Electrochemical and spectroscopic properties of 1:2 Ni complexes of 1,3-substitued (CH3, OCH3) phenyl-5-phenylformazans

Article abstract of DOI:10.1016/j.electacta.2008.02.048

In this study, new 1:2 nickel complexes of 1-[o-, m-, p-(methyl, metoxyphenyl)]-3-(p-metoxyphenyl)-5-phenylformazans were synthesized. Their structures were elucidated and spectral behaviors were investigated with the use of elemental analysis, GC-mass, ¹H NMR, ¹³C NMR, IR, and UV-vis spectra. The redox characteristics of these compounds have been investigated in nonaqueous dimethylsulfoxide at platinum and ultramicro platinum (10 μm) electrodes. Through controlled potential electrolysis, the oxidation products of each class of compounds can be separated and identified. The oxidation mechanism is suggested and it is proved. It was observed the oxidation mechanism take place in a single step two-electron or one-electron transfer to a disproportionation or dimerization reactions following the radical formation step. Eventually the relation between their absorption properties and electrochemical properties was examined.

Full text of DOI:10.1016/j.electacta.2008.02.048

Electrochimica Acta 53 (2008) 5597–5607
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Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Electrochemical and spectroscopic properties of 1:2 Ni complexes of
1,3-substitued (CH3, OCH3) phenyl-5-phenylformazans
Habibe Tezcan∗, Elif Uzluk, Mehmet Levent Aksu
Department of Chemistry, Gazi University, Faculty of Gazi Education, Teknikokullar, 06500 Ankara, Turkey
a r t i c l e i n f o
a b s t r a c t
Article history:
In this study, new 1:2 nickel complexes of 1-[o-, m-, p-(methyl, metoxyphenyl)]-3-(p-metoxyphenyl)-
5-phenylformazans were synthesized. Their structures were elucidated and spectral behaviors were
investigated with the use of elemental analysis, GC–mass, 1H NMR, 13 C NMR, IR, and UV–vis spectra.
The redox characteristics of these compounds have been investigated in nonaqueous dimethylsulfoxide
at platinum and ultramicro platinum (10 ␮m) electrodes. Through controlled potential electrolysis, the
oxidation products of each class of compounds can be separated and identified. The oxidation mecha-
nism is suggested and it is proved. It was observed the oxidation mechanism take place in a single step
two-electron or one-electron transfer to a disproportionation or dimerization reactions following the
radical formation step. Eventually the relation between their absorption properties and electrochemical
properties was examined.
Received 26 October 2007
Received in revised form 5 February 2008
Accepted 9 February 2008
Available online 10 March 2008
Keywords:
Formazan nickel(II) complexes
Platinum electrodes
Cyclic voltammetry
Oxidation mechanism
Spectroscopy
© 2008 Elsevier Ltd. All rights reserved.
Substituent effect
1. Introduction
As seen, many studies have been worked with formazans. But
electrochemical studies of their nickel(II) complexes have not been
Derivatives of formazans were synthesized and structural fea-
tures, tautomeric and photochromic isomers were investigated
[1–4]. Metal complexes of formazans were synthesized and
structural determination, magnetic properties, complex stability
constant determination and spectroscopic characterization and the
formation of Fe complexes depending upon the pH value were
investigated [5–11].
Formazans form tetrazolium salt when they are oxidized [12].
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 and this feature enabled the deter-
mination of activity on tumor cell [13,14]. This feature caused an
increasinginterest inthe chemistryandespeciallyelectrochemistry
of formazans.
The study related to the redox behavior of formazans was car-
ried out and it was claimed that ditetrazolium salts are reduced
to both mono- and diformazans by one-electron transfer. The first
one-electron transfer results in the formation of a tetrazolium rad-
ical and this radical was undergoing the disproportion reaction
[15]. Formazans are oxidized in a single step two-electron trans-
fer followed by a deprotonation reaction forming corresponding
tetrazolium cation [16].
reported in the literature. In this study, seven different formazan
and their nickel(II) complexes of formazans have been synthesized
(Scheme 1). Their structures were elucidated by elemental analy-
ses, GC–mass, FTIR, and their spectral behaviors were investigated
using 1H NMR, 13C NMR, FTIR and UV–vis spectral data. The effect
of substituents on ꢀmax values was determined.
The biological activity of formazan makes the knowledge of
its oxidation potentials and possible mechanisms are very impor-
tant. Furthermore in this study is determined peak potentials (Eox
and Ered), diffusion coefficient (D), number of electrons transferred
(n) and heterogeneous rate constant (ks) with the use of cyclic
voltammetry, lineer sweep voltammetry and chronoamperome-
try. Separation and identification of the intermediates and the
final products were made through controlled potential electroly-
sis (CPE). A mechanistic scheme for the oxidation of formazan to
tetrazolium salt was proposed based upon these data (Scheme 3).
2. Experimental
2.1. Reagents
All chemicals were obtained from Merck and Fluka except
sodium hydroxide and [Ni(CH3COO)2·4H2O] that were purchased
from Sigma–Aldrich. Deionized water (Millipore, Milli-Q) was
used for synthesis; the organic solvents: CH3OH, CH3COCH3,
dimethylsulfoxide (DMSO, 99.9%) and 1,4-dioxane were used for
∗
Corresponding author. Tel.: +90 312 2021108; fax: +90 312 2227037.
E-mail address: habibe@gazi.edu.tr (H. Tezcan).
0013-4686/$ – see front matter © 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2008.02.048

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H. Tezcan et al. / Electrochimica Acta 53 (2008) 5597–5607
Scheme 1. The structure of the formazans and their nickel(II) complexes synthesized.
potential E0 vs. Fc0/Fc+. All measurements were carried out with
1 × 10−5 mol of the reactant in 10 mL dry oxygen-free solvent wi−th
electrochemical and spectroscopic measurements. Dimethylsul-
foxide was used as solvent. During the solvent purification, all the
processes were performed under a dry oxygen-free argon atmo-
sphere. Fractionation was carried out using a 120 cm column filled
with glass spirals at a recoil ratio 50:1. Purified solvent was stored
under argon in the dark. DMSO (Merck) was boiled four times
with calcium hydride (Merck) for 14 h (5 g/L) and subsequently
fractionated at 14 Torr. Finally, the main fraction was carefully
fractionated.
0.1 mol dm−3 tetrabuthylammonium tetrafluoroborate (TBA+BF4
)
as supporting electrolyte. All solutions were deaerated for 10 min
with pure argon. All the measurements were taken at room
temperature, 25 ◦C. The voltage scan rate range during the CV mea-
suruments was 10–10,000 mV/s. Elemental analyses were carried
out using a LECO-CHNS-932 elemental analyser. Mass spectra were
recorded on an AGILENT 1100 MSD mass spectrometer.
