A novel ferrocenic copper(II) complex Salen-like, derived from 5-chloromethyl-2-hydroxyacetophenone and N-ferrocenmethylaniline: Design, spectral approach and solvent effect towards electrochemical behavior of Fc+/Fc redox couple

Article abstract of DOI:10.1016/j.jorganchem.2017.08.016

This paper reports the synthesis, spectroscopic characterizations and electrochemical behavior of the obtained tetradentate copper (II)-Schiff base complex with its two wings as ferrocenylaniline moieties. This new ferrocenic derivative with two ferroceny

Full text of DOI:10.1016/j.jorganchem.2017.08.016

Accepted Manuscript
A novel ferrocenic copper(II) complex Salen-like, derived from 5-chloromethyl-2-
hydroxyacetophenone and N-ferrocenmethylaniline: Design, spectral approach and
+
solvent effect towards electrochemical behavior of Fc /Fc redox couple
Yasmina Ouennoughi, Houssam Eddine Karce, Djouhra Aggoun, Touhami Lanez,
Ramiro Ruiz-Rosas, Brahim Bouzerafa, Ali Ourari, Emilia Morallon
PII:
S0022-328X(17)30505-3
10.1016/j.jorganchem.2017.08.016
JOM 20069
DOI:
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 23 May 2017
Revised Date: 22 August 2017
Accepted Date: 23 August 2017
Please cite this article as: Y. Ouennoughi, H. Eddine Karce, D. Aggoun, T. Lanez, R. Ruiz-Rosas, B.
Bouzerafa, A. Ourari, E. Morallon, A novel ferrocenic copper(II) complex Salen-like, derived from 5-
chloromethyl-2-hydroxyacetophenone and N-ferrocenmethylaniline: Design, spectral approach and
+
solvent effect towards electrochemical behavior of Fc /Fc redox couple, Journal of Organometallic
Chemistry (2017), doi: 10.1016/j.jorganchem.2017.08.016.
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Graphical abstract
ACCEPTED MANUSCRIPT
H3C
O
OH
H3C
CH3
N
N
O
CH2
CH
Cu
Fe
Fe
+
N CH2
O
H2C N
CH3
CH N CH , I
2
3
Fe
CH3
-
e-
R
R
Fe
Fe
+
e-
Effect of Solvent
Polarity
Effect of
Substituant R
Effect of
Scan rates

ACCEPTED MANUSCRIPT
II
A Novel Ferrocenic Copper Complex Salen-Like, Derived from 5-Chloromethyl-
2
-hydroxyacetophenone and N-Ferrocenmethylaniline: Design, Spectral
+
Approach and Solvent Effect towards Electrochemical Behavior of Fc /Fc Redox
Couple.
1
1,2
1,*
Yasmina OUENNOUGHI , Houssam Eddine KARCE , Djouhra AGGOUN , Touhami LANEZ
2
4
3
1,*
4
,
Ramiro RUIZ-ROSAS , Brahim BOUZERAFA , Ali OURARI , Emilia MORALLON
1
Laboratoire d'Electrochimie, d'Ingénierie Moléculaire et de Catalyse Rédox (LEIMCR),
Département Génie des Procédés, Faculté de Technologie, Université Ferhat ABBAS Sétif-1, Sétif
9000 - Algeria.
1
2
Université El-Oued, VTRS Laboratoire, B.P.789, 39000, El Oued, Algeria.
Laboratoire de Préparation, Modification des Matériaux Polymériques Multiphasiques (LMPMP),
3
Faculté de Technologie, Université Ferhat ABBAS Sétif-1, Sétif 19000 -Algeria.
4
Instituto Universitario de Materiales, Universidad de Alicante, Ap. 99. E-03080 Alicante, Spain.
Abstract
This paper reports the synthesis, spectroscopic characterizations and electrochemical behavior
of the obtained tetradentate copper(II)-Schiff base complex with its two wings as ferrocenylaniline
moieties. This new ferrocenic derivative with two ferrocenylaniline entities surrounding the
copper(II)-Salen complex (5) was synthesized by reacting N-ferrocenmethyl-N-phenyl-5-
aminomethyl-2-hydroxyacetophenone (3) with one half equivalent of 1,2-diaminoethane in absolute
ethanol. As for the intermediates involved in this synthetic sequence, the compound (3) was prepared
in tetrahydrofuran (THF) by reacting N-ferrocenmethylaniline (2) with 5-chloromethyl-2-
hydroxyacetophenone (1) in presence of sodium hydrogenocarbonate (NaHCO ). The compound (3)
3
condensed on the diamine yields the Schiff base ligand (4). Under a nitrogen atmosphere and stirring
at 50 °C, the ligand dissolved in absolute ethanol with stoichiometric amount of copper acetate
monohydrate (Cu(COOCH ) · H O) formed the expected tetradentate Cu(II)-Schiff base complex
3
2
2
(5) after 6 hours of reaction. All these synthesized compounds gave yields varying from 50 to 90%.
Their purities were estimated from elemental analysis while the molecular structures were elucidated
1
13
with FT-IR, UV–Vis, H- and C- NMR. For the obtained copper complex, it is tetracoordinated in a
distorted square planar environment with two nitrogen and two oxygen atoms as donor sites. The
electrochemical behaviors of the synthesized compounds were explored using cyclic voltammetry.
This investigation showed an important anodic shifting of the E and E potential waves for the
pc
pa
+
Fc /Fc redox couple of compound (3) in dichloromethane (DC) medium, compared to other polar
1

