Co(II), Ni(II), Cu(II), and Cd(II) complexes of some new arylazopyrazolopyrimidine derivatives

Article abstract of DOI:10.1081/SIM-120024314

The metal ion complexes [Co(II), Ni(II), Cu(II) and Cd(II)] of some azopyrazolopyrimidine derivatives have been prepared and characterized using elemental analyses, molar conductivity, electronic and IR spectra. The azopyrazolopyrimidines are characterized by a high tendency towards complex formation with the neutral molecules coordinating to the metal ion as bidentate ligands. It is concluded that the ligands are bonded to the metal ion through the α- or β-nitrogen atom of the arylazo group and the carbonyl oxygen of the pyrazolone ring where six- or five-membered chelate rings are formed. The apparent formation constants of the complexes in solution have been determined.

Full text of DOI:10.1081/SIM-120024314

SYNTHESIS AND REACTIVITY IN INORGANIC AND METAL-ORGANIC CHEMISTRY
Vol. 33, No. 8, pp. 1351–1366, 2003
Co(II), Ni(II), Cu(II), and Cd(II) Complexes of
Some New Arylazopyrazolopyrimidine Derivatives
H. M. A. Salman,1 A. A. Mohamed,1 S. A. Ibrahim,2,
*
and A. A. Ahmed1
1Department of Chemistry, Faculty of Science,
South Valley University, Qena, Egypt
2Department of Chemistry, Faculty of Science,
Assiut University, Assiut, Egypt
ABSTRACT
The metal ion complexes [Co(II), Ni(II), Cu(II) and Cd(II)] of some
azopyrazolopyrimidine derivatives have been prepared and characterized
using elemental analyses, molar conductivity, electronic and IR spectra.
The azopyrazolopyrimidines are characterized by a high tendency
towards complex formation with the neutral molecules coordinating to
the metal ion as bidentate ligands. It is concluded that the ligands are
bonded to the metal ion through the a- or b-nitrogen atom of the arylazo
group and the carbonyl oxygen of the pyrazolone ring where six- or five-
membered chelate rings are formed. The apparent formation constants of
the complexes in solution have been determined.
*
Correspondence: S. A. Ibrahim, Department of Chemistry, Faculty of Science,
Assiut University, Assiut, Egypt; E-mail: saidib@acc.aun.edu.eg.
1351
DOI: 10.1081/SIM-120024314
0094-5714 (Print); 1532-2440 (Online)
www.dekker.com
Copyright D 2003 by Marcel Dekker, Inc.

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Salman et al.
INTRODUCTION
Azo compounds constitute a very interesting and important class of
compounds from both the theoretical and practical points of view. This is
due to their interesting structural features,[1] unusual spectroscopic prop-
erties,[2] as well as their use as disperse dyes[3] and in the manufacture of
polyamide and polyester fibers.[4]
On the other hand, a literature survey reveals that azo compounds
containing heterocyclic nitrogen atoms were applied for the separation and
the spectrophotometric determination of metal cations.[5–8] Moreover, azo
compounds containing the pyrazolone moiety were also applied for the
spectrophotometric[9,10] determination of some metal cations. In view of
these findings, this article reports the preparation and characterization of the
Co(II), Ni(II), Cu(II) and Cd(II) complexes of some 8-[p-substituted
phenylazo]-2,4-dimethylpyrazolo[1,5-a]pyrimidine-7(6H)ones. The dyes
used are represented schematically in Figure 1.
EXPERIMENTAL
Preparation of Arylazopyrazolopyrimidine Derivatives
The preparation of arylazopyrazolopyrimidine derivatives includes the
following two steps.
Preparation of Arylazopyrazolinone Derivatives
3-Amino-4-(arylazo) pyrazolin-5-one derivatives were prepared simi-
larly to the literature method.[11] The appropriate amine (aniline, p-toludine,
p-anisidine, p-chloroaniline and p-nitroaniline, 0.02 mol) was dissolved in
Figure 1. R = H (L1); CH3 (L2); OCH3 (L3); Cl (L4); NO2 (L5).

