New heme-dioxygen and carbon monoxide adducts using pyridyl or imidazolyl tailed porphyrins

Article abstract of DOI:10.1016/j.poly.2012.11.011

Inspired by the chemistry relevant to dioxygen storage, transport and activation by metalloproteins, in particular for heme/copper oxidases, and carbon monoxide binding to metal-containing active sites as a probe or surrogate for dioxygen binding, a series of heme derived dioxygen and CO complexes have been designed, synthesized, and characterized with respect to their physical properties and reactivity. The focus of this study is in the description and comparison of three types heme-superoxo and heme- CO adducts. The starting point is in the characterization of the reduced heme complexes, [(F ₈)Fe^II], [(P^Py)Fe^II] and [(P ^Im)Fe^II], where F₈, P^Py and P ^Im are iron(II)-porphyrinates and where P^Py and P ^Im possess a covalently tethered axial base pyridyl or imidazolyl group, respectively. The spin-state properties of these complexes vary with solvent. The low temperature reactions between O₂ and these reduced porphyrin Fe^II complexes yield distinctive low spin heme-superoxo adducts. The dioxygen binding properties for all three complexes are shown to be reversible, via alternate argon or O₂ bubbling. Carbon monoxide binds to the reduced heme-Fe^II precursors to form low spin heme-CO adducts. The implications for future investigations of these heme O₂ and CO adducts are discussed.

Full text of DOI:10.1016/j.poly.2012.11.011

Polyhedron 58 (2013) 190–196
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Polyhedron
journal homepage: www.elsevier.com/locate/poly
New heme–dioxygen and carbon monoxide adducts using pyridyl
or imidazolyl tailed porphyrins
⇑
Yuqi Li, Savita K. Sharma, Kenneth D. Karlin
Department of Chemistry, The Johns Hopkins University, Baltimore, MD 21218, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
Available online 20 November 2012
Inspired by the chemistry relevant to dioxygen storage, transport and activation by metalloproteins, in
particular for heme/copper oxidases, and carbon monoxide binding to metal-containing active sites as
a probe or surrogate for dioxygen binding, a series of heme derived dioxygen and CO complexes have
been designed, synthesized, and characterized with respect to their physical properties and reactivity.
The focus of this study is in the description and comparison of three types heme–superoxo and heme–
CO adducts. The starting point is in the characterization of the reduced heme complexes, [(F8)FeII],
[(PPy)FeII] and [(PIm)FeII], where F8, PPy and PIm are iron(II)–porphyrinates and where PPy and PIm possess
a covalently tethered axial base pyridyl or imidazolyl group, respectively. The spin-state properties of
these complexes vary with solvent. The low temperature reactions between O2 and these reduced
porphyrin FeII complexes yield distinctive low spin heme–superoxo adducts. The dioxygen binding
properties for all three complexes are shown to be reversible, via alternate argon or O2 bubbling. Carbon
monoxide binds to the reduced heme–FeII precursors to form low spin heme–CO adducts. The implica-
tions for future investigations of these heme O2 and CO adducts are discussed.
Keywords:
Base tethered heme complexes
Reduced heme–FeII complexes
Heme–FeIII–superoxo complexes
Heme–FeII–carbonyl complexes
1,2H NMR spectroscopy
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
strongly binding to five-coordinated ferrous species to form stable
six-coordinate heme–FeII carbonyl complexes. Determination of
Heme–O2 chemistry has a long history, due to its direct rela-
tionship to hemoglobin and myoglobin O2-carrier proteins, and
other (per)oxidases (e.g., catalase, chloroperoxidase) or oxygenases
(e.g., cytochrome P-450 monooxygenase) [1,2]. Our own interest is
in relationship to heme/copper oxidases such as cytochrome c
oxidase (CcO). CcO is responsible for O2-reduction to water as a ter-
minal step of respiratory chain of mitochondria and many aerobic
bacteria [3–7]. The most well established enzyme dioxygen inter-
mediate is proposed to be ferric-superoxo species which is
generated upon initial O2-reaction with the fully reduced heme–
iron. . .Cu center [8]. This forms prior to O–O bond cleavage and
has deservedly attracted considerable interest [9–14]. In our syn-
thetic research program inspired by cytochrome c oxidase heme–
copper chemistry [15–19], one approach we take is to add copper
complexes to pre-formed heme-superoxo species [16–18,20]; thus
it is in our strong interest to also fully understand the nature of
these iron(II)–dioxygen adducts, the ferric-superoxo species
[13,14]. Carbon monoxide has been widely used on metallopro-
teins as a surrogate for O2-binding to the active site [21–25] and
indeed there exist biological heme CO-sensors [26,27]. CO favors
either m(C–O) or Fe–(CO) stretching vibrations can provide
insights into the heme–iron environment electronic structure
[23,24,28,29]. Laser photoejection of CO and the study of the kinet-
ics of CO-rebinding or ‘‘flash and trap’’ experiments to probe the
fast reactions between O2 and heme FeII complexes also provides
a great deal of fundamental information about the active-site nat-
ure and/or the O2-binding process [25,30–33]. We have even used
heme–carbonyl complexes and photoejection of CO, to study trans-
fer of carbon monoxide to copper(I) complexes whereupon the CO
ligand returns to iron [33–36]; this process has relevance to CcO
(bio)chemistry [22]. Given this history of chemically or biochemi-
cally derived heme–CO complexes, further insights into synthetic
aspects and physical properties of heme–CO adducts are still
needed, and as mentioned, this information is of value for the
study of heme–Cu complex chemistry. In this paper we report
new iron(II) and iron(III) complexes of an imidazolyl tailed tetra-
arylporphyrin PIm and a new ligand and iron complexes with a pyr-
idyl tailed porphyrin PPy. Reduced species (PIm)FeII and (PPy)FeII
have been synthesized and characterized. Then, we describe O2
and CO reactivity towards these species, along with [(F8)FeII],
where F8 is the same porphyrinate used in PIm and PPy, however
lacking a covalently tethered axial base ligand for iron. The CO
and O2 binding to [(F8)FeII] will be compared and contrasted with
that observed for [(PPy)FeII] and [(PIm)FeII] (Fig. 1).
