Resonance Raman Spectroscopic and Emission Studies on Nickel,Palladium,and Platinum Tetraphenylporphyrins in Noncoordinating and Coordinating Solvents

Luminescence spectra and Resonance Raman (RR) spectra are reported for nickel, palladium, and platinum complexes of tetraphenylporphyrin (TPP) in methylene chloride and piperidine. For NiTPP the RR frequencies in the range of 900 to 1700 cm⁻¹ are reported for 4-coordinate low-spin (methylene chloride) and 6-coordinate high-spin (piperidine) forms. The core size expansion for the high-spin complex results in large decreases in several high-frequency porphyrin skeletal modes. For PdTPP and PtTPP the luminescence spectra and RR frequencies are reported for methylene chloride and in methylene chloride/piperidine mixtures. It has been reported previously that PdTPP and PtTPP do not coordinate significantly with pyridine because of a large ligand-field splitting. However, in a strongly ligating solvent like piperidine, PdTPP and PtTPP apparently form 5-coordinate low-spin complexes, as judged by spinorbit coupling parameters and luminescence spectra. The RR frequencies remain essentially the same in methylene chloride and piperidine, because for Pd and Pt the spin states are low-spin in both solvents.