Nickel and copper complexes with few amide-based macrocyclic and open-chain ligands

Article abstract of DOI:10.1016/j.ica.2011.08.017

The present work shows three new amide-based ligands H₂L ¹, H₂L² and H₂L³ and their nickel and copper complexes. The X-ray structural analysis substantiate that the ligands constitute a squ

Full text of DOI:10.1016/j.ica.2011.08.017

Inorganica Chimica Acta 377 (2011) 144–154
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Inorganica Chimica Acta
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Nickel and copper complexes with few amide-based macrocyclic and open-chain
ligands
⇑
Megha Munjal, Sushil Kumar, Savita K. Sharma, Rajeev Gupta
Department of Chemistry, University of Delhi, Delhi 110 007, India
a r t i c l e i n f o
a b s t r a c t
The present work shows three new amide-based ligands H2L1, H2L2 and H2L3 and their nickel and copper
complexes. The X-ray structural analysis substantiate that the ligands constitute a square-based basal
plane around the metal center. The crystal structures also show interesting solid state packing due to
hydrogen-bonding and various weak Cꢀ ꢀ ꢀH interactions. The solution-based spectral studies support
the solid-state geometry observed for these complexes. The electrochemical results show that the
Ni3+/2+ and Cu3+/2+ redox couple primarily depends on the N4 donors composed of Namide and Namine
atoms. It was observed that the ligands H2L1 and H2L2 are better suited to stabilize the Cu(III) species
whereas ligand H2L3 is ideal for the stabilization of Ni(III) species. On the basis of electrochemical find-
ings, transient Ni3+ species were generated and characterized by the absorption spectroscopy.
Ó 2011 Elsevier B.V. All rights reserved.
Article history:
Received 10 March 2011
Received in revised form 21 July 2011
Accepted 10 August 2011
Available online 19 August 2011
Keywords:
Nickel
Copper
Amide-based ligands
Spectral studies
Electrochemistry
1. Introduction
macrocyclic ligands [64,65] and their open-chain analogues
(Scheme 1) [66–70]. The macrocyclic ligands and some of their
Amide-based macrocyclic ligands as well as their open-chain
analogues are important since they carve up the properties of oli-
gopeptides and polyamine moieties. Such ligands have been ex-
plored extensively for preparing transition metal complexes with
implications in the field of coordination, bioinorganic and oxida-
tion chemistry [1–17]. The coordination chemistry of nickel and
copper ions has received much attention due to their presence in
the redox cycles of several metalloenzymes [18–29]. Thus, a large
number of high-valent nickel and copper complexes with amide-
based ligands are available in literature and constitute an impor-
tant area of research [30–63]. Several such compounds are also
important as biomimetic models in addition to their potential
application as catalytic oxidants. Prominent among them are the
Ni2+/3+ and Cu2+/3+ complexes of amidate containing ligands from
the groups of Margerum [30–35], Collins [36–39], Ruiz-Garcia
and Journaux [40–45], Holm and Kruger [46–50], Mukherjee [51–
56], and Lampeka and Gavrish [57–63]. These examples have suc-
cessfully demonstrated the stabilizing effect of Namidate donors in
bringing out the high oxidation state of a metal ion. In addition,
such studies also showcase that the ligand architecture plays a cru-
cial role in the redox regulation of a metal center and subsequent
stabilization and/or isolation of the product. The present study is
in continuation of our earlier work on the coordination chemistry
and redox investigation of nickel and copper ion with amide-based
open-chain analogues have been shown to stabilize 2+ as well as
3+ oxidation state of a metal ion in a somewhat rare square-planar
geometry [64–66]. Recently, we have extended our work on 12-
membered macrocyclic chemistry and have expanded the ring size
of the macrocycle to 13-membered macrocyclic ligands. The
expansion of the ring size from 12-membered to 13-membered
has helped in the optimal placement of the Ni(II) ion within the
macrocyclic cavity of ligands (Scheme 1) [71]. In addition, several
macrocyclic ligands carrying various electronic substituents
(–CH3, –Cl, and –OCH3) have been shown to significantly alter
the redox properties and subsequent generation/stability of the
M3+ species [65,71]. In all aforementioned complexes, the basic
coordination environment was kept identical with subtle differ-
ences in the ligand framework to evaluate their architectural effect
on the structural and electrochemical properties of the metal ions.
The present work designs few amide-based ligands where the
primary coordination framework has been kept identical while
altering the peripheral parts (Scheme 2). In particular, ligands
H2L1, H2L2, and H2L3 have been synthesized which differ by the
placement of an arene ring between two amide donors. Notably,
ligands H2L1 and H2L2 are macrocyclic whereas ligand H2L3 is an
open-chain analogue. Interestingly, placement of an arene ring
and/or inclusion of –CH2– group (with sp3 carbon center) that
separates two amide functionalities; may alter the electronic com-
munication between the deprotonated Namidate fragments with the
o-phenylene ring. Further, inclusion of this methylene group will
also induce asymmetrical coordination from both amide donors.
⇑
Corresponding author. Fax: + 91 11 2766 6605.
E-mail address: rgupta@chemistry.du.ac.in (R. Gupta).
0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2011.08.017

M. Munjal et al. / Inorganica Chimica Acta 377 (2011) 144–154
145
applied according to the standard text [74]. Cyclic voltammetric
and coulometric experiments were performed using a CHI electro-
chemical analyzer (Model No: CHI 1120 A). The cell contained a
glassy carbon or a platinum working electrode, a Pt wire auxiliary
electrode, and a saturated calomel electrode (SCE) or Ag/Ag+ as the
reference electrode. A salt bridge (containing supporting electro-
lyte tetra-n-butylammonium perchlorate, TBAP) was used to con-
nect the reference electrode with the electrochemistry solution
[75]. The solutions were ca. 1 mM in complex and ca. 0.1 M in sup-
porting electrolyte, TBAP. Under our experimental conditions, the
E1/2 value for the couple Fc+/Fc was found to be 0.40 V in CH3CN
versus SCE [76].
O
N
N
N
N
O
Ni
O
O
N
N
N
N
O
O
N
N
N
N
O
Ni
A
Ni
C
B
R
R
R
R
N
N
N
N
O
O
N
N
N
N
O
O
N
N
O
M
Ni
2.3. Crystallography
Ni
N
N
M = Ni Cu
Single crystals suitable for the X-ray diffraction studies were
grown by the vapor diffusion of diethyl ether to a DMF (complex
2) or CH2Cl2 solution (complex 5) of the compound. The intensity
data for complexes 2 and 5 were collected on an Oxford XCalibur
CCD diffractometer equipped with graphite monochromatic Mo
R R R R
R = -H
D
E
F
H
I
R = -H
L
R = -CH3
R = -C2H5
R = -CH2Ph R
P
R = -Cl
R = -Me
R = -Cl
R = -Me
M
N
Q
J
K
R = -OMe G
R = -OMe O
Ka radiation (k = 0.71073 Å) at 150(2) K [77]. A multi-scan
Scheme 1. Metal complexes discussed in text.
absorption correction was applied [77]. The structures were solved
by the direct methods using SIR-92 and refined by full-matrix
least-squares refinement techniques on F2 using SHELXL97. The
hydrogen atoms were placed into the calculated positions and in-
cluded in the last cycles of the refinement. All calculations were
done using the WinGX software package [78]. For complex 2, dis-
order was observed for the C7, C8, C9, and C10 atoms. These atoms
were occupying two crystallographically independent positions
which could be located from the additional residual electron den-
sity. The site occupancy factors were refined with the help of free
variable PART instruction. Refinement of the site occupancy factors
for the disordered atoms gave the value 0.52 and 0.48, respectively.
