Preparation and infrared spectroelectrochemical studies of five-coordinate (por)Fe(OC(=O)R) compounds (por = TPP, OEP; R = CCl3, CH2Cl)

Article abstract of DOI:10.1016/j.ica.2017.09.014

Three five-coordinate iron acetate porphyrin complexes, (por)Fe(OC(=O)R) (por = porphyrinato dianion; R = CCl₃, CH₂Cl), have been synthesized and characterized. The crystal structure of (TPP)Fe(OC(=O)CCl₃) (TPP = tetraphen

Full text of DOI:10.1016/j.ica.2017.09.014

Inorganica Chimica Acta 469 (2018) 183–188
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Research paper
Preparation and infrared spectroelectrochemical studies
of five-coordinate (por)Fe(OC(=O)R) compounds (por = TPP, OEP;
R = CCl3, CH2Cl)
Nan Xu a, , Beiqi Yan a, Dennis Awasabisah b, Douglas R. Powell b, George B. Richter-Addo b,
⇑
⇑
a Department of Chemistry, The Pennsylvania State University, Altoona College, 3000 Ivyside Park, Altoona, PA 16601, USA
b Department of Chemistry and Biochemistry, University of Oklahoma, 101 Stephenson Parkway, Norman, OK 73019, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 6 July 2017
Received in revised form 1 September 2017
Accepted 6 September 2017
Available online 12 September 2017
Three five-coordinate iron acetate porphyrin complexes, (por)Fe(OC(=O)R) (por = porphyrinato dianion;
R = CCl3, CH2Cl), have been synthesized and characterized. The crystal structure of (TPP)Fe(OC(=O)CCl3)
(TPP = tetraphenylporphyrinato dianion) has been determined by X-ray crystallography. The redox
behaviors of the (por)Fe(OC(=O)R) compounds have been investigated by cyclic voltammetry and infrared
spectroelectrochemistry. Analysis of the data reveals that the first oxidations of the (por)Fe(OC(=O)R)
compounds are porphyrin-centered processes to yield
p
-radical cations of the form (porꢀ+)Fe(OC(=O)R),
and the first reductions may facilitate the dissociation of the axial acetate ligands from the iron centers.
Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
Numerous iron porphyrin complexes have been prepared as
models for the studies of important roles that heme enzymes play
in biological processes. The (por)Fe(O-ligand) (por = porphyrinato
dianion) cofactor is an essential constituent of several heme pro-
teins such as heme catalase with tyrosine axial ligands [1–4], the
HasA [5,6] and IsdB [7] heme-binding proteins, and in some natural
2.1. General
All reactions were performed under a nitrogen atmosphere
using standard Schlenk glassware and/or in an Innovative Technol-
ogy Labmaster 100 Dry Box. Hexane and CH2Cl2 were distilled from
CaH2 under nitrogen just prior to use.
mutant hemoglobins such as Hb M Boston [a58(E7)His?Tyr] (i.e.,
alkoxide ligation) and Hb M Milwaukee [b67(E11)Val?Glu] (i.e.,
carboxylate ligation) [8]. Consequently, there is continued interest
in the preparation and examination of the redox behavior of such
(por)Fe(O-ligand) species, especially for synthetic analogues that
allow probing of porphyrin and axial O-ligand substitutions on
the properties of these species.
2.2. Chemicals
Trichloroacetic acid (99%), chloroacetic acid (99%) and NBu4PF6
(98%) were purchased from Aldrich Chemical Company. The oxo-
dimer precursors [(por)Fe]2(l-O) (por = TPP, OEP) [12,13] were
prepared by published methods and their identities confirmed by
In this paper, we report the preparation, characterization, elec-
trochemistry, and IR spectroelectrochemistry of three (por)Fe(OC
(=O)R) (por = TPP or OEP; R = CCl3 or CH2Cl) compounds. Only a
few other reports of the electrochemistry of iron porphyrin acetate
complexes have been published [9–11]. Further, and to the best of
our knowledge, this is the first report describing the infrared spec-
troelectrochemistry of these compounds.
IR spectroscopy.
2.3. Instrumentation
Electrochemical measurements were performed using a BAS
CV-50W instrument (Bioanalytical Systems, West Lafayette, IN).