ACCEPTED MANUSCRIPT
media like acetonitrile (AN), dimethylformamide (DMF) and dimethylsulfoxide (DMSO). This
behavior was also observed for the tetradentate copper(II)-Salen complex with its different metallic
centers.
Key words: N-Ferrocenmethylaniline, 5-Chloromethyl-2-hydroxyacetophenone, Organometallic
copper(II)-Salen, Cyclic voltammetry, Solvent effects.
Corresponding authors: Tel: +213.776.28.15.33; Fax: +213.36.61.11.54, E-mail address:
alourari@yahoo.fr
(A. Ourari); Tel: +213.697.78.81.45; Fax: +213.36.61.11.54, E-mail address:
aggoun81@yahoo.fr (Dj. Aggoun).
1
. Introduction
In recent years, more consideration and a large number of papers have been published on the
synthesis and characterization of substituted derivatives of ferrocene as typical organometallic
species [1-3]. These kinds of compounds are currently receiving much consideration due to their
increasing role in the rapid growing area of materials science. Therefore, the addition of
cyclopentadienyl rings of ferrocene moiety into an organic compound, not only improves the
electrochemical and spectroscopic behavior, but also increases the application range of these
compounds [4,5]. This is due to the fact that ferrocenyl derivatives are stable compounds, nontoxic,
and have useful redox properties. From a redox perspective, ferrocene derivatives are also interesting
as monoelectronic reservoir complexes capable to act as mediators in the redox-catalytic reactions.
Ferrocenyl Schiff base ligand may also be considered as one of the potential precursors for the
ferrocenic compounds that have been studied in this context [6-9]. Accordingly, several groups have
reported diverse redox properties for various monoferrocenyl and diferrocenyl of bi-, tri- tetra- or
polydentate Schiff bases ligands but most of them have essentially focused their efforts on the
tetradentate Schiff base ligands [10]. These compounds display excellent biological activities in
2

ACCEPTED MANUSCRIPT
which potent therapeutics applications were proved like those applied against cancer [11],
antimicrobial [12], antibacterial [13], antifungal [14] and DNA photo-cleavage activity [15].
Schiff bases were found to have good coordinating properties to synthesize various
complexes using transition metal ions. The electropolymerization of these molecular materials yield
the expected redox polymers with an electronic acceptable conductivity. These materials, with their
high potential in chemistry, biology and medicine, have been found to be useful in several
applications in a wide range of applications such as biochemistry [16], electrochemistry [17],
spectroscopy [18] and in the other fields of analytical [19], nonlinear optics (NLO) [20], catalysis
[21] and electrocatalysis [22].
Our interest, in this subject, is to synthesize novel substituted ferrocene derivatives containing
appropriate coordinating spheres which make them efficient Schiff base ligands leading easily to
their corresponding complexes of transition metals, electropolymerizable with its aniline unit
(Scheme 1). Thus, ferrocenylmethylation was recognized as a powerful method for the preparation
of various ferrocene derivatives [23]. This reaction typically uses stable starting materials such as N-
ferrocenmethylammonium salts [24,25] that are usually applied as alkylating agents for the
introduction of ferrocenmethyl group into various substrates. The most often employed
ferrocenylmethylation reagents are certainly ferrocenylmethylation of aniline [26].
As a continuation of our studies with iminic compounds and modified electrodes, a new N O2
2
donor
type
Schiff
base
ligand
like
N,N-Bis[N-ferrocenmethyl-N-phenyl-5-
aminomethylsalicylidynemethy]-1,2-diimino ethane (H L) and its copper complex were prepared
2
and characterized by routine NMR, FT-IR, UV–Vis absorption spectroscopy, CHN elemental
analysis and cyclic voltammetry. The Cyclic voltammograms of the synthesized copper complex
(Cu(II)L) were conducted in different solvents such as DMF, DMSO, AN and DC. In this case, it
must be noted that the redox system of ferrocene was systematically observed in all cases as a rapid
II
I
and very stable redox couple. As for the Cu /Cu redox system, it displays a quasi-reversible
3

ACCEPTED MANUSCRIPT
behavior in the same media. The electrochemical reactions involved in these media are expressed by
chemical equations illustrated below in the Scheme 1.
CH3
H3C
H
CH2 N CH , I
O
3
N
+
Fe
CH3
H
+
HCHO
HCl
OH
H3C
H
N
O
CH2
+
Fe
Cl CH2
OH
(1)
(2)
THF
H3C
O
CH2
N
CH2
OH
Fe
(3)
+
1/2 NH CH CH NH
2
2
2
2
H3C
CH3
N
N
CH2
Fe
Fe
N CH2
OH HO
H2C N
(4)
Cu(OAc) . H O
2
2
H3C
CH3
N
N
O
CH2
Cu
Fe
Fe
N CH2
O
H2C N
(5)
Scheme 1. Reaction pathways leading to the formation of the compounds: 1, 2, 3, 4 and 5.
4

