Acid-base and spectroelectrochemical properties of doubly N-confused porphyrins

Article abstract of DOI:10.1021/ic000441j

The cis-doubly N-confused porphyrin, H₂N₂CP, containing two adjacent confused pyrrole rings has been investigated from the point of view of its acid-base and electrochemical behavior in dichloromethane. This novel porphyrin isomer can form two metal-carbon bonds in the central core, stabilizing metal ions in unusually high oxidation states. Furthermore, the two outside N-pyrrole atoms remain available for acid-base and specific solvent interactions. Protonation of the pyrrole N atoms proceeds according to two successive steps, while only a single deprotonation step has been observed in the presence of bases. Similarly, in the case of the silver and copper complexes the protonation and deprotonation of the outer pyrrole rings have been detected, confirming the structure of the metalated species as M^III-HN₂CP. The electrochemical reduction of the metal ions (III/II redox process) and oxidation of the macrocycle ring have been detected respectively at -0.9 and 1.4 V based on spectroelectrochemical measurements in conjunction with the acid/base equilibrium studies. Additional waves observed around -0.5 and 1.3 V have been assigned to redox processes involving water molecules associated with the doubly N-confused porphyrins.

Full text of DOI:10.1021/ic000441j

2020
Inorg. Chem. 2001, 40, 2020-2025
Acid-Base and Spectroelectrochemical Properties of Doubly N-Confused Porphyrins
Koiti Araki,*,† Herbert Winnischofer,† Henrique E. Toma,† Hiromitsu Maeda,‡
Atsuhiro Osuka,‡ and Hiroyuki Furuta‡
Instituto de Qu´ımica, Universidade de Sa˜o Paulo, C. Postal 26077, CEP 05513-970,
Sa˜o Paulo (SP), Brazil, and Department of Chemistry, Graduate School of Science, Kyoto University,
Kyoto 606-8502, Japan
ReceiVed April 21, 2000
The cis-doubly N-confused porphyrin, H2N2CP, containing two adjacent confused pyrrole rings has been investigated
from the point of view of its acid-base and electrochemical behavior in dichloromethane. This novel porphyrin
isomer can form two metal-carbon bonds in the central core, stabilizing metal ions in unusually high oxidation
states. Furthermore, the two outside N-pyrrole atoms remain available for acid-base and specific solvent
interactions. Protonation of the pyrrole N atoms proceeds according to two successive steps, while only a single
deprotonation step has been observed in the presence of bases. Similarly, in the case of the silver and copper
complexes the protonation and deprotonation of the outer pyrrole rings have been detected, confirming the structure
of the metalated species as MIII-HN2CP. The electrochemical reduction of the metal ions (III/II redox process)
and oxidation of the macrocycle ring have been detected respectively at -0.9 and 1.4 V based on
spectroelectrochemical measurements in conjunction with the acid/base equilibrium studies. Additional waves
observed around -0.5 and 1.3 V have been assigned to redox processes involving water molecules associated
with the doubly N-confused porphyrins.
Introduction
Pd2+ complexes have been reported.11,12 This porphyrin isomer
has two adjacent, inverted pyrrole rings (Figure 1), such that it
coordinates metal ions through two nitrogens and two carbon
atoms. Functional density calculations have suggested the
possibility of the N2CP ligand to stabilize metal ions in the 4+
oxidation state.13 It is important to notice that the outside
N-pyrrole atoms are available for acid-base and specific solvent
interactions, reflecting in a direct, strong influence on the
porphyrin properties. In order to improve our understanding on
the chemistry of N-confused porphyrins, we carried out a
detailed acid-base and spectroelectrochemical investigation of
the free-base species, H2N2CP, and its copper and silver
complexes, i.e., [CuIIIHN2CP] and [AgIIIHN2CP], respectively
(Figure 1).
The preparation and properties of new porphyrin analogues
have been recently focused on the literature.1 In particular, the
N-confused porphyrin (NCP) was the first reported “true
porphyrin isomer” containing one inverted pyrrole ring.2,3 The
presence of an inner carbon and outward-pointing nitrogen atoms
conferred unique properties to this ligand.4-10 For example, it
can act as a dianionic or trianionic macrocyclic ligand, generat-
ing neutral complexes with Ni2+ and Ag3+ ions.4,7,8 The Ni2+
complex can be easily oxidized to the Ni3+ species, but the
reduction of the AgIIINCP complex in CH2Cl2 did not lead to
the Ag2+ complex. Instead, there is evidence of reduction of
the macrocyclic ligand,4 confirming the high capability of NCP
to stabilize metal ions in unusually high oxidation states.
