Direct incorporation of a ferric ion in the porphyrinogen core: Tetrakis(cyclohexyl)iron porphyrinogen anion with different conformers and its reaction with iodine

Article abstract of DOI:10.1021/ic050890p

Et₄N[L″Te^III]·3DCM (1) is directly synthesized by adding ferric chloride into a solution of a lithium salt of tetrakis(cyclohexyl)porphyrinogen (L″). [L″]^4- is a good chelating ligand for both Fe(III) and Fe(II) ions. It is an avid proton scavenger but not a reducing agent. 1 showed a magnetic moment (μ_eff) of 4.3 μ_B in the solid, which changed to 6.0 μ_B in solution. This change in spin state is common for all iron porphrinogens. 1 showed polymorphism, and with pyridine in the lattice, it changed to Et₄N[L″Te^III]·DCM _0.5Py_1.5 (2), possessing two different conformers. Calculation of these conformers at the density functional theory level showed the relative energies of all d orbital changes in three conformers, highlighting the influence of the disposition of a peripheral ligand. Iodine oxidation of 1 yielded [L″^γγFe^III][I ₃·I₂⁺·I₃^-] (3) with the introduction of two C_α-C_α bonds with concomitant reduction of Fe(III) to Fe(II). Its μ_eff (5.4 μ_B) in the solid changed to 4.8 μ_B in solution, suggesting a high spin state (S = 2) for Fe(II).

Full text of DOI:10.1021/ic050890p

Inorg. Chem. 2005, 44, 7699−7701
Direct Incorporation of a Ferric Ion in the Porphyrinogen Core:
Tetrakis(cyclohexyl)iron Porphyrinogen Anion with Different Conformers
and Its Reaction with Iodine
Dibyendu Bhattacharya, Soumen Dey, Suman Maji, Kuntal Pal, and Sabyasachi Sarkar*
Department of Chemistry, Indian Institute of Technology, Kanpur 208 016, India
Received June 2, 2005
Et4N[L′′FeIII]
‚
3DCM (1) is directly synthesized by adding ferric
ylporphyringen) in 80% yield.3 The use of an excess of FeCl2
led to the isolation of the meso-octaethyliron(III) porphy-
rinogen complex3 similar to the structurally characterized [Li-
(MeCN)][L′FeIII] (L′ ) meso-octamethylporphyrinogen)
complex, where the stoichiometry between L′ and FeCl2 as
1:2 was used.4 This was thought to be a disproportionation-
type reaction and has recently been shown to be due to the
oxidation of [L′Fe(II)]2- by an excess of FeCl2 present in
the reaction mixture with the precipitation of elemental iron.5
It is reported that direct incorporation of a trivalent late-
transition-metal ion like Fe(III) into the porphyrinogen core
is not possible.2b
chloride into a solution of a lithium salt of tetrakis(cyclohexyl)-
porphyrinogen (L′′). [L′′ 4
(III) and Fe(II) ions. It is an avid proton scavenger but not a
reducing agent. 1 showed a magnetic moment ( eff) of 4.3 µB in
-
]
is a good chelating ligand for both Fe-
µ
the solid, which changed to 6.0 µB in solution. This change in
spin state is common for all iron porphrinogens. 1 showed
polymorphism, and with pyridine in the lattice, it changed to Et4N-
[L′′FeIII]
‚DCM0.5Py1.5 (2), possessing two different conformers.
Calculation of these conformers at the density functional theory
level showed the relative energies of all d orbital changes in three
conformers, highlighting the influence of the disposition of a
peripheral ligand. Iodine oxidation of 1 yielded [L′′∆∆FeIII][I3
(3) with the introduction of two CR CR bonds with concomitant
reduction of Fe(III) to Fe(II). Its µeff (5.4 B) in the solid changed
In contrast to the behavior of iron porphyrin1b and the
earlier prediction on the direct nonincorporation of Fe(III)
into the porphyrinogen core,2 we report, herein, the synthesis
of Et4N[L′′FeIII]‚3DCM (1) by directly using a Fe(III) ion.
On the basis of the electrochemistry of 1, its irreversible ring
oxidation was followed using elemental iodine, resulting in
+
-
‚I2 ‚I3 ]
−
µ
to 4.8 µB in solution, suggesting a high spin state (S
Fe(II).
) 2) for
+
-
the isolation of [L′′∆∆FeIII][I3‚I2 ‚I3 ] (3).
