OXYGEN COMPLEX FORMATION BY COBALT(II)-TRIS (2-AMINOETHYL) AMINE+

Abstract

The reversible oxygenation of the Co(II) complex of tris(2-aminoethyl)amine (TREN, L) has been studied in some detail. The equilibrium constant K _O2 =10^26.92 M^?2 atm^?1, corresponding to the quotient [H⁺] [L₂Co₂(O₂) (OH)³⁺]/[Co²⁺]² [L]² P_O2 was determined by potentiometric equilibrium measurements of hydrogen ion concentration. Values for the thermodynamic constants, ΔH° =–63 ± 9 kcal/mole and ΔS° =–100 ± 15 cal/deg. mol, were calculated from the temperature dependence of the equilibrium constant. Oxygen stoichiometry, measured with a polarographic sensor, indicated the formation of a binuclear (peroxo bridged) complex, and the potentiometric equilibrium data indicated the presence of a second, μ-hydroxo, bridge. Measurement of the kinetics of the fast reaction between the cobalt(II)-TREN complex and dioxygen gave the value of the second order rate constant for the formation of the dioxygen complex as k ₁ =2.8 × 10⁺³ sec^?1 mol^?1. The first order rate constant for the decomposition of the dioxygen complex measured by stopped-flow was found to be k _?2 =0.7 sec^?1. Kinetic and equilibrium data are discussed with respect to the probable structure and mechanism of formation of the dioxygen complex, and are compared with similar data previously reported for analogous complexes. The oxygen complex reported is unique with respect to its extremely slow rate of conversion to inert cobalt(III) complexes.