Effect of steric strains in the macroring on the structure and properties of molecular complexes of (chloro)[5,15-(p-butoxyphenyl)-2,8,12,17-tetramethyl- 3,7,13,17-tetrabutylporphinato]aluminum

Article abstract of DOI:10.1134/S1070363206100276

The chelate (Cl)AlP was prepared by complexation of porphine (P) with aluminum(III) chloride in refluxing pyridine. Equilibrium coordination of nitrogen-containing ligands (L = 2-methylimidazole, imidazole, pyridine, 3,5-dimethylpyrazole, dimethylformamide) with (Cl)AlP in benzene was studied by spectrophotometric titration and computer simulation. Quantitative and qualitative characteristics of the reaction were obtained. The structure of the mixed-ligand complex formed by intermolecular interaction of the metal porphyrin with a base was determined spectrophotometrically and by quantum-chemical calculations. An effect of additional molecular ligand and of steric strain in the macroring on the stability of the complex was noted. The stability constant (K _s) increases with an increase in the basicity (K _BH ⁺) of the extra ligand and is proportional to the shift of the main bands (?λ) in the electronic absorption spectra. The geometric and energy characteristics of hexacoordinated aluminum porphyrin were calculated by the PM3 method. Correlations were found between the calculated energy of the interaction of the aluminum atom with the base molecule (E _b) and stability of the mixed-ligand complexes (Cl)Al(L)P. The cis and trans effects in the complexes (Cl)Al(L)P were analyzed. The dependence of the strength of the Al-L bond on the nature of the porphyrin and the basicity of the additional molecular ligand was determined from the experimental data and calculation results. Nauka/Interperiodica 2006.

Full text of DOI:10.1134/S1070363206100276

ISSN 1070-3632, Russian Journal of General Chemistry, 2006, Vol. 76, No. 10, pp. 1660 1667.
Pleiades Publishing, Inc., 2006.
Original Russian Text S.V. Zaitseva, S.A. Zdanovich, O.I. Koifman, 2006, published in Zhurnal Obshchei Khimii, 2006, Vol. 76, No. 10, pp. 1732 1739.
Effect of Steric Strains in the Macroring on the Structure
and Properties of Molecular Complexes
of (Chloro)[5,15-(p-butoxyphenyl)-2,8,12,17-tetramethyl-
3,7,13,17-tetrabutylporphinato]aluminum
S. V. Zaitsevaa, S. A. Zdanovicha, and O. I. Koifmana,b
St. Petersburg State University, St. Petersburg, Russia
Received April 25, 2006
Abstract The chelate (Cl)AlP was prepared by complexation of porphine (P) with aluminum(III) chloride
in refluxing pyridine. Equilibrium coordination of nitrogen-containing ligands (L = 2-methylimidazole, imida-
zole, pyridine, 3,5-dimethylpyrazole, dimethylformamide) with (Cl)AlP in benzene was studied by spectro-
photometric titration and computer simulation. Quantitative and qualitative characteristics of the reaction were
obtained. The structure of the mixed-ligand complex formed by intermolecular interaction of the metal
porphyrin with a base was determined spectrophotometrically and by quantum-chemical calculations. An
effect of additional molecular ligand and of steric strain in the macroring on the stability of the complex was
+
noted. The stability constant (Ks) increases with an increase in the basicity (KBH ) of the extra ligand and is
proportional to the shift of the main bands ( ) in the electronic absorption spectra. The geometric and energy
characteristics of hexacoordinated aluminum porphyrin were calculated by the PM3 method. Correlations
were found between the calculated energy of the interaction of the aluminum atom with the base molecule
(Eb) and stability of the mixed-ligand complexes (Cl)Al(L)P. The cis and trans effects in the complexes
(Cl)Al(L)P were analyzed. The dependence of the strength of the Al L bond on the nature of the porphyrin
and the basicity of the additional molecular ligand was determined from the experimental data and calculation
results.
DOI: 10.1134/S1070363206100276
Growing interest in macrocyclic complexes with
R2
R2
highly charged metal cations is primarily due to the
possibility of their use as redox catalysts [1 4].