Recently, the synthesis and structural characterization of “cis-
doubly N-confused porphyrin (N2CP)” and its Cu3+, Ag3+, and
Experimental Section
2-Ethoxy-5,10,15,20-tetrakis(pentafluorophenyl)-3,7-diaza-21,22-di-
carbaporphyrin (H2N2CP) was previously obtained11 by the reaction of
N-confused 5-(pentafluorophenyl)dipyrromethane, pentafluorobenzal-
dehyde, and Bu4NBr in 0.5% EtOH-CHCl3, in the presence of BF3-
OEt2 at room temperature, after oxidation with DDQ and purification
by silica gel column chromatography. [AgHN2CP] and [CuHN2CP]
were obtained by metalation of free-base H2N2CP with silver acetate
and copper acetate, respectively. These reactions were carried out at
room temperature for 8 h (Ag) or 1 h (Cu).
The UV-vis spectra were recorded on a Hewlett-Packard model
8453 diode array spectrophotometer. The solutions of H2N2CP, [AgHN2-
CP], and [CuHN2CP] and their protonated and deprotonated derivatives
were stable at room temperature. A cutoff filter below 280 nm was
used to prevent the photolysis of dichloromethane (DCM) and
consequent formation of HCl, which interfered in the measurements.
* Author to whom correspondence should be addressed. E-mail:
koiaraki@iq.usp.br. Phone: 55 11 818-3887. Fax: 55 11 815-5579.
† Universidade de Sa˜o Paulo.
‡ Kyoto University.
(1) Sessler, J. L. Angew. Chem., Int. Ed. Engl. 1994, 33, 1348.
(2) Furuta, H.; Asano, T.; Ogawa, T. J. Am. Chem. Soc. 1994, 116, 767.
(3) Chmielewski, P. J.; Latos-Grazynski, L.; Rachlewicz, K.; Glowink,
T. Angew. Chem., Int. Ed. Engl. 1994, 33, 779.
(4) Furuta, H.; Ogawa, T.; Uwatoko, Y.; Araki, K. Inorg. Chem. 1999,
38, 2676.
(5) Ishikawa, Y.; Yoshida, I.; Akaiwa, K.; Koguchi, E.; Sasaki, T.; Furuta,
H. Chem. Lett. 1997, 453.
(6) Ariga, K.; Kunitake, T.; Furuta, H. J. Chem. Soc., Perkin Trans. 2
1996, 667.
(7) Chmielewski, P. J.; Latos-Grazynski, L. Inorg. Chem. 1997, 36, 840.
(8) Chmielewski, P. J.; Latos-Grazynski, L.; Glowiak, T. J. Am. Chem.
Soc. 1996, 118, 5690.
(9) Chmielewski, P. J.; Latos-Grazynski, L. J. Chem. Soc., Perkins Trans.
2 1995, 503.
(10) Furuta, H.; Ishizuka, T.; Osuka, A.; Ogawa, T. J. Am. Chem. Soc.
1999, 121, 2945.
(11) Furuta, H.; Maeda, H.; Osuka, A. J. Am. Chem. Soc. 2000, 122, 803.
(12) Furuta, H.; Maeda, H.; Osuka, A.; Yasutake, M.; Shinmyozu, T.;
Ishikawa, Y. Chem. Commun. 2000, 1143.
(13) Furuta, H.; Maeda, H.; Osuka, A. J. Org. Chem. 2000, 65, 4222.
10.1021/ic000441j CCC: $20.00 © 2001 American Chemical Society
Published on Web 03/27/2001

Properties of Doubly N-Confused Porphyrins
Inorganic Chemistry, Vol. 40, No. 9, 2001 2021
Figure 1. Structures of (A) H2N2CP and (B) MIIIHN2CP.
Figure 2. (A) Changes in the spectrum of free-base H2N2CP in DCM
(2.1 × 10-5 M) after addition of successive aliquots of less than
stoichiometric amount of trifluoroacetic acid. (B) Changes in the
spectrum of the protonated species obtained after addition of more
trifluoroacetic acid.
