Soret-excited resonance Raman (RR) spectra are reported for some 40 nonplanar metalloporphyrin complexes. These include (i) copper β-octabromo-meso-tetra(para-X-phenyl)porphyrins, Cu [ Br 8 T (p- X - P ) P ] ( X = CH 3, H , F , CF 3, NO 2); (ii) copper β-octachloro-meso-tetra(para-X-phenyl)porphyrins, Cu [ Cl 8 T (p- X - P ) P ] ( X = H , CF 3); (iii) zinc β-octabromo-meso-tetraphenylporphyrin, Zn [ Br 8 TPP ], and zinc β-octasubstituted meso-tetrakis(pentafluorophenyl)porphyrins, Zn [ Y 8 TPFPP ] ( Y = CH 3, Cl , Br ); (iv) nickel β-octabromo-meso-tetra(para-X-phenyl)porphyrins, Ni [ Br 8 T (p- X - P ) P ] ( X = CH 3, H , F , Br , COOMe , CF 3, NO 2), with and without bis-ligated axial pyridine and imidazole ligands; (v) nickel β-octachloro-meso-tetra(para-X-phenyl)porphyrins, Ni [ Cl 8 T (p- X - P ) P ] ( X = CH 3, H , F , COOMe , CF 3, NO 2), again with and without bis-ligated axial pyridine ligands; and (vi) nickel octaethyltetraphenylporphyrin, Ni [ OETPP ], and palladium octaethyltetraphenylporphyrin, Pd [ OETPP ]. The spectra lead to the following conclusions. For a particular nonplanar porphyrin ligand, the high-frequency RR marker bands ν2 and ν4 downshift with increasing size of the coordinated metal ion, including a low- to high-spin transition for nickel porphyrins. In contrast, for a particular coordinated metal ion, these frequencies downshift with increasing nonplanarity of the porphyrin ligand, which in turn correlates with decreasing metal–nitrogen distances or the 'core size'. Similar results were also found for β-octaalkyl-meso-tetraphenylmetalloporphyrins by Shelnutt and coworkers. Thus, it appears that this complicated multiple-valued core size dependence of the high-frequency RR marker bands may be a generally shared property of all saddled metalloporphyrins. Variations in the electronic character of the metalloporphyrins resulting from variations in the para-substituents on meso-aryl groups do not result in significant shifts in the RR marker bands. The symmetric C meso – C phenyl stretching frequency (ν1) at approximately 1240 cm-1 is prominent in all the RR spectra except those of the low-spin Ni saddled porphyrins, for which they are relatively suppressed, and possible electronic-structural implications of this effect are discussed. In the low-frequency regions of the RR spectra of the compounds studied, the strongest band is roughly around 300 cm-1 and appears to be relatively safely assignable as γ16, a pyrrole tilting mode that strongly resembles the saddling mode in terms of atomic displacements.

HALVORSEN, I., STEENE, E., & GHOSH, A. (2001). Resonance Raman marker bands of β-octahalogeno-meso-tetraarylmetalloporphyrins. Journal of Porphyrins and Phthalocyanines, 05(10), 721–730.