High spin d5 complexes of tris(6-hydroxymethyl-2-pyridylmethyl)amine (H3L): hepta-coordinated [Mn(H3L)]Cl2 and linear trinuclear [Fe3L2](ClO4)3

Abstract

Reaction of MnCl₂·4H₂O with H₃L (H₃L = tris(6-hydroxymethyl-2-pyridylmethyl)amine) in methanol gives hepta-coordinated [Mn(H₃L)]Cl₂ involving attachment of Mn(II) to all four nitrogens and three hydroxymethyl arms. Reaction of H₃L with Fe(ClO₄)₂·6H₂O in CH₃CN in the presence of NaO₂CC₆H₅ in an attempt to make [Fe^IIIOH(H₃L)(O₂CC₆H₅)](ClO₄), a putative model for soybean lipoxygenase-1, instead gave rise to the linear triiron(III) complex [Fe₃L₂](ClO₄)₃ with all three hydroxymethyl arms deprotonated and forming three alkoxide bridges between each Fe(III) centre. The central Fe(III) is hexa-coordinated to only the alkoxide bridges and flanked by two hepta-coordinated iron(III) centres analogous to the Mn(II) complex. [Fe₃L₂](ClO₄)₃ exhibits two reversible 1e^– reductions to mixed-valence [Fe₃L₂]²⁺ and [Fe₃L₂]⁺ forms. Structure data and magnetochemistry on [Fe₃L₂](ClO₄)₃ reveals the tightest Fe–O–Fe angle (87.4°) and shortest Fe⋯Fe distance (2.834 Å) yet found for any weakly antiferromagnetically-coupled high spin alkoxide-bridged di- or triiron(III) system and challenges current theories involved in correlating the extent/nature of magnetic interactions in such systems based on Fe–O(bridge) distances and Fe–O–Fe angles. The central hexa-alkoxide coordinated Fe(III) is novel and shows a remarkable resistance towards reduction to Fe(II).