Download IJCA 49A(2) 171-175

Indian Journal of Chemistry
Vol 49A, February 2010, pp. 171-175
Synthesis, characterization and ferromagnetic
behavior of [CoIII(en)2(RC6H4NH2)I]I2.H2O
complexes
K Anbalagan*, A S Ganeshraja & S Kandasamy
Department of Chemistry, Pondicherry University,
Kalapet, Pondicherry 605 014, India
Email: [email protected]
Received 24 August 2009; revised and accepted 21 January 2010
Mixed ligand cobalt(III) complexes containing aryl amine
(RC6H4NH2) ligand have been synthesized using a modified route
developed in this study. A wide range of complexes,
[CoIII(en)2(L)I]I2.H2O, (where L = RC6H4NH2; R = m-OMe, p-F,
H, m-Me, p-Me, p-OEt and p-OMe) show promising reactivities
and physical properties due to the presence of electron
donor/acceptor groups in the aryl ligand. The complexes have
been characterized by elemental analysis, FT-IR, UV-vis and
powder X-ray diffraction patterns. FT-IR ν(M-X) and UV-vis
spectral data are compatible with trans-form of the complexes.
Magnetic studies yield magnetic remanence Mr = 7.071 × 10-3 –
3.069 × 10-5 emu/g and intrinsic coercivity, -Hci = 141-1156 Oe.
Linear regression plot of Mr versus σ yields negative slope,
implying that electron withdrawing group in RC6H4NH2 (σ > 0)
enhances ferromagnetic character while the electron donating
groups (σ < 0) reduce ferromagnetism. The study provides
evidence that substitution at the aryl ligand, remote from the
Co(III) center, effectively displays and imparts characteristic
ferromagnetic properties. Cobalt(III) complexes containing
RC6H4NH2 ligand illustrate a simple molecular fabrication
technique, which can be suitably integrated into a system, to have
some control on the behavior of metal complexes. A modified
ferromagnetic character due to charge build up on the metal centre
by ligating nitrogen atom, which in turn makes the molecule into a
centre of systematic variation in magnetization, has been
observed.
Keywords: Coordination chemistry, Ferromagnetism, Cobalt,
Aryl amines
IPC Code: Int. Cl.9 C07F15/06
Transition metal complexes with aromatic amines as
ligands such as aniline, RC6H4NH2, and its derivatives
have been known for many years. The physical
properties of these compounds are often used as
models in more complicated biological systems1.
Recently, aryl/alkyl amine-cobalt(III) complexes have
been synthesized and structurally characterized by
spectroscopic and X-ray crystallographic analysis;
such complexes exhibit distortion in octahedral
symmetry2. In order to understand the substituent
effects due to varying chemical structures of
cobalt(III) complexes on the spectral, powder X-ray
diffraction patterns and magnetic properties, a series
of cobalt(III)-aryls containing electron-donating or
withdrawing substituents attached in 3- and
4-positions in the aniline ring of the ligand have been
synthesized.
Pazderski et al. reported3 the effect of auxiliary
ligand
in
trans-[Co(py)4Cl2]Cl.6H2O
and
mer-[Co(py)3Cl3] complexes on the spectral
properties including 15N NMR/1H NMR. Synthesis of
aryl amine cobalt(III) complexes is heavily dependent
on appropriate conditions and has been unsuccessful
due to the steric effect of both the existing chelate in
the coordination sphere and incoming aryl ligand
during substitution. Linear polyamines and aryl
amines are of great interest4 from several perspectives
due to systematic change in their substitution, redox
reactions, and magnetic behavior, more interestingly,
in biological systems. Similarly, metal complexes
containing polyamine/N-alkyl ligands in Co(II)5 and
Co(III)6 complexes illustrate the steric hindrance in
original coordinating modes7. Some of the
modifications carried out in the ligated polyamine
leading to N-benzylation8 of Co(III) complexes are
scarce9. However, they illustrate characteristic
structural features in reactivities.
In this study, we have synthesized and
characterized seven aryl amine-cobalt(III) complexes,
[CoIII(en)2(L)I]I2 (where L = RC6H4NH2; R = m-OMe,
p-F, H, m-Me, p-Me, p-OEt and p-OMe) to
understand the geometrical distortion and the linear
dependence of ferromagnetism due to the presence of
aryl ligand in the coordination sphere of cobalt(III)
complexes. The aniline ligand present in the
coordination sphere imparts novel magnetic
properties10. Variations in the powder X-ray
diffraction patterns have also been discussed.