2.2. Measurements
2.3. Controlled potential electrolysis (CPE)
The UV–vis spectra of the formazans synthesized in this study
were obtained with UNICAM UV2-100 UV–visible spectropho-
tometer using 1 cm quartz cells in 10−5 mol L−1 DMSO and 325 nm
lamps in the range of 250–600 nm. IR spectra were recorded on
a MATT-SON 100-FTIR spectrophotometer between 4000 and
400 cm−1 using KBr pellets. 1H NMR spectra were performed on a
Bruker AVANCE DPX-400 MHz and 13C NMR 100 MHz spectropho-
tometer using CDCl3 and d6-DMSO, 10−4 mol L−1. Electrochemical
studies were carried out with a computerized CHI Instrument
660 B system in a conventional three-electrode cell. A platinum
electrode (PE) (CHI102) and a 10 ␮m-platinum ultramicrodisc
electrode (UME) (CHI107) were used as a working electrode. The
electrodes were cleaned by electrochemical potential cycling and
washed with excess dimethylsulfoxide. A platinum wire was used
as the auxiliary electrode. The reference electrode was a silver
wire in constant contact with 0.1 M AgNO3 in dimethylsulfoxide.
Ferrocene (Fc) was used as an internal standard to measure formal
CPE experiments were carried out in dry dimethylsulfoxide
containing 0.1 mol dm−3 tetrabuthylammonium tetrafluoroborate
(TBA+BF4−) as supporting electrolyte. Compounds 1b and 7b are
reported here as examples. The potential was controlled at the
current plateau of the oxidation wave. As working electrode, a
platinum electrode was used. The progress of the electrolysis was
followed by recording periodically the decrease in current with
time. From time to time the anode was removed from the cell,
sprayed with pure acetone and burned in a direct flame, cooled
and replaced in the cell. After the electrolysis was completed, the
cell was disconnected from the circuit and the solvent was evap-
orated in vacuum. The residue was shaken with dry ether and the
supporting electrolyte was filtered off.
The ethereal layer was evaporated in turn. The obtained residue
was chromatographed on thin layer silica gel plates using chloro-
form as an eluent.

H. Tezcan et al. / Electrochimica Acta 53 (2008) 5597–5607
5599
The main oxidation product obtained was scraped off the plate
and extracted with dimethylsulfoxide, filtered and evaporated in
vacuum.
2.4.6. 1-(o-,m-,p-Methoxyphenyl)-3-(p-methoxyphenyl)-5-
phenylformazans (5a–7a)
5a–7a were synthesized by the reaction of p-
methoxybenzaldehyde (1.35 g, 0.01 mol), phenylhydrazine (1.08 g,
0.01 mol), o-, m-, p-methoxyaniline (1.28 g, 0.01 mol), concen-
trated HCl (5 mL) and sodium nitrite (0.75 g) in methanol, at
0–5◦ C a method similar to that for 1,3,5-triphenylformazan. Each
compound was recrystallized from methanol.
2.4. Synthesis of formazans
2.4.1. 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, 0.01 mol),
aniline (0.93 g, 0.01 mol), concentrated HCl (5 mL) and sodium
nitrite (0.75 g) in a methanol, at 0–5 ◦C alike literature [3,4]. Cherry
red colored crystals: mp 172–173 ◦C; yield 78%. IR: 3069 cm−1 (aro-
matic C H), 3050–3000 cm−1 (N H), 1600 cm−1 (aromatic C C),
1500 cm−1 (C N), 1450 cm−1 (N N), 930–905 cm−1 (CNNC). 1H
NMR: 8.32–7.27 ppm (15H, aromatic H), 1.18 ppm (1H, azo H). 13C
NMR: 148.80 ppm [1C, (imino-C) C N], 141.98–119.54 ppm (8C,
other carbons). Elemental analysis: calcd. for C19 H16 N4: C, 76.00;
H, 5.33; N, 18.66. Found: C, 75.97; H, 5.29; N, 18.69%. Calcd. M: 300;
found mass: m/z (eV) 301.10, 273.10, 223.00, 195.10, 105.05.
2.4.7. Selected data for 5a
Dark purple colored crystals: mp 196–197 ◦C; yield 71%.
IR: 3490–3400 cm−1 (aromatic C H), 2900–2800 cm−1 (N H),
1600 cm−1 (aromatic C C), 1510 cm−1 (C N), 1250 cm−1 (CH3),
1250 cm−1 (OCH3), 1020 cm−1 (N N), 800–610 cm−1 (CNNC). 1H
NMR: 8.26–6.83 ppm (13H, aromatic H), 1.31 ppm (1H, azo H), 4.01,
3.96 ppm (6H, OCH3). 13C NMR: 160.36 ppm [1C, (imino-C) C N],
151.86–55.98 ppm (16C, other carbons). Elemental analysis: calcd.
for C21H20N4O (5a): C, 70.00; H, 5.55; N, 15.55. Found: C; 70.06,
H; 5.51, N; 15.62%. Calcd. M: 360.00; found mass: m/z (eV) 361.10,
255.10, 225.10, 122.10.
2.4.2. 1-(o-, m-,
2.4.8. Selected data for 6a
p-Tolylphenyl)-3-(p-methoxyphenyl)-5-phenylformazans (2a–4a)
2a–4a were synthesized by the reaction of p-
methoxybenzaldehyde (1.35 g, 0.01 mol), phenylhydrazine (1.08 g,
0.01 mol), o-, m-, p-tolylaniline (1.07 g, 0.01 mol), concentrated HCl
(5 mL) and sodium nitrite (0.75 g) in a methanol, at 0–5◦ C a method
similar to that for 1,3,5-triphenylformazan. Each compound was
recrystallized from methanol.
Red dark-purple colored crystals: mp 205–206.5 C; yield 42%.
IR: 3490–3400 cm−1 (aromatic C H), 3030 cm−1 (N H), 1600 cm−1
(aromatic C C), 1505 cm−1 (C N), 1210 cm−1 (CH3), 1210 cm−1
(OCH3), 1030 cm−1 (N N), 810–580 cm−1 (CNNC). 1H NMR:
8.36–6.58 ppm (13H, aromatic H), 1.30 ppm (1H, azo H), 3.91,
3.84 ppm (6H, OCH3). 13C NMR: 164.50 ppm [1C, (imino-C) C N],
160.02–55.90 ppm (16C, other carbons). Elemental analysis: calcd.
for C21H20N4O (6a): C, 70.00; H, 5.55; N, 15.55. Found: C; 70.10,
H; 5.48, N; 15.70%. Calcd. M: 360.00; found mass: m/z (eV) 361.05,
255.05, 226.05, 123.10.
