Mononuclear copper (II)-superoxo complexes 2(X)-OO center dot having triplet (S = 1) ground states were obtained via reaction Of O-2 with the copper(I) starting materials 1(X) supported by tridentate tigands L-X [1-(2-p-X-phenethyl)-5-(2-pyridin-2-ylethyl)-1,5-diazacyclooctane; X = CH3, H, NO2] in various solvents. The superoxo complexes 2(X)-OO center dot mimic the structure [tetrahedral geometry with an end-on (eta(1))-bound O-2(center dot-)] and the aliphatic C-H bond activation chemistry of peptidylglycine alpha-hydroxylating monooxygenase and dopamine beta-monooxygenase.