L
2+
NH2
NH2
Co
NH2
III
NH2
I
trans - [CoIII(en)2(L)I]I2
172
INDIAN J CHEM, SEC A, FEBRUARY 2010
Experimental
CoCl2.6H2O (99%), aryl amines (RC6H4NH2) and
KBr (spectral grade) were purchased from Sigma
Aldrich. HI (AR), 1,2-diamino ethane (LR) and all
other chemicals were purchased from SD. Fine
Chemicals (India). All the solvents and 1,2-diamino
ethane were distilled and purified before use. Water
was triply distilled over an all-glass apparatus.
Elemental analyses were performed using an
Elementar Vario EL III-Germany instrument. The FTIR spectral investigations in the range 4000-400 cm-1
were carried out on a Thermo Nicollet-6700 FT-IR
instrument (KBr pellet method) and far-IR spectra
were obtained using polyethylene pellet technique.
The crystalline powders were mixed with
polyethylene in a 1% (w/w) mixture and pressed into
pellet at 10 ton pressure for 8 min. to obtain a
transparent pellet, which was inserted in the apparatus
to record vibrational spectrum in the range 400 to
50 cm-1 with a resolution 1 cm-1.
Electronic absorption spectral studies were carried
out using Shimadzu (model 2450) double beam
spectrophotometer with quartz cells with complex
(2 × 10-4M) in neat water.
Magnetic measurements were carried out using
vibrating sample magnetometer (VSM) in powder
form on Lakeshore-7404 (sample vibration frequency
82.5 Hz, dynamic range (1 × 10-7) to 103 emu, max.
field at 1′′ gap: 15 kOe). The VSM technique was
used to observe the hysteresis curve for the
compounds [CoIII(en)2(RC6H4NH2)I]2+ in dc magnetic
field.
Powder X-ray diffraction (PXRD) patterns were
recorded in the range of 2θ = 10 -70° by step scanning
using 2θ increments of 0.02° and a fixed counting
time of 5 s/step with X’PERT diffractometer
employing Cu-Kα radiation. Wavelength of the X-ray
source was λ = 1.541 Å. The acceleration voltage was
50 kV with 40 mA current.
Trans-iodo(R-aniline)bis(1,2-diamino
ethane)
cobalt(III) iodide complex was prepared from
trans-[Co(en)2I2]I by a modified method as described
in the literature11. A paste of the complex was
prepared in a mortar with 1g of trans-[Co(en)2I2]I
crystals in 3-4 drops of deionized water. To this,
about 1.5 mL of R-aniline was added in drops
for ~20 min with vigorous grinding. The grinding was
continued for half an hour and the color was found to
change from dark green to reddish brown. The
reaction mixture was allowed to stand overnight in the
dark for completion of the reaction. Finally, the solid
obtained was washed 3-4 times using pure ethanol.
The final complex was recrystallized by dissolving in
5-10 mL of deionized water, acidified with a few
drops of HI and preheated to 70°C. The crystals were
filtered, washed with ethanol and kept in a desiccator.
A similar procedure was used to synthesize the other
complexes by changing time of addition and
concentration of ligand. Yield and elemental analysis
data for the [CoIII(en)2(RC6H4NH2)I]I.H2O complexes
were as follows: R = m-OCH3; yield: 0.722 g, 72%,
Anal.: Calc. (Found): C, 18.85 (19.10); H, 3.88
(3.20); N, 10.01 (9.82)%. p-F; yield: 0.80 g, 80%,
Calc. (Found): C, 17.43 (17.01); H, 3.51 (3.13); N,
10.19 (10.01)%. H; yield: 0.782 g, 78%, Calc.
(Found): C, 17.91 (18.02); H, 3.76 (3.20); N, 10.46
(10.23)%. m-CH3; yield: 0.764 g, 76%, Calc. (Found):
C, 19.29 (19.03); H, 3.97 (3.58); N, 10.25 (10.02)%.
p-CH3; yield: 0.844 g, 84%, Calc. (Found): C, 19.29
(18.74); H, 3.97 (3.72); N, 10.25 (10.04)%. p-OC2H5;
yield: 0.752 g, 75%, Calc. (Found): C, 20.16 (19.70);
H, 4.09 (3.61); N, 9.82 (9.62). p-OCH3; yield, 0.691 g,
69%, Calc. (Found): C, 18.85 (19.02); H, 3.88 (3.90);
N, 10.01 (9.73)%.
