Chromium(III) and chromium(IV) tetraphenylporphine complexes

Article abstract of DOI:10.1023/A:1021630228084

Stable chromium complex (AcO)CrTPP was synthesized through the reaction of meso-tetraphenylporphine with chromium(II) acetate in boiling phenol. Coordination properties of chromium porphyrin in reaction with imidazole and pyridine in o-xylene were studied by electronic absorption spectroscopy and computer modeling. A single-electron oxidation of chromium(III) complex was found to be affected by peroxide compounds. The stability of an extra complex depends on the basic properties of the extra ligand and oxidation number of the central metal atom. The complex stability correlates with the calculated energy of formation of the metal-extra ligand bond. The geometrical structure and energy parameters of hexacoordinated chromium porphyrins were calculated using the quantum-chemical method. The effect of the cis and trans position of ligands in the composition of a macrocyclic compound was established to be significant only in the extra complexes (AcO)CrTPP.

Full text of DOI:10.1023/A:1021630228084

Russian Journal of Coordination Chemistry, Vol. 28, No. 12, 2002, pp. 843–847. Translated from Koordinatsionnaya Khimiya, Vol. 28, No. 12, 2002, pp. 900–905.
Original Russian Text Copyright © 2002 by Zaitseva, Zdanovich, Golubchikov.
Chromium(III) and Chromium(IV) Tetraphenylporphine
Complexes
S. V. Zaitseva, S. A. Zdanovich, and O. A. Golubchikov
Institute of Chemistry of Nonaqueous Solutions, Russian Academy of Sciences,
ul. Akademicheskaya 1, Ivanovo, 153045 Russia
Received June 14, 2001
Abstract—Stable chromium complex (AcO)CrTPP was synthesized through the reaction of meso-tetraphe-
nylporphine with chromium(III) acetate in boiling phenol. Coordination properties of chromium porphyrin in
reaction with imidazole and pyridine in o-xylene were studied by electronic absorption spectroscopy and com-
puter modeling. A single-electron oxidation of chromium(III) complex was found to be affected by peroxide
compounds. The stability of an extra complex depends on the basic properties of the extra ligand and oxidation
number of the central metal atom. The complex stability correlates with the calculated energy of formation of
the metal–extra ligand bond. The geometrical structure and energy parameters of hexacoordinated chromium
porphyrins were calculated using the quantum-chemical method. The effect of the cis and trans position of
ligands in the composition of a macrocyclic compound was established to be significant only in the extra com-
plexes (AcO)CrTPP.
The interest shown in porphyrins with high-valent
metal cations is explained by the possibility of using
them as efficient catalysts in redox processes and as ini-
tiators in radical polymerization reactions [1–4]. The
d2sp3-hybridization of a metal atom in a porphyrin
complex makes acidoligands X act as σ- and π-electron
donors, which leads to the formation of stable complexes
capable of reacting with the nitrogen-containing bases.
However, such processes, in particular, for complexes
with high-valent metal cations, have been poorly studied.
EXPERIMENTAL
The reaction (AcO)CrTPP + nL = (AcO)Cr(L)nTPP
(L = Im, Py) was carried out in freshly prepared o-
xylene (special purity grade) dried with metallic
sodium [5]; o-xylene was used due to its relatively low
reactivity.
Synthesis of (AcO)CrTPP. H2TPP (0.1 g,
0.16 mmol) was boiled with chromium(III) acetate
(0.2 g, 0.87 mmol) in phenol for 6 h. The extent of the
reaction was monitored spectrophotometrically. The
reaction mixture was cooled, dissolved in chloroform,
repeatedly washed with warm water, and placed on a
column with Al2O3. A dry complex was obtained after
repeated chromatography and vacuum removal of
CHCl3. The yield was 50%. The compound was identi-
fied through elemental analysis, as well as electronic
and 1H NMR spectroscopies.
