Versatile Reactivity of a Solvent-Coordinated Diiron(II) Compound: Synthesis and Dioxygen Reactivity of a Mixed-Valent FeIIFeIII Species

A new, DMF-coordinated, preorganized diiron compound [Fe-2(N-Et-HPTB)(DMF)(4)](BF4)(3) (1) was synthesized, avoiding the formation of [Fe(N-Et-HPTB)](BF4)(2) (10) and [Fe-2(N-Et-HPTB)(mu-MeCONH)](BF4)(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-2(N-Et-HPTB)(MeCN)(4)](BF4)(3) (2), substitution to afford [Fe-2(N-Et-HPTB)(mu-RCOO)](BF4)(2) (3, R = Ph; 4, RCOO = 4-methyl-2,6-diphenyl benzoate]), one-electron oxidation by (Cp2Fe)(BF4) to yield a Robin-Day class II mixed-valent diiron(II,III) compound, [Fe-2(N-Et-HPTB)(mu-PhCOO)(DMF)(2)](BF4)(3) (5), two-electron oxidation with tris(4-bromophenyl)aminium hexachloroantimonate to generate [Fe-2(N-Et-HPTB)Cl-3(DMF)](BF4)(2) (6), reaction with (2,2,6,6-tetramethylpiperidin-1-yl)oxyl to form [Fe-5(N-Et-HPTB)(2)(mu-OH)(4)(mu-O)(DMF)(2)](BF4)(4) (7), and reaction with dioxygen to yield an unstable peroxo compound that decomposes at room temperature to generate [Fe-4(N-Et-HPTB)(2)(mu-O)(3)(H2O)(2)](BF4)center dot 8DMF (8) and [Fe-4(N-Et-HPTB)(2)(mu-O)(4)](BF4)(2) (9). Compound 5 loses its bridging benzoate ligand upon further oxidation to form [Fe-2(N-Et-HPTB)(OH)(2)(DMF)(2)](BF4)(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, Mossbauer spectroscopy, EPR spectroscopy, resonance Raman spectroscopy, and cyclic voltammetry.