Cl
Al
R1
C4H9O
R1
N
One of elementary reactions in catalysis is extra
coordination [3, 4]. To develop effective catalysts, it
is necessary to study this reaction in detail with sim-
ple model systems metal porphyrin base. However,
such reactions have not been comprehensively studied
up to now.
N
N
N
R1
OC4H9
R1
R2
R2
(Cl)AlP, R1 = C4H9; R2 = CH3.
To study systematically the influence of the struc-
ture of porphyrin and nature of the central metal atom
on the coordination properties of macrocyclic com-
pounds, we examined in this study the interaction of
(chloro)[5,15-(p-butoxyphenyl)-2,8,12,17-tetramethyl-
coordination of nitrogen-containing bases L with
aluminum porphyrin occurs in one step and virtually
instantaneously. Formation of the Al L bond in the
macroring, as in the complex of zinc with a similar
porphyrin [9], is accompanied by a bathochromic shift
and a change in the intensity of the main absorption
bands of the chromophore (Fig. 1) [10]. Such changes
in the electronic spectrum of the metal porphyrin are
associated, as a rule, with an increase in the electron
density on the central metal atom and, corresponding-
ly, on the nitrogen atoms of the porphyrin, with the
destabilization of the molecular orbital a2u, no change
3,7,13,17-tetrabutylporphinato]aluminum
(Cl)AlP
with nitrogen-containing bases (2-methylimidazole,
2-MeIm; imidazole, Im; pyridine, Py; 3,5-dimethylpy-
razole, DMP; dimethylformamide, DMF) in benzene.
The interactions were studied by spectrophotomet-
ric titration [5] and computer simulation [6 8].
Spectrophotometric titration data show that the
1660

EFFECT OF STERIC STRAINS IN THE MACRORING
(a)
1661
408
421
1
(b)
2
A
3
4
5
6
A
0.30
0.26
0.22
7
8
9
10
5
10
15
20
103, M
c
11
12
400
420
440
, nm
Fig. 1. (a) Variation of the electronic absorption spectrum of (Cl)AlP in benzene with addition of imidazole. Imidazole
2
concentration, M: (1) 0 and (12) 6.3 10 ; (2 11) intermediate concentrations. (b) Curve of spectrophotometric titration of
(Cl)AlP with imidiazole ( = 407 nm).
in the level of a1u, and decrease in the configuration
interaction of the excited state of the Eu type [10].
porphine), are ascending and are described by the
+
equations logKs = 1.2583 + 0.0002pKBH (r = 0.971)
2
and logKs = 0.0189
0.098
+ 1.2428 (r =
The intermolecular interaction of the macrocyclic
compound with the nitrogen-containing base yields a
complex (Cl)Al(L)P [metal porphyrin coordinates one
base molecule (Fig. 2)].
0.997), respectively. It should be noted that the linear-
ity of the dependences is disturbed owing to an in-
log [(Ae A0)/(A
Ae)]
DMF
2
1.5
From data in Table 1, we estimated the stability of
the molecular complexes, which increases in the order
(Cl)Al(DMF)P < (Cl)Al(DMP)P < (Cl)Al(Py)P <
(Cl)Al(2-MeIm)P < (Cl)Al(Im)P. With an increase in
temperature by 20 K, this order is not altered, but the
stability constants decrease by a factor of 1.6 2. We
found correlations between Ks, basicity of the extra
DMP
Py
2-MeIm
1
0.5
0
2
2.2 2.4 2.6 2.8 3.0 3.2
1.8
0.5
log cL
Im
+
ligand (characterized by pKBH and determined poten-
1
tiometrically in water at 298 K [11]), and shift ( ) of
the main absorption bands in the electronic absorption
spectra of the complexes (Table 1). The correlations
obtained, in contrast to the similar dependences for
the complexes (Cl)AlTPP (TPP is meso-tetraphenyl-
1.5
Fig. 2. Plots of log[(A
the intermolecular interaction of (Cl)AlP with nitrogen-
containing bases in benzene.