This work continues the systematic study of coordi-
nation properties of chromium porphyrins. The synthe-
sis of (AcO)CrTPP (I), the single-electron oxidation of
Cr(III) to Cr(IV) by peroxide compounds, and the
mechanism of the extra coordination of chromium(III)
and chromium(IV) tetraphenylporphine ((AcO)CrTPP,
O=CrTPP) by imidazole (Im) and pyridine (Py),
respectively, are considered.
For C46H31CrN4O2
anal. calcd. (%):
C 73.93;
C 74.02;
H 6.48;
H 6.55;
N 7.84.
N 7.70.
Found (%):
X
8
10
7
9
12
14
The electronic absorption spectra of the complex in
chloroform (λmax, nm (logε)): 605 (3.90), 565 (3.99),
523 (3.65), 450 (5.20), 396 (4.34), 358 (4.18); in acetic
acid: 602 (3.99), 564 (4.12), 525 (3.70), 448 (5.31), 397
(4.61), 363 (4.40), which correspond to the literature
data [6]; and in o-xylene: 604 (3.89), 564 (3.95), 525
(3.69), 448 (5.22), 398 (4.40), 365 (4.23).
11
6
N(22)
13
N(23)
5
Cr
4
15
N(21)
3
N(24)
19
16
1
2
17
20
1H NMR in CDCl3 (δ, ppm): 1.78 (m, o-H), 1.26 (m,
p-H), 0.88 (t, CH3).
18
The metalloporphyrin solutions in o-xylene were
prepared just before they were used. The optical density
(X)CrTPP (X = OAc, O)
1070-3284/02/2812-0843$27.00 © 2002 åÄIä “Nauka /Interperiodica”

844
ZAITSEVA et al.
log((As – A0)/(A∞ – A0))
A
0.64
18
A
1
0.9
1
(a)
(b)
0.56
0.48
0.40
0.4
2
–0.1
–0.6
–1.1
–1.6
–5.2
logcL
–4.2
–3.2
–2.2
0
10
20
c
Im × 104, mol/l
Fig. 2. Plots of log(A – A )/(A – A ) vs. logc for extra
s
0
∞
0
L
coordination of (AcO)CrTPP with (1) Im and (2) Py.
λ, nm
440 450 460 470
Fig. 1. (a) (1–18) Electronic absorption spectra of
(AcO)CrTPP in o-xylene in the presence of imidazole with
Quantum-chemical calculations were performed by
using both the Fletcher–Reeves and the Polak–Raivier
methods [11–14].
The electronic absorption spectra were recorded
using SF-26 and Specord M-400 spectrophotometers;
1H NMR spectra were taken on a Bruker AC-500 spec-
trophotometer (500 MHz, HMDS as internal standard)
in CDCl3.
0
–3
c
changing from 0 to 2.07 × 10 mol/l, c(AcO)CrTPP =
Im
−5
0.48 × 10 mol/l and (b) the respective titration curve at
λ = 445 nm.
of the studied solutions with different concentrations of
Im and Py was measured at λ = 445 and 455 nm for
(AcO)CrTPP and at λ = 432 and 544 nm for O=CrTPP,
respectively.
RESULTS AND DISCUSSION
The experimental procedure and the calculation of
stability constants (Kst) of the extra complexes are
described in [7–10]. For the ternary equilibrium sys-
tem, Kst of the (X)Cr(L)TPP extra complexes (X =
OAc, O; L = Im, Py) was calculated as
The formation of the Cr–L σ-bond in the extra com-
plex of chromium porphine results in a decrease in the
effective positive charge on the chromium atom. There-
fore, one could expect an increase in the energy of the
a1u(π), a2u(π)
eg(dπ) transition. However, the reverse
situation is observed. The introduction of extra ligand L
into the metalloporphyrin solution and the further
increase in the ligand concentration causes a batho-
chromic shift (∆λ) of the absorption bands in the elec-
tronic absorption spectra of the complex (Fig. 1a). This
occurs due to the formation of a dative Cr–L π-bond
that diminishes the electron density on the dxz and dyz
orbitals of Cr(III).
