Reductive and oxidative electrochemical study and spectroscopic properties of nickel(II) complexes with N2O2 Schiff bases derived from (±)-trans-N,N′-bis(salicylidene)-1,2-cyclohexanediamine

Article abstract of DOI:10.1016/j.electacta.2013.12.073

Cyclic voltammetry, chronopotentiometry, chronocoulometry, controlled potential electrolysis and UV VIS were used in investigation of ligands and Ni(II) complexes with (±)-trans-N,N′-bis(salicylidene)-1,2- cyclohexanediamine substituted in orto- and/or para- positions of phenolate anions with tert-butyl and/or methoxy groups. Irreversible reduction and oxidation of ligands were shown. No influence of solvent on reduction of complexes, which results in Ni(I) complexes, was observed, however, an effect of solvent and substituents of phenolate anions on oxidation processes was proved. Anodic process in (CH₃)₂SO always results in appropriate Ni(III)-phenolate complex. Oxidation of orto- and para- substituted complexes in CH₂Cl₂ results in Ni(II)-mono-phenoxyl radicals, which undergo delocalisation and subsequently are oxidised to Ni(II)-bis-phenoxyl radicals. Last oxidation step of radicals produce long-lived Ni(II)-bis-phenoxonium cations. Complexes without substituents in para-and/or orto- position having formed Ni(II)-mono-phenoxyl radicals undergo dimerization. Mechanisms of electrode processes were studied and D and E_f values for processes of reduction and I^st step of oxidation of complexes were calculated.

Full text of DOI:10.1016/j.electacta.2013.12.073

Accepted Manuscript
Title: Reductive and oxidative electrochemical study and
spectroscopic properties of nickel(II) complexes with N2O2
Schiff bases derived from
( )-trans-N,Nꢀ-bis(salicylidene)-1,2-cyclohexanediamine
Author: D. Tomczyk L. Nowak W. Bukowski K. Bester P.
Urbaniak G. Andrijewski B. Olejniczak
PII:
S0013-4686(13)02513-9
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.electacta.2013.12.073
EA 21874
To appear in:
Electrochimica Acta
Received date:
Revised date:
Accepted date:
27-9-2013
19-11-2013
12-12-2013
Please cite this article as: D. Tomczyk, L. Nowak, W. Bukowski, K.B.
</sup>,P. Urbaniak, G. Andrijewski, B. Olejniczak, Reductive and oxidative
electrochemical study and spectroscopic properties of nickel(II) complexes with N2O2
Schiff bases derived from (pm)-trans-N,Nꢀ-bis(salicylidene)-1,2-cyclohexanediamine.,
Electrochimica Acta (2013), http://dx.doi.org/10.1016/j.electacta.2013.12.073
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Reductive and oxidative electrochemical study and spectroscopic properties of
nickel(II) complexes with N2O2 Schiff bases derived from (±)-trans-N,N’-
bis(salicylidene)-1,2-cyclohexanediamine.
by D. Tomczyk*1), L. Nowak1), W. Bukowski2), K. Bester2),
P. Urbaniak1), G. Andrijewski1), B. Olejniczak1)
1)Department of Inorganic and Analytical Chemistry,
University of Łódź, 91-403, ul.Tamka 12, Poland
*e-mail: tomczyk@chemia.uni.lodz.pl
2
) Faculty of Chemistry, Rzeszów University of Technology,
35-959 Rzeszów, Al. Powstańców W-wy 6, Poland
Abstract
Cyclic voltammetry, chronopotentiometry, chronocoulometry, controlled potential electrolysis and UV VIS were
used in investigation of ligands and Ni(II) complexes with (±)-trans-N,N’-bis(salicylidene)-1,2-
cyclohexanediamine substituted in orto- and/or para- positions of phenolate anions with tert-butyl and/or
methoxy groups. Irreversible reduction and oxidation of ligands were shown. No influence of solvent on
reduction of complexes, which results in Ni(I) complexes, was observed, however, an effect of solvent and
substituents of phenolate anions on oxidation processes was proved. Anodic process in (CH3)2SO always results
in appropriate Ni(III)-phenolate complex. Oxidation of orto- and para- substituted complexes in CH2Cl2 results
in Ni(II)-mono-phenoxyl radicals, which undergo delocalisation and subsequently are oxidised to
Ni(II)-bis-phenoxyl radicals. Last oxidation step of radicals produce long-lived Ni(II)-bis-phenoxonium cations.
Complexes without substituents in para-and/or orto- position having formed Ni(II)-mono-phenoxyl radicals
undergo dimerization. Mechanisms of electrode processes were studied and D and Ef values for processes of
reduction and Ist step of oxidation of complexes were calculated.
Keywords: salen, phenoxonium cation, UV VIS, cyclic voltammetry, delocalization
1. Introduction
During last years complexes of nickel(II) with N2O2 Schiff bases have been intensively
investigated taking into account their optical properties as well as their application as catalysts or
electrocatalysts of numerous organic redox reactions [1]. The possibility of electrochemical
formation of stable oxidised or reduced forms of complexes enables their utilisation in
electrocatalytic oxidation and reduction. They play an important role in reduction of alkyl and
aryl halides [1d ,2] and in epoxidation of olefins [3]. They are used in asymmetric catalysis of
many organic compounds [3b, 4], e.g. aminoacids The dependence of redox properties of
complexes on the substituents in phenolate anions or imine bridge and on the solvent gives an
opportunity to apply the complexes in specific homogenic and heterogenic catalysis [1a,5].
Heterogenic catalysis is possible due to an ability of anodic electropolymerisation of some
transition metal complexes with salen derivatives with unsubstituted para positions of the
aldehyde moieties, in weak donor solvents [6]. Considerable ease of preparation of electrodes
modified with polymers obtained from such monomers enables their utilisation as sensors [7].
Papers dealing with electrochemical properties of Ni(II)-N2O2 Schiff bases complexes
show that their reduction leads to Ni(I) complexes [8], while oxidation process may result in
oxidation of metal ion [9] or ligand [10], or both metal ion and ligand [11], which depends
mainly on solvent and substituents in phenolate anions and/or imine bridge.
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Investigation of Ni(II) salen-type ligands has also a biological aspect because of the
presence and significant role of Ni(I) and Ni(III) ions for activity of methyl coenzyme M
reductase, of carbon monoxide oxydoreductase and several hydrogenases [12]. This is the reason
for wide analysis of nickel(II) ion complexes with N2O2 Schiff bases in metalloprotein modelling
and metalloenzyme-mediated catalysis.
Our research is concentrated on solutions of complexes of Ni(II) with trans isomers of
four derivatives of (±)-trans-N,N’-bis(salicylidene)-1,2-cyclohexanediamine with phenolate
anions substituted with tert-butyl and/or methoxy groups in CH2Cl2, CH3CN and (CH3)2SO. We
have focused on determination of mechanism and kinetic parameters both of oxidation and
reduction of complexes in solvents possessing different solvation properties.
2. Experimental
2.1. Materials
Salicylaldehyde, 99% and (±) trans-1,2-Diaminocyclohexane, ferrocene (Fc) were
obtained from Aldrich, nickel(II) acetate tetrahydrate Ni(AcO)2·4H2O (reagent grade), ethanol
(EtOH) 95%, acetone (analytical grade) and potassium hexacyanoferrate(III) (analytical grade)
were purchased from POCH. Dimethyl sulphoxide ((CH3)2SO), methylene chloride (CH2Cl2) and
acetonitrile (CH3CN) were of HPLC grade, from Baker. Tetraethylammonium perchlorate
(TEAP) and tetrabutylammonium perchlorate (TBAP) and were of analytical grade, from Fluka.
All of them were used as received. Solutions of ligands and complexes were prepared directly
before measurements.
2.2. Syntheses
2.2.1. Reagents
3,5-Di-tert-butylsalicylaldehyde and 5-metoxy-3-tert-butyl salicylaldehyde were obtained
by standard method [13a], 3-tert-butyl salicylaldehyde was obtained according to [13b].
2.2.2. Ligands
Salen ligands were obtained in the stoichiometric reaction of salicylic aldehyde and trans-
1,2-diaminocyclohexane in 96% ethanol solution according to [13c]. Reactions were carried out
in 50 ml three-neck round-bottomed flask, equipped with reflux condenser, dropping funnel,
magnetic stirrer and heating mantle. The solution of trans-1,2-diaminocyclohexane (0,57 ml, 5
mmol) in EtOH (10 ml) was slowly added to a hot solution of appropriate aldehyde (10mmol) in
EtOH (20 ml). The reaction mixture was heated at reflux for 1.5 h. After cooling to room
temperature, the yellow precipitate that formed was filtered off and washed with cold EtOH (5
ml). The ligands were used without further purification.
(±)-trans-N,N’-bis(salicylidene)-1,2-cyclohexanediamine (H2salcn)
Anal. Calc. for C20H22N2O2: C, 74.51%; H, 6.88%; N, 8.69%; C/N = 8.57. Found: C,
74.59%; H, 7.07%; N, 8.73%; C/N = 8.54. 1H-NMR (CDCl3): δ = 13.29 (bs, 2H), 8.25 (s, 2H),
7.25 (m, 2H), 7.13 (m, 2H), 6.88 (d, J = 8.25Hz, 2H), 6.78 (t, J = 7.5Hz, 2H), 3.31 (m, 2H), 1.88
(m, 4H), 1.73(m, 2H), 1.47 (m, 2H). 1H-NMR (CD2Cl2): δ = 13.21 (bs, 2H), 8.28 (s, 2H), 7.22
(m, 2H), 7.18 (m, 2H), 6.82 (m, 2H), 6.78 (m, 2H), 3.33 (m, 2H), 1.94 (m, 2H), 1.86 (m, 2H),
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1.71 (m, 2H), 1.48 (m, 2H). F. Wt. calc./found: 322.4 / ESI-MS: m/z 323.3 ([M+1]+,100). Yield:
1.47 g, 91%, mp=120-122oC.
(±)-trans-N,N’-bis(3-tert-butylsalicylidene)-1,2-cyclohexanediamine (H2salcn(Bu))
Anal. Calc. for C28H38N2O2: C, 77,38%; H, 8,81%; N, 6,45%; C/N = 11.99. Found: C, 76,77%;
H, 8,98%; N, 6,42%; C/N = 11,95. 1H-NMR (CDCl3): δ = 13.80 (bs, 2H), 8.31 (s, 2H), 7.25 (m,
2H), 7.08 (m, 2H), 6.72 (t, J = 7.65Hz, 2H), 3.37 (m, 2H), 1.98 (m, 2H), 1.88 (m, 2H), 1.76 (m,
2H), 1.47 (m, 2H), 1,40 (s, 18H). Yield: 1.63 g, 75%, mp=139-142oC.
(±)-trans-N,N’-bis(5-methoxy-3-tert-butylsalicylidene)-1,2-cyclohexanediamine
(H2salcn(BuOMe))
Anal. Calc. for C30H42N2O2: C, 72.84%; H, 8.56%; N, 5.66%; C/N = 12.87. Found: C, 72.63%;
H, 8.45%; N, 5.68%; C/N = 12.78. 1H-NMR (CDCl3): δ = 13.40 (bs, 2H), 8.24 (s, 2H), 6.90 (m,
2H), 6.48 (m, 2H), 3.69 (s, 6H), 3.32 (m, 2H), 1.99 (m, 2H), 1.89 (m, 2H), 1.77 (m, 2H), 1.47 (m,
2H), 1.39 (s, 18H). Yield: 1.33 g, 54%, mp=146-149oC.
(±)-trans-N,N’-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexanediamine (H2salcn(2Bu))
Anal. Calc. for C36H54N2O2: C, 79.07%; H, 9.95%; N, 5.12%; C/N = 15.44. Found: C, 78.98%;
H, 10.25%; N, 5.17%; C/N = 15.27. 1H-NMR (CDCl3): δ = 13.70 (bs, 2H), 8.29 (s, 2H), 7.29 (d,
J = 2.40Hz, 2H), 6.97 (d, J = 2.30Hz, 2H), 3.31 (m, 2H), 1.94 (m, 2H), 1.87 (m, 2H), 1.73 (m,
2H), 1.47 (m, 2H), 1.40 (s, 18H), 1.23 (s, 18H). Yield: 2.19 g, 80%, mp=178-181oC.
2.2.3. Complexes
N
O
N
O
Ni
5
5'
R2
R2
3
3'
R1
R1
R1 = H
R2 = H
R2 = H
R2 = t-Bu
R2 = OMe
[Ni(salcn)]
R1 = t-Bu
R1 = t-Bu
R1 = t-Bu
[Ni(salcn(Bu))]
[Ni(salcn(2Bu))]
[Ni(salcn(BuOMe))]
Scheme 1. Complexes described in this study
Nickel(II) complexes were obtained similarly to [13d]. Reactions were carried out under
nitrogen atmosphere in three-neck round-bottomed flask, equipped with reflux condenser,
dropping funnel, magnetic stirrer and heating mantle. A solution of Ni(AcO)2·4H2O (0.497g, 2
mmol) in hot water (2 ml) was slowly dropped to the solution of appropriate Schiff base ligand (2
mmol) in hot EtOH (10 ml). The resulting solution was refluxed for 2h and a dark precipitate
appeared. After cooling to room temperature the precipitate was collected by filtration and
washed with cold EtOH (2 ml). The complex was dried under vacuum at 50oC.
