Corrole numbering

The seiectivities of metal complexes of cofacial porphyrinoids (porphyrins, corroles, and phthalocyanines) reported in the literature by mid-2007 are summarized in Fig. 18.15. The data are organized by the type of catalyst as well as in order of decreasing Mav Whereas ORR catalysis by certain cofacial porphyrins, such as (FTF4)Co2 and (DPY)Co2 (Y = a, B) has been smdied extensively by a number of groups, and the values of av are known with high degree of confidence, those for most other 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. " " ... 

Figure 2. Transformation of porphyrin to corrole and the numbering schemes.

The iron porphyrins and related compounds constitute an extremely important class of coordination complex due to their chemical behaviour and involvement in a number of vital biological systems. Over recent years a vast amount of work on them has been published. Chapter 21.1 deals with the general coordination chemistry of metal porphyrins, hydroporphyrins, azaporphyrins, phthalocyanines, corroles, and corrins. Low oxidation state iron porphyrin complexes are discussed in Section 44.1.4.5 and those containing nitric oxide in Section 44.1.4.7, while a later section in this chapter (44.2.9.2) is mainly concerned with iron(III) and higher oxidation state porphyrin complexes. Inevitably however, a considerable amount of information on iron(II) complexes is contained in that section as well as in Chapter 21.1. Therefore in order to prevent excessive duplication, the present section is restricted to highlighting some of the more important aspects of the coordination chemistry of the iron(II) porphyrins while the related unusually stable phthalocyanine complexes are discussed in the previous section. 

A number of five-coordinate Co(lll) complexes have also been crystallographically characterized, with the corrole complex (4) and the thiolate complex (5) providing... 

The review presented here has a more synthetic focus than its predecessors. Detailed information about the physical properties of the corrin-related macrocycles is, therefore, not included here. Instead, the reader is referred to the earlier reviews, as well as to a number of relevant papers, for detailed descriptions of the physical properties of corrin-type systems.Still, in this chapter, a complete, up-to-date discussion of corrole and heterocorrole synthesis and metalation properties will be presented. Also, two sections will be devoted to other synthetic contracted porphyrins, including isocorroles, and several systems that contain fewer than four pyrrole-like subunits in their macrocyclic framework. 

Figure 2.1.1 Structure of corrole showing the atom numbering scheme...

It is perhaps not surprising that metallocorroles may be prepared from preformed metal-free corroles as well as from linear pyrrolic precursors. In fact, the former metal insertion approach has allowed a considerable number of metallocorroles to be prepared, including complexes containing mono-, di-, tri-, and tetravalent metal cations (as discussed above in Sections 2.1.2.1.1-2.1.2.1.3). The following section will describe examples of the latter approach to metallocorroles, that is, via insertion of a metal center into a pre-formed corrole ring. 

The numbering schemes of porphyrins and corroles are essentially the same (Fig. 4). Throughout this chapter, the protonated macrocycles are denoted by the addition of Hn to the appropriate abbreviation, where n = the number of protons in the N4 core. The abbreviations cor and por are used to denote the bare correlate and porphyrinate macrocycles, respectively. The 5, 10, 15, and 20 positions on the ring are referred to as meso positions, while the 2, 3, 7, 8, 12, 13, 17, and 18 ring positions are referred to by the shorthand p (they are p-pyrrole carbon atoms). 

As noted above, the complexes of corroles with metals of the first transition series comprise the majority of reported metallocorroles. Many investigations have focused on the usual bioinorganic suspects, Cu and Fe. A number of computationally heavy reports on the electronic and molecular structures and spectroscopic properties of Cu corroles have been produced over the last few years, while the field has seen numerous attempts to formalize consensus electronic structural descriptions of Co and Fe corroles. There is also a rich literature describing the catalytic utility of high-valent Cr and Mn complexes with oxo, imido, or nitrido ligands, and a small amount of work has been performed on Ti and V corroles. Ni corroles have been reported in the literature as well, but investigations of their properties have often been folded into larger studies. 

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]. 

Fig. 13 The crystal structure of a highly saddled copper [3-octabrominated corrole complex (left) and the predicted saddling angles for a number of mero-triaryl-substituted copper corroles. Reprinted with permission from [171]. Copyright 2010 American Chemical Society...

Significant work has been performed on the corrole complexes of four second-row transition metals Mo, Ru, Rh, and Ag. The spectroscopic properties of a number of (oxo)Mo(V) corroles have been examined in detail. Meanwhile, a number of dimeric Ru corroles have been synthesized and their electrochemical properties have been reported monomeric Ru-NO corroles have also been disclosed. The syntheses of both Rh(I) and Rh(III) corroles have been reported by a number of groups, and the excited-state EPR spectroscopy of the Rh(III) derivatives has been presented. High-valent Ag(III) corroles have been studied extensively with regards to their facile demetalation. 

Fig. 1. 20 (a) Corrin, corrole, and corphin macrocycles. Notice how C-20 is missing and the numbering of the macrocycle reflects this, (b) Coenzyme vitamin B. ... 

Many successM methods have been reported for the synthesis of porphyrinoids. Symmetric porphyrin synthesis is achieved by cyclic tetramerizalion using a-free pyrroles and aldehyde [6]. a-Hydroxymethylpyrroles are also employed suc-cessfiiUy in the cyclic tetramerization method [7]. For the synthesis of unsymmetric porphyrinoids, oligomeric pyrroUc units are condensed under acidic conditions. These mediods are categorized by the number of pyrroles (or five-membered heterocycles) in the units. The [2 + 2] and [3 -h 1 ] methods are common in the synthesis of porphyrins [8] the [3 - - 2] and [3+1 + 1] methods are employed for the synthesis of sap-phyrins [9] and the [2 + 2] method provided porphyrins and corroles [10]. For the preparation of 7t-expanded porphyrinoid precursors by using these methods, monomeric or oligomeric equivalents of isoindoles are required and these preparations are first introduced. 

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