Novel potentially five-coordinate pyridyl-pendant dioxocyclam [1-(2-pyridyl)methyl-5,7-dioxo-1,4,8,11-tetraazacyclotetradecane (H(2)L) and its homologs (6-methyl and 6,6-dimethyl derivatives)] have been synthesized to study nickel(II) complexation. A purple nicker(II) complex with a deprotonated amide (NiHL) was isolated from aqueous equimolar solution of H(2)L and Ni(ClO4)(2). A yellow nicker(II) complex with two deprotonated amides (Nit) was crystallized from an H2O/CH3CN solution of H(2)L and Ni(OH)(2). The X-ray crystal study of Nit showed a square-planar nickel(II) complex with the pyridyl-pendant remaining uncoordinated. It is concluded from the visible absorption and NMR study of Nit in aqueous solution that the four-coordinate Nit is in equilibrium with a five-coordinate square-pyramidal nickel(II) complex with the apical coordination of the pyridyl-pendant. A voltammetric study disclosed a low nickel(II/III) redox potential of + 0.29 V vs SCE for NiL at pH 9.5 and 25 degrees C with 0.10 M Na2SO4. The nickel(II) complex Nit absorbed an equimolar amount of O-2 at pH 9.5 and 25 degrees C, and the O-2 was activated to cleave plasmid DNA.
