Synthesis, thermoanalysis, and thermal kinetic thermogravimetric analysis of transition metal Co(II), Ni(II), Cu(II), and Zn(II) complexes with 2-(2-Hydroxyphenyl)benzimidazole (HL)

Article abstract of DOI:10.1021/je101107w

Four complexes with general formulas M^IIL₂· CH₃OH (M = Co, Zn) and M^IIL₂ (M = Ni, Cu) were synthesized by transition metal ions M^II (Co, Ni, Cu, Zn) reacting with 2-(2-hydroxyphenyl)benzimi

Full text of DOI:10.1021/je101107w

ARTICLE
pubs.acs.org/jced
Synthesis, Thermoanalysis, and Thermal Kinetic Thermogravimetric
Analysis of Transition Metal Co(II), Ni(II), Cu(II), and Zn(II) Complexes
with 2-(2-Hydroxyphenyl)benzimidazole (HL)
‡
,†
§
†
†
†
Min Jiang, Jun Li,* Yong-qian Huo, Yun Xi, Jun-feng Yan, and Feng-xing Zhang
†
Key Laboratory of Synthetic and Natural Functional Molecule Chemistry of Ministry of Education, College of Chemistry & Materials
Science, Northwest University, Xian, Shaanxi 710069, China
‡
School of Chemistry and Environment Science, Shaanxi University of Technology, Hanzhong, Shaanxi, 723000, China
§
College of Chemistry & Chemistry Engineering, Yanan University, Yan'an, Shaanxi, 716000, China
II
II
ABSTRACT: Four complexes with general formulas M L CH OH (M = Co, Zn) and M L (M = Ni, Cu) were synthesized by
2
3
3
2
II
transition metal ions M (Co, Ni, Cu, Zn) reacting with 2-(2-hydroxyphenyl)benzimidazole (HL). Their thermal properties were
studied; the probable thermal decomposition mechanism for the first step of the Co(II) and Zn(II) complexes was suggested, and
the kinetic parameters were also given.
1
. INTRODUCTION
sulfuric acid, by ethylenediaminetetraacetic (EDTA) titrations.
Molar conductance measurements were made with a DDS-307
conductivity meter. IR spectra were obtained with a Bruker EQ
UINOX-550 spectrophotometer in the region of (4000 to 400)
The fluorescent compound 2-(2-hydroxyphenyl)benzimi-
dazole (HPBI) (Figure 1) is useful as a laser dye, high energy
radiation detector, molecular energy storage system, and fluor-
escent probe.
fer mechanism about it and its analogues have been extensively
studied, which showed a high Stokes shift and a great thermal
and photophysical stability.
The bidentate ligand HPBI is structurally similar to N-sub-
stituted salicylaldimines, which have been widely used in transi-
tion metal coordination chemistry. Moreover, HPBI comprises
two groups of relevance to the coordination of metal centers in
biological systerms, namely, phenolate (tyrosine) and imidazole
-
1
1
-3
cm . The thermoanalysis curves were obtained using a
The excited state intramolecular proton trans-
NETZCH STA449C thermal analysis instrument. The atmo-
-
1
4
-7
sphere was nitrogen, with a flow rate of 30 mL min . The rate
3
-
1
of heating was (15, 10, or 5) ꢀC min , and the sensitivity of the
3
instrument is 0.1 μg. The sample mass is (2 to 4) mg. The
thermal kinetic thermogravimetric (TG) analysis adopted the
kinetic software from NETZSCH.
2
.2. Synthesis of 2-(2-Hydroxyphenyl)benzimidazole
(HL). A solution of 2.32 g (19 mmol) of salicylaldehyde in 15
mL EtOH was added to a solution of 2.05 g (19 mmol) of o-
phenylenediamine in 25 mL of EtOH under stirring and heating.
The resulting orange solution was refluxed for 1 h and then
cooled to room temperature. After 12 h in the refrigerator, the
orange solution was filtered, and 15 mL of ether was added to the
solution. The solution was placed in the open air for 2 days; some
needle orange crystals appeared. The crystals were filtered and
then air-dried. The yield was 52 % and melting point (251 to
252) ꢀC.