ACCEPTED MANUSCRIPT
2
. Experimental
2.1. Materials and physicochemical characterization
All reagents were obtained from commercial sources and were used as received without any
further purification. The purity of the synthesized compound was checked by TLC using aluminum
plates, precoated with silica gel (60F, Merck). FT-IR spectra were recorded as KBr disks on a
PerkinElmer 1000 FTIR Spectrophotometer while the UV-Visible spectra were obtained on a
1
UNICAM UV-300 spectrophotometer using DMF solutions (A cell length of 1 cm). The HNMR
spectra were recorded on a Bruker AC300 Y 400 at 25 °C using tetramethylsilane (TMS) as an
1
internal reference. All chemical shifts H are given in ppm using deuterated CDCl and DMSO-d6 as
3
solvents. The microanalysis was performed with LECO TruSpec Micro CHNS elemental micro
3
analyzer. Cyclic voltammetry experiments were carried out in a Metrohm cell of 5 cm using
voltalab 40 (PGZ 301) potentiostat/galvanostat. A planar, circular glassy carbon (GC) electrode (3
mm in diameter) was employed as a working electrode, a platinum wire as an auxiliary electrode and
a saturated calomel electrode (SCE) as a reference electrode. The ferrocene/ferricenium redox couple
0
/+
(
FcH ) was located at E = 0.420 V, where E was calculated from the average of the oxidation
1/2 1/2
and reduction peak potentials. This redox couple was also used as reference in this study. Tetra-n-
butylammonium perchlorate (TBAP) was used as supporting electrolyte and dichloromethane (DC),
acetonitrile (AN), dimethylformamide (DMF) and dimethylsulfoxyde (DMSO) as solvents in all
electrochemical experiments. All solutions were nitrogen purged prior to and during the experiments.
2.2. Synthesis of starting materials
Compound 1: 5-Chloromethyl-2-hydroxyacetophenone (1)
This compound was synthesized according to the procedure described in the literature [27 a].
Briefly, 170 mg (0.125 mmol) of 2-hydroxyacetophenone were added to a mixture of 10 mL of
concentrated hydrochloric acid containing 675 mg (0.225 mmol) of paraformaldehyde. The mixture
was maintained at room temperature under stirring for 48 hours until a precipitate is formed. The
solid product was collected by suction filtration, washed with a solution of sodium hydrogen
carbonate (NaHCO , 50 %), then with water until neutrality, dried under vacuum and then
3
recrystallized in a mixture of toluene and petroleum ether yielding 80 % of white crystals.
-1
-1
1
UV-Vis: λmax(n) (nm), εmax(n) [M .cm ]: λ
(265), εmax(1) [6030]; λmax(2) (327), εmax(2) [1139]. H
max(1)
NMR (CDCl ): δ (ppm) = 12.33 (1H, s, OH), 7.37 (3H, m, Ar-H), 4.69 (2H, s, CH ), 2.66 (3H, s,
3
H
2
5

ACCEPTED MANUSCRIPT
13
CH ). C NMR (CDCl ): δ (ppm) = 204.3 (1C, C=O), 161.1 (1C, C-O), 137.3, 132.3, 128.9, 120.8,
3
3
C
1
18.6 (5C, Aromatic carbons), 46.3 (1C, CH ), 28.4 (1C, CH ).
2 3
Compound 2: N-ferrocenmethylaniline (2)
This compound was also synthesized according the literature [27 b]. 40 g of ferrocenylmethyl
trimethylammonium iodide (0.104 mol), in distilled water (300 mL) was added to a solution of
aniline (55.6 g, 0.597 mol) in distilled water (300 mL). The mixed solution was stirred at reflux of
water for 2 hours. Then, the product was extracted with toluene and recrystallized in a mixture of
ethanol and water to give 20.09 g (0.069 mol) as yellow crystals (66 % yield).
-1
-1
UV-Vis (CH Cl ): λ
(nm), εmax(n) [M .cm ]: λ
(250), εmax(1) [15100]; λmax(2) (290), εmax(2)
2
2
max(n)
max(1)
1
[
4260]. H NMR (CDCl ): δ (ppm) = 7.31(1H, s, H-N), 6.70-7.30 (5H, m, Ar-H), 4.33 (2H, s, C H
3 H 5 4
13
(α,α’)), 4.24 (5H, s, C H ), 4.20 (2H, s, C H
), 4.01 (2H, s, CH ); C NMR (CDCl ): δ (ppm) =
2 3 C
5
5
5
4 (β,β’)
1
48.4 (1C, C -N), 112.9, 117.6, 129.4 (5C, Aromatic carbons), 86.6 (1C, C H , ipso), 68.6 (5C,
p
h
5
4
C H ), 68.2 (2C, C H , C ), 68.0 (2C, C H , C ), 43.5 (1C, CH ). Microanalysis: Calc. (Found)
5
5
5
4
β
5
4
α
2
(
C H NFe): C70.13% (69.80), H 5.88% (6.02), N 4.81% (5.32).
17 17
Compound 3: 5-(N-ferrocenylmethyl-N-phenyl)aminomethyl)-2-hydroxyacetophenone (3)
The 5-(N-ferrocenmethyl-N-phenyl)aminomethyl)-2-hydroxyacetophenone (3) was prepared
according to the following experimental procedure: A mixture of 300 mg (0.257 mmole) of N-
ferrocenmethylaniline (2) and one equivalent of NaHCO (84 mg) in 6 mL of THF was placed in a
3
three-necked flask surmounted by a condenser. To this mixture, 182 mg (0.257 mmole) of 5-
Chloromethyl-2-hydroxyacetophenone (1), dissolved in 6 mL of THF, were added drop wisely. This
mixture was then heated at 50 °C under stirring and nitrogen atmosphere for at least 6 hours. The
resulting suspension containing NaCl was filtered, washed with diethyl ether and the solvents were
removed under reduced pressure. The pale yellow solid obtained was recrystallized in a mixture of
ethanol and petroleum ether to yield compound 3. Its purity was checked by TLC using silica gel
plates eluted with a combination of dichloromethane/heptane (8/2, v/v) as eluent. Yield 381 mg
(79%).
-1
-1
UV-Vis (CH Cl ): λ
(nm), εmax(n) [M .cm ]: λ (255), εmax(1) [2140]; λmax(2) (307), εmax(2)
max(1)
2
2
max(n)
[
304]; λmax(3) (337), εmax(3) [285]. 1H NMR (DMSO-d6): δ (ppm) = 11.75 (1H, s, OH), 7.80-6.50 (m,
H
8
H, Ar-H), 4.48 (2H, s, N-CH -Ph), 4.41 (2H, s, N-CH -Fc), 4.26 (2H, t, C H4(α,α’)), 4.18 (5H, s,
2 2 5
1
3
C H ), 4.09 (2H, t, C H
), 3.32 (3H, s, CH ). C NMR (DMSO-d6): δ (ppm) = 204.9 (1C,
3 C
5
5
5
4 (β,β’)
C=O), 160.4 (1C, C-O), 148.8 (1C, C -N), 135.6, 130.6, 130.2, 129.8, 121.1, 118.4, 117.0, 113.7
ph
(
10C, Aromatic carbons), 85.4 (1C, C H , ipso), 69.9 (5C, C H ), 69.4 (2C, C H , C ), 68.4 (2C,
5 4 5 5 5 4 β
6

