Control over the oxidative reactivity of metalloporphyrins. Efficient electrosynthesis of meso,meso-linked zinc porphyrin dimer

Article abstract of DOI:10.1039/c1dt11330e

The electrochemical oxidation of zinc(ii) 5,15-p-ditolyl-10-phenylporphyrin at its first oxidation potential leads to the formation of the corresponding meso-meso porphyrin dimer as the main product. The number of electrons abstracted, the addition of the hindered base 2,6-lutidine as well as operating in DMF, instead of a CH₂Cl₂/CH₃CN mixture are the key parameters to obtain high yields of the desired coupling product. Indeed, when the electrolyses are carried out in the CH₂Cl ₂/CH₃CN mixture, the unexpected zinc(ii) 5-chloro-10,20-p-ditolyl-15-phenyl porphyrin is produced as a by-product, the chlorine atom originating from the CH₂Cl₂ solvent. The monomer and the dimer are characterised by electrochemical analysis. The signature of the dimer is clearly distinguished on the cyclic voltammogram of the monomer on condition of the prior addition of 2,6-lutidine as a hindered base, indicating that the dimerisation process is thus strongly accelerated. Besides, unprecedented X-ray crystallographic structures of the monomer and the meso-meso dimer are presented and their respective structural parameters are compared.

Full text of DOI:10.1039/c1dt11330e

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PAPER
Control over the oxidative reactivity of metalloporphyrins. Efficient
electrosynthesis of meso,meso-linked zinc porphyrin dimer†
Abdou K. D. Dime, Charles H. Devillers,* He´le`ne Cattey, Benoˆıt Habermeyer and Dominique Lucas*
Received 14th July 2011, Accepted 6th October 2011
DOI: 10.1039/c1dt11330e
The electrochemical oxidation of zinc(II) 5,15-p-ditolyl-10-phenylporphyrin at its first oxidation
potential leads to the formation of the corresponding meso–meso porphyrin dimer as the main product.
The number of electrons abstracted, the addition of the hindered base 2,6-lutidine as well as operating
in DMF, instead of a CH2Cl2/CH3CN mixture are the key parameters to obtain high yields of the
desired coupling product. Indeed, when the electrolyses are carried out in the CH2Cl2/CH3CN mixture,
the unexpected zinc(II) 5-chloro-10,20-p-ditolyl-15-phenyl porphyrin is produced as a by-product, the
chlorine atom originating from the CH2Cl2 solvent. The monomer and the dimer are characterised by
electrochemical analysis. The signature of the dimer is clearly distinguished on the cyclic
voltammogram of the monomer on condition of the prior addition of 2,6-lutidine as a hindered base,
indicating that the dimerisation process is thus strongly accelerated. Besides, unprecedented X-ray
crystallographic structures of the monomer and the meso–meso dimer are presented and their respective
structural parameters are compared.
salts15 and finally m-chloroperbenzoic acid16‡ leads principally
Introduction
to meso–meso and/or b-meso dimers, depending on the metal
Owing to their essential role in photosynthetic processes and
inserted in the macrocycle. This wide variety of oxidising agents
also due to their potential integration into energy converting
demonstrates that the choice of the right reactant is not straightfor-
devices, multiporphyrin arrays have attracted considerable atten-
ward. Accordingly, the ease of oxidation of the metalloporphyrin
tion over the two last decades.1 Numerous spacers have been
may change to a large extent as a function of the metal inside
used to hold the porphyrin units together, including groups such
and/or the substituents around the macrocycle. At this point,
as phenyl, thiophene, ethynyl and aniline.2 Besides, the direct
oxidative C–C coupling, i.e. without any spacer, appears as
an interesting alternative, shortening the distance between the
oxidation of the monomer by electrochemical techniques seems
an interesting green alternative, since it is possible to modulate
the “oxidative power” of the working electrode thanks to the
porphyrin macrocycles and thus increasing the communication
fine tuning of its potential. Very few examples dealing with
between them. Demonstrating the growing interest in this kind
the electrosynthesis of directly linked meso–meso dimers and/or
oligomers starting from porphyrin monomers exist.17–19 Some
publications have reported the electrochemical synthesis of dimers
starting from a tris-meso-substituted monomer but without any
experimental detail.20 To the best of our knowledge, the highest
yield described for the electrochemical meso–meso coupling of
two 5,10,15-meso-substituted monomer units is 47.5%.17 Quite
surprised by this moderate yield, we questioned ourselves about
the ways it could be improved. Our own contributions in this
field are related to magnesium porphine, the totally unsubstituted
porphyrin, for which electrochemical oxidation was demonstrated
to form meso–meso-linked oligomers in solution19 and polymers
on the electrode surface.21 The addition of a hindered base in
the reaction medium was found to avoid demetallation of the
acid sensitive magnesium cation, thanks to the capture of free
of multiporphyrinic assembly, a great number of studies have
been reported on the chemical synthesis of directly connected
dimers, trimers and oligomers.3–6 Starting with a 5,10,15-meso-
substituted porphyrin monomer, the oxidative C–C coupling using
chemical reagents such as Ag+,7 thallium(III) trifluoroacetate,8
tris(4-bromophenyl)aminiumhexachloroantimonate (BAHA),9,10
DDQ,11,12 hypervalent iodine (PhICl2 and PhIF2,13 PhI(OCOCF3)2
(PIFA)4,14 or PhI(OAc)2, (PIDA)4), mixtures of Ag+ and Au3+
Institut de Chimie Mole´culaire de l’Universite´ de Bourgogne, Universite´
de Bourgogne, CNRS UMR 5260, 9 avenue Alain Savary, 21078, Dijon
Cedex, France. E-mail: charles.devillers@u-bourgogne.fr, dominique.lucas@
u-bourgogne.fr; Fax: +33380396065; Tel: +33380399125
† Electronic supplementary information (ESI) available: Characterisation
data of 1-Zn, 2-Zn, 1-Zn-Cl, 1-H2. CCDC reference numbers 837061 and
837062. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c1dt11330e
‡ In this case, the C–C bond formation is driven by m-chloroperbenzoic
acid as the oxidant and uses a MnIV(O) porphyrin catalyst facilitating both
the proton elimination and electron transfer.
