Binding of catechols to iron(III)-octaethylporphyrin: An experimental and DFT investigation

Article abstract of DOI:10.1002/ejic.201000707

The synthesis, X-ray structure, and properties of high-spin, five-coordinate catecholate complexes of (octaethylporphyrinato)iron(III), Fe^III(OEP)(L) (L: catecholate monoanion), are reported here for the first time. In these complexes, catechol

Full text of DOI:10.1002/ejic.201000707

FULL PAPER
DOI: 10.1002/ejic.201000707
Binding of Catechols to Iron(III)–Octaethylporphyrin: An Experimental and
DFT Investigation
Arvind Chaudhary,[a] Ranjan Patra,[a] and Sankar Prasad Rath*[a]
Dedicated to Prof. S. S. Krishnamurthy on the occasion of his 70th birthday
Keywords: Catecholate binding / Hydrogen bonds / Spectroelectrochemistry / Density functional calculations / Structure
elucidation
The synthesis, X-ray structure, and properties of high-spin,
five-coordinate catecholate complexes of (octaethylporphyr-
inato)iron(III), FeIII(OEP)(L) (L: catecholate monoanion), are
reported here for the first time. In these complexes, catechol
binds in an η1-fashion as an axial ligand, which is supported
by DFT calculations. The Fe–O–C angles of FeIII(OEP)(Hcat),
FeIII(OEP)(4-tBu-Hcat), FeIII(OEP)(4-NO2-Hcat), and FeIII-
(OEP)(sal) are 119.5, 125.1, 122.2, and 124.3°, respectively.
FeIII(OEP)(Hcat) has the smallest Fe–O–C angle in addition
to the smallest dihedral angle of 26.2° between the planes of
the porphyrin and axial catechol ligand among all phenolate
complexes of iron(III)–porphyrins. In comparison to those of
FeIII(OEP)(OPh) and FeIII(OEP)(4-tBu-Hcat), the Fe–O bond
in FeIII(OEP)(Hcat) is elongated by 0.064 and 0.038 Å,
respectively. This is due to the H-bonding interactions in
FeIII(OEP)(Hcat) and not caused by steric hindrance. In the
1H NMR spectra of the complexes, the signals of the ortho-
and para-protons of catechol are shifted upfield, whereas
those of the meta-protons are shifted downfield. The alter-
nating shift pattern observed is due to negative and positive
spin densities on the catechol carbon atoms and is indicative
of π-spin delocalization on the catecholato ligand. Electro-
chemical data reveal that the complexes undergo three one-
electron oxidations and a single one-electron reduction.
Based on spectroelectrochemical and DFT studies, the first
oxidation is assigned to a catechol-to-semiquinone transfor-
mation and the second and third oxidations are found to be
porphyrin-ring-centered.
also been reported.[10] The binding of benzene-1,2-diols
(catechols) as substrates to the iron atom in nonheme en-
zymes, such as pyrochatechase and protocatechuate-3,4-di-
oxygenase, is well documented.[11] We sought to determine
the nature of the binding of catechols with FeIII–porphyrins
as no report on the structure and characterization of such
species appears to exist. There are very few structural re-
ports for catecholate-bound metalloporphyrins; just two ex-
amples are known with SnIV–tetraphenylporphyrins[12a,12b]
and one each with ZrIV–[12c] and NbV–tetraphenylpor-
phyrins.[12d]
Introduction
Catalase is a common enzyme found in nearly all living
organisms exposed to oxygen, in which it catalyzes the de-
composition of hydrogen peroxide to water and oxygen.[1–3]
Catalase has one of the highest turnover numbers of all
enzymes; one molecule of catalase can convert millions of
molecules of hydrogen peroxide to water and oxygen. (Phen-
olato)(porphinato)iron(III) complexes merit attention on
the basis of tyrosine coordination to (porphinato)iron(III)
centers in catalase[1–7] and certain mutant hemoglobin. In
case of catalase, the role of the axial phenolato ligand on
the catalytic activity has been poorly investigated.[4] How-
ever, it is thought that the axial phenolato ligand must mod-
ulate the redox potential of the iron atom because of chemi-
cal similarity. Phenolato binding to FeIII–porphyrins has
been known for quite some time as the η1-O binding mode
presented in I[8] and II.[9] The bidentate η2-O,O binding of
the tropolonate anion to iron–porphyrins, seen in III, has
[a] Department of Chemistry, Indian Institute of Technology
Kanpur,
Kanpur 208016, India
This state of development has prompted us to undertake
the task of binding catechols to FeIII–porphyrins. In the
present work, we have successfully prepared stable cate-
View this journal online at
E-mail: sprath@iitk.ac.in
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201000707.
wileyonlinelibrary.com
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A. Chaudhary, R. Patra, S. P. Rath
FULL PAPER
cholate complexes of (octaethylporphyrinato)iron(III), tions, the previously reported five-coordinate FeIII(OEP)-
which structurally mimic the active site of the resting state (OPh) shows a Soret band at 391 and three Q-bands at 490,
of catalase. Synthesis, structure, and properties of the new 520, and 603 nm, which suggests a coordination number of
heme analogues FeIII(OEP)(L) (L: Hcat, 4-NO2-Hcat and five for the complexes reported here.[8a,14] In our complexes,
4-tBu-Hcat) are reported, in which catechol binds in an η1- the positions of the charge-transfer band (around 600–
fashion as an axial ligand. The binding of salicylaldehyde 650 nm) and Soret band (around 390 nm) are found to be
to FeIII–octaethylporphyrin has also been attempted, which dependent on the acidity of the axial ligand. For example,
also eventually yields η1-coordination of the ligand. All the the addition of an electron-donating substituent (tBu) to
complexes reported here have been successfully charac- the catechol redshifts the Soret band of the complex to
terized by single-crystal X-ray structure determinations. 393 nm, whereas the charge-transfer band blueshifts to
This study provides the basis for a discussion on the axial 608 nm. The reverse trends are obtained by adding an elec-
catecholate binding to iron–porphyrins especially regarding tron-withdrawing (NO2) substituent onto the catechol.
