Synthesis, spectroscopic and electrochemical studies on bis-[1,3-substituted (Cl, Br) phenyl-5-phenyl formazanato]nickel(II) complexes

Article abstract of DOI:10.1016/j.saa.2007.10.010

In this study, new 1:2 Ni complexes of 1,3-substituted phenyl-5-phenylformazans were synthesized with -Cl, -Br substituents in the o-, m-, p-positions of the 1-phenyl ring and -NO₂ group in the m-position of the 3-phenyl ring. Their structures were elucidated and spectral behaviors were investigated with the use of elemental analysis, GC-Mass, ¹H NMR, ¹³C NMR, FTIR, UV-vis spectra. Furthermore electrochemical properties such as number of electrons transferred (n), diffusion coefficients (D) and possible reaction mechanism of the compounds were determined with the use of cyclic voltammetry, ultramicrodisc electrode and chronoamperometry. The relation between their absorption properties and electrochemical properties was examined. A linear correlation was obtained between Hammett substituent coefficients with λ_max values.

Full text of DOI:10.1016/j.saa.2007.10.010

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Spectrochimica Acta Part A 70 (2008) 973–982
Synthesis, spectroscopic and electrochemical studies on bis-[1,3-substituted
(Cl, Br) phenyl-5-phenyl formazanato]nickel(II) complexes
Habibe Tezcan∗, Elif Uzluk1, M. Levent Aksu
Department of Chemistry, Faculty of Gazi Education, Gazi University, Teknikokullar, 06500 Ankara, Turkey
Received 23 July 2007; received in revised form 11 September 2007; accepted 4 October 2007
Abstract
In this study, new 1:2 Ni complexes of 1,3-substituted phenyl-5-phenylformazans were synthesized with –Cl, –Br substituents in the o-, m-,
p-positions of the 1-phenyl ring and –NO2 group in the m-position of the 3-phenyl ring. Their structures were elucidated and spectral behaviors were
investigated with the use of elemental analysis, GC–Mass, 1H NMR, 13C NMR, FTIR, UV–vis spectra. Furthermore electrochemical properties
such as number of electrons transferred (n), diffusion coefficients (D) and possible reaction mechanism of the compounds were determined with the
use of cyclic voltammetry, ultramicrodisc electrode and chronoamperometry. The relation between their absorption properties and electrochemical
properties was examined. A linear correlation was obtained between Hammett substituent coefficients with λmax values.
© 2007 Elsevier B.V. All rights reserved.
Keywords: Formazan dyes; Nickel(II) complexes; Spectroscopy; Substituent effect; Cyclic voltammetry; Ultramicrodiscelectrode; Chronoamperometry
1. Introduction
claimed that ditetrazolium salts are reduced to both mono and
diformazans by 1 electron transfers. The first 1 electron trans-
fer results in the formation of a tetrazolium radical. He claimed
that the reaction key leading to the simultaneous generation for
mono and diformazans is the disproportionation reaction. It was
also claimed that in polarographic study of formazans there were
two irreversible diffusion controlled processes, each one with 1
electron transfers [9].
It was reported that formazans are oxidized in a single 2
electron transfer followed by a deprotonation reaction form-
ing corresponding tetrazolium cation [10]. In a study of the
reduction of tetrazolium salts into formazans with superoxide
ions, claimed to be the cause of aging and various diseases
in human body, there was 1 electron transfer at −0.20 V
(Ag/AgCl) and one 1e−/1H+ transfer at −0.40 V (Ag/AgCl)
[11].
In a previous study the electrochemical behavior of nitro
formazan derivatives was investigated and the number of the
electrons transferred and diffusion coefficients were determined
in a similar manner to the present study. An EC mechanism
was proposed involving 1 electron transfer radical formation
followed by a tetrazolium anion and cation formation with a
disproportional reaction (TT + e− + H+ ⇔ TT•, TT• ⇔ T+ + T)
[12].
Formazans and their metalcomplexesare coloredcompounds
*
due to ␲–␲ transitions of ␲-electrons in formazan skeleton
–N N–C N–NH– which caused intensive interest among the
scientist. There have been numerous formazans synthesized
up to now and their structural features, tautomeric and pho-
tochromic isomers were investigated [1,2]. Their derivatives
with electron donating and withdrawing group attached to 1,3,5-
phenyl ring were synthesized and the effects of substituents on
the absorption λmax values were examined [3,4].
The redox behaviors of formazans were evaluated in detail.
Formazans form tetrazolium salt when they are oxidized [5].
Tetrazolium salts are reduced back to formazans by the enzymes
in the cell and stain the tissue. Tetrazolium–formazan system is
classified as a marker of vitality [6]. This feature enabled the
determination of activity on tumor cell [7], which caused an
increasing interest in the chemistry, and especially electrochem-
istry of formazans.
The most comprehensive study related to the redox behav-
ior of formazans was carried out by Umemoto [8]. Here it was
∗
Corresponding author. Tel.: +90 312 2028260; fax: +90 312 2227037.
