The effects of point of substitution on the electrochemical behavior of new manganese phthalocyanines, tetra-substituted with diethylaminoethanethiol

Article abstract of DOI:10.1016/j.ica.2010.06.003

The syntheses and comparative studies of the spectral, voltammetry and spectroelectrochemical properties of new manganese phthalocyanine complexes, tetra-substituted with diethylaminoethanethio at the peripheral (complex 3a) and non-peripheral positions (

Full text of DOI:10.1016/j.ica.2010.06.003

Inorganica Chimica Acta 363 (2010) 3229–3237
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
The effects of point of substitution on the electrochemical behavior of new
manganese phthalocyanines, tetra-substituted with diethylaminoethanethiol
Isaac Adebayo Akinbulu, Tebello Nyokong *
Department of Chemistry, Rhodes University, Grahamstown 6140, South Africa
a r t i c l e i n f o
a b s t r a c t
Article history:
The syntheses and comparative studies of the spectral, voltammetry and spectroelectrochemical proper-
ties of new manganese phthalocyanine complexes, tetra-substituted with diethylaminoethanethio at the
peripheral (complex 3a) and non-peripheral positions (complex 3b) are reported. Solution electrochem-
Received 4 September 2009
Received in revised form 21 January 2010
Accepted 4 June 2010
III ꢀ2
II ꢀ2
istry of complex 3a showed quasi-reversible metal-based (Mn Pc /Mn Pc , E1/2 = ꢀ0.07 V vs. Ag|AgCl)
Available online 17 June 2010
II ꢀ2
II ꢀ3
and ring-based (Mn Pc /Mn Pc , E1/2 = ꢀ0.78 V vs. Ag|AgCl) reductions, but no ring-based oxidation.
However, complex 3b showed weak irreversible ring-oxidation signal (E = +0.86 vs. Ag|AgCl). Reversible
p
Keywords:
Manganese
Diethylaminoethanethiol
Phthalocyanine
Cyclic voltammetry
Spectroelectrochemisrty
III ꢀ2
II ꢀ2
II ꢀ2
II ꢀ3
metal-based (Mn Pc /Mn Pc , E1/2 = ꢀ0.04 V vs. Ag|AgCl) and ring-based (Mn Pc /Mn Pc , E1/2
=
ꢀ0.68 V vs. Ag|AgCl) reductions were also observed for complex 3b. Spectroelectrochemistry was used
III ꢀ2
II ꢀ2
to confirm these processes. Reduction process involving the metal (Mn Pc /Mn Pc ) was associated
with the formation of manganese -oxo complex in complex 3a.
Ó 2010 Elsevier B.V. All rights reserved.
l
1
. Introduction
served in this work. The formation of l-oxo complex in DMF and in
the presence of oxygen is common for MnPc complexes, hence in
The present and potential applications of metallophthalocya-
this work the effects of changing the substituent position on the
nine (MPc) complexes have attracted unprecedented research
interest in these macrocycles. Their uses in electronic devices, as
gas sensors, as photosensitizers, in non-linear optics, in electro-
chromic devices and in Langmuir–Blodgett films have been
reported [1,2]. Incorporation of different metals into the phthalo-
cyanine cavity and changes in the nature and position of substitu-
ent (peripheral and non-peripheral), give MPc complexes with
different chemical, physical, electrochemical, spectroscopic, elec-
trocatalytic, photophysical and photocatalytic properties. The pos-
sibility of tuning these properties, to form MPc complexes with
specific requirements, needed for different applications, promotes
the versatility of these macrocycles.
formation of l-oxo MnPc species is discussed.
In the present work, we report on the syntheses and the ef-
fects of the point of substitution on the spectral, voltammetric
and spectroelectrochemical properties of new MnPc complexes
containing diethylaminoethanethio, tetra-substituted at the
peripheral (complex 3a) and non-peripheral (complex 3b) posi-
tions (Scheme 1). The electrochemical behavior of complexes
3a and 3b is compared with that of the octa-substituted deriva-
tives {manganese(III) (acetate) octakis-(2-diethylaminoethanethi-
o) phthalocyanine, OAcMnODEAETPc(b)}, reported before [17].
The formation of different redox products is highly influenced
by the nature of substituents in MnPc complexes [18]; hence it
is important to increase the range of substituted MnPc
complexes.
The electrochemical properties of manganese phthalocyanine
(
MnPc) complexes have attracted attention [3–5]. MnPc complexes
show interesting electrochemical behavior with oxidation states of
The choice of the substituent employed in this work (diethy-
laminoethanethio) is influenced by the possibility of forming poly-
mers and self-assembled monolayers of the complexes on suitable
electrodes, thus promoting their uses as electrocatalysts and in the
fabrication of electrochemical sensors. The nitrogen group in the
substituent is oxidizable, hence can generate radicals necessary
for initiating polymer formation [19–22].
I
IV
IV
the central Mn ion ranging from Mn to Mn [6–16]. Mn Pc spe-
cies rarely occurs, but has been reported recently [11,12]. The
II ꢀ2
reduction of Mn Pc species has been extensively contested, with
I
ꢀ2
some authors proposing metal reduction to Mn Pc species and
II ꢀ3
others ring reduction forming Mn Pc species. The latter was ob-
MPc complexes containing diethylaminoethanethio on periph-
eral positions (either tetra- or octa-substituted) have been re-
ported in the literature [23–27]. The central metals which have
been employed in the literature include Ni(II), Zn(II) or Co(II).