The electrochemistry was carried out in dichloromethane using a
computer-driven PAR model 283 potentiostat/galvanostat and model
270/250 Research Electrochemistry Software version 4.30. A conven-
tional three-electrode cell consisting of a platinum disk working
electrode, Ag/Ag+ (0.010 M) reference electrode in acetonitrile, and a
coiled platinum wire auxiliary electrode was employed. Dichlo-
romethane was distilled over CaH2 immediately before use. TBAPF6
Aldrich was used as electrolyte without further purification. All
potentials were converted to the SHE scale by adding 0.503 V to the
experimental data.
Table 1. Absorption Bands (nm) of the Neutral, Protonated,
Deprotonated Free-Base and Metalated H2N2CP
H2N2CP
345, 424, 467, 580 (sh) 623, 670, 717 (sh)
351, 455, 527 (sh), 594, 662, 757
361, 386 (sh), 464, 514, 599, 655, 698, 759
343, 406, 454 (sh), 480, 510, 600, 660, 721
366, 419 (sh) 440, 480, 517, 573, 620, 674
367, 469, 520, 570, 614, 660, 764
367, 428, 474, 498, 560, 620, 678
367, 417 (sh), 440, 480 (sh), 519 (w), 567, 612, 674
368, 467, 520, 566, 612, 646, 765
H3N2CP+
H4N2CP2+
HN2CP-
AgHN2CP
AgH2N2CP+
AgN2CP-
CuHN2CP
CuH2N2CP+
CuN2CP-
The spectroelectrochemical data were collected using a previously
described homemade thin-layer cell4,14,15 and a PAR model 173
potentiostat/galvanostat in parallel with a HP-8453A spectrophotometer.
Results and Discussion
355, 425, 471, 498, 552, 607, 687
The H2N2CP molecule is an isomer of perfluorinated meso-
tetraphenylporphyrin compound, exhibiting two adjacent in-
verted pyrrole rings. In this case, a fully conjugated π structure
is not possible, in contrast with porphyrins and analogous
N-confused porphyrin species containing only one inverted
pyrrole ring. Its electronic spectrum exhibits three major sets
of bands in the 300-800 nm range, e.g., at 345, 424, and 670
nm, as one can see in Figure 2, keeping some similarity with
the N, Soret (B), and Q bands in the porphyrin analogues.
The spectral data for the several species are shown in Table
1, for comparison purposes.
The free-base H2N2CP species can be readily protonated, as
observed by introducing small amounts of HCl or trifluoroacetic
acid vapor into the DCM solution. However, since there are
two pyrrolic nitrogen atoms available for protonation, a careful
procedure is necessary in order to distinguish between the single
and double protonation steps. To address this point, aliquots of
trifluoroacetic acid (TFA, diluted in DCM), containing less than
the stoichiometric amount of acid to monoprotonate the doubly
N-confused porphyrin, were added to the H2N2CP solution, and
the electronic spectra recorded in the visible-UV region. As
the amount of TFA increased, the characteristic absorption bands
at 345 and 424 nm for free-base species were shifted to 351
and 454 nm, while the composed Q type band at 670 nm was
split into two main bands at 662 and 757 nm (Figure 2A).
Simultaneous isosbestic points were observed at 353, 433, 553,
and 683 nm, confirming the involvement of only two species
in equilibrium. Addition of more TFA aliquots enhanced the
absorption bands at 464 and 757 nm and led to the rise of a
new band at 514 nm (Figure 2B). Simultaneous isosbestic points
at 396, 449, and 690 nm indicated the presence of a second
acid/base equilibrium process. The spectra obtained after
addition of an excess of TFA or concentrated sulfuric acid were
almost identical to that shown in Figure 2B, confirming that no
further protonation is possible. Consequently, one can infer that
the protonation of free-base H2N2CP occurs in two successive
steps involving the two available pyrrole N atoms. This is
analogous to the acid-base behavior of the NCP molecule in
which two protonation steps were also observed. The basicity
of N2CP seems to be much higher, however, because in order
to protonate NCP an excess of TFA was required.2 In contrast
with the Lash et al.16 results, the absorption spectra of the
samples obtained by using HCl or TFA were identical, showing
no modulation effect induced by the counterion.
(14) Araki, K.; Toma, H. E. J. Photochem. Photobiol., A 1994, 83, 245.
(15) Toma, H. E.; Araki, K.; Silva, E. O. Monatsh. Chem. 1998, 129, 975.

2022 Inorganic Chemistry, Vol. 40, No. 9, 2001
Araki et al.
