Journal of Porphyrins and Phthalocyanines

Theoretical calculations were performed for studying the electronic molecular structure of axial subphthalocyanine (SubPc) derivatives. The intermolecular SubPc interactions were also investigated at theoretical level using adequate models. These models represent the interaction of one Cl(OR)-SubPc with the surrounding molecules through the Cl atom as well as the interaction of the Cl-SubPc with an incoming diiminoisoindole group in the first steps of the open ring reaction. The SubPc complexes with more electronegative atoms than Cl heteroatoms in apical positions are more pyramidal than Cl-SubPc. The B–X and B–Np are the most sensitive macrocycle positions with respect to the axial substitutions in SubPcs. The B–X bond has an elevated polarity and seems to correlate with the chemical reactivity of these compounds. The theoretical calculations of the near-frontier orbitals reveal an outstanding energetic and structural homogeneity along the studied series which is in correspondence with the fact that the axial ligand has only a small influence on the position of the maximum bands in UV-vis spectra as well as in the first half-wave potentials for oxidative processes. The study of the intermolecular interactions give information about their influence on the molecular structure of the axial complexes of the SubPcs. The characterization of the interaction of one Cl-SubPc with a pyrrol containing group may be useful in the understanding of the mechanism of the ring expansion reactions. In general, the present results indicate that the apical position may be a critical reactive center both for redox and axial substitution processes.

The synthesis and characterization of new alkali-soluble osmium tetraphenylporphyrinates, complexes of the 5,10,15,20-tetrakis(4-carboxyphenyl)porphyrin Os(tH₄cpp)L₂ and Os(tH₄cpp)LL' (2a–j), is described. 2a–j are obtained from the corresponding new osmium porphyrinates Os(tmecpp)L₂ and Os(tmecpp)LL'(1a–k) by alkaline saponification and precipitation with hydrochloric acid. In fact, the osmium(II) porphyrinates 2c–f as well as 2i–j are oxidized to the osmium(III) porphyrinates Os(tH₃cpp)L₂(3c–f) and Os(tH₃cpp)LL'(3i–j) in the presence of air, as confirmed by UV-vis spectroscopy.

Synthesis of 3,4-dichloropyrrole and reaction with formaldehyde and p-toluene sulfonic acid catalysis, followed by oxidation with Ag₂O allows a low yield entry into 2,3,7,8,12,13,17,18-octachloroporphyrin. The extremely low solubility of the free base requires work-up in the presence of metallo acetates for in situ formation of metal complexes. Utilizing this method, zinc(II) and nickel(II) complexes of the title compound were prepared as the first examples of a novel porphyrin class bearing only β-halogeno substituents. In addition, the first crystal structure of a 3,4-dihalopyrrole, 3,4-dichloropyrrole, is reported.

A series of β-halogenated cationic metalloporphyrins were analyzed by LDI-TOF and ESI-MS. Although LDI-TOFMS reveals to be a good tool for the characterization of this family of metal-complexes, including the redox state of the metal, ESI-MS indicates in addition the relative tendency of such metal-complexes to be reduced.

The structures of the charged porphyrins and their dimers have been investigated with computational methods. Dimers have been formed based on electrostatic attraction of the opposite charges on two different porphyrin monomers, tetra ammonium porphyrin (TAP) and tetra carboxy porphyrin (TCP). Semi-empirical quantum mechanical calculations have been employed to explore the most stable ground-state structures of TCP, TAP and their hetero-dimers. Dimeric structures analyzed are all in face-to-face fashion indicating the strong electrostatic attraction between the two porphyrin rings. Calculations have also predicted that the protons transfer from ; groups to -COO^- groups when the interplanar separation is shorter than 3.7 Å.