*
Corresponding author. Tel.: +27 46 6038260; fax: +27 46 6225109.
E-mail address: t.nyokong@ru.ac.za (T. Nyokong).
0
020-1693/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2010.06.003

3
230
I.A. Akinbulu, T. Nyokong / Inorganica Chimica Acta 363 (2010) 3229–3237
NC
NC
N
K CO3
NC
NC
+
HS
2
NO2
DMF
R
1
a
2
a
NC
N
+
HS
NC
2
K CO3
NC
DMF
NC
NO2
R
1
b
2b
R
OAc
N
N
NC
NC
Mn (CH COO)2
N
3
R
R
N
N
Mn
N
N
Diethylaminoethanol
N
R
2
a
3
a
R
R
R
OAc
NC
N
N
N
Mn (CH COO)2
3
N
N
Mn
N
N
N
NC
Diethylaminoethanol
R
R
2
b
R
3b
N
R
=
S
Scheme 1. Synthetic route for complexes 3a and 3b.
No manganese derivatives have been reported. It is important to
note that synthesis of MPc complex containing relatively bulky
substituents is dependent on the nature of the central metal
2.2. Electrochemical studies
All electrochemical experiments were performed using Autolab
potentiostat PGSTAT 302 (Eco Chemie, Utrecht, The Netherlands)
driven by the general purpose Electrochemical System data pro-
cessing software (GPES, software version 4.9). Square wave voltam-
metric analysis was carried out at a frequency of 10 Hz, amplitude:
[
12], hence the synthesis of MnPc derivatives is important. In
addition, the electrochemistry of MPc complexes containing the
ligand of interest in this work has also not received much
attention.
5
0 mV and step potential: 5 mV. A conventional three-electrode
system was used. The working electrode was a bare glassy carbon
electrode (GCE), Ag|AgCl wire and platinum wire were used as the
pseudo reference and auxiliary electrodes, respectively. The poten-
tial response of the Ag|AgCl pseudo-reference electrode was less
than the Ag|AgCl (3 M KCl) by 0.015 ± 0.003 V. Prior to use, the elec-
trode surface was polished with alumina on a Buehler felt pad and
rinsed with excess millipore water. All electrochemical experi-
ments were performed in freshly distilled dry DMF containing
2
. Experimental
2.1. Materials
Potassium carbonate, manganese(II) acetate (OAc), 2-(diethyl-
aminoethanethiol) hydrochloride and 2-diethylaminoethanol were
obtained from Sigma–Aldrich. Tetrabutylammonium tetrafluoro-
borate (TBABF
4
) (Aldrich) was used as the electrolyte for electro-
4
TBABF as supporting electrolyte.
chemical experiments. Aluminum oxide, (WN-3 neutral for
column chromatography) was purchased from Sigma–Aldrich.
Dimethylformamide (DMF) and dichloromethane (DCM) were ob-
tained from Merck, methanol (MeOH) and dimethylsulfoxide
Spectroelectrochemical data were obtained using a home-made
optically transparent thin-layer electrochemical (OTTLE) cell (con-
taining Pt grit working and auxiliary electrodes, and a Ag|AgCl
pseudo-reference electrode) connected to a Bioanalytical Systems
(BAS) CV 27 voltammograph.
(
DMSO) and were distilled before use.

I.A. Akinbulu, T. Nyokong / Inorganica Chimica Acta 363 (2010) 3229–3237
3231
2.3. Equipment
1.20 g (67%). Anal. Calc. for C56
68
H N
12
4
S MnOAc: C, 57.93%; H,
5
.86; N, 14.48; S, 11.03). Found: C, 57.65; H, 5.96; N, 13.98; S,
UV/Vis spectra were recorded on Cary 500 UV/Vis/NIR spectro-
11.33%. UV–Vis (DMF): kmax (nm) (log ): 770 (5.4), 696 (4.7),
510 (4.5), 357 (5.0); IR (KBr) vmax/cm ; 2967–2805 (Aliph-CH ),
2
1719, 1568, 1466, 1379, 1235, 1190, 1067, 915, 877 (Mn–O) 766,
738, 592.
e
ꢀ1
photometer. IR (KBr discs) was recorded on Bruker Vertex 70-Ram
II spectrophotometer. Elemental analysis was performed using
1
Vario Elementar Microcube EL111. H nuclear magnetic resonance
1
(
3
H NMR, 400 MHz) was obtained in CDCl using Bruker EMX 400
NMR spectrometer.
3
. Results and discussion
2
.4. Synthesis
.4.1. 4-(2-Diethylaminoethanethiol) phthalonitrile (2a) and 3-(2-
3
.1. Synthesis and characterization
2
The synthetic pathways for complexes 3a and 3b are shown in
diethylaminoethanethiol) phthalonitrile (2b)
-Nitrophthalonitrile (compound 1a) and 3-mitrophthalonitrile
compound 1b), Scheme 1, were synthesized according to reported
Scheme 1. The phthalonitriles, compounds 2a and 2b, were ob-
tained via a based-catalyzed (K CO ) nucleophilic aromatic substi-
tution reaction. Cyclotetramerization of 2a and 2b occurred in the
presence of manganese(II) acetate to form the targeted complexes:
3
plished using column chromatography on alumina. The complexes
were soluble in solvents such as DMF, DCM and DMSO.