ACCEPTED MANUSCRIPT
C H , C ), 53.3 (1C, CH ), 50.8 (1C, CH ), 28.6 (1C, CH ). Microanalysis: Calc. (Found)
5
4
α
2
2
3
(
C H O NFe): C71.08 % (71.53), H 5.74% (5.59), N 3.19% (2.52).
26 25 2
Compound 4: N,N-Bis[(N-Ferrocenemethyl-N-phenyl)-5-aminomethylsalicylidynemethyl]-1,2-
diiminoethane (4)
The tetradentate Schiff base ligand (4) was synthesized according to the literature [28]. Briefly,
to a solution of 0.1 g (0.23 mmole) of 5-(N-Ferrocenmethyl-N-phenyl)aminomethyl-2-
hydroxyacetophenone (3) in 7 mL of absolute methanol, placed in a 100 mL three-necked flask
surmounted with a condenser, was slowly added a solution of 0.067 g (0.23 mmole) of 1,2-
diaminoethane in 8 mL of the same solvent. This mixture was heated to 65 °C under stirring and
nitrogen atmosphere for 4 hours. The precipitate obtained was filtered, washed with diethyl ether and
dried under reduced pressure to afford 80 mg of compound 4 as a yellow solid. Its yield was
estimated to 78% and the purity was checked by silica gel TLC plates using only dichloromethane as
unique eluent.
-1
-1
UV-Vis: λmax(n) (nm), εmax(n) [M .cm ]: λ
(255), εmax(1) [22550], λmax(2) (310), εmax(2) [3970],
max(1)
1
λmax(3) (335), εmax(3) [3190], λmax(4) (405), εmax(4) [410]. H NMR (DMSO-d6): δ (ppm) = 15.67 (2H,
H
s, 2OH), 7.50-6.50 (m, 16H), 4.48 (4H, s, N-CH -Ph), 4.42 (4H, s, N-CH -Fc), 4.28 (4H, t, C H
4
2
2
5
1
3
(α,α’)), 4.21 (10H, s, C H ), 4.12 (4H, t, C H
), 3.38 (6H, s, 2CH ), 2.32 (4H, t, 2CH ). C NMR
3 2
5
5
5
4 (β,β’)
(
DMSO-d6): δ (ppm) = 172.9 (2C, C=N), 161.7 (2C, C-O), 148.0 (2C, C -N), 134.6, 130.8, 129.7,
C
p
h
1
28.8, 127.1, 118.7, 117.6, 115.9, 112.7 (20C, Aromatic carbons), 84.6 (2C, C H , ipso), 69.0 (10C,
5 4
C H ), 68.5 (4C, C H , C ), 67.5 (4C, C H , C ), 52.7 (2C, CH ), 49.8 (2C, CH ), 27.7 (2C, CH ).
5
5
5
4
β
5
4
α
2
2
2
1
5
4.7 (2C, CH ). Microanalysis: Calc. (Found) (C H Fe N O · 3H O): C68.08 % (67.90), H
3
54 50
2
4
2
2
.92% (5.57), N 5.88% (6.02).
Compound 5: Copper complex Cu(II)L (5)
Following the same way, a methanolic solution (5 mL) containing 33.8 mg of copper acetate
monohydrate (Cu(CH COO) ·H O) salt (0.17 mmol) was added slowly drop wise to a solution (5
3
2
2
mL) of 124.8 mg (0.34 mmol) of the ligand (H L) (4) placed in a 50 mL three-necked flask
2
surmounted by a condenser. The mixture was then heated to reflux, under nitrogen atmosphere and
stirring for at least 6 hours. The mixture was cooled to room temperature and the solvent was
reduced under vacuum, the resulting precipitate was filtered, washed with diethylether and air-dried
providing 98 mg (yield 62 %) of the copper complex.
7

