Resonance Raman Frequencies and Core Size for Low- and High-Spin Nickel Porphyrins

Resonance Raman (RR) spectra are reported with B- and Q-band excitation for nickel(II) complexes of octaethylporphyrin (OEP), protoporphyrin IX dimethyl ester (PP), and meso-tetraphenylporphine (TPP) in methylene chloride (4-coordinate, low spin) and piperidine (pip) (6-coordinate, high spin). The large core size expansion accompanying the formation of the 6-coordinate species (1.96-2.04 A) is reflected in large decreases, up to 40 cm^-1 in the positions of high-frequency porphyrin skeletal modes. For NiOEP and NiPP, these are in near-quantitative accord with the core size correlations obtained previously for iron porphyrin complexes, although certain deviations due to differential coupling with the vinyl modes of protoporphyrin are noted. Contributions of a minority 5-coordinate complex to the RR spectrum of NiTPP in piperidine, previously noted on the basis of photolysis effects, are evaluated quantitatively from titration data. Formation of a monopiperidine adduct, detected previously via a RR study of NiTPP(pip)₂ photolysis, is examined for nickel meso-tetrakis(p-cyanophenyl)porphine. Equilibrium constants for successive addition of piperidine ligands, K₁ = 0.4 and K₂ = 2.5 M^-1, are evaluated from optical titration data, and the absorptivities of the 5- and 6-coordinate species are found to be nearly the same, consistent with both having a high-spin configuration. The frequency of the 5-coordinate v₄ RR band is likewise found to be much closer to the 6-coordinate than to the 4-coordinate frequency.