One-pot dodecanitration of Zn(II) and Ni(II) meso-tetrakis-(2,6-dichlorophenyl) porphyrin, and extreme redox properties of the obtained complexes

Article abstract of DOI:10.1016/S0022-328X(01)01382-1

Introduction of 12 nitro substituents on Zn(II) and Ni(II) [meso-tetra-(2,6-dichlorophenyl)porphyrin = TDCPP] was performed by adapting a recently described method for polynitration of metalloporphyrins, allowing the one-pot synthesis of metallododecanitr

Full text of DOI:10.1016/S0022-328X(01)01382-1

Journal of Organometallic Chemistry 643–644 (2002) 522–524
www.elsevier.com/locate/jorganchem
Note
One-pot dodecanitration of Zn(II) and Ni(II)
meso-tetrakis-(2,6-dichlorophenyl) porphyrin, and extreme redox
properties of the obtained complexes
Magali Palacio, Anne Juillard, Philippe Leduc, Pierrette Battioni, Daniel Mansuy *
UMR 8601, Uni6ersite´ Paris V, 45 rue des Saints-Pe`res, 75270 Paris 06, France
Received 3 August 2001; accepted 9 October 2001
Abstract
Introduction of 12 nitro substituents on Zn(II) and Ni(II) [meso-tetra-(2,6-dichlorophenyl)porphyrin=TDCPP] was performed
by adapting a recently described method for polynitration of metalloporphyrins, allowing the one-pot synthesis of metallodode-
canitroporphyrins from commercially available compounds. The corresponding complexes exhibit the highest reduction potentials
ever described for such metalloporphyrins, which are shifted by more than +1.6 V when compared with those of Zn and
Ni(TDCPP). The Ni (dodecanitroporphyrin) complex underwent four successive, reversible one-electron reductions between
+470 and −555 mV (vs. SCE). The one-electron reduction product of the Ni dodecanitroporphyrin is stable for hours at room
temperature even in aerobic conditions; its visible and EPR spectra indicate a Ni (porphyrin radical anion) structure. © 2002
Elsevier Science B.V. All rights reserved.
Keywords: Nitro substituents; Metallododeca nitroporphyrins; EPR spectra; Radical anion
HNO −CF SO H
3
3
3
Iron, manganese and ruthenium complexes of meso-
tetraarylporphyrins have been used as oxidation cata-
lysts that mimic some of the properties of cytochromes
P450 [1–6]. It has been shown that metalloporphyrins
bearing many electron-withdrawing b-substituents such
as halogeno or nitro groups, are particularly efficient
catalysts for monooxygenation of hydrocarbons includ-
ing alkanes [4,6–9]. In an effort to obtain metallopor-
phyrins with significantly altered redox potentials and
unusual reactivities, we have recently developed a very
powerful nitrating system based on red fuming HNO3
and the superacid CF3SO3H in the presence of
(CF3SO2)2O. Application of this system to Zn[meso-
tetra-(2,6-dichlorophenyl)porphyrin=TDCPP) allowed
us to prepare the corresponding Zn(b-octanitropor-
phyrin), as well as all the Zn (porphyrins) bearing
between one and seven b-nitro substituents in high
yields, from titration of Zn (TDCPP) with increasing
amounts of HNO3 and CF3SO3H in the presence of
(CF3SO2)2O Eq. (1)) [10].
Zn(TDCPP) ꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢁ Zn[TDCP(NO2)xP]
(1)
(CF SO ) O
2 2
15x58
3
We here describe an application of this system for the
one-pot dodecanitration of the Zn(II) and Ni(II) com-
plexes of TDCPPH2, by using a modified version of the
previously described method. This led to the synthesis
of metalloporphyrins that exhibit the highest reduction
potentials described so far for Zn(II) and Ni(II)
porphyrins.
During our study of the best conditions for polyni-
tration of the b-positions of the tetrapyrrole ring of
Ni(II) or Zn(II) (TDCPP), we have observed that the
use of too high concentrations of CF3SO3H or
(CF3SO2)2O led to a partial nitration of the meso-
phenyl rings besides the expected nitration of the te-
trapyrrole b-positions. This led us to use the following
conditions that result in
a one-pot synthesis of
Ni[TDCm(NO2)P(NO2)8P] in which eight b-nitro sub-
stituents have been introduced on the tetrapyrrole ring
of Ni(TDCPP) and a meta-nitro substituent on each
meso-aryl ring of Ni(TDCPP) (Fig. 1). In a first step,
Ni(TDCPP), 5×10−3 M in CH3NO2, was treated with
380 equivalents of red fuming HNO3 in the presence of
* Corresponding author.
0022-328X/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved.
PII: S0022-328X(01)01382-1

