Electrochemistry as an attractive and effective tool for the synthesis and immobilization of porphyrins on an electrode surface

Article abstract of DOI:10.1002/chem.201404314

Magnesium(II) 10-phenyl-5,15-p-ditolylporphyrin is easily and cleanly transformed by electrolysis. A nitro group is first introduced at the free meso position by anodic substitution. Hydrogenation into the amine is then carried out electrocatalytically under ambient conditions with water as a hydrogen supplier. The synthesized porphyrin under the nickel(II) form can be covalently grafted onto a platinum electrode by electrochemical reduction of the diazonium cation, generated in situ by a reaction of the nickel(II) aminoporphyrin with sodium nitrite and trifluoroacetic acid. The electrosynthesized thin film gives an electrochemical response typical of a porphyrin material. Films grown under our conditions have a maximum surface coverage of approximately 5×10^-10 mol cm^-2. The modified electrode exhibits a reproducible electrochemical behavior and a good level of stability over potential cycling and exposition to air. Being choosy: Selective approaches based on electrochemistry have been developed to yield different porphyrins. The final synthesized porphyrin is covalently attached to the electrode surface through the electroreduction of the corresponding diazonium generated in situ in organic media (see figure; RE, CE, WE=ref, counter and working electrode, respectively). The results demonstrate that electrochemistry is an efficient alternative tool for the synthesis of designed porphyrins and their use in surface modification.

Full text of DOI:10.1002/chem.201404314

DOI: 10.1002/chem.201404314
Full Paper
&
Molecular Electrochemistry
Electrochemistry as an Attractive and Effective Tool for the
Synthesis and Immobilization of Porphyrins on an Electrode
Surface
Seydou HebiØ, Abdou K. D. DimØ, Charles H. Devillers,* and Dominique Lucas*[a]
Abstract: Magnesium(II) 10-phenyl-5,15-p-ditolylporphyrin is
easily and cleanly transformed by electrolysis. A nitro group
is first introduced at the free meso position by anodic substi-
tution. Hydrogenation into the amine is then carried out
electrocatalytically under ambient conditions with water as
a hydrogen supplier. The synthesized porphyrin under the
nickel(II) form can be covalently grafted onto a platinum
electrode by electrochemical reduction of the diazonium
cation, generated in situ by a reaction of the nickel(II) ami-
noporphyrin with sodium nitrite and trifluoroacetic acid. The
electrosynthesized thin film gives an electrochemical re-
sponse typical of a porphyrin material. Films grown under
our conditions have a maximum surface coverage of approx-
imately 5”10À10 molcmÀ2. The modified electrode exhibits
a reproducible electrochemical behavior and a good level of
stability over potential cycling and exposition to air.
or sensing.[6] In this context, the preparation of thin films of
porphyrins on solid substrates calls for efficient methods to
ensure that the specific properties of the molecule (physical
and chemical) are transferred to the material. Among other
techniques the electrochemical reduction of aryl diazonium
salts has been proven to be efficient in forming organic mono-
layer films covalently grafted to the electrode surface.[7] The at-
tractiveness of this approach arises from the strong attach-
ment of the film to the surface, the ability to control the layer
thickness, and its tolerance toward other chemical functions
on the aryl moiety. Concerning the mechanism, it is generally
believed that a one-electron reduction of the aryl diazonium
ion and the subsequent elimination of dinitrogen produce aryl
radicals prone to attack the electrode material and result in
the covalent bonding of the aryl group to the surface.[7a,8]
There are only two reports in the literature on the electrograft-
ing of porphyrin from diazonium precursors.[7d,9] In all of them,
the starting compound is derived from tetraphenylporphyrin,
Introduction
Owing to their essential role in natural processes such as pho-
tosynthesis and oxygen transport in blood,[1] porphyrins have
been the subject of intense research for many decades. At the
same time, the synthesis of porphyrins still remains a delicate
process, particularly in the case of complex substitution pat-
terns. Although electrochemistry presents several advantages
as a tool for synthesizing complex organic molecules,[2] its use
in porphyrin synthesis is rather limited and concerns the direct
functionalization of the primarily formed macrocycle. Examples
of reactions performed electrochemically are the metalation of
the porphyrin ring,[3] the oxidative coupling, which affords di-
verse types of directly linked polyporphyrins,[4] or the nucleo-
philic anodic substitution by which a nucleophile is introduced
on a peripheral position of the macrocycle by replacement of
a hydrogen atom.[5] However, none of these examples employs
the electrochemical method in successive reactions that com-
prise a synthetic sequence.
+
to which the N2 function is added onto the para position of
In addition to these aspects of molecular synthesis, there is
great interest in the integration of porphyrins into molecular
devices designed for energy conversion, molecular electronics,
one or several phenyl substituents. Therefore, in the resulting
modified electrode, the porphyrin is not grafted by itself onto
the surface, but through a para-phenylene spacer. Starting
with this observation, our conviction was that a direct linking
could provide unique advantages that result from a more im-
mediate interaction between the porphyrin and the electrode
surface (for instance, a faster electron transfer to the electroac-
tive porphyrin moiety). The main objective of the work pre-
sented herein was to achieve such a goal.