For cyclic voltammetry, a three-electrode cell with a 3 mm diame-
ter Pt disk working electrode, a Pt wire counter electrode, and a Ag/
AgCl reference electrode were utilized. The solutions for all electro-
chemical experiments were 1.0 mM in analyte, in 10 mL CH2Cl2
solution containing 0.1 M NBu4PF6 as support electrolyte. The solu-
tions were deaerated by purging with prepurified nitrogen for
⇑
Corresponding authors.
E-mail
addresses:
nxx103@psu.edu
(N.
Xu),
grichteraddo@ou.edu
(G.B. Richter-Addo).
http://dx.doi.org/10.1016/j.ica.2017.09.014
0020-1693/Ó 2017 Elsevier B.V. All rights reserved.

184
N. Xu et al. / Inorganica Chimica Acta 469 (2018) 183–188
10 min before each set of measurements, and then the nitrogen
atmosphere was maintained during the measurements which were
performed at room temperature (ꢀ20 °C). Scan rates of 200–
1000 mV/s were used, but no significant differences in the cyclic
voltammogram profiles were noted.
Infrared spectra for the synthetic work were collected on a Bio–
Rad FT–155 FTIR spectrometer. For infrared spectroelectrochem-
istry, the experimental setup consisted on the same cell utilized
for the cyclic voltammetry measurements, with the 3 mm Pt disk
working electrode serving as the mirror for the fiber optic mid-IR
dip probe, as described previously [14–16]. The resulting IR spectra
refinement. The structure was solved by direct methods and
refined by full-matrix least-squares methods on F2 [22,23]. Hydro-
gen atom positions were initially determined by geometry and
refined using a riding model. Non-hydrogen atoms were refined
with anisotropic displacement parameters. Hydrogen atom dis-
placement parameters were set to 1.2 times the displacement
parameters of the bonded atoms. A total of 505 parameters were
refined against 9144 data to give wR(F2) = 0.1638 and S = 1.012
for weights of w = 1/[
[F2o + 2F2c]/3. The final R(F) was 0.0655 for the 5690 observed,
[F > 4 (F)], data.
r
2(F2) + (0.0680 P)2 + 1.9000 P], where P =
r
were recorded using
a
Bruker Vector 22 FTIR spectrometer
The largest shift/s.u. was 0.003 in the final refinement cycle. The
final difference map had maxima and minima of 0.616 and
ꢂ0.876 e/Å3, respectively. Displacement ellipsoids in Fig. 1 are
drawn at the 35% probability level. Details of the crystal data and
refinement are given in Table 1. CCDC 1444107 contains the crys-
tallographic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/conts/retrieving.html, or by email-
ing deposit@ccdc.cam.ac.uk, or by contacting The Cambridge Crys-
tallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK;
fax: +44 1223-336-033
equipped with a liquid nitrogen cooled MCT detector (Remspec
Corporation, Sturbridge, MA, USA). UV–Vis spectra were recorded
on a Hewlett-Packard model 8453 diode array instrument.
2.4. Synthesis
Three iron porphyrin acetate compounds were prepared in a
manner similar to that used for the preparation of (por)Fe(OC
(=O)R) analogues [17]. The following is representative:
(TPP)Fe(OC(=O)CCl3 (1). To a CH2Cl2 solution (15 mL) of [(TPP)
Fe]2(l-O) (0.033 g, 0.024 mmol) was added trichloroacetic acid
(0.010 g, 0.06 mmol). The mixture was stirred for 45 min, during
which time the color of the solution changed from green to brown.
The solvent was reduced to ꢀ3 mL and hexane (10 mL) was added.
The solution was slowly concentrated under reduced pressure until
precipitation of the product occurred. The dark brown precipitate
was collected by filtration, washed with hexane (2 ꢁ 15 mL), and
dried in vacuo to give (TPP)Fe(OC(=O)CCl3) (0.020 g, 0.024 mmol,
50% isolated yield). Slow evaporation of a CH2Cl2/cyclohexane
(1:1 ratio; 4 mL) solution of the product at room temperature gave
suitable crystals for X-ray diffraction studies. IR (CH2Cl2, cmꢂ1):
t
OCO(asym) = 1704. IR (KBr, cmꢂ1):
tOCO(asym) = 1709. UV–vis (k
(relative
(14), 571 (8), 681 (4) nm.
e
, mMꢂ1cmꢂ1), 2 ꢁ 10ꢂ5 M in CH2Cl2): 411 (138), 509
The other iron acetate porphyrin complexes were generated
similarly using the respective porphyrin and acid precursors.