Fe–O bond character, Fe–O–C angle, and redox potential.
EPR measurements show similar spectral features in
Spectroelectrochemical investigations and density func- both solid and solution state; Figure 1 shows two represen-
tional theory (DFT) calculations have been used to study tative spectra of the complexes. Both spectra are axially
various aspects of the electronic structures of the com- symmetric for FeIII(OEP)(Hcat) with gЌ = 5.96 and gʈ =
plexes.
2.01 for the frozen toluene solution and gЌ = 5.97 and gʈ
= 1.99 for the powder. The solution magnetic moment for
Results and Discussion
The free octaethylporphyrin (H2OEP) and [FeIII
-
(OEP)]2O were prepared according to literature pro-
cedures.[13] A solution of [FeIII(OEP)]2O with excess L (L:
H2cat, 4-NO2-H2cat, 4-tBu-H2cat, and Hsal) in THF was
heated to reflux to form FeIII(OEP)(L), which after cooling
to room temperature was concentrated to complete dryness.
From a solution of this residue, dark brown crystals of the
product were deposited by slow diffusion. The crystals were
collected by filtration and structurally characterized.
Scheme 1 shows the synthetic strategy and list of the com-
plexes reported in the paper along with their abbreviations.
The UV/Vis spectrum of FeIII(OEP)(Hcat) in chloroform
shows a Soret band at 391 nm and three Q-bands at 500,
529, and 612 nm. Similar spectral features are also observed
Figure 1. X-band EPR spectrum recorded in toluene (at 120 K) of
in the other complexes reported here. Under similar condi- (A) FeIII(OEP)(Hcat) and (B) FeIII(OEP)(4-tBu-Hcat).
Scheme 1.
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Binding of Catechols to Iron(III)–Octaethylporphyrin
FeIII(OEP)(Hcat) at 295 K in purified dichloromethane, cal- Table 1. Selected bond lengths [Å] and angles [°] for FeIII(OEP)(L).
culated by Evan’s method,[15] was found to be 5.91 BM.
FeIII(OEP)(L)
Similar results are obtained for the other iron(III)–cate-
cholate complexes. These results suggest high-spin states of
iron (S = 5/2) in both solid and solution phase for all of
the complexes.
L:
Hcat
4-tBu-Hcat 4-NO2-Hcat sal
Fe1–N1
Fe1–N2
Fe1–N3
Fe1–N4
Fe1–O1
N(1)–Fe–N(2) 87.61(7)
N(1)–Fe–N(3) 156.81(7)
N(1)–Fe–N(4) 87.41(7)
N(2)–Fe–N(3) 87.55(7)
N(2)–Fe–N(4) 155.71(7)
N(3)–Fe–N(4) 87.74(7)
N(1)–Fe–O(1) 101.66(6)
N(2)–Fe–O(1) 103.34(7)
N(3)–Fe–O(1) 101.53(7)
N(4)–Fe–O(1) 100.95(7)
2.0645(17) 2.075(3)
2.0576(18) 2.055(3)
2.0603(17) 2.069(3)
2.0551(18) 2.055(3)
1.9120(14) 1.874(2)
2.058(3)
2.052(4)
2.057(4)
2.052(4)
1.909(3)
87.76(13)
2.058(3)
2.063(3)
2.065(3)
2.058(3)
1.912(3)
88.26(10)
87.18(12)
154.79(13)
87.53(12)
87.26(12)
154.91(13)
87.16(12)
100.04(12)
101.90(12)
105.16(12)
103.17(12)
124.3(4)
Crystallographic Characterization of FeIII(OEP)(Hcat)
159.04(10) 156.44(15)
87.59(10)
87.85(11)
87.58(14)
87.60(15)
Dark brown crystals of FeIII(OEP)(Hcat) were obtained
from the slow diffusion of acetonitrile into a THF solution
of the complex in air in the presence of a slight excess of
H2cat. The molecule crystallizes in the triclinic crystal sys-
157.41(10) 157.06(15)
88.14(10)
100.64(9)
87.75(14)
104.76(14)
100.42(10) 100.11(14)
100.31(10) 98.80(14)
102.17(10) 102.79(14)
¯
tem with a P1 space group, and a perspective view is shown
in Figure 2. This is the first structural characterization of
the binding of catechols to FeIII–porphyrins. In the asym-
metric unit, one full molecule of the complex along with
one free catechol molecule is present. Selected bond lengths
and angles are given in Table 1. The complex has a five-
coordinate square-pyramidal geometry, in which the iron
atom is displaced by 0.42 Å from the N4 plane of the por-
phyrin ring, a distance within the range of 0.39–0.54 Å and
seen for other high-spin five-coordinate iron(III)–por-
phyrins.[8] The average Fe–N and Fe–O distances are found
to be 2.059(2) and 1.912(1) Å, respectively. The Fe–O–C
bond angle is 119.5(1)°, which is the smallest angle reported
for any phenolato complex of FeIII–porphyrin. It is worth
noting that the Fe–O–C bond angle for FeIII(OEP)(OPh) is
142.2(3)°.[8a]
Fe–O1–C37
119.48(13) 125.13(19) 122.2(3)
2.616(2), and 2.705(2) Å, respectively. A bond of the latter
type generates a dimer between two free catechol molecules,
whereas the first two types connect between coordinated
and free catechol molecules in the crystal lattice. Figure 3b
shows the packing diagram for FeIII(OEP)(Hcat).
Figure 2. A perspective view of FeIII(OEP)(Hcat) showing 50%
thermal contours for all non-hydrogen atoms at 100 K (H atoms
have been omitted for clarity).