Results and discussion
Micro elemental analysis data are in accordance
with the formula, [CoIII(en)2(RC6H4NH2)I]I2.H2O
complex. The complexes were found to be
trans-isomers by the synthetic method and spectral
assignments. Trans-[Co(en)2I2]I + RC6H4NH2 →
trans-[CoIII(en)2(RC6H4NH2)I]I2 ligand replacement
was followed using UV-vis repeated scan spectra
which indicated a shift in λmax at 416.9 nm to a longer
wavelength. A single low-energy absorption band at
ca. 601 nm is characteristic of the trans-form.
IR spectra of [CoIII(en)2(RC6H4NH2)I]I2.H2O
complexes in the 3500-400 cm-1 ranges closely
resemble each other in absorption patterns. IR data of
the complexes shows the peaks at 3500 to 3400 cm-1,
which may be attributed to the antisymmetric and
symmetric ν(O-H) stretching modes of lattice-bound
water. The stretching absorption bands at
1650-1620 cm-1 confirm bending modes of ν(O-H)
due to hydrogen bonding in the complexes,
suggesting hydrated form of molecule. The
considerable shift in the prominent antisymmetric
ν(N-H) stretching modes at ~3318-3005 cm-1 (metalamine at ~3300 cm-1) and deformation mode at
~1632-1497 cm-1 (metal-amine ~1600 cm-1) shows
NOTES
Co-amine coordination. Non-splitting of ν(NH2)
asymmetric deformation frequencies at 1593 cm-1
indicates the trans-character of the complex12. The
characteristic absorptions at 1490-1430 cm-1 and
1400-1350 cm-1 are assigned to ν(CH2) bending, at
1290-1250 cm-1 to wagging and at 920-860 cm-1 due
to ν(CH2) rocking modes. Aryl amine ligand stretches
observed at 1507 and 1455 cm-1 agree well with
reported literature values for iso-structural cobalt(III)
and iridium(III) complexes13,14.
The vibrational modes appearing in the far-IR
region are indicative of the coordination arising from
metal-ligand stretching. The stretching modes
observed for [CoIII(en)2(RC6H4NH2)I]I.H2O closely
resemble that of (4BzpipdH)2CoX4 (X = Br, I)
complexes.
The
far-IR
spectra
of
the
trans-[CoIII(en)2(RC6H4NH2)I]I2.H2O
complexes
reveal intense absorption bands at ~320 cm-1 assigned
to ν(Co-I) stretching vibration (assuming C4V
symmetry of the trans-[MN4IN] chromophore)3. This
ν(Co-I) parameter is lower than that for ν(Rh-Cl)
(364-362 cm-1) and ν(Ir-Cl) (335 cm-1) in
trans-[Mpy4Cl2]Cl (M = Rh/Ir), following the rule
that energies of metal-halogen stretching modes
depend on central atom mass. Therefore, the bands at
320 and 280 cm-1 may be assigned to the v(M-X)
stretching vibrations15, 320-294 cm-1 to ν(N-M-N)
stretching bands and 490-458 cm-1 peaks to ν(M-N)
stretching. Weak absorption peaks in the region
1335-1290 cm-1 are more complex to identify.
However, a band at 1335-1290 cm-1 is altered by
substitution in the coordination sphere.
Electronic absorption spectra of [CoIII(en)2
(RC6H4NH2)I]I2.H2O complexes in neat water are
identical in shape and differ only in position of
absorption maximum and extinction coefficient
values.
The UV-vis spectra of the complexes closely
resemble one another and show bands at ~232 nm,
~ 280 nm, ~344 nm, ~ 465 nm and 601 nm. The two
higher energy bands can be assigned due to ligand-tometal charge transfer transitions, while lower energy
bands represent d-d transitions. Regular spin allowed
transitions in cobalt(III) complexes are 1A1 → 1T1 and
1
A1 → 1T2. These split into components for complexes
belonging
to
lower
symmetry
such
as
CoIII(en)2(RC6H4NH2)I2+ as 1A1→ 1A2 + 1Ea and
1
A1 → 1B2 + 1Eb. Among the four transitions only the
1
A1 → 1B2 is magnetically forbidden11,16. The two higher
energy visible bands are centered at 328-350 nm and at
173
461-470 nm, while the low energy band is expected to
appear at 600 nm (E band). However, the latter is
obscured by high energy band at the present
experimental condition.
