Selective Radical Transfer in a Series of Nonheme Iron(III) Complexes

A series of nonheme iron complexes, Fe^III(BNPA^Ph2O)(L_ax)(L_eq) (L_ax/eq = N₃^-, NCS^-, NCO^-, and Cl^-) have been synthesized using the previously reported BNPA^Ph2O^- ligand. The ferrous analogs Fe^II(BNPA^Ph2O)(L_ax) (L_ax = N₃^-, NCS^-, and NCO^-) were also prepared. The complexes were structurally characterized using single crystal X-ray diffraction, which shows that all the Fe^III complexes are six-coordinate, with one anionic ligand (L_ax) in the H-bonding axial site and the other anionic ligand (L_eq) in the equatorial plane, cis to the L_ax ligand. The reaction of Fe^III(BNPA^Ph2O^-)(L_ax)(L_eq) with Ph₃C^• shows that one ligand is selectively transferred in each case. A selectivity trend emerges that shows ^•N₃ is the most favored for transfer in each case to the carbon radical, whereas Cl^• is the least favored. The NCO and NCS ligands showed an intermediate propensity for radical transfer, with NCS > NCO. The overall order of selectivity is N₃ > NCS > NCO > Cl. In addition, we also demonstrated that H-bonding has a small effect on governing product selectivity by using a non-H-bonded ligand (DPA^Ph2O^-). This study demonstrates the inherent radical transfer selectivity of nonhydroxo-ligated nonheme iron(III) complexes, which could be useful for efforts in synthetic and (bio)catalytic C-H functionalization.