Study of the coordination properties of 1,2-bis(2,6-dimethphenylamino) glyoxime and determine the stability constant of its complexes with Ni(II), Cu(II) and Zn(II) metal ions in solution

Article abstract of DOI:10.1007/s10953-011-9733-z

This study aims to investigate the nature and type of complexes formed in solution between 1,2-bis(2,6-dimethphenylamino) glyoxime (DPG) and the ions Ni²⁺, Cu²⁺ and Zn²⁺. Potentiometric titration was used to follow the for

Full text of DOI:10.1007/s10953-011-9733-z

J Solution Chem (2011) 40:1618–1628
DOI 10.1007/s10953-011-9733-z
Study of the Coordination Properties
of 1,2-Bis(2,6-dimethphenylamino) Glyoxime
and Determine the Stability Constant of Its Complexes
with Ni(II), Cu(II) and Zn(II) Metal Ions in Solution
Hayati Sari · Faisal N. Al-Obaidi · Mustafa Macit ·
Hasan Atabey
Received: 18 May 2010 / Accepted: 17 February 2011 / Published online: 8 September 2011
© Springer Science+Business Media, LLC 2011
Abstract This study aims to investigate the nature and type of complexes formed in solu-
tion between 1,2-bis(2,6-dimethphenylamino) glyoxime (DPG) and the ions Ni2+, Cu2+ and
Zn2+. Potentiometric titration was used to follow the formation of complexes.
The complexes formed were studied through the determination of stability constants of
these complexes in mixed ethanol–aqueous solution at 25 0.1 °C and ionic strength of
0.1 mol·dm−3 NaCl. The basicity of the ligand was also assessed by the determination of
the dissociation constants of the ligand. All of the constants were determined by computer
refinement of pH–volume data using the SUPERQUAD program. The species distribution
diagrams were also calculated.
Comparison with other vic-dioximes was made to provide reliable support for the forma-
tion of the proposed complexes in solution.
Keywords Vic-dioxime · Potentiometric titration · Metal complexes · Stability constants ·
SUPERQUAD · Dissociation constants
1 Introduction
The basis for the wide use of vic-dioxime metal complexes is their outstanding stability
[1]. Vic-dioximes, together with their complexes, are of interest for many researchers in
different fields of chemistry. They have enormous importance in coordination chemistry, as
many complexes of vic-dioximes with transition metal ions have been isolated and charac-
terized [2]. The presence of the acidic hydroxyl group and the basic azomethine group in
H. Sari ( ) · H. Atabey
ꢀ
Faculty of Science and Arts, Department of Chemistry, Gaziosmanpas¸a University, 60250 Tokat, Turkey
e-mail: hsari@gop.edu.tr
F.N. Al-Obaidi
Department of Chemistry, College of Science, The University of Mustansiriya, Baghdad, Iraq
M. Macit
Department of Chemistry, Ondokuzmayıs University, 55139 Samsun, Turkey