A )/(A
A )] vs. logc for
e L
e
0
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 76 No. 10 2006

1662
ZAITSEVA et al.
Table 1. Thermodynamic parameters of the intermolecular interaction of (chloro)[5,15-(p-butoxyphenyl)-2,8,12,17-tetra-
methyl-3,7,13,17-tetrabutylporphinato]aluminum with nitrogen-containing bases in benzene
1
Ks, l mol
logKs
(298 K)
,
H,
kcal
S,
kcal
+
Extra ligand
pKBH
nm
298 K
308 K
318 K
Im
2-MeIm
Py
DMP
DMF
1510.0 295.0
124.0 11.40
88.3 10.30
37.9 4.15
904.0 90.40
90.3 10.60
62.2 6.49
26.9 5.66
10.2 0.85
609.0 60.90
66.9 3.23
39.8 2.94
19.5 3.96
7.7 1.24
3.1789
2.0934
1.9459
1.5786
1.1789
6.65
5.89
5.29
4.27
0.92
13
10
9
7.5
4.5
8.557
5.809
7.489
6.256
6.361
14.17
9.92
16.23
13.77
15.95
15.1 1.63
crease in the steric strain of the macroring in the metal
porphyrin [12].
Scheme 1.
Cl
With an increase in the deformation of the porphy-
rin in going from (Cl)AlTPP to (Cl)AlP, the aromatic-
ity of the macroring decreases and, as a result, its
basicity increases. A decrease in the number of phenyl
substituents and introduction of alkyl groups into
positions of the macroring also lead to an increase
in the basicity. As a result, the stability of molecular
complexes of (Cl)Al(L)P becomes 2 3 times lower
than the stability of (Cl)Al(L)TPP (Table 1, [12]).
N2
N1
Al
N4
N3
the lowest internal stresses of the macroring [10, 13]
shows that the molecule of (Cl)AlP has a nonplanar
distorted structure. The Al atom is shifted to one of
the meso-C atoms and deviates from the mean N4
plane by 0.4985 toward the acido ligand. The pyr-
role fragments are inclined alternately up and down
(Fig. 3). The macroring is unsymmetrical: The dis-
tances between the opposite internal N atoms (N1 N3,
N2 N4) and between the opposite external carbon
atoms (Cm1 Cm3, Cm2 Cm4) are not equal (Table 2).
The geometries of the pyrrole rings are also different:
Four CNC bond angles are 107.25 , 107.09 , 108.73 ,
and 108.52 . The skeleton atoms of the porphyrin
macroring deviate from the mean molecular plane
(Fig. 4).
It is interesting that, despite higher formal charge
and small ionic radius, aluminum porphyrin (Cl)AlP
forms less stable complexes than its zinc analog [9].
In this case, the stability is influenced not only by the
macroring strain but also by the presence of the acido
ligand in the mixed-ligand complx (Cl)Al(L)P. It
should be noted that the Al Cl bond, which is largely
covalent, decreases the effective positive charge on
the Al atom, which also results in destabilization of
the molecular complexes of the aluminum porphyrin.
From the experimental data, we determined the
thermodynamic characteristics of the intermolecular
interaction ( H, S). The complexation of (Cl)AlP
with L is an exothermic reaction with low negative
values of S (Table 1). The negative entropy charac-
terizes a decrease in the number of the reacting spe-
cies and an increase in the ordering in the system.
The total effective charge (qeff) on the coordinated
N atoms is 0.172 e, and the charge on the metal atom
is +0.621 e.
In molecular complexes of (Cl)AlP with nitrogen
bases, the coordination core becomes bipyramidal
(Scheme 2). Such changes are accompanied by an
By the PM3 quantum-chemical method, we opti-
mized the structures of (Cl)AlP and its molecular
complexes and determined the geometric and energy
characteristics of the complexes.
Scheme 2.
Cl
N2
N1
The compound (Cl)AlP has a pyramidal structure
of the coordination core with a rhombic base, with
approximate symmetry C4v (Scheme 1). This conclu-
sion is well consistent with X-ray structural data [10].
