Crystallographic, spectroscopic and electrochemical characterization of pyridine adducts of magnesium(II) and zinc(II) porphine complexes

Article abstract of DOI:10.1016/j.crci.2013.01.015

A new purification method of magnesium(II) and zinc(II) porphine complexes (MgP and ZnP, respectively) by crystallization of their respective pyridine adducts is described. Pure MgP and ZnP can be regenerated by removal of the coordinated pyridine ligands by heating at 200 C under vacuum. X-ray crystallographic structures of the pyridine adducts are presented for the first time. NMR analyses of the adducts reveal the coordination of two pyridine molecules. Electrochemical as well as UV-vis absorption spectroscopy analyses in DMF of MgP·(Py)₂, ZnP·(Py)₂, MgP and ZnP indicate that pyridine adducts are totally dissociated. Besides, oxidation peaks of these complexes are totally irreversible, revealing a high reactivity of the oxidized species. Electrolyses at the first oxidation potential lead to the formation of the meso-meso (ZnP)₂ and (MgP)₂ dimers, oligomers and polymers on the electrode surface, as attested by MALDI-TOF mass spectrometry and UV-vis absorption spectroscopy analyses of the crude solution.

Full text of DOI:10.1016/j.crci.2013.01.015

C. R. Chimie 16 (2013) 540–549
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´
Full paper/Memoire
Crystallographic, spectroscopic and electrochemical characterization of
pyridine adducts of magnesium(II) and zinc(II) porphine complexes
*
´
´ `
Charles H. Devillers , Abdou K.D. Dime, Helene Cattey, Dominique Lucas
Institut de chimie mole´culaire de l’universite´ de Bourgogne, UMR CNRS 6302, faculte´ des sciences Mirande, Aile B-Chimie, 9, avenue Alain-Savary, BP 47870,
21078 Dijon cedex, France
A R T I C L E I N F O
A B S T R A C T
Article history:
A new purification method of magnesium(II) and zinc(II) porphine complexes (MgP and
ZnP, respectively) by crystallization of their respective pyridine adducts is described. Pure
MgP and ZnP can be regenerated by removal of the coordinated pyridine ligands by
heating at 200 8C under vacuum. X-ray crystallographic structures of the pyridine adducts
are presented for the first time. NMR analyses of the adducts reveal the coordination of two
pyridine molecules. Electrochemical as well as UV-vis absorption spectroscopy analyses in
DMF of MgPÁ(Py)2, ZnPÁ(Py)2, MgP and ZnP indicate that pyridine adducts are totally
dissociated. Besides, oxidation peaks of these complexes are totally irreversible, revealing
a high reactivity of the oxidized species. Electrolyses at the first oxidation potential lead to
the formation of the meso-meso (ZnP)2 and (MgP)2 dimers, oligomers and polymers on the
electrode surface, as attested by MALDI-TOF mass spectrometry and UV-vis absorption
spectroscopy analyses of the crude solution.
Received 30 November 2012
Accepted after revision 24 January 2013
Available online 1 March 2013
Keywords:
Magnesium and zinc porphines
Unsubstituted porphyrins
Pyridine adducts
X-ray diffraction
Cyclic voltammetry
NMR spectroscopy
´
ß 2013 Academie des sciences. Published by Elsevier Masson SAS. All rights reserved.
1. Introduction
advances in their characterization and application.’’
Although several studies are reported on free base (H2P)
For many decades, porphyrins have been the subject of
intense researches due to their versatile and outstanding
physico-chemical properties and also to their implication
in essential natural processes, like photosynthesis, blood
oxygen transport [1]. The progress in porphyrin chemistry
has led to the synthesis of complex structures, adding new
specifically designed fragments on the periphery of the
porphyrin core. Conversely, the totally unsubstituted
porphyrin called porphine has attracted much less
attention despite its great fundamental interest. Recently,
Senge published the first review on porphines, ‘‘Porphyrin
(porphine) a neglected parent compound with potential’’
[2]. The conclusion was: ‘‘The true potential of porphines
and their metal complexes is just emerging and hopefully
the advances made in their synthesis will now spawn
and metallated porphines (MP) (in the solid state [3] or in
solution [3g,4]), currently, the major part concerns
theoretical studies owing to the structural simplicity
and high symmetry of this macrocycle, resulting in less
power/memory demanding calculations [5]. Though por-
phine is the common basic unit of porphyrins, its physico-
chemical properties and electrochemical reactivity have
been poorly explored principally due to its only recent
commercial availability. Furthermore, despite its structur-
al simplicity, its synthesis is not straightforward since the
common Adler et al. [6] or Lindsey et al. [7] synthetic
methods starting directly from pyrrole and formaldehyde
do not work. However, a concise and efficient synthesis
(three steps, 17% overall yield starting from pyrrole) of
magnesium(II) porphine (MgP), was recently described by
Lindsey et al. [8]. This magnesium(II) complex exhibits
good solubility in common organic solvents contrary to the
free base porphine. As its electrochemical properties had
not yet been described in detail (only its first oxidation
* Corresponding author.
