Orotic acid complexes of Co(II), Ni(II), Zn(II) and Cd(II) with imidazole: Syntheses, spectroscopic and thermal studies

Article abstract of DOI:10.1007/s10973-005-7479-9

The [Co(HOr)(H₂O)₂(im)₂] (1), [Ni(HOr)(H₂O)₂(im)₂] (2), [Zn(H ₂O)₂(im)₄](H₂Or)₂ (3) and [Cd(HOr)(H₂O)(im)₃] (4) complexes (H₃Or: orotic acid, im: imidazole) were synthesized and characterized by elemental analysis, magnetic and conductance measurements, UV-vis and IR spectra. The thermal behaviour of the complexes was also studied by simultaneous thermal analysis techniques (TG, DTG and DTA). The orotate ligand (HOr^2-) coordinated to the Co(II), Ni(II) and Cd(II) ions are chelated to the deprotonated pyrimidine nitrogen (N₍₃₎) and the carboxylate oxygen, while do not coordinate to the Zn(II) ion is present as a counter-ion (H ₂Or^-). The first thermal decomposition process of all the complexes is endothermic deaquation. This stage is followed by partially (or completely) decomposition of the imidazole and orotate ligands. In the later stage, the remained organic residue exothermically burns. On the basis of the first DTG_max, the thermal stability of the complexes follows order: 2, 176°C>1, 162°C>4, 155°C>3, 117°C in static air atmosphere. The final decomposition products which identified by IR spectroscopy were the corresponding metal oxides.

Full text of DOI:10.1007/s10973-005-7479-9

Journal of Thermal Analysis and Calorimetry, Vol. 89 (2007) 2, 555–559
OROTIC ACID COMPLEXES OF Co(II), Ni(II), Zn(II) AND Cd(II) WITH
IMIDAZOLE
Syntheses, spectroscopic and thermal studies
O. Z. Yeêilel1*, H. Ölmez2 and H. ¸budak2
1Department of Chemistry, Faculty of Arts and Sciences, Eskiêehir Osmangazi University, 26480 Eskiêehir, Turkey
2Department of Chemistry, Faculty of Arts and Sciences, Ondokuz MayÏs University, 55139 Kurupelit, Samsun, Turkey
The [Co(HOr)(H2O)2(im)2] (1), [Ni(HOr)(H2O)2(im)2] (2), [Zn(H2O)2(im)4](H2Or)2 (3) and [Cd(HOr)(H2O)(im)3] (4) complexes
(H3Or: orotic acid, im: imidazole) were synthesized and characterized by elemental analysis, magnetic and conductance measure-
ments, UV-vis and IR spectra. The thermal behaviour of the complexes was also studied by simultaneous thermal analysis tech-
niques (TG, DTG and DTA). The orotate ligand (HOr2–) coordinated to the Co(II), Ni(II) and Cd(II) ions are chelated to the
deprotonated pyrimidine nitrogen (N(3)) and the carboxylate oxygen, while do not coordinate to the Zn(II) ion is present as a coun-
ter-ion (H2Or–). The first thermal decomposition process of all the complexes is endothermic deaquation. This stage is followed by
partially (or completely) decomposition of the imidazole and orotate ligands. In the later stage, the remained organic residue exo-
thermically burns. On the basis of the first DTGmax, the thermal stability of the complexes follows order: 2, 176°C>1, 162°C>4,
155°C>3, 117°C in static air atmosphere. The final decomposition products which identified by IR spectroscopy were the corre-
sponding metal oxides.
Keywords: imidazole complexes, orotic acid complexes, thermal analysis
Introduction
and the carboxyl group. Although, the most potential
coordination sites are the deprotonated carboxyl group
and its adjacent nitrogen, when they coordinate to
metal ion, the complexation ability of the exocylic car-
bonyl oxygen is greatly enhanced [8]. The complexes
of HOr2– of Mn(II), Co(II), Ni(II), Zn(II), Mg(II)
([M(HOr)(H2O)4]×H2O) were reported [4, 5, 9–12].
