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 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.
Stolzenberg
A M
AM
Department of Chemistry, West Virginia University, Morgantown, West Virginia 26506, USA.
Summers
J S
JS
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
D000473:Alkanes; D005609:Free Radicals; D009942:Organometallic Compounds; D011166:Porphyrins; D011522:Protons; D011725:Pyridines; D014050:Toluene; D003035:Cobalt