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observed reflections: R1 = 0.0612, wR2 = 0.1067, R indices for all
data: R1 = 0.0866, wR2 = 0.1169, goodness of fit = 1.155, Dr =
3
˚
0.355/-0.476 e.A .
Crystal data for 2-Zn [C86H66N8O2Zn2, C3H6O], dark-red prism
crystal: 0.23 ¥ 0.13 ¥ 0.10 mm3, M = 1432.29 g mol-1, triclinic,
¯
˚
space group P1 (No. 2), Z = 2, a = 10.◦9880(2) A, b = 16.0826(4)
◦
˚
˚
A, c = 22.0181(5) A, a = 73.0542(8) , b = 81.9980(12) , g =
◦
3
-3
˚
74.2166(12) , V = 3577.32(14) A , Dc = 1.330 g cm , m = 0.729
mm-1, T = 115(2)K, F(000) = 1492, Mo-Ka radiation (l = 0.71073
˚
A), mixture of f rotations and j scans, 28 953 reflections collected
on a Nonius Kappa CCD diffractometer (index ranges: h = 14;
k = 20; l = 28) measured in the range 1.5◦ £ q £ 27.51◦, with
16 167 independent (Rint = 0.0237) and 13 792 observed reflections
[I ≥ 2s(I)], 943 refined parameters, 18 restraints, R indices for
observed reflections: R1 = 0.0451, wR2 = 0.1038, R indices for all
data: R1 = 0.0572, wR2 = 0.1120, goodness of fit = 1.080, Dr =
3
˚
0.625/-0.557 e.A .
X-ray equipment and refinement. Diffraction data were col-
lected on a Nonius KappaCCD diffractometer equipped with a
nitrogen jet stream low-temperature system (Oxford Cryosystems).
The X-ray source was graphite-monochromated Mo-Ka radiation
˚
(l = 0.71073 A) from a sealed tube. Data were reduced by using
Scheme 1 Structures of the monomer (1-H2, 1-Zn), 5-chloro substituted
porphyrin, meso–meso dimer (2-H2, 2-Zn).
DENZO22 software without applying absorption corrections; the
missing absorption corrections were partially compensated by
the data scaling procedure in the data reduction. The structure
was solved by direct methods using the SIR9223 program and
refined with full-matrix least-squares on F2 using the SHELXL9724
program with the aid of the WIN-GX25 program suite. All non-
hydrogen atoms were refined with anisotropic thermal parameters.
Hydrogen atoms attached to carbon atoms were included in
calculated positions and refined as riding atoms. For compound
2-Zn, one of the acetone groups linked to the porphyrin was found
to be disordered over two positions and refined with an occupation
factor of 0.68/0.32. Only the minor components of the disordered
acetone group were isotropically refined. Moreover, all bonds and
angles of the porphyrin linked acetone groups were restrained to
be identical (SAME24 instruction).
protons released during the formation of new C–C bonds. In the
course of this research we felt that the study of a model porphyrin
bearing a unique free meso position could be helpful towards a
better understanding and a finer control of the oxidative reactivity
of porphine. We turned our attention towards the tri-meso-
substituted zinc(II) 5,15-p-ditolyl-10-phenylporphyrin model 1-
Zn (Scheme 1) and its reaction towards the meso–meso dimer
2-Zn. Although the synthesis and characterisation of 1-Zn and
2-Zn have been recently reported,16 their X-ray crystallographic
structures are presently unknown: these are presented below. The
voltammetric behaviour of both compounds with or without the
presence of an additive base are analysed.
