TIMEN^mes: An Iron Nitride Complex

Abstract

Coordination compounds of iron in high oxidation states have been invoked as reactive intermediates in biocatalyses. Iron(IV) ferryl species are examples of highly reactive compounds that have long been known to be at the catalytic centers of oxygenases. Supported by X-ray diffraction studies on nitrogenase, the iron nitride moiety has recently been suggested to be present at the site of biological nitrogen reduction. As a result, well-characterized high-valent iron complexes have been sought as biomimetic models for transformations mediated by iron-containing enzymes. To gain understanding of iron nitride reactivity and the possible role of such species in biocatalysis, insight into the molecular and electronic structure of complexes stabilizing the [FeN] synthon is highly desirable. Whereas significant progress has been made in the synthesis and spectroscopic elucidation of Fe=NR and Fe?N species, X-ray crystallographic characterization of a complex with a terminal Fe?N functionality has not been accomplished. The first mononuclear Fe(IV)=O entity crystallographically characterized was stabilized in an octahedral environment provided by a macrocyclic tetra-N-methylated cyclam ligand. Similar cyclam derivatives also allow the stabilization and detailed spectroscopic characterization of octahedral Fe(V) and Fe(VI) nitride complexes in unusually high oxidation states. Recently, Peters and Betley developed a stunningly redox-rich iron system employing the tripodal tris(phosphino)borate ligand system (PhB^RP₃ ^?), which stabilizes tetrahedral L₃Fe=N_x species in oxidation states ranging from +I to +IV. Remarkably, this ligand system enabled the first room-temperature spectroscopic characterization of a terminal Fe(IV) nitride species. Concentration-dependent coupling to the Fe(I)–N₂–Fe(I) dinuclear product, however, prevents crystallization of this nitride species.