EFFECT OF COORDINATING ANIONS ON THE KINETICS AND THERMODYNAMICS OF OXYGEN BINDING BY A MACROCYCLIC COBALT(II) COMPLEX

The macrocyclic cobalt(II) complexes (X)L2Co+ (X− = Cl−, SCN−; L2 = C-meso-5, 7, 7, 12, 14, 14-hexamethyl-1, 4, 8, 11-tetraazacyclotetradecane) react with O2 to yield (X)L2CoO2+. The kinetics of the forward reaction in aqueous solution at 25 °C (X− = Cl−, k = 1.80 × 106 M−1 s−1; X− = SCN−, k = 7.29 × 106 M−1 s−1) are similar to those for the unsubstituted, presumably diaquo complex L2Co2+ (k = 5.00 × 106 M−1 s−1). The dissociation of O2 from (X)L2CoO2+ takes place with the rate constants of 3300 s−1 and <500 s−1 for the chloro and thiocyanato complexes, respectively. The accelerated autoxidation of L2Co2+ in the presence of Cl− is attributed to catalysis of the redox steps following oxygen binding. In the case of SCN− the autoxidation is also promoted by the large equilibrium constant for O2 binding by (SCN)L2Co+. The compound L2Co(ClO4)2 crystallizes in the monoclinic space group P21/n with two molecules in a unit cell of dimensions a = 8.396 (1) Å, b = 9.149 (1) Å, c = 15.385 (2) Å, and β = 97.449 (6)°. The data refined to a final value of the weighted R factor of 0.0421. The cobalt ion is located at the crystallographic center of inversion, which coincides with the center of the equatorial plane containing the four nitrogens. The two axial positions are occupied by the perchlorates. © 1990, American Chemical Society. All rights reserved.