Oxidative coupling of porphyrins using copper(ii) salts

Article abstract of DOI:10.1039/c1cc13596a

A method for radical coupling of porphyrins using copper(ii) salts as one-electron oxidants was developed. A Zn(ii)-porphyrin bearing an aminophenyl group yielded porphyrin oligomers, and two tri-arylporphyrins were oxidized to form doubly and triply link

Full text of DOI:10.1039/c1cc13596a

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COMMUNICATION
Oxidative coupling of porphyrins using copper(II) saltswz
Bradley J. Brennan,a Michael J. Kenney,a Paul A. Liddell,a Brian R. Cherry,b Jian Li,c
Ana L. Moore,*a Thomas A. Moore*a and Devens Gust*a
Received 16th June 2011, Accepted 18th July 2011
DOI: 10.1039/c1cc13596a
A method for radical coupling of porphyrins using copper(II)
salts as one-electron oxidants was developed. A Zn(II)-porphyrin
bearing an aminophenyl group yielded porphyrin oligomers, and
two tri-arylporphyrins were oxidized to form doubly and triply
linked dimers. Bromination of doubly linked dimers gave macro-
cycles with twisted skeletons.
porphyrin would permit spectroscopic studies in solution,
construction of devices by spin coating, mixing with other active
materials, etc. We therefore searched for a solution-compatible
oxidative chemical coupling method.
A variety of agents for one-electron oxidation of porphyrin
rings have been reported by Osuka and coworkers. Examples
include tris(4-bromophenyl)aminium hexachloroantimonate
(BAHA),9 AgPF6,10 and combinations of 2,3-dichloro-5,6-
dicyano-1,4-benzoquinone (DDQ) with scandium(III) salts.12,13
Although these reagents are generally effective they can be
expensive, and BAHA has been shown to halogenate
porphyrins.9,11
Porphyrins are a major class of chromophores for artificial photo-
synthesis,1–3 and are useful in photonics,4 organic electronics,5
biomedicine,6 and other areas. New synthetic methods can
facilitate tuning the absorption spectra of porphyrins for better
solar spectral coverage, adjusting their redox potentials for
different energy conversion applications, and modifying their
substituents for linkage to other moieties.
Here, we describe a method for using copper(II) perchlorate
or tetrafluoroborate salts in acetonitrile for porphyrin oxidative
radical coupling. Advantages are the use of an inexpensive
commercially available salt, air stability of the oxidizer, and
simplicity, as only a single chemical oxidant is required.
Development of this method was based upon the observation
that both porphyrins14 and amines15 form cation radicals
upon treatment with Cu(II). Acetonitrile was chosen as a
solvent because the stability of Cu(I) ions in this solvent makes
Cu(II) a strong one-electron oxidizer.16 The redox potential of
Cu(II) in acetonitrile (B0.95 V vs. SCE) renders it competent
to oxidize both typical zinc porphyrins and anilines, which
have redox potentials on the order of 0.78 V17 and 0.90 V,18
respectively.
Recently, we reported a new class of porphyrin and porphyrin-
fullerene polymers with potential applications in organic photo-
voltaics. Synthesis involved electrochemical oxidative coupling
of an arylamino group to a meso-position on the macrocycle
(Scheme 1).7,8 The semiconductive polymer films grown on
conductive glass featured broad absorption bands throughout
the visible and stable electrochemical properties, making them
promising materials for solar energy applications. However,
because the preparation method yielded only an insoluble film,
exploitation of these materials was limited. A soluble polymeric
The oxidative coupling reaction mechanism of a monomer
such as 2 is presumably similar to that of the electrochemical
process,7 but with Cu(II) removing electrons. Radical coupling
of two oxidized porphyrins via the meso-H and phenylamino
positions followed by loss of two protons yields the dimer.
Continued growth to form a polymer occurs at both ends of
the dimer. Two equivalents of oxidant are needed to form each
new bond. Because each unit of the polymer is more easily
oxidized than the monomer, all porphyrins are left in an
oxidized state at the end of the reaction, and complete reaction
requires 3 equivalents of Cu(II).
Scheme 1 Oxidative porphyrin polymerization. The [O] denotes an
oxidizing agent (electrochemical or chemical).
a Department of Chemistry and Biochemistry, Center for Bioenergy
and Photosynthesis, Arizona State University, Tempe, AZ 85287,
USA. E-mail: amoore@asu.edu, tom.moore@asu.edu, gust@asu.edu
b Magnetic Resonance Research Center, Arizona State University,
Tempe, AZ 85287, USA
Upon addition of 3.5 equivalents of Cu(II) tetrafluoroborate
to a suspension of 2 in acetonitrile at room temperature, all of
the porphyrin dissolved and the color of the mixture changed
from red to deep green. After 5 h the reaction was quenched
with aqueous potassium ferrocyanide to reduce excess Cu(II)
and oxidized porphyrins, and to precipitate the porphyrin
products. The porphyrins were demetallated with neat trifluoro-
acetic acid and later precipitated from dichloromethane with
c School of Materials, Arizona State University, 7700 South River
Parkway, Tempe, Arizona 85284, USA
w This article is part of the ChemComm ‘Artificial photosynthesis’ web
themed issue.
z Electronic supplementary information (ESI) available: Experimental
details, synthetic procedures, NMR data. See DOI: 10.1039/
c1cc13596a
c
10034 Chem. Commun., 2011, 47, 10034–10036
This journal is The Royal Society of Chemistry 2011