(±)-trans-N,N’-bis(salicylidene)-1,2-cyclohexanediaminenickel(II) [Ni(salcn)]
Anal. Calc. for C20H20N2NiO2: C, 63.37%; H, 5.32%; N, 7.39%; Ni, 15,48%C/N = 8.57.
Found: C, 63.36%; H, 5.95%; N, 7.40%; C/N = 8.56. 1H-NMR (CD2Cl2): δ = 7.41 (s, 2H), 7.20
(m, 2H), 7.13 (m, 2H), 6.84 (d, J = 8.55Hz, 2H), 6.53 (m, 2H), 3.11 (m, 2H), 2.42 (m, 2H), 1.91
(m, 2H), 1.34 (m, 4H). (CDCl3 caused relatively fast decomposition of the complex, hence the
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spectrum was recorded in CD2Cl2 and that is why the spectrum of H2salcn were also taken in this
solvent). F. Wt. calc./found: 379.1 / ESI-MS: m/z 379.1 ([M]+,100). Yield: 0,57 g, 75%.
(±)-trans-N,N’-bis(3-tert-butylsalicylidene)-1,2-cyclohexanediaminenickel(II)
[Ni(salcn(Bu))]
Anal. Calc. for C28H36N2NiO2: C, 68.45%; H, 7.39%; N, 5.70%; Ni, 11,95%; C/N = 12,01.
Found: C, 68.19%; H, 7.83%; N, 5.66%; C/N = 12.05. 1H-NMR (CDCl3): δ = 7.40 (s, 2H), 7.22
(dd, J = 7.30Hz, 1.75Hz, 2H), 6.98 (dd, J = 7.80Hz, 1.70Hz 2H), 6.47 (t, J = 7.55Hz, 2H), 3.01
(m, 2H), 2.43 (m, 2H), 1.91 (m, 2H), 1.41 (s, 18H), 1.31 (m, 4H). F. Wt. calc./found: 491.4 / ESI-
MS: m/z 491.4 ([M]+, 100), 513.4 ([M+22]+, 25). Yield: 0,89 g, 91%.
(±)-trans-N,N’-bis(5-methoxy-3-tert-butylsalicylidene)-1,2-cyclohexanediaminenickel(II)
[Ni(salcn(BuOMe))]
Anal. Calc. for C30H40N2NiO4: C, 65.35%; H, 7.31%; N, 5.08%; Ni, 10.65%; C/N = 12,86.
Found: C, 65.20%; H, 7.70%; N, 5.05%; C/N = 12.91. 1H-NMR (CDCl3): δ = 7.36 (s, 2H), 6.96
(m, 2H), 6.35 (m, 2H), 3,68 (s, 6H), 3.04 (m, 2H), 2.43 (m, 2H), 1.91 (m, 2H), 1.39 (s, 18H), 1.32
(m, 4H). F. Wt. calc./found: 551.4 / ESI-MS: m/z 550.4 ([M-1]+, 100). Yield: 0,90 g, 82%.
(±)-trans-N,N’-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexanediaminenickel(II)
[Ni(salcn(2Bu))]
Anal. Calc. for C36H52N2NiO2: C, 71.65%; H, 8.68%; N, 4.64%; Ni, 9.73%; C/N = 15.44. Found:
C, 71.20%; H, 9.19%; N, 4.61%; C/N = 15.44. 1H-NMR (CDCl3): δ = 7.39 (s, 2H), 7.29 (d,
J = 2.35Hz, 2H), 6.88 (d, J = 2.15Hz, 2H), 2,96 (m, 2H), 2.44 (m, 2H), 1.92 (m, 2H), 1.41 (s,
18H), 1.31 (m, 4H), 1.26 (s, 18H). F. Wt. calc./found: 603.6 / ESI-MS: m/z 602.6 ([M-1]+, 100),
625.5 ([M+22]+, 35).Yield: 0,89 g, 74%.
2.3. Instruments and procedures
The full CHN analysis was performed using a Vario EL III analyzer.
FT-IR spectra were recorded in the 490-4000 cm-1 range, using a Nicollet 8700
spectrometer and a standard KBr pellet technique.
Mass spectra were recorded using a Varian 500-MS LC Ion Trap.
NMR spectra were recorded using a Bruker AM 500 MHz spectrometer.
UV – Vis spectra were recorded at room temperature on a PU 8630 Philips
spectrophotometer in a standard quartz (0.995 cm) cells, in the range from 190 to 1100 nm, using
solutions of the ligands (10-5 mol·dm-3) and complexes (10-5 mol·dm-3, 10-4 mol·dm-3) in
(CH3)2SO, CH3CN, CH2Cl2. UV – Vis measurements of the solution during (with 10% time
intervals) and after electrolysis were carried out under argon atmosphere with the use of a flow
quartz cell connected with a three electrode electrochemical cell by teflon tube.
Magnetic studies were performed at 298 K on a magnetic susceptibility balance of
Sherwood Scientific Ltd.
Cyclic voltammetry (CV) measurements were performed using AUTOLAB PGSTAT 10
Eco Chemie with a three-electrode cell. A Pt disk electrode (MINERAL) with an area of 0.1865
cm2 was used as working electrode. The area of the Pt disk electrode was determined
voltammmetrically using potassium hexacyanoferrate(III) solution as a standard [14]. A Pt plate
was used as a counter electrode and Ag/AgCl (1 mol·dm-3 NaCl) as a reference electrode,
connected to the bulk of the solution by a Luggin capillary. Measurements were carried out for
solutions of ligands (10-3 mol·dm-3) or complexes (10-3 mol·dm-3) in (CH3)2SO/TEAP (0.1
mol·dm-3), CH3CN/TEAP (0.1 mol·dm-3), CH2Cl2/TBAP (0.1 mol·dm-3). Voltammograms were
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recorded in the potential range from -2.1 V to 2.2 V, at scan rates between 0.02 and 1 V·s-1.
Before measurements the solutions were deoxidized with argon.
Chronopotentiometry (CP), chronocoulometry (CC) and controlled potential electrolysis
were carried out in (CH3)2SO/TEAP (0.1 mol·dm-3) solution containing 10-3 mol·dm-3 [Ni(salcn)],
and CH2Cl2/TBAP (0.1 mol·dm-3) solution containing 10-3 mol·dm-3 [Ni(salcn(2Bu))] or
[Ni(salcn(BuOMe))] complexes.
Chronopotentiometric measurements were performed using single current step method
with different step duration (20 ÷ 60s) and at different currents (±10-5 ÷ ± 10-4A).
Double potential step chronocoulometry curves were recorded for different step durations
(10-3 ÷ 60s), where the steps started from the potential value where no current was observed to
the value slightly higher/lower than potential oxidation/reduction peak, respectively, recorded in
voltammetric curve.
Potential controlled electrolysis (oxidation and reduction) was carried out at potential
values higher than first and higher than second anodic peak potential, and lower than cathodic
peak potential, in three electrode cell, using Pt gauze as a working electrode, Pt plate as a counter
electrode and an Ag/AgCl (1 mol·dm-3 NaCl) as a reference electrode, using AUTOLAB
PGSTAT 10. Three electrode electrochemical cell was connected with a flow quartz cell of the
spectrophotometer.
All potentials are reported vs. to a Ag/AgCl (1 mol·dm-3 NaCl) reference electrode.
Under the experimental conditions used (scan rate 0.05 V·s-1) formal potential Ef and ΔE
for redox couple Fc/Fc+ counted as follows: in (CH3)2SO: Ef = 0.45 V, ΔE = 0.09 V; in CH3CN:
Ef = 0.48 V, ΔE = 0.09 V; in CH2Cl2: Ef = 0.44 V, ΔE = 0.10 V.
All the electrochemical and spectroscopic measurements were carried out under argon
atmosphere at 295K.
The Pt electrode was cleaned in concentrated HNO3 and aqueous suspension of 0.05μm
alumina micropolish before each measurement.
3. Results and discussion
3.1. Spectral and magnetic studies
3.1.1. Infrared spectra
IR spectra of free ligands and their complexes exhibits absorption bands connected with
stretching and bending vibration of C-H, C=C, C=N, C-N, C-O and O-H groups. Tab. 1 presents
only absorption bands that are characteristic for Schiff bases and their complexes.
IR spectra of free ligands in the range of frequencies 2500-2700 cm-1 exhibit broad bands
of medium intensity. They are connected with stretching vibrations of O-H groups. Their
significant shift towards lower frequencies, as compared to values expected for O-H groups, is a
result of intramolecular interactions of O-H groups with N atoms [15], which is enabled by flat
structure of salicylaldehyde moieties of ligands [16]. The lack of this band in spectra of
complexes proves deprotonation of O-H groups as a result of complex formation [15c]. The bands
at frequencies in the range 1620-1624 cm-1 in ligand spectra are connected with C=N stretching
vibration. These vibration were also observed for similar ligands in the range 1590-1650 cm-1
[15b.c,17]. The shift of characteristic imine bands in complexes by 16-19 cm-1 towards lower
frequencies occurs because electron pairs on azomethine nitrogen take part in nickel ion
coordination [15b,18].
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Medium bands in the range 1380-1396 cm-1 in ligand spectra are corresponding to C-N
stretching frequencies. These bands are placed in the range which is characteristic for this type
Schiff bases (1350-1410 cm-1) [15b,18a,19]. Strong bands recorded for H2salcn and H2salcn(Bu)
as well as medium bands for H2salcn(2Bu) and H2salcn(BuOMe) are connected to C-O stretching
frequencies and are placed in the range 1280-1340 cm-1, described in literature for similar ligands
[15b,18a,19a]. In complexes, these bands are shifted towards higher frequencies [15b,18a,19a]
because both azomethine nitrogen atoms and phenolate oxygen atoms contribute to coordination.
The C-N stretching frequencies shift by 9-12 cm-1 and C-O stretching frequencies by 21-39 cm-1.
Table 1. Infrared spectra frequencies (cm-1) for ligands and complexes.
ligand/complex
H2salcn
C=N
C-N
C-O
1640 (s)
1624 (s)
1636 (s)
1619 (s)
1638 (s)
1622 (s)
1639 (s)
1620 (s)
1381 (m)
1390 (m)
1380 (m)
1392 (m)
1382 (m)
1394 (m)
1383 (m)
1396 (m)
1280 (s)
1319 (s)
1274 (s)
1308 (s)
1285 (m)
1314 (s)
1289 (m)
1311 (s)
[Ni(salcn)]
H2salcn(Bu)
[Ni(salcn(Bu))]
H2salcn(2Bu)
[Ni(salcn(2Bu))]
H2salcn(BuOMe)
[Ni(salcn(BuOMe))]
3.1.2. NMR spectra
All of free ligands in 1H NMR spectra exhibit broad singlets in the range 13.29-13.80 ppm
corresponding to hydrogen bonded phenolic proton. The singlets in the range 8.24-8.31 ppm are
attributable to CH=N imine protons. Protons of aromatic rings appear in the range 6.48-7.29 ppm.
In case of H2sal, a ligand with unsubstituted aromatic rings four signals are observed which
corresponds to eight protons. Other ligands under investigation possess one or two substituents in
positions 3- and/or 5- and give three or two signals, respectively to number of substituents.
Protons of bridge group neighbouring to nitrogen atoms, N-CH<, are observed as a multiplet
signal in the range 3.31-3.37 ppm. The multiplets in the range 1.47-1.99 ppm in spectra of each
ligand are connected to protons of four methylene groups of cyclohexane ring. The spectra of
H2salcn(Bu), H2salcn(BuOMe) and H2salcn(2Bu) show in the range 1.23-1.40 ppm one, one and
two signals, respectively, from protons of tert-Bu groups. In the spectrum of H2salcn(BuOMe) at
1
3.69 ppm is also observed a single resonance from methoxy group protons. Analogical H NMR
spectra were recorded and described for similar Schiff bases [20].
1
In the H NMR spectra of the complexes, the signals corresponding to hydrogen bonded
phenolic proton, observed in the range 13.29-13.80 ppm, have gradually disappear. Furthermore
the signals of CH=N and N-CH< groups protons as well as the ones corresponding to aromatic
rings have moved towards lower δ values.
3.1.3. UV-VIS spectra
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Electronic spectra of ligands and complexes were recorded in the range 190-1100 nm, in
(CH3)2SO, CH3CN and CH2Cl2. In (CH3)2SO and CH3CN only H2salcn and [Ni(salcn)] dissolves.
All the complexes under investigation dissolve in CH2Cl2. The solutions obtained for ligands are
all brightly yellow, while those of complexes are green-orange-brownish, except acetonitrile
solution of [Ni(salcn)], which is yellow.
Table 2. Absorption spectra of free ligands in UV-VIS region.
