Cis-influence of hydroporphyrin macrocycles on axial ligation equilibria and alkyl exchange reactions of alkylcobalt(III) porphyrin complexes

Stability constants are reported for the coordination of pyridine and subst ituted pyridines to the alkylcobalt(III) complexes of octaethylporphyrin (O EP), t-octaethylchlorin (OEC), and ttt-octaethylisobacteriochlorin (OEiBC) in toluene solution. The stability constants correlate with the base streng th of the nitrogenous ligand, A cis-influence of the macrocycle saturation level on the stability constants is observed. Stability constants for coord ination of a given pyridine ligand to an alkylcobalt(III) complex are rough ly 10 times smaller than the stability constants for the corresponding coba lt(II) complex. Analysis of a thermodynamic cycle demonstrates that this le ads to decreased stability of the complex with respect to Co-C bond homolys is upon ligand coordination, a "base-on" effect. Alkyl exchange occurs betw een cobalt complexes of different tetrapyrroles. Equilibrium data establish that the exchange is nonstatistical and that the Co-C bond is stabilized b y increasing the saturation of the tetrapyrrole macrocycle.