Electrochemical metallation with Ni(II) and Al(III) of 5,10,15,20- tetrakis(p-hydroxyphenyl)porphyrin: Effect of ultrasound

Article abstract of DOI:10.1016/j.electacta.2013.03.028

The electrochemical synthesis of Ni(II) and Al(III) metalloporphyrins by reaction of the dianion radical of the 5,10,15,20-tetrakis(p-hydroxyphenyl) porphyrin (TpOHPP) with Ni(II) and Al(III) ions is reported. The electrosynthesis conditions were established by cyclic voltammetry of TpOHPP. The electrosynthesis process was carried out in an undivided cell at the potential corresponding to the formation of the porphyrin dianion radical. A sacrificial anode of Ni or Al was used to electrogenerate the ions. The experiments were carried out in aprotic and protic solvents. The effect of the application of 20 kHz ultrasound on the electrochemical process was evaluated. The electrosynthesis products were characterized by spectroscopy and electrochemical techniques. The results indicate the formation of metalloporphyrins in the different solvents in silent conditions and with ultrasound. The application of 20 kHz ultrasound and protic solvents enhance the reaction yields without affecting the structure of the obtained products.

Full text of DOI:10.1016/j.electacta.2013.03.028

Electrochimica Acta 98 (2013) 82–87
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Electrochimica Acta
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Electrochemical metallation with Ni(II) and Al(III) of
5,10,15,20-tetrakis(p-hydroxyphenyl)porphyrin: Effect of ultrasound
Elsa N. Aguileraa,∗, Leonor M. Blanco Jerezb, Jorge L. Alonsob
a Corporación Mexicana de Investigación en Materiales, S.A. de C.V., Ciencia y Tecnología No. 790, Fracc. Saltillo 400, C.P. 25290, Saltillo, Coah., Mexico
b Laboratorio de Electroquímica, Facultad de Ciencias Químicas, Universidad Autónoma de Nuevo León, Guerrero y Progreso S/N, Col. Trevi˜no, C.P. 64570
Monterrey, N.L., Mexico
a r t i c l e i n f o
a b s t r a c t
Article history:
The electrochemical synthesis of Ni(II) and Al(III) metalloporphyrins by reaction of the dianion radical of
the 5,10,15,20-tetrakis(p-hydroxyphenyl)porphyrin (TpOHPP) with Ni(II) and Al(III) ions is reported. The
electrosynthesis conditions were established by cyclic voltammetry of TpOHPP. The electrosynthesis pro-
cess was carried out in an undivided cell at the potential corresponding to the formation of the porphyrin
dianion radical. A sacrificial anode of Ni or Al was used to electrogenerate the ions. The experiments were
carried out in aprotic and protic solvents. The effect of the application of 20 kHz ultrasound on the elec-
trochemical process was evaluated. The electrosynthesis products were characterized by spectroscopy
and electrochemical techniques. The results indicate the formation of metalloporphyrins in the differ-
ent solvents in silent conditions and with ultrasound. The application of 20 kHz ultrasound and protic
solvents enhance the reaction yields without affecting the structure of the obtained products.
© 2013 Elsevier Ltd. All rights reserved.
Received 2 February 2013
Accepted 5 March 2013
Available online 13 March 2013
Keywords:
5,10,15,20-Tetrakis(p-
hydroxyphenyl)porphyrin
Nickel porphyrins
Aluminum porphyrins
Electrosynthesis
Ultrasound
1. Introduction
[5–8]. Therefore, the application of an electric field will produce a
␲ anionic radical, which reacts with the positive metallic ions to
Research on porphyrin and metalloporphyrin has evolved from
the first synthesis by Fisher in 1920 to their use as selective cata-
lysts in molecular electronic devices, photodynamic therapy agents
and others applications in material chemistry [1,2]. Since then, the
porphyrins and their complex salts have been obtained by the inter-
action of free base porphyrins and a metal salt in suitable reaction
conditions.
Template synthesis and direct synthesis are the methods cur-
rently used to prepare metalloporphyrins [3]. In direct synthesis,
which is the most widely used method, free base porphyrins react
with metallic salt ions under drastic reaction conditions [4].
Electrolysis is commonly used to synthesize coordination com-
pounds. Advantages of electrolysis include higher yields and higher
purity of the resulting compounds than those obtained by classic
methods. Also, electrosynthesis is carried out under milder condi-
tions and with lower reaction times than those of classic methods
[3].
form the metalloporphyrin.
The solvent plays a very important role in metalloporphyrin
synthesis. Kadish et al. reported that the reaction of cobalt acetate
with meso-tetrakis(pentafluorophenyl)porphyrin (TF5PP)H2 leads
to the formation of different metalloporphyrin products for dif-
ferent solvents [7]. The spectroscopic, electrochemical and redox
properties of metalloporphyrins are also affected by the solvent.
Kadish et al. found that nickel metalloporphyrins can be oxidized
or reduced on three one-electron-transfer steps depending on the
solvent [8].
Another important aspect to consider is the metal ion used for
porphyrin metallization. The compatibility of the porphyrin ring
dimensions with the metal cation ionic radius is crucial for met-
alloporphyrin formation. Stable complexes are formed only when
both sizes are similar [9].
In recent years, ultrasound irradiation has been used in some
chemical reactions to improve yields, selectivity and reaction rate
without using harsh conditions [10]. These advantages attributed
to the special sonochemical effect, which is primarily resulted from
“hot spots” formed during acoustic cavitations (i.e., the formation,
growth, and implosive collapse of bubbles in liquids). In particular,
the behavior of localized intensity of temperature and pressure in
the reaction determines their function feature [11]. The imploding
of bubbles creates locally high pressure up to 1000 bar and temper-
ature up to 5000 K. These extraordinary conditions permit access
to a range of chemical reaction space normally nor accessible,
The porphyrin ring system can be reduced or oxidized during
the electrolysis process. Electrochemical studies by cyclic voltam-
metry (CV) show that porphyrins can be reduced or oxidized in two
one-electron-transfer steps to form ␲ anionic or cationic radicals
∗
Corresponding author. Tel.: +52 844 411 32 00x1349.
E-mail addresses: eaguilera.gonzalez@gmail.com, elsa.aguilera@comimsa.com
(E.N. Aguilera).
0013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.electacta.2013.03.028

