Synthesis of pure iron(II) mesotetraphenylchlorin complexes via a versatile general method of iron insertion into chlorins

Article abstract of DOI:10.1016/j.ica.2013.01.012

An original synthesis of pure (tetraphenylchlorinato)iron(III) chloride (1) is reported. Insertion of iron is achieved using 1.2 equivalents of cadmium acetate in DMF at 110 °C, followed by a transmetallation reaction (in situ) using 8 equivalents of iron(II) chloride tetrahydrate in acetone at 60 °C. The ¹H NMR shifts at 25 °C of the pyrrole protons varied from 60 to 100 ppm. The absence of the signal at 80 ppm related to undesired (tetraphenylporphirinato)iron(III) chloride is a purity criterion and shows the importance of the method used. Reaction of (tetraphenylchlorinato)iron(III) chloride (1) with four equivalents of tert-butyl isocyanide, 2,6-xylyl isocyanide and dimethylphenylphosphine in the presence of zinc amalgam in dichloromethane (DCM) afforded pure Fe^II(TPC)(t-BuNC)₂ (2), Fe^II(TPC)(2,6-xylylNC)₂ (3) and Fe ^II(TPC)[P(Me)₂Ph]₂ (4) respectively.

Full text of DOI:10.1016/j.ica.2013.01.012

Inorganica Chimica Acta 396 (2013) 30–34
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Note
Synthesis of pure iron(II) mesotetraphenylchlorin complexes via a versatile
general method of iron insertion into chlorins
Marwan Kobeissi a, , Khalil Cherry a, Mahmoud K. Faraj b, Gérard Simonneaux c
⇑
a Laboratoire de Synthèse Organique, équipe de Physiotoxicité Environnementale. Lebanese University, Faculty of Sciences, Section V, Lebanon
b School of Chemistry, Lebanese International University, Al Mousaytbeh, P.O. Box 146404, Mazraa, Beirut, Lebanon
c Ingénierie Chimique et Molécules pour le Vivant, UMR CNRS 6226, Campus de Beaulieu, Université de Rennes 1, 35042 Rennes, France
a r t i c l e i n f o
a b s t r a c t
Article history:
An original synthesis of pure (tetraphenylchlorinato)iron(III) chloride (1) is reported. Insertion of iron is
achieved using 1.2 equivalents of cadmium acetate in DMF at 110 °C, followed by a transmetallation reac-
tion (in situ) using 8 equivalents of iron(II) chloride tetrahydrate in acetone at 60 °C. The 1H NMR shifts at
25 °C of the pyrrole protons varied from 60 to 100 ppm. The absence of the signal at 80 ppm related to
undesired (tetraphenylporphirinato)iron(III) chloride is a purity criterion and shows the importance of
the method used.
Reaction of (tetraphenylchlorinato)iron(III) chloride (1) with four equivalents of tert-butyl isocyanide,
2,6-xylyl isocyanide and dimethylphenylphosphine in the presence of zinc amalgam in dichloromethane
(DCM) afforded pure FeII(TPC)(t-BuNC)2 (2), FeII(TPC)(2,6-xylylNC)2 (3) and FeII(TPC)[P(Me)2Ph]2 (4)
respectively.
Received 5 September 2012
Received in revised form 24 December 2012
Accepted 4 January 2013
Available online 1 February 2013
Keywords:
Iron chlorins
Phosphines
Isocyanides
Diamagnetic complexes
Transmetallation
Cadmium
Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction
as isocyanides and phosphorus derivatives to heme proteins is part
of our long-term goal to establish correlations between the chem-
Investigations of the structural and spectroscopic properties of
model compounds such as iron(III) chlorins and isobacteriochlorins
[1–19] have been useful in understanding the active site of numer-
ous heme enzymes such as heme d found in a terminal oxidase
complex [20–23] or a catalase, hydroxyperoxidase II [24], all from
Escherichia coli. In sulfmyoglobin, a nonfunctional form of myoglo-
bin, the porphyrin macrocycle has been reduced to a chlorin by
addition of a sulfur atom to a pyrrole ring [25]. Various hemopro-
teins such as myoglobins [26,27], horseradish peroxidase and cyto-
chrome b5 [27] have also been reconstituted with iron chlorin
prosthetic groups. A chlorin is a hydroporphyrin with one reduced
pyrrole double bond. There are currently not enough results avail-
able of the physical properties of iron chlorins to explain their bio-
logical properties and their electronic ground state is still
problematic [28]. In contrast, much more information is available
with iron porphyrins [29].
ical and the electronic structures of heme derivatives [30,31].