E-mail addresses: habibe@gazi.edu.tr (H. Tezcan), euzluk@yahoo.com
Metal complexes of formazans are colored compounds. That
is why they have been subjected to thorough investigation. It
(E. Uzluk).
1
Tel.: +90 312 2028260.
1386-1425/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2007.10.010

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H. Tezcan et al. / Spectrochimica Acta Part A 70 (2008) 973–982
0.01 mol), aniline (0.93 g, 0.01 mol) concentrated HCl (5 ml)
and sodium nitrite (0.75 g) in methanol at 0–5 ◦C according to
literature [2–4]. There was a cherry red colored (1a) compound
formed. Elemental analysis for C19H16N4 (1a): Calc. (%): C,
76.00; H, 5.33; N, 18.66. Found (%): C, 75.97; H, 5.29; N,
18.69. Mass: m/z (eV), Calc. M: 300.0, Found: M: 301.1 (M+).
Other peaks: 273.10, 223.00, 195.10, 105.05.
2.2.2. Synthesis of 1-(o-, m-, p-chlorophenyl)-3-
(m-nitrophenyl)-5-phenylformazans (2a–4a)
m-Nitrobenzaldehyde (1.51 g, 0.01 mol) was dissolved in
methanol (25 ml) and reacted with phenylhydrazine (1.08 g,
00.01 mol) to give m-nitrobenzaldehyde phenylhydrazone. The
m-nitrobenzaldehyde phenylhydrazone (2.41 g, 0.01 mol) was
dissolved in methanol (45 ml). In another flask o-, m-, p-
chlorobenzenediazonium chloride solutions were prepared
using o-, m-, p-nitroaniline (1.27 g, 0.01 mol). This solution was
added to the m-nitrobenzaldehyde phenylhydrazone solution.
The solution was stirred for 2 h at the same temperature and kept
inacupboardfor3days. Eachcompoundwasrecrystallisedfrom
methanol. There were red-brown colored (2a), red-brown col-
ored (3a) and dark violet-red colored (4a) compounds formed.
Elemental analysis for C19H14N5O2Cl (2a–4a): Calc. (%): C,
60.08; H, 3.69; N, 18.44. Found: (%) C, 60.13; H, 3.73; N, 18.50.
Mass: m/z (eV), Calc. M: 379.5, Found: M: 380.1 (M+). Other
peaks: 274.0, 139.0, 126.0.
Scheme 1. The structure of the formazans (a) and their Ni(II) complexes (b)
synthesized.
was determined that the formation of Fe complexes was depen-
dent upon the pH value and it was observed that there were two
different isomers formed at pH 2 and pH 7–8 [13]. Synthesis,
structural determination, stability and formation constants and
spectroscopic characterization of metal complexes of formazans
were investigated [14–19].
In this study, total seven different novel formazans and their
Ni(II) complexes with various substituents on 1- and 3-phenyl
rings have been synthesized (Scheme 1). Their structures were
elucidated and their spectral behaviors were investigated with
the use of elemental analysis, GC–Mass, 1H NMR, 13C NMR,
FTIR, UV–vis spectra. The effect of substituents on λmax values
was determined.
The biological activity of formazan makes the knowledge
of its oxidation potentials and possible mechanisms very impor-
tant. This study is related to the determination of peak potentials,
diffusion coefficients (D) and a number of electrons transferred
(n) with the use of cyclic voltammetry, ultramicrodisc electrodes
and chronoamperometry. A mechanistic scheme for the oxida-
tion of formazan to tetrazolium salt was proposed based upon
these data (Scheme 3).
2.2.3. Synthesis of 1-(o-, m-, p-bromophenyl)-3-
(m-nitrophenyl)-5-phenylformazans (5a–7a)
m-Nitrobenzaldehyde (1.51 g, 0.01 mol) was dissolved in
methanol (25 ml) and reacted with phenylhydrazine (1.08 g,
00.01 mol) to give m-nitrobenzaldehyde phenylhydrazone. The
m-nitrobenzaldehyde phenylhydrazone (2.41 g, 0.01 mol) was
dissolved in methanol (45 ml). In another flask o-, m-, p-
bromobenzenediazonium chloride solutions were prepared
using o-, m-, p-nitroaniline (1.27 g, 0.01 mol). This solution was
added to the m-nitrobenzaldehyde phenylhydrazone solution.
The procedure was as alike as early in Section 2.2.2. There
were dark orange-red (5a), dark red colored (6a) and orange-
red colored 7a formed. Elemental analysis for C19H14N5O2Br
(5a–7a), Calc. (%): C, 53.77; H, 3.30; N, 16.51. Found:
(%) C, 53.70; H, 3.27; N, 16.55. Mass: m/z (eV), Calc.
M: 424.0, Found M: 425.0 (M+). Other peaks: 318.3, 240.0,
170.1.