M. Palacio et al. / Journal of Organometallic Chemistry 643–644 (2002) 522–524
523
Fig. 1. One-pot dodecanitration of Zn(II) and Ni(II).
CF3SO3H and (CF3SO2)2O [HNO3:CF3SO3H:(CF3-
SO2)2O molar ratio=1:0.12:0.06] for 5 days at 30 °C,
according to the previously described procedure for
b-octanitration of Zn and Ni(TDCPP) [10]. Intermedi-
ate formation of Ni[TPCP(NO2)8P] was observed by
visible spectroscopy, as shown by a shift of the Soret
peak of the starting metalloporphyrin from 412 to 470
nm. Then, CH3NO2 was removed by distillation under
vacuum and the resulting solid redissolved in
(CF3SO2)2O containing a small amount of CH3NO2
(5:1 v/v). The reaction mixture was then treated with
190 equivalents of red fuming HNO3 for 24 h at 20 °C.
In this medium, meta-nitration of the meso-(2,6-
dichloro)phenyl substituents of the metalloporphyrin
readily occurred leading to the dodecanitrated complex,
Ni[TDCm(NO2)P(NO2)8P] as a major product in a 36%
yield based on starting Ni(TDCPP) (Fig. 1).
aryl rings after nitration of the most reactive position
(ortho to a Cl substituent). As expected, the signal of
the para-hydrogens of the meso-aryl rings is a doublet
(J=8.8 Hz), while that of the meta-hydrogens is more
complex (mass if between 7.77 and 7.85 ppm, 4H)
because the Ni porphyrin complex exists as a mixture
of atropoisomers, due to a restricted rotation of the
meso-aryl substituents of the tetrapyrrole ring.
Ni[TDCm(NO2)P(NO2)8P] was also prepared by nitra-
tion of the b-pyrrole positions of Ni[meso-tetra-(3-ni-
tro-2,6-dichlorophenyl)porphyrin=TDCm(NO2)PP]
[11] by the HNO3–CF3SO3H–(CF3SO2)2O system de-
scribed recently [10]. This confirms the structure pro-
posed for the product obtained by one-pot nitration of
Ni(TDCPP).
Polynitration of Zn (TDCPP) by the same method
led to the corresponding Zn[TDCm(NO2)P(NO2)8P]
complex in a similar yield. The spectral characteristics
of this complex were very similar to those of
Ni[TDCm(NO2)P(NO2)8P] (Table 1).
Reduction of Ni[TDCm(NO2)P(NO2)8P] was studied
by cyclic voltammetry in CH2Cl2 containing 0.1 M
Bu4NPF6. Four successive, reversible one-electron re-
ductions of this complex were observed at +470, +
110, −420 and −555 mV (vs. saturated calomel
electrode: SCE). By comparison, Ni(TDCPP) was only
able to undergo a one-electron reduction at a much
more negative potential (−1215 mV) [12] (Table 2).
Under identical conditions, the corresponding Zn com-
plex, Zn[TDCm(NO2)P(NO2)8P], underwent four suc-
cessive, reversible one-electron reductions at +370, 0,
−635 and −820 mV, whereas Zn(TDCPP) was only
This complex was characterized by a red shifted
Soret peak at 466 nm (m=143 cm−1 mM−1) and a and
b bands at 652 (7.8) and 605 (9.8) nm, as expected for
a b-octanitro Ni(II)porphyrin [10]. Its mass spectrum
(MALDI-TOF) was in agreement with the proposed
structure, with a molecular cluster at m/z=1485.7,
whose isotopic distribution well corresponds to that
calculated for NiC44H8N16O24Cl8. Its elemental analy-
sis, after lyophilization in a CH2Cl2–benzene mixture,
was in agreement with a NiC44H8N16O24Cl8, C6H6, 0.5
1
CH2Cl2 formula. Finally, its H-NMR spectrum clearly
showed the loss of all the b-hydrogens of Ni(TDCPP)
and the appearance of two sets of signals at 8.31 (4H,
doublet, J=8.8 Hz) and 7.77–7.85 (4H) ppm corre-
sponding to the two hydrogens remaining on the meso-
Table 1
Spectral characteristics of Ni and Zn[TDCm(NO2)P(NO2)8P]
a
c
Complex
Visible spectrum u m in nm (m in cm−1
1H-NMR bl (ppm)
Mass spectrum molecular
ion m/z
mM−1
)
Ni[TDCm(NO2)- 466, (143)
P(NO2)8P]
Zn[TDCm(NO2)- 463, (163)
P(NO2)8P]
605, (10)
600, (13)
652, (7.8)
642, (9)
8.31 (d, J=8.8 Hz,4H para) 7.77–7.85 (m,
4H, meta)
8.3 (d, J=8.7 Hz,4H para) 7.71–7.88 (m, 4H, 1493.6
meta)
1485.7
a In CH2Cl2.
b In CDCl3.
c MS (MALDI-TOF): position of the most intense peak of the isotopic cluster corresponding to the molecular ion.