Characterizations of the complexes were carried out using IR
and UV–Vis spectroscopies as well as elemental analysis. The re-
sults obtained were consistent with the predicted structures
shown in Scheme 1. The formation of complexes 3a and 3b was
4
2
3
(
procedure [28]. Compound 2a was synthesized using the method
reported for the synthesis of 1,2-bis-(diethylaminoethanethiol)-
a and 3b, respectively. Purification of 3a and 3b was accom-
4
,5-dicyanobenzene [29] with some modifications as follows: com-
pound 1a (3.05 g, 17.63 mmol) was dissolved in anhydrous DMF
150 ml) under nitrogen and 2-diethylaminoethanethiol hydro-
chloride (9 g, 53.01 mmol) was added. After stirring for 10 min, fi-
nely ground anhydrous K CO (29.25 g, 212.04 mmol) was added
(
2
3
in portions over 2 h with stirring. The reaction mixture was stirred
at room temperature for 48 h under nitrogen. Then the solution
was poured into ice (900 g). The precipitate was filtered off,
washed with water, until the filtrate was neutral. The product
confirmed by the disappearance of the sharp C„N vibration at
2
ꢀ1
229 cm of 2a and 2b.
Fig.
1
shows the UV–Vis spectra of complexes 3a
ꢀ
1
was then dried in air. Yield: 3.42 g (75%). IR (KBr)
v
max/cm
CN), 1582,
542, 1471, 1384, 1289, 1225, 1198, 1142, 1072, 987, 906, 825,
:
ꢀ6
ꢀ6
(
9.48 ꢁ 10 M) and 3b (8.82 ꢁ 10 M) in DMF. Absorption due
3
1
7
3
4
2
084–3018 (Ar–C–H), 2968–2807 (Aliph-CH ), 2229 (v
to the Q-band of complex 3a appeared at 739 nm. The UV–Vis
spectrum of complex 3b showed a red-shifted Q-band (770 nm)
with respect to that of complex 3a. Red-shifting of the Q-band is
normal with non-peripheral substitution in MPc complexes. In-
crease in steric limitations, caused by non-peripheral substitution,
increases the conformational stress on MPc complexes. Such con-
formational stress results in shift in position of Q-band [30].
It has been suggested [31,32] that equilibrium exists between
MnPc species in air and in DMF as shown by Eqs. (1)–(5).
1
32, 609, 521. H NMR (CDCl
.15–3.12 (t, 2H, SCH ), 2.81–2.77 (t, 2H, NCH
H, CH C), 1.06–1.03 (t, 6H, CH ) ppm.
Compound 2b, Scheme 1, was synthesized following the meth-
od described above for compound 2a, using 3-nitrophthalonitrile
3
) d = 7.66–7.54 (m, 3H, Ar–H),
2
2
), 2.62–2.57 (qnt,
2
3
(
compound 1b) in place of 4-nitrophthalonitrile (compound 1a).
ꢀ1
2
Yield: 2.74 g (60%). IR (KBr) vmax/cm : 2969–2814 (Aliph-CH ),
2
1
229 (
065, 1029, 987, 855, 787, 730, 547, 439. H NMR (CDCl
), 2.82–2.79 (t,
C), 1.06–1.02 (t, 6H, CH ) ppm.
vCN), 1565, 1520, 1445, 1372, 1290, 1231, 1193, 1133,
1
II
III
3
)
PcMn þ O $ PcMn ðO Þ
2
2
ð1Þ
ð2Þ
ð3Þ
ð4Þ
ð5Þ
d = 7.73–7.55 (m, 3H, Ar–H), 3.22–3.19 (t, 2H, SCH
H, NCH ), 2.61–2.58 (dd, 4H, CH
2
2
2
2
3
III
II
III
III
PcMn ðO
2
Þ þ PcMn $ PcMn —O
2
—Mn Pc2
2
.4.2. Manganese(III) (acetate) 2(3),9(10),16(17),23(24)-tetrakis-(2-
III
III
IV
PcMn —O
2
—Mn Pc $ 2PcMn O
diethylaminoethanethio) phthalocyanine (3a, Scheme 1)
Complexes 3a was synthesized following the method reported
for the octa-substituted complex {manganese(III) (acetate)
octakis-(2-diethylaminoethanethio) phthalocyanine, (OAcMnO-
DEAETPc(b)} [17] with some modifications. A mixture of com-
pound 2a (0.40 g, 1.54 mmol) and manganese(II) acetate (0.065 g,
IV
II
III
III
2
PcMn O þ 2PcMn $ 2PcMn —O—Mn Pc
II
III
III
4PcMn þ O $ 2PcMn —O—Mn Pc ðNet equationÞ
2
0
1
.38 mmol) was refluxed in 2-diethylaminoethanol (1.2 ml) for
2 h under nitrogen. Thereafter, the mixture was cooled to room
Thus in general the electronic absorption spectra in the visible
III
II
region for MnPc complexes may be attributed to Mn Pc, Mn Pc
and -oxo MnPc species in air. The presence of the -oxo MnPc
temperature and treated with excess MeOH:H
2
O (1:1) to precipi-
l
l
tate the crude purple product. The product was filtered and dried
in air. Purification was carried out using column chromatography
with neutral alumina as column material and DCM/MeOH (10:1)
species may be confirmed by monitoring the spectral transforma-
tions of an MnPc complex in DMF solution when not de-aerated
and when de-aerated with dry N
work. Thus in Fig. 1, the weak band around 636 nm in complex 3a
is associated with the formation of MnPc -oxo complex, which is
2
gas. This is discussed later in this
as eluent. Yield: 1.28 g (72%). Anal. Calc. for C56
c.H O: C, 57.04; H, 5.77; N, 14.26; S, 10.87). Found: C, 56.76; H,
.72; N, 14.10; S, 11.41%. UV–Vis (DMF): kmax (nm) (log ): 739
5.1), 662 (4.5), 500 (4.4), 432 (5.5), 365 (4.7); IR (KBr)
68 12 4
H N S MnOA-
2
l
5
e
common with MnPc complexes in DMF in the presence of oxygen
[31,32]. Although, Mn(II)OAc was the metal salt employed for syn-
thesis, the positions of Q-bands in both complexes (Fig. 1) confirm
(
v
max
/
ꢀ1
cm ; 2966–2802 (Aliph-CH
2
), 1597, 1504, 1450, 1389, 1325,
III ꢀ2
1
067, 924, 878 (Mn–O), 822, 769, 741.