Arylazopyrazolopyrimidine Complexes
1353
dilute HCl (20 mL, 3N) and the resulting solution was cooled in an ice bath
to 5 °C. To this solution was added dropwise with stirring a cold solution of
sodium nitrite (1.6 g, 0.023 mol in 20 mL H2O). The resulting diazonium
salt was added slowly with stirring to a cold solution of 3-aminopyrazolin-
5-one (1.98 g, 0.02 mol) in 40 mL (1:1) water: ethanol (the latter compound
was prepared[12] by the reaction of ethyl cyanoacetate with hydrazine
hydrate and sodium ethoxide). After the completion of the addition, stirring
was continued for 30 min, then a solution of sodium acetate (20 g in 60 mL
H2O) was added until complete precipitation of the dye. The precipitated
solid was filtered, washed with cold water and recrystallized from ethanol
to give the corresponding arylazopyrazolinone derivative.
Preparation of Arylazopyrazolopyrimidine Derivatives
Each of the resulting azo dyes from the above step was refluxed for
4 h with acetylacetone in the molar ratio 1:1 in 30 mL of acetic acid.[13]
The reaction mixture was then cooled to room temperature whereby a
solid compound was precipitated. The precipitated solid was filtered,
washed with cold ethanol and recrystallized from ethanol to give the
corresponding arylazopyrazolopyrimidine derivative. The purity of the
compounds was checked by elemental analyses and IR spectra.
Preparation of the Solid Complexes
The solid complexes were synthesized by mixing hot methanolic
solutions ( ꢀ 30 mL) of the respective ligand (L1, L2, L3, L4 and L5) and
the metal salt (CoCl2Á6H2O, NiCl2Á6H2O, CuCl2Á2H2O and CdCl2Á2H2O) in
the proper metal:ligand ratio (1:1 for Cd with L1, L2, L3 and Cu with L4
and L5; both 1:1 and 1:2 for Co, Ni, with L3 and 1:2 for Co, Ni, Cu with L1
and L2). The mixture was refluxed on a water bath for 4 h and then
concentrated to a small volume and left at room temperature for at least
12 h, whereby the complexes separated in crystalline form. The solid
complexes were filtered and washed several times with cold methanol,
dried and preserved in a desiccator over dry P2O5.
Preparation of Solutions of the Complexes and Ligands
Standard metal solutions: High purity metal salts were dissolved in the
appropriate volume of methanol to obtain 10–2 M stock solutions. Ligand
solutions: Methanolic stock solutions (10–2 M) were prepared by dissolving
the accurately weighed amounts in the required volume of methanol.

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Salman et al.

Arylazopyrazolopyrimidine Complexes
1355

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Salman et al.
Physical Measurements
The IR spectra of all the ligands and their complexes were recorded on
both Shimadzu IR-408 and Nicolet FT-IR 560 spectrophotometers as KBr
discs. The absorption spectra of the various solutions prepared were
recorded on a Shimadzu 2401 PC spectrophotometer using 1 cm matched
silica cells at 25 °C. Temperature control was achieved using an ultra-
thermostat of 0.05 °C accuracy. All conductance measurements reported
were performed using an S-506 conductance bridge. All measurements were
carried out at 25 °C.
RESULTS AND DISCUSSION
Elemental analysis data, together with the color, decomposition
temperature and molar conductance values of the ligands and the prepared
solid complexes are reported in Table 1. Examination of the data of this
table reveals that the molar conductance values of 10–3 M methanolic
solution of each of the various complexes synthesized were found to be in
the range 21–70 Ohm–1 mol–1 cm2. Since it has been reported[14] that the
reasonable range of the molar conductance values for 10–3 M solution of
1:1 electrolytes in methanol is 80–115 Ohm–1 mol–1 cm2, one may
conclude that the various complexes under investigation are all non-
electrolytes. This indicates that the chlorine atoms are all inside the
coordination sphere of the complexes. Therefore, these complexes may be
formulated as [M(II)(L)nCl2]ÁxH2O where L = L1–L5, n = 1 or 2 and x = 0 –
4 (cf. Table 1). It should be noted that the relatively high molar
conductance values of some complexes are due to the probable solvolysis
and dissociation of the complexes in methanol.[14] The complexes reported
in Table 1 are those that gave satisfactory elemental analyses. Several
attempts were made to prepare other complexes in a pure form but
were unsuccessful.
Spectrophotometric investigation of various complexes was performed
to inspect the formation and composition of the most stable complexes
possibly formed in solution. The absorption spectra of the free ligands L1,
L2, L3, L4 and L5, are characterized by a main absorption band in the
visible region located at 420, 430, 440, 400 and 450 nm, respectively
(Table 2). This band is assigned to an intramolecular charge transfer taking
place within the ligand molecule. It is suggested that the aryl moiety is the
donor center of CT and the heteroaromatic system is the acceptor one.
Generally, it is evident that coordination of ions to the ligand causes a blue-
shift of the CT band. This can be considered as evidence for instantaneous