Al
N3
N4
L
Comparison of the optimized structure of (Cl)AlP
with the averaged structure of metal porphyrins with
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 76 No. 10 2006

EFFECT OF STERIC STRAINS IN THE MACRORING
1663
Table 2. Some geometric and energy characteristics of molecular complexes of (chloro)[5,15-(p-butoxyphenyl)-2,8,12,17-
tetramethyl-3,7,13,17-tetrabutylporphinato]aluminum
Interatomic distances,
Eb,
kcal mol
Extra ligand
1
N1 N3
N2 N4
lav. Al Np
Al L
Al Cl
Al Ct Cm1 Cm3 Cm2 Cm4
4.1300
4.1689
4.1701
4.1671
4.1653
4.1581
4.1407
4.1902
4.1888
4.1909
4.1856
4.1945
2.1299
2.1242
2.1218
2.1235
2.1214
2.1188
2.3047
2.3439
2.3489
2.3431
2.3461
2.3513
0.4685
0.3847
0.3685
0.3672
0.3637
0.3402
7.2185
7.2381
7.2389
7.2423
7.2402
7.2383
6.6757
6.6745
6.6707
6.6708
6.6725
6.6769
Im
2-MeIm
Py
DMP
DMF
9.13
7.03
4.99
2.45
9.59
2.4984
2.5212
2.5217
2.5476
2.6498
increase in the in-plane and torsion strains in the
metal porphyrin (Table 2; Figs. 3, 4).
have structures of the type (Cl)Al(L)P. This result is
well consistent with the experimental data. The base
molecule is coordinated from the side opposite to the
acido ligand (Fig. 3).
According to the calculations, the molecular com-
plexes of the sterically strained aluminum porphyrin
(Cl)AlP
(Cl)Al(2-MeIm)P
Fig. 3. Structure of (chloro)[5,15-(p-butoxyphenyl)-2,8,12,17-tetramethyl-3,7,13,17-tetrabutylporphinato]aluminum and its
molecular complex with 2-methylimidazole, calculated by the PM3 method.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 76 No. 10 2006

1664
ZAITSEVA et al.
(Cl)AlP
(Cl)AlP + Py
Z,
Z,
0.25
1
2
.
0.05
0
10
20
0.05
0
10
20
Atom no.
0.15
0.15
Atom no.
(Cl)AlP + Im
Z,
0.20
(Cl)AlP + DMF
Z,
0.20
10
20
0.00
0.20
0
10
20
0.00
0.20
0
Atom no.
Atom no.
(Cl)AlP + 2-Me-Im
(Cl)AlP + DMF
Z,
0.10
0.05
0.00
Z,
0.15
20
20
10
0
0
10
0.05
0.25
0.05
0.25
Atom no.
Atom no.
Fig. 4. Deviation Z from the mean molecular plane of the skeleton atoms of the porphyrin macroring in (chloro)[5,15-(p-
butoxyphenyl)-2,8,12,17-tetramethyl-3,7,13,17-tetrabutylporphinato]aluminum and its molecular complexes with nitrogen-
containing bases, according to PM3 calculations. (1) N and (2) C atoms.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 76 No. 10 2006

EFFECT OF STERIC STRAINS IN THE MACRORING
1665
IAl L
PerN
1
4
11.81
2-MeIm
Im
2.58
2
3
4
Py
11.80
5
DMP
2.48
1
1.5
2
2.5
3
3.5
DMF
log Ks
11.79
0
2
4
6
8
pKBH+
Fig. 5. Correlation between the Al L bond length and
stability of the molecular complex: (1) (Cl)Al(DMF)P,
(2) (Cl)Al(DMP)P, (3) (Cl)Al(Py)P, (4) (Cl)Al(2-
MeIm)P, and (5) (Cl)Al(Im)P.
Fig. 6. Correlation between the perimeter of the N
4
+
coordination plane of the macroring and pK
of the
BH
nitrogen-containing base.