[a] Dr. S. HebiØ,+ Dr. A. K. D. DimØ,+ Dr. C. H. Devillers, Prof. Dr. D. Lucas
Institut de Chimie MolØculaire de l’UniversitØ de
Bourgogne (ICUMB), UMR CNRS 6302
Électrochimie, MatØriaux MolØculaires, et Dispositifs (EMMD)
UniversitØ de Bourgogne, 9 Avenue Alain Savary
BP 47870, 21078 Dijon Cedex (France)
E-mail: charles.devillers@u-bourgogne.fr
dominique.Lucas@u-bourgogne.fr
[+] These authors contributed equally to this work.
In this report, we will detail the selective and efficient trans-
formation of a magnesium porphyrin that possesses an unsub-
stituted meso position, magnesium(II) 10-phenyl-5,15-p-ditolyl-
porphyrin (Mg(PDTP); see the chemical structure in Scheme 1),
into the corresponding amino-functionalized free base. The
synthesis operates by nitration and hydrogenation, both of
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201404314. It includes experimental proce-
dures, characterization data, NMR spectra, electrochemical data, and UV/
Vis spectra.
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sively increased from 0.00 V
versus a saturated calomel elec-
trode (SCE), at which point no
current flowed through the cir-
cuit to 0.70 V, the point at which
the reaction proceeded. Finally,
the electrolysis was stopped
after an uptake of two Faradays
per mole of Mg(PDTP), in ac-
cordance with the stoichiometry
of the expected reaction.
Scheme 1. Structure of the synthesized porphyrins. The 10-phenyl-5,15-p-ditolylporphyrinyl group is abbreviated
as PDTP.
After appropriate treatment of
which are carried out electrochemically, with excellent yields.
Later on, in the form of the nickel(II) complex, the aminopor-
phyrin is converted into the corresponding diazonium deriva-
tive. In situ reduction of the latter on platinum leads to elec-
trode modification. The modified electrode response favors
a direct linking to the surface of the nickel(II) porphyrin
through the meso position.
the crude electrolyzed solution (see all details in the Experi-
mental Section and the Supporting Information), Mg(PDTPÀ
NO2) was isolated in 91% yield (Scheme 2), which is compara-
ble to what is obtained with classical methods. Nitration at the
meso position of porphyrins has been inventoried with a variety
of reagents and conditions: concentrated HNO3 in H2SO4,
À
MeOH, or CHCl3; NaNO2 in CF3COOH; NO2 in combination
with an oxidizer, I2, Cu2+, or Ag+.[15] Nevertheless, no reagent
seems to be appropriate for universal use, and the reaction
often lacks selectivity: the mono-nitrated product may be sub-
ject to subsequent nitration, either directly on the macrocycle
or on peripheral phenyl substituents. Conversely, in the elec-
trochemical nitration of Mg(PDTP), by means of potential con-
trol, the reaction is easily constrained at the first degree of
substitution, and demetalation (accidental release of the mag-
nesium) does not occur under these basic conditions.
The mechanism of the electrochemical reaction was investi-
gated by voltammetry. Figure 1 shows the cyclic voltammo-
gram of Mg(PDTP) in the potential range in which it is oxidized
and its evolution upon consecutive addition of lutidine and
NaNO2.
Results and Discussion
Nitration at the meso position
Most commonly the nitration of aromatic compounds is realiz-
ed by means of aromatic electrophilic substitution. The usual
+
reactant is concentrated nitric acid that forms the NO2 ion as
the attacking entity.[10] This reaction can also be performed
electrochemically by means of anodic substitution.[11] The ad-
vantages provided by the electrochemical method are the pos-
sibility of milder reaction conditions (in particular the use of
a nonacidic medium), the minimization of waste (replacement
of a chemical oxidizer by the anode), and in certain cases,
better selectivities. The use of this method in the porphyrin
series has already been reported once. Zinc octaethylporphyrin
was converted into a mixture of the meso-mono- and -dinitrat-
ed derivatives by means of anodic oxidation in the presence of
an excess amount of sodium nitrite.[12] Thus the rather limited
information on this subject led us to study the reaction on
Mg(PDTP). The choice of this porphyrin representative was de-
liberate. In Mg(PDTP) a single meso position is available for re-
action, and magnesium as the central metal atom is known to
favor oxidation.[13]
With only Mg(PDTP) in acetonitrile that contained tetraethyl-
ammonium hexafluorophosphate (TEAPF6) as the supporting
electrolyte, two chemically reversible peak systems are ob-
served (solid blue line, Figure 1), which led, respectively, to the
cation radical (peak system O1/R1, E1/2 =0.68 V) and the dicat-
ion (peak system O2/R2, E1/2 =1.03 V). Thus the response is typ-
ical of metalloporphyrins with a non-electroactive metal that
undergoes two ring-centered one-electron transfers in
oxidation.[13]
The addition of 10 molar equivalents of lutidine in the ana-
lyzed solution induced important modifications in the CV (red
dotted line, Figure 1). Although the first oxidation peak (O1’)
The electrochemical nitration of Mg(PDTP) was carried out
at a controlled potential in a separated compartment cell in
the presence of a slight excess
amount of NaNO2 (3.4 equiv
versus Mg(PDTP)). Lutidine was
also incorporated prior to elec-
trolysis (10 equiv). Indeed, in our
recent work on porphyrin anodic
substitution, we experienced
that this hindered base favors
the elimination of the proton at
the reactive meso position.[5a,14]
Under these conditions, the
working potential was progres- Scheme 2. Nitration reaction at the meso position of Mg(PDTP).
