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 = L(1)1, L(2)2 or L(3)3) consisting of a bridging ligand (tdim = 1,1'-tetramethylenediimidazole) and a quadridentate Schiff base ligand have been prePared and characterized, where H-2L(n) = RN(CH2)2NR' [R = R' = MeC(OH)=CHCMe=, n = 1; R = R'= o-HOC6H4CH=,n = 3; R = MeC(OH)=CHCMe=, R'= o-HOCH4CH=,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 reversible 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, Mossbauer and ESR spectra.