Study of nonplanar distortions of Ni(II)-mesomethylporphyrins using spectroscopy of resonance Raman scattering

Abstract

The role of mesomethyl substitution and of steric interaction of peripheral substituents in the formation of nonplanar distortions of the porphyrin macrocycle is studied for a series of Ni(II)-porphyrins. Resonance Raman spectra are recorded for Ni(II)-5-methyl-2,3,7,8,12,13,17,18-octaethylporphyrin (1); Ni(II)-2,5,8,12,18-pentamethyl-3,7,13,17-tetraethylporphyrin (2); and Ni(II)-3,5,7,12,17-pentamethyl-2,8,12,17-tetraethylporphyrin (3). The obtained spectra are compared with spectra of Ni-octaethylporphyrin (Ni-OEP) and Ni-etioporphyrin II. It is found that introducing one methyl group into a mesoposition leads to significant low-frequency distortions of the structure-sensitive lines of resonance Raman scattering. Specifically, frequenciesν 3 ,ν 11 ,ν 19 ,ν 2 , andν 10 decrease by 10–44 cm−1 while frequencyν 4 remains practically unchanged. It is shown that the observed spectral variations are caused by nonplanar distortions of the structure of Ni-porphyrins. The same tendency of the line displacement for tetragonal Ni-OEP and for compounds 1, 2, and 3 indicates that CH3-mesosubstituted Ni-porphyrins have a “corrugated” structure. The degree of nonplanarity is modulated by alkyl groups on pyrrole rings and increases in the porphyrin series 3→2→1. All mesomethyl-substituted Ni-porphyrins have spectral indications that the considered compounds are structurally heterogeneous in solution due to the presence of conformers with different degrees of planarity.