(TPP)Fe(OC(=O)CH2Cl) (2). Isolated yield: 60%. IR (CH2Cl2,
cmꢂ1):
UV–vis (k (relative
(36), 412 (109), 507 (8), 573 (9), 656 (7) nm.
t tOCO(asym) = 1692.
OCO(asym) = 1684. IR (KBr, cmꢂ1):
e
, mMꢂ1cmꢂ1), 1 ꢁ 10ꢂ5 M in CH2Cl2): 351
(OEP)Fe(OC(=O)CCl3) (3) [18]. Isolated yield: 65%. IR (CH2Cl2,
cmꢂ1):
UV–vis (k (relative
(98), 506 (7), 533 (7), 634 (3) nm.
t tOCO(asym) = 1711.
OCO(asym) = 1704. IR (KBr, cmꢂ1):
e
, mMꢂ1cmꢂ1), 8 ꢁ 10ꢂ6 M in CH2Cl2): 381
2.5. X-ray crystallography
A black plate-shaped crystal of (TPP)Fe(OC(=O)CCl3) (1) of
dimensions 0.28 ꢁ 0.26 ꢁ 0.05 mm was selected for structural
analysis. Intensity data for this compound were collected using a
diffractometer with a Bruker APEX ccd area detector [19,20] and
graphite-monochromated Mo K
a radiation (k = 0.71073 Å). The
sample was cooled to 100(2) K. Cell parameters were determined
from a non-linear least squares fit of 8508 peaks in the range
2.21 < h < 27.37°. A total of 62935 data were measured in the range
1.689 < h < 28.456° using / and
x oscillation frames. The data were
corrected for absorption by the semi-empirical method [21] giving
minimum and maximum transmission factors of 0.833 and 0.967.
The data were merged to form a set of 9144 independent data with
R(int) = 0.1583 and a coverage of 100.0%.
The monoclinic space group P21/n was determined by system-
atic absences and statistical tests and verified by subsequent
Fig. 1. (a) Molecular structure of (TPP)Fe(OC(=O)CCl3) (1). Hydrogen atoms are
omitted for clarity, and thermal ellipsoids are drawn at the 35% probability level. (b)
Perpendicular displacements (in units of 0.01 Å) of the porphyrin core atoms from
the 24-atom mean porphyrin plane. Selected bond lengths (in Å) and angles (in
deg): Fe–N1 = 2.044(3), Fe–N2 = 2.046(3), Fe–N3 = 2.046(3), Fe–N4 = 2.054(3), Fe–
O1 = 1.917(2), \Fe–O1–C45 = 133.2(2).

N. Xu et al. / Inorganica Chimica Acta 469 (2018) 183–188
185
Table 1
spin species. A saddle distortion has been noted for the closely
related (TPP)Fe(OC(=O)CF3) [25]. However, the compound (TTP)Fe
(OC(=O)CH3) has a domed porphyrin core [17], whereas the (OEP)
Fe(OC(=O)CCl3) [18] has varied porphyrin core deformations
depending on the crystal form obtained with different solvates.
The relative positioning of adjacent porphyrins is shown in Fig. 2,
where a lateral shift of 3.76 Å and a porphyrin mean plane separa-
tion of 3.67 Å are observed [24].
As seen in Figs. 1 and 2, the trichloroacetate ligand in 1 binds to
the Fe center through one of the acetate O atoms. The Fe–O bond
distance is 1.917(2) Å, and the axial \Fe–O–C angle is 133.2(2)°.
The Fe atom is displaced by 0.44 Å from the 24-atom mean por-
phyrin plane towards the acetate ligand. This apical Fe displace-
ment is similar to that determined in high-spin (TPP)Fe(OC(=O)
CH3) (+0.477(2) Å) [26] and in (TPP)Fe(OC(=O)CF3) (+0.45 Å) [27],
and is in the range (0.39–0.62 Å) determined for five-coordinate
high-spin ferric porphyrins as documented by Scheidt and cowork-
ers [28]. Indeed, the Fe–N(por) and Fe–O bond parameters are also
indicative of (TPP)Fe(OC(=O)CCl3) being a high-spin species in the
crystal.