J Solution Chem (2011) 40:1618–1628
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Fig. 1
1,2-bis(2,6-dimethylphenylamino)
glyoxime
the vic-dioxime molecule makes it a suitable ligand for coordination with a variety of metal
ions. The great stability of some vic-dioximes, such as dimethyl glyoxime (DMG), is asso-
ciated with the formation of metal-metal bonds [3]. It has been proposed that nickel-nickel
bonding is an essential characteristic of the (DMG) complex [4], and this can justify the
enormous distinction in aqueous solubility of the Ni-DMG and Cu-DMG complexes, where
no such bonding occurs in the latter complex; in other words, they have different crystal
structures. Solubility depends on the nature of the ligand, in addition to metal-metal bond,
and the influence of the ligand may be so large that it conceals the influence of the metal-
metal bond [4]. Ni-DMG is more soluble than the corresponding complexes for the alicyclic
vic-dioximes, which can be linked to the nature of the ligand [5]. There are other various
applications of vice-dioxime such as chemical analysis [6], medicine [7], biochemistry and
other fields. Vic-dioxime complexes are linked to the chemistry of vitamin B12; therefore
they acquired considerable attention [8]. Some of these vic-dioximes show photo-physical
and photo-chemical properties [9–12], which make them quite significant materials. Other
vic-dioximes have the ability to stabilize high oxidation states of transition metals such as
Cu(III) [13] and Co(IV) [14]. The stability of such species has been ascribed to the fact
that the oxime group possesses some pi donor character, another factor which enhances the
stability is the negative charge initiated on the oximatio-oxime. This negative charge will
eventually neutralize the charge on the metal ion [13]. Different types of dioximes such as
pyridine-dioxime were also reported to stabilize nickel (III, IV) [15].
This study is concerned with the coordination competence of DPG in mixed aqueous
solutions.
2 Experimental
2.1 Chemicals
NaOH, KOH, NaCl, HCl, ethanol, CuCl2·2H2O, ZnCl2·7H2O and NiCl2·6H2O were pur-
chased from Merck and potassium hydrogen phthalate from Fluka. All reagents were of
analytical quality and were used without further purification. For the solutions, CO2-free,
double-distilled deionized water was obtained with an aquaMAX™-Ultra water purification
system (Young Lin Inst.). Its resistivity was 18.2 Mꢀ·cm−1
.
2.2 The Synthesis of 1,2-Bis(2,6-dimethphenylamino) Glyoxime (DPG)
1,2-Bis(2,6-dimethphenylamino)glyoxime was synthesised as described in the literature
[16]. To a stirred solution of anti-dichloroglyoxime 10 mmol (1.57 g) [17, 18] in absolute
ethanol (30 mL), at −10 °C, was added drop-wise 20 mmol (2.42 g) of 2,6-dimethylaniline
in absolute ethanol (30 mL) over a period of 1 hour. The reaction mixture was stirred for
3 hours at −10 °C. After adjusting the pH of the mixture to 5–6 with 0.1 mol·dm−3 KOH

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Table 1 Summary of the
experimental parameters for the
potentiometric stability constant
measurements
+
−3
2+
2+
2+
System:
DPG with H , Cu , Ni and Zn in water
Solution composition: [L]/mol·dm = 0.001–0.002
−3
[M]/mol·dm = 0.001
−3
ionic strength/mol·dm = 0.1 (NaCl)
Metal:Ligand ratio
1 : 2
−3
−1
[NaOH]/mol·dm = 0.025
[KHP]/mol·kg = 0.05 (at 25 °C, pH = 4.005) [22]
Volume increments/mL = 0.03
pH range = 3–11
a
Number of titration points per
log
β
= −13.98
00-1
10
titration
t/°C = 25.0 0.1
b
Number of titrations per
a
n
= 230
= 3
metal:ligand system
M: metal ion, L: ligand,
β: overall stability constant
tot
b
n
tit
solution at 0 °C, and further stirring for 1 hour and dilution with 100–120 mL of water, the
reaction mixture was left overnight at 0 °C. The resulting precipitate was collected by filtra-
tion and then recrystalized from ethanol–water (1:1). The crystalline product was filtered,
washed with water and dried in vacuum. Yield, 72%. Elemental Analysis: Calculated: C,
1
66.26; H, 6.75; N, 17.18; Found: C, 66.48; H, 6.94; N, 17.32. H-NMR (DMSO-d6): 10.1
(2H, s), 7.3 (2H, s), 6.6–7.1 (6H, m), 2.1–2.2 (12H, s) ppm. IR (KBr): 3390 (N–H), 3250
(O–H), 1620 (C=N), 960 (N–O) cm−1
.
2.3 Titration Procedure
Stock solutions (2.00×10−3 mol·dm−3) used in the titrations were prepared in aqueous solu-
tions at 25 °C. Solutions of metals ions (0.001 mol·dm−3) were prepared from CuCl2·2H2O,
ZnCl2·7H2O and NiCl2·6H2O as received and standardized with ethylenediaminetetraacetic
acid (EDTA) [19]. Potassium hydrogen phthalate (KHP) was prepared as buffer solution
(0.05 mol·kg−1) and used for calibration of the combination pH electrode according to the
method of Molspin [20]. NaOH (0.025 mol·dm−3) was standardized with primary standard
KHP solution by pH-metric titration. HCl stock solutions, used for obtaining pH 3.0, were
prepared from concentrated HCl and their concentration were determined with standardized
NaOH. All potentiometric titrations were carried out on solutions in 100 mL double-walled
glass vessel using the Molspin automatic titration system, which interfaces to a PC, with
a 10 cm3 syringe, a SenTix 20 pH combination electrode (WTW, Weilheim); the tempera-
ture was controlled at 25.0 0.1 °C by circulating water from a constant-temperature bath.
The cell was equipped with a magnetic stirrer and a tightly fitting cap, which contained
three holes for the combined electrode, nitrogen gas and automatic burette. The pH was
measured between 3.0–11.0 under nitrogen atmosphere. The ligand concentrations varied in
the range 1.00–2.00 × 10−4 mol·dm−3. Summaries of the titration conditions are given in
Table 1. The pH-metric data was used to find the dissociation and stability constants using
the SUPERQUAD computer program [21]; the standard deviations (σ values) computed by
SUPERQUAD refer to random errors.

