Using pseudohalides (NCS–, N3–) as a probe for the active site of (µ-alkoxo)diiron(III) complexes and to reveal a novel asymmetrical structure

Abstract

A series of (µ-alkoxo)diiron(III) complexes [Fe₂(L)Cl₄]Cl·2H₂O 1, [Fe₂(L)(NCS)_n(Cl)_4–n]Cl· 4H₂O 2–5 (n = 1–4) and [Fe₂(L)(N₃)_n(Cl)_4–n] NO₃·3H₂O 6–9 (n = 1–4), where HL is N,N,N′,N′-tetrakis(2-benzimidazolylmethyl)-2-hydroxy-1,3-diaminopropane, have been synthesized and their structures, magnetic and redox properties and Mössbauer spectra have been investigated. Three complexes 1, 3 and 9 were characterized by single crystal structure analysis. The structures of 1 and 9 are geometrically symmetric, but 3 is asymmetric. The Mössbauer spectra of all complexes are typical of high-spin (µ-alkoxo)diiron(III) complexes. Complexes 2–9 exhibit intense IR bands (2022–2081 cm^–1) which are characteristic of terminally bound thiocyanate (N-bound) and azide ligands. The E_1/2 and the ΔE_pc of 1, 3 and 9 increase with π-donor effectiveness of the exogenous ligands in the following order: NCS^– < Cl^– < N₃^–. The –J (300–4.2 K) of 1, 3 and 9 are in the range of 13.3–16.4 cm^–1, indicating that the iron(III) sites are antiferromagnetically coupled.