Features of formation of mixed-ligand complexes of aluminum tetraphenylporphine

Article abstract of DOI:10.1023/A:1012306228553

Formation of extra complexes of aluminum tetraphenylporphine was studied by spectrophotometric titration. The effect of the nature of acido ligands on the stability of mixed-ligand complexes of aluminum tetraphenylporphine was determined. The stability co

Full text of DOI:10.1023/A:1012306228553

Russian Journal of General Chemistry, Vol. 71, No. 1, 2001, pp. 132 136. Translated from Zhurnal Obshchei Khimii, Vol. 71, No. 1, 2001,
pp. 145 149.
Original Russian Text Copyright
2001 by Zaitseva, Zdanovich, Ageeva, Golubchikov.
Features of Formation of Mixed-Ligand Complexes
of Aluminum Tetraphenylporphine
S. V. Zaitseva, S. A. Zdanovich, T. A. Ageeva, and O. A. Golubchikov
Institute of Solution Chemistry, Russian Academy of Sciences, Ivanovo, Russia
Ivanovo State Chemical Engineering University, Ivanovo, Russia
Received November 11, 1999
Abstract Formation of extra complexes of aluminum tetraphenylporphine was studied by spectrophotometric
titration. The effect of the nature of acido ligands on the stability of mixed-ligand complexes of aluminum
tetraphenylporphine was determined. The stability constant (log Kst) of sterically unstrained complexes
+
(Cl)Al(L)TPP and (OH)Al(L)TPP increases linearly with increasing basicity of the extra ligand (log KBH );
in the case of sterically distorted complexes (OAc)Al(L)TPP and (Acac)Al(L)TPP changes in log Kst and
+
log KBH vary in the same direction. The geometries and energy characteristics of six-coordinate complexes
of aluminum porphyrins were calculated quantum-chemically. The calculated enthalpies and Gibbs energies
of formation of the complexes are consistent with the experiment. The possibility of the bidentate coordination
of acetate and acetylacetonate in the porphyrin extra complexes was proved.
The interest in aluminum porphyrins is primarily
due to their possible application in redox processes
as catalysts [1 4]. One of elementary stages of such
catalytic reactions is extra coordination [3, 4]. How-
ever, this process with aluminum porphyrins was not
studied systematically. Quantum-chemical calculations
for optimizing chemical processes and structures of
various compounds have been described in [5 7]. In
this work we used the ZINDO/1 method to calculate
the geometric and energy parameters of aluminum
porphyrins and their extra complexes.
The equilibrium in the system metal porphyrin
extra ligand is established instantaneously, which is
proved by spectroscopic experiments. Chloroform is
a very weak solvating agent for aluminum as a central
atom in a metal porphyrin, and the effect of specific
interaction between the Al atom and CHCl3 molecule
on the extra coordination equilibrium can be neglected.
The spectrophotometric method allows determina-
tion of the concentrations of light-absorbing com-
A specific feature of synthesis of aluminum com-
plexes in high-boiling solvents is formation of
(X)MTPP complexes as a result of the reaction between
aluminum salts and a porphyrin (in particular, tetra-
phenylporphine). The aluminum complexes (X)AlTPP
were isolated by liquid chromatography and recrystal-
lization.
The formation of extra complexes is accompanied
by a significant shift of bands in the absorption spec-
trum of metal porphyrins and by a change in their rel-
ative intensities (Fig. 1). This fact allows us to study
spectroscopically the extra coordination:
Fig. 1. (a) Variation of the electronic absorption spectra of
(Acac)Al(MeIm)TPP in chloroform with increasing MeIm
concentration (direction of changes is denoted by arrows).
Concentration of MeIm added, M: initial 0, first portion
(X)AlTPP + L
(X)Al(L)TPP.
(1)
4
3
X = Cl , OH , AcO , Acac ; L = N-methylimidazole
(MeIm), imidazole (Im), pyridine (Py), 3,5-dimethyl-
pyrazole (DMP), dimethylformamide (DMF).
1.26 10 , and final content 3.2 10 . (b) Correspond-
ing curve of spectrophotometric titration at (Acac)AlTPP
5
concentration of 0.9 10 M and
555 nm.
1070-3632/01/7101-0132$25.00 2001 MAIK Nauka/Interperiodica