ACCEPTED MANUSCRIPT
-1
-1
UV-Vis: λmax(n) (nm), εmax(n) [M .cm ]: λ
(255), εmax(1) [23450]; λmax(2) (310), εmax(2) [3650];
max(1)
λmax(3) (372), εmax(3) [1650]. Microanalysis: Calc. (Found) (C H Fe N O Cu): C 67.54% (67.87),
54
48
2
4
2
H 5.04% (5.40), N 5.83% (5.65).
3
. Results and discussion
3.1. Physicochemical and analytical characterization
Microanalysis data obtained for the copper complex confirmed that it is a mononuclear
complex. Therefore, the ligand:metal ratio was found to be 1:1 and the resulting copper complex can
be formulated as Cu(II)L (5) (See Scheme 1). The ligand with its copper complex seems to be stable
at room temperature and they are also soluble in common organic solvents such as DC, MeOH,
EtOH, DMF and DMSO. The synthesis of compounds (1-4) including the copper complex
(compound 5) was carried out through known chemical reactions as shown in Scheme 1. Regarding
the compound 1, it was prepared according to a recently published procedure involving the
chloromethylation reaction of 2-hydroxyacetophenone. On the other hand, compound 2 was prepared
by reacting aniline with (ferrocenmethyl)trimethylammonium iodide. As for compound (3), it was
also synthesized via an alkylation of N-ferrocenmethylaniline (2) method with compound 1. Thus,
compound 3 reacted with ethylenediamine to afford the expected ligand as tetradentate Schiff base
H L (4) which, upon treatment with copper acetate salt yielded its corresponding copper complex.
2
The structures of these synthesized compounds were confirmed by different spectroscopic techniques
such as UV-Vis, FT-IR and NMR spectral data. Additionally, the elemental analysis data were found
to be in good agreement with the proposed structures. Details of some spectral data are given below.
3.2. Spectroscopic characterization
∑
FT-IR spectra
The FT-IR spectra of the starting products, the free ligand as well as its copper complex are
listed in Table 1, together with assignments of most of the major bands. Generally, in order to study
the binding modes of the Schiff base ligand to the metal ions in their complexes form, the FT-IR
spectrum of the free ligand was compared to the spectrum of its corresponding copper complex.
As it can be seen, the FT-IR data of all compounds exhibit similar features indicating the
analogy of their molecular structures. All these synthesized compounds, except compound 2 and the
8

ACCEPTED MANUSCRIPT
-1
copper complex, show a broad absorption band in the 3200-3800 cm range which may be ascribed
to the O–H groups (intramolecular hydrogen bonding). The spectra of all synthesized compounds are
-
1
-1
also dominated by bands between 3100–3050 cm as ν (Aromatic, C-H) and 2950–2840 cm for the
-1
ν (Aliphatic, C-H) stretching vibrations. The absorption bands observed at 1640 and 1637 cm were
attributed to the C=O stretching vibrations for compounds 1 and 3, respectively. Also, the spectrum
-1
of 3 showed an absorption band at 1313 cm due to the newly formed CH -N group [28]. As for the
2
-
1
tetradentate Schiff base ligand 4, its FT-IR spectrum displayed a characteristic band at 1618 cm
indicating the formation of an iminic function, currently known as azomethine group or Schiff base
HC=N). The characteristic frequencies of this group exhibit an obvious shifting to lower wave
(
-1
numbers (1598 cm ) in the spectrum of its Cu(II) complex. This bathochromic effect could be
explained by an increase in the electronic delocalization through the metal centre, newly coordinated
yielding the copper complex. Additional data supporting the formation of this complex was further
-1
-1
provided by the appearance of other new indicative bands between 410-430 cm and 510-520 cm
that are assignable to ν (N-Cu) and ν (O-Cu), respectively. The remaining characteristic band at 478
-1
cm in the free ligand could be assigned to the Fe–Cp stretching frequency. For the copper complex,
no significant changes were observed including its ferrocenic moieties for which the two
cyclopentadienyl rings were coordinated to the bivalent iron ion via their five molecular orbitals
[12,14].
−
1
Table 1 Characteristic FT-IR bands (4000–400 cm ) of 1, 2, 3, 4 and 5.
Compounds
ν(O-H)
3426
-
ν(C-H) (arom.)
3013
ν(C-H) (aliph.)
2951
ν(C=O)
ν(C=N)
ν(CH
2
-N)
ν(C-H)(Cp)
-
ν(Fe-Cp)
-
ν (M-O) ν (M-N)
1
2
3
4
5
1640
-
-
-
-
-
3089
2920
-
-
753
819
819
820
490
478
478
477
-
-
-
-
3450
3430
-
3085
2927
1637
-
1310
1380
1381
3090
2920
-
-
1618
1598
-
-
3098
2925
503
422
∑
Spectroscopic UV-Vis characterization
The electronic spectra were recorded using freshly prepared solutions in DC at a concentration of
-4
1
0 M. The common band, seen in the spectrum of the derivative 2 and in all the synthesized
compounds, was observed at higher energy (200–300 nm) and it was attributed to π-π* transitions
associated to the benzene ring (intraligand charge transfer ILCT). While the weaker one, appeared at
lower energy (300–400 nm) in the spectra of compounds 2 and 3. This band is probably related to n-
9

