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

Oxo- and chloroacetato-bridged tetra-nuclear iron(III) complex [Fe4O2(ClCH2COO)8(bpy)2]·H2O, 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) Å, b = 13.742(5), c = 20.437(5) Å, α = γ = 90.000(5)°, β = 99.792(5)°, V = 2664.9(18) Å3 and Z = 2. The tetra-nuclear entity consists of a [Fe4(μ3-O)2]8+ unit comprising four FeIII 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 E1/2 = +0.061 V vs Ag/AgCl (ΔEp = 0.082 V) and the other is irreversible at EP.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 FeIII ions. The exchange coupling constant Jbb (body–body interaction) is indeterminate due to prevailing spin frustration, but the ‘wing-body’ antiferromagnetic interaction (Jwb) was evaluated as −115 cm−1, using the spin Hamiltonion model H = −Jwb (S1⋅S2 + S2⋅S1 + S1′⋅S2′ + S2′⋅S1) – Jbb(S2⋅S2′).