E-mail address: charles.devillers@u-bourgogne.fr (C.H. Devillers).
1631-0748/$ – see front matter ß 2013 Acade´mie des sciences. Published by Elsevier Masson SAS. All rights reserved.
http://dx.doi.org/10.1016/j.crci.2013.01.015

C.H. Devillers et al. / C. R. Chimie 16 (2013) 540–549
541
potential was mentioned [9]), our first contributions in this
field were dedicated to the electrochemical behavior of
MgP in CH2Cl2 [10], pyridine [11] and CH3CN [12].
In the preliminary synthesis of MgP according to
Lindsey’s procedure [8], we failed to reproduce the final
purification step consisting in crystallization of the crude
solution containing magnesium(II) porphine with an
ethanol–water mixture. In our case, there was no other
way to obtain MgP in a pure form than preparative
chromatography. So, for practical purposes, a chromatog-
raphy-free procedure was highly desirable.
Additionally, to the best of our knowledge, the
electrochemical characterization of zinc(II) porphine
(ZnP) has never been done before (only its first oxidation
potential was mentioned [9a,9b]). Bearing in mind that
MgP can oligomerize [10] and polymerize [12] upon
oxidation, we were wondering if the zinc complex could
behave similarly.
porphyrins are stable below 360 8C [15], we did not
observe any degradation of MgP at this temperature.
Although ZnP is much less soluble in organic solvents
than MgP, the same behavior was observed, i.e. addition of
an excess of pyridine leads to ZnPÁ(Py)2 adduct in
crystalline form.
2.2. Crystallographic structures of MgPÁ(Py)2 and ZnPÁPy
2.2.1. Crystallographic structure of MgPÁ(Py)2
To our best knowledge, only two pyridine-magnesium
porphyrins adducts have been described. The first one, the
bis(pyridine)-octaethylporphyrin magnesium complex
[14a] was reported in 1977 while the second one, the
bis(pyridine)-tetraphenylporphyrin magnesium complex
[14b], was published sixteen years later. Nevertheless, a
functionalized pyridine adduct, the bis(4-methylpyridine)-
magnesium porphyrin adduct was also described in 1984
by Mc Kee et al. [16]. Suitable crystals for X-ray diffraction
studies of MgPÁ(Py)2 were obtained from slow diffusion of
cyclohexane into a CH2Cl2/Py (4/1 v/v) solution containing
MgP. This latter is the second example of metallated
porphine crystallographic structure, the first one being the
nickel(II) complex [17] and joins the very restricted family
of magnesium porphyrin crystallographic structures since
only 28 X-ray structures are recorded to date on CCDC
database [11,14b,16,18]. Three-dimensional molecular
views of MgPÁ(Py)2 are presented in Fig. 1. Selected data
are also given in Table 1 and are compared with the
bis(pyridine) adducts of MgOEP (MgOEPÁ(Py)2) [14a] and
MgTPP (MgTPPÁ(Py)2) [14b]. The centrosymmetric macro-
cycle is almost planar (side view, Fig. 1), the Mg(II) ion,
lying in the mean plane, formed by the 20 C and four
equatorial N (NP) atoms of the porphine (mean planeP,
We, thus, report here a new method to purify MgP and
ZnP by crystallization, their respective crystallographic
structures as pyridine adducts and a comparison of their
spectroscopic and electrochemical properties.
2. Results and discussion
2.1. Synthesis of MgPÁ(Py)2 and ZnPÁ(Py)2
As a result of their good solubility in a broad variety of
organic solvents,
a common issue with magnesium
porphyrins stems from their final purification step, i.e.
precipitation or crystallization [8,13]. Thus, one solution
consists of the precipitation with water of the magnesium
complex dissolved in a miscible organic solvent such as
acetone, methanol, ethanol, DMF, DMSO, acetonitrile, THF.