ACCEPTED MANUSCRIPT
π* transitions, implicated in the promotion of the lone pair electron of the carbonyl oxygen atom to
the anti-bonding π orbital [29]. Moreover, a novel band, appearing at 375 nm in the spectrum of the
copper complex, could be ascribed to LMCT and MLCT charge transfer (See Fig. 1).
2,5
2,0
1,5
1,0
0,5
0,0
2
3
4
5
300
400
Wavelength (nm)
Fig. 1. UV-Vis spectra of the synthesized compounds: 2, 3, 4 and 5.
1
13
∑
H and C NMR characterization
1
13
The H and C NMR chemical shifts of the synthesized compounds with suggestible assignments
have been listed in the experimental section in which the required evidences confirmed the proposed
1
13
structures shown in Scheme 1. The H NMR and C NMR spectra of the ferrocenyl derivative (3)
have been illustrated in Fig. S1.
1
The H NMR spectrum of compound 3 was recorded in DMSO-d at room temperature using
6
TMS as internal standard (Fig. S1A). As it can be seen, the integral intensities of each signal were
found to be in good agreement with the number of different types of protons of the studied structure.
The most important peak of this compound was attributed to the singlet centered at 11.75 ppm
confirming the presence of the phenolic (O-H) protons. In the case of the ligand 4, these phenolic
protons appear at 15.67 ppm. This deshielding effect is imputable to the formation of strongest
intramolecular hydrogen bonds [30].
1
0

ACCEPTED MANUSCRIPT
As for the aromatic protons, they resonate as multiplet between 6.50 and 7.80 ppm. The
resonances appearing at 4.40 and 4.47 ppm were assigned to the aliphatic protons of the two
methylene (N–CH ) groups. The ferrocenyl entities exhibit three signals in the range 4.00-4.30 ppm
2
which are assigned to the free C H ring, with its two types of protons H and H of the substituted
5
5
α
β
cyclopentadienyl ring (C H ). The singlet observed at 4.18 ppm was ascribed to the five protons of
5
4
free cyclpentadienyl ring whereas the two distinct signals, respectively located at 4.26 (triplet: Hα,α’
)
and 4.09 ppm (triplet: Hβ,β’), were attributed to the substituted ring. These assignments are consistent
with those recently reported in the literature [6] for similar ferrocenyl derivatives. All of these
assignments are in good accordance with almost the same chemical shifts in the case of the Schiff
base ligand (compound 4). As for the singlet observed at 3.32 ppm, it was attributed to the methyl
protons directly linked to the carbonyl function (O=C–CH ). The singlet observed at 3.38 ppm for
3
the ligand and its chemical shift is due to the azomethine function (N=C–CH ) linked to its methyl
3
group. A new signal of both methylene groups, bonded to two azomethine linkage (C=N-CH -CH -
2
2
N=C), was observed as triplet at 2.32 ppm in the free ligand.
1
3
The proton decoupled C NMR spectrum of compound 3 seems to be in good accordance
1
with H NMR data above mentioned and confirming its structure by chemical shifts clearly shown by
its spectrum. This structure has been confirmed by 18 observed resonances (See Fig. S1B) where
carbon atoms resonances seen between 204.9–148.8 ppm are associated to carbonyl, phenoxy, and
anilinic groups. As for the aromatic carbon atoms (C ), they resonate in the 135.6–113.7 ppm
sp2
range. In the spectrum of compound 3, signals of the aliphatic carbon atoms (C ), appear in the
sp3
region of stronger fields between 53.3–28.6 ppm. The ferrocenyl carbon atoms were located in the
region δ 86.0 to δ 67.0. As for the carbon atoms of unsubstituted cyclopentadienyl ring (C H ), they
5
5
appeared as a strong peak at δ 69.9 while other ferrocenyl peaks such as those of α,α’ and β,β’
carbons founded in the same ring (C H ), produced rather chemical shifts at δ 68.4 and δ 69.4,
5
4
respectively. The ipso carbon of the substituted cyclopentadienyl ring appears at δ 85.4. Aromatic
and aliphatic carbon atoms resonate at almost the same chemical shifts for the same carbon atoms of
compounds 3 and 4 but, in case of ligand (4), it displays a novel peak located at 14.72 ppm which is
due to the methylene group. These results are in good accordance with the spectra of similar
structures reported in literature [31].
1
1

