Download Influence of the benzyl-substitution in porphyrin macrocycles on its

Influence of the benzyl-substitution in porphyrin macrocycles on its
ability to extra-ligand axial coordination
Galina Mikhailovna Mamardashvili+*, Olga Valentinovna Maltceva,
Chizhova Natalya Vasil’evna
G.A. Krestov Institute of Solution Chemistry of the Russian Academy of Sciences,
Akademicheskaya St.1, 153045 Ivanovo, Russia
Тел.: (4932)33-69-90. Е-mail: [email protected]
_______________________________________________________
*
Leading directions; + Corresponding author
Keywords: zinc tetrabenzoporphyrin complexes, axial ligands, nitro-substitution, spectrophotometric
titration
Abstract
Axial coordination processes of the extra-ligands (pyridine and 1-methilmidasol) with zinc(II)
tetrabenzoporphyrin and it’s tetra-nitrosubstituted analogue were investigated in toluene by the
spectrophotometric titration and computer simulating methods. It was established, that the expansion of
the aromatic π-system by addition of additional cycles results in the increasing of the porphyrinic
macrocycle binding ability with axial ligands, in comparison with zinc(II) aryl- and alkyl-substuted
porphyrins. The coordination process is accompanied by the red shifts of the absorption bands in the UVVis spectra of the reactionary system. It was shown, that the influence of the nito-substitution in
tetrabenzoporphyrinic macrocycle on the axial coordination processes is less as compared with the
influence of similar substitution in the octaethylporphirin.
Introduction
Axial coordination of extra-ligands on the coordinatively unsaturated metal cations of the
porphyrinates is significant property of tetrapyrrolic aromatic macrocycles, underlying in the
basis of the majority functions of biologically active metalloporphrins [1-3]. It is known, that
axial bond strength of extra ligands on metalloporphyrins in solutions mainly depends on the
basicity of the ligand and the solvent, and on the nature of central metal cation and functional
substitution on periphery of porphyrinic macrocycle [4-5].
Present work is devoted to the investigation of the tetrapyrrolic macrocycle chemical
modification influence on the axial coordination of the N-containong ligands. Therefore we
investigated the binding ability of the zinc(II)tetrabenzoporphyrin (ZnP1) and it’s tetranitrosubstituted analogue (ZnP2) towards two nitrogen-contained extra ligands (pyridine and 1methilmidasol) in toluene. Porphyrins (ZnP3-5) were investigated earlier and were used as object
of comparison.
Experimental
In this work we used commercially available ligands pyridine and 1-methilmidasol
(Aldrich-Sigma). Zinc(II)tetrabenzoporphyrin (ZnP1) and Zinc(II) tetranitrotetrabenzoporphyrin
(ZnP2) were synthesized as described in [8-9].
Complexation processes of porphyrins with amines were studied by the
spectrophotometric method, using spectrophotometer Carry 100. NMR 1Н – spectra of the
porphyrins and their complexes were obtained using spectrometer Bruker AV III-500.
Stability constants Kу of zinc(II) porphyrin complexes (A) with the appropriate substrate
(B) were calculated from spectrophotometric titration data using the equation:
А
А
i, 
o, 
[ A  B]
1
2 ) ( моль / л) 1
1
Kу 
 [ B] (

[ A] [ B]
А
А
o, 
i, 
1
2
Where 1 – decreasing wavelength, 2 – increasing wavelength, [А] – porphyrin
concentration, [В] – substrate concentration, Аo – maximal change of solution absorbance on
current wavelength, Аi - maximal change of solution absorbance on current wavelength at
current concentration.
Geometric optimization of the studied porphyrins and its pyridine and imidazole axial
complexes was carried using DFT/B3LYP BLYP/6-31G (d, p) method and Gaussian software.
Results and discussion
The expansion of the aromatic π-pophyrinic system by insertion of extra cycles
(tetrabenzo-substitution) or increasing it’s aromaticity by aza-substitution results in changes in
the majority of physicochemical properties of the metalloporphyrins and their ligands [1].
It is well known, that formation of axial porphyrin complexes (their structure,
composition, stability) depends on nature of the coordinating metal. The Zn(II) cation has
electron-deficit 4pz and 4dz2 orbitals, which acts in donor-acceptor interaction with electrondonor ligands. Upon mono-extracomplexes formation occurs red shift of the Sore band and Qbands in the UV-Vis spectra of the reactionary system. There is a clear correlation between the
dependence of the shift from the basicity of a axial ligand: the more is the ligand basicity, the
higher is the extracomplex stability, and the more red shift of the absorption bands in UV-Vis
spectra are observed [1,5]. Upon axial coordination of pyridine molecules to Zn-mesotetraphenylporphyrin (ZnP3) or to Zn-(β-oktaethyl)pophin (ZnP4) the value of the shift is 10 nm.
In case of the imidazole or methylimidazole extracoordination the shift is about 11-12 nm.
Investigation of the zinc tetrabenzoporphyrin complexation with pyridine (L1) and
methylimidazole by the spectrophotometric method in toluene shows, that in a wide
concentration range of the ligand (Cligand is from 0 to 810-4 mol/l) spectral changes upon the
complexation occur with preservation of one family of isosbestic points. This fact indicates the
formation of one type of complexes in ratio of 1:1(fig. 1). The most visible spectral changes are
in Sore band region. The red shift of the Sore band is 7- 8 nm.
a)
b)
Fig.1. а) Spectral changes in Sore band region of ZnP(1) solution at MeIm titration from 0 to 410-4
mol/l, Сporphyrin.=5.110-6 mol/l, b) Spectrophotometric titration curves of ZnP(1) by methilimidazole in
toluene on «increasing» and «decreasing» wavelength 20оС.
