Ligand noninnocence in FeNO corroles: insights from β-octabromocorrole complexes

Abstract

The first FeNO octabromocorroles have been synthesized including four β-octabromo-meso-tris(p-X-phenyl)corrole derivatives Fe[Br₈TpXPC](NO) (X = CF₃, H, CH₃, OCH₃) and the β-octabromo-meso-tris(pentafluorophenyl)corrole complex, Fe[Br₈TPFPC](NO). The last complex, which proved amenable to single-crystal X-ray structure determination, exhibits the geometry parameters: Fe–N(O) 1.643(8) Å, N–O 1.158(9) Å, and a FeNO angle of 176.4(6)°. The more electron-deficient complexes exhibit increased instability with respect to NO loss and also higher infrared NO stretching frequencies (ν_NO). Interestingly, DFT calculations and IR marker bands indicate a noninnocent {FeNO}⁷-(corrole˙²⁻) formulation for all FeNO corroles, both β-H₈ and β-Br₈, with essentially the same degree of corrole radical character. Instead, an electron-deficient corrole appears to exert a field effect resulting in reduced Fe-to-NO backdonation, which accounts for both the increased instability with respect to NO loss and the higher ν_NO's.