Coordination properties of sterically stressed zincporphyrins

Article abstract of DOI:10.1023/A:1009538009930

Spectrophotometric titration and computer simulation were used to study how the nature of porphyrin and extra ligand affect the formation of extra complexes of zincporphyrins in o-xylene. The compounds under study were zincporphyrins (ZnP) with different

Full text of DOI:10.1023/A:1009538009930

Russian Journal of Coordination Chemistry, Vol. 27, No. 3, 2001, pp. 152–157. Translated from Koordinatsionnaya Khimiya, Vol. 27, No. 3, 2001, pp. 168–173.
Original Russian Text Copyright © 2001 by Zaitseva, Zdanovich, Ageeva, Golubchikov, Semeikin.
Coordination Properties of Sterically Stressed Zincporphyrins
S. V. Zaitseva, S. A. Zdanovich, T. A. Ageeva, O. A. Golubchikov, and A. S. Semeikin
Institute of Chemistry of Nonaqueous Solutions, Russian Academy of Sciences,
ul. Akademicheskaya 1, Ivanovo, 153045 Russia
Ivanovo State Academy for Chemistry and Technology, Ivanovo, Russia
Received July 20, 2000
Abstract—Spectrophotometric titration and computer simulation were used to study how the nature of porphyrin and
extra ligand affect the formation of extra complexes of zincporphyrins in o-xylene. The compounds under study were
zincporphyrins (ZnP) with different substituents and phenyl radicals in meso-positions (zinc-5,15-(p-butyloxyphenyl)-
2,8,12,18-tetramethyl-3,7,13,17-tetraethylporphyrin (ZnP1), zinc-5,15-(p-butyloxyphenyl)-2,8,12,18-tetramethyl-
3,7,13,17-tetrabutylporphyrin (ZnP2), zinctetraphenylporphine (ZnP3), and zinc complexes with overlapped porphyrin
(ZnP4). N-Methylimidazole, imidazole, pyridine, 3,5-dimethylpyrazole, and dimethylformamide were used as extra
ligands (L). The strength of Zn–L bonding was found to decrease in extra complexes (L)ZnP in the series of ZnP as
follows: ZnP4 > ZnP1 > ZnP2 > ZnP3. It was established that the stability constant (logKst) for sterically nonstressed
complexes (L)ZnP4 linearly increases with growth in the extra ligand basicity ( logKBH+ ) and is proportional to the shift
of the main absorption bands (∆λ) in the electronic spectra of extra complexes of zinctetraphenylporphine. For spatially
distorted (L)ZnP1, (L)ZnP2, and (L)ZnP3, the values of logKst and logKBH+ , as well as logKst and ∆λ, change symbat-
ically. The geometric structure and energy characteristics of pentacoordinated zincporphyrins were calculated by quan-
tum-chemical methods. Correlations were established between the calculated values of the energy of the interaction of
the central metal atom with the extra ligand molecule and the stability of the extra complexes of zincporphyrins.
The aim of this work was a systematic study of the
regularities of extracoordination of metalloporphyrins,
namely, of the effect of porphyrin and extra ligand nature
on the formation of sterically stressed extra complexes.
We studied the coordinating properties of zinc com-
plexes with porphyrins of different structures (ZnP).
N
Zn
N
N
N
R2
R1
R2
BuO
N
Zn
ZnP4
N
R1
R1
N
Spectrophotometric titration [1] and computer sim-
ulation [2–4] were used to investigate extracoordina-
tion. It was established, on the basis of experimental
and calculated data, how the strength of a bond between
zinc and an additional molecular extra ligand L
depends on the porphyrin nature and basicity of L.
N
R2
OBu
R1
R2
ZnP1: R1 = Et, R2 = Me; ZnP2: R1 = Bu, R2 = Me
CH3O
CH2
EXPERIMENTAL
CH2
CH3O
The reaction ZnP + nL = ZnLnP under study was run in
o-xylene, which does not form stable solvate com-
plexes with metalloporphyrins and is regarded as a neu-
tral solvent.