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(as that is not the purpose of this work), the same electrode re-
action seemed to occur with Mg(PDTP). Peak O2’ in particular
would be the voltammetric trace of the dimer formed near the
electrode surface.
Upon supplemental addition of an excess amount of NaNO2
(3.4 equiv) the cyclic voltammogram of Mg(PDTP) underwent
further modifications (black dashed line curve on Figure 1): the
first oxidation peak (O1’’) is slightly shifted towards a lower po-
tential (Ep,O1’ÀEp,O1’’ =67 mV). In fact, NO2À was oxidized simulta-
neously to Mg(PDTP) at the potential level of peak O1’’. Ac-
cordingly we have verified that NaNO2 alone in acetonitrile
(ACN) resulted in a chemically irreversible peak at Epa =0.54 V.
That is coincident with the shoulder in the increase of peak
O1’’. In agreement with previous reports, this oxidation of
À
NO2 is expected to produce radical NO2.[15d,16] Moreover, on
Figure 1. Cyclic voltammogram of Mg(PDTP) (0.5 mm) in 0.1m TEABF4/
CH3CN: alone (solid blue line), and after consecutive addition of 10 equiv of
lutidine (red dotted line) and 3.4 equiv of NaNO2 (black dashed line). Work-
ing electrode: Pt disk (Ø=2 mm); sweep rate: 100 mVsÀ1; initial potential:
0.00 V versus SCE.
the basis of the CV in Figure 1, by inverting the potential scan
direction after O1’’, no cathodic peak was detected in the re-
verse sweep, which indicates a fast reaction of Mg(PDTP +),
C
but different from dimerization. Conversely, by continuing the
oxidative scan after O1’’, two well-defined reversible peak sys-
tems, O2’’/R2’’ and O3’’/R3’’, appeared at E1/2 =0.89 and 1.20 V.
These systems correspond to the stepwise oxidation of
Mg(PDTPÀNO2) (see below), which demonstrates that this
product was formed instantaneously in the vicinity of the
electrode.
did not change its position, the peak current is multiplied by
1.25. Additionally, in the anodic scan, peak O1’ is followed by
a partially fused and less intense peak O2’ (Ep,O2’ =0.80 V). In
fact, this evolution is highly reminiscent of what was previously
observed with Zn(PDTP) by the addition of a base.[4a] The new
features on the cyclic voltammogram were proven to have
come from the rapid formation of the simply linked dimer at
the electrode by oxidative coupling at the hydrogenated meso
position (see Scheme 3). Although we have no absolute proof
As presented in Scheme 4, two mechanisms can be consid-
ered for the reaction. These reaction pathways were initially
proposed for the nitration of magnesium octaethylporphyrin
by nitrite in combination with an oxidant and both were sug-
gested to be relevant.[15d,17] In
path A, the metalloporphyrin is
transformed into its p-cation
+
C
radical Mg(PDTP)
, on which
À
the addition of NO2 proceeds.
Subsequent loss of an electron
and a proton led to the final
meso-nitrated metalloporphyrin
Mg(PDTPÀNO2). Path B implies
a parallel formation of nitrogen
+
C
C
dioxide NO2 and Mg(PDTP) by
À
Scheme 3. Dimerization from oxidation at the meso position.
concomitant oxidation of NO2
and Mg(PDTP), followed by the
coupling of these two radicals
and loss of a proton to give
Mg(PDTPÀNO2).
Conversion into amine
Transformation of porphyrins
that bear a nitro function in the
meso position into the corre-
sponding amino derivatives is
frequently encountered in por-
phyrin chemistry. Two main
routes are mentioned using:
a) NaBH4 as a reducer with palla-
dium on carbon as a catalyst;[18]
Scheme 4. Mechanistic possibilities for electrochemical nitration of Mg(PDTP) at the meso position.
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and b) SnCl2 (reducer) in concentrated HCl.[19] Furthermore, in
the last thirty years, electrocatalytic hydrogenation has been
recognized to be an appealing method for the selective hydro-
genation of unsaturated organic substrates.[20] The method
provides mild and environmentally friendly conditions. Indeed,
the reaction occurs in a mixture of MeOH and H2O (the latter
of which supplies hydrogen) at room temperature and ambi-
ent pressure and avoids using costly and/or deleterious metals
(Pd, Sn). In particular, nitrated aromatics can be converted into
the corresponding amines with competitive chemical and fara-
daic yields.[21] The availability of Mg(PDTPÀNO2) provided us
with the opportunity to test this electrosynthetic method on
nitrated porphyrins as it had never been attempted before.
