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

Dutta, Sujit K. ; Ghosh, Meenakshi ; Biswas, Papu ; Florke, Ulrich ; Saal, Cristoph ; Haase, Wolfgang ; Nag , Kamalaksha (2007) Formation of oxo-bridged tetrairon(III) complexes mediated by oxygen activation. Structure, spectroscopy, magnetism and electrochemistry New Journal of Chemistry (1). pp. 93-101. ISSN 1144-0546

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Official URL: http://pubs.rsc.org/en/content/articlelanding/2007...

Related URL: http://dx.doi.org/10.1039/B610649H

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 [FeII2L(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 [FeII2L(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 [{FeIII2L(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 [{FeIILFeIII}2(µ-O)2]2+, followed by two one-electron two-proton coupled reactions to [FeIILFeII]2+.

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