the formation of Mn Pc complexes. This is expected since the
III
II
Mn /Mn oxidation potential is appreciably negative, making the
II ꢀ2
2.4.3. Manganese(III) (acetate) 1(4),8(11),15(18).22(25)-tetrakis-(2-
Mn Pc species very sensitive to air, resulting in the formation
III ꢀ2
diethylaminoethanethiol) phthalocyanine (3b, Scheme 1)
of Mn Pc
species during synthesis and purification [12].
Complex 3b was synthesized (Scheme 1) and purified following
the same method described for complex 3a, using compound 2b
Although, the solution was dearated with purified argon before
spectral characterization, the possibility of the formation of MnPc
(
0.40 g, 1.54 mmol) as the starting material in place of 2a.Yield:
l-oxo complex may not be completely ruled out. The tendency

3
232
I.A. Akinbulu, T. Nyokong / Inorganica Chimica Acta 363 (2010) 3229–3237
2
1
0
.5
2
3b
.5
3
a
1
.5
0
3
50
450
550
650
750
850
Wavelength(nm)
Fig. 1. UV–Vis spectral of complexes 3a (9.48 ꢁ 10ꢀ M), and 3b (8.82 ꢁ 10ꢀ6 M) in DMF.
6
to form the MnPc
l
-oxo complex (band around 636 nm) was sig-
The peaks at 500 and 432 nm (complex 3a) and 510 and 478 nm
(complex 3b) are associated with charge transfer in MPc com-
plexes [14]. The complexes have well-resolved B-bands: 365 and
357 nm for 3a and 3b, respectively.
nificantly reduced, almost absent, in complex 3b. This may be
attributed to restriction to coplanar association of the rings, via
oxo-bridge, caused by the sterically crowded nature of the non-
ꢀ6
ꢀ5
peripheral position. The prevalence of MnPc
l-oxo species was also
Fig. 2a (3.16 ꢁ 10
to 2.21 ꢁ 10 M) and Fig. 2b (1.26 to
ꢀ6
observed during spectral transformations involving OAcMnO-
DEAETPc(b) containing the same substituents at the peripheral
positions, but octa-substituted [17].
8.82 ꢁ 10 M) show the effect of changing concentration on the
spectra of complexes 3a and 3b, respectively. There was no signif-
icant increase in intensity of the peak associated with the MnPc
(a)
4
.5
3
3
.5
2
3
2
1
0
2
1
0
y = 0.1202x + 0.122
R² = 0.9986
.5
1
.5
2
.5
0
0
5
10
15
20
25
Concentration (M) (10-6
)
.5
1
.5
0
3
50
450
550
Wavelength (nm)
650
750
850
2
1
0
.5
2
(b)
2
.5
2
y = 0.2509x -0.2699
R² = 0.9986
1
0
.5
1
.5
0
.5
1
2
3
4
5
6
7
8
9
10
Concentration (M) (10-6
)
.5
0
3
50
450
550
Wavelength (nm)
650
750
850
Fig. 2. UV–Vis spectral of (a) complex 3a at different concentration (3.16 ꢁ 10ꢀ6 to 2.21 ꢁ 10ꢀ5 M), and (b) complex 3b at different concentration (1.26–8.82 ꢁ 10ꢀ6 M).
Solvent: DMF. Insets: plots of absorbance versus concentration.

I.A. Akinbulu, T. Nyokong / Inorganica Chimica Acta 363 (2010) 3229–3237
3233
l
-oxo complex in complex 3a as concentration increased, suggest-
which is the octa-substituted derivative of 3a); unlike complex
3a that does not undergo any oxidation process. Expectedly, the
OAcMnODEAETPc(b) complex is more difficult to reduce than com-
plex 3a (Table 1, considering first reduction). The ease of oxidation
and difficulty in reduction is a result of the plurality of the sulfur
containing substituents. Assignments are confirmed below for 3a
using spectroelectrochemistry.
