Macrobicyclic cobalt complexes

The E values for macrobicyclic cobalt complexes in aqueous solutions differ by 1400 V (from -550 to 840 mV). For nonmacrocyclic cobalt complexes with diamines and triamines, the E values change from -630 to 280 mV. The potentials for [Co(NH3)6] and [Co(en)3] ions occupy an intermediate position in this range [4, 344]. 

Redox potentials (mV) of macrobicyclic cobalt complexes in acetonitrile. 

Thioetheric macrobicyclic cobalt complexes were also reduced via a second stage to form a cobalt (I) complex. The stabilization of the cobalt(I) oxidation state was attributed to the increased ligand cavity owing to enlargement of a carbon-donor atom distance along with partial transfer of the electron density from the cobalt ion onto the sulphur atoms [339]. 

Reactions of electron self-exchange in macrobicyclic cobalt complexes... 

Electron self-exchange reactions in macrobicyclic cobalt complexes have intensively been investigated. The rate constant of such reactions obtained for a variety of complexes, listed in Table 52, differ by several orders of magnitude (from 0.011 and 0.02 for the [CoCdiMesAMHsar)] and [Co(diAMHsar)]° cations to 2.8x10 for the hexathioether macrobicyclic [Co(diMEsar-S6)] + cation). The available data allow one to determine certain rules for the variation in the rate of electron self-exchange in macrobicyclic cobalt complexes. 

At the same time, several authors have given much attention to the deviations from Eq. (104). Endicott and Ramasami have studied in detail the nonadiabaticity of the electron-transfer cross-reactions with the participation of macrobicyclic cobalt complexes [363, 366, 367, 372],... 

Substituents in the macrobicyclic cobalt complexes affect their photochemical properties. These effects are more pronounced for solutions of the complexes exhibiting high redox potentials and low electron-transfer rates (Tables 42 and 52). Clathrochelates with lower redox potentials and higher electron-transfer rates are less photochemically active. As mentioned above, the [Co(diNOsar-H)]2+ cation displays the highest photochemical activity among the other complexes studied spectral changes are observed on its irradiation... 

The scheme of the photochemical reactions that occur in the presence of oxygen indicates that macrobicyclic cobalt complexes can be employed as efficient catalysts for photochemical electron-transfer processes. 

With a platinum catalyst, methylviologen is undoubtedly a better electron transfer agent than macrobicyclic cobalt complexes. To the contrary, when colloidal RU2O3 (5 x IQ- moM i) deposited on silicon surface is employed as a catalyst, the rate of hydrogen production in the case of [Co(Clsar)]3+ cation is 55 pmolmin-h whereas for methylviologen it is 11 pmol min-i under the same experimental conditions. 

The BF4 anion in the clathrochelate [CoDma(BF)2](BF4) complex can readily be replaced by another large inorganic anion (e.g. PFe ) via an exchange reaction occurring in aqueous-acetonitrile solution in the presence of a great excess of the substituting anion salt [39]. The reduction of the [CoDma(BF)2](BF4) clathrochelate with Nal solution in acetone yielded a macrobicyclic cobalt(II) CoDma(BF)2 complex. The synthesis of the latter via a template condensation on the Co2+ ion was not yet successful. 

The corresponding macrobicyclic cobalt(II) [Co(sep)](ZnCl4) H20 complex was prepared by reduction of the [Co(sep)]Cl3 complex with zinc dust in aqueous HCl [95], A similar procedure was used for the synthesis of the dithionate [Co(sep)](S206) salt in the presence of Li2S206 [99], The optically active R- and S-isomers of cobalt sepulchrate were obtained from the optically active parent A-[Co(en)3]Cl3 and A-[Co(en)3]Cl3 complexes [94-95],... 

The UV-vis spectra of the macrobicyclic cobalt(II) oximehydra-zonates contain two CTBs in the visible region (at 18 000-19 000 and at 24 000-25 000 cm-i). In the spectra of cobalt(III) complexes, a shoulder at 18 000t20 000 cm-i and a peak at 24 000-30 000 cm-i were observed. All the absorption bands for the tris-dioximate complexes occur at slightly higher energy than the corresponding lines in spectra of the oximehydrazonate compounds [186]. 

