Ligand topology effects on olefin oxidations by bio-inspired [Fe ^II(N₂Py₂)] catalysts

Linear tetradentate N₂Py₂ ligands can coordinate to an octahedral Fe^II center in three possible topologies (cis-α, cis-β, and trans). While for the N,N'-bis(2-pyridylmethyl)-l,2- diaminoethane (bpmen) complex, only the cis-α topology has been observed, for N,N'-bis(2-pyridylmethyl)-1,2-diamino-cyclohexane (bpmcn) both cis-α and cis-β isomers have been reported. To date, no facile interconversion between cis-α and cis-β topologies has been observed for iron(II) complexes even at high temperatures. However, this work provides evidence for facile interconversion in solution of cis-α, cis-β, and trans topologies for [Fe(bpmpn)X₂] (bpmpn = N, N'-bis(2-pyridylmethyl)-1,3- diaminopropane; X = triflate, CH₃CN) complexes. As reported previously, the catalytic behavior of cis-α and cis-β isomers of [Fe(bpmcn)(OTf)₂] with respect to olefin oxidation depends dramatically on the geometry adopted by the iron complex. To establish a general pattern of the catalysis/ topology dependence, this work presents an extended comparison of the catalytic behavior for oxidation of olefins of a family of [Fe(N₂py₂)] complexes that present different topologies. ¹⁸O labeling experiments provide evidence for a complex mechanistic land-scape in which several pathways should be considered. Complexes with a trans topology catalyze only non-water-assisted epoxidation. In contrast, complexes with a cis-α topology, such as [Fe(bpmen)X₂] and [Fe(α-bpmcn)-(OTf)₂], can catalyze both epoxidation and cis-dihydroxylation through a water-assisted mechanism. Surprisingly, [Fe(bpmpn)X₂] and [Fe(β-bpmcn)-(OTf)₂] catalyze epoxidation via a water-assisted pathway and cis-dihydroxylation via a non-water-assisted mechanism, a result that requires two independent and distinct oxidants.