Macrocycle, metalation

Perspectives for fabrication of improved oxygen electrodes at a low cost have been offered by non-noble, transition metal catalysts, although their intrinsic catalytic activity and stability are lower in comparison with those of Pt and Pt-alloys. The vast majority of these materials comprise (1) macrocyclic metal transition complexes of the N4-type having Fe or Co as the central metal ion, i.e., porphyrins, phthalocyanines, and tetraazaannulenes [6-8] (2) transition metal carbides, nitrides, and oxides (e.g., FeCjc, TaOjcNy, MnOx) and (3) transition metal chalcogenide cluster compounds based on Chevrel phases, and Ru-based cluster/amorphous systems that contain chalcogen elements, mostly selenium. 

Aiaki K, Toma HE. 2006. Supramolecular porphyrins as electrocatalysts. In Zagal JH, Bedioui F, Dodelet J-P, editors. N4-Macrocyclic Metal Complexes. New York Springer, p. 255. 

The octaethylporphyrin (OEP) complex cocrystalline with C60, namely Pdn(OEP)-C6o 1.5C6H6 is one of the first examples of the anti-formed macrocyclic metal complexes of OEPs.180,181... 

Studies on the electrocatalytic activity of metal porphyrins are limited in comparison with those on other classes of macrocyclic metal complex. Among the few porphyrin complexes tested, cobalt porphyrins have been demonstrated to be efficient electrocatalysts for the reduction of C02 to CO... 

Considering their generally similar dimensions, there are not expected to be major differences between the solvation of the open-chain and macrocyclic metal complexes arising from size differences. Nevertheless. 

Subsequently, Backvall and coworkers developed triple-catalysis systems to enable the use of dioxygen as the stoichiometric oxidant (Scheme 3) [30-32]. Macrocyclic metal complexes (Chart 1) serve as cocatalysts to mediate the dioxygen-coupled oxidation of hydroquinone. Polyoxometallates have also been used as cocatalysts [33]. The researchers propose that the cocatalyst/BQ systems are effective because certain thermodynamically favored redox reactions between reagents in solution (including the reaction of Pd° with O2) possess high kinetic barriers, and the cocatalytic mixture exhibits highly selective kinetic control for the redox couples shown in Scheme 3 [27]. 

Template syntheses of P macrocycles are a new area. In fact, a 1978 review93 of template synthesis made no mention of P macrocycles. Template syntheses have been developed by Stelzer and co-workers.94 Firstly, two molecules of the bidentate secondary phosphine are complexed with a nickel(II) or palladium(II) salt (Scheme 6) and the resultant secondary phosphine complex is then condensed with a diketone to form the macrocyclic metal complex. Unfortunately, these macrocycles are strong field ligands and no method has yet been devised to remove the metal from the ring. On the other hand, Cooper and co-workers95 have used a template synthesis to produce a [l4]aneP2N2 macrocycle (Scheme 7). 

Two further miscellaneous classes of macrocyclic metal complexes are included in this section because they have structural features such as aza linkages and a high degree of unsaturation associated with fused aromatic rings. Ogawa and coworkers have shown that when 2,4-di-chloro-l,10-phenanthroline is fused with ammonium tetrachlorozincate, a macrocyclic zinc complex is produced. The metal can be removed subsequently by acidic treatment (Scheme 57).244 The free macrocycle can also be prepared by non-template methods, which however do not appear to be as effective as they require reaction of the dichlorophenanthroline with 2,9-diaminophen-anthroline.245... 

Plieger, P. G., Tasker, P. A., Galbraith, S. G., Zwitterionic macrocyclic metal sulfate extractants containing 3-dialkylami-nomethylsalicylaldimine units. Dalton Trans. 2004, 313-318. 

The preferential coordination geometry of the template metal ion determines the nature of the macrocycle that is formed, e.g., Ni2 + and Cu2+ are particularly effective template ions for the synthesis of N-donor macrocycles containing 4 nitrogen atoms that can be arranged in a plane and, hence, form a square planar macrocycle-metal complex. 

In subsequent investigations (1,2) macrocyclics were converted into macrocyclic metallic complexes. A particularly long-lived homogenous oxidation catalyst substrate, (VI), has been prepared and is described (3). 

In a manner similar to that used to prepare 112, the reaction of the diformyl-tripyrrane 114 with o-phenylenediamine was found by Sessler and coworkers to result in the synthesis of a pentaazamacrocycle 115 (Scheme 13) [59], An X-ray structure of a derivative of 115 is shown in Fig. 19. Unfortunately, no structurally characterized metal complexes of 115 have been reported to date. However, oxidation of 115 in the presence of cadmium(II) was found to give the aromatic pentaaza-macrocycle metal complex 116, which has been characterized by X-ray diffraction [60]. The properties and chemistry of these tripyrroledimethine-derived texaphyrins is reported in the next section. 

In the macrocyclic structure the metal binding site within the ligand is more encapsulated and the entropy is decreased upon metal incorporation. As a result the stability of the majority of macrocyclic metal chelates is higher than that of acyclic complexes (Table 3) [27,28]. Generally the macrocyclic complexes exhibit a higher kinetic stability. 

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