(As – A0)/(A∞ – A0)
1 – (A – A0)/(A∞ – A0)
-------------------------------------------------------
=
Kst
1
------------------------------------------------------------------------
×
,
(cL – c0MP)(As – A0)/(A∞ – A0)
where c0MP is the initial metalloporphyrin (MP) concen-
tration in a solution, As is the optical density of an equi-
The destabilization of the energy levels of the
ground and excited states of a metallophophyrin as a
librium solution, A0 is the optical density of the MP result of extra coordination changes the configurational
interaction and electronic structure of the chro-
mophore. This causes a substantial shift and increase in
the intensity of the main bands in the electronic absorp-
tion spectra of the extra complex. This spectral feature
of the reaction of the extra coordination of Im and Py
by chromium porphyrins makes it possible to use the
spectrophotometric method in studies of mixed-ligand
complexes.
Chromium porphyrins mainly occur in solutions as
hexacoordinated compounds and, thus, one can suggest
that in o-xylene the sixth coordination site of chromium
is occupied by a water or solvent molecule. The
solution, and A∞ is the optical density of an equilibrium
solution with the maximum extra ligand concentration.
The relative error of the Kst determination did not
exceed 10%.
Although the electronic absorption spectra of the
extra complexes retain an isobestic point, which sug-
gests that the solution under study contains two colored
compounds, metalloporphyrins with one and two extra
ligands have very similar electronic absorption spectra.
A shift of the isobestic point is commensurable with the
experimental error, and, therefore, the number of added
ligands was determined by the Bent–French method [7]. (AcO)Cr(H2O)TPP complex can form when the water
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 28 No. 12 2002

CHROMIUM(III) AND CHROMIUM(IV) TETRAPHENYLPORPHINE COMPLEXES
845
A
A
(a)
(b)
1
0.14
10
0.10
0.06
0
20
40
60
c
Im × 104, mol/l
450
550
650
λ, nm
Fig. 3. (a) (1–10) Electronic absorption spectra of O=CrTPP in o-xylene in the presence of imidazole with c changing from 0 to
Im
0
–3
–5
4.36 × 10 mol/l, cO=CrTPP = 0.76 × 10 mol/l and (b) the respective titration curve at λ = 544 nm.
concentration in a solvent does not exceed 10–3 mol/l
[15, 16]. However, the content of H2O in o-xylene dried
with metallic sodium is about 10–7 mol/l, which is
insufficient for H2O coordination with metalloporphy-
rin. Note that (CH3)2C6H4 is “inactive” and poorly sol-
vates the chromium atom [17]; the solvation mainly
involves the peripheral part of a macrocyclic ligand. The
energy of the Cr–(CH3)2C6H4 bond formation calculated
by the ZINDO1 quantum-chemical method is equal to
+37.47 kJ/mol, while that of the Cr–H2O, Cr–Im, and
Cr–Py bond is –116, –191.68, and –178.27 kJ/mol,
respectively. This is why the energy spent on replacing
a solvent molecule by a nitrous base is so insignificant
and is commensurable with the measurement error. For
this reason, we consider this process as a simple addi-
tion reaction of imidazole and pyridine when these
occupy the sixth coordination site of the chromium
atom.
2.7 × 10–5–3.2 × 10–3 mol/l and are equal to 3.36 × 103
and 7.845 × 103 mol–1 l for (AcO)Cr(Im)TPP and
(AcO)Cr(Py)TPP, respectively. When going from imi-
dazole to pyridine, the Cr–L bond in the extra complex
becomes weaker (Tables 1 and 2) due to the decrease in
the basic properties of the extra ligand (pKImH+ = 6.65,
pKPyH+ = 5.29).
As is seen from the data in Table 1, the bathochro-
mic shift ∆λ is related to Kst of the extra complex. The
value of ∆λ can be used to qualitatively estimate the
Cr–L bond strength and, hence, the displacement of the
metal atom from the plane of the coordination center
(N4).
As was already noted above, the extra coordination
reaction with chromium(III) porphyrins was conducted
in o-xylene, which was washed with Na2S2O3 and dried
with metallic sodium according to the standard proce-
dure [5]. In this solvent, the electronic absorption spec-
On the basis of the results obtained, it was estab-
lished that (AcO)CrTPP coordinates one molecule of
Im or of Py (tanα = 1.0 and 0.99, respectively) (Fig. 2).