Also we describe our electrochemical oxidation tests on 1-Zn
scrutinising the reaction selectivity according to the experimental
conditions. Finally we draw the decisive parameters by which the
reaction has been rendered quantitative into the formation of the
meso–meso dimer.
Reagents and instrumentation
Porphyrin 1-Zn was synthesised according to known procedures.26
Our data (1H NMR, 13C NMR, UV-Visible absorption, and
MALDI-TOF mass spectra) were consistent with those described
in reference 26 (see ESI†). Tetraethylammonium hexafluorophos-
phate (TEAPF6, Fluka puriss., electrochemical grade, ≥99.0%)
and 2,6-lutidine (Aldrich, purified by redistillation, ≥99%) were
used as received. Tetra-n-butylammonium hexafluorophosphate
(TBAPF6) was synthesised by mixing stoichiometric amounts of
tetra-n-butylammonium hydroxide (Alfa-Aesar, 40% w/w aq. sol.)
and hexafluorophosphoric acid (Alfa-Aesar, ca. 60% w/w aq. sol.).
After filtration, the salt was recrystallised three times in ethanol
and dried at 80 ◦C for at least two days. CH2Cl2 (Carlo Erba
99.5%), CH3CN (SDS, Carlo Erba, HPLC gradient 99.9%) and
DMF (SDS, Carlo Erba, purity (GC) 99.9%) were distilled from
P2O5, CaH2 and CaH2 respectively.
Experimental
Crystallography
X-ray crystallographic study. Crystal data for 1-Zn
[C40H28ZnN4], red prism crystal: 0.18 ¥ 0.15 ¥ 0.15 mm3,
M = 630.03 g mol-1, monoclinic, space group P21/c (No. 14),
˚
˚
˚
Z = 4, a = 22.8237(7) A, b = 9.3135(3) A, c = 15.0499(5) A,
◦
3
-3
˚
b = 107.2963(14) , V = 3054.47(17) A , Dc = 1.370 g cm , m =
0.840 mm-1, T = 115(2)K, F(000) = 1304, Mo-Ka radiation (l =
˚
0.71073 A), mixture of f rotations and j scans, 12 577 reflections
collected on a Nonius Kappa CCD diffractometer (index ranges:
h = 29; k = -11/12; l = 19) measured in the range 1.0◦ £ q £
27.48◦, with 6935 independent (Rint = 0.0354) and 5272 observed
reflections [I ≥ 2s(I)], 408 refined parameters, R indices for
UV-visible absorption spectra were obtained with a Varian UV-
vis spectrophotometer Cary 50 scan using quartz cells (Hellma).
In spectroelectrochemical experiments, a UV-vis immersion probe
930 | Dalton Trans., 2012, 41, 929–936
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(Hellma, l = 2 mm) was connected through a fibre optic to the
same spectrophotometer.
Mass spectra were obtained on a Bruker ProFLEX III spec-
trometer (MALDI-TOF) using dithranol as a matrix.
NMR spectra were measured on a BRUKER 300, 500 or
600 MHz spectrometer (Avance III Nanobay, Avance III, Avance
II, respectively). The reference was the residual non-deuterated
solvent.
Elemental analyses (C, H, N, S) were carried out on a Flash EA
1112 Thermo Electron analyser.
Rf 0.48 (Al2O3, CH2Cl2/n-hexane, 50/50, v/v); MALDI-TOF
1
MS (dithranol): [M]+ = 1254.28; H NMR (THF-d8, 500 MHz,
298 K): d (ppm) 8.88 (s, b-Pyrr, 8H), 8.53 (d, 3J = 4.7 Hz,
b-Pyrr, 4H), 8.26–8.29 (m, o-Ph, 4H), 8.08 (d, 3J = 8.1 Hz,
o-Tol, 8H), 8.01 (d, 3J = 4.7 Hz, b-Pyrr, 4H), 7.76–7.80 (m,
m-and p-Ph, 6H), 7.47 (d, 3J = 8.0 Hz, m-Tol, 2H), 2.59 (s,
CH3, 12H); lmax (CH2Cl2)/nm (log e) 419 (5.40), 455 (5.40),
559 (4.76); Elemental analysis (Found: C, 74.08; H, 4.62; N,
8.26%. Calc. for C80H54Zn2N8·2H2O: C, 74.25; H, 4.52; N,
8.66%).
All electrochemical manipulations were performed using
Schlenk techniques in an atmosphere of dry oxygen-free argon at
room temperature (T = 20 ◦C 3 ◦C). The supporting electrolyte
was degassed under vacuum before use and then dissolved to a con-
centration of 0.1 mol L-1. Voltammetric analyses were carried out
in a standard three-electrode cell, with an Autolab PGSTAT 302 N
potentiostat, connected to an interfaced computer that employed
Electrochemistry Nova software. The reference electrode was a
saturated calomel electrode (SCE) separated from the analysed
solution by a sintered glass disk filled with the background
solution. The auxiliary electrode was a platinum wire separated
from the analysed solution by a sintered glass disk filled with
the background solution. For all voltammetric measurements, the
working electrode was a platinum disk electrode (Ø = 2 mm). In
these conditions, when operating in a mixture of CH2Cl2/CH3CN
4/1 (0.1 M TEAPF6) the formal potential for the Fc+/Fc couple
was found to be +0.40 V vs. SCE.