III
ing the dominance of the Mn Pc species. Beer’s law was obeyed
within the range of concentration (Fig. 2) investigated for both
complexes.
3.2. Voltammetric studies
Comparative study of the voltammetry properties of the com-
plexes was carried out using cyclic and square wave voltammetries.
3
.2.2. Complex 3b
The square wave and cyclic voltammetry profiles of 1 ꢁ 10
ꢀ
4
M
3.2.1. Complex 3a
of complex 3b, in freshly distilled dry DMF containing 0.1 M
Figs. 3a and b, respectively, show the square wave and cyclic
TBABF
respectively. Three redox processes can be identified. Process III
(E = +0.86 V vs. Ag|AgCl) is a weak irreversible ring-oxidation pro-
cess assigned to the either the formation of Mn Pc /Mn Pc or
4
, as supporting electrolyte, are shown in Fig. 4a and b,
ꢀ4
voltammetry profiles of 1 ꢁ 10 M of complex 3a in freshly dis-
tilled dry DMF containing 0.1 M TBABF as supporting electrolyte.
Fig. 3b shows two redox processes. Process II is a quasi-reversible
4
p
III ꢀ1
III ꢀ2
III ꢀ2
II ꢀ2
IV ꢀ2
III ꢀ2
metal reduction process, assigned to Mn Pc /Mn Pc
species
1/2 = ꢀ0.07 V vs. Ag|AgCl), Table 1, in comparison with literature
33]. Although, process II has a cathodic to anodic current ratio of
Mn Pc /Mn Pc species using the data in Table 1. Using redox
potentials is never enough to definitely assign redox processes.
The irreversibility of oxidation processes of MPc complexes con-
taining sulfur substituents is well established [34,35]. Process II
(E1/2 = ꢀ0.04 V vs. Ag|AgCl) is reversible with a cathodic to anodic
(
[
E
near unity, a peak separation larger than that of ferrocene standard
ꢀ1
(
90 mV vs. Ag|AgCl), at the same scan rate (100 mV s
vs.
Ag|AgCl), was obtained. Process I is a quasi-reversible ring reduc-
current ratio of near unity and cathodic to anodic peak separation
III ꢀ2
tion, with large peak separation (171 mV vs. Ag|AgCl) and is as-
of 87 mV vs. Ag|AgCl. It is assigned to metal reduction, Mn Pc /
II ꢀ2
II ꢀ3
II ꢀ2
signed to Mn Pc /Mn Pc species (E1/2 = ꢀ0.78 V vs. Ag|AgCl).
No ring-based oxidation was identified in complex 3a. Table 1
shows the presence of oxidation process in OAcMnODEAETPc(b),
Mn Pc . Process I (E1/2 = ꢀ0.68 V vs. Ag|AgCl), assigned to ring-
II ꢀ2 II ꢀ3
based reduction (Mn Pc /Mn Pc ), is also reversible with catho-
dic to anodic current ratio of near unity and peak separation of
(a)
25µA
I
II
-
1.5
-1
-0.5
0
0.5
1
1.5
Potential/Vvs.Ag|AgCl
(b)
II
1
8 µA
I
-
1.5
-1
-0.5
0
0.5
1
1.5
Potential/Vvs.Ag|AgCl
Fig. 3. (a) Square wave and (b) cyclic voltammetry profiles of 1 ꢁ 10 M of complex 3a in freshly distilled dry DMF containing 0.1 M TBABF
ꢀ
4
4
supporting electrolyte. SWV
ꢀ
1
parameters: Step potential: 5 mV, amplitude: 50 mV vs. Ag|AgCl, frequency: 10 Hz. Scan rate: 100 m Vs
.

3
234
I.A. Akinbulu, T. Nyokong / Inorganica Chimica Acta 363 (2010) 3229–3237
Table 1
Summary of peak and half wave potentials, in volt vs. Ag|AgCl, of the new complexes in comparison with that of previously reported thio derivatised MnPc complexes. Values
were recorded in DMF containing TBABF unless otherwise stated.
4
II ꢀ2
II ꢀ3
MnIIIPc /Mn Pc
ꢀ2
II ꢀ2
MnIIIPc /MnIIIPc
ꢀ1
ꢀ2
MnIVPc /MnIIIPc
ꢀ2
ꢀ2
MnIVPc /MnIVPc
ꢀ1
ꢀ2
Complex
Mn Pc /Mn Pc
Reference
3
3
a
b
ꢀ0.78
ꢀ0.68
ꢀ0.78
ꢀ0.07
ꢀ0.04
ꢀ0.12
ꢀ0.057
ꢀ0.46
ꢀ0.26
ꢀ0.16
ꢀ0.16
ꢀ0.14
TW
TW
17
6
11
13
38
38
3
+0.86d
+0.78
+1.34
MnODEAETPc(b)
MnTMPyPc
a
a
MnOPTPc
ꢀ1.24
ꢀ0.98
ꢀ1.45
ꢀ1.34
ꢀ0.69
+0.47
+0.75
MnTBMPca
+0.83
b
MnTHHTPc(
a
)
+1.03
+1.00
b
MnTHHTPc(b)
MnPcc
+0.87
a
b
c
Values recorded in DCM, using TBABF
Values (V vs. SCE) recorded in DMSO containing tetrabutylammonium perchlorate.
