Download Synthesis and characterization of macrocyclic complexes of Co (II

Indian Journal of Chemical Technology
Vol. 19, September 2012, pp. 351-356
Synthesis and characterization of macrocyclic complexes of
Co (II), Ni (II) and Cu (II)
C Surendra Dilip*, V Sivakumar & J Joseph Prince
Anna University of Technology, Tiruchirappalli 620 024, India
Received 27 April 2011; accepted 21 June 2012
The ligand 3,8-dimethyl-5,6-benzo-4,7-diazadeca-3,7-diene-2,9-dione dioxime [(DOH)-bzo] has been synthesized by
the condensation of o-phneylenediamine with diacetyl monoxime in a 1:2 mole ratio. It has been isolated in the solid state
and characterized by CHN data, and IR and NMR spectral studies. Six coordinate Co (II) complex, four coordinate square
planar Ni (II) complex and five coordinate square pyramidal Cu (II) complex are synthesized by the reaction of the
respective metal chlorides. All these complexes are characterized by infrared, electronic spectroscopy and conductivity
measurements. Indirect evidence for the formation of intra-molecular hydrogen bond in these complexes is obtained by
replacing the hydrogen bonded proton by BF2 group by treating the complex [Cu (DO) (DOH) bzo) Cl] with BF3OEt2.
Keywords: 1H NMR spectra, IR spectra, Macrocyclic complexes, Metal complexes
In living systems, metal ions play an important role as
a catalyst for many biological reactions. The activity
of such metal ions depends on their confinement
within approximately planar, tetra dentate and totally
enclosed macrocyclic ligands, which themselves are
highly conjugated species1-3. Quadridentate tetraaza
macrocyclic ligands fulfil the above requirements and
hence serve as effective models for biological
systems. The study of transition metal complexes of
macrocyclic ligands as model for biological system
stems mainly from the difficulties encountered in
isolating the biological systems. These are due to their
high molecular weights, extreme molecular
complexity, limited solubility and instability in
aerobic conditions. The macrocylic complexes of
transition metal ions cannot serve as biological
mimics of the primary bioinorganic systems but can
act as simple and an efficient model to extract the
essential details of the biological systems4, 5.
A large number of macrocylic ligands synthesized
are highly conjugated and yield complexes that
exhibit extensive electron delocalization. The
extensive conjugation dictates the co-planarity of the
ligand. The complexes of such chelates are therefore
used to study ligand bonding and electronic
transmission induced by the ligand to the metal ion6-8.
The study has a wider scope of establishing the
______________
*Corresponding author.
E-mail: [email protected]
inverse relationship between the cation cavity
provided by the ligand and the size of the metal ion in
stabilizing the complexes. The trend is followed from
Sc to Zn in determining the stability of the complexes.
The present study intends to arrive at the “cationcavity best fit” in a planar array without disturbing the
planarity of the ligand structure. It is expected that the
cation size related modifications of the ligand would
be evident. This would help to pursue the effect of
conjugation on the physiochemical properties of the
given metal ions. The study thus aims at synthesizing
Co(II), Ni(II) and Cu(II) complexes of the hitherto
unreported ligand 5,10-dimethyl-2, 3, 4; 7, 8 ; 11, 12,
13-tribenzo-6,9-diaza-1,14-diamine. The complexes
are characterized by CHN analysis, conductivity
measurement, IR, UV-Visible spectral studies and
estimation of Ni, Co, and Cu.
Experimental Procedure
Reagents
Cobalt (II) chloride (AR, BDH), copper (II) chloride
(AR, BDH) and nickel (II) chloride (AR, BDH) were
used as such for the synthesis of metal complexes. For
the synthesis of ligands (L), o-phenylenediammine
(LOBA) and diacetylmonoxime were used wherein ophenylenediamine was recrystallized from hot water
using activated charcoal prior to use.
Solvents
Commercial grade ethyl alcohol was refluxed for
7h over lime and the fraction boiling at 78-79oC was
INDIAN J. CHEM. TECHNOL., SEPTEMBER 2012
352
collected and used. Double distilled water was
obtained by distilling water over alkaline KMnO4 in
an all glass apparatus.
asymmetric deformation vibrations, respectively, of
the methyl groups11.