(
histidine). In the past, some metal complexes with HPBI
derivatives have been synthesized and structurally characterized
8
-13
14-16
by other groups
and by us,
and some of them displayed
good photoluminescence and magnetic properties. However, the
thermal stability and decomposition steps of these complexes
have been rarely reported.
In this paper, four complexes of HPBI with divalent transition
II
metal ions M (Co, Ni, Cu, Zn) were prepared, with general
formula M L CH OH (M = Co, Zn) and M L (M = Ni, Cu),
and the thermal properties of these complexes were studied. To
the complexes of CoL CH OH and ZnL CH OH, the ki-
netics mechanism of the first-step decomposition reaction is also
II
II
2
3
3
2
2.3. Synthesis of the Complexes. The HL (0.420 g, 2.0
mmol) and KOH (0.112 g, 2.0 mmol) were dissolved in a
sufficient amount of methanol, and then CoCl 6H O (0.238
2
3
3
2
3
3
2
3
2
g, 1.0 mmol) in 20 mL methanol was added to this solution with
stirring. The product began to crystallize from the solution
almost immediately. After 1 h at room temperature, the solid
was filtered off, washed with methanol, and air-dried. The yield
was 76 %.
reported.
2. EXPERIMENTAL SECTION
2
.1. Materials and Methods. CoCl 6H O, NiCl 6H O,
2
3
2
2
3
2
CuCl 2H O, ZnCl , methanol, ethanol, KOH, salicylaldehyde,
2
3
2
2
and o-phenylenediamine were all analytical reagent or guaran-
teed reagent grade from China. Elemental analyses (C, H, and N)
were performed by a Vario EL-CHNOS instrument. The metal
content in the complexes was determined, after destroying the
organic matter with aqua regia and then with concentrated
Special Issue: John M. Prausnitz Festschrift
Received: October 26, 2010
Accepted: January 24, 2011
Published: February 10, 2011
r 2011 American Chemical Society
1185
dx.doi.org/10.1021/je101107w
|J. Chem. Eng. Data 2011, 56, 1185–1190

Journal of Chemical & Engineering Data
ARTICLE
The syntheses of the Ni, Cu, and Zn complexes are similar to
that of the Co complex, and the metal salts are NiCl 6H O,
The lack of knowledge of the analytical form of g(R) can be
circumvented quite easily by combining eqs 2 and 4:
2
3
2
CuCl 2H O, and ZnCl , respectively. Yields: 58 % (Ni), 83 %
2
3
2
2
E
(
Cu), and 70 % (Zn).
log β ¼ - 0:4567 þ constant
ð5Þ
2
.4. Theory Base of the Kinetics Estimation of the Thermal
RT
Decomposition. According to nonisothermal kinetic models,
the differential and integral forms of a rate equation are generally
expressed, as follows:
For a number of experiments with different tempreture pro-
grams, β, we can write for the same extent of reaction R
ꢀ
ꢁ
dR
dT
A
-Ea
RT
AE
E
¼ β fðRÞ exp
ð1Þ
log β ¼ log
- 2:315 - 0:4567
ð6Þ
RgðRÞ
RT
The plot of log β versus 1/T for a given value of R must give
the activation energy E. The procedure was suggested by both
AE
gðRÞ ¼ βR pðxÞ
ð2Þ
19
20
Ozawa and Flynn and Wall. We can use the method for
determining E which does not depend on a knowledge of the
analytical form of g(R).
where x = E/RT, E is the activation energy, A the pre-exponential
factor, R the gas constant, R the degree of conversion, β = dT/dt
the heating rate, g(R) and f(R) are appropriate functions of R,
and p(x) represents the integral
21
Another method is the ASTM E698. It uses a model-free
estimate for E which is evaluated from Kissinger's plot of ln
2
-1 22
(
β/T ) against T
,
where T is the temperature corres-
m
m
m
Z
¥
expð - xÞ
ponding to the maximum of dR/dT. However, the pre-expro-
nential factor is evaluated on the assumption of first-order
reactions as follows:
dx
ð3Þ
2
x
x
A difficulty in nonisothermal kinetic methods is that there is no
ꢀ
ꢁ
17
exact analytical solution of the p(x) function. Doyle has
suggested that log p(x) is an approximately linear function of
x, that is, of 1/T.
