Unusual properties of tetraphenylporphyrin copper complex

Article abstract of DOI:10.1134/S107042800903018X

5,10,15,20-Tetraphenylporphyrin reacted with copper powder in boiling xylene on exposure to air (but not in an inert atmosphere) to give bright red copper complex which differed in properties from the initial porphyrin and its complexes with other metals.

Full text of DOI:10.1134/S107042800903018X

ISSN 1070-4280, Russian Journal of Organic Chemistry, 2009, Vol. 45, No. 3, pp. 449–451. © Pleiades Publishing, Ltd., 2009.
Original Russian Text © V.A. Nefedov, A.M. Pavlov, 2009, published in Zhurnal Organicheskoi Khimii, 2009, Vol. 45, No. 3, pp. 458–460.
Unusual Properties of Tetraphenylporphyrin Copper Complex
V. A. Nefedov and A. M. Pavlov
Nesmeyanov Institute of Organometallic Compounds, Russian Academy of Sciences,
ul. Vavilova 28, Moscow, 119991 Russia
e-mail: viktornefed@pochta.ru
Received June 25, 2007
Abstract—5,10,15,20-Tetraphenylporphyrin reacted with copper powder in boiling xylene on exposure to air
(but not in an inert atmosphere) to give bright red copper complex which differed in properties from the initial
porphyrin and its complexes with other metals. The copper complex was stable to oxidation with Cu2+ ions
(during chromatography), and it readily underwent demetalation by the action of LiAlH4 in tetrahydrofuran.
DOI: 10.1134/S107042800903018X
Syntheses of various meso-substituted porphyrins
[1] generally involve two problems. One of these is
related to the fact that the formation of porphyrins
from pyrrole and aldehydes under acidic conditions is
almost uncontrollable, and the second problem is dif-
ficult isolation procedure. Chromatographic isolation
of porphyrins is often accompanied by their oxidation.
Therefore, synthesis of compounds that are stable to
oxidation and chromatographically mobile, attracts
researchers’ attention. There are no published data
indicating that porphyrin copper complexes could be
promising in this respect. Although porphyrins are
readily identified on thin-layer chromatograms by their
color, by-products are often colorless or weakly col-
ored, so that special reagents are necessary to detect
and remove such impurities.
such as acetate, phthalimide, and phenylacetylide.
These are common reactions intrinsic to other metal
salts. Tetraphenylporphyrin copper complex turned out
to be also formed in the reaction with copper powder
provided that it was performed on exposure to atmos-
pheric oxygen rather than in an inert atmosphere. Ad-
dition of 1–2 ml of acetonitrile to a xylene solution
completes the formation of the copper complex in
a few minutes. Partly hydrogenated derivatives, such
as chlorin and bacteriochlorin, react in a similar way.
No copper complex is formed under argon. The reac-
tion involves oxidation of tetraphenylporphyrin rather
than copper, for copper oxide does not react under
analogous conditions. Other metals, e.g., zinc powder,
failed to react with tetraphenylporphyrin.
Tetraphenylporphyrin copper complex II is a crys-
talline substance which gives rise to bright red solu-
tions in common solvents. Its electronic spectrum
consists of the only long-wave absorption band with its
maximum at λ 540 nm; by contrast, the free ligand and
complexes with other metals are characterized by up to
We have found that porphyrin copper complexes
possess a number of properties that differentiate them
fairly strongly from both metal-free porphyrins and
porphyrin complexes with other metals. Tetraphenyl-
porphyrin (I) readily reacts with various copper salts,
Scheme 1.
Ph
Ph
N
HN
N
N
N
N
Cu, Δ
Ph
Ph
Cu
Ph
Ph
(1) LiAlH4, THF
(2) H2O
NH
N
Ph
Ph
I
II
449