The conditions chosen for the electrolysis of Mg(PDTPÀNO2)
were similar to those reported for the reaction of nitrobenze-
ne;[21f] among others, Devarda copper constituted the cathode
material. The electrolysis was managed in two steps. First, in
the pure supporting electrolyte solution the cathode was po-
larized at À1.20 V versus SCE over 20 min, thus provoking hy-
drogen evolution and allowing the electrode surface to reach
saturation in adsorbed H2. Then, the porphyrin was added
(70 mg, 0.11 mmol) into the cathodic compartment. The work-
ing potential was set at À0.96 V, at which point the electrolysis
occurred. Finally, the current dropped to zero after a charge of
6 F per one mole of Mg(PDTPÀNO2), as expected for the pre-
dicted reaction.
from single cells,[6b] efficient materials for electrocatalytic oxida-
tion of organics[6a,d] including methanol, ethanol, hydrazine,
and formaldehyde). Apart from the diazonium electrografting
method, the literature mentions a wide range of approaches
for the functionalization of surfaces by porphyrins. That in-
cludes chemisorption,[22] physisorption,[23] self-assembly
through tethered functional groups,[24] thermolysis,[25] click
chemistry,[26] oxidation of a peripheric terminal alkyne,[27] and
anodic polymerization.[28] Otherwise the surface modification
by electrochemical reduction of aryldiazonium salts offers sev-
eral advantages:[7c,29] a) the ability to control the deposit (e.g.,
thickness, mono- or multilayer character of the film); b) com-
patibility with a wide range of materials and a great variety of
reaction media; c) stability of the organic layer provided by the
covalent attachment to the surface; d) ease of preparation of
the diazonium salt from the amine precursor; and e) tolerance
toward many kinds of functional groups on the aryl backbone.
Therefore, in view of these advantages, the availability of
Ni(PDTPÀNH2) provided the opportunity to test in this manner
the direct grafting of the porphyrin onto electrode surfaces.
Electrografting and properties of the Pt-modified electrode
In situ generation of a porphyrin diazonium and subsequent
electrografting on Pt
The nickel porphyrin diazonium Ni(PDTPÀN2+) was generated
in 0.1m TEABF4/CH3CN by reaction of Ni(PDTPÀNH2) with an
excess amount of sodium nitrite (2 equiv NaNO2) in the pres-
ence of a strong organic acid (0.5m trifluoroacetic acid (TFA)).
Multiple reaction conditions were attempted before finding
the most adequate one (see the Experimental Section for de-
tails). In particular, a certain proportion of water (0.5%) must
be incorporated to the reaction medium, probably to make
NaNO2 more soluble. The concentration of TFA was adjusted to
avoid the possible demetalation of nickel porphyrin by means
of acidolysis. The order of the TFA addition is also critical: the
reaction worked much better when TFA was added afterwards
instead of before or at the same time as NaNO2.
The magnesium aminoporphyrin thus produced has been
proven to be sensitive to air. This high susceptibility to oxida-
tion originates both from the peripheric amine and the inner
magnesium, which together accentuate the electron-richness
of the macrocycle. Therefore, immediately after the electro-
chemical reaction, Mg(PDTPÀNH2) was converted into the cor-
responding air-stable free base H2(PDTPÀNH2) (Scheme 5). De
facto, concentrated hydrochloric acid was added to the crude
electrolyzed solution and further neutralization with sodium
acetate afforded H2(PDTPÀNH2). After other treatments (wash-
ing by liquid–liquid extraction, column chromatography)
H2(PDTPÀNH2) was isolated in its pure state in 80% yield,
which is in the same range as the usual synthetic methods.
The free base was then treated by nickel acetate in DMF under
reflux conditions to afford the nickel(II) aminoporhyrin
Ni(PDTPÀNH2) in view of its conversion into the corresponding
diazonium derivative, the use of which allowed immobilization
of the nickel porphyrin on electrode surfaces.
Once the diazotation reaction was completed, the diazoni-
um cation was reduced on the platinum electrode by means
of multiple scan voltammetry. Figure 2b presents the cyclic vol-
tammogram of the in situ generated compound recorded on
a platinum electrode at 50 mVsÀ1 with a potential scan that
was initially made to go down.[30] The first scan shows a sharp
peak at Epc =0.20 V that corresponds to the irreversible one-
electron reduction of the in situ generated diazonium cation,
similarly to what is generally ob-
Thin films of nickel porphyrins have several attractive appli-
cations (e.g., microsensors for detection of nitric oxide release
served for aromatic diazonium
molecules.[7c] Over the following
scans the cathodic peak current
rapidly decreased in intensity in
accordance with the formation
of an insulating film blocking
electron transfer between the
electrode and Ni(PDTPÀN2+) in
solution. Then, after completion
Scheme 5. Electrocatalytic hydrogenation of the nitro group followed by in situ demetalation.
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of this voltammetric deposition
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sible reduction and oxidation
couples centered at E1/2 =À1.35
and À0.94 in DMF and at
+1.28 V in CH2Cl2.[31] In particu-
lar, the reductive response (ex-
plored in DMF) is not so straight-
forward, since the redox systems
have unequal intensities (peak
area ratio Rf2/Rf3=0.36 at
1 VsÀ1). But the sum of the Rf2
and Rf3 peak areas is close to
that of peak Of1 (peak area ratio
Of1/(Rf2+Rf3)=1.07). These vol-
tammetric features show impor-
tant similarities to the typical be-
havior of nickel porphyrins in so-
lution. As a relevant representa-
tive, nickel tetraphenylporphyrin,
Ni(TPP),[32] gives rise to a one-
electron oxidation in CH2Cl2
(E1/2 =1.05 V versus SCE) and
a one-electron reduction in DMF
(E1/2 =À1.17 V versus SCE), both
chemically reversible.[33] There-
Figure 2. a) Reaction scheme for in situ generation of the nickel(II) 5-phenyl-10,20-bis-p-tolylporphyrin diazonium
from Ni(PDTPÀNH2) followed by electrochemical grafting on the platinum surface. b) The first five grafting scans
on Pt of 1 mm Ni(PDTPÀNH2) after diazotation in 0.1m TEABF4/CH3CN recorded at 50 mVsÀ1 at (20Æ3)8C.
