SYNTHESIS AND CHARACTERIZATION OF NEUTRAL HEXANUCLEAR IRON-SULFUR CLUSTERS CONTAINING STAIR-LIKE [FE-6(MU(3)-S)(4)(MU(2)-SR)(4)] AND NEST-LIKE [FE-6(MU(3)-S)(2)(MU(2)-S)(2)(MU(4))(MU(2)-SR)(4)] CORE STRUCTURES

The binuclear iron complex [{Fe(''EtN2S2'')}(2)] (1b, ''EtN2S2'' = N,N '-diethyl-3,7-diaZanonane-1,9-dithiolate) was prepared from the free l igand and ferrous bis[bis(trimethylsilyl)amide] in toluene. In dichlor omethane 1b reacts with the [Fe4S4I4](2-) cubane cluster to displace t wo iodo ligands and to form the neutral hexanuclear cluster [{Fe(''EtN 2S2'')}(2)Fe4S4I2] (2), which is isolated as black crystals in 87% yie ld. As elucidated by an X-ray structure analysis, 2 contains the novel hexanuclear stair-like [Fe-6 (mu(3)-S)(4)(mu(2)-SR)(4)] core, which e xhibits crystallographic inversion symmetry. The compound crystallizes as a solvate with two molecules of CH2Cl2 per formula unit in the mon oclinic 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 po lar solvents DMF, 1,2-propylenecarbonate, and DMSO, 2 dissolves with d ecomposition and formation of the cluster [{Fe(''EtN2S2'')}(2)Fe4S5] ( 3), which is isolated as black needles from DMF. 3.2DMF crystallizes i n the triclinic space group P (1) over bar with a = 950.9(1) pm, b = 1 086.0(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 co ntains a nest-like e-6(mu(4)-S)(mu(3)-S)(2)(mu(2)-S)(2)(mu(2)-SR)(4)] core of idealized C-2v symmetry that is closely related to that of oth er well-known clusters, e.g., the cluster anion [Fe6S9(SR)(2)](4-). Th e zero-field Fe-57 Mossbauer spectrum of 3 is in accordance with four Fe2.5+S4 centers (delta = 0.46 mm/s; Delta E-Q = 1.14 mm/s) and two N2 S3-bound high-spin Fe2+ sites (delta = 0.83 mm/s; Delta E-Q = 3.64 mm/ s). A total cluster spin of 0 is deduced from the Mossbauer spectrum a t 4.2 K and 5.3 T, which yields magnetic splitting from the applied fi eld only. For 2, three subspectra are observed in the Mossbauer spectr um (a, delta = 0.45 mm/s, Delta E-Q = 1.05 mm/s; b, delta = 0.55 mm/s, Delta E-Q = 1.61 mm/s, c, delta = 0.80 mm/s; Delta E-Q = 3.83 mm/s) r eflecting 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 reve rsibly oxidized in dichloromethane solution at -75 mV (vs SCE). Wherea s 2 shows only an irreversible redox behavior in N,N-dimethylimidazoli din-2-one solution, 3 in the same solvent can be quasi-reversibly redu ced in two consecutive steps at -830 and -1630 mV (vs SCE) to the dian ion, which consists entirely of Fe(II).