ε'II
ε’III
ε’IV
ligand
solvent
ε'I
λI
λII
λIII
λIV
CH2Cl2
CH3CN
(CH3)2SO
255
254
2.08 317
2.13 315
0.76
0.84
0.77
416sh
412sh
414sh
0.01
0.02
0.02
213 4.80
H2salcn
262sh 2.05 316
259
262
268
2.00 322
2.04 330
2.06 335
0.74
0.78
0.76
420sh
426sh
429sh
0.02
0.03
0.03
CH2Cl2
CH2Cl2
CH2Cl2
H2salcn(Bu)
H2salcn(2Bu)
H2salcn(BuOMe)
λ (nm), ε’ = 10-4·ε (dm3⋅mol-1⋅cm-1), sh = shoulder
Electronic spectra of free ligands in CH2Cl2 are very similar to each other in the number
of absorption bands and in their energies and extinction coefficients because of identical structure
of salicylaldehyde moiety (tab. 2). Small differences in absorption band energies result from
different substituents in aromatic rings. In case of H2salcn there was no influence of CH2Cl2 and
CH3CN on character of ligand electronic spectra, in (CH3)2SO solution high and medium energy
bands were not observed. Based on literature data recorded for benzene [21] and similar Schiff
bases in different solvents [15b,18a,22] one can conclude that bands at wave lengths λI – λIII
correspond to π→π* transitions First two strong absorption bands are connected with π→π*
transitions of the phenolic chromophores. Absorption bands in the range of λIII: 315-335 nm are
assigned to π→π* transitions in azomethine chromophores. A shoulder in the range of λIV: 412-
429 nm is attributed to n→π* transitions within molecular orbitals of azomethine chromophore
and phenolic ring, i.e. transitions of dipolar zwitterionic keto-amine tautomeric structures of
ligands [23]. Substitution of the aromatic ring of the ligand results in bathochromic shift, which is
strongest for methoxy group, which has the most activating influence on the ligand aromatic ring
among substituents under investigation.
Table 3. Absorption spectra of complexes in UV-VIS region, T = 293 K.
complex
solvent
CH2Cl2
λ1(ε'1) λ2(ε'2) λ3(ε'3) λ4(ε’4) λ5(ε'5) λ6(ε'6) λ7(ε'7) λ8(ε'8) λ9(ε'9)
257 316 339 380sh 402 441sh 546br
(5.17) (0.78) (0.72) (0.40) (0.62) (0.37) (0.01)
255 317 340 383sh 400 440sh 544br
(3.08) (5.85) (5.27) (0.78) (0.74) (0.39) (0.62) (0.25) (0.01)
268sh 320 342 390sh 409 445sh 567sh
(4.31) (0.94) (0.90) (0.50) (0.68) (0.38) (0.04)
265 322sh 346 394sh 417 444sh 554br
CH3CN
(CH3)2SO
CH2Cl2
213
244
[Ni(salcn)]
[Ni(salcn(Bu))]
(6.34) (0.81) (0.84) (0.41) (0.72) (0.43) (0.01)
7
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[Ni(salcn(2Bu))]
CH2Cl2
267
324sh 349
404sh 422
449sh 560br
(7.89) (0.84) (0.93) (0.49) (0.76) (0.50) (0.02)
[Ni(salcn(BuOMe))] CH2Cl2
263 335 356 420sh 439 465sh 576br
(6.53) (1.00) (0.91) (0.62) (0.91) (0.67) (0.02)
λ (nm), ε’ = 10-4·ε (dm3⋅mol-1⋅cm-1), sh = shoulder, br = broad
UV-VIS spectra of complexes in CH2Cl2, CH3CN and (CH3)2SO are summarised in tab. 3.
Electronic spectra of all the complexes are similar. Strong absorption bands in the range: λ1 ÷λ4,
observed also in free ligand spectra, are connected with intraligand π→π* transitions, while those
at the lengths λ5 and λ6, unobserved for free ligands, correspond to CT transitions. The bands of
lower values of molar absorption coefficients at higher wavelengths above λ6 correspond to d-d
transitions in Ni(II) ion in low spin square planar complex [15 z 5]. Typical broad bands of low
intensity in the range: 546-576 nm (λ9) result from transitions from four d orbitals of lower energy.
None of complexes exhibits absorption over 600 nm, and such a behaviour is expected for low
spin square planar complexes of d8 ion [8-2].
Electronic spectra of [Ni(salcn)] in CH2Cl2 and CH3CN generally do not differ from each
other. However, in (CH3)2SO one can observe a shift of bands corresponding to d-d transitions
towards longer waves, which may result from weaker ligand field because of distortion of square
symmetry by solvent of stronger solvation properties [24]. The shifts are not significant (with
exception of λ9) which indicates weak interactions of metal ion with solvent. The fact may be
caused by steric effect of cyclohexane group in imine bridge impeding coordination by solvent
molecule in one of axial positions in the complex. This effect was shown by Castro et al [25] for
the complex of Ni(III) ion with salcn2-. Another factor indicating deformation of square
symmetry of the complex in (CH3)2SO may be an increase of molar absorption coefficients as
compared to corresponding values in other solvents. Considerable difference is observed for ε'9
which, together with more significant shift of λ9 [fig. 3, dashed line], may indicate some
contribution of solvent in coordination of Ni(II) ion and, accordingly, a presence of other than
square forms of the complex. Octahedral Ni(II) complexes with similar ligands exhibit weak
absorption bands slightly over 600 nm [26].
In spectra recorded for CH2Cl2 solutions of complexes substituted in phenolate moiety
one can observe shift of absorption bands towards lower energies in comparison to unsubstituted
complexes. The shift increases as follows: [Ni(salcn)] < [Ni(salcn(Bu))] < [Ni(salcn(2Bu))] <
[Ni(salcn(BuOMe))] and the order may depict increasing distortion of square coordination
symmetry around Ni(II) ion due to steric effect resulting from the presence of substituents in
ligand phenolate moiety. In the same sequence molar absorption coefficients increase confirming
such a character of changes. Similar alteration of electronic spectra with changes of ligand
symmetry were observed also for macrocyclic Ni(II) complexes [27]. The greatest influence on
the complex symmetry deformation is to be seen in the presence of two different substituents
(-Bu and -OMe) in each phenyl ring, as we can see for [Ni(salcn(BuOMe))], showing the greatest
shifts of absorption bands towards near infrared as well as the highest values of ε. The lowest
deformation is observed where each aromatic ring is singly substituted.
3.1.4. Magnetic properties
8
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Magnetic susceptibility measurements carried out in temperature 273K confirmed the
surmise that all the complexes are diamagnetic. The result proves that they are low spin
complexes of square planar d8 symmetry.
3.2. Electrochemical studies
3.2.1. Reductive electrochemical study
Ligands
Voltammetric studies of the ligands were carried out at scan rates from v = 0.02 to
0.5 V·s-1, in two potential ranges: from 0.4 to -2.1 V and from -2.1 to 2.2 V, where the second
range comprises both oxidation and reduction areas. Cathodic peak potential values, Epc, are
summarised in tab. 4. Exemplary curve is presented in fig. 1 with dashed line.
Fig. 1. Cyclic voltammogram at Pt in CH2Cl2 + 0.1 M TBAP, v = 0.05 V⋅s-1, vs. Ag/AgCl; T = 293 K; dashed line
– 10-3 mol·dm-3 H2salcn(Bu), solid line – 10-3 mol·dm-3 [Ni(salcn(Bu))].
H2salcn in (CH3)2SO undergoes irreversible reduction. In the reverse curve no anodic
peak is observed in either of potential ranges and scan rates. At the scan rate v = 0.05 V·s-1
cathodic peak potential counts for Epc = -1.80 V and this peak is attributed to intramolecular
reductive coupling of the two imine groups in the same molecule [28]. The result of the reaction
is piperazine formation. Shono et al proved such a process for a series of substituted
N,N’-bis(benzylidene)ethylenediamines in DMF [28b], and Zolezzi et al did the same for a group
of similar Schiff-bases in (CH3)2SO [28c]. The process of piperazine formation is connected to
self-protonation reactions, where phenolic hydroxyl groups are donors of protons [28b].
In CH3CN H2salcn is reduced at slightly lower potential value (tab. 4), also irreversibly.
The increased difficulty of ligand reduction probably results from more impeded process of
intramolecular proton transfer in the solvent of lower relative permittivity as compared to
(CH3)2SO. Consequently, reduction of H2salcn is most difficult in CH2Cl2, the solvent of the
weakest dipole character among being under our investigation. In the last solvent peaks are lower,
wider and more shifted towards lower potential values, indicating considerable impediment in
piperazine formation.
Voltammetry for other ligands in reduction range also exhibits irreversible redox
processes. Reverse curves lack of anodic peaks, and cathodic peaks are not well shaped. During
continuous scanning with given scan rate cyclic curves change initially and then their shape do
not alter. Lower Epc values (tab. 4) for ligands with substituted aromatic rings as compared to
H2salcn indicate impediment of redox processes caused by electron donating substituents. This
9
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result depicts ligand reduction efficiency measured by the ability of piperazine formation.
Chemical reaction is verified by removal of proton from phenolic hydroxyl groups, and the
process depends on electron density on oxygen atoms of hydroxyl groups. In the sequence
presented in tab. 4 of ligands substituted in orto- and/or para- positions to phenolic hydroxyl
groups, acidity of hydroxyl groups decreases gradually making reduction of ligand more difficult.
Table 4. Values obtained for the ligands and complexes from reductive study, T = 293 K.
Epc[V]
(v=0.05 V·s)
Ef[V]
106·Dred (CV) 106·Dred(CC) 106·Dred
(
CP)
ligand / complex
H2salcn
solvent
[cm2·s-1]
[cm2·s-1]
[cm2·s-1]
a
CH2Cl2
CH3CN
(CH3)2SO
-1.91
-1.84
-1.80
a
a
a
CH2Cl2
CH2Cl2
CH2Cl2
-1.94
-2.01
-2.05
H2salcn(Bu)
H2salcn(2Bu)
H2salcn(BuOMe)
CH2Cl2
CH3CN
(CH3)2SO
-1.73±0.03
-1.71±0.02
-1.64±0.02
8.02
7.98
3.38
7.65
7.54
3.02
7.83
8.01
3.83
[Ni(salcn)]
-1.75±0.03
-1.80±0.04
-1.82±0.03
8.72
9.65
9.86
8.43
9.93
9.62
8.29
9.64
9.33
CH2Cl2
CH2Cl2
CH2Cl2
[Ni(salcn(Bu)]
[Ni(salcn(2Bu))]
[Ni(salcn(BuOMe))]
a Cyclic voltammograms do not show the corresponding anodic wave.
Complexes
Voltammetric studies on complexes in reduction range were carried out in analogical
conditions. [Ni(salcn)] in (CH3)2SO and CH3CN is reduced in single step and, as showed by
Castro et al [8], the process corresponds to one-electron reduction of central ion in square
complex. The process is reversible in the range of scan rates v: 0.02 ÷ 0.35 V·s-1. Data of
electronic spectra recorded for solution after electrolysis with controlled potential of working
electrode are summarised in tab. 5 and are in agreement with those obtained previously [8].
Furthermore, in voltammetric curves recorded both in (CH3)2SO and CH3CN no peaks connected
with two-electron reduction of imine ligand groups were recorded. The fact indicates that the lack
of protons of hydroxyl group shifts reduction of ligand towards lower potentials. Such a shift of
reduction potential of ligand imine groups was observed also for other complexes, e.g. complex
of salen with Cu(II) [28c].
All the complexes were studied voltammetrically in reduction range in CH2Cl2.
Voltammetric curves (exemplary curve see fig. 1 solid line) of each complex is characterised by
one reduction peak, very well shaped, in comparison to peaks recorded for free ligands, which
proves that these signals do not result from ligand reduction. In case of continuous scanning after
initial changes in shape, the curves overlap each other. The fact that peaks occur at similar
potentials to those of [Ni(salcn)] in (CH3)2SO and CH3CN proves that cathodic process is
connected with reduction of coordinated Ni(II) ion. The analysed couples are reversible in scan
10
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range v from 0.02 to 0.30 V·s-1. At these scan rates the cathodic and anodic potential difference
(ΔE) is comprised in the range observed for Fc+/Fc couple and indicates one-electron process. For
curves recorded in over mentioned range of v, the ratio ipa/ipc is constant and counts for 1 and the
dependence ipc = f(v1/2) is linear, which indicates simple diffusion controlled electrode process. At
scan rates higher than v = 0.30 V·s-1 voltammetric curves gain quasi-reversible character and the
dependence ipc = f(v1/2) deviates from linearity indicating both kinetic and diffusion control of the
process.
In order to analyse the products of cathodic process for each of complexes exhaustive
electrolysis was carried out at the potentials values lower than reduction peak potential
(-1.85÷-1.95 V, depending on the complex) and electronic spectra of solutions before and after
electrolysis were compared. As a result of electrolysis solutions turned green-yellow, with
exception of [Ni(salcn(BuOMe))] which turned orange. The character of electronic spectra after
electrolysis (tab. 5) is analogical to those of [Ni(salcn)] in (CH3)2SO and CH3CN after the same
process and comparable to spectra of similar Ni(I) compounds described in literature [8,16b,29]
indicates reduction of investigated complexes to Ni(I) species, where tetra-coordinated structure
remains unchanged. Transitions d-d are shifted towards longer waves as compared to Ni(II)
complexes (λ9 – tab. 3), which is a result of a weaker ligand field of Ni(I) ion. Higher molar
absorption coefficients than those of Ni(II) complexes (ε9 – tab. 3) indicate greater contribution of
covalent bonding in reduced forms of complexes in interactions between central ion and donor
atoms of ligands [8,16b,29].
