The redox chemistry of copper tetraphenylporphyrin revisited

Article abstract of DOI:10.1142/S108842461250112X

Mixing Cu(II)Cl₂ with free base tetraphenylporphyrin ((H ₂(TPP)) in dry deaerated 1,2-dichloroethane surprisingly yields a mixture of Cu(II)(TPP) and Cu(III)(TPP). The later is long-lived in the absence of water and slowly decomposes within days to yield Cu(II)(TPP). Furthermore, in the presence of air, a slightly different Cu^III porphyrin species forms, presumably, Cu(III)(TPP)^?+, in which the Q-band is subject to a 10 nm red shift. The new Cu(III)(TPP) and Cu(III)(TPP) ^?+ differ in terms of absorption, fluorescence, electron paramagnetic resonance (EPR), and X-ray photoemission spectroscopy (XPS) when compared to the well-characterized Cu(II)(TPP), Cu(II)(TPP)^?+, and Cu(II)(TPP)²⁺ species.

Full text of DOI:10.1142/S108842461250112X

Journal of Porphyrins and Phthalocyanines
J. Porphyrins Phthalocyanines 2012; 16: 1124–1131
DOI: 10.1142/S108842461250112X
Published at http://www.worldscinet.com/jpp/
The redox chemistry of copper tetraphenylporphyrin revisited
Magal Saphier*a,b, TovaYifrahc, Israel Zilbermanna,c, Oshra Saphierb, Alexander I.
Shamesd, Natalya Froumine, Dan Meyersteinc,f and Dirk M. Guldi*g
a Chemistry Dept., Nuclear Research Centre Negev, Beer-Sheva, Israel
b Sami Shamoon College of Engineering, Beer-Sheva, Israel
c Chemistry Dept., Ben-Gurion University of the Negev, Beer-Sheva, Israel
d Physics Dept., Ben-Gurion University of the Negev, Beer-Sheva, Israel
e Materials Engineering Dept., Ben-Gurion University of the Negev, Beer-Sheva, Israel
f Biological Chemistry Dept., Ariel University Center of Samaria, Ariel, Israel
g Chemistry and Pharmaceutical Dept., Friedrich-Alexander-Universitat Erlangen-Nuernberg-Germany
Received 13 March 2012
Accepted 5 May 2012
ABSTRACT: Mixing Cu(II)Cl2 with free base tetraphenylporphyrin ((H2(TPP)) in dry deaerated
1,2-dichloroethane surprisingly yields a mixture of Cu(II)(TPP) and Cu(III)(TPP). The later is long-
lived in the absence of water and slowly decomposes within days to yield Cu(II)(TPP). Furthermore,
in the presence of air, a slightly different CuIII porphyrin species forms, presumably, Cu(III)(TPP)•+,
in which the Q-band is subject to a 10 nm red shift. The new Cu(III)(TPP) and Cu(III)(TPP)•+ differ in
terms of absorption, fluorescence, electron paramagnetic resonance (EPR), and X-ray photoemission
spectroscopy (XPS) when compared to the well-characterized Cu(II)(TPP), Cu(II)(TPP)•+, and Cu(II)
(TPP)2+ species.
KEYWORDS: tetraphenylporphyrin, copper, redox, spectroscopy.
central metal ion in metalloporphyrins is a cofactor of
INTRODUCTION
many enzymes [1]. Some of the copper(II)porphyrins
The porphyrins and their metal complexes play key
studied in the past have been fully characterized [2–7].
functions in many biological processes. The resulting
Gouterman et al. [3]. documented, for example, the
metalloporphyrins exist in a broad variety of plants and
luminescence spectra, excitation spectra, lifetime, and
animals and are involved in metabolic processes, such as
quantum yields of several Cu(II) porphyrins at 77 K.
hydroxylation, oxygen transport, electron transport, and
Important is the fact that these copper(II)porphyrins lack
oxidizing catalytic reactions [1]. Common to most of
the typical fluorescence features seen in closed-shell
these metalloporphyrins are copper and or iron cations.
metalloporphyrins. According to the literature [8–12],
Importantly, porphyrins facilitate the formation and
the oxidation of Cu(II)(TPP) by one electron is ligand
stabilization of a broad range unusual oxidation states.
centered and yields Cu(II)(TPP)•+. Inamo et al. [8] studied
Such metastable species are key intermediates in energy
the electron transfer reactions between Cu(II)(TPP)
sources for cells and in the entire biological system.
complexes and various oxidizing reagents in acetonitrile.
