Protonation of Planar and Nonplanar Porphyrins: A Calorimetric and Computational Study

Abstract

Herein, we report the first calorimetric study of the protonation of planar and nonplanar free-base porphyrins: H 2 OETPP (strongly saddled by its substituents), H 2 T(tBu)P (strongly ruffled by its substituents), and the nominally planar porphyrins (npPs) H 2 OEP, H 2 TPP, H 2 T(nPe)P, and H 2 T(iPr)P. The observed enthalpies of protonation in solution (ΔH 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 H 2 T(tBu)P, and −70.9 kcal mol −1 for H 2 OETPP. The corresponding enthalpies of protonation (ΔH 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 H 2 OETPP 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 (ΔH 1 ) are significantly more positive than the enthalpies of the second protonations (ΔH 2 ). In addition, the structural strain energies for the first protonations (ΔE st (1)) are also significantly more positive than ΔE 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 ΔH 1 and ΔH 2 being similar; so, the monocation is no longer an unstable intermediate.

Nonplanar porphyrins: synthesis, properties, and unique functionalities

“…Calorimetric studies on the diprotonation were performed for a series of planar and nonplanar free base porphyrins. 278 The enthalpies of protonation in solution (Δ H protsoln ) for formation of the dications were determined to be −52.0 kcal mol −1 for H 2 160 , and −70.9 kcal mol −1 for H 2 45 in 1,1,2,2-tetrachloroethane containing 2% TFA (Table 6). H 2 45 is known to be highly saddle-distorted 279 and H 2 160 is ruffled.…”

“…Protonation enthalpies (DH protosoln ) and DFT-calculated enthalpies for the stepwise protonation (DH 1 and DH 2 )278 …”

Redox‐Mediated Single‐Molecule Transistor with A Subthreshold Swing Down To 120 mV Decade−1

“…To investigate the changes in conjugation patterns and transport pathways by protonation at two sites on the porphyrin ring, we studied the protonation process at macroscopic and singlemolecule levels. The different nonplanarity [55,56] demonstrated the different protonation mechanisms of STPP and OSTPP. By referring to the previous reports, [57,58] we chose methanol (MeOH) to stabilize probable monoprotonated products and p-Toluenesulfonic acid (TsOH) as acid.…”

π‐Conjugated Metal Free Porphyrin as Organic Cathode for Aluminum Batteries

“…[13] However, the influence of the acidic electrolyte on the chemical properties of the porphyrins has not been evaluated yet, where the tetrapyrrole skeletons tend to gain two protons in an acidic environment to form a dication. [14] As our group has a longstanding interest in the development of porphyrin-based electrode materials, [15] we envisioned that porphyrins substituted with diphenyl amino-phenyl (TDPP) could serve as promising (RAB)-active material. This system has been chosen in particular because of: i) the bipolar nature of porphyrin material that can either donate or accept electrons at its mesomeric core.…”