Protonation Of Planar And Nonplanar Porphyrins: A Calorimetric And Computational Study

Herein, we report the first calorimetric study of the protonation of planar and nonplanar free-base porphyrins: H2OETPP (strongly saddled by its substituents), H2T(tBu)P (strongly ruffled by its substituents), and the nominally planar porphyrins (npPs) H2OEP, H2TPP, H2T(nPe)P, and H2T(iPr)P. The observed enthalpies of protonation in solution (Delta H-protsoln) for formation of the dications in 1,1,2,2-tetrachloroethane with 2% trifluoroacetic acid are -45 +/- 1 kcal mol(-1) for the npPs, -52.0 kcal mol(-1) for H2T(tBu)P, and -70.9 kcal mol(-1) for H2OETPP. The corresponding enthalpies of protonation (Delta H-DFT) obtained from DFT calculations (-27 +/- 5, -42, and -63 kcal mol(-1) respectively) reproduce this trend. The much more negative enthalpy of protonation seen for H2OETPP is consistent with this molecule being pre-deformed into the saddle structure favored by porphyrin dications. Except for OETPP, the calculated enthalpies of the first protonations (Delta H-1) are significantly more positive than the enthalpies of the second protonations (Delta H-2). In addition, the structural strain energies for the first protonations (Delta E-st(1)) are also significantly more positive than Delta E-st(2). According to the calculations, the monocations thus have higher proton affinities than the corresponding free-base porphyrins due to a structural strain effect, which is consistent with the generally elusive nature of the porphyrin monocation. The recent observations of monocations for free-base porphyrins with a high degree of saddling can be rationalized in terms of Delta H-1 and Delta H-2 being similar; so, the monocation is no longer an unstable intermediate.