Synthetic metalloporphyrins are often studied as models
The oxidation reactions of copper(II)porphyrins
for biological systems and as catalysts. Copper(II) as the
complexes were shown to take place at the porphyrin
without impacting the formal oxidation state of copper,
that is, +2. An oxidation potential of 0.57 V vs. Fe/Fe+
*Correspondence to: Magal Saphier, email: magal_saphier@
yahoo.com, tel: +972 50-6244216, fax: +972-8-6567750; Dirk
was reported for the Cu(II)(TPP)/Cu(II)(TPP)•+ couple.
During the oxidation, a drastic lowering of intensity and
a blue shift of the Soret band and a broadening of the
M. Guldi, email: guldi@chemie.uni-erlangen.de, tel: +49 (0)-
9131-8527340, fax: +49 (0)-9131-8528307
Copyright © 2012 World Scientific Publishing Company

THE REDOX CHEMISTRY OF COPPER TETRAPHENYLPORPHYRIN REVISITED 1125
Q-band is discernable. Such spectral patterns evolved
wire. Tetra-butyl ammonium perchlorate (0.1 M) was
as characteristic fingerprints for the formation of
Cu(II)(TPP)•+ [11–12]. Photoinduced electron transfer
processes in fullerene-based electron donor–acceptor
conjugates with porphyrins and metalloporphyrins as
light harvester have been extensively studied in recent
years [13–16]. In these conjugates, porphyrins and
metalloporphyrins function as excited state electron
donors, while fullerenes serve as electron acceptors
due to their remarkable redox and structural properties.
The majority of the metalloporphyrins that were probed
in these conjugates are based on zinc porphyrins
[13–16], zinc porphyrins were as well used as excited
state electron donors in catalytic systems [17]. As
metalloporphyrins in the photoinduced electron transfer
processes mostly closed-shell, diamagnetic, metal ions
like cadmium(II), magnesium(II), and zinc(II), which
are all strongly fluorescent, are employed [18–22]. On
the contrary, the use of open-shell, paramagnetic centers,
such as copper(II), iron(II)(high-spin), maganese(II),
iron(III), or chromium(III) is scare. This difference
comes predominantly from the existence of triplet-
multiplet states due to the mixing of spin multiplicity of
the paramagnetic metal centers and the porphyrin. The
thrust of this work was to probe the nature of copper
complexes with tetraphenylporphyrins in aprotic media
under different conditions as a potential replacement for
Zn(TPP) as exited state electron donor in electron donor–
acceptor conjugates. Indeed, a new Cu(III)(TPP) was
prepared and characterized. The complex acted like a
closed-shell, diamagnetic metalloporphyrin and showed
strong fluorescence. Moreover, the oxidized states of the
new complex were studied using EPR, XPS, absorption
spectroscopy, and electrochemistry.
used as supporting electrolyte.
Spectroscopic studies
The UV-vis spectra were measured using an
Agilent 8453 diode array UV-vis Spectrophotometer.
Emission spectra were recorded with an SLM 8100
Spectrofluorometer. EPR spectra were recorded using
a Bruker EMX-220 digital X-band spectrometer. XPS
spectra were measured using an ESCALAB 250.
RESULTS AND DISCUSSION
Complexation
Dry anhydrous Cu(II)Cl2 reacted with tetrapheny-
lporphyrin H2(TPP) in organic non-polar solvents like
1,2 dichloroethane (DCE), dichloromethane (DCM) or
toluene in the presence of 0–5% dry acetonitrile, under
air, or under inert conditions. The reaction resulted
in a drastic color change, that is, from purple to green
(Fig. 1). In the presence of water, the new, green solution
turned red with an absorption spectrum in full agreement
with Cu(II)(TPP). The latter assignment is based on
data taken from the literature [3]. The dissolution of
anhydrous Cu(II)Cl2 and H2(TPP) in DCE under argon
atmosphere resulted in a novel complex with red shifted
absorption bands relative to the air exposed solution.
Figure 2 illustrates an experiment, in which upon
mixing Cu(II)Cl2 and H2(TPP) under Ar for 24 h, two
different species of copper metalloporphyrins, that is,
Cu(II)(TPP) and presumably Cu(III)(TPP), are present
EXPERIMENTAL
Materials
All chemicals used were of AR grade purchased
from Sigma/Aldrich. As a source for copper ions, dry
anhydrous CuCl2, CuCl and Cu(CH3CN)4PF6 salts were
used. Toluene, C2Cl2H4 (DCE), and CCl2H2 (DCM) in
the presence/absence of anhydrous CH3CN were used
as solvents. 5,10,15,20-tetraphenyl-21H,23H-porphyrin
(H2(TPP) was used as the porphyrin source.