E.N. Aguilera et al. / Electrochimica Acta 98 (2013) 82–87
83
0.350
0.300
0.250
0.200
0.150
0.100
0.050
0.000
- 0.050
- 0.100
- 0.150
-2000
-1500
-1000
-500
0
500
1000
1500
2000
2500
3000
E (mV)
Fig. 1. Cyclic voltammograms of TpOHPP and TPP in TBAP/DMF at 0.5 V/s at a glassy carbon disk electrode (E vs. Fc/Fc+); the arrow indicates the sweep direction.
which allows for the synthesis of a wide variety of unusual materials
[12,13].
obtained by precipitation with water, were washed with CH2Cl2
and vacuum dried.
This paper reports the results of electrochemical metallization
of 5,10,15,20-tetrakis(p-hydroxyphenyl)porphyrin with Ni(II) and
Al(III) in different solvents. We also report the effect of ultrasound
on the metalloporphyrin electrosynthesis process.
2.3. Characterization
Metalloporphyrins were analyzed using various techniques. The
atomic absorption technique was carried out in a GBC spectrom-
eter (932AA model). The samples were previously dissolved in
concentrated HNO3 to destroy the organic material. The UV-Vis
spectra of the starting material and the electrosynthesis prod-
ucts were obtained in a Varian UV spectrophotometer (Cary 100
CONC model). The samples (0.01 mg/mL) were analyzed in the
200–800 nm spectral region. FTIR analysis was carried out in a
Bruker IR spectrophotometer (Tensor 27 model) using a KBr tablet.
Finally, the 1H NMR and 13C NMR studies were carried out in a
Varian spectrometer (Mercury 200 model) and a 400 MHz Bruker
spectrometer (Avance DPX model), respectively.
2. Experimental
2.1. Electrochemical study
An electrochemical study was carried out to determine the
formation potential of the porphyrin dianion radical. Linear and
voltammetric techniques were applied in a conventional three-
electrode cell using an Epsilon potentiostat/galvanostat (BAS).
Porphyrin voltammograms were obtained using a glassy carbon
disk as the working electrode, a platinum disk as the auxiliary elec-
trode (both from BAS) and a silver wire as the pseudo-reference
electrode. MeOH and AN were used as solvents. The glassy carbon
disk was polished before each experiment with alumina/methanol
paste. Different sweep rates were applied to 10 mL of 2 mM TpOHPP
with 0.1 M TBAP (Sigma–Aldrich) as the supporting electrolyte. All
the potential values are reported with reference to the Fc/Fc+ sys-
tem.
3. Results and discussion
3.1. Porphyrin electrochemical study
Fig. 1 depicts the electrochemical behavior of TpOHPP and
5,10,15,20-tetraphenylporphyrin (TPP) in TBAP/DMF at 0.5 V/s.
TpOHPP shows three reduction processes. The first two pro-
cesses, labeled (A) and (B) in the figure, occur at Epc1 = −75 mV and
Epc2 = −222 mV; a less defined third process (labeled (C)) occurs
at Epc3 = −703 mV TpOHPP also shows three oxidation processes,
labeled (a)–(c).
As reported in the literature [5–8], porphyrins experience two
reductions and two oxidations by transfer of two electrons. There-
fore, the processes shown for TpOHPP are likely associated with
the porphyrin ring oxidation and reduction. However, the chem-
2.2. Metalloporphyrins electrosynthesis
The metalloporphyrins electrosynthesis was carried out at con-
trolled potential. An undivided electrochemical cell containing
75 mL of a 0.1 mM porphyrin solution with 0.1 M TBAP as the sup-
porting electrolyte was used. The working electrode was a glassy
carbon cylinder (BAS) with a large surface area. A nickel and alu-
minum sheets of 2 × 3 cm2 were used as the sacrificial anodes and
a silver wire was used as the reference electrode. To evaluate the
solvent effect, the electrosynthesis was carried out in DMF and
MeOH at room temperature for nickel metalloporphyrins and AN
and MeOH at room temperature for aluminum metalloporphyrins.
For ultrasound-assisted electrosynthesis, a 1/8-in. probe (or horn)
was introduced in the reaction mixture, and a Fisher Scientific Soni-
fier (500 Model) was used to generate a 20 kHz ultrasound signal.
The solvent was removed in a rotative evaporator and the products,
Table 1
Potential for the formation of (TpOHPP)2− (Epc) and applied potential (Eapp) for
metalloporphyrin electrosynthesis (E vs. Fc/Fc+).
Solvent
Epc (mV)
Eapp (mV)
DMF
MeOH
AN
−222
−73
−235
−93
−585
−600