R2
O
R1
R2
R1 O
N
N
The changes in the electronic absorption spectra
(EAS) that depend on the extra ligand concentration
make it possible to apply the spectral method for study-
ing extracoordination. The details of the experiment
and the procedures of calculating the stability constants
(Kst) for the extra complexes are described in [1, 5–8].
R1
R2
Zn
N
N
R1
R2
ZnP3: R1 = Me, R2 = Bu
1070-3284/01/2703-0152$25.00 © 2001 åÄIä “Nauka /Interperiodica”

COORDINATION PROPERTIES OF STERICALLY STRESSED ZINCPORPHYRINS
153
The quantum-chemical calculations were performed by
A
jointly using the Fletcher–Reeves and the Polak–
Raivier methods [9]. A specified gradient 0.04 kJ/mol
was used for the count termination.
0.11
The investigated porphyrins (5,15-(p-butyloxyphe-
nyl)-2,8,12,18-tetramethyl-3,7,13,17-tetraethylporphy-
rin (H2P1), 5,15-(p-butyloxyphenyl)-2,8,12,18-tetrame-
thyl-3,7,13,17-tetrabutylporphyrin (H2P2) and tetraphe-
nylporphine (H2P4)) were synthesized following the
procedures in [10–12]. The overlapped porphyrin H2P3
was obtained according to [13].
0.10
0.09
(b)
A
0
5
10 15 20
cL × 104, mol/l
Zincporphyrins ZnP1, ZnP2, ZnP3, and ZnP4 were
synthesized by boiling respective porphyrins with a
tenfold excess of zinc acetate in benzene for 40–
50 min. The complexes were purified by chromatogra-
phy on Al2O3 of activity degree II (with chloroform
used as eluent) and by further recrystallization from
chloroform.
Electronic absorption spectra of the complex solu-
tions were recorded on a Specord M400 spectropho-
tometer. Data on the EAS of synthesized zincporphy-
rins in benzene (λ, nm (logε)) are given below (except
for ZnP3 spectrum taken in xylene):
(a)
540
580
λ, nm
1
Fig. 1. (a) Changes in EAS of solutions of ZnP in o-xylene
in the presence of L with a change in c from 0 to 1.9 ×
L
0
−3
–5
10 mol/l, cZnP1 = 0.9 × 10 mol/l and (b) the correspond-
ing curve of spectrophotometric titration at λ = 550 nm.
Band no.
I
II
III
ZnP1
ZnP2
ZnP3
ZnP4
570.0(3.98) 534.0(4.31) 407.0(5.18)
575.0(3.00) 541.0(4.25) 412.0(5.08)
587.0(4.22) 549.0(4.43) 410.0(5.34)
590.0(3.49) 550.0(4.23) 422.5(5.57)
plexes as calculated by quantum-chemical methods.
Data in Table 1 make it possible to estimate the strength
of bonding of the additional ligand with zincporphyrin
and to obtain stability series for the extra complexes.
The stability of the extra complexes with MeIm and Im
The extra ligands used were N-methylimidazole drop in the series of ZnP as follows: ZnP4 > ZnP2 >
ZnP1 > ZnP3. This can be explained by the influence of
the porphyrin ligand nature on the extracoordination
process. In the case of ZnP4, the phenyl rings draw the
electron density from the zinc atom and thus increase
its fractional positive charge. Accordingly, the Zn–L
bond becomes stronger. A decrease in the number of
phenyl substituents and introduction of alkyl groups in
the β-position of a porphyrin macrocycle in ZnP1 and
ZnP2 result in an increase in the electron density on the
nitrogen atoms in the coordination center and in a
decrease in the positive charge on the zinc atom, which
in turn weakens the strength of the Zn–L bond. Thus it
becomes clear that the extra complexes (L)ZnP1 and
(L)ZnP2 are less stable than (L)ZnP4. The low stability
constant of the (L)ZnP3 as compared to those of the
remaining members of the series is explained by the
substantial distortion of the porphyrin macrocycle as a
result of the steric stresses caused by a “cap” rather than
by the effect of substituents in the porphyrin macrocy-
cle. In the case of the extra complexes of zincporphy-
rins with DMF, the series is as follows: ZnP4 > ZnP2 >
ZnP3 > ZnP1. The stability constants for ZnP complexes
with Py are nearly equal (except for (L)ZnP3) (Table 1);
i.e., the nature of the porphyrin ligand in this case pro-
(MeIm), imidazole (Im), pyridine (Py), 3,5-dimeth-
ylpyrazole (DMP), and dimethylformamide (DMF).