Figure 3. Changes in the spectrum of H2N2CP (2.1 × 10-5 M in DCM)
as small aliquots of triethylamine were added into the solution.
The protonation sites were investigated by recording 1H NMR
spectra in the presence of 1 equiv and an excess (20 equiv) of
TFA. The two broad signals at 15.28 and 13.37 ppm were
assigned to the outer NH protons, strongly suggesting that the
first protonation occurs at the peripheral pyrrole N atom. The
addition of an excess of acid sharpened and shifted those peaks
to 12.70 and 12.06 ppm. The possible association of the
conjugate base with the protonated doubly N-confused porphyrin
species was investigated, using CHCl2CO2H (10 equiv) as probe.
In this case, the peak of the dichloroacetate was found at 5.73
ppm, while the inner NH and CH peaks were found at 4.62
(NH), 2.39, and 1.48 ppm, respectively. The NMR signal of
the counteranion of the protonated meso-tetrakis(3′,5′-di-tert-
butylphenyl)porphyrin sits above the porphyrin ring and was
shifted around 3 ppm to higher field,16 while the signal of our
probe ion remained almost unchanged. This is evidence that
the dichloroacetate (or TFA) should be around the periphery of
N2CP ring rather than above.
Figure 4. Spectra in DCM of the starting M-HN2CP (solid line), the
protonated M-H2N2CP+ (dotted line), and the deprotonated M-N2-
CP- (dashed line) species obtained after the addition of TFA and
triethylamine, respectively, to (A) AgHN2CP and (B) CuHN2CP.
N-confused silver porphyrin complex, even in the presence of
a 10 times excess of TFA.
The [AgHN2CP] solution in DCM (Figure 4A) exhibits major
absorption bands at 366, 440, and 620 nm. Addition of TFA
shifted the Soret type band to 469 nm, with significant changes
in all other absorption bands. The spectrum of the deprotonated
species generated by addition of triethylamine exhibited its
characteristic bands at 367, 428, 560, 620, and 678 nm, in
addition to the enhancement of the absorption shoulder at 500
nm (Figure 4A). The [CuHN2CP] derivative exhibited a very
similar spectral behavior (Figure 4B), displaying the charac-
teristic absorption bands at 367, 440, and 612, while the
protonated species absorption bands were found at 368, 467,
and 612 nm. The absorption bands of the deprotonated species
were found at 355, 425, and 607 nm, in addition to the
enhancement of the shoulders at 471 and 498 nm.
As shown above, the acid-base and 1H NMR studies
confirmed the presence of one outer N atom in M-HN2CP and
two N atoms available for protonation in the free-base species,
consistent with the structure shown in Figure 1.
Cyclic Voltammetry and Spectroelectrochemistry. (a)
H2N2CP. The cyclic voltammograms (Figure 5) of H2N2CP,
[AgHN2CP], and [CuHN2CP] exhibited four sets of waves
around 1.5, 1.3, -0.5, and -1.0 V, as denoted by IV, I, II, and
III, respectively. The assignment was carried out based on the
cyclic voltammetric behavior after addition of excess of TFA
or triethylamine, and on the spectroelectrochemistry results
detailed below.
It is interesting to note that H2N2CP can undergo deproto-
nation reactions by addition of mild bases such as triethylamine,
or strong bases like tetrabutylammonium hydroxide and sodium
ethoxide. But, whatever the base used in the reaction, the final
spectrum was exactly the same (Figure 3). The deprotonation
reaction leads to the decay of the N band at 345 nm and
enhancement of the Soret type band at 480 nm, and also at 510
nm (shoulder). The Q type bands were observed at 600, 660,
and 721 nm. When the bases were added in a stepwise manner,
the spectral changes were consistent with a single equilibrium,
in contrast with the protonation process which occurred in two
successive steps generating the H3N2CP+ and H4N2CP2+ species.
1
In order to determine the amount of H+ ions involved, the H
NMR spectrum of a H2N2CP solution was recorded after
addition of an excess of Proton Sponge, revealing the band
assigned to the inner N-H proton. Therefore, the deprotonation
reaction involved only the removal of the outer N-H group.
The acidity of the inner N-H bond should be very low,
presumably lower than that of methanol, since even sodium
methoxide was unable to react with it.