Synthesis and structural characterization of mixed-
ligand transition metal complexes of HOr2– with
mono- and bidentate N-donor ligands such as am-
mine [6, 13, 14], imidazole [15], 1,10-phenanthroline
[16, 17], 2,2’-bipyridine [17, 18] and ethylene-
diamine [12, 19] appear in recent literature. We re-
ported earlier the syntheses and spectrathermal
studies of 1,10-phenanthroline complexes of Co(II),
Ni(II), Cu(II) and Cd(II) orotates [20],
[Zn(H2O)2(phen)2](H2Or)2×2.125H2O [21],
Orotic acid (1,2,3,6-tetrahydro-2,6-dioxo-4-pyrimidine-
carboxylic acid, vitamin B13, H3Or, Fig. 1) and its salts
and other derivatives play an important role in the me-
tabolism of pyrimidine nucleotides [1, 2] and are found
in cell and body fluids of many living organisms. Metal
orotates are also widely applied in medicine. Metal
orotates have been used e.g. as uricosurica (for en-
hanced excretion of uric acid) and for electrolyte substi-
tution (in heart and liver protection) [3]. Platinum, palla-
dium and nickel orotates with wide variety of
substituents have been screened as therapeutic agents
for cancer [4–7]. Besides biologically importance,
orotic acid is also an interesting ligand in coordination
chemistry. It may coordinate through the two nitrogen
atoms of the pyrimidine ring, the two carbonyl oxygens
[Cu(HOr)(tea)]×H2O [22], [Cu(HOr)(H2O)(bipy)] [18],
[Cu(HOr)(H2O)(en)]×H2O, [Co(HOr)(en)2](H2Or)×5H2O
[12], [Ni(HOr)(H2O)(tea)]×H2O [23],
[Co(HOr)(H2O)3(na)]×3H2O [24] and Co(II), Ni(II) and
Cu(II) complexes of neutral orotic acid [25] (phen=
1,10-phenanthroline; tea=triethanolamine; bipy=
2,2’-bipyridine; en=ethylenediamine; na=nicotinamide).
In the present paper, we describe the syntheses,
spectroscopic and thermal behaviour of the new
orotic acid complexes of Co(II), Ni(II), Zn(II) and
Cd(II) with imidazole.
Fig. 1 Structures of the ligands: H3Or=orotic acid, im=imidazole
*
Author for correspondence: yesilel@ogu.edu.tr
1388–6150/$20.00
Akadémiai Kiadó, Budapest, Hungary
Springer, Dordrecht, The Netherlands
© 2007 Akadémiai Kiadó, Budapest

YEŸILEL et al.
Table 1 Analytical data and conductance values of 1–4
Found (Calc.)/%
N
Complexes
F.W./g mol–1
Yield/%
Colour
L/S cm2 mol–1
C
H
[Co(HOr)(H2O)2(im)2] (1)
C11H14N6O6Co
34.04
(34.30)
3.86
(3.66)
21.99
(21.82)
385.20
384.96
683.91
488.74
86
84
60
67
red
24
26
[Ni(HOr)(H2O)2(im)2] (2)
C11H14N6O6Ni
33.72
(34.32)
3.75
(3.67)
21.66
(21.83)
blue
[Zn(H2O)2(im)4](H2Or)2 (3)
C22H26N12O10Zn
39.22
(38.64)
4.03
(3.83)
24.05
(24.58)
colourless
colourless
103
30
[Cd(HOr)(H2O)(im)3] (4)
C14H16N8O5Cd
33.45
(34.41)
3.27
(3.30)
23.45
(22.93)
Experimental
ble in air. 2 was also structurally characterized [15].
The elemental analyses conformed to their proposed
formula (Table 1). The molar ratio M:im:HOr:H2O in
the complexes are 1:2:1:2 for 1 and 2, 1:4:2:2 for 3
and 1:3:1:1 for 4. In the complexes, imidazole acts as
a monodentate ligand through its nitrogen atom,
while dianionic orotate ligand (HOr2–) coordinates to
the Co(II), Ni(II) and Cd(II) ions as a bidentate,
through carboxylate oxygen and deprotonated nitro-
gen atom of pyrimidine ring. But, monoanionic
orotate (H2Or–) behaves as a counter ion in 3. These
results were supported by conductivity measure-
ments. The molar conductance values (L) of the com-
plexes are 24, 26, 103 and 30 Scm2 mol–1 for 1, 2, 3
and 4, respectively, indicating that 1, 2 and 4 are a
non electrolyte, while 3 is a 1:2 electrolyte in water.