2.4.3. Selected data for 2a
Red-pink colored crystals: mp 187–188 ◦C; yield 69%. IR:
3480–3400 cm−1 (aromatic
C
H), 2920–2800 cm−1 (N H),
1600 cm−1 (aromatic C C), 1500 cm−1 (C N), 1220 cm−1 (CH3),
1030 cm−1 (N N), 800–620 cm−1 (CNNC). 1H NMR: 8.21–6.97 ppm
(13H, aromatic H), 1.60 ppm (1H, azo H), 2.56 ppm (3H, CH3),
3.90 ppm (3H, OCH3). 13C NMR: 160.46 ppm [1C, (imino-C) C N],
149.93–23.42 ppm (16C, other carbons). Elemental analysis: calcd.
for C21H20N4O2 (2a): C, 73.25; H, 5.81; N, 16.28. Found: C, 73.21;
H, 5.76; N, 16.24%. Calcd M: 344.00; found mass: m/z (eV): 345.10,
239.10, 225.00, 119.00.
2.4.9. Selected data for 7a
Dark purple colored crystals: mp 187–188 ◦C; yield 72%. IR:
3490 cm−1 (aromatic C H), 3050–3000 cm−1 (N H), 1600 cm−1
(aromatic C C), 1495 cm−1 (C N), 1250 cm−1 (CH3), 1250 cm−1
(OCH3), 1080 cm−1 (N N), 810–510 cm−1 (CNNC). 1H NMR:
8.12–6.87 ppm (13H, aromatic H), 1.31 ppm (1H, azo H), 3.90,
3.82 ppm (6H, OCH3). 13C NMR: 162.84 ppm [1C, (imino-C) C N],
160.37–55.70 ppm (14C, other carbons). Elemental analysis: calcd.
for C21H20N4O (6a): C, 70.00; H, 5.55; N, 15.55. Found: C; 69.88,
H; 5.58, N; 15.57%. Calcd. M: 360.00; found mass: m/z (eV) 361.15,
256.05, 225.35, 122.00.
2.4.4. Selected data for 3a
Purple-red colored crystals: mp 210–211 ◦C; yield 40%. I−R1:
3490 cm−1 (aromatic C H), 3020–3010 cm−1 (N H), 1620 cm
(aromatic C C), 1495 cm−1 (C N), 1200 cm−1 (CH3), 1020 cm−1
(N N), 850–600 cm−1 (CNNC). 1H NMR: 8.34–6.40 ppm (13H,
aromatic H), 1.30 ppm (1H, azo H), 2.44 ppm (3H, CH3),
3.88 ppm (3H, OCH3). 13C NMR: 160.80 ppm [1C, (imino-C) C N],
147.36–22.80 ppm (16C, other carbons). Elemental analysis: calcd.
for C21H20N4O2 (3a): C, 73.25; H, 5.81; N, 16.28. Found: C, 73.19;
H, 5.85; N, 16.34%. Calcd M: 344.00; found mass: m/z (eV): 345.50,
240.10, 226.00, 120.05.
2.5. Synthesis of nickel(II) complexes
2.5.1. 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 literature [3] was dissolved in dioxane (20 mL). In
another flask [Ni(CH3COO)2·4H2O] salt (0.0025 mol, 0.625 g) was
dissolved in ethanol (22 mL) under reflux with constant stirring
at 25 ◦C and formazan is added to it stirring with a glass rod in
1–2 min. There was a no color change or precipitation observed
during this process. The mixture was stirred with a magnetic
stirrer at 30–35 ◦C under reflux. The precipitation started after
an hour and the color turned into orange from red after 2 h
and brown after 3 h. The stirring process was continued for 8 h.
The color remained brown. The mixture was kept in the cup-
board for 4 days. The light-brown precipitate was filtered off and
washed with 10 mL 0.5 M NaOH, water and methanol. The com-
pound was dried in stove at 40 ◦C for 24 h and recrystallized
from methanol. Light-brown colored crystals: mp 300 ◦C; yield
85%. IR: 3098–3028 cm−1 (aromatic C–H), 2963 cm−1 (Ni-ligand),
1600 cm−1 (aromatic C C), 1500 cm−1 (C N), 1410 cm−1 (N N),
2.4.5. Selected data for 4a
Dark purple colored crystals: mp 242–243 ◦C; yield 74%.
IR: 3450–3440 cm−1 (aromatic C H), 2950–2820 cm−1 (N H),
1600 cm−1 (aromatic C C), 1500 cm−1 (C N), 1250 cm−1 (CH3),
1010 cm−1 (N N), 840–540 cm−1 (CNNC). 1H NMR: 8.16–6.78 ppm
(13H, aromatic H), 1.58 ppm (1H, azo H), 2.45 ppm (3H, CH3),
3.90 ppm (3H, OCH3). 13C NMR: 160.39 ppm [1C, (imino-C) C N],
148.88–21.56 ppm (14C, other carbons). Elemental analysis: calcd.
for C21H20N4O2 (4a): C, 73.25; H, 5.81; N, 16.28. Found: C, 73.28;
H, 5.82; N, 16.20%. Calcd M: 344.00; found mass: m/z (eV): 345.05,
240.10, 226.50, 119.95.

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H. Tezcan et al. / Electrochimica Acta 53 (2008) 5597–5607
780–550 cm−1 (CNNC). 1H NMR: 8.02–7.28 ppm (30H, aromatic H),
3.72–1.58 ppm [3H, solvent (CH3OH)]. 13C NMR: 174.81 ppm [1C,
(imino-C) C N], 163.97–118.50 ppm (17C, other carbons).
[1C, (imino-C) C N], 175.02–115.00 ppm (29C, other carbons). Ele-
mental analysis calcd. for NiC42H38N8O4 (5b): C, 70.72; H, 5.33; N,
15.71. Found: C, 70.78; H, 5.20; N, 15.76%. M.: 776.70. Mass: m/z
(eV), found M.: 777.10. Other peaks: 402.00, 344.85, 226.10.