The strong bands appearing at 1260, 1180 and
1130cm-1 are assigned to ν(C-C), ν(C-N) vibrations or
to the coupled vibrations of these modes. The metal
sensitive strong as well as sharp band at 765cm-1 may
be attributed to C=N-O deformation vibrations. The
metal dependent strong vibration as well as sharp
bands appearing at 990, 920, 910cm-1 can be ascribed
to the deformation vibrational modes of the ligand.
The proton magnetic resonance spectrum of the
ligand in DMSO-d6 gives sharp singles at 1.96 and
2.66 ppm corresponding to the two types of methyl
protons. The aromatic protons give broad multiplets
centred at 7.8 ppm and the two oxime protons give a
sharp singlet at 10.86ppm12.
Synthesis of ligand
To a solution of o-phenylenediamine (10.8g,
0.1mol) mixed in 100 mL warm distilled water, a
solution of diacetylmonoxime (20.2g, 0.2mol) was
added and refluxed for 1 h till yellow crystals are
separated out. The reaction mixture was then cooled
to 5oC and filtered. The yellow crystals (yield 80%
and m.p. 176oC) were recrystallized in dry ethanol.
Synthesis of complexes
To a solution of [(DOH)2 bzo] (0.409 g, 5 mmol)
mixed in ethanol, a hot ethanol solution of metal salts
[Co(II) chloride (1.4g, 5 mmol), Ni(II) chloride
(0.95g, 5 mmol) and Cu(II) chloride (0.554g, 5
mmol)] were added and refluxed for 3 h. The resultant
solution was cooled overnight and the crystals which
separated out were filtered through sintered crucible
washed with cooled ethyl alcohol and dried over
anhydrous CaCl2.
Characterization of complexes
The metal-ligand complexes were synthesized at
1:1 mole ratio and obtained in pure form. Hence,
these were not further purified by recrystallization.
The results of the percentages of metal present in the
complexes determined gravimetrically are given in
Table 113.
Results and Discussion
Characterization of ligand
Molar conductance
The recrystallized pure ligand melts at 276oC as
reported in the literature7,8. The infrared spectrum of
the ligand shows no characteristic absorptions
assigned to either C=O or NH2 groups, which is due
to the formation of ligand. The sharp band of medium
intensity occurred at 1575cm-1 is attributed to C=N,
while a similar band at 1630cm-1 is assigned to C=C.
The strong as well as sharp band at 1220cm-1 is due to
the N-O stretching vibrations9,10.
The CH stretching vibrations result in a weak band
at 3260cm-1 and a broad band at around 2900cm-1 is
assigned to OH. A very strong band of medium
broadness at 1430cm-1 and a weak band at 1430cm-1
are assigned to the ring models of O-disubstituted
benzenes. The sharp bands of medium intensity at
1375cm-1 and 1445cm-1 are due to the symmetric and
At room temperature the observed molar
conductances of the complex in DMF are in the range
of 10-20 ohm-1cm2mole-1. Hence, from the
conductivity measurements, it is concluded that the
chloride ions are covalently bonded to metal ions.
Based on the metal-ligand ratio calculated analytically
and the nature of the electrolytes given by the
conductance measurements, compositions are
assigned for the prepared complexes. Molar
conductivity results of [Co(DO)(DOH)bzo)Cl] and
[Cu(DO)(DOH)bzo)Cl] complexes in DMF are found
to be 27 and 32 ohm-1cm2mol-1 respectively,
indicating both to be neutral complexes. This shows
that chloride acts as a ligand and not as a simple ion14.
The molar conductance of [Ni(DO)(DOH)bzo)]Cl
complex is 82 ohm-1cm2mol-1 indicating that it is a 1:1
Table 1Analytical data of complexes
Complex
Colour
Yield
%
Mol.