Results and discussion
Crystallography
As they were not previously reported, the crystallographic
structures of the starting monomer 1-Zn and the meso–meso
dimer 2-Zn were investigated in the course of this work. It is
noteworthy that only six zinc(II) porphyrin meso–meso dimers are
structurally described in the literature.5,27–29 Suitable crystals for X-
ray diffraction studies were obtained by slow diffusion of n-hexane
into a CH2Cl2 solution of 1-Zn and evaporation of a CD3COCD3
solution containing 2-Zn. The three-dimensional molecular views
are presented in Fig. 1 and 2 and the most representative data
Bulk electrolyses were performed in a two or three compartment
cell with glass frits of medium porosity with an Amel 552
potentiostat coupled with an Amel 721 electronic integrator. A
platinum wire spiral (l = 53 cm, Ø = 1 mm) was used as the
working electrode, a platinum plate as the counter electrode and a
saturated calomel electrode as the reference electrode. Electrolyses
were followed by TLC and UV-visible absorption measurements.
Electrosynthesis of 2-Zn
Electrolyses were carried out under argon, at room temperature,
in 32 mL of DMF containing 0.1 M of TEAPF6, 20.0 mg (31.74
mmol, c = 9.92 ¥ 10-4 M) of 1-Zn26 and 10 eq. of 2,6-lutidine (37 ml)
in a two compartment cell under vigorous stirring (w = 1350 rpm).
The applied potential was Eapp = 0.85 V/ECS and the initial current
was 1.75 mA. At the end of the oxidation step (Dt = 80 min.), 2.5
faraday per mol of 1-Zn were transferred with a final current of
1.5 mA. To extinguish the residual traces of cation radicals in
solution (reduction step), a potential of 0.00 V/ECS was applied
in the same cell configuration but with the auxiliary electrode
protected from the electrolysed solution with a sintered glass disk
filled with the background solution. The initial current was -0.2
mA and 0.039 faraday per mol of 1-Zn was transferred. The
solution mixture was then evaporated to dryness under reduced
pressure. The resulting crude solid was dissolved in a minimum of
CH2Cl2 and this solution was washed with 4 ¥ 250 mL of distilled
water to remove the supporting electrolyte. The organic phase
was evaporated to dryness. The crude product was then purified
by column chromatography (Alumina, CH2Cl2/n-heptane/Et3N
(69.9/30/0.1)). The first collected fraction contained 2-Zn which
was finally obtained as a brown/purple solid in 93.5% yield after
recrystallisation from EtOH/H2O.
Fig. 1 Front and side views of 1-Zn crystallographic structure (H atoms
were omitted for clarity; plot of 1-Zn with 30% probability).
Fig. 2 Ortep view of 2-Zn crystallographic structure (H, disordered atoms
and one free deuterated acetone molecule were omitted for clarity; plot of
2-Zn with 30% probability).
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Table 1 Selected structural parameters for 1-Zn and 2-Zn
1-Zn
2-Zn
Zinc(II) coordination sphere
d(Zn–O)
Tetracoordinated (no axial ligand)
Pentacoordinated (axial acetone-d6)
2.2113(34)/2.1823(24) A
a
˚
d(Zn-mean plane)b
d(Zn–N)
0.0100(15) A
0.268(1)/0.283(1) A
˚
˚
˚
˚
2.034(3) < d < 2.039(3) A
2.045(2) < d < 2.075(2) A
56.70(9)◦/61.89(7)◦
69.10(5)◦/72.57(5)◦
66.44(5)◦/72.23(6)◦
Ph torsion anglesc
Tol torsion angles
76.23(12)◦
84.59(9)◦/88.69(9)◦
˚
˚
d(Cmeso–CPh or Tol
d(Cmeso–Cmeso
“meso-meso” torsion angled
)
1.501(4) < d < 1.504(5) A
1.494(3) < d < 1.503(3) A
a
a
˚
)
1.507(3) A
72.76(26)◦
a Not relevant. b Mean plane formed by 4 nitrogen atoms of one porphyrin unit. c Porphyrin mean plane–phenyl group or tolyl mean plane angle.
d Considered torsion angle: C1–C20–C21–C22.
are given in Table 1 (see also ESI† for complete X-ray data). As
a notable difference between the two compounds, the central zinc
cation is tetracoordinated in 1-Zn by four pyrrolic nitrogen atoms,
whereas in 2-Zn each zinc atom additionally supports an axial
deuterated acetone molecule. The corresponding bond lengths,
Electrochemistry
All the potential values cited in this manuscript will be referred to
the saturated calomel electrode (SCE).