Materials and instrumentation
All chemicals used were analytical reagent products.
Synthesis of the complexes
The divalent metal complexes of HOr2–, [M(HOr)(H2O)4]
×H2O (M=Co(II), Ni(II), Zn(II) and Cd(II)), were pre-
pared according to the conventional methods described in
[4, 5, 12]. The [M(HOr)(H2O)4]×H2O complexes
(M=Co(II), Ni(II), Zn(II) and Cd(II), 2 mmol, 0.62 g)
were dissolved in hot distilled water (20 mL) with stirring
at 60°C. The im ligand (8 mmol, 0.55 g), which dissolved
in the ethanol (10 mL), was added to the solutions of the
[M(HOr)(H2O)4]×H2O with continuous stirring for ap-
proximately 5 h at 60°C in a temperature-controlled bath.
The reaction mixture was then cooled to room tempera-
ture. The crystals formed were filtered and washed with
10 mL of water and ethanol and dried in air.
Elemental analyses for C, H and N were carried out
at the Tubitak Marmara Research Centre. Magnetic sus-
ceptibility measurements at room temperatures were
performed using a Sherwood Scientific MXI model
Gouy magnetic balance. UV-vis spectra were obtained
for the aqueous solutions (10–3 M) of the complexes
with a Unicam UV2 spectrometer in the range
900–190 nm. IR spectra were recorded in the
4000–400 cm–1 region with a Mattson 1000 FT-IR spec-
trometer using KBr pellets. Thermal analysis curves
(TG, DTG and DTA) were recorded simultaneously in a
static air atmosphere with a Rigaku TG8110 thermal
analyser. The heating rate was 10°C min–1 and the DTG
sensitivity was 0.05 mg s–1. Conductance values were
measured on 522 CRISON conductimeter.
UV-Vis spectra
The lmax values of the intraligand and d–d transitions
displayed, assignment of d–d transitions and the ef-
fective magnetic moment values of the complexes are
given in Table 2. 1 and 2 exhibit magnetic moment
values of 4.38 and 2.92 BM which correspond to
three and two unpaired electrons, respectively, which
are consistent with a weak field octahedral geometry
as expected. 3 and 4 are diamagnetic.
The intense bands in the 200–300 nm range are
assigned to the intraligand p®p* and n®p* transi-
tions, and the bands with low intensity between 350
and 800 nm are due to d–d transitions. The assign-
ments of the d–d transitions corresponded to an octahe-
dral coordination geometry of the metal(II) ions and
the values of the D0 parameter were found using
Tanabe and Sugano diagrams [26]. 1 displays one peak
in the visible region of the electronic spectrum, while 2
shows three maxima in the 350–900 range. 3 and 4 did
not display any peak in the visible region, but only ex-
hibit the intraligand transitions. In the spectrum of 1,
the lmax value of the absorption band is 493 nm
(e=26 L mol–1 cm–1). This value may be assigned to the
Results and discussion
Analytical results, the molar conductivity values and
the compositions of the complexes are listed in Ta-
ble 1. 1–4 were synthesized in good yields and with
high purity and found to be nonhygroscopic and sta-
4
4T1g®4T1g (P) d–d transition. The T1g®4A2g, and
556
J. Therm. Anal. Cal., 89, 2007

OROTIC ACID COMPLEXES WITH IMIDAZOLE
Table 2 Electronic spectra and magnetic moment values of 1–4
Complexes
lmax/nm
e/cm–1 mol–1
Assignment of d–d transitions
4T1g®4T1g(P)
D0/cm–1
meff/(B.M.)
[Co(HOr)(H2O)2(im)2] (1)
493
26
–
4.38
379
624
890
26
7
4
3A2g®3T1g(p)
3A2g®3T1g
3A2g®3T2g
[Ni(HOr)(H2O)(im)2] (2)
11.230
2.92
[Zn(H2O)2(im)](H2O)2 (3)
[Cd(HOr)(H2O)(im)3] (4)
–
–
–
–
–
–
Dia.
