Ni(Me2bqb)] retains a remarkable close relation-
The FT-IR spectral data of the complexes are listed in Section 2.
The amidic NH exhibits a band at 3321–3339 cmꢀ1 in the IR spec-

S. Meghdadi et al. / Polyhedron 29 (2010) 2225–2231
2229
E
2
tra of H2Me2bqb and the H2Me2bpb ligands. The lack of N–H
stretching bands in the IR spectra of these two complexes confirms
that the ligands are coordinated in their deprotonated form [48]. A
further indication of the formation of deprotonated complexes is
the rather large decrease in the carbonyl stretching frequency
exhibited by the ligands upon complex formation. The two sharp
C@O stretching vibration bands at 1688 cmꢀ1 for H2Me2bqb and
1677 and 1666 cmꢀ1 for H2Me2bpb are shifted to lower frequen-
cies, which is in accordance with the data reported for the related
complexes [4,5]. The observation of a sizable increase in the fre-
quency of C–N stretching vibration of medium intensity for the
pc
E
1
pc
E
2
pa
coordinated Me2bqb2ꢀ (1587 cmꢀ1) and Me2bpb2ꢀ (1564 cmꢀ1
)
relative to the free ligands (1525 and 1510 cmꢀ1, respectively)
gives additional support to the coordination of these amides in
their deprotonated form. This is presumably due to the resonance
enhancement in the deprotonated amide which in turn leads to the
strengthening of C–N bond [48].
E
1
pa
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
Potential / V
The absorption spectral data are reported in Section 2. The
H2Me2bqb ligand exhibits
15 845 L molꢀ1 cmꢀ1), two overlapping bands at 319 nm
17 460 L molꢀ1 cmꢀ1) and 286 nm (
distinct absorption band at 241 nm (
a
shoulder at 330 nm
(
(
e
e
=
=
Fig. 3. Cyclic voltammogram of H2Me2bqb, c = 7.8 ꢂ 10ꢀ4 M in CH2Cl2 at 298 K.
Scan rate, 100 mV sꢀ1
.
e
= 21 440 L molꢀ1 cmꢀ1), and a
e
= 90 960 L molꢀ1 cmꢀ1). The
peak positions of the ligand undergo a red shift of 33–94 nm
zene ring is the expected site of oxidation, generating a cation
radical [54]. The irreversible reduction peak at ꢀ1.81 V (Fig. 3) is
attributed to the first step reduction of quinoline rings and is
shifted to the more positive values in the corresponding complexes
(Table 3).
when complex (1) is formed. [Ni(Me2bqb)] exhibits two broad
bands at 424 nm
(e ) and 535 nm (e =
= 5838 L molꢀ1 cmꢀ1
2362 L molꢀ1 cmꢀ1) which are assigned to the ligand field transi-
tions with CT admixture.
The UV–Vis spectra of H2Me2bpb ligand displays a shoulder at
Similar behavior is observed for H2Me2bpb ligand. In general
the redox couples of this ligand are shifted to more cathodic poten-
tials relative to those of H2Me2bqb. The presence of a quinolic ring
makes the reduction potential more positive for H2Me2bqb relative
to H2Me2bpb ligand.
290 nm
(
e
= 12 250 L molꢀ1 cmꢀ1
)
and two bands at 268 nm
(e
= 14 950 L molꢀ1 cmꢀ1) and 230 nm (
e
= 19 550 L molꢀ1 cmꢀ1).
A red shift of 1–34 nm is observed in the spectrum of the coordi-
nated ligand in Ni(Me2bpb). An additional new shoulder at
427 nm appears in the spectrum of Ni(Me2bpb) complex which is
assigned to the ligand field transitions with CT admixture, and is
consistent with the square-planar structure of the complex having
slight tetrahedral distortion (vide supra) [53]. It is reasonable to as-
sume that there is a larger tetrahedral distortion in (1) relative to
(2) leading to the splitting of the electronic states and appearance
of additional absorption bands.
The cyclic voltammogram of the [NiII(Me2bqb)] (1) was mea-
sured in dichloromethane solvent and the representative voltam-
mogram is shown in Fig. 4. The first quasi-reversible oxidation
process at E1/2 = 1.31 V (Fig. 4 and Table 3), is mainly ligand cen-
tered. The oxidized form of the deprotonated ligand is stabilized
by coordination to Ni(II). This is presumably due to the increased
delocalization of electrons in the more planer conformation, ac-
quired by the coordinated ligand. The observation of a similar re-
dox process in our previous investigations [50,5] gives further
support to this assignment. The second reversible oxidation pro-
cess at E1/2 = 0.78 V (Fig. 4 and Table 3), is due to the NiII/III redox
process, which is in agreement with similar reports on carboxamido
Ni(II) complexes [50,55]. The final electrochemically irreversible
3.4. Electrochemistry
The electrochemical behavior of the two ligands H2Me2bqb and
H2Me2bpb and their corresponding Ni(II) complexes, in dichloro-
methane solutions with 0.1 M [N(n-Bu)4]PF6 as the supporting
electrolyte, was studied at a glassy carbon working electrode.