Based on the experimental findings it could be concluded that the axial coordination
property of the Zn-tetrabenzoporhyrin towards the nitrogen-containing bases is less, than the
aryl- or alkyl-substituted porphyrins. However, stability constants, calculated by the standard
method and depicted in the table 1, indicate, that by stability of the complexes ZnTBP-L are
similar to the analogue complexes, which are formed by aryl- and alkyl-porphyrins. Computer
simulation data confirms the experimental results (table 2, fig. 1).
Table.1. Stability constants of the complexes ZnP-L (Кassoc., l/mol) in toluene.
Ligand
L1
L2
ZnP1
8200
104600
ZnP2
6400
93000
ZnP3
5800
3600
ZnP4
5600
ZnP5
Error in calculating Кassoc. is 57%.
Table 2. Geometric parameters of the axial complexes ZnP-L
Porphyrin
ZnP1
ZnP3
Ligand
L1
L2
L1
Zn-N (Ǻ)а
2.108 (2.108)
2.108 (2.120)
2.086 (2.084)
ZnN(ligand) (Ǻ)б
2.162
2.121
2.195
Zn P (Ǻ)в
0.406
0.426
0.423
L2
2.089 (2.085)
2.142
0.426
а
Zn-N - average value of coordination bonds in coordinate center
б
ZnN(ligand) – length of donor-acceptor bond between porphyrin and ligand
в
Zn P – distance, on which Zn cation escapes from coordinate center in axial coordination
Table 3. Calculated formation enthalpies of ZnP1-ZnP3 porphyrins and their methilimidazole axial
complexes
ZnP3
ZnP1
ZnP2
Hf(ZnP),
(kcal/mol)
Hf(ZnP∙L1),
(kcal/mol)
ΔgH°,
(kcal/mol) *
Hf(ZnP∙L2),
(kcal/mol)
ΔgH°,
(kcal/mol)*
307.76
250.97
266.14
328.91
260.73
281.93
-9.08
-20.47
-14.44
325.83
256.87
276.70
-11.62
-23.79
-19.13
*Enthalpies of chemical reactions in ideal gas phase were calculated by Guess law on the basis of
formation enthalpies of reagents
The obtained data (tables 2,3) let us suppose, that increasing of a aromaticity degree of
the porphyrin macrocycle leads to increasing of the sizes of porphyrin coordination center. As a
result, the bonds in coordination center Zn-N extend and become less stable. But at the same
time the stability of additional donor-acceptor interaction Zn-L increases.
Fig.2. NMR 1Н-spectrum of axial complex of ZnP1-MeIm in DMSO.
As it was shown in our earlier papers [6,11], the insertion of nitro-groups in mesoposition of the Zn-oktaethylporphyrin results in significant increasing of the macrocycles binding
ability towards N-containing organic molecules. Bulky nitro-groups in meso-positions cause the
deformation of porphyrin macrocycle. As a result the stability of axial complexes increases on
one degree.
Calculation results show, that similar to Zn oktaethylporphyrin the insertion of four NO2substitutes into tetrabenzoporphyrin macrocycle leads to a deformation of the macrocycle. As a
result of tetranitro-substitution, the tetrabenzoporphyrin macrocycle gains saddle-point
conformation (fig. 3). The main factor, characterizing the features of conformation structure of
porphyrin molecule, is steric interaction between hydrogens of the benzo- substituents and NO2.
ZnP4
ZnP5
ZnP1
ZnP2
Fig.3. Optimized structures of ZnP1, ZnP2, ZnP4 and ZnP5 molecules.
The comparison of ΔgH° values for ZnP1-L1 (-23.79 ccal/mol) and ZnP2-L1 (-14.44 ccal/mol)
and increasing of the distance between zinc atom and bounded nitrogen atom of the ligand in this
complexes indicate on decreasing of stability of axial binding of tetranitro-substituted macrocycle in
comparison with unsubstituted.
Spectrophotometric titration data of ZnP2 by L1-L2 ligands showed, that in axial complexes
formation the red shift of the Sore band is about 5-6 nm, and calculated stability constants of
ZnP2-L1 and ZnP2-L2 complexes are less than stability constants for ZnP1-L1 and ZnP1-L2.
This is due to the influence of functional substitution in tetrabenzoporphyrin macrocycle on it’s
ability to extracoordination. According to table 1 data, if the coordinating property of ZnP2 to
pyridine upon nitro-substitution decreases by 20%, stability constants of methylimidazole complexes for
both porphyrins ZnP1 and ZnP2 are practically comparable.
ZnP2-L1
ZnP2-L2
Fig.4. Optimized structures of axial complexes of ZnP2-L1 and ZnP2-L2.
Calculation results show, that in the case of ZnР2-L2 complex the formation of two very
weak intermolecular bonds between oxygen atom of porphyrin nitro-group and hydrogen atoms
of ligand are observed. This interaction is weak, but resulting in changing of porphyrin
macrocycle structure. It transforms from saddle into grooved (fig. 4). The bond lengths of the
first Н…О bond and for the second are is 3,36 Å and 3,84 Å, respectively. These lengths in
pyridine are 4.42 and 4.49 Ǻ respectively. Apparently, decreasing of stability of axial binding of
ZnP2 porphyrin with L2 ligand by macrocycle deformation is compensated by formation of two
additional intermolecular bonds between porphyrin and ligand (HO) in ZnP2-L2 complex.
Thus, the expansion of aromatic porphyrin π-system by insertion of extra aromatic cycles
results in increasing of binding property of Zn-porphyrin macrocycles towards to nitrogencontaining axial ligands.
Spectrophotometric and 1Н NMR investigations were carried out on the equipment of the
Center of Collective Using of the Verkhnevolzhskiy Regional Center of Physicochemical
Investigations”.
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