⇑
Corresponding author.
E-mail address: karlin@jhu.edu (K.D. Karlin).
0277-5387/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2012.11.011

Y. Li et al. / Polyhedron 58 (2013) 190–196
191
Fig. 1. Porphyrinate–FeII complexes used in the present study.
found at d 41 and 60 ppm (Fig. S2a) also indicating a high-spin fer-
rous complex (d6, S = 2), thus as a five coordinate species with an
axial ligand tailed imidazole. However in THF, a diamagnetic 1H
NMR spectrum was observed and the 2H NMR of the pyrrole-deu-
terated analog [(PIm-d8)FeII] (which allows direct visualization of
only the pyrrole resonances in the diamagnetic region, unob-
structed by other proton resonance of the entire porphyrinate,
see Fig. S2b) shows that these hydrogens are shifted upfield rela-
tive to the S = 2 complexes described above, to d 8.4, 10.9 and
12.6 ppm (Fig. S2b). This indicates that in THF as solvent, [(PIm)FeII]
is a low-spin ferrous complex (d6, S = 0), possessing two axial li-
gands, one the imidazole donor which is part of PIm and on the
other side of the heme plane, a strongly bound THF molecule.
2. Results and discussion
2.1. Heme–FeII complexes and solvent dependent spin-states
The reduced FeII hemes employed are: (i) [(F8)FeII] (F8 = tetra-
kis(2,6 difluorophenyl)porphyrinate), which was previously de-
scribed [14,18,19], (ii) a recently reported porphyrinate PIm [37],
which until now has only been used for heme nitric oxide chemis-
try; here we develop the reduced heme [(PIm)FeII] and small mole-
cule (O2, CO) binding chemistry, and (iii) [(PPy)FeII], a completely
new porphyrinate which employs a tailed pyridyl moiety as axial
base. Workable quantities of heme complexes [(F8)FeII], [(PPy)FeII]
and [(PIm)FeII] have been prepared and characterized by ESI-MS,
UV–Vis and 1,2H NMR spectroscopies (see Section 4).
We previously reported that the spin state of [(F8)FeII] is depen-
dent on the solvent used [14]. Based on 2H NMR spectra, where
pyrrole-H positions have been replaced by deuterium [14,38–42],
[(F8)FeII] exhibits downfield resonances in acetone as solvent (Ta-
ble 1), revealing a high-spin FeII (d6, S = 2) complex state; this indi-
cates pentacoordination with one axially ligated acetone molecule.
In tetrahydrofuran (THF), downfield resonances are also observed,
thus the complex is five- or six-coordinate with one or two axial
THF molecules; a crystal structure of [(F8)FeII(THF)2] has been
determined [43]). However, we find that acetonitrile (but only at
low temperature, i.e., 233 K) and pyridine at all temperatures act
as ‘strong’ ligands. Pyrrole 2H resonances are found in diamagnetic
region at d 10.1 ppm for MeCN (233 K) and d 8.9 ppm for pyridine
as solvent (293 K), indicating a low-spin, six-coordinate ferrous
center (d6, S = 0) is present and thus two axial solvent derived mol-
ecules ligate.
2.2. Reversible heme–Fe–superoxo formation
We previously reported on the complex [(THF)(F8)FeIII(O2Åꢀ)],
kmax = 416, 536 nm (in tetrahydrofuran solvent), as a six coordinate
low-spin ferric superoxo complex that shows a diamagnetic NMR
spectrum; the pyrrole hydrogens or deuterium were found at d
8.9 ppm. It was generated by reversible binding of O2 to [(F8)FeII]
at low temperature (193 K) and where bubbling the solution with
argon gas reversed the binding, regenerating [(F8)FeII] (Scheme 1)
[14]. Titrations indicating 1:1 uptake of O2 for each [(F8)FeII] pre-
cursor also strongly supports the [(THF)(F8)FeIII(O2Åꢀ)] formulation.