A structure model based on statistically disordered carbon atoms
improved the accuracy of C–C bonds of interest. The crystallo-
graphic data collection and structure refinement parameters are
compiled in Table 1.
O
NH HN
O
O
NH HN
O
O
NH HN
O
N
N
N
N
N
N
H2L1
H2L2
H2L3
Scheme 2. Ligands used in this work.
Finally, in continuation with our earlier work, the emphasis has
been given to understand the effect of ligand architecture on the
structure, properties and redox chemistry of the metal ion.
2.4. Synthesis of ligands
2.4.1. H2L1
LiBr (0.76 g, 9 mmol), Na2CO3 (5.0 g, 40 mmol) and N,N0-bis(chlo-
roacetyl)propylenediamine (1.0 g, 5 mmol) were taken in MeCN
(150 mL) and the mixture was refluxed with stirring for 1 h. To this
mixture was added N,N0-dimethyl ethylenediamine (0.4 g, 5 mmol)
dissolved in MeCN (50 mL). The resulting mixture was refluxed un-
der stirring for an additional 48 h. The mixture was cooled, filtered
and the solvent was removed under reduced pressure to afford a
sticky solid. The crude product was recrystallized by layering the
CHCl3 solution with petroleum ether (boiling range 60–80 °C).
Yield: 0.8 g (75%). Mp 195–197 °C. FT-IR spectrum (KBr, selected
2. Experimental
2.1. Chemicals and reagents
All reagents were obtained from the commercial sources and
used as received. The solvents were purified as reported earlier
[64–70]. N,N0-Bis(chloroacetyl)propylenediamine was synthesized
according to a literature report [72].
2.2. Physical methods
peaks, cmꢁ1):
m = 3327 (mNH), 1645 (m
C@O). 1H NMR spectrum
The microanalysis data were obtained from the Elementar Anal-
ysen Systeme GmbH Vario EL-III instrument. The NMR measure-
ments were made using an Avance Bruker (300 MHz) and/or Jeol
Delta (400 MHz) instruments. The infra-red spectra (either as KBr
pellet or as a mull in mineral oil) were recorded with the Perkin-
Elmer FT-IR-2000 spectrometer. The absorption spectra were
recorded using the Perkin-Elmer Lambda-25 spectrophotometer.
The X-band electron paramagnetic resonance spectra were
recorded with a Jeol JES-FA 200 spectrometer (fitted with quartz
Dewar for measurements at 120 K). Solution conductivity mea-
surements were performed with the digital conductivity bridge
from the Popular Traders, India (Model number: PT-825). Solution
magnetic susceptibility measurements were made by the Evan’s
NMR spectral method [73]. The diamagnetic corrections were
(300 MHz, CDCl3, 25 °C): d = 1.81 (m, 2H, –CH2–, Ha), 2.33 (s, 6H,
CH3), 2.59 (s, 4H, –CH2–, Hc), 3.05 (s, 4H, –CH2C(O)–, Hd), 3.48 (m,
4H, –CH2–CH2–, Hb), 7.85 (s(broad), 2H, NH). MS (EI+, m/z): Calc.
for C11H22N4O2 242.32. Found: 243.897 [(H2L1+H+)].
2.4.2. N,N0-Bis(chloroacetyl)-2-aminobenzylamine
2-Aminobenzylamine (1.07 g, 9 mmol) was dissolved in dichlo-
romethane (130 mL) and kept under stirring on an ice-bath. A solu-
tion of chloroacetyl chloride (2.68 g, 18 mmol) dissolved in
dichloromethane (10 mL) was added drop-wise followed by the
addition of pyridine (0.75 g, 9.0 mmol). The resulting reaction mix-
ture was stirred for 12 h at room temperature. A white product was
obtained after removal of the solvent followed by washing with
water. The product was filtered; washed with water followed with

146
M. Munjal et al. / Inorganica Chimica Acta 377 (2011) 144–154
N–H). MS (EI+, m/z): Calc. for C15H22N4O2 290.36. Found:
Table 1
290.3207 [(H2L2+H+)].
Crystallographic data collection and structural refinement parameters for complexes
[Cu(L1)(H2O)] (2) and [Ni(L3)]ꢀCH2Cl2 (5).
2.4.4. H2L3
Complex
[Cu(L1)(H2O)] (2)
[Ni(L3)]ꢀCH2Cl2 (5)
A mixture of dimethylamine (0.49 g, 10.9 mmol), K2CO3 (7.5 g,
5.4 mmol), and N,N0-bis(chloroacetyl)-2-aminobenzylamine (1 g,
3.6 mmol) were taken in MeCN (10 mL). The resulting mixture
was refluxed for 12 h. The solvent was removed under reduced
pressure to afford a colorless oil. Yield: 1.07 g (50%). FT-IR spec-
Formula
Formula weight
T (K)
C
11H22N4O3Cu
C16H24N4O2Cl2Ni
434.00
150(2)
0.71073
monoclinic
P21/c
321.87
150(2)
0.71073
orthorhombic
Pbca
k (Å)
Crystal system
Space group
Unit cell dimensions
a (Å)
b (Å)
c (Å)
trum (Film, selected peaks, cmꢁ1): 3244 (
1589 (
m
NH), 2948, (
mCH), 1681,
C@O). 1H NMR spectrum (CDCl3, 300 MHz, 25 °C): d = 2.4
8.9522(4)
16.6759(7)
19.0465(9)
90
90
90
2843.4(2)
8
1.504
8.0386(5)
17.1921(11)
14.2759(9)
90
101.796(6)
90
1931.3(2)
4
1.493
m
e
0
(s, 6H, –CH3), 2.5 (s, 6H, ACH3 ), 4.40 (s, 2H, –CH2–, H ), 4.45 (s,
4H, –CH2C(O)–, Hc/d), 7.50 (m, 4H, Aromatic–H, Hf/g/h), 9.5 (s
(broad), 2H, NH).
a
(°)
b (°)
c
(°)
V (Å3)
2.5. Synthesis of metal complexes
Z
Dcalc (g cmꢁ3
)
2.5.1. [Ni(L1)] (1)
Absorption coefficient
(mmꢁ1
F(000)
h (°)
1.546
1.298
The ligand H2L1 (0.10 g, 0.413 mmol) was dissolved in MeOH
(5 mL) and treated with solid NaOH (0.036 g, 0.909 mmol) under
)
1352
904
3.12 to 26.50
ꢁ11 6 h 6 6,
ꢁ13 6 k 6 20,
ꢁ23 6 l 6 13
7926
2.92 to 25.00
ꢁ9 6 h 6 9,
ꢁ18 6 k 6 20,
ꢁ15 6 l 6 16
8532
magnetic stirring.
