title: Cis-influence of hydroporphyrin macrocycles on axial ligation equilibria and alkyl exchange reactions of alkylcobalt(III) porphyrin complexes.
abstract: Stability constants are reported for the coordination of pyridine and substituted pyridines to the alkylcobalt(III) complexes of octaethylporphyrin (OEP), t-octaethylchlorin (OEC), and ttt-octaethylisobacteriochlorin (OEiBC) in toluene solution. The stability constants correlate with the base strength of the nitrogenous ligand. A cis-influence of the macrocycle saturation level on the stability constants is observed. Stability constants for coordination of a given pyridine ligand to an alkylcobalt(III) complex are roughly 10 times smaller than the stability constants for the corresponding cobalt(II) complex. Analysis of a thermodynamic cycle demonstrates that this leads to decreased stability of the complex with respect to Co-C bond homolysis upon ligand coordination, a "base-on" effect. Alkyl exchange occurs between cobalt complexes of different tetrapyrroles. Equilibrium data establish that the exchange is nonstatistical and that the Co-C bond is stabilized by increasing the saturation of the tetrapyrrole macrocycle.
authors.affiliation: Department of Chemistry, West Virginia University, Morgantown, West Virginia 26506, USA.
authors.forename: A M
authors.initials: AM
authors.lastname: Stolzenberg
authors.forename: J S
authors.initials: JS
authors.lastname: Summers
mesh_terms: D000473:Alkanes / Q000138:chemical synthesis* / Q000737:chemistry*; D001665:Binding Sites; D003035:Cobalt / Q000737:chemistry*; D005609:Free Radicals / Q000737:chemistry; D009682:Magnetic Resonance Spectroscopy; D009942:Organometallic Compounds / Q000138:chemical synthesis* / Q000737:chemistry*; D011166:Porphyrins / Q000737:chemistry*; D011522:Protons; D011725:Pyridines / Q000737:chemistry*; D013237:Stereoisomerism; D013816:Thermodynamics; D014050:Toluene / Q000737:chemistry
chemical_list: D000473:Alkanes; D005609:Free Radicals; D009942:Organometallic Compounds; D011166:Porphyrins; D011522:Protons; D011725:Pyridines; D014050:Toluene; D003035:Cobalt