Confirmation that this complex is best described as an iron(III)-
We further investigated the related properties for the new com-
plexes [(PPy)FeII] and [(PIm)FeII] using 1,2H NMR spectroscopies. Ta-
ble 1 provides the chemical shifts for the pyrrole-H resonances for
[(F8)FeII], [(PPy)FeII] and [(PIm)FeII] in acetone and THF at 293 K. In
acetone and THF, the pyrrole-H resonance for [(PPy)FeII] are para-
magnetically shifted to d 46, 48 ppm in acetone (Fig. S1a) while
in THF they are found at d 50 and 58 ppm (Fig. S1b). These values
are similar to the pyrrole hydrogen shifts observed in [(F8)FeII],
suggesting [(PPy)FeII] is a high-spin ferrous complex (d6, S = 2), a
five-coordinate species with an axial pyridyl donor derived from
the porphyrinate ligand PPy. As is well known for such species,
the iron would be expected to be well out of the plane of the pro-
phyrinate toward the side with the pyridyl group to which it coor-
dinates. For [(PIm)FeII], in acetone, the pyrrole hydrogens were
Table 1
Chemical shifts of the pyrrole-H of [(F8)FeII], [(PPy)FeII] and [(PIm)FeII] at various
temperatures in various solvents.
Solvent
THF
Acetone
Ref.
dpyrrole ([(F8)FeII], 293 K) (ppm)
dpyrrole ([(PPy)FeII], 293 K) (ppm)
dpyrrole ([(PPy)FeII], 193 K) (ppm)
dpyrrole ([(PIm)FeII], 293 K) (ppm)
dpyrrole ([(PIm)FeII], 193 K) (ppm)
56
48
[14]
50, 58
99, 88
8.4, 10.9, 12.6
9.5
46, 48
86, 92
41, 60
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Scheme 1.

192
Y. Li et al. / Polyhedron 58 (2013) 190–196
Fig. 3. (a) 1H NMR spectrum [(PPy)FeII] (dpyrrole = 88, 99 ppm) and 2H NMR spectrum
of [(PPy)FeIII(O2Åꢀ)] (dpyrrole = 9.1 ppm) at 193 K in THF. (b) 2H NMR spectrum of
[(PIm)FeII] (dpyrrole = 9.5 ppm) and [(PIm)FeIII(O2Åꢀ)] (dpyrrole = 9.8 ppm) at 193 K in
THF.
Fig. 2. (a) UV–Visible spectra of the heme iron (III)–superoxo complex [(PPy)FeIII
cmꢀ1): 423 (247.4), 534 (23.0)] (Fig. 2b). The reversible O2-binding
to [(PIm)FeII] in CH2Cl2 also required bubbling argon through the
[(PIm)FeIII(O2Åꢀ)] containing solution while warming to room tem-
perature (Fig. S3b).1
NMR spectroscopic studies were also performed on these base
tailed heme–superoxo complexes [(PPy)FeIII(O2Åꢀ)] and [(PIm)FeIII
(O2Åꢀ)]. By directly bubbling O2 through a THF solution of [(PPy)FeII]
(d 99, 88 ppm, high-spin d6) in a septa closed NMR tube, 1H NMR
experiments at 193 K revealed formation of a diamagnetic spec-
(O2Åꢀ)] (red, kmax = 419, 535 nm) formed after bubbling O2 into
a solution of
[(PPy)FeII] (black, kmax = 417, 524, 553 nm) at 193 K in THF. Subsequent bubbling of
argon through the [(PPy)FeIII(O2Åꢀ)] solution reverses the O2 binding to give back
[(PPy)FeII] (blue, kmax = 417, 524, 553 nm). (b) UV–Visible spectra of the heme
iron(III)–superoxo complex [(PIm)FeIII(O2Åꢀ)] (red, kmax = 423, 534 nm) after bub-
bling O2 into solution of [(PIm)FeII] (black, kmax = 417, 525, 553 nm) at 193 K in THF.
Argon bubbling through the [(PIm)FeIII(O2Åꢀ)] solution reversed O2 binding to give
back [(PIm)FeII] (blue, kmax = 417, 525, 553 nm). (Colour online.)
trum, with pyrrole hydrogen shifts occurring at
d 9.1 ppm
superoxide species, in terms of its electronic structure, come from
(Fig. 3a). Confirmation of the assignment came from 2H NMR spec-
troscopy employing a pyrrole-deuterated analog [(PPy-d8)FeII] as
the starting complex. All of this data is consistent with the formu-
lation of a low-spin FeIII (d5), six coordinate heme–superoxo com-
plex formulated as [(PPy)FeIII(O2Åꢀ)]. As is known from classical
studies on hemoglobin and model compounds [9,21], the unpaired
resonance Raman spectroscopy:
m
(O–O) = 1178 cmꢀ1
[D(
18O2)
ꢀ64 cmꢀ1],
m
(Fe–O) = 569 cmꢀ1
[D
(
18O2) ꢀ24 cmꢀ1] [44,45].