A
solution of [Ni(H2O)6](ClO4)2 (0.15 g,
Index ranges
0.413 mmol) dissolved in MeOH (6 mL) was added drop-wise to
the ligand solution. The resulting orange–yellow solution was stir-
red for 2 h at room temperature. The solvent was removed under
reduced pressure and the crude compound was isolated after
washing with diethyl ether. The recrystallization was achieved by
dissolving the crude compound in MeCN and subjecting to the va-
por diffusion of diethyl ether at room temperature. The orange
crystalline product thus obtained was filtered, washed with diethyl
ether and dried under vacuum. Yield: 0.07 g (56%). Anal. Calc. for
Reflections collected
Independent reflections
2945 (0.0660)
3405 (0.0445)
(Rint
)
Data completeness
Refinement method
99.9%
99.9%
full-matrix least-
squares on F2
2945/91/192
full-matrix least-
squares on F2
3405/0/230
Data/restraints/
parameters
Goodness-of-fit (GOF) on 0.889
1.067
C
11H24N4O4Ni (including 2H2O, 335.03): C, 39.44; H, 7.22; N,
16.72. Found: C, 39.59; H, 7.50; N, 17.02%. FT-IR spectrum (KBr, se-
lected peaks, cmꢁ1):
= 3369 ( OH), 2922 ( CH), 1602 ( C@O). Con-
ductivity (DMF, ꢂ1 mM solution, 298 K): KM = 12
ꢁ1 cm2 molꢁ1
UV–Vis spectrum [kmax, nm (
, Mꢁ1 cmꢁ1)] (in CH3CN): 213
(23480), 257 (15700), 470 (430). MS (EI+, m/z): Calc. for
F2
Final R indices [I > 2
r
(I)] R1 = 0.0531,
wR2 = 0.0656
R1 = 0.1454,
R1 = 0.0428,
wR2 = 0.1119
R1 = 0.0545,
wR2 = 0.1155
0.695 and ꢁ0.625
m
m
m
m
R indices (all data)
X
.
wR2 = 0.0792
0.594 and ꢁ0.372
e
Largest difference in
peak and hole (e ÅAꢁ3
0
)
C
11H20N4O2Ni 299.00. Found: 299.21 ([Ni(L1)]+H+). 1H NMR spec-
trum (300 MHz, DMSO-d6, 25 °C): d = 1.16 (m, 2H, –CH2–, Ha),
1.98 (m, 2H, –CH2–, He), 2.30 (m, 2H, –CH2–, Hb), 2.21 (s, 6H –
0
CH3), 2.34 (d, 2H, –CH2–, Hb ), 2.48 (d, 2H, –CH2C(O)–, Hc), 3.22
diethyl ether and dried. Yield: 1.07 g (50%). Mp 125 °C. FT-IR spec-
trum (KBr, selected peaks, cmꢁ1): 3260 (
NH), 3076, ( CH), 1665
C@O). 1H NMR spectrum (CDCl3, 300 MHz, 25 °C): d = 4.27 (s, 2H,
–CH2C(O)–), 4.32 (s, 2H, –CH2C(O)–), 4.55 (d, 2H, –CH2–), 7.55
(m, 4H, Aromatic–H), 8.7 (s(broad), 2H, NH).
(m, 2H, –CH2CH2–, Hf), 3.65 (d, 2H, –CH2C(O)–, Hd) ppm. 13C
NMR spectrum (75 MHz, DMSO-d6, 25 °C]: d = 32.38 (–CH2–),
42.55 (–CH3), 60.50 (–CH2CH2–), 49.09 (–NCH2–), 67.67 (–
C(O)CH2–), 176.38 (C@O) ppm.
m
m
(m
2.5.2. [Cu(L1)(H2O)] (2)
2.4.3. H2L2
The ligand H2L1 (0.10 g, 0.413 mmol) was dissolved in DMF
(10 mL) under magnetic stirring and treated with solid NaH
(0.021 g, 0.908 mmol) under dinitrogen atmosphere. The resulting
mixture was stirred until H2 evolution was ceased. To this mixture
was added a solution of copper triflate (0.149 g, 0.413 mmol) dis-
solved in DMF (10 mL). The resulting deep purple solution was stir-
red for 3 h at room temperature. The solvent was evaporated under
reduced pressure and the crude product was isolated after washing
with diethyl ether. The recrystallization was achieved by diffusing
vapors of diethyl ether to a DMF solution of the crude product. The
crystalline product thus obtained was filtered, washed with diethyl
ether and dried under vacuum. Yield: 0.034 g (26%). Anal. Calc. for
LiBr (0.4 g, 4 mmol), Na2CO3 (2.0 g, 0.016 mmol) and N,N0-
bis(chloroacetyl)-2-aminobenzylamine (1.0 g, 5 mmol) were taken
in MeCN (140 mL) and the mixture was refluxed with stirring for
1 h. To this was added N,N0-dimethylethylenediamine (0.18 g,
2 mmol) dissolved in MeCN (20 mL). The resulting mixture was
further refluxed under stirring for an additional 48 h. The mixture
was cooled, filtered and the solvent was removed under reduced
pressure to afford the crude product. This product was further
purified by the column chromatography using silica gel (100–
200 mesh) with 10% MeOH–CHCl3 (Rf = 0.45). A white product
was obtained after the removal of solvent. Yield: 0.260 g (43%).
Mp 138–140 °C. An analytically pure product (ꢂ82% yield) can also
be obtained by layering CHCl3 solution of the product with the hex-
C
11H22N4O3Cu (321.86): C, 41.05; H, 6.89; N, 17.41. Found: C,
40.75; H, 7.22; N, 17.65%. FT-IR spectrum (KBr, selected peaks,
cmꢁ1):
= 3461 ( OH), 2927 CH), 1586 ( C@O). Conductivity
(DMF, ꢂ1 mM solution, 298 K): KM = 12
ꢁ1 cm2 molꢁ1. UV–Vis
spectrum [kmax, nm (
, Mꢁ1 cmꢁ1)] (in DMF): 454 (sh, 115), 505
(145), 568 (sh, 125). UV–Vis spectrum [kmax, nm (
, Mꢁ1 cmꢁ1)]
(in CH3CN): 484 (sh, 90), 570 (110). UV–Vis spectrum [kmax, nm
, Mꢁ1 cmꢁ1)] (in MeOH): 542 (495). MS (EI+, m/z): Calc. for
anes. FT-IR spectrum (KBr, selected peaks, cmꢁ1): 3232 (
m
NH), 3062,
m
m
(m
m
2949, 2839, (
m
CH), 1668 (
m
C@O). 1H NMR spectrum (CDCl3, 300 MHz,
X
0
25 °C): d = 2.40 (s, 3H, –CH3), 2.44 (s, 3H, ACH3 ), 2.58 (s, 4H, –CH2–
CH2–, Hb), 3.06 (s, 2H, –CH2C(O)–, Hc), 3.18 (s, 2H, –CH2C(O)–, Hd),
4.49 (d, 2H, –CH2–, He), 7.31 (m, 2H, Ar–Hg), 7.10 (m, 1H, Ar–Hh),
8.48 (d, 1H, Ar–Hf), 7.81 (s (broad), 1H, N–H), 9.84 (s (broad), 1H,
e
e
(e

M. Munjal et al. / Inorganica Chimica Acta 377 (2011) 144–154
147
C11H22N4O3Cu 321.86. Found: 322.14 ([Cu(L1)(H2O)]+H+). leff
cmꢁ1): 2949, 2782 (
m
CH), 1618, 1587 (
m
C@O). Conductivity (DMF,
(CH3CN, 298 K) = 1.94 lB
.
ꢂ1 mM solution, 298 K): KM = 10
X
ꢁ1 cm2 molꢁ1. UV–Vis spec-
trum [kmax, nm (e
, Mꢁ1 cmꢁ1)] (in DCM): 558 (sh, 100), 470
2.5.3. [Ni(L2)] (3)
(210), 350 (570). UV–Vis spectrum [kmax, nm (
e
, Mꢁ1 cmꢁ1)] (in
The ligand H2L2 (0.10 g, 0.344 mmol) was dissolved in MeOH
(5 mL) and treated with solid NaOH (0.03 g, 0.758 mmol) under
CH3CN): 555 (sh, 790), 470 (170), 340 (450). UV–Vis spectrum
[kmax, nm (e
, Mꢁ1 cmꢁ1)] (in DMF): 550 (sh, 730), 470 (140), 340
magnetic stirring.
