Divalent metalloporphyrin

Platinum porphyrin complexes can be prepared by reaction with PtCl2(PhCN)2. Purification of the final complex is by medium pressure liquid chromatography on alumina. The strongly phosphorescent platinum(II) porphyrin complexes are efficient sensitizers for stilbene isomerization. The quantum yields for the cis to trans process are greater than unity because of a quantum chain process in which the metalloporphyrin serves both as an energy donor and an acceptor.1110 Picosecond laser spectroscopy has been used to obtain time-resolved excited-state spectra of platinum octaethylporphyrin complexes, and to probe the excited-state energy levels.1111 Tetrabenzoporphyrin complexes have been prepared for platinum in both the divalent and tetravalent oxidation states. The divalent complex shows strong phosphorescence at 745 nm.1112... 

The dimers have turned out to be a very interesting and useful class of metalloporphyrins which was extended to dimers containing two divalent metal ions with Rh-Rh single (see Sect. 3.3), Re-Re triple, and Mo-Mo or W-W quadruple bonds [49,222-224]. These species react with many donor and acceptor molecules (see the following Sects. 3.22 and 3.3). The reactions with typical neutral donor ligands may be seen in Scheme 1, the oxidation reactions in Scheme 2. 

Scheme 4.1 Formal oxidation states of a divalent M and the porphyrin ligand For in species generated from a neutral metalloporphyrin [(Por)M(II)].

In 10-2 M HC1, [In(tmpyp)]5+ has bands at 424 nm (e = 390,000), 518 nm (e = 3690), 558 nm (c = 23,200), and 597 nm (e = 4660). The spectrum is similar to that of indium(III)-tetraphenylporphyrin.16 The [In(tmpyp)]5+ is fully water soluble, and rapid acid solvolysis occurs only above 3 M HC1 levels. Refluxing a divalent metal chloride with H2tmpyp in water, and keeping the pH between 3 and 5, is the general method used to prepare the Cu2+, Zn2+, Ni2+, Mn3+, Fe3+, and Co3+ compounds.6 The 642-nm free-base porphyrin band disappears when the incorporation reaction is complete. The high-pH conditions (a) reduce the concentration of unreactive centrally protonated15 porphyrins, (b) minimize the extent of acid catalyzed metalloporphyrin solvolysis reactions,17 and (c) increase the concentration of the often more reactive hydrolyzed metal ion forms.18... 

Radiolytic studies in aqueous solutions demonstrated reduction of Fe P to Fe P, of Co P to Co P to Co P/ of Mn P to Mn"P/ and in 2-propanol reduction of Rh P to Rh P via an unstable Rh P/ Trivalent metalloporphyrins generally contain a halide ion and/or a solvent molecule as axial ligands. Reduction of these complexes to form the divalent metal center often is accompanied by release of the axial ligands. 

When divalent metal ions (e.g. Co(ll), Ni(Il), Cu(II)) are chelated, the resulting tetracoordinate chelate has no residual charge. While Cu(II) and Ni(II) in their porphyrin complexes have generally low affinity for additional ligands, the chelates with Mg(II), Cd(II) and Zn(II) readily combine with one more ligand to form pentacoordinated complexes with square-pyramidal structure (Fig. 11a). Some metalloporphyrins (Fe(II), Co(II), Mn(II)) are able to form distorted octahedral (Fig. 11b) with two extra ligand molecules (Biesaga et al., 2000). 

Figure 1. Representative polyoxometalates in polyhedral notation. A. the isopolyanion decatungstate (Wio032 ) (C>4h point group symmetry) and B. the heteropolyanion family, (TM)PWii039 , where TM is a first row divalent transition metal ion, P is the heteroatom (Cs point group symmetry). In the latter class of complexes, which constitute functional oxidatively resistant inorganic metalloporphyrin analogs, P is one of many elements that can function as the heteroatom. The darker octahedron on the surface and the veiy dark internal tetrahedron of B represent the TM ion and the heteroatom, respectively. In polyhedral notation a complementary notation to ball-and-stick or bond representations, the vertices of the polyhedra, principally W06 octahedra, are the nuclei of the oxygen atoms. The metal atoms lie inside each polyhedron.

Soon after Guldi et al. published on their system, Stod-dart and coworkers published a report wherein dynamic chemistry was used to form self-assembled oligomers that were found to be capable of binding and, possibly even, sorting SWNTs by size. Their system is composed of 5,15-fran.y-pyridyl metalloporphyrin, 13, which can act as a linear divalent ligand and cw-protected Pd or Pt complexes (Figure 13). While neither the Pd/Pt ligand nor the porphyrin displayed an ability to solubilize SWNTs on its own, they were found to completely solubilize SWNTs in aqueous acetonitrile solutions when used in conjunction with one another. 

Closed shell metalloporphyrins such as those having divalent ions (e.g., magnesium or zinc) readily undergo oxidation to radical cations/ These closed shell ionic metalloporphyrins can be considered to be porphyrin dianions, and therefore they are oxidized more readily than are the free porphyrins themselves. The redox potentials of metalloporphyrins increase with increasing electronegativity of the metal ion. Metalloporphyrins with trivalent or tetravalent metal ions are particularly difficult to oxidize because of the high electropositive character of the metal center. 

Fig. 14 Schematic representation of the experimental STM junction technique for measuring conductance (a) through flat configuration of the free metalloporphyrin and metalloporphyrin Co (II)-5,15-diphenylporphyrin (Co-DPP) with a divalent cobalt center, which allows coordination to pyridinyl ligands affording a hexacoordinate system (b) through flat configuration of the metallophthalocyanine self-assembled to pyridinium molecule and pyridinyl ligand. Reprinted with permission from [174], Copyright 2014 American Chemical Society...

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