The cyclic voltammogram (CV) of 1-Zn in CH2Cl2/CH3CN
(4/1 v/v) 0.1 M TBAPF6 (solid line, top of Fig. 3) exhibits
two monoelectronic reversible oxidations leading respectively to
the cation-radical (peak system O1/R1, E1/2 = 0.72 V; see Table
2) and the dication (O2/R2, E1/2 = 1.15 V) and one reversible
monoelectronic reduction leading to the radical anion species
(R3/O3, E1/2 = -1.415 V). In that way, the response of 1-Zn is
representative of the classical behaviour of metalloporphyrins with
a non-electroactive metal, which undergoes two reversible one-
electron transfers, both in oxidation and reduction.30 For 1-Zn,
the second reduction of the porphyrin is not observed since it falls
beyond the solvent reduction.
˚
˚
d(Zn–O1) = 2.2113(34) A and d(Zn2–O2) = 2.1823(24) A, can be
correlated to the ones reported for axial ligation of methanol in a
similar porphyrin dimer compound (d(Zn–O) = 2.248 and 2.297
29
˚
A). This change in the coordination number of zinc profoundly
influences the porphyrin geometry. In 1-Zn, the macrocycle is
almost planar including the central Zn atom (distance of Zn to
the mean plane formed by the four pyrrolic nitrogen atoms =
˚
0.0100(15) A). On the contrary, in 2-Zn, each porphyrin unit
deviates markedly from planarity and the Zn atoms are clearly out
of plane (distance from the four nitrogen atom-containing mean
˚
plane = 0.268(1) and 0.283(1) A, for Zn1 and Zn2 respectively).
As an immediate consequence, the zinc atom is more distant
from the pyrrolic nitrogen atoms in 2-Zn (2.045(2) £ d(Zn–N) £
˚
˚
2.075(2) A) than in 1-Zn (2.034(3) £ d(Zn–N) £ 2.039(3) A). The
torsion angle between the tolyl groups and the porphyrin plane
is also dependent on the steric hindrance of the axially ligated
acetone molecule. This torsion angle is close to orthogonality in 1-
Zn: (torsion angles equal to 84.59(9)◦ and 88.69(9)◦) but undergoes
a substantial reduction in 2-Zn (from 66.44(5)◦ to 72.57(7)◦). In
a similar way, from 1-Zn to 2-Zn, to accommodate the acetone
molecule, the phenyl groups are forced to rotate around the Cmeso
–
Cipso bond (in 1-Zn, this torsion angle is equal to 76.23(12)◦ against
56.70(9)◦ and 61.89(7)◦ in 2-Zn).
Certain geometric features are not sensitive to the transforma-
tion into the dimer, being nearly identical from one structure to
the other. These include the Cmeso–Cipso bond length, which value
falls into a relatively narrow range (in 1-Zn, 1.501(4) < d(Cmeso
–
)
˚
Ctolyl or phenyl) < 1.504(5) A; in 2-Zn, 1.494(3) < d(Cmeso–Ctolyl or phenyl
˚
< 1.503(3) A), typically intermediate between a single and double
bond. Particular to 2-Zn, the meso–meso bond connecting the
two aromatic macrocycles lies in the same interval (d(C20–C21) =
Fig. 3 Cyclic voltammograms of 1-Zn (top) without (solid line) or
with (dotted line) 10 molar eq. of 2,6-lutidine and 2-Zn (bottom) in
CH2Cl2/CH3CN (4/1 v/v) 0.1 M TBAPF6. Concentration: 5 ¥ 10-4
for 1-Zn and 2.5 ¥ 10-4 M for 2-Zn; WE: Pt Ø = 2 mm, n = 100 mV s-1.
M
˚
1.507(3) A). The torsion angle (C1–C20–C21–C22) between the
two porphyrin macrocycles is 72.76(26)◦ and appears to be the
smallest among the meso–meso zinc porphyrin dimers described in
the literature. The resultant intramolecular metal–metal distance
The CV of 1-Zn undergoes drastic changes in basic conditions
(dotted line, top of Fig. 3). Indeed, when 10 molar equivalents of
2,6-lutidine as a non-nucleophilic base are added to the solution,
the first oxidation process (peak O1¢) becomes almost irreversible
˚
is 8.3037(4) A, being the shortest Zn–Zn distance reported to date
over this family of compounds.