A
solution of [Ni(H2O)6](ClO4)2 (0.125 g,
(420). 1H NMR spectrum (400 MHz, DMSO-d6): d 2.65 (12H, d, –
c
0
0.344 mmol) dissolved in MeOH (6 mL) was added drop-wise to
the ligand solution. The resulting orange–yellow solution was stir-
red for 2 h at room temperature. The solvent was removed under
reduced pressure and the crude compound was isolated after wash-
ing with diethyl ether. The recrystallization was achieved by dis-
solving the crude compound in MeCN and diffusing diethyl ether.
This afforded orange crystalline product that was filtered, washed
with diethyl ether and dried under vacuum. The compound 3 was
found to co-crystallize with one equivalent of NaClO4. The presence
of NaClO4 was established by the microanalysis results, FT-IR spec-
trum, and conductivity measurement. It may also be noted that sev-
eral attempts gave the identical observation. Yield: 0.08 g (67%).
Anal. Calc. for C15H26N4O9ClNiNa (including 3H2O and 1NaClO4,
523.52): C, 34.41; H, 5.01; N, 10.70. Found: C, 34.12; H, 4.96; N,
CH3 and ACH3 ); d 3.05 (2H, s, –CH2C(O), H ); d 3.40 (2H, s, –
CH2C(O), Hd); d 3.80 (2H, s, –CH2–, He); d 6.75 (1H, t, J = 6.6 HZ,
0
Hh); d 6.90 (1H, d, J = 6.9 HZ, Hg ); d 7.05 (1H, t, J = 7.8 HZ, Hg); d
7.45 (1H, d, J = 7.8 HZ, Hf) ppm. 13C NMR spectrum (100 MHz,
0
0
DMSO-d6, 25 °C): d = 46.69 (Ce), 51.2 (Ca ), 46.69 (Ca), 73.58 (Cb ),
74.41 (Cb), 126.61 (Ch), 130.98 (Cg ), 131.66 (Cg and Cf), 139.41
0
0
0
(Cd ), 148.41(Cd), 176.40(Cc ), 177.24(Cc) ppm.
2.5.6. [Cu(L3)] (6)
This compound was synthesized following the procedure for
complex 2 with following reagents: H2L3 (0.15 g, 0.51 mmol),
NaH (0.028 g, 1.1 mmol), CuCl2 (0.06 g, 0.51 mmol). The product
was isolated as blue colored powder. Recrystallization was
achieved by diffusing diethyl ether to a CH2Cl2 solution of the
crude product at 5 °C. Yield: 0.13 g (72%). Anal. Calc. for
10.72%. FT-IR spectrum (KBr, selected peaks, cmꢁ1):
OH), 2925 ( CH), 1618, 1581 ( C@O), and 1091 ( ClO4). Conductivity
(DMF, ꢂ1 mM solution, 298 K): KM = 38
ꢁ1 cm2 molꢁ1. UV–Vis
spectrum [kmax, nm (
, Mꢁ1 cmꢁ1)] (in CH3CN): 450 (160), 238
(8900). UV–Vis spectrum [kmax, nm (
, Mꢁ1 cmꢁ1)] (in DMF): 282
(7520), 306 (4020), 380 (160), 458 (140). MS (EI+, m/z): Calc. for
15H20N4O2Ni 347.04. Found: 347.22 ([Ni(L2)+H+]). 1H NMR spec-
trum (400 MHz, DMSO-d6, 25 °C]: d = 2.45 (s, 3H, –CH3), 2.51 (s,
m = 3425
(m
m
m
m
C
15.5H23N4O2ClCu (including 0.5CH2Cl2): C, 46.97; H, 5.85; N,
14.13. Found: C, 47.14; H, 5.74; N, 14.37%. FT-IR spectrum (KBr
disk, selected peaks, cmꢁ1):
= 2981, 2888 ( CH), 1601, 1583
X
e
m
m
e
(
14
m
C@O). Conductivity (DMF, ꢂ1 mM solution, 298 K): KM
=
X
ꢁ1 cm2 molꢁ1. UV–Vis spectrum [kmax, nm (
e
, Mꢁ1 cmꢁ1)] (in
C
DCM): 630 (sh, 110), 520 (270), 410 (420). UV–Vis spectrum
[kmax, nm (
, Mꢁ1 cmꢁ1)] (in CH3CN): 540 (190), 400 (310). UV–
Vis spectrum [kmax, nm (
, Mꢁ1 cmꢁ1)] (in DMF): 550 (150), 410
e
b
d
0
3H, ACH3 ), 2.63 (m, 2H, –CH2, H ), 2.91 (d, 1H, –CH2C(O)–, H ),
e
2.98 (d, 1H, –CH2C(O)–, He), 3.41 (d, 1H, –CH2–, Hf), 3.62 (m, 2H,
(250). leff (CH3CN, 298 K) = 1.89 lB
.
0
–CH2–, Hc), 3.82 (d, 1H, –CH2C(O)–, He ), 3.90 (d, 1H, –CH2C(O)–,
0
0
Hd ), 4.48 (d, 1H, –CH2–, Hf ), 6.74 (m, 1H, Hg), 6.96 (m, 2H, Hh),
7.77 (m, 2H, Hi) ppm. 13C NMR spectrum (100 MHz, DMSO-d6,
25 °C]: d = 45.79 (–CH2–), 46.48 (–CH3), 47.42 (–CH3), 58.12
(–CH2C(O)–), 58.84 (CH2C(O)–), 66.06 (–NCH2–), 121.10 (Ar-k),
123.37 (Ar-n), 125.95 (Ar-m), 128.65 (Ar-l), 132.59 (Ar-j), 142.18
(Ar-i), 173.19 (C@O), 175.36 (C@O) ppm.
3. Results and discussion
3.1. Ligand design and synthesis
All three ligands H2L1, H2L2 and H2L3 have been designed to pro-
vide two Namide and two Namine coordinations to the metal center in
an identical donor environment (Scheme 2). While ligands H2L1
and H2L2 are macrocyclic in nature, the ligand H2L3 is an open-
chain analogue. Interestingly, ligands H2L2 and H2L3 are originated
from 2-aminobenzyl amine and thus a flexible –CH2– group (with
sp3 carbon center) separates two amide functionalities. Inclusion of
this group will induce asymmetric coordinations from both amide
donors and may also interrupt the electronic communication
between the Namidate fragment and o-phenylene ring. All three li-
gands were synthesized in two steps in good overall yield. The
FT-IR spectra of the ligands show the mN–H stretching frequency
in the range of 3230–3250 cmꢁ1 and the mC@O frequency is in the
range of 1665–1685 cmꢁ1 [79]. The asymmetry in ligands H2L2
and H2L3 is clearly visible by the observation of different chemical
shifts for the methyl and methylene groups in the NMR spectra.
2.5.4. [Cu(L2)] (4)
This compound was synthesized following the procedure for
complex 2 with following reagents: H2L2 (0.10 g, 0.344 mmol),
NaH (0.018 g, 0.758 mmol), CuCl2 (0.046 g, 0.344 mmol). The prod-
uct was isolated as blue colored powder. Recrystallization was
achieved by diffusing diethyl ether to a DMF solution of the crude
product at room temperature. Yield: 0.076 g (62%). Anal. Calc. for
C
15H25N4O4.5Cu (including 2.5H2O): C, 45.39; H, 6.35; N, 14.12.