ACCEPTED MANUSCRIPT
3
.3. Electrochemical characterization
Due to its well defined reversible system, experimentally obtained at the accessible potentials,
+
the ferrocenium/ferrocene redox couple Fc /Fc (Fc = Fe(C H ) ) has been established as one of the
5
5 2
internal standards for calibration of electrochemical measurements and extensively investigated as an
interesting redox mediator currently involved in the electrochemical studies [32-34].
In this study, we have explored the changes in the oxidation potential of ferrocenium/ferrocene
redox couple using cyclic voltammetry (CV). All the synthesized derivatives were studied in the
presence of tetra-n-butylammonium perchlorate (TBAP) as a supporting electrolyte with the use of
four aprotic organic solvents commonly utilized in the electrochemical measurements:
dichloromethane (DC), acetonitrile (AN), N,N-dimethylformamide (DMF) and dimethylsulfoxide
(DMSO). Therefore, this study was restricted to the use of only three solvents (DMF, DMSO and
DC) for the Schiff base ligand (4) and its copper complex because of the low solubility of these
compounds in acetonitrile medium.
‹
Cyclic voltammetry characterization of Ferrocenyl derivatives
The electrochemical properties of the synthesized compounds were studied by cyclic
-1
-3
voltammetry 10 M TBAP and 2.10 M of each sample in different solvents using a glassy carbon
electrode.
The potential was scanned within the potential range of +1.6 to -2.2 V at 100 mV/s. Cyclic
+
voltammograms of all studied compounds showed reversible redox couples of Fc /Fc at about E ≈
1
/2
0.45 V. In the next step, the CV was also adopted to investigate the electrochemical response of this
+
system (Fc /Fc) for all compounds between 0.2 and 0.8 V using various scan rates (Figure S2). This
redox couple was systematically observed in all voltammograms, recorded in the following pure
solvents DC, AN, DMF and DMSO. The electrochemical data for these compounds are summarized
in Table S1. The main values shown in this table are i /i ratios, the peak-to-peak separation ∆Ep
pa pc
(
Epa and Epc) for various scan rates and E1/2 as the half-wave potential values (E = (Epa+Epc)/2).
1/2
As it can be seen in Table S1, the peak current ratio (i /i ) seems to converge rapidly to unity and it
pa pc
is therefore independent of the scan rate in all the explored media. The values of ∆Ep are higher than
the ideal value of 60 mV for the Nernstian monoelectronic and reversible process system indicating
some irreversibility of this process. As expected, the ∆Ep value seems to be sensibly increased in
DMSO producing a well defined redox system. The values of the half-wave potential (E ) were also
1
/2
found to be significantly shifted to the more anodic potentials and are also independent of the scan
rate.
1
2

ACCEPTED MANUSCRIPT
‹
Effect of scan rates
Figure 2A, recorded in acetonitrile medium, showed that the redox peak currents of the ferrocenic
system (i , i ) of compound 3 increase with increasing the scan rate. Their two anodic (E ) and
pa pc
pa
cathodic (E ) potential waves also displayed an obvious shifting to the positive and negative
pc
potentials respectively as the scan rate increases. The peak to peak separation seems to be greater
than 60 mV in all voltammograms even, at higher scan rates, which is an indication of fundamental
slowness of the electron-transfer process. In addition, it was found that the obtained peak currents
present a linear dependence versus the square root of the scan rate (Fig. 2B) as it is expected for a
1
/2
controlled diffusion process. The linear dependency of the anodic peak current with ν was also
studied with all the synthesized compounds in different media as presented in figure S3.
200
150
100
-
1
H3C
1
5
0 mV.s
O
AN
-1
00 mV.s
CH2
N
CH2
OH
Fe
(3)
50
90
(B)
0
60
30
-50
ipa
ipc
0
-
30
-100
-150
-200
(
A)
-60
-
90
0
5
10
15
20
25
1/2 -1 1/2
(mV.s )
V
200
400
600
800
1000
1200
1400
E(mV)
-3
Fig. 2(A) Cyclic voltammograms recorded on glassy carbon electrode with 2.10 mol/L compound
-1
III and 10 mol/L TBAP in acetonitrile solution, (B) Anodic and the cathodic peak currents as
function of square root of scan rate.
Detailed study of this effect for the copper complex is illustrated in Fig. 3. The
voltammograms, recorded for the copper complex in the range between -0.4 and -1.5 V, indicate the
presence of quasi-reversible process for the Cu(II)/Cu(I) system. This figure shows that each peak
1
3

ACCEPTED MANUSCRIPT
position is not merely dependent on the scan rate but they are all critically dependent on solvent
nature.
20
0
0
-
50
-
-
-
-
20
40
60
80
-
-
-
100
150
200
DC
DMF
-
100
-
1 200
-1 000
-800
-600
-400
-200
-1600
-1400
-1200
-1000
-800
-600
E(mV)
E(mV)
5
0
0
10
20
30
40
DC
DMF
DMSO
-
-
-
-
-
5
-
10
15
DMSO
-
-1600
-1400
-1200
-1000
-800
-600
-400
-200
5
6
7
8
9
10
1
/2
1/2
E(mV)
V (mV/s)
Fig. 3 Cyclic voltammograms, recorded on glassy carbon electrode, showing the effect of the scan
rate and the cathodic peak heights as function of the square root of the scan rate for copper
-3
-1
complex in various media (2.10 mol/L) (10 mol/L TBAP).
‹
Effect of the Solvent on the Electrochemical Behaviour of Compound 3
Cyclic voltammograms of compound 3 recorded in various solvent are shown in Fig. 4. It has
been demonstrated that E1/2 shifts towards the more anodic potentials according to the following
order: AN<DC<DMF<DMSO. This order indicates that the oxidation reaction of ferrocene
derivative to its ferrocenium cation is more and more difficult to be done from AN to DMSO.
In the present system, the electrochemical oxidation of ferrocene derivative corresponds to a
reversible change between the neutral reduced form and the oxidized cationic form. The oxidized
form is more sensitive to the interactions with the solvent molecules than the reduced neutral form.
1
4