Values (V vs. SCE) recorded in DMF containing tetraethlyammonium perchlorate. Where OPTPc = octa pentathio phthalocyanine; TBMPc = tetra benzylmercapto
4
, TW = this work.
phthalocyanine (peripheral); TMPyPc = tetra mercaptopyridine phthalocyanine; THHTPc = tetrakis (6-hydroxyhexylthio) phthalocyanine;ODEAETPc = octakis-(2-diethy-
laminoethanethio) phthalocyanine.
d
Assignment not confirmed by spectroelectrochemistry.
(a)
III
3
3µA
I
II
-
1.5
-1
-0.5
0
0.5
1
1.5
Potential/V vs. Ag|AgCl
(b)
III
1
5µA
II
I
-
1.5
-1
-0.5
0
0.5
1
1.5
Potential/Vvs.Ag|AgCl
Fig. 4. (a) Square wave and (b) cyclic voltammetry profiles of 1 ꢁ 10ꢀ M of complex 3b in freshly distilled dry DMF containing 0.1 M TBABF
4
supporting electrolyte. SWV
4
ꢀ
1
parameters: Step potential: 5 mV, amplitude: 50 mV vs. Ag|AgCl, frequency: 10 Hz. Scan rate: 100 m Vs
.
9
0 mV vs. Ag|AgCl. The presence of weak oxidation signal, which
difficult to reduce than 3b. This observation clearly defines the im-
pact of point of substitution on the voltammetry properties of
these complexes. All the processes were confirmed by
spectroelectrochemistry.
was absent in complex 3a, coupled with a reversible metal and
ring-based reduction processes, suggest better electron transfer
processes in complex 3b than in 3a. However, complex 3a is more

I.A. Akinbulu, T. Nyokong / Inorganica Chimica Acta 363 (2010) 3229–3237
3235
3
3
.3. Spectroelectrochemical studies
points were also observed at 555 and 669 nm, suggesting the pres-
ence of two species, the starting Mn Pc and the electrogenerated
III ꢀ2
.3.1. Complex 3a
MnPc
l-oxo species. Fig. 5b shows spectral changes observed with
Fig. 5a and b show the spectral changes observed, on the appli-
2
N de-aerated solution of complex 3a in DMF. The initial spectrum
cation of potential slightly more negative of process II (E1/2
=
(bold solid line) is the same as that discussed in Fig. 5a. Upon
reduction, the position of Q-band shifted to higher energy (from
743 to 692 nm) and the charge transfer bands between 450 and
ꢀ
0.07 V, Fig. 3), in the presence of oxygen and when de-aerated
with nitrogen, respectively. The
l-oxo species was not observed
in the first spectrum in Fig. 5a unlike Fig. 1. This shows that the
ratios of the various MnPc species in solution are dependent on
condition such as the presence of electrolyte, among others. The
Q-band (743 nm) of the initial spectrum (bold solid line) was dif-
ferent from that shown in Fig. 1 (739 nm), due to the presence of
electrolyte used for spectroelectrochemical study. Upon reduction,
a drastic decrease in intensity of the Q-band and collapse of the
charge transfer bands between 450 and 550 nm were observed,
550 nm collapsed. These spectral transformations are consistent
III ꢀ2
with the reduction of Mn Pc
Mn Pc species [14]. Attempts to preclude the formation of the
species and the formation of
II ꢀ2
MnPc
oxygen in the OTTLE cell resulted in the formation of MnPc
-oxo species of weak intensity (second spectrum in Fig. 5b). Inter-
l-oxo species were not completely successful, trace of
l
estingly, the intensity of the peak due to the
l-oxo species
II ꢀ2
decreased while that due to Mn Pc species became dominant
II ꢀ2
III ꢀ2
suggesting the formation of Mn Pc species [14]. The B bands
as reduction of Mn Pc species continued (Fig. 5b). The presence
shifted to lower wavelengths. However, since spectral transforma-
tions occurred in an oxygen-rich environment, peak due to
of diffuse isobestic points supports the fact that more than two
III ꢀ2
II ꢀ2
species were present (the Mn Pc , Mn Pc and MnPc l-oxo spe-
II ꢀ2
II ꢀ2
Mn Pc species was not observed. There was direct conversion
cies). The complete absence of the Mn Pc species during spectral
II ꢀ2
of the Mn Pc species to the MnPc l-oxo complex, evidenced by
transformations in Fig. 5a, as result of its direct conversion to the
the emergence of intense peak at 636 nm. Interestingly, isobestic
l-oxo complex, unlike that in Fig. 5b, clearly shows the effect of
(a)
3
50
400
450
500
550
600
650
700
750
800
850
Wavelength (nm)
(b)
3
50
400
450
500
550
600
650
700
750
800
850
Wavelength(nm)
Fig. 5. UV–Vis spectral changes observed for complex 3a during controlled potential electrolysis at the potential more negative of process II (ꢀ0.10 V vs. Ag|AgCl) (a) in the
presence of oxygen and (b) in N de-aerated solution. Bold solid line (spectrum before electrolysis) and bold dashed line (spectrum after electrolysis). Electrolyte = DMF
containing 0.1 M TBABF
2
4
.

