Electrochemistry of Macrobicyclic (Hexaamine)coball(III) Complexes. Metal-Centered and Substituent Reductions

The electrochemistry of a wide range of macrobicyclic hexaamine (‘cage’) and polyamine complexes of coball(III) is reported. All of the substituted [Co(sar)]³⁺ and [Co(absar)]³⁺ cage complexes exhibit chemically reversible Co(III)/Co(II) couples (sar = sarcophagene = 3,6,10,13,16,19-hexaazabicyclo[6.6.6]icosane, absar = 3,6,10,13,15,18-hexaazabicyclo[6.6.5]nonadecane). The Co(II) oxidation state for these complexes is substitutionally inert and undergoes irreversible reduction at very negative potentials in nonaqueous solvents (<-2 V). Quasi-reversible behavior is apparent with all the complexes under study m both aqueous (ClO₄^- and CF₃SO₃^- media) and nonaqueous (acetone and acetonitnle, CF₃SO₃^- media) solvents. However, in aqueous perchlorate media, at a mercury interface, these reductions are complicated by adsorption processes. The potential of the Co(III)/Co(II) couple is markedly dependent on the nature of substituents on the cage ligand and varies by ~0.6 V for the apically substituted [Co(sar)^3+/2+ series. Other factors that influence the potential are preferred cavity size of the cage ligand and outer-sphere medium effects. For each series of structurally similar ligands with constant medium and electrode, the heterogeneous rate constants are the same within experimental erro. In addition to the metal-centered electrochemistry, many of the complexes exhibit redox processes of electroactive organic substituents. These processes are utilized in selected electroorganic syntheses of new cage complexes. The pH dependence of the Co(III)/Co(II) couple is a useful probe to the acidity of various functional groups, and some applications in determining pK_a values are described.