J Solution Chem (2011) 40:1618–1628
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−3
Fig. 2 Species distribution diagram of DPG, at 25 °C in aqueous NaCl (I = 0.1 mol·dm
)
3 Results and Discussion
3.1 Dissociation Constants of the Ligand (DPG)
The deprotonation equilibrium is as seen in the following equations (charges are omitted for
simplicity).
LHn ꢀ LHn−1 + H
and the deprotonation constants (Kn) are given as;
Kn = [LHn−1][H]/[LHn]
The distribution diagram shows that the DPG ligand exists in solution as different species
depending on pH (see Fig. 2). There are four possible species that might occur under the ex-
perimental conditions. This is in accordance with the other vic-dioximes, where they expect
to exhibit four pKas as they associate to both azomehtine and hydroxyl groups (see Table 2).
In the DPG ligand, the nitrogen of the imino groups is not basic enough to be determined
under our experimental conditions. The four pKas values of the DPG were determined po-
tentiometrically (see Table 2); two of them are related to the oxime nitrogen and expressed
as LH4 and LH3. pKas values of DPG calculated in this study are within the expected values
in comparison with other similar ligands in literature [23, 24].
It is quite useful to study the ligand forms in solution prior to the complex formation
process; this will assist in predicting the possible species types which will materialize at
certain conditions.
Figure 3 shows the titration curve of the DPG, together with the titration curves of
dimethylglyoxime (DMG) and dichloroglyoxime (DCG) [25]. It can be noticed that the
acidity extent of the DCG is comparable to DPG, as both of them differ from the DMG by
replacement of a methyl group by an electron withdrawing group. DPG is even more basic
than DMG, which indicates that the dimethyl phenyl amino moiety shows no electron with-
drawing properties within the ligand. In other words, the dimethyl phenyl amino moiety does