step, the modified Pt electrode was sonicated in DMF, or in
CH2Cl2, and transferred to a blank electrolyte solution.
fore, on the basis of the closeness of the potentials, we
assume that the integrity of the porphyrin skeleton is retained
in the electrodeposited film. Clearly, for the latter, the oxidation
potential is higher, but this effect is typically encountered for
densely packed porphyrin monolayers.[34] Considering the split-
ting of the reduction signal, one possible explanation might
be the existence in the film of two different electroactive por-
phyrin sites, which would be expressed at distinct potentials.
As this phenomenon seems to be specific to DMF (no splitting
of the oxidation signal in CH2Cl2), we suggest that, as a chemi-
cal differentiation between the two forms, some of the nickel
porphyrins might be axially ligated by DMF and others pre-
served from this coordination. It is known from literature that
nickel porphyrins, and more specifically Ni(TPP), can form ad-
ducts with nitrogenous bases,[35] and the coordinative ability of
DMF is well established.[36]
Characterization of the Pt-modified electrode
Cyclic voltammetry investigation
After rinsing, the Pt-modified electrode was characterized by
cyclic voltammetry. The cyclic voltammograms reproduced in
Figure 3a and b show three well-defined and chemically rever-
Control experiments were carried out using a polished Pt
electrode, which was treated either by depositing 10 mL of
a 1 mm Ni(PDTPÀNH2) solution and then dried under argon, or
by soaking the Pt electrode for 24 h in this solution. The result-
ing electrodes did not show any voltammetric evidence of the
attachment of porphyrin.
As expected for surface-immobilized electroactive species,
for systems 1, 2, and 3 the peak currents are linearly depen-
dent on the scan rates (see Figure 3a’ and b’), and the ratio be-
tween the oxidation and reduction peak currents at any given
scan rate is close to one. On the first cycle (not shown), the in-
tensity of the reduction peaks is 10 and 12% larger than on
the second one, probably because of the desorption of some
noncovalently bonded porphyrin molecules. After the first
cycle, the voltammograms are persistent for more than 70
cycles. The film redox processes were found to be strongly de-
pendent on both the medium and the scan rate used for anal-
Figure 3. Cyclic voltammograms of a derivatized electrode a) reduction in
0.1m TEABF4/DMF and b) oxidation in 0.1m TEABF4/CH2Cl2 of Ni(PDTP)/Pt
film prepared by thirty grafting cycles at a Pt electrode: second scan (solid
line) and fifteenth scan (dotted line). Conditions: n=1 VsÀ1 at (20Æ3)8C.
a’,b’) Plots of anodic and cathodic peak currents versus scan rate.
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ysis. The oxidation couple is chemically irreversible in CH3CN
and not observed in DMF, whereas the reduction couple does
not appear in CH2Cl2 and is chemically irreversible in CH3CN.
Such dependence on the solvent of the modified electrode re-
sponse was also mentioned by Downard, Moutet, and co-work-
ers in the case of similar covalently bonded Ni–porphyrin
films.[9]
in which Q is the charge obtained from the integration of the
baseline corrected area under the oxidation or reduction peak,
F is the Faraday constant (96485 CmolÀ1), n the number of
electrons exchanged per electroactive molecule (here n=1 for
both oxidation and reduction), and S the geometric area of the
electrode (0.031 cm2).
Figure 4b presents the variation of G as a function of the
number of grafting cycles. The larger increase is obtained on
the first deposition voltammetric scan. Thereafter G continues
to increase, but from scan to scan the variation becomes
weaker. Finally, after 40 cycles, G reaches a value that nears the
limit of Gmax =(5.02Æ0.06)”10À10 molcmÀ2. This value com-
pares quite well with those reported previously for other elec-
trochemically fashioned covalently bonded porphyrin
films.[37,9,27] With regard to porphyrins grafted on surfaces, a re-
lation has been established between Gmax and the orientation
of the porphyrin plane relative to the surface.[25,38] In our case,
the Gmax value is in accordance with a monolayer structure of
the film and a roughly perpendicular arrangement of the por-
phyrin to the surface.
However, when Ni(PDTP) films were subjected to continuous
cycling over the range 0.00 to À1.6 V in DMF at scan rate
ꢀ1 VsÀ1, the redox system Of2/Rf2 progressively decreased in
current intensity before completely disappearing after 60
cycles, whereas the redox system Of3/Rf3 remained persistent
(see the Supporting Information). This result seems to confirm
that systems 2 and 3 are associated to distinct surface-bound
species.
The stability of the Pt-modified electrodes when exposed to
air was also investigated. As can be seen in Figure 4a, the
Water contact-angle characterization
The water contact-angle characterization of modified electro-
des gave additional evidence of the surface modification.[39] In
our case, the contact angle of a water droplet was increased
from 50 to 758 by the electrode functionalization (see
Figure 5). This result demonstrates that the porphyrin deposi-
tion provides a hydrophobic layer on the platinum substrate.
