Some Very Rapid Reactions of Porphyrins in Aqueous Solution
The rate of addition of two water molecules to α,β,γ,δ-tetra(4-N-methylpyridyl)porphinatonickel(II) to form the six-coordinate diaquo complex has been studied by the laser Raman temperature-jump method. The rate constant for the addition of the first water molecule with the concomitant spin change is 2.5 × 10 M s at 25 °C (0.1 M KCl). The laser temperature-jump apparatus has also been used to study the rate of protonation of the metal-free porphyrin. The kinetics are consistent with a mechanism in which the free-base porphyrin exists in solution as a mixture of buckled and planar forms. The relatively low concentration buckled form, with its exposed nitrogen atom, is more susceptible to proton attack. The rate constants for the ring buckling and subsequent proton addition are (25 °C, μ = 0.7 M) PH ⇌ *PH (K = 4.6 × 10 s , k = 1.2 × 10 s ), *PH + H ⇌ PH (K = 5.0 × 10 M s , k = 8.0 × 10 s ). The activation energy for the ring buckling is ~2.5 kcal mol . The analysis of the amplitudes of the relaxation curves leads to pK values consistent with those obtained from spectrophotometric titrations. © 1976, American Chemical Society. All rights reserved. 5 -1 -1 4+ 4+ 7 -1 9 -1 4+ + 5+ 10 -1 -1 6 -1 -1 2 2 1 -1 2 3 2 -2
Journal of the American Chemical Society
Pasternack, Robert F.; Sutin, Norman; and Turner, Douglas H., "Some Very Rapid Reactions of Porphyrins in Aqueous Solution" (1976). Faculty Articles Indexed in Scopus. 2689.