The effects of solvent on dioxygen binding to cobalt(II) picket fence porphyrins

Abstract

The O 2 affinities on base adducts of four atropisomers of picket fence porphyrin Co(TpivPP), and the corresponding α 4 complex containing valeramido pickets instead of pivalamido Co(α 4-TvalPP) were measured in several solvents at O or −15 °C. The O 2 affinities of the α 4 complexes are the lowest in DMF which is the most polar of the solvents used, while those of the other isomers are the highest in DMF. This observation was explained in terms of direct and indirect interactions between the solvent and the bound O 2. The trans-α 2 complex shows higher O 2 affinity in dichloromethane than those in aromatic solvents because of the preferential solvation of the deoxy complex in the solvents. The variation of the O 2 affinities of this system to solvents is considerably smaller than those of ‘flat porphyrin’ complexes. This result suggested that the pocket polarity introduced by the amide groups weakens the solvent–solute interaction on the O 2 affinities of this system and also that the solvation of the oxy state rather than the deoxy state predominantly affects the O 2 affinities. It was concluded that the enhanced O 2 uptake by the picket fence may be due to the stabilization of the oxy state by intramolecular interactions rather than to destabilization of the deoxy state by inhibiting solvation for the active site.