FEATURES OF FORMATION OF MIXED-LIGAND COMPLEXES
133
Table 1. Thermodynamic characteristics of extra coordination of nitrogen-containing ligands by aluminum porphyrin
complexes in chloroform
3
3
Extra
ligand
Kst 10 ,
Eb,
,
Extra
ligand
Kst 10 ,
Eb,
kJ mol
,
Complex
Complex
1
1
M (298 K) kJ mol
nm
M (298 K)
nm
(Cl)AlTPP
MeIm
Im
Py
DMP
DMF
MeIm
Im
Py
DMP
DMF
0.35 0.02
4.0 0.3a
360.7
332.7
395.4
415.1
486.2
326.8
334.8
355.7
370.3
397.5
11.0
16.0
9.0
7.0
4.0
12.5
17.0
11.0
10.5
5.0
(OAc)AlTPP
MeIm
Im
Py
DMP
DMF
MeIm
Im
Py
DMP
DMF
0.24 0.02
12 1a
329.3
323.0
342.3
371.6
396.0
233.5
225.1
293.7
302.1
383.3
11.0
15.5
11.0
8.0
0.15 0.01a
0.075 0.001
0.034 0.001
0.67 0.02
15.0 0.3a
0.23 0.01a
0.12 0.01
0.048 0.001
0.38 0.01
4.8 0.3a
4.0
(OH)AlTPP
(Acac)AlTPP
10.0
14.5
7.0
6.0
5.5
0.38 0.03a
0.31 0.01
0.066 0.003
0.065 0.005a
0.046 0.002
0.040 0.001
a
Data of [14].
Table 2. Bond lengths (d, ) in extra complexes of aluminum porphyrin
Complex
L
M N1
M N2
M N3
M N4
M X
M L
M Ct
(Cl)AlTPP
None
MeIm
Im
2.176
2.176
2.169
2.165
2.164
2.174
2.182
2.175
2.182
2.180
2.173
2.173
2.188
2.180
2.175
2.186
2.180
2.172
2.188
2.180
2.179
2.186
2.180
2.176
2.176
2.183
2.182
2.171
2.170
2.182
2.193
2.192
2.183
2.188
2.172
2.172
2.186
2.176
2.175
2.180
2.173
2.173
2.188
2.177
2.175
2.183
2.175
2.177
2.176
2.167
2.177
2.165
2.164
2.174
2.181
2.184
2.172
2.180
2.181
2.184
2.187
2.174
2.179
2.186
2.136
2.177
2.187
2.177
2.181
2.187
2.191
2.176
2.176
2.178
2.177
2.180
2.173
2.182
2.195
2.185
2.188
2.191
2.183
2.190
2.185
2.179
2.178
2.181
2.172
2.176
2.188
2.181
2.180
2.183
2.174
2.179
2.420
2.457
2.459
2.462
2.467
2.424
2.197
2.220
2.218
2.197
2.274
2.277
2.378
2.419
2.419
2.380
2.407
2.413
2.220
2.391
2.390
2.230
2.417
2.461
0.552
0.237
0.236
0.326
0.474
0.542
0.577
0.296
0.275
0.310
0.313
0.372
0.621
0.343
0.338
0.381
0.405
0.352
0.684
0.360
0.351
0.434
0.545
0.559
2.335
2.335
2.507
2.665
6.737
Py
DMP
DMF
None
MeIm
Im
(OH)AlTPP
(AcO)AlTPP
(Acac)AlTPP
2.340
2.347
2.411
2.992
3.522
Py
DMP
DMF
None
MeIm
Im
2.360
2.355
2.409
2.341
4.003
Py
DMP
DMF
None
MeIm
Im
Py
DMP
DMF
2.571
2.497
2.561
2.994
5.110
pounds, i.e., metal porphyrins and their extra com-
plexes, since their dilute solutions obey the Bouguer
Beer law. The experimental technique and procedure
for calculation of the stability constants of extra com-
plexes (Kst) are similar to those described in [8 11].
Using the Bent French method [12], we have found
that the aluminum complexes of tetraphenylporphine
can bind a single molecule of the ligand.
To optimize the geometry of the aluminum por-
phyrin molecules, we combined the conjugated gra-
1
dient methods [13]. The gradient of 0.04 kJ mol 1 A
was taken as a criterion for calculation completion.
The experimental and calculated characteristics of
the extra coordination are given in Tables 1 and 2.
Table
1
shows that the extra complexes
(X)Al(L)TPP are stable compounds having a noticea-
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 71 No. 1 2001

