Fig. 1. Reactions of Cu(II) complex 1 and the Cu(I) complex 3 obtained by reduction of 1 with ascorbate.

For the formation of oxalate complex 4 from 3, CO₂ was previously reported to be required. We show here that the reaction requires ascorbate or dehydroascorbic acid (DHA), and oxygen. If ascorbate and DHA are absent, oxidation of 3 in air produces 6. All reactions were conducted in DMF, except for the removal of oxalate from 4 (Note: 1, 3, and 4 represent the same compounds as in ref. ¹⁶).