Copper(I) complexes supported by a series of N-3-tridentate ligands bearing a rigid cyclic diamine framework such as 1,5-diazacyclooctane (L8, eight-membered ring), 1,4-diazacyclohexane (L7, seven-membered ring), or 1,4-diazacyclohexane (L6, six-membered ring) with a common 2-(2-pyridyl)ethyl side arm were synthesized and their reactivity toward O-2 were compared. The copper(I) complex of L8 preferentially provided a mononuclear copper(II) end-on superoxide complex S as reported previously [Itoh, S., et al. J. Am. Chem. Soc. 2009, 131, 2788-2789], whereas a copper(I) complex of L7 gave a bis(mu-oxido)dicopper(111) complex O at a low temperature (-85 degrees C) in acetone. On the other hand, no such active-oxygen complex was detected in the oxygenation reaction of the copper(I) complex of L6 under the same conditions. In addition, O-2-reactivity of the copper(I) complex supported by an acyclic version of the tridentate ligand (LA, PyCH2CH2N(CH3)CH2CH2CH2N(CH3)(2); Py = 2-pyridyl) was examined to obtain a mixture of a (mu-eta(2):eta(2)-peroxido)dicopper(II) complex P-s and a bis(mu-oxido)dicopper(III) complex O. Careful inspection of the crystal structures of copper(I) and copper(II) complexes and the redox potentials of copper(I) complexes has revealed important geometric effects of the supporting ligands on controlling nudearity of the generated copper active-oxygen complexes.