134
ZAITSEVA et al.
Fig. 2. Structure of (OH)Al(MeIm)TPP calculated by the ZINDO/1 quantum-chemical method.
ble mutual effect of ligands. The strength of binding
an extra ligand by aluminum porphyrins can be esti-
mated. Depending on the nature of the ligand, the sta-
bility of the extra complexes (X)Al(L)TPP decreases
in the following orders:
of the macrocycle upon formation of the extra com-
plex. This is proved by calculations of optimized
structures of the molecules of aluminum porphyrins
(Table 2). The formation of more stable complexes
(OH)Al(L)TPP can be explained, first of all, by
the purely electrostatic Al OH interactions. The ex-
cessive positive charge remains on aluminum atoms,
which favors the formation of strong donor acceptor
Al L bonds.
OH > OAc > Cl
Acac ; L is Im, DMF; (2)
OH > OAc > Cl > Acac ; L is Py, DMP. (3)
It should be noted that extra complexes formed
by aluminum porphyrins are less stable than their zinc
analogs [9, 10, 17] despite a higher formal charge
and a smaller ionic radius of aluminum. This fact
seems to have several reasons. First, X AlTPP bonds
are covalent rather than ionic. Second, aluminum cat-
ions are displaced significantly from the N4 plane of
the porphyrin toward acido ligands X, which is proved
by quantum-chemical calculations (Table 2). This shift
is retained even in the extra complexes (Fig. 2).
Obviously, the effective positive charge on the alu-
minum atom decreases and the metal porphyrin com-
plex is destabilized as the covalence of Al X bonds
increases. Series (2) and (3) well agree with the pub-
lished data [15, 16]. However, with MeIm the usual
trend is broken:
OH > Cl , Acac > OAc ; L is MeIm.
(4)
This is due to the steric hindrance to forma-
tion of the Al MeIm bond. The low stability of
(OAc)Al(L)TPP is caused by a significant steric strain
of the porphyrin (Table 2) and by a high polarizability
of the electronic shell of OAc resulting in a notice-
able decrease in the effective positive charge on
the metal atom and, as a consequence, in weakening
of the Al L bond.
The values of Kst of the aluminum tetraphenylpor-
phine extra complexes depend also on the basicity
of the molecular ligand. It was found that the stabil-
ity of (X)Al(L)TPP increases in the order DMF <
DMP < Py < Im, as the basicity of the extra ligand
increases (Table 1). Complexes with MeIm fall out
of this trend owing to strong solvation of this ligand
in chloroform. According to the quantum-chemical
calculations, the energy of a hydrogen bond between
A sharp decrease in Kst of (Acac)Al(L)TPP seems
to be due to the bidentate coordination of the acido
ligand to the metal porphyrin and to the distortion
1
MeIm and CHCl3 is 54.8 kJ mol , whereas for
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 71 No. 1 2001