Figure 3a displays aspects of H-bonding interactions be-
tween oxygen atoms of the coordinated and free catechol
molecules in the crystal lattice. The hydrogen atoms bound
to catecholate oxygen atoms in the structure are directly
located in difference Fourier maps, whereas all other pro-
tons were added in calculated positions. The catecholate
oxygen atom O1 is engaged in H-bonding interactions with
O3 of the free catechol molecule present in the asymmetric
unit. The uncoordinated catechol oxygen atom O2 also
forms H-bonds with O4 of the symmetry-related catechol
molecule. Catechol oxygen atoms are also engaged in H-
bonding interactions with the symmetry-related catechol
molecule forming a dimer, which eventually bridges be-
tween two iron–porphyrins in the crystal lattice. Thus, three
Figure 3. (A) Hydrogen-bonding interactions between FeIII(OEP)-
(Hcat) and free catechol molecules in the crystal lattice (hydrogen
atoms and ethyl substituents are omitted for clarity). (B) Crystal
packing diagram of FeIII(OEP)(Hcat).
Crystallographic Characterization of FeIII(OEP)(4-tBu-
Hcat)
Dark brown crystals of FeIII(OEP)(4-tBu-Hcat) were
types of H-bonds are observed: O(4)···H–O(2), O(1)···H– grown by slow diffusion of cyclohexane into a tetra-
O(3), and O(4)···H–O(3), and these distances are 2.928(2), hydrofuran solution of the complex in the presence of a
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A. Chaudhary, R. Patra, S. P. Rath
FULL PAPER
slight excess of 4-tBu-H2cat in air at room temperature. The
molecule crystallizes in the triclinic crystal system with a
¯
P1 space group, and the asymmetric unit contains one full
molecule of the complex. A perspective view of FeIII
-
(OEP)(4-tBu-Hcat) is shown in Figure 4, and selected bond
lengths and angles are given in Table 1. The average Fe–N
and Fe–O bond lengths are 2.063(3) and 1.874(2) Å, respec-
tively, and the Fe–O–C angle is 125.1(2)°. The uncoordi-
nated OH group of the tert-butylcatechol has orientational
disorder and is distributed between two possible ortho sites
with 62% and 38% occupancies. The occupancies were de-
termined by refinement, and the refined occupancies were
then fixed in subsequent cycles of refinement.
Figure 5. Perspective view of FeIII(OEP)(4-NO2-Hcat) showing
50% thermal contours for all non-hydrogen atoms at 100 K (H-
atoms have been omitted for clarity). Only the major orientation
of 4-nitrocatechol is shown.
Crystallographic Characterization of FeIII(OEP)(sal)
Dark brown crystals of FeIII(OEP)(sal) were grown by
slow diffusion of hexane into a THF solutions of the com-
plex in the presence of a slight excess of salicylaldehyde at
room temperature in air. The complex crystallizes in the
monoclinic space group C2/c, and a perspective view is
shown in Figure 6. The average Fe–N bond length is
2.061(3) Å, and the Fe–O distance is 1.912(3) Å. The Fe–
O–C angle of 124.3(4)° is similar to that previously reported
for FeIII–porphyrins coordinated with phenols.[8] Salicyl-
aldehyde can bind with the iron atom in two possible modes
and thus has orientational disorder with 54.4 and 45.6%
occupancies. The occupancies were originally determined
by refinement, which was then fixed in subsequent cycles of
refinement.
Figure 4. A perspective view of FeIII(OEP)(4-tBu-Hcat) showing
50% thermal contours for all non-hydrogen atoms at 100 K (H-
atoms have been omitted for clarity).
Crystallographic Characterization of FeIII(OEP)(4-NO2-
Hcat)
Dark brown crystals of FeIII(OEP)(4-NO2-Hcat) were
grown by slow diffusion of cyclohexane into a chloroform
solution of the complex in the presence of a slight excess
of 4-NO2-H2cat at room temperature in air. The complex
crystallizes in the monoclinic crystal system with a C2/c
space group, and the asymmetric unit contains one full mo-
lecule of the complex. The iron atom binds in an η1-fashion
with either of the two hydroxy groups of 4-nitrocatechol,
but to an unequal extent. As a result, the nitro and the
uncoordinated hydroxy groups have two possible orienta-
tions with 85 and 15% occupancies. These two possible co-
ordination modes of 4-NO2-Hcat are also seen in solution
(vide infra). A perspective view of the molecule is shown in
Figure 6. Perspective view of FeIII(OEP)(sal) showing 50% thermal
contours for all non-hydrogen atoms at 100 K (H-atoms have been
omitted for clarity). Only the major orientation of salicylaldehyde
is shown.
Several structurally characterized iron(III) porphinate
Figure 5, and selected bond lengths and angles are given in complexes with phenolato ligands have been reported pre-
Table 1. The Fe–N distances are in the narrow range of viously.[8,10] Table 2 compares the key structural parameters
2.052–2.058 Å and the Fe–O distance is 1.909(3). These dis- of these complexes along with the catecholato complexes
tances fall within the spread of literature values observed reported here. As seen from the table, FeIII(OEP)(Hcat) has
for high-spin iron(III) porphyrinates containing axial the smallest Fe–O–C angle and the smallest dihedral angle
phenoxido ligands.[8] The complex has a five-coordinate between the planes of the porphyrin and the axial catechol
square-pyramidal geometry, in which the iron atom is dis- ligand among all phenolato complexes of iron(III)–por-
placed by 0.41 Å from the N4 plane of the porphyrin ring, phyrins reported so far. The average Fe–N distance in-
a distance in the range of 0.39–0.54 Å and seen for other creases in the order: FeIII(OEP)(4-NO2-Hcat) Ͻ FeIII
high-spin five-coordinate iron(III)–porphyrins.[8] (OEP)(Hcat) Ͻ FeIII(OEP)(4-tBu-Hcat), whereas the Fe–O
-
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Eur. J. Inorg. Chem. 2010, 5211–5221

Binding of Catechols to Iron(III)–Octaethylporphyrin
Table 2. Selected geometric parameters for phenoxido complexes of FeIII–porphyrins.