The silver and copper complexes of H2N2CP also are prone
to react with acids and bases, generating the protonated and
deprotonated species, as can be seen in Figure 4. The protonation
of the inner carbon atoms was ruled out on the basis of the 1H
NMR, because only a broad peak at 15 ppm, assigned to the
outer N-H protons, was found in the spectrum of doubly
The voltammogram of H2N2CP is shown in Figure 5A. When
trifluoroacetic acid was added to the solution, wave I shifted
anodically from 1.24 to about 1.4 V generating a broad oxidation
wave with maximum at 1.46 V. Concomitantly, wave II was
intensified and shifted from -0.5 to 0.0 V, while wave III was
shifted from -1.04 to -0.20 V and a new wave appeared at
-1.17 V. In contrast, the addition of triethylamine did not
(16) Lash, T. D.; Richter, D. T.; Shiner, C. M. J. Org. Chem. 1999, 64,
7973.

Properties of Doubly N-Confused Porphyrins
Inorganic Chemistry, Vol. 40, No. 9, 2001 2023
Figure 5. Cyclic voltammograms of 2.0 mM solutions of H2N2CP,
AgHN2CP, and CuHN2CP in dichloromethane with TBAPF6 as
electrolyte.
change the voltammetric behavior in the 0.0 to -1.3 V range.
Nevertheless, waves I and IV were shifted from 1.24 and 1.45
V to 0.79 and 1.24 V, respectively.
Figure 6. Spectroelectrochemistry of a 1.0 × 10-5 M H2N2CP solution
in acetonitrile, TBAPF6 0.10 M. In all experiments the potential was
stepped and the spectral changes registered as a function of the time:
(A) 0.50-1.20 V; (B) 1.20-1.44 V; and (C) 1.44-2.10 V.
In view of the similarity of the voltammetric behavior in the
absence (shown in Figure 5A) and presence of triethylamine,
the cathodic wave II was assigned to the reduction of small
amounts of water present in solution, generating OH- ions.
Consequently, wave III was assigned to the first ring reduction
of the HN2CP- species. The composite wave I of H2N2CP
(Figure 5A) was assigned to the first ring oxidation (∼1.24 V)
of the neutral H2N2CP species coupled with water oxidation,
while wave IV (1.45 V) was assigned to the second monoelec-
tronic ring oxidation process. The oxidation reactions of the
deprotonated species were found in less positive potentials, at
0.79 and 1.24 V, respectively. Upon biprotonation, the first
oxidation process (wave I coupled with water oxidation) shifted
to more positive potentials, generating a broad irreversible wave
with maximum at 1.46 V. As expected, the addition of TFA
shifted wave II to ∼0.0 V and the first reduction of H4N2CP2+
species was found at -0.20 V. The new reversible wave
observed at -1.17 V was attributed to the second reduction of
H4N2CP2+ species.
Unfortunately, the complete removal of water could not be
achieved. However, the addition of water to the solution led to
the intensification of waves I and II, while waves III and IV
became less reversible because of the enhancement of the
corresponding coupled chemical reactions. This was particularly
effective for the water reduction reaction by the reduced
metalated species, reinforcing our assignment.
The spectroelectrochemistry of H2N2CP, in DCM solution
containing 0.1 M TBAPF6, is shown in Figure 6. When the
potential was stepped from 0.5 to 1.20 V (Figure 6A), the
absorption bands shifted to 351, 459 (sh), and 482 nm, while
the absorbance at 640 nm decreased slightly in parallel with
the rise of a band in 750 nm. These spectral changes are very
similar to those observed for the first protonation of free-base
to H3N2CP+ species (Figure 2A). By applying 1.44 V, the rise
of a band at 512 nm, accompanied by several other incipient
changes, very similar to those observed for the protonation of
H3N2CP+ to H4N2CP2+ species, was observed. Therefore, at
1.3-1.44 V (wave I) the electrochemical process should be
generating H+ ions, in order to account for the observed
spectroelectrochemical changes. Since no wave was observed
in the cyclic voltammograms of the pure electrolyte solution,
we suggest that the H2N2CP species has water molecules
associated with it, probably via hydrogen bonding with the
nitrogen atoms of the inverted pyrrole rings. Oxidation of water
is expected at 1.3 V and above, generating O2 and H+,
responsible for the successive protonation reactions observed
in the spectroelectrochemistry.
According to the spectroelectrochemical measurements, the
oxidation of H2N2CP, or, more exactly, its diprotonated form,
occurs only at more positive potentials. When the potential was
stepped from 1.44 to 2.10 V (Figure 6C), the bands at 365,
466, 510, and 740 nm faded, giving rise to two broad bands
around 323 and 610 nm.