2.5.2. Bis [1-(o-,m-,p-tolylphenyl)-3-(p-methoxyphenyl)-
5-phenylformazanato]nickel(II) complexes (2b–4b)
2.5.8. Selected data for 6b
Green colored crystals: mp 273 ◦C; yield 64%. IR:
3480–3420 cm−1 (aromatic C–H), 2980 cm−1 (Ni-ligand),
1580 cm−1 (aromatic C C), 1500 cm−1 (C N), 1240 cm−1 (CH3),
1027 cm−1 (N N), 835–620 cm−1 (CNNC). 1H NMR: 8.42–6.80 ppm
(26H, aromatic H), 3.92 ppm (12H, OCH3). 13C NMR: 185.56 ppm
[1C, (imino-C) C N], 177.42–113.32 ppm (29C, other carbons).
Elemental analysis calcd. for NiC42H38N8O4 (6b): C, 70.72; H, 5.33;
N, 15.71. Found: C, 70.65; H, 5.28; N, 15.67%. M.: 776.70. Mass: m/z
(eV), found M: 777.03. Other peaks: 401.80, 345.20, 225.10.
2a–4a (0.690 g, 0.002 mol) obtained in Section 2.3 were dis-
solved in dioxane (15 mL). In another flask [Ni(CH3COO)2·4H2O]
(0.001 mol, 0.250 g) salt was dissolved in ethanol (15 mL) and for-
mazans solution was 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. Following the procedure outlined for
compound 1b. Each product was recrystallized from methanol.
2.5.3. Selected data for 2b
Light green-brown colored crystals: mp 244–245 ◦C; yield
69%. IR: −31480–3410 cm−1 (aromatic C H), 2937 cm−1 (Ni-ligand),
2.5.9. Selected data for 7b
Green colored crystals: mp 271–272 ◦C; yield 60%. IR:
1580 cm
(aromatic C C), 1518 cm−1 (C N), 1241 cm−1 (CH3),
3450–3440 cm−1 (aromatic
C
H), 3058 cm−1 (Ni-ligand),
1027 cm−1 (N N), 839–589 cm−1 (CNNC). 1H NMR: 8.23–6.92 ppm
(26H, aromatic H), 2.50 ppm (6H, CH3), 3.90 ppm (6H, OCH3), 3.82,
1.22 ppm (2H, CH3OH). 13C NMR: 186.87 ppm [1C, (imino-C) C N],
175.28–114.73 ppm (29C, other carbons). Elemental analysis calcd.
for NiC42H38N8O2 (2b): C, 64.89; H, 4.89; N, 14.42. Found: C, 64.93;
H, 4.87; N, 14.47%. M: 744.70. Mass: m/z (eV), found: M.: 745.00.
Other peaks: 418.05, 360.00, 226.05.
2950–2820 cm−1 (N H), 1600 cm−1 (aromatic C C), 1500 cm−1
(C N), 1250 cm−1 (CH3), 1010 cm−1 (N N). 1H NMR:
8.21–6.68 ppm (26H, aromatic H), 3.89 ppm (12H, OCH3). 13C
NMR: 182.62 ppm [1C, (imino-C) C N], 175.24–117.38 ppm (25C,
other carbons). Elemental analysis calcd. for NiC42H38N8O4 (7b):
C, 70.72; H, 5.33; N, 15.71. Found: C, 70.80; H, 5.30; N, 15.78%.
M.: 776.70. Mass: m/z (eV), found M: 777.50. Other peaks: 401.55,
345.65, 225.50.
2.5.4. Selected data for 3b
All the synthesized compounds (1a–7a, 1b–7b) were purified
by repeated crystallization, dried under reduced pressure and the
purity was checked by thin layer chromatography.
Orange-brown colored crystals: mp 238 ◦C; yield 63%.
IR: 3475–3410 cm−1 (aromatic
C
H), 2950 cm−1 (Ni-ligand),
1580 cm−1 (aromatic C C), 1500 cm−1 (C N), 1241 cm−1 (CH3),
1027 cm−1 (N N), 840–590 cm−1 (CNNC). 1H NMR: 8.45–6.83 ppm
(26H, aromatic H), 2.45 ppm (6H, CH3), 3.80 ppm (6H, OCH3), 3.83,
1.22 ppm (2H, CH3OH). 13C NMR: 185.87 ppm [1C, (imino-C) C N],
177.60–117.05 ppm (29C, other carbons). Elemental analysis calcd.
for NiC42H38N8O2 (3b): C, 64.89; H, 4.89; N, 14.42. Found: C, 64.85;
H, 4.95; N, 14.35%. M: 744.70. Mass: m/z (eV), found M: 744.80.
Other peaks: 419.15, 359.10, 225.00.
2.6. Oxidation product of 1b
Elemental analysis calculated: C, 76.25; H, 5.02; N, 18.73. Found:
C, 76.80; H, 5.13; N, 18.57%. Mass: m/z (eV), calculated M: 598.00.
Mass spectrum shows the main fragments at m/z 599.10, parent
493.00; 388.00; 311.00; 234.00; 157.00.
2.7. Oxidation product of 7b
2.5.5. Selected data for 4b
Light green-brown colored crystals: mp 295 ◦C; yield 61%.
Elemental analysis calculated: C, 73.47; H, 5.54; N, 16.32. Found:
C, 73.28; H, 5.49; N, 16.20%. Mass: m/z (eV), calculated M: 343.00.
Mass spectrum shows the main fragments at m/z 344.00, parent
224.00; 135.00; 131.00; 105.00.
IR: 3450–3440 cm−1 (aromatic
C
H), 3062 cm−1 (Ni-ligand),
2950–2820 cm−1 (N H), 1600 cm−1 (aromatic C C), 1500 cm−1
(C N), 1250 cm−1 (CH3), 1010 cm−1 (N N). 1H NMR: 8.15–6.38 ppm
(13H, aromatic H), 2.50 ppm (6H, CH3), 3.70 ppm (6H, OCH3), 3.74,
1.39 ppm (2H, CH3OH). 13C NMR: 174.80 ppm [1C, (imino-C) C N],
169.11–118.21 ppm (25C, other carbons). Elemental analysis calcd.
for NiC42H38N8O2 (4b): C, 64.89; H, 4.89; N, 14.42. Found: C, 64.79;
H, 4.83; N, 14.33%. M: 744.70. Mass: m/z (eV), found M: 745.10. Other
peaks: 419.50, 359.10, 225.15.
3. Result and discussion
3.1. Synthesis and spectral properties of formazans and their
nickel(II) complexes
This study was carried out in four stages. In the first and two
steps, formazans and their nickel(II) complexes were synthesized.