Dia.
4T1g®4T2g transitions were not observed due to low en-
ergy and shift to the IR region [27]. Therefore, the D0
value of 1 could not be calculated. The UV-vis spec-
trum for 2 exhibits three weak d–d absorption bands
1594 cm–1, while the very broad and strong one band
at 1617 and 1618 cm–1 in 1 and 2, respectively, which
are prescribed to overlapped C=O and C=C stretch-
ing, and bending water and amine modes. The me-
dium-intense band at 1531–1581 cm–1 region can be
attributed to the imidazole (–C=N–) stretching vibra-
centered at 379 (e=26
L
mol–1 cm–1), 624
(e=7 L mol–1 cm–1) and 890 nm (e=4 L mol–1 cm–1)
which support octahedral geometry. These values were
tion. The nOH
vibration bands which appeared at
assigned to A2g®3T1g,(P), A2g®3T1g and A2g®3T2g,
respectively. The D0 value for 2 was calculated as
11.230 cm–1. The colourless 3 and 4 do not show any
d–d bands as expected.
2500 cm–1 in (atchid)e free orotic acid have not been ob-
served in all the complexes. The disappearance of the
bands was attributed to the deprotonation. The weak
bands in the region 400–500 cm–1 are due to M–N
stretching vibrations [29]. As a result, the IR spec-
trum of 3 indicates that the orotic acid is monoanionic
and counter ion as supported by conductivity mea-
surements, while the IR spectra of 1, 2 and 4 show
that orotic acid is dianionic and coordinated to the
metal ions as a bidentate via deprotonated N(3) of py-
rimidine and the carboxylate oxygen.
3
3
3
IR spectra
The main IR group frequencies of the metal com-
plexes are presented in Table 3. OH stretching bands
of aqua ligands appear in the 3475, 3462, 3500 and
3325 cm–1 for 1, 2, 3 and 4, respectively. In 1, 2 and 4,
the disappearance of the nN
band at 3160 cm–1
(3) H
which was observed in the free orotic acid [6], was at-
tributed to the deprotonation and coordination of N(3)
to the metal ions. However, the N(3)H stretching band
was clearly observable as strong at 3144 cm–1 in the
IR spectrum of 3. The band in the 3136–3222 region
in the complexes can be attributed NH stretching vi-
bration of the imidazole ligand. This data substanti-
ates the suggestion that the imidazole molecules coor-
dinate to the metal ions in their neutral form. The
carbonyl groups appear as two main peaks at 1709
[nC= O +nC =O ] and 1671 cm–1 [nC =O +nC=C ] in the
Thermal analyses
The TG-DTG and DTA curves of the complexes are
shown in Figs 2–5. The endothermic peak of
1 (DTGmax=171°C) in the temperature range of
103–186°C, corresponds to the loss of the two moles
aqua ligands (found 10.59, calcd. 9.35%). In the tem-
perature range of 187–380°C, 1 involves the consecu-
tive decompositions of neutral imidazole ligands by
giving endothermic effects (DTGmax=228 and 301°C).
A good agreement between the experimental and cal-
culated values was observed for the mass loss (found
35.10; calcd. 35.35%). The following stage involves
(acid)
(2)
free H3Or [28]. The carbonyl stret(6c) hing modes in
3 and 4 were observed at 1674, 1613 cm–1 and 1635,
Table 3 IR spectra of 1–4 (cm–1)
Complexes
nOH
nN +nNH
*
nN
nC=O +nC
nC = O +nC= C
nC=N
dN H +dNH *
(1) im
(1)H
(2) = O
(3)
im
asit
(6)
1380*m,
1406w
[Co(HOr)(H2O)2(im)2] (1)
3475m
3222*s
3220*s
3075w
3050w
3053m
3017m
1617vs,b
1618vs,b
1531s
1382*m,
1407w
[Ni(HOr)(H2O)2(im)2] (2)
[Zn(H2O)2(im)](H2O)2 (3)
[Cd(HOr)(H2O)(im)3] (4)
3462m
3500s
1543s
1581s
1534s
1384*w,
1425m
3144, 3247*m
3136*m
1674vs
1635vs
1613vs
1594vs
1387*s,
1440m
3325m
w – weak, m – medium, s – strong, vs – very strong, b – broad
J. Therm. Anal. Cal., 89, 2007
557

YEŸILEL et al.
