Iron(iii) activation hits a [4 + 4] macrocycle

Abstract

The tetranuclear complex [Fe^III₂(L′)(OH)(CH₃O)]₂, 1, has been synthesised from the reaction of either ferrous [in excess as 4 ∶ 1 or stoichiometric 2 ∶ 1 iron(II) ∶ H₄L] or ferric ions [4 ∶ 1 iron(III) ∶ H₄L] with the large macrocycle, H₄L, using aerobic conditions in methanol in the presence of triethylamine. The structure of 1 was determined by single-crystal X-ray diffraction. These reaction conditions lead to the modification of the original macrocycle through the incorporation of a methylene group between two amine groups to give an imidazolidine ring in (L′)⁴⁻. The controlled addition of formaldehyde into the reaction system results in a significantly improved yield of 1, suggesting that it is involved in the reaction mechanism. The (L′)⁴⁻ macrocycle binds to two, well-separated, iron(III) centres [Fe(1)⋯Fe(1a) > 8 Å]. Each iron(III) centre is further linked via hydroxy and methoxy bridges to equivalent iron(III) centres contained in a second macrocycle. Overall this gives a structure containing two {Fe(OH)(CH₃O)Fe} dimers [Fe(1)⋯Fe(2) ca. 3.2 Å] sandwiched by two (L′)⁴⁻ macrocycles. The complex was further characterised by SQUID magnetic measurements and can be interpreted in terms of two isolated antiferromagnetically coupled Fe(III) dimers (J = −23.75 K).