Synthesis, structural and magnetic properties of oxo-, chloroacetato-bridged tetra-nuclear iron(III) complex

Abstract Oxo- and chloroacetato-bridged tetra-nuclear iron(III) complex [Fe 4 O 2 (ClCH 2 COO) 8 (bpy) 2 ]·H 2 O, where bpy = 2,2′-bipyridine, has been synthesized and characterized on the basis of X-ray crystallography, elemental analysis, cyclic voltammetric, UV–vis and IR spectroscopic techniques. X-ray diffraction analysis reveals that the complex crystallizes in the monoclinic space group P2/n with a  = 9.629(5) A, b  = 13.742(5), c  = 20.437(5) A, α  =  γ  = 90.000(5)°, β  = 99.792(5)°, V  = 2664.9(18) A 3 and Z  = 2. The tetra-nuclear entity consists of a [Fe 4 ( μ 3 -O) 2 ] 8+ unit comprising four Fe III atoms with a “butterfly” arrangement. Each pair of iron(III) atoms occupy the “hinge” or “body” sites, and “wing-tip” sites, respectively. It undergoes two stepwise one electron reductions, one is quasi-reversible at E 1/2  = +0.061 V vs Ag/AgCl (Δ E p  = 0.082 V) and the other is irreversible at E P.C  = −0.38 V at a scan rates 0.1 V s −1 . Variable-temperature magnetic susceptibility data reveals strong antiferromagnetic exchange interactions among the four high-spin Fe III ions. The exchange coupling constant J bb (body–body interaction) is indeterminate due to prevailing spin frustration, but the ‘wing-body’ antiferromagnetic interaction ( J wb ) was evaluated as −115 cm −1 , using the spin Hamiltonion model H  = − J wb ( S 1 ⋅ S 2  +  S 2 ⋅ S 1  +  S 1′ ⋅ S 2′  +  S 2′ ⋅ S 1 ) – J bb ( S 2 ⋅ S 2′ ).