Electrochemical studies
A 273 EG&G Potentiostat/Galvanostat was used. The
working electrode used in all of the experiments was a
glassy carbon electrode (A = 0.07 cm2), which was finely
polished after each experiment. The pseudo-reference
electrode was a silver wire. Its potential was calibrated
for each experiment versus the ferrocene/ferrocenium
standard couple. The counter electrode was a platinum
Fig. 1. Photographs of DCE solutions containing 5%vol
acetonitrile of free base porphyrin (H2(TPP) (a), copper(II)
metalloporphyrin (Cu(II)(TPP)) (b), and copper(III)metall-
oporphyrin (Cu(III)(TPP)) (c)
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1126 M. SAPHIER ET AL.
Fig. 2. Absorption spectra obtained from a mixture of Cu(II)Cl2 and H2(TPP) in deaerated DCE several minutes after mixing
(i.e. mostly H2(TPP) (a), after 24 h in a sealed vial (i.e. a mixture of Cu(II)(TPP) and Cu(III)(TPP)) (b) and (d), and after 27 days
(i.e. Cu(III)(TPP)•+) (c)
Fig. 3. Absorption spectra of H2(TPP) (a), Cu(II)(TPP) (b), and Cu(III)(TPP)•+ (c) in aerated DCM
in solution. During a month, the slow penetration of
oxygen into the reaction mixture is the inception for a
Cu(III)(TPP) to Cu(III)(TPP)•+ transformation, for which
the dominant absorption features appear blue shifted,
though the solution color remains green. Adding a
slightly acidic (HClO4) aqueous solution to both Cu(II)
(TPP) and Cu(III)(TPP)•+ (dissolved in DCE) solutions,
causes demetalation of the TPP. Interestingly, when
anhydrous Cu(I)Cl or Cu(I)(CH3CN)4PF6 and H2(TPP)
were mixed in deaerated DCE, no complex formation
was discernable. Only when Cu(I) salts were added under
air complex formation was seen, Cu(I)Cl reacted rather
slowly to afford the aforementioned green solutions,
while Cu(I)(CH3CN)4PF6 reacts much faster to give the
same complex.
Ground state absorption spectra
The absorption spectra of H2(TPP), Cu(II)(TPP), and
the new species formed in the presence of O2, presumably
Cu(III)(TPP)•+, are compared in Fig. 3. Cu(III)(TPP)•+
with its green color turned in the absence of acetonitrile,
on a timescale of minutes into a product with red as the
dominating color. Clear isosbestic points attest the clean
transformation (Fig. 4). The absorption data of the four
porphyrins/metalloporphyrins are listed in Table 1.
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THE REDOX CHEMISTRY OF COPPER TETRAPHENYLPORPHYRIN REVISITED 1127
Fig. 4. Absorption spectra obtained upon mixing 265 mg/l H2(TPP) in air exposed DCM and a 10% molar excess of Cu[CH3CN]4PF6
after different times – see figure legend for details
In toluene and in the absence of acetonitrile, Cu(III)
(TPP)•+ precipitated as green crystals. The solution, on
the other hand, remained red, showing the characteristic
fingerprints of Cu(II)(TPP). Dissolving the green solid in
DCE resulted in a green solution with absorption features
resembling those of Cu(III)(TPP)•+. The absorption
spectra of the new green Cu(TPP) species differ from
those reported for Cu(II)(TPP)•+ and Cu(II)(TPP)2+ in
the literature [15, 18]. The reaction products of mixing
excess Cu(I)(CH3CN)4PF6 and H2(TPP) in aerated
DCE solutions is shown in Fig. 5 as a function of time.
Importantly, the 540 nm absorption band due to Cu(II)
(TPP) increased as a function of time. This initial step of
Cu(II)(TPP) generation is followed by the evolution of
Table 1. The UV-vis absorption data of the four TPP analogs
in DCM
Compound
H2(TPP)
Soret bands
Q-bands
417 (22.6)*
514 (1.0),
548 (0.46)
588 (0.34),
645 (0.24)
Cu(II)(TPP)
Cu(III)(TPP)•+
Cu(III)(TPP)
418 (20.2)
438 (7.6)
445 (~8)
540 (1.0)
600 (0.35)
607 (0.38)
578 (0.14)
650 (1.0)
660 (1.0)
*Relative absorptions are in parentheses, for Cu(II)(TPP)
540nm = 9 × 103 M-1.cm-1 and for Cu(III)(TPP)•+ e650nm = 1.4 × 104
M-1.cm-1 and for Cu(III)(TPP)e660nm ~ 3.5 × 104 M-1.cm-1
e
Fig. 5. Absorption spectra of a mixture of Cu(I)(CH3CN)4PF6 and H2(TPP) in aerated DCE 1.5 min (a), 35 min (b) and 220 min (c)
after mixing
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1128 M. SAPHIER ET AL.
