Dioxygen activation proceeds via O-O bond scission of peroxodi-iron(iii) to high-spin oxodi-iron(iv) in soluble methane mono-oxygenase (sMMO). Recently, we have shown that reversible O-O bond scission of peroxodi-iron(iii) to high-spin oxodi-iron(iv) is attained with a bis-tpa type dinucleating ligand, 6-hpa. In this study, a new carboxylate-containing dinucleating ligand, 1,2-bis[2-(N-2-pyridylmethyl-N-glycinylmethyl)-6-pyridyl]ethane (H 2BPG2E) and its μ-oxodiaquadi-iron(iii) complexes [Fe2(μ-O)(H2O)2(BPG2E)]X 2 [X = ClO4 (2a) or OTf (2b)] were synthesized to mimic a common carboxylate-rich coordination environment in O2-activating non-heme di-iron enzymes including sMMO. The crystal structures of 2a and 2b revealed that BPG2E prefers a syn-diaqua binding mode. 2b catalyzed the epoxidation of alkenes with H2O2. A new purple species was formed upon reaction of 2b with H2O2, and characterized by the elemental analysis and spectral and kinetic studies. These clearly showed that the purple species was a μ-oxo-μ-peroxodi-iron(iii), and converted to high-spin μ-oxodioxodi-iron(iv) via rate-determining reversible O-O bond scission. In comparison of BPG2E with 6-hpa, it is shown that the carboxylate donor stabilizes the Fe-O-O-Fe structure of the peroxo complex due to the structural effect to retard O-O bond scission. This may shed light on the roles of carboxylate donors in the dioxygen activation of non-heme di-iron enzymes. This journal is © the Partner Organisations 2014.