Synthesis and Characterization of Neutral Hexanuclear Iron Sulfur Clusters Containing Stair-like...

The binuclear iron complex [{Fe("EtN(2)S(2)")}(2)] (1b, "EtN(2)S(2)" = 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 [Fe(4)S(4)I(4)](2-) cubane cluster to displace two iodo ligands and to form the neutral hexanuclear cluster [{Fe("EtN(2)S(2)")}(2)Fe(4)S(4)I(2)] (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 [Fe(6)(&mgr;(3)-S)(4)(&mgr;(2)-SR)(4)] core, which exhibits crystallographic inversion symmetry. The compound crystallizes as a solvate with two molecules of CH(2)Cl(2) per formula unit in the monoclinic space group P2(1)/n with a = 1570.5(2) pm, b = 1060.2(1) pm, c = 1604.0(2) pm, beta = 114.93(1) degrees, and Z = 2. In the aprotic polar solvents DMF, 1,2-propylenecarbonate, and DMSO, 2 dissolves with decomposition and formation of the cluster [{Fe("EtN(2)S(2)")}(2)Fe(4)S(5)] (3), which is isolated as black needles from DMF. 3.2DMF crystallizes in the triclinic space group P&onemacr; with a = 950.9(1) pm, b = 1086.0(1) pm, c = 2381.5(2) pm, alpha = 101.81(1) degrees, beta = 91.94(1) degrees, gamma = 97.01(1) degrees, and Z = 2. The neutral compound contains a nest-like [Fe(6)(&mgr;(4)-S)(&mgr;(3)-S)(2)(&mgr;(2)-S)(2)(&mgr;(2)-SR)(4)] core of idealized C(2)(v)() symmetry that is closely related to that of other well-known clusters, e.g., the cluster anion [Fe(6)S(9)(SR)(2)](4-). The zero-field (57)Fe Mössbauer spectrum of 3 is in accordance with four Fe(2.5+)S(4) centers (delta = 0.46 mm/s; DeltaE(Q) = 1.14 mm/s) and two N(2)S(3)-bound high-spin Fe(2+) sites (delta = 0.83 mm/s; DeltaE(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, delta = 0.45 mm/s, DeltaE(Q) = 1.05 mm/s; b, delta = 0.55 mm/s, DeltaE(Q) = 1.61 mm/s, c, delta = 0.80 mm/s; DeltaE(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).