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The influence of alkyl bridge substitution on the porphyrin solubility
&S”LAR
LIQUIDS
Journal of Molecular Liquids 91 (2001) 189-191 www.elsevier.nl/locate/molliq
The influence
of alkyl bridge
G.M. Mamardashvili,
substitution
N.Zh. Mamardashvili,
on the porphyrin
O.A. Golubchicov
solubility
and B.D. Berezin
Institute of Solution Chemistry of the Russian Academy of Sciences 153045, Academicheskaya st., 1, Ivanovo, Russia 1. INTRODUCTION The influence of the alkyl substitution in the porphyrins on their solubility in the organic solvent considerably depends on which porphyrin position the hydrogen atoms are substituting the alkyl groups: in pyrrol rings (2,3,7,8,12,13,17,18), mezo-position ((5,10,12,20) or NHgroups of proper porphyrins. The macrocycle solubility values can increase or decrease ten times depending on the alkyl length and the substitution position. The influence of pyrrol alkyl substitution in the porphyrins on their solubility in benzene
and a series of alcohols has been the aim of a preceding paper [ 11. For the majority of octalkylporphyrins with alkyl substitutes in the 2,3,7,8,12,13,17,18 position the solubility values in benzene are 1W3 mol/. The number of alkyl groups, their lengths (corn CHJ- up to C&Iv) and the relative positions in the case of octalkylpoxphyrinisomers cause the difference between the solubility values in benzene of not more than two or three times, apparently, as consequence of the changes of molecule interactions in the crystals. The solubility values in the alcohols (methanol, ethanol, I-propanol) are far less than in benzene, and more important, they depend on the porphyrin structure. 2,3,7,8,12,13,17,1&octamethylporphin is an exception. It is hardly soluble in most organic solvents and has very low solubility values in benzene. In all other cases the alkyl (C~HS- and so on) substitution in the pyrrol rings - complete or incomplete, symmetric or asymmetric - leads to a strong increase of the macrocycle solubility. As this takes place with more than one species of the alkyl substitutes in the porphyrin molecules the alkyl length (in the limits C~HS-C~H~ investigated by us) results in a lesser effect on the solubility than the various positions of the substitutes in the macrocycle do in the case of the isomers. The influence of the mezo- and N-alkyl substitution in porphyrins on solubility in organic solvents has not been investigated quantitatively previously. For the investigation of the influence of the mezo-al@1 substitution we took a series of the octametbylporphyrins with the different alkyl substitutes in 5,15-positions of 13octamethylporphyrin. The above mentioned porphyrin solubility in benzene, ethanol and lpropanol at 298.14 K has been studied. 2. EXPERIMENTAL The porphyrin: 2,3,7,8,12,13,17118-octamethylporphin (P-octamethylporphin) (I); 5,15dimethyl+octamethylporphin (II); j~lj-diethyl-P-octamethylporphin (III); 5,15dipropyl-Poctamethylporphin (IV) and 5,15-dibuthyl-P-octamethylporphin (V) were synthesized by the 0167.7322/01/$ - see front matter 0 2001 Elsevier Science B.V. All rights reserved. PII SO167-7322(01) 00161-l
190
Standard method [ 11. The solvents were rectified by known methods; the water content in the solvents does not exceed O,Ol% in benzene and 0,03 % in ethanol and 1-propanol. The solubility has been studied by the method of isothermal saturation. The porphyrin concentration was measured spectrophotometrically. The method error was 5 %. 3. RESULTS AND DISCUSSION The octamethylporphin is hardly soluble (essentially insoluble) in organic solvents. The introduction of two methyl groups into the mezo-position has no profound effect on the macrocycle solubility. II as well as I are insoluble in most organic solvents, specifically in the alcohols, at room temperature. The solubility values in benzene are four times greater for II than for I, evidencing the insignificant difference in the energy of the crystalline lattice. The changes are attributable, at least partially, to the change of the electronic structure of I when introducing two bridge methyl groups, as revealed in the W-vis spectra. The first band in the spectrum of II is shifted batochromically when compared to that of I while its intensity decreases ten times (Table 1). Table 1 W-vis spectra of the alkyl porphyrines in benzene Compound h run, (lg E) II I 563 (3.72) 619 (2.95) Porphine 570 (4.14) 620 (4.03) I 574 (4.17) 625 (2.98) II 572 (3.14) 624 (2.97) III 575 (3.14) 625 (2.96) IV 572 (3.19) 626 (2.97) V
III 520 527 538 540 539 540
(4.48) (4.15j (3.21) (3.19) (3.27) (3.18)
IV 490 501 505 507 506 509
(4.20) (4.39) (3.77) (3.78) (3.77) (3.77)
hicreasing the alkyl length up to CrHs- in 5,15-positions of porphyrins raises the III solubility in benzene to 7,3x10” mol/l, that is by around two orders (Table 2). The further increase of the alkyl length up to C4H9 in mezo-position causes the solubility value to raise further. 515dipropyl substitution increases the solubility of IV approximately two times when compared with III in benzene and alcohols. 5,15dibutyl substitution increases the solubility of V in the alcohols three times, whereas it decreases somewhat the solubility in benzene. Consequently, not the influence of the crystalline lattice energy but of the macrocycle solvation on the III-V solubility is here evident. With increasing the alkyl length in mezo-positions of porphyrins (from CsHr) more favourable conditions appear for porphyrin solvation in the alcohols (ethanol and 1-propanol), and less favourable ones for solvation in benzene. Unfortunately, the X-ray structure or A Hsmr_ data for mezo-alkylporphyrins are not available. There is no way of inferring the influence of a given type of substitution on the macrocycle crystalline lattice strength.
