A new general method for selective β-polynitration of porphyrins; preparation and redox properties of Zn-porphyrins bearing one through to eight β-nitro substituents and X-ray structure of the first Zn β-pernitro porphyrin

Article abstract of DOI:10.1039/b004160m

Selective β-polynitration of Zn-5,10,15,20-tetrakis(2,6-dichlorophenyl)porphyrin, Zn(TDCPP), is achieved by controlled titration with the HNO₃-CF₃SO₃H-(CF₃SO₂)₂O system, and affords a full series of Zn porphyrins bearing one through to eight β-nitro groups in high yield and exhibiting a wide range of reduction potentials (from -920 to +155 mV vs. SCE); an X-ray structure of the first reported β-pernitrated Zn porphyrin Zn(TDCPN₈P)(EtOH)₂·2EtOH confirms the synthetic methodology.

Full text of DOI:10.1039/b004160m

A new general method for selective b-polynitration of porphyrins; preparation
and redox properties of Zn-porphyrins bearing one through to eight b-nitro
substituents and X-ray structure of the first Zn b-pernitro porphyrin†
Magali Palacio,a Virginie Mansuy-Mouries,a Guillaume Loire,a Karine Le Barch-Ozette,a Philippe Leduc,a
Kathleen M. Barkigia,b Jack Fajer,b Pierrette Battionia and Daniel Mansuy*a
a UMR 8601, Universite´ Paris V, 45 Rue des Saints-Pe`res, 75270 Paris Cedex 06, France.
E-mail: daniel.mansuy@biomedicale.univ.paris5.fr
b Department of Applied Science, Brookhaven National Laboratory, Upton, New York 11973-5000, USA
Received (in Basel, Switzerland) 19th May 2000, Accepted 28th July 2000
First published as an Advance Article on the web
Selective b-polynitration of Zn–5,10,15,20-tetrakis(2,6-di-
chlorophenyl)porphyrin, Zn(TDCPP), is achieved by con-
trolled titration with the HNO3–CF3SO3H–(CF3SO2)2O
system, and affords a full series of Zn porphyrins bearing
one through to eight b-nitro groups in high yield and
exhibiting a wide range of reduction potentials (from 2920
to +155 mV vs. SCE); an X-ray structure of the first reported
b-pernitrated Zn porphyrin Zn(TDCPN8P)(EtOH)2·2EtOH
confirms the synthetic methodology.
TDCPNxP porphyrin required a different nitrating system2 for
each value of x.
We have now developed a nitrating system based on HNO3
and the superacid CF3SO3H in the presence of (CF3SO2)2O that
acts as a general nitrating agent for the selective synthesis of the
full TDCPNxP series. We describe here the successful use of
this nitrating procedure to prepare all eight members of the
Zn(TDCPNxP) series in high yield, present the first X-ray
structure of a b-pernitroporphyrin, Zn(TDCPN8P), prepared by
this method, and report the strikingly wide range of redox
potentials now accessible via the eight members of the
Zn(TDCPNxP) series.
Titration of Zn(TDCPP) with 1, 2, 3, 4 or 6 equiv. of red
fuming HNO3 and CF3SO3H (1+1)4 selectively led to
Zn(TDCPNxP) with x = 1, 2, 3, 4 or 6, respectively with yields
between 50 and 70% (Table 1). However, the use of 5 equiv. of
HNO3–CF3SO3H led to a complex mixture of Zn(TDCPNxP)
with x = 4, 5 and 6. Moreover, low yields of Zn(TDCPN8P)
were obtained (ca. 25%, Table 1) when using the same
system.
