Ligand Topology Effects on Olefin Oxidations by Bio‐Inspired [FeII(N2Py2)] Catalysts

AbstractLinear tetradentate N2Py2 ligands can coordinate to an octahedral FeII center in three possible topologies (cis‐α, cis‐β, and trans). While for the N,N′‐bis(2‐pyridylmethyl)‐1,2‐diaminoethane (bpmen) complex, only the cis‐α topology has been observed, for N,N′‐bis(2‐pyridylmethyl)‐1,2‐diaminocyclohexane (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 landscape 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.