191
Table 2 Solubility of the alkylp Porpyrin
phyrins (S2’* 10’ 9mole Benzene
Ethanol
n-Propanol
I
2.0
insohtble
insoluble
II
9.1
insoluble
insoluble
III
730
18
25
N
1340
34
58
V
474
45
85
Changes in the UV-vis spectra are not observed when two methyl groups in mezo-positions of Roctamethylporphyrins are substituted by ethyl or larger alkyl groups (compare spectra of III-V and II, Table 1). This evidences our assumption that the strong increase of solubility of III-V is connected with a modification in the macrocycle crystalline structure. The packing type of molecules in the crystals changes, thus the intermolecular distances and molecular interaction forces change. It may be concluded that as a result of modifications of the electronic and crystalline structure of the macrocycles, the modifications of the packing type of the molecules (in crystals), the deterioration of solvation conditions (in arens), the mezo-alkyl substitution exert a more pronounced effect on the macrocycle solubility than the pyrrol substitution. The introdcution of the alkyl groups with lengths from CH3 to C4Hg at the mezo-positions of the macrocycle increases their solubility more than the alkylation of the pyrrol positions independent of the number and position of R-alkyl groups.
REFERENCES
1. G.M. Trofimenko and B.D. Berezin, Koord. Khimiya V. 23, No. 3. (1997) 234. 2. N.Zh. Mamardashvili, .%A. Zdanovich and O.A. Golubchicov, Russian Journal of Organic Chemistry V. 32, No 5. (1996) 788.
LIQUIDS
Journal of Molecular Liquids 91 (2001) 189-191 www.elsevier.nl/locate/molliq
The influence
of alkyl bridge
G.M. Mamardashvili,
substitution
N.Zh. Mamardashvili,
on the porphyrin
O.A. Golubchicov
solubility
and B.D. Berezin
Institute of Solution Chemistry of the Russian Academy of Sciences 153045, Academicheskaya st., 1, Ivanovo, Russia 1. INTRODUCTION The influence of the alkyl substitution in the porphyrins on their solubility in the organic solvent considerably depends on which porphyrin position the hydrogen atoms are substituting the alkyl groups: in pyrrol rings (2,3,7,8,12,13,17,18), mezo-position ((5,10,12,20) or NHgroups of proper porphyrins. The macrocycle solubility values can increase or decrease ten times depending on the alkyl length and the substitution position. The influence of pyrrol alkyl substitution in the porphyrins on their solubility in benzene
and a series of alcohols has been the aim of a preceding paper [ 11. For the majority of octalkylporphyrins with alkyl substitutes in the 2,3,7,8,12,13,17,18 position the solubility values in benzene are 1W3 mol/. The number of alkyl groups, their lengths (corn CHJ- up to C&Iv) and the relative positions in the case of octalkylpoxphyrinisomers cause the difference between the solubility values in benzene of not more than two or three times, apparently, as consequence of the changes of molecule interactions in the crystals. The solubility values in the alcohols (methanol, ethanol, I-propanol) are far less than in benzene, and more important, they depend on the porphyrin structure. 2,3,7,8,12,13,17,1&octamethylporphin is an exception. It is hardly soluble in most organic solvents and has very low solubility values in benzene. In all other cases the alkyl (C~HS- and so on) substitution in the pyrrol rings - complete or incomplete, symmetric or asymmetric - leads to a strong increase of the macrocycle solubility. As this takes place with more than one species of the alkyl substitutes in the porphyrin molecules the alkyl length (in the limits C~HS-C~H~ investigated by us) results in a lesser effect on the solubility than the various positions of the substitutes in the macrocycle do in the case of the isomers. The influence of the mezo- and N-alkyl substitution in porphyrins on solubility in organic solvents has not been investigated quantitatively previously. For the investigation of the influence of the mezo-al@1 substitution we took a series of the octametbylporphyrins with the different alkyl substitutes in 5,15-positions of 13octamethylporphyrin. The above mentioned porphyrin solubility in benzene, ethanol and lpropanol at 298.14 K has been studied. 2. EXPERIMENTAL The porphyrin: 2,3,7,8,12,13,17118-octamethylporphin (P-octamethylporphin) (I); 5,15dimethyl+octamethylporphin (II); j~lj-diethyl-P-octamethylporphin (III); 5,15dipropyl-Poctamethylporphin (IV) and 5,15-dibuthyl-P-octamethylporphin (V) were synthesized by the 0167.7322/01/$ - see front matter 0 2001 Elsevier Science B.V. All rights reserved. PII SO167-7322(01) 00161-l