A slightly modified system consisting of red fuming HNO3 in
the presence of < 1 equiv. of CF3SO3H and (CF3SO2)2O, in
order to limit side reactions [demetallation and destruction of
Zn(TDCPP)], allowed us to prepare each compound of the
Zn(TDCPNxP) series in much higher yields. Thus, titration of
Zn(TDCPP) with increasing amounts of the HNO3–CF3SO3H–
(CF3SO2)2O (1+0.12+0.06) mixture selectively led to each
compound of the Zn(TDCPNxP) series (x = 1–7) with yields
between 78 and 95%. Formation of the desired porphyrin was
followed by thin-layer chromatography and addition of the
nitrating agent was stopped after complete formation of the
target product. The pernitrated Zn(TDCPN8P) was selectively
obtained with a satisfactory 50% yield by the same method
under the conditions described in Table 1. The full range of b-
nitro products obtained with this system contrasts sharply with
Iron and manganese meso-tetraaryl porphyrins bearing elec-
tron-withdrawing b-substituents have been shown to be effi-
cient catalysts for cytochrome P450-mimetic oxygenations of
hydrocarbons.1 In an effort to obtain metalloporphyrins with
significantly altered redox potentials and unusual reactivities,
we have attempted to synthesize metalloporphyrins bearing as
many electron-withdrawing b-nitro substituents as possible.2
3
Nitration of TDCPPH2 with red fuming HNO3 led us to
mixtures of b-pentanitro- and b-hexanitro-TDCPPH2.2a More
recently, we reported that a more powerful nitrating agent,
consisting of a mixture of HNO3 and (CH3CO)2O in the
presence of K10 montmorillonite, reacted with Zn(TDCPP) to
afford
the
corresponding
b-heptanitroporphyrin,
Zn(TDCPN7P), in 50% yield.2b However all the nitrating
systems that we have used so far suffered from two main
drawbacks, (i) they did not pernitrate the porphyrin to the b-
octanitro level, and (ii) they did not act as general nitrating
agents to produce all the desired b-polynitroporphyrins,
Zn(TDCPNxP) (x = 1–8), as the number of nitrating equiva-
lents was increased. Indeed, selective formation of any one
† Electronic supplementary information (ESI) available: ORTEP plots for
1, bond distances and displacements from the plane of the porphyrin core.
See http://www.rsc.org/suppdata/cc/b0/b004160m/
Table 1 Synthesis and reduction potentials of Zn(TDCPNxP)
x
1
2
3
4
5
6
7
8
Synthesis method
Yield (%)
b
HNO3–CF3SO3H (1+1)a
60
78
62
90
67
86
63
93
—
60
95
53
90
25
50
HNO3–CF3SO3H–(CF3SO2)2O (1+0.12+0.06)c
80
E1/2 (first reduction)/mV vs. SCEd
2920
2750
2595
2470
2300
2150
0
+155
a Yields of Zn(TDCPNxP) after addition of x equiv. of red fuming HNO3 and CF3SO3H (1+1) to 5.2 3 1025 M Zn(TDCPP) in CH2Cl2 at 20 °C and 0.5, 1,
2, 4, 5, 24, 48 and 72 h reaction time for x = 1 to 8, respectively. For x = 7 and 8, 10 and 40 equiv. of nitrating agent were added, respectively. b Only case
in which Zn(TDCPNxP) was not largely predominant. c Yields of Zn(TDCPNxP) after titration of Zn(TDCPP) with the HNO3–CF3SO3H–(CF3SO2)2O
mixture. For 1 @ x @ 5, reactions at 20 °C in CH2Cl2 for 24 h after addition of 1.1, 3.5, 6.5, 9 and 11 equiv. of HNO3, respectively. For x = 6, 7 and 8, reactions
d
at 30 °C in MeNO2 for 1, 2 and 5 days and after addition of 18, 150 and 380 equiv. of HNO3, respectively. E1/2 for the first one-electron reduction of
Zn(TDCPNxP), 1023 M in CH2Cl2 containing 0.1 M NBu4PF6; E1/2 for Zn(TDCPP) = 21285 mV.2b
DOI: 10.1039/b004160m
Chem. Commun., 2000, 1907–1908
This journal is © The Royal Society of Chemistry 2000
1907

of the reduction potential and thus readily explains why the p-
anion radical of Zn(TDCPN8P) forms under such mild reducing
conditions, even in the presence of O2.
