Corroles, 259 reduction

Capar C, Hansen LK, Conradie J, Ghosh A (2010) P-octabromo-me.s0-tris(pentafluoro-phenyl)corrole reductive demetalation-based synthesis of a heretofore inaccessible, perhalo-genated free-base corrole. J Porphyr Phthalocyanines 14 509-512... 

Octaethyl and tris(pentafluorophenyl) corroles, known as oec and tpfc, respectively, are also efficient for the stabilization of P(VI) phosphorus [53,54]. The electron-rich oec reacts with PCI3 to form (oec)P=0 40 that can be further derived into dihydrido 41a, dimethyl 41b and diphenyl 41c compounds by reduction with LiAlH4 and reactions with methyl and phenyl Grignard reagents. 

The simple porphyrin category includes macrocycles that are accessible synthetically in one or few steps and are often available commercially. In such metallopor-phyrins, one or both axial coordinahon sites of the metal are occupied by ligands whose identity is often unknown and cannot be controlled, which complicates mechanistic interpretation of the electrocatalytic results. Metal complexes of simple porphyrins and porphyrinoids (phthalocyanines, corroles, etc.) have been studied extensively as electrocatalysts for the ORR since the inihal report by Jasinsky on catalysis of O2 reduction in 25% KOH by Co phthalocyanine [Jasinsky, 1964]. Complexes of all hrst-row transition metals and many from the second and third rows have been examined for ORR catalysis. Of aU simple metalloporphyrins, Ir(OEP) (OEP = octaethylporphyrin Fig. 18.9) appears to be the best catalyst, but it has been little studied and its catalytic behavior appears to be quite distinct from that other metaUoporphyrins [CoUman et al., 1994]. Among the first-row transition metals, Fe and Co porphyrins appear to be most active, followed by Mn [Deronzier and Moutet, 2003] and Cr. Because of the importance of hemes in aerobic metabolism, the mechanism of ORR catalysis by Fe porphyrins is probably understood best among all metalloporphyrin catalysts. 

One example of a tin porphycene has been reported, but as yet no organometallic derivatives have been reported." A small number of tin corrole complexes are known including one organotin example, Sn(OEC)Ph, prepared from the reaction of Sn(OEC)Cl with PhMgBr. A crystal structure of Sn(OEC)Ph shows it to have both shorter Sn—N and Sn—C bonds than Sn(TPP)Ph2, with the tin atom displaced 0.722 A above the N4 plane of the domed macrocycle (Fig. 6). The complex undergoes reversible one-electron electrochemical oxidation and reduction at the corrole ring, and also two further ring oxidations which have no counterpart in tin porphyrin complexes. " " ... 

In this section, we present material dealing with the direct oxidation and reduction of a variety of organocobalt species, including complexes with more than one cobalt center, electrodes functionalized with cobalt complexes, cobalt-containing SchifF-base complexes, cobalt porphyrins and corroles, and macrocyclic tetraamines. 

No ligand centered reduction has been observed in a cathodic region up to — 2.0 V (vs SCE) the HOMO-LUMO energy separation in corrole resulted then to be larger than 3.3 V, i.e. much greater than that present in porphyrins. This is consistent with the lower skeletal symmetry of the corrole structure with respect to porphyrin which is expected to increase the separation between HOMO and LUMO [46]. 

The reduction peaks do not vary if the electrochemistry is carried out in the presence of excess PPh3. It is known that Co2+ porphyrinates can coordinate donor molecules along the z axis and the lack of occurrence of such reaction in the case of corroles has been attributed to the negative charge of the electrogenerated Co2+ complex. 

The electrochemical and spectroelectrochemical data for rhodium corrolate have been explained with the existence of monomers and dimers in solution and the reductions have been proposed to occur according to the mechanism shown in Fig. 23a. 

The first example reported in the literature is the cyclization of dihydrobilin to octadehydrocorrin [51-54]. The reaction is catalyzed by the presence of nickel or cobalt salts. As in the case of corrole and its metal complexes such ring closure reaction has been carried out in alcoholic solution, it is oxidative and base catalyzed. It has been demonstrated that the formation of the corrin ring is part of an equilibrium where the oxidative ring closure is coupled with a reductive ring opening reaction [55]. 

Corrphycenes and their metal complexes undergo four distinct one-electron redox steps, two are reduction steps and two are oxidation steps. A comparison with porphyrins and porphycenes indicates that the first reduction potentials of the free base and of metallo-corrphycenes are between those of porphycene 38, the easiest to reduce molecules, and those of porphyrins 2. The oxidation potentials of corrphycenes and porphyrins, however, are quite similar (00IC2850). The synthesis of hemi-porphycene 70 (Scheme 33) has been achieved (05ACI3047) from corrole 8 (R=C6H5). 