Found: C, 45.47; H, 6.10; N, 14.31. FT-IR spectrum (KBr, selected
peaks, cmꢁ1):
= 3401 ( OH), 2923, 2860 ( CH), 1562 ( C@O). Con-
ductivity (DMF, ꢂ1 mM solution, 298 K): KM = 8
ꢁ1 cm2 molꢁ1
UV–Vis spectrum [kmax,
, Mꢁ1 cmꢁ1)] (in DMF): 570 (180),
m
m
m
m
X
.
nm (e
UV–Vis spectrum [kmax, nm (
e
, Mꢁ1 cmꢁ1)] (in MeOH): 580 (270).
, Mꢁ1 cmꢁ1)] (in CH3CN): 560 (280).
UV–Vis spectrum [kmax, nm (
e
leff (CH3CN, 298 K) = 1.97 lB
.
3.2. Synthesis and characterization of metal complexes
2.5.5. [Ni(L3)] (5)
This compound was synthesized following the procedure for
complex 2 with following reagents: H2L3 (0.07 g, 0.23 mmol),
NaH (0.013 g, 0.55 mmol), NiCl2 (0.03 g, 0.24 mmol). The filtrate
was concentrated under reduced pressure to one fourth of its ori-
ginal volume and diethyl ether was added to precipitate the crude
product as orange red powder. Recrystallization was achieved by
diffusing diethyl ether to a CH2Cl2 solution of the crude product
at 5 °C. Yield: 0.058 g (69%). Anal. Calc. for C15.5H23N4O2ClNi
(including 0.5CH2Cl2): C, 47.55; H, 5.92; N, 14.31. Found: C,
47.76; H, 6.09; N, 14.46%. FT-IR spectrum (KBr disk, selected peaks,
The nickel and copper complexes (Scheme 3) were synthesized
by treating the deprotonated form of ligand with the necessary
metal salt. The usual work up and recrystallization (vapor diffusion
of diethyl ether to CH3CN or DCM solution of the crude product)
afforded highly crystalline product in moderate yield. The solution
conductivity [80] confirms the non-electrolytic nature for all com-
plexes, whereas the elemental analysis results authenticate the
purity of the bulk samples. In the FT-IR spectra, the absence of
mN–H stretching and the observed bathochromic shift for mC@O
(compared to free ligand) validate the involvement of

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M. Munjal et al. / Inorganica Chimica Acta 377 (2011) 144–154
Table 2
Selected bond lengths and bond angles for complexes [Cu(L1)(H2O)] (2) and
H2
[Ni(L3)]ꢀCH2Cl2 (5).
O
O
N
N
N
N
O
O
N
N
N
N
O
[Cu(L1)(H2O)] (2)
[Ni(L3)]ꢀCH2Cl2 (5)
Ni
1
Cu
Bond lengths (Å)
Cu–N1
Cu–N2
Cu–N3
Cu–N4
1.921(3)
1.941(3)
2.071(3)
2.057(4)
2.243(3)
Ni–N1
Ni–N2
Ni–N3
Ni–N4
1.862(2)
1.888(2)
1.993(2)
1.998(2)
2
Cu–O3
Bond angles (°)
N1–Cu–N2
N1–Cu–N3
N1–Cu–N4
N2–Cu–N3
N2–Cu–N4
N3–Cu–N4
O3–Cu–N1
O3–Cu–N2
O3–Cu–N3
O3–Cu–N4
94.01(1)
151.2(1)
84.4(1)
84.4(1)
158.2(1)
86.7(1)
110.64(13)
102.45(12)
97.72(13)
98.46(12)
N1–Ni–N2
N1–Ni–N3
N1–Ni–N4
N2–Ni–N3
N2–Ni–N4
N3–Ni–N4
91.03(10)
84.01(10)
169.70(11)
170.56(10)
83.87(10)
102.24(10)
O
N
N
O
O
N
N
N
N
O
M
M
N
N
M = Ni (5); Cu(6)
M = Ni (3); Cu(4)
Scheme 3. Synthesized metal complexes.
deprotonated Namidate group in the bonding [79]. In case of com-
amine nitrogen donors. The
(86.70°) is ca. 7° smaller than the
(94.00°) which is most likely due to the involvement of five- and
six-membered chelate rings, respectively. The coordinated water
N
amine–Cu–Namine bond angle
amide–Cu–Namide angle
plex 2, coordinated water molecule clearly shows the mO–H stretch-
N
ing at 3461 cmꢁ1; whereas few complexes also have water as the
solvent of crystallization which absorbs in the region of 3369–
3425 cmꢁ1. The solution magnetic moment [73,74] for the com-
0
molecule forms a Cu–Owater bond length of 2.243 ÅA. The Cu(II) ion
0
plexes 2 (1.94 lB), 4 (1.97 lB), and 6 (1.89 lB) clearly support the
is displaced by 0.431 ÅA from the N4 equatorial plane towards the
coordinated water molecule. The geometry around the Cu(II) ion
monomeric nature of the complexes as the magnetic moment val-
ues were falling in the range typically observed for the Cu(II) ion in
a magnetically dilute environment [65,70].
is distorted square-pyramidal as the
cation of the distortion of the five-coordinate geometry towards
square-pyramidal ( = 0) or trigonal bipyramidal ( = 1), is 0.116.
s value [81], which is an indi-
s
s
3.3. Molecular structures
The solid-state packing of complex 2 is quite interesting where
hydrogen atoms of the coordinated water molecule are engaged in
connecting individual molecules together. The first hydrogen atom
(H3A) of the coordinated water molecule (O3) makes a hydrogen-
bond with the Oamide (O1) group of the neighbor molecule resulting
in the generation of a chain (Fig. 2a). The second hydrogen atom
(H3B) of the coordinated water forms a hydrogen-bond with the
Oamide (O2) group from the adjacent chain thus connecting two
chains together (Fig. 2b). The heteroatom separations, O3ꢀ ꢀ ꢀO1
and O3ꢀ ꢀ ꢀO2 were found to be 2.720 and 2.713 Å, respectively.
3.3.1. Crystal structure of complex 2
The molecular structure of complex 2 is shown in Fig. 1 whereas
the selected bond lengths and angles are provided in Table 2. The
Cu(II) ion adopts a distorted square-pyramidal geometry where
ligand constitute a N4 basal plane and a water molecule serves as
the fifth ligand. The Cu(II) ion is coordinated by two deprotonated
Namide (average: 1.931 Å) and two neutral Namine atoms (average:
2.064 Å). The Cu–Namide bond distances are shorter by ca. 0.13 Å
than the Cu–Namine bond distances (Table 2). This difference of
0.13 Å may be attributed to the anionic coordination from the
deprotonated amidate nitrogen atoms compared to the neutral
3.3.2. Crystal structure of complex 5
The unit cell of complex 5 contains a monomeric unit and a
CH2Cl2 as the solvent of crystallization. The molecular structure
of complex 5 is presented in Fig. 3 whereas Table 2 contains the
bond distances and angles. The crystal structure shows that the
deprotonated ligand, [L3]2ꢁ is holding the Ni(II) ion in a somewhat
distorted square-planar environment. The Ni(II) ion is coordinated
by two deprotonated Namide and two neutral Namine atoms. The
average Ni–Namide distance (1.870 Å) is shorter than the average
Ni–Namine distance (1.996 Å) by 0.13 Å. A similar observation has
been noted for complex 2 as well as several structurally character-
ized complexes from our laboratory as shown in Scheme 1 [64–71].
