Autoxidation of a dicopper(I) macrocyclic complex: evidence for a mixed-valence intermediate

Interaction of O2 with dicopper(I) complexes of 20-membered N4 macrocycles L1 and L2{23,24-dioxa-3,7,14,18-tetra-azatricyclo[18.2.1.1 9,12]tetracosa-1 (22),2,7,9,11,13,18,20-octaene and its 5,5,16,16-tetramethyl derivative} proceeds via an intermediate red-brown species, whose electronic and e.s.r. spectra suggest a mixed-valence formulation. This red-brown species can also be generated by conproportionation between dicopper(I) and dicopper(II) complexes of these macrocycles. The mixed-valence product cannot be isolated in the solid state, as concentration of the solutions leads to disproportionation with precipitation of a solid [Cu4L12(OH)]3+ cluster. Cyclic voltammetry revealed that all redox processes associated with the copper centres in these complexes are irreversible, probably due to a chemical process such as transfer of the hydroxo bridging ligand at each redox stage. It is believed that the pair of irreversible reduction waves observed in the cyclic voltammogram of the di-µ-hydroxo-dicopper(II) complex of L1 correspond to sequential one-electron reductions of the dicopper centre.