Metallocorroles

In the case of the successful Ni(II) cyclization, the initial report (1964) indicated that the reaction could be carried out by irradiating with a 200 W tungsten lamp. It was later determined (in 1966) that, in contrast to the case of the metal-free dideoxybiladienes, light did not serve to activate this particular metal-dependent cyclization. Conversely, the presence of a base was found to be essential. As part of this 1966 study, it was also found that cyclization of the metal(II)dideoxybiladienes-ac in the presence of di-f-butyl peroxide afforded only small yields of a mixture of macrocycles. The above results led to the suggestion that the cyclization of metal(II)dideoxybiladienes-ac does not proceed via a free-radical mechanism. It was thus proposed that the initial reaction involved abstraction of a proton from the central (C(10)) methylene group of the dideoxybiladiene-ac. 

Johnson, et al. showed in 1965 that the hydrobromide salts of dideoxybila-dienes-ac (e.g., 2.67-2.69) could be converted directly to the corresponding nickel(II) 

Trivalent cobalt corroles may also be prepared from dideoxybiladienes-ac as 

Trivalent metallocorrole complexes containing metals other than cobalt have also been prepared from hydrobromide salts of dideoxybiladienes-ac. For instance, in 1988, Boschi, et al. reported two approaches to the formation of in-plane trivalent rhodium complexes. The first involved reacting octamethylbiladiene-ac 2.106 with 

The second approach to the generation of trivalent rhodiumcorrole complexes involved reacting dideoxybiladiene-ac 2.106 in methanol with tetracarbonyldi-p-chlororhodium(I) (Rh2(CO)4Cl2). ° After addition of PPh3, a Rh(III) corrole 2.134 

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Metallocorroles (M = Cu, Ni or Pd) can also be alkylated under the same conditions as the metal-free corroles23,24 to give the N2i-alkylated products together with a small amount of C3 alkylated product ( f = Pd). Allyl halides or bulky alkyl halides react with nickel corroles also at the 3-position. 

In 1992, Kadish and coworkers [76] reported results for the first comprehensive study of the electrochemical oxidation and reduction of some metallocorroles, including (OMC)Co(PPh3) (16), where OMC denotes the trianion of 2,3,7,8,12,13,17,18-octamethylcorrole. 

The most successful approach is to complete the macrocycle by A-D linkage formation (Scheme 69).239,240 The ring closure is effected by light or oxidants, and the presence of an appropriate metal template facilitates metallocorrole synthesis.239-242... 

The chemistry of corroles and their metal complexes has been reviewed by different authors in the past [7-11] and apart from necessary clarifications this paper will deal with the contributions to the chemistry of metallocorrolates and corrinoids published in the last ten years. 

Fig. 9 The Periodic Table of Metallocorrolates. The asterisk marks the elements successfully introduced into a corrole ring...

Thus, the historical development of the chemistry of metallocorrolates until 1980 includes complexes with Cu2+, Ni2+, Pd2+, Fe3+, Co3+, Rh+, Mo5+ and Cr5+. The palladium complex has been isolated as its pyridinium salt since the neutral species was too unstable to be isolated or spectroscopically characterized [19]. The nickel complex was non-aromatic, with one of the potentially tautomeric hydrogens displaced from nitrogen to carbon in such a way as to interrupt the chromophore. In contrast the electronic spectrum of the paramagnetic copper complex is similar to those of the fully conjugated lV(21)-methyl derivatives [11],... 

In the last ten years, several metal ions have been inserted into the corrole moiety leading to the Periodic Table of Metallocorrolates shown in Fig. 9. 

Two major strategies have been considered in order to obtain metallocorrolates the first one consists of the reaction of the preformed macrocycle with metal ions, while the second involves the oxidative, base induced cycliz-ation of the open-chain tetrapyrrolic precursor dihydrobilin (1,19-dideoxybiladiene-a,c) in buffered alcoholic solution in the presence of metal salts. By proper tuning of the experimental conditions (reaction time, solvent, metal carrier) several metal complexes have been obtained and characterized. 

Table 1. Metallocorrolates prepared by reaction of the metal free macrocycle with different metal carriers...

The reactions of dihydrobilin (1,19-dideoxybiladiene-a, c) with transition metals are strongly influenced by the nature of the metal ion. Thus with Mn(OAc)3 or FeClj the corresponding metallocorrolates have been obtained in high yield, in the presence of chromium or ruthenium salts the reaction product isolated has been the metal free macrocycle, while coordination of rhodium requires the presence of an axial ligand such as a phosphine, arsine or amine [21]. Neutral pentacoordinated rhodium complexes have thus been obtained. Although analysis of the electronic spectra of the reaction mixtures demonstrated that cyclization of the open-chain precursor and formation of metallocorrolates occur even in the absence of extra ligands, no axially unsubstituted rhodium derivative has been reported. 

Table 2 lists the metallocorrolates obtained by this synthetic procedure. 

The data reported in the literature relative to the IR spectra of metallocorrolates have been used mainly to characterized their axial coordination. Selected data are reported in Table 5. 

The electronic spectra of corroles and metallocorrolates are mainly determined by transitions within the it system of the macrocycle. 

The presence of a Soret band in the optical spectra of all metallocorrolates can then be considered as a proof of their aromaticity. 