wt
MP
°C
µ eff
BM
[Co(DO)(DOH)bzo)Cl]
Pink
65
1480
280
1.90
[Ni(DO)(DOH)bzo)]Cl
Green
70
1458
295
4.81
[Cu(DO)(DOH)bzo)Cl]
Pale blue
70
1468
270
2.8
Elemental analysis [Found (Calcd.)], %
Λm
−1
2
−1
M
C
H
N
Cl Ω cm mol
27
7.56
64.84
3.42
13.17
7.54
(7.51)
(64.86) (3.79) (13.15) (7.53)
82
7.05
66.50
4.41
13.10
7.44
(7.00)
(66.51) (4.77) (13.08) (7.41)
7. 05
67.21
4.15
14.01
7.60
32
(7.45)
(67.32) (4.55) (14.00) (7.05)
DILIP et al.: SYNTHESIS & CHARACTERIZATION OF MACROCYCLIC COMPLEXES OF METALS
353
electrolyte. This value falls within the range of molar
conductance values of 1:1 electrolytes dissolved in
DMF and confirms the formation of the proposed
cationic complexes in the solution and thus the
chloride is a simple ion and not a ligand. The molar
conductivity data is provided in Table 1.
symmetric and asymmetric deformation vibrations of
the methyl groups. A unique feature in this study is the
occurrence of a new strong and sharp band at 1090cm-1
in the complex, that is not observed in the ligand and
which is assigned to ν(NO).
Infrared spectra
The electronic absorption spectra are often very
helpful in the evaluation of results furnished by other
methods of structural investigation. The electronic
spectral measurements were used for assigning the
stereochemistry of metal ions in the complexes based
on the positions and number of d–d transition peaks.
The electronic absorption spectra of the Schiff base
and its Co(II), Ni(II) and Cu(II) complexes are
recorded at room temperature using acetone as solvent
and the results are listed in Table 2.
The magnetic moment value of the Co(II) complex
is found to be 4.81BM which suggests a high spin sixcoordinated octahedral arrangement of ligand
molecules around the metal ion (Fig. 1). The complex
shows two characteristic peaks at around 17856 and
21734cm–1, assigned to 4T1g(F) → 4A2g(F) and
4
T1g(F)→ 4T1g(P) transitions respectively. The Ligand
Field Stabilisation Energy values are calculated from
transition energy ratio diagram using the ratio ν2/ν1
which lies at 1.21. The inter electronic repulsion
parameter of the complex B′ is found to be 972cm-1
which is lower than that of the free ion value
(B=1148cm-1), suggesting the delocalisation of metal
electrons into metal-ligand molecular orbital18.
The IR spectra of all metal complexes are
compared with those of the free ligands. On
comparison, the band positions of various vibrations
are ascertained. In the infrared spectrum of the ligand,
the band at 1385-1400cm-1 is due to asymmetric
(C=N) stretching vibration15.
The presence of ν(C=N-C) asymmetric stretching
vibration is confirmed by a band at 1520cm–1. In the
infrared spectrum of metal complexes, the ν(C=N)
stretching vibrations are observed at around
~1480cm–1 and are due to the coordination of nitrogen
from C=N to the metal. The free ligand shows a
strong peak at 1520cm–1, which is characteristic of the
imine ν(C=N) group. The ν(C-C) stretching vibrations
are affected upon complexation and are situated at a
frequency significantly different than that of the free
ligands. Coordination of the ligands to the metal
centre through the nitrogen atom is expected to reduce
the electron density in the methine and imine link, and
hence lower are the ν(C-C) and ν(C=N) absorption
frequencies. The peaks due to ν(C-C) are slightly
shifted to lower frequencies and appear at around
1576–1579cm–1, indicating the coordination of imine
nitrogen to the metal. Hence, the shifts in the –C=N
and C-C stretches of the metal complex provide
support for ML6-type coordination16.
The characteristic infrared absorption bands of the
ligand are shifted, lowered in intensity on formation
of the complex and new vibrational bands
characteristic of the complex appear. A broad band
of medium intensity, occurring at 2900cm-1 in
the free ligand and shifted to 3120cm-1 in
[Cu(DO)(DOH)bzo)Cl], is attributed to the ν’(OH) of
the hydrogen bonding17.
The ν(OH) of the complex is observed at 1196 cm-1.