Further, the Ni–Namide distances for complex 5 are bit longer than
that of similar nickel complexes as shown in Scheme 1. This obser-
vation suggests that the inclusion of the flexible methylene group
leads to weaker donor capacity of the amidate fragments. For com-
C2
C3
C1
O2
O1
N2
N1
Cu
C11
C4
O3
N3
N4
C10
C5
parison, structurally characterized Ni(II) complexes
D and P
C9
(Scheme 1) show the Ni–Namide distances in the range of
1.801–1.845 Å [64,66]. The Ni(II) ion is optimally placed within
the N4 basal plane with the deviation of only 0.008 Å. Notably,
the displacement of Ni(II) ion from the N4 basal plane is 0.126
and 0.072 Å for complexes D and P, respectively [64,66]. This
comparison suggests that the inclusion of methylene group in
the ligand caused the expansion of chelate ring and resulted in
C6
C7
C8
Fig. 1. Thermal ellipsoidal (40% probability) representation of complex
[Cu(L1)(H2O)] (2). The atoms C7, C8, C9, and C10 were occupying two crystallo-
graphically independent positions and only the atoms with site occupancy factor
higher than 0.5 are shown for clarity. The hydrogen atoms are omitted for clarity.

M. Munjal et al. / Inorganica Chimica Acta 377 (2011) 144–154
149
(a)
(b)
Fig. 2. (a) Linear 1D chain formation in complex 2 via hydrogen bonding of Oamide atom with hydrogen atoms of H2O molecule attached to the metal center. (b) Chain
structure extended along two-dimension via hydrogen bonding; view along b-axis.
C8
with the hydrogen atoms of the CH2Cl2 molecule via hydrogen
bonding. Further, one of the Cl atoms of the CH2Cl2 molecule
C9
C7
C6
interacts with the hydrogen atom of the methylene group of the
complex. In addition, there are several weak CHꢀ ꢀ ꢀO interactions
between the hydrogen atoms of the methyl groups and Oamide
atoms. Several such interactions connect two individual chains in
the bc plane if viewed along the a axis (Fig. S1, Supporting
Information).
C10
O2
C5
C11
O1
N1
N2
3.4. NMR spectral studies for nickel complexes
C4
C12
C13
C14
Ni
To investigate the solution state structure of Ni(II) complexes 1,
3, and 5, their 1H and 13C NMR spectral studies were performed
(Figs. S2–S6, Supporting Information). In these complexes, the
Ni(II) ion is in square-planar geometry leading to a diamagnetic
ground state. Assignment of the peaks has been made through con-
sideration of the relative peak area and comparison to that of free
ligand. The NMR spectra also prove that the metallation occurred
with the fully deprotonated form of the ligand as the peaks corre-
sponding to N–Hamide group of the ligand were absent.
For complex 1, the twisting of –CH2CH2– fragment of the
N,N0-dimethylethylenediamine resulted in different chemical
environment of the protons attached which were observed as
multiplets at 1.98 and 3.22, respectively (labeled as e and f). The
multiplet observed in the up-field region is characteristic of the
–CH2– group (a and a0 protons) of the trimethylene part in a chair
conformation [63,82,83]. In the present case, their relative order
(dax < deq) and separation are similar to those of other macrocyclic
complexes [84,85] and six-membered organic rings [86]. Further,
the trimethylenediamine fragment of the ligand is forming a six-
membered chelate ring and is in a chair conformation and the axial
protons (b0) are slightly de-shielded as compared to the equatorial
protons (b). Two sets of signals (labeled as b and b0) were observed
C3
C2
N3
N4
C1
C15
Fig. 3. Thermal ellipsoidal (50% probability) representation of complex
[Ni(L3)]ꢀCH2Cl2 (5). The solvent molecule and hydrogen atoms are omitted for
clarity.
the optimal placement of metal ion within the N4 basal plane.
Around Ni(II) center, theNamide–Ni–Namide bond angle is 91.03°
whereas theNamine–Ni–Namine bond angle is 102.24°. Two lateral
five-membered chelate planes NiN1C4C3N3 and NiN2C12C13N4 make
an angle of ca. 16° with each other and suggest the relative twist-
ing of these two planes. In addition, the methylene fragment (atom
C5) is displaced by 0.706 Å out of the N2C11C6N1Ni plane that is
part of six-membered chelate ring.
The CH2Cl2 molecules present in the unit cell stitch together the
mononuclear units resulting in the formation of a chain running
along the bc plane (Fig. 4). The Oamide atoms O1 and O2 interact

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M. Munjal et al. / Inorganica Chimica Acta 377 (2011) 144–154
(a)
(b)
Fig. 4. Linear 1D chain formation in complex 5 via hydrogen bonding of (a) Oamide atom with hydrogen atoms of CH2Cl2 molecule and (b) chlorine atom of CH2Cl2 molecule
with hydrogen atoms of methylene fragment of the complex; view along the a-axis.
as doublet and triplet at 2.30 and 2.34 with the geminal coupling
DEPT-135 and APT spectra along with the 13C NMR spectra. All
these spectra are shown in Figs. S3, S5, and S6 (Supporting
Information). APT was used to differentiate CH2 signals from the
CH and CH3 signals where methine (CH) and methyl (CH3) signals
appear in negative mode and the quaternary (C) and methylene
(CH2) signals in positive mode [86]. The APT spectrum shows that
the carbon atoms labeled as a, a0 l, m, n and k resonate in the neg-
ative mode for CH3 and CH, while the carbon atoms labeled as b, c,
d, g, i, j, e and f for CH2 and C resonate in the positive mode.
Similarly, DEPT-135 yielded the CH and CH3 signals in positive
mode and CH2 in negative mode whereas the carbons labeled as
i, j, e and f are absent due to quaternary nature. In the 13C NMR
spectrum of the complex 3, 14 carbon signals corresponding to
each carbon in the complex are clearly visible except for the –
CH2CH2– fragment which coincidently resonate at the same posi-
tion (Fig. S3).
0
constant J(HbHb ) of ca. 12.3 Hz. Another interesting aspect of the
proton spectrum was the observation of the AB-type geminal cou-
pling between the protons attached to the –C(O)CH2– fragment
[63–65,82,83]. The signals for the pair of protons from the –
C(O)CH2– fragment (c and d) were found to appear at 2.48 and
3.65 ppm with the geminal coupling constant, J(HdHe) of
14.85 Hz. The observation of the AB-type coupling suggests the ri-
gid nature of the macrocyclic ligands that places protons c and d in
different chemical environment. In general, the proton NMR spec-
tra of the transition metal complexes of aza-macrocyclic ligands
are complicated due to the frozen conformation of the chelate rings
and the spectrum of the present complex is no exception [63–
65,82,83]. Interestingly, in contrast to complex proton NMR spec-
trum, the 13C NMR spectrum of this complex was simple and dis-
played the signals at their respective positions.