[d]
Complex
Fe–N[a]
Fe–O[b]
Fe–O–C[c]
∆Fe4N
Φ[e]
Ref.
FeIII(OEP)(Hcat)
2.059(2)
2.063(3)
2.054(4)
2.061(3)
2.061
2.051
2.060
2.071
2.059
2.122
1.912(1)
1.874(2)
1.909(3)
1.912(3)
1.848(4)
1.926(3)
1.887(2)
1.816(4)
1.868
119.5(3)
125.1(2)
122.2(3)
124.3(4)
142.2(3)
122.8(3)
125.5(2)
170.6(4)
132.54
0.42
0.39
0.41
0.44
0.47
0.43
0.44
0.48
0.46
0.69
26.2
34.6
31.4
32.9
96
28.7
39.2
81.7
39.9
87
this work
this work
this work
FeIII(OEP)(4-tBu-Hcat)
FeIII(OEP)(4-NO2-Hcat)
FeIII(OEP)(sal)
this work
FeIII(OEP)(OPh)
[8a]
FeIII(OEP){O-2,6-(CF3CONH)2C6H3}
FeIII(OEP)(O-2-CF3CONHC6H4)
FeIII(OEP){O-2,6-iPr2C6H3}
FeIII(TPP)(O-2,2Ј-Cl2C6H3)
FeIII(OEP)(tropolone)
[8a]
[8a]
[8a]
[8d]
[10]
2.067
120.75
2.064
1.847(5)
1.842
1.842
1.896
120.50
146.0(5)
129.97
132.92
126.99
[8b]
[8c]
[8c]
[8c]
FeIII(TPP)(O-2-CF3CONHC6H4)
FeIII(TPP)(OPh)·C6H6
2.071
2.077
2.103
2.093
0.44
0.47
0.58
0.58
73.3
34.9
39.9
38.5
FeIII(TPP)(OPh)·C7H8
FeIII(TPP)(O-2-ClC6H4)
[a] Average value in Å. [b] Value in Å. [c] Value in °. [d] Displacement of the iron atom from the mean plane containing the four porphyri
nitrogen atoms in Å. [e] Dihedral angle between the plane of the porphyrin (24 atoms) and the axial ligand.
distance decreases in the same order with an exception in which are temperature-dependent, are shifted both upfield
the case of FeIII(OEP)(Hcat), in which the Fe–O distance is and downfield indicative of π-spin delocalization. It is ex-
the longest.
pected that there should be one meso resonance, two meth-
ylene resonances, and one methyl resonance for five-coordi-
It is appropriate to compare FeIII(OEP)(Hcat) and FeIII
-
(OEP)(OPh) since the former only has an additional unco- nate FeIII(OEP)(L). As seen in Figure 7 and Table 3, the
ordinated OH group. However, their X-ray structures reveal two methylene peaks arise in the narrow downfield region
that FeIII(OEP)(Hcat) is engaged in H-bonding between between δ = 35 and 46 ppm, the lone meso signal is ob-
oxygen atoms of coordinated and free catechol. Compared served in the upfield region at δ ≈ –37 ppm. The resonances
with those of FeIII(OEP)(OPh) and FeIII(OEP)(4-tBu- for the methyl protons are found in the diamagnetic region
Hcat), the Fe–O bond in FeIII(OEP)(Hcat) is elongated by at δ ≈ 6.5 ppm since they are far from the paramagnetic Fe
0.064 and 0.038 Å, respectively. The elongation of the Fe–
O distance is certainly due to the H-bonding interactions
and not caused by steric hindrance. The IR stretching fre-
quencies for OH of FeIII(OEP)(Hcat) and free H2cat were
observed at 3440 and 3451 cm–1 respectively. This shift of
ν(O–H) (11 cm–1) supports the presence of O–H···O hydro-
gen bonds observed by X-ray analysis. In contrast to the
large Fe–O–C bond angle of 142.2(3)° for FeIII(OEP)(OPh),
the Fe–O–C angles of FeIII(OEP)(Hcat), FeIII(OEP)(4-tBu-
Hcat), FeIII(OEP)(4-NO2-Hcat), and FeIII(OEP)(sal) are
119.5, 125.1, 122.2, and 124.3°, respectively. The relatively
small Fe–O–C angles observed are notable since a small Fe–
O–C bond angle is unsuitable for orbital overlap. In ad-
dition, such remarkable bending is not forced by crystal
packing or H-bonding [with the exception of FeIII
-
(OEP)(Hcat) where H-bonding is present], and thus the co-
ordinated oxygen atom must have reduced s-character. In
contrast, the smaller Fe–O–C bond angles for FeIII
-
(OEP)[O-2,6-(CF3CONH)2C6H3] and FeIII(OEP)(O-2-
CF3CONHC6H4) are explained by their NH···O H-
bonds.[8a]
1H NMR Study
1H NMR spectra of CDCl3 solutions of the FeIII
(OEP)(L) cmplexes were obtained at 295 K and are shown
-
Figure 7. 1H NMR spectra at 295 K of CDCl3 solutions of (A)
FeIII(OEP)(sal), (B) FeIII(OEP)(4-NO2-Hcat), (C) FeIII(OEP)-
in Figure 7. The signals are broad, and the chemical shifts, (Hcat), and (D) FeIII(OEP)(4-tBu-Hcat).
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1
Table 3. H NMR chemical shifts of FeIII(OEP)(L) measured in CDCl3 at 295 K.