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E.N. Aguilera et al. / Electrochimica Acta 98 (2013) 82–87
ical structure of TpOHPP has OH groups that can also oxidize or
reduce. To identify and differentiate the reduction and oxidation
processes of the porphyrin ring and the OH groups, we performed
the cyclic voltammetry of TPP, which is similar to TpOHPP, but lacks
OH groups. The conditions for this cyclic voltammetry were the
same as that TpOHPP.
TPP shows two reduction and two oxidation processes. The
reduction processes, labeled (Aꢀ) and (Bꢀ), occur at Epc1 = –20 mV
and Epc2 = −219 mV; the oxidation processes ((aꢀ) and (bꢀ)) take
place at Epa1 = 1636 mV and Epa2 = 1914 mV. These processes
should be the porphyrin ring redox processes, because TPP does not
contain other constituents that can be reduced or oxidized. There-
fore, the processes observed in TpOHPP (A and B) correspond to the
porphyrin ring reduction, and are associated with the formation of
a dianion radical, according to the following equations:
P(PhOH)4 + e− → P−(PhOH)4 Epc1 = −75 mV
P−(PhOH)4 + e− → P2−(PhOH)4 Epc2 = −222 mV
(1)
(2)
Fig. 2. UV–vis spectra in DMF of electrosynthesis products obtained in silent con-
ditions and with ultrasound application.
TpOHPP also showed an oxidation process at Epa = 2.336 V that is
not present in TPP (c). This process can be associated with phenolic
TpOHPP groups.
For negative potential sweeps, the reduction of each hydroxyl
likely leads to the formation of a phenoxide ion by losing a proton
as shown in Eq. (3).
For positive potential sweeps, the phenoxide ions may oxidize
to form quinones, as shown in Eq. (4).
PhO− → PhO + e− Epa = 2336 mV
(4)
PhOH → PhO− + H+
(3)
Fig. 3. IR spectra of the Ni electrosynthesis products in DMF and MeOH in silent conditions and with ultrasound, and of the commercial Ni-TpOHPP.

