Linear Free Energy Correalations for Reversible Dioxygen Binding of a Cobalt-Porphyrin Complex: A Solvent Effect Study

Article abstract of DOI:10.1021/j150643a019

The dioxygen uptake ability of Co(T(p-CH3O)PP)B, where T(p-CH3O)PP is the dianion of 5,10,15,20-tetrakis(p-methoxphenyl)porphyrin and B is pyridine, piperidine, and N-methylimidazole, was studied in several solvents.Log K_O2 (standard state of 1 torr of O2) and ΔH_O2 were found to depend on both specific and nonspecific solute-solvent interactions.Linear correlations of log K_O2 and ΔH_O2 were found with electrophilic solvating ET(30) factors, the Taft ?^* function, and the polarity function (ε - 1)/(ε + 2), where ε is the solvent dielectric constant.The correalation coefficients of the trends were ca. 0.99 for the solvents methylene chloride, chloroform, and toluene.Comparison of log K_O2 correlations with ET(30) and ?^* values indicated that the ET(30) correlations were more precise when excluding methanol from the trend whereas the ?^* correlation gave a better overall fit.Reactions in N,N'-dimethylformamide were included in log K_O2 trends, those in chlorobenzene fit the log K_O2 vs.ET(30) and ΔH_O2 vs. (ε - 1)/(ε + 2) plots.Deviations from the observed trends were explained in terms of ? bonding between solute and solvent in chlorobenzene and o-nitrotoluene, solvent coordination to the sixth coordination site of the precursor complex requiring it to dissociate prior to reaction with dioxygen in N,N'-dimethylformamide and methanol, and hydrogen-bonding effects of the solvent for reactions in methanol.The general trends are attributed to stabilization of the charge separated product, Co^δ+-O^δ--O and this interpretation was further substantiated by dioxygen uptake studies in salt solutions.