Complex
m-H
p-H
o-H
meso
CH2
CH3
[a]
FeIII(OEP)(Hcat)
74.5, 58.6
–80.5
–33.8
–35.7
–38.1
–36.8
38.2, 43.4
6.4
5.8
5.7
6.9
FeIII(OEP)(4-tBu-Hcat)
FeIII(OEP)(4-NO2-Hcat)
FeIII(OEP)(sal)
96.7 (A),94.2 (A), 70.8 (B)
67.7 (A), 67.3 (A), 64.2 (B)
80.4, 76.4
–107.6 (B)
–74.6 (B)
–78.5
–98.5
37.9, 36.8, 36.4, 35.6
42.5, 41.2, 35.9
40.3, 36.3
[a]
–85.5
[a] Not detected.
centre. For a typical five-coordinate high-spin complex of signals in the downfield region are observed from the two
1
FeIII(OEP)Cl, the H NMR spectrum in CDCl3 comprises nonequivalent meta-protons. However, for form B, one
two methylene resonance at δ = 40.9 and 44.4 ppm and one downfield and one upfield proton resonance are observed
meso resonance at δ = –56.8 ppm.[16] Chemical shifts of the due to one meta- and one para-proton, respectively. Signals
CH3 protons of the peripheral ethyl substituent are also arising from the ortho-protons are too broad to be ob-
sensitive to spin states; S = 5/2 complexes exhibit the CH3 served. Figure 7b and d show the 1H NMR spectra of FeIII
-
signals further downfield, because the unpaired electrons (OEP)(4-NO2-Hcat) and FeIII(OEP)(4-tBu-Hcat) at 295 K,
can be delocalized through σ-bonds.[17] Thus, the position- in which A and B can be identified easily, and the reso-
ing of the methyl, methylene, and meso signals provide more nances are listed in Table 3.
evidence for the high-spin (S = 5/2) nature of the complexes
in solution.[17,18]
Molecular orbital calculations of FeIII(OEP)(Hcat) by
using DFT[19] clearly show that the electron spin densities
are also spread over the catechol ligand (Figure 8a). Fig-
ure 8b shows the Mulliken spin densities of the catechol
carbon atoms of the complex, in which the spin densities
are positive at ortho and para positions and negative at the
Scheme 2.
meta position. Thus, the ortho- and para-protons show up-
field shifts, whereas meta-protons show downfield shifts.
The alternating shift pattern, which is the opposite sign of
the chemical shifts for meta- vs. ortho- and para-protons, is
also indicative of π-spin delocalization on the catecholato
ligand. The ortho-protons of the axial catechol ligand are
closest to the paramagnetic Fe centre and have extremely
small T1 (spin-lattice relaxation time) values because of the
dipolar contact from the high-spin FeIII ion. As a result,
signals arising from the ortho-protons are very broad, and
some of them could not be located. Similar observations are
also reported where thiolates are used as axial ligands.[20]
Catechol can bind to iron(III)–porphyrins in the mono-
dentate η1-O binding mode (IV), the bidentate mononeg-
ative η2-O,O binding mode (V), and the bidentate binega-
tive η2-O,O binding mode (VI), shown in Scheme 3. When
catechol binds in the η2-fashion in any six-coordinate FeIII
complex, bite angles are observed within the narrow range
of 80–85°.[12c,12d,21] The η2-binding of tropolone with FeIII
–
porphyrin is the only known example, in which the bite an-
gle was found to be outside this range (73°).[10] For the
FeIII–porphyrins reported here, catechol only binds in the
η1-fashion. To bind catechol in the η2-binding mode (as in
V and VI) the bite angles need to be increased, which also
increases the repulsion further. Single-point energy calcula-
tions have been performed by using DFT for all probable
binding modes of catechol (Scheme 3) in FeIII(OEP)(Hcat).
For the η1-coordination mode (IV), atom coordinates were
taken directly from the single-crystal X-ray structure of the
molecule. However, for the η2-coordination modes (V and
Figure 8. (A) Singly occupied HOMO of FeIII(OEP)(Hcat) and (B)
Mulliken spin densities of the catechol carbon atoms.
In complexes with catechol, the iron center can bind
either of the two available oxygen atoms, and therefore two
1H NMR peaks in the downfield region are observed from
the two meta-protons, whereas one signal in the upfield re-
gion arises from the para-proton. For substituted catechols,
the iron center also can bind with either of the two catechol
oxygen atoms as shown in Scheme 2. As a result of the sub-
stitution, the two forms become inequivalent, which can be
seen in the X-ray structure of the molecule in the solid (vide
1
supra) as well as in solution. For form A, two H NMR Scheme 3.
5216
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© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Binding of Catechols to Iron(III)–Octaethylporphyrin
VI), the atom coordinates were taken from the single-crystal Spectroelectrochemical Study
X-ray structure of FeIII(OEP)(tropolone)[10] with the re-
Bulk oxidation of dichloromethane solutions of FeIII
-
(OEP)(L) at a constant potential under nitrogen have been
performed. The gradual UV/Vis spectral change during the
1e-oxidation process is shown for FeIII(OEP)(4-tBu-Hcat)
in Figure 10, in which the intensities of Soret and charge-
transfer bands at 392 and 615 nm, respectively, decrease in
intensity, whereas the corresponding peaks at 380 and
638 nm increase in intensity. Similar spectral changes are
also observed for the other complexes and are assigned to
the catechol-to-semiquinone oxidation based on previously
reported observations.[8f] Solid- and solution-phase IR spec-
tra of the complexes were recorded before and after the oxi-
dation. The 1265 and 1422 cm–1 bands are assigned to the
ν(CO) bands of FeIII(OEP)(4-tBu-Hcat) and the 1e-oxid-
ized species, respectively. The ν(CO) bands of M(cat) and
M(sq) complexes generally appear in the ranges of 1250–
1275 and 1400–1500 cm–1, respectively.[22] According to
these criteria, the ν(CO) band of the 1e-oxidized species is
assigned to the semiquinone, which is further justified by
our DFT calculations (vide infra).
quired change in the bite angle of catechol. The calculation
shows that the η1-binding mode of catechol (IV), as ob-
served in the X-ray structure of FeIII(OEP)(Hcat), is found
to be energetically much more favorable than the two η2-
binding modes (V and VI).
