Iron-arylimide clusters [Fem(NAr)nCl4]2- (m, n=2, 2; 3, 4; 4, 4) from a ferric amide precursor:: Synthesis, characterization, and comparison to Fe-S chemistry

Tetrahedral FeCl[N(SiMe3)(2)](2)(THF) (2), prepared from FeCl3 and 2 equiv of Na[N(SiMe3)(2)] in THF, is a useful ferric starting material for the synthesis of weak-field iron-imide (Fe-NR) clusters. Protonolysis of 2 with aniline yields azobenzene and [Fe-2(mu-Cl)(3)(THF)(6)](2)[Fe-3(mu-NPh)(4)Cl-4] (3), a salt composed of two diferrous monocations and a trinuclear dianion with a formal 2 Fe(III)/1 Fe(IV) oxidation state. Treatment of 2 with LiCl, which gives the adduct [FeCl2{N(SiMe3)(2)}(2)](-) (isolated as the [Li(TMEDA)(2)](+) salt), suppresses arylamine oxidation/iron reduction chemistry during protonolysis. Thus, under appropriate conditions, the reaction of 1:1 210 with arylamine provides a practical route to the following Fe-NR clusters: [Li-2(THF)(7)][Fe-3(mu-NPh)(4)Cl-4] (5a), which contains the same Fe-NR cluster found in 3; [Li(THF)(4)](2)[Fe-3(mu-N-p-Tol)(4)Cl-4] (5b); [Li(DME)(3)](2)[Fe-2(mu-NPh)(2)Cl(4)J (6a); [Li-2(THF)(7)][Fe-2(mu-NMeS)(2)Cl-4] (6c). [Li(DME)(3)](2)[Fe-4(mu(3)-NPh)(4)Cl-4] (7), a trace product in the synthesis of 5a and 6a, forms readily as the sole Fe-NR complex upon reduction of these lower nuclearity clusters. Products were characterized by X-ray crystallographic analysis, by electronic absorption, H-1 NMR, and Mossbauer spectroscopies, and by cyclic voltammetry. The structures of the Fe-NR complexes derive from tetrahedral iron centers, edge-fused by imide bridges into linear arrays (5a,b; 6a,c) or the condensed heterocubane geometry (7), and are homologous to fundamental iron-sulfur (Fe-S) cluster motifs. The analogy to Fe-S chemistry also encompasses parallels between Fe-mediated redox transformations of nitrogen and sulfur ligands and reductive core conversions of linear dinuclear and trinuclear clusters to heterocubane species and is reinforced by other recent examples of iron- and cobalt-imide cluster chemistry. The correspondence of nitrogen and sulfur chemistry at iron is intriguing in the context of speculative Fe-mediated mechanisms for biological nitrogen fixation.