Complexes of zinc 5,15-di(ortho-methoxyphenyl)octaalkylporphyrinate with nitrogen-containing bases

Article abstract of DOI:10.1134/S1070328406070049

The formation of molecular complexes of zinc 5,15-di(ortho-methoxyphenyl)- 2,8,12,18-tetramethyl-3,7,13,17-tetrabutylporphyrinate (ZnP) with nitrogen-containing bases L (L is imidazole, 2-methylimidazole, pyridine, 3,5-dimethylpyrazole, or dimethylformami

Full text of DOI:10.1134/S1070328406070049

ISSN 1070-3284, Russian Journal of Coordination Chemistry, 2006, Vol. 32, No. 7, pp. 481–488. © Pleiades Publishing, Inc., 2006.
Original Russian Text © S.V. Zaitseva, S.A. Zdanovich, O.I. Koifman, 2006, published in Koordinatsionnaya Khimiya, 2006, Vol. 32, No. 7, pp. 504–511.
Complexes of Zinc
5,15-Di(ortho-methoxyphenyl)octaalkylporphyrinate
with Nitrogen-Containing Bases
S. V. Zaitsevaa, S. A. Zdanovicha, and O. I. Koifmanb
a Institute of Solution Chemistry, Russian Academy of Sciences, Akademicheskaya 1, Ivanovo, 153045 Russia
b Ivanovo State University of Chemical Technology, Ivanovo, Russia
Received June 23, 2005
Abstract—The formation of molecular complexes of zinc 5,15-di(ortho-methoxyphenyl)-2,8,12,18-tetrame-
thyl-3,7,13,17-tetrabutylporphyrinate (ZnP) with nitrogen-containing bases L (L is imidazole, 2-methylimid-
azole, pyridine, 3,5-dimethylpyrazole, or dimethylformamide) in benzene is studied by spectrophotometry and
computer simulation. The nature of an additional molecular ligand affects the stability of the macrocyclic com-
plex (L)ZnP. The stability constant of this complex increases linearly with an increase in the basicity of the extra
ligand and is proportional to the shift of the main absorption bands in the electronic spectra (Q(0–1), B (Soret
band)). The molecular structures of zinc porphyrinate and its extra complexes are optimized by the PM3 quan-
tum-chemical method. Their geometric and energy parameters are calculated. Correlations between the calcu-
lated energies of interaction of the central metal atom with the nitrogen atom of the extra ligand and the stability
of (L)ZnP are found. The dependence of the zinc−extra ligand bond strength on the basicity of the additional
molecular ligand is determined on the basis of the experimental and calculated data. The coordination proper-
ties of the compound under study are found to be affected by steric strains.
DOI: 10.1134/S1070328406070049
INTRODUCTION
Quantum-chemical calculations were performed by
the PM3 method [2–4] with the complete geometry
optimization. The condition for the end of counting was
a specified gradient of 0.04 kJ/(mol Å). Calculations
were carried out using the PC version [9] of the Gamess
program package [10]. The initial data were prepared
and the results of geometry calculations were processed
using the ChemCraft program developed by
G.A. Zhurko. In the initial approximation, angles and
bonds of the averaged structure of metalloporphyrins
were used for the porphyrin macrocycle [11]. The bond
lengths and angles for the phenyl and alkyl substituents
of porphyrin and nitrogen-containing bases were taken
from [12]. In the initial approximation, the butyl sub-
stituents were specified as trans isomers in a hindered
conformation, and the mean plane of the phenyl frag-
ments was arranged at a right angle to the mean plane
of the macrocycle. The orientation of aromatic bases
was specified in such a way that the mean plane of the
extra ligand was perpendicular to the mean plane of the
porphyrin macrocycle (for dimethylformamide, it was
parallel) and the nitrogen atom with a lone electron pair
was directed toward the coordination center. The orien-
tation of the mean plane of the extra ligand relative to
the phenyl substituents was varied with an increment of
30°. In all cases, the mean planes of the base (except for
dimethylformamide) and phenyl fragments became vir-
tually planar during calculation.
With the aim to systematically study the regularities
of extra coordination of metalloporphyrins with steri-
cally strained structures, we investigated the formation
of complexes of zinc 5,15-di(ortho-methoxyphenyl)-
2,8,12,18-tetramethyl-3,7,13,17-tetrabutylporphyrinate
(ZnP) with nitrogen-containing ligands L (L is imid-
azole (Im), 2-methylimidazole (2-Me-Im), pyridine
(Py), 3,5-dimethylpyrazole (DMP), and dimethylform-
amide (DMF)) in benzene at 298 K.
