Frontier Molecular Orbital Contributions to Chlorination versus Hydroxylation Selectivity in the Non-Heme Iron Halogenase SyrB2

The ability of an Fe-IV=O intermediate in SyrB2 to perform chlorination versus hydroxylation was computationally evaluated for different substrates that had been Studied experimentally. The pi-trajectory for H atom abstraction (Fe-IV=O oriented perpendicular to the C-H bond of substrate) was found to lead to the S = 2 five-coordinate HO-Fe-III-Cl complex with the C-center dot of the substrate, pi-oriented relative to both the Cl- and the OH- ligands. From this ferric intermediate, hydroxylation is thermodynamically faVored, but chlorination is intrinsically more reactive due to the energy splitting between two key redox-active d pi* frontier Molecular orbitals (FMOs). The splitting is determined by the differential ligand field effect of Cl- OH- versus on the Fe center. This makes chlorination effectively competitive with hydroxylation. Chlorination versus hydroxylation selectivity is then determined by the orientation of the substrate with respect to the HO-Fe-Cl plane that controls either the Cl- or the OH- to rebound depending on the relative pi-overlap with the substrate C radical. The differential contribution of the two FMOs to chlorination versus hydroxylation selectivity in SyrB2 is related to reaction mechanism that involves two asynchronous transfers: electron transfer from the substrate radical to the iron center followed by late ligand (Cl- or OH-) transfer to the substrate.