Very few structural reports of catecholato-bound metallo-
porphyrins exist. Two examples of the η1-binding mode of
catechol have been observed for SnIV–tetraphenylpor-
phyrins in a six-coordinate species.[12a,12b] Two more exam-
ples have been reported where the catechol oxygen atom
binds the metal center in both η1- and η2-modes to form
seven-coordinate species, one with ZrIV–tetraphenylpor-
phyrin[12c] and other with NbV–tetraphenylporphyrin.[12d]
In both cases, the larger size of the metal ions as well as
very large metal displacements from the mean porphyrin
plane (1.06 and 1.02 Å, respectively) facilitate the η2-bind-
ing mode and the increase in coordination number.[12c,12d]
Cyclic Voltammetric Study
Cyclic voltammetric experiments were carried out at
25 °C under nitrogen in CH2Cl2 with 0.1  tetrabutyl-
ammonium hexafluorophosphate (TBAH) as the support-
ing electrolyte. Two representative cyclic voltammograms
are shown in Figures 9 and S1 (see Supporting Infor-
mation), and the potentials are given in the Experimental
Section. Electrochemical data reveal that the complexes un-
dergo three one-electron oxidations and a single one-elec-
tron reduction. The first oxidation is assigned to the cate-
chol-to-semiquinone oxidation, and the second and third
oxidations are porphyrin-ring-centered (vide infra) and
quasi-reversible under the experimental conditions.
Figure 10. UV/Vis spectra in CH2Cl2 during the 1e-oxidation of
FeIII(OEP)(4-tBu-Hcat), with an applied potential of 0.75 V. Ar-
rows indicate the increase and decrease of band intensity.
2e-Oxidations of FeIII(OEP)(L) at a fixed anodic poten-
tial under nitrogen have also been performed, during which
the initial brown solution changed to green. Figure 11
shows the UV/Vis spectral change of FeIII(OEP)(4-NO2-
Hcat) upon oxidation at a constant potential of 1.28 V, in
which the Soret band at 386 nm blueshifts to 357 nm with
a decrease in intensity and generates a porphyrin dication,
[FeIII(OEP)(L)]++, which is confirmed by DFT. These spec-
tral changes are characteristic of the presence of a por-
phyrin π-cation radical.[23] Similar spectral features are also
observed during the controlled oxidations of the other com-
plexes reported here. Molecular orbital calculations have
been performed by using DFT for both the 1e- and 2e-oxid-
ized products of FeIII(OEP)(Hcat). The HOMOs of both of
the oxidized species are presented in Figure 12, which
clearly demonstrates the removal of the first electron mostly
from the catechol part of the molecule, whereas the second
electron is removed from the porphyrin ring, as observed
experimentally.
Figure 9. Cyclic voltammograms of (A) FeIII(OEP)(4-NO2-Hcat)
and (B) FeIII(OEP)(4-tBu-Hcat) in CH2Cl2 (scan rate 100 mV/s)
with 0.1  TBAH as the supporting electrolyte and Ag/AgCl as the
reference electrode.
Eur. J. Inorg. Chem. 2010, 5211–5221
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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5217

A. Chaudhary, R. Patra, S. P. Rath
FULL PAPER
study of [Fe(OEP)(Hcat)]2+ shows that the HOMO also has
a mixed character with both A1u and A2u generated from
pseudo-Jahn–Teller distortion (Figure 12c).
Conclusions
Synthesis, X-ray structures, and properties of new heme
analogues FeIII(OEP)(L) (L: Hcat, 4-NO2-Hcat, 4-tBu-
Hcat, and sal) are reported, in which catechol binds in an
η1-fashion as an axial ligand. DFT calculations show that
the η1-binding mode, as observed in the X-ray structure, is
energetically much more favorable than the η2-binding
modes. FeIII(OEP)(Hcat) has the smallest Fe–O–C angle of
119.5(1)° and the smallest dihedral angle of 26.2° between
the planes of the porphyrin and the axial catechol ligand of
all phenolato complexes of iron(III)–porphyrins reported.
Figure 11. UV/Vis spectra in CH2Cl2 during the 2e-oxidations of
FeIII(OEP)(4-NO2-Hcat), with an applied potential of 1.28 V. Ar-
rows indicate the increase and decrease of band intensity.
In comparison with those of FeIII(OEP)(OPh) and FeIII
-
-
(OEP)(4-tBu-Hcat), the Fe–O bond length in FeIII
(OEP)(Hcat) is elongated by 0.064 and 0.038 Å, respec-
tively, which is due to the H-bonding interactions with the
coordinated catechol oxygen atom and not steric hindrance.
The solid- and solution-state spectral observations are char-
1
acteristic of high-spin complexes. In the H NMR spectra,
the signals of the ortho- and para-protons of the catechol
ligands are shifted upfield, whereas those of the meta-pro-
tons are shifted downfield. The alternating shift pattern,
which has the opposite sign of the chemical shifts for meta-
vs. ortho- and para-protons, is due to negative and positive
spin densities on the catecholate carbon atoms and is also
indicative of π-spin delocalization. The iron center can bind
with either catechol oxygen atom, which is observed in both
solid- and solution-state investigations. Electrochemical
data reveal that the complexes undergo three one-electron
oxidations and a single one-electron reduction. The first
oxidation is assigned as the catechol-to-semiquinone oxi-
dation and the second and third oxidations are porphyrin-
ring-centered. These assignments are further supported by
theoretical calculations. Spectroelectrochemical and DFT
studies have been used to assign the electroninc structures
of the oxidized species.
Figure 12. Singly occupied HOMOs of (A) [FeIII(OEP)(Hcat)]+·,
(B) [FeIII(OEP)(Hcat)]2+, and (C) porphyrin macrocycle of [FeIII
(OEP)(Hcat)]2+ showing both A1u and A2u contributions.
