Kinetic H/D/T Isotope and Solid State Effects on the Tautomerism of the Conjugate Porphyrin Monoanion

Using dynamic NMR spectroscopy, the kinetic isotope effects of the degenerate single hydron transfer in the conjugate 15N-labeled porphyrin anion Por-H- dissolved in organic solvents have been measured as a function of temperature. Por-H- was generated by dissolving the labeled parent compound porphyrin together with the phosphazene base 1,1,1,5,5,5-hexakis(dimethylamino)-3-[(1,1,2,2-tetramethylpropyl)imino]-3-[[tris(dimethylamino)phosphoranyliden]amino]-1λ5,3λ5,5λ5,1,4-triphosphazadiene (tHept-P4) in organic solvents. In addition, by reaction of porphyrin with the base 1,1,1,5,5,5-tetrakis[tris(dimethylamino)phosphoranylidene]amino]phosphonium fluoride (P5+F-) Por-H- could be embedded into the solid state and its tautomerism followed by 15N CPMAS NMR. Surprisingly, within the margin of error, the degeneracy of the tautomerism was not lifted in the solid state and the rate constants of the proton transfer were identical in the liquid and the solid. The kinetic isotope effects at 240 K (extrapolated) are given by kH/kD = 34 and kH/kT ≈ 152. The size and the temperature dependence of the isotope effects indicates a proton tunneling mechanism as in the parent porphyrin where this process is nondegenerate. However, the proton tunneling contribution is much larger in the anion as there the process is degenerate. These findings are reproduced in terms of a modified Bell tunnel model for both systems.