βE
RTm2
E
A ¼
exp
ð7Þ
RTm
The method occupies an intermediate position between the
model-fitting and the model-free methods.
Using the kinetic software from NETZSCH, the multivariate
nonlinear regression method is used to fit the kinetic curves. The
5 mechanism functions applied to describe thermal decomposi-
tion in solids are shown in Table 1.
23
log pðxÞ ¼ - 2:315 - 0:4567x
ð4Þ
18
However, Vyazovkin and Dollimore has considered that the
application is restricted (x > 13).
1
3. RESULTS AND DISCUSSION
3.1. Structural Characterization. The structure of ligand HL
is shown in Figure 2a. It is only a fake plane for a very small
dihedral angle (θ = 2.2ꢀ) which was found between the benzi-
midazole plane and the phenolate plane. Meanwhile, there is a
Figure 1. Structure of 2-(2-hydroxyphenyl)benzimidazole (HPBI).
Table 1. Selected 15 Mechanism Functions for the Evaluation of the Kinetic Equations
mechanisms
symbol
f(R)
g(R)
-ln(1 - R)
random nucleartion; unimolecular decay (first order)
formal chemical reaction (second order)
formal chemical reaction (nth order)
F1
(1 - R)
(1 - R)
(1 - R)
2
-1
F2
(1 - R)
n
[1 - (1 - R)1-n]/(1 - n)
Fn
D1
D2
D3
D4
R2
R3
B1
Bna
C1
A2
A3
An
-
1
2
one-dimensional diffusion (parabola law)
two-dimensional diffusion (Valensi equation)
three-dimensional diffusion (Jander equation)
three-dimensional diffusion (Ginst-Broun equation)
phase boundary reaction (contracting cylinder)
phase boundary reaction(contracting sphere)
autocatalysis, ramiform nucleation
1/2R
-1/ln(1 - R)
R
(1 - R)ln(1 - R) þ R
2
/3
1/3 -1
1/3 2
3/2(1 - R) [1 - (1 - R)
]
[1 - (1 - R)
]
-
1/3
-1
2/3
3/2[(1 - R)
- 1]
(1 - 2R/3) - (1 - R)
1
/2
1/2
2(1 - R)
3(1 - R)
1 - (1 - R)
2
/3
1/3
1 - (1 - R)
(1 - R)R
ln[R/(1 - R)]
n
a
autocatalysis, ramiform nucleation
(1 - R) R
first order with autocatalysis
(1 - R)(1 þ KR)
2(1 - R)[-ln(1 - R)]
3(1 - R)[-ln(1 - R)]
n(1 - R)[-ln(1 - R)]
1
2
/2
1/2
two-dimensional nucleation and growth
three-dimensional nucleation and growth
n-dimensional nucleation and growth
[-ln(1 - R)]
/3
n-1)/n
1/3
[-ln(1 - R)]
(
1/n
[-ln(1 - R)]
1
186
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Journal of Chemical & Engineering Data
ARTICLE
intramolecular hydrogen bond between the phenolic OH group
and the N atom of benzimidazole ring with a corresponding
O1 N2 distance of 2.578(2) Å.
Figure 2b displays the one-dimensional zigzag chain structure
of HL constructed by intermolecular hydrogen bonding interac-
tions. The imidazole group donates a hydrogen bond to the
The broad absorption bands of νOH are not observed in the IR
spectra of the four complexes, indicating M-O bond formation.
There is a shift of the CdN vibration to lower frequencies [(1622
3
3 3
-
1
to 1626) cm ] which is assigned to N-metal coordination. The
conclusive evidence of bonding of the oxygen and nitrogen atoms
to the metal ions is confirmed by the appearance of bands at
-
1
-1
phenolate O atom of the adjacent molecule [N(1A) O(1B)
∼435 cm and ∼470 cm in all four complexes which are
assigned to ν( and ν vibrations, respectively. A new
3
3 3
2
.909(2) Å], resulting in a one-dimensional zigzag chain. The
M-O)
(M-N)
-
1
structure gives proof of the good thermal stability of HL.