The coordination properties of ZnP were studied by
spectrophotometric titration [1] and computer simula-
tion [2–4].
EXPERIMENTAL
The reaction ZnP + nL = Zn(L)nP was conducted in
benzene, which forms no stable solvate complexes with
metalloporphyrins and is a neutral solvent.
Electronic absorption spectra were recorded on
SF-26 and Specord M40 instruments. 1H NMR spectra
were obtained on a Bruker AC-200 spectrometer
(200 MHz) using hexamethyldisilane (HMDS) in deu-
terated chloroform as the internal standard. The exper-
imental procedure and calculation of stability constants
for the extra complexes (Ks) are presented in detail in
[1, 5–8].
481

482
ZAITSEVA et al.
5,15-di(o-Methoxyphenyl)-2,8,12,18-tetrame-
However, since it is difficult to separate the atropo-
thyl-3,7,13,17-tetrabutylporphyrin (I) was synthe- isomers and their spectra are identical, we studied their
sized by the condensation of the corresponding dipyr- mixture.
rolylmethane
with
ortho-methoxybenzaldehyde
according to published procedures [13–15]. Electronic
RESULTS AND DISCUSSION
absorption spectrum (benzene; λmax, nm (logε )): 622.0
1
(3.60), 574.0 (3.87), 542.0 (3.60), 410.0 (5.02). ç
The formation of the bond between the nitrogen
atom of a base (L) and the zinc atom of metalloporphy-
rin II is accompanied by the bathochromic shift and a
change in the intensity of the main absorption bands in
the electronic absorption spectrum of the chromophore
(Fig. 1) [11]. Such changes are caused by the electron
density redistribution in the macrocycle and destabili-
zation of the a2u molecular orbital at constant a1u level
[11]. As a result, the configurational interaction of the
excited state of the Eu type decreases and, as a conse-
quence, the main bands B and Q(0–1) become more
intense in the electronic absorption spectra of zinc por-
phyrinates (λB = 425.2 nm and λQ(0–1) = 551.2 nm,
Fig. 1). The bathochromic shift is caused by an increase
in the energy of the MO of the a2u type.
NMR (δ, ppm): 10.19 (meso-H), 3.99 (m, Bu-H), 2.53
(s, Me-H), 3.66 (s, OCH3), 2.44 (s, NH).
Zinc
5,15-di(o-methoxyphenyl)-2,8,12,18-tet-
ramethyl-3,7,13,17-tetrabutylporphyrinate (ZnP,
II) was synthesized as follows. Porphyrin I (10 mg)
was dissolved in benzene (50 ml), and a tenfold excess
of zinc acetate was added to the resulting solution. The
reaction mixture was refluxed for 40–50 min. The com-
pleteness of the reaction was confirmed by disappear-
ance of absorption bands of porphyrin at 622.0, 574.0,
542.0, and 410.0 nm and by the appearance of new
absorption bands of its metal complex at 574.7, 540.0,
and 413.9 nm. After the reaction completion, the reac-
tion mixture was cooled, and salt excess was removed
by extraction with water. A benzene solution of the zinc
complex was concentrated by evaporation and chro-
matographed on a column packed with silica gel (Sili-
cagel L 100/250) using chloroform as the eluent. Chlo-
roform was evaporated, and a crystalline precipitate
was purified by recrystallization from a chloroform–
methanol mixture. The yield of the complex was 95%.
Electronic absorption spectrum (benzene; λmax, nm
It was shown by spectral studies that the ZnP com-
plex is prone to add one molecule of a nitrogen-contain-
ing base (Fig. 2). Compounds with different basicities
were used as extra ligands. To complete the process
(ZnP was completely transformed into extra complex),
we chose different intervals of base concentrations
(cM–Im = 0–0.30 × 10–3, cIm = 0–0.20 × 10–3, cPy = 0–
0.30 × 10–3, cDMP = 0–1.20 × 10−2, and cDMF = 0–15.4 ×
10–2 mol/l). The experiment was finished when the
electronic absorption spectrum of the solution did not
change with time.
(logε )): 574.7 (4.04), 540.0 (4.22), 413.9 (5.18).