Studies of the electronic structures with simple iron chlorins
which might serve as models for naturally occurring iron chlorin
proteins have been more limited due to the synthetic difficulty of
preparing pure compounds due to the oxygen sensitivity of re-
duced macrocycle [4–16]. For the preparation of iron(III) deriva-
tives as models for hemoproteins, the key step is the obtention
of pure Fe(TPC)Cl (1). Our work was focused on finding optimal
conditions allowing the production of pure Fe(TPC)Cl. This purity
was then tested by preparing the first iron(II) diamagnetic com-
plexes of TPC.
2. Experimental
2.1. General procedures and materials
Several considerations have prompted us to investigate iron
chlorin models. A deeper understanding of the electronic ground
state will require precise structural and spectroscopic features for
the prosthetic group. The study of the binding of small ligands such
As a precaution against the formation of the
l-oxo dimer
[Fe(TPC)]2O [10,40], all reactions were carried out in dried solvents
in Schlenk tubes under an Argon atmosphere. Solvents were dis-
tilled from appropriate drying agents and stored under argon.
The mesotetraphenylchlorin (TPC) was prepared by literature
methods [41]. tert-Butyl isocyanide, 2,6-xylyl isocyanide and
P(Me)2Ph are commercially available.
⇑
Corresponding author. Tel.: +961 (70)222005; fax: +961 (1)306044.
E-mail addresses: marwan.kobeissi@liu.edu.lb (M. Kobeissi), mahmoud.faraj@
liu.edu.lb (M.K. Faraj), gerard.simonneaux@univ-rennes1.fr (G. Simonneaux).
0020-1693/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.ica.2013.01.012

M. Kobeissi et al. / Inorganica Chimica Acta 396 (2013) 30–34
31
2.2. Physical measurements
crystallization. Fine crystals were collected by filtration after
3 days. Yield 0.1 g (85%). Dark red powder. UV–Vis (CHCl3): kmax
UV–Vis spectra were recorded on an Uvikon 941 spectropho-
tometer in dichloromethane. 1H NMR spectra were recorded on a
Bruker AC 300P spectrometer in CD2Cl2 or CDCl3 at 300 MHz. Tet-
ramethylsilane was used as the internal reference. Mass spectrom-
etry was performed by the Centre Régional de Mesures Physiques
de l’Ouest, CRMPO, Rennes, France.
(nm) 427 (e e
110 dm3 mmolÀ1 cmÀ1), 608 ( 17). 1H NMR (d, CD2Cl2,
ppm) 8.19 (2H, s, Hpyr); 8.18 (2H, d, Hpyr); 7.85 (2H, d, Hpyr); 7.95,
7.76 (8H, m, Ho); 7.6 (m, 12H, Hm+p); 4.02 (s, 4H, Hpyrroline); À0.07
(s, 18H, Hligand).
FAB MS (m/z): [MÀ2t-BuNC]+ 670. IR
m
(CN) 2117 cmÀ1 (Nujol).
Abbreviations used: TPC = 7,8-dihydro-5,10,15,20-tetraphenylpor-
phyrin dianion or mesotetraphenylchlorin, TPP = 5,10,15,20-tetra-
phenylporphyrin dianion or mesotetraphenylporphyrin, Fe(TPC)Cl =
(7,8-dihydro-5,10,15,20-tetraphenylporphyrin) iron(III) chloride or
(tetraphenylchlorinato) iron(III) chloride, Fe(TPP)Cl = (5,10,15,20-tet-
raphenylporphyrin) iron(III) Chloride or (tetraphenylporphyrin-
2.5. Bis(2.6-xylyl isoyanide) tetrakis(phenyl)-chlorinatoiron(II) (3)
2.5.1. FeII(TPC)(2,6-xylylNC)2 (3)
A solution of Fe(TPC)Cl (0.1 g, 0.14 mmol) in 15 mL of dichloro-
methane was reduced under argon by Zn–Hg amalgam at room
temperature. After a reaction time of 60 min, the solution was then
filtered and 4 equiv. of 2,6-xylyl isocyanide added by a syringe to
the Fe(TPC) species. The solution was then stirred for 30 min. Hex-
ane (40 mL) was added gradually and the solution set aside for
crystallization. Fine crystals were collected by filtration after
3 days. Yield 0.09 g (69%). Dark red powder. UV–Vis (CHCl3): kmax
ato)iron(III)
20-tetraphenylporphyrin)
Fe(TPC)(2,6-xylylNC)2 = (7,8-dihydro-5,10,15,20-tetraphenylporphy-
rin) iron(II) bis-(2,6-xylyl)isocyanide, Fe(TPC)[P(Me)2Ph]2
chloride,
Fe(TPC)(t-BuNC)2 = (7,8-dihydro-5,10,15,
iron(II) bis-(tert-butyl)isocyanide,
=
(7,8-dihydro-5,10,15,20-tetraphenylporphyrin) iron(II) bis-dimeth-
ylphenylphosphine, 2,6-xylylisocyanide = 2-isocyano-1,3-dimeth-
ylbenzene, P(Me)2Ph = dimethylphenylphosphine.