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Table 2 Dissociation constants
of the DPG ligand with other
related ligands at 25 °C in
Ligand
DPG
Species
pK
Reference
a
LH
4
6.09 0.03
9.30 0.03
9.98 0.02
10.41 0.04
Present work
aqueous NaCl
(I = 0.100 mol·dm
−3
)
LH
LH
LH
3
2
a
BPG
LH
LH
LH
LH
2.83
6.06
6.97
9.51
[23]
[24]
[24]
4
3
2
b
BBPGH2
LH
LH
LH
LH
3.46
5.89
6.77
9.76
4
3
2
a
BPG: 1,2-bis(4-benzyl-
piperidine)glyoxime
c
BMPG2
LH
LH
LH
LH
2.91
6.79
4
3
2
b
BBPGH2: 1,2-bis(4-benzyl-
piperazine)glyoxime
7.97
c
BMPGE: 1,2-bis(4-methyl-
10.00
piperazine)glyoxime
−3
Fig. 3 Titration graph of DPG, DMG and DCG at 25 °C in aqueous NaCl (I = 0.1 mol·dm
)
not practically add any acidity as a result of its withdrawing properties. The high basicity
of DPG, in comparison with DMG, is unexpected; again, comparing the basicity of DMG
and DPG showed the diminishing electron withdrawing properties of the dimethyl phenyl
group, this probably due to the electron donating properties of the methyl groups attached to
phenyl ring in the molecules. This might compensate the effect of the electron withdrawing
properties of the dimethyl phenyl amino group and boost the basicity of the whole DPG lig-
and, through pushing electron density into the whole molecule. This may be investigated by

J Solution Chem (2011) 40:1618–1628
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2+
2+
2+
Table 3 Stability constant data for the complexation of Cu , Ni and Zn with DPG at 25 °C in aqueous
−3
p
q
r
NaCl (I = 0.1 mol·dm ) β
= [M L H ]/[M] [L] [H]
pqr
p q r
a
pqr
Ni(II)
Cu(II)
Zn(II)
log
10
β
pqr
M(II)-DPG complexes
101
111
121
131
141
18.41 0.06
27.66 0.07
36.73 0.04
45.38 0.04
51.41 0.05
17.15 0.05
25.90 0.02
32.37 0.05
38.83 0.03
42.92 0.04
7.50 0.04
−2.17 0.04
−12.04 0.05
–
18.05 0.02
26.47 0.02
–
41.72 0.02
47.94 0.03
–
1–11
1–21
1–31
1–41
–
−0.63 0.04
−10.75 0.08
–
−0.34 0.02
–
−19.99 0.04
a
p: number of metal ions, q: number of hydrogen ions (positive values) or hydroxide ions (negative values),
r: number of ligands, in the complexes
using other similar ligands with no donating groups. The higher basicity of DPG in compari-
son to DMG lessens the dissociation of the hydroxyl groups in the molecule. But, during the
coordination of the ligand with the metal ion, this process enhances the dissociation of the
hydroxyl group resulting in the generation of hydrolyzed species (see Table 3) and proton
release.
3.2 Stability Constants of the Complexes
The overall stability constants (βpqr ) are defined as follows (charges are omitted for sim-
plicity):
p
q
r
pM + qL + rH ꢀ MpLqHr ,
βpqr = [MP LqHr ]/[M] [L] [H]
where M is a metal ion, L is the ligand, H is the proton, and p, q and r are the respective
stoichiometric coefficients.
Figure 4 shows the titration curve of the M-DPG complex system. As seen in Fig. 3, the
inflection points are different for each metal ion, M.
The bi-dentate nature of the DPG ligand imposes a cis-structure of the coordinated lig-
and; this makes clear the extent of the coordinating power of the ligand and indicates its
affinity towards metal ions. Thus the trans-form the DPG cannot act as a bi-dentate ligand,
yet has less steric strain. The cis form can also be found in one of the following structures,
syn(α), anti(β) and amphi(γ ) [26]. The anti form seems to be the most appropriate structure
for coordination with metal ions.
Generally the main mode of coordination of vice-dioximes with the transition metals
takes a square planar geometry through the two nitrogen atoms in the oxime chain [9, 27–
29], leading to a five membered ring chelate. Other, six membered ring chelates may also
be formed [30] but to a lesser extent. Coordination in this manner enhances the dissociation
of the hydroxyl groups, and this assists the formation of symmetrical intra-hydrogen bond-
ing in the case of bis-vic-dioxime complexes [31]. Vic-dioximes also form complexes with
structures other than square planar [32].

