Oxidative degradation of zinc porphyrin in comparison with its iron analogue

Article abstract of DOI:10.1002/chem.201001073

(Figure Presented) Solar conversion: Zinc hydroxyisoporphyrin (2), under sunlight and argon, is reduced back to zinc meso-tetraphenylporphyrin (1), but stays stable in oxygen in the absence of light. This behavior is in contrast to its iron analogue and s

Full text of DOI:10.1002/chem.201001073

COMMUNICATION
DOI: 10.1002/chem.201001073
Oxidative Degradation of Zinc Porphyrin in Comparison with Its Iron
Analogue
[
a]
Jagannath Bhuyan and Sabyasachi Sarkar*
Isoporphyrins, derivatives of porphyrins possessing a satu-
rated meso-carbon, are known to be reactive intermediates
in the biosynthesis of chlorophyll and byproducts of heme
very distinctive role of a specific metal in metallo-macromo-
lecules in their degradation processes.
Compound 1 in CH Cl reacts with nitrogen dioxide (NO
2
2
[1]
oxidation. They are unique because of their distinctive
near-IR electronic absorption with interesting redox behav-
and O2) to yield the zinc hydroxyisoporphyrin (2;
[6]
Scheme 1), which was isolated like its iron analogue. The
[
2]
[9]
ior. Based on the proposal of isoporphyrin as a tautomer
molecular structure of 2 is shown in Figure 1. The presence
[3]
of porphyrin by Woodward, Dolphin and co-workers re-
ported the formation of metalloisoporphyrin by electro-
of the counteranion nitrate and the tetrahedral meso-carbon
(C5) confirms the cationic nature of 2. One oxygen atom of
counteranion nitrate is hydrogen bonded with the oxygen
[4]
chemical oxidation of zinc meso-tetraphenylporphyrin (1).
These tetraaryl metalloisoporphyrins were never isolated in
the pure state, although they have been known to exist in
[5]
solution for the last four decades. Therefore, structural fea-
tures of such macromolecules remained unexplored. While
investigating the joint role of biorelevant molecules like O2
and NO in creating oxidative stress we found meso-hydrox-
ylation of an iron porphyrin that follows degradation similar
[6]
to that of heme proteins. Heme oxygenase catalyzes the
degradation of heme to biliverdin by meso-hydroxylation as
the first step using oxygen along with nicotinamide adenine
dinucleotide phosphate (NADPH) and cytochrome P450 re-
Scheme 1. Synthesis of hydroxy isoporphyrin (2) from 1 and the regener-
[7]
À
ation of 1 using thiol. The counteranion NO
3
in 2 is not shown.
ductase. In such a degradation reaction, the redox role of
iron ion has not been properly stressed. For chlorophyll, the
degradation may be photooxidative or enzymatic as known
along with different environmental stress-related degrada-
[8]
tion. As most of the isoporphyrin chemistry in solution is
studied by non-redox zinc systems, we now investigate zinc
porphyrin chemistry similar to those studies conducted with
[6]
iron systems. Based on this, herein we report the first
structural characterization of an air-stable tetraarylzinciso-
porphyrin. Interestingly, it does not decompose to a biliver-
din-type complex like its iron analogue, which stresses the
[
a] J. Bhuyan, Prof. S. Sarkar
Department of Chemistry, Indian Institute of Technology Kanpur
Kanpur-208016, U.P. (India)
Fax : (+91)512-259-7265
E-mail: abya@iitk.ac.in
Figure 1. ORTEP view of 2 showing 50% probability thermal ellipsoids.
Hydrogen atoms except those at O1 and O2 are omitted for clarity. Se-
lected bond lengths and angles: C5ÀO1=1.430(4) ꢀ, ZnÀO2=
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201001073.
2.064(3) ꢀ; C5-O1-H1=109.4(3)8.
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[
11]
atom of meso-hydroxyl group of the isoporphyrin and an-
other oxygen atom of the nitrate is hydrogen bonded with
the oxygen atom of the water molecule coordinated to the
zinc atom. (Figure S7 in the Supporting Information).
high potential. In the present case such reversible reduc-
tion under the influence of pyridine may be related to its co-
ordination to zinc, presumably replacing the coordinated
water in 2.
[12–13]
In 2 all of the carbon atoms attached to C5 show bond-
length elongation (Figure S8 in the Supporting Informa-
DFT calculations
ruption of delocalization around the C5 atom in the
HOMO. The narrow HOMO–LUMO gap corroborates the
of 2 (Figure 3) clearly show the dis-
[9]
tion). The C4ÀC5 and C5ÀC6 bond lengths are around
1.505 ꢀ, equivalent to a single bond, showing the point of
disruption of aromaticity in 2. There is also an increase in
the C5ÀC21 bond length. The C4-C5-C6 angle decreases to
1178 compared with the other three meso-carbon angles
(
1258). Interestingly, upon hydroxylation the macrocyclic
structure became significantly ruffled with the meso-carbon
atoms moving above and below by 0.42, À0.22, 0.169, and
À0.20 ꢀ at C5, C10, C15, and C20, respectively, but such
[6]
ruffling was not observed in the iron isoporphyrin. The
loss of its aromaticity is also corroborated by a distinctive
NMR spectrum, in which the b-H peaks of pyrrole are ob-
served in the region d=6–7 ppm (Figure S4 in the Support-
ing Information).
The electrochemistry of 2 differs much from the starting
complex 1. Its first reduction potential (irreversible) appears
at a very low potential, Epc (À115 mV vs. Ag/AgCl) (Fig-
ure S6 in the Supporting Information) to suggest its ease for
reduction under mild conditions. Thus, compound 2 can be
reduced to the starting complex 1 with ease even using thio-