After the oxidation step, the solution was renewed in order
to investigate the spectroelectrochemical behavior at the cathodic
region. When the potential was stepped from 0.00 to -0.70 V
(Figure 7), the absorbance changes were identical to that for
the formation of the HN2CP- species (Figure 3). Consequently,
the redox process corresponding to wave II can be assigned to
the reduction of water molecules generating H2 and OH- ions,
responsible for the removal of protons from the doubly
N-confused porphyrin ring.

2024 Inorganic Chemistry, Vol. 40, No. 9, 2001
Araki et al.
Figure 8. Spectroelectrochemistry of a 1.5 × 10-5 M solution of
AgHN2CP in DCM, [TBAPF6] ) 0.10 M, 25 °C: (A) potential stepped
from 1.00 to 1.60 V; (B) further spectral changes as a function of the
time at 1.60 V.
Figure 7. Spectroelectrochemistry of a 8.0 × 10-6 M H2N2CP solution
in acetonitrile, TBAPF6 0.10 M. In all experiments the potential was
stepped and the spectral changes registered as a function of the time:
(A) 0.00 to -0.70 V and (B) -0.70 to -1.20 V.
triethylamine shifted the redox waves in a way similar to that
observed for the free base. The ring reduction of the protonated
[CuH2N2CP]+ was shifted to -0.5 V, and the CuIII/IIN2CP
potential remained almost unchanged at -0.9 V, but wave III
remained unchanged at 1.3 V. The addition of triethylamine
did not change the voltammogram in the negative region, but
the oxidation potential was lowered to 1.4 V. A similar behavior
was observed for the silver such that the ring oxidation of the
[AgN2CP]- species was found at 1.0 V, while the ring reduction
of [AgH2N2CP]+ species was found at -0.6 V. Unfortunately
the corresponding silver ion reduction could not be determined
because of the appearance of a strong catalytic cathodic current
next to the estimated reduction potential.
The spectroelectrochemical behavior of the silver complexes
was very similar to that observed for the free-base species. After
stepping the potential from 1.00 to 1.60 V, the spectral changes
(Figure 8A) were very similar to those observed for the
protonation reaction (Figure 4). This result suggests that the
oxidation of adventitious water molecules generates O2 and H+
ions and protonates the silver and copper complexes, shifting
their potential to the more positive side. In addition, the oxidized
species acts as catalyst for that coupled oxidation reaction,
precluding the observation of the first monoelectronic ring
oxidation in its normal potential. After that, a decrease in the
absorbance at 470 nm and in the bands around 600 nm,
concomitantly with the increase in the absorbance at 310 and
425 nm, was observed (Figure 8B). These changes were
assigned to the one electron ligand oxidation of protonated
[AgH2N2CP]+ species.
Finally, when the potential was stepped from -0.70 to -1.20
V, the strong band at 480 nm and the weaker bands below and
above 500 nm merged, giving rise to very broad spectral features
(Figure 7B). These changes are consistent with the reduction
of the HN2CP- species, generating a π radical anion. No further
reduction processes could be observed at more negative
potentials due to the exceedingly high currents involved.
The electrochemical behavior described above for H2N2CP
is quite different from that observed for NCP.4 In this case three
reversible oxidation processes after an irreversible oxidation
reaction and an irreversible reduction wave were observed in
the same experimental conditions. Interestingly, no one of those
electrochemical processes could be related to reactions involving
water molecules as for doubly N-confused porphyrin.
(b) AgHN2CP and CuHN2CP. The voltammetric profiles
of AgHN2CP and CuHN2CP were similar, exhibiting waves II
and III at -0.50 and -1.08 V, respectively. Interestingly, the
effect of the coordination of Ag3+ and Cu3+ ions on the redox
potentials seems to be rather small, in contrast with the typical
behavior of metalloporphyrins,17 in which case a significant shift
of the redox processes is usually associated with the metalation
of the porphyrin ring. Wave III is less reversible than in the
H2N2CP, suggesting the occurrence of a chemical reaction
coupled with the electrochemical process. In fact, the addition
of small amounts of water made this wave completely irrevers-
ible, exhibiting a cathodic wave whose intensity increased as a
function of the water content.