2. Experimental
2.1. Chemicals
All chemicals were obtained from Merck and Fluka except
sodium hydroxide and [Ni(CH3COO)2·4H2O] that were pur-
chased from Sigma–Aldrich. All chemicals and solvents used in
the syntheses were of reagent grade and were used without fur-
ther purification. Deionized water (Millipore, Milli-Q) was used
for synthesis; the organic solvents, CH3OH, CH3OCH3, DMSO
and dioxane were used for electrochemical and spectroscopic
measurements.
2.3. Synthesis of the nickel(II) complexes
2.3.1. Synthesis of bis (1,3,5-triphenyl
formazanato)nickel(II) complexes (1b)
1,3,5-Triphenylformazan (1a) (1.500 g, 0.005 mol) obtained
as outlined in Section 2.2 was dissolved in dioxane (20 ml). In
another flask [Ni(CH3COO)2·4H2O] salt (0.0025 mol, 0.625 g)
was dissolved in ethanol (25 ml) under reflux with constant stir-
ringat25 ◦Candformazansolutionwasaddedtoit. Therewasno
color change or precipitation observed during this process. The
mixture was stirred for 8 h with a magnetic stirrer at 30–35 ◦C
under reflux. The precipitation started after an hour and the color
2.2. Preparation of formazans (1a–7a)
2.2.1. Synthesis of 1,3,5-triphenylformazan (1a)
1,3,5-Triphenylformazan was synthesized by the reaction
of benzaldehyde (1.06 g, 0.01 mol), phenylhydrazine (1.08 g,

H. Tezcan et al. / Spectrochimica Acta Part A 70 (2008) 973–982
975
Scheme 2. Synthesis of formazans and their Ni(II) complexes.
turned into orange from red after 2 h and brown after 3 h. The
2.3.3. Synthesis of bis [1-(o-, m-, p-bromophenyl)-3-
(m-nitrophenyl)-5-phenyl formazanato]nickel(II) complexes
(5b–7b)
color remained brown. The mixture was kept in the cupboard for
4 days. The light-brown precipitate was filtered off and washed
with 10 ml 0.5 M NaOH, water and methanol. The compound
was dried in stove at 40 ◦C for 24 h and recrystallized from
methanol.
Formazans (5a–7a) (0.848 g, 0.002 mol) prepared in Section
2.2 was dissolved in ethanol (10 ml) and dioxane (25 ml). In
another flask [Ni(CH3COO)2·4H2O] (0.001 mol, 0.250 g) salt
was dissolved in ethanol (15 ml) and formazan solution was
added to it. Following a similar procedure as in Section 2.3.2
and the brown colored (5b), light green-brown (6b) and brown
colored (7b) were filtered off and recrystallized from dioxane.
Elemental analysis for C40H34N10O6Br2Ni, M: 969.27. Calc.
(%): C, 49.57; H, 3.54; N, 14.44. Found: (%) C, 49.52; H,
3.57; N, 14.48. Mass: m/z (eV), Calc. M: 969.27. Found M:
969.40 (M+). Other peaks: 481.15, 423.90, 240.90. The reaction
scheme is given in Scheme 2 and the experimental parameters
are tabulated in Table 1.
2.3.2. Synthesis of bis-[1-(o-, m-, p-chlorophenyl)-3-
(m-nitrophenyl)-5-phenyl formazanato]nickel(II) complexes
(2b–4b)
Formazans (2a–4a) (0.760 g, 0.002 mol) obtained as
described in Section 2.2 were dissolved in dioxane (25 ml). In
another flask [Ni(CH3COO)2·4H2O] (0.001 mol, 0.250 g) salt
was dissolved in ethanol (15 ml) and formazan solutions were
added to it. The mixtures were stirred at 30–35 ◦C under reflux
for 8 h. The resulting precipitates were kept in the cupboard
for 3 days. The brown colored (2b), light green colored (3b) and
brown colored (4b) products were filtered off, washed with water
and methanol, recrystallized from methanol and dried in stove
at 40 ◦C for 24 h. Elemental analysis for C40H34N10O6Cl2Ni,
M: 880.37. Calc. (%): C, 54.57; H, 3.89; N, 15.91. Found:
(%) C, 54.53; H, 3.92; N, 17.85. Mass: m/z (eV), Calc. M:
880.37. Found M: 880.40 (M+). Other peaks: 437.20, 378.05,
240.05.
2.4. Physical measurements
The UV–vis spectra of the formazans synthesized in this
study were obtained with UNICAM UV2-100 UV–vis spec-
trophotometer using 1 cm quartz cells in 10−5 mol l−1 DMSO
using a 325 nm lamp in the range of 250–600 nm. The IR spectra
were obtained on a MATT-SON 100-FT-IR spectrophotometer

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H. Tezcan et al. / Spectrochimica Acta Part A 70 (2008) 973–982
Table 1
assolvent, ionicstrengthwasmaintainedat0.1 M, withTBATFB
as supporting electrolyte. The concentration of all species was
1 × 10−4 M.
