Cobalt placement

While there have been a considerable number of structural models for these multinuclear zinc enzymes (49), there have only been a few functional models until now. Czamik et al. have reported phosphate hydrolysis with bis(Coni-cyclen) complexes 39 (50) and 40 (51). The flexible binuclear cobalt(III) complex 39 (1 mM) hydrolyzed bis(4-nitro-phenyl)phosphate (BNP-) (0.05 mM) at pH 7 and 25°C with a rate 3.2 times faster than the parent Coni-cyclen (2 mM). The more rigid complex 40 was designed to accommodate inorganic phosphate in the in-temuclear pocket and to prevent formation of an intramolecular ju.-oxo dinuclear complex. The dinuclear cobalt(III) complex 40 (1 mM) indeed hydrolyzed 4-nitrophenyl phosphate (NP2-) (0.025 mM) 10 times faster than Coni-cyclen (2 mM) at pH 7 and 25°C (see Scheme 10). The final product was postulated to be 41 on the basis of 31P NMR analysis. In 40, one cobalt(III) ion probably provides a nucleophilic water molecule, while the second cobalt(III) binds the phosphoryl group in the form of a four-membered ring (see 42). The reaction of the phosphomonoester NP2- can therefore profit from the special placement of the two metal ions. As expected from the weaker interaction of BNP- with cobalt(in), 40 did not show enhanced reactivity toward BNP-. However, in the absence of more quantitative data, a detailed reaction mechanism cannot be drawn. 

The recyclability of some of these solid supports makes these processes truly eco-friendly green procedures. Previous cartography of the oven using a cobalt chloride aqueous solution to determine the best location for placement of vessels (high electric field density) enables accurate and reproducible MW experiments [30]. 

Cobalt oxide on silica-alumina in the presence of alkyl aluminum also yields high cis-1,4 structure polymers. An all 1,2 polybutadiene can be prepared with n-butyllithium modified with bis-piperidino ethane. The atactic polymer can be formed in hexane at —5 to +20°C temperature [111]. The 100% 1,2 placement was postulated to proceed according to the following scheme [111]. First a complex forms between the base and butyllithium ... 

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