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⁺.