phenol (Figure 2, Scheme 1). However, upon addition of
pyridine the first irreversible reduction was now shifted to a
reversible reduction at À40 mV (vs. Ag/AgCl). Such dramat-
ic effect of pyridine to facilitate reduction may be due to its
coordination to zinc, but its exact role to bring such changes
is yet to be ascertained. Interestingly, the formation of a
stable iron b-pyridiniumtetraphenylporphyrin, a very unsta-
ble iron isoporphyrin and an iron porphodimethene as the
reaction products of iron porphyrin cation radical with pyri-
Figure 3. Relative energy-level diagram for the selected molecular orbi-
tals of 2. Isosurface cutoff value=0.04.
[10]
dine is reported. Being iron complexes, these are prone to
have metal- and ligand-centered redox reactions. Recently,
the electrochemistry of some iron phorphodimethene com-
plexes showed that the ligand-based redox reaction requires
appearance of a low-energy electronic transition in the near
infrared region (Figure 2).
Nitrogen dioxide may form by the reaction of free O and
2
NO during malfunction of oxygenation and NO synthase re-
[14]
actions. This is also a product of the spontaneous decay of
peroxynitrite, which is the most damaging reactive species
produced by the reaction between superoxide ion and nitric
[15]
oxide.
Most metallated macromolecules present in the
natural systems function as antioxidants to smother the on-
slaught of reactive oxygen and nitrogen species. It is now
known that metalloporphyrin and metallocorrole complexes
have high utility to catalyze the decomposition of these re-
[16]
active oxygen and nitrogen species.
The degradation of closed-shell metalloporphyrin by non-
biological oxidizing agents, such as cerium(IV) or thallium-
[
17]
AHCTUNGTRENNUNG( III) salts, led to the formation of several products. Under
such drastic oxidizing conditions, less amount of biorelevant
degraded product was obtained. Further, there are reports
that magnesium porphyrins are photochemically oxidized to
produce the corresponding ring-opening compound that
Figure 2. Reduction of 2 to 1 in dichloromethane by thiophenol (trace
pyridine; time interval of each scan, 2 min).
10650
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Oxidative Degradation of Zinc Porphyrin
COMMUNICATION
mimics the major biochemical process of chlorophyll degra-
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[19]
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under bright sunlight exposure. The present study shows
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dized porphyrin ring, such as that present in 2 does not un-
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[6]
and unlike its iron analogue it does not undergo degrada-
tion in aerobic conditions. It readily responds to chemical or
photoreduction to revert back to the starting complex 1.
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the reactions must involve complimentary changes in the
redox states of the coordinated iron concomitant with the
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under oxidative stress may lead to the formation of oxidized
[9] See the Supporting Information for synthetic details. Crystallograph-
3
ic data for 2: C44
H
31
N
5
O
5
Zn,
M
r
=775.09, 0.20ꢂ0.10ꢂ0.08 mm ,
monoclinic, a=12.910(5) ꢀ, b=11.244(5) ꢀ, c=24.241(5) ꢀ, a=
3
9
0.005(5)8, b=92.287(5)8, g=90.005(5)8, V=3516.0(2) ꢀ , space
group P21/c, Z=4 R
2805 reflections, 8660 unique, Rint =0.0838, R
[10] K. Rachlewicz, L. Latos-Grazynski, Inorg. Chem. 1995, 34, 718–727.
1
=0.0603 lAHCTUNTGENRNUG
2
1
2
2
, but to induce chlorophyll-like degradation the presence of
[18,20]
[
[
11] D. Bhattacharya, S. Sarkar, Eur. J. Inorg. Chem. 2010, 3429–3435.
12] DFT calculations have been carried out by employing a B3LYP
hybrid functional using the Gaussian 03 program. Molecular orbitals
were visualized by using GView. The 6-31G*+ basis set was used
for C, N, O, and H atoms, and the effective core potential basis set
LanL2DZ was used for the Zn atom. The geometry of 2 was taken
from the crystal structure.
strong light, such as sunlight is essential.
In summary, zinc hydroxyisoporphyrin (2) has been struc-
turally characterized as the oxidative product of zinc meso-
tetraphenylporphyrin (1) after exposure to NO (NO and
2
O ). Compound 2 does not respond to degradation reactions
2
in the same way as its iron analogue and requires strong
light exposure under oxygen to degrade.
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Experimental Section
The synthetic details, characterization (UV/Vis, NMR spectra, cyclic vol-
tammetry, X-ray structural data), and DFT details of 2 are provided in
the Supporting Information. CCDC-772725 contains the supplementary
crystallographic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
Acknowledgements
J.B. acknowledges a Doctoral Research Fellowship from the CSIR, New
Delhi and S.S. thanks the DST, New Delhi for funding the project.
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Keywords: density functional calculations · hydroxylation ·
porphyrinoids · X-ray crystallography · zinc
2
Chem. Eur. J. 2010, 16, 10649 – 10652
ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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10651

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Received: April 22, 2010
Published online: August 4, 2010
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