Cobalt bipyridine complexes

A chromophore such as the quinone, ruthenium complex, C(,o. or viologen is covalently introduced at the terminal of the heme-propionate side chain(s) (94-97). For example, Hamachi et al. (98) appended Ru2+(bpy)3 (bpy = 2,2 -bipyridine) at one of the terminals of the heme-propionate (Fig. 26) and monitored the photoinduced electron transfer from the photoexcited ruthenium complex to the heme-iron in the protein. The reduction of the heme-iron was monitored by the formation of oxyferrous species under aerobic conditions, while the Ru(III) complex was reductively quenched by EDTA as a sacrificial reagent. In addition, when [Co(NH3)5Cl]2+ was added to the system instead of EDTA, the photoexcited ruthenium complex was oxidatively quenched by the cobalt complex, and then one electron is abstracted from the heme-iron(III) to reduce the ruthenium complex (99). As a result, the oxoferryl species was detected due to the deprotonation of the hydroxyiron(III)-porphyrin cation radical species. An extension of this work was the assembly of the Ru2+(bpy)3 complex with a catenane moiety including the cyclic bis(viologen)(100). In the supramolecular system, vectorial electron transfer was achieved with a long-lived charge separation species (f > 2 ms). 

The first example of facile reversible trans-to-cis isomerization of the azo group through a combination of photoirradiation and a redox cycle has been achieved in an azobenzene-attached tris(bipyridine)cobalt system [56-58]. This combination makes possible both forward and backward isomerization in response to irradiation from a single light source (Scheme 1). The Co11 complex, 70-2BF4, with trans-azobenzene moieties affords the cis form in... 

The addition of trace levels (>1M) of bis(bipyridine)cobalt(II) to O2-saturated solutions of aldehydes in acetonitrile initiates their rapid autooxidation to carboxylic acids. 0 Figure 6-1 illustrates the CoIKbpy)2 -induced autooxidation of hexanal [CH3(CH2)4CH(O)] for 02-saturated (8.1 mM) and air-saturated (1.6 mM) acetonitrile. The apparent reaction dynamics for the catalyzed auto-oxidation of PhCH(O) and of CH3(CH2)4CH(O) during the first hour of their auto-oxidation is summarized in Table 6-1. The initial reaction rates appear to be first order in catalyst concentration, first order in substrate concentration, and first order in O2 concentration (Fig. 6-1). However, within one hour the autooxidation process is almost independent of catalyst concentration. Although the Fellfbpy) and Mnii(bpy)complexes also induce the auto-oxidation of aldehydes, they are much less effective initiators, and the propagation dynamics are much slower. 

Kubel, F., and Strahle, J., Polymeric dimethyl- and diphenylglyoximato complexes of cobalt and iron with 4,4 -bipyridine as a bridging ligand. The crystal structure of bis(dimethylglyoximato)4,4 -bipyridine cobalt(Il), Z. Naturforsch. B, 37, 272-275 (1982). 

For the approach detailed above to be successful, the metal binding domain must form robust complexes, yet retain synthetic flexibility. Although substituted phenanthroline and bipyridine easily form self-assembled receptors (12), the substitution stability of terpyridine (Fig. 2) makes this ligand more suited for the creation of libraries. The choice of metal also permits some flexibility in library creation. Iron complexes form easily through the addition of ammonium iron(II) sulfate hexahydrate, but can be cleaved by the addition of chelating guests (11,13), Cobalt complexes are also easily formed using cobalt(II)... 

Vinyl substituted bipyridine complexes of ruthenium 9 and osmium 10 can be electropolymerized directly onto electrode surfaces The polymerization is initiated and controlled by stepping or cycling the electrode potential between positive and negative values and it is more successful when the number of vinyl groups in the complexes is increased, as in 77 A series of new vinyl substituted terpyridinyl ligands have recently been synthesized whose iron, cobalt and ruthenium complexes 72 are also susceptible to electropolymerization... 

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