Cuprous complexes and dioxygen. IX. Formation of a unique trimeric species Cu3L and Ligand Oxidation (L cis, cis‐1, 3, 5‐cyclohexanetriamine)

The complexation of Cu+ by the potentially tripod like ligand cis, cis‐1, 3, 5 cyclohexanetriamine (chta) has been studied potentiometrically in aqueous acetonitrile (an). The expected tetracoordinated species Cu (chta) ⇌ (an)+ was formed only at rather high pH with log K (Cu (an)+ + chta ⇌ Cu (chta) · (an)+) = 6.94. Quite unexpectedly the most stable complex in neutral solution was the trimetric species Cu3 (chta) with log K (3 Cu+ + 2 chta ⇌ Cu3 (chta) ) = 31.75. In addition, the ternary complexes Cu (LH2) · (an)3+ and Cu (LH) · (an)2+ (L = chta) are formed at low pH. From model considerations, Cu3 (chta) must contain two ligand molecules with all amino groups in equatorial position, linked by three linearly coordinated Cu+‐ions. Cu3 (chta)3+2 shows no measurable reactivity towards dioxygen. At pH values above 9, very rapid O2‐uptake due to Cu (chta) · (an)+ is observed. In this reaction, Cu+‐autoxidation is stoichiometrically coupled to ligand oxidation, followed by a much slower Cu‐catalyzed secondary reaction of the primary oxidation product of chta. Hydrogen peroxide and likely also superoxide, are involved in the coupled Cu+/ligand oxidation.