In the third step, their structures were elucidated then spectro-
scopic features and the effect of substituents on the absorption
properties of compounds were investigated. In the fourth step,
electrochemical properties were investigated. The intermediate
hydrazones (c) were synthesized at pH 5–6 and the syntheses of
formazans were carried out at pH 9–12. Nickel(II) complexes of for-
mazans were synthesized at pH 6–8 according to the literature [5]
in order to prevent the formation of different isomers. The reaction
scheme is given in Scheme 2.
2.5.6. Bis [1-(o-,m-,p-methoxyphenyl)-3-(p-methoxyphenyl)-
5-phenylformazanato]nickel(II) complexes (5b–7b)
5a–7a (0.690 g, 0.002 mol) prepared in Section 2.3 were dis-
solved in dioxane (15 mL) in another flask Ni(CH3COO)2·4H2O
(0.001 mol, 0.250 g) salt was dissolved in ethanol (15 mL) and for-
mazans were added to it. Following the procedure outlined for
compound 1b. Each product was recrystallized from methanol.
2.5.7. Selected data for 5b
Pink-brown colored crystals: mp 225–226.5 ◦C; yield 73%.
IR: 3490–3455 cm−1 (aromatic
C
H), 2964 cm−1 (Ni-ligand),
Elemental analysis and mass spectroscopic data are corrobo-
rated the structures proposed in Scheme 1. When one examines
1H NMR data shows that the aromatic-H peak for Ni-TPF (1b) are
observed at ı = 8.02–7.28 ppm. In nickel(II) complexes of formazans
1607 cm−1 (aromatic C C), 1509 cm−1 (C N), 1223 cm−1 (CH3),
1027 cm−1 (N N), 839–616 cm−1 (CNNC). 1H NMR: 8.28–6.41 ppm
(26H, aromatic H), 4.02 ppm (12H, OCH3). 13C NMR: 186.87 ppm

H. Tezcan et al. / Electrochimica Acta 53 (2008) 5597–5607
5601
Scheme 2. Synthesis of formazans and their nickel(II) complexes.
2b–4b; 5b–7b where position of 3-phenyl ring is substituted
with p-OCH3 while –CH3, –OCH3 is each substituted to o-, m-, p-
positions of the 1-phenyl ring, the aromatic-H signals (8.23, 8.45
and 8.15 ppm; 8.28, 8.42 and 8.21 ppm, respectively) are shifted
towards lower fields according to Ni-TPF (1b). There is a shift
towards to lower fields in the case of electron donating groups
such as CH3 and OCH3 this is not at the same dimension with that
of the electron withdrawing groups. This can be explained by the
fact that the electron donating effects of CH3 and OCH3 can be
complexes are in lower fields compared with those of correspond-
ing formazans. While C N peaks observed between 164.50 and
148.80 ppm in formazans are shifted to 186.87–174.80 ppm in
nickel(II) complexes. This result is in accordance with the inser-
tion of an electron withdrawing group Ni2+ into the system. Other
C peaks can also be evaluated in a similar manner [19].
The C N stretching band of Ni-TPF (1b) was observed at
1500 cm−1. The band related to the o-,m-, p-substituted CH3
(2b–4b) on the 1-phenyl ring were observed at 1518, 1500 and
1500 cm−1. These peaks apperared at 1509, 1500 and 1500 cm−1
in the case of o-, m-, p-OCH3 substitution (5b–7b). The C N bands
observed at 1510–1495 cm−1 in formazans were observed to shift to
1518–1500 cm−1 in nickel(II) complexes. The N H band observed
at 3050–2800 cm−1 in formazans was observed to generally disap-
pear in their nickel(II) complexes. However, new peaks observed
at 3062–2937cm−1. These peaks were attributed to metal–ligand
bonds. This is the verification of the formation of nickel(II) com-
neglected against the strong electron withdrawing effect of Ni2+
.
When aromatic-H peaks nickel(II) complexes are compared with
the corresponding formazans they were observed to shift towards
to lower fields. This is in accordance with the fact that the elec-
trons in the structure are withdrawn with the insertion of Ni2+ into
formazans structure [8,18,19]. N–H peak observed at 1.60–1.18 ppm
in formazans disappeared in nickel(II) complexes. This is a further
proof that metal ion was inserted in place of proton of N–H group.
Also the peaks appeared at 3.90–1.22 ppm were attributed to the
solvent (CH3OH). This was an expected outcome. These data are
supported by the IR results.
plexes as a result of insertion of Ni2+ in place of an H atom in N
H
[7,20]. These results confirm the formula given in Scheme 1b–c.
However, the N H band were weakly observed in compounds 4b,
7b. These results are in agreement with the literature [6]. N N band
observed at 1410 cm−1 in Ni-TPF (1b) this bond was seen to shift to
1027 cm−1 in o-, m-, p-CH3 substituted nickel(II)complexes(2b–4b)
and 1027, 1027 and 1054 cm−1 o-, m-, p-OCH3 substituted nickel(II)
The ı values for the imino-C (C N) carbon for Ni-TPF (1b) was
observed at 174.81 ppm. The attachment of an electron donating
group to the 1-phenyl ring such as CH3 and OCH3 caused the ı
values showed a sligth increase. C N carbon peaks of nickel(II)

5602
H. Tezcan et al. / Electrochimica Acta 53 (2008) 5597–5607
Table 1
UV–visible absorption maxima of the nickel(II) complexes formazans 1b–7b (DMSO, 10−5 mol/L)
Compound
ꢀmax1 (nm)
␧ (× 103 M−1 cm−1
)
ꢀmax2 (nm)
␧ (× 104 M−1 cm−1
)
ꢀmax3 (nm)
␧ (× 104 M−1 cm−1
)
Wavelength shift (ꢁꢀmax)
1b
2b
3b
4b
5b
6b
7b
611.0
654.0
647.0
644.0
661.0
649.0
642.0
6.78
5.16
5.76
3.95
3.12
5.20
3.67
448.0
458.0
454.0
452.0
460.0
464.0
456.0
5.00
3.40
4.70
8.80
3.76
5.30
1.80
275.0
307.0
299.0
–
311.0
322.0
274.0
7.80
11.34
11.20
–
15.60
12.60
13.92
–
43
36
33
50
38
31
Column 8: ꢁꢀmax = ꢀmax1 (Ni-TPF), ꢀmax1 (substituted nickel(II) complexes of formazans).
complexes (5b–7b) N N band was observed to shift towards the
lower frequencies in Ni-TPF (1b) complex than its respective for-
mazan (1a). This band also shifted to lower frequencies in CH3
and OCH3 substituted formazans nickel(II) complexes compared
to Ni-TPF (1b). The results are in agreement with the literature
[7,20]. Other aromatic C H, C C and CNNC stretching peaks in
the complexes and the peaks related to CH3, OCH3 in the cor-
responding compounds were observed in their expected region
[20].
depicted in Fig. 1a and b. In the complexes containing CH3 and
OCH3 the amount of shifts observed in absorption values (ꢁꢀmax
was observed to follow the order of o- > m- > p-. If we think that elec-
tron withdrawing effect results only from the inductive effect which
diminishes as we move from the center this order is an expected
outcome.