Figure 4. a) Cyclic voltammograms of a derivatized electrode in
CH2Cl2 +0.1m TEABF4 on a platinum gauze working electrode (geometric
area S=1 cm2) recorded at 1 VsÀ1 at (20Æ3)8C: after grafting (solid line)
and after two months (dashed line). b) Plots of surface concentration versus
number of grafting cycles.
Figure 5. Photographs of a water droplet (10 mL) on the surface of a platinum
plate working electrode before and after electrodeposition. The values of
the contact angles are shown inside the images.
signal of the modified electrode persisted over time. No
change could be detected with electrodes left on a laboratory
bench top for two months or with electrodes exposed to vigo-
rous ultrasonic cleaning in dimethylformamide, dichlorome-
thane, or acetonitrile. These results point to a strong covalent
bonding of the porphyrin moiety on the platinum surface
rather than a mere adsorption.
Conclusion
We have reported new electrochemical approaches for the
functionalization of electrode surfaces by a porphyrin that
bears a free meso position. In this manner, the nitro group is
first introduced on the macrocyclic ring. In a second step, elec-
trocatalytic hydrogenation affords the corresponding amino
derivative. Both reactions are accomplished with competitive
yields, thus making the electrochemical method very appealing
in such circumstances. The aminoporphyrin, under the form of
its nickel complex, is a convenient precursor suited for the
electrografting of the porphyrin onto a platinum electrode sur-
face. The process involves converting the aminoporphyrin into
its diazonium cation through its reaction with sodium nitrite
and trifluoroacetic acid. Further reduction by potential cycling
delivers the modified electrode. The grafted film demonstrates
Surface concentrations of electroactive nickel porphyrin
groups were estimated on the basis of the charge associated
with the film oxidation or reduction, in CH2Cl2 or DMF, respec-
tively. Both redox processes gave similar results. The associated
surface coverages (G) are calculated from Faraday’s law
[Eq. (1)]:
G ¼ Q=nFS
ð1Þ
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53 cm, Ø=1 mm) as the counter electrode, and a saturated calo-
mel electrode as the reference electrode. The Devarda working
electrode was prepared by co-deposition of Ni and Devarda alloy
particles on a stainless-steel grid (3 cm”4 cm).[40] Electrolyses were
followed by TLC and UV/Vis absorption measurements.
a well-behaved electrochemical response, which indicates that
the porphyrin structure is retained. Although further character-
ization will be necessary to obtain a formal proof, a direct
bonding of the porphyrin to the electrode surface (with no
linker between them) has been achieved. The specific elec-
trode properties that are anticipated from such a particularity
might be essential for developments in sensing and electro-
catalysis.
Synthesis
Magnesium(II)
5,15-ditolyl-10-phenylporphyrin
(Mg(PDTP)):
H2(PDTP) (200 mg, 0.35 mmol) and MgBr2 (2.60 g) were dissolved in
CH2Cl2 (50 mL) that contained triethylamine (4 mL, 80 equiv). The
reaction mixture was stirred for 10 min at room temperature. The
solution was then washed five times with distilled water (250 mL).
After evaporation of the solvent, purification by column chroma-
tography (alumina, 0 to 1% MeOH in CH2Cl2), and recrystallization
from MeOH/H2O, Mg(PDTP) was obtained in 80% yield (166 mg).
Experimental Section
Reagents and instrumentation
Tetraethylammonium hexafluorophosphate (TEAPF6; Fluka puriss.,
electrochemical grade, ꢀ99.0%) and 2,6-lutidine (Aldrich, ꢀ99%)
were used as received. Tetraethylammonium tetrafluoroborate
(TEABF4) was synthesized by the following method. Typically, in
a 500 mL Erlenmeyer flask, tetrafluoroboric acid (84.28 g; HBF4;
Sigma Aldrich, 48% H2O) was mixed with a solution of tetraethy-
lammonium hydroxide (193.84 g; TEAOH; Alfa Aesar, 35% H2O).
The reaction mixture was continuously stirred under an atmos-
phere of air. Then, a white precipitate formed after cooling the
flask in an ice bucket and was filtered using a Buchner apparatus.
Finally, the residue was crystallized from MeOH under reflux condi-
tions, then cooled in a freezer at À188C, filtered on a Buchner, and
dried at 110 8C in the stove for at least two days before use. CH2Cl2
(Carlo Erba 99.5%), CH3CN (SDS, Carlo Erba, HPLC gradient 99.9%),
and triethylamine (99%, Acros) were distilled from P2O5, CaH2, and
CaH2 respectively. H2(PDTP) was prepared as described in the
literature.[4a,14]
3
1H NMR (CD2Cl2, 300 MHz, 298 K): d=2.73 (s, 6H; CH3), 7.59 (d, J=
7.8 Hz, 4H; m-tol), 7.71–7.79 (m, 3H; m-and p-Ph), 8.12 (d, 3J=
3
7.8 Hz, 4H; o-tol), 8.20–8.23 (m, 2H; o-Ph), 8.88 (d, J=4.5 Hz, 2H;
3
3
b-Pyrr), 8.93 (d, J=4.5 Hz, 2H; b-Pyrr), 9.03 (d, J=4.3 Hz, 2H; b-
3
Pyrr), 9.35 (d, J=4.3 Hz, 2H; b-Pyrr), 10.19 ppm (s, 1H; b-Pyrr); UV/
Vis (CH2Cl2): lmax =420 (100), 558 (4.03), 595 nm (1.73); MALDI-TOF
+
C
MS (dithranol): m/z: 587.97 [M]
.
