Mössbauer Spectroscopic Study Of An Iron(III) Complex With Crown Porphyrin
Mössbauer Spectroscopic Study Of An Iron(III) Complex With Crown Porphyrin. Pankratov D.A., Stukan R.A., Al'Ansari Ya.-F., Savinkina E.V., Kiselev Yu.M. //Russian Journal of Inorganic Chemistry. 2011. V. 56. № 10. P. 1603-1608
A complex of 57Fe with 5-{4-[((4′-hydroxybenzo-15-crown-5)-5′-yl)diazo]phenyl}-10,15,20-triphenylporphyrin was studied by Mössbauer spectroscopy. The presence of signals of two types in the spectra (a doublet and an extended absorption band over a wide velocity range) suggests that the Fe atoms occupy two structurally different positions in this complex. The dependences of the doublet asymmetry on temperature and the angle between the normal to the sample plane and the γ-ray beam were studied. The isomer shift δ of the doublet in the temperature range from 360 to 5 K changes from 0.25 to 0.41 mm/s, while the quadrupole splitting remains virtually unchanged (Δ ≈ 0.65 mm/s). The relaxation-type absorption over a wide velocity range, the relative area of which strongly varies with temperature, can be described by a broad singlet with the following parameters: δ = 0.30–0.44 mm/s and Γ = 2.8–3.38 mm/s. According to the δ values, both signals are due to Fe(III) derivatives.
The structures and properties of metalloporphyrins (MPs), such as hemoglobin, chlorophyll, and vitamin В12, involved in vital processes depend on the complexing metal and the peripheral substituents. A great number of MPs are chemically inert and thermally stable, have high molar absorption coefficients in the UV-Vis and near-IR ranges, and show reversible redox transitions. This accounts for the interest in comprehensive study of the corresponding properties using a variety of physicochemical techniques.
In the last few years, it has been found, through variation of the peripheral substituents and metal cations in MPs, that they can form stable supramolecular ensembles with electronic and ionic conduction channels, which are very sensitive to weak impacts. This opens up wide scope for the design of novel (e.g., luminescent) materials suitable for use in modern facilities for various purposes.
One of the most topical areas in the chemistry of MPs is modification of the porphyrin molecule with additional fragments (e.g., crown ethers capable of housing various metals); this substantially extends the scope for the formation of supramolecular metal complex systems with desired properties.
Here, we studied the properties of an iron metalloporphyrin with mesotetraphenylporphyrin as the porphyrin component covalently bonded to benzo-15-crown-5 providing an extra coordination site.
Thus, according to the Mössbauer spectra, the X-ray amorphous complex of 57Fe with 5-{4-[(4-hydroxybenzo-15-crown-5)-5-yldiazo]phenyl}-10,15,20-triphenylporphyrin contains the iron atoms in two different structural positions: in the porphyrin and crown-ether macrocycles.
The iron atoms in the “rigid” porphyrin environment are manifested in the Mössbauer spectra as an asymmetric doublet with the parameters corresponding to high-spin iron(III).
The presence of iron(III) atoms in the large crow-nether cavity is manifested as an extended absorption over a wide velocity range, which is of relaxation character.
The Mossbauer spectra of the complex studied show some peculiar features: the temperature independence of the doublet asymmetry and the extremely low temperature at which relaxation phenomena are blocked.
 |
 |
 |
 |
 |
 |