1 was synthesized by adding a stoichiometric amount of
anhydrous FeCl3 into a solution of a lithium salt of tetrakis-
(cyclohexyl)porphyrinogen (L′′) in a THF medium.6 X-ray
structure analysis of 1 revealed that Fe(III) has been
incorporated into this porphyrinogen core.7 Its room-tem-
perature magnetic moment in the solid state has been found
to be 4.3 µB at 298 K (3.8 µB at 6 K). When magnetic
measurement was carried out in DMF or in a DCM medium
by NMR,8 the spin state of S ) /2 changed to S ) /2,
yielding µeff ) 6.0 µB. This change in the spin state of S )
3/2 (in solid) to S ) 5/2 (in solution) is a general phenomenon
observed in all characterized iron(III) porphyrinogen sys-
tems.9 The space-filling model of 1 showed no room for
Ferrous is known to be the effective oxidation state of
iron for its incorporation in the biosynthesis of heme.1a In
the synthesis of iron porphyrin, it was shown that it is Fe-
(II) that is incorporated into porphyrin to make room for
introduction of Fe(III).1b The metalation of meso-octaalky-
lporphyrinogen has been shown to follow a similar trend.2a
The introduction of iron into the core of meso-octaalkylpor-
phyrinogen has been carried out several times, and in each
case, it was stated that an excess of ferrous ions would lead
to the formation of iron(III) porphyrinogen.
3
5
The synthesis of Li2(THF)4LFeII has been reported by the
reaction between FeCl2 and Li4L‚4THF (L ) meso-octaeth-
(3) (a) Judd, J.; Jacoby, D.; Floriani, C.; Chiesi-Villa, A.; Rizzoli, C. Inorg.
Chem. 1992, 31, 1306. (b) A very similar stoichiometric reaction
yielded the corresponding Ru(II) complex. See: Bonomo, L.; Stern,
C.; Solari, E.; Scopelliti, R.; Floriani, C. Angew. Chem., Int. Ed. 2001,
40 (8), 1449-1452.
* To whom correspondence should be addressed. E-mail: abya@iitk.ac.in.
(1) (a) Elder, G. H. In Iron in biochemistry and medicine II; Jacobs, A.,
Worwood, M., Eds.; Academic Press: London, 1980; pp 245-292.
(b) Husza´nk, R.; Horva´th, O. Chem. Commun. 2005, 2, 224-226.
(2) (a) Floriani, C.; Floriani-Moro, R. In The Porphyrin Handbook;
Kadsish, K. M., Smith, K. M., Guilard, R., Eds.; Academic Press:
New York, 2000; Vol. 3, p 389. (b) A hexacoordinated V complex
using V(III) as the starting material has been reported. See: Jubb, J.;
Gambarotta, S. J. Am. Chem. Soc. 1993, 115, 10410-10411.
(4) Jacoby, D.; Floriani, C.; Chiesi-Villa, A.; Rizzoli, C. J. Chem. Soc.,
Chem. Commun. 1991, 220.
(5) Bachmann, J.; Nocera, D. G. J. Am. Chem. Soc. 2005, 127, 4730-
4743.
(6) Detail syntheses of 1-3 are in the Supporting Information.
10.1021/ic050890p CCC: $30.25
Published on Web 10/06/2005
© 2005 American Chemical Society
Inorganic Chemistry, Vol. 44, No. 22, 2005 7699

COMMUNICATION
Figure 2. Comparison of molecular orbital energy level diagrams (a-c
numbering is identical with that in Figure 1; only d orbitals are shown).
c) were different from each other and from that of the
molecule present in 1 (Figure 1, top, conformer a). Density
functional theory (DFT) calculation of these conformers
showed an interesting perturbation in the molecular orbital
energy level (Figure 2). The relative energy levels of all d
orbitals change drastically in the three conformers, highlight-
ing the influence of the disposition of a peripheral ligand
(flipping of the cyclohexyl ring). The electrostatic surface
potentials of all three conformers differ,10 suggesting the
possibility of solvent interaction with 1, which may affect
the conformation of the molecule in such a way as to lower
Figure 1. Conformer of the anion of 1 (top, a), conformers of the anion
of 2 (top, b and c), and the structure of the anion of 3 (bottom) (see the
Supporting Information for ORTEP plots). Color code: C, gray; N, blue;
Fe, brown; I, pink.
solvent coordination, which could have been responsible for
2
2
the energy of the dx -y orbital, resulting in the change in
the spin state in solution. Iron incorporation in porphyrin
and porphyrinogen does not follow the same chemistry. For
porphyrin, its deviation from planarity with out-of-plane
displacement of Fe(III) in pentacoordination and planar
tetracoordination of Fe(II) and their relative stability even
in an aqueous medium suggested that protonated porphyrin
does not possess any extra stability. The metalation and
demetalation of porphyrin are generally reversible and pH-
dependent. In contrast, for porphyrinogen its ligational role
toward metal ions is centered on likelihood of the extremely
basic tetralithiated [L′′]4- ion, which is an avid proton
scavenger, to change to the stable H4L′′. [L′′]4- is not
reducing but a good chelating ligand. The addition of 1 equiv
of anhydrous FeCl2 or FeCl3 to it resulted in immediate
complexation to generate the respective complex anion. The
addition of 2,2′-bipyridine or [Bu4N]3[Fe(III)(CN)6] into a
solution of [L′′FeII]2- did not show any color changes,
confirming the absence of any free or dissociable Fe(II) ion.