(nm) 427 (e e 11).
80 dm3 mmolÀ1 cmÀ1), 610 (
1H NMR (d, CD2Cl2, ppm) 8.29 (s, 2H, Hpyr); 8.27 (d, 2H, Hpyr);
7.85 (d, 2H, Hpyr); 7.99; 7.81 (m, 8H, Ho); 7.66 (m, 12H, Hm+p);
6.67 (t, 2H, Hp ligand); 6.49 (d, 4H, Hm ligand); 4.12 (s, 4H, Hpyrro-
line); 0.79 (s, 12H, CH3 ligand). FAB MS (m/z): [MÀ2(2,6-xylylNC)]+
2.3. Tetrakis(phenyl)-chlorinatoiron(III) chloride (1)
2.3.1. FeIII(TPC)Cl (1)
670. IR m
(CN) 2111 cmÀ1 (Nujol).
0.5 g of TPC (0.81 mmol) and 0.26 of cadmium acetate dihydrate
(0.97 mmol, 1.2 equiv.) were mixed in a Schlenk under argon
atmosphere. 45 mL of distilled DMF were then introduced by a syr-
inge. The mixture was heated and stirred at 110 °C for 20 min. The
reaction was monitored by UV–Vis spectroscopy. DMF was evapo-
rated under vacuum. In situ and under argon, 200 mg of solid so-
dium ascorbate, 1.29 g of iron(II) chloride tetrahydrate
(6.48 mmol, 8 equiv.) and 50 mL of pure acetone (99%) was added
by a syringe,. This mixture was heated at 60 °C for 2 h. The reaction
was monitored by UV–Vis spectroscopy and TLC (eluent: pentane
then chloroform). Acetone was evaporated under vacuum and
the residue obtained was purified by column chromatography on
neutral alumina, using a mixture of CHCl3/CH3OH (95/5) as eluent.
Two drops of HCl (12 N) were added (to avoid the formation of the
2.6. Bis(dimethylphenylphosphine) tetrakis(phenyl)-chlorinatoiron(II)
(4)
2.6.1. FeII(TPC)[P(Me)2Ph]2 (4)
A solution of (TPC)FeCl (0.1 g, 0.14 mmol) in 15 mL of dichloro-
methane was reduced under argon by Zn–Hg amalgam at room
temperature. After a reaction time of 60 min, the solution was then
filtered and 4 equiv. of dimethylphenylphosphine added by a syr-
inge to the Fe(TPC) species. The solution was then stirred for
30 min. Hexane (40 mL) was added gradually and the solution
set aside for crystallization. Fine crystals were collected by filtra-
tion after 3 days. Yield: 0.094 g (70%). Dark red powder. UV–Vis
(toluene): kmax (nm) 370 (
569 ( 7.9), 609 ( 17.1), 654 (
298 K): Chlorin: 8.2 (s, 2H, Hpyrr); 7.8 (m, 4H, Ho); 7.6 (m, 20H,
pyrr + Ho,m,p); 4.35 (s, 4H, Hpyrroline). Ligand: 6.9 (t, 2H, Hp); 6.7
e
13.7 dm3 mmolÀ1 cmÀ1), 444 (
4.3). 1H NMR (d, CDCl3, ppm,
e 62.4)
FeIII–
ness. 0.4 g was obtained. Yield = 80%. Dark green powder. UV–Vis
kmax (nm) ( 74); 415 (
, 10À3 dm3 MÀ1 cmÀ1) (CH2Cl2): 385 (
89); 600 ( 16.4); 646 ( 4.5); 740 (
4.9). FAB MS (m/z): [M]+ 706.2.
loxo dimer complex), and the sample was evaporated to dry-
e
e
e
e
e
e
H
e
e
e
(t, 4H, Hm); 4.9 (t, 4H, Ho); À2.35 (s, 12H, CH3). FAB MS (m/z):
[M]+ 946.3, [MÀP(Me)2Ph]+ 808.3, [MÀ2P(Me)2Ph] 670.3.
