Thermodynamic and kinetic effects of 12-membered macrocycles. Polarographic studies of 1,4,7,10-tetra-azacyclododecanecopper(II)

Abstract

Thermodynamic parameters and reaction rates for complex formation between Cu²⁺ and 1,4,7,10-tetra-azacyclododecane (tcdd) have been determined polarographically. The stability constants (log K 24.8 at 25 °C) for the 1 : 1 complex is more than 10⁴-fold greater than that for the corresponding open-chain tetra-amine, 3,6-diazaoctane-1,8-diamine (3,6NH-od). The enhanced stability with tcdd is entirely due to a more favourable entropy change, which is in sharp contrast to the 14-membered macrocyclic effect attributed to a more favourable enthalpy change by Hinz and Margerum. The rate constants are 1.84 × 10⁶ and 1.80 × 10^–1 dm³ mol^–1 s^–1 for the reaction of [Cu(O₂CMe)]⁺ with Htcdd⁺ and H₂tccd²⁺, respectively. The sluggishness with which the diprotonated species reacts suggests that the rate-determining step is an associative process, probably proton loss preceding second-bond formation. The increased stability constant of the 12-membered macrocyclic complex is reflected most in the dissociation rate constants.