The structure and O-2-reactivity of copper(I) complexes supported by novel ligands, Pye2 (1,3,5-triethyl-2,4-bis((N-benzyl-N-(2-(pyridin-2-yl)ethyl)-)aminomethyl )benzene), Pye3 (1,3,5-triethyl-2,4,6-tris((N-benzyl-N-(2-(pyridin-2-yl)ethyl)) aminomethyl) benzene), (Me)Pym2 (1,3,5-triethyl-2,4-bis((N-benzyl-N-(6-methylpyridin-2-ylmethyl))aminome thyl)benzene), and (Me)Pym3 (1,3,5-triethyl-2,4,6-tris((N-benzyl-N-(6-methylpyridin-2-ylmethyl)) aminomethyl) benzene) have been examined. The ligands are designed to construct mono-, di-, and trinuclear copper(I) complexes by connecting two or three pyridylalkylamine metal-binding sites to a 1,3,5-triethylbenzene spacer. Thus, the reaction of the ligands with [Cu-I(CH3CN)(4)]X (X = PF6, CF3SO3) or (CuCl)-Cl-I gave the expected mononuclear copper(I) complexes [CuI(Pye2)(CF3-SO3)] (1) and [Cu-I(Pye3)](CF3SO3) (2), dinuclear copper(I) complex [Cu-2(I)(MePym(2))(Cl)](CuCl2)-Cl-I (3), and trinuclear copper( I) complex [ CuI 3( MePym3)( CH3CN) 3](CF3SO3) 3 (4), the structures of which were determined by X-ray crystallographic analysis. The mononuclear copper(I) complexes, 1 and 2, exhibit a distorted three-coordinate T-shape structure and a trigonal planar structure, respectively, which are very close to the coordination geometry of the CuA site of PHM (peptidylglycine alpha-hydroxylating monooxygenase) and the CuB site of CcO (cytochrome c oxidase). Notably, 1 and 2 showed a significantly high oxidation potential (990 mV vs SCE), thus showing virtually no reactivity toward O-2. On the other hand, the metal centers of the dinuclear and trinuclear copper(I) complexes, 3 and 4, exhibit a distorted trigonal planar geometry and a trigonal pyramidal geometry, respectively. In contrast to the mononuclear copper(I) complexes, these dinuclear and trinuclear copper(I) complexes reacted with O2 to induce an aromatic ligand hydroxylation reaction involving an NIH-shift of one of the ethyl substituents on the benzene spacer. The NIH- shift of the alkyl substituent on the aromatic ring is strong evidence of the electrophilic aromatic substitution mechanism, although the active oxygen intermediate could not be directly detected during the course of the reaction. The biological relevance of the copper(I) complexes is also discussed on the basis of structure and O-2-reactivity.