Formation of oxo-bridged tetrairon(iii) complexes mediated by oxygen activation. Structure, spectroscopy, magnetism and electrochemistry

Abstract

The reaction involving 2,6-diformyl-4-methylphenol, 1,3-diaminopropane, iron(II) perchlorate and sodium acetate in the ratio (2 : 2 : 2 : 3) in methanol produces the acetate-bridged macrocyclic diiron complex [Fe^II₂L(μ-O₂CCH₃)(CH₃OH)₂](ClO₄) (1). The direct reaction between the perchlorate salt of the macrocyclic ligand [LH₄](ClO₄)₂ and iron(II) perchlorate in the presence of a mixture RCO₂H and triethylamine also produces in solution the diiron complex [Fe^II₂L(μ-O₂CR)]⁺ (where R = H, CH₃, C₂H₅, (CH₃)₃C and C₆H₅), which on exposure to air readily transforms to the corresponding oxo-bridged tetrairon(III) complex [{Fe^III₂L(μ-O₂CR)}₂(μ-O)₂](ClO₄)₂ (2–6). The X-ray crystal structures of the acetate (3) and benzoate-bridged (6) complexes have been determined. Variable temperature (4–300 K) magnetic susceptibility measurements carried out for 3 and 6, have been analyzed in terms of molecular field approximation. The dominant antiferromagnetic exchange interaction (J) occurring through the oxo-bridged diiron(III) is −115 cm⁻¹ for 3 and −109 cm⁻¹ for 6, while the magnitude of spin exchange coupling occurring through the phenoxide bridges (J′) is −4 cm⁻¹ for both 3 and 6. Complexes 2–6 exhibit identical proton NMR spectra in CD₃CN, indicating that the capped carboxylate ligand gets detached from the metal centres. The Fe⋯H distances estimated from relaxation time (T₁ and T₂) measurements have been found to agree well with the crystallographic distances. Complexes 2–6 exhibit identical electrochemical behaviour and undergo reduction in three steps involving a first two-electron transfer reaction to [{Fe^IILFe^III}₂(μ-O)₂]²⁺, followed by two one-electron two-proton coupled reactions to [Fe^IILFe^II]²⁺.