In conclusion, we have developed a new general method for
selective b-polynitration of Zn(TDCPP) in high yield based on
controlled titration of the starting porphyrin with HNO3–
CF3SO3H–(CF3SO2)2O. The method leads to a complete series
of porphyrins bearing 1–8 b-nitro groups with a wide span of
reduction potentials that range from 2920 to +155 mV. Similar
results were obtained with Ni(TDCPP) and methods for
selective demetallation of the Zn and Ni polynitroporphyrins
have been recently developed. Use of metallo–b-poly-
nitroporphyrins in catalysis, and application of the nitration
method to other polyaromatic molecules are under investiga-
tion.
This work was supported by CNRS, Universite´ Paris V and
the Division of Chemical Sciences, U.S. Department of Energy
under Contract no. DE-ACO2-98CH10886 at BNL.
Fig. 1 Edge-on-view of Zn(TDCPN8P)(EtOH)2 1. Thermal ellipsoids have
been reduced to 1% to illustrate the planarity of the porphyrin skeleton and
the orientations of the substituents.
Notes and references
the classical nitrating HNO3–H2SO4 system which was found to
be efficient only for the preparation of the first four members of
the Zn(TDCPNxP) series (yield ca. 80%).
1 For general reviews see: B. Meunier, Chem. Rev., 1992, 92, 1411; D.
Mansuy, Coord. Chem. Rev., 1993, 125, 129; R. A. Sheldon, Metal-
loporphyrins in Catalytic Oxidations, M. Dekker, New York, 1994; J. T.
Groves and Y. Z. Han, in Cytochrome P450: Structure, Mechanism and
Biochemistry, ed. P. Ortiz de Montellano, Plenum Press, New York,
1995, p. 3; D. Dolphin, T. G. Traylor and L. Xie, Acc. Chem. Res., 1998,
31, 155.
2 (a) J. F. Bartoli, P. Battioni, W. R. De Foor and D. Mansuy, J. Chem. Soc.,
Chem. Commun., 1994, 23; (b) K. Ozette, P. Leduc, M. Palacio, J. F.
Bartoli, K. M. Barkigia, J. Fajer, P. Battioni and D. Mansuy, J. Am. Chem.
Soc., 1997, 119, 6442; (c) K. Ozette, P. Battioni, P. Leduc, J. F. Bartoli
and D. Mansuy, Inorg. Chim. Acta, 1998, 272, 4.
3 TDCPP = dianion of 5,10,15,20-tetrakis(2,6-dichlorophenyl)porphyrin,
SCE = saturated calomel electrode.
4 The HNO3–CF3SO3H system has been previously used for nitration of
aromatic compounds: C. L. Coon, W. G. Blucher and M. E. Hill, J. Org.
Chem., 1973, 38, 4243; G. A. Olah, Angew. Chem., Int. Ed., 1993, 32,
767.
All the Zn(TDCPNxP) compounds (x = 1–8) were com-
1
pletely characterized by elemental analysis and UV–VIS, H
NMR spectroscopy and mass spectrometry. Additional details
of the syntheses and full characterizations will be presented
elsewhere. It is of note that Zn(TDCPNP), Zn(TDCPN7P) and
Zn(TDCPN8P) are single pure compounds while the other
members of the Zn(TDCPNxP) series are mixtures of regio-
isomers with different relative positions of the b-nitro sub-
stituents.
The molecular structure of the first b-pernitrated porphyrin to
be reported, Zn(TDCPN8P)(EtOH)2 1, was confirmed by an X-
ray crystallographic study of 1·2EtOH obtained by crystalliza-
tion from CH2Cl2–EtOH.5 As shown in Fig. 1, the Zn is axially
coordinated by two molecules of EtOH with Zn–O distances of
2.272(10) Å and average Zn–N distances to the pyrrole
nitrogens of 2.075(8) Å. The Zn–O and Zn–N distances are,
respectively, the shortest and longest bond distances reported to
date for hexacoordinated Zn porphyrins with oxygen donor
axial ligands.6 The tight axial bonds and the expanded
porphyrin core are readily attributable to the multiple electron-
withdrawing groups of 1. In spite of the 12 peripheral
substituents, which generally result in severe skeletal distor-
tions,7 the macrocycle of 1 is essentially planar with an average
deviation from the 24-atom porphyrin plane of only 0.035 Å,
and maximum displacements of only 0.08 and 0.07 Å at any one
b or meso carbon, respectively. Macrocycle planarity is
achieved by orienting all the peripheral substituents nearly
orthogonal to the porphyrin plane: the two crystallographically
independent phenyl rings align at 80 and 90° to the porphyrin
plane whereas the four independent nitro groups subtend angles
of 76, 82, 87, 77° to the same plane.