The color, molar conductance, and elemental analysis of these
complexes are shown in Table 2.
band at ∼1030 cm can be found in the IR spectra of the Co(II)
and Zn(II) complexes, which is assigned to the ν in
C-O
methanol. By referring to the coordination forms of methanol
molecules in the Cu(II) with the 2-(2-hydroxy-3-methylphenyl)
-
1
The molar conductivities of these complexes in 0.001 mol L
3
2
-1
11
DMF solutions are in the range of (0.385 to 1.24) S cm mol ,
benzimidazole complex and the weak bending vibration of O-
3
3
24
-1
26
indicating that these compounds are nonelectrolytes. All four
metal complexes displayed low solubility in water and common
organic solvents except for dimethylformamide (DMF) and di-
methyl sulfoxide (DMSO). The bivalent metal ions bond two HL
anions in all four complexes. The Co and Zn complexes contain a
H O bridges around (1740 to 1720) cm in the IR spectra
3
3
of the Co(II) and Zn(II) with HL complexes, the methanol
molecule in the Co(II) or Zn(II) complexes is hydrogen-bonded
to the O atom of HL rather than coordinated to the metal center.
From these results, it can be concluded that the HL is
coordinated to the metal ions via the phenolate oxygen and the
imidazole nitrogen atoms, and the proposed structures of com-
CH OH molecule.
3
The IR spectra of HL exhibits a strong ν
stretching band
N-H
-
1
at 3327 cm , and a broad absorption band at (3000 to 2800)
plexes is depicted in Figure 3.
-
1
25
II
3.2. Thermal Decomposition of M L
cm is attributed to the intramolecular hydrogen bond. The
2
CH
3
II
OH (M = Co, Zn)
3
-
1
II
(1633, 1266, and 727) cm bands are attributed to the stretch-
and M L
TG curve (Figure 4) shows three main steps corresponding to
the loss of one CH OH molecule (calcd 6.28 %, found 6.24 %),
2
(M = Ni, Cu). For the complex Co L CH OH, the
3
2 3
ing vibration of νCdN, νC-O, and the four adjacent H atoms in
the phenyl ring, respectively.
3
then to the loss of one ligand molecule, and finally to the slow
decomposition process to give the metal oxide (calcd 15.85 %,
found 16.76 %). The DSC curve (Figure 5) shows an endother-
mic peak related to the CH OH loss at 130 ꢀC and then one big
3
exothermic peak to give the metal oxide.
II
For the complex Zn L CH OH, the TG curve (Figure 4)
2
3
3
shows the first step is the loss of one CH OH molecule (calcd
3
6
.21 %, found 6.27 %), followed by the loss of one ligand
molecule that contains two steps, and then to the decomposition
to give the metal oxide (calcd 15.72 %, found 16.61 %). In the
DSC curve (Figure 5), the endothermic process appears due to
the loss of CH OH and is followed by two exothermic peaks, and
3
around 300 ꢀC decomposition starts, giving ZnO.
In contrast with the former complexes, the TG curves of
complexes NiL and CuL are quite simple, from the starting of
2
2
decomposition at 300 ꢀC to give the final metal oxide (for NiL :
2
calcd 15.67 %, found 15.83 %; for CuL : calcd 16.51 %, found
2
1
7.45 %). There are two poorly resolved processes. According to
this, the DSC curves (Figure 5) show a big exothermic peak.
II
The four complexes show two general formulas M L
2
3
II
CH OH (M = Co, Zn) and M L (M = Ni,Cu). For the Co(II)
3
2
and Zn(II) complexes, the fact that the temperature of losing one
methanol molecule is not high (∼130 ꢀC) confirms that the
methanol molecule does not coordinate to the metal ions. These
two complexes are stable between (160 and 300) ꢀC. The Ni and
Cu complexes are stable up to around 400 ꢀC. All of these four
Figure 2. (a) Structure of HL; the dashed line denotes the intramole-
cular hydrogen bond. (b) One-dimensional zigzag chain of HL formed
by intermolecular hydrogen bonds.