Here, we show that new heme–superoxo complexes form using
the porphyrinates with a tailed pyridyl (PPy) or imidazolyl (PIm) as
an axial ligand. Upon O2 addition via bubbling to a 193 K THF solu-
tion of [(PPy)FeII], with kmax /mMꢀ1 cmꢀ1): 417 (210.5), 524
(e
electron of the low-spin FeIII ion is antiferromagnetically coupled
with the unpaired electron of the superoxide radical anion O2
(22.0), 553 nm (8.7), there is an immediate change to give a new
Åꢀ
,
species which is stable at this temperature, the heme–superoxo
making the complex diamagnetic. Upon bubbling O2 through a
solution of pyrrole-deuterated analog [(PIm-d8)FeII] in THF (d
9.5 ppm, low-spin d6), 2H NMR spectroscopy indicates the pyrrole
hydrogens resonate at d 9.8 ppm (Fig. 3b), again suggesting a low-
spin FeIII (d5), six-coordinate heme–superoxo complex formulated
as [(PIm)FeIII(O2Åꢀ)]. These features are similar to previously de-
scribed complex [(THF)(F8)FeIII(O2Åꢀ)], where we excluded the pos-
sibility of formation of other possible heme–Fe/O2 adducts, such as
complex formulated as [(PPy)FeIII(O2Åꢀ)] (Scheme 1), with kmax
(e/
mMꢀ1 cmꢀ1): 419 (157.0), 535 (17.6) (Fig. 2a). Upon bubbling ar-
gon through this solution, UV–Vis monitoring suggests that O2-
binding is slowly reversed giving back [(PPy)FeII] (Fig. 2a). The
reversible O2-binding was also observed when the chemistry was
carried out in the non-coordinating solvent CH2Cl2, but the reversal
occurs much more slowly; Ar bubbling and warming the solution
to RT is required to remove all of the O2 (Fig. S3a). Similar chemis-
try occurred when employing [(PIm)FeII] as the starting complex.
When O2 is added to the solution of [(PIm)FeII] in THF at 193 K [kmax
1
Note: the shift of the Soret band for heme–superoxo complexes, as compared to
(e
/mMꢀ1 cmꢀ1): 417 (258.6), 525 (28.6), 553 nm (7.2)], a complex
formulated as [(PIm)FeIII(O2Åꢀ)] is reversibly formed [kmax /mMꢀ1
their Fe(II) precursors, can be to lower or higher energies (or even unchanged),
depending on the particular ligand system, or the solvent [58,59].
(e

Y. Li et al. / Polyhedron 58 (2013) 190–196
193
ported for [(F8)FeII], namely [(F8)FeII(CO)] [43] (Scheme 2). These
species were detected by UV–Vis spectroscopy (Fig. S4a and b)
and confirmed by infrared (IR) (Fig. S5a–c) and 1H NMR spectro-
scopic studies (Fig. S6a and b) (see Supporting Information).
Table 2 provides the C–O stretching frequencies for [(F8)FeII(CO)],
[(PPy)FeII(CO)] and [(PIm)FeII(CO)] in various solvents at 293 K. After
bubbling CO into the solution of [(PPy)FeII] in THF at 293 K, the UV–
Vis absorption band of [(PPy)FeII] shifts to kmax /mMꢀ1 cmꢀ1): 420
(e
(175.1), 538 (17.6) (Fig. S4a) while solution cell IR spectroscopy in
various solvents leads to the detection of a single CO stretch at
m
(C–O) = 1985 cmꢀ1 in THF (Fig. S5b), a typical value for reduced
heme–Fe–CO complexes [43]. The diamagnetic 1H NMR spectrum
observed for [(PPy)FeII(CO)] shows the pyrrole hydrogens to be
observed at d 8.8 ppm (Fig. S6a) indicative of a six coordinate
low-spin FeII (S = 0, d6). The three peaks at d 4 ꢁ6 ppm are assigned
as axial pyridine hydrogens which are shifted upfield relative to
their free ligand (i.e., PPy) values, confirming the pyridyl group is
bound to the iron atom. [(PIm)FeII(CO)] was prepared same manner
in THF as solvent and similar results are observed for [(PIm)FeII
(CO)] with
(Fig. S4b), the same value for
a
kmax
(
e
/mMꢀ1 cmꢀ1): 421 (279.5), 538 (25.1)
(C–O) = 1985 cmꢀ1 (Table 2)
m
(Fig. S5c) and a diamagnetic 1H NMR showing the pyrrole hydro-
gens at d 8.5 ppm and imidazole hydrogens shifted upfield to d 3
ꢁ5 ppm (Fig. S6b), again evidence for a low-spin FeII (S = 0, d6)
six coordinate [(PIm)FeII(CO)] complex. Somewhat surprisingly,
while one might expect that a stronger axial trans donor (e.g.,
imidazolyl > pyridyl > THF) would lead to a measurably reduced
the C–O stretching frequency, the data here shows that does not
seem to be the case. However, this finding has precedent; a lack
Scheme 2.
of correlation of m(C–O) with base strength is observed for a number
Table 2
of other cases [28,29,43].