However, generally, the finely divided amorphous suspen-
sion obtained is difficult to filtrate on standard sintered
glass frits since it has a trend to seal it. This behavior was
also observed in our case with MgP. Besides, grease might
be difficult to remove in these conditions. Inspired by the
fact that several crystallographic structures of magnesium
porphyrin–pyridine adducts were described [14], we
found that the addition of an excess of pyridine into a
solution of MgP in dichloromethane led during the
evaporation of CH2Cl2 to the formation of shiny deep
purple crystals. Upon addition of pyridine, the solution
color changes from pink to deep purple, which evidences
the formation of a pyridine-MgP adduct. These crystals
were filtered, thoroughly washed with n-hexane and dried
under vacuum at 45 8C. Starting from pure MgP, the
crystallization yield was 79%. The interest of this method
stems from the high purity of the pyridine-magnesium
˚
d(Mg–NP) = 2.071(2) and 2.072(2) A). The Mg–NPy (from
˚
pyridine) distance is 2.337(2) A which is slightly shorter
compared to the MgTPPÁ(Py)2 complex (d(Mg–
˚
N
Py) = 2.3764(2) A) but very similar to the MgOEPÁ(Py)2
˚
adduct (d(Mg–NPy) = 2.3366(8) A). Both pyridine ligands
are lying in the same mean plane (front view, Fig. 1). The
latter is nearly orthogonal to the porphine mean planeP
with a dihedral angle of 88.45(6)8 as a result of the absence
of steric hindrance at the macrocycle periphery. This
orthogonality is nearly kept in the case of the MgOEPÁ(Py)2
adduct (85.438) but is clearly lost for the MgTPPÁ(Py)2
complex (71.838). The RMS deviation from the planarity,
i.e. the root mean square of the distances of the 24 carbon
and nitrogen porphyrin ring atoms from the mean plane
formed by these atoms, is very low for MgPÁ(Py)2
˚
˚
(0.0242 A), increases slightly for MgOEPÁ(Py)2 (0.0319 A)
˚
and significantly for MgTPPÁ(Py)2 (0.0810 A). Thus, though
MgTPPÁ(Py)2 is less substituted than MgOEPÁ(Py)2, this
complex exhibits the most important degree of nonpla-
narity. As no additional solvent is included in the
crystallographic cell of these porphyrins, it is also possible
to directly compare their respective density. So, whereas
MgOEPÁ(Py)2 and MgTPPÁ(Py)2 have similar density (1.257
and 1.262, respectively), MgPÁ(Py)2 exhibits a significant
increase in this value (1.372), which could result from the
absence of steric strains at the periphery of the porphine
core and a higher magnesium/carbon ratio. Besides, each
porphine MgPÁ(Py)2 obtained but also from
a easy
handling and removal from the sintered glass frits of the
crystalline material. Furthermore, when starting with
crude MgP, it is possible to synthesize pure MgPÁ(Py)2
in one or several crystallization steps, depending on the
purity of the initial MgP. Ultimately, pure MgP can be
totally recovered (100% yield) from pure MgPÁ(Py)2 by
simple heating at 200 8C under vacuum (P ꢀ 5.10À2 mbar)
during 24 h. In agreement with the fact that metallated

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C.H. Devillers et al. / C. R. Chimie 16 (2013) 540–549
Fig. 1. ORTEP views of MgPÁ(Py)2: front view (top left); side view (top center), general view (top right) and 2D network (bottom, H atoms have been omitted
for clarity reasons) of MgPÁ(Py)2. Thermal ellipsoids are scaled to the 50% probability level.
MgPÁ(Py)2 molecule interacts with two other ones through
structures of zinc(II) porphyrins pyridine adducts are
described. For example, to date 100 structures are reported
when drawing a single pyridine molecule coordinated to
ZnTPP as a substructure in CCDC. An interesting point
stems from the rarity of the bis-pyridine-coordinated
ZnTPP X-ray structures recorded in this database. Indeed,
only two examples are reported among the 100 aforemen-
tioned. Thus, statistically, in the particular case of ZnTPP,
the zinc(II) cation principally exhibits a pentacoordinated
˚
C(9) atoms (d(C(9)–C(9)) = 3.315(4) A) leading to a 2D
network (bottom, Fig. 1). These particular interactions
p-p
between carbon atoms of porphine cores are not observed
in the other MgOEPÁ(Py)2 and MgTPPÁ(Py)2 complexes.
Besides, adjacent porphyrins interact via
p-stacking
between the pyridine ligands, the distance between the
pyridine centro¨ıd Ctpyr of one molecule with the carbon of
the second molecule located at its vertical Ctpyr. . .C13 is
˚
equal to 3.547(4) A.
coordination sphere. A similar trend, though less
pronounced, is also observed for ZnOEP with 11 examples
when drawing one pyridine coordinated on ZnOEP as a
substructure in CCDC. However among these, only one
structure presents an additional coordinated pyridine.
2.2.2. Crystallographic structure of ZnPÁPy
In contrast to magnesium(II) porphyrin pyridine
adducts, numerous examples of X-ray crystallographic

C.H. Devillers et al. / C. R. Chimie 16 (2013) 540–549
543
Table 1
Comparison of structural parameters extracted from the X-ray crystallographic structures of MgPÁ(Py)2, MgOEPÁ(Py)2[14a] and MgTPPÁ(Py)2[14b].
MgPÁ(Py)2
MgOEPÁ(Py)2 [14a]
MgTPPÁ(Py)2 [14b]
˚
d(Mg–NPy) (A)
2.337(2)
2.3366(8)
2.3764(2)
0.000
a
˚
d(Mg–mean planeP) (A)
0.000
0.000
˚
d(Mg–NP) (A)
2.071(2) < d < 2.072(2)
88.45(6)8
0.0242
2.041(1) < d < 2.0644(6)
2.069(3) < d < 2.074(3)
mean planeP/mean planePy angle
85.438
0.0319
1.257
71.838
0.0810
1.262
b
˚
RMS deviation (A)
density (g/m3)
1.372
a
The mean plane of the porphyrin (mean planeP) is calculated with the 20C and 4N atoms of the porphyrin ring.
b
The RMS deviation corresponds to the root mean square of the distances of the 24 carbon and nitrogen porphyrin ring atoms from the mean plane
formed by these atoms.
