Porphycenes metal complexes

In contrast to the rectangular cavity of 2,7,12/17-tetra-substituted porphycenes 38, which favors the formation of metal complexes with small cations, the larger cavity of etioporphycenes is able to accommodate even bigger cations favored by steric interactions of the alkyl chains at the (l-positions. 

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). 

As mentioned earlier, the poor solubility and crystallinity characteristics of the metal complexes derived from porphycene 3.2 inspired the synthesis of a number of P-alkyl-substituted analogs. Not surprisingly, these P-substituted systems showed metal coordination properties similar to those of their unsubstituted counterpart. For instance, the 2,7,12,17-tetrapropylporphycene 3.20 has been shown to form stable complexes with Co(II), Ni(II), Cu(II), Pd(II), and Pt(II) (i.e., 3.63-... 

The A,7V -bridged porphycenes 3.119-3.121 were found to exhibit little tendency to act as viable metal-chelating agents." By contrast, the mono-A-methylpor-phycene 3.110 has been demonstrated to be capable of forming stable metal complexes." " For instance, treatment of A(l)-methylporphycene 3,110 with a metal... 

Of all the porphyrin isomers, porphycene and its derivatives have been studied most thoroughly. DFT calculations predict [33] that porphycene 1 as a free base is the most stable of all isomers, including porphyrin 2. The reason for this exceptional stability of 1 is a very strong intramolecular double NH---N hydrogen bond (HB), due to, first, the rectangular shape of the inner cavity that leads to a nearly linear arrangement of the three atoms, and, second, a small N-N distance (2.63 A in 1 [21] as compared to 2.90 A in 2 [34]). In metal complexes, the relative stabilities of porphycene and porphyrin are reversed, since the larger, square-shaped cavity in the latter is much better suited to the accommodation of a metal ion. 

This chapter deals with the provision of suitable starting materials for investigations in the noble metal porphyrin field (Sects. 2.1-2.4) and the establishment of their structures (Sect. 2.5). Most of these investigations are devoted to complexes with synthetic porphyrin ligands which are soluble in organic solvents [e.g. (OEP)- or (TPP) complexes Sect. 2.1]. Special mention is made to some novel porphycene derivatives (Sect. 2.2), to phthalocyanine systems (Sect. 2.3), and to water-soluble porphyrin complexes (Sect. 2.4). 

Like porphyrin, porphycene in its neutral form can be considered as being a doubly protonated version of a dianionic ligand. Thus, it was hoped that porphycene would act as an effective cheland, mimicking the rich metalation chemistry of the porphyrins. Thanks to the efforts of Vogel and others, much of this promise has now been transformed into experimental reality. Thus, in spite of the fact that the por-phycenes possess a central cavity that is smaller than that of the porphyrins, porphycene has indeed been found to form stable, neutral complexes with a wide range of divalent metal cations.Additionally, one example of a porphycene complex formed from a monovalent metal has been reported, as have several examples of complexes of porphycene with higher-valent metals. 

The rich metalation chemistry of the porphycenes was alluded to in Vogel s seminal publication in the area. In it, it was noted that the all-unsubstituted porphycene 3.2 forms stable cobalt(II), copper(II), nickel(II), and zinc(II) complexes O.e., 3.57-3.60), when treated with the appropriate divalent metal salt (Scheme 3.1.10). Later it was shown that porphycene 3.2 also forms stable complexes with divalent palladium and platinum (3.61 and 3.62). Unfortunately, at present no X-ray structural information is available for any of these latter complexes. 

The octa-alkyl porphycenes are also able to support the formation of metal(II) complexes. Examples of these include the nickel(II) complex of octamethylpor-phycene (3.69) and the nickel(II) and zinc(II) complexes of octaethylporphycene (3.70 and 3.71). As shown by single crystal X-ray diffraction data, incorporation of divalent zinc into octaethylporphycene results in the formation of a nearly planar structure (Figure 3.1.13). Considering the somewhat non-planar nature of the starting metal-free system 3.28, the insertion of this cation apparently serves to enforce a marked increase in ligand planarity. 

It is interesting to note that 9,10,19,20-tetrapropylporphycene 3.39 does not form a stable complex with zinc(II). Similarly, 2,7,12,17-tetrapropylporphycene 3.20 forms only a moderately stable Zn(II) complex. By contrast, 2,3,6,7,12,13,16,17-octaethylporphycene 3.28 forms highly stable complexes with Zn(II). These results indicate that the potential exists to adjust considerably the metal coordination properties of the porphycenes simply by varying the peripheral substituents. ... 

In addition to forming complexes with divalent cations, the porphycenes act as ligands for certain trivalent metal cations. In many cases this chemistry parallels that seen in the porphyrin series. For instance, the (x-oxo-diiron(lll)porphycenates 3.72 and 3.73 were prepared from the corresponding free-base porphycene 3.20 or 3.23 using a procedure analogous to that used to prepare the corresponding p-oxo-diiron(III)porphyrinates. Thus, the reaction of 3.20 or 3.23 with Fe(acac)3 in... 

Several other examples of porphycene complexes with trivalent metals have also appeared in the literature.3.4.12,29,29-32 These include the aluminum(III) complexes 3.75a-d and 3.76, as well at the manganese(III), cobalt(III) iron(III), indium(III), and gallium(III) complexes 3.77-3.81 (Figure 3.1.16). Interestingly, a p-oxo-dialuminum(III)porphycenate 3.85 analogous to the p-oxo-diiron(III)porphycenates 3.72 and 3.73 has also been prepared. This was accomplished by heating the aluminum(III)porphycenato hydroxide complex 3.76 to 350 °C under reduced pressure (Scheme 3.1.12). 

Porphycenes have also been shown to form stable complexes with tetravalent metal cations. Examples of tetravalent systems include the tin(IV) and germanium(IV) complexes 3.82-3.84 (Figure 3.1.16). In each of these complexes, two axial chloride anions were found to be coordinated to the metal atom. While it is... 

The reaction between porphycene, and second and third row transition-metal carbonyls has also been investigated. Here, it was found, for instance, that the reaction between decacarbonyldirhenium (Re2(CO)io) and tetrapropylporphycene 3.20 in decalin leads to the formation of the bis[tricarbonylrhenium(I)] complex 3.86 (Scheme 3.1.13), wherein the two metal centers are bound in a sitting-atop fashion (as judged from a single crystal X-ray diffraction analysis Figure 3.1.17). On the other hand, when porphycene 3.20 is reacted with Ru3(CO)i2 in... 

The stretched porphycenes have shown some promise as ligands for the com-plexation of transition metal cations. For instance, when [22]tetradehydroporphyrin-(2.2.2.2) 4.115 is treated with di-p-chlorobis[dicarbonylrhodium(I)] in dichloro-methane using potassium carbonate as base, a bis[Rh(CO)2] complex is formed. ... 

Notably, this metal-mediated concept, namely, coordination of a fullerene-pyridine ligand by a macrocyclic r-system, is very general and has been successfully extended to zinc complexes of phthalocyanines, porphycenes and corrphycene macrocycles. Interestingly, the binding strength reveals a close resemblance to the oxidation potential of the macrocyclic s -system porphycene > porphyrin. 

---