Experimental and 1H NMR data of the Ni(II) complexes formazans (1b–7b)
investigated
Compound
Color
m.p. (◦C)
Yield (%)
1H NMR dataa
Aromatic H/δ (ppm)
3. Results and discussion
1b
2b
3b
4b
5b
6b
7b
Light brown
Pink-brown
Light green
Brown
Pink-brown
Light green
Brown
300
240
305
310
260
281.5
298
85
79
65
63
82
63
60
8.02–7.28(30H)
8.82–7.30(26H)
8.45–7.12(26H)
8.78–7.22(26H)
8.84–7.24(26H)
8.50–7.02(26H)
8.62–7.30(26H)
The analytical data of the Ni complexes of formazan indicate
1:2 metal to ligand stoichiometry. All the isolated complexes
are stable in air, in anhydrous EtOH and MeOH, but soluble in
DMSO and DMF. Attempts to propose the structure of the iso-
latedcomplexescomefromfullinvestigationusingthefollowing
studies.
a
The 1H NMR spectra were recorded with 400 MHz (in CDCl3).
3.1. 1H NMR spectra
between 4000 and 400 cm−1 using KBr pellets. 1H NMR spectra
were performed on a Bruker AVANCE DPX-400 MHz and 13
C
1
When one examines H NMR data listed in Table 1 shows
NMR 100 MHz spectrophotometer using CDCl3 and d6-DMSO,
10−4 mol l−1. The elemental analyses were carried out by the
use of LECO-CHNS-932 elemental analyzer. Mass spectra were
recorded using an AGILENT 1100 MSD mass spectrometer.
Electrochemical studies were carried out on a computerized
CHI Instrument 660 B system in a conventional three-electrode
cell. A platinum electrode (PE) (CHI102) and a 10 ␮m-platinum
ultramicro electrode (UME) (CHI107) were used as a work-
ing electrode. The electrodes were cleaned by electrochemical
potential cycling and washing with excess dimethyl sulfoxide.
A platinum wire was used as the auxiliary electrode. The refer-
ence electrode was a silver wire in contact with 0.1 M AgNO3
in dimethyl sulfoxide. All solutions were deaired for 10 min
with pure argon. All the measurements were taken at room
temperature, 25 ◦C. The supporting electrolyte, tetrabutylam-
monium tetrafluoroborate (TBATFB) was purchased from Fluka
(21796-4) and was used without purification. DMSO was used,
that the aromatic-H peak for Ni-TPF (1b) are observed at
δ = 8.02–7.28 ppm. In Ni(II) complexes formazans (2b–4b;
5b–7b) where position of the 3-phenyl ring is substituted with
m-NO2 while –Cl, –Br is each substituted to o-, m-, p-positions
of 1-phenyl ring, the aromatic-H signals 8.82, 8.45, 8.78 ppm;
8.84, 8.50, 8.62 ppm, respectively are shifted towards lower
fields according to Ni-TPF (1b). This is in good accordance with
the electron withdrawing properties of these substitutes. When
aromatic-H peaks Ni(II) complexes are compared with the cor-
responding formazans they were observed to shift towards lower
fields. This is in accordance with the fact that the electrons in
the structure are withdrawn with the insertion of Ni2+ into for-
mazans structure [16,22,23]. N–H peak observed in formazans
wereabsentinNi(II)complexes. Thisthefurtherproofthatmetal
is replaced in place of proton of N–H group. These data are
supported with IR results.
Table 2
13C NMR data of the Ni(II) complexes formazans (1b–7b) investigated (in CDCl3 and DMSO-d6)
Compound
δ (ppm)
Imino-C (C N)
174.81
Other carbons
1b
2b
163.97, 161.86, 158.67, 157.90, 155.03, 153.47, 152.20, 150.21, 149.01, 148.61, 142.21, 139.03, 130.42, 127.90,
126.33, 120.59, 118.50 (total 20C)
167.81, 163.90, 150.94, 149.69, 147.96, 146.72, 145.62, 144.53, 143.75, 143.12, 141.72, 141.09, 140.62, 139.75,
137.75, 137.00, 134.50, 133.25, 132.75, 131.25, 130.00, 128.75, 127.00, 126.25, 125.10, 124.50, 123.00, 122.50,
122.00, 120.15, 116.00, 113.50, 108.20 (total 34C)
172.06, 164.80, 160.00, 153.05, 149.44, 148.89, 148.33, 147.22, 145.56, 144.17, 143.33, 142.50, 141.39, 140.00,
139.17, 137.50, 136.39, 135.28, 134.44, 133.89, 132.50, 131.11, 130.00, 128.06, 127.67, 125.10, 124.17, 123.33,
122.78, 121.67, 121.39, 119.78, 116.28 (total 34C)
172.36, 163.85, 149.44, 148.33, 146.67, 145.00, 144.44, 143.33, 142.22, 141.11, 139.89, 137.78, 136.67, 135.47,