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Table 2 Potential values for 1-Zn and 2-Zn in the presence or absence of base in CH2Cl2/CH3CN (4/1 v/v) 0.1 M TBAPF6
1st oxidation 2nd oxidation 1st reduction
2nd reduction
Compounds
Epa
Epc
E1/2
Epa
Epc
E1/2
Epc
Epa
E1/2
Epc
Epa
1-Zn
0.75
0.75
0.72
0.72
0.69
0.65
0.66
0.66
0.72 (60)
—
0.69 (60)
0.69 (60)
1.18
0.88
0.88
0.88
1.12
0.81
0.81
0.81
1.15 (60)
—
0.845 (70)
0.845 (70)
-1.38
-1.38
-1.35
-1.35
-1.45
-1.45
-1.40
-1.40
-1.415 (70)
—
—
-1.44
-1.44
—
—
-1.51
-1.51
1-Zn + basea
2-Zn
-1.415 (70)
—
—
2-Zn + basea
a + base means in the presence of 10 or 20 equivalents of 2,6-lutidine vs. 1-Zn or 2-Zn, respectively. Values in brackets correspond to DE = (Epa + Epc)/2.
with an ca. 1.35 fold increase of the intensity along with the
appearance of a new peak at Ep(O2¢) = 0.88 V. When the positive
potential scan is reversed just after peak O1¢, one reduction peak
(R1¢) is seen at a lower potential (Ep(R1¢) = 0.65 V) than the
reduction peak (R1) of 1-Zn (Ep(R1) = 0.69 V). Furthermore,
when the positive potential scan is reversed after peak O2¢, one
additional peak (R2¢) is observed at ca. 0.81 V. These additional
features induced by the addition of base are proved to come
from the rapid formation at the electrode of the simply linked
dimer 2-Zn. Thus, the base strongly accelerates the rate of the
dimer formation. Indeed, the voltammetric analysis of 2-Zn under
the same experimental conditions affords the same set of peaks
O1¢/R1¢ and O2¢/R2¢ (bottom of Fig. 3). These discriminated
oxidations are associated with the successive reactions of the two
porphyrin rings yielding respectively a cation radical and a bis-
cation radical as established previously.6,10,14 Thus, the oxidation
of the first porphyrin unit makes the second one harder to oxidise.
For two chemically equivalent and non-interacting redox enti-
ties, the theoretical DE1/2 value should be 35.6 mV,31 whereas in
our case DE1/2 = 155 mV which is in agreement with a strong
electronic communication between these two redox centers.6,28
A third irreversible highly intense oxidation peak appears at
Ep(O3–O4¢) = 1.28 V, corresponding probably to the simultaneous
second oxidation of both porphyrin units (bis-dication). In the
negative potential direction, the CV voltammogram exhibits two
monoelectronic reversible reductions at E1/2(R6¢) = -1.40 and
window caused by the solvent reduction. It should be noted that
the addition of a base (2,6-lutidine) in a solution of 2-Zn does not
change its voltammetric trace, contrary to what is observed with
1-Zn. This implies a relative stability, at least on the time scale of
the CV, of the oxidised and reduced forms of 2-Zn.
Electrosynthesis of meso,meso dimer 2-Zn
In the search for a medium suited to the preparation and isolation
of this compound, tetraethylammonium hexafluorophosphate was
selected as the supporting electrolyte (TEAPF6) affording an
easy separation from the electrosynthesised organic products by
extraction in water.32,33 Due to the relatively low solubility of this
organic salt in pure CH2Cl2, a mixture of CH2Cl2/CH3CN (4/1
v/v) was primarily chosen allowing a suitable 0.1 M concentration
to be reached.
As summarised in Table 3, the following experimental param-
eters were systematically varied with regard to the selectivity of
the formation of 2-Zn: the addition of 2,6-lutidine, the cell setup
(anode and cathode in a one or two separated compartments)
and, in the case of the unseparated cell assembly, the total
charge. The detected products are the unreacted monomer 1-Zn,
the demetallated monomer 1-H2 (ESI†) the unexpected 5-chloro
derivative 1-Zn-Cl and the targeted simply linked dimer 2-Zn (see
structures in Scheme 1). The relative distribution of all of them at
the end of each electrolysis is established by 1H NMR spectroscopy.
The electrolyses are operated in two stages. First, the potential is
set at Ew = 0.75 V vs. SCE until 1-Zn is completely exhausted,
after which the working potential is moved to 0.00 V to reduce the
cation radicals possibly generated in the first period.
E
1/2(R7¢) = -1.485 V corresponding to the successive first reduction
of each porphyrin ring (anion radical and bis-anion radical). As
for 1-Zn, the second reduction stage of the porphyrin rings is
not observed in cyclic voltammetry due to the restricted potential
Table 3 Tested conditions for the electrosynthesis of 2-Zn starting with 1-Zna
Experimental conditions
# of
1st step: oxidation
Faraday vs.
2nd step: reductionf
Faraday vs.