Suitable crystals for X-ray diffraction studies of ZnPÁPy
were obtained from slow diffusion of cyclohexane into a
solution of ZnP in CH2Cl2/Py (4/1 v/v). Three dimensional
molecular views of ZnPÁPy are given in Fig. 2. Selected data
are also given in Table 2 and are compared with ZnTPPÁPy
[19] and ZnOEPÁPy [20] adducts. Given the numerous
examples of mono pyridine zinc(II) porphyrin adducts, it is,
thus, not surprising to obtain a similar pentacoordinated
coordination sphere around the zinc cation. Contrary to
MgPÁ(Py)2, ZnPÁPy macrocycle as well as its ZnOEPÁPy and
ZnTPPÁPy homologues (see Table 2) deviate markedly from
the planarity since the Zn(II) atom is clearly out of the
mean plane formed by the 20 C and four N atoms of the
side view, Fig. 2). As a result of an higher Zn–mean planeP
˚
distance than its ZnTPPÁPy (0.435 A) and ZnOEPÁPy
˚
(0.396 A) congeners, the Zn–N(porphyrin) (d(Zn–NP))
distances are the longer ones and are intermediate
˚
between 2.071(2) and 2.083(2) A. However, the Zn–NPy
(from pyridine) distance appears to be the shorter one
˚
˚
˚
(2.127(2) A vs 2.2002 A and 2.153 A for ZnOEPÁPy and
ZnTPPÁPy, respectively) in agreement with a stronger
interaction of the pyridine with the zinc(II) atom in ZnPÁPy.
Contrary to ZnOEPÁPy and ZnTPPÁPy, the mean plane
formed by the nitrogen and carbon atoms of the pyridine
molecule (mean planePy) is almost orthogonal to the mean
planeP (89.75(5)8). The RMS deviation from the planarity,
i.e. the root mean square of the distances of the 24 carbon
˚
porphyrin (mean planeP) (d(Zn–mean planeP) = 0.483 A,
Fig. 2. ORTEP views of ZnPÁPy: front view (top left); side view (top center), general view (top right) and 2D network (bottom, H atoms have been omitted for
clarity reasons) of ZnPÁPy. Thermal ellipsoids are scaled to the 50% probability level.

544
C.H. Devillers et al. / C. R. Chimie 16 (2013) 540–549
Table 2
Comparison of structural parameters extracted from the X-ray crystallographic structures of ZnPÁPy, ZnOEPÁPy [20] and ZnTPPÁPy.[19].
ZnPÁPy
ZnOEPÁPy [20]
ZnTPPÁPy [19]
˚
d(Zn–NPy) (A)
2.1305(1)
2.2002(5)
2.153(4)
a
˚
d(Zn–mean planeP) (A)
0.483
0.396
0.435
˚
d(Zn–NP) (A)
2.0737(1) < d < 2.0820(1)
2.0602(5) < d < 2.0751(5)
2.065(4) < d < 2.076(4)
mean planeP/mean planePy angle
89.75(5)8
0.0873
1.516
85.078
0.0675
1.254
70.118
0.1153
1.438
b
˚
RMS deviation (A)
density (g/cm3)
a
The mean plane of the porphyrin (mean planeP) is calculated with the 20 C and 4 N atoms of the porphyrin ring.
b
The RMS deviation corresponds to the root mean square of the distances of the 24 carbon and nitrogen porphyrin ring atoms from the mean plane
formed by these atoms.
and nitrogen porphyrin ring atoms from the mean plane
Due to the high symmetry of these porphine deriva-
tives, 1H NMR spectra are very simple. Their respective
˚
formed by these atoms, is 0.0873 A, an intermediate value
˚
between those of ZnOEPÁPy (0.0675 A) and ZnTPPÁPy
meso and b-pyrrolic singlets have similar chemical shifts
˚
(0.1153 A). Similarly to MgPÁ(Py)2, ZnPÁPy exhibits the
(Table 3). The structure of the solid-state metalloporphine
adducts of MgPÁ(Py)2 and ZnPÁ(Py)2 is preserved in the
deuterated acetone solution since the corresponding
coordinated pyridine signals are shielded compared with
the simple pyridine molecule (Fig. 3). These significant
high field shifts as well as the integration of these signals
agree well with the coordination of two pyridine molecules
on the magnesium(II) and zinc(II) metals undergoing the
influence of the shielding cone of these porphines.