The complex ion, CoIII(en)2(RC6H4NH2)I2+, shows
two intense bands in the UV region. These are
interpreted on the basis of LCAO-MO theory to be
charge transfer bands from p-orbitals of halide ion to
mainly dz2 orbital of the cobalt(III) ion. The shorter
wavelength transition pπ → dz2 is centered at ~ 287 nm
while the other pσ → dz2 is centered at ~ 345 nm and
the π → π* transition due to RC6H4NH2 ligand is at
~ 232 nm. High energy charge transfer band is
obscured in some cases or such transition is not
detected at this concentration17. Aniline systems show
optical transition18 from HOMO to LUMO, π → π* at
250 nm. However, acceptor/donor groups attached to
the ring alter the energies of LUMO/HOMO levels.
Intensities of absorption due to d-d bands of
CoIII(en)2(RC6H4NH2)I2+ are parallel with that of
magnitude19 of ε for Co(en)(NH3)3CN2+ ion. It is
reported that molar extinction coefficient of d-d band
increases11 by a factor of about ten when X is
substituted by Cl → Br → I. More importantly, a
weak band at ca. 601 nm is characteristic of the trans
structures. In addition, cobalt(III) complexes of lower
symmetry containing aromatic amines exist preferably
in trans-form and the cis-form is less probable11.
Powder X-ray diffraction pattern of the
trans-[CoIII(en)2(RC6H4NH2)I]I.H2O complexes have
not been reported previously. However, there are
previous studies19,20 on cobalt-ammine chloride and
[Co(NH3)5(OH2)]I3. The unit cell parameters
evaluated from powder X-ray diffraction patterns are
given in Table 1. The dhkl and 2θ values observed are
found to be in good agreement with that of values
indexed for monoclinic analogue of [Co(en)2Cl2]ClO4
(JCPDS: 00-023-1605, 00-023-1606). The prominent
peaks observed between 2θ = 18.5 to 45º were
matched with that of peaks of the reference21,22.
Table 1 presents unit cell dimensions (a, b, c) values,
interfacial angles (α, β, γ), cell volume and particle
size (<d> in nm). When iodide is replaced with
RC6H4NH2 in the coordination sphere of
[CoIII(en)2I2]I, the particle size is expected to be
enhanced, which is evident from the tabulated data.
Thus, the X-ray diffraction pattern of cobalt(III) aryl
amine complexes can be indexed with monoclinic
system
in
comparison
with
analogous
[Co(en)2Cl2]ClO4.
174
INDIAN J CHEM, SEC A, FEBRUARY 2010
Table 1 ―Powder X-ray diffraction parameters for trans[CoIII(en)2(RC6H4NH2)I]I2.H2O complexesa
R in aniline
Unit cell dimensions (Å)
a
b
c
10.34
7.98
9.51
10.94
8.46
8.12
4.32
8.20
18.39
12.74
8.20
8.90
3.75
28.47
23.61
12.95
7.65
9.03
13.68
7.36
9.83
m-OMe
p-F
H
m-Me
p-Me
p-OEt
p-OMe
Interfacial angle (β°)
Vol. (Å 3)
Cystallite size (nm)
99.12
98.75
99.86
98.94
98.92
98.86
99.01
776.22
742.58
647.12
918.92
2487.02
883.63
976.81
71
89
79
71
89
98
87
a
α = 90°, γ = 90°; 2θ = 10 - 70°; step scanning 2θ increments of 0.02°; counting time 5s/step; X- ray source λ = 1.541 Å;
acceleration voltage 50 kV with 40 mA current.
Table 2 ―Magnetic characteristics obtained from
vibrating sample magnetometer for
trans-[CoIII(en)2(RC6H4NH2)I] I2.H2O complexesa
R in
aniline
m- OCH3
p- F
H
m- CH3
p-CH3
p-OC2H5
p-OCH3
Hammett’s
substitution
constant, σ
Intrinsic
coercivity,
−Hci (Oe)
Remanent
magnetization,
Mr ×10-3 (emu/g)
+ 0.115
+ 0.062
0.000
- 0.069
- 0.170
- 0.250
- 0.268
570
708
415
248
1157
141
596
7.071
4.530
6.040
3.167
1.086
0.861
0.031
Vibrating frequency 82.5 Hz; dynamic range (1×10-7) to
103 emu; max. field at 1” gap: 15 kOe in dc magnetic field at
27°C.