The reduction of the sample by stepping the potential from
0.00 to -0.70 V led to spectral changes (Figure 9A) very similar
to those observed in the deprotonation reaction of AgHN2CP,
indicating that the electrochemical process at -0.50 V (wave
II) is associated with the reduction of water. Further reduction
at -1.00 V led to minor changes (Figure 9B) in the spectrum,
a quite different spectroelectrochemical behavior when com-
pared with the free base in the same potential range. We believe
that the increase in the absorption at 318 nm and the decrease
In the positive potential side, a broad wave I with features
that suggested the occurrence of an electrochemical process with
coupled chemical reaction was observed in the CV’s of the silver
and copper complexes. The first ring reduction wave was
observed at -1.09 V besides an additional wave around -0.9
V, assigned to MIII/IIN2CP couple. The addition of TFA or
(17) Kadish, K. M. Prog. Inorg. Chem. 1986, 34, 435.

Properties of Doubly N-Confused Porphyrins
Inorganic Chemistry, Vol. 40, No. 9, 2001 2025
Figure 9. Spectroelectrochemistry of a 2.0 × 10-5 M solution of
AgHN2CP in DCM, [TBAPF6] ) 0.10 M, 25 °C: (A) potential stepped
from 0.00 to -0.70 V; (B) further spectral changes as a function of
the time at -1.00 V.
at 366, 474, 617, and 676 nm can be assigned to the AgIII/IIN2CP
redox reaction, in contrast with AgIIINCP.4 This is followed by
wave III, which probably is associated with the bielectronic
reduction of the macrocycle ring, by comparison with free-base
doubly N-confused porphyrin electrochemistry. The redox
potential of about -0.95 V for the Ag(III/II) redox pair
confirmed the high degree of stabilization of the metal ion in
the unusual +3 oxidation state.
Figure 10. Spectroelectrochemistry of a 1.3 × 10-5 M solution of
CuHN2CP in DCM, [TBAPF6] ) 0.10 M, 25 °C: (A) potential stepped
from 0.90 to 1.50 V and changes associated with the protonation of
CuHN2CP; (B) oxidation of CuH2N2CP+ species at 1.50 V; and (C)
spectral changes after stepping the potential from 0.00 to -0.70 V.
The changes were followed as a function of the time.
The copper complex exhibited a behavior similar to that of
AgHN2CP, as shown in Figure 10. When the potential was
stepped from 0.90 to 1.50 V, the 439 nm band faded and an
absorption band with maxima at 467 and 481 and a broad band
at 785 appeared, concomitantly with the decrease in the
absorbance at 608 and 674 nm (Figure 10A). This process is
analogous to that observed in the protonation of CuHN2CP to
CuH2N2CP+ and was attributed to the oxidation of water. A
second process was detected, in which the 467 and 481 nm
bands decreased, leading to an increase of the absorbance at
325 and 690 nm, as shown in Figure 10B. This process was
assigned to the oxidation of CuH2N2CP+ species. The reduction
of the sample by stepping the potential from 0.00 to -0.70 V
led to spectral changes (Figure 10C) very similar to those for
the deprotonation reaction of CuHN2CP to CuN2CP-, supporting
the occurrence of water reduction. No further changes could be
observed at more negative potentials because of current
overflow.
removal of the sole outer H+ ion. A similar behavior was
observed for the silver and copper complexes. The spectroelec-
trochemistry in conjunction with the acid/base equilibrium
studies showed that the first reduction and oxidation waves
observed in the cyclic voltammograms (starting at 0.5 V)
involves small amounts of water, probably associated with the
outer N and N-H atoms of the inverted pyrrole rings. The
exceptional influence of adventitious water on the electrochemi-
cal and spectroelectrochemical behavior was unexpected, con-
sidering the special care in using a dry solvent (DCM was
distilled over CaH2 immediately before use) and the use of a
sealed electrochemical cell, in addition to our previous studies
on the electrochemistry of NCP and AgNCP. Consequently, the
interaction between the water molecules and doubly N-confused
porphyrins seems to be quite strong influencing the redox
reactions of water. The monoelectronic oxidation waves were
found around 1.4 V and the reduction of the metal ions around
-0.9 V. The more intense wave III in the CV’s was assigned
to the reduction of the doubly N-confused ligand.
Final Remarks
The H2N2CP molecule can be reversibly protonated in two
successive steps at the pyrrole N atoms. From the results
obtained with mild and strong bases and 1H NMR spectroscopy
results, the deprotonation process should involve only the
Acknowledgment. The financial support of the Brazilian
agencies FAPESP and CNPq is gratefuly acknowledged.
IC000441J