Eiichi KIMURA*, Masatoshi SASADA*, Mitsuhiko SHIONOYA, Tohru KOIKE*, Hiromasa KUROSAKI** and Motoo SHIRO*** (*Hiroshima Univ., **Kumamoto Univ., ***Rigaku Corp.)
[J. Bioinorg. Chem. 2, 74 (1997)]
A novel pyridyl-pendant dioxocyclam has been synthesized to study its nickel(II) complexation and reactivity with molecular oxygen. A purple nickel(II) complex with a deprotonated amide and a yellow nickel(II) complex with two deprotonated amides were isolated and the structure of the latter complex was determined by X-ray analysis. It was concluded from the visible absorption and NMR studies of this complex in aqueous solution that the four-coordinate nickel(II) complex is in equilibrium with a five-coordinate square-pyramidal nickel(II) complex with the apical coordination of the pyridyl-pendant. An electrochemical study disclosed a low nickel(II/III) redox potential of +0.29 V vs SCE for this complex at pH 9.5 and 25 °C. The nickel(II) complex absorbed an equi-molar amount of O2 at pH 7.9 and 25 °C, and the O2 was activated to cleave supercoiled pBR 322 DNA.