Crystal data and structure refinement for 1.
Formula
Formula weight
T (K)
Crystal system
Space group
Unit cell dimensions
a (Å)
b (Å)
c (Å)
a, c (°)
C46H28Cl3FeN4O2
830.92
100(2)
monoclinic
P21/n
11.300(3)
22.877(6)
14.761(4)
90, 90
106.053(6)
3667.1(17)
4, 1
1.505
1700
0.678
0.967 and 0.833
1.689 to 28.456
62935
9144 [Rint = 0.1583]
9144/0/505
wR2 = 0.1638
R1 = 0.0655
1.012
b (°)
Volume (Å3)
Z, Z’
Dcalc (g/cm3)
F(0 0 0)
Absorption coefficient (mmꢂ1
Max. and min. transmission
h range for data collection (°)
Reflections collected
Independent reflections
)
Data / restraints / parameters
wR (F2 all data)
R (F observed data)
Goodness-of-fit on F2
3.2. Electrochemistry
Observed data [I > 2
r
(I)]
5690
0.616 and –0.876
Largest difference of peak and hole (e/Å3)
The redox behaviors of the (por)Fe(OC(=O)R) compounds 1–3
have been examined by cyclic voltammetry at room temperature.
The resulting cyclic voltammograms are shown in Figs. 3–5. As
shown in Fig. 3, the (OEP)Fe(OC(=O)CCl3) compound (i.e., 3)
undergoes two reversible oxidations, occurring at Eo’ox1 = +0.55
and Eo’ox2 = +1.04 V versus the Cp2Fe0/+ redox couple.
3. Results and discussion
3.1. Preparation and crystallography
The separations in peak potentials (i.e.,
DE) for each of the oxi-
The (por)Fe(OC(=O)R) compounds (por = OEP or TPP; R = CCl3 or
CH2Cl) were prepared in a manner similar to that used for the
preparation of other (por)Fe(OC(=O)R) analogues. Reactions of
dations are near-identical to that for the known reversible Cp2Fe0/+
couple under our electrochemical cell setup conditions, indicating
that both the first and second oxidations are reversible single-
electron transfer processes. In addition, the cathodic to anodic peak
current ratios (ipc/ipa) for both oxidations are ꢀ1.0, suggesting that
the two oxidations are chemically reversible at this scan rate. Fur-
the respective acetic acids with the
l-oxo dimers [(por)Fe]2(l–O)
generated, after appropriate workup, the desired products in 50–
65% isolated yields (Eq. (1)). These compounds are moderately
stable as solids in air, as judged by the lack of changes in their IR
spectra over several days.
thermore, plots of ipa versus t
1/2 for the first and second oxidations
are linear, indicative of diffusion-controlled processes for both
oxidations. In contrast to the well-behaved oxidations, the
reduction of 3 is quite ill-defined under our conditions, showing
a broad irreversible reduction peak at Epc = ꢂ0.99 V; this broad fea-
ture is observed when either the positive or negative scan direction
is initiated first. The electrochemistry of the related compounds
½ðporÞFeꢃ2ð ꢂOÞ þ 2RCð¼OÞOH!2ðporÞFeðOðCð¼OÞRÞ þ H2O
l
ð1Þ
1: por = TPP, R = CCl3
2: por = TPP, R = CH2Cl
3: por = OEP, R = CCl3 [18]
The IR spectra of the (por)Fe(OC(=O)R) compounds
display bands attributed to the coordinated carboxylates; for 1
(ʋOCO(asym) 1704 cmꢂ1 (CH2Cl2) and 1709 cmꢂ1 (KBr)), for 2
(ʋOCO(asym) 1684 cmꢂ1 (CH2Cl2) and 1692 cmꢂ1 (KBr)), and for 3
(ʋOCO(asym) 1704 cmꢂ1 (CH2Cl2) and 1711 cmꢂ1 (KBr)). Clearly,
the ʋOCO(asym) band positions are more reflective of the carboxy-
late R substitution rather than the type of porphyrin macrocycle
(i.e., TPP vs. OEP). In line with this, iron porphyrin analogues with
more electron-rich carboxylate ligands (OC(=O)CHꢂ3 , OC(=O)C6H5ꢂ,
OC(=O)CH2CHꢂ3 ) have much lower
ʋOCO(asym) bands in the
1651–1670 cmꢂ1 range [17].
