Oxygen Activation by 3d Metal Complexes of Open Chain and Macrocyclic Schiff Base Ligands

Abstract

A big number of metal complexes of the types 1 - 4 ¹⁾ (M = Cu, Ni, Co, Fe, Mn; R¹ = Me, Ph, OEt; R² = H, COMe, COOEt, CN) have been compared with respect to i) the uptake of dioxygen (measured by precision gas volumetry), and ii) their catalytic activity for the oxidation of hydroquinones and olefines in different solvents (toluene, pyridine). The time dependent oxygen uptake can be described by the following general characteristics (x = n_dioxygen / n_metal):

• i) no significant oxygen uptake (x < 0.05 after 10 hours); observed with the most complexes of Cu(II) and Ni(II) but also with the Co(II)-, Fe(II)-, and Mn(II)-complexes of 1, R¹ = OEt, R² = CN, in pyridine;

• ii) irreversible oxygen uptake with a definite and time stable limiting value of x = 0.25, indicating the formation of a µ-oxo derivative; observed with the Fe(II)-complexes 2, R¹ = Me, R² = COOEt, COMe, and 3, R² = COOEt, as well as with some Cu(I)-complexes;

• iii) irreversible oxygenation with a definite and time stable limit of x = 0.5, indicating the formation of a oxo- or µ-peroxo derivative respectively; observed with the Fe(II)- and Mn(II)-complexes 1, R¹ = Me, R² = COOEt, COMe in pyridine, and 1, R₁ = OEt, R² = CN in toluene;

• iv) irreversible oxygenation with a definite limit of x = 1, indicating the formation of a superoxo derivative (detectable by EPR spectroscopy); observed with the most electrochemically formed Ni(I) complexes 1 and 2;

• v) reversible oxygenation with a definite and time stable limit between 0 ≤ x ≤ 1, depending on the special complex and the strength and concentration of additional bases, often characterized by a sharp maximum in the time dependent oxygenation curve; observed with the most Co(II) complexes 1, 2 (R² ≠ H), and 3 (R² ≠ H);

• vi) slow continuous oxygen uptake without a definite and time stable limiting value (x > 1), indicating an irreversible oxidation of the ligand or the solvent; observed with the Co(II)-complexes 2 (R² = H) and 4 and the Fe(II)-complexes 4 with special axial bases (e.g. imidazole).