134.44, 133.89, 132.78, 131.11, 130.23, 129.79, 128.89, 127.78, 125.56, 122.22, 120.38, 119.44, 117.22, 115.00, 110.00
(total 30C)
164.87, 163.32, 150.15, 149.61, 149.00, 148.65, 147.11, 146.15, 145.38, 144.42, 143.07, 141.73, 140.96, 140.19,
139.42, 137.88, 135.70, 135.00, 134.42, 133.27, 131.92, 130.96, 129.42, 128.65, 127.50, 126.54, 125.00, 124.04,
122.50, 121.54, 118.85, 115.58, 107.52 (total 34C)
170.38, 164.86, 152.86, 151.07, 148.57, 146.43, 145.71, 144.98, 143.57, 142.14, 141.07, 139.29, 138.57, 136.43,
135.10, 134.28, 133.57, 131.78, 130.71, 130.00, 129.28, 127.78, 127.50, 126.79, 126.07, 125.00, 120.00, 119.28,
117.14, 115.00, 112.88, 111.07, 108.57 (total 34C)
167.47, 156.50, 149.38, 149.00, 148.17, 147.50, 146.53, 145.97, 145.00, 144.50, 143.25, 142.80, 141.53, 140.82,
140.10, 139.50, 139.38, 138.50, 136.65, 135.00, 134.81, 129.17, 126.10, 124.64, 122.50, 120.00, 117.22, 113.90, 110.88
(total 30C)
175.31
175.36
174.80
172.38
175.02
174.96
3b
4b
5b
6b
7b

H. Tezcan et al. / Spectrochimica Acta Part A 70 (2008) 973–982
977
Table 3
The IR data of the Ni(II) complexes formazans (1b–7b) (in KBr, cm−1
)
Compound
Aromatic C–H
Aromatic C
C
C
N
N
N
N–Ni
CNNC structural vibration
1b
2b
3b
4b
5b
6b
7b
3098–3025
3482–3384
3482–3428
3480–3410
3491–3411
3490–3464
3490–3411
1600
1589
1607
1607
1598
1580
1571
1500
1527
1563
1536
1527
1536
1527
1410
1241
1276
1277
1232
1214
1276
3089
2938
2987
2945
3010
2980
2965
780–550
813–652
830–679
857–679
813–652
800–650
813–643
3.2. 13C NMR spectra
The C N bands observed at 1530–1520 cm−1 in formazans
were observed to shift to 1563–1527 cm−1 in Ni(II) complexes.
The N–H band observed at 3120–2850 cm−1 in formazans was
observed to disappear in their Ni(II) complexes. However new
peaks appeared at 3089–2938cm−1. These peaks were attributed
to metal–ligand bonds. This is the verification of the formation
of Ni(II) complexes as a result of replacement of Ni2+ in place
of H atom in N–H [17]. These results confirm the formula given
in Scheme 1. However the N–H band was weakly observed in
compound (3b). These results are in agreement with literature
[15]. N N band observed at 1410 cm−1 Ni-TPF (1b). This band
was seen to shift to 1241, 1276 and 1277 cm−1 in o-, m-, p-
Cl substituted Ni(II) complexes (2b–4b) and 1232, 1214 and
1276 cm−1 o-, m-, p-Br substituted Ni(II) complexes (5b–7b).
N N band was observed to shift towards the lower frequencies
in Ni-TPF (1b) than their respective formazan (1a). This band
also shifted to lower frequencies in Cl and Br substituted for-
mazans. Ni(II) complexes compared to Ni-TPF (1b). The results
are in agreement with literature [17]. Other aromatic C–H, C C,
CNNC skeleton stretching peaks in all complexes were observed
in their expected regions [24]. The changes in bands belonging
to C N, N–H and N N groups taking 6a and 6b as an example
are shown in Fig. 1.
As seen from Table 2, the δ value for C N carbon of Ni-
TPF complex (1b) is 174.81 ppm. The attachment of a second
electron withdrawing group (–Cl, –Br) to o-, m-, p-position
of the 1-phenyl ring while 3-phenyl ring contains an elec-
tron withdrawing group m-NO2 does not cause a significant
change in δ values. C N carbon peaks of Ni(II) complexes
are in lower fields compared with those of corresponding
formazans. While C N peaks observed between 149.61 and
150.82 ppm in formazans are shifted to 172.38–175.36 ppm
in Ni(II) complexes. This result is in accordance with the
replacement of an electron withdrawing Ni2+ into the sys-
tem. Other C peaks can also be evaluated in a similar manner
[17].
3.3. IR spectral studies
As seen in Table 3, the C N stretching band of Ni-TPF (1b)
was observed at 1500 cm−1. The band related to the o-,m-,
p-substituted Cl (2b–4b) on 1-phenyl ring were observed at
1527, 1563and1536 cm−1. Thesepeaksappearedat1527, 1536,
1527 cm−1 in the case of o-, m-, p-Br substitution (5b–7b).
Fig. 1. Comparison the IR bands of compounds (6a and 6b).