hexanes to give free base porphyrin oligomers in 72% yield.
These were soluble in dichloromethane, chlorobenzene, and
tetrahydrofuran.
Analysis of the oligomers by MALDI-TOF mass spectro-
metry showed a series of peaks consistent with the polymeric
structure shown in Scheme 1. The polydispersity, determined
by HPLC-GPC, was 1.37 with an average molecular weight of
4343, which represents B6.8 porphyrin units. The reason for
obtaining relatively short oligomers has not been determined,
but solubility of the growing polymer could play a role. No
optimization of the reaction conditions has been attempted.
Fig. 1 compares the UV-visible-NIR absorption spectrum of
the oligomeric product with those of monomer 1 and model
compound 3 (Fig. 2). The UV-visible portion of the spectrum
resembles that of 3, providing evidence for the proposed
molecular structure given in Scheme 1. It is also similar to
the spectrum of the electropolymerized analog.7 The Soret
band shifts from 413 nm in the monomer to 418 nm in the
model compound and oligomeric product, with the Q-bands of
the model and product also shifting in similar ways. Overall
the spectrum of the oligomeric material is broadened relative
to the spectra of 1 and 3. In the near-IR region there are minor
peaks that reflect the formation of head-to-head oxidative
couplings, suggesting that some fused porphyrin dimers with
the general structure shown in 4 (Fig. 2) are present. Related
structures have been reported to form during oxidative
coupling reactions using other reagents by Osuka and
co-workers.9 Similar spectral features were not observed in
the electrochemically-polymerized porphyrin on conductive
glass. Chromatographic separation followed by UV-visible-
NIR analysis indicates that many of the longer oligomeric
Scheme 2 Oxidative dimerization of porphyrins. Treatment of 5 with
Cu(ClO4)2ꢀ6H2O in acetonitrile followed by aqueous K4Fe(CN)6
yielded 6 and 7. Similar conditions were employed for conversion
of 9 to 10 and 11. Treatment of the mixture of 6 and 7 with acid
converted 7 to 8, but left 6 unchanged (see text).
chains have one or more of these head-to-head linkages
embedded within them.
The potential of Cu(II) as a chemical oxidizer for porphyrin
reactions was explored further (Scheme 2) by treating porphyrin
5 with 3.3 equivalents of Cu(II) perchlorate in acetonitrile.
After 3.5 h at ambient temperature all starting material was
consumed, and the reaction mixture was treated with aqueous
potassium ferrocyanide to reduce any remaining Cu(II) and
oxidized porphyrin. The major products were the meso-meso,
b-b triply connected dimer 6 and the meso-b doubly connected
dimer 7.
Chromatographic separation of 6 and 7 was difficult, and was
only carried out on a small scale. However, it was found that
demetallation of 6 required much stronger acidic conditions than
removal of the copper from 7. To facilitate chromatographic
separation, this difference was exploited by selective demetal-
lation of the doubly connected product. The mixture of 6 and 7
was dissolved in 2 : 1 trifluoroacetic acid : dichloromethane and
H2SO4 was added to 18% v/v. After 1.5 h, demetallation to
produce 8 was almost complete, but 6 was essentially unaffected.
The isolated yields after silica gel chromatography of the
resulting mixture were 29% and 34% for 6 and 8, respectively.
The absorption spectra of 5 and dimers 6, 7 and 8 in
dichloromethane are shown in Fig. 3. Monomer 5 shows a
Soret band at 410 nm and Q-band absorption at 534 nm and
567 nm. In the triply linked dimer 6, the Soret is broadened
and has a maximum at 409 nm. Longer-wavelength absorp-
tions are seen at 557, 577, 911 (sh) and 1002 nm. Doubly
linked dimer 7 features a Soret band at 418 nm with additional
absorbances at 487 (sh), 553, 629, 702 (sh) and 768 nm. When
the metal is removed from 7 to yield 8, the Soret shifts to 424 nm,
and longer wavelength absorptions are found at 499, 561,
611, 739(sh), and 814 nm.
Fig. 1 UV-Visible-NIR spectra of 1 (line), 3 (dot), and porphyrin
oligomers (dash) in THF. The spectra have been normalized at the
porphyrin absorption maximum.
Fig. 2 Porphyrin model compound 3 and proposed dimer 4.
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 10034–10036 10035