Comparison of C–O stretching frequencies for carbonyl adducts of [(F8)FeII], [(PPy)FeII]
and [(PIm)FeII] in various solvents at 293 K.
Solvent
3. Conclusions
THF
Acetone
CH2Cl2
Ref.
In summary, we have described here the synthesis of hemes
with covalently tethered axial base ligands for iron, either a pyridyl
or imidazolyl group. Reduced iron(II) porphyrinates have been syn-
thesized and compared to a closely related iron(II) complex not
possessing the tailed base, [(F8)FeII]. Comparison of the solution
properties of the three iron(II) complexes and their O2 or CO ad-
ducts has been carried out using multinuclear NMR, UV–Vis and
IR spectroscopies. The spin state of iron in the complexes [(F8)FeII],
[(PPy)FeII] and [(PIm)FeII] depends on solvent employed. We have
generated and characterized two new low-spin O2-adducts, the
six-coordinate heme–Fe–superoxo complexes [(PPy)FeIII(O2Åꢀ)]
and [(PIm)FeIII(O2Åꢀ)], and two new low-spin six coordinate
heme–Fe–CO complexes [(PPy)FeII(CO)] and [(PIm)FeII(CO)], with
tailed base as the axial ligand. The reversibility of O2-binding to re-
duced complexes has been established by UV–Vis spectroscopy,
following either oxygenation by O2-bubbling or deoxygenation
via purging solutions of [(PPy)FeIII(O2Åꢀ)] or [(PIm)FeIII(O2Åꢀ)] with
argon gas.
The longer term goal of our research, the main reason for
synthesizing such complexes with PPy and PIm, is to utilize derived
iron(II) or Fe(III)–superoxo compounds to generate new heme–
FeIII–((hydro)peroxo)–CuII species [20,52,53], of possible relevance
to the active site chemistry occurring in cytochrome c oxidase
[20,54]. We have already shown that addition of a strong ‘base’
(e.g., dicyclohexylimidazole) to a heme–peroxo–copper assembly
can have a large influence with respect to subsequent O–O cleav-
age when protons and/or electrons are added (where a phenol
was used as a hydrogen atom source) [19,20]. The new heme–FeII–
CO species will be employed in the initiation of CO flash photolysis
investigations using laser pulses to photoeject CO and observe
either CO (as an O2-surrogate) or O2 (flash-and-trap) rebinding
([(F8)FeII(C–O)]) (cmꢀ1
)
1980
1985
1985
1973
1980
1980
1979, 2040
1985
1985
This work
This work
This work
m
m
m
C–O
C–O
C–O
([(PPy)FeII(C–O)]) (cmꢀ1
([(PIm)FeII(C–O)]) (cmꢀ1
)
)
a binuclear peroxo bridged diiron(III) compound Fe–O–O–Fe, based
on the pyrrole hydrogen resonance assignments [14,46,47].
Given the close similarity of certain of the features for the three
FeIII(O2Åꢀ) complexes reported here, the porphyrinate which they
share in common, their NMR spectroscopic properties, and the res-
onance Raman features previously described for [(THF)(F8)FeIII
(O2Åꢀ)] (see above), we can conclude that they are all low-spin
six-coordinate with end-on (and not side-on) binding of the super-
oxo moiety. The O–O and Fe–O stretching frequencies observed for
[(THF)(F8)FeIII(O2Åꢀ)] are consistent with other known six-
coordinate end-on bound species, model complexes and hemoglo-
bin [48–50]. Further, these parameters do not match those known
for a side-on bound example, (TPP)Fe(O2) (TPP = tetraphenyl-
porphyrinate) whose infra-red spectroscopic properties were
studied in an Ar matrix at 15 K [51].2
2.3. Stable heme–Fe carbonyl formation
Carbon monoxide (CO) reacts immediately with the reduced
synthetic heme complexes, [(PPy)FeII] and [(PIm)FeII] to yield six-
coordinate low-spin heme–Fe carbonyl species similar to that re-
We have recently obtained resonance Raman data for [(PIm)FeIII
2
(O2Åꢀ)], and the O–O and Fe–O stretching frequencies are essentially identical to
those for [(THF)(F8)FeIII(O2Åꢀ)] and for heme proteins, K.D. Karlin and co-workers,
unpublished observations.

194
Y. Li et al. / Polyhedron 58 (2013) 190–196
[32,33,35,36], to probe how the kinetics and thermodynamics of
binding to heme or copper is affected by the ligand environment.
