Iron nitrosyls of a pentadentate ligand containing a single carboxamide group: Syntheses, structures, electronic properties, and photolability of NO

Three iron complexes of a pentadentate ligand N,N-bis(2-pyridylmethyl)amine-N-ethyl-2-pyridine-2-carboxamide (PaPy3H, H is the dissociable amide proton) have been synthesized. All three species, namely, two nitrosyls [(PaPy3)-Fe(NO)](ClO4)(2) (2) and [(PaPy3)Fe(NO)](ClO4) (3) and one nitro complex [(PaPy3)Fe(NO2)](ClO4) (4), have been structurally characterized. These complexes provide the opportunity to compare the structural and spectral properties of a set of isostructural {Fe-NO}(6,7) complexes (2 and 3, respectively) and an analogous genuine Fe(III) complex with an "innocent" sixth ligand ([(PaPy3)Fe(NO2)](ClO4), 4). The most striking difference in the structural features of 2 and 3 is the Fe-N-O angle (Fe-N-O = 173.1(2)degrees in the case of 2 and 141.29(15)degrees in the case of 3). The clean H-1 NMR spectrum of 2 in CD3CN reveals its S = 0 ground state and confirms its {Fe-NO}(6) configuration. The binding of NO at the non-heme iron center in 2 is completely reversible and the bound NO is photolabile. Mossbauer data, electron paramagnetic resonance signal at g approximate to 2.00, and variable temperature magnetic susceptibility measurements indicate the S = 1/2 spin state of the {Fe-NO}(7) complex 3. Analysis of the spectroscopic data suggests Fe(II)-NO+ and Fe(II)-NO. formulations for 2 and 3, respectively. The bound NO in 3 does not show any photolability. However, in MeCN solution, it reacts rapidly with dioxygen to afford the nitro complex 4, which has also been synthesized independently from [(PaPy3)Fe(MeCN)](2+) and NO2-. Nucleophilic attack of hydroxide ion to the N atom of the NO ligand in 2 in MeCN in the dark gives rise to 4 in high yield.