Distribution of productsg
Entry Solvent
compartmentsd Presence of basee Eapp (V/SCE) 1-Zn
Eapp (V/SCE) 1-Zn
1-Zn 1-H2 1-Zn-Cl 2-Zn
1
2
3
4
5
6
DCM/ACNb
3
3
2
2
2
2
no
0.75
0.75
0.75
0.75
0.75
0.85
1.50
1.50
1.74
2.50
2.50
2.50
0.00
0.00
0.00
0.00
0.00
0.00
0.56
0.49
0.17
0.54
0.57
0.11
—
—
8%
—
—
—
26% 4%
70%
95%
89%
91%
85%
>95%
DCM/ACNb
DCM/ACNb
DCM/ACNb
DCM/ACNb
DMFc
yes
yes
yes
no
—
—
—
6%
—
5%
3%
9%
9%
—
yes
a Conditions common to all experiments: initial amount of 1-Zn (m = 4.8 mg, c = 5.0 ¥ 10-4 M); electrode materials (both Pt for anode and cathode).
b Experiments performed in dichloromethane (DCM)/acetonitrile (ACN) 4/1 v/v 0.1 M TEAPF6. c Experiments performed in DMF 0.1 M TEAPF6. d
2
or 3 compartments: the anode and the cathode are in the same compartment or in separated compartments, respectively. e “presence of base”: 10 molar
1
equivalents of 2,6-lutidine vs. 1-Zn. f This step was performed in a three compartment cell. g The distribution of products was calculated from the H
NMR spectrum of the crude electrolysed solution by integration of characteristic 1H NMR signals of 1-Zn, 1-H2, 1-Zn-Cl and 2-Zn (see an example of
calculation in ESI†). The precision of these measurements is estimated to be ca. 2%.
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The formation of the 5-chloro substituted porphyrin 1-Zn-Cl
was unforeseen. This compound was unambiguously identified
by an independent synthesis to obtain its complete spectroscopic
data (ESI†). The preparation was conducted through a clean
reaction of 1-Zn with N-chlorosuccinimide (ESI†). Particularly, in
1H NMR spectroscopy, 1-Zn-Cl displays a characteristic pattern
of four doublets at 8.80, 8.85, 8.94 and 9.69 pp (3J = 4.7 Hz; each
signal integrating for 2 protons), corresponding to the b-pyrrolic
hydrogen atoms. These signals are systematically observed in the
electrolysis product when the oxidation of 1-Zn is carried out in
the CH2Cl2/CH3CN medium, in a yield ranging from 3 to 9%
(entries 1 to 5 in Table 3).
The mechanism of formation of 1-Zn-Cl is not easy to clarify.
The only possible source of the exogenous chlorine atom is
dichloromethane but, presently, there is no indication of the
identity of the porphyrin reactive intermediate and whether the
reaction proceeds in an homolytic or heterolytic fashion. Notably,
a similar chlorination reaction was previously described by Shi
and Liebeskind in the course of their studies on the oxidative
coupling of 5,10,15-trisubstituted free base porphyrin with DDQ
as oxidant.11 Although the authors claimed that the chlorinated
product might be formed from a Cl radical generated in the
reaction conditions, the origin of this radical was not stated
between CH2Cl2 and DDQ.
Fig. 4 Electrolysis of a 5 ¥ 10-4 M solution of 1-Zn with 10 equiv. of
2,6-lutidine followed by UV-vis spectroscopy (l = 2 mm, 0.1 M TEAPF6 in
DCM/ACN 4/1 v/v, Eapp = 0.75 V vs. SCE, -1.5 electrons, WE: Pt wire,
WE and counter-electrode are in separated compartments).
prominent. This kind of absorption is typical for cation radical
species.35 Accordingly, in the first period of the electrolysis, in
agreement with the applied potential and the oxidation potentials
of 2-Zn mentioned above, no 2-Zn is generated, but its derivated
cation radical 2-Zn∑+ is. Confirming this assumption, analysis
by RDE voltammetry of the electrolysed solution evidences the
characteristic wave of the latter species (E1/2,O¢2 = 0.845 V and
E1/2,R¢1 = 0.69 V). It fully justifies the need for the second reduction
stage of electrolysis allowing to recover the dimer in its neutral
form. Also it must be outlined that the charges, involved in the
two periods of electrolysis of this experiment and measured by
integration of the current, are in agreement with the balance of
electrons in the equations of the reactions, going from 1-Zn to
2-Zn∑+, and from 2-Zn∑+ to 2-Zn (Scheme 2).
Concerning our reaction tests, in entries 1 and 2 of Table 3, the
electrolyses are carried out in a three compartment cell without
and with the prior addition of 2,6-lutidine (10 molar equiv. with
respect to the initial amount of 1-Zn). In the first experiment, the
1
desired dimer 2-Zn is obtained in 70% H NMR yield, together
with 1-H2 (26%) and 1-Zn-Cl (4%). In fact, the free base porphyrin
accounts for the fraction of 1-Zn acting as a base to capture the
protons released by the coupling process (2 H+ per bond between
porphyrins).19,21 Interestingly no detectable trace of demetallated
meso–meso dimer 2-H2 was observed in the 1H NMR spectrum and
the MALDI-TOF mass spectrum of the crude solution, meaning
that 2-Zn is apparently less sensitive to acidolysis than 1-Zn.