190
Standard method [ 11. The solvents were rectified by known methods; the water content in the solvents does not exceed O,Ol% in benzene and 0,03 % in ethanol and 1-propanol. The solubility has been studied by the method of isothermal saturation. The porphyrin concentration was measured spectrophotometrically. The method error was 5 %. 3. RESULTS AND DISCUSSION The octamethylporphin is hardly soluble (essentially insoluble) in organic solvents. The introduction of two methyl groups into the mezo-position has no profound effect on the macrocycle solubility. II as well as I are insoluble in most organic solvents, specifically in the alcohols, at room temperature. The solubility values in benzene are four times greater for II than for I, evidencing the insignificant difference in the energy of the crystalline lattice. The changes are attributable, at least partially, to the change of the electronic structure of I when introducing two bridge methyl groups, as revealed in the W-vis spectra. The first band in the spectrum of II is shifted batochromically when compared to that of I while its intensity decreases ten times (Table 1). Table 1 W-vis spectra of the alkyl porphyrines in benzene Compound h run, (lg E) II I 563 (3.72) 619 (2.95) Porphine 570 (4.14) 620 (4.03) I 574 (4.17) 625 (2.98) II 572 (3.14) 624 (2.97) III 575 (3.14) 625 (2.96) IV 572 (3.19) 626 (2.97) V
III 520 527 538 540 539 540
(4.48) (4.15j (3.21) (3.19) (3.27) (3.18)
IV 490 501 505 507 506 509
(4.20) (4.39) (3.77) (3.78) (3.77) (3.77)
hicreasing the alkyl length up to CrHs- in 5,15-positions of porphyrins raises the III solubility in benzene to 7,3x10” mol/l, that is by around two orders (Table 2). The further increase of the alkyl length up to C4H9 in mezo-position causes the solubility value to raise further. 515dipropyl substitution increases the solubility of IV approximately two times when compared with III in benzene and alcohols. 5,15dibutyl substitution increases the solubility of V in the alcohols three times, whereas it decreases somewhat the solubility in benzene. Consequently, not the influence of the crystalline lattice energy but of the macrocycle solvation on the III-V solubility is here evident. With increasing the alkyl length in mezo-positions of porphyrins (from CsHr) more favourable conditions appear for porphyrin solvation in the alcohols (ethanol and 1-propanol), and less favourable ones for solvation in benzene. Unfortunately, the X-ray structure or A Hsmr_ data for mezo-alkylporphyrins are not available. There is no way of inferring the influence of a given type of substitution on the macrocycle crystalline lattice strength.
191
Table 2 Solubility of the alkylp Porpyrin
phyrins (S2’* 10’ 9mole Benzene
Ethanol
n-Propanol
I
2.0
insohtble
insoluble
II
9.1
insoluble
insoluble
III
730
18
25
N
1340
34
58
V
474
45
85
Changes in the UV-vis spectra are not observed when two methyl groups in mezo-positions of Roctamethylporphyrins are substituted by ethyl or larger alkyl groups (compare spectra of III-V and II, Table 1). This evidences our assumption that the strong increase of solubility of III-V is connected with a modification in the macrocycle crystalline structure. The packing type of molecules in the crystals changes, thus the intermolecular distances and molecular interaction forces change. It may be concluded that as a result of modifications of the electronic and crystalline structure of the macrocycles, the modifications of the packing type of the molecules (in crystals), the deterioration of solvation conditions (in arens), the mezo-alkyl substitution exert a more pronounced effect on the macrocycle solubility than the pyrrol substitution. The introdcution of the alkyl groups with lengths from CH3 to C4Hg at the mezo-positions of the macrocycle increases their solubility more than the alkylation of the pyrrol positions independent of the number and position of R-alkyl groups.
REFERENCES
1. G.M. Trofimenko and B.D. Berezin, Koord. Khimiya V. 23, No. 3. (1997) 234. 2. N.Zh. Mamardashvili, .%A. Zdanovich and O.A. Golubchicov, Russian Journal of Organic Chemistry V. 32, No 5. (1996) 788.