Such investigations should provide fundamental information rele-
vant to other synthetic chemical systems (and thus potentially cat-
alytic processes) as well as to biological systems using iron and/or
Cu in the utilization of molecular oxygen.
the solution IR cell to obtain the carbonyl C–O stretching frequen-
cies. The measurements were taken in transmission mode and
averaged by 32 scans with 2 cmꢀ1 resolutions.
4.3. Synthesis
The reduced heme–FeII complexes [(F8)FeII] and [(d8-F8)FeII] were
prepared following previously reported procedures [14,15,41,55].
The free porphyrin ligands PPy and PIm were synthesized using pub-
lished procedures [37] but with modifications presented here. The
precursor porphyrins F6(NO2) (=5,10,15-tris-(2,6-difluoro-phenyl)-
20-(2-nitro-phenyl)-porphyrine) [56], F6(NO2)-d8 (=5,10,15-
Tris-(2,6-difluoro-phenyl)-20-(2-nitro-phenyl)-porphyrine-d8) [57]
and F6(NH2) (=2-[10,15,20-tris-(2,6-difluoro-phenyl)-porphyrin-5-
yl]-phenylamine) [56] were synthesized using the procedures in
the references cited. F6(NH2)-d8 (=2-[10,15,20-Tris-(2,6-difluoro-
phenyl)-porphyrin-5-yl]-phenylamine-d8) were prepared as re-
portedforF6(NH2)[56]butemployingF6(NO2)-d8 inplaceofF6(NO2).
4. Experimental
4.1. Materials
All reagents and solvents purchased and used were of commer-
cially available quality except as noted. All the air sensitive com-
pounds and reactions were handled under an Ar atmosphere by
applying Schlenk techniques or prepared in an MBraun glovebox
filled with nitrogen (O2, H2O < 1 ppm). Dichloromethane (CH2Cl2)
was purified and dried over an activated alumina column under
a nitrogen atmosphere. Acetone was distilled over Drierite (97%
CaSO4, 3% CoCl2) under Ar. Tetrahydrofuran (THF) was distilled
over sodium/benzophenone under Ar. All solvents were degassed
with argon bubbling for 30 min or by three freeze/pump/thaw cy-
cles before transferring into the glovebox. Dioxygen gas (4.4 Grade)
was purchased from Air Gas East and dried by passage through a
column of CaSO4. Carbon monoxide gas (2.3 Grade) was used as re-
ceived from Air Gas East and dried by passage through an R&D sep-
aration oxygen/moisture trap model OT3-4.
. A
PPy mixture of 3-(30-pyridyl) propionic acid (1.56 g,
10.3 mmol) and 100 mL CHCl3 were placed in a 500 mL round bot-
tom flask and heated to 60 °C. Thionyl chloride (SOCl2) (10 mL) was
added and the solution stirred for 4 h at 60 °C. The resulting mix-
ture was evacuated to remove excess SOCl2 and solvent. The solid
residue was re-dissolved in 20 mL CH2Cl2 at room temperature. To
this, the F6(NH2) (1.90 g, 2.58 mmol) was dissolved in 25 mL CH2
Cl2 and the solution was added dropwise, followed by the addition
of 1.5 mL pyridine. The resulting mixture was stirred for 30 min.
The excess pyridine and solvent were removed under vacuum.
The resulting solid mixture was re-dissolved in 250 mL CH2Cl2
and washed with deionized H2O several times. The organic layer
was saved and dried over anhydrous MgSO4 followed by filtration
of MgSO4 and solvent removal using rotary evaporation. The prod-
uct obtained was purified by column chromatography (silica, ethyl
acetate/hexane = 4:1). Yield: 1.56 g, 70%. 1H NMR (400 MHz,
CDCl3): d 8.99–8.73 (m, 8H, pyrrole-H), 8.64 (m, 1H, amimophe-
nyl), 8.06 (m, 1H, aminophenyl), 7.93 (m, 1H, pyridyl), 7.72(m,
5H, diflurophenyl (3H) and aminophenyl (2H)), 7.52 (m, 1H, pyri-
dyl), 7.38 (m, 6H, diflurophenyl), 6.68 (s, 1H, –NH–C@O), 6.53
(m, 1H, pyridyl), 6.42 (m, 1H, pyridyl), 2.3–2.1 (m, 4H, -CH2-CH2-
Pyridyl), ꢀ2.78 (s, 2H, NH pyrrole) (Fig. S6). ESI-MS (m/z): 871
(M+H+)+.
4.2. Methods
All the UV–Vis measurements were carried out by using a Hew-
lett Packard 8453 diode array spectrophotometer with a quartz
cuvette (path length = 10 mm). The spectrometer was equipped
with a HP Chemstation software and Unisoku thermostated cell
holder for low temperature experiments. In a typical reaction,
the quartz cuvette was usually filled with 2.7 mL (1lM) starting
solution of heme–FeII complex prepared and sealed in the glovebox
and then taken to the benchtop where it was cooled in the thermo-
stated cell holder. Then, O2 or CO was bubbled through cold solu-
tion via a 10-inch needle hooked to the gas tank.