Single crystals of (TPP)Fe(OC(=O)CCl3) were obtained by slow
evaporation of a CH2Cl2/cyclohexane solution of the complex at
room temperature. The molecular structure and the porphyrin
atom displacements for (TPP)Fe(OC(=O)CCl3) are shown in Fig.1
(a) and (b), respectively.
The porphyrin core is saddled, where diagonally positioned pyr-
role moieties have their atoms above the porphyrin mean plane,
and the other pyrrole atoms are below the plane [24]. There are
only three other five-coordinate (por)Fe(acetate) compounds
whose crystal structures have ben reported, and these are high-
Fig. 2. Relative positions of adjacent porphyrin macrocycles in the crystal structure
of 1.

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N. Xu et al. / Inorganica Chimica Acta 469 (2018) 183–188
(TPP)Fe(OC(=O)CH3) also exhibits a reversible reduction [9]. It thus
appears that the presence of the trichloroacetate ligand in 3 prob-
ably enhances follow-up decomposition of the reduced product.
The cyclic voltammograms of the tetraphenylporphyrin com-
pounds, namely (TPP)Fe(OC(=O)CCl3) 1 and (TPP)Fe(OC(=O)CH2Cl)
2, were also recorded. As shown in Fig. 4a and 5 a, and when the
full oxidation scan is recorded, the first oxidations appear less fully
reversible. In addition, 1 undergoes an apparent reversible second
oxidation at Eo’ox2 = + 1.01 V, but with a daughter peak at + 0.11 V.
That the second oxidations of the trichloroacetate complexes and 1
and 3 are more reversible than that of the chloroacetate complex 2
is suggestive of the second oxidations of 1 and 3 also being largely
porphyrin centered.
Interestingly, however, when only the first oxidations are
probed, i.e., the when the direction of the scans is reversed just
after the first oxidation, well-defined reversible first oxidations
are observed at Eo’ox1 = + 0.72 V (for 1) and Eo’ox1 = + 0.69 V (for
2) as shown in Figs. 4b and 5b. It is not surprising that the first oxi-
dation potential of 1 is slightly higher than that of 2, since 1 has the
more chloro-substituted axial acetate.
Fig. 3. Cyclic voltammogram of 0.1 mM (OEP)Fe(OC(=O)CCl3)
containing 0.1 NBu4PF6 at a scan rate of 0.2 V/s.
3 in CH2Cl2
Further, the first oxidation of the OEP trichloroacetate deriva-
tive 3 (at Eo’ox1 = +0.55 V) is lower than that of the related TPP
derivative 1 (at Eo’ox1 = +0.72 V), reflecting the fact that the OEP
macrocycle is more electron-rich than the TPP macrocycle. Given
the greater effect of macrocycle substitution, compared with axial
acetate identity, on the oxidation potentials, we surmise that the
first oxidations are largely porphyrin macrocycle based. The elec-
trochemical data are summarized in Table 2.
As with the (OEP)Fe(OC(=O)CCl3) compound 3 (Fig. 3), the
trichloroacetate compound (TPP)Fe(OC(=O)CCl3) 1 undergoes an
irreversible reduction with a broad Epc value of ꢂ0.76 V (Fig. 4a),
indicative of rapid decomposition of the reduction products on
the cyclic voltammetry timescale. We speculated that this lack of
the reversibility of the first reductions of (OEP)Fe(OC(=O)CCl3)
and (TPP)Fe(OC(=O)CCl3) was due to the dissociation of the Cl3C
(C=O)Oꢂ anions after the reductions. Similar ligand dissociations
after the first reductions of five-coordinate iron porphyrins have
been reported [29,30]. On the other hand, we found that the first
reduction of the chloromethyl compound (TPP)Fe(OC(=O)CH2Cl)
was quite well-defined at the scan rate of 0.2 V/s, showing
reversible behavior (Eo’ = ꢂ0.78 V) (Fig. 5c), which indicates that
the Fe–O bond stays intact in the reduction process. Similar
reversible reductions of the non-halogenated axially coordinated
(TPP)Fe(OC(=O)CH3) and (OEP)Fe(OC(=O)CH3) compounds have
been reported previously [9,10].
Fig. 4. Cyclic voltammograms of 0.1 mM (TPP)Fe(OC(=O)CCl3)
containing 0.1 NBu4PF6 at a scan rate of 0.2 V/s.