Electrosynthesis
Magnesium(II)
5-nitro-10,20-ditolyl-15-phenylporphyrin
(Mg(PDTPÀNO2)): The electrolysis was carried out under argon at
room temperature in CH3CN (170 mL) that contained TEAPF6
(0.1m), Mg(PDTP) (100 mg, 0.17 mmol), 2,6-lutidine (200 mL,
10 equiv), and NaNO2 (40 mg, 0.58 mmol) under vigorous stirring
(w=1350 rpm). The applied potential was Eapp =0.70 V versus SCE.
At the end of the electrolysis, 2 Faraday per mole of Mg(PDTP)
were transferred. The solution mixture was then evaporated to dry-
ness under reduced pressure. The resulting crude solid was dis-
solved in a minimum of CH2Cl2, and this solution was washed five
times with distilled water (750 mL) to remove the supporting elec-
trolyte. The organic phase was evaporated to dryness. The crude
product was then purified by column chromatography (alumina, 0
to 2% MeOH in CH2Cl2) and recrystallized from MeOH/H2O to give
Mg(PDTPÀNO2) in 91% yield (98 mg). 1H NMR (CD2Cl2, 300 MHz,
UV/Vis absorption spectra were recorded using a Varian Cary UV/
Vis spectrophotometer 50 scan using quartz cells (Hellma). In the
spectroelectrochemical experiments, a UV/Vis immersion probe
(Hellma, l=2 mm) was connected through a fiber optic to the
same spectrophotometer.
Mass spectra were obtained using a Bruker ProFLEX III spectrome-
ter (MALDI-TOF) with dithranol as a matrix or using a Bruker Micro-
ToF Q instrument in ESI mode.
3
NMR spectra were measured using a Bruker 300 MHz spectrometer
(Avance III Nanobay, Avance III, Avance II, respectively). The refer-
ence was the residual non-deuterated solvent.
298 K): d=2.72 (s, CH3; 6H), 7.59 (d, J=7.7 Hz, 4H; m-tol), 7.71–
3
7.81 (m, 3H; m-and p-Ph), 8.08 (d, J=7.4 Hz, 4H; o-tol), 8.16–8.20
3
(m, 2H; o-Ph), 8.82 (s, 4H; b-Pyrr), 8.98 (d, J=4.5 Hz, 2H; b-Pyrr),
All electrochemical manipulations were performed using Schlenk
techniques under an atmosphere of dry oxygen-free argon at
room temperature (T=(20Æ3)8C). The supporting electrolyte was
degassed under vacuum before use. Voltammetric analyses were
carried out in a standard three-electrode cell using an Autolab
PGSTAT 302N potentiostat connected to an interfaced computer
that employed Electrochemistry Nova software. A double-junction
SCE with background electrolyte between the two frits was used
as reference electrode. The auxiliary electrode was a platinum wire
in an independent compartment filled with the background elec-
trolyte and separated from the analyzed solution by a sintered
glass disk. For all voltammetric measurements, the working elec-
trode was a platinum disk electrode (Ø=2 mm). Under these con-
ditions, when operating in CH3CN (0.1m TEAPF6) or in CH2Cl2 (0.1m
TEABF4), the formal potential for the Fc+/Fc couple was found to
be +0.40 or +0.52 V versus SCE, respectively.
9.25 ppm (d, 3J=4.5 Hz, 2H; b-Pyrr); UV/Vis (CH2Cl2): lmax =426
+
C
(100), 565 (7.97), 619 nm (6.12); MS (ESI-MS): m/z: 634.21 [M+1]
.
5-Amino-10,20-ditolyl-15-phenylporphyrin (H2(PDTPÀNH2)): The
synthesis protocol was inspired by the work of J. Lessard et al.[21d]
In practice, the activated Devarda electrode (3 cm”4 cm) was
placed in the cell that contained MeOH (170 mL) with H2O (1.5%)
and KOH (0.27m) under argon. The electrolysis was carried out
under argon at room temperature under vigorous stirring (w=
1350 rpm). The applied potential was Eapp =À1.20 V versus SCE to
generate hydrogen at the surface of the electrode for at least
20 min. After the addition of Mg(PDTPÀNO2) (70 mg) in one por-
tion, 6 Faradays per mole of Mg(PDTPÀNO2) were transferred at
À0.96 V versus SCE. At the end of the electrolysis, concentrated hy-
drochloric acid (5 mL) was added, and this mixture was stirred for
1 min. After evaporation of the solvent, the colored residue was
dissolved in CH2Cl2. The precipitated KCl was then removed by fil-
tration. The green filtrate was evaporated to dryness and neutral-
ized with saturated sodium acetate solution (100 mL). The mixture
was extracted with CH2Cl2. The organic phase was washed twice
Bulk electrolyses were performed in a three-compartment cell sep-
arated by glass frits of medium porosity using an Amel 552 poten-
tiostat coupled with an Amel 721 electronic integrator. A platinum
grid was used as the working electrode, a platinum wire spiral (l=
Chem. Eur. J. 2015, 21, 8281 – 8289
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8287
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
with distilled water (250 mL). After evaporation of the solvent, pu-
rification by column chromatography (alumina, CH2Cl2), and recrys-
tallization by evaporation of CH2Cl2, H2(PDTPÀNH2) was obtained in
and dried using argon gas. The modified electrodes were charac-
terized by cyclic voltammetry in TEABF4/CH2Cl2 (0.1m) and TEABF4/
DMF (0.1m).
