Influence of the Nature of Porphyrin and Extraligand on the Stability of Zinc Extracomplexes

Svetlana V. Zaitzeva*, Sergey A. Zdanovich, Tatiana A. Ageeva, Alexandr S. Ocheretovi andOleg A. GolubchikovInstitute of the Solution Chemistry of Russian Academy of Sciences, Akademicheskaya, 1, 153045,Ivanovo, Russian FederationTel.: +7 0932 378512, E-mail: svz@ihnr.polytech.ivanovo.suReceived: 15 March 2000 / Accepted: 29 April 2000 / Published: 9 June 2000Abstract: The mutual influence of ligands within the structure of Zn-porphyrins on the ther-modynamic stability of the latter was investigated.Keywords: Extracoordination, complex, metalloporphyrins, stability.IntroductionThe influence of the nature of porphyrin and extraligand on the process of formation of extracom-plexes of zinc porphyrins in o-xylene was studied by a spectrophotometrical titration method and com-puter simulation. For the research the following metalloporphyrins were used: zinc 5,15-(para-butyloxyphenyl)-2,8,12,18-tetramethyl-3,7,13,17-tetraethylporphyrin ZnP(I), zinc 5,15-(para-butyl-oxyphenyl)-2,8,12,18-tetramethyl-3,7,13,17-tetrabutylporhyrin ZnP(II), superimposed zinc porphyrinZnP(III) and zinc tetraphenylporphyrin ZnP(IV). N-methylimidazole (MeIm), imidazole (Im), pyridine(Py), 3,5-dimethylpyrazole (DMP) and dimethylformamide (DMF) were used as extraligands (L). Thestrength of Zn-L bonding was noted to be lower within the series of extracomplex stuctures: ZnP(IV) >ZnP(I) > ZnP(II) > ZnP(III). The stability constant (lgKs) in the case of sterically unhindered com-plexes was found to increase linearly with the increase of basisity of the extraligand (lgK