Fig. 2 TG, DTG and DTA curves of [Co(HOr)(H2O)2(im)2]
Fig. 4 TG, DTG and DTA curves of [Zn(H2O)2(im)4](H2Or)2
Fig. 3 TG, DTG and DTA curves of [Ni(HOr)(H2O)2(im)2]
Fig. 5 TG, DTG and DTA curves of [Cd(HOr)(H2O)(im)3]
loss of 6.19% (calcd. 5.27%). The anhydrous 3 is ther-
mally stable up to about 206°C. In the temperature
range of 206–321°C, the endothermic peak of 3
(DTGmax=281°C), is related to the release of imidazole
ligands and partial decomposition of orotates. This de-
composition stage is similar to the other imidazole
complexes [31–34]. In the last stage, the strong exo-
thermic mass loss process occurs in a single step. Dur-
ing this stage, the remaining organic part is abruptly
burnt (DTGmax=504°C). The final decomposition prod-
uct, ZnO, was identified by IR spectroscopy.
The first decomposition stage of 4, in the tem-
perature range of 127–189°C, corresponds to mass
loss of the aqua ligand (found 4.82, calcd. 3.68%).
The following stages between 190–362°C are related
to the endothermic decompositions of the three moles
of neutral imidazole ligands (DTGmax=236, 275 and
307°C, found 41.18, calcd. 41.79%). The exothermic
peak at 386°C is associated with the phase transition
of the CdHOr compound. In the last stage
(363–532°C), the organic residue is abruptly burnt
(DTGmax=529°C). The final decomposition product is
CdO (found 71.90, calcd. 73.73%).
the decomposition of the orotate ligand by exothermic
effect (DTGmax=387°C). In the last stage the strong
exothermic peak (DTGmax=465°C) is associated with
the burning of the organic residue, leading finally to
the Co3O4. The endothermic peak at 901ºC (DTGmax
)
is related to the conversion of Co3O4 to CoO. The over-
all mass loss (found 83.45, calcd. 80.60%) agrees with
the proposed structure well.
The thermal decomposition of 2 (Fig. 3) is simi-
lar with that of the isomorphous 1 [30] (Fig. 2). The
first stage of the thermal decomposition of 2 starts at
about 120°C with the release of the two moles aqua
ligand in two steps (DTGmax=184, 204°C, found
10.23, calcd. 9.50%). In the second stage, two moles
of imidazole ligand are released, as inferred from the
agreement between the theoretical and experimental
value for the mass loss (found 36.77, calcd. 35.35%).
The following stage involves the extremely exother-
mic decomposition of orotate ligand and organic resi-
due, leading finally to the NiO.
The thermal behavior of 3 is different from 1, 2
and 4, because of 3 has different structure from the
complexes. In 3, the orotates were not coordinated to
the Zn(II) ion and act as the counter ions. In the first
decomposition stage, endothermic removal of the two
aqua ligands occurs at 117°C (DTGmax) with a mass
On the basis of the first DTGmax, the thermal sta-
bility of 1–4 follows order: Ni(II), 176°C>Co(II),
162°C>Cd(II), 155°C>Zn(II), 117°C.
558
J. Therm. Anal. Cal., 89, 2007

OROTIC ACID COMPLEXES WITH IMIDAZOLE
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The Co(II), Ni(II), Zn(II) and Cd(II)-orotic acid (H3Or)
complexes with imidazole were synthesized and char-
acterized. The HOr coordinated to the Co(II), Ni(II)
and Cd(II) ions are chelated to the deprotonated pyrim-
idine nitrogen (N(3)) and the carboxylate oxygen, while
the H2Or is present as a counter-ion in Zn(II) complex.
The thermal decomposition pathway of the complexes
has been predicted by the help of thermal analysis (TG,
DTG and DTA). The compounds decompose in three
thermal stages: (i) dehydration (ii) release of imidazole
ligands and decomposition of orotate (iii) remained or-
ganic residue burns. The final decomposition products
corresponding metal oxides.
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J. Therm. Anal. Cal., 89, 2007
559