Synthesis and Characterization of Neutral Hexanuclear Iron Sulfur Clusters Containing Stair-like [Fe6(μ3-S)4(μ2-SR)4] and Nest-like [Fe6(μ3-S)2(μ2-S)2(μ4-S)(μ2-SR)4] Core Structures

Abstract

The binuclear iron complex [{Fe(“EtN₂S₂”)}₂] (1b, “EtN₂S₂” = N,N‘-diethyl-3,7-diazanonane-1,9-dithiolate) was prepared from the free ligand and ferrous bis[bis(trimethylsilyl)amide] in toluene. In dichloromethane 1b reacts with the [Fe₄S₄I₄]^2- cubane cluster to displace two iodo ligands and to form the neutral hexanuclear cluster [{Fe(“EtN₂S₂”)}₂Fe₄S₄I₂] (2), which is isolated as black crystals in 87% yield. As elucidated by an X-ray structure analysis, 2 contains the novel hexanuclear stair-like [Fe₆(μ₃-S)₄(μ₂-SR)₄] core, which exhibits crystallographic inversion symmetry. The compound crystallizes as a solvate with two molecules of CH₂Cl₂ per formula unit in the monoclinic space group P2₁/n with a = 1570.5(2) pm, b = 1060.2(1) pm, c = 1604.0(2) pm, β = 114.93(1)°, and Z = 2. In the aprotic polar solvents DMF, 1,2-propylenecarbonate, and DMSO, 2 dissolves with decomposition and formation of the cluster [{Fe(“EtN₂S₂”)}₂Fe₄S₅] (3), which is isolated as black needles from DMF. 3·2DMF crystallizes in the triclinic space group P1̄ with a = 950.9(1) pm, b = 1086.0(1) pm, c = 2381.5(2) pm, α = 101.81(1)°, β = 91.94(1)°, γ = 97.01(1)°, and Z = 2. The neutral compound contains a nest-like [Fe₆(μ₄-S)(μ₃-S)₂(μ₂-S)₂(μ₂-SR)₄] core of idealized C_2v symmetry that is closely related to that of other well-known clusters, e.g., the cluster anion [Fe₆S₉(SR)₂]^4-. The zero-field ⁵⁷Fe Mössbauer spectrum of 3 is in accordance with four Fe^2.5+S₄ centers (δ = 0.46 mm/s; ΔE_Q = 1.14 mm/s) and two N₂S₃-bound high-spin Fe²⁺ sites (δ = 0.83 mm/s; ΔE_Q = 3.64 mm/s). A total cluster spin of 0 is deduced from the Mössbauer spectrum at 4.2 K and 5.3 T, which yields magnetic splitting from the applied field only. For 2, three subspectra are observed in the Mössbauer spectrum (a, δ = 0.45 mm/s, ΔE_Q = 1.05 mm/s; b, δ = 0.55 mm/s, ΔE_Q = 1.61 mm/s, c, δ = 0.80 mm/s; ΔE_Q = 3.83 mm/s) reflecting different coordination environments of the iron atoms rather than different oxidation states. The electrochemical properties of 1b, 2, and 3 were determined by cyclic voltammetry. 1b can be quasi-reversibly oxidized in dichloromethane solution at −75 mV (vs SCE). Whereas 2 shows only an irreversible redox behavior in N,N‘-dimethylimidazolidin-2-one solution, 3 in the same solvent can be quasi-reversibly reduced in two consecutive steps at −830 and −1630 mV (vs SCE) to the dianion, which consists entirely of Fe(II).