Roles of the Distinct Electronic Structures of the {Fe(NO)₂}⁹ and {Fe(NO)₂}¹⁰ Dinitrosyliron Complexes in Modulating Nitrite Binding Modes and Nitrite Activation Pathways

Abstract

Nitrosylation of [PPN]2[(ONO)2Fe(η2-ONO)2] [1; PPN = bis(triphenylphosphoranylidene)ammonium] yields the nitrite-containing {Fe(NO)}7 mononitrosyliron complex (MNIC) [PPN]2[(NO)Fe(ONO)3(η2-ONO)] (2). At 4 K, complex 2 exhibits an S = 3/2 axial EPR spectrum with principal g values of g⊥ = 3.971 and g∥ = 2.000, suggestive of the {FeIII(NO−)}7 electronic structure. Addition of 1 equiv of PPh3 to complex 2 triggers O-atom transfer of the chelating nitrito ligand under mild conditions to yield the {Fe(NO)2}9 dinitrosyliron complex (DNIC) [PPN][(ONO)2Fe(NO)2] (3). These results demonstrate that both electronic structure [{FeIII(NO−)}7, S = 3/2] and redox-active ligands ([RS]− for [(RS)3Fe(NO)]− and [NO−] for complex 2) are required for the transformation of {Fe(NO)}7 MNICs into {Fe(NO)2}9 DNICs. In comparison with the PPh3-triggered O-atom abstraction of the chelating nitrito ligand of the {Fe(NO)2}9 DNIC [(1-MeIm)2(η2-ONO)Fe(NO)2] (5; 1-MeIm = 1-methylimidazole) to generate the {Fe(NO)2}10 DNIC [(1-MeIm)(PPh3)Fe(NO)2] (6), glacial acetic acid protonation of the N-bound nitro ligand in the {Fe(NO)2}10 DNIC [PPN][(η1-NO2)(PPh3)Fe(NO)2] (7) produced the {Fe(NO)2}9 DNIC [PPN][(OAc)2Fe(NO)2] (8), nitric oxide, and H2O. These results demonstrate that the distinct electronic structures of {Fe(NO)2}9/10 motifs [{Fe(NO)2}9 vs {Fe(NO)2}10] play crucial roles in modulating nitrite binding modes (O-bound chelating/monodentate nitrito for {Fe(NO)2}9 DNICs vs N-bound nitro as a π acceptor for {Fe(NO)2}10 DNICs) and regulating nitrite activation pathways (O-atom abstraction by PPh3 leading to the intermediate with a nitroxyl-coordinated ligand vs protonation accompanied by dehydration leading to the intermediate with a nitrosonium-coordinated ligand). That is, the redox shuttling between the {Fe(NO)2}9 and {Fe(NO)2}10 DNICs modulates the nitrite binding modes and then triggers nitrite activation to generate nitric oxide