Interestingly, this influence is more pronounced in the
case of ZnPÁ(Py)2, in agreement with a stronger interaction
of nitrogen atoms with zinc(II) cation. Attribution of the
pyridine signals was solved by means of 2D techniques
(1H-1H COSY, 1H-13C HSQC).
higher density (1.516) compared with ZnOEPÁPy (1.254)
and ZnTPPÁPy (1.438). Interestingly, each ZnPÁPy unit
interacts with four neighboring porphine molecules
˚
p-p stacking (N(2)–C(23) = 3.177(5) A; C(6)–
through
˚
˚
C(24) = 3.345(5) A; C(25)–C(18) = 3.374(4) A) leading to a
2D network (Fig. 2, bottom).
2.3. NMR characterization of MgPÁ(Py)2 and ZnPÁ(Py)2
NMR spectra of MgPÁ(Py)2 and ZnPÁ(Py)2 were recorded
in deuterated acetone and are compared with MgP, ZnP
and pyridine in the same solvent. Partial 1H NMR spectra
are presented in Fig. 3. Surprisingly, contrary to ZnPÁPy
crystals grown from slow diffusion of cyclohexane in a
CH2Cl2–pyridine mixture, NMR characterization of the
2.4. Electrochemical characterization of MgPÁ(Py)2 and
ZnPÁ(Py)2
crystals synthesized by dissolving ZnP in
a CH2Cl2/
pyridine mixture followed by rapid evaporation of
CH2Cl2/pyridine mixture, filtration, washing with n-
hexane and drying for 24 h under vacuum at 45 8C, reveals
a bis-pyridine adduct (ZnPÁ(Py)2) in deuterated acetone.
As the electrochemical behaviors of MgPÁ(Py)2 and
ZnPÁ(Py)2 were not, to our best knowledge examined
before, we studied these pyridine adducts in DMF with
Fig. 3. Partial 1H NMR spectra, from top to bottom, of ZnPÁ(Py)2, ZnP, MgPÁ(Py)2, MgP and Pyridine (Py) (300 MHz, 298 K, CD3COCD3). The peak marked by
an asterisk corresponds to CH2Cl2 traces.

C.H. Devillers et al. / C. R. Chimie 16 (2013) 540–549
545
Table 3
1H NMR chemical shifts of porphine compounds and pyridine (300 MHz,
CD3COCD3, 298 K).
Proton signals
(ppm)
ZnPÁ(Py)2
ZnP
MgPÁ(Py)2
MgP
Py
d
meso
10.43
9.63
7.10
6.47
5.56
10.45
9.64
10.42
9.59
7.51
7.02
7.51
10.41
9.58
b
-pyrrolic
Ha
Hb
Hc
7.76
7.35
8.59
tetraethylammonium hexafluorophosphate (TEAPF6) as
the supporting electrolyte. Although ZnPÁ(Py)2 is poorly
soluble in common organic solvents, contrary to
MgPÁ(Py)2, its solubility was sufficient in DMF for
electrochemical studies. Their cyclic voltammograms
(CV) are presented in Fig. 4 and are compared with MgP
and ZnP which have been synthesized by the removal of
coordinated pyridine molecules from MgPÁ(Py)2 and
ZnPÁ(Py)2 (Section 3). At first sight, there is no difference
between the CVs of ZnPÁ(Py)2 and ZnP and between
MgPÁ(Py)2 and MgP. This result is not surprising since DMF
is known to be a strongly coordinating solvent. Thus, these
pyridine adducts are instantaneously and totally dissoci-
ated in solution as previously reported for octaethylpor-
phyrin magnesium pyridine adduct in other coordinating
solvents [21]. Besides, in DMF, UV-vis absorption spectra of
ZnPÁ(Py)2 and ZnP as well as those of MgPÁ(Py)2 and MgP
are identical, confirming this full pyridine dissociation.
Usually, metalloporphyrins with a non-electroactive met-
al, like Mg(II) or Zn(II), undergo reversibly two ring-
centered reductions and two ring-centered oxidations
Fig. 4. Cyclic voltammograms of ZnPÁ(Py)2, ZnP, MgPÁ(Py)2 and MgP (c ꢀ
5 Â 10À4 M, DMF 0.1 M TEAPF6, RT, WE: Pt Ø = 2 mm, CE: Pt, RE: SCE,
n
= 100 mV sÀ1).
observed on rotating disk electrode, is indicative of a
multiple electron transfer (n > 2) coupled with chemical
reactions. This behavior agrees well with the high
reactivity of the electrogenerated cation radicals, as
already reported for MgP in other solvents [10–12].
Besides, this superior ratio for ZnPÁ(Py)2 compared with
MgPÁ(Py)2 can be correlated with a higher first oxidation
potential leading to enhanced reactivity of the initially
formed cation radical. Remarkably, the experimentally
determined HOMO-LUMO gap, i.e. the potential difference
between the first oxidation and the first reduction (2.27
and 2.23 V for ZnPÁ(Py)2 and MgPÁ(Py)2, respectively) falls
into the diagnostic criterion values (2.15 Æ 0.15 V) reported
for TPP and OEP derivatives [22]. Thus, the primary electron
transfers of ZnPÁ(Py)2 and MgPÁ(Py)2 only affect the
macrocycle (Table 4).