the characteristic Q-band patterns of p-radical cations of
porphyrins in full agreement with the literature. Finally,
on a time scale of minutes a slow decomposition sets in.
Hereby, a new absorption band at 650 nm was formed,
which is presumably attributed to Cu(III)(TPP)•+.
The latter then slowly yielded Cu(III)(TPP) with an
absorption band at 660 nm, due to O2 consumption by an
excess of Cu(I)(CH3CN)4PF6.
18]. A representative cyclic voltammogram of Cu(II)
(TPP), that is, a mixture of Cu(II)Cl2 and H2(TPP), in
deaerated DCE and tetra-butyl ammonium perchlorate as
supporting electrolyte is shown in Fig. 6. The cathodic
part of the voltammogram reveals four distinct peaks
that are ascribable to the reductions of {Cu(III)(TPP·)+
D Cu(II)(TPP·)2+} to {Cu(III)(TPP) D Cu(II)(TPP·)•+},
to Cu(II)(TPP), to Cu(I)(TPP), and to Cu(0)(TPP). As
there was no anodic stripping of the reduced product in
the anodic cycle, it is safe to assume that no demetalation
takes place during the time frame of our electrochemical
experiments. When CH3CN was, however, added to the
DCM solution (Fig. 7), monovalent copper rather than
divalent copper is stabilized. As a consequence, the
Electrochemical measurements
It is known that copper(II)metalloporphyrins are
oxidized in acetonitrile in two reversible steps to the
corresponding Cu(II)(TPP)•+ and Cu(II)(TPP)2+ [15,
Fig. 6. Cyclic voltammogram of Cu(I)(CH3CN)4PF6 and H2(TPP) in deaerated DCE. Working electrode: glassy carbon; reference
electrode: silver wire; v = 0.1 V/s, Ar saturated
Fig. 7. Cyclic voltammograms of Cu(I)(CH3CN)4PF6 and H2(TPP) in deaerated DCE in the absence (a) and presence of 4 vol%
CH3CN (b). Working electrode: glassy carbon; reference electrode: silver wire; v = 0.1 V/s, Ar saturated mixed in DCE
Copyright © 2012 World Scientific Publishing Company
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THE REDOX CHEMISTRY OF COPPER TETRAPHENYLPORPHYRIN REVISITED 1129
Fig. 8. ESR spectra of Cu(III)(TPP) in the absence (a) and presence (b) of a drop of water. The differential spectrum (c) gives the
characteristic ESR signal of Cu(II)(TPP)
cathodic part of the voltammogram is best described by
a single two-electron reduction, Cu(III)(TPP) to Cu(I)
(CH3CN)4+ (~+0.63 V vs. Fc/Fc+) and Cu(I) (CH3CN)4+ to
Cu0, with anodic stripping of the copper metal occurring
as expected.
charge on the metal by the transfer of electron density
from the ligand in the case of a d8 square planar complex
can sometimes result in binding energies in the range
normally associated with Cu(I) compounds. Reid
et al. [23] studied the electronic structure of copper
metalloporphyrins using the XPS technique. This study
points out that in addition to the Cu(II)(TPP), there
is evidence of the presence of a second copper ion
oxidation state. The XPS spectrum of Cu(III)(TPP),
as spectroscopically identified prior to precipitation by
solvent evaporation, is shown in Fig. 9. Before etching
the surface, the Cu(II)(TPP) species was identified. The
Electron Spin Resonance (ESR)
Cu(II) has a d9 electron configuration and the unpaired
electron is responsible for a characteristic ESR signal.
In contrast, Cu(III) has a low spin d8 planar electron
configuration and is ESR silent. Cu(II)(TPP·)•+ is
known to be paramagnetic in solution but diamagnetic
(S=0) in the solid state due to spin coupling in the
metalloporphyrin p-radical cations [19, 20]. The green
DCE solution was indeed ESR silent, and the signal
appeared when a drop of water was added to the solution
resulting in an immediate color change from green to red
(Fig. 8). The UV-vis absorption patterns of the solutions
before and after adding the water match the spectra of
the new species Cu(III)(TPP) and that of the well-known
Cu(II)(TPP).
