Tetrabenzoporphyrins metal complexes

The ideal monomer for the synthesis of a symmetrically substituted tetrabenzoporphyrin by an analogous tetramerization would be expected to be the isoindole 96. This substance has not been prepared, but the expectation of its usefulness is confirmed by the observation that 1,3,4,7-tetra-methylisoindole is an excellent precursor, in spite of the fact that it appears to be structurally unsuitable in that it has one a-methyl group too many. Thus, heating 1,3,4,7-tetramethylisoindole with nickel(II) acetate in 1,2,4-trichlorobenzene under reflux provides the nickel(II) octamethyltetra-benzoporphyrin (97) in 71% yield.123 The transition metal ion has a template effect since without it only a small amount (5%) of the metal-free tetrabenzoporphyrin is formed. Various other metal complexes (Mg, Mn, Fe, Co, Cu) have been made this way.123 124 An alternative route involves fusing the... 

The so-called shish kebab or Schaschlik approach allows the construction of stacked macrocyclic metal complexes such as phthalocyaninatometal complexes, metal complexes of tetrabenzoporphyrin, and octaethylporphyrin with variable distances between the planar macrocycles. The macrocycles are linked together by bisaxially metal-bonded bridging ligands thereby combining them... 

The metallation of TPPH2 in D20 or H20 has been studied, and the rate difference (kn/kD = 2.3) shown to be due to differences in the concentration of OH and OD complexes, and not due to intrinsic differences in the reactivity of TPPH and TPPD.1149 The synthesis and properties of porphyrins bearing cationic or anionic substituents are areas of active interest, and zinc complexes of the ligands (172), tetrabenzoporphyrin and octamethylporphyrin have been studied.1150-1154... 

Several syntheses of tetrabenzoporphyrin complexes make use of similar template strategies to those used in phthalocyanine synthesis, so they will be discussed here. An isoindole has been found to undergo a very complex series of reactions promoted by a range of metal ions in refluxing 1,2,4-trichlorobenzene to produce tetrabenzoporphyrin complexes (Scheme 55).240 In contrast, an extremely simple synthesis of tetrabenzoporphyrin can be achieved from 2-acetylbenzoic acid, using zinc as templating metal (equation 44).241... 

Here we review our work aimed at correlating the reactivity of a series of M(II) N4-ligands, see Figure 12.1, (M = Co, Fe, Mn and N4 = porphyrin (P), phthalocyanine (Pc), teraphenylporphyrin (TPP), tetrabenzoporphyrin (TBP) and tetraazaporphyrin (TAP)) towards the electrocatalytic oxidation of 2-mercaptoethanol. Different effects will be analysed, namely the role of the metal atoms, the role of the N4 functionalisation, solvent and the impact of the adsorption on the electrode on the electrochemical activity. The whole machinery of DFT and the notions of hardness, chemical potential, intramolecular hardness and elec-trophilicity are used to better quantify these effects and discriminate between the examined molecular complexes. 

Because of space limitations, we will only briefly discuss the most basic application in the area of porphyrin optical spectra. Much work has already been done in this area, both with TDDFT and related methods. The TDDFT applications include free base porphin and its ss-octahalogenated derivatives, the porphyrinato-porphyrazinato-zirconiumhV) complex, NiP, NiPz, NiTBP, and NiPc, zinc phthalocyanine, chlorophyll a, zinc complexes of porphyrin, tetraazaporphyrin, tetrabenzoporphyrin, phthalocyanine, phenylene-linked free-base and zinc porphyrin dimers, metal bis(porphyrin) complexes a series of porphyrin-type molecules, and many more. We refer to ref. 75 for an extensive discussion of TDDFT calculations on the spectra of porphyrins and porphyrazines, as well as their interpretation. For further theoretical work on porphyrines, we mention ref. 76 and other papers in that special issue. 

Contrary to the majority of phthalocyaninatometal complexes, tetrabenzo-porphyrinatometal complexes have to be prepared from tetrabenzoporphyrin TBPH2 by treatment with metal salts in a suitable solvent. The preparation of tetrabenzoporphyrin mostly follows one of the routes given in Scheme 2. Tetrabenzoporphyrinatozinc (TBPZn) is prepared first [7]. Subsequent removal of zinc from TBPZn e.g. by treatment with trifluoroacetic acid leads to TBPH2 [8]. 

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