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

► Tetra-nuclear oxo- and chloroacetato-bridged iron(III) complex, from [Fe3O]+7 core. ► The tetra-nuclear entity consists of a [Fe4(μ3-O)2]8+ “bent-butterfly” unit. ► X-ray crystallography displays C2 axis perpendicular to the central Fe2O2 rhomboid. ► Four iron(III) centers exhibits strong antiferromagnetic exchange interactions. ► FeIII atoms on body/body site suffers frustration in the spin alignment with Jbb=0. 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=+0.061V vs Ag/AgCl (ΔEp=0.082V) and the other is irreversible at EP.C=−0.38V at a scan rates 0.1Vs−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 −115cm−1, using the spin Hamiltonion model H=−Jwb (S1⋅S2+S2⋅S1+S1′⋅S2′+S2′⋅S1) –Jbb(S2⋅S2′).