524
M. Palacio et al. / Journal of Organometallic Chemistry 643–644 (2002) 522–524
Table 2
positive potentials led to starting Ni[TDCm(NO2)-
P(NO2)8P] in a quantitative yield. The radical anion of
the Ni polynitroporphyrin is remarkably stable at r.t.
for hours even in the presence of dioxygen, in contrast
to what was observed for the radical anion of Ni(TD-
CPP) which was only produced at very low redox
potentials under highly controlled anaerobic conditions
[13]. As far as the extreme redox properties of Zn and
Ni polynitroporphyrins are concerned, it is noteworthy
that the four-electron reduced species of Ni[TDCm-
(NO2)P(NO2)8P] was obtained at a much higher redox
potential, −555 mV, than the one-electron reduced
species of Ni(TDCPP) (−1215 mV, see Table 2). The
structure and chemical properties of the two-, three-
and four-electrons reduced species derived from Zn and
Ni[TDCm(NO2)P(NO2)8P] are currently under study.
Redox potentials for the successive, reversible one-electron reductions
of Ni and Zn[TDCm(NO2)P(NO2)8P]
a
Complex
Redox potentials (mV vs. SCE)
First
Second Third Fourth
One-electron reductions
Ni(TDCPP)
Ni[TDCm(NO2)P(NO2)8P] +470
Zn(TDCPP)
Zn[TDCm(NO2)P(NO2)8P] +370
−1215 n.o.
+110
−1285 n.o.
n.o.
−420 −555
n.o. n.o.
−635 −820
n.o.
0
a Conditions:
1 mM complex in CH2Cl2 containing 0.1 M
Bu4NPF6; sweep rate: 50 mV s−1; n.o., reduction not observed in the
electroactivity domain of CH2Cl2.
able to undergo a one-electron reduction at −1285 mV
[12] (Table 2). These results illustrate the remarkable
properties of the dodecanitro complexes as electron
reservoirs. They exhibit an unusually high redox poten-
tial for the first reversible reduction of the metallopor-
phyrin, the values observed for the Ni and Zn
[TDCm(NO2)P(NO2)8P] complexes (+470 and +370
mV, respectively) being the highest reduction potentials
described so far for Ni and Zn porphyrins. Introduc-
tion of the twelve nitro substituents thus led to a
positive shift of the reduction potentials of Zn(DCPP)
and Ni(TDCPP) of +1615 and +1685 mV, respec-
tively. Therefore, it was particularly easy to obtain the
radical anions derived from the one-electron reduction
of the Zn(II) and Ni(II) polynitroporphyrin complexes,
either by preparative electrochemistry in CH2Cl2 at
room temperature (r.t.), or by chemical reduction using
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