)
3.2. Cyclic voltammetry
There were three peaks observed in the UV–vis spectra of
nickel(II) complexes of formazans. The absorptions peaks ꢀmax1
the nickel(II) complex, ꢀmax2 formazan unit and ꢀmax3 hydrazone
unit were attributed [8]. The ꢀmax1 observed at 482–531 nm in
formazans shifted to 611–661 nm in their nickel(II) complexes
(Table 1). This is in accordance with the fact that the electrons
in the structure are withdrawn with the insertion of Ni2+ into
complex structure. These results are in compliance with litera-
ture [5,6,8–11]. When ꢁꢀmax values are compared according to
the type of the substituents it was observed that the amount
of shift was higher when 1-pheny ring was attached with elec-
tron donating groups such as CH3 and OCH3. These results are
The cyclic voltammetric behavior of the nickel(II) complexes of
formazans were studied in anhydrous DMSO (dimethylsulfoxide)
at room temperature. The cyclic voltammograms of substituted
(–CH3 and –OCH3) nickel(II) complexes (2b–7b) are compared with
the parent compound Ni-TPF (1b). Figs. 2–4 show a typical cyclic
voltammograms of these compounds.
A cyclic voltammogram of Ni-TPF (1) is shown in Fig. 2. The
scan showed one anodic wave and one cathodic wave. A cyclic
sweep in the −1.60 to +0.00 V range shows an anodic peak at
Eox1 = −1260.6 mV and a cathodic peak at Ered1 = −798.1 mV. As seen
from Fig. 2, Ni-TPF (1b) gives one-electron transfer. This complex
probably gives dimerization or disproportionation reaction follow-
ing the radical formation step at −1260.6 mV. However, it seems
that the substituted groups (–CH3 and –OCH3) have quite effect
upon the electrochemical behavior of the nickel(II) complexes of
formazans in Figs. 3 and 4.
As seen from Fig. 3 and Table 2, compounds 2b–4b caused
major changes in both the peak potential and the peak currents
as compared with the parent Ni-TPF (1b). o- and p-CH3 sub-
Fig. 1. Absorption spectra of (a) 1.0 × 10−5 M DMSO solutions of compounds 2b–4b
compared to Ni-TPF (1b); (b) 1.0 × 10−5 M DMSO solutions of compounds 5b–7b
compared to Ni-TPF (1b).
Fig. 2. Cyclic voltammogram of a DMSO solution of 1.0 × 10−5 M Ni-TPF (1b) in the
−
presence of 0.1 M TBA+BF4 at Pt electrode. Potential scan rate: 100 mV s−1
.

H. Tezcan et al. / Electrochimica Acta 53 (2008) 5597–5607
5603
Fig. 3. Representative cyclic voltammogram in DMSO (25 ◦C, ionic strength, 0.1 M TBA+BF4−; ꢂ, 100 mV s−1): (a) 1.0 × 10−5 M compound 2b and (b) 1.0 × 10−5 M 4b.
Fig. 4. Representative cyclic voltammogram in DMSO (25 ◦C, ionic strength, 0.1 M TBA+BF4−; ꢂ, 100 mV s−1): (a) compound 1.0 × 10−5 M 5b and (b) 1.0 × 10−5 M 7b.
stituted nickel(II) complexes (2b, 4b) directly give tetrazolium
cations (TPT+) by a single step two-electron transfer. Compound
3b behaves similarly towards oxidation; they are oxidized by one-
electron process. This complex (3b) probably gives dimerization
or disproportionation reaction following the radical formation
step.
The scans of complexes 2b and 4b showed two anodic waves
and one cathodic wave. On oxidation of –CH3 substituted nickel(II)
complexes, oxidation peaks were observed more cathodic poten-
tial in the o-, m-, p-CH3 (complexes 2b–4b) compared with Ni-TPF
(1b). A cyclic sweep in the −1.80 to +0.00 V range shows two
anodic peaks at Eox1 = −1623.8 mV, Eox2 = −590.4 mV and a cathodic
peak at Ered1 = −807.4 mV for compound 2b (Fig. 3a). In the same
range, two anodic peaks (Eox1 and Eox2) are appeared at −1510.6
and −842.9 mV, respectively. A cathodic peak (Ered1) is observed at
−814.6 mV for compound 4b (Fig. 3b). In the same range, an anodic
peak (Eox1) is obtained at −1570.0 mV and a cathodic peak (Ered1
)
is observed at −840.0 mV for compound 3b. However, the inten-
sities of the first peak (Eox1) for the complex 3b are much higher
compared with the o-CH3 substituted nickel(II) complex (2b). This
shows that the electron transfer rate is much lower in o-CH3 substi-
tuted nickel(II) complex 2b. The intensity of the first peak (Eox1) is
much higher for the p-CH3 substituted nickel(II) complex 4b com-
pared with those of o- and m-CH3 substituted nickel(II) complexes.
The intensity of the first peak changes as p- > m- > o-CH3 substituted
nickel(II) complexes. These results also are in accordance with the
spectroscopic data.
As seen from Fig. 4 and Table 2, compounds 5b–7b caused
amount changes in both the peak potential and the peak currents
as compared with the parent Ni-TPF (1b). o-, m-, p-OCH3 substi-
Table 2
Voltamperometric results of nickel(II) complexes (1b-7b)
Compound
Eox1 (mV)
Ipox1 (␮A)
Eox2 (mV)
Ipox2 (␮A)
Ered1 (mV)
Ipred1 (␮A)
ꢁEp (mV)
ks (cm s−1
)
1b
2b
3b
4b
5b
6b
7b
−1260.6
−1623.8
−1570.0
−1510.6
−1576.8
−1590.2
−1607.5
3.086
3.445
6.260
8.765
6.027
5.456
5.101
–
−590.4
–
−842.9
−956.2
−795.7
−1008.0
1.789
1.799
–
3.280
24.880
20.879
12.260
−798.1
−807.4
−842.2
−814.6
−757.9
−890.7
−795.6
2.657
12.070
11.780
12.220
14.070
12.457
14.044
−462.5
−820.9
−727.8
−696.0
−818.9
−699.5
−811.9
11.964 × 10−3
1.714 × 10−3
6.464 × 10−3
5.371 × 10−3
3.785 × 10−3
4.605 × 10−3
2.298 × 10−3
Column 8: ꢁEp: Eox1–Ered1 (mV). Cyclic voltammograms were obtained in DMSO at 25◦ C at platinum electrode, ionic strength 0.1 M (TBA+BF4−), sweep speed: 100 mV s−1
.