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−3
Fig. 4 Titration graph for M-DPG at 25 °C in aqueous NaCl (I = 0.1 mol·dm
)
The ligand DPG forms only mono binary complexes in solution despite the experimen-
tal conditions potentially allowing formation of a bis-complex. The immense size and the
structure of the ligand might be the primary factor preventing formation of the bis-complex.
The relatively large DPG molecule will occupy more space of the inner coordination sphere,
leaving insufficient room for a second molecule of the ligand to be attached. The structural
shape of the DPG ligand imposes strain on coordination, as the two 2,6-dimethyl amino
moieties are situated nearby within the molecule, where there is a possible contact between
them which eventually affects the coordination.
The relative high stability constants of the main complexes in all metal ion used in this
research (see Table 3) indicate that steric hindrance has less influence than expected. This
may be explained on the basis that the DPG ligand is capable of adapting its shape as it
approaches a metal ion, where it keeps the steric hindrance at its minimal.
Intra-complex hydrogen bonding would not be possible, but inter-molecular hydrogen
bonding is quite likely; this would be encouraged by the increased acidity of the hydroxyl
group upon coordination. This type of hydrogen bonding increases the solubility of the
complex in solution.
For the Ni-DPG system, the complex formation spans between pH = 4 to pH = 12, and
a variety of species are traced (see Fig. 5). At the early stages and up to pH 8.75 NiH4L
and NiH3L are almost the only species present. NiH4L decreases at approximately pH = 7;
while NiH3L can be found in solution up to almost pH = 9. The NiH2L species can be found
in solution in basic medium and starts to develop in significant amounts just above pH = 8,
and at pH = 8.75 it forms about 40% of the total metal ion concentration. At the same pH
(8.75) and up to pH = 9.25, NiHL developed too but to a lesser extent, forming less than
30%. NiL species occurred at slightly higher pH than NiHL. So the species which can be
traced between pH 8.75–9.00 are: NiLH3, NiLH2, NiLH, and NiL. After pH = 9 and up to
pH just above 11, two more hydrolyzed species come into solution and they are NiLH−2 and
NiH−3, and above pH = 11 only the species NiLH exists.
−3

J Solution Chem (2011) 40:1618–1628
1625
−3
Fig. 5 Species distribution diagram for the Ni-DPG system, at 25 °C in aqueous NaCl (I = 0.1 mol·dm
)
−3
Fig. 6 Species distribution diagram for the Cu-DPG system, at 25 °C in aqueous NaCl (I = 0 mol·dm
)
It is expected that the distribution curve in the case of the Cu-DPG system would be
similar to that of the Ni-DPG system. Figure 5 shows the resemblance between the two
systems, but different species in the case of Cu-DPG formed at relatively lower pH than
the Ni-DPG system. This is expected, as Cu(II) in aqueous solution is more acidic than
the corresponding Ni(II) [33]; however, this actually doesn’t necessitate higher stability of
Cu-complexes over the Ni-complexes. Figure 6 also shows that the Cu-species span over a
wider pH range than the Ni-species, especially in the acidic region.