1 in CH2Cl2
3.3. Infrared spectroelectrochemistry
In order to further investigate the redox processes of these com-
pounds, difference IR spectra were recorded using a fiber-optic dip
probe [14–16] at room temperature while the electrode was held
at the potential just past the potential of the forward peak. To
the best of our knowledge, there are no reports describing the
infrared spectroelectrochemistry of five-coordinate iron acetate
porphyrin complexes.
Table 2
Redox potentials (Eo
, in V ) for oxidations and reductions of the
vs Cp2Fe0/+
0
Fig. 5. Cyclic voltammograms of 0.1 mM (TPP)Fe(OC(=O)CH2Cl)
containing 0.1 NBu4PF6 at a scan rate of 0.2 V/s.
2
in CH2Cl2
(por)Fe(OC(=O)R) compounds in CH2Cl2 containing 0.1 M NBu4PF6.
Compounds
1st oxid.
2nd oxidation
reduction
(TPP)Fe(OC(=O)CCl3) 1
(TPP)Fe(OC(=O)CH2Cl) 2
(OEP)Fe(OC(=O)CCl3) 3
0.72
0.69
0.55
1.01
1.13*
1.04
ꢂ0.76*
ꢂ0.78
ꢂ0.99*
(por)Fe(OC(=O)CH3) (por = OEP, TPP) have been reported; the
acetate (OEP)Fe(OC(=O)CH3) compound in CH2Cl2 undergoes
two reversible oxidations and a reversible reduction [10], and
*
Irreversible redox process; the forward peak potential Ep is reported instead.

N. Xu et al. / Inorganica Chimica Acta 469 (2018) 183–188
187
For (OEP)Fe(OC(=O)CCl3) 3, the electrode potential was held at
+0.85 V to generate a measurable quantity of the first oxidation
product on the electrode surface. The difference IR spectrum is
shown in Fig. 6a. The ʋOCO(asym) band at 1704 cmꢂ1 of the starting
compound disappeared, and a new ʋOCO(asym) at 1776 cmꢂ1
formed assigned to the oxidation product. In addition, a new band
at 1535 cmꢂ1 appeared in the spectrum assigned to the formation
of an OEP-containing p-radical cation, indicating that the first oxi-
dation occurs at the porphyrin ring, a spectral conclusion that is
consistent with the results from the cyclic voltammograms. Char-
acteristic IR bands in the region of 1520–1570 cmꢂ1 are diagnostic
for OEP-containing p-radical cations as detailed elsewhere [31,32].
The reduction of (OEP)Fe(OC(=O)CCl3) 3 was also probed by IR
spectroelectrochemistry, where the electrode potential was held
at -1.17 V. As shown in Fig. 6b, the reduction of 3 results in
loss of ʋOCO(asym) at 1704 cmꢂ1 of the starting compound and
the appearance of a new band at 1687 cmꢂ1 was assigned to
the accumulation of the dissociated Cl3C(C=O)Oꢂ anion at the
electrode surface. This assignment was further confirmed by
the
ʋ ) of independently synthesized
OCO(asym) (at 1687 cmꢂ1
[(15-crown-5)Na](OC(=O)CCl3) [33–36] added to CH2Cl2 contain-
ing the support electrolyte.
The IR difference spectra of the first oxidations for the
(TPP)Fe(OC(=O)CCl3) 1 and (TPP)Fe(OC(=O)CH2Cl) 2 complexes
show that the ʋOCO(asym) bands of the oxidized products shifted
by + 72 and + 76 cmꢂ1, respectively, from their neutral precursors
(Figs. 7a and 8a). In addition, an IR band shift from 1600 cmꢂ1 to
ꢀ1585 cmꢂ1 for 1, and 1600 cmꢂ1 to ꢀ1588 cmꢂ1 for 2, are
observed in the difference IR spectra of the two complexes. Similar
shifts are detected for one-electron porphyrin centered oxidation
of tetraphenyl-based metalloporphyrins, due to the phenyl mode
Fig. 7. Difference IR spectra showing the products from the (a) first oxidation, and
(b) first reduction of (TPP)Fe(OC(=O)CCl3) 1 in CH2Cl2 containing 0.1 M NBu4PF6,
with the potentials held at + 0.91 V and ꢂ0.87 V, respectively, vs the Cp2Fe0/+
couple.
