Nickel(I) and nickel(III) complexes of substituted tetraaza macrocycles formed by pulse radiolysis and electrochemistry of nickel(II) precursors

Abstract

The square-planar nickel(II) complexes of the ligands 8-methyl-8-nitro-1,3,6,10,13,15-hexaazatricyclo(13.1.1.1/sup 13,15/)octadecane, 8-amino-8-methyl-1,3,6,10,13,15-hexaazatricyclo(13.1.1.1/sup 13,15/)octadecane, 3,7-bis(2-aminoethyl)-1,3,5,7-tetraazabicyclo(3.3.1)nonane, and 9-methyl-9-nitro-1,4,7,11-tetraazacyclotridecane (I-IV) react rapidly with hydroxyl radicals and aquated electrons (e/sub aq/). The initial transient products of these reactions decay via first-order kinetics within a few milliseconds in neutral aqueous solution at 22/degrees/C in all cases. Electronic spectra and decay rate constants, as well as formation rate constants, are reported for all transients. Reaction of the nitro-substituted complexes with e/sub aq/ led to electron addition to the nitro group rather than to the metal center; otherwise, a Ni/sup I/ transient is observed. Following reaction with OH, the product of the initial decay remains a Ni/sup III/ species. This is more long-lived, and stabilization of Ni/sup III/ by axial coordination of the pendant amine in II is indicated. No notable stabilization of Ni/sup I/ or Ni/sup III/ from the presence of the bicyclic azamethylene football in I-III occurs. Cyclic voltammetry in acetonitrile identified both one-electron oxidation and one-electron reduction processes for the nickel(II) complexes, as well as nitro group reduction, where this group was pendant to the macrocycle. 34 references, 3 figures, 3 tables.