Table 2. Color, Molar Conductance, and Elemental Analysis of the Complexes
Λ
m
calcd (found)/%
compounds
Co(HL) CH
color
pink
S cm mol-1
2
C
H
N
M
3
3
2
3
3
OH
1.24
63.66 (63.38)
62.68 (62.71)
65.72 (65.31)
64.78 (64.59)
4.35 (4.23)
4.30 (4.26)
3.82 (3.61)
3.76 (3.69)
11.00 (10.86)
10.86 (10.77)
11.79 (11.46)
11.31 (11.31)
11.59 (11.72)
12.61 (13.28)
12.03 (11.85)
13.28 (12.95)
Zn(HL)
2
CH
3
OH
white
1.05
3
Ni(HL)
2
pale yellow
brown
0.385
0.625
Cu(HL)
2
1
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Journal of Chemical & Engineering Data
ARTICLE
Figure 3. Proposed coordinated modes for the metal complexes with
HPBI.
Figure 6. Isoconversional plots for the first-stage thermal decomposi-
II
II
tion of Co L CH OH (a) and Zn L CH OH (b).
II
II
2
3
3
2
3
3
Figure 4. TG curve of Co L CH OH (a), Zn L CH OH (b),
2
3
3
2
3
3
II
II
2 2
-1
Ni L
(c), and Cu L (d). Heating rate: 10 ꢀC min , nitrogen flow
3
-
1
II
at a 30 mL min rate.
Table 3. Activation Energy Values of Co L CH OH (a) and
3
2
3
3
II
Zn L CH OH (b) Obtained from the Kinetic Analysis of the
Isoconversional Lines of (Figure 5) by Means of Equation 6
2
3
3
a
b
E/kJ mol-
1
R
E/kJ mol
-1
R
3
3
0
0
0
0
0
0
0
0
0
0
.02
.05
.10
.20
.30
.40
.50
.60
.70
.80
106.4
111.9
105.1
104.9
99.6
0.02
0.05
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
0.90
0.95
0.98
137.5
106.6
104.8
91.8
84.1
82.9
80.9
79.4
78.3
77.4
77.2
75.8
76.5
100.7
99.6
98.6
98.5
97.5
II
II
Figure 5. DSC curves of Co L
2
CH
3
OH (a), Zn L
2
CH
3
OH (b),
3
3
0.90
0.95
97.8
II
II
2 2
-1
Ni L
(c), and Cu L (d). Heating rate: 10 ꢀC min , nitrogen flow at
3
-
1
104.5
113.1
a 30 mL min rate.
3
0
.98
complexes have good stability except losing the methanol molecule,
and the Ni complex shows the best thermal stability. The good
thermal stability is attributed to the formed six-membered ring with
the metal ions(Figure 3) and the strongcoordinated bond between
HL and metal ions.
The area of the endothermic peak in the DSC (Figure 5) for
II
the loss of one methanol molecule from the complexes Co L
2
3
II
CH OH and Zn L CH OH was measured and calculated. The
3
2
3
3
-
1
results are (48 and 60) kJ mol , respectively. They have nearly
3
1
188
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Journal of Chemical & Engineering Data
ARTICLE
tried to use a single reaction mechanism tofit the curves. The results
II
were different. To the first stage of Co L CH OH, the model Fn
2
3
3
(
n = 1.02) was designated as the best fit (Figure 7a). The Arrhenius
parameters were E = 109.5 kJ mol and log A = 12.2 s . But for
the Zn L CH OH, the fit result was not good. When we used a
multireaction mechanism to fit, the result obtained was satisfactory
Figure 7b). There is a pair of parallel reactions. The mech-
anisms and Arrhenius parameters were Fn (n = 0.87), E = 85.3
kJ mol , log A = 8.6 s , and An (n = 1.79), E = 83.7 kJ mol ,
log A = 8.5 s , respectively. Using the ASTM E698 method, the
-1
-1
3
II
2
3
3
(
-1
-1
-1
3
3
-1
-1
Arrhenius parameters obtained are E = 106.1 kJ mol , log A =
3
II
-1
1
1.5 s for the first-step decomposition of Co L CH OH and
2 3
1
3
-
-1
E = 80.9 kJ mol , log A = 8.2 s for the first-step decomposition
3
II
of Zn L CH OH. By comparing with the former two methods,
2
3
3
the resulting predictions appear to confirm the kinetic scheme for
II
the first-step decomposition of Co L CH OH, but it cannot
2
3
3
disclose the activation energy variations that may accompany the
complex (e.g., multistep) kinetics of the first-step decomposition of
II
Zn L CH OH.