For C54H64ZnN4O2
An analysis of the experimental data made it possi-
ble to calculate the stability constants of extra com-
plexes (L)ZnP that formed in benzene
anal calcd., (%):
Found, %:
C 74.91,
C 75.03,
H 7.40,
H 7.45,
N 6.47.
N 6.42.
It should be mentioned that zinc porphyrinate II
exists as two atropoisomers.
K2s95 , l/mol
L
2-Me-Im
Im
806 41.7
3920 615
934 112
640 87.1
77.8 1.11
CH3
O
CH3
O
Bu
Bu
Me
Me
Me
N
N
N
Py
Zn
N
DMP
DMF
Me
Bu
Bu
α,α-atropoisomer
and to establish the dependence of Ks on the basicity of
the extra ligand (Kex+ ), whose value was determined
CH3
O
Bu
Bu
by the potentiometric method in water at 298 K [16]
(Fig. 3). The stability of extra complexes changes as
follows: (DMF)ZnP < (DMP)ZnP < (2-Me-Im)ZnP <
(Py)ZnP < (Im)ZnP. Despite the higher basicity of
2-Me-Im as compared to that of Py, the stability con-
stant of (2-Me-Im)ZnP is somewhat lower than Ks of
the (Py)ZnP complex. It is most likely that the bonding
of 2-Me-Im with metalloporphyrin is complicated by
Me
Me
N
N
N
Zn
N
Me
Me
O
Bu
Bu
CH3
α,β-atropoisomer
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COMPLEXES OF ZINC
483
413.9
A
1
2
3
4
5
6
7
8
425.2
A
(b)
0.90
0.75
0.60
10
9
0.001
0.002
0.003
cL, mol/l
(a)
×4
551.2
540.0
574.7
450
550 λ, nm
–5
Fig. 1. (a) Electronic absorption spectra of a solution of ZnP (c
= 2.0 × 10 mol/l) with imidazole in benzene at different con-
ZnP
–3
centrations of Im: (1) 0, (10) 4.0 × 10 , and (2−9) intermediate imidazole concentrations; (b) the corresponding titration curve, λ =
411 nm.
small steric hindrance caused by the methyl group in compound is nonsymmetric. The N(1)–N(3) and
position 2 of the base.
N(2)−N(4) distances in the ZnN4 coordination mode
are different. The CNC bond angles in the pyrrole rings
also differ: 105.6°, 107.8°, 107.3°, and 107.6°.
The obtained results make it possible to consider the
dependence of the stability of extracomplexes on the
shift (Dl) of the main absorption bands (λQ(0–1) and λB
for metalloporphyrin and its extra complex, respec-
tively) in their spectra. The electronic absorption spec-
tra are presented in Fig. 4. This dependence is linear
with the regression equation logKs = 0.2647∆λ +
0.5891 and correlation coefficient r = 0.9994. Thus, the
spectral changes during extra complex formation can
be used for the qualitative estimation of its stability.
The geometric and energy characteristics of the
interaction of metalloporphyrin with the extra ligand
were obtained by the PM3 quantum-chemical method.
The calculated metalloporphyrin molecule is nonpla-
nar. The phenyl substituents form an angle of 86°–88°
with the plane of the coordination center. The slope of
the line of the O–C bond in the OCH3 group to the plane
of the phenyl fragment is 1°–2°. An analysis of the tab-
A comparison of the optimized structure of zinc por-
phyrinate with the averaged structure of metal porphy-
rinates with the lowest internal strains of the macrocy-
cle [11, 17] shows that a molecule of the ZnP complex
is strained and has the “saddle” configuration. The N4
rhombus lies in the base of the Zn coordination polyhe-
dron [11]. The ZnN4 coordination mode is distorted due
to the intermolecular interaction of the ZnP atropoiso-
mers with the L nitrogen-containing bases, which
agrees with the X-ray diffraction data [11].
L
N(1)
N(2)
Zn
N(4)
N(3)
The appearance of the above structures is accompa-
ulated data shows that the coordination cavity of the nied, as a rule, by an increase in the planar and torsional
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484
ZAITSEVA et al.
logKeq
Aeq – A0
--------------------
A∞ – Ap
lg
Im
3
2
5
1
0.9
3.5
Py
tanα = 1
0.4
DMP
2-Me-Im
–0.1
–0.6
–1.1
–2
0
2
4
2.5
1.5
logcL
4
DMF
0.5
2.5
4.5
6.5
8.5
pKex
+
Fig. 3. Plot of logKeq of the (L)ZnP extra complexes vs.
basicity of the added ligands.
