Versatile Reactivity of a Solvent-Coordinated Diiron(II) Compound: Synthesis and Dioxygen Reactivity of a Mixed-Valent Fe [superscript II] Fe [superscript III] Species

Abstract

A new, DMF-coordinated, preorganized diiron compound [Fe[subscript 2](N-Et-HPTB)(DMF)[subscript 4]](BF[subscript 4])[subscript 3] (1) was synthesized, avoiding the formation of [Fe(N-Et-HPTB)](BF4)2 (10) and [Fe2(N-Et-HPTB)(μ-MeCONH)](BF[subscript 4])[subscript 2] (11), where N-Et-HPTB is the anion of N,N,N′,N′-tetrakis[2-(1-ethylbenzimidazolyl)]-2-hydroxy-1,3-diaminopropane. Compound 1 is a versatile reactant from which nine new compounds have been generated. Transformations include solvent exchange to yield [Fe[subscript 2](N-Et-HPTB)(MeCN)[subscript 4]](BF[subscript 4])[subscript 3] (2), substitution to afford [Fe[subscript 2](N-Et-HPTB)(μ-RCOO)](BF[subscript 4])[subscript 2] (3, R = Ph; 4, RCOO = 4-methyl-2,6-diphenyl benzoate]), one-electron oxidation by (Cp[subscript 2]Fe)(BF[subscript 4]) to yield a Robin–Day class II mixed-valent diiron(II,III) compound, [Fe[subscript 2](N-Et-HPTB)(μ-PhCOO)(DMF)[subscript 2]](BF[subscript 4])[subscript 3] (5), two-electron oxidation with tris(4-bromophenyl)aminium hexachloroantimonate to generate [Fe[subscript 2](N-Et-HPTB)Cl[subscript 3](DMF)](BF[subscript 4])[subscript 2] (6), reaction with (2,2,6,6-tetramethylpiperidin-1-yl)oxyl to form [Fe[subscript 5](N-Et-HPTB)[subscript 2](μ-OH)[subscript 4](μ-O)(DMF)[subscript 2]](BF[subscript 4])[subscript 4] (7), and reaction with dioxygen to yield an unstable peroxo compound that decomposes at room temperature to generate [Fe[subscript 4](N-Et-HPTB)2(μ-O)[subscript 3](H[subscript 2]O)[subscript 2]](BF[subscript 4])·8DMF (8) and [Fe[subscript 4](N-Et-HPTB)[subscript 2](μ-O)[subscript 4]](BF[subscript 4])[subscript 2] (9). Compound 5 loses its bridging benzoate ligand upon further oxidation to form [Fe[subscript 2](N-Et-HPTB)(OH)[subscript 2](DMF)[subscript 2]](BF[subscript 4])[subscript 3] (12). Reaction of the diiron(II,III) compound 5 with dioxygen was studied in detail by spectroscopic methods. All compounds (1–12) were characterized by single-crystal X-ray structure determinations. Selected compounds and reaction intermediates were further examined by a combination of elemental analysis, electronic absorption spectroscopy, Mössbauer spectroscopy, EPR spectroscopy, resonance Raman spectroscopy, and cyclic voltammetry.