All NMR spectra were recorded in 7 inch, 5 mm o.d. NMR tubes.
1H NMR was performed on Bruker 300 or 400 MHz NMR instru-
ment while 2H NMR were carried out on a Varian 500 MHz NMR
instrument equipped with a tunable deuterium probe to enhance
deuterium detection. The 1,2H chemical shifts are calibrated to nat-
ural abundance deuterium or proton solvent peaks. In a typical
reaction, the NMR tube was loaded with 0.5 mL (10 mM) solution
of heme–FeII prepared and sealed in glovebox then cooled in a cold
bath. Then O2 was bubbled very slowly through cold solution via a
10-inch needle hooked to a syringe filled with O2. CO was bubbled
through room temperature solution via a 10-inch needle hooked to
the carbon monoxide tank.
Electrospray ionization mass spectrometry (ESI-MS) spectra
were acquired using a Finnigan LCQ Duo ion-trap mass spectrom-
eter equipped with an electrospray ionization source (Thermo
Finnigan, San Jose, CA). The heated capillary temperature was
250 °C and the spray voltage was 5 kV. Spectra were recorded con-
tinuously after injection.
PPy-d8. The pyrrole deuterated porphyrin ligand PPy-d8 was pre-
pared using a procedure identical to that described above for PPy
,
but employing the pyrrole deuterated porphyrin F6(NH2)-d8 in-
stead of F6(NH2). 2H NMR (400 MHz, CHCl3): d 8.99–8.73 (m, 8H,
pyrrole-D). ESI-MS (m/z): 879 (M+H+)+.
(PPy)FeIIICl. The ligand PPy (1.44 g, 1.5 mmol) was dissolved in
20 mL THF under an argon atmosphere. Iron(II) chloride tetrahy-
drate (7 g, 55.2 mmol) was added and the solution was heated to
reflux at 60 °C under argon for 3 h. After cooling to room temper-
ature, the solution was exposed to air and stirred for 3 h. The sol-
vent was removed by rotary evaporation and the residue obtained
was re-dissolved in 100 mL CH2Cl2 followed by filtering the insol-
uble solid present. The solution was stirred with HCl (1 M, 100 mL)
for 3 h and then neutralized using solid NaHCO3. The organic layer
was washed with 100 mL saturated NaHCO3 then NaCl water solu-
tion and dried over anhydrous MgSO4. The desired product was
purified by column chromatography (silica, CH2Cl2/MeOH = 98:2).
Yield: 1.08 g, 68%. UV–Vis [nm]: CH2Cl2, 334, 413, 574; THF, 335,
415, 565. 1H NMR (400 MHz, THF-d8): d 80 (s, br, pyrrole-H). ESI-
MS (m/z): 924 (M–Clꢀ)+.
Infrared spectroscopy (IR) spectra were collected using a Ther-
mo Scientific Nicolet Nexus 670 FT-IR spectrophotometer. A solu-
tion of heme–FeII was prepared the same way as for NMR
spectroscopy and then injected into a 25
lm path length solution
(PPy)FeII. The degased solution of PPyFeIIICl (500 mg, 0.5 mmol)
in 40 mL CH2Cl2 was added to the degassed 50 mL saturated Na2S2
O4 (aq) solution under an argon atmosphere. The two solutions were
mixed using argon bubbling for 30 min in an additional funnel. The
reaction mixture was allowed to sit for 20 min until the two layers
flow cell (using two CaF2 windows: one with a sensitized thin film
and the other without a thin film) via a 1 mL syringe to take an IR
spectrum as background. Then, CO was bubbled into the stock
solution in the NMR tube and in a similar manner loaded into

Y. Li et al. / Polyhedron 58 (2013) 190–196
195
separated. The organic layer was separated and passed through
anhydrous Na2SO4 powder loaded in a filter tube (one end connect-
ing to the additional funnel and the other end connecting to a
Schlenk flask) under an argon atmosphere. Then the solvent was
removed and dried in vacuo for 3 h. The resulting solid was kept
in glove box. Yield: 488.6 mg, 92%. UV–Vis [nm] in THF: 417,
524, 553. 1H NMR (400 MHz, THF-d8): d 50, 58 (s, br, pyrrole-H)
(Fig. S1). ESI-MS (m/z): 924 (M–eꢀ)+.
76%. UV–Vis [nm] in THF, 414, 541. 1H NMR (400 MHz, THF-d8):
d 80 (s, br, pyrrole-H). ESI-MS (m/z): 975 (M–OHꢀ)+.
(PIm)FeII. A degassed solution of PImFeIII(OH) (500 mg, 0.5 mmol)
dissolved in 40 mL CH2Cl2 was added to the degassed 50 mL satu-
rated Na2S2O4 and Na2CO3 (aq) solution under an argon atmosphere.
