Synthesis and characterization of a monomeric and a dihydroxo-bridged dimeric complexes of iron(III) with α-benzoinoxime

The reaction of α-benzoinoxime, H2BNO with FeCl3 in the presence of Et3N as a base gives the mononuclear Fe(III) complex, Fe(HBNO)3 (1). Treatment of 1 with a methanolic solution of KOH at room temperature leads to a dinuclear Fe(III)–Fe(III) complex, [Fe(HBNO)2OH]2 (2). The complexes were initially characterized on the basis of their elemental, mass and thermal analyses. The IR studies were useful in assigning the coordination mode of the benzoinoxime ligand to the iron metal. In addition, the presence of a hydroxo-bridge in the dimeric complex 2 is inferred from the IR spectral studies. Room-temperature Mössbauer studies indicated octahedral, high-spin iron(III). Variable-temperature magnetic susceptibility measurements supported the existence of the μ-dihydroxo-bridging structure core, FeIII(μ-OH)2FeIII in the dinuclear complex 2. Theoretical modelling of the magnetic data indicated a weak antiferromagnetic spin exchange between the iron(III) centers (J = −8.35 cm−1, g = 2.01, ρ = 0.02 and TIP = 1.7 × 10−4 cm3 mol−1 for H = −2JS1 · S2). The electronic spectra of the complexes revealed two bands due to d–d transitions and one band assignable to an oxygen (pπ) → Fe(dπ∗) LMCT transition observed in each complex. An additional charge-transfer transition, assignable to μ-hydroxo(pπ) → Fe(dπ∗), was observed for the dimeric complex 2. The structural and vibrational behaviors of these complexes have been elucidated with quantum mechanical methods.

Graphical abstractA mononuclear Fe(III) complex, Fe(HBNO)3 (1), and a dinuclear Fe(III)–Fe(III) complex, [Fe(HBNO)2OH]2 (2), have been synthesized and characterized. Theoretical modelling of the magnetic data indicate a weak antiferromagnetic spin exchange between the iron(III) centers of the dimeric complex (J = −8.35). The structural and vibrational behaviors of the two complexes have been elucidated with a quantum mechanical PM3 routine.Figure optionsDownload full-size imageDownload as PowerPoint slide