2.4. Bis(tert-butyl isocyanide) tetrakis(phenyl)-chlorinatoiron(II) (2)
2.4.1. FeII(TPC)(t-BuNC)2 (2)
3. Results and discussion
A solution of Fe(TPC)Cl (0.1 g, 0.14 mmol) in 15 mL of dichloro-
methane was reduced under argon by Zn–Hg amalgam at room
temperature. After a reaction time of 60 min, the solution was fil-
tered and 4 equiv. of tert-Butyl isocyanide added by a syringe to
the Fe(TPC) species. The solution was then stirred for 30 min. Hex-
ane (40 mL) was added gradually and the solution set aside for
3.1. Syntheses
The insertion of iron in the TPC was performed by a transmetal-
lation reaction using the cadmium complex Cd–TPC as intermedi-
ate (Scheme 1).
H
H
H
H
H
Cl
H
H
H
H
H
H
H
FeN
N
CdN
N
N
N
N
N
FeCl2,4H2O/Acetone
HCl
Cd(OAc)2,2 H2O
DMF
H
H
N
N
N
N
80 %
TPC
Cd-TPC
Scheme 1. Synthesis strategy of FeIII(TPC)Cl (1).
Fe(TPC)Cl (1)

32
M. Kobeissi et al. / Inorganica Chimica Acta 396 (2013) 30–34
Table 1
for ligand binding to metal complexes generally increase with
increasing saturation of the macrocycle [37,38].
Percentages of oxidized macrocycle using classical insertion methods.
One major difficulty may be encountered in preparing iron(II)
complexes of chlorins. The oxidation of the chlorin ring to a por-
phyrin ring may occur, as previously reported with imidazole li-
gands [15]. This problem was solved by the iron insertion
method presented above.
FeCl2.4H2O
FeBr2
Transmetallation via Cd
Yield = 75%
FeTPPCl = more than 25%
Yield = 75%
FeTPPCl = 10–20%
Yield = 80%
FeTPPCl (not detected)
This method developed in our laboratory, is adapted from a pro-
cedure used by Scherz with bacteriochlorophyll a [32]. The proce-
dure allows one to obtain a pure compound and overcomes the
problem of oxidation of the TPC that gives non negligible amounts
of Fe(TPP)Cl together with the desired product (1) [35].
Obviously, this reaction is the key-step to master, in order to
prepare iron(II) and especially iron(III) complexes of sufficient pur-
ity for structural and spectroscopic studies.
Using classical methods of iron insertion such as FeCl2 or FeBr2
under reflux in DMF gave huge amounts of oxidized macrocycle
(Table 1).
The Cadmium complex Cd–TPC is easily accessible with the
method (acetate/DMF) and could be subject to transmetallation
in good yields (80%) into other metal complexes under mild con-
3.2. 1H NMR spectroscopy
A representative 1H NMR spectrum of [Fe(TPC)]Cl (1) is shown
in Fig. 1. The non-equivalent pyrrole (pyr) proton resonances for
(1) at 87, 77 and 68 ppm (298 K) are within the range of 50–
100 ppm found for the pyrrole resonances in other high-spin chlo-
rin complexes (S = 5/2) [11]. These values are much further down-
field than the values of 10–40 ppm found for spin-admixed (S = 3/
2, 5/2) complexes such as [Fe(TPP)]CF3SO3 and [Fe(TPP)]ClO4 sug-
gesting a pure high-spin state for our paramagnetic complex (1)
[34].
The three pyrrole type resonances that are strongly shifted
downfield are indicative of a strong sigma contact shift at these
positions. This is in accordance with a half-filled dx2
of iron(III) and a delocalisation through the
rin [35].
It is important to notice the absence of the shift at 80 ppm re-
lated to Fe(TPP)Cl, which was always present when classical means
of iron insertion were used [35].
electron
r orbitals of the chlo-
Ày2
ditions. The ease of transmetallation of Cd(TPC) complexes is
i
probably due to the large ionic radius of cadmium (rM
)
Cd2+(0.95 Å) with respect to that of Fe2+ (0.65 Å). In addition, it
is well know that the cadmium does not completely fit into
the core of the macrocycle and is found slightly displaced above
its plane [33]. This explains the average stability of the cadmium
complex which is demetallated at pH 6–7. Accordingly, these
complexes were not isolated and the transmetallation is per-
formed in situ.