FEATURES OF FORMATION OF MIXED-LIGAND COMPLEXES
135
1
imidazole it is as low as 21.7 kJ mol . It is pos-
sible that the steric distortion of extra complexes
(X)Al(MeIm)TPP makes a certain destabilizing con-
tribution.
The value of log Kst for sterically unstrained alu-
minum porphyrins increases linearly with increasing
negative logarithms of the acid dissociation constants
+
of the protonated molecular ligands L pKBH (Fig. 3a):
+
For (Cl)Al(L)TPP log Kst = 0.36pKBH + 1.29 (cor-
relation coefficient r = 0.957 at the confidence level
+
of 0.95); for (OH)Al(L)TPP log Kst = 0.35pKBH
+
0.81 (r = 0.959). In the case of sterically distorted
+
extra complexes the dependence of log Kst on pKBH
is not linear (Fig. 3b): for (OAc)Al(L)TPP log Kst =
2
+
)
+
0.062(pKBH
0.156pKBH + 1.81 (r = 0.993);
2
+
for (Acac)Al(L)TPP log Kst = 0.055(pKBH
0.137pKBH (r = 0.979).
)
+
The shift of the main absorption bands in the elec-
tronic spectra of metal porphyrins ( ) can be used
as a measure of the stability of the aluminum mixed-
ligand complexes. We have found a correlation be-
tween log Kst and
on the basis of the experi-
mental data given in Table 1. A linear dependence is
typical for the sterically unstrained complexes: for
(Cl)Al(L)TPP log Kst = 0.175
for (OH)Al(L)TPP log Kst = 0.1961
+ 0.694 (r = 0.984);
+ 0.5837 (r =
Fig. 3. Correlation of the stability of extra complexes
0.955). In the case of distorted structures the linear
dependence is broken.
of aluminum tetraphenylporphine with the extra ligand
basicity: (1) (OH)AlTPP, (2) (Cl)AlTPP, (3) (OAc)AlTPP,
and (4) (Acac)AlTPP.
The computer simulation based on quantum-chemi-
cal calculations gives insight into the origin of changes
in stability of aluminum tetraphenylporphine extra
complexes depending on the nature of acido and extra
ligands and also allows determination of the most
energy-favorable arrangement of the ligands relative to
the macrocycle plane (Fig. 2). According to our cal-
culations, the energy of the Al L bond (Eb) in the ex-
tra complexes varies in parallel with their stability
(Table 1).
ordination cavity, and Ct N, Al X, and Al L dis-
tances. In the mixed-ligand aluminum complexes with
large acido ligands (AcO , Acac ), which are capable
of bidentate coordination, porphyrin macrocycles are
significantly distorted, which destabilizes the extra
complexes of aluminum porphyrins. Finally, it should
be noted that the change in stability of the extra com-
plexes affects the Al N bond lengths insignificantly.
Thus, a cis effect of ligands in extra complexes of
aluminum porphyrins is manifested relatively weakly.
Table 2 shows that even a qualitative interrelation
between Kst and the size of the coordination cavity
of the coordinated porphyrin is lacking.
The calculated characteristics of changes in the struc-
ture of aluminum porphyrin upon extra coordination
illustrate specific features of this process (Table 2).
Aluminum porphyrins are square-pyramidal with
the metal atom displaced from the N4 plane toward
the acido ligand (Fig. 2). The distance Al Ct (Ct is
the center of the porphyrin ring) increases in the order
OH , Cl , AcO , Acac , which hampers the extra co-
ordination. Such a deformation is retained in extra
complexes; however, the Al Ct distance shortens sig-
nificantly (Table 2), the distance being the shorter,
the greater is Kst for the complex. The lengths of Al L
bonds vary in the same way. No correlation was found
between the macrocycle distortion, i.e., the size of co-
EXPERIMENTAL
The electronic absorption spectra were recorded on
a Specord M-400 spectrophotometer.
(Cl)AlTPP was obtained by the known procedure
[18] by refluxing H2TPP and AlCl3 in a 1 : 6 molar
ratio in pyridine (10 ml per 1 g of AlCl3) for 6 h.
Electronic absorption spectrum in CHCl3, max, nm
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 71 No. 1 2001

136
ZAITSEVA et al.
(log ): 592.0 (3.56), 549.0 (4.20), 510.0 sh, 466.0
(3.45), 418.0 (4.58).
8. Karmanova, T.V., Koifman, O.I., and Berezin, B.D.,
Koord. Khim., 1983, vol. 9, no. 6, pp. 772 776.
9. Koroleva, T.A., Koifman, O.I., and Berezin, B.D.,
Koord. Khim., 1983, vol. 7, no. 8, pp. 2007 2012.
10. Karmanova, T.V., Cand. Sci. (Chem.) Dissertation,
Ivanovo, 1985.
11. Tipugina, M.Yu., Lomova, T.N., and Ageeva, T.A.,
(OH)AlTPP. H2TPP and AlCl3 in a 1 : 6 molar ra-
tio were refluxed in phenol [14] for 1 h. Electronic
absorption spectrum in CHCl3, max, nm (log ):
569.0 (3.45), 556.0 (3.91), 516.0 (2.77), 425.0 (5.44),
404.0 (4.28).
Zh. Obshch. Khim., 1999, vol. 69, no. 3, pp. 459 463.
(OAc)AlTPP and (Acac)AlTPP were synthesized
by the procedure in [14, 19]. Electronic absorption
spectrum in CHCl3, max, nm (log ): (OAc)AlTPP
595.0 (3.96), 552.0 (4.25), 420.0 (5.11); (Acac)AlTPP
589.0 (3.59), 547.0 (4.61), 412.0 (5.75).
12. Bulatov, M.I. and Kalinkin, I.P., Prakticheskoe ruko-
vodstvo po fotokolorimetricheskim i spektrofotometri-
cheskim metodam analiza (Manual on Photocolori-
metric and Spectrophotometric Methods of Analysis),
Leningrad: Khimiya, 1968.
13. Fletcher, R., Methods of Optimization, New York:
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