Cobaloxime, isomerization

Another type of isomerization, that of cis-trans isomerization of an axial Co substituent has also been observed to occur in a single crystal-to-single crystal manner in cobaloximes. Compound 42 was monitored by x-ray crystallographic determination of the unit cell dimensions at 9 points over 2 h [121]. Molecular... 

Vitamin B12 is known for its ability to catalyze molecular rearrangements. A variety of cobalt chelates are logical models for vitamin B12, and their stoichiometric and catalytic activities in a variety of reactions,403 particularly olefin isomerizations, were studied intensively.404-411 Noncatalytic isomerization reactions based upon the synthesis of alkylcobalt chelates as model intermediates were favored. A variety of catalytic oxidations of substrates such as hydroquinone, azo compounds, phosphines, and olefins were also investigated.412-415 Copolymerization of a-methylstyrene and other monomers with oxygen in the presence of CoTPP led to alternating polyperoxides.416 418 Cobaloximes were found to catalyze... 

It was found that the 2-cyanoethyl (2-ce) group bonded to the cobalt atom is isomerized to the 1-ce group on exposure to visible light only in the solid state as shown in Scheme 5.1 [1]. The reachon is not observed in solutions probably because the comphcated coupling reactions between the produced alkyl radicals would occur. Since the crystals are usuahy decomposed as the reaction proceeded, the reaction rate is estimated from the change of the IR spectra due to the C-N triple bond. Several KBr discs of a cobaloxime complex were irradiated with a xenon lamp under the same conditions and the IR spectra were measured one by one at a constant interval. The spectra due to the stretching vibration mode of the cyano... 

Fig. 5.4 Isomerization rates with exposure time for the powdered samples of (2-ce) (tpp) cobaloxime obtained in the rapid and slow crystallizations...

When the axial base ligand was replaced with pyridine (py), two crystal forms, I and n, were obtained under the same conditions [5]. Figure 5.9 shows the molecular structures of the two forms. The 2-ce group of I has an approximately parallel conformation to the cobaloxime plane, whereas that of II has a perpendicular conformation to the plane. Moreover, the 2-ce group of II has a disordered structure. The isomerization rates of the two crystal forms were measured in the same way as those of the two forms of the 3mpy crystals. Form I with parallel conformation has significantly a faster rate than form n with perpendicular conformation, as shown in Fig. 5.10. 

Fig. 5.12 Isomerization rates of six cobaloxime crystals with different axial base ligands...

Fig. 5.17 Isomerization rates of pure and mixed crystals of (2-ce)(3mpy)cobaloxime...

The isomerization rates of the 2-ce to 1-ce groups in the cobaloxime complex crystals were found to be controlled by three factors (1) the size of the reaction... 

The reaction rates of A are greater than those of B in the three crystals, although the volumes of the cavities of A are smaller than those of B. Such differences are caused by differences in conformations of the 2-ce groups between A and B molecules, since A molecules have parallel conformations, whereas B molecules have perpendicular conformations. Although the different reaction rates of the two con-formers were already observed in the polymorphic crystals of the (2-ce)(pyridine) cobaloxime complex, the reactions in the polymorphic crystals cannot proceed under exactly the same conditions, because different crystals are used. Since the two conformers in the present photoreaction have exactly the same conditions, it was made clear that the 2-ce group with the parallel conformation is isomerized to the 1-ce group faster than that with the perpendicular conformation. The difference in reactivity between the two conformers in the solid state can be made clear in the same way as that in a solution if the two conformers were included in a crystal. 

It was found that the cobaloxime complex with the 4-cyanobutyl (4-cb) group was isomerized only to the 3-cyanobutyl (3-cb) group with retention of the single crystal form. The NMR data indicated that about 6 % of 4-cb group was transformed to the... 

Properties and Reactions.—Physicochemical studies of axial ligation of pyridine to cobalt corroles have been carried out/ and the kinetics and mechanism of cobalt-carbon bond cleavage in the alkylation of tin and mercury ions have also been studied. The mechanism of action of the vitamin coenzyme has been reviewed/ and several studies of the mechanism of cobalt-carbon bond cleavage and isomerization reactions of the model cobalt cobaloxime system have been published. ... 

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