To study the electronic and geometrical structures of metallocorrolates further, rhodium derivatives have also been investigated by XPS [38]. The study has been extended to rhodium derivatives of mcso-tetraphenylporphyrin and oc-taethylporphyrin in order to compare different coordinative structures. Selected data are reported in Table 10. 

All resonances in the proton NMR spectra of diamagnetic metallocorrolates show a strong upheld shift due to anisotropic effects caused by the macrocycle ring current [44]. This is another demonstration of the aromatic character of the corrole ring [21]. Spectral properties of several derivatives of octamethylcorrole are reported in Table 14. 

Table 14. H NMR data of some diamagnetic metallocorrolates. According to Ref. [25].

In spite of the above, and in spite of what might have been inferred from the early papers, the Cu and Co oxacorroles 2.17 and 2.18 were apparently successfully made by the Johnson group. These, in turn, were later demetalated successfully using sulfuric acid to give the metal-free species 2.19. This metal-free corrole was reported to react further with various metal salts to afford the corresponding meta-lated derivatives. Unfortunately, however, specific details as to which metals were used and the properties of the resulting metallocorroles were never presented in the open literature. They are thus not known to the present authors. 

The dideoxybiladiene-ac approach to corroles was also applied to the preparation of metallocorroles. The first report of this sort of reaction appeared in 1964. It described the preparation of the nickel(II) corrole 2.105 from the corresponding metal(II) dideoxybiladiene-ac 2.103 (Scheme 2.1.23). Interestingly, this approach did not work in the case of zinc(II) dideoxybiladiene-ac (2.104). This latter result was rationalized in terms of the known coordination chemistry of Zn(II). In particular, it was postulated that the presumed tetrahedral coordination of the zinc(II) center served to lock the reactive termini of the biladiene into positions that were unsuited for cyclization. 

It was found that addition of hydroxide anion in dimethylformamide or dimethylsulfoxide to metal(II) corrole complexes results in the appearance of much sharper absorption bands relative to the starting compounds. These findings were considered consistent with the idea that an anionic, 18 Jt-electron aromatic corrole complex (e.g., 2.119) is formed as the result of what appears to be a formal deprotonation process (Scheme 2.1.25). That deprotonation actually occurs was inferred from acid-base titrations involving nickel(II) and copper(II) corroles. The conclusion that these species are anionic aromatic compounds came from an appreciation that their electronic spectra resemble those recorded for divalent metallo-porphyrins. In any event, the anion that results was found to be quenched upon acidification, regenerating the corresponding non-aromatic metallocorroles. ... 

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. 

To date, only one example of a monovalent metallocorrole has been reported. It was reported in 1976 by Grigg, et al. and involves a rhodium corrole, which was obtained in 36% yield as the result of reacting free-base diethyl-hexamethyl corrole 2.6 with Rh2(CO)4Cl2. Unlike the trivalent corrole complex 2.134, obtained earlier by the treatment of a dideoxybiladiene-ac with this same metal salt, the complex isolated in this instance analyzed as being the monovalent Rh(CO)2Corrole, 2.157 (Scheme 2.1.42). This complex was later prepared in 72% yield,although it was... 

Formally, pentavalent neutral metallocorroles have been prepared by Murakami and coworkers.The first of these was the oxomolybdenum(V) corrole derivative 2.179. ° This complex was prepared by heating free-base corrole 2.82 with molybdenum pentachloride in oxygen-free decalin (Scheme 2.1.56). Alternatively, molybdenum hexacarbonyl (Mo(CO)e) could be used as the metal source. In both cases, oxidation to the oxomolybdenum complex 2.179 was believed to occur during workup (involving chromatography on neutral alumina followed by recrystallization). In this way, complex 2.179 was isolated in c. 40% yield. Similar yields of the oxochromium(V) complex 2.180 could be achieved via the reaction of 2.82 with anhydrous chromium(II) chloride in DMF. Here too, spontaneous oxidation during workup was used to afford the formally pentavalent oxo-complex 2.180. 

Oxidations of metallocorroles have also been investigated by Vogel and co-workers." For instance, these researchers found that treatment of a-phenyl-cobalt(III) corrole 2.182 with Fe(C104)3 resulted in oxidation to cation 2.191 (Scheme 2.1.64). They also found that iron(III) salts could be used to effect oxidation of nitrosyl iron(III) corrole 2.192. In this case, however, it was a ic-cation radical species (i.e., 2.193), which was obtained upon by treatment with, for instance, iron(III) chloride (Scheme 2.1.65)." Similar oxidations of tetravalent complexes have also been carried out by Vogel and coworkers (see Scheme 2.1.60)." ... 

Alkylation reactions involving other electrophiles are known. For instance, treatment of the anion of nickel(II) corrole 2.105 with allyl bromide was found to afford a mixture of two compounds that proved to be C-allyl metallocorroles. The major product was found to be the 3-allyl-3-methyl-nickel(II) corrole 2.218, and the minor component the meso-d d y derivative 2.219 (Scheme 2.1.74). In the first instance, the regioselectivity of the addition (i.e., at the 3-position) was specifically confirmed by treating the 3,17-diethylcorrole 2.217 with allyl bromide this, as expected, afforded the 3-allyl-3-ethyl derivative 2.220. 

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