Reduction mechanism for each of the complex in applied solvents was also examined based
on one step chronopotentiometric measurements. There were different time pulses (20 ÷60s) and
different currents (-1·10-5 ÷ -1·10-4 A) applied. Sand equation [30] for a process occurring
according to E-mechanism at constant concentration of electroactive substance arises that the
expression iτ1/2 is independent of current. The values of iτ1/2 for each complex, regardless the
solvent, are comparable (they differ maximally of 10% from average value, e.g. -6.14·10-5 A·s for
[Ni(salcn(Bu))]) and indicate simple E-mechanism of the electrode process.
Kinetics of reduction processes was examined by determination of diffusion coefficients
(Dred), a measure of velocity of charge transport towards electrode surface.
Their values were calculated independently from voltammetry, chronopotentiometry and
chronocoulometry measurements. The values Dred from voltammetry were calculated on the basis
of the analysis of voltammetric curves recorded at different sweep rates within the
electrochemical reversibility region. These values were determined from the slope of the
ipc = f(v1/2) plot basing on the Randles-Ševčik equation. The Dred values from
chronopotentiometry were calculated from the curves recorded at different currents using the
Sand equation, knowing of the constant value of the iτ1/2 product.
Double step chronocoulometry was carried out with potential step from -1.5 to maximally -
1.95 V, in dependence on the complex, with different time pulses (0.001 ÷ 60s). Diffusion
coefficients were estimated on the basis of the Cottrell equation from the slops of dependence
Q = f(t1/2), for such pulse times, where linearity was kept, indicating diffusion character of the
process. Average values of Dred from each of over mentioned methods are summarised in tab. 4.
The same magnitude (ca 10-6 cm2⋅s-1) in all applied solvents results from diffusion character of
charge transport to electrode surface. [Ni(salcn)] exhibits the smallest value of Dred in (CH3)2SO,
indicating the most hindered charge transport in solution of greatest viscosity [30]. In CH2Cl2 o-
and p-substituted complexes exhibit slightly lower values of Dred to [Ni(salcn)], suggesting steric
effect of four substituents on hindrance of charge transport towards electrode surface.
11
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Based on voltammetric curves in the range of process reversibility formal potentials (Ef) of
Ni(II)ligand/Ni(I)ligand redox couples were calculated as a measure of reductive properties of
complexes and stabilisation of low valent nickel ion (tab. 4). All the Ef values for
Ni(II)ligand/Ni(I)ligand couples are lower at over (-0.2 V) than those calculated for
Ni(ClO4)2/NiClO4 couple in CH3CN and (CH3)2SO (Ni(ClO4)2 does not dissolve in CH2Cl2).
Such an observation indicates slightly higher stability of Ni(II) complex to corresponding Ni(I)
complex [28c]. Ef values for [Ni(salcn)]/[Ni(salcn)]- couple are varied depending on solvent. The
highest value is observed in (CH3)2SO and in this solvent Ni(I) ion is stabilised to the greatest
extent. Similar relation of E1/2 values in (CH3)2SO and CH3CN was obtained by Castro et al [8].
Higher Ef value in the solvent of highest solvation properties may suggest changes in
coordination structure around central ion, but this suggestion is in contradiction to spectroscopic
studies (tab. 5), which for [Ni(salcn)]- do not differ in CH2Cl2, CH3CN and (CH3)2SO and
indicates undisturbed square symmetry around reduced coordination centre of the complexes. In
CH2Cl2 the highest value of Ef exhibits the complex without any ligand substituents and in this
complex stabilisation of Ni(I) ion is most efficient. Electron donor substituents increase electron
density on central ion in complexes and diminish the share of π-backbonding in metal-to-ligand
interactions, which results in decrease of crystal field splitting, and also in decrease of strength of
metal-to-ligand bonding. This way they impede reduction of coordinated Ni(II) ions and reduce
stabilisation of low valent central ions, which is shown in Ef values presented in tab. 4. The
presence of both tert-butyl and methoxyl substituents in one ligand shifts Ef values to greatest
extend towards more negative values and causes the weakest stabilisation of Ni(I) ion.
Table 5. Absorption spectra of complexes after reduction in 500÷1100 nm region, T = 293 K.
λ (nm) (ε (dm3⋅mol-1⋅cm-1))
complex
solvent
CH2Cl2
CH3CN
806 (1510)
804 (1480)
628sh (4280)
625sh (4310)
626sh (4090)
618sh (4060)
620sh (3840)
623sh (3490)
[Ni(salcn)]
(CH3)2SO 807 (1460)
800 (1370)
[Ni(salcn(Bu))]
[Ni(salcn(2Bu))]
CH2Cl2
CH2Cl2
803 (1520)
805 (1440)
[Ni(salcn(BuOMe))] CH2Cl2
sh = shoulder
3.2.3. Oxidative electrochemical study
Ligands
Voltammetric curves in ligand solutions were recorded in potential range -0.5 ÷ 2.2 V, at
scan rates v from 0.02 to 0.5 V·s-1 and in the range including both oxidation and reduction regions
(2.2 ÷ -2.1 V). Regardless the potential range and scan rate ligands undergo one-step, irreversible
oxidation at anodic peak potential over 1.1 V (tab. 6). With increasing v a shift of potential to
higher potentials and an increase of currents were observed. Upon successive cycling at given v
the curves preserved their character. Phenol and its derivatives in anhydrous medium undergo
two-electron, irreversible oxidation to quinine [31]. The process is preceded by formation of
phenoxyl radicals. Voltammetric curves of two-electron process is characterised by one anodic
peak at over 1 V and a significantly lower cathodic peak [31c]. Reverse curves in the range
12
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-0.5 ÷ 2.2 V do not exhibit any cathodic response, which indicates higher irreversibility of
oxidation of salen derivatives. Typical curve is presented in fig. 2 with dashed line.
Fig. 2. Cyclic voltammogram at Pt in (CH3)2SO + 0.1 M TEAP, v = 0.05 V⋅s-1, vs. Ag/AgCl; T = 293 K; dashed
line – 10-3 mol·dm-3 H2salcn, solid line – 10-3 mol·dm-3 [Ni(salcn)].
Literature data dealing with oxidation of similar compounds show phenoxyl radicals as products
of this reaction [22b]. The peak observed in curves recorded in our studies may indicate oxidation
of phenolic hydroxyl groups in ligand molecules. However, irreversible character of charge
transport makes it impossible to establish number of electrons exchanged in the process. A
variety of Epa values for H2salcn in different solvents, as well as a variety of Epa values for all the
ligands in CH2Cl2, though not significant, indicate an influence of solvent and substituents in
H2salcn on oxidation process. The lowest value of Epa obtained in (CH3)2SO depicts the easiest
electrode process in this solvent. Lower Epa values for substituted ligands towards to the
unsubstituted analogue result from an easier oxidation process with increasing electron density in
orto- and/or para- positions in phenolic rings.
Table 6. Values obtained for the ligands and complexes from oxidative study, T = 293 K.
c
ΔEpa(II-I)
n(I+II)
nIII
Ef (I)
[V]
Ef (II)
[V]
106·Doxi
EpaI
EpaII
EpaIII
[V]a,b
[cm2·s-1]d
ligand / complex
H2salcn
solvent
[V]a
[V]a
[V]a
f
e
e
e
CH2Cl2
CH3CN
1.18
1.15
(CH3)2SO 1.13
f
e
f
e
e
e
CH2Cl2
CH2Cl2
CH2Cl2
1.12
1.09
1.03
H2salcn(Bu)
H2salcn(2Bu)
H2salcn(BuOMe)
g
g
CH2Cl2
CH3CN
(CH3)2SO 0.78
0.97g 1.37g
0.42g
0.17g
1.81g
1.79g
-
-
1
-
-
0.93g 1.10g
[Ni(salcn)]
d
0.74±0.01
3.78
f
0.35
0.54
0.38
-
-
CH2Cl2
CH2Cl2
CH2Cl2
0.91
0.85
0.70
1.26
1.39
1.08
1.82
1.73
1.49
[Ni(salcn(Bu)]
2
2
1.8 0.81±0.01 1.35±0.01
0.66±0.01 1.04±0.01
9.89
9.70
[Ni(salcn(2Bu))]
2
[Ni(salcn(BuOMe))]
a Parameter obtained at v=0.05 V·s-1;
b ΔEpa(II-I) = Epa(II) - Epa(I);
13
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c n(I+II) – total number of electrons exchanged in Ist and in IInd oxidation step;
d average values obtained for one-electron oxidation process based on: cyclic voltammetry,
chronopotentiometry and chronocoulometry;
e single-step process;
f cyclic voltammograms do not show the corresponding cathodic wave;
g complex is adsorbed on electrode surface and/or a value characteristic only for 1st cycle;
Complex [Ni(salcn)] in (CH3)2SO
Voltammetric curves for solutions of complexes were recorded in the same potentials
ranges and the same scan rates. For [Ni(salcn)] dissolved in (CH3)2SO upon continuous scanning
at given scan rate recorded curves retained their character, which suggested absence of any
adsorption on electrode surface. In the range of positive potentials votammograms at each of
applied scan rates (up to v = 0.40 V·s-1) exhibited only one oxidation peak and one reduction
peak, where the potential peak difference was corresponding to that observed for the Fc+/Fc
couple, indicating one-electron reversible process (fig. 2, solid line). Castro et al [25] described
oxidation of [Ni(salcn)] as oxidation to octahedral Ni(III) complex, where axial positions are
occupied by solvent molecules, although the authors observed one or two oxidation peaks,
depending on applied scan rate, which was explained by the presence of two isomers. The result
of our chronopotentiometric measurements, carried out analogically to over described reduction
processes, giving equal iτ1/2 values, proves E electrode mechanism. The magnitude of diffusion
coefficients (tab. 6), obtained by use of three over mentioned methods, indicates diffusion
character of charge transport towards electrode surface. Values of Doxi, analogical to those
presented by Castro et al [25], comparable to Dred (tab. 4) in (CH3)2SO confirm the reason of
lower value of Dred in this solvent, i.e. higher viscosity of (CH3)2SO. Formal potential Ef of
[Ni(III)(salcn)]+/[Ni(II)(salcn)] couple (tab. 6), analogical to E1/2 [25] and much lower in
comparison to Ni(III)/Ni(II) couple, obtained in the same condition for Ni(ClO4)2 solution (using
polarisation curves method [14] because of lack of reversibility of the process,
Ef(Ni(III)/Ni(II)) = 1.04 V), indicates significant stabilisation of Ni(III) ion by salcn and solvent. In
order to compare oxidation results with those obtained for other complexes in CH2Cl2 electrolysis
with controlled potential of working electrode was carried out at potential higher than Ef (0.8 V)
as well as spectroscopic measurement were accomplished (fig. 3).
Fig. 3. Electronic spectra of [Ni(salcn)] in (CH3)2SO before electrolysis - dashed line, after electrolysis – solid
line; left - c = 10-4 mol·dm-3; right - c = 5·10-4 mol·dm-3; T = 293 K.
The solution of the complex after electrolysis changed colour from green-orange-brownish
into more brownish. The spectrum recorded after electrolysis (fig. 3 solid line) exhibits broad
band at 994 nm (580 dm3⋅mol-1⋅cm-1) and a shoulder at 605 (540) on an intense band in visible
range. According to previous research [9,10b,32] octahedral Ni(III) ion complexes with similar
14
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ligands are characterised by very broad band in the range 880 – 1070 nm, although in some
spectra the band is invisible. Next, characteristic and intensive band is to be seen in the range
530 - 750 nm, sometimes only as a shoulder on very intensive CT band occurring at
360 - 420 nm. The value of molar absorption coefficient of the band at 994 nm
(510 dm3⋅mol-1⋅cm-1) (fig. 3 solid line) indicates, that it results from d-d transitions in Ni(III) ion.
The shift of the band in comparison to the bands of d-d transitions in Ni(II) ion (located below
600 nm, tab. 3) into NIR range is connected with lower energy of an octahedral complex to a
square one for d7 ion. Intense absorption in visible range results from permitted LMCT
transitions, connected to a change of coordination symmetry. Other bands on the borderline
UV-VIS are also changed. A band at 320 nm (12960), connected to π→π* transition in the ligand
increased towards to corresponding band in Ni(II) complex (λ4 tab. 3), a band at 342 nm
connected to CT transition in square complex disappeared and a new shoulder appeared at
446 nm (3450), indicating changes in coordination sphere after oxidation to Ni(III) ion complex.
Complexes [Ni(salcn(2Bu))] and [Ni(salcn(BuOMe))] in CH2Cl2
Voltammetric studies in CH2Cl2 solutions brought another results. In potential range
(0 ÷ 1.6 V) [Ni(salcn(2Bu))] undergoes two-step oxidation (fig. 4, dotted line) and the process
does not concern central ion oxidation, but in its Ist step is connected, as shown by Shimazaki and
Stack et al for rac_[Ni(salcn(2Bu))] [32b,33], to one-electron oxidation of phenolate anion of the
ligand to phenoxyl radical.
Fig. 4. Cyclic voltammogram at Pt in CH2Cl2 + 0.1 M TBAP, v = 0.05 V⋅s-1, vs. Ag/AgCl; T = 293 K; dotted line
– 10-3 mol·dm-3 [Ni(salcn(2Bu))], solid and dashed line – 10-3 mol·dm-3 [Ni(salcn(BuOMe))].
The process under investigation is stable, i.e. the curves does not change upon continuous
scanning at given scan rate, which means that the products do not adsorb onto electrode surface.