3
236
I.A. Akinbulu, T. Nyokong / Inorganica Chimica Acta 363 (2010) 3229–3237
the presence of oxygen on the electronic absorption spectra of
MnPc complexes in the visible region, as discussed previously.
There was no clear formation of the MnPc
Fig. 6a on reduction as was the case for complex 3a. This implies
restriction to coplanar association of the rings, via -oxo-bridge,
in complex 3b, thus preventing the growth of the MnPc -oxo
complex. This is expected because of the sterically hindered nature
of the non-peripheral position. This shows that the extent of the
l-oxo species in
The prevalence of MnPc
l-oxo species was also observed for the
l
OAcMnODEAETPc(b) complex, which is the octa-substituted deriv-
ative of complex 3a, but was not too evident for complex 3b below.
The number of electrons involved cannot be accurately deter-
l
mined as a result of the contribution of the
reduction of the Mn Pc using spectroelectrochemistry was not
successful due to complications arising from the formation of the
l
-oxo complex. Further
occurrence of the l-oxo complex in MnPc complexes depends lar-
gely on the position of substituent.
II ꢀ2
A shift in the position of Q-band with the same intensity is char-
acteristic of metal-based electro reduction process. Specifically,
blue-shift in Q-band (778–713 nm in this regard) is consistent with
MnPc
l-oxo species in Fig. 5. But in Table 1 we have assigned pro-
cess I to ring-based process in comparison with complex 3b below.
III ꢀ2
II ꢀ2
the reduction of Mn Pc to Mn Pc [14]. These spectral transfor-
3
.3.2. Complex 3b
The spectral transformations shown in Fig. 6a were obtained on
mations (Fig. 6a) confirm the assignment of process II in Fig. 4 to
Mn Pc /Mn Pc species. The number of electron transfer was
calculated to be approximately 1 using Eq. (6)
III ꢀ2
II ꢀ2
the application of potential more negative of process II (Fig. 4). The
Q-band (778 nm) of the initial spectrum (bold solid line) is differ-
ent from that shown in Fig. 1 (770 nm). The difference is usually
associated with presence of electrolyte used for spectroelectro-
chemical studies. There was a blue-shift in Q-band (778 to
Q ¼ nFVC;
ð6Þ
where, n, F, V and C are the number of electrons transferred, Fara-
day’s constant, volume and concentration of the electroactive spe-
cies, respectively. The species in Fig. 6a was further reduced on
the application of potential more negative of process I (Fig. 4),
Fig. 6b. There was a decrease in the intensity of the new peak and
7
13 nm) upon reduction and a slight decrease in intensity of the
charge transfer band around 511 nm. The B band decreased in
intensity. The color of the complex also changed from red to green.
(a)
3
50
400
450
500
550
600
650
700
750
800
850
Wavelength (nm)
(b)
3
50
400
450
500
550
600
650
700
750
800
850
Wavelength (nm)
Fig. 6. UV–Vis spectral changes observed for complex 3b during controlled potential electrolysis at (a) ꢀ0.10 V (process II), (b) ꢀ0.75 V (process I) vs. Ag|AgCl. Bold solid line
spectrum before electrolysis) and bold dashed line (spectrum after electrolysis). Electrolyte = DMF containing 0.1 M TBABF . The final spectrum in (a) is the starting spectrum
in (b).
(
4

I.A. Akinbulu, T. Nyokong / Inorganica Chimica Acta 363 (2010) 3229–3237
3237
increase in the absorption intensity between 400 and 600 nm
531 nm), which are consistent with ring-based redox process in
MPc complexes [36–38]. These spectral transformations are a con-
[2] N.B. Mckeown, in: Phthalocyanine Materials, Cambridge University Press,
998.
1
(
[
[
3] A.B.P. Lever, P.C. Minor, J.P. Wilshire, Inorg. Chem. 20 (1981) 2550.
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firmation that process I (E1/2 = ꢀ0.68 V vs. Ag|AgCl, Fig. 4) is due
[5] S. Sievertsen, B. Moubaraki, K.S. Murray, H. Homborg, Z. Anorg. Allg. Chem. 620
(1994) 682.
II ꢀ2
II ꢀ3
to the formation of Mn Pc /Mn Pc species. The number of elec-
trons transferred was estimated to be approximately 1 using Eq. (6).
The origin of process III (+0.86 V, Fig. 4b) was investigated on
the application of potential slightly more positive of +0.86 V. There
was gradual degradation of the complex (figure not shown) as re-
ported previously for other thio-derivatised MPc complexes [39].
Therefore, we can not categorically assign process III to either ring
[
6] N. Sehlotho, M. Durmu sß , V. Ahsen, T. Nyokong, Inorg. Chem. Commun. 11
2008) 479.
[7] J. Zhu, F. Gu, J. Zhag, Mater. Lett. 61 (2007) 1296.
(
[
8] W. Posiuk-Bronikoswska, M. Krajewska, I. Pfis-Kabulska, Polyhedron 18 (1998)
61.
5
[
9] S. Knecht, K. DÜrr, G. Schmid, L.R. Subramanian, M. Hanack, J. Porphyr.
Phthalocyan. 3 (1999) 292.