As a proof of high purity of (1), A representative 1H NMR spec-
trum of FeII(TPC)(t-BuNC)2 (2) is shown in Fig. 2. The peaks for the
phenyl protons of the chlorin ring are fully assigned by proton
COSY experiments (Fig. 3). For isocyanide axial ligands,
measurements of the relative intensities and relative line-widths
fully determine the assignment. The shift of the isocyanide ligand
is unchanged in the presence of excess ligand. Hence axial ligand
dissociation is not expected to be significant at ambient
temperature.
Another important factor of the reaction is the use of acetone as
solvent for the transmetallation together with the counteranion
chloride. In fact, during the transmetallation, CdCl2 and the Fe–
TPC complex are in equilibrium with the starting materials and
the driving force of the reaction is the very low solubility of CdCl2
in acetone that shifts the equilibrium in the direction of formation
of the products.
This procedure has proven its efficiency with other chlorins dif-
ferently substituted on the meso positions of the phenyl groups.
The synthesis of symmetrical bis-isocyanides and bis-phos-
phine (2–4) complexes in the iron(II) state was via a variation of
our method, previously reported for the synthesis of Fe(TPP)(CNR)2
and Fe(TPP)(phosphine)2 (Scheme 2) [36].
Starting from Fe(TPC)Cl, the reduction of Fe(III) to Fe(II) was car-
ried out with zinc amalgam under argon. The compounds were ob-
tained as crystalline solids in 70–80% yield and characterized by 1H
NMR. The stability of the reduced complexes was satisfactory at
ambient temperature so that it was not necessary to add excess
ligand in the solution to record NMR spectra. This result was ex-
pected since it was previously reported that the stability constants
The 1H NMR spectrum of Fe(TPC)(t-BuNC)2 (2) displays two
groups of signals corresponding to the chlorin ring protons: 8.19,
8.18 and 7.85 (Hpyr); 7.80 (Ho); 7.56–7.6 (Hm+p); 4.02 (Hpyrroline
and to the ligand (À0.07 ppm). These chemical shifts are very
similar to those found for Fe(TPC)(PMe2Ph)2 (4) and are as ex-
pected for diamagnetic iron(II) chlorin derivatives. The protons of
the ligand are shifted to high field (vs. free ligand) due to the ring
current shift of the macrocycle. Similar results were obtained with
complex (3).
)
3.3. IR spectroscopy
In the IR spectrum of the new complex FeII(TPC)(t-BuNC)2 (2),
The
m
(C„N) stretching frequency of CNR is decreased upon
t-Bu
N
Ar
H
Ar
C
H
H
Ar
H
Ar
H2
H2
N
N
H
Zn-Hg amalgam/CH2Cl2
H
N
N
H
H
Ar
Fe
Fe
N
N
N
then 4 equiv. of Ligand
here t-BuNC
85 %
N
Ar
H1
Ar
C
N
Ar
Cl
H3
t-BU
FeIII(TPC)Cl
FeII(TPC)(t-BuNC)2 2
Scheme 2. Synthesis of FeII complexes.

M. Kobeissi et al. / Inorganica Chimica Acta 396 (2013) 30–34
33
Fig. 1. 1H NMR spectrun of Fe(TPC)Cl (1) in CD2Cl2 at 298 K.
Fig. 2. 1H NMR of Fe(TPC)(t-BuNC)2 (2) in CD2Cl2 at 298 K.
coordination of the isocyanide to the metal, decreasing from
2130 cmÀ1 for the free ligand to 2117 cmÀ1 in (2.) This suggests
that the observed isocyanide stretching frequency is influenced
by the reduction of the porphyrin ring (cis-macrocycle influence).
frequency due to an increase in the p⁄ population of the CN bond
of the iron(II) complex.
In summary, a simple and highly efficient synthesis of pure FeIII-
TPCCl (1) has been developed. The purity of this compound was
unprecedented and allowed the synthesis of iron(II) complexes
(2–4). This methodology will open a new perspective in the syn-
thesis and correct characterization of highly pure iron(II) and iro-
n(III) complexes of chlorins as models of hemoproteins.
Thus, isocyanide ligands bonded to iron(II) chlorins (
2117 cmÀ1) have lower CN stretching frequencies than their corre-
sponding Fe(TPP)(t-BuNC)2 (
(C„N) 2129 cmÀ1) [39]. As expected,
the more electron donating the macrocycle the lower the (CN)
m(C„N)
m

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M. Kobeissi et al. / Inorganica Chimica Acta 396 (2013) 30–34
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Acknowledgments
We thank the team Ingénierie Chimique et Molécules pour le
Vivant (Rennes-France) where the work was done. We are also
grateful to the Lebanese University (UL) and the Lebanese Interna-
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Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.ica.2013.01.012.
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