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H. Tezcan et al. / Spectrochimica Acta Part A 70 (2008) 973–982
Table 4
UV–vis absorption maxima of the Ni(II) complexes formazans (1b–7b) (DMSO, ×10−5 mol/l)
Compound
Abbreviation
λmax1 (nm) (Abs)
λmax2 (nm) (Abs)
λmax3 (nm) (Abs)
Chemical shift ꢀλmax
1b
2b
3b
4b
5b
6b
7b
Ni-TPF
611.0 (0.067)
621.0 (0.032)
619.0 (0.063)
624.0 (0.058)
622.0 (0.054)
618.0 (0.064)
625.1 (0.047)
481.0 (0.684)
481.0 (0.740)
415.6 (0.548)
426.0 (0.402)
479.0 (0.617)
413.4 (0.452)
415.8 (0.354)
275.0 (0.722)
305.0 (1.338)
–
275.0 (1.423)
304.0 (1.176)
298.0 (1.164)
355.0 (0.648)
–
10
8
13
11
7
Ni-o-CNF
Ni-m-CNF
Ni-p-CNF
Ni-o-BNF
Ni-m-BNF
Ni-p-BNF
14
Column 6: ꢀλmax = λmax1(Ni-TPF) − λmax1 (substituted Ni(II) complexes formazans).
3.4. Substituent effect on the UV–vis absorption λmax values
As seen from Table 4 there were three peaks observed in
the UV–vis spectra of the Ni(II) complexes of formazans. The
absorptions peaks are: λmax1 the Ni(II) complex unit, λmax2 for-
mazan unit and λmax3 hydrazone unit were attributed [8,20,21].
The λmax1 values observed at 479–486 nm in formazans shifted
to 611–625.1 nm in Ni(II) complexes. This is an expected
outcome as a result of the replacement of the Ni(II) ion in
the structure. These results are in agreement with literature
[15,16,18,20]. When ꢀλmax values are compared according to
the type of the substituents it was observed that the value of shift
is almost the same with the presence of Cl and Br. This was an
expected outcome since the electron withdrawing inductive and
electron donating resonance effects approximately cancel each
other. The small difference (1 nm) between them may be the
manifestation of the difference in their respective electronega-
tivity values. When λmax values are compared according to the
positionsofthesubstituentsatthe1-phenylringofthecomplexes
containing Cl and Br followed the order of p- > o- > m-positions.
If we think that electron withdrawing effect results only from the
inductive effect which diminishes as move from the center and
resonance effect will be dominant o-position and p-position are
expectedoutcome. TheseresultsaredepictedinFig. 2aandb. All
these shift values (λmax) are illustrated in Fig. 3 for comparative
purpose.
Fig. 2. Absorption spectra of: (a) 1.0 × 10−5 M DMSO solutions of compounds
(2b–4b) compared to Ni-TPF (1b) and 1.0 × 10−5 M DMSO solutions of com-
pounds (5b–7b) compared to Ni-TPF (1b).
3.5. Mass spectra
The mass spectra of formazans (1a–7a) and their complexes
(1b–7b) were recorded and their moleculer ion peaks confirm
the suggested formula of these formazans and their complexes
(Scheme 1). The calculated and found values of the moleculer
weigths of the formazans and their nickel complexes are given
in Section 2. Elemental analysis and mass spectroscopic data
corroborated the structures proposed in Scheme 1.
3.6. Cyclic voltammetry
The electrochemistry of formazans substituted with o-, m-,
p-Cl and –Br at the 1-phenyl and m-NO2 at the 3-phenyl ring
were discussed in detail in our previous study. The cyclic voltam-
mograms of their corresponding Ni(II) complexes are given in
Figs. 4 and 5.
Fig. 3. The comparison of the absorption values of substituted Ni(II) complexes
formazans (2b–7b) against Ni-TPF (1b).

H. Tezcan et al. / Spectrochimica Acta Part A 70 (2008) 973–982
979
and a much sharper peak at −1605 mV for the o-Cl substi-
tuted formazan complex (2b). This suggests that the complex
give tetrazolium cation by two successive electron transfers.
The same situation exists for m-Cl substituted formazan Ni(II)
complex (3b). However, the intensities of the first and second
peaks are much lower compared with to the o-Cl substituted
Ni(II) complex. This shows that the electron transfer rate is much
lower in m-Cl substituted Ni(II) complexes. The appearance of
the voltammogram for p-Cl substituted formazan Ni(II) com-
plex (4b) is highly different. The first peak observed around
−1000 mV for o-, m-Cl is absent in p-Cl Ni(II) complex. The
intensity of the second peak is much higher for the p-Cl substi-
tuted complex compared with those of o-and m-Cl substituted
complexes. This may be explained by the decreasing inductive
effect going from o- to p-position.
