A key requirement fbr the generation of active copper catalysts for olefin aziridination has been identified by examination of a copper(II) complex supported by a new tridentate. mer-coordinating, pyridyl-appended diazacyclooctane ligand. Combined structural, spectroscopic, and electrochemical studies suggest that the high catalytic reactivity of complex 4 (shown) is rooted in facile access to an equatorial coordination site, occupied by a labile trifluoroacetate ligand in its X-ray crystal structure.