The bands observed at 1330 and 1400cm-1 indicates the
Electronic absorption spectra
Fig. 1Proposed structure of ligand and its complex, where M= Co (II)
Table 2Electronic spectral data and ligand field parameters of complexes
Complex
ν1, cm-1
ν2, cm-1
B, cm-1
B’, cm-1
β
Β, %
ν2/ ν1
LFSE, kcal.mol-1
[Co(DO)(DOH)bzo)Cl]
[Ni(DO)(DOH)bzo)]Cl
[Cu(DO)(DOH)bzo)Cl]
17856
19240
16638
21734
-
1148
1030
1060
972
946
923
0.84
0.91
0.87
13.34
08.16
12.93
1.21
-
25.34
23.50
32.65
354
INDIAN J. CHEM. TECHNOL., SEPTEMBER 2012
The Ni(II) complex is high spin with a room
temperature magnetic moment value of 2.8 BM,
indicating a spin-free square planar configuration. The
appearance of a band at 19240cm–1 due to 3A2g(F)
→3T1g(P) transition favours an square planar geometry
for the Ni(II) complex. The absence of any band below
10,000 cm–1 eliminates the possibility of a tetrahedral
environment in this complex. The inter electronic
repulsion parameter of the complex B′ is observed at
946cm-1, which is lower than that of the free ion value
(B=1030cm-1). The spectrum also shows a band at
24,556cm−1 that may be attributed to ligand–metal
charge transfer19.
The Cu(II) complex exhibits a magnetic moment of
1.90 BM at room temperature. The energy level
diagram for ligand field of distorted Oh, D4h symmetry
predicts three transitions, viz. 2B1g→2A1g(νI) 2B1g→2B2g
(ν2) and 2B1g→2Eg (ν3). However, only one broad band
(weak and split) at 16,638cm–1 is generally observed in
the visible region for many Cu(II) complexes as a
result of overlap of these three bands. Hence, the
absorption maximum of 2Eg→2T2g transition
corresponds to 10Dq value from which LFSE can be
evaluated for the five coordinate square pyramidal
geometry. The broadness of the band is due to the
ligand field and the John-Teller effect. The inter
electronic repulsion parameter for the Cu(II) complex
(B'=923cm-1) is less than the free ion value
(B=1060cm-1), suggesting a considerable overlap and
also delocalisation of the metal and ligand orbital. The
β value indicates that the covalent character of metal
ligand sigma bond is very low20.
gav = 1/3[g|| + 2γ⊥] and
gav = 2(1–2l/ 10Dq)
This value is found to be less than that of the free
Cu(II) ion (–832cm–1), which also supports covalent
character of M–L bond in the complex21. The G value
of 3⋅82 indicates negligible exchange interaction of
Cu–Cu in the complex. The covalency parameter α2 is
calculated (α 2 = 0⋅82) using the following equation:
α2Cu = –(A||/0⋅036) + (g|| – ⋅0023) + 3/7(g⊥–2⋅0023) + 0⋅04.
If the value of α2 = 0⋅5, then it indicates complete
covalent bonding, while a value of α2= 1⋅0 suggests
complete ionic bonding. The observed value of α2 =
0⋅82, of the complex is less than unity, indicating that
the complex has some covalent character in the ligand
environment22.
Cyclic voltammetric study
The cyclic voltammogram of Cu(II) complex (0⋅01
M) in 0.1 M TBATFB in DMSO on Pt electrode,
v=0.1 V S—1, (vs. Ag|Ag+ electrode) is represented in
ESR spectra
The ESR spectra of copper complex provide
information on importance in studying the metal ion
environment. The X-band ESR spectra of the Cu(II)
complex, recorded in DMSO at liquid nitrogen
temperature (77K) and at room temperature (300 K),
are shown in Figs 2a and b.
The spectrum of the copper complex at room
temperature shows one intense absorption band in the
high field and is isotropic due to the tumbling motion
of the molecules. However, this complex in the frozen
state shows four well resolved peaks with low field
region. The copper complex exhibits the g|| value of
2⋅31 and g⊥ value of 2⋅16. These values indicate that
the ground state of Cu(II) is predominantly dx2–y2.