0.15
0.10
0.05
0.00
-0.05
σ
-0.10
-0.15
-0.20
-0.25
a
Magnetic hysteresis loops are the experimental
evidence, giving the most distinctive fingerprint of
ferromagnetism. The VSM technique was used to
observe
the
hysteresis
curves
of
III
2+
Co (en)2 (RC6H4NH2)I in dc magnetic field. The
hysteresis loops of the cobalt(III) complexes were
obtained by plotting moment/mass (emu/g) versus
field (Oe). Hysteresis loops measured for the
complexes imply the existence of ferromagnetic
behavior. The intrinsic coercivity and remanent
magnetization values calculated for the complexes
from VSM analysis are listed in Table 2. The results
indicate that all the trans-CoIII(en)2(RC6H4NH2)I2+
complexes show ferromagnetic behavior with
significant magnetization. A closer look at the
magnetic data reveal very clearly that the remanent
magnetization varies consistently, which is in tune
with
the
nature
of
the
complex
ion,
CoIII(en)2(RC6H4NH2)I2+. More precisely, it depends
on the presence of the sixth ligand in the coordination
environment, that is, R of RC6H4NH2 ligand, which in
turn depends on electron withdrawing/donating group
attachment. Although, R is remotely placed with
reference to the metal centre, the magnetic property
-0.30
-1
0
1
2
3
4
5
6
7
8
-3
Mr (x10 ) (emu/g)
Fig. 1 — Regression plot of remanent magnetization, Mr (emu/g)
versus
Hammett’s
substituent
constant,
σ,
for
trans-[CoIII(en)2(RC6H4NH2)I]I2.H2O complexes at 27°C,
(slope = 61.11).
depends shows the existence of some relationship
with Hammet’s substitution constant (σ).
The linear plot of remanent magnetization (Mr) versus
Hammet’s substitution constant σ (Fig. 1) indicates that
Mr of trans-CoIII(en)2(RC6H4NH2)I2+ complexes increases
linearly with respect to an enhancement in Hammet
constant as Mr = 3.069 × 10-5 – 7.071 × 10-3 emu/g when
σ = –0.268–0.115 (Table 2). Further, intrinsic coercivity,
-Hci = 141-1156 Oe indicates the existence of magnetic
behavior in the complexes. The room temperature
magnetic hysteresis loop measurements suggest that
the cobalt(III) complexes are ferromagnetic and the
magnetic character varies in tune with the basicity of
the axial ligand as observed for some of the low spin
iron(III) complexes22,23. However, linear relationship
between the magnetic properties of the complexes
versus substituent in aryl ligand is more complicated
than that anticipated earlier. Some investigations have
proved correlation between demagnetization property
NOTES
of the mixed ligand nicotinate N-oxide metal(II)
complexes24. Likewise there is a clear correlation
between d-d band positions and electronic effects of
the substituents. In addition, metal complexes with
aryl amine have been reported to confirm the
correlation of physicochemical properties of the
complexes with the electronic nature of the
substituents25-27.
The
above
study
shows
that
CoIII(en)2(RC6H4NH2)I2+ complexes are ferromagnetic
and establish a linear dependence of remanent
magnetization with respect to RC6H4NH2 ligand in
terms of Hammett’s σ constant. Linear regression
analysis of Mr versus σ yields negative slope
indicating that the electron withdrawing group in
RC6H4NH2 (σ > 0) enhances ferromagnetic character
and that electron donating group in RC6H4NH2 (σ < 0)
reduces the ferromagnetism. Some of the Fe(III) and
Cu(II) complexes are found to show the
demagnetization28 effect whenever the ligand field
strength is increased. Therefore, it can be concluded
that coordination environment of the metal centre of a
transition metal ion can greatly influence the
ferromagnetic character of the complex.
Acknowledgement
KA is thankful to the Department of Science and
Technology, New Delhi, (SR/S1/IC-13/2006/
6.3.2007) and the Council of Scientific and Industrial
Research [01(2140)/07/EMR-II/30.3.2007], New
Delhi, for financial support through major research
projects. The authors thank CIF, Pondicherry
University for providing instrumental facilities.
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