The second peak showed a shift of 200 mV towards anodic
direction for p-Cl substituted Ni(II) complex compared to o-, m-
Cl substituted complexes and appears at −1396 mV. This shift
may be the result of higher electronegativity of Cl (this shift is
much lower in Br complexes as will be outlined shortly). This
is the indicative of the fact that p-Cl complex directly gives a
tetrazolium cation by a single step 2 electron transfer. On the
other hand o- and m-substituted complexes give two successive
1 electron transfer giving tetrazolium cation via radical ion. The
intensity of the first peak changes as o- > m- > p-Cl substituted
Ni(II) complexes. This is in accordance with the yield of p-Cl
substituted complex was the highest followed by m-Cl and o-Cl
substituted Ni(II) complexes. These results also are in accor-
dance with the spectroscopic data. The same arguments made
for Cl are valid for Br as well. The only exception is that the shift
in the sharp peak observed in the reverse scan is smaller with
the p-substituted Br compared to p-Cl substituted Ni(II) complex
which was attributed to lower electronegativity of Br compared
to Cl. The results are in good accordance with the spectro-
scopic data. All the cyclic voltammetric data are tabulated in
Table 5.
Fig. 4. Cyclic voltammogram of DMSO solutions of 1.0 × 10−4 M compounds
(1b–4b) in the presence of 0.1 M TBATFB at Pt electrode. Potential scan rate:
100 mV s−1
.
Fig. 5. Cyclic voltammogram of DMSO solutions of 1.0 × 10−4 M compounds
(1b, 5b–7b) in the presence of 0.1 M TBATFB at Pt electrode. Potential scan
3.7. Ultramicro disc and chronoamperometric results
rate: 100 mV s−1
.
The number of electrons transferred was found with the use
of chronoamperometric, Cottrell equation and ultra micro Pt
disc electrode (UME) steady state current [20]. The real surface
area of the Pt electrode was found to be 2.58 cm2 with the use
ferrocene. If it values are plotted against t−1/2 the resulting slope
As seen from Fig. 4 Ni-TPF (1b) gave no anodic peak in the
forward direction. However shows that the substituted groups
have a quite effect upon the electrochemical behavior of the for-
mazan Ni(II) complexes. There are a broad peak at −967 mV
Table 5
Voltamperometric results in DMSO at 25 ◦C in platinum electrode, ionic strength 0.1 M (TBATFB), sweep speed: 100 mV s−1. Epa1: anodic, Epa2: anodic
Compound
Abbreviation
First peak
Second peak
Epa1 (mV)
Ip1 (A)
Epa2 (mV)
Ip2 (A)
1b
2b
3b
4b
5b
6b
7b
Ni-TPF
–
−967.0
−1008.0
–
−873.0
−893.5
–
–
–
−1605
−1585
−1396
−1461
−1494
−1338
–
Ni-o-CNF
Ni-m-CNF
Ni-p-CNF
Ni-o-BNF
Ni-m-BNF
Ni-p-BNF
1.637 × 10−5
1.876 × 10−6
–
2.884 × 10−6
1.400 × 10−7
1.680 × 10−5
2.282 × 10−5
1.315 × 10−5
1.338 × 10−5
6.823 × 10−6
5.822 × 10−6
–

980
H. Tezcan et al. / Spectrochimica Acta Part A 70 (2008) 973–982
Table 6
Some of the parameters calculated for Ni(II) complexes formazans
Compound
Abbreviation
C* (ml)
Cottrell slope (S)
n
n net
D (cm2/s)
˙
Iss (A)
1b
2b
3b
4b
5b
6b
7b
Ni-TPF
7.8
8.0
9.1
8.8
8.3
8.5
7.4
4.417 × 10−10
2.782 × 10−10
2.358 × 10−10
3.917 × 10−10
4.543 × 10−10
7.082 × 10−10
4.375 × 10−10
1.249 × 10−5
1.457 × 10−5
1.417 × 10−5
1.822 × 10−5
2.194 × 10−5
6.875 × 10−6
1.053 × 10−5
0.88
1.85
1.82
1.87
2.48
1.52
0.66
1
2
2
2
2
2
1
1.667 × 10−6
4.864 × 10−5
3.356 × 10−7
6.166 × 10−5
5.720 × 10−5
1.079 × 10−6
2.303 × 10−6
Ni-o-CNF
Ni-m-CNF
Ni-p-CNF
Ni-o-BNF
Ni-m-BNF
Ni-p-BNF
will be from which n could easily be calculated. The calculated
n values are given in Table 6.
Both the complexes of Cl and Br substituted formazans gave
clear S-shaped steady state currents with UME (Figs. 6 and 7).
The number of electrons transferred was found to be in accor-
dance with the CV data. The diffusion coefficients were also
found within expected dimensions. The results of UME and
chronoamperometric data are tabulated in Table 6.
Looking at the UME results (Figs. 6 and 7) there is a 1 elec-
tron transfer wave for Ni-TPF (1b). Ni-TPF (1b) is fragmented.
Ni2+ and TF− anion (X) occurred in solution (Scheme 3). TF−
anion gives a single 1 electron transfer giving a formazan rad-
ical (TF•). These radicals probably give a disproportionation
reaction to give a formazan and TTC+ cation or a dimerization
reaction resulting a diformazan. Mechanism of the oxidation of
compound (1b) is shown in Scheme 3A and B. These results are
in agreement with literature [8,12].