RESULTS AND DISCUSSION
Binding of ligands L with zincporphyrins is accom-
panied in all cases by a bathochromic shift and by a
change in the intensities of the main absorption bands
of a chromophore (Fig. 1). We believe that this occurs
due to an increase in the electron density on the zinc
cation and, respectively, on the porphyrin nitrogen
atoms. Growth in the fractional negative charge on the
N atoms leads to destabilization of the a2u MO with an
unchanged position of the a1u level. As a result, the con-
figurational interaction of the excited state of the Eu
type decreases, which is a substantial reason for
increasing the intensity of the first band in the EAS of
zincporphyrins. The bathochromic shift probably
occurs due to an increase in the a2u energy.
It was found that the ZnP1, ZnP2, and ZnP3 com-
plexes can each add one molecule of MeIm, Im, DMP,
Py, or DMF. In the case of ZnP4, coordination of two
DMF molecules occurs.
Tables 1 and 2 present Kst for (L)ZnP complexes that
were obtained on the basis of EAS data, as well as some
geometric and energy characteristics of these com- duces almost no effect on the extracoordination pro-
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 27 No. 3
2001

154
ZAITSEVA et al.
Table 1. Thermodynamic characteristics of extracoordina-
tion of nitrogen-containing ligands by zincporphyrins in
o-xylene
cess. The reason for such a behavior of zincporphyrin
remains unclear.
The stability of extra complexes of zincporphyrins
is also influenced by the nature of the extra ligand. A
growth in its basicity (Table 3) makes the Zn–L bond
stronger (Table 2) and the Kst of the complexes increase
(Table 1). Thus, for sterically nonstressed complexes
ZnP4, extra complexes with MeIm and Im are the most
stable. Linear dependence can be observed between
298
st
× 10–3,
Extra
ligand
Eb, Zn–L,
K
Complex
ZnP1
∆λ, nm
kcal/mol–1
mol/l
MeIm
Im
Py
DMP
DMF
MeIm
Im
0.477 ± 0.001
4.4 ± 0.1
–16.01
–15.94
–4.524
–12.85
0.049
5.0
12.0
9.0
10.0
2.5
1.14 ± 0.06
1.64 ± 0.03
0.019 ± 0.001
0.56 ± 0.01
6.7 ± 0.4
the values of logKst for (L)ZnP4 and pKBH+ for extra
ligand L:
logKst = 0.2564pKBH+ + 2.4589
ZnP2
5.0
17.0
10.0
10.5
4.0
4.0
8.0
with correlation factor r = 0.963 (Fig. 2). Despite the
high basic properties of MeIm, complexes ZnP1, ZnP2,
and ZnP3 react with it with difficulty. This can most
probably be explained by steric hindrances and by dis-
