THE ORTHO EFFECT IN LIGATION OF IRON TETRAPHENYLPORPHYRINS

Kinetics and equilibria of binding of 1-methylimidazole, 1,2-dimethyli midazole, isonitriles, and carbon monoxide to iron(II) tetraphenylporp hyrin and iron(II) tetramesitylporphyrin bearing various ligands in th e sixth position have been determined. Those fifth and sixth ligand co mbinations which result in steric repulsion between the porphyrin and one of the ligands (e.g. 1,2-dimethylimidazole) show much greater bind ing constants and slower dissociation rates with the iron(II) tetrames itylporphyrin than to iron(II) tetraphenylporphyrin. Thus in the bis-( 1,2-dimethylimidazole) complex the rate of dissociation from the iron( II) tetraphenylporphyrin system is about 10(4) times that for the iron (II) tetramesitylporphyrin. These large differences are attributed to a conformational effect in which phenyl groups with ortho substitutent s destabilize the domed form of the metalloporphyrin. Possible consequ ences of this effect on catalytic and ligation behavior are discussed.