1
80% yield (51 mg). H NMR (CD2Cl2, 300 MHz, 298 K): d=À0.78 (s,
3
2H; NH), 2.68 (s, 6H; CH3), 5.81 (s, 2H; NH2), 7.51 (d, J=7.71 Hz,
Water contact-angle characterization
4H; m-tol), 7.62–7.71 (m, 3H; m-and p-Ph), 7.94 (d, 3J=7.92 Hz,
4H; o-tol), 8.02–8.05 (m, 2H; o-Ph), 8.39 (d, 3J=4.53 Hz, 2H; b-
This technique allows one to discriminate between the polar or
nonpolar nature of the liquid–solid interactions at the interface.
One can thus deduce the hydrophilic or hydrophobic character of
a surface. The method involves measuring the angle of the tangent
of the profile of a drop deposited on the substrate with the sub-
strate surface. In essence, a drop of ultrapure water (10 mL) is de-
posited by using a syringe on the surface of the same platinum
plate electrode before and after its modification.
3
3
Pyrr), 8.40 (d, J=4.89 Hz, 2H; b-Pyrr), 8.51 (d, J=4.89 Hz, 2H; b-
Pyrr), 8.62 ppm (d, 3J=4.62 Hz, 2H; b-Pyrr); UV/Vis (CH2Cl2): lmax
(loge)=425 (5.46), 542 (3.83), 580 (4.27), 622 (3.53), 686 nm (4.24);
+
C
MALDI-TOF MS (dithranol): m/z: 581.98 [M]
.
Synthesis of nickel(II) 5-amino-10,20-ditolyl-15-phenylporphyrin
(Ni(PDTPÀNH2)): The porphyrin Ni(PDTPÀNH2) was synthesized by
metalation of H2(PDTPÀNH2) with Ni(OAc)2·4H2O according to
known procedures.[19c,22] The porphyrin H2(PDTPÀNH2) (333 mg,
0.573 mmol) was dissolved in DMF (20 mL), and Ni(OAc)2·4H2O
(717 mg, 2.882 mmol) was added in one portion. The reaction mix-
ture was then degassed with argon for 10 min and heated under
reflux conditions in a preheated oil bath (1708C) for 15 min under
argon. After cooling at room temperature, deionized water (30 mL)
was added, and the resulting solid was isolated by filtration and
washed with water. This residue was finally purified by column
chromatography (SiO2, CH2Cl2) and recrystallized from a CH2Cl2/
MeOH mixture to yield Ni(PDTPÀNH2) in 84% yield (310 mg,
Acknowledgements
The authors would like to acknowledge and give thanks to the
UniversitØ de Bourgogne, the CNRS, and the Conseil RØgional
de Bourgogne (CRB) for funding. S.H. is particularly grateful to
the CRB for providing his research grant (convention no.
B190CVHCP-2013). We are thankful for Sophie Fournier’s tech-
nical assistance. The authors thank Jean Lessard for his kind
advice on the best way to perform electrocatalytic hydrogena-
tion. The authors are very thankful to Dr. Benoit Habermeyer
and PorphyChem Company for the generous gift of a PDTP
sample.
1
0.486 mmol). H NMR (CD3COCD3, 300 MHz, 300 K): d=2.60 (s, 6H;
3
CH3), 7.50 (d, J=7.9 Hz, 4H; m-tol), ꢁ7.51 (s, 2H; NH2), 7.66 (m,
3
3H; m-and p-Ph), 7.79 (d, J=7.9 Hz, 4H; o-tol), 7.89 (m, 2H; o-Ph),
3
3
8.26 (d, J=5.0 Hz, 2H; b-Pyrr), 8.33 (d, J=4.8 Hz, 2H; b-Pyrr), 8.37
(d, 3J=5.0 Hz, 2H; b-Pyrr), 9.26 ppm (d, 3J=4.8 Hz, 2H; b-Pyrr);
13C{1H} NMR (CD3COCD3, 125 MHz, 300 K): d=144.5, 143.1, 142.9,
140.3, 138.1, 137.9, 137.1, 134.9, 133.7 (two peaks), 133.4, 132.8,
129.3, 128.3, 128.0, 127.7, 127.1, 122.7, 121.5, 21.6 ppm; Rf =0.48
(SiO2, CH2Cl2/n-pentane, 50/50 v/v); UV/Vis (CH2Cl2): lmax (loge)=
291 (4.24), 359 (3.94), 425 (5.30), 542 (3.92), 599 nm (4.13); HRMS
(ESI-MS): m/z calcd for C40H29N5Ni: 637.1771 [M]+; found: 637.1765.
Keywords: diazonium · electrochemistry · electrografting ·
electrosynthesis · porphyrins
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Electrografting of the nickel(II) 10,20-ditolyl-15-phenylporphyrin
diazonium generated in situ: Platinum electrodes (Pt disk Ø=
2 mm and Pt plate of S=1 cm2, geometric area) were modified by
electroreduction of Ni(PDTPÀN2+). The porphyrin diazonium was
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Published online on April 17, 2015
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