-
Ghosh and co-workers applied DFT to investigate the
energetics, molecular structures, and spin-density profiles of
metalloporphyrin π-cation radicals.[24c] Metallo-OEP (OEP
= octaethylporphyrin) derivatives form A1u-type cation rad-
icals, whereas metallo-TPP (TPP = meso-tetraphenylpor-
phyrin) derivatives form A2u-type cation radicals.[24] The
common practice of describing these radicals in terms of
the A1u/A2u dichotomy is often not justified, because the
actual picture is further complicated by the pseudo-Jahn–
Teller effect.[24d] Interestingly, it was concluded that not all
metalloporphyrin π-cation radicals are subject to pseudo-
Jahn–Teller distortion. For example, by employing energy-
difference criteria, it was suggested that metallooctaethyl-
porphyrin should always be pseudo-Jahn–Teller-distorted,
whereas metallo-meso-tetrahalogenoporphyrin radicals
should be not.[24a] The pseudo-Jahn–Teller distortion leads
to structures with lower symmetries and mixed character
Experimental Section
Materials: Reagents and solvents were purchased from commercial
sources and purified according to standard procedures before use.
Grade-I neutral alumina was used for column chromatography.
H2OEP was prepared according to a literature method,[13a] and iron
was inserted by a reported procedure under an inert gas, which
produced the corresponding FeIII(OEP)Cl in excellent yield.[25]
[FeIII(OEP)]2O was prepared as described previously.[13b] The com-
plexes FeIII(OEP)(L) were prepared according to a general method;
details are given below for a representative case.
Preparation of FeIII(OEP)(Hcat)·H2cat: [Fe(OEP)]2O (100 mg,
0.082 mmol) was dissolved in tetrahydrofuran (25 mL). Catechol
(45 mg, 0.41 mmol) in tetrahydrofuran (5 mL) was added, and the
mixture was heated under N2 to reflux for 2 h. As the reaction
proceeded, the greenish-red solution changed to dark brown. After
with respect to the A1u and A2u components. Our DFT completion of the reaction, the solution was cooled and concen-
5218 Eur. J. Inorg. Chem. 2010, 5211–5221
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© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Binding of Catechols to Iron(III)–Octaethylporphyrin
trated to complete dryness to obtain a dark brown solid. The re-
sulting solid was dissolved in tetrahydrofuran and layered with ace-
tonitrile at room temperature in air. After standing for 7–8 d, dark
brown crystalline solids of FeIII(OEP)(Hcat)·H2cat had deposited,
which were collected by filtration and dried. Yield: 43 mg, 70%.
C48H55FeN4O4 (807.36): calcd. C 71.40, H 6.86, N 6.94; found C
71.49, H 6.95, N 6.89. UV/Vis (chloroform): λmax (ε) = 391
= 5.97, gʈ = 1.99. E1/2(ox) = 0.94, 1.42 V; E1/2(red) = –0.85 V. µeff
(295 K, in CH2Cl2) = 5.95 µB.
Instrumentation: UV/Vis spectra were recorded with a PerkinElmer
UV/Vis spectrometer. Elemental (C, H, and N) analyses were per-
formed with a CE-440 elemental analyzer. IR spectra were recorded
in the range 4000–400 cm–1 with a Bruker Vertex 70 Spectropho-
tometer. EPR spectra were obtained with a Bruker EMX EPR
spectrometer. Cyclic voltammetric studies were performed with a
BAS Epsilon electrochemical workstation in dichloromethane with
0.1  TBAH as the supporting electrolyte, Ag/AgCl as the reference
electrode, and Pt wire as the auxiliary electrode. The concentration
of the compounds was in the order of 10–3 . The ferrocene/ferro-
cenium couple occurs at E1/2 (∆Ep) = +0.45 V (65 mV) vs. Ag/AgCl
under the same experimental conditions. 1H NMR spectra were
recorded with a JEOL 500 MHz instrument. The spectra for para-
magnetic molecules were recorded over a 100 kHz bandwidth with
64 K data points and a 5 µs 90° pulse. For a typical spectrum be-
tween 2000 and 3000 transients were accumulated with a 50 ms
delay time. The residual 1H resonances of the solvents were used
as a secondary reference.
(1.6ϫ105),
500
(1.73ϫ104),
529
(1.65ϫ104),
612
(9.86ϫ103 –1 cm–1) nm. IR (KBr): ν = 3440 (OH) cm–1. EPR
˜
(120 K): solid: gЌ = 5.97, gʈ = 1.99; solution: gЌ = 5.96, gʈ = 2.01.
E1/2(ox) = 0.88, 1.11, 1.44 V; E1/2(red) = –0.80 V. µeff (295 K, in
CH2Cl2) = 5.91 µB.
The following compounds were prepared by using similar pro-
cedures.
Preparation of FeIII(OEP)(4-NO2-Hcat): Yield: 47 mg, 80%.
C42H48FeN5O4 (742.30): calcd. C 67.95, H 6.51, N 9.43; found C
67.87, H 6.60, N 9.35. UV/Vis (chloroform): λmax (ε) = 389
(2.54ϫ105),
499
(2.54ϫ104),
527
(2.52ϫ104),
621
(1.4ϫ104 –1 cm–1) nm. IR (KBr): ν = 3462 (OH) cm–1. EPR
˜
(120 K): solid: gЌ = 5.96, gʈ = 2.01; solution: gЌ = 5.98, gʈ = 1.99.
E1/2(ox): 0.93, 1.08, 1.43 V; E1/2(red): –0.94 V. µeff (295 K, in
CH2Cl2) = 5.94 µB.
X-ray Structure Solution and Refinement: Crystals were coated with
light hydrocarbon oil and mounted in the 100 K dinitrogen stream
of a Bruker SMART APEX CCD diffractometer equipped with a
CRYO Industries low-temperature apparatus, and intensity data
were collected by using graphite-monochromated Mo-Kα radiation
(λ = 0.71073 Å). The data integration and reduction were processed
with SAINT[26] software. An absorption correction was applied.[27]
Structures were solved by direct methods using SHELXS-97 and
were refined on F2 with full-matrix least-squares techniques by
using the SHELXL-97 program package.[28] Non-hydrogen atoms
were refined anisotropically. In the refinement, hydrogen atoms
were treated as riding atoms by using SHELXL default parameters.