Arylazopyrazolopyrimidine Complexes
1357
Table 2. lmax (nm) and emax  10–4 (cm2 mol–1) values of visible absorption bands
of 10–4 M solutions of the ligands and their (1:1) metal complexes.
Free ligand
Co complex
Ni complex
Cu complex
Cd complex
lmax
emax
lmax
emax
lmax
emax
lmax
emax
lmax
emax
L1
L2
L3
L4
420
430
440
400
3.20
3.00
2.60
0.80
405
410
415
415
2.45
2.55
2.50
1.10
400
405
410
412
2.45
2.40
2.50
1.10
395
405
410
379
410sh
410
2.45
2.65
2.45
1.50
1.20
1.40
420
420
430
385
410
435
2.70
2.90
2.70
1.50
1.30
1.70
L5
450
1.60
425
1.50
425
1.30
sh = shoulder.
complex formation between the azo derivatives under investigation and the
metal ions. The observed blue-shift in the CT band on complexation can be
attributed to the less facile intramolecular charge transfer interaction in the
complexed ligand relative to that in the free ligand. This results from the
unavailability of the lone pair of electrons of the azo group nitrogen as it is
involved in the coordination to the metal ion, and hence the conjugation is
shortened. The intramolecular CT interaction within the complexed ligand
is represented schematically in Figure 2.
The stoichiometric ratio of the various complexes studied was
examined by two different methods: the molar ratio method[15] and Job’s
continuous variation method.[16] In all cases a solution of the azo dye of the
same concentration as in the complex solution was used as a blank. The
corresponding results are represented graphically in Figures 3 and 4.
The data from the continuous variation method were applied to the
determination of the apparent formation constant values (Kf) of the various
Figure 2. 1M2+ : 1L Complex; bonding from azo group could be exerted either by
a- or b-nitrogen atom.

1358
Salman et al.
Figure 3. Molar Ratio Method as applied to Cu(II)-ligand complexes. a: L1,
l = 308 nm; b: L2, l = 382 nm; c: L3, l = 395 nm; d: L4, l = 342 nm; e: L5,
l = 425 nm.
complexes formed in solution.[17] The Kf values obtained are listed in Table 3.
Examination of these results reveals that the stability of 1:1 and 1:2
[M(II):ligand] complexes increases in the general order: Cd(II) < Co(II) <
Ni(II) < Cu(II). This is in agreement with the general order of stability of
the complexes of these metal ions which was established by Grinberg and
Yatsimirski[18] and by Irving and Williams.[19]
Electronic Spectra
The electronic absorption spectra of methanolic solutions of the various
complexes have been recorded in the wave length range 250–600 nm and
the values of lmax (nm) and emax are given in Table 4.
The complexes display one or more intense band in the UV region
(below 350 nm) due to intraligand electronic transitions (mainly p–p*). In
the visible region, the spectra are characterized by an intense band in the

Arylazopyrazolopyrimidine Complexes
1359
Figure 4. Job’s Continuous Variation Method as applied to Cu(II)-ligand com-
plexes. a: L1, l = 350 nm; b: L2, l = 357 nm; c: L3, l = 395 nm; d: L4, l = 360 nm;
e: L5, l = 435 nm.
range 374–432 nm. This band is due to CT interaction within the
complexed ligand (mainly ligand to metal LMCT). It is worthy to note
here that in case of the Cu(II)–L1, Cu(II)–L2 and Cu(II)–L3 complexes, the
visible band is accompanied by a broad shoulder at the longer wave length
side with lmax around 509–520 nm (nmax 19.64–19.23 kk). This broad
structured shoulder trailing off in the low energy region is indicative of a
tetragonally distorted octahedral configuration of the copper complexes.
Since a four-coordinated square-planar complex is strongly favored for a d9

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Salman et al.