With the base as an additive (entry 2), the demetallation no
longer occurs. 2-Zn is formed in high yield (95%) though a
small amount of 1-Zn-Cl is still obtained (5%). To the best
of our knowledge, the oxidative coupling of metalloporphyrins
has never been managed in basic conditions; on the contrary
these conditions are believed to strongly disfavour this reaction.34
Although not argued in these reports, such a behaviour could stem
from deactivation of the oxidant by formation of a coordination
or charge transfer complex with the base.§ Under the conditions
of entry 2, with a nearly complete selectivity in the formation of
2-Zn, the UV-vis. spectrum of the reacting solution of 1-Zn has
been monitored as a function of the electrolysis progress, with the
exception of the Soret band which could not be observed due to
a massive absorption in this region resulting in saturation of the
measured signal (see Fig. 4). It follows that the initial Q band of
1-Zn (lmax = 549 nm) is progressively substituted by the band at
lmax = 566 nm of 2-Zn. Besides this change at the Q band, a diffuse
and large absorption band between 600 and 900 nm becomes
Scheme 2 Reaction equations for the first and second steps of electrolysis.
“B” is a base.
The influence of the cell assembly on the selectivity in the
formation of 2-Zn was tested. Experiments 3, 4, 5 and 6 were
managed with the counter and working electrodes unseparated
from each other in a single compartment, motivated in part
by easier operation of the electrolyses (larger currents, shorter
electrolysis durations), thanks to a lesser ohmic drop. After
optimisation (entries 3 and 4, Table 3), we found that 2.5 faraday
per mol of 1-Zn (entry 4, Table 3) were necessary to fully exhaust
the starting monomer reactant. Under these conditions, and after
the systematic second electrolysis step managed in the same
manner as in the other experiments, 2-Zn was identified as the
major product (91%) but always associated with 1-Zn-Cl (9%).
With the operation in the single compartment cell, the necessity
of the base was questioned, since protons released during the C–
C bond formation could be potentially reduced at the platinum
§ To illustrate the extinction of the oxidative power of a chemical agent
by action of a base, reduction of Ag+ is shifted from E1/2 = 0.65 V/SCE
in CH2Cl2 0.1 M TBAPF6 to ca. E1/2 = –0.25 V/SCE in pyridine 0.1 M
TBAPF6.33
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cathode. Therefore, the experiment of entry 4 (Table 2) was
reproduced but without the base (entry 5, Table 2). The targeted
meso–meso dimer 2-Zn is then obtained in a good 85% yield but
6% of free-base 1-H2 and 9% of 1-Zn-Cl are also formed as side-
products, thus confirming the beneficial effect of the base.
Finally, to avoid the systematic formation of the chlorinated
by-product, we chose to exclude the use of CH2Cl2. DMF was
chosen as the substitute providing a satisfactory solubility of 1-
Zn. Keeping the optimized conditions of entry 4 but with DMF
in pure form as solvent (entry 6), the electrosynthesis of 2-Zn was
obtained as the sole detectable product with 1H NMR yield higher
than 95%. This electrosynthesis was then scaled up (20.0 mg of 1-
Zn, c = 9.92 ¥ 10-4 M) and after purification by silica gel column
chromatography, 2-Zn was isolated in 93.5% yield.
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Conclusions
The efficient electrosynthesis of the meso,meso-linked zinc
porphyrin dimer from a zinc(II) 5,10,15-tris-aryl-substituted-
porphyrin monomer has been achieved. With dichloromethane
as the reaction solvent, an unexpected chloro meso-substituted
derivative is generated as a by-product. The addition of the
hindered base 2,6-lutidine as well as operating in dimethylfor-
mamide are the key parameters to obtain high yields of the
desired coupled product, thus providing crucial information to
any chemist interested in this kind of compound. Studies are in
progress to extend the scope of this reaction. We now want to apply
these optimised conditions to electron-deficient zinc porphyrin
monomers which are difficult not to say impossible to couple
by chemical routes. Besides, we want to extend this method to
porphyrin monomer free bases or those metallated with metals
other than zinc.
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This work echoes our recently reported results on the redox
reactivity of the totally unsubstituted porphyrin, i.e. magnesium
porphine (MgP). These studies have revealed that MgP is readily
oligomerised and/or polymerised by electrochemical oxidation19,21
via the formation of new meso–meso links. Nevertheless, for
porphine, due to the multiplicity of reactive sites and the absence
of steric constraint around the macrocycle, the process was too
fast to be analysed in detail by the usual electrochemical methods.
This process is by far much simpler in the case of 1-Zn, involving
the formation of only one C–C bond. Thus, this tri-substituted
porphyrin is opportune to model porphine in the very first steps
of its oligomerisation/polymerisation, and this work is our first
contribution in this line of research. A complete and detailed
electrochemical analysis of the C–C coupling mechanism is to
follow.
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Acknowledgements
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The authors would like to thank the Centre National de la
Recherche Scientifique, the Conseil Re´gional de Bourgogne and
the Universite´ de Bourgogne for financial support. The authors
are grateful to Sophie Dalmolin for technical support, and David
Holland for her nice and priceless expertise.
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©