One-electron reduction of Cu3EL (L3- = tris(beta-diketiminate)cyclophane, and E = S, Se) affords [Cu3EL](-), which reacts with CO2 to yield exclusively C2O42- (95% yield, TON = 24) and regenerate Cu3EL. Stopped-flow UV/visible data support an A -> B mechanism under pseudo-first-order conditions (k(obs), (298K) = 115(2) s(-1)), which is 10(6) larger than those for reported copper complexes. The k(obs) values are dependent on the countercation and solvent (e.g., k(obs) is greater for [K(18-crown-6)](+) vs (Ph3P)(2)N+, and there is a 20-fold decrease in k(obs) in THF vs DMF). Our results suggest a mechanism in which cations and solvent influence the stability of the transition state.