The characterization of free base porphyrin 2,3,7,8,12,13,17,18-octakis(hexyl-thio) tetraazaporphyrin (H₂OHTTAP) and its zinc(II) complexes [Zn(II)OHTTAP] containing eight thioether groups at the β-pyrrole positions of the macrocycle was reported. Results obtained by cyclic voltammetry and differential pulse voltammetry indicated a five-electron reduction in five steps for each complex. They were oxidized in two single-electron-transfer steps to yield π-cation radicals and dications and reduced in three single-electron-transfer steps to yield π-anion radicals, dianions and trianions, respectively. The redox property of H₂OHTTAP was unusual as compared to porphyrins (PPs) and phthalocyanines (Pcs). Each process was monitored by in situ thin-layer spectroelectrochemistry, which indicated that only the ligand was electroactive. The existence of the eight hexylthio groups was responsible for the intrastack interactions and enhanced intracolumnar and intercolumnar electron motions, resulting in improved conductivity.

The ¹H NMR spectra of the five-coordinated rhodium phthalocyanine complex Me(RPc)Rh(III) (1, RPc = dianion of 1,4,8,11,15,18,22,25-octa-n-pentylphthalocyanine) in toluene indicate that it exists as a discrete monomer at low concentrations. The complex forms a dimer aggregate reversibly in the concentration range of 2.6–10.2 mM at -90 < T < 22 °C. The dimerization constants at -60, -80, and -90 °C are 2.0 × 10², 5.7 × 10², and 1.0 × 10³M^-1, respectively. In chloroform, aggregation of the complex occurs at ~0.3 mM, reflecting the lower solvating power of chloroform for this complex. The ¹H NMR spectra of the chloroform solutions indicate that at concentrations greater than 1.0 mM, two discrete dimers in the ratio of 87:13 are reversibly formed. The ¹H NMR spectra of the major isomer, 2a, suggest that it is the 45 °-staggered cofacial dimer, a dimer with a D_4d symmetry. The ¹H NMR spectra of the minor isomer, 2b, suggest that it is a slipped cofacial dimer with a C_2h symmetry, which is formed by slipping a (RPc)Rh plane of the eclipsed dimer (a D_4h dimer) along a line connecting two opposing meso nitrogens of (RPc)Rh. Interestingly, the solution conformation of 2b is similar to that of the basic stacking unit found in the crystal structure of 1.

Hematoporphyrin IX, H₂HMP, 8,13-bis(1-hydroxyethyl)-3,7,12,17-tetramethyl-21H,23H-porphine-2,18-dipropionic acid and protoporphyrin IX, H₂PP, 8,13-divinyl-3,7,12,17-tetramethyl-21H,23H-porphine-2,18-dipropionic acid were efficiently immobilized on niobium oxide grafted on a silica gel surface, SiO₂/Nb₂O₅, by the -COO–Nb bond formed between the porphyrin carboxyl groups and the grafted Nb₂O₅. These immobilized porphyrins, SiO₂/Nb₂O₅/H₂HMP and SiO₂/Nb₂O₅/H₂PP, were further reacted with Co(II) in dimethylformamide, resulting in SiO₂/Nb₂O₅/CoHMP and SiO₂/Nb₂O₅/CoPP metallated complexes. The UV-vis spectra of the solid materials showed changes of the Q-bands (a_2u → e_g transition) upon metallation, indicating that by incorporation of Co(II) in the porphyrin ring the local symmetry changed from D_2h to D_4h. These materials, when incorporated in carbon paste electrodes, presented the property of electrocatalyzing O₂ reduction. Rotating disk experiments were performed in order to estimate the number of electrons involved in the process. It was observed that, for both modified electrodes, O₂ was reduced to water in a four-electron process. Amperometric studies showed the potentiality of both modified electrodes as sensors for the determination of dissolved dioxygen. The response time was less than 3 s. A linear response for both systems was obtained between 2 and 12 ppm.

The complexes of formula [Fe(Pc)(diazole)₂]·nH₂O (Pc = phthalocyaninato dianion; diazole = pyrazole, 3(5)-methylpyrazole, indazole and 1-phenylimidazole) have been prepared and characterized. The Mössbauer spectra of the bisadducts of [Fe(Pc)] with diazoles show values of isomer shift close to those of the corresponding complexes with pyridines, but somewhat smaller values of quadrupole splitting.