The process is reversible in scan rate range v: 0.02 ÷ 0.12 V·s-1 both on Ist and IInd step. ΔE values
for each step comprise in rages analogical to those observed for Fc+/Fc redox couple indicating
one-electron processes. At higher scan rates the process is controlled both diffusionally and
kinetically. Based on measurements under diffusional conditions Ef values were calculated for
[Ni(salcn(2Bu))]+/[Ni(salcn(2Bu))] and [Ni(salcn(2Bu))]2+/[Ni(salcn(2Bu))]+ redox couples
[tab. 6, Ef (I) and Ef (II)]. The difference in formal potential of this two oxidation steps counts for
0.54 V (tab. 6 presents differences between IInd and Ist anodic peak potentials at v = 0.05 V·s-1)
and indicates delocalised system after one-electron oxidation [10a,33,34]. Delocalised radical
active centres taking part in IInd oxidation step possess lower electron density in comparison to
15
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undelocalised system, which impedes next oxidation step. So, if the radical were not delocalised,
Ef(II) would be comparable to Ef(I), and on voltammetric curve only one peak of double height
would be observed [26c, 34,35].
Fig. 5. Electronic spectra of: (black__) - [Ni(salcn(BuOMe))]; (black__) - [Ni(salcn(BuOMe))]+; (black_ _) -
[Ni(salcn(BuOMe))]2+; (black....) - [Ni(salcn(BuOMe))]3+; (grey__) - [Ni(salcn(2Bu))]+;
left = c = 10-5 mol·dm-3; right – c = 2·10-4 mol·dm-3; T = 293 K.
Delocalised character of the [Ni(salcn(2Bu))]+ radical is also indicated by an electronic spectrum
of the solution recorded after electrolysis at potential E > Ef(I) (0.9 V). The solution turned
willow-green. Electronic spectrum in NIR range, as was previously proved by Shimazaki et al
[32b, 33b] and Stack et al [33a], is characterised by bands at 1094 nm (7120) and 896sh (2860)
(fig. 5, right, grey__), connected to phenoxyl radicals [36]. These bands indicate IVCT ligand –
ligand transitions [34] and entirely delocalised system - Class III in Robin-Day classification
system [37]. The most important feature of entirely delocalised radical system is an intense,
narrow band at ca. 2000 nm [34, 32b, 33b]. Spectra presented in literature [10,33,26c] indicate,
that the Robin-Day Class III system, apart from this band, always shows also a narrow band at ca.
1000 nm of lower intensity, which enables an analysis of radical coupling based on spectra
recorded for wavelength up to 1100 nm. Another feature of electronic spectra of phenoxyl
radicals is absorption in the range 300 - 450 nm connected to π→π* transitions in phenoxyl
radicals [10b,26c,38]. Electronic spectrum of [Ni(salcn(2Bu))] recorded after electrolysis at
E > Ef(I) (0.9 V) is characterised in this range by following bands: 385(9350), 399sh(8360),
450(4580), analogical to reported previously [33]. On the spectra recorded during the electrolysis
the disappearance of CT and d-d electron transition bands from square planar Ni(II) ion complex
and increase of characteristic bands of [Ni(salcn(2Bu)]+ was observed. Such behaviour was
noticed after ~ 40% pass of the electrolysis. [Ni(salcn(2Bu))]+ radicals are stable in CH2Cl2
solution, after a few hours the electronic spectrum preserves its character. The stability of radicals
is also indicated by independence of the Ist couple from the IInd one observed in voltammetric
measurements. Voltammetric curves recorded in potential range limited to the first step at
different scan rates exhibited one reversible redox couple with Ef value equal to Ef(I).
Voltammetric curves recorded for [Ni(salcn(BuOMe))] (fig. 4, solid line) in potential
range (0 ÷ 1.4 V) (analysed towards Fc+/Fc couple) exhibit also two reversible one-electron redox
couples, but here the difference between their formal potentials for oxidation process (tab. 6,
ΔEpa(II-I) = 0.38 V at v = 0.05 V·s-1) is lower than it was observed for [Ni(salcn(2Bu))], which
indicates lower delocalisation in case of [Ni(salcn(BuOMe))]+ than in [Ni(salcn(2Bu))]+. The
[Ni(salcn(BuOMe))]+ radical is also stable and may exist independently on IInd oxidation step.
Voltammograms recorded in potential range limited to Ist step are similar to those comprising
both oxidation steps.
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Lower delocalisation degree of monoradical is also indicated by electronic spectra of
[Ni(salcn(BuOMe))]+ solution obtained after electrolysis at E > Ef(I) (0.7 V), which, in NIR range
(fig. 5, right, black__), exhibits bands: 980(3040), 838sh(2760), shifted towards higher energies
as compared to [Ni(salcn(2Bu))]+ spectrum. Similar differences in VIS NIR spectra of
compounds after one-electron oxidation, resulting from varied degree of radical delocalisation
was observed for Ni(II) and Pd(II) complexes [10a, 33b]. The spectrum in the range of
characteristic π→π* transitions in phenoxyl radicals [10b,26c, 38] exhibits bands at 328(18310),
401sh(5750) and 424(6830) (fig. 5, black__). On the spectra recorded at the first stage of
electrolysis process (0-30%) the decrease and moving toward lower values of wavelength of the
initial complex were observed. Such behaviour is connected with CT and d-d transitions in Ni(II)
ion (fig. 6, grey lines, range 300-500 nm) what indicate the proceeding concentration decrease of
unoxidized complex. On the successive spectra within this region (50-100% of the process)
(fig. 6, black lines, range 300-500 nm) there are not any changes in location of the bands
observed, but their characteristic is changed what suggest the presence of phenoxyl radicals
(401sh and 424 nm, π→π* transitions of phenoxyl radicals). Simultaneously the disappearance of
typical bands characteristic for the initial complex (439 and 465sh) was noticed. The increase in
absorption bands originated from π→π* transitions of phenoxyl radicals indicates the increase of
its concentration during the electrolysis process, however the differences between the absorbation
values of consecutive spectra are relatively small in comparison with the ones observed within
the NIR region. Such nature of changes in the 300-500 nm range, may be connected with the very
close location of two characteristic bands originated from different species.
After 10% of electrolysis process, on NIR spectra (fig. 6, second grey line, range 700-1100 nm)
there are clear ligand-ligand IVCT bands observed. This confirm the existence of phenoxyl
radicals at the first stage of the electrolysis process. The significant increase in absorption of
these bands (fig. 6, grey and black lines, range 700-1100 nm) reflects the increase in
concentration of phenoxyl radicals during the electrolysis. The absence of other bands shows, that
at E > Ef(I) there are no other forms of complexes in the oxidation process.
The solution after electrolysis turned yellow. Lower degree of delocalisation shown in
voltammetric measurements as well as the character of analysed spectra may indicate mixed
Class III/II system in Robin-Day classification [37,39]. In case of compounds of Class I, ions
interact with each other preserving their individual characteristics, those of Class III are entirely
delocalised, and those of Class II are crystallographically distinct, but exhibit significant
similarity of chemical properties, which is to be seen in spectra in broad absorption band of low
energy in near infrared connected to IVCT transition.
Fig. 6. Electronic spectra of [Ni(salcn(BuOMe))] during electrolysis at E > Ef(I): grey – 0%, 10%, 20%, 30%,
40% electrolysis; black – 50%, 60%, 70%, 80% electrolysis, c = 2·10-4 mol·dm-3; T = 293 K.
17
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EpaI values presented in tab. 6 indicate, that the complex oxidation potential of first anodic
peak corresponding to ligand oxidation yielding mono-phenoxyl radical is lower than potential of
oxidation of free ligand, which means, that oxidation occurs easier for the complex than free
ligand. Such a dependence was also shown for phenol derivatives and correspondent free
phenolate anions [31c]. Taking into consideration the fact that the ligands after oxidation undergo
a delocalisation, which is indicated by one oxidation peak, while their complexes do not undergo
the process, the considerably lower value of EpaI for complexes than that of free ligand results
probably from significant stabilisation of the radical by metal ion, which consequently must take
part in delocalisation. Previous works [10b,26c,40] proved the contribution of central ion in
delocalisation in the whole system after one-electron oxidation of similar complexes.
Canonical forms of phenoxyl radicals show higher electron density in o- i p- positions, so
one can suppose that oxidation of [Ni(salcn(2Bu))] and [Ni(salcn(BuOMe))] may occur in both
positions. The result of the process depends on substituents. Lower Ef value for
[Ni(salcn(BuOMe))] to this for [Ni(salcn(2Bu))] (tab. 6) indicates a distinct influence of more
electron donating substituent on facilitation of phenolate anion oxidation. Also, the research
shows that the substituents influence radical delocalisation process. The [Ni(salcn(2Bu))]+ radical
is more delocalised than [Ni(salcn(BuOMe))]+ and it can result from stronger stabilising effect,
caused by steric effect of four bulky substituents as well as by higher symmetry of the whole
system.
Chronopotentiometric analysis of electrode mechanisms of Ist step of oxidation processes
showed simple E processes.
Based on kinetic studies of this processes by use of cyclic voltammetry, chronocoulometry
and chronopotentiometry, Doxi values were determined. Values obtained with these independent
methods were coincidental and tab. 6 presents average values. The magnitude of this values as
well the fact that Doxi values for both complexes do not differ significantly, which are expected
for diffusion process in structurally and energetically similar systems, correspond to such a
charge transport.
Analysis of products of [Ni(salcn(2Bu))] and [Ni(salcn(BuOMe))] oxidation at E > Ef(II)
was carried out as described above based on electronic spectra made for solution obtained as
results of electrolysis at potentials 1.4 and 1.1 V, respectively. The solutions turned dark yellow
and bright green, respectively. [Ni(salcn(BuOMe))]2+ electronic spectrum is characterised by
disappearance of ligand – ligand IVCT bands in near infrared and double increase of bands in
π→π* transition range in phenoxyl radicals (400sh(11040), 420(12880)) (fig. 5, right, black_ _, )
as compared to corresponding bands for [Ni(salcn(BuOMe))]+. The loss of bands in NIR
indicates lack of intervalence ligand – ligand transitions. In this mono-phenoxyl radical complex,
that are not influenced by other factors, the reason of absence of IVCT transitions must be lack of
differences in aromatic groups. Further, higher bands in π→π* transition range in phenoxyl
radicals indicate the presence of greater number of radicals. The facts that the intensity of π→π*
bands in phenoxyl radicals increased twice and the bands in NIR disappeared indicate a presence
of bis-phenoxyl radical complex. Analogical spectra were reported in literature for similar
bis-phenoxyl radical species [26c,41].
In [Ni(salcn(2Bu))]2+ spectrum also one can observe changes indicating oxidation at
E = Ef(II) of second phenolate anion to radical yielding bis-phenoxyl radical complex.
18
Page 18 of 27

Fig. 7. Electronic spectra of [Ni(salcn(BuOMe))] during electrolysis at E > Ef(II): 0%, 50%, 80%, 100%
electrolysis, c = 2·10-4 mol·dm-3; T = 293 K.
The spectra recorded during the electrolysis process in case of the both investigated complexes
(fig.7), show successive disappearance of ligand-ligand IVCT bands within NIR region and the
increase in the π→π* phenoxyl radicals transition bands. There were no other bands observed,
what allows to exclude the existence of other forms of complexes during oxidation process at
E > Ef(II).
Voltammetric curves recorded for [Ni(salcn(BuOMe))] complex (fig. 4, dashed line) in
potential range (0 ÷ 1.8 V) exhibit next, IIIrd, oxidation peak at potential 1.495 V (v = 0.05 V·s-1).
The process is irreversible and, as a result, for two first steps the ratio ipc/ipa is lower than unity.
Electronic spectrum of solution of [Ni(salcn(BuOMe))] oxidised at E higher than IIIrd peak
(1.6 V) changed its character as compared to spectra recorded after oxidation at lower potentials
and possesses bands at: 415 (4960), 308sh (14880), 258 (39530) (fig. 5, black....). In space of two
overlapping bands at 400sh and 420 nm (fig. 5, right, black_ _, ) resulting from π→π* transitions
in bis-phenoxyl radical species one can see only one band of lower intensity at 415 nm, and the
band at 328 nm shifted towards shorter wavelengths. On the spectra recorded during the
experiment (fig. 8), the decrease of the absorption band at 400sh and 420 nm was observed up to
40 % progress of the process. This fact indicate that the concentration of bis-phenoxyl radical
decreases. After 80% pass of the electrolysis there are only one clear band at 415 nm. During the
whole electrolysis process at this potential, there were no other changes observed (fig.8).
Fig. 8. Electronic spectra of [Ni(salcn(BuOMe))] during electrolysis at E > Ef(III): 0%, 10%, 40%, 60%, 80%,
100% electrolysis, c = 2·10-4 mol·dm-3; T = 293 K.
Reports dealing with oxidation of derivatives of phenol and phenolate anions [38a,42]
prove one-electron oxidation of this compounds to phenoxyl radicals, which may undergo
dimerization or, at higher potentials, they are oxidised to very short-lived phenoxonium cations.