The two solutions were mixed by argon bubbling for 30 min in an
additional funnel. The reaction mixture was allowed to sit for
20 min until the two layers separated. The organic layer was sepa-
rated and passed through anhydrous Na2SO4 powder loaded in a
filter connecting to the additional funnel (one end connecting to
the additional funnel and the other end connecting to a Schlenk
flask) under an argon atmosphere. Then the solvent was removed
and dried by vacuum for 3 h. The resulting solid was kept in glove
box. Yield: 442 mg, 90%. UV–Vis [nm] in THF: 417, 524, 553. 1H
NMR (400 MHz, acetone-d6): d 41, 60 (s, br, pyrrole-H) (Fig. S2).
ESI-MS (m/z): 975 (M–eꢀ)+.
(PPy)FeII-d8. The pyrrole deuterated heme–FeII (PPy)FeII-d8 was
prepared using identical procedure to that described above for
(PPy)FeII, but employing pyrrole deuterated porphyrin PPy-d8 in-
stead of PPy 2H NMR (400 MHz, THF): d 50, 58 (s, br, pyrrole-D).
.
ESI-MS (m/z): 932 (M–eꢀ)+.
F6(NHCOBzCH2Cl). The porphyrin F6(NH2) (1.3 g, 1.77 mmol, 1
equiv) was dissolved in 100 mL THF under an argon atmosphere.
Triethylamine (1.04 mL, 4 equiv) was added and the solution was
cooled to 0 °C followed by the addition of 3-(chloromethyl)-ben-
zoylchloride (0.4 mL, 1.5 equiv) dropwise. The solution was stirred
for 3 h at 0 °C. Then excess triethylamine and solvent were
removed under a vacuum. The resulting product was purified by
column chromatography (silica, CH2Cl2). Yield: 1.2 g, 77%. 1H
NMR (400 MHz, CDCl3): d 8.76 (m, 8H, pyrrole-H), 8.15 (m, 1H,
amimophenyl), 7.88 (m, 1H, aminophenyl), 7.73 (m, 4H, para
diflurophenyl (3H) and aminophenyl (1H)), 7.58(m, 1H, aminophe-
nyl), 7.34 (m, 6H, ortho diflurophenyl), 6.78 (m, 2H, benzyl–C@O),
6.72 (s, 1H, –NH–C@O), 6.34 (m, 1H, benzyl–C@O), 6.14 (m, 1H,
benzyl–C@O), 3.68 (s, 1H, benzyl-CH2-Cl), ꢀ2.72 (s, 2H, NH pyr-
role). ESI-MS (m/z): 891 (M+H+)+.
(PIm)FeII-d8. A procedure identical to that given above for
(PIm)FeII was used but employing pyrrole deuterated PIm-d8 instead
of PIm 2H NMR (400 MHz, THF): d 8.4, 10.9, 12.6 (s, pyrrole-D)
.
(Fig. S2). ESI-MS (m/z): 983 (M–eꢀ)+.
Acknowledgement
This work was supported by the National Institutes of Health
(R01 GM 060353 to K.D.K).
Appendix A. Supplementary data
PIm
. The porphyrin F6(NHCOBzCH2Cl) (1.1 g, 1.24 mmol, 1
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.poly.2012.11.011.
equiv) was dissolved in a solvent mixture of 150 mL dry toluene
and 150 mL dry ethanol under an argon atmosphere. Imidazole
(25 g, 300 equiv) and NaI (741 mg, 4 equiv) were added and the
solution was heated to 62 °C and stirred in the dark and under
an argon atmosphere for 12 h. The solution was then cooled to
RT and solvent was removed by rotary evaporation. The solid res-
idue obtained was re-dissolved in 50 mL CH2Cl2 and was twice
washed with 400 mL deionized H2O. The organic layer was saved
and dried over anhydrous MgSO4 and this was followed by filtra-
tion of the MgSO4 and solvent removal using rotary evaporation.
The product obtained was purified by column chromatography in
the dark (silica, 0.1% triethylamine, 1% MeOH/CH2Cl2). Yield:
800 mg, 70%. 1H NMR (400 MHz, CDCl3): d 8.86–8.78 (m, 9H, pyr-
role (8H) and aminophenyl (1H)), 8.11 (dd, 1H, benzyl), 7.84–
7.71 (m, 4H, para diflurophenyl (3H) and aminophenyl (1H)),
7.52 (m, 1H, aminophenyl), 7.46–7.25 (m, 8H, meta diflurophenyl
(6H) and benzyl (2H)), 6.78 (s, 1H, –NH–C@O), 6.35–6.24 (m, 4H,
benzyl (1H), aminophenyl (1H) and imidazolyl (2H)), 6.08 (s, 1H,
imidazolyl), 3.73 (s, 2H, –CH2–benzyl), ꢀ2.79 (s, 2H, NH pyrrole)
(Fig. S6). ESI-MS (m/z): 923 (M+H+)+.
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