Arylazopyrazolopyrimidine Complexes
1361
Table 4. Electronic spectral data of methanolic solutions of the various
metal complexes.
Intraligand band
LMCT band
d–d band
a
b
a
b
a
b
Complex
lmax
(emax
)
lmax
(emax
)
lmax
(emax)
[Co(L1)2Cl2]Á2H2O
[Ni(L1)2Cl2]Á3H2O
[Cu(L1)2Cl2]Á2H2O
[Cd(L1)Cl2]Á3H2O
[Co(L2)2Cl2]Á2H2O
[Ni(L2)2Cl2]Á4H2O
[Cu(L2)2Cl2]Á2H2O
322
324
330
326
249
326
251
327
254
328
254
255
254
331
255
334
254
240
256
241
–
(2,312)
(1,585)
(2,462)
(992)
407
396
403
415
504
407
412
(3,500)
(2,140)
(4,202)
(2,187)
(4,300)
(4,000)
(4,675)
–
–
–
–
520
–
(4,500)
–
(3,325)
(2,652)
(3,852)
(2,405)
(2,400)
(1,375)
(1,900)
(3,750)
(2,345)
(1,302)
(3,600)
(2,330)
(1,912)
(3,597)
(3,377)
(1,682)
–
–
–
–
520
–
(5,000)
[Cd(L2)Cl2]Á4H2O
424
(3,450)
–
–
[Co(L3) Cl2]Á2H2O
[Co(L3)2Cl2]Á2H2O
[Ni(L3) Cl2]Á4H2O
415
415
410
–
(2,107)
(3,977)
(2,722)
–
–
–
–
–
–
–
[Ni(L3)2Cl2]Á2H2O
414
–
(3,925)
–
–
–
(557)
(912)
509
510
–
[Cu(L3)Cl2]Á2H2O
[Cu(L3)2Cl2]Á3H2O
[Cd(L3)Cl2]
[Cu(L4)Cl2]
[Cu(L5)Cl2]Á4H2O
406
408
432
374
379
(1,737)
(4,077)
(4,325)
(2,345)
(310)
–
–
–
–
–
almax (nm).
bemax in cm2 mol–1.
configuration, the usual distortion is that expected by the Jahn–Teller effect.
This will result in a transition from t2g (xz or yz) to eg (x2 À y2).[20]
Infrared Spectra
The IR spectra of all of the free ligands, L1, L2, L3, L4 and L5
(Table 5), display a broad band covering the range 3250–2900 cm–1. This
band includes mixed vibrations of n(NH) and n(CH) (aliphatic and
aromatic). Therefore, changes in this band upon complexation could not
be used for diagnostic purposes. Moreover, this band was found to be
completely hidden under the strong band appearing in the range 3450–
3350 cm–1 region due to n(OH) of water molecules which is seen in the

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Salman et al.
Table 5. Selected IR bands (cm–1) of the free ligands and their metal complexes.a
Compound
Free L1
n(OH)
n(NH + CH)
n(C=O)
n(N=N) n(Ar–N=N)
–
3,150–2,920 b 1,640 vs 1,440 s
1,170 m
1,130 w
1,140 w
1,160 w
1,140 w
1,180 m
1,130 w
1,130 w
1,130 w
1,120 w
1,180 m
1,130 w
1,130 w
1,110 w
1,120 w
1,110 w
1,130 w
1,130 w
[Co(L1)2Cl2]Á2H2O 3,450 s
[Ni(L1)2Cl2]Á3H2O 3,350 s
[Cu(L1)2Cl2]Á2H2O 3,350 s
–
–
–
–
1,620 s
1,630 s
1,620 s
1,420 m
1,400 m
1,425 m
[Cd(L1)Cl2]Á3H2O
3,400 m
–
1,630 m 1,430 m
Free L2
3,200–2,915 b 1,642 vs 1,440 s
[Co(L2)2Cl2]Á2H2O 3,400 s
[Ni(L2)2Cl2]Á4H2O 3,400 s
[Cu(L2)2Cl2]Á2H2O 3,450 s
–
–
–
–
1,615 s
1,620 s
1,620 s
1,430 m
1,430 m
1,420 m
[Cd(L2)Cl2]Á4H2O
3,450 m
–
1,610 m 1,430 m
Free L3
3,140–2900 b 1,630 vs 1,430 s
[Co(L3) Cl2]Á2H2O 3,450 s
–
1,600 s
1,600 s
1,620 s
1,615 s
1,600 s
1,600 s
1,420 m
1,420 m
1,420 m
1,420 m
1,415 m
1,415 m
[Co(L3)2Cl2]Á2H2O 3,450 s
–
[Ni(L3)Cl2]Á4H2O
3,400 s
–
[Ni(L3)2Cl2]Á2H2O 3,450 s
–
[Cu(L3)Cl2]Á2H2O
3,400 s
–
[Cu(L3)2Cl2]Á3H2O 3,400 s
–
3,075 b
[Cd(L3)Cl2]
–
1,620 m 1,410 m
n(C=N) azomethine
1,607 m
1,580 w
Free L4
–
3,200–3,922 b 1,642 s
3,198–2,920 b 1,635 s
3,210–2,920 b 1,645 s
[Cu(L4)Cl2]
Free L5
–
–
3,400 s
1,610 m
1,585 w
[Cu(L5)Cl2]Á4H2O
–
1,625 s
ab = broad; vs = very strong; s = strong; m = medium; w = weak.
solid metal complexes. The presence of water molecules in the prepared
solid complexes is supported by the results of elemental analyses of these
complexes (Table 1). The free ligands L1–L3, which were found to exist
mainly in the true azo form, show a very strong sharp band with definite
peaks at 1640, 1642 and 1630 cm–1, respectively, due to the n(C=O) of the
pyrazolone moiety.[21,22]
In the IR spectra of the corresponding complexes this band occurs but
with a pronounced shift to lower frequencies indicating the coordination of
the carbonyl oxygen of the pyrazolone ring to the metal ion. The strong
band appearing in the IR spectrum of the free ligands L1, L2 and L3 at
1440, 1440 and 1430 cm–1, respectively, is due to the azo group stretching
vibration n(N=N).[23,24] This band is found at lower frequencies in the IR
spectra of the metal complexes indicating the participation of the azo group
in complex formation. Furthermore, the band of medium intensity occurring