19
Page 19 of 27

Consequently, oxidation process of phenoxyl radicals to phenoxonium cations is irreversible and
the peak on reverse curve rarely is observed [43]. Being very short-lived species phenoxonium
cations may undergo further reaction yielding, in dependence on substituents, benzoquinone
derivatives (if the substituent was OH group in orto- or para- position) or benzoquinone methide
derivatives (if the substituent was alkyl group with at least one ionisable hydrogen), or they
undergo a reaction with nucleophile. Unusual stability of phenoxonium cations originating from
natural compounds in CH3CN and CH2Cl2 solutions were observed by Webster et al
[38b,42c,43,44]. These and authors [38a,b,43,44a,45] indicated very similar differences in
electronic spectra of phenoxyl radical and phenoxonium cation to those presented in
fig. 5 (black__; _ _; ....), which, taking into consideration the lack of following reaction because
of conditions of the experiment as well as the presence of substituents in orto- i para- positions
without ionisable hydrogen, allows to assume that oxidation of bis-phenoxyl radical complex in
IIIrd step yields phenoxonium cation complex. Oxidation of bis-phenoxyl radical is a decisively
irreversible process (ΔE ~1 V), but the presence of a peak on reverse voltammetric curve
indicates significant stability of phenoxonium cation in Ni(II) complex, all the more so as the
peak was also observed at low scan rates. The solution after electrolysis turned bright green and
was stable during spectroscopic measurements, which confirms that phenoxonium cation
complex with Ni(II) is long-lived. Such properties result from steric effect of bulky substituents
in orto- i para- positions [38a]. The lack of reversibility of the process makes its analysis more
difficult, but the ratio of the area under the peak of IIIrd step to that under a peak of Ist lub IInd
step, counted for curves recorded at scan rates that assure reversibility of Ist and IInd step, counts
for 2.0 ± 0.15, which indicates two-electron oxidation of [Ni(salcn(BuOMe))]2+ in IIIrd step and
suggests a presence of bis-phenoxonium cation complex. Quantitative analysis of electronic
spectra for this oxidation step did not bring any conclusive results because comparison of spectra
of phenoxonium cation and bis-phenoxonium cation complex was impossible.
Analogical voltammetric curves in the potential range to 2.2 V were recorded for
[Ni(salcn(2Bu))]. However, in case of this complex a determination of number of electrons
exchanged in IIIrd step is encumbered with an error because the peak (tab. 6 ) is placed near the
borderline of supporting electrolyte decomposition. The value was estimated at 1.8, and, in spite
of its inaccuracy, it indicates with great probability that the process is connected with both
aromatic groups in the complex.
Also, the electronic spectrum after oxidation of [Ni(salcn(2Bu))] at potential higher than IIIrd
oxidation peak, i.e. 1.9 V, indicates the presence of phenoxonium cation complex in solution.
After this process the solution turned green of lower intensity than that obtained as a result of
oxidation after IInd anodic step. The spectra taken during the electrolysis process showed similar
changes as it was noticed for spectra for the same process for [Ni(salcn(BuOMe))] at potential
value after III oxidation peak.
Complexes [Ni(salcn)] and [Ni(salcn(Bu)] in CH2Cl2 and CH3CN
Voltammetric studies of [Ni(salcn))] and [Ni(salcn(Bu))] in oxidation range gave another
result.
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Fig. 9. Cyclic voltammogram at Pt 10-3 mol·dm-3 [Ni(salcn(Bu))] in CH2Cl2, v = 0.05 V⋅s-1, μ = 0.1 M TBAP, vs.
Ag/AgCl; T = 293 K
Voltammetric curve of [Ni(salcn(Bu))] in CH2Cl2 (fig. 9) in potential range where three-
step oxidation of [Ni(salcn(2Bu))] and [Ni(salcn(BuOMe))] occurs, exhibits in first scan, apart
from two oxidation peaks at: EpaI = 0.91, EpaII = 1.26, a shoulder placed in between at potential
1.02 V. The IIIrd step peak appears at EpaIII = 1.82 V and is also preceded by a shoulder. In
reverse curve two reduction peaks are to be seen, at potentials: 0.89 and 0.55 V and a shoulder
between them at E = 0.82. All next scans differ from the first one and the lack of repeatability in
following scans is observed for each of applied scan rates. The working electrode already after
first scan was covered with yellow-green layer, that did not dissolve during cathodic process and
in following polarisation scans, which excludes monomeric character of the film [6a]. As phenol,
phenolate anions and their derivatives with unoccupied orto- and/or para- positions tend to
dimerise [38a,42a,b,] and taking into account previous reports dealing with polymerisation of
Ni(II) complexes with salen and its dimethyl derivative [6,46], one can suppose that the oxidation
product is polymeric. Such a product of electrooxidation may justify the changes in following
polymerisation scans. Curves recorded in the range limited to Ist oxidation step including the
shoulder (to 1.2 V) also were not repetitive which, as showed visual inspection of the electrode
surface, was connected with film deposition. Consequently, current responses in further process
may originate from reagents in solution being in contact with platinum electrode and/or modified
electrode, depending on the film formed, i.e. whether it is electroactive and whether its structure
enables physical transport of species to electrode surface, or they may as well originate from
deposited film itself if it is electroactive. In this respect one can conclude that only the Ist peak in
first scan is not connected with the process occurring on the surface of modified electrode but
results from oxidation of a complex on platinum electrode surface. Considering the result of
oxidation of complexes [Ni(salcn(2Bu))] and [Ni(salcn(BuOMe))], similar values of anodic
potential to that of Ist anodic peak for [Ni(salcn(Bu))], as well as great structural similarity of
complexes described in this paper, Ist step of oxidation of [Ni(salcn(Bu))] may be connected with
its oxidation to phenoxyl radical. Then, dimerisation of radicals may occur resulting in formation
neutral molecules, as well as reaction of radicals with next complex molecule [38a] and
subsequently following coupling steps. As there is possibility of reaction of monomer on
different conducting phases and film oxidation, which result in misshapen peaks in voltammetric
curve, an assignment of two following peaks to particular oxidation steps, as it was done for
[Ni(salcn(2Bu))] and [Ni(salcn(BuOMe))], may not justified. However, similar potential values
may suggest further oxidation of phenoxyl radicals, which did not undergo coupling process
according to the scheme presented for [Ni(salcn(2Bu))] and [Ni(salcn(BuOMe))].
In voltammetric curves for [Ni(salcn)] in CH2Cl2 and CH3CN solutions, recorded in the
whole potential range, in first scan the amount of main anodic peaks is analogical to
21
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[Ni(salcn(Bu))]. Lack of repeatability of recorded scans whatever scan rate was applied and
presence of deposit on working electrode surface, similarly to the effect observed for
[Ni(salcn(Bu))], indicates electropolymerisation process. Voltammetric curves differ from each
other both in current and peak potential values. Analysis of polymeric films is a subject of our
present studies, but based on hitherto existing results from Ist step anodic peak potential values
(tab. 6) one can conclude that [Ni(salcn)] is the species that is the easiest to oxidise in CH3CN
solution. The fact results from higher polarity of the solvent, which interacts stronger with radical
than CH2Cl2 and this way brings better stabilisation. In CH2Cl2 solutions the oxidation process is
easiest for [Ni(salcn(Bu))] and the process occurs at even lower potential than oxidation of
[Ni(salcn)] in CH3CN, which indicates significant influence of electron donating substituent on
removal of electron from phenolate moiety.
4. Conclusion
Ni(II) ion complexes with (±)-trans-N,N’-bis(salicylidene)-1,2-cyclohexanediamine
substituted in orto- and/or para- positions of phenolate anions tert-butyl and/or methoxy groups
were synthesised, to study an influence of substituents and solution on symmetry of the
complexes and on electrode processes. Composition of ligands and complexes was determined
based on spectroscopic measurements and elemental analysis. Ligands undergo irreversible
electrochemical reaction. They are reduced to piperazine and the process is easiest in case of
H2(salcn) dissolved in (CH3)2SO, whereas oxidation is easiest for H2(salcn(BuOMe)) in CH2Cl2,
H2(salcn) the highest value of Ef is observed in (CH3)2SO. Spectroscopic measurements of solid
complexes and their solutions in (CH3)2SO, CH3CN and CH2Cl2 proved insignificant influence of
the medium and substituents on deformation of square symmetry of synthesised complexes.
Electrochemical and UV VIS studies indicated lack of influence of solvent on reduction process.
Complexes are reduced to Ni(I) complexes according to E mechanism and afterwards they retain
their symmetry. Among complexes under investigation, best stabilisation is observed in
[Ni(salcn)]— and this complex is most stable in (CH3)2SO. Kinetic parameters represented by D
showed an influence of bulky substituents on charge transport towards electrode surfaces as well
as limitation of the transport in (CH3)2SO, a solvent of greatest viscosity among those under
investigation. Both substituents and solvent influence electrode oxidation processes. One-electron
oxidation of metal ion occurs only in the solvent of the strongest solvation properties. In CH3CN
and CH2Cl2 oxidation is related to coordinated and the process is influenced by the presence and
positions of substituents of phenolate anions. Complexes possessing substituents in orto- and
para- positions are oxidised in three steps to products in succession: mono-phenoxyl radicals,
bis-phenoxyl radicals and bis-phenoxonium cations. Two first oxidation steps occur reversibly
according E mechanism, in the easiest way in case of [Ni(salcn(BuOMe))]. Mono-phenoxyl
radicals undergo delocalisation process, to the highest degree it occurs for [Ni(salcn(2Bu))]+.
Oxidation to bis-phenoxonium cations is irreversible. Kinetic studies in this group of complexes
did not show any influence of substituents on charge transport towards electrode surface. In case
of complexes with substituents in para- and/or orto- positions also ligand is oxidised, where after
first anodic step resulting mono-phenoxyl radicals undergo dimerisation and further coupling
with modification of electrode surface with polymeric film. The process occurs in easiest way in
solution of [Ni(salcn(Bu))].
References
22
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[1]
(a) K.C. Gupta, A.K. Sutar, Catalytic activities of Schiff base transition metal complexes, Coord. Chem.
Rev. 252 (2008) 1420.
(b) J.P. Collin, J.P. Sauvage, Electrochemical reduction of carbon dioxide mediated by molecular catalysts,
Coord. Chem. Rev. 93 (1989) 245.
(c) H. Yoon, T.R. Wagler, K.J. O'Connor, C.J. Burrows, High turnover rates in pH-dependent alkene
epoxidation using NaOCl and square-planar nickel(II) catalysts, J. Am. Chem. Soc. 112 (1990) 4568.
(d) C. Amatore, A. Jutand, Activation of carbon dioxide by electron transfer and transition metals.
Mechanism of nickel-catalyzed electrocarboxylation of aromatic halides, J. Am. Chem. Soc. 113 (1991)
2819.
(e) E. Szłyk, A. Surdykowski, M. Barwiolek, E. Larsen, Spectroscopy and stereochemistry of the optically
active copper(II), cobalt(II) and nickel(II) complexes with Schiff bases N,N’-(1R,2R)- (-)-1,2-
cyclohexylenebis(3-methylbenzylideneiminato) and N,N’- (1R,2R)- (-)-1,2-cyclohexylenebis(5-
methylbenzylideneiminato), Polyhedron 21 (2002) 2711.
(f) E. Szłyk, M. Barwiołek, R. Kruszynski, T.J. Bartczak, Synthesis and spectroscopic studies of the
optically activecopper(II), cobalt(II) and nickel(II) complexes with Schiff bases N,N’-(1R,2R)(-)-1,2-
cyclohexylenebis(3-methoxybenzylideneiminato), N,N’-(1R,2R)(-)-1,2-cyclohexylenebis (5-
methoxybenzylideneiminato) and X-ray diffraction structure ofthe [Cu(II)(1R,2R)(-)chxnbis(5-
methylbenzylideneiminato)2], Inorganica Chimica Acta 358 (2005) 3642.
(g) M. Barwiolek, E. Szlyk, A. Surdykowski, A. Wojtczak, New nickel(II) and copper(II) complexes with
unsymmetrical Schiff bases derived from (1R,2R)(−)cyclohexanediamine and the application of Cu(II)
complexes for hybrid thin layers deposition, Dalton Trans. 42 (2013) 11476.
[2]
[3]
(a) K.P. Healy, D. Pletcher, The chemistry of electrogenerated transition metals species - the insertion of
olefins into a nickel(I) carbon bond, J. Organomet. Chem. 161 (1978) 109.
(b) P. Vanalabhpatana, D.G. Peters, Stoichiometric reduction of secondary alkyl monohalides by
electrogenerated nickel(I) salen in the presence of oxygen and water: Prospects for the formation of ketones,
J. Electroanal. Chem. 593 (2006) 34.
(c) M.P. Foley, P. Du, K.J. Griffith, J.A. Karty, M.S. Mubarak, K. Raghavachari, D.G. Peters,
Electrochemistry of substituted salen complexes of nickel(II): Nickel(I)-catalyzed reduction of alkyl and
acetylenic halides, J. Electroanal. Chem. 647 (2010) 194.
(a) M.D. Hobday, T.D. Smith, N,N'-ethylenebis(salicylideneiminato) transition metal ion chelates, Coord.
Chem. Rev. 9 (1972) 311.
(b) L. Canali, D.C. Sherrington, Utilisation of homogeneous and supported chiral metal(salen) complexes in
asymmetric catalysis, Chem. Soc. Rev. 28 (1999) 85.
