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

Dongho Kim, Y. Oliver Su, Thomas G. Spiro

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Abstract

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.

Original languageEnglish
Pages (from-to)3988-3993
Number of pages6
JournalInorganic Chemistry
Volume25
Issue number22
DOIs
Publication statusPublished - 1986 Jan 1

Fingerprint

Porphyrins
Nickel
porphyrins
nickel
piperidine
Photolysis
Titration
Raman scattering
Methylene Chloride
Equilibrium constants
titration
photolysis
Iron
Raman spectra
Ligands
minorities
methylene
adducts
esters
absorptivity

All Science Journal Classification (ASJC) codes

  • Physical and Theoretical Chemistry
  • Inorganic Chemistry

Cite this

Kim, Dongho ; Su, Y. Oliver ; Spiro, Thomas G. / Resonance Raman Frequencies and Core Size for Low- and High-Spin Nickel Porphyrins. In: Inorganic Chemistry. 1986 ; Vol. 25, No. 22. pp. 3988-3993.
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Resonance Raman Frequencies and Core Size for Low- and High-Spin Nickel Porphyrins. / Kim, Dongho; Su, Y. Oliver; Spiro, Thomas G.

In: Inorganic Chemistry, Vol. 25, No. 22, 01.01.1986, p. 3988-3993.

Research output: Contribution to journalArticle

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T1 - Resonance Raman Frequencies and Core Size for Low- and High-Spin Nickel Porphyrins

AU - Kim, Dongho

AU - Su, Y. Oliver

AU - Spiro, Thomas G.

PY - 1986/1/1

Y1 - 1986/1/1

N2 - 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.

AB - 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.

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