Crystal data and data collection parameters are given in Table 4.
CCDC-782059, -782058, -782061, and -782060 contain the supple-
mentary crystallographic data for this paper. These data can be
Preparation of FeIII(OEP)(4-tBu-Hcat): Yield: 46 mg, 75%.
C46H57FeN4O2 (753.38): calcd. C 73.33, H 7.62, N 7.43; found C
73.22, H 7.54, N 7.38. UV/Vis (chloroform): λmax (ε) = 393
(2.26ϫ105),
496
(2.24ϫ104),
526
(2.18ϫ104),
608
(1.36ϫ104 –1 cm–1) nm. IR (KBr): ν = 3438 (OH) cm–1. EPR
˜
(120 K): solid: gЌ = 5.95, gʈ = 2.01; solution: gЌ = 5.97, gʈ = 1.99.
E1/2(ox) = 0.75, 1.12, 1.44 V; E1/2(red) = –0.73 V. µeff (295 K, in
CH2Cl2) = 5.93 µB.
Preparation of FeIII(OEP)(sal): Yield: 59 mg, 90%. C43H49FeN4O2
(709.32): calcd. C 72.81, H 6.96, N 7.89; found C 72.75, H 7.08, N
7.82. UV/Vis (chloroform) λmax (ε)
=
394 (2.5ϫ105), 495
(1.9ϫ104), 526 (1.8ϫ104), 613 (9.92ϫ103). IR (KBr): ν = 1723
˜
(CO) cm–1. EPR (120 K): solid: gЌ = 5.95, gʈ = 2.01; solution: gЌ
Table 4. Crystal data and data collection parameters.
FeIII(OEP)(Hcat)
FeIII(OEP)(4-tBu-Hcat)
FeIII(OEP)(4-NO2-Hcat)
FeIII(OEP)(sal)
T [K]
100(2)
C48H55FeN4O4
807.81
100(2)
C46H57FeN4O2
753.81
100(2)
100(2)
C86H98Fe2N8O4
1419.42
monoclinic
C2/c
20.624(5)
21.201(5)
16.897(5)
90
98.296(5)
90
7311(3)
Mo-Kα
0.71073
4
Empirical formula
Formula mass
Crystal system
Space group
a [Å]
b [Å]
c [Å]
C42H48FeN5O4
742.70
monoclinic
C2/c
21.0337(18)
20.9809(18)
16.7705(14)
90
95.940(2)
90
7361.2(11)
Mo-Kα
0.71073
8
triclinic
triclinic
¯
¯
P1
P1
12.6927(8)
13.6188(9)
13.7795(9)
66.3110(10)
68.6410(10)
84.7360(10)
2027.2(2)
Mo-Kα
0.71073
2
12.388(2)
13.315(2)
14.035(2)
100.138(3)
110.308(3)
104.825(3)
2007.2(6)
Mo-Kα
0.71073
2
α [°]
β [°]
γ [°]
V [Å3]
Radiation
λ [Å]
Z
dcalcd. [g cm-3]
1.323
0.423
1.247
0.418
1.340
0.460
1.290
0.455
µ [mm-1]
F(000)
858
7420
531
1.008
0.0457
0.0535
0.1168
806
7337
495
1.035
0.0615
0.0833
0.1582
3144
8572
493
1.014
0.0785
0.1492
0.2103
3016
6777
508
1.077
0.0714
0.1080
0.1924
No. of unique data
No. of parameters refined
GOF on F2
R1[a] [IϾ2σ(I)]
R1[a] (all data)
[b]
wR2 (all data)
[a] R1 = Σ||Fo| – |Fc||/Σ|Fo|. [b] {Σ[w(Fo – Fc ) ]/Σ[w(Fo2)2]}1/2
.
2
2 2
Eur. J. Inorg. Chem. 2010, 5211–5221
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5219

A. Chaudhary, R. Patra, S. P. Rath
FULL PAPER
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B3LYP hybrid functional by using the Gaussian 03, revision B.04,
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calculations were performed for [FeIII(OEP)(Hcat)]+ and [FeIII
-
(OEP)(Hcat)]2+ where all the coordinates were taken directly from
the single-crystal X-ray structure of FeIII(OEP)(Hcat). Single-point
energy calculations were performed for FeIII(OEP)(Hcat), in which
the mode of catechol binding with Fe varies between IV, V, and VI
as shown in Scheme 3. For the η1-coordination mode (IV), the
atom coordinates are taken directly from the single-crystal X-ray
structure of the molecule. However, for the η2-binding modes (V
and VI), the atom coordinates are taken from the single-crystal X-
ray structure of FeIII(OEP)(tropolone),[10] tropolone was replaced
by catechol, and the bite angle was fixed manually at 82°. For the
binegative η2-coordination mode (VI), negative charge was bal-
anced by a hydronium ion. The total energy was calculated by sim-
ply adding the energy obtained for VI with the energy of the hy-
dronium ion. For IV and V, however, the total energy was adjusted
by adding the energy of the molecules with the energy of the water
molecules. The calculation shows that the η1-mode of binding (IV)
of catechol is found to be energetically much more stable than η2-
binding modes (V and VI) by 12.9 and 94.2 kcalmol–1, respectively,
in the gas phase. Single-point solvent calculations were also per-
formed by using the CPCM[31] approach, which is an implementa-
tion of the conductor-like screening solvation model (COSMO)[32]
in Gaussian 03; THF was used as solvent (dielectric constant =
7.58). The calculation shows that the η1-mode of binding (IV) of
catechol is also energetically more stable than η2-binding modes
(V and VI) by 58.1 and 79.6 kcalmol–1, respectively. No geometry
optimizations were performed for any of these molecules.
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[12]
[13]
[14]
Supporting Information (see footnote on the first page of this arti-
cle): Cyclic voltammograms of FeIII(OEP)(Hcat) and FeIII(OEP)-
(sal) (Figure S1).
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Received: June 29, 2010
Published Online: October 19, 2010
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