linking the aryl rings to the macrocycles. Rotational barriers
for p-substituted phenyl rings on metallated porphyrins are on
the order of 14–18 kcal molꢂ1, depending on the metal.20
Barriers for mesityl rings, however, are 426 kcal molꢂ1 19–21
.
Thus, the magnitude and the equality of the barriers for
interconversion of resonances in 13 observed for both the
mesityl rings and the rings bearing substituents only para to
the porphyrin macrocycle proves that the measured barrier is
that for equilibration of the distorted macrocyclic skeleton of
the molecule through the planar conformation.
In summary, we have described a general method for one-
electron porphyrin oxidation using copper(II) perchlorate or
copper(II) tetrafluoroborate in acetonitrile. The oxidation
reaction was used to polymerize a porphyrin monomer via
an aminophenyl moiety and to dimerize porphyrins bearing a
meso-hydrogen. The simplicity and efficiency of the oxidations,
coupled with the use of inexpensive, readily available
reagents, shows that this method could be widely applicable in
porphyrin oxidative coupling.
Fig. 3 UV-Visible-NIR spectra of 5 (solid), 6 (dot), 7 (dash) and 8
(dash-dot-dot) in dichloromethane. The spectra have been normalized
at the Soret band absorption maxima.
A similar coupling reaction was carried out with porphyrin 9.
A
mixture of copper-containing triply linked (10) and
doubly linked (11) dimers was obtained. After acid treatment
as described above, pure 10 (31%) and free base dimer 12
(47%) were isolated.
This work was supported by a grant from the U. S.
Department of Energy (DE-FG02-03ER15393).
The absorption spectra of these compounds in dichloro-
methane were very similar to those of the trifluoroacetanilide
series. Monomer 9 features a Soret absorption at 410 nm and
Q-band absorption at 534 and 567 nm. Dimer 10 has maxima
at 409, 557, 577, 909 (sh) and 1002 nm. Free base dimer 12,
shows peaks at 425, 499, 561, 610, 739(sh), and 812 nm.
These dimers have strong absorption throughout much of
the UV-visible-NIR region. The absorption above 450 nm
relative to the Soret absorption is greatly enhanced compared
with that of the corresponding monomer. Thus, molecules of
this type are potentially useful light harvesters for various
applications, assuming that their other properties are suitable.
Bromination of the dimers was regioselective. Upon treatment
with 1 equivalent of N-bromosuccinimide, 12 gave compounds
13 and 14 in 32% and 29% yields, respectively. In the
1H-NMR spectrum of 13, the resonances of the mesityl methyl
groups ortho to the porphyrin macrocycle and all of the
aromatic proton resonances of the various aryl groups were
broad, and there were twice as many as expected for a
structure of C2h symmetry. The meso aryl rings of porphyrins
are twisted out of the plane of the macrocycle due to steric
hindrance. The observed anisochrony indicates not only slow
rotation of the aryl groups about their bonds to the porphyrins,
as expected,19–21 but also that bromine substitution causes the
doubly linked macrocycles to distort out of plane, leading to a
C2 or Ci structure. The distortion is ascribed to steric
hindrance between the bromine atoms and the nearby
b-hydrogen atoms on the adjacent macrocycle. Upon warming,
each pair of broadened resonances coalesces to a single
resonance and sharpens. From coalescence temperatures and
chemical shift differences, the activation barriers were estimated22
to be DG323z = 15.0 ꢁ 0.5 kcal molꢂ1 for the mesityl rings and
DG297z = 15.7 ꢁ 0.5 kcal molꢂ1 for the aryl rings bearing the
ester moieties. In principle, coalescence could be due either to
equilibration of the macrocyclic skeleton between the distorted
conformations (wherein the bromine atoms and hydrogen
atoms pass one another), or to rotation about the bonds
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10036 Chem. Commun., 2011, 47, 10034–10036
This journal is The Royal Society of Chemistry 2011