Tetraoxacorrole cation 2.55a has been subjected to preliminary investigations in terms of its reactions with nucleophiles. Interestingly, nucleophilic attack appears to occur predominantly at the 5-position of the corrole ring. For instance, treatment of 2.55a with NaOH in dimethylsulfoxide (DMSO) followed by neutralization with HCl afforded 5-oxotetraoxacorrole 2.64 (Scheme 2.1.13). When treated with sodium borohydride, reduction of 2.55a was found to produce the 5-hydro-tetraoxacorrole 2.65 in 86% yield (Scheme 2.1.14). Corrologen 2.65 proved to be... 

The cobalt(II) corrole anion prepared as above was characterized primarily by electron spin resonance (esr) and absorption spectroscopy. When prepared via sodium film reduction, the cobalt(II) corrole oxidizes rapidly to the corresponding Co(III) corrole on exposure to air. When prepared by the other methods, it is moderately stable in air in the presence of a reducing agent. Attempts to prepare the neutral form of the initial Co(II) corrole anion, by protonation with perchloric acid, resulted in formal oxidation to the Co(III) derivative. Interestingly, further protonation of the Co(III) corrole with perchloric acid led to what appeared to be a protonated Co(III) corrole. Certainly, the absorption spectrum of this species is similar to that of the corresponding neutral nickel(II) corrole complex. However, the exact nature of this protonated material has not been fully elucidated. 

In work along somewhat related lines, Murakami, et al. investigated the hydroxide-mediated reduction of 8,12-diethyl-2,3,7,13,17,18-hexamethyl-cobalt(III) corrole 2.184. This involved treating 2.184 with -butyl ammonium hydroxide in the presence of ethyl vinyl ether in an aprotic solvent such as dichloromethane, benzene, or DMF. Using a similar approach, these researchers also found that iron(III) corrole 2.185 could be reduced to give the corresponding iron(II) corrole (Scheme 2.1.61). In this work, it was determined that olefins, on oxygenation with hydroxide ion, can act as the requisite electron donors for reduction. 

The one-electron reduction of cobalt(III) corroles to cobalt(II) corroles can also be effected electrochemically. As expected, increasing the donating ability of the axially coordinating ligand decreases the ease of reduction. It also affects the extent to which the reaction can be made to be reversible under normal laboratory conditions. For instance, reductions involving pyridine cobalt(III) corroles are gen-... 

Gross and coworkers have used Co(tpfc)(PPh3) as a test bed for ring-substitution reactions it was on this compound that they first demonstrated the facility of electrophilic chlorosulfonation as a corrole functionalization method [132]. They also found that doubly reduced [(tpfc)Co]2- can catalyze the reduction of C02 over sodium amalgam in acetonitrile, although the catalytic reaction... 

It was observed in a 2005 article that Co(II) porphyrin-Co(III) corrole dimers are more effective dioxygen reduction electrocatalysts than analogous Co(III)-Co(III) corrole dimers or monomeric Co(III) corroles [145], The heterodimers operated effectively at lower overpotentials and promote complete reduction to water (the average number of electrons transferred per 02 molecule approaches 4 in the best porphyrin-corrole catalyst). It was suggested that the inferior catalytic performance of the corrole homodimers could be due to a reduction in the basicity of the activated intermediate when two Co(III) moieties are involved, leading to a less favorable 4-electron reduction. Heterobimetallic catalysts containing formally Co (IV) corroles were also examined as potential dioxygen reduction catalysts [146]. 

While the Co corrole-Fe/Mn porphyrin dyads were active electrocatalysts, the homobimetallic Co porphyrin-corrole dyads operate at lower overpotentials and favor 4-electron reduction to a greater extent. This was attributed to the locus of reduction in each complex apparently, the porphyrin is reduced first in the homometallic dimer, whereas the corrole is the site of the first reduction in the heterometallic dimers. A later article presented EPR and spectroelectrochemical evidence supporting the assignment of a Co(III) Ji-cation electronic configuration for the oxidized derivatives of monomeric Co triarylcorrolates, suggesting that the dimeric five-coordinate Co corrole catalysts are also Co(III) 7i-cation radicals [147]. Further study is needed to elucidate this point. 

An article published in 2011 explored the oxygen reduction reactivity of Co hangman corroles [153], Along with novel catalytic activity, this paper described some potentially useful synthetic procedures. For example, H3tpfc was obtained in... 

Murakami Y, Aoyama Y, Hayashida M (1980) Hydroxide-promoted reduction of the corrole complexes of cobalt(III) and iron(III) in the presence of olefin. J Chem Soc Chem Comm 501-502... 

Grodkowski J, Neta P, Fujita E, Mahammed A, Simkhovich L, Gross Z (2002) Reduction of cobalt and iron corroles and catalyzed reduction of C02. J Phys Chem A 106 4772 -778... 

Jerome F, Gros CP, Tardieux C, Barbe JM, Guilard R (1998) Synthesis of a face-to-face porphyrin-corrole a potential precursor of a catalyst for the four-electron reduction of dioxygen. New J Chem 22 1327-1329... 

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