NEFEDOV, PAVLOV
450
four absorption bands in the region λ 500–650 nm.
Likewise, tetraazaporphyrin copper complex [1] dis-
plays only one absorption band in the long-wave
region (λmax 600 nm). The presence of only one ab-
sorption band suggests that its molecule contains either
a single chromophore or several similar chromophores,
whereas molecules of other porphyrins comprise chro-
mophores involving both coordination and ionic, as
well as covalent, bonds between the metal and pyrrole
nitrogen atom.
between nonpolar substrate and polar oxidant rather
than high stability toward oxidation.
Ionic copper(II) salts are reagents that make it pos-
sible to detect by chromatography not only porphyrins
but also concomitant products in their synthesis. Com-
pounds containing pyrrole fragments immediately turn
black upon contact with Cu(BF4)2 and, judging by in-
solubility of the products in organic solvents, undergo
polymerization. The chromatograms were treated by
drawing cross bands with a solution of copper salt
[usually Cu(BF4)2] in acetonitrile: black color appeared
in places where contact with pyrrole compounds oc-
curred. Pyrrole itself (even in the vapor phase) under-
goes instantaneous polymerization to give deep black
insoluble polymers in the presence of even traces of
copper salts applied, e.g., to paper or fabric (vapor
dyeing).
Porphyrin metal complexes are characterized by
higher chromatographic mobility and stability as com-
pared their complexes with other metals, which is im-
portant for the isolation of porphyrins. Demetalation of
porphyrin metal complexes, including tetraphenylpor-
phyrin complex II, is usually accomplished by the ac-
tion of strong acids (e.g., H2SO4), which is not always
convenient. We found that demetalation of copper
complex II can be performed under considerably
milder conditions, by treatment with LiAlH4 in THF.
During the process the mixture turned blue, presum-
ably due to intermediate formation of tetraphenylpor-
phyrin radical anion. The subsequent treatment with
water gave metal-free tetraphenylporphyrin. Demetala-
tion of tetraphenylporphyrin zinc complex under anal-
ogous conditions occurred to an insignificant extent.
Presumably, increase in the oxidative stability of the
copper complex is accompanied by increase in its abil-
ity to undergo reduction.
Covalent character of the copper–nitrogen bond is
confirmed by high chromatographic mobility of the
copper complex which is readily eluted even with
benzene–heptane mixtures, and its Rf values strongly
exceed those of tetraphenylporphyrin itself and its
complexes with other metals. Tetraphenylporphyrin is
very sensitive to acids adsorbed by silica gel from
laboratory atmosphere: a sample of tetraphenylporphy-
rin applied to silica gel from solution turns green
almost immediately. By contrast, tetraphenylporphyrin
copper complex gives a stable red spot. The ability of
tetraphenylporphyrin to undergo oxidation with cop-
per(II) ions directly on a chromatographic plate was
reported by us previously [2]. We found that only
copper complex II is capable of passing through
a Cu(BF4)2 band applied onto the plate, whereas tetra-
phenylporphyrin I and its zinc complex were instantly
adsorbed to form, respectively, bright green and almost
black zones. Porphyrins can be regenerated from these
zones by treatment with diethylamine or pyridine
vapor, and just the initial porphyrins are formed in
such a way, though tetraphenylporphyrin gives rise to
traces of the copper complex. Analogous elution of
tetraphenylporphyrin zinc complex through a Cu(BF4)2
band also gave almost black chromatographically im-
mobile complex, and treatment of the latter with di-
ethylamine recovered initial red–violet zinc complex.
Copper complexes of tetraphenylporphyrins having
substituents in the benzene rings, e.g., 3-methoxy
derivatives, are also stable to oxidation and are charac-
terized by enhanced chromatographic mobility as com-
pared to metal-free porphyrins. This may be interesting
from the preparative viewpoint. Unlike tetraphenylpor-
phyrin and its para-substituted derivatives which sepa-
rate from reaction mixtures as crystalline materials,
meta-substituted analogs having hydroxy, methoxy, or
acetoxy groups remain in solution. Transformation of
such porphyrins into the corresponding copper com-
plexes facilitates their isolation by chromatography
due to enhanced mobility and stability to oxidation.
Nevertheless, we can speak about the stability of
the copper complex to oxidation only under the above
conditions. We previously noticed that perylene, which
is known to be readily oxidized with Cu2+ ions, some-
times passed through the oxidative zone without for-
mation of purple radical cation. The latter was formed
only when the superimposed spots were slightly
heated. Analogous superposition of spots of the copper
complex and copper salt gives rise to a grayish–brown
color after slight heating. The subsequent elution with
toluene gradually extracts red copper complex from
the spot. The fact that the copper complex remains
unchanged during chromatography through a zone
containing an oxidant indicates the absence of contact
Like porphyrins, dissolution of zinc phthalocyanine
in acetonitrile in the presence of Cu(BF4)2 leads to the
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 45 No. 3 2009