The number of electrons transferred is two for all o-, m-, p-Cl
substituted Ni(II) complexes based on UME and chronoamper-
ometric. This is in accordance with CV results. The appearance
of two distinctive peaks in cyclic voltammograms of o-, m-
Cl substituted Ni(II) complexes suggest that the compounds
give two successive 1 electron transferred giving formazan
radical and tetrazolium cation. Mechanism of the oxidation
of o-, m-Cl substituted Ni(II) complexes (2b, 3b) are shown
in Scheme 3C. These results are in agreement with literature
[11]. However the presence of only one peak in the case of p-
substituted Ni(II) complex (4b) is most probably due to the fact
of the compound giving a 2 electron transfer resulting directly
into the tetrazolium cation. Mechanisms of the oxidation of
compounds (4b) are shown in Scheme 3D. These results are
in agreement with literature [10]. The situation for o-, m-Br
substituted Ni(II) complexes (5b, 6b) are the same with Cl-
substituted Ni(II) complexes. Mechanism of the oxidation of
o-, m-Br substituted Ni(II) complexes (5b, 6b) are shown in
Scheme 3C. These results are in agreement with literature [11].
However p-Br substituted Ni(II) complex (7b) appears to make
1 electron transfer. This complex probably gives dimerization
or disproportionation reaction following the radical formation
step. Mechanism of the oxidation of compound (7b) is shown
in Scheme 3A, B. These results are in agreement with literature
[8,12].
Fig. 6. UME curves of Ni-TPF (1b) and compounds (2b–4b) at 10 ␮m-platinum
ultramicroelectrode. Potential scan rate: 10 mV s−1
.
Fig. 7. UME curves of Ni-TPF (1b) and compounds (5b–7b) at 10 ␮m-platinum
ultramicroelectrode. Potential scan rate: 10 mV s−1
Fig. 8. The λmax values against the Hammett substituent coefficients-σ.
.

H. Tezcan et al. / Spectrochimica Acta Part A 70 (2008) 973–982
981
Scheme 3. Possible oxidation mechanisms of Ni(II) complexes formazans.
4. Conclusions
tigated whether the Hammett substituent coefficients could be
used to evaluate the total effect of the two different substituents,
upon the λmax values and thus the color. Hammett substituent
coefficients-σ have been related with λmax values for only one
substituent in the structure up to now. The total σT(σ1 + σ2 = σT)
and related λmax values are in Table 7.
Nickel(II) complexes of formazans were prepared and char-
acterized. Their absorption and redox behavior were discussed.
Their peak potentials, diffusion coefficients and a number of
electrons transferred were determined. Mechanisms of oxida-
tion of compounds showed the effect of changing the type and
the position of the substituent on the rings. It was also inves-
As seen from Fig. 8 there is a linear correlation between
Hammetsubstituentcoefficients-σT andλmax values. Thisshows
Table 7
The total σT (σ1 + σ2) and related λmax values
Substitution position
Compound
Abbreviation
σꢀ values
σT (total effect)
λmax1 (nm)
m-
1b
3b
6b
H
H: 0
0
1.08
1.08
611.0
619.0
618.0
m-Cl, m-NO2
m-Br, m-NO2
m-Cl: 0.37; m-NO2: 0.71
m-Br: 0.37; m-NO2: 0.71
p-
4b
7b
p-Cl, m-NO2
p-Br, m-NO2
p-Cl: 0.24; p-NO2: 0.71
p-Br: 0.26; p-NO2: 0.71
0.95
0.97
624.0
625.1

982
H. Tezcan et al. / Spectrochimica Acta Part A 70 (2008) 973–982
that it is possible to use the summation of Hammet substituent
coefficients-σT (σ1 + σ2 = σT) to evaluate the absorptional fea-
tures of the compounds.
[11] T. Oritani, N. Fukuhara, T. Okajima, F. Kitamura, T. Ohsaka, Inorg. Chim.
Acta 357 (2004) 436–442.
[12] G. Go¨kce, Z. Durmus, H. Tezcan, H. Yılmaz, E. Kılıc, Anal. Sci. 21 (2005)
1–4.
[13] M. Szymczyk, W. Czajkowski, R. Stolarski, Dyes and Pigments 42 (1999)
227–235.
Acknowledgement
[14] Y. Gok, M. Tufekci, E. Ozcan, Synth. React. Inorg. Met. -Org. Chem. 23
(1993) 861–873.
[15] D.A. Brown, H. Bo¨gge, G.N. Lipunova, A. Mu¨ller, W. Plass, K.G. Walsh,
Inorg. Chim. Acta 280 (1998) 30–38.
We are very grateful to the Gazi University Research Fund
for providing financial support for this project (no. 04/2004-13).
[16] Y.M. Issa, M.S. Rizk, W.S. Tayor, M.H. Soliman, J. Indian Chem. Soc. 70
(1993) 5–7.
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