Organocobalt complex stability

This paper evaluates the role of the hydrido complex in various carbon-carbon double bond hydrogenations and carbon-halogen bond hydrogenolyses, and the relationship between the two reaction types. Evidence for free radicals, as well as organocobalt complexes ( metal-complexed radicals), as intermediates will be presented. The relationship of organocobalt complex stability to catalytic hydrogenation will also be discussed. 

In addition to the work described here, experiments with styrenes and a,)8-unsaturated acids, esters and ketones, as well as related halides (24), suggest certain generalizations concerning the effect of structure on organocobalt complex stability and catalysis. Table IX lists several organocobalt complexes, whose preparations, either by reaction of the halide with pentacyanocobaltate(II) or by addition of hydrido complex to the corresponding unsaturated substrate, have been attempted. In many cases, both methods were used. 

This reaction leads to an organocobalt complex, RColnX2, which is reduced to the corresponding RConX. Owing to the low stability of the cobalt(I) species, the reaction proceeds in one step (equation 29). 

Hydroxy- and alkoxy-cobalt(III) complexes are also known to react with reagents which can form stabilized carbanions on loss of a proton [67], such as malononitrile, to form organocobalt complexes (Eqn. 27, R = H or CH3, R = CN, R" = CN, CONH2, etc.). 

Brookhart s group has discovered a new hydrido organocobalt complex in which the cobalt atom is in a high oxidation state. They thus reported a rare example of an organometallic cobalt derivative with a +v formal oxidation state for the cobalt center 10. This species contained additional silyl ligands that stabilized such high oxidation state complexes. These species were remarkably resistant toward reductive elimination. The 2.28 A H-H and the... 

The homogeneous catalytic [2+2+2]-cycloaddition of alkynes and nitriles was first discovered by Yamazaki and Wakatsuki [3] using the phosphine-stabilized cobalt(III) complex (Stmcture 5). At the same time, Bdnnemann and co-workers [5] observed the co-cyclization (eq. (2)) at cobalt catalysts prepared in situ, as well as using phosphine-free organocobalt(I) diolefm complexes. 

Organocobalt Cluster Complexes. XIII. NMR Evidence Concerning the Stabilization of Nonacarbonyltricobalt carbon Substituted Carbonium Ions, D. Seyferth, G. H. Williams, and D. D. Traficante, J. Amer. Chem. Soc., 96, 604(1974). 

The stability constants for the coordination of pyridine or substituted pyridines to various alkylcobalt porphyrin systems have been reported. This study has been done in order to observe the m-influence of hydroporphyrin macrocycles on axial position and alkyl exchange reactions. The alkyl exchange reactions of organocobalt(lll) porphyrins with a cobalt(ll) complex of a distinguishable porphyrin or tetrapyrrolc have been studied. The equilibrium constants for the alkyl transfer have been reported. The exchange of the axial ligand is reversible and it follows a bimolecular mechanism. Thermodynamic and activation parameters for homolytic Co-C bond dissociation have been obtained on the (TAP)CoC(CH3)2CN complex (TAP = tetraanisylporphirinato). ... 

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