The quasi-reversible character of these redox reactions indicates that some geometrical changes are taking place without changing the clathrochelate framework. As in the case of trinuclear cobalt complexes, macrobicyclic nickel dioximates are characterized by a high oxidation state stabilization in comparison with other nickel complexes. For example, for Ni(HDm)2 bis-dioximate, two processes (oxidation and reduction) were observed for nickel(II) ion [80],... 

Kinetically inert low-spin cobalt (III) clathrochelates are reversibly reduced by accepting one electron to yield kinetically labile cobalt(II) complexes. In the case of the usual amines (for instance, ammonia), the reduction is, as a rule, accompanied by irreversible decay of the amine cobalt complex. This reaction is slower for chelating amines macrocyclic and especially macrobicyclic amines produce complexes with cobalt(II) ion that are stable over a long time. This fact facilitates the study of the reduction of cobalt(III) complexes to cobalt(II) ones. In most cases, the reactions of macrobicyclic ligands do not interfere with this process. 

The reactions induced by irradiation of aqueous-methanol solutions of the macrobicyclic cobalt(III) complexes occur by Scheme 122. The resulting effect is the photocatalytic oxidation of methanol by oxygen. 

Macrobicyclic cobalt compounds satisfy all these requirements. Their additional advantage is that the redox potentials and electron-transfer rates may be varied on introduction of different apical substituents, by changing the charge of the complexes via protonation or deprotonation, or by altering steric factors. This allows one to select the most suitable complexes as ETAs. 

Chen D, Martell AE (1991) The synthesis of new binucleating polyaza macrocyclic and macrobicyclic ligands dioxygen affinities of the cobalt complexes. Tetrahedron 47 6895-6902... 

Complexes containing encapsulated metal ions (clathrochelates ) with the formula [M(dioxime)3(BR)2] are known with iron(II) 135, cobalt(ll) 136, cobalt(III) 137, and ruthenium(ll) 138 (Fig. 37) [205-220]. Generally, these macrobicyclic complexes are prepared by template synthesis from a mixture of... 

Sargeson and co-workers have reported the use of [Pt(en)3]4+ in template reactions to produce the platinum(IV) complexes of the macrobicyclic ligands sep and (N02)2sar (170).477 These reactions are analogous to those that occur around cobalt(III). However, in contrast to the [Co((N02)2sar)]3+ system, reduction of the pendant dinitro groups did not yield amines, but hydroxyamine groups.478... 

Sepulchrates, the polyaza cage macrobicycles analogous to the cryptates, were first synthesized in 1977.178 The cobalt(lII) complex shown in Figure 3 (19) is the octaazasepulchrate analog of the [2.2.2] cryptand (Figure 3 16), and is commonly written [Co(sep)]3+ (sep = sepulchrate). 

This compound was isolated as a polymeric gel. Macrobicyclic fragments in the polymer chain are linked by Si-O-Si bridging fragments. It was claimed [39] that this clathrochelate may be either a low-spin cobalt(II) compound or a mixture of cobalt(II) and cobalt(III) compounds. However, taking into account the polymeric nature of the complex and the difficulties encountered in its isolation and investigation, one may conclude that the data reported [39] are not sufficient for its unambiguous identification. 

Synthesis of macrobicyclic MDma(BR)2 complexes (where M is nickel (paramagnetic), iron and cobalt ions R is CeHs and re-C4H9) was reported by Umland and coworkers [38]. However, the formation of nickel complexes of this type was not further confirmed. 

The resultant ligand readily formed semiclathrochelate [M(P(Hpox)2(pox))] complexes (where M is nickel, copper, cobalt, or zinc(II) ions) by the interaction of metal salts, e.g., perchlorates, with P Hpox) i ligand either in dry ethanol or in acetonitrile. Attempts to isolate analogous iron(II) and manganese(II) complexes gave no desired results [91, 92]. Direct synthesis of the macrobicyclic... 

Homobinuclear macrobicyclic copper, cobalt, and iron(II) complexes and a heteronuclear iron(II)/cobalt(II) clathrochelate of the [MiM2(trom)] type arise from interaction of a sodium complex of trom ligand with the corresponding metal salts. 

With pentaazamacrobicyclic clathrochelates, not only the nitrogen atoms of the side units but also capping apical nitrogen atoms take part in the coordination. Synthesis of such complexes was performed by interaction of the presynthesized macrobicyclic ligand (Scheme 94) with copper, zinc, cobalt, and nickel(II) perchlorates in boiling methanol [204]. 

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