THERMOCHEMISTRY OF SOLUTION OF Fe(III) AND Mn(III) COMPLEXES
297
ing the molecular structure, which is manifested in the
electronic absorption spectra (Table 4).
state is broken down [15]. Apparently, the process
observed with the Fe(III) and Mn(III) complexes in
piperidine and DMF (Table 4) is intramolecular
breakdown of the macrocycle. Molecular oxygen may
also exert certain effect on the process, as the phytol
substituent containing a bond can facilitate oxida-
tive transformations. This assumption is confirmed by
the significantly larger negative value of intH0 of
Fe(III) and Mn(III) complexes with chlorophyll ligand
in piperidine as compared with the pheophorbid com-
plexes containing no phytol substituent. In the other
cases, when dissolution of metal porphyrins in elec-
tron-donor solvents is not accompanied by essential
changes in the electronic spectra, the increase in
intH0 as compared to noncoordinating solvents is due
to specific solvation of the central atom of the metal
porphyrin with solvent molecules.
V’yugin [13] also gives attention to large negative
heats of specific solvation of (X)MnTPP in piperidine.
Other processes, along with extra coordination of
piperidine molecules, are suggested to occur. Karma-
nova et al. [14] have examined the ESR and electronic
absorption spectra of the complexes (X)MnTPP in
piperidine and concluded that under these conditions
the manganese oxidation state changes from Mn(d4)
to Mn(d5); it is noted that the Mn(d5) state in porphy-
rin complexes is unstable and exists only in strongly
electron-donor media. In this state the central atom
can take up two molecular ligands.
Comparison of the enthalpies of interaction ( intH0)
of Fe(III) and Mn(III) porphyrin complexes with elec-
tron-donor solvents (Table 5) and the electronic ab-
sorption spectra shows that very large values of intH0
of AcFeChP with DMF and pyridine and of AcFePPb,
AcMnChP, and AcMnPPb with piperidine correlate
with significant changes in the spectra. For example,
a solution of AcMnChP in piperidine has an absorp-
tion band at 474 nm only, and the Soret band (about
400 nm) is absent, which suggests decomposition of
the macrocycle. In some cases (AcFeChP in DMF,
AcFeChP, AcFePPb in piperidine), the Soret band is
blurred and weak. In the spectrum of AcFePPb in
DMF, the Soret band is present but the remaining part
of the visible spectrum is not resolved.
To check this interpretation, we studied the thermo-
chemistry of solution of the Fe(III) chlorin e6 complex
in various solvents. Chlorin taken as trimethyl ester
IV contains no phytol substituent and no cyclopenta-
none ring. Table 5 shows that the enthalpy character-
istics of interaction of AcFeChn e6 with electron-
donor solvents are related to specific solvation of the
central atom without side reactions, similar to ClFePP.
EXPERIMENTAL
Porphyrin ligands for synthesis of Fe(III) and
Mn(III) complexes were prepared as described in [18,
19]. Acetatoiron(III) porphyrins were prepared by
heating (90 C, 1 2 h) the corresponding porphyrin
ligand with a tenfold molar excess of iron powder in
acetic acid. The reaction completion was judged from
the disappearance of the bands of the initial ligand
from the electronic absorption spectrum. The solvent
In the chemistry of chlorophyll and its analogs with
other metals, reactions are known involving intramo-
lecular oxidation of the macrocycle with cleavage of
the cyclopentanone ring or complete breakdown of the
macrocycle [15]. An example is the so-called Molish
phase test: Addition of an alkaline agent to a solution
of chlorophyll or its derivative containing a cyclopen- was distilled off in a vacuum, and the residue was dis-
tanone substituent leads first to appearance and then to
disappearance of a color [16]. A similar phenomenon
was described in [17], namely: In reaction of FeCl3
with chlorophyll, a brown color first appears and then
disappears. This is due either to cleavage of the cyclo-
pentanone ring only or to total degradation of the
molecule. It is interesting that the reaction with FeCl3
is known from the analytical chemistry as a qualita-
tive reaction for phenols. In the reaction of FeCl3 with
chlorophyll, the ketone group apparently transforms
into the enol group, which reacts with FeCl3 similarly
to phenols, insofar as the enol form of chlorophyll is
an analog of phenols.
solved in chloroform, filtered, and chromatographed
on alumina (Brockmann grade IV) with chloroform as
eluent. The eluate was evaporated, and the complex
was precipitated with hexane. Chloroiron(III) porphy-
rins were prepared by heating FeCl2 with the corre-
sponding ligand in chloroform methanol (1 : 1) in an
inert atmosphere. Manganese(III) porphyrin com-
plexes were prepared from Mn(II) acetate and por-
phyrin ligand in acetic acid as described for acetato-
iron(III) porphyrins. The electronic absorption spectra
(band positions and intensities) of the products agreed
with published data [6, 20].
The enthalpies of solution were determined on a
precision calorimeter with a variable-temperature
isothermal jacket [21]. Prior to experiments, samples
were finely divided and vacuum-dried at 350 K to
constant weight. The electronic absorption spectra
Studies of protolytic dissociation of some chloro-
phyll complexes in strong proton-donor media re-
vealed cases when the metal ion is released from the
complex and the ligand macrocycle in the transition
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 71 No. 2 2001
Berezin