Formation of the molecular complexes is accompa-
nied by an increase in the size of the coordination
cavity N4 (Table 2) and in the effective charge on N
atoms by 0.055 0.113 e. The charge of the metal
atom decreases by 0.019 0.034 e. The deviation of
the Al atom from the N4 plane (Al Ct) decreases in
the order Im > 2-MeIm Py > DMP > DMF. In the
case of sterically strained (Cl)AlTPP, the trend is
opposite [12]. For (Cl)Al(L)TPP and (Cl)Al(L)P, the
correlations between the length of the Al L bond in
the complex and its stability are largely similar. The
stronger the Al L bond, the higher Ks (Tables 1, 2).
However, as in the above-described cases, for (Cl)Al
of the trans effect in the molecular complexes is con-
firmed by variation of the Al Cl and Al Ct distances
and by an increase in the negative charge on Cl (by
0.047 0.065 e), observed with formation of the Al L
bond (Table 2).
The calculation results obtained show that the ener-
gy consumption (Eb) for formation of a bond between
the Al atom and the N atom of the base increases in
the order (Table 2) (Cl)Al(Im)P < (Cl)Al(2-MeIm)P <
(Cl)Al(Py)P < (Cl)Al(DMP)P < (Cl)Al(DMF)P. The
stability of the molecular complex correlates with Eb
(FIg. 7): logKs = 0.0014(Eb)2 + 0.022Eb + 0.4107,
with the correlation coefficient r = 0.982. An increase
in Eb corresponds to a decrease in the Al L bond
strength.
(L)P, in contrast to (Cl)Al(L)TPP, the correlation is
2.607 log Ks
nonlinear: lAl L = 2.502(1
e
)
1.208, r =
0.994 (Fig. 5).
The good agreement between the experimental and
calculated data (Tables 1, 2) allows the use of com-
puter simulation for predicting characteristics of the
extra coordination reaction and the structures of the
molecular complexes.
We found that the Al L bond length correlates
with the Al Ct distance (Table 2). The correlation is
ascending and is described by the equation lAl L
=
2
62.563(lAl Ct
)
48.792lAl Ct + 12.009, with the
correlation coefficient r = 0.9909. The perimeter of
the N4 coordination plane of the macroring (PerN ) of
molecular complexes linearly correlates with 4 the
+
Our results show that the stability of metal por-
phyrin extra complexes depends on the nature of the
extra ligand and central metal atom and on the struc-
ture and extent of steric strain of the macroring. For-
mation of molecular complexes is accompanied by
changes in the electronic and geometric structure and,
as a consequence, by an increase in the steric strain
of aluminum porphyrin.
basicity of the extra ligand: PerN = 0.0017pKBH
+
4
11.791, r = 0.9686 (Fig. 6). The size of the N4 plane
in the molecular complex increases with an increase in
+
pKBH of the extra ligand.
We found that the strength of the Al NP bond
in aluminum porphyrins decreases in the series of
bases DMF > DMP > 2-MeIm > Py > Im (Table 2).
In this case, the interaction of the aluminum atom
with the nitrogen atom of the extra ligand becomes
weaker; as a result, the stability of the metal porphyrin
decreases (Tables 1, 2). Thus, the cis effect of ligands
in the extra complexes is fairly strong. The occurrence
EXPERIMENTAL
The complexation equilibrium, (Cl)AlP + nL =
(Cl)Al(L)nP, was studied in benzene at 298, 308,
and 318 K.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 76 No. 10 2006

1666
ZAITSEVA et al.
mean plane of the macroring. The orientation of aro-
matic bases was set so that the mean plane of the extra
ligand was perpendicular (for dimethylformamide,
parallel) to the mean plane of the porphyrin macro-
ring, and the nitrogen atom with the lone electron
pair was directed toward the coordination center. The
orientation of the mean plane of the extra ligand rela-
tive to phenyl substituents was varied with a step of
30 ; in the course of calculations, the mean planes of
the base (except dimethylformamide) and phenyl frag-
ments became virtually parallel in all the cases.