In order to obtain more information about the oxidation
of ZnP and ZnPÁ(Py)2, spectroelectrochemical experiments
were performed in DMF 0.1 M TEAPF6. Solutions of these
zinc complexes were electrolyzed on a large area platinum
spiral and electrolyses were stopped for an amount of
electricity corresponding to two electrons transferred per
porphine molecule. For purpose of comparison, these
experiments were reproduced with MgPÁ(Py)2 and MgP.
No difference on the evolution of the UV-vis spectra
between ZnP and ZnPÁ(Py)2 and between MgPÁ(Py)2 and
delivering, respectively, the
p
-anion radical and
p-
dianion, and the -cation radical and
p
p
-dication [22].
This common behavior is indeed observed for ZnPÁ(Py)2
and ZnP in the negative potential range, with two
successive monoelectronic reduction waves at E1/
2 = À1.44 V and E1/2 = À1.81 V (peaks R1 and R2, respec-
tively, Fig. 4). For MgPÁ(Py)2 and MgP, the first reduction
peak is reversible (E1/2 = À1.48 V, peak R1) whereas the
second one is not clearly distinguished due to the
proximity of the solvent reduction though a reduction at
Epc ꢀ À1.98 V (peak R2, Fig. 4) is discernible. In the positive
potential direction, all the oxidation peaks of the
metallated porphines are irreversible. Moreover, the first
oxidation peak (O1) is significantly higher than the first
mono-electronic and reversible reduction peaks (R1). This
marked difference in intensity (ratio ipa(O1)/ipc(R1) = 3.5
and 2.5 for ZnPÁ(Py)2 and MgPÁ(Py)2, respectively), also
Table 4
Potential values (E1/2 or Ep for irreversible systems) for ZnPÁ(Py)2, ZnP, MgPÁ(Py)2 and MgP (in V vs SCE). When redox systems are reversible,
values are given in brackets.
DEp = (Epa - Epc)
2nd reduction
1st reduction
peak R1
1st oxidation
peak O1
2nd oxidation
peak O2
3rd oxidation
peak O3
DE(ox1/red1) =
(Epa(ox1)–E1/2(red1))
peak R2
ZnPÁ(Py)2
ZnP
À1.81 (100)
À1.81 (100)
ꢀ À1.98
À1.44 (75)
À1.44 (75)
À1.48 (75)
À1.48 (75)
0.83
0.83
0.75
0.75
0.93
0.93
0.93
0.93
1.03
1.03
2.27
2.27
2.23
2.23
MgPÁ(Py)2
MgP
ꢀ À1.98

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C.H. Devillers et al. / C. R. Chimie 16 (2013) 540–549
MgP was observed. Consequently, only the UV-vis
evolutions during the electrolysis of ZnPÁ(Py)2 and
MgPÁ(Py)2 are presented in Fig. 5. After each electrolysis,
the platinum working electrode was totally covered by a
black/purple solid material, in agreement with an
electropolymerization process occurring during the oxida-
tion of these metallated porphines, as already established
for MgP [10,12]. On both spectra, two major trends are
noted:
continuously maintain a low concentration of these
products in the electrolyzed solution.
3. Experimental
3.1. X-ray crystallographic study
CCDC-912742 & 912743 contains the supplementary
crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_re-
quest.cif.
ꢁ
ꢁ
the progressive decrease in the initial bands at 307 and
532 nm for ZnPÁ(Py)2 and at 309 and 536 nm for
MgPÁ(Py)2;
Crystal data for MgPÁ(Py)2 [C30H22MgN6], red prism
the emergence of one band at 440 and 441 nm for
ZnPÁ(Py)2 and MgPÁ(Py)2, respectively.
crystal:
0.18 Â 0.10 Â 0.08
mm3,
M = 490.85 g/mol,
monoclinic, space group C2/c (No. 15), Z = 4,
˚
˚
˚
As previously reported for the zinc(II) porphine model,
a = 12.7579(9) A,
b
m
b = 15.0501(12) A,
c = 12.3850(8) A,
3
= 92.071(4)8, V = 2376.5(3) A , Dcalcd = 1.372 g/cm3,
˚
i.e. 5,15-ditolyl-10-phenyl zinc(II) porphyrin [23] and for
MgP [10] and as confirmed by MALDI-TOF mass spec-
trometry analyses of the electrolyzed solutions, these new
Soret bands correspond to the signature of meso-meso
dimers (ZnP)2 and (MgP)2. Indeed, chemical or electro-
chemical oxidation of magnesium and zinc porphyrins
having at least one unsubstituted meso position is known
to lead to meso-meso linked porphyrin dimers with their
specific 30 to 40 nm spaced double Soret bands features
[24]. It should be noted that the highest energy Soret band
of the meso-meso dimer is located at nearly (within a few
nm) the same wavelength than the monomer one. As
MgPÁ(Py)2 and ZnPÁ(Py)2 Soret band maxima appear at 400
and 401 nm in DMF, respectively, and considering that
their corresponding dimer highest energy Soret band
absorbs at the same wavelength, the wavelength gap
between both Soret bands is 41 and 39 nm, respectively.