(c) R. Irie, K. Noda, Y. Ito, N. Matsumoto, T. Katsuki, Catalytic asymmetric epoxidation of
unfunctionalized olefins, Tetrahedron Lett. 31 (1990) 7345.
(d) D. Chatterjee, A. Mitra, Olefin epoxidation catalysed by Schiff-base complexes of Mn and Ni in
heterogenised-homogeneous systems, J. Mol. Cat. A: Chem. 144 (1999) 363.
(e) R.I. Kureshy, N.H. Khan, S.H.R. Abdi, S.T. Patel, P. Iyer, E. Suresh, P. Dastidar, Chiral Ni(II) Schiff
base complex-catalysed enantioselective epoxidation of prochiral non-functioalised alkanes, J. Mol. Cat. A:
Chem. 160 (2000) 217.
[4]
[5]
(a) S. P. Flanagan, P.J. Guiry, Substituent electronic effects in chiral ligands for asymmetric catalysis, J.
Organomet. Chem. 691 (2006) 2125.
(b) T. Achard, Y.N. Belokon, J.A. Fuentes, M. North, T. Parsons, Influence of aromatic substituents on
metal(II)salen catalysed, asymmetric synthesis of α-amino acids, Tetrahedron 60 (2004) 5919.
(a) C.E. Dahm, D.G. Peters, Catalytic reduction of α, ω-dihaloalkanes with nickel(I) salen as a
homogeneous-phase and polymer-bound mediator, J. Electroanal. Chem. Interfacial Electrochem. 406
(1996) 119.
(b) N.T.S. Phan, D.H. Brown, P. Styring, A faciale method for catalyst immobilisation on silica: nickel-
catalysed Kumada reactions in mini-continuous flow and batch reactors, Green Chem. 6 (2004) 526.
(a) M. Vilas-Boas, C. Freire, B. de Castro, A.R. Hillman, Electrochemical Characterization of a novel salen-
type modified electrode, J. Phys. Chem. B 102 (1998) 8533.
[6]
(b) C.E. Dahm, D.G. Peters, J. Simonet, Electrochemical and spectroscopic characterization of anodically
formed nickel salen polymer films on glassy carbon, platinum, and optically transparent tin oxide electrodes
in acetonitrile containing tetramethylammonium tetrafluoroborate, J. Electroanal. Chem. 410 (1996) 163.
23
Page 23 of 27

[7]
(a) P.H. Aubert, A. Neudeck, L. Dunsch, P. Audebert, P. Capdevielle, M. Maumy, Electrochemical
synthesis and structural studies of copolymers based on the electrooxidation of pyrrole and some salen
compounds, J. Electroanal. Chem. 470 (1999) 77.
(b) J. Tedim, A. Carneiro, R. Bessada, S. Patrίcio, A.L. Magalhães, C. Freire, S.J. Gurman, A.R. Hillman,
Correlating structure and ion recognition properties of [Ni(salen)]-based polymer films, J. Electroanal.
Chem. 610 (2007) 46.
(c) M.F.S. Teixeira, T.R.L. Dadamos, An electrochemical sensor for dipyrone determination based on
nickel-salen film modified electrode, Procedia Chem. 1 (2009) 297.
(d) M. Vilas-Boas, E.M. Pereira, C. Freire, A.R. Hillman, oxidation of ferrocene derivatives at a
poly[Ni(saltMe)] modified electrode, J. Electroanal. Chem. 47 (2002) 538.
[8]
[9]
F. Azevedo, C. Freire, B. de Castro, Reductive electrochemical study of Ni(II) complexes with N2O2 Schiff
base complexes and spectroscopic characterisation of the reduced species. Reactivity towards CO,
Polyhedron 21 (2002) 1695.
(a) C. Freire, B. de Castro, EPR characterisation of nickel(II) complexes with N2O2 Schiff base ligands
derived from naphthaldehyde and their pyridine adducts, Polyhedron 17 (1998) 4227.
(b) B. de Castro, C. Freire, EPR and electrochemical study of nickel(III) complexes of bis(3,5-
dichlorosalicylaldehyde)-o-phenylenediimine. Evidence for adduct formation with pyridines, Inorg. Chem.
29 (1990) 5113.
[10]
(a) Y. Shimazaki, N. Arai, T.J. Dunn, T. Yajima, F. Tani, C.F. Ramogida, T. Storr, Influence of chelate
effect on the electronic structure of one-electron oxidized group 10 metal(II)-(disalicylidene)diamine
complexes, Dalton Trans. 40 (2011) 2469.
(b) O. Rotthaus, O. Jarjayes, F. Thomas, C. Philouze, C.P.D. Valle, E. Saint-Aman, J.-L. Pierre, Fine tuning
of the oxidation locus, and electron transfer, in nickel complex of pro-radical ligands, Chem. Eur. J. 12
(2006) 2293.
[11]
[12]
S. Özalp-Yaman, V.T. Kasumov, A.M. Önal, Electrochemistry of nickel(II) complexes with N,N’-bis(3,5-
di-tert-butylsalicylidene)polymethylenediamines, Polyhedron 24 (2005) 1821.
(a) G. Diekert, B. Klee, R.K. Thauer, Nickel, a component of factor F 430 from Methanobacterium
thermoautotrophicum , Arch. Microbiol. 124 (1980) 103-106.
(b) A. Pfaltz, B. Jaun, A. Fassler, A. Eschenmoser, R. Jaenchen, H.H. Gilles, G. Diekert, R.K. Thauer, Zur
Kenntnis des Faktors F430 aus methanogenen Bakterien: Struktur des porphinoiden Ligandsystems , Helv.
Chim. Acta 65 (1982) 828.
(c) S.P.J. Albracht, E.-G. Graf, R.K. Thauer, The EPR properties of nickel in hydrogenase from
Methanobacterium thermoautotrophicum, FEBS Lett. 140 (1982) 311.
[13]
(a) J.F. Larrow, E.N. Jacobsen, Y. Gao, Y. Hong, X. Nie, C.M. Zepp, A practical method for the large-scale
preparation of [N,N'-Bis(3,5-di-tertbutylsalicylidene)-1,2-cyclohexanediaminato(2-)]manganese(III)
chloride, a highly enantioselective epoxidation Catalyst, Org. Chem. 59 (1994) 1939.
(b) G. Casiraghi, G. Casnati, G. Puglia, G. Sartori, G. Terenghi, Selective reactions between phenols and
formaldehyde. A novel route to salicylaldehydes, J. Chem. Soc., Perkin Transactions 1, 9 (1980) 1862.
(c) R.H. Bailes, M. Calvin, The Oxygen-carrying synthetic chelate compounds. VII. Preparation, J. Am.
Chem. Soc. 69 (1947) 1886.
(d) E.T.G Cavalheiro, F.C.D. Lemos, J.Z. Schpector, E.R. Dockal, The thermal behaviour of nickel, copper
and zinc complexes with the Schiff basas cis- and trans-N,N’-bis(salicylidene)-1,2-ciclohexadiamine
(Salcn), Thermochimica Acta, 370 (2001) 129.
[14]
[15]
L. Nowak, G. Andrijewski, D. Tomczyk, N. Cichomski, Modification of glassy carbon electrode with
phenanthroline complexes of manganese(II) immobilized in Nafion layer, Pol. J. Chem. 81 (2007) 493.
(a) H.H. Freedman, Intramolecular H-Bonds. I. A spectroscopic study of the hydrogen bond between
hydroxyl and nitrogen, J. Am. Chem. Soc. 83 (1961) 2900 2905.
(b) O. Signorini, E.R. Dockal, G. Castellano, G. Oliva, Synthesis and characterization of aquo[N,N′-
ethylenebis(3-ethoxysalicylideneaminato)]dioxouranium(VI), Polyhedron 15 (1996) 245 -255.
(c) J.A. Faniran, K.S. Patel, J.C. Bailar Jr., Infrared spectra of N,N′-bis(salicylidene)-1,1-
(dimethyl)ethylenediamine and its metal complexes, J. Inorg. Nucl. Chem. 36 (1974) 1547.
(a) E. Szłyk, A. Wojtczak, E. Larsen, A. Surdykowski, J. Neumann, An optically active nickel(II) Schiff
base coordination compound N,N’-(1R,2R)-(-)-1,2-cyclohexylenebis (2-
[16]
hydroxyacetophenonylideneiminato)nickel(II), Inorg. Chim. Acta 293 (1999) 239.
(b) B. de Castro, C. Freire, M.T. Duarte, M.F. M. da Piedade, I.C. Santos, trans-[N,N’-
Bis(salicylidene)cyclohexane-1,2-diaminato]nickel(II)-chloroform (1/1), Acta Cryst. C57 (2001) 370.
24
Page 24 of 27

(c) F. Azevedo, C. Freire, B. de Castro, M. Convery, D. Domingues, M.T. Duarte, K. Nielsen, I.C. Santos,
Electrochemical and structural studies of nickel(II) complexes with N2O2 Schiff base ligands 2. Crystal and
molecular structure of N,N′-l,2-ethane-1,2-diyl-bis(2- hydroxyacetophenonylideneiminate)nickel(II), N, N′-
1,2-cis cyclohexane-1,2-diyl-bis(2-hydroxyacetophenonylideneiminate)- nickel(II) and N,N′-1,2-benzene-
1,2-diyl-bis(3,5-dichlorosalicylideneiminate)nickel(II), Inorg. Chim. Acta, 219 (1994) 43.
(a) A. Seminara, S. Giuffrida, A. Musumeci, I. Fragalá, Polynuclear complexes of lanthanides with nickel
and copper schiff bases as ligands, Inorg. Chim. Acta 95 (1984) 201.
(b) Chun-Wah Ho, Wing-Chi Cheng, Ming-Chuan Cheng, Shie-Ming Peng, Kin-Fai Cheng and Chi-Ming
Che, Preparation and reactivities of chiral manganese(III) and copper(II) complexes of binaphthyl Schiff
bases, J. Chem. Soc., Dalton Trans. (1996) 405.
[17]
[18]
(a) J. R. Zamian, E.R. Dockal, Tetradentate Schiff base oxovanadium(IV) complexes, Trans. Met. Chem. 21
(1996) 370.
(b) G.A. Kolawole, K.S. Patel, The stereochemistry and mass spectra of oxovanadium(IV) complexes of
N,N′-bis(3-etoxysalicylidene) trimethylenediimine and N,N′-bis(7-methylsalicylidene) trimethylenediimine,
J. Coord. Chem. 14 (1986) 235.
[19]
[20]
(a) G.C. Percy, D.A. Thornton, Infrared spectra of N-aryl salicylaldimine complexes substituted in both aryl
rings, J. Inorg. Nucl. Chem. 35 (1973) 2319.
(b) X. Wang, X.M. Zhang, H.X. Liu, Synthesis properties and structure of vanadium(IV) schiff base
complex (VO)[Salphen] CH3CN, Polyhedron 14 (1995) 293.
(a) G. Condorelli, I. Fragalá, S. Giuffrida, A. Cassol, Lanthanide compounds with Schiff bases and with
copper(II) Schiff base complexes as ligands, Z. Anorg. Allg. Chem. 412 (1975) 251.
(b) F.A. Bottino, P. Finocchiaro, E. Libertini, Synthesis and characterization of schff base complexes with
zinc halides, J. Coord. Chem. 16 (1988) 341.
[21]
[22]
R.M. Sliverstein, G.C. Bassler, T.C. Morril, Spectrometric identification of organic compounds, 5th Ed.,
John & Son, New York, 1991, p. 289
(a) S. Zolezzi, A. Decinti, E. Spodine, Syntheses and characterization of copper(II) complexes with Schiff-
base ligands derived from ethylenediamine, diphenylethylenediamine and nitro, bromo and methoxy
salicylaldehyde, Polyhedron 18 (1999) 897.
(b) V.T. Kasumov, F. Köksal, Synthesis, spectroscopy, and electrochemistry of copper(II) complexes with
N,N’-bis(3,5-di-t-butylsalicylideneimine)polymethylenediamine ligands, Spectrochim. Acta Part A 61
(2005) 225.
(c) V.T. Kasumov, Ş. Özalp-Yaman, E. Taş, Synthesis spectroscopy, and electrochemical behaviors of
t
nickel(II) complexes with tetradentate shiff bases derived from 3,5-Bu2 -salicylaldehyde, Spectrochim. Acta
Part A 62 (2005) 716.
[23]
G.L. Estiú, A.H. Jubert, J. Costamagna, J. Vargas, UV-visible spectroscopy in the interpretation of the
tautomeric equilibrium of N,N′(bis-3,5-di-bromo-salicyliden)-1,2-diaminobenzene and the redox activity of
its Co(II) complex. A quantum chemical approach., J. Mol. Struct. Theochem. 367 (1996) 97.
M. Calvin, C.H. Barkelew, The Oxygen-carrying synthetic chelate compounds. IV. Magnetic properties1, J.
Am. Chem. Soc. 68 (1946) 2267.
I.C. Santos, M. Vilas-Boas, M.F.M. Piedade, C. Freire, M.T. Duarte, B. de Castro, Electrochemical and X-
ray studies of nickel(II) Schiff base complexes derived from salicylaldehyde. Structural effects of bridge
substituents on the stabilisation of the +3 oxidation state, Polyhedron 19 (2000) 655.
(a) H. Ohtsu, K. Tanaka, Equilibrium of Low- and High-Spin States of Ni(II) Complexes controlled by the
donor ability of the bidentate ligands, Inorg. Chem. 43 (2004) 3024.