Similarly, in the case of [L′′FeIII]2-, the addition of [Bu4N]3H-
[FeII(CN)6] did not produce any prussian blue precipitate,
suggesting that the complexed Fe(III) is not dissociated
readily. However, both of the anions are susceptible to
hydrolysis (Scheme 1). For the formation of [L′′FeIII]- in
the presence of excess FeCl2, the reaction involves two redox
couples. In THF, the cyclic voltammetric response of the
[L′′FeIII]-/[L′′FeII]2- couple is irreversible with Epa at
-1.05V vs Ag/AgCl. In the same solvent, the electrochemi-
cal response of FeCl2 showed an irreversible redox response
2
2
altering the energy of the dx -y orbital, resulting in the change
in the spin state. Recrystallization of 1 in DCM in the
presence of pyridine retained the molecular structure of 1
with the presence of pyridine in the lattice to yield Et4N-
[L′′FeIII]‚DCM0.5Py1.5 (2). However, the conformational
orientation of the molecules in 2 differed in comparison to
that found in 1. In the asymmetric unit of 2, the orientations
of the two molecules (see Figure 1, top, conformers b and
(7) Crystal data for Et4N[L′′FeIII]‚3DCM (1): a ) 13.640(5) Å, b )
18.548(5) Å, c ) 20.750(5) Å, R ) 90(5)°, â ) 90(5)°, γ ) 90(5)°,
V ) 5250 Å3, orthorhombic, space group P222, Z ) 4, Dcalc ) 1.298
g cm-3, T ) 100 K. Of a total of 35 396 reflections collected, 12 997
were independent (Rint ) 0.049). Final R1 ) 0.065 [I > 2σ(I)], wR2
) 0.1514, GOF ) 0.999. Crystal data for Et4N[L′′FeIII]‚DCM0.5Py1.5
(2): a ) 11.696(5) Å, b ) 16.749 (5) Å, c ) 24.767(5) Å, R ) 90.663-
(3)°, â ) 96.540(5)°, γ ) 95.869(5)°, V ) 4794 Å3, triclinic, space
group P1h, Z ) 2, Dcalc ) 1.266 g cm-3, T ) 100 K. Of a total of
32 274 reflections collected, 22 809 were independent (Rint ) 0.047);
GOF ) 1.041, final R1 ) 0.094 [I > 2σ(I)], wR2 ) 0.205. Crystal
data for [L′′∆∆FeIII][I3‚I2+‚I3-] (3): a ) 11.742(5) Å, b ) 13.001(5)
Å, c ) 17.686(5) Å, R ) 85.904(5)°, â ) 72.965(5)°, γ ) 72.100-
(5)°, V ) 2456 Å3, triclinic, space group P1h, Z ) 2, Dcalc ) 2.297 g
cm-3, T ) 100 K. Of a total of 16 338 reflections collected, 11 658
were independent (Rint ) 0.017); GOF ) 1.067, final R1 ) 0.0746 [I
> 2σ(I)], wR2 ) 0.2199.
(8) Evans, D. F. J. Chem. Soc. 1959, 2003.
(9) We follow the synthesis of Fe(III) containing three other porphyrinogen
(octaethyl, octamethyl, and methylethyl substituted) using the present
synthetic procedure to isolate deep red tetraethylammonium salt of
the corresponding iron(III) porphyrinogen. For the octaethylporphy-
rinogen, Floriani et al. reported a solid-state magnetic moment value
of 4.35 µB at 293 K.3 The same complex synthesized by the present
method showed the room-temperature magnetic moment 4.35 µB,
identical to that. However, in DMF, its magnetic behavior changed to
that of high-spin Fe(III), showing µeff ) 6.2 µB. For iron(III)
butylporphyrinogen, the solid-state magnetic moment value was µeff
) 4.05 µB (SQUID measurement), which followed Curie-Weiss law
(µeff ) 3.8 µB at 6 K). Upon measurement in DMF, it changes to a
high-spin type, showing µeff ) 6.1 µB.
(10) Calculations were carried out with the CAChe 5.03 program of Fujitsu
Ltd., using a ZINDO semiempirical method.
7700 Inorganic Chemistry, Vol. 44, No. 22, 2005

COMMUNICATION
Scheme 1
around -0.05 V vs Ag/AgCl along with the deposition of
the reduced metallic iron on the electrode. The small excess
of free Fe(II) in the THF solution of [L′′FeII]2- thus responds
to oxidize the complex anion to the relatively more stable
Fe(III) species. Because of the difference in the ligational
nature between [L′′FeII]2- and the THF-coordinated Fe(II)
ion, the observed redox reaction between them may not be
classified as the “normal” disproportionation reaction of the
transition-metal ions.