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−3
Fig. 7 Species distribution diagram for the Zn-DPG system, I = 0.1 mol·dm NaCl, at 25 °C
Cu-DPG species formed in solution over a wide range of pH values (3–11+), where
at lower pH (3–5) only CuLH4 and CuLH3 are formed and dominate the solution. At pH
exceeding 5, the complex CuLH2 starts to form and within the range of pH = 5–9, and in
addition to the above species, the complex CuL forms reaching a maximum of 55% of the
total metal ion at pH = 9.
The hydrolyzed species CuLH−1 and CuLH−2 and formed in solution between pH = 9.5
and 10 with about 30% of each. At pH higher than 10.5, CuLH will be the dominant
−3
species in the solution.
Considering Fig. 7, which shows the distribution diagram of the Zn-DPG system, differ-
ent species are formed at wide pH range too. Species are formed at almost the same pH as
in the Ni-DPG system, but higher than for Cu-DPG. This again reflects the acidity of the
partially complexed ion.
Zn-DPG species formed in solution over a wide range of pH (4–11); at pH (4–5), the
only species present in solution is ZnLH4 with a maximum percentage above 95% of the
total concentration of the metal ion at pH = 4. The species ZnLH3 starts to form just above
pH = 5 and reaches its maximum percentage of 80% of the metal ion at pH = 7. Above
pH = 7 and up to pH = 9 the main species in the solution are the ZnHL with a maximum
percentage of 62% at pH = 8, and the other species is ZnL with a maximum percentage of
60% at pH = 8.8. The other two hydrolyzed species ZnLH−2 and ZnLH−4 are formed in the
solution at pH range 8.75–11 with maxima of 65% at pH = 9.5 and more than 95% at pH
above 11 respectively.
Hydrolyzed species occur as a result of the dissociation of the hydroxyl groups of the
ligand and/or hydrolysis of water molecules in the coordination sphere of the complexed
ion. The partially coordinated metal ion in aqueous solution would undergo hydrolysis at
pH less than for the aquated ion, as the coordination between the ligand and the metal ion
in solution will influence the water molecules coordinated to the ion, whereby it eventually
facilitates hydrolysis. At the high pH applied in this work, it is highly expected that such a
process would take place; as the pK of Ni(II) is about 7.6 [33].
The mixed solvent affects the dissociation of the aquated metal ion, raising the dissocia-
tion constant [34]. On the other hand, the relatively big size of the ligand may influence the

J Solution Chem (2011) 40:1618–1628
1627
coordinated water molecules resulting in weakening of the coordination with the metal ion.
This may further decrease the hydrolysis of the aquated metal ion in the binary complex.
The process of hydrolysis of the aquated binary metal complex, which entails the dissocia-
tion of a proton from one or more of the water molecules, takes place at a relatively higher
pH than hydrolysis of the aquated metal ion [M(H2O)6]2+ (M is Ni, Cu, or Zn). This is due
to the decrease of the coordination power of the metal ion as a result of partial neutralization
by coordination with the molecule.
On the other hand, the hydrolyzed species may be accounted for by the dissociation
of hydroxyl groups in the ligand. The pKas of the hydroxyl groups are 9.98 and 10.41
respectively (see Table 2) and these values suggest that the hydrolysis can occur as a result of
the hydroxyl groups dissociation. But, it is hard to judge exactly the source of the hydrolyzed
species in solution; it may be a result of the both processes, each contributing to a different
degree.
So the possibility exists for the formation of hydrolyzed species under our experimental
conditions, by one or both of these processes. The hydrolysis process is influenced by pH,
the strength of coordination bonding between metal ion and the ligand, and the availability
of water molecules on the complexed ion.
The order of the stability of the complexes under investigation is Ni > Zn > Cu (Table 3)
which contravenes to Irving-William series [34]. The outstanding stability of the nickel com-
plex over that of copper can be explained in terms of the possible linkage between nickel-
nickel ions in the solution [35]. This type of linkage can not be considered as proper bonding
as in the case of solid complexes [35]. Still it has its own impact on coordination, despite
that there is no trace of poly-nuclear complexes recorded under our experimental conditions.
This is due to the low concentration of such complexes, which makes them undetectable by
means of the computer program.
All Zn-DPG complexes formed in solution are more stable than the corresponding Cu-
DPG complexes; this is again not expected on the light of Irving William order [34]. This
can be explained in terms of the possible existence of aquated Zn-DPG complexes in the
form of a tetrahedral arrangement with a [Zn-(DPG)(H2O)2]2+ structure. So the additional
stability initiated was through the entropy factor, which was raised by the loss of more water
molecules from the aquated zinc ion through coordination.
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