2
3
3
The loss of one methanol molecule reaction of the two
complexes which have the same coordination form undergoes
different reaction methanisms may be attributed to the different
center atom.
4
. SUMMARY
The complexes M L CH OH (M = Co, Zn) and M L
2
II
II
2
3
3
(
M = Ni, Cu) were prepared and characterized with HL as the
ligand. These complexes showed good thermal stability around
00 ꢀC except that a methanol molecule was lost in Co(II) and
3
Zn(II) complexes, indicating strong coordinated bonds between
the metal ion and ligand. The methanol molecule has a similar
environment in the Co(II) and Zn(II) complexes due to them
having almost the same temperature of endothermic peak. The
nonisothermal kinetic analysis of the first-stage thermal decom-
position of Co(II) and Zn(II) complexes showed a single
reaction mechanism for the first-stage thermal decomposition
II
II
Figure 7. First step of TG curve of Co L CH OH (a) and Zn L
-
2
3
3
2
3
-1
CH OH (b) at the heating rate of (15, 10, and 5) ꢀC min as the best
3
3
fit to mechanism functions. The solid lines are the fitted curve.
the same temperature for the endothermic peak, and the
methanol molecule is proposed to have the same coordination
form in the two complexes.
.3. Kinetic Analysis of the First-Stage Decomposition of
Co L CH OH and Zn L CH OH. A nonisothermal kinetic
analysis of the first-stage thermal decomposition was carried
out using the data elaborated from the TG curves performed at
II
of Co L CH OH but a multistep mechanism of the first-stage
2
3
3
II
thermal decomposition of Zn L CH OH.
2
3
3
3
II
II
2
3
3
2
3
3
’
AUTHOR INFORMATION
Corresponding Author
*Fax: 00 86 29 88303798. Tel.: 0086 29 88302604. E-mail
address: junli@nwu.edu.cn.
-
1
different heating rates of (5, 10, and 15) ꢀC min
.
3
The isoconversion lines obtained in plotting log β versus the
reciprocal of temperature (eq 6) in Figure 6, at the same degree
of conversion, yield a series of activation energies, as shown in
Funding Sources
We are very grateful for the financial support from the National
Nature Science funds (20971103) and the International Coop-
eration Project of Shaanxi Province (2008KW-33).
Table 3.
II
For the first-step decomposition of Co L CH OH, from R =
2
3
3
0
.2 to R = 0.9, the activation energy value is approximately
-1
invariable, about 99 kJ mol , but at R < 0.2 and R > 0.9, there is
3
-
1
-1
little fluctuation (5 kJ mol ∼10 kJ mol ). The results ap-
3
3
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II
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2
3
3
II
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Munoz, J. M. Pulsed Liquid Lasers from Proton Transfer in the Excited
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first-step decomposition of Zn L CH OH, in the process of
2
3
3
losing a methanol molecule, the activation energy decreases
-
1
-1
monotonically from 137 kJ mol to 76 kJ mol . The ob-
3
3
tained dependencies probably indicate a multistep mechanism of
the first-stage thermal decomposition of Zn L CH OH.
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regression method has used to fit the kinetic curves. For the target
II
27
2
3
3
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II
stage of the TG curves of Co L CH OH and Zn L CH OH, we
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3
3
2
3
3
1
189
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Journal of Chemical & Engineering Data
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