[24]
[25]
[26]
(b) S. Mukhopadhyay, D. Mandal, D. Ghosh, I. Goldberg, M. Chaudhury, Equilibrium studies in solution
involving nickel(II) complexes of flexidentate Schiff base ligands: isolation and structural characterization
of the planar red and octahedral green species involved in the equilibrium, Inorg. Chem. 42 (2003) 8439.
(c) O. Rotthaus, F. Thomas, O. Jarjayes, C. Philouze, E. Saint-Aman, J.-L. Pierre, Valence tautomerism in
octahedral and square-planar phenoxyl-nickel(II) complexes: Are imino nitrogen atoms good friends?,
Chem. Eur. J. 12 (2006) 6953.
[27]
[28]
L. Sabatini, L. Fabbrizzi, Fitting of nickel(II) ion into two 14-membered tetraaza macrocycles. Blue-to-
yellow conversion and the oxidation and reduction behavior, Inorg. Chem. 18 (1979) 438.
(a) A. Ahmed Isse, A. Gennaro, E. Vianello, Electrochemical reduction of Schiff base ligands H2salen and
H2salophen, Electrochim. Acta 42 (1997) 2065-2071.
(b) N. Kise, H. Oike, E. Okazaki, M. Yoshimoto, T. Shono, Synthesis of nitrogen-containing macrocycles
with reductive intramolecular coupling of aromatic diimines, J. Org. Chem. 60 (1995) 3980.
25
Page 25 of 27

(c) S. Zolezzi, E. Spodine, A. Decinti, Electrochemical studies of copper(II) complexes with Schiff-base
ligands, Polyhedron, 21 (2002) 55.
[29]
(a) M.P. Suh, I.S. Kim, S.-J. Cho, W. Shin, Syntheses of square-planar nickel-(II) and -(I) complexes of an
octaaza macrohexacyclic ligand and crystal structure of the nickel(II) complex, J. Chem. Soc., Dalton Trans.
(1994) 2765.
(b) D.J. Szalda, E. Fujita, R. Sanzenbacher, H. Paulus, H. Elias, Syntheses and properties of nickel(I) and
nickel(II) complexes of a series of macrocyclic N4 ligands: Crystal structures of C-RSSR-
[NiIHTIM](ClO4), C-RSSR-[NiIIHTIM](ClO4)2, C-RRSS-[NiIIHTIM](ClO4)2, and [NiIITIM](ClO4)2
(HTIM = 2,3,9,10-tetramethyl-1,4,8,11-tetraazacyclotetradecane, TIM = 2,3,9,10-Tetramethyl-1,4,8,11-
tetraazacyclotetradeca-1,3,8,10-tetraene), Inorg. Chem. 33 (1994) 5855.
[30]
[31]
(a) A.J. Bard, L.R. Faulkner, Electrochemical methods. Fundamentals and applications, 2nd edition, John
Wiley & Sons, New York, 2001.
(a) J. A. Richards, P.E. Whitson, D.H. Evans, Electrochemical oxidation of 2,4,6-tri-tert-butylphenol, J.
Electroanal. Chem. 63 (1975) 311.
(b) D.H. Evans, P.J. Jiménez, M.J. Kelly, Reversible dimerization of phenoxyl radicals formed by anodic
oxidation of phenolates. A quantitative study by cyclic voltammetry, J. Electroanal. Chem. 163 (1984) 145.
(c) R.D. Webster In situ electrochemical-ATR-FTIR spectroscopic studies on solution phase 2,4,6-tri-
substituted phenoxyl radicals, Electrochem. Comm. 5 (2003) 6.
[32]
(a) C. Freire, B. de Castro, Spectroscopic characterisation of electrogenerated nickel(III) species. Complexes
with N2O2 Schiff-base ligands derived from salicylaldehyde, J. Chem. Soc. , Dalton Trans. (1988) 1491.
(b) Y. Shimazaki, F. Tani, K. Fukui, Y. Naruta, O. Yamauchi, One-electron oxidized nickel(II)-
(disalicylidene)diamine complex: Temperature-dependence tautomerism between Ni(III)-phenolate and
Ni(II)-phenoxyl radical states, J. Am. Chem. Soc. 125 (2003) 10512.
(c) G.E. Kirvan, D.W. Margerum, Characterization of bis( tripeptido)nickelate(III) complexes in aqueous
solution, Inorg. Chem. 24 (1985) 3245.
(d) L.F. Mehne, B.B. Wayland, Properties of Ni(DPG)2X (DPG = diphenylglyoximato; X = Br, I) in the
presence of donor molecules and in the solid, Inorg. Chem. 14 (1975) 881.
(e) M.A.A.F. de C.T. Carrondo, B. de Castro, A.M. Coelho, D. Domingues, C. Freire, J. Morais,
Electrochemical and structural studies of nickel(II) complexes with N2O2 Schiff base ligands derived from
2-hydroxy-1-naphthaldehyde. Molecular structure of N,N′-2,3-dimethylbutane-2,3-diyl-bis(2-hydroxy-1-
naphthylideneiminate) nickel(II), Inorg. Chim. Acta 205 (1993) 157.
(f) F. Azevedo, M.A.A.F.de C.T. Carrondo, B. de Castro, M. Convery, D. Domingues, C. Freire, M.T.
Duarte, K. Nielsen, I.C. Santos, Electrochemical and structural studies of nickel(II) complexes with N2O2
Schiff base ligands 2. Crystal and molecular structure of N,N′-l,2-ethane-1,2-diyl-bis(2-
hydroxyacetophenonylideneiminate)nickel(II), N, N′-1,2-cis cyclohexane-1,2-diyl-bis(2-
hydroxyacetophenonylideneiminate)- nickel(II) and N,N′-1,2-benzene-1,2-diyl-bis(3,5-
dichlorosalicylideneiminate)nickel(II), Inorg. Chim. Acta 219 (1994) 43.
[33]
[34]
(a) T. Storr, E.C. Wasinger, R.C. Pratt, T.D. Stack, The geometric and electronic structure of a one-oxidized
nickel(II) bis(salicylidene)diamine complex, Angew. Chem. Int. Ed. 46 (2007) 5198.
(b) Y. Shimazaki, T.D. Stack, T. Storr, Detailed evaluation of the geometric and electronic structures of one-
electron oxidized group 10 (Ni, Pd, and Pt) metal(II)-(disalicylidene)diamine complexes, Inorg. Chem. 48
(2009) 8383.
D.M. D’Alessandro, F.R. Keene, Current trends and future challenges in the experimental, theoretical
andcomputational analysis of intervalence charge transfer (IVCT) transitions, Chem. Soc. Rev. 35 (2006)
424.
[35]
[36]
J.B. Flanagan, S. Margel, A.J. Bard, F.C. Anson, Electron transfer to and from molecules containing
multiple, noninteracting redox centers. Electrochemical oxidation of poly(vinylferrocene), 100 (1978) 4248.
(a) K. Ray, T. Petrenko, K. Wieghardt, F. Neese, Joint spectroscopic and theoretical investigations of
transition metal complexes involving non-innocent ligands, Dalton Trans. (2007) 1552.
(b) K. Ray, T. Weyhermüller, F. Neese, K.Wieghardt, Electronic Structure of square planar bis(benzene-1,2-
dithiolato)metal complexes [M(L)2]z (z = 2−, 1−, 0; M = Ni, Pd, Pt, Cu, Au): An experimental, density
functional, and correlated ab initio study, Inorg. Chem. 44 (2005) 5345.
(c) F. Thomas, O. Jarjayes, C. Duboc, C. Philouze, E. Saint-Aman, J.-L. Pierre, Intramolecularly hydrogen-
bonded versus copper(II) coordinated mono- and bis-phenoxyl radicals, Dalton Trans. (2004) 2662.
26
Page 26 of 27

(d) D. Herebian, E. Bothe, F. Neese, T. Weyhermüller, K. Wieghardt, Molecular and electronic structures of
bis-(o-diiminobenzosemiquinonato)metal(II) complexes (Ni, Pd, Pt), their monocations and -anions, and of
dimericdications containing weak metal-metal bonds, J. Am. Chem. Soc. 125 (2003) 9116.
M.B. Robin, P. Day, Mixed-valence chemistry: a survey and classification, Inorg. Chem. Radiochem. 10
(1967) 247.
(a) H.M. Peng, B.F. Choules, W.W. Yao, Z. Zhang, R.D. Webster, P.M.W. Gil, Long-lived radical cations
as model compounds for the reactive one-electron oxidation product of vitamin E, J. Phys. Chem. B 112
(2008) 10367.
[37]
[38]
(b) G.J. Wilson, C.Y. Lin, R.D. Webster, Significant differences in the electrochemical behavior of the α-,
ß-, γ-, and δ-tocopherols (vitamin E), J. Phys. Chem. B 110 (2006) 11540.
(c)H.M. Chang, H.H. Jaffe, Uses of the CNDO method in spectroscopy. Doublet states, Chem. Phys. Lett.
23 (1973) 146.
(d) L.J. Johnstonn, N. Mathivanan, F Negri, W.Siebrand, Assignment and vibrational analysis of the 600 nm
absorption band in the phenoxyl radical and some of its derivatives1, Can. J. Chem. 71 (1993) 1655.
(e) P. Hapiot, A. Neudeck, J. Pinson, H. Fulcrand, P. Neta, C. Rolando, Oxidation of caffeic acid and related
hydroxycinnamic acids, J. Electroanal. Chem. 405 (1996) 169.
[39]
[40]
D. Tomczyk, G. Andrijewski, L. Nowak, P. Urbaniak, D. Sroczyński, Spectrocopic and electrochemical
properties of mononuclear Mn(III) complex and of binuclear di-µ-oxo bridged Mn(III) and Mn(IV) complex
with isocyclam, Inorg. Chim. Acta 390 (2012) 70.
J. Müller, A. Kikuchi, E. Bill, T. Weyhermüller, P. Hildebrandt, L. Ould-Moussa, K. Wieghardt, Phenoxyl
radical complexes of chromium(III), manganese(III), cobalt(III), and nickel(II), Inorg. Chim. Acta 297
(2000) 265.
[41]
[42]
R.C. Pratt and T. D.P. Stack, Intramolecular charge transfer and biomimetic reaction kinetics in galactose
oxidase model complexes, J. Am. Chem. Soc. 125 (2003) 8716.
(a) H. Lund, O. Hammerich, Organic electrochemistry,4th Ed., Marcel Dekker, New York,2001, p. 589.
(b) A.J. Wain, J.D. Wadhawan, R.R. France, R.G. Compton, Biphasic redox chemistry of α-tocopherol:
Evidence for electrochemically induced hydrolysis and dimerization on the surface of and within femtolitre
dreplets immobilized onto graphite electrodes, Phys. Chem. Chem. Phys. 6 (2004) 836.
(c) R.D. Webster, Voltammetric studies on the α-tocopheroxyl (Vitamin E) radical in acetonitrile,
Electrochem. Comm. 1 (1999) 581.
(d) W.W. Yao, H.M. Peng, R.D. Webster, Electrochemistry of α-tocopherol (Vitamin E) and α-tocopherol
quinone films deposited on electrode surfaces in the presence and absence of lipid multilayers, J. Phys.
Chem. C 113 (2009) 21805.
(e) J. Malyszko, M. Karbarz, Electrochemical oxidation of trolex and α-tocopherol in acetic acid: A
comparative study, J. Electroanal. Chem. 595 (2006) 136.
(f) H. Eickhoff, G. Jung, A. Rieker, Oxidative phenol coupling—tyrosine dimers and libraries containing
tyrosyl peptide dimers, tetrahedron 57 (2001) 353.
[43]
[44]
S. Chen, H.M. Peng, R.D. Webster, Infrared and UV-vis spectra of phenoxonium cations produced during
the oxidation of phenols with structures similar to vitamin E, Electrochim. Acta 55 (2010) 8863.
(a) Y.-T. Wang, K.J. Jin, S.H. Leopold, J. Wang, H.-L. Peng, M.S. Platz, J. Xue, D.L. Philips, S.A. Glover,
M. Novak, Characterization of reactive intermediates generated during photolysis of 4-acetoxy-4-aryl-2,5-
cyclohexadienones: oxenium ions and aryloxy radicals, J. Am. Chem. Soc. 130 (2008) 16021.
(b) L.L. Williams, R.D. Webster, Electrochemically Controlled Chemically Reversible Transformation of α-
Tocopherol (Vitamin E) into Its Phenoxonium Cation, J. Am. Chem. Soc. 126 (2004) 12441.
R. Petrucci, P. Astolfi, L. Greci, O. Firuzi, L. Saso, G. Marrosu, A spectroelectrochemical and chemical
study on oxidation of hydroxycinnamic acids in aprotic medium, Electrochem. Acta 52 (20070 2461.
(a) M. Vilas-Boas, C. Freire, B. de Castro, P.A. Christensen, A.R. Hillman, New insights into the structure
and properties of electroactive polymer films derived from [Ni(salen)], Inorg. Chem. 36 (1997) 4919.
(b) L.A. Hoferkamp, K.A. Goldsby, Surface-modified electrodes based on nickel(II) and copper(II)
bis(salicylaldimine) complexes, Chem. Mat. 1 (1989) 348.
[45]
[46]
27
Page 27 of 27