Resonance Raman marker bands of β-octahalogeno-meso-tetraarylmetalloporphyrins

Soret-excited resonance Raman (RR) spectra are reported for some 40 nonplanar metalloporphyrin complexes. These include (i) copper beta -octabromo-meso-tetra(para-X-phenyl)porphyrins, Cu[Br8T(p-X-P)P] (X = CH3, H, F, CF3, NO2); (ii) copper beta -octachloro-meso-tetra(para-X-phenyl)porphyrins, Cu[Cl8T(p-X-P)P] (X = H, CF3); (iii) zinc beta -octabromo-meso-tetraphenylporphyrin, Zn[Br8TPP], and zinc beta -octasubstituted mesotetrakis(pentafluorophenyl)porphyrins, Zn[Y8TPFPP] (Y = CH3, Cl, Br); (iv) nickel beta -octabromo-meso-tetra(para-X-phenyl)porphyrins, Ni[Br8T(p-X-P)P] (X = CH3, H, F, Br, COOMe, CF3, NO2), with and without bis-ligated axial pyridine and imidazole ligands; (v) nickel beta -octachloro-meso-tetra(para-X-phenyl)porphyrins, Ni[Cl8T(p-X-P)P] (X = CH3, H, F, COOMe, CF3, NO2), 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 nu (2) and nu (4) 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 beta -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 (nu (1)) 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 nu (16), a pyrrole tilting mode that strongly resembles the saddling mode in terms of atomic displacements. Copy-right (C) 2001 John Wiley & Sons, Ltd.