UNUSUAL PROPERTIES OF TETRAPHENYLPORPHYRIN COPPER COMPLEX
451
formation of dark red radical cation. The latter is
thermally unstable, and heating of the solution to 60–
70°C results in its complete decoloration. It is better to
use a mixture of acetonitrile with nitromethane as sol-
vent in analogous oxidation of metal-free phthalocy-
anine. However, copper phthalocyanine does not
change under the same conditions, i.e., it is as resistant
to oxidation with Cu2+ ions as tetraphenylporphyrin
copper complex.
of Cu(BF4)2 and copper complex II was heated to 30–
40°C, weak gray color appeared. The subsequent
elution with toluene gradually removed the initial red
complex from the spot.
Demetalation of tetraphenylporphyrin copper
complex II with LiAlH4. Copper complex II, 0.34 g
(0.05 mmol), was dissolved in 20 ml of THF, and 0.2 g
of LiAlH4 was added under stirring. The original
crimson–red solution turned blue. After 10 min, 5 ml
of isopropyl alcohol was added, the mixture was
diluted with 20 ml of toluene, acidified with aqueous
acetic acid, and diluted with water, the organic phase
was separated, and the solvent was removed. Accord-
ing to the TLC data, complex II almost completely
disappeared, and tetraphenylporphyrin was formed. By
chromatography (toluene–ethyl acetate, 4:1) we isolat-
ed 0.29 g (94%) of tetraphenylporphyrin I. Under anal-
ogous conditions, demetalation of tetraphenylporphy-
rin zinc complex occurred by 3–5%.
EXPERIMENTAL
5,10,15,20-Tetraphenylporphyrinatocopper(II)
(II). a. Reaction with copper powder. Copper powder,
0.6 g, was added to a solution of 0.3 g of tetraphenyl-
porphyrin I in 10 ml of o-xylene. The mixture was
heated for 1 h under reflux. During that time, the initial
crimson–red fluorescing porphyrin was converted into
bright red nonfluorescing copper complex (TLC, silica
gel, toluene) which was eluted with toluene almost at
the front. The xylene solution was separated by decant-
ing, the solvent was removed, and the residue was
subjected to chromatography using toluene as eluent.
The solvent was removed from the red eluate, and the
residue was washed with hexane. Yield 0.31 g (91%).
Electronic absorption spectrum (heptane): λmax 540 nm.
Found, %: C 77.88; H 4.47; Cu 9.30; N 8.28. C44H30N4.
Calculated, %: C 77.91; H 4.46; Cu 9.37; N 8.26.
Isolation of tetraphenylporphyrin. A chromato-
graphic column containing a layer with copper(II) tetra-
fluoroborate on the top was prepared by applying
a solution of Cu(BF4)2 in 2–3 ml of ethyl acetate and
subsequent eluting with toluene. A light blue zone was
thus formed at the top. A reaction mixture containing
tetraphenylporphyrin and by-products was applied to
the column from toluene solution. Tetraphenylporphy-
rin underwent oxidation (green zone), while by-prod-
ucts were transformed into black insoluble compounds.
The column was then eluted with toluene containing
several drops of pyridine, and bright-red nonfluoresc-
ing copper complex and then crimson tetraphenyl-
porphyrin (exhibiting strong red fluorescence) were
washed off. The eluate was washed with water and
subjected to repeated chromatography. Copper com-
plex II thus isolated was dissolved in THF and treated
with LiAlH4, and the blue solution was diluted with
isopropyl alcohol and water, extracted with toluene,
and passed through a thin layer of Al2O3. We isolated
tetraphenylporphyrin I which was identified by com-
paring with an authentic sample. Electronic absorption
spectrum (toluene), λmax, nm: 510, 545, 595, 650.
b. Reaction with copper salts. As described in a,
heating of 0.3 g of tetraphenylporphyrin and copper(II)
acetate, phthalimide, or phenylacetylide in DMF, fol-
lowed by chromatographic isolation, gave tetraphenyl-
porphyrin copper complex II in 90–95% yield. No
complex II was formed by heating tetraphenylporphy-
rin I with powdered copper(II) oxide.
Oxidation of tetraphenylporphyrin with cop-
per(II) ions on a chromatogram. Tetraphenylporphy-
rin was applied to a chromatographic plate to obtain
a crimson band, and a band containing Cu(BF4)2 was
applied from a solution in acetonitrile to the same plate
in front of the tetraphenylporphyrin band. Elution with
toluene gave a bright green band which was treated
with a drop of pyridine or with pyridine or diethyl-
amine vapor. The plate was dried, and the bright red
complex was washed off from the violet band (com-
plex of pyridine with copper tetrafluoroborate) with
toluene–hexane. The initial tetraphenylporphyrin was
then eluted with toluene.
REFERENCES
1. Porfiriny, struktura, svoistva, sintez (Porphyrins. Struc-
ture, Properties, and Synthesis), Enikolopyan, I.S., Ed.,
Moscow: Nauka, 1985, p. 205.
Copper complex II passed through the Cu(BF4)2
2. Nefedov, V.A., Russ. J. Org. Chem., 2007, vol. 43,
without change. When a plate with superimposed spots
p. 1163.
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