Interestingly, the Soret band at 440 nm (during the
oxidation of ZnPÁ(Py)2) is narrower than the one at
441 nm (corresponding to the oxidation of MgPÁ(Py)2).
This behavior could stem from a lower solubility of the zinc
derivatives compared with the magnesium ones as already
observed for the monomer. Indeed, the adsorption on the
electrode surface of the less soluble zinc oligomers would
= 0.108 mmÀ1, T = 115(2) K, F(000) = 1024, Mo K
a
radia-
scans, 4122
˚
tion (l
= 0.71073 A), mixture of
f
rotations and
w
reflections collected on a Nonius Kappa CCD diffractometer
(index ranges: h = Æ16; k = –19/17; l = Æ16) measured in the
range 1.08 ꢂ u ꢂ 27.498, with 2695 independent (Rint = 0.0323)
and 2053 observed reflections [I ꢃ 2s(I)], 169 refined param-
eters,
R indices for observed reflections: R1 = 0.0623,
wR2 = 0.1091,
R
indices for all data: R1 = 0.0892,
3
˚
wR2 = 0.1225, goodness of fit = 1.168, Dr = 0.283/À0.248 e.A .
Crystal data for ZnPÁ(Py) [C25H17ZnN5], red prism
crystal: 0.15 Â 0.10 Â 0.05 mm3, M = 452.81 g/mol, mono-
˚
clinic, space group P21/c (No. 14), Z = 4, a = 9.5746(4) A,
˚
˚
b = 14.6935(6) A,
c = 14.6410(6) A,
V = 1984.44(14) A , Dcalcd = 1.516 g/cm3,
T = 115(2) K, F(000) = 928, Mo K radiation (
mixture of rotations and w scans, 7847 reflections
b = 105.542(2)8,
3
= 1.261 mmÀ1
˚
l = 0.71073 A),
,
˚
m
a
f
collected on a Nonius Kappa CCD diffractometer (index
ranges: h = Æ12; k = À18/19; l = Æ18) measured in the range
1.08 ꢂ u ꢂ 27.498, with 4495 independent (Rint = 0.0305) and
3688 observed reflections [I ꢃ 2s(I)], 280 refined parameters,
R indices for observed reflections: R1 = 0.0426, wR2 = 0.0787,
R indices for all data: R1 = 0.0609, wR2 = 0.0877, goodness of
3
˚
fit = 1.143, Dr = 0.534/À0.350 e.A .
Fig. 5. Electrolyses of 5 Â 10À4 M solutions of ZnPÁ(Py)2 (A) and MgPÁ(Py)2 (B) followed by UV-vis absorption spectroscopy (l = 1 mm, 0.1 M TEAPF6 in DMF,
Eapp = 0.83 and 0.75 V for ZnPÁ(Py)2 and MgPÁ(Py)2, respectively, -2.0 electrons, WE: Pt spiral, CE: Pt, RE: SCE).

C.H. Devillers et al. / C. R. Chimie 16 (2013) 540–549
547
3.2. X-ray equipment and refinement
552 potentiostat coupled with an Amel 721 electronic
integrator. A platinum wire spiral (l = 53 cm, Ø = 1 mm,
S = 16.65 cm2) 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 UV-vis absorption measurements.
Diffraction data were collected on a Nonius KappaCCD
diffractometer equipped with a nitrogen jet stream low-
temperature system (Oxford Cryosystems). The X-ray
source was graphite-monochromated Mo K
˚
(l = 0.71073 A) from a sealed tube. The structure was
a radiation
solved by direct methods using the SIR92 [25] program and
refined with full-matrix least-squares on F2 using the
SHELXL97 [26] program with the aid of the WIN-GX [27]
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.
3.4. Synthesis of MgPÁ(Py)2
An amount of 50.0 mg of MgP (1.50 Â 10À4 mol) is
dissolved in 12 mL of CH2Cl2. Then, 500
mL of pyridine
(6.195 Â 10À3 mol) are added to this solution. CH2Cl2 is
then evaporated yielding deep purple shiny crystals in the
remaining pyridine. These crystals are filtered and
thoroughly rinsed with n-hexane. Crystals are then dried
at 45 8C under vacuum (P ꢀ 5.10À2 mbar) for 24 h. 58.4 mg
of pure MgPÁ(Py)2 crystals are obtained (79% yield).
This experimental procedure can be repeated one or
several times for purification of crude MgP.
3.3. Reagents and instrumentation
Magnesium porphine (MgP) and zinc porphine (ZnP)
were synthesised according to known procedures [8]. Our
data (1H NMR, 13C NMR, UV-vis absorption, and MALDI-
TOF mass spectrum) were consistent with those described
in reference [8]. Tetraethylammonium hexafluoropho-
sphate (TEAPF6, Fluka puriss., electrochemical grade,
ꢃ 99.0%), pyridine (99%+, Acros) were used as received.