The proton NMR spectrum of complex 3 is quite complicated
since the complex as a whole is asymmetrical in nature (Fig. S3,
Supporting Information). In this case a number of coupling be-
tween various protons is clearly observable. The –C(O)CH2– frag-
ments labeled as (d/d0 and e/e0) and the –CH2– group labeled as f
and f0 display AB-type coupling [63–65,82,83]. Thus, three sets of
doublet at 2.98 and 3.82 ppm for e/e0; 2.91 and 3.90 ppm for d/d0;
3.41 and 4.48 ppm for f/f0 were observed with coupling constant
The proton NMR spectrum of complex 5 is simple due to non-
macrocyclic nature of the complex (Fig. S4, Supporting Informa-
tion). For this complex, the methyl groups (CH3 and CH3 ) are coin-
cidentally resonating at 2.65 ppm. The methylene group proton e
appeared at 3.8 ppm while the methylene protons of –CH2C(O)–
group (protons c and d) were observed at 3.05 and 3.40 ppm due
to non-equivalent nature. The 13C NMR spectrum of this complex
is quite straightforward and supports the suggested asymmetric
structure of the compound.
0
e0
f0
e
d
f
0
J(H H ), J(H Hd ) and J(H H ) of ca. 15.90, 15.45 and 16.05 Hz,
respectively. Further, the N–CH3 groups were observed at two dif-
ferent positions (2.45 and 2.51 ppm) due to the asymmetric nature
of complex. Similarly, the –CH2CH2– fragment of N,N0-dimethyleth-
ylenediamine, displays two sets of signals at 2.63 and 3.62 ppm as
multiplets (labeled as b and c). The proton NMR spectrum could
only be interpreted after carrying out the 2D-correlation,
3.5. EPR spectral studies for copper complexes
To further understand the geometry around the metal ion in
copper complexes, X-band EPR spectra were recorded as polycrys-
talline solid and in frozen DMF solution at 120 K (Figs. 5 and S7–S9,

M. Munjal et al. / Inorganica Chimica Acta 377 (2011) 144–154
151
Supporting Information). The powder EPR spectrum of complex 2
0.6
0.5
0.4
0.3
0.2
0.1
0.0
strongly justifies the square-pyramidal geometry observed crystal-
lographically as an axial signal was noted (Fig. 5a). The g|| and g\
values were obtained at 2.185 and 2.085, respectively. This spec-
trum is typical of a square-pyramidal Cu(II) ion with the unpaired
electron in the dx2–y2 orbital [87]. It is well documented in the lit-
erature that the tetragonally coordinated Cu(II) complexes show
features with g|| > 2.1 > g\ > 2.0 [87,88]. In the DMF solution, a sim-
ilar axial spectrum was observed; however, superimposed hyper-
fine coupling was clearly observable (Fig. S7). The low-
temperature powder spectra for complexes 4 and 6 displayed iso-
tropic signal with large
DHp–p values (90–115 G). The isotropic g
values were found to be 2.106 and 2.120 for complexes 4 and 6,
respectively. In contrast, frozen solution EPR spectra were clearly
anisotropic with the superimposition of hyperfine coupling from
the copper nucleus (Fig. 5b, S8 and S9).
200
300
400
500
600
700
λ(nm)
Fig. 6. UV–Vis spectra of NiII complexes 1 (—), 3 (---) and 5 (ꢀꢀꢀ) in CH3CN.
3.6. Absorption spectral studies
square-planar geometry in the presence of various potential li-
gands. The representative spectra in CH3CN for Ni(II) and Cu(II)
complexes are shown in Figs. 6 and 7, respectively.
The Ni(II) complexes 1, 3 and 5 are deep yellow to orange in
color whereas Cu(II) complexes 2, 4 and 6 were purplish-blue to
deep blue. The absorption spectra of all complexes were studied
in different solvents such as DMF, DCM and CH3CN. The kmax as
well as other prominent features were found to remain invariant
to change in solvent polarity or the coordinating ability. In addi-
tion, there is no change in the spectral features of the complexes
(except complex 2) on addition of potential axial ligands such as
Clꢁ, pyridine, and 4-dimethylaminopyridine. These results strongly
suggest that most complexes retain their four-coordinate
For nickel complexes, an absorption maximum was obtained
between 450 and 470 nm with low extinction coefficient value.
For complex 5, however, the absorption maximum is accompanied
with a low-energy shoulder at ca. 555 nm. These absorption bands
clearly point towards the square-planar geometry in the solution
as also seen for our earlier Ni(II) complexes [64–67,69,70] as well
as literature examples [89–91]. For a diamagnetic Ni(II) ion with
d8 electronic configuration in a square-planar environment, three
transitions are expected: from four lower-lying fully occupied d
orbitals to the upper empty d orbital [41,42]. However, the lower
orbitals are often so close in energy that individual transitions to
the upper d orbital is indistinguishable and that results in a single
band [41,42] as noted for the present complexes. Comparison of
the kmax values for nickel complexes shows almost identical ligand
field strength in all cases albeit a somewhat distorted donor envi-
ronment in case of complex 5. The reason can be well understood
in terms of higher flexibility in complex 5 as compared to rigid
macrocyclic complexes 1 and 3.
2.185
(a)
2.085
For copper complexes, the kmax was observed as a broad feature
between 540 and 570 nm with
e values in the range of 100–280
Mꢁ1 cmꢁ1. These low-energy bands are typical d–d transition for a
2.14
5
2.28
4
3
2
1
0
2.44
(b)
2.08
2.04
1.98
2000
3000
4000
300
400
500
600
700
800
900
H (G)
(nm)
Fig. 5. EPR spectra of (a) complex 2 in the polycrystalline state at 120 K and (b)
complex 6 in DMF at 120 K.
Fig. 7. UV–Vis spectra of CuII complexes 2 (—), 4 (---) and 6 (ꢀꢀꢀ) in CH3CN.

152
M. Munjal et al. / Inorganica Chimica Acta 377 (2011) 144–154
Cu(II) ion in a tetragonal environment and actually consists of three
individual transitions (2B1g ? 2A1g 2B1g ? 2B2g; and 2B1g ? 2E1g
1.30 V
;
in D4h symmetry) [41,42]. The complex 2 being square-pyramidal
(cf. crystal structure of complex 2) showed a broad peak and under-
lying features are clearly noticeable. Interestingly for this complex, a
shift in the kmax was noticed when the spectra were recorded in dif-
ferent solvents; for example, the kmax changed from 570 nm in
CH3CN to 542 nm in MeOH to 505 nm in DMF. These experiments
point out that the axial position occupied by the coordinated water
molecule (cf. crystal structure of complex 2) is displaced by the
solvent molecule.
1.18 V
0.64 V
0.84 V
1.37 V
3.7. Electrochemical studies
1.29 V
The cyclic voltammetric (CV) and controlled potential electrol-
ysis (coulometry) studies were performed for all complexes to
investigate the extent of stabilization of M3+ state as well as the
possible ligand oxidation. In addition, these results may also help
in understanding the accessibility of higher oxidation states and
the effect of ligand architecture on the redox potential. Reversible,
quasi-reversible and irreversible responses were observed when
the CV studies were performed in CH3CN on the glassy carbon or
platinum working electrode in the potential range of 0.2–2.0 V
(Figs. 8–10).
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
Potential (V) vs SCE
Fig. 9. Cyclic voltammograms of Ni(II) 3 (—) and Cu(II) 4 (ꢀꢀꢀ) complexes in CH3CN.
Complex ꢂ1 mM solution; TBAP as supporting electrolyte ꢂ100 mM; platinum
working electrode, Pt wire auxiliary electrode; scan rate = 100 mV/s.
0.79 V
3.7.1. Nickel complexes
The complex 1 showed only one quasi-reversible oxidative re-
sponse at 0.79 V with peak-to-peak separation (DEp) of 120 mV.
