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

Abstract

AbstractSeveral unsymmetrically substituted salen‐type cobalt(II) Schiff‐base complexes CoL [H2L = 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 t½ ≈ 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 Kpy was determined for monoadduct formation with pyridine (CoL + pyCoL·py). An O2‐sensitive optode was used to determine the Henry constant, KH, for the system O2/DMF in the temperature range 298–228 K. The formation of 1:1 adducts of complexes 1–7 with O2 in DMF, as characterized by the equilibrium constant K, was followed spectrophotometrically in the temperature range 298–228 K. The parameters ΔHo, ΔSo, and K are reported. At 298 K, K ranges 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 O2‐saturated DMF, confirm that the 1:1 adducts CoL·O2 are cobalt(III) superoxo compounds. The symmetrically substituted salen complex8 [H2L = 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 O2 in DMF at ambient temperature. The correlation of the data obtained for K, Kpy, and the oxidation potential E½ is discussed. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)