CH2Cl2 (Carlo Erba 99.5%) was distilled over P2O5; DMF
(SDS, Carlo Erba, purity (GC) 99.9%) was distilled under
vacuum on CaH2.
MgPÁ(Py)2: HRMS (ESI/TOF) m/z calcd for C20H13N4Mg:
333.0985; found: 333.0983 [M–2Py + H]+; 1H NMR
(CD3COCD3, 300 MHz, 298 K):
4H), 9.59 (s, -Pyrr, 8H), 7.55–7.47 (m, o- and p-Py, 6H),
7.05-7.00 (m, m-Py, 4H); 13C NMR (CD3COCD3, 75 MHz,
d (ppm) 10.42 (s, meso,
b
298 K):
133.1 (
nm (log
d
b
(ppm) 150.6 (
-Pyrr), 124.4 (m-Py), 106.4 (meso); (lmax (DMF)/
e) 309 (4.26), 380 (4.67), 400 (5.79), 500 (3.28), 536
a-Pyrr), 149.7 (o-Py), 136.8 (p-Py),
UV-vis 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 (Hellma, l = 1 mm) was connected
through a fibre optic to the same spectrophotometer.
High-resolution mass spectra (HRMS) were recorded on
a MicrOTOF Q Bruker instrument in ESI (positive mode) at
(4.23) 572 (3.46); Elemental analysis (Found: C, 70.81; H,
4.75; N, 16.52%. Calc. for C20H12N4MgÁ2 C5H5NÁH2O: C,
70.85; H, 4.69; N, 16.37%).
MgP of higher purity than the starting one can be
recovered by heating at 200 8C under vacuum for 24 h
(100% yield from MgPÁ(Py)2).
`
´
the Plateforme d’analyse chimique et de synthese mole-
3.5. Synthesis of ZnPÁ(Py)2
´
culaire de l’universite de Bourgogne (PACSMUB).
1H and 13C NMR spectra were measured on a BRUKER
300 MHz spectrometer (Avance III Nanobay). 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.
The experimental procedure performed for MgPÁ(Py)2
was reproduced on 50.0 mg of ZnP (1.338 Â 10À4 mol)
yielding 56.9 mg of pure ZnPÁ(Py)2 crystals (80% yield).
ZnPÁ(Py)2: HRMS (ESI/TOF) m/z calcd for C20H12N4Zn:
372.0348; found: 372.0357 [M–2Py]+; 1H NMR
All electrochemical manipulations were performed
using Schlenk techniques in an atmosphere of dry
oxygen-free argon at room temperature (T = 20 Æ 3 8C).
The supporting electrolyte was degassed under vacuum
before use and then dissolved to a concentration of 0.1 mol/L.
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 Electro-
chemistry 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
DMF (0.1 M TEAPF6), the formal potential for the Fc+/Fc
couple was found to be +0.44 V vs SCE.
(CD3COCD3, 300 MHz, 298 K):
4H), 9.63 (s, -Pyrr, 8H), 7.17–7.05 (m, p-Py, 2H), 6.53-6.40
(m, m-Py, 4H), 5.56 (br s, o-Py, 4H); 13C NMR (CD3COCD3,
75 MHz, 298 K): (ppm) 150.7 ( -Pyrr), 147.8 (o-Py), 137.0
-Pyrr), 124.1 (m-Py), 105.5 (meso); (lmax
) 307 (4.26), 350 (4.04), 381 (4.65), 401
(5.70), 496 (3.29), 532 (4.22) 566 (3.50); Elemental
analysis (on ZnPÁPy crystals) (Found: C, 64.92; H, 3,71;
N, 15,09%. Calc. for C20H12N4ZnÁC5H5NÁ0.5H2O: C, 65.02; H,
3.93; N, 15.16%).
d (ppm) 10.43 (s, meso,
b
d
b
e
a
(p-Py), 133.1 (
(DMF)/nm (log
ZnP of higher purity than the starting one can be
recovered by heating at 200 8C under vacuum for 24 h
(100% yield from ZnPÁ(Py)2).
4. Conclusion
Addition of a pyridine excess into a concentrated CH2Cl2
solution of MgP and ZnP affords MgPÁ(Py)2 and ZnPÁ(Py)2
adduct crystals. This treatment is, thus, a new method
for purification of these metallated porphines. X-ray
Bulk electrolyses were performed in a three-compart-
ment cell with glass frits of medium porosity with an Amel

548
C.H. Devillers et al. / C. R. Chimie 16 (2013) 540–549
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ˇ
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the platinum electrode surface. Our efforts are now
focused on the characterization of the ZnP-based polymer,
which may have interesting applications in molecular
devices such as non-linear optical devices, sensors or
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The authors would like to thank the Centre national de
´
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technical support and to Dr Fanny Chaux for carrying out
the ESI-MS analyses.
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