Tetraphenylporphyrin , protonation

Mass transfer for this technique was examined by studying the protonation and aggregation kinetics of 5,10,15,20-tetraphenylporphyrin (H2TTP) at the dodecane-aqu-eous interface [61]. The rate law for the diffusion-controlled protonation of H2TTP at the interface was derived. 

However for the tetraphenylporphyrin diacid the increase in conjugation resulting from interaction of the almost coplanar phenyl TT-systems with the porphyrin 7r-system results in a red shift of the visible bands ( ). For the individual atropisomers of H PF,THA the 4,0 isomer exhibits a net blue shift upon protonation while the diacid of the trans-2,2 isomer is significantly red shifted. Small spectral shifts are seen for the cis-2,2 and 3t1 isomers. This has in turn been interpreted to mean that in the excited singlet state "stabilities are shifted such that trans-2,2 > 3,1 > cis-2,2 > 4,0 for this diacid. A comparison of the spectral shifts observed upon protonation is given in Table IV. 

It was reported that cobalt-tetraphenylporphyrin complex (CoTPP) coated on an electrode catalyzes electrocatalytic proton reduction,215 but the activity was not very high. We have found that metal porphyrins and metal phtahlocyanines when incorporated into a polymer membrane coated on an electrode show high activity in electrocatalytic proton reduction to produce H2.22,235 Some data are summarized in Table 19.2. It was shown that this catalyst is more active than a conventional platinum base electrode. 

A light-driven compound containing a photosensitizing tetraphenylporphyrin group linked to a diiron azadithiolate moiety (25) has been synthesized as a model compound of iron-only hydrogenases. This compound reduces protons photochemically to hydrogen.392... 

Fig. 8 Dependence of the equilibrium between the trans tautomers in a series of 2-substituted tetraphenylporphyrins (3-R). Arrows indicate the preferred direction of proton transfer. Data cited after [66]... 

In the case of P-substituted 2-hydroxy-5,10,15,20-tetraphenylporphyrin, an equilibrium was detected in solution, which involved three tautomers aromatic hydroxy (13b-l), enol (13b-2), andketo (13b-3, Fig. 10) [82], For a sample of this compound with all exchangeable protons replaced with deuterons, the ratio of the three species 13b-l 13b-2 13b-3 obtained from integration of a II NMR spectrum recorded at 220 K was 2 3 5. In contrast, for the 2-amino derivative 3-NH2 (Fig. 8), no imino tautomers analogous to 13b-3 were detected [66],... 

Figure 7.16 In contrast to -alkyl substituted porphyrins found in nature, the artificial raeso-tetraphenylporphyrins do not form stable dimers. In crystal structures the molecules are well separated by the phenyl rings (see Figures 7.26 and 7.27) which are oriented perpendicular to the porphyrin macrocycle (a). Upon protonation with HCl and FeCf, the diprotonated porphyrin dication is formed. In its crystal structure the porphyrin ring is puckered and the phenyl rings are approximately in the porphyrin plane (b). Aggregation can now...

The W arrangement in all the CH2-C-CH3 groups in a chain requires that one of the methylene protons and the methyl carbon should be in the trans state, that is, that the main-chain backbone should be in the repeated tt conformation. Similar 47(HH) connectivities were reported for the telomers of MMA prepared by radical polymerization in the presence of cobalt(II) tetraphenylporphyrin as chain transfer reagent.269,286... 

The Metallophlorin Spectrum. This is the spectrum of a metalloporphyrin in any oxidation state, which has added an extra substituent at one methine bridge. It is also found very often in various oxidation or protonation products of tetraphenylporphyrins. It is very similar to the spectrum of metallo-bilatrienes with an opened macro-cyclic ring [Wasser (195) e.g. Peychal-Heyling (147)]. 

Deronzier and Latour ° have extended their study of autoxidation of Ti porphyrins to include the influence of axially coordinated tdt. In Ti (tdt)(tpp), where ttp = tetraphenylporphyrin, the dithiolene ligand experiences the strong porphyrin ring currents as indicated by the upfield shift of tdt ortho and meta protons to 5.14 and 5.83 p.p.m., respectively. Lowering the temperature below 200 K was sufficient in a peroxo complex, Ti (02)(tpp), to block rotation of the axial ligand. However, rotation of tdt around the C2 axis (ignoring the Me on tdt) was still too rapid on the NMR timescale even at 180 K. Although the Ti d y orbital can interact with the tdt S atoms in a manner similar to the peroxo complex, the orbital overlap is much weaker and the resultant barrier to rotation is lower. 

---