[
[
10] D.K. Rittenberg, L. Baarrs-Hibbe, A. Böhm, J.S. Miller, J. Mater. Chem. 10 (2000)
III ꢀ1
III ꢀ2
IV ꢀ2
III ꢀ2
241.
oxidation (Mn Pc /Mn Pc
oxidation.
) or metal (Mn Pc /Mn Pc )
11] G. Mbambisa, P. Tau, E. Antunes, T. Nyokong, Polyhedron 26 (2007) 5355.
[12] C.C. Leznoff, L.S. Black, A. Heibert, P.W. Causey, D. Christendat, A.B.P. Lever,
Inorg. Chim. Acta. 359 (2006) 2690.
[
[
13] B.O. Agboola, K.I. Ozoemena, T. Nyokong, Electrochim. Acta 52 (2007) 2520.
14] M.J. Stillman, T. Nyokong, in: C.C. Leznoff, A.B.P. Lever (Eds.), Phthalocyanines:
Properties and Applications, vol. 1, VCH, New York, 1989 (Chapter 3).
15] I.A. Akinbulu, T. Nyokong, Electrochim. Acta 55 (2009) 37.
4
. Conclusions
The syntheses and effect of point of substitution on the spectral,
[
[
16] N. Nombona, P. Tau, N. Sehlotho, T. Nyokong, Electrochim. Acta 53 (2008)
voltammetric and spectroelectrochemical properties of new man-
ganese phthalocyanine complexes, peripherally (complex 3a) and
non-peripherally (complex 3b) substituted with diethylaminoe-
thanethio group are reported. The differences in spectral properties
were explained in terms of position of substituent and existence of
3139.
[17] I.A. Akinbulu, T. Nyokong, Polyhedron 28 (2009) 2831.
[
[
[
18] B.O. Agboola, K.I. Ozoemena, T. Nyokong, Electrochim. Acta 51 (2006) 4379.
19] Y. Tse, P. Janda, H. Lam, A.B.P. Lever, Anal. Chem. 67 (1995) 981.
20] K.L. Brown, J. Shaw, M. Ambrose, H.A. Mottola, Microchem. J. 72 (2002) 285.
[21] E. Trullund, P. Ardies, M.J. Aguirre, S.R. Biaggio, R.C. Rocha-Filho, Polyhedron 19
2000) 2303.
[
(
the MnPc l-oxo complex. The difference in the position of substi-
22] S. Zhang, W. Sun, Y. Xian, W. Zhang, L. Jin, K. Yamamoto, S. Tao, J. Jin, Anal.
Chim. Acta 399 (1999) 213.
tuent resulted in differences in voltammetry properties in terms of
the number of observed processes, half wave potentials and catho-
dic to anodic peak separations. Based on these criteria, complex 3b
was considered to have better voltammetry properties in terms of
reversibility of the redox processes. Spectroelectrochemical studies
were employed to confirm cyclic and square wave voltammogram
assignments. Reduction of Mn Pc to Mn Pc species was com-
plicated by the emergence of the MnPc -oxo species for 3a but not
for 3b. The presence of the MnPc -oxo species was significantly
[23] M. Idowu, T. Nyokong, Polyhedron 28 (2009) 416.
[24] L. Zhang, J. Huang, L. Ren, M. Bai, L. Wu, B. Zhai, X. Zhou, Bioorg. Med. Chem. 16
(
2008) 303.
[
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25] Z.A. Bayir, Dyes Pigm. 65 (2005) 235.
26] S. Gursoy, A. Cihan, M. Kocak, B. Makbule, O. Bekaroglu, Monatshefte fuer
Chemie 132 (2001) 813.
III ꢀ2
II ꢀ2
[27] S. Dabak, G. Gumus, A. Gul, O. Bekaroglu, J. Coord. Chem. 38 (1996) 287.
[
[
28] B.N. Acher, G.M. Fohlen, J.A. Parker, J. Keshavayya, Polyhedron 6 (1987) 1463.
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l
l
[30] I. Chambrier, M.J. Cook, P.T. Wood, Chem. Commun. (2000) 2133.
[
[
[
31] J. Obirai, T. Nyokong, Electrochim. Acta 50 (2005) 3296.
affected by point of substitution on the complex and the concen-
tration of oxygen in the solution of the complex being investigated.
32] A.B.P. Lever, J.P. Wilshire, S.K. Quan, Inorg. Chem. 20 (1981) 761.
33] A.B.P. Lever, E.R. Milaeva, G. Speier, in: C.C. Leznoff, A.B.P. Lever (Eds.),
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34] A.R. Ozkaya, A.G. Gurek, A. Gul, O. Bekaroglu, Polyhedron 16 (1997) 1877.
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Acknowledgements
[
[
This work was supported by the Department of Science and
Technology (DST) and National Research Foundation (NRF) of
South Africa through DST/NRF South African Research Chairs Initia-
tive for Professor of Medicinal Chemistry and Nanotechnology and
Rhodes University.
[36] M.J. Stillman, in: C.C. Leznoff, A.B.P. Lever (Eds.), Phthalocyanines: Properties
and Applications, vol. 3, VCH Publishers, New York, 1993 (Chapter 5).
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37] J. Mack, M.J. Stillman, J. Porphyr. Phthalocyan. 5 (2001) 67.
38] M. Kandaz, A. Koca, Polyhedron 28 (2009) 2933.
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4