The spin-orbit coupling constant l (− 486cm–1) is
calculated using the relationship as shown below:
Fig. 2 ESR spectra of Cu (II) complex in DMSO solution at (a)
300 K and (b) 77 K
DILIP et al.: SYNTHESIS & CHARACTERIZATION OF MACROCYCLIC COMPLEXES OF METALS
Fig. 3. (1⋅0 to –1⋅2 V potential range) shows a welldefined redox process corresponding to the formation
of Cu(II)/Cu(III) couple at Epa = 0⋅22 V and the
associated cathodic peak at Epc = 0⋅18 V. This couple
is found to be reversible with ∆Ep ≈0⋅04 V and the
ratio of anodic to cathodic peak currents (Ipc/Ipa = 1)
corresponds to a simple one-electron process. The
complex also shows a quasi-reversible peak in the
negative region which is characteristic of the
Cu(II)→Cu(I) couple at Epc = –0⋅47 V, with
associated anodic peak at Epa = –0⋅34 V for
Cu(I)→Cu(II) oxidation23. The cyclic voltammogram
of Ni(II) complex (0⋅01 M) in 0.1 M TBATFB in
DMSO on Pt electrode, v=0.1 V S—1, (vs. Ag|Ag+
electrode) (in the absence of molecular oxygen) at
room temperature in the potential range of 1⋅0 to –1⋅0
V and at a scan rate 50 mVs–1 indicate a quasireversible one-electron process.
A noteworthy feature is observed in the cyclic
voltammogram of Ni(II) complex. During the forward
scan it shows two cathodic reduction peaks, one at
+0⋅45Vand another at –0⋅61 V that are attributed to
the reduction of Ni(II) →Ni(I) and Ni(I) →Ni(0)
respectively. During the reverse scan it shows two
anodic oxidation peaks, one at –0⋅26 V and another at
+0⋅62 V which are attributed to the oxidation of Ni(0)
→Ni(I) and Ni(I) →Νi(II) respectively24.
Intramolecular hydrogen bonding
The free ligand is a neutral molecule which upon
complexation loses one of the oxime protons with the
concomitant formation of an intramolecular hydrogen
bond. All these complexes also exhibit two strong and
sharp bands at 1220 and 1075 cm-1, assigned to ν(NO)
and ν’(NO) respectively. The appearance of only one
band at 1220 cm-1 assigned to ν(NO) in the free
ligand shows that the two NO- linkages are identical,
while the appearance of two bands assigned to ν(NO)
and ν’(NO) in the complexes, indicate the presence of
two unequal –NO linkages. Indeed, upon
complexation, there are 2-NO-linkages (C=N-O-H
and C=N-O…H) due to the formation of hydrogen
bonding.
Studies on cobaloximes by Schruazer25 also
confirm the presence of two unequal NO-linkages due
to hydrogen bonding. Disappearance of the ν(OH)
band in the complexes is a further evidence for the OH-O bond formation.
Indirect evidence for the intramolecular hydrogen
bonding is obtained by replacing the hydrogen bonded
355
Fig. 3Cyclic voltammogram of 1 mM Cu (II)-complex
proton in [Cu(DO)(DOH)bzo)Cl] by the BF2 group,
by treating the complex with BF3.OEt2. This reagent
readily reacts with the O–H–O bridge in the complex,
affording the stable complex containing the O-BF2-O
bridge. Schrauzer replaced the two hydrogen bonded
protons in bis(dimethyl glyoximato) Nickel(II) by the
BF2 group by treating with BF3·OEt2 for the first time.
Later, Schrauzer and Windgassen replaced the
hydrogen bonded proton in methyl (aquo) cobaloxime
by the BF2 group using the same method26.
Conclusion
In the present investigation the quadridentate
ligands employ coordinates readily with Co(II) in the
equatorial position and readily afford the synthesis of
six coordinate octahedral complex. Ni(II) forms a
stable square planar complex with the pseudomacrocyclic ligand. Cu(II) forms square pyramidal
five coordinate complexes in which the metal anion
occupies one of the apical sites. The ligand readily
affords synthesis of 6 coordinate complexes wherein
the apical sites can be altered and it seems to be a
convenient vehicle to explore the influence the ligand
conjugation on the electronic properties of the metals.
However, detail EPR and electronic spectral
investigation are needed to examine the influence of
structure and conjugation on the geometry and
reactivity of transition metals.
The synthesis of octahedral complexes to the
family of closely related monondendate ligands such
as Lewis bases and detailed spectral investigations
would enable the construction of spectrochemical
series of the axial ligands. A detailed electrochemical
investigation of such a series of complexes would
throw more light on the influence of axial and
equatorial ligands on the metal ion.
356
INDIAN J. CHEM. TECHNOL., SEPTEMBER 2012
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