1 in other solvents responded with a reversible reduction
of Fe(III) in its cyclic voltammetric study followed by an
irreversible oxidation of coordinated porphyrinogen. In DCM,
the oxidation process is easier (+0.43 V vs Ag/AgCl) than
that in DMF (+0.59 V vs Ag/AgCl). Interestingly, in DCM,
two irreversible oxidation processes were observed, and the
second one is rather ill-defined and appeared at +1.2 V vs
Ag/AgCl. It is known that metalated porphyrinogen is
capable of oxidation involving CR-CR bond formation with
apparent two- and four-electron dehydrogenase-type reac-
tions.11 To follow the predicted oxidation of 1 by an
electrochemical study, we oxidized 1 in DCM by iodine to
isolate the oxidized product 3. Its X-ray structure confirmed
that the porphyrinogen has been oxidized with the introduc-
tion of two CR-CR bond formations (instead of one) with
concomitant reduction of the coordinated Fe(III) to Fe(II),
which, in turn, ligated with one iodide ion, resulting in
pentacoordination of Fe(II) (Figure 1, bottom).7 Oxidation
by transition-metal ions such as Cu(II) has extensively been
done where the copper cluster anions formed as the byprod-
uct in such a reaction invariably trap the oxidized cationic
species.2a A cleaner oxidation was reported with the [Fe-
(cp)2]+ cation.5 The isolation of 3 suggested that the
irreversible redox chemistry must be a complex process. Its
room-temperature magnetic moment in the solid showed µeff
) 5.4 µB, which is higher than that expected for a high spin
Fe(II) state (S ) 2), but in solution (DMF or DCM), its
magnetic moment showed a value (µeff ) 4.8 µB) expected
for Fe(II) in a high spin state (S ) 2). The lattice of 3
possesses a DCM molecule and one I2 unit along with two
I3 units. In the solid state at room temperature, this compound
showed an EPR signal with g value ) 2.03 , which could
Figure 3. Molecular orbital energy level diagram and frontier molecular
orbital plot of d orbitals for 3.
+
-
I2 I3 interaction.12 The frontier molecular orbital analysis
(Figure 3) showed the population of four unpaired electrons
essentially in four d orbitals.13,14 A thermogravimetric study
of 3 showed that at 164 °C it loses one molecule of DCM
and one molecule of I2 under Ar and above that temperature
it is not thermally stable. Oxidation of 1 by iodine in other
solvents or by other halogens such as Br2 or other oxidants
behaved differently, and details for these will be published
later.
Acknowledgment. D.B. thanks IIT Kanpur for a Senior
Teaching Fellowship, S.D., S.M., and K.P. thank CSIR, New
Delhi, India, for SRF, and S.S. thanks the DST, New Delhi,
India, for funding the project.
Supporting Information Available: Details of synthesis, cyclic
voltammograms of 1 in different solvents, ORTEP plots with
thermal ellipsoids, SQUID magnet measurements for 1 and 3,
complete ref 13, and X-ray crystallographic files for 1-3 (CIF
format). This material is available free of charge via the Internet at
http://pubs.acs.org.
IC050890P
(12) Considering the oxidation state of iron coordinated with one iodide
ion, the cation in 3 has a monopositive charge, which requires one
-
-
I3 as a counteranion, leaving formally the I2+/I3 ion pair in the
crystal.
(13) Pople, J. A.; et al. Gaussian 03, revision B.04; Gaussian, Inc.:
Pittsburgh, PA, 2003 (see the Supporting Information).
(14) DFT calculations have been carried out by employing a B3LYP hybrid
functional using the Gaussian 03 program; molecular orbitals were
visualized using G View. The 6-31g** basis set was used for the C,
N, and H atoms, and MidiX was used for the I atom; the effective
core potential basis set LANL2DZ was used for the Fe atom. The
geometry was taken from the crystal structure. Because spin restriction
is lifted, there is no common spatial wave function for each R and â
spin pair, and two manifolds of “orbitals” are drawn separately. For
three conformers (parts a-c in Figure 2) and for 3 (Figure 3), the â
spinator bears 90% d character.
+
-
have originated from an ion pair like I2 I3 . The reduction
of the magnetic moment and the loss of the EPR signal in
solution are consistent with the disappearance of ion pair
(11) Angelis, S. D.; Solari, E.; Floriani, C.; Chiesi-Villa, A.; Rizzoli, C. J.
Am. Chem. Soc. 1994, 116, 5691-5701.
Inorganic Chemistry, Vol. 44, No. 22, 2005 7701