Aeq – A0
--------------------
Fig. 2. Plots of log
versus logcL for extra coordi-
A∞ – Ap
nation of ZnP with (1) 2-Me-Im, (2) Im, (3) Py, (4) DMP,
and (5) DMF in benzene (A , A , and A are the absor-
bances of the solution of the initial metalloporphyrin and its
equilibrium mixtures with the extra ligand at the working
0
eq
∞
the complex, increases (table). The values correspond-
ing to the shift of the zinc ion from the coordination
plane N4 (Zn–Ct) for the extra complex with
2-methylimidazole and dimethylformamide do not
obey the general dependence, which can be explained
by the existence of many bulky substituents near the
nitrogen atom of the extra ligand. The cis-effect of the
ligands is observed in (L)ZnP. An increase in the stabil-
ity of the bond between a metal and a base is accompa-
nied by a decrease in the stability of the bond between
the zinc atom and the nitrogen atom of porphyrin
(Zn−NP).
–5
wavelength λ = 411 nm; c
= 1.59–2.09 × 10 , c
=
ZnP
–3
MÂ−Im
–3
–3
0–3.0 × 10 , c = 0–2.0 × 10 , c = 0–3.0 × 10 , c
=
Im
Py
–2
DMP
–2
0–1.2 × 10 , c
= 0–1.54 × 10 ).
DMF
distortions of the macrocyclic ligand (table, Fig. 5). In
this case, the slope angle of the phenyl fragments to the
N4 plane is 80°–84°. The slope of the line of the O–C
bond in the OCH3 group to the plane of the phenyl frag-
ment increases to 3°–5°.
Extra complexes that formed can have different
structure due to different orientations of extra ligand
(which adds to ZnP) with respect to the functional
groups in the phenyl susbtituents of zinc porphyrinate
(Fig. 6). For the α,α-atropoisomer, the arrangement of
the extra ligand on the side of the functional groups in
the phenyl substituents is more favorable (Fig. 6a, 1).
This is associated with the fact that the metal atom in
uncoordinated zinc porphyrinate shifts from the macro-
cycle plane toward the functional groups. Otherwise,
the addition of a molecule of a nitrogen-containing base
needs energy expenses to pull the metal ion into the
coordination cavity.
In the case of the α,β-atropoisomer and the coordi-
nation of imidazole, 2-methylimidazole, and pyridine,
the geometric and energy characteristics depend insig-
nificantly on the orientation of the extra ligand with
respect to the functional groups in the phenyl substitu-
ents (Fig. 6b; 1, 2). The orientation of the molecule pre-
sented in Fig. 6c, 2 is the most favorable for dimeth-
ylpyrazole and dimethylformamide.
Note that some dependence is also observed
between the distortion of the macrocycle (in other
words, a change in the sizes of the coordination cavity)
and the Zn–NL bond length. An increase in the stability
logKeq
Im
3.5
Py
DMP
2-Me-Im
DMF
1.5
4
6
8
10
12
∆λ, nm
It was found that in (L)ZnP, the stability of the
σ-Zn–NL bond increases in the series of nitrogen-con-
taining bases L: DMP < Py < Im; in this case, Zn–Ct
distance, Ct is the center of the N4 macrocycle cavity in
Fig. 4. Relationship between logKeq of the (L)ZnP extra
complexes and the shift of the main absorption bands
(Q(0−1) and B) in the electronic absorption spectra.