This is clearly seen from the spectrophotometric titra-
tion curve of (AcO)CrTPP with imidazole (Fig. 1b).
The monotonous nature of the curve is not violated,
which suggests that the mixed-ligand complexes form
in a one-stage reaction. The (AcO)CrTPP titration
curve with pyridine is similar. After storage, the equi-
librium systems did not change their electronic absorp-
tion spectra, which also confirms the common mecha-
nism of the extra coordination process and the forma-
tion of a stable reaction product (AcO)Cr(L)TPP.
Table 1. Thermodynamic parameters of extra coordination of
nitrogen-containing ligands to chromium porphyrins in o-xylene
298
st
mol–1
–Eb,
kJ/mol
K
× 10–3,
Complex
∆λ, nm
l
(AcO)CrTPP
(AcO)Cr(Im)TPP
(AcO)Cr(Py)TPP
O=CrTPP
3.36 ± 0.23
191.68
178.27
6
4
0.785 ± 0.028
The stability constants of extra complexes of chro-
mium(III) porphyrins were determined at 298 K in the
O=Cr(Im)TPP
O=Cr(Py)TPP
1.75 ± 0.12
1.01
0.86
22
19
concentration interval cIm = 1.5 × 10–5–2 × 10–3, cPy
=
0.258 ± 0.029
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 28 No. 12 2002

846
ZAITSEVA et al.
Table 2. Selected geometrical parameters of chromium porphyrins obtained by quantum-chemical calculations
Bond length, Å
Complex
Cr–N(21) Cr–N(22) Cr–N(23) Cr–N(24) Cr–NL Ct*–Cr Cr–X N(21)–N(23) N(22)–N(24)
(AcO)CrTPP
2.1656
2.0924
2.2283
2.1008
2.0996
2.1006
2.1654
2.0962
2.1285
2.1007
2.1005
2.1000
2.1735
2.0937
2.1118
2.1005
2.0998
2.1006
2.1734
0.5229 2.3913
4.2112
4.1852
4.3324
4.0437
4.0484
4.0429
4.2110
4.1936
4.2578
4.1225
4.1239
4.1221
(AcO)Cr(Im)TPP
(AcO)Cr(Py)TPP
O=CrTPP
2.0984 2.1604 0.0456 2.2389
2.1302 2.3051 0.0431 2.3926
2.1004
0.5699 1.5867
O=Cr(Im)TPP
O=Cr(Py)TPP
2.1000 4.5746 0.5581 1.5871
2.0999 4.7636 0.5713 1.5869
* Ct is the center of a macrocycle plane.
trum of the (AcO)CrTPP complex contains a Soret while the second product does not exhibit these bands
band (B(0,0)) at λmax = 448 nm due to the charge-trans- in the long-wave region.
fer transition [18]. However, in the electronic absorp-
The electronic absorption spectrum of (AcO)CrTPP
tion spectrum of the complex solution in o-xylene dried
in o-xylene with a peroxide concentration 10–6–10–5 mol/l
with NaOH [19] to the equilibrium content of water,
contains bands with λmax = 371.0, 431.9, 544.7, and
10−3 mol/l, this band was observed at 431.9 nm. The
604.0 nm, which is typical of O=CrTPP [23–25].
shift of the Soret band may occur due to the coordina-
tion of a water molecule and the formation of
As in the case of (AcO)CrTPP, O=CrTPP coordina-
tion with Im and Py molecules is accompanied by a
bathochromic shift of the basic absorption bands
(Fig. 3). However, the value of ∆λ is considerably
higher in this case (Table 1). Obviously, this is due to
the fact that extra ligand coordination occurs simulta-
neously with the reduction of metalloporphyrins.
(AcO)Cr(H2O)TPP.
