Synthesis and magnetic properties of iron(III) complexes bridged by 1,1′-tetramethylenediimidazole exhibiting an infinite zigzag-chain structure

Three polymeric iron(III) complexes [Fe(tdim)Lⁿ]ClO₄ (L = L¹ 1, L² or L³ 3) consisting of a bridging ligand (tdim = 1,1′-tetramethylenediimidazole) and a quadridentate Schiff base ligand have been prepared and characterized, where H₂Lⁿ = RN(CH₂)₂NR′ [R = R′ = MeC(OH)=CHCMe=, n = 1; R = R′ = o-HOC₆H₄CH=, n = 3; R = MeC(OH)=CHCMe=, R′ = o-HOC₆H₄CH=, n = 2]. Magnetic susceptibility data (4.2-300 K) showed that 1 is low spin (S = 1/2), 2 of spin equilibrium (S = 5/2 ⇌ 1/2), and 3 essentially high spin (S = 5/2) and exhibiting an unusual behaviour below ca. 100 K. The spin equilibrium of 2 occurs over a wider temperature range than that of the corresponding mononuclear complex [Fe(mim)₂L²]ClO₄ (mim = 1-methylimidazole). A crystal structure analysis of 3 confirmed that it has an infinite zigzag-chain structure and the iron(III) ion is six-co-ordinated by N₂O₂ donor atoms of the quadridentate ligand, an imidazole nitrogen atom of a bridging ligand and a nitrogen atom of the adjacent bridging ligand. The crystal structure also shows that 3 suffers from disorder and there are two configurations A and B with respect to the orientation of the imidazole moiety, having populations of 0.65 and 0.35, respectively. The unusual magnetic behaviour is rationalized by the crystal structure, Mössbauer and ESR spectra.