Results and Discussion
The binding of ligands L with Zn-porphyrins in all cases is accompanied by bathocromic shifts and with changes of intensity of the chromophore basic absorption bands (Figure 2).It is our point of view that this is due to the increase of electron density at the zinc cation and at the porphyrin nitrogen atoms.
The growth of a fractional negative charge at N atoms establishes the destabilisation of the a 2u mo- lecular orbital at a 1u .constant level.As a result the configuration interaction of the E u -type excited state decreases, and that is sufficient reason for amplification of the first band in the electronic absorption spectra (EAS) of Zn-porphyrins.The bathochromic shift, probably, is caused by the increase in energy of the a 2u -type MO.During the study complexes ZnP(I), ZnP(II), ZnP(III) were found to have the property of attaching RQHPROHFXOHRI0_,P,P'033\'0))RUZnP(IV) the coordination of two molecules of extraligand is observed only in the case of DMF.K s values obtained, calculated characteristics of geometric structure and energy parameters of molecules of Zn-porphyrins are presented in Tables 1 and 2. The comparison of the data in Table 1 enables us to estimate the strength of additional ligand binding with Zn-porphyrins and to establish a series for their stability.In the case of formation of extracomplexes of =QSRUSK\ULQVZLWK0_,PDQG,PWKHVHULHVLVZnP(IV) > ZnP(II) ≈ ZnP(I) > ZnP(III).This is explained by the influence of porphyrin ligand nature on the extracoordination process.In the case of ZnP(IV), the electron density withdrawal from the zinc atom by phenyl rings increases its partial positive charge.Hence, the Zn-L bonding becomes stronger.The decrease of the number of phenyl substi- tutions and the introduction of alkyl-groups in the β-position of the porphyrin macrocycle for ZnP(I), ZnP(II), leads to an increase of electron densities at nitrogen atoms in the coordination center and to the decrease of positive charge at the zinc atom that reduces the strength of Zn-L bonding.Thus, extracomplexes (L)ZnP(I), (L)Zn(II) are less stable in comparison with (L)ZnP(IV).The low value of the stability constant of extracomplexes (L)ZnP(III) in comparison with the remaining members of the series is explained not only by the influence of substituents on the porphyrin macrocycle, but by its distortion as a result of steric strain caused by the "cover".
Stability constants of zinc complexes with Py are practically the same (Table 1) and the nature of porphyrin ligand, in this case, has practically no influence on the process of extracoordination.The reason of such anomalous behavior of pyridine remains unexplainable.The nature of the extraligand has an effect on the stability of Zn-porphyrin extracomplexes.The strength of Zn-L bonds (Table 2) increases and the K s values of complexes raise (Table 1) with the increase of the extraligand basisity.A linear correlation between lgK s and jD \g values with the regression equation lgK s ÂjD \g + 2.4589 and a correlation coefficient r = 0.963, is observed for the sterically non-strained ZnP(IV) complex (Figure 3).Table 2. Some geometric characteristics of pentacoordinated Zn-porphyrins.On the basis of quantum -chemical calculations for N-containing extraligand structures the linear correlation between their basisity and the energy of protonation E p (Figure 4) is obtained.Calculated jD \g + values compared with data from literature [14] in most cases is notedto be not more than 10% higher than values of jD \g + (calcul) and jD \g + * (Table 3, Figure 5).In our opinion it allows us to use calculated energy of protonation for the evaluation of the basisity of nitrogen-containing compounds.The value of bathochromic shift û, may be used for qualitative characteristic of Zn-L bond strength, and consequently, also of value of metal atom exit from the coordination center plane.The use of quantum -chemical methods for calculations of Zn-porphyrins structures, allows us to explain the reasons of modification in stabilities of their extracomplexes as the function of the porphyrin and extraligand natures.Characteristics calculated show that the energy of Zn-L bonding formation (E b ) of extracomplex varies non-linearly with its stability (Table 1).The increase of E b in the series (L)ZnP(I) to (L)ZnP(IV) shows the increase in their stabilities.
Zn-porphyrin extracomplexes have square-pyramidal coordination structures with the metal atom emerging slightly from the N 4 plane towards the extraligand (Figure 7).On the basis the data in Table 2 the correlations between an exit of metal from a plane of macrocycle and length of Zn-L bonding are obtained.We found out that in series of nitrogen-containing bases Im > DMP > Py > DMF the length of Zn-Ct in extracomplexes decreases, the stability of Zn-N bonding increases at coordination center (Table 2), the interaction of zinc atom with nitrogen atom of extraligand weakens and as the consequence, the overall stability of the metalloporphyrin is reduced (Table 1).Thus the cis-effect of ligands within the structure of Zn-porphyrin extracomplexes is shown to be strong.Finally, it is necessary to point out that some dependence is observed between distortion of a macrocycle ( i.e. the size of a coordination cavity) and length of Zn-L bonding.The increase of Zn-L bonding strength causes the increase of steric stresses in the metalloporphyrin (Table 2).

Conclusions
On the basis of experimental and calculated data the dependence between the stability of extracomplexes Zn(L)P and the nature of porphyrin and the basicity of additional molecular ligands was obtained.
Correlations between D s values, the magnitude of basic absorption band displacements ∆λ in EAS of metalloporphyrins and calculated bonding energy of zinc atom with nitrogen atom of extraligand (E b ) were found.

=QWHWUDSKHQ\OSRUSK\ULQ
It was synthesized by boiling tetraphenylporphyrin with a tenfold excess of zinc acetate in benzene during 40-50 min.The zinc complex was purified by chromatography on Al 2 O 3 (activity II) using chloroform as eluent, followed by recrystallization from chloroform.EAS (benzene) λ max nm, (lgε): 590.0 Changes in metalloporphyrin EAS with the concentration of extraligand allowed us to use a spectral method for study of extracoordination.The experimental technique and calculations of stability constants of extracomplexes (K s ) are discussed in more detail in the literature [1,[9][10][11][12].The quantumchemical calculations were carried out by a combination of conjugated gradient methods [13].The condition of termination of the optimization was given by a gradient of 0.04 kJ/mol.

Figure 3 .
Figure 3. Dependency of extracomplex stabilities of Zn-porphyrins on the basisity of added extraligands.

Figure 4 .
Figure 4.The dependence of protonation energy of nitrogen-containing molecules on their basisity.

Table 1 .
Thermodynamic characteristics of extracoordination reactions of nitrogen-containing ligands by zinc complexes of porphyrins in o-xylene.

Table 3 .
Calculated values of protonation energy and basisity of nitrogen-containing molecules.