absorption changes [37]. The oxidized tetraarylporphyrin p-radical
cations have characteristic IR bands in the region of 1270–
1295 cmꢂ1 [31]. Unfortunately, we could not identify this band
because of the strong absorption of our solvent system in this
Fig. 8. Difference IR spectra showing the products from the (a) first oxidation, and
(b) first reduction of (TPP)Fe(OC(=O)CH2Cl) 2 in CH2Cl2 containing 0.1 M NBu4PF6,
with the potentials held at + 0.92 V and ꢂ1.03 V, respectively, vs the Cp2Fe0/+
couple.
region. However, the OCO shifts of the two TPP derivatives are
comparable with that of the OEP derivative. We thus assign the
first oxidations of (TPP)Fe(OC(=O)CCl3) and (TPP)Fe(OC(=O)CH2Cl)
to also be porphyrin based.
Fig. 6. Difference IR spectra showing the products from (a) the first oxidation, and
(b) first reduction of (OEP)Fe(OC(=O)CCl3) 3 in CH2Cl2 containing 0.1 M NBu4PF6,
with the potential held at + 0.85 V and -1.17 V, respectively, vs the Cp2Fe0/+ couple.

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N. Xu et al. / Inorganica Chimica Acta 469 (2018) 183–188
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lier in Fig. 6, the reduction of (TPP)Fe(OC(=O)CCl3) 1 results in the
loss of the ʋOCO(asym) band at 1704 cmꢂ1 and the appearance of a
new band at 1688 cmꢂ1, suggesting the dissociation of the acetate
anion from the iron center of the porphyrin (Fig. 7b).
In the case of the reduction of (TPP)Fe(OC(=O)CH2Cl), the differ-
ence IR spectrum also shows the disappearance of the original
ʋ
OCO(asym) band at ꢀ1684 cmꢂ1, but the appearance of two new
bands at 1643 cmꢂ1 and 1624 cmꢂ1 (Fig. 8b). It is interesting that
the cyclic voltammogram of (TPP)Fe(OC(=O)CH2Cl) 2 shows a
reversible reduction on the cyclic voltammetry timescale
(Fig. 5c). These two new ʋOCO(asym) bands indicate that the reduc-
tion product undergoes a slow but detectable decomposition on
the IR spectroelectrochemical timescale. In addition, the solution
IR spectrum of the salt [(15-crown-5)Na](OC(=O)CH2Cl) shows a
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ʋ
OCO(asym) band at ꢀ1620 cmꢂ1 in CH2Cl2. Thus, the 1624 cmꢂ1
band is assigned to the ʋOCO(asym) band of the dissociated
chloroacetate anion and the 1643 cmꢂ1 band is assigned to the
ʋOCO(asym) band of the bound chloroacetate anion still attached
to Fe in the reduction product. The IR data thus suggests that the
(TPP)Fe(OC(=O)CH2Cl) 2 compound undergoes a fast electron
transfer followed by a structural change that involves loss of the
chloroacetate anion from the metal center. Consistent with the
cyclic voltammetry results, we find that axial ligand loss after
the first reduction of (TPP)Fe(OC(=O)CH2Cl) 2 is much slower than
those of the trichloroacetate compounds (TPP)Fe(OC(=O)CCl3) 1
and (OEP)Fe(OC(=O)CCl3) 3 in this study.
4. Conclusion
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We have prepared three five-coordinate iron acetate
porphyrins: (TPP)Fe(OC(=O)CCl3) 1, (TPP)Fe(OC(=O)CH2Cl) 2, and
(OEP)Fe(OC(=O)CCl3) 3 [18]. We have also determined the X-ray
crystal structure of (TPP)Fe(OC(=O)CCl3). The electrochemistry and
IR spectroelectrochemistry of theses iron acetate porphyrin com-
plexes have been investigated. The first reversible oxidations of
these compounds are porphyrin based, resulting in +71–76 cmꢂ1
shifts of the ʋOCO(asym) bands of the acetate groups. The reductions
of these compounds result in eventual dissociation of the acetate
anions from the Fe centers.
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Acknowledgements
We are grateful to Penn State Univ. Altoona for support (to NX),
and to the National Science Foundation (CHE-1566509 and CHE-
1213674 to GBRA) for funding for this research.
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