tortion of the porphyrin macrocycle during extracoordi-
nation. The values of logKst for (L)ZnP1 and (L)ZnP2
Py
1.8 ± 0.1
1.9 ± 0.1
DMP
DMF
MeIm
Im
0.15 ± 0.02
0.46 ± 0.02
0.93 ± 0.01
0.24 ± 0.02
0.042 ± 0.003
110 ± 8
15 ± 2
1.8 ± 0.1
1.5 ± 0.2
0.6 ± 0.01
ZnP3
ZnP4
–0.0299
–0.0298
–0.0256
–0.0097
–290.21
–152.77
–147.73
–145.36
1.90
and of pKBH+ change symbatically and are related by
the equations
Py
3.0
2.0
DMF
MeIm
Im
Py
DMP
DMF
logKst = –0.038pK2BH+ + 0.558pKBH+ + 1.741,
15.5
14.5
11.0
10.0
15.0
r = 0.984;
logKst = –0.069pK2BH+ + 0.936pKBH+ + 0.536,
r = 0.979,
Table 2. Bond lengths (Å) and characteristics of coordination core in (L)ZnP extra complexes
Complex
ZnP1
ZnP1 + MeIm
ZnP1 + Im
ZnP1 + Py
ZnP1 + DMP
ZnP1 + DMF
ZnP3
ZnP3 + MeIm
ZnP3 + Im
ZnP3 + Py
ZnP3 + DMF
ZnP4
ZnP4 + MeIm
ZnP4 + Im
ZnP4 + Py
ZnP4 + DMP
ZnP4 + DMF
ZnP4 + 2DMF
Zn–N(1) Zn–N(2) Zn–N(3) Zn–N(4)
Zn–NL
Ct*–Zn
N(1)–Ct N(3)–Ct N(1)–N(3)
2.025
2.126
2.125
2.121
2.088
2.069
2.019
2.035
2.032
2.033
2.030
2.055
2.070
2.101
2.069
2.085
2.055
2.011
2.055
2.109
2.108
2.107
2.087
2.058
2.383
2.538
2.522
2.519
2.497
2.043
2.101
2.105
2.095
2.104
2.043
2.045
2.074
2.086
2.086
2.085
2.103
2.021
2.019
2.037
2.033
2.034
2.029
2.011
2.105
2.096
2.101
2.095
2.011
2.055
2.064
2.115
2.115
2.112
2.101
2.049
2.376
2.474
2.484
2.486
2.498
2.043
2.096
2.069
2.100
2.053
2.042
2.040
0.049
0.387
0.459
0.385
0.408
0.097
0
2.024
2.091
2.075
2.085
2.048
2.066
0
2.073
2.049
2.064
2.051
2.063
2.018
0
4.097
4.141
4.139
4.137
4.111
4.085
4.038
3.696
3.707
3.705
3.709
4.066
4.094
4.118
4.093
4.091
4.066
4.066
2.100
2.099
2.124
2.100
6.871
1.980
1.978
1.990
2.016
0.855
0.834
0.839
0.825
0.002
0.408
0.404
0.400
0.429
0.006
0
1.847
1.853
1.851
1.855
2.055
2.029
2.062
2.031
2.040
2.055
2.011
1.849
1.854
1.853
1.855
2.011
2.065
2.056
2.062
2.051
2.011
2.055
2.095
2.096
2.119
2.203
7.324
7.919
7.621
* Ct is the center of the coordination cavity of a porphyrin macrocycle.
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 27 No. 3
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COORDINATION PROPERTIES OF STERICALLY STRESSED ZINCPORPHYRINS
155
logKst
logKst
Ep, kcal/mol
4
130
4.9
r = 0.968
3
110
90
2
3.7
2
1
3
2.5
0.5
1
3.5
6.5
0.5 2.5 4.5 6.5
70
0
+
KÇç
2
4
6
8
+
KÇç
Fig. 2. Plot of logK vs basicity of extra ligand L for extra-
st
Fig. 3. Energy of protonation of nitrogen-containing mole-
cules vs their basicity.
