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

Nitrosylation of [PPN](2)[(ONO)(2)Fe(eta(2)-ONO)(2)] [1; PPN = bis(triphenylphosphoranylidene)-ammonium] yields the nitrite-containing {Fe(NO)}(7) mononitrosyliron complex (MNIC) [PPN](2)[(NO)Fe(ONO)(3)(eta(2)-ONO)] (2). At 4 K, complex 2 exhibits an S = 3/2 axial EPR spectrum with principal g values of g(perpendicular to) = 3.971 and g(parallel to) = 2.000, suggestive of the {FeIII(NO-)}(7) electronic structure. Addition of 1 equiv of PPh3 to complex 2 triggers 0-atom transfer of the chelating nitrito ligand under mild conditions to yield the {Fe(NO)(2)}(9) dinitrosyliron complex (DNIC) [PPN][(ONO)(2)Fe(NO)(2)] (3). These results demonstrate that both electronic structure [{Fe-III(NO-)}(7), S = 3/2] and redox-active ligands ([RS]- for [(RS)(3)Fe(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-Melm)(2)(eta(2)-ONO)Fe(NO)(2)] (5; 1-Melm = 1-methylimidazole) to generate the {Fe(NO)(2)}(10) DNIC [(1-Melm)(PPh3)Fe(NO)(2)] (6), glacial acetic acid protonation of the N-bound nitro ligand in the {Fe(NO)(2)}(10) DNIC [PPN][(eta(1)-NO2)(PPh3)Fe(NO)(2)] (7) produced the {Fe(NO)(2)}(9) DNIC [PPN][(OAc)(2)Fe(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 pi 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.