Eox: oxidation; Ered: reduction, ks/cm s−1 values.

5604
H. Tezcan et al. / Electrochimica Acta 53 (2008) 5597–5607
Table 3
Voltamperometric data of nickel(II) complexes (1b–7b) in DMSO at platinum electrode and 25◦ C, ionic strength 0.1 M (TBA+BF4−), scan rates: 10, 1000 and 10,000 mV s−1
Comp
Scan rate (mV/s)
Eox1 (mV)
Ipox1 (␮A)
Eox2 (mV)
Ipox2 (␮A)
Ered1 (mV)
Ipred1 (␮A)
1b
10
1,000
10,000
10
1,000
10,000
10
1,000
10,000
10
1,000
10,000
10
1,000
10,000
10
1,000
10,000
10
−1477.0
−1168.3
−1632.0
−1656.5
−1536.0
−1005.0
−1593.4
−1510.0
−1223.0
−1530.0
−1441.7
−1145.6
−1559.5
−1515.4
−1171.8
−1597.6
−1390.5
−1234.5
−1597.7
−1483.0
−1132.6
3.923
3.430
9.499
–
–
–
–
–
–
−803.5
−900.5
−1144.5
−1053.6
−1096.0
−1094.6
−875.9
−789.0
−1100.3
−863.4
−847.7
−941.7
−885.0
−897.6
−1077.5
−895.6
−810.0
−978.6
−953.5
−862.3
−986.2
5.287
23.110
42.190
9.599
26.670
87.490
6.239
17.890
77.900
8.667
32.740
100.700
10.300
36.560
122.500
9.100
2b
3b
4b
5b
6b
7b
3.367
−1091.8
4.268
21.000
–
–
–
–
6.749
–
–
1.299
27.820
–
–
44.560
68.990
3.490
2.890
66.890
2.792
77.130
140.000
2.499
63.760
103.800
6.780
23.560
56.890
2.381
69.380
79.010
−1306.6
–
–
–
–
−978.3
–
–
−936.0
−1215.4
–
–
–
–
–
–
43.240
89.100
8.306
32.820
95.010
−1259.5
−1215.4
–
3.244
30.000
–
1,000
10,000
Eox: oxidation; Ered: reduction.
Table 4
Some of the parameters calculated for nickel(II) complexes (1b–7b)
Compound
C (mM)
Iss (× 10−10 A)
Cottrell slope (S × 10−5
)
n
n net
D (× 10−6 cm2 s−1
)
1b
2b
3b
4b
5b
6b
7b
7.8
8.1
8.0
7.9
8.0
8.1
7.5
4.417
2.056
4.300
4.610
5.227
6.040
2.454
1.249
1.182
1.395
1.863
1.963
2.096
1.273
0.88
1.63
1.10
1.85
1.79
1.74
1.71
1
2
1
2
2
2
2
1.667
0.330
1.392
0.755
0.846
0.966
0.424
tuted nickel(II) complexes (5b–7b) directly give tetrazolium cations
(TPT+) by a single step two-electron transfer.
At the oxidation peaks were observed more cathodic potential
in the o-, m-, p-OCH3 complexes 5b–7b compared with Ni-TPF (1b).
The scans of compounds 5b–7b showed two anodic waves and
one cathodic wave. In the −1.80 to +0.00 V range, two anodic
peaks (Eox1 and Eox2) are appeared at −1576.8 and −956.2 mV,
respectively, and a cathodic peak (Ered1) is observed at −757.9 mV
−
Fig. 5. UME curves of DMSO solutions of 1.0 × 10−5 M compounds Ni-TPF, 1b (a); 2b (b); 4b in the presence of 0.1 M TBA+BF4 at 10 ␮m-platinum ultramicro electrode.
Potential scan rate: 10 mV s−1
.

H. Tezcan et al. / Electrochimica Acta 53 (2008) 5597–5607
5605
for compound 5b (Fig. 4a). In the same range, two anodic peaks
at Eox1 = −1590.2 mV, Eox2 = −795.7 mV and a cathodic peak at
Ered1 = −890.7 mV for compound 6b. In the same range, two anodic
peaks (Eox1 and Eox2) are appeared at −1607.5 and −1008.0 mV,
respectively, and a cathodic peak (Ered1) is observed at −795.6 mV
for compound 7b (Fig. 4b). However, the intensities of the first peak
(Eox1) for the complex 6b is much lower compared with the o-OCH3
substituted nickel(II) complex. This shows that the electron trans-
fer rate is much lower in o-OCH3 substituted nickel(II) complex.
The intensity of the first peak (Eox1) is much lower for the p-OCH3
substituted nickel(II) complex compared with those of o- and m-
OCH3 substituted nickel(II) complexes. This may be explained by
the decreasing inductive effect going from o- to p-position. The
intensity of the first peak changes as o- > m- > p-OCH3 substituted
nickel(II) complexes. This is in accordance with the yield of p-OCH3
substituted nickel(II) complex was the highest followed by m-OCH3
and o-OCH3 substituted nickel(II) complexes. These results also
are in accordance with the spectroscopic data. –OCH3 group more
cathodic potential was shifted according to –CH3 group when sub-
stitutents were compared each other. This result is agreement with
–OCH3 group is a stronger electron-donating group according to
–CH3 group.
Consequently, peak potentials are dependent upon the type and
position of substituents. All the cyclic voltammetric data are tabu-
lated in Tables 2 and 3.
3.3. Lineer sweep voltammetry and chronoamperometry
The number of electron transferred (n) and the diffusion
coefficients (D) were determined by the ultramicroelectrode CV
technique of Baranski et al. [17]. The diffusion coefficients (D) of
the compounds 1b–7b were calculated from the Cottrell equa-
tion after the n values had been obtained. The D values are
tabulated in Table 4. The number of electrons transferred was
found to be in accordance with the CV data. The diffusion
coefficients were also found within expected dimensions. The
results of UME and chronoamperometric data are tabulated in
Table 4.
3.3.1. The calculation of the number of electrons transferred
The number of electrons transferred was obtained with the use
of chronoamperometric Cottrell equation and ultramicro Pt disc
electrode (UME) steady state current [17]. The real surface area of
the Pt electrode was found to be 2.58 cm2 with the use ferrocene.
