Macrocyclic Dioxo Pentaamines: Novel Ligands for 1:1 Ni(II)-O2 Adduct Formation

Article abstract of DOI:10.1021/ja00331a018

Substituted and unsubstituted macrocyclic dioxo pentaamines, 1,4,7,10,13-pentaazacyclohexadecane-14,16-dione, form stable 1:1 square-pyramidal complexes with Ni(II) and Cu(II), which possess two deprotonated amide donors in equatorial positions.The high-spin Ni(II) complexes show a very low Ni(II,III) redox potential of +0.24 V vs.SCE and are easily oxidized chemically or electrochemically to Ni(III) complexes.Moreover, the Ni(II) complexes react with molecular oxygen to form 1:1 Ni(II)-O2 adducts in aqueous solution, as established by a combination of polarographic, spectrophotometric, and manometric methods.Comparative studies with relevant pentadentate macrocyclic polyamine complexes revealed that the two deprotonated amide groups and the fifth amine donor incorporated in the 16-membered macrocyclic frame are essential for the formation of the Ni(II)-O2 adducts.The oxygen uptake reaction is first order in and in -2L>⁰ in aqueous solutions, and the second-order rate constant is 1.7*10² s^-1 M^-1 at 35 deg C.The attack of O2 at the sixth coordinate site is competitively inhibited by imidazole.The O2 complexation constant K_O2 is determined to be 1.9*10⁴ M^-1 at 35 deg C by potentiometric titration.Novel features of the Ni(II)-O2 adducts are discussed.