E.N. Aguilera et al. / Electrochimica Acta 98 (2013) 82–87
85
Fig. 4. 1H NMR spectrum (400 MHz) in DMSO-d6 of the Ni electrosynthesis products obtained in silent conditions: (a) in DMF and (b) in MeOH.
These results indicated that dianion radical (TpOHPP)2− can be
electrogenerated in situ at potential values slightly more negative
than the second peak potential and its reaction with the electro-
generated Ni(II) and Al(III) ions allows obtaining the corresponding
metalloporphyrins.
Table 1 shows the potential for the formation of (TpOHPP)2−
(Epc) and applied potential (Eapp) for metalloporphyrin elec-
trosynthesis using different solvents. The formation of (TpOHPP)2−
using MeOH as a solvent takes place at a less negative potential than
the potential for which this radical is formed when using DMF. In
contrast, formation of (TpOHPP)2− in the presence of AN occurs at a
more negative potential than in the presence of DMF. These results
indicated that using different solvents can affect the rate and effi-
ciency of the electrochemical metallation process with TpOHPP. In
this case, the formation of (TpOHPP)2− is more favorable in MeOH.
As reported in the literature [14–16], factors affecting stabil-
ity of the ionic forms of the macrocyclic compounds (both cationic
Fig. 5. UV–vis spectrum of the Al electrosynthesis product synthesized in AN.

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E.N. Aguilera et al. / Electrochimica Acta 98 (2013) 82–87
Fig. 6. 1H NMR spectrum (400 MHz) in acetone-d6 of the Al electrosynthesis product synthesized in AN with ultrasound application.
and anionic) yielded by proton transfer are: electronic factors (or
polarization), structural (or steric) and solvation factors. These fac-
tors are particularly significant for weak acids and bases with rigid
reaction centers, as porphyrins. The contributions of these factors
on the overall stabilization of the ionic species depend on the group
to which belong the porphyrin macrocycle and the solvent.
Porphyrins are usually subdivided into classical (with moder-
ate distortion of the planarity of the macrocycle and well-known
properties), and non-classical (with non-traditional structures
and properties) [14]. The lightly non- planarity of meso-
tetraphenylporphyrins makes the NH centers are more accessible
to an attack of a base or a deprotonation. Donor solvents, with elec-
tron properties pronounced, like DMSO and DMF which have high
donor number (DN 29.8 and 26.6), increase the acidic properties of
classical porphyrins [14], and make deprotonation of the NH acids
thermodynamically more favorable than donor solvents with poor
properties as AN or MeOH (DN 14.1 and 19). However, MeOH is
a protic solvent with high acceptor number (AN 41.5) and tends
to form hydrogen bonds with lonely electron pairs of highly elec-
tronegative elements (such as O or N). The methanol protons are
bonded with the OH groups of phenols substituents of TpOHPP,
which may share electron pairs more easily than the N of the por-
phyrin macrocycle (sterically more hindered). The formation of
these hydrogen bonds provides delocalization of electron density
excess of ring substituents increasing the acidity of the NH proton.
This fact makes more easy the porphyrin deprotonation. It has also
been reported in the literature that the effect of solvation, stabi-
lizes the ionic species of porphyrins (either anionic or cationic) [15].
Hence, the formation of radicals mono and dicationic of TpOHPP is
more favorable in MeOH and require less negative potential values.
3.2. Metalloporphyrin electrosynthesis
3.2.1. Nickel metalloporphyrins
Fig. 2 depicts the UV–vis spectra of electrosynthesis products
with Ni (II) using DMF in silent conditions and with ultrasound
Fig. 7. IR spectrum of the Al electrosynthesis product.