X-ray Photoelectron Spectroscopy (XPS)
XPS is a surface analytical technique appropriate for
studying the valence state and the chemical environment
of atomic species. Healy et al. [21]. published the XPS
spectra of planar four-coordinate Cu(III) complexes.
The authors report that great care must be exercised
when interpreting XPS studies of copper compounds
in the di- or trivalent states. Reduction of the effective
Fig. 9. Cu2p-XPS spectra of a blend of Cu(II)TPP and Cu(III)-
TPP (a) and of Cu(III)TPP (b) precipitated from a green DCE
solution after evaporating the solvent prior (a) and after (b)
surface etching
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1130 M. SAPHIER ET AL.
spectral features, in full agreement
with previously published results of
copper tetraphenylporphyrin [20],
have characteristic broad shakeup
satellites shifted by approximately
9 eV to higher binding energies.
Additional lower binding energies
at 933.7 and 953.4 eV reveal the
coexistence of another copper
valence state, which is dominant
below the surface after etching, when
the characteristic broad shakeup
satellite pattern almost disappears.
The nitrogen XPS of the Cu(II)(TPP)
(Fig. 10a), as expected from the Jahn–
Teller effect, shows two different
chemical surroundings of nitrogen
atoms, while the other copper
valence state (Fig. 10b) shows only
one surrounding of nitrogen atoms,
Fig. 10. N1s XPS spectra of Cu(II)(TPP) (a) and (Cu(III)(TPP) (b). It is highly
improbable that the second valence state depicted is Cu(I), as monovalent copper
prefers a tetrahedral geometry that increases the difference between the nitrogen
atoms. Moreover, the analysis, as already mentioned, was performed in under air
indicating the existence of the square planar symmetry
characteristic of the d8 Cu(III).
Fluorescence measurements
Figure 11 shows the fluorescence spectra recorded at
450 nm excitation wavelength of Cu(II)(TPP), and the new
Cu(III)(TPP) species. Cu(III)(TPP) gives rise to a much
more intense fluorescence at 680 nm when excited at 450
nm relative to the weak fluorescing Cu(II)(TPP). With the
formation of Cu(III)(TPP·)+ in the presence of oxygen the
emission is quenched by approximatively 20% (Fig. 12).
Fig. 11. Emission spectra from 450 nm excited wavelength of
(a) Cu(II)(TPP), and (b) Cu(III)(TPP) in DCM
CONCLUSION
Our results indicate the formation of a stable, EPR-
silent, new copper tetraphenylporphyrin in non-polar
organic solvents. The newly formed copper complex
reveals Q and Soret absorption bands that are shifted to
the red relative to well known Cu(II) tetraphenylporphyrin
and fluorescence at ambient conditions in the visible part
of the spectrum. XPS results suggest that the new green
copper tetraphenylporphyrin is Cu(III)(TPP) a notion
that is further supported by electrochemical experiments.
A clear isosbestic point develops when the new Cu(III)
(TPP) is reduced to Cu(II)(TPP) indicating a 1 to 1 ratio
between Cu(III)(TPP) and Cu(II)(TPP). Demetalation,
on the other hand, is possible by adding a slightly acidic
(HClO4) aqueous solution. In light of the aforementioned
we propose the following mechanism:
Fig. 12. Emission spectra from 450 nm excited wavelength of
1. Cu2+ + H2TPP
2. 2Cu2+ + H2TPP
3. Cu+ + 4CH3CN



Cu(II)(TPP) + 2H+
Cu(III)(TPP) + Cu+ + 2H+
(a) Cu(III)(TPP) and (b) Cu(III)(TPP)+. in DCM
+
binding energy of the predominant spectral feature at
935.8 eV is indicative of the Cu2+2p3/2 oxidation state,
and the peak at 955.4 eV is the Cu2+2p1/2 component that
is shifted by 19.8 eV from the Cu2+2p3/2 peak. Both these
Cu(I)(CH3CN)4
4. 2Cu(III)(TPP) + H2O
→
2Cu(II)(TPP) + 2H+ + O2
Cu(III)(TPP)•+
air
  →
←  
5. Cu(III)(TPP)
argon
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THE REDOX CHEMISTRY OF COPPER TETRAPHENYLPORPHYRIN REVISITED 1131
Divalent copper reacts directly with H2(TPP)
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