Previous efforts to synthesize a cupric superoxide complex possessing a thioether donor have resulted in the formation of an end-on trans-peroxo-dicopper(II) species, [{(Ligand)Cu-II}(2)(mu-1,2-O2(2-))](2+). Redesign/modification of previous N3S tetradentate ligands has now allowed for the stabilization of the monomeric, superoxide product possessing a S(thioether) ligation, [((N3S)-N-DMA)Cu-II(O-2(center dot-))](+) (2S), as characterized by UVvis and resonance Raman spectroscopies. This complex mimics the putative Cu-II(O-2(center dot-)) active species of the copper monooxygenase PHM and exhibits enhanced reactivity toward both O-H and C-H substrates in comparison to close analogues [(L)Cu-II(O-2(center dot-))](+), where L contains only nitrogen donor atoms. Also, comparisons of [(L)CuII/I]n(+) compound reduction potentials (L = various N-4 vs (N3S)-N-DMA ligands) provide evidence that (N3S)-N-DMA is a weaker donor to copper ion than is found for any N-4 ligand-complex.