EFFECTS OF AXIAL ANIONS AND SOLVENT ON THE REDOX BEHAVIOR OF NICKEL-COMPLEXES WITH C-FUNCTIONALIZED TETRAAZAMACROCYCLES

The redox behaviour of Ni(III)/Ni(II) complexes with unsubstituted cyclam (L1) and with carbon-functionalized analogues (L2, L3) was studied in CH3CN + 0.1 M TEAP. The Ni(III)/Ni(II) redox couple was shifted slightly to more negative potentials in C-functionalized cyclams, indicative of an effective, although small, interaction between the side arms and the nickel. Steric effects of the side arms on the central nickel cyclam core can be ruled out as an explanation of this slight potential shift because in pure aqueous solvent all complexes gave the same Ni(III)/Ni(II) redox potential. Also, the axial coordination of monodentate Lewis bases - such as chloride - on Ni(II) and Ni(III) cyclams was analysed and compared. In CH3CN, the general trend was that coordination-decoordination of such Lewis bases with Ni(III)/Ni(II) had much more dramatic effects on the Ni(III)/Ni(II) redox couple than the pendant arm borne by one carbon atom in the cyclam ring. Specifically, chloride anions coordinated more strongly than perchlorate with both Ni(II) and Ni(III). In addition, the coordination constant of chloride was higher with Ni(III) than with Ni(II). These distinct coordination effects faded away when enough water was added to acetonitrile.