As established by cyclic voltammetry, all eight
Zn(TDCPNxP) undergo a reversible one-electron reduction
leading to the formation of p-anion radicals. Formation of such
radicals8 is confirmed by controlled-potential electrolysis of
Zn(TDCPN8P) at 0 V vs. SCE in CH2Cl2 which yields a species
that exhibits new absorption bands centered at 900 nm and a
singlet EPR spectrum centered at g = 2.00.9 Table 1 compares
the reduction potentials of each ZnP/ZnP2 couple for all eight
nitro derivatives. The potentials increase in a linear manner, the
introduction of each additional b nitro group causing a shift of
ca. +150 mV. Thus, the incorporation of eight b-nitro groups in
Zn(TDCPP) leads to a strikingly large positive shift of ca. 1.4 V
5 Crystal data for 1·2EtOH (two EtOHs are bound to the Zn and two EtOHs
are in the lattice): C52H36Cl8N12O20Zn, M = 1497.90, triclinic, space
¯
group P1 (no. 2), dark blue blocks, a = 11.792(3), b = 11.993(2), c =
12.279(4) Å, a = 106.04(2), b = 90.24(3), g = 106.87(3)°, V =
1590.4(7) Å3, m = 4.343 mm21, Dc = 1.564 g cm23, Z = 1, T = 293 K,
4210 reflections measured, 3973 unique, R1 = 0.094, wR2(F2) = 0.317
(3971 data), GOF = 1.031. Data were measured on an Enraf-Nonius
CAD4 diffractometer with graphite-monochromated Cu-Ka radiation (l
= 1.54178 Å). The structure was solved by direct methods (SIR92) and
refined by full-matrix least squares (SHELXL-93) on F2. CCDC
182/1732. See http://www.rsc.org/suppdata/cc/b0/b004160m/ for crys-
tallographic files in .cif format.
6 For hexacoordinated Zn porphyrins, see Cambridge Structural Data Base,
F. H. Allen and O. Kennard, Chem. Des. Automat. News, 1993, 8(1), 1;
F. H. Allen and O. Kennard, Chem. Des. Automat. News, 1993, 8(1),
31.
7 For examples of severely nonplanar dodecasubstituted Zn porphyrins,
see: K. M. Barkigia, M. D. Berber, J. Fajer, C. J. Medforth, M. W. Renner
and K. M. Smith, J. Am. Chem. Soc., 1990, 112, 8851; M. O. Senge,
J. Porphyrins Phthalocyanines, 1998, 2, 93; K. M. Barkigia, D. J. Nurco,
M. W. Renner, D. Melamed, K. M. Smith and J. Fajer, J. Phys. Chem. B,
1998, 102, 322.
8 K. M. Kadish, Prog. Inorg. Chem., 1986, 34, 435; K. M. Kadish, E. Van
Caemelbecke and G. Royal, in The Porphyrin Handbook, ed. K. M.
Kadish, K. M. Smith and R. Guilard, Academic Press, New York, 1999,
vol. 8, pp. 1–97.
9 For properties of metalloporphyrin p-anion radicals, see: R. H. Felton, in
The Porphyrins, ed. D. Dolphin, Academic Press, New York, 1978, vol.
5, pp. 53–125; P. Bhyrappa and V. Krishnan, Inorg Chem., 1991, 30, 239;
M. W. Renner, L. R. Furenlid, K. M. Barkigia, A. Forman, H. K. Shim,
D. J. Simpson, K. M. Smith and J. Fajer, J. Am. Chem. Soc., 1991, 113,
6891.
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