4
2
1
complex (1) (L)ZnP , (2) (L)ZnP , and (3) (L)ZnP .
calc
Çç
logKst
logKst
+
pK
10
8
6
r = 0.953
3.1
4
3
4
2
2
1
4
1
1
1.1
2
5
9
13
8
14
2
4
6
8
∆λ, nm
exp
+
pK
Çç
3
4
1
Fig. 5. Dependence of logK for (1) ZnP , (2) ZnP , (3) ZnP ,
st
2
Fig. 4. Correlation between calculated and experimental
pKBH+ for nitrogen-containing ligands.
and (4) ZnP on a shift of band II in the EAS of these extra
complexes.
Zn
N
C
H
4
Fig. 6. Structure of ZnP extra complex with DMF calculated by the quantum-chemical method.
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 27
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2001

156
ZAITSEVA et al.
Table 3. Protonation energies and basicity of nitrogen-con-
r = 0.932;
(L)ZnP3: logKst = –0.069∆λ2 + 0.917∆λ + 0.055,
r = 0.979.
taining molecules
pKBH+
Epcalcd
kcal/mol
,
Extra ligand
MeIm
experiment* calculation
The value of a bathochromic shift can serve as a
qualitative characteristic of the Zn–L bond strength.
The results of the quantum-chemical calculations make
it possible to explain the reasons for the change in the
stability of the extra complexes of sterically stressed
zincporphyrins depending on the nature of the porphy-
rin and extra ligand. Calculations show that the energy
of the Zn–L bond formation (Eb) for the extra complex
changes symbatically with its stability (Table 1). An
increase in Eb when changing from (L)ZnP1 to (L)ZnP4
points to an increase in the complex stabilities in this
series.
7.33
6.65
5.29
6.46
5.52
3.76
3.7
8.22
7.46
4.60
5.93
5.21
3.13
3.01
1.99
0.73
0.8
92.73
95.27
Im
Py
104.8
3,4-Dimethylpyridine
3-Methylpyridine
2-Formylpyridine
3-Formylpyridine
3-Cyanopyridine
2-Nitropyridine
Pyrazine
100.38
102.79
109.74
110.14
113.56
119.96
114.55
108.85
123.3
1.35
0.81
0.6
Zincporphyrins have a square–pyramidal structure
of the coordination core with a metal atom slightly
extending from the N4 plane toward the extra ligand
(Fig. 6). Data presented in Table 2 indicate that the
metal extension from the macrocycle plane correlates
with the Zn–L bond length. It was found that a decrease
in the basicity of the nitrogen-containing bases in the
series Im > DMP > Py > DMF with increasing strength
of the σ bond Zn–N(1–4) is accompanied by a decrease
in the Zn–Ct distance (Table 2). In this case, interaction
of the zinc atom with the nitrogen atom of ligand L
weakens and, as a consequence, the stability of the
extra complex drops (Table 1). Thus, a cis-position of
the ligands incorporated in the composition of the extra
complexes of the zincporphyrins produces a significant
effect. One should note that an increase in the Zn–L
bond strength leads to a growth in the steric stresses in
metalloporphyrins (Table 3). Alkyl substituents in ZnP1
and ZnP2 and a cap in ZnP3 also induce a macrocycle
deformation, which becomes even greater when going
to the extra complexes and, thus, results in their desta-
bilization.
DMP
4.27
–
3.99
0.92
DMF
* The values of pKBH+ were obtained by the potentiometric
method in water at 298 K [13].
respectively, (Fig. 2).
On the basis of quantum-chemical calculations of
the electronic structure of the nitrogen-containing
ligands, a linear correlation was established between
their basicity and energy of protonation (Ep) (Fig. 3).
Comparison of the pKBH+ values produced from this
correlation with the respective data in the literature [14]
shows that the discrepancy does not exceed 10% in
most cases (Table 3, Fig. 4). Therefore, we believe that
the calculated value of the protonation energy can be
used to estimate the basicity of the nitrogen-containing
compounds.
Comparison of the stability constants for the extra
complexes with an absorption band shift in the EAS of
zincporphyrins during extracoordination allowed us to
detect the following correlation between these values:
an increase in Kst is accompanied by an increase in the
shift of absorption band II (∆λ) (Table 1).
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2001