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COMPLEXES OF ZINC
485
Selected geometric and energy characteristics of extra complexes (L)ZnP (L = Im, 2-Me-Im, Py, DMP, DMF)
α,α-tropoisomer
Bond lengths, Å
α,β-atropoisomer
Bond lengths, Å
Orientation of
extra ligand
Extra ligand
EZn–N
,
EZn–N ,
L
L
kcal/mol
kcal/mol
N(1)–(3)
N(2)–(4)
N(1)–N(3)
N(2)–(4)
Zn–NL Zn–Ct
Zn–NL Zn–Ct
4.0881*
4.1235*
4.1207
4.1529
4.1387
4.1393
4.1208
4.1538
4.1364
4.1373
4.1191
4.1497
4.1379
4.1367
4.1201
4.1535
4.1362
4.1346
4.1380
4.1224
4.1080
4.1545
0.0289*
4.0870*
4.1233*
4.1249
4.1524
4.1234
4.1534
4.1189
4.1552
4.1212
4.1512
4.1213
4.1503
0.0329*
Im
2-Me-Im
Py
1
2
1
2
1
2
1
2
1
2
2.0962 0.3729
2.0990 0.4007
2.1277 0.3947
2.1326 0.4165
2.1219 0.3655
2.1257 0.4010
2.2033 0.3450
2.2236 0.3650
2.2293 0.4248
2.2334 0.4402
–16.42
–15.31
–13.12
–12.26
–14.06
–12.44
–7.52
2.0983 0.3692
2.0981 0.3691
2.1318 0.3969
2.1313 0.3886
2.1248 0.3641
–15.53
–15.81
–12.60
–12.92
–13.10
DMP
4.1204
4.1513
4.1176
4.1450
4.1145
4.1472
4.1126
4.1466
2.1021 0.3340
2.2090 0.3356
2.2325 0.4138
2.2339 0.4172
–4.49
–7.17
–0.14
–0.74
–3.74
DMF
–1.08
–0.02
* Data for ZnP.
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486
ZAITSEVA et al.
(a)
Atom no.
10 15 20 25
Atom no.
10 15 20 25
0.2
0
–0.2
–0.4
–0.6
–0.8
0
5
5
5
5
–0.2
–0.4
–0.6
ZnP + Im
ZnP
Atom no.
10 15 20 25
Atom no.
10 15 20 25
0.2
0
–0.2
–0.4
–0.6
–0.8
5
5
0
–0.2
–0.4
–0.6
ZnP + Py
ZnP + 2-Me-Im
Atom no.
10 15 20 25
Atom no.
0.2
0
10 15
25
0
–0.2
–0.4
–0.6
–0.2
–0.4
–0.6
–0.8
ZnP + DMP
ZnP + DMF
(b)
Atom no.
Atom no.
0
5
10 15 20 25
0
5
5
5
10 15 20 25
–0.2
–0.4
–0.6
–0.2
–0.4
–0.6
ZnP
ZnP + Im
Atom no.
10 15 20 25
Atom no.
10 15 20 25
0.2
0
–0.2
–0.4
–0.6
–0.8
0
5
5
–0.2
–0.4
–0.6
ZnP + 2-Me-Im
ZnP + Py
Atom no.
Atom no.
10 15 20 25
0.2
0
0.2
0
10 15
25
–0.2
–0.4
–0.6
–0.8
–0.2
–0.4
–0.6
–0.8
ZnP + DMP
ZnP + DMF
Fig. 5. Deviation from the mean plane of the molecule (∆Z) of the skeletal atoms of the porphyrin macrocycle of ZnP and (L)ZnP
in the energetically favorable orientation: (a) α,α-atropoisomer and (b) α,β-atropoisomer according to the PM3 quantum-chemical
calculation data (ꢀ are nitrogen atoms, ꢁ are carbon atoms).
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 32 No. 7 2006

COMPLEXES OF ZINC
487
(a)
(2-Me-Im)ZnP
1
2
(b)
(2-Me-Im)ZnP
(c)
1
2
1
2
(DMF)ZnP
Fig. 6. Structures of the (a) α,α- and (b, c) α,β-atropoisomers of the (L)ZnP extra complexes calculated by the PM3 quantum-chem-
ical method with (1, 2) different orientations of the extra ligand.
of the Zn–NL bond induces an increase in steric strains agree well (table). This allows one to use the data of
in metalloporphyrin (table, Fig. 5).
quantum-chemical calculations to predict specific fea-
tures of complex formation of electron-donor com-
pounds with zinc porphyrinates and structures of extra
complexes that formed.
It can be concluded that the stability of the macrocy-
clic zinc extra complex is determined by the nature of
For extra complexes under study, the experimental
stability constants (see above), calculated lengths of the
bonds between the zinc and nitrogen atoms of the extra
ligand (Zn−NL), and energies of these bonds (EZn–N
)
L
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ZAITSEVA et al.
9. Granovsky, A.A., www http://classic.chem.msu.su/
gran/gamess/index.html.
10. Schmidt, M.W., Baldridge, K.K., Boatz, J.A., et al.,
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Moscow: Nauka, 1985.
12. Comprehensive Organic Chemistry, Barton, D. and
Ollis, W.D., Eds., NewYork: Pergamon, 1979, vols. 1, 2,
and 8.
the extra ligand (nitrogen-containing base) and defor-
mational distortions of porphyrin.
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