Roles of carboxylate donors in O-O bond scission of peroxodi-iron(III) to high-spin oxodi-iron(IV) with a new carboxylate-containing dinucleating ligand

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(2)BPG(2)E) and its mu-oxodiaquadi-iron(III) complexes [Fe-2(mu-O)(H2O)(2)(BPG(2)E)]X-2 [X = ClO4 (2a) or OTf (2b)] were synthesized to mimic a common carboxylate-rich coordination environment in O-2-activating non-heme di-iron enzymes including sMMO. The crystal structures of 2a and 2b revealed that BPG(2)E 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 mu-oxo-mu-peroxodiiron(III), and converted to high-spin mu-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-OO-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.