Quantitative Evaluation of Lewis Acidity of Metal Ions with Different Ligands and Counterions in Relation to the Promoting Effects of Lewis Acids on Electron Transfer Reduction of Oxygen

• Published in: Journal of organic chemistry Vol. 68; no. 12; pp. 4720 - 4726
• Main Authors: Ohkubo, Kei, Menon, Saija C, Orita, Akihiro, Otera, Junzo, Fukuzumi, Shunichi
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 13.06.2003
• Subjects: Chemical reactivity; Chemistry; Exact sciences and technology; Organic chemistry; Reactivity and mechanisms; Lewis acid; Fluorescence spectrum; Acridine derivatives; EPR spectrometry; Organic ligand; Metal ion; Electron transfer; Acidity; Experimental study; Complexes; Chemical reduction; Hyperoxides; Binding energy; Counter ion; Metalloporphyrin; Rate constant; Catalyst activity; Cobalt II Complexes
• ISSN: 0022-3263; 1520-6904
• DOI: 10.1021/jo034258u

The g zz values of ESR spectra of superoxide (O2 •-) complexes of metal ion salts acting as Lewis acids with different ligands and counterions were determined in acetonitrile at 143 K. The binding energies (ΔE) of O2 •-/Lewis acid complexes have been evaluated from deviation of the g zz values from the free spin value. The ΔE value is quite sensitive to the difference in the counterions and ligands of metal ion salts acting as Lewis acids. On the other hand, the fluorescence maxima of the singlet excited states of 10-methylacridone/Lewis acid complexes are red-shifted as compared with that of 10-methylacridone, and the relative emission energies (Δhνf) vary significantly depending on the Lewis acidity of metal ion salts with different counterions and ligands. The promoting effects of Lewis acids were also examined on electron transfer from cobalt(II) tetraphenylporphyrin to oxygen in acetonitrile at 298 K, which does not occur in the absence of Lewis acids under otherwise the same experimental conditions. Both ΔE and Δhνf values are well correlated with the promoting effects of Lewis acids on the electron transfer reduction of oxygen. Such correlations indicate that ΔE and Δhνf values can be used as quantitative measures of Lewis acidity of metal ion salts with different ligands and counterions. The Lewis acidity thus determined can also be applied to predict the promoting effects of Lewis acids on organic synthesis.