log Ks
0.6
0.5
0.4
5
3
0.3
0.2
0.1
4
2
1
5,15-( p-Butoxyphenyl)-2,8,12,18-tetrabutyl-
3,7,13,17-tetramethylporphyrin was prepared by con-
densation of the corresponding dipyrrolylmethane
with p-butoxybenzaldehyde, following published
protocols [21 23]. Electronic absorption spectrum
(chloroform), max, nm (log ): 632.0 (3.72), 549.0
12
7
2
0
3
8 Eb
Fig. 7. Correlation of the stability of molecular com-
plexes (Cl)Al(L)P with E : (1) (Cl)Al(Im)P, (2) (Cl)Al
1
b
(3.80), 517.0 (4.12), 413.0 (5.10). H NMR spectrum,
(2-MeIm)P, (3) (Cl)Al(Py)P, (4) (Cl)Al(DMP)P, and
(5) (Cl)Al(DMF)P.
, ppm: 10.07 (m”s°-H), 2.79 m (Bu H), 3.39 s
(Me H),4.00 m (OCH2), 2.27 s (NH).
Variation of the electronic absorption spectrum of
the metal porphyrin with varying the extra ligand con-
centration allows spectroscopic monitoring of the
extra coordination. Measurements were performed at
= 407 nm. The electronic absorption spectra were
recorded on SF-26 and Specord M-400 spectrometers,
(Chloro)[5,15-(p-butoxyphenyl)-2,8,12,17-tetra-
methyl-3,7,13,17-tetrabutylporphinato]aluminum.
A 10-mg portion of 5,15-(p-butoxyphenyl)-2,8,12,18-
tetrabutyl-3,7,13,17-tetramethylporphyrin was dis-
solved in 30 ml of anhydrous porphyrin, and a 50-fold
excess of AlCl3 was added. The mixture was refluxed
for 6 h. After cooling, the complex was precipitated
by adding 100 ml of water and 30 ml of acetic acid,
filtered off, washed with warm water, dissolved in a
minimal amount of CHCl3, and chromatographed on
alumina (Brockmann grade II, eluent CHCl3). After
repeated chromatographic purification, the dry com-
plex was isolated by removing CHCl3 in a vacuum;
yield 61%. Electronic absorption spectrum (benzene),
max, nm (log ): 578.0 (4.48), 539.0 (4.41), 408.0
(5.38). Found, %: C 76.1; H 8.15; N 6.0. C60H76Al
ClN4O2. Calculated, %: C 76.04; H 8.08; Al 2.85;
Cl 3.74; N 5.91; O 3.38.
1
and the H NMR spectra, on a Bruker AC-200 spec-
trometer (200 MHz, external reference HMDS) in
CDCl3. The experimental procedure and the procedure
for calculating the stability constants of the extra
complexes (Ks) are described in detail in [5, 14 17].
The number of the added ligands was determined by
the Bent French method [5].
The quantities H and S for the extra complexa-
tion were determined by least-squares treatment of the
dependences of lnKs on 1/T.
Quantum-chemical calculations were performed by
the PM3 method [6 8] with full geometry optimiza-
tion. The calculations were performed until a preset
gradient, 0.06 kJ mol
ACKNOWLEDGMENTS
1
1, was attained. The PC
version [18] of the Gamess program package [19] was
used. Preparation of the initial data and processing of
the results of geometry calculation were performed
using ChemCraft program (author A.A. Zhurko). In
the starting approximation, we used for the porphyrin
macroring the bond angles and lengths in the averaged
structure of metal porphyrins [10]; the bond lengths
and angles in phenyl and alkyl substituents of the por-
phyrin and in nitrogen bases were taken from [20].
The butyl substituents in the starting approximation
were set in the form of trans isomers in the staggered
conformation. The mean plane of the phenyl frag-
ments was oriented at a right angle relative to the
The study was supported by the Basic Research
Program of the Russian Academy of Sciences Theo-
retical and Experimental Study of the Nature of
Chemical Bond and Mechanisms of the Most Import-
ant Chemical Reactions and Processes and by the
Russian Foundation for Basic Research (project no.
06-03-32537-a).
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