E.N. Aguilera et al. / Electrochimica Acta 98 (2013) 82–87
87
Table 2
metallation (qi) divided by the total charge (Q) used in the pro-
cess of electrolysis. The results show that the application of 20 kHz
ultrasound significantly increased the metallation efficiency with
Ni(II) and Al(III) in the different solvents. Table 2 also shows that
MeOH provides better electrolytical efficiencies than DMF and AN
under silent conditions.
Efficiency of electrochemical metallation process of TpOHPP with Ni(II) and Al(III).
Ni(II) metallation
Al(III) metallation
Condition
Electrolytical efficiency
(%)
Condition
Electrolytical efficiency
(%)
DMF
DMF-US
MeOH
82
99
92
95
AN
66
82
74
80
AN-US
MeOH
MeOH-US
4. Conclusions
MeOH-US
The electrochemical reduction of TpOHPP to the dianion rad-
ical and its further chemical reaction with the Ni(II) and Al(III)
ions lead to metalloporphyrins in protic and aprotic solvents with
efficiencies higher than 60%. Application of 20 kHz ultrasound
enhances the electrochemical metallation process efficiency with-
out affecting the product chemical structure. The solvent used in
metalloporphyrin electrosynthesis affects the process efficiency.
Protic solvents enhance the reaction yields.
application. The absorption pattern for products using MeOH
as solvent is the same. The electrosynthesis products have two
absorption bands (beta and alpha bands). This result indicates a
symmetrical change of porphyrin macrocycle (D4h–D2h) because
of the metal insertion in the porphyrin cavity.
IR spectra of the products obtained in DMF and MeOH in silent
conditions and with ultrasound application, and the commercial
Ni-TpOHPP are showed in Fig. 3. The electrosynthesis products
show the same absorption pattern than commercial Ni-TpOHPP
(stretching vibrations of the O H group of phenolic groups at
3425 cm−1, folding and torsion vibrations of C N bonds at 1170
and 1001 cm−1).
Acknowledgement
The authors gratefully acknowledge funding provided by CONA-
CyT project No. 83253.
Fig. 4 shows the 1H NMR spectra of products obtained using DMF
and MeOH as solvent in silent conditions .The peak at ı = 12.47 ppm
corresponding to the internal protons of the porphyrin macrocycle
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H bond is absent. This confirms that Ni insertion in the por-
phyrin core shifts the pyrrolic protons, and the same product is
obtained. All these results suggest the formation of the same metal-
loporphyrin using different solvents, with and without ultrasound
application.
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(T(p-Me2N)F4PP)M where T(p-Me2N)F4PP is the dianion of meso-
The UV–vis spectra of the electrosynthesis products obtained in
AN and MeOH show a Soret band and four Q bands similar to those
of a free-base porphyrin spectrum. Fig. 5 shows one of these spectra.
However, in the 1H NMR spectra (see Fig. 6) the N H bond peak of
the pyrrolic groups is absent, indicating the possible coordination
of aluminum ions in the porphyrin core.
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Fig. 7) show the stretching vibrations of O H groups. This fact
suggests that the aluminum atom is not bound by oxygen in
the product obtained. Also, the decrease in the C N bond sig-
nal intensity suggests insertion of aluminum in the porphyrin
cavity.
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and
M = Co(II), Cu(II), or Ni(II). Structures of (T(p-Me2N)F4PP)Co and (meso-
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Product spectrometric analysis showed aluminum content of
3%, which corresponds to a possibl−e metal/ligand ratio of 1:1 for an
aluminum complex with one ClO4 ion as a counterion.
The characterization results show the obtention of Ni(II) and
Al(III) metalloporphyrins in protic and aprotic solvents. Apparently,
˚
the porphyrin ring dimensions (3.7 A) are compatible with the ionic
˚
radius of Ni and Al (0.78 and 0.50 A), hence stable complexes are
formed.
Table 2 shows the efficiency values of the electrochemical met-
allation process of TpOHPP with Ni(II) and Al(III) under silent
conditions and with ultrasound application. The efficiency values
were calculated as qi/Q ratio-net charge used for the porphyrin