Synthesis, Structure, and Properties of an Fe(II) Carbonyl [(PaPy₃)Fe(CO)](ClO₄): Insight into the Reactivity of Fe(II)−CO and Fe(II)−NO Moieties in Non-Heme Iron Chelates of N-Donor Ligands

An Fe(II) carbonyl complex [(PaPy3)Fe(CO)](ClO4) (1) of the pentadentate ligand N,N-bis(2-pyridylmethyl)amine-N-ethyl-2-pyridine-2-carboxamide (PaPy3H, H is the dissociable amide proton) has been synthesized and structurally characterized. This Fe(II) carbonyl exhibits its νCO at 1972 cm-1, and its 1H NMR spectrum in degassed CD3CN confirms its S = 0 ground state. The bound CO in 1 is not photolabile. Reaction of 1 with an equimolar amount of NO results in the formation of the {Fe−NO}7 nitrosyl [(PaPy3)Fe(NO)](ClO4) (2), while excess NO affords the iron(III) nitro complex [(PaPy3)Fe(NO2)](ClO4) (5). In the presence of [Fe(Cp)2]+ and excess NO, 1 forms the {Fe−NO}6 nitrosyl [(PaPy3)Fe(NO)](ClO4)2 (3). Complex 1 also reacts with dioxygen to afford the iron(III) μ-oxo species [{(PaPy3)Fe}2O](ClO4)2 (4). Comparison of the metric and spectral parameters of 1 with those of the previously reported {Fe−NO}6,7 nitrosyls 3 and 2 provides insight into the electronic distributions in the Fe(II)−CO, Fe(II)−NO, and Fe(II)−NO+ bonds in the isostructural series of complexes 1−3 derived from a non-heme polypyridine ligand with one carboxamide group.