0.90 V
A single oxidative response is expected for this complex as the li-
gand, being redox innocent, is not involved in electron transfer
process [64]. The coulometric experiment confirmed the one-elec-
tron nature of this response. For this complex, the oxidized species
0.97 V
[1ox +
] was generated after oxidation at 1.0 V that resulted in the
formation of a green species. This species displayed new absorp-
tion spectral features at 595, 475, and 385 nm (Fig. S10a, Support-
ing Information). For reference, complex 1 showed only single
transition at 470 nm. Notably, the spectral features for the oxidized
1.64 V
1.51 V
2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4
species [1ox + are similar to that of species [Aox +
] ] supported with
Potential (V) vs SCE
12-membered aliphatic-based macrocyclic ligand (Scheme 1) [64].
Complexes 3 and 5 exhibited two consecutive oxidative pro-
cesses as both metal and the attached ligand take part in the redox
process. In case of complex 3, the first oxidative response was irre-
versible with Epa of 0.84 V while the second response is reversible
Fig. 10. Cyclic voltammograms of Ni(II) 5 (—) and Cu(II) 6 (ꢀꢀꢀ) complexes in CH3CN.
Complex ꢂ1 mM solution; TBAP as supporting electrolyte ꢂ100 mM; platinum
working electrode, Pt wire auxiliary electrode; scan rate = 100 mV/s.
in nature with E1/2 value of 1.34 V (DEp = 70 mV). In case of com-
plex 5, the first oxidative response is reversible (E1/2 = 0.85 V) with
peak to peak separation (DEp) of 100 mV whereas the second re-
sponse is irreversible with Epa value of 1.51 V. All these responses
were one-electron in nature as confirmed by the controlled poten-
tial electrolysis experiments. For complexes 3 and 5, the first redox
response (E1) is suggested to be metal-centered leading to a Ni(III)
state [64–66]. The second response, E2 for both complexes 3 and 5
has been tentatively assigned as the ligand-based oxidation gener-
0.51 V
0.73 V
ating a cation radical [64–66]. The species [3ox +
] was generated
after setting the potential at 0.95 V that produced a brick-red solu-
tion. The absorption spectrum of this species shows two absorption
maxima at 380 and 513 nm in addition to several weak features
(Fig. S10b). The spectral features for this species are somewhat
similar to our earlier Ni3+ species with macrocyclic ligands
[64,65,71] as well as to that of Ni3+ species reported by Journaux
and Ruiz-Garcia [89–91]. For complex 5, the electrochemical oxi-
dation at the first oxidative response resulted in the generation
of a reddish-brown solution of [5ox]+. This species displayed the
absorption maximum at 780 nm and a shoulder at ca. 480 nm
0.60 V
0.85 V
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
Potential (V) vs SCE
(Fig. S10c). Importantly, the absorption spectrum of species [5ox +
is similar to that of [Pox +
supported with an open-chain ligand
(Scheme 1) [66]. Qualitatively, the oxidized species [3ox]+ was most
stable compared to either [1ox + or [5ox]+. We postulate that the
]
]
Fig. 8. Cyclic voltammograms of Ni(II) 1 (—) and Cu(II) 2 (ꢀꢀꢀ) complexes in CH3CN.
Complex ꢂ1 mM solution; TBAP as supporting electrolyte ꢂ100 mM; platinum
working electrode, Pt wire auxiliary electrode; scan rate = 100 mV/s.
]

M. Munjal et al. / Inorganica Chimica Acta 377 (2011) 144–154
153
extra flexibility induced by the methylene spacer in [5ox +
]
may
amide-based ligands H2L1, H2L2 and H2L3. The crystal structure
analysis for representative complexes 2 and 5 substantiate that
the ligand constitute a square-based basal plane around the metal
center. The structural studies show that the Ni(II) complex 5 re-
mains square-planar whereas the geometry in Cu(II) complex 2 is
square-pyramidal with water molecule serving as the fifth ligand.
The spectral studies support that the solid-state geometry for these
complexes is retained in the solution. The electrochemical results
suggest that the Ni3+/2+ and Cu3+/2+ redox couple primarily depends
on the N4 donors composed of two Namide and two Namine atoms. It
was observed that the ligands H2L1 and H2L2 are better suited to
stabilize the Cu(III) species whereas ligand H2L3 is ideal for the
stabilization of Ni(III) species. On the basis of electrochemical find-
ings, transient Ni3+ species were generated in situ and character-
ized by the absorption spectroscopy.
have increased the distortion at the nickel center and diminished
the possible delocalization of the unpaired spin accumulated on
the Ni(III) ion; whereas in case of [3ox]+ the rigid macrocyclic nat-
ure of the ligand may have resisted the distortion introduced by
the methylene fragment [64,65,71]. Moreover, such delocalization
of the unpaired spin to the attached ligand is least feasible in [1ox +
]
due to non-aromatic nature of the ligand.
3.7.2. Copper complexes
Complex 2 showed only one quasi-reversible oxidative re-
sponse at 0.56 V with peak to peak separation (DEp) of 90 mV. As
expected for a perfect innocent system, the single oxidative re-
sponse is metal-based [64] as also noted for the nickel complex
1. In case of complexes 4 and 6, two quasi-reversible to irreversible
electrochemical responses were observed. For complex 4, the first
oxidative response is irreversible in nature with an Epa value of
0.64 V while the second quasi-reversible response was noted at
Acknowledgements
R.G. thanks Department of Science & Technology (DST) and the
University of Delhi under the scheme to strengthen R&D for the
financial support. Authors sincerely thank CIF-USIC facility of this
university for the crystallographic data collection and instrumental
facilities; Dr. Rüdiger W. Seidel at Max-Planck-Institut für Kohlenf-
orschung for his help in the crystallographic solution; and Profes-
sor S Pal (University of Hyderabad) for providing EPR spectra.
S.K. and S.K.S., respectively, thank UGC and CSIR for their
fellowships.
E1/2 value of 1.24 V (DEp = 110 mV). For complex 6, two irrevers-
ible responses (E1 and E2) appeared at 0.97 and 1.64 V, respectively.
For complexes 5 and 6, the first (E1) and second (E2) oxidative re-
sponses are tentatively assigned to be metal- and ligand-centered,
respectively [64–66]. Both E1 and E2 redox processes were one-
electron changes as concluded by the coulometric studies. In case
of copper complexes, the electrochemical oxidation at E1 potential
did not result in appreciable change in the absorption spectrum
when compared to that of original Cu(II) complexes. This observa-
tion suggests that either these complexes are reluctant to oxida-
tion or oxidation was accompanied with significant structural or
geometrical changes that were not fully supported with the pres-
ent ligand system.
It is worth mentioning here that the observed chemical irre-
versibility (during CV measurements) for some of the nickel and
copper complexes suggests a possible chemical follow-up process
centered on the complex. Such a process may lead to the decompo-
sition of the complex as elegantly shown by Collins and co-workers
[36–39] in several of their high-valent metal complexes supported
with amide-based macrocyclic ligands. It would be interesting to
explore the decomposition pathways of the metal complexes after
electrochemical and/or chemical oxidation; however, is a matter of
future studies.
Appendix A. Supplementary material
CCDC 814925 and 814926 contain the supplementary crystallo-
graphic data for complexes 2 and 5. These data can be obtained
free of charge from The Cambridge Crystallographic Data Centre
via www.ccdc.cam.ac.uk/data_request/cif. Supplementary data
associated with this article can be found, in the online version, at
doi:10.1016/j.ica.2011.08.017.
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