ACCEPTED MANUSCRIPT
Therefore, the power of such interactions, involved between the cation and solvent molecules could
be strongly influenced by the redox properties (donor number) of the solvent medium. As
consequence, this behavior provides a strong electronic contribution from the solvent molecules
influencing the oxidation reaction to yield the ferrocenic cations with an important increase of their
E values [35-37].
1
/2
Compound: 3
4
3
1
5
0
5
0
-
-
15
30
DMSO
AN
DC
DMF
2
00
300
400
500
600
700
800
E(mV)
Fig.4 Cyclic voltammograms, recorded on glassy carbon electrode, showing the effect of the
-3
-1
solvent medium on the electrochemistry of compound 3 (2.10 mol/L) (10 mol/L TBAP) using
–
1
1
00 mV.s as scan rate.
+
‹
Effect of the Nature of Ferrocenyl derivatives on the Electrochemistry of Fc /Fc System in
DMSO Medium
+
Representative cyclic voltammograms of Fc /Fc in DMSO medium at 100 mV/s showing the
effect of radical nature (Fc-R), are shown in Fig. 5. Ferrocenyl system exhibits reversible oxidation
with all the synthesized ferrocenic compounds 2, 3, 4 and 5. This reversibility exhibits some slight
changes according to the nature of the studied compound. The anodic and cathodic peak separation
(∆Ep) of these compounds was found to be variable in the range between 76 and 88 mV. The ∆Ep
value of compound 2 is in good accordance with Nernstian system. This observation indicates one-
electron step oxidation and suggests an easy electron transfer as it was previously reported in similar
studies [38]. The high ∆Ep values, determined for other compounds can be attributed to the
uncompensated resistance effect [39]. It was also observed that the peak currents ratio is near to the
unity (i /i = 1.0) namely for the copper complex and also for some others like compound 3. The
pa pc
1
5

ACCEPTED MANUSCRIPT
difference in the half-wave potentials of ferrocene and its substituted derivatives could be explained
based on the electron-withdrawing inductive effect of the substituents [40].
Fc
2
3
4
5
45
30
15
0
1/2
Ipa / Ipc Epa-Epc (mV) E (mV)
Fc
2
1.06
1.08
1.00
1.25
7
6
420
68
399
433
437
441
3
92
92
88
4
5
1.00
-
15
30
DMSO
-
200
300
400
500
600
700
800
E(mV)
Fig.5 Cyclic voltammograms, recorded on glassy carbon electrode, showing the effect of the
+
-3
Ferrocenyl derivative nature on the cyclic voltammetry of Fc /Fc system in DMSO medium (2.10
-1
–1
mol/L) (10 mol/L TBAP) ) using 100 mV.s as scan rate.
4
. Conclusion
The ferrocenmethylaniline with other starting materials were successfully synthesized. So,
four ferrocenyl derivatives were obtained via reactions sequence leading to the Schiff base ligand
with its mononuclear copper complex. These resulting compounds were characterized by the usual
1
13
physicochemical methods, such as elemental analysis, UV-Vis, FT-IR, and H and C NMR. The
stoichiometry of the synthesized copper complex has been proposed based on elemental analysis
data. The electrochemical behaviors of all the synthesized compounds were investigated by cyclic
voltammetry using glassy carbon electrode in different solvents as dichloromethane, acetonitrile,
+
dimethylformamide and dimethylsulfoxide. The redox couple Fc /Fc of these compounds 2, 3, 4 and
5
, show a reversible and a well defined monoelectronic system particularly in DMSO solvent
compared to other solvents. Their half wave potentials (E ) exhibit a good stability. This indicates
1
/2
that the ferrocenic couple could be used as an electrochemical reference avoiding any use of classical
1
6

ACCEPTED MANUSCRIPT
electrode materials. All the anilinic derivatives, synthesized in this study, may also be considered as
electropolymerizable monomers which could be used in the fabrication of modified electrodes with
their polyanilinic redox polymers.
Acknowledgement
The authors would like to thank the MESRS and DG–RSDT (Ministère de l’Enseignement
Supérieur et de la Recherche Scientifique et la Direction Générale de la Recherche et du
Développement Technologique- Algérie) for their financial support. Financial support from the
Ministerio de Economía y Competitividad (Spain) and FEDER funds (MAT2016-76595-R) is also
gratefully acknowledged.
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[
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0

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Highlights
‹
‹
‹
‹
New ferrocenic compounds derived from Ferrocenmethylaniline and Cu-Salen.
UV-Vis, FT-IR, Elemental analysis, NMR and Cyclic voltammetry.
+
Studied of Fc /Fc redox couple using four aprotic organic solvents.
+
Redox properties of ferrocenyl derivatives and their effect on the Fc /Fc system.