The approximate concentrations of the compounds were 10ꢀ3 to
10ꢀ4 M. The electrochemical data obtained in this study are sum-
marized in Table 3.
E
2
pc
As reported in our previous studies, the carboxamide ligands
are electroactive in the potential range of 1.5 to ꢀ2 V in different
solvents [50,5]. Fig.
3 shows the cyclic voltammogram of
III/II
Ni
H2Me2bqb ligand. The electrochemically irreversible anodic peak
at +1.56 V, is due to the oxidation of 1,2-diaminobenzene ring
[50,5]. Using spectroelectrochemistry and EPR spectroscopy, Wieg-
E
1
pc
hardt et al., established that the
p system in the 1,2-diaminoben-
Table 3
Redox potentials of H2Me2bqb and H2Me2bpb ligands and corresponding complexes.a
Compound
Epc
1
Epa
1
Epc2
Epa
2
Epc NiIII/II
Epa NiII/III
H2Me2bqb
0.21
1.56
1.34
1.37
1.41
ꢀ1.81
ꢀ2.05
ꢀ1.32
ꢀ1.57
0.02
ꢀ
ꢀ
H2Me2bpb
ꢀ0.41
1.24
1.28
ꢀ0.14
[NiII(Me2bqb)]
[NiII(Me2bpb)]
ꢀ0.44
0.75
0.77
0.81
0.86
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
Potential / V
a
Potentials are vs. Fc+/0 in 0.1 M TBAH, T = 298 K. Scan rate 100 mV sꢀ1
.
Fig. 4. Cyclic voltammogram of [NiII(Me2bqb)], c = 2.7 ꢂ 10ꢀ3 M in CH2Cl2 at 298 K.
Approximate concentrations: 10ꢀ3 to 10ꢀ4 M. In the text E1/2 = (Epc + Epa)/2.
Scan rate, 100 mV sꢀ1
.

2230
S. Meghdadi et al. / Polyhedron 29 (2010) 2225–2231
E
2
(2) The coordination geometry around Ni(II) center in complex
(1) is distorted square-planar. In solid state the nickel com-
plex is stabilized by inter-ligand -stacking interactions
pc
p
causing the complex to deviate significantly from C2 symme-
try of the free complex. The overall solid state structure of
(1) is related to that of [NiII(bqb)] and [NiII(Mebqb)], but
has a triclinic instead of the monoclinic symmetry of the
latter.
(3) In dichloromethane solutions, the Ni(II) complexes are capa-
ble of undergoing three 1eꢀ redox processes, one metal-and
two-ligand centered. The redox potentials of the coordinated
ligands and the metal center are influenced by the inductive
effect from the methyl substituents, structural distortions,
and charge delocalization by the fused phenyl rings in
Me2bqb.
III/II
E
1
pc
Ni
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
Potential / V
5. Supplementary data
Fig. 5. Cyclic voltammogram of [NiII(Me2bpb)], c = 4.8 ꢂ 10ꢀ4 M in CH2Cl2 at 298 K.
Scan rate, 100 mV sꢀ1
.
CCDC 759583 contains the supplementary crystallographic data
for 1. These data can be obtained free of charge via http://
www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ,
UK; fax: (+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.
reduction process observed at Epc of ꢀ1.32 V, is attributed to the
one step reduction of the quinoline rings. The second step reduc-
tion of the quinoline rings is obscured due to the potential scan-
ning limitations of dichloromethane solvent.
The cyclic voltammogram of [NiII(Me2bpb)] (2) in dichloro-
methane solution (Fig. 5) shows features similar to those observed
for [NiII(Me2bqb)] (1). It is interesting to note that, the first ligand
centered and second NiII/III oxidation processes at E1/2 = 1.345 and
0.815 V are shifted to more positive values relative to those ob-
served for [NiII(Me2bqb)] (1) (1.305 and 0.78 V), demonstrating
the weaker electron–donating character of Me2bpb2ꢀ ligand in
(2) relative to Me2bqb2ꢀ in (1). This behavior conforms to the
structural data obtained from X-ray structure analysis (Table 2).
The average bond lengths of Ni–Nam (1.848 Å) and Ni–Nqu
(1.960 Å) in [NiII(Me2bqb)] (1) are shorter than Ni–Nam (1.920 Å)
and Ni–Npy (2.02 Å) in [NiII(Me2bpb)] (2) complex. For complex 1,
the occurrence of the first quasi-reversible oxidation process at
lower potential (1.305 V) relative to complex 2 (1.345 V) is most
Acknowledgements
Partial support of this work by the Isfahan University of Tech-
nology Research Council is gratefully acknowledged. Special thanks
are also due to Professor Shadpour Mallakpour for stimulating dis-
cussion. The authors also wish to express gratitude to Professor Pe-
ter C. Ford of UCSB for giving access to the mass spectrometry
facilities at the Department of Chemistry and Biochemistry.
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