Under the action of different oxidative agents, chro-
mium(III) porphyrins are known to easily enter the
redox reactions and form -yl complexes of Cr(IV) and
Cr(V) [20–22]. Xylenes and toluene of reactive quality
contain sufficient amount of organic peroxides to com-
pletely convert (AcO)CrTPP into O=CrTPP. The
removal of a solvent with NaOH does not make it pos-
sible to eliminate the hydroperoxides it contains, and,
thus, the oxidation of chromium(III) is more than prob-
able. A single-electron oxidation under different condi-
tions gives two products, namely, the π-cation-radical
On the basis of the data obtained, it was established
that O=CrTPP adds one molecule of Im or Py (tanα =
1.00 and 1.01, respectively) (Fig. 4). The titration curve
(Fig. 3b), as with (AcO)CrTPP, falls smoothly main-
taining its monotonous nature. The stability of the
mixed-ligand complexes formed depends on the basic-
ity of the extra ligands and decreases when going from
imidazole to pyridine (Table 1). The stability constants
of O=Cr(L)TPP are lower than those of the respective
(AcO)Cr(L)TPP complexes (Table 1). We believe that
the reason for this lies in that the covalently bonded π-
donor acidoligand O2– in O=CrTPP can significantly
increase the electron density in the chromium d orbit-
als, which results in a decrease in its effective positive
charge and, correspondingly, in the formation of a less
stable Cr–NL bond. The stability can also decrease due
to steric factors, namely, due to the displacement of a
metal from the N4 plane (Table 2) and the reduction of
chromium(IV) tetraphenylporphine occurring simulta-
neously with extra coordination.
.
+
(AcO)CrTPP or O=CrTPP [22]. In addition to max-
ima in the range of 400–550 nm, the first product con-
tains absorption bands in the range of 600–840 nm,
log((As – A0)/(A∞ – A0))
1.5
1
2
1.0
0.5
0
–5
–4
–3
–2
logcL
–0.5
–1.0
The quantum-chemical calculations revealed that
the metalloporphyrins under study have nonplanar
structures with slightly raised and lowered (in pairs)
pyrrole fragments (Fig. 5). The chromium atom is dis-
placed from the N4 plane toward the acidoligand (Table 2).
The extra coordination of the Cr(III) complex is fol-
lowed with substantial changes in the geometrical
parameters of the coordination center of the porphyrin
ligand (Table 2).
–1.5
–2.0
–2.5
Fig. 4. Plots of log((A – A )/(A – A )) vs. logc for extra
s
0
∞
0
L
coordination of O=CrTPP with (1) Im and (2) Py.
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 28 No. 12 2002

CHROMIUM(III) AND CHROMIUM(IV) TETRAPHENYLPORPHINE COMPLEXES
(a) (b)
847
Fig. 5. Structures of (a) (AcO)Cr(Im)TPP and (b) O=Cr(Im)TPP calculated by the ZINDO1 quantum-chemical method.
richeskim metodam analiza (Practical Guide on Photo-
colorimetric and Spectrophotometric Methods of Analy-
sis), Leningrad: Khimiya, 1968.
The obtained data showed a correlation between Kst
and the length of the Cr–NL bond, whose strengthening
increases the stability of the mixed-ligand complexes
(Tables 1 and 2). It was established that the energy of
formation of the M–L bond in a porphyrin complex
changes symbatically with its stability (Table 1). At the
same time, no correlation was found between the metal
displacement from the N4 plane and the Cr–NL bond
length (Table 2).
8. Karmanova, T.V., Koifman, O.I., and Berezin, B.D.,
Koord. Khim., 1983, vol. 9, no. 6, p. 772.
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Analysis of the obtained geometrical parameters of
cis effects of ligands in the Cr(III) and Cr(IV) com-
plexes appear in different ways. A sufficiently strong
trans effect of ligands is observed only in the case of
(AcO)Cr(Im)TPP (Table 2). The cis effect is confirmed
by the change in the M–N(21–24) bonds during the inter-
action of Cr(III) with the extra ligand molecule, while it
is almost absent for Cr(IV) porphyrins (Table 2).
Thus, one can conclude that, unlike the
(AcO)Cr(L)TPP complexes, the coordination of Im and
Py to O=CrTPP does not cause significant distortions in
a macrocycle.
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