Reactivity and Structural and Physical Studies of Tetranuclear Iron(III) Clusters Containing the [Fe₄(μ₃-O)₂]⁸⁺ “Butterfly” Core: an Fe^III₄ Cluster with an <i>S</i> = 1 Ground State

The initial use of di-2-pyridyl ketone oxime [(py)2CNOH] in iron(III) carboxylate chemistry has yielded the tetranuclear complex [Fe4O2Cl2(O2CMe)2{(py)2CNO}4] (1). Compound 1 can be synthesized either by the 1:2:1 molar ratio reaction between FeIII, MeCO2-, and (py)2CNOH (complex 1a) or by the 1:3 molar ratio reaction between [Fe3O(O2CMe)6(H2O)3]Cl and (py)2CNOH (complex 1b). The presence of N3- in both reaction mixtures has afforded the tetranuclear complex [Fe4O2(N3)2(O2CMe)2{(py)2CNO}4] (2), which can be alternatively synthesized by the reaction of 1 with N3-. Compound 1a crystallizes in the tetragonal space group I4̄ with (at 25 °C) a = 35.06(2) Å, b = 35.06(2) Å, c = 13.255(6) Å, V = 16293(2) Å3, and Z = 8. Compound 1b crystallizes in the monoclinic space group P21/c with (at 25 °C) a = 22.577(7) Å, b = 17.078(6) Å, c = 17.394(6) Å, β = 93.50(1)°, V = 6694(4) Å3, and Z = 4. Compound 2 crystallizes in the triclinic space group P1̄ with (at 25 °C) a = 13.658(9) Å, b = 15.815(9) Å, c = 17.29(1) Å, α = 97.08(3)°, β = 98.55(3)°, γ = 112.12(3)°, V = 3355(4) Å3, and Z = 2. The structures of 1 and 2 contain the [Fe4(μ3-O)2]8+ core comprising four FeIII ions in a “butterfly” disposition and two μ3-O2- ions, each bridging three FeIII ions forming the “wings” of the “butterfly”. The Mössbauer spectra of 1b and 2 consist of composite quadrupole-split doublets, with parameters typical for high-spin FeIII in octahedral environments. Magnetic susceptibility measurements on 1a revealed antiferromagnetic interactions between the S = 5/2 ferric ions, with best-fit parameters being Jwb = −40.2 cm-1 and Jbb = −59.4 cm-1 (H = −2∑JiJj) for wingtip−body and body−body interactions, respectively, yielding an S = 1 ground state. Both wingtip−body and body−body interactions are well determined.