Cobalt( II ) Complexes with Substituted Salen‐Type Ligands and Their Dioxygen Affinity in N , N ‐Dimethylformamide at Various Temperatures

Several unsymmetrically substituted salen‐type cobalt( II ) Schiff‐base complexes CoL [H 2 L = 1,6‐bis(2‐hydroxyphenyl)‐3,3‐dimethyl‐2,5‐diaza‐1,5‐hexadiene ( 1 ); 1,6‐bis(2‐hydroxyphenyl)‐3,3‐dimethyl‐2,5‐diaza‐1,5‐heptadiene ( 2 ); 1‐(3‐ tert ‐butyl‐2‐hydroxy‐5‐methylphenyl)‐6‐(2‐hydroxyphen‐yl)‐3,3‐dimethyl‐2,5‐diaza‐1,5‐heptadiene ( 3 ); 1‐(2‐hydroxyphenyl)‐6‐methyl‐2,5‐diaza‐1,5‐nonadien‐8‐one ( 4 ); 1‐(3‐ tert ‐butyl‐2‐hydroxy‐5‐methylphenyl)‐6‐methyl‐2,5‐di‐aza‐1,5‐nonadien‐8‐one ( 5 ); 1‐(2‐hydroxyphenyl)‐3,3,6‐trimethyl‐2,5‐diaza‐1,5‐nonadien‐8‐one ( 6 ); 1‐(3‐ tert ‐butyl‐2‐hydroxy‐5‐methylphenyl)‐3,3,6‐trimethyl‐2,5‐diaza‐1,5‐nonadien‐8‐one ( 7 )] were prepared and characterized by their UV/Vis absorption spectra, magnetic moments, and oxidation potentials. Except for complex 4 (irreversible oxidation with t ½ ≈ 3 h), complexes 1 – 3 and 5 – 7 are remarkably resistant against irreversible auto‐oxidation in air‐saturated N , N ‐dimethylformamide (DMF) at ambient temperature. To characterize the Lewis acidity of the cobalt center in 1 – 7 , the equilibrium constant K py was determined for monoadduct formation with pyridine (CoL + pyCoL · py). An O 2 ‐sensitive optode was used to determine the Henry constant, K H , for the system O 2 /DMF in the temperature range 298–228 K. The formation of 1:1 adducts of complexes 1 – 7 with O 2 in DMF, as characterized by the equilibrium constant K   O   2, was followed spectrophotometrically in the temperature range 298–228 K. The parameters Δ H o , Δ S o , and K   O   2are reported. At 298 K, K   O   2ranges from 21.9 M –1 ( 5 ) to 155 M –1 ( 7 ). The overall spectroscopic information, including EPR spectra obtained with frozen solutions of 3 and 7 in O 2 ‐saturated DMF, confirm that the 1:1 adducts CoL · O 2 are cobalt( III ) superoxo compounds. The symmetrically substituted salen complex 8 [H 2 L = 1,6‐bis(3‐ tert ‐butyl‐2‐hydroxy‐5‐methylphenyl)‐3,3,4,4‐tetramethyl‐2,5‐diaza‐1,5‐hexadiene in 8 ] is shown to catalyze the oxidation of triphenylphosphane and 2,6‐di‐ tert ‐butylphenol by O 2 in DMF at ambient temperature. The correlation of the data obtained for K   O   2, K py , and the oxidation potential E ½ is discussed. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)