Scheme 3. (a) Possible oxidation mechanism of compound 1b (Ni-TPF) and compound 3b. (b) Possible oxidation mechanism of compounds 2b, 4b and 5b–7b.

5606
H. Tezcan et al. / Electrochimica Acta 53 (2008) 5597–5607
−
Fig. 6. UME curves of DMSO solutions of 1.0 × 10−5 M compounds 5b (a); 7b (b) in the presence of 0.1 M TBA+BF4 at 10 ␮m-platinum ultramicro electrode. Potential scan
rate: 10 mV s−1
.
Fig. 7. The correlation between the ꢀmax and the Eox1/mV (Ag/AgCl) values.
If it values are plotted against t−1/2 from the resulting slope n could
easily be calculated. The number electrons calculated are given in
Table 4.
Fig. 8. The correlation between the ꢀmax and the Ered1/mV (Ag/AgCl) values.
As seen from Fig. 5 and Table 4, there is a one-electron transfer
wave for Ni-TPF (1b). Ni-TPF (1b) is fragmented. Ni2+ and for-
mazan anion (TPF−, X) are formed in solution (Scheme 3). TPF−
anion gives a single one-electron transfer giving a formazan radi-
cal (TPF•). These radicals give a disproportionation reaction to give
a formazan (TPF) and tetrazolium cation (TPT+) or a dimerization
reaction resulting a diformazan. Mechanism of the oxidation of
compound 1b is shown in Scheme 3. These results are in agreement
with literature [15].
Both the nickel(II) complexes of –CH3 and –OCH3 substi-
tuted formazans gave clear S-shaped steady state currents with
UME (Figs. 5 and 6). As seen from Fig. 5, there are one-electron
transfer wave for compounds 2b and 4b. The numbers of elec-
trons transferred are two for o-, p-CH3 substituted nickel(II)
complexes in UME based cyclic voltammograms of based UME
and chronoamperometric (Table 4). This is accordance with CV
results (Fig. 3). The appearance of one distinctive peak in o-, p-
CH3 substituted nickel(II) complexes suggest that the compounds
give two-electron transfer. Complexes 2b and 4b give one-step
Fig. 9. The ꢀmax values against Hammett substituents coefficients-ꢃT.
two-electron transfer giving directly tetrazolium cations. Mech-
anisms of the oxidation of compounds 2b and 4b are shown in
Scheme 3b. These results are in agreement with the literature
[16].
Table 5
The total ꢃT (ꢃ1 + ꢃ2) and related ꢀmax values
Substituent position
Compound
Abbreviation
ꢃꢀ (s)
ꢃT (total effect)
ꢀmax1 (nm)
m-
1b
3b
6b
H
H: 0
0
611.0
647.0
649.0
m-CH3, p-OCH3
m-OCH3, p-OCH3
m-CH3: −0.06; p-OCH3: −0.12
−0.18
m-OCH3: 0.10; p-OCH3: −0.12
−0.02
p-
4b
7b
p-CH3, p-OCH3
p-OCH3, p-OCH3
p-CH3: −0.14; p-OCH3: −0.12
p-OCH3: −0.12; p-OCH3: −0.12
−0.26
−0.24
644.0
642.0

H. Tezcan et al. / Electrochimica Acta 53 (2008) 5597–5607
5607
As seen from Figs. 5, there are one-electron transfer wave for
compound 3b. The number of electrons transferred is one for m-CH3
substituted nickel(II) complexes cyclic voltammograms of based
UME and chronoamperometric (Table 4). This is accordance with
CV results (Fig. 3). The appearance of one distinctive peak in m-CH3
substituted nickel(II) complex suggest that the compound gives
one-electron transfer. This complex (3b) is fragmented. Ni2+ and
substituted formazan radicals are occurred in solution (Scheme 3a).
These radicals give a disproportionation reaction to give a substi-
tuted formazan and tetrazolium cation or a dimerization reaction
result a diformazan. Mechanism of the oxidation of m-CH3 substi-
tuted nickel(II) complex (3b) is shown in Schemes 3a. These results
are in agreement with the literature [15].
It was also investigated whether the Hammett substituent coef-
ficients could be used to evaluate the total effect of the two different
substituents, upon the ꢀmax values and thus the color. Hammett
substituent coefficients ꢃ have been related with ꢀmax values for
only one substituent in the structure up to now. The total ꢃT
(ꢃ1 + ꢃ2 = ꢃT) and related ꢀmax values are at Table 5. ꢀmax values
have been correlated to Hammett subsituent coefficients (ꢃT). Rep-
resentative ꢀmax–ꢃT plots are illustrated in Fig. 9.
There were lineer correlation between Hammett substituent
coefficients-ꢃ with ꢀmax values for the nickel(II) complexes.
Acknowledgement
As seen from Fig. 6, there are one-electron transfer wave for com-
pounds 5b–7b. The numbers of electrons transferred are two for o-,
m-, p-OCH3 substituted nickel(II) complexes cyclic voltammograms
of based UME and chronoamperometric (Table 4). This is accor-
dance with CV results (Fig. 4). The appearance of one distinctive
peak in o-, m-, p-OCH3 substituted nickel(II) complexes suggest that
the compounds give two-electron transfer. Complexes 5b–7b give
one-step two-electron transfer giving directly tetrazolium cations.
Mechanisms of the oxidation of compounds 5b–7b are shown in
Scheme 3b. These results are in agreement with the literature [16].
We are very grateful to the Gazi University Research Fund for
providing financial support for this Project (No. 04/2004-13).
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5.
Nickel(II) complexes of formazan were synthesized. The prod-
ucts obtained were purified by repeated crystallization from
methanol and the purity was checked by TLC. Spectroscopic charac-
teristics of the prepared complexes are discussed. Determined peak
potentials (Eox and Ered), diffusion coefficients, number of elec-
trons transferred (n) and heterogeneous rate constants (ks) were
obtained with the use of cyclic voltammetry, lineer sweep voltam-
metry and chronoamperometry. The oxidation products prepared
by controlled potential electrolysis (CPE) were isolated and identi-
fied by different techniques: GC–mass and elemental analysis. The
oxidation mechanism was occurred in a single step two-electron
or one-electron transfer to a disproportionation or dimerization
reactions following the radical formation step.
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The spectral and redox properties were compared. There was a
relatively good correlation between the ꢀmax and peak potentials
(Eox1 and Ered1 values) (Figs. 7 and 8).
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