Iron−Arylimide Clusters [Fe<i>_m</i>(NAr)<i>_n</i>Cl₄]^2- (<i>m</i>, <i>n</i> = 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 [Fe2(μ-Cl)3(THF)6]2[Fe3(μ-NPh)4Cl4] (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 2/LiCl with arylamine provides a practical route to the following Fe−NR clusters: [Li2(THF)7][Fe3(μ-NPh)4Cl4] (5a), which contains the same Fe−NR cluster found in 3; [Li(THF)4]2[Fe3(μ-N-p-Tol)4Cl4] (5b); [Li(DME)3]2[Fe2(μ-NPh)2Cl4] (6a); [Li2(THF)7][Fe2(μ-NMes)2Cl4] (6c). [Li(DME)3]2[Fe4(μ3-NPh)4Cl4] (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, 1H NMR, and Mössbauer 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.