Arylazopyrazolopyrimidine Complexes
1363
Figure 5. 1M:1L Complexes; bonding from azo group could be exerted either by a-
or b-nitrogen atom.
at 1170, 1180, and 1180 cm–1 in the spectra of the ligands L1, L2 and L3,
respectively, is assignable to the Ar–N=N stretching vibration.[25] This
band acquires also a shift towards lower frequencies and a decrease in
intensity on complexation confirming the coordination of the azo group to
the metal ion.
It is noteworthy that the appearance of these two bands n(N=N) and
n(Ar–N=N) in the IR spectra of these three ligands and their metal
complexes is further convincing evidence that the ligands L1, L2 and L3
exist in the solid states in the true azo form. Based on the foregoing
discussion, it may be concluded that the ligands L1, L2 and L3 act as neutral
bidentate chelating agents where the bonding sites are the aryl azo a- or
b-nitrogen and carbonyl oxygen of the pyrazolone ring to form stable
six- or five-membered chelate rings. The following bonding mode shown in
Figures 5 and 6 is suggested for these three ligands in their complexes.
Figure 6. 1M:2L Complexes (L = L1, L2 and L3); bonding from azo group could be
exerted either by the a- or b-nitrogen atom.

1364
Salman et al.
Figure 7. 1 : 1 Complex; bonding from azomethine group could be exerted either
by the a- or b-nitrogen atom.
The IR spectra of the free ligands L4 and L5 are characterized also by a
strong band at 1642 and 1645 cm–1, respectively, which is assignable to
n(C=O) of the pyrazolone moiety.[21,22] This band acquires a marked shift
to lower frequency (Table 5) indicating a decrease in the bond order of the
C=O group as a result of the coordination of the oxygen atom to the metal
ion. The azomethine, n(C=N),[22] band appearing at 1607 (L4) and 1610
(L5) was shifted to lower frequency in the IR spectra of the complexes
relative to its position in the spectra of the free ligand. The lowering of
n(C=N) frequency indicates that the C=N bond order decreases and that the
nitrogen atom coordinates to metal.[26]
It is worth to mention that the appearance of a n(C=N) band and the
absence of both n(N=N) and n(Ar–N=N) bands in the IR spectra of these
two ligands and their metal complexes are convincing evidence that L4
and L5 in the solid state exist in the hydrazone (keto) form. Therefore, it
may be concluded that the complexes of these two ligands are formed
through the coordination of the carbonyl oxygen of the pyrazolone ring
and the hydrazone azomethine a- or b-nitrogen atom to form a stable six-
or five-membered chelate ring. The bonding sites of these two ligands are
the carbonyl oxygen and the azomethine